ZnO cathode buffer layers for inverted polymer solar cells

This article provides an overview of the design, fabrication and characterization of the most widely used cathode buffer layers (CBLs) constructed using pristine zinc oxide (ZnO), doped-ZnO, and ZnO-based composites as well as the surface modified ZnO-based CBLs for the improvement of power conversion efficiency (PCE) and long-term device stability of inverted polymer solar cells (PSCs). To achieve high PCE in inverted PSCs, the selection of an appropriate material to form high quality CBLs so as to optimize the electron collection and transport is particularly important. ZnO has been the most extensively studied material for CBL of inverted PSCs in view of its relatively high electron mobility, optical transparency, ease of being synthesized with low cost solution methods at low temperature, versatile morphologies, and being environmentally stable. It is pointed out in this review that the electronic processes at the interface between the ZnO CBL and polymer active layer play an important role in determining the solar cells performance. This review attempts to deliver better understanding with regard of the impacts of (1) morphology, (2) thickness, (3) nanostructures, (4) doping, (5) surface modification and (6) composition/ hybrids of ZnO CBLs on the inverted PSCs performance. Well understanding the interfacial processes in PSCs is believed also a benefit to the emerging perovskite solar cells in view of their similar energy levels and device structures.


Introduction
Harnessing the abundant and renewable solar energy has been recognized as a promising way to address the quickly growing world's energy consumption and the increasing concerns about rapid increase in green-house gas emissions from the consumption of fossil fuels as the primary energy sources. Therefore, solar energy utilization is currently a leading research area for a CO 2 -free and renewable energy supply. Many approaches have been proposed and studied extensively to harvest solar energy including solar thermal energy, photocatalytic and photoelectrochemical water splitting, and solar cells. [1][2][3][4][5][6][7][8][9] Among these technologies, the photovoltaic technology of converting sunlight directly into electricity is considered as one of the most promising and mature technologies. [7][8][9][10] Although silicon or other inorganic semiconductor solar cells and thin-film solar cells have been the most used solar cell systems because of their advantages of high efficiency and reliability, the high cost and slow energy payback time of these solar cells have limited a Institute of Functional Nano & Soft Materials Laboratory (FUNSOM), their widespread applications. In recent years, polymer solar cells (PSCs) have attracted extensive interest as a potential alternative to conventional silicon-based solar cells, mainly due to their inherent advantages of being low-cost, and compatible with a flexible substrate and solution-based roll-to-roll processing technique. 8,9,[11][12][13][14] In addition, the rapid energy payback time (EPBT) and low carbon emissions are also the major motivation for PSCs in future energy marketplace. 15,16 Conventional PSC device architecture consists of a bulkheterojunction (BHJ) active layer sandwiched between a transparent conducting electrode, such as indium tin oxide (ITO) glass, and a low-work-function metal electrode (which usually uses the Al material). The BHJ active layer is achieved by a blend of the p-type polymer donor and n-type fullerene acceptor materials dissolved in common solvent, and subsequent phase segregation results in the formation of two interpenetrated percolated networks during the annealing process after spin-coating. In such active layers, only excitons formed within a distance of B20 nm from the p-type polymer donor/n-type fullerene derivative acceptor interface can reach the interface and then dissociate into free charge carriers. 13,[17][18][19] The design criterion for the BHJ is to maximize the donor/acceptor interfacial area to provide possibly more exciton dissociation sites and form two interpenetrated continuous charge transport pathways in each material towards the corresponding electrodes. 13,17,18 Another outstanding merit of BHJ-based polymer solar cells is that the composite active

Qifeng Zhang
Qifeng Zhang is currently working at the University of Washington as a Research Assistant Professor. His research interests involve engineering applications of nanostructured materials on electrical devices including solar cells, UV light-emitting diodes (LEDs), fieldeffect transistors (FETs), and gas sensors. His current research is focused on the synthesis of nanomaterials and the application of nanomaterials in electronic and optoelectronic devices, such as dye-sensitized solar cells (DSCs) and organic/inorganic hybrid solar cells. Qifeng Zhang can be reached at qfzhang@u.washington.edu. layer can be processed from a solution in a single step, making the fabrication of devices fully compatible with the roll-to-roll processing techniques. 13,18,20 Thus, the concept of BHJ was considered to establish the cornerstones of polymer solar cells. 21 In recent years, significant progress has been achieved on the improvement of the performance of PSCs.
The state of the art power conversion efficiencies (PCEs) of single junction PSCs with conventional structure exceeding 9%, [22][23][24] and higher than 10% 25 for tandem PSCs have been achieved in small area devices, which promises them a bright future in commercialization.
However, in spite of high PCEs, the PSCs with conventional architecture have suffered from some drawbacks that hinder the solar cells from practical applications. One of the major drawbacks of the conventional structure PSCs is the lack of long-term stability when exposed to air. [26][27][28][29] This is, on one hand, because the use of a low-work-function metal (e.g., Al) such as cathode, which is sensitive to air and moisture and leads to the oxidization of the cathode quickly. On the other hand, in the conventional structure PSCs, the acidic PEDOT:PSS interfacial layer directly contacts the ITO glass and can etch the ITO resulting in a degradation of the device performance. [26][27][28]30 This has inspired the emergence of inverted structure polymer solar cells, in which the nature of charge collection is reversed. 12,[31][32][33] Fig. 1a is a schematic showing an inverted polymer solar cell with a stacked structure of a transparent electrode (e.g., ITO, FTO and AZO), an interfacial layer named as the cathode buffer layer (CBL), a BHJ active layer, an anode buffer layer (ABL) usually made of PEDOT:PSS, and a metal electrode with high-workfunction such as Ag and Au. The structure of conventional PSCs is also schematically presented in Fig. 1b for comparison. It should be noted that in literature, the CBL is also named as the electron selective layer (ESL), the electron collection layer (ECL) or the electron transport layer (ETL), and the ABL is named as the hole selective layer (HSL), the hole collection layer (HCL) or the hole transport layer (HTL). For the sake of clarity and ease in presentation, the CBL and the ABL are used in this review unless specified otherwise.
In the inverted device architecture, the contact between the ITO and PEDOT:PSS is avoided, and meanwhile Al for the top electrode adopted in conventional PSCs is replaced with an air stable high-work-function metal, such as Au or Ag. 31,34 As a result, the inverted PSCs exhibit greatly improved ambient stability as compared to the PSCs with a conventional structure (Fig. 2), overcoming one big hurdle for possible widespread applications of PSCs. 32,33,[35][36][37][38] Moreover, in the case of inverted solar cells, the high-work-function metal anode, such as Ag, can be prepared using either the coating or printing technology. These technologies are compatible with all solution processing methods and can therefore greatly simplify the fabrication process and lower the manufacturing cost of solar cells. [39][40][41] Another advantage of the inverted geometry is that it possesses higher flexibility in terms of the design of multi-junction or tandem solar cells compared to the conventional one. 13,25 In addition, the vertical phase separation mechanism, which describes the P3HT prone to accumulate on top of the electrode and the fullerene derivative at the bottom, also makes the inverted device configuration more advantageous compared to the normal configuration. 21,34,[42][43][44] Despite the inverted PSCs having a lot of apparent merits, one shortcoming of the inverted device geometry is that the PCE is usually inferior to those of conventional PSCs in the initial stages of the study for inverted PSCs. But the study in recent years has overturned this situation and demonstrated that inverted PSCs can possess both long-term stability and high PCEs. 25,[45][46][47][48][49][50] It has been reported that, through device engineering and using of new polymers, the state of the art PCEs of single junction inverted PSCs exceeding 10% have been achieved in small area devices, which are comparable to and even exceed the efficiencies of conventional ones. 47,49,[51][52][53][54] The study of the high PCE, stability and the roll-to-roll process has become three major directions in polymer solar cells. 25,30,55,56 The inverted PSCs seem to be the best candidates that meet the requirements for high efficiency, good stability, low cost and being compatible with the existing roll-to-roll process. 12,34,55,56 The working mechanism of inverted PSCs can be described as follows. Under illumination, photo-generated excitons diffuse to the donor-acceptor (such as P3HT:PCBM) interface, where the  excitons dissociate due to a driving force in view of the energy level difference between the donor and the acceptor. The separated electrons will diffuse through the acceptor layer and are collected by the CBL, and ultimately reach the cathode electrode. 37 The CBLs in inverted devices are to (1) form an electron selective and transport interlayer, (2) block the reverse holes flowing from the donor polymer to the cathode, (3) adjust the energetic barrier between the active layer and the cathode, and (4) prevent the chemical or physical reaction between the active layer and the cathode. To achieve high power conversion efficiency in inverted polymer solar cells, the selection of an appropriate material to form the high quality CBL so as to optimize the electron collection and transport is particularly important. The key requirements for an ideal CBL include being good at electron extraction and transport, having a suitable energy level that facilitates electron transport, and having high transparency, good compactness, and high stability.
Recently, inorganic-organic organometal perovskite solar cells have emerged and are acknowledged as a new promising photovoltaic technology with respect to their tremendous progress in power conversion efficiency and potentially low-cost. In the past two years, the PCEs of perovskite solar cells have advanced from 15% to 19% and have now exceeded 20% on the basis of both planar and mesoscopic cell configurations. [57][58][59][60][61][62][63][64][65][66] However, perovskite solar cells also face the device stability problems, and this will determine their practical applications. 57,[67][68][69] Similar to the polymer solar cells, it has been found that the selection and modification of a suitable interface layer could substantially improve the efficiency and stability of perovskite solar cells. 68,[70][71][72][73] Therefore, the fundamental understanding of the rapid progress of interfacial engineering made in polymer solar cells would be beneficial to the development of perovskite solar cells, in addition to advancing the polymer solar cell itself. Compared to the existing photovoltaic (PV) technologies, perovskite solar cells exhibit very short energy payback time (EPBT). 74,75 Although perovskite solar cells are the youngest number in the PV family, they may become the most environmentally sustainable PV technology due to their potential for high efficiency, rapid EPBT and stable performance. 74,75 Over the past few years, many semiconducting metal oxides (MOs) including zinc oxide (ZnO), 29,31,33,37,76,77 zinc tin oxide (ZTO), 78 titanium sub-oxide (TiO x ), 35,36,[79][80][81][82] aluminum oxide (Al 2 O 3 ), 83 and niobium pentoxide (Nb 2 O 5 ) 84 have been studied to serve as the CBL in inverted PSCs. Besides the metal oxides, several polymers, such as hydrophilic conjugated 2,7-carba-zole-1,4-phenylene alternating copolymers PCP-NOH or PCP-EP, 85 cationic biopolymer poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA), 86 poly(amido amine) (PAMAM, generation 2), 86 poly(ethyleneimine) (PEI) 87 and conjugate polyelectrolyte poly-[(9,9-bis(3 0 -(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9ioctylfluorene)] (PFN), 45,51,88 and PFPA1 89 have also been studied for inverted PSCs to work as efficient CBL materials. In addition, some other materials such as ZnS, 90 CdS, 91 102 sodium hydroxide (NaOH), 103 zwitterions, 104 amino acid (serine), 105 amino-functionalized fluorene oligomers, 106 ionic liquids (ILs), 53 and the low work function metals (such as Al, Mg and Ca) 107 have also been investigated as CBLs in inverted PSCs. Another low work function metal, lanthanum (La), can also serve as a cathode interlayer material for PSCs with conventional structure. 108 Graphene as a promising two-dimensional nanomaterial with outstanding electronic, optical, thermal, and mechanical properties has also been used in inverted PSCs to serve as the CBL. [109][110][111] For inverted PSCs, the device performance critically relies on the type of the CBL material and the condition of the interface between the CBL and the BHJ active layer. Among the materials mentioned above, ZnO is the most extensively investigated one for CBLs in the inverted PSCs, mainly due to its suitable energy levels, high electron mobility, good transparency, environmental stability and low cost. 13,29,31,32,37,55,76,112,113 The energy levels of ZnO (conduction band bottom and valence band top) are at around À4.4 eV and À7.8 eV, respectively. Such band positions allow ZnO to function well for electron collection and hole blocking. The relatively high electron mobility of ZnO makes it a suitable material for cathode buffer layers to reduce the charge recombination. The good transparency in the whole visible spectrum benefits in lowering the optical loss and the band edge cut-off of ZnO at around 375 nm can block UV light and accordingly protect the organic materials from photodegradation under UV light irradiation. 33,43 For single junction inverted PSCs, the state of the art PCEs of B10% have been achieved by using ZnO or ZnO-based CBLs. 46,47,49,52,53,114 Another merit of ZnO as the CBL material is that it can be easily processed via a solution method with subsequent thermal treatment at relatively low temperatures. This makes the ZnO fully compatible to all solution roll-to-roll fabrication on flexible plastic substrates which is the predominant advantage of polymer solar cells. 29,55,[115][116][117][118] Almost all of the reported inverted PSCs fabricated using the roll-to-roll processing technique are based on ZnO thin layers as CBLs. 40,41,56,116,119,120 Due these features and merits mentioned above, ZnO has been identified as a good material to serve as the CBL in inverted PSCs.
Shirakawa et al. were the first group to demonstrate the application of a ZnO film as the cathode interfacial layer in an organic photovoltaic cell with a stacked structure of ITO/ZnO/ C60/PAT6/Au. 17 The first inverted polymer solar cells based on a sol-gel processed ZnO thin film on ITO as the CBL and a P3HT:PCBM BHJ active layer was pioneered by White and co-workers in 2006. 31 Significant progress has been made in the past few years in this area, with the research focused mainly on the preparation method and modification of ZnO buffer layers. However, due to the use of different polymers for the construction of solar cells and different methods for the fabrication of the cathode buffer layer, it is hard to make a fair comparison of the effects of the cathode buffer layer. In recent years, many papers have reviewed the photovoltaic materials of active layers and electrode buffer layers, cell architectures, interfacial layer engineering, and the device operation mechanisms of polymer solar cells. 7,8,[12][13][14]19,25,30,37,44,55,56,[121][122][123][124][125][126][127] However, as one of the most extensively used CBL in inverted PSCs, so far no paper has provided a comprehensive review on the progresses and perspectives of ZnO and ZnO based CBLs. This review article intends to fill this gap and provide a comprehensive overview of the design, fabrication, and characterization of ZnO, doped-ZnO, and ZnO-based composite CBLs as well as their surface chemistry and morphology modification for inverted PSCs. The development of ZnO cathode buffer layers will make an important contribution to the fabrication of PSCs with high power conversion efficiency and long-term stability at a large scale for their practical applications.

ZnO as a good fit for CBLs in inverted PSCs
In the polymer solar cells with inverted geometry, a ZnO film inserted between the BHJ active layer and the cathode can function as a CBL to extract and transport electrons, and simultaneously block the reverse flow of holes from the donor polymer to the cathode. The good fit of ZnO as a CBL in inverted PSCs first of all originates from its suitable energy levels. Fig. 3 shows a schematic diagram of the energy levels and the transport directions of electrons and holes in an inverted structure solar cell consisting of a ZnO cathode buffer layer and a P3HT:PCBM active layer. It can be seen that, on one hand, the conduction band bottom of ZnO at À4.4 eV is lower than the lowest unoccupied molecular orbital (LUMO) of the fullerene derivative acceptor (for example, À3.8 eV and À3.74 eV for PCBM and ICBA, respectively), meaning that ZnO CBLs can help in extracting and collecting electrons in the fullerene derivative acceptor. 37,128 It has been reported that the electron transport from PCBM to ZnO does not result in a significant energy loss. 31 On the other hand, the valence band top of ZnO at BÀ7.8 eV is lower than the highest occupied molecular orbital (HOMO) of the polymer donor (for example, À5.0 eV for P3HT). As a result, the reverse flow of holes from the polymer donor to the ITO cathode is blocked by the great energy barrier at the P3HT/ZnO interface.
This means that the ZnO CBL can prevent the generation of leakage current at the polymer/ITO interface. That is why the inverted PSCs with ZnO CBLs have been found to dramatically improve photovoltaic performance compared to those without a ZnO CBL. 33,43,77,129,130 In a study done by Liu et al., it was reported that the devices with and without ZnO CBLs presented different shapes of the J-V curve, as shown in Fig. 4. 77 Such a difference in the J-V curve was attributed to the converse directions of charge transport, suggesting the function of ZnO CBLs in transporting electrons and blocking holes. 77 Yang et al. have also studied inverted PSCs constructed with solution processed ZnO thin films as a CBL, PsiF-DBT:PCBM blend as a BHJ active layer, a MoO 3 thin film as an anode buffer layer (ABL), and an Au electrode as an anode. 33 It was found that, due to the use of ZnO, the series resistance (R S ) of solar cells decreased from 56 O cm 2 to 13 O cm 2 and the shunt resistance (R SH ) increased from 600 O cm 2 to 1400 O cm 2 giving rise to a dramatic enhancement in the solar cell PCE, which increases from 1.67% to 3.80%. A high R SH means a low current leakage or energy loss across the solar cell. 33,131 Table 1 shows a summary of device characteristics of representative inverted PSCs with or without ZnO CBLs. The values shown in brackets are obtained for the solar cells without CBLs, exhibiting much lower efficiencies than those using CBLs for the reason of severe charge recombination. These results are great evidence that the ZnO films can efficiently work as hole-blocking layers to prevent the current leakage.

ZnO
CBLs fabricated with sol-gel processing. The sol-gel method using a mixture of zinc acetate [Zn(OAC) 2 ] and 2-ethanolamine in methoxyethanol (MEA) as precursors is a widely used approach for the fabrication of ZnO CBLs for inverted PSCs. ZnO thin films obtained using this method present good surface quality, excellent optical transmittances and electrical properties. Moreover, this is a low-cost process that does not involve any expensive and/or vacuum instruments. 29,132,161 In addition, the features of low temperature and solution processing make the sol-gel method compatible to roll-to-roll manufacturing for the fabrication of inverted PSCs on flexible plastic substrates, which is one of the important advantage of PSCs. 29,55,115 In the sol-gel method, the post heat treatment process is adopted to convert the spin-coated gel film to crystalline ZnO and is a factor which can significantly affect the optical and electrical properties of ZnO films as well as the photovoltaic performance of the inverted PSCs. First, the heat treatment temperature is required to be higher than the boiling points of the solvents (2-methoxyethanol and MEA at 125 1C and 170 1C, respectively) to get the solvents completely evaporated. 161,163 Second, it has been demonstrated that, when heated in air, Zn(OAC) 2 begins to thermally decompose at 190 1C, its thermal decomposition completed at around 310 1C, and the formation of crystalline ZnO at the temperature between 300 1C and 400 1C. A higher annealing temperature ranging from 400-500 1C is identified as the most efficient temperature for the thermal decomposition of Zn(OAC) 2 to form crystalline ZnO. 164 The electron mobility of sol-gel processed ZnO films as well as the device performance of inverted PSCs based on these ZnO films as the CBL would be promoted by improving the crystallinity of ZnO. However, there have been conflicting reports of the optimized annealing temperature of sol-gel processed ZnO CBLs for inverted PSCs in the literature. 29,33,38,137,142,165,166 The optimal annealing temperature reported in the literature range from 100 1C to 450 1C. 26,30,35,134,140,160,161 Zhang et al. reported that the inverted PSCs using sol-gel processed ZnO CBLs exhibit a clear improvement in the device performance upon increasing the ZnO sintering temperature from 100 1C to 350 1C. 137 It was observed that R S of the inverted devices decreased with an increased sintering temperature and it reached a minimal value when the sintering temperature increased from 100 1C to 350 1C, as a result an improved device performance (PCE = 2.58%) was achieved. Such a performance improvement was ascribed to the improved ZnO quality in view of a relatively high sintering temperature, which effectively promoted the electron mobility of sol-gel processed ZnO films by improving the crystallization of ZnO. However, the need of a high annealing temperature above 300 1C hampers the advantage of PSCs compatible with flexible substrates, such as polyethylene terephthalate (PET), which usually cannot withstand a high temperature treatment process. Additionally, the high annealing temperature processes will also lead to the degradation of the conductivity of the cathode (i.e., ITO) and thus degrades the device performance. 137,166 In order to overcome these constraints, the sol-gel derived ZnO CBLs annealed at a relatively low temperature have been studied. 29 29 They found that the ZnO films annealed at 200 1C were able to function as an efficient electron transporting layer in inverted solar cells, leading to a PCE as high as 6.33% for the inverted devices constructed with the PCDTBT:PC 70 BM active layer. In addition, the inverted devices based on those low temperature (130 1C, 150 1C and 200 1C) annealed ZnO buffer layers exhibit a promising long-term stability. For example, the PCEs retain above 70% of their original value even after being exposed to air at room temperature for more than 30 days (without encapsulation). 29 In sharp contrast, for the unencapsulated solar cells with the PCDTBT:PC 70 BM active layer and in the conventional structure, the PCE decreases significantly under the same conditions. 29 Some research revealed that the ZnO films annealed at 200 1C were amorphous instead of being crystalline, and as a result their electron transporting properties (B1.3 Â 10 À4 -2.6 Â 10 À5 cm 2 V À1 s À1 ) are not as good as those of the crystalline ZnO (B1 cm 2 V À1 s À1 ). 33,38,142 However, it seems because the electron mobility of this amorphous ZnO is comparable to that of most organic materials for PSCs, the device  142 Recently, Jagadamma et al.
reported the use of sol-gel driven amorphous ZnO CBLs annealed at a lower temperature (B100 1C) for inverted PSCs to minimize the degradation to flexible plastic substrates. 140 They demonstrated that the sol-gel driven amorphous ZnO layers annealed at such a low temperature delivered the solar cell performance comparable to the best devices based on ZnO films prepared at substantially higher temperature. In their results, the inverted PSCs based on these amorphous ZnO CBLs and constructed using low band-gap polymer donors on glass/flexible PET substrates show performances of PTB7:PC 71 BM (PCE: 6.5% (glass)/5.6% (PET)) and PBDTTPD:PC 71 BM (PCE: 6.7% (glass)/5.9% (PET)). 140 More recently, Morvillo et al. investigated the performance of the inverted PSCs based on sol-gel deposited ZnO CBLs annealed at different temperatures (100,150,200,250 and 300 1C) for 5 or 10 min. 166 They found that the sheet resistance of the ITO increased linearly with the increase of annealing temperatures and times, which, in turn, contributes to the reduction of the FF and the PCE of the corresponding devices. As a result, the highest PCE (7%) was achieved for the device fabricated using a ZnO CBL annealed at 150 1C for only 5 min. Therefore, recently, most of the sol-gel processed ZnO CBLs adopted heat treatment at a relative low temperature (o200 1C) due to the comparable performance and compatible with the solution processing technique of inverted PSCs fabricated on flexible substrates. During the post annealing process, the annealing environment, such as humidity and vacuum, can also affect the electrical properties of ZnO CBLs to some extent. 167 Lin et al. found that a thermal treatment (150 1C) of the solution processed ZnO CBLs in a vacuum could help further reduce the defects and increase the electron mobility in the low temperature processed ZnO film (up to 0.36 cm 2 V À1 s À1 ), and thus improve the electron extraction in inverted PSCs. 167 Besides the annealing temperature and annealing environment, the concentration of the ZnO precursor is another key factor largely affecting the quality of ZnO CBLs derived from a sol-gel route. In general, a relatively low sol concentration (B0.1-0.3 M) is more prone to yield a ZnO film with high quality, reflecting on denser films, lower surface roughness, and higher transmittance. 38,132,137,167 This is because a low sol concentration can form small nano-sized ZnO colloids and small grain sizes. As a result, the inverted devices based on dense ZnO CBLs exhibit a low reverse current and contact resistance between the ZnO layer and the active layer owing to the full coverage of the ITO surface and an intimate contact between the ZnO and the active layer, and as a result the overall R S is reduced, giving rise to enhanced photovoltaic performance. 132,137 On the contrary, a relatively high sol concentration (B0.5-0.75 M) is usually prone to result in a ZnO film with larger surface roughness and even voids. The rough surface and the existence of voids in ZnO buffer layers would result in an increase in R S , due to the inferior contact between the ZnO buffer layer and the photoactive layer. 37,132,137 However, it has also been demonstrated that ZnO buffer layers having both dense and smooth surfaces and a large surface area could be obtained by using a slow ramp annealing method to heat treat the wet gel film derived from the ZnO sol with relatively high concentrations (i.e., 0.75 M). 138,139 It was explained that a low heating rate in the ramp annealing process provided sufficient time for the gel particles to structurally relax and pile up, resulting in a dense and undulated film. 138,139 More recently, it was reported that the type of ZnO precursor could also affect the properties of the sol-gel processed ZnO CBL, and thus impacts the device efficiency and stability. 168 It has been reported that the inverted PSCs with ZnO layers derived from diethylzinc are more stable than the devices with ZnO layers prepared by decomposing zinc acetate under long term illumination. 168 The difference in device stability was attributed to the different chemical nature of ZnO surfaces, specifically the presence of interstitial zinc (Zn i ) defects at the ZnO surface and their subsequent interactions with the adjacent active layer. It was explained that the ZnO layers derived from diethylzinc (deZn) possessed less Zni defects than the ZnO layers deposited from zinc acetate (Zn(OAC) 2 ). 168 Overall, to achieve a high quality ZnO CBL using the solution processed sol-gel method, the type of ZnO precursor as well as the concentration, and the post heat treatment temperature and environment should be considered comprehensively.
2.2.2 ZnO CBLs derived from pre-fabricated ZnO nanoparticle suspensions or aqueous ZnO hydrate. Besides the sol-gel method, another widely used solution processing method to deposit ZnO CBLs is spin-coating the pre-fabricated ZnO nanoparticle (NP) suspensions or colloids, synthesized using a hydrolysis and condensation of zinc acetate dihydrate under low temperature. 12,32,77,119,[143][144][145][146]169 The resulting ZnO NPs demonstrated good electron mobility (B0.066 cm 2 V À1 s À1 ) even though no additional post-thermal treatment was applied. 170 Hau et al. prepared ZnO NP CBLs (50 nm in thickness) with a dense and homogenous surface (Fig. 5) by spin-coating pre-fabricated colloidal ZnO NPs. 12,32 They found that the inverted devices with the ZnO NP CBLs exhibited performance comparable to that of devices based on sol-gel processed dense ZnO CBLs treated at a high temperature (400 1C). Qin et al. found that there were a large number of hydroxyl (-OH) groups formed on the ZnO NPs synthesized by hydrolysis and condensation of zinc acetate dehydrate. 146 It was suggested that the surface hydroxyl groups could depredate the photoactive layer by elevating the photocatalytic activity of ZnO, and hence deteriorate the device performance. Fortunately, the hydroxyl groups could be effectively detached from the ZnO NPs by annealing at a low temperature, B150 1C. 146 The low temperature treatment and the scalable processing capability make the colloidal ZnO NPs well compatible with the solution processing technique for inverted PSCs fabricated on flexible substrates. Krebs  co-workers reported the fabrication of flexible organic tandem solar modules with a PCE as high as B6% by using ZnO NP CBLs combined high throughput roll-to-roll compatible processing. 120 However, the ZnO NPs in suspension are not very stable and are sensitive to the moisture. Therefore, an appropriate ligand is usually required to be attached onto the NPs to stabilize them to avoid aggregation. To prevent the aggregation of ZnO NPs and to fabricate ZnO films with a smooth surface are important for the achievement of high performance inverted PSCs. 108,[132][133][134]171 Therefore, a major challenge of this method is to achieve spatial distribution of individual NPs and gain high electron mobility without the high temperature annealing process. [172][173][174] The improvement of spatial distribution of ZnO NPs by using polymeric or organic molecules as a matrix/surfactant has been reported by several groups. 144,[172][173][174] However, it should be noted that the removal of the surfactant material from ZnO films is required to optimize the properties of ZnO CBLs as well as the device performance, due to the fact that surfactant materials may reduce the electron transport by introducing additional resistance. 172,173 Besides the surfactant, the solvent of the ZnO NP solution also plays an important role in the ZnO NP dispersion and the morphology of ZnO NP films. Li et al. reported the study of the control of ZnO NP aggregation in solution by adjusting the solvent ratio (chloroform vs. methanol). 171 They found that when the ratio of chloroform : methanol was 1 : 1 (v/v), the superior dispersity of ZnO NPs in solution and a consequent ZnO film with compact and homogeneous surface could be obtained. As a result, a remarkable PCE of 8.54% in inverted PSCs based on the PTB7:PC 71 BM active layer was achieved. Besides the use of colloidal ZnO NPs, Chen et al. achieved ultrathin (B4 nm) ZnO CBLs by spin coating an aqueous zinc oxide hydrate (ZnOÁxH 2 O) solution. 175 They demonstrated that a post annealing temperature can be as low as 80 1C due to the low energy metal-ammine dissociation and hydroxide condensation/ dehydration chemistry. The inverted devices with the ultrathin ZnO CBLs and PCDTBT:PC 71 BM photoactive layers show a high PCE, up to 6.48%, which is comparable to those devices based on ZnO CBLs deposited by the sol-gel process using zinc acetate and annealed at 200 1C. 175 2.2.3 ZnO CBLs fabricated using the atomic layer deposition method. Besides the above-mentioned solution approaches, there are several other methods for the fabrication of ZnO CBLs, including atomic layer deposition (ALD), 129,148-150 spray pyrolysis, 43,152,153 metal organic chemical vapor deposition (MOCVD), 154 mist pyrolysis chemical vapor deposition (MPCVD), 155 radiofrequency sputtering (RF), 156 magnetron sputtering, 157,158 and fine-channel mist-spray coating. 159 Among these methods, the ALD technique is regarded as a very promising approach for fabricating the ZnO CBLs for inverted PSCs in view of the capability of ALD in producing a high surface quality film at a relatively low temperature and controlling the film thickness precisely. 148 Wang et al. deposited the ZnO CBL on the plastic substrate, ITO/PEN, using the ALD method at low temperatures (45 1C and 80 1C) for flexible inverted PSCs. 129 They found that the devices with ZnO grown at 80 1C presented enhanced photovoltaic performance compared to the devices with ZnO grown at 45 1C, for the reason that the former achieved a smoother and more hydrophobic surface than the latter. The authors suggested that the smoother and more hydrophobic surface of ZnO was beneficial to the contact quality between the ZnO and the active layer, thus leading to reduced series resistance and improved electron transport to the ITO electrode. It is worth pointing out that, besides the morphology, the ALD temperature also affects the electrical properties of the grown ZnO films. 176 Chang and Tsai found that both the electron mobility and electron concentration of the ZnO CBL increased with increasing deposition temperature for the reason of increased crystallinity. 149 In their results, the best inverted device was achieved by using a ZnO CBL (60 nm in thickness) deposited at 90 1C and exhibited a PCE of 4.1%, high R SH , and low R S . 149 An atmospheric atomic layer deposition (AALD) method was also developed to grow ZnO thin layers in ambient atmosphere to avoid the slow and vacuum-based process in conventional ALD. 150 It has been demonstrated that the AALD ZnO layers possessed compact surface, high electron mobility (3.4 AE 0.1 cm 2 V s À1 ) and good transmittance to visible light. As a result, the inverted BHJ P3HT:PCBM PSCs with AALD ZnO CBLs exhibited a PCE of 3.6% and good device stability in air. Compared with the conventional ALD route, such an AALD method allows the ZnO layer to be deposited more rapidly in air, and thus has higher potential for implementation in a throughput roll-toroll process.

The impacts of the morphology of ZnO CBLs on the performance of inverted PSCs
The photovoltaic performance of the inverted PSCs depends critically on the electrical properties of the interface between the ZnO CBL and the active layer. The condition of the interface is largely related to the contact quality and the contact area. 13,37,132,177 Therefore, the control of the morphology of ZnO CBLs is an important aspect to tune the contact quality so as to achieve the full potential of inverted PSCs. In a work done by Liang et al., 132 the ZnO CBL was prepared using a sol-gel method and its morphology was controlled by adjusting the concentration of the precursor sol. Fig. 6a-f exemplify the morphological change when the concentration of the precursor sol changes from 0.05 to 1 M. The PCE of 3.3% was obtained for the device with the P3HT:PCBM active layer and based on a dense and homogenous ZnO film driven from 0.1 M sol (Fig. 6c); such an efficiency is 32% higher than that for the device made using a rough ZnO film derived from a 1 M sol. The PCE enhancement arises from the higher FF while J sc only increases slightly and V oc remains almost unchanged. R S , series resistance, was found to increase by increasing the surface roughness and the content of voids in ZnO films. According to these experimental observations, Liang et al. suggested that the dense ZnO CBL with a homogenous surface allowed the formation of an intimate contact between the ZnO CBL and the BHJ photoactive layer, however a rough surface including voids led to an inferior contact and an increase in the contact resistance. 132 In addition, the uniform coverage of the ZnO CBL derived from 0.1 M sol is crucial for separating the ITO cathode from contacting the active layer, and blocking the reverse flow of holes from the active layer to the ITO electrode. These results demonstrated that the ZnO layer with a dense and homogenous surface benefited from establishing a better contact between the ZnO and the active layer, resulting in the low R S of the device.
Besides the compactness, the surface area of ZnO CBLs, which is mainly determined by the morphology, is another important issue that impacts the device performance by affecting the interfacial area between the CBL and the active layer and therefore affecting the electron collection efficiency. 43,138,139,151 Sekine et al. first reported the use of unique nano-ridge (also named as nano-ripple) ZnO films having both a large surface area and a dense surface in inverted PSCs to serve as a CBL (Fig. 7a). 139 By adopting sol-gel processing and using a ramp annealing method at a rate of 50 1C min À1 from room temperature to 275 1C, they fabricated the nano-ridge ZnO films with a peak height of around 120 nm and a valley to valley distance of about 500 nm, as shown in Fig. 7b and c. 139 It was suggested that  the low heating rate of the ramp annealing method provided sufficient time for the gel particles to structurally relax and pile up, resulting in a dense and undulated film. 139 In their results, the inverted solar cells fabricated based on nano-ridge films showed an average PCE of 3.87%, while the devices with planar films only showed an efficiency of 3.06%. The enhancement resulted in a low leakage current due to the improvement in the electron collection efficiency and the hole blocking capability of the nano-ridge film, resulting in higher FF, while V oc and J sc were found to remain almost unchanged. The better charge extraction was thought to result from the large surface area of the ZnO nano-ridge film, which makes the device possess a large ZnO/BHJ active layer interfacial area for electron collection and shortens the travelling distance of charges. Meanwhile, the ZnO nano-ridge film is also very dense, making it effective in blocking the hole from transporting from the active layer to the ITO cathode, and thus reducing the charge recombination and leading to a low leakage current.
To meet the full potential of ZnO nano-ripple CBLs for high performance inverted PSCs, the ripple size and density should be controlled. By adding ZnO nanoparticles in zinc acetate solutions and using a ramp annealing method, Lim et al. prepared ZnO nano-ripple films with controlled ripple size and density (Fig. 8). 138 It has been found that the surface structure of ZnO nano-ripples with B70 nm high nanoripples gives the highest PCE of B 3.2%. They demonstrated that the homogenous surface and a higher contact area between ZnO and the active layer contributed to the enhanced photovoltaic performance of the inverted PSC. Besides adding ZnO NPs, it has been found that the ripple morphology can also be tuned via controlling the concentration of the Zn(OAC) 2 sol-gel solution, annealing ramp rate, and the spin-coating parameters. 140,142,[178][179][180] This is because the surface morphology of the sol-gel derived ZnO thin films depends largely on the solvent drying kinetics. 142 Although many reports have demonstrated that the ripple morphology of ZnO CBLs could result in a high contact area between the ZnO and the active layer and thus enhanced the solar cell performance, there are some other reports showing that the ripple ZnO CBLs with a large surface area however harm the device performance compared to the smooth ZnO layers. 140,179 Ma et al. reported that nano-ripple ZnO CBLs with a high surface roughness could lead to high surface energy and a small donor/acceptor (D/A) interfacial area in the active layer deposited on the top of ZnO CBLs, and thus lower the J sc . 179 In addition, it has been suggested that the large surface area of ZnO CBLs could also result in more trap-assisted recombination at the active layer/ZnO interface compared to the case of smooth ZnO CBLs, and accordingly reduces the FF of solar cells. As a result, the best inverted PSCs were produced with the smoothest ZnO layer which gives the largest D/A interfacial area, and the lowest ZnO/active layer interfacial area. 179 The different or even contrary effect of the large surface area of nano-ripple ZnO CBLs on the device performance may be because the dominant factors influencing on the device performance, such as surface area, surface energy and surface defect, are different in various research studies. Therefore, to fully achieve the potential of the nano-ripple ZnO CBLs possessing a large surface area, the surface defects of the nano-ripple ZnO should be passivated and the surface energy should be optimized.
Besides the 3D ZnO nano-ridge films, there are several other ZnO films with a large surface area fabricated to serve as CBLs in inverted PSCs. Schumann et al. reported that the preparation of ZnO CBLs using spray-pyrolysis (SP) and electro-deposition (ED) methods achieved different surface morphologies. 43 In their work,   148 They demonstrated that the device performance strongly depended on the thickness of ZnO CBLs and there was a critical value for the thickness, which was at around 10 nm. For ZnO CBLs with thickness below this critical value the device demonstrated poor performance and the current-voltage ( J-V) curves presented an ''S-shaped'' kink. For devices with ZnO layers from 10 nm and up to 100 nm in thickness, the ''S-shaped'' kink completely disappeared, and the thickness difference of ZnO CBLs only caused a little variation in the PCE and FF. Such a ''S-shaped'' J-V curve was suggested to be a result of the low conductivity of such ultra-thin ZnO layers, which had a large surface-to-bulk ratio and might cause considerable defects at grain boundaries; these defects act as electron traps reducing the electrical conduction of the CBL. 41,148 Recently, a number of studies have ascribed the low conductivity of ZnO layers to the presence of oxygen at the grain boundaries or at the surface. 41,128,181,182 A similar phenomenon of ''S-shaped'' kink in the J-V curve in inverted devices with ultrathin ZnO CBLs (several nanometers) has also been found in Liang's 132 and Sharma's 179 work. In addition, the film thickness and the preparation method are two main factors affecting the coverage of ZnO buffer layers. A full and conformal coverage of ZnO layers is crucial to separate the cathode from the active layer, and block the reverse flow of holes from the active layer to the cathode. 132,140,183,184 For the ZnO buffer layer derived from the sol-gel method, it has been found that a very thin ZnO layer (a few nanometers in thickness) is difficult to achieve a full coverage on the ITO surface, and thus leaving a portion of uncovered ITO in direct contact with the active layer forming current leakage pathways. 132,140,184 Therefore, for ZnO buffer layers derived from the sol-gel method, a certain thickness (around ten nanometers) is necessary to obtain a full coverage on the cathode. Cheun et al. found that, when the thickness of ZnO layers increased from 10 nm to 100 nm, similar grain sizes and surface morphology were observed, resulting in little variation in the device performance. 148 More recently, Hu et al. fabricated ZnO CBLs with varying thicknesses (from 0 to 1500 nm) and different surface morphology and work function. 154 They found that, when using a P3HT:PCBM active layer, the optimized inverted device with a ZnO thickness of 80 nm showed a PCE of 2.93% using a P3HT:PCBM active layer. Based on the UPS spectra, Loser et al. found that both the valence band (VB) and the conduction band (CB) shifted slightly when the thickness of the ZnO layer increased from 3.0 to 42 nm. 185 In their results, the device performance only slightly varies as the ZnO thickness changes. Overall, a few tens of nanometers of the ZnO layer is suitable to obtain a uniform and dense surface so as to fully cover the cathode surface and completely separate the cathode from contacting the active layer and thus blocking the reverse hole flow, while not harming its electrical properties, transmittance, and mechanical robustness. It has been demonstrated that the inverted PSCs with different thicknesses of ZnO CBLs exhibit good long-term device stability, which is much better than that of the inverted PSCs without ZnO CBLs as well as that of various conventional PSCs. 38 The transmittance of CBLs is another important factor that affects the device performance by influencing the light harvest of the BHJ photoactive layer. 76,132,135,155 In order to obtain a high device performance, the light absorption of the active layer should be maximized. In inverted PSCs, incident light comes from the transparent cathode side (i.e., ITO or FTO) where the CBL was prepared on. Therefore, a CBL with high transmittance in the visible region is important in order to make the active layer gain high optical absorption of the active layer. The transmittance of ZnO films not only depends on the film thickness, but is also affected by the crystal size, defects and crystallinity, which are related to the preparation methods and post-treatment parameters. [186][187][188] High quality ZnO thin films possess an excellent transparency in the whole visible range and give rise to minimize optical losses. Through an optimization of the heat treatment process, Liang et al. fabricated high transparent ZnO layers on the ITO cathode with the thickness ranging from several nm to B55 nm using a sol-gel process. 132 They found that the transmittance of ITO/ZnO layers increased with increasing ZnO thickness, and is even higher than the bare ITO when the ZnO thickness is over 10 nm (Fig. 9). The transmittance enhancement of ITO/ZnO layers was attributed to the antireflection  38,141,189 Therefore, the high transmittance of ITO/ZnO layers will certainly benefit to the light absorption of the active layer in inverted PSCs. In addition, in Fig. 9, it can be seen that the sharp absorption edge of the transmittance spectra of ZnO modified ITO exhibits a gradual red shift toward B380 nm (corresponding to an energy gap, B3.2 eV of ZnO) by increasing the ZnO film thickness. 38,132,141 The band edge cut-off of ZnO at B380 nm can help in blocking the UV light and thus protecting the organic materials from the photo-degradation induced by UV light. 33,43 In Liang's work, a new parameter, ''relative optical transmittance'' (T R ), was put forward to quantify the overall effect of the transmittance of ZnO CBLs on the optical absorption of the given photoactive layer in inverted PSCs. The T R was calculated using the following formula: 132 where t l is the measured transmittance spectra of the ITO/ZnO buffer layer at l wavelength, I l is the measured absorption spectra intensity of the active layer at l wavelength, and l 1 B l 2 is the measurement arrange, usually corresponding to 300-900 nm. According to the definition of ''relative optical transmittance'', T R reflects the influence of ZnO CBL transmittance on the opticalabsorption of the active layer. The higher T R means less optical loss and higher optical-absorption of the photo-active layer, leading to better device performance. In Liang's work, although the transmittance of ITO/ZnO CBLs decreased at UV and increased at the wavelength of around 450 nm by increasing the thickness of ZnO films, the calculated T R presented very small variation. 132 This indicates that the change in the transmittance of ZnO CBLs in their work just makes a negligible impact on the light harvest of the photo-active layer and device performance. In other words, the light harvest and the device performance are only affected when there is an obvious change in the transmittance of ZnO CBLs. Kyaw et al. reported that the improvement in optical transmittance of ZnO CBLs led to an increase in the J sc of devices from 5.986 to 8.858 mA cm À2 without sacrificing the V oc and FF of the solar cell. 76 Recently, it has been demonstrated that the light-absorption of the active layer can also be enhanced by the ZnO nano-ripple structure due to its good light-scattering properties, which contribute to increasing the J sc of the device. 178,190,191 From these results, we can see that the charge extraction at the ZnO/active layer interface and the photovoltaic performance of the inverted devices are a combined effect relating to both the thickness and morphology of the ZnO layer.

Doping of ZnO CBLs in inverted polymer solar cells
In inverted PSCs, the thickness of ZnO CBLs is typically only a few tens of nanometers in order to ensure good conductivity so as to reduce the serial resistance and improve device performance. 38,162 However, such a thin CBL makes the devices suffer from low mechanical robustness and reduced protective properties against a chemical or physical reaction between the active layer and the electrode. 162,192 Therefore, such a thin ZnO CBL imposes a challenge for the manufacture of PSCs with scalable processes, such as the roll-to-roll manufacturing. 47,162,192 Doping is an efficient way to enhance the conductivity of ZnO so as to overcome the thickness limitation. Through doping, an increase in the electrical conductivity of ZnO CBLs and the photovoltaic performance of inverted PSCs has been observed. 162,[192][193][194][195][196][197] The group-III elements, such as boron (B), 154 aluminum (Al), 162,192,195,[197][198][199] gallium (Ga) 193,200 and indium (In) 194 have been intensively investigated for the doping of ZnO CBLs. It was explained that group-III elements could work as an n-type dopant for ZnO to replace Zn sites and generate free electrons. Besides aluminium and group-III elements, group-II elements (alkaline earth), such as magnesium (Mg), strontium (Sr) and barium (Ba) doped ZnO (named as MZO, ZnSrO and ZnBaO, respectively) layers have also been introduced into inverted PSCs. 48,201 Besides the metals, the fullerene derivatives have been used to dope ZnO for forming an efficient CBL in inverted PSCs. 49,114 The photovoltaic performance of inverted devices based on these doped ZnO CBLs in literature is summarized in Table 2.
Among the various doping methods, aluminium doping is a well-known approach to enhance the conductivity of ZnO layers for inverted PSCs. This seems to be because the doping of ZnO with aluminium can be easily processed and the aluminiumdoped ZnO (AZO) possesses better electrical properties than that of an intrinsic ZnO (i-ZnO) reference. 162,192 It has been reported that the inflection (S-shaped) I-V curve of inverted PSCs could be removed by introducing an AZO CBL. [195][196][197] Stubhan et al. and co-workers studied the thickness dependence of the performance of inverted PSCs based on both i-ZnO and AZO CBLs with thicknesses from B30 to 120 nm. 162,192 They demonstrated that an increase of the thickness of AZO CBL from B30 to more than 100 nm did not hamper the solar cell performance, while the devices based on 100 nm i-ZnO films suffered from severely decreased FF, V oc and PCE due to an increase in the series resistance. Considering that a thick CBL has a relatively good mechanical strength robust which benefits the inverted devices to be fabricated using a roll-to-roll method, the authors suggested that the doping of ZnO with aluminium opens the possibility to incorporate a thick CBL into the inverted devices without sacrificing their efficiency. More recently, based on a facial and low temperature (125 1C) solution processed Al doped ZnO electron accepting/hole blocking layer, Jagadamma et al. reached high performance inverted PSCs yielding a remarkable PCE in excess of 10% on glass substrates and 8% on plastic substrates. 47 They found that the average PCE of inverted devices using the PTB7:PC 70 BM active layer decreased marginally from 8.8% to 8.1%, while the thickness of AZO CBLs increased from 22 nm to 75 nm. The very slight decrease of the device performance by increasing the AZO thickness indicates that a relatively thick AZO can serve as an efficient CBL material for inverted PSCs. A thicker CBL with better mechanical strength is more suitable for manufacturing PSCs by scalable processes, such as the roll-to-roll manufacturing. 47,162,192 For the inverted PSCs with AZO CBLs, it is worth noting that the device performances are strongly affected by the crystallinity and stoichiometry of AZO. 192,203 This is because the electronic properties of AZO CBLs, such as the work function, conductivity, mobility as well as surface states, are massively determined by the stoichiometry of AZO. 153,163 Tsai and coworkers have investigated the effects of the Al content (from 0 to 12 mol%) on the electronic properties of AZO layers and device performances of inverted PSCs with a structure of ITO/ AZO/P3HT:PCBM/PEDOT:PSS/Ag (Fig. 10a). 203 They found that the carrier concentration, mobility and resistivity of AZO thin films strongly depended on the Al molar ratio (Fig. 11), which further impacted the performance of inverted devices with AZO CBLs. In their results, to achieve the best device performance, the optimum Al content of AZO CBLs is 6 mol%. Compared with the devices based on pure ZnO layers, the device with optimum AZO CBLs exhibits highest PCE of 3.78%, enhanced by B13.9% via improving J sc from 10.73 to 11.12 mA cm À2 and FF from 53.7% to 60.8%. 203 At the optimum Al doping concentration, the authors demonstrated that the band structure of ZnO was modified with an upward energy shift in the Fermi level (Fig. 10b), which facilitates the electron collection and can also serve as an efficient hole blocking layer. It is believed that the tunable band-gap Doping of ZnO with group-III elements such as boron (B), gallium (Ga), or indium (In) for doping ZnO CBLs is another effective way to improve the performance of inverted PSCs. 154,193,194,200 Kyaw et al. investigated the correlation between the device performance and indium contents of the sol-gel derived indiumdoped ZnO (IZO) CBL in inverted PSCs. 194 Compared to the device with i-ZnO CBLs, the photovoltaic performance of the devices using appropriately doped IZO CBLs, especially in terms of J sc , was enhanced, due to a combined effect of improvement in charge collection and higher optical transmittance of the electrode/IZO buffer layer stack. The device with 1 at% IZO CBLs showed a maximum PCE of 3.3% compared to the PCE of 2.94% for the device with i-ZnO CBLs. Gallium (Ga) has also been used as a dopant to enhance the electrical conductivity of ZnO CBLs. Shin et al. investigated the Ga-doped ZnO (GZO) CBL for the inverted PSCs. 193 By employing a GZO CBL, the PCE of the fabricated devices was improved by about 110% compared to the devices with i-ZnO CBLs, which was ascribed to the higher electron conductivity and smooth surface morphology of the ZnO film with Ga doping.
The doping of ZnO with group-II elements (alkaline earth), such as magnesium (Mg), strontium (Sr) and barium (Ba), doped ZnO (named as MZO, ZnSrO and ZnBaO, respectively) films have also been studied for inverted PSCs to serve as CBLs. 48,201 Pachoumi et al. reported an improved performance and stability of inverted PSCs by employing the sol-gel processed amorphous Sr and Ba doped ZnO CBLs prepared at a relatively low temperature. 201 The inverted PSCs based on Sr and Ba doped ZnO CBLs exhibited higher performance and better stability than the devices with pure ZnO CBLs. In addition, the common requirement of light soaking for reaching a maximum photovoltaic performance of inverted device based on i-ZnO CBLs was successfully circumvented. The improvements of device performance were attributed to the doping of ZnO with Sr or Ba reducing the electron trapping on the surface associated with oxygen absorption. The authors suggested that the Sr or Ba doping could effectively suppress/reduce the oxygen adsorption in mobile oxygen vacancy sites on the ZnO surface. 201 For doped ZnO CBLs, the band gap structure is a key parameter that could affect the device performance. The band gap and energy level dependence on the doping content enables researchers to adjust the optical and electrical properties of the doped ZnO CBLs for achieving better device performance. 48,192,198 For example, it is well established that the substitutional doping of Mg 2+ into the Zn site leads to an increase in the bandgap of ZnO. 48,204,205 By using Mg doped ZnO (ZMO) CBLs, Yin et al. fabricated inverted PSCs with high performance, as shown in Fig. 12a. 48 They developed a solution processed ZMO CBL with tunable band gaps and energy levels (Fig. 12b), good optical transmittance and electron transporting abilities. With an increase of the Mg content (x = 0.1-0.6), the CB edge value of ZMO CBLs are finely adjusted in the range of BÀ4.3 to À3.9 eV (inset in Fig. 12c) to approach the LUMO of PC 71 BM. The corresponding band gap of ZMO films increased from 3.3 to 3.7 eV, which was confirmed by the gradual blue shift of the fundamental absorption edge as the Mg content increases (Fig. 12c). A favorable CB level of ZnO matching well with the PC 71 BM acceptor would improve the electron extraction/ transportation in inverted devices. The electron mobility of ZMO films was found to be close to that of pure ZnO films, which is in favor of electron extraction/transportation in the inverted PSCs. Moreover, ZMO layers also exhibit a more hydrophobic    Fig. 12d), which are much higher than those of devices with an i-ZnO CBL (PCE = 7.1%) under the same conditions. 48 Macleod and co-workers have also reported an improved PCE in inverted PSCs when a sol-gel processed Mg-ZnO is used in place of ZnO as CBLs. 205 It has been found that, as the Mg content increased, the FF and V oc increased substantially for both the devices based on P3HT:PCBM and P3HT:ICBA systems. The maximum PCEs were achieved when the Mg content in the precursor was 10 mol%, and subsequently decreased for higher Mg content due to decay in J sc . They have also found that the increase in Mg content led to a decrease in the work function and an increase in the bandgap. The increased bandgap in Mg-ZnO is ascribed to a downshift in the valence band maximum, rather than an upshift in the conduction band minimum. The improved device performances were partially ascribed to the reduced recombination caused by substituting Zn with Mg. However, it has been suggested that the mechanisms for the improvement of inverted devices with Mg doped ZnO CBLs remain an open question. 205 More systematic investigation needs to be done to elucidate the dominating factors with regard to the improvement in the performance of inverted PSCs when using Mg-ZnO CBLs.
Besides the above-mentioned metals, fullerene derivatives have also been used to dope ZnO for forming high quality CBLs in inverted PSCs. 49,114 Liao et al. reported a fullerene derivative doped ZnO (ZnO-C60) nanofilm as the CBL of inverted PSCs. 114 For the inverted device with ZnO-C60 CBLs, the PCE remarkably improved from 7.64% to 9.35% for the PTB7-th:PC 71 BM active layer, from 6.65% to 8.21% for PTB7:PC 71 BM, and from 5.26% to 6.60% for P3HT:ICBA, compared to the devices with pristine ZnO CBLs. It has been demonstrated that the ZnO-C60 CBL can enhance the electron collection by producing a fullerene derivative rich cathode surface and promote electron conduction at the interface and in the bulk of ZnO-C60. The high content of fullerene at the surface of ZnO-C60 CBLs offers a better contact with the fullerene derivative in the active layer, and thus facilitates electron collection. In another work also done by Liao et al., a dual-doped ZnO with indium and fullerene derivative (BisNPC60-OH) was fabricated by sol-gel processing and applied as CBLs in inverted PSCs (Fig. 13). 49 They found that this dual doped ZnO, InZnO-BisC60, buffer layer showed dual and opposite gradient dopant concentration profiles, being rich in fullerene derivative at the cathode surface in contact with the active layer and rich in indium at the cathode surface in contact with the ITO surface (Fig. 13b). For the InZnO-BisC60 film, the surface conductivity was improved by a factor of 270

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(from 0.015 to 4.06 S cm À1 ), and the electron mobility was enhanced by a factor of 132 (from 8.25 Â 10 À5 to 1.09 Â 10 À2 cm 2 V À1 s À1 ). As a result, with this InZnO-BisC60 film as the CBL, the PTB7-Th-PC 71 BM inverted device exhibited a preferable PCE, 10.31%, significantly higher than the 8.25% for pristine ZnO.

One dimensional ZnO nanostructures for inverted polymer solar cells
For polymer solar cells, the bulk-heterojunction (BHJ) active layer was considered to establish the cornerstones of a device with high performance. This is because the structure of the BHJ can maximize the donor/acceptor interfacial area to provide possibly more exciton dissociation sites and form continuous pathways for the transport of electrons and holes. 13,18,21 However, there are still some shortcomings of the BHJ active layer that limit the photovoltaic performance of polymer solar cells.
One of the limitations is that the very thin thickness of the BHJ active layer leads to low optical absorption and thus non-ideal device performance. [206][207][208] The use of a very thin active layer is, on one hand, because the photo-generated charges in the active layer need to transport in the percolation network of the polymer donor and the acceptor, and on the other hand the carrier mobility in polymers is relatively low. 206,209 The charge recombination in a thick active layer will increase because of an increase in the diffusion length of carriers, leading to a decrease in device performance. Thus, the optimal thickness of the active layer is determined by a trade-off between maximizing the optical absorption and meanwhile allowing the charges to transport efficiently from the BHJ active layer to the electrode. It has reported that an optimal active layer thickness is typically about only B100-200 nm. 185,[206][207][208] Research effort on the methods of light absorption enhancement has been driven by these limitations, including using photonic crystals, 210 plasmonic enhancement, [211][212][213][214] and textures. 215,216 Besides the methods mentioned above, another way to enhance the light harvesting of the photoactive layer is to increase the effective thickness of the photoactive layer and meanwhile avoid the increase of charge recombination in the case of a thick active layer. 217,218 To increase the effective thickness of the photoactive layer, the use of a vertically aligned one dimensional (1D) ZnO nanostructure in PSCs to provide a direct electron transport pathways so as to enhance charge carrier collection and transport has been investigated. 213,214 Fig. 14 shows a schematic diagram of inverted PSCs with a 1D ZnO nanostructure. In the case of inverted PSCs with ZnO nanorods (NRs), after the charge    217 They found that the PCE and FF of devices were improved from 1.8% and 0.38 to 2.7% and 0.50, respectively, while the I sc and V oc remained almost constant when the length of ZnO nanorods increased from 0 to 350 nm. The improvement in PEC was mainly ascribed to the enhancement in FF, resulting from introducing ZnO nanorods as direct pathways for more efficient electron transport and collection. It has been demonstrated that the ZnO NR array in the ZnO NR/P3HT:PCBM system mainly works as an electron collector/transport pathway, rather than an acceptor material to form heterojunctions with the donor polymer in view of a much larger interface between the P3HT and PCBM than the interface between the P3HT and ZnO NRs. 77,217,239 In conventional and inverted PSCs, holes are also commonly limiting charge carriers. Although ZnO nanorods may not improve the hole transport, the extraction and transport away from electrons would reduce the probability of electron-hole recombination.

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In order to fully achieve the potential of 1D ZnO nanostructures in inverted PSCs, an electrical coherence at the organic photoactive layer and ZnO interface needs to be provided. Therefore, the infiltration of polymers into the space between ZnO NRs is a critical issue that greatly affects the device performance. Several methods have been developed to improve the contact between the active layer and ZnO nanostructures, including the surface modification of ZnO nanostructures and optimizing the post-treatment processes. 218,226,230,232 Fig. 16 shows the SEM images of the crosssection of inverted devices with the ZnO nanowire arrays with or without surface modification. Obviously, the surface modification of ZnO NPs effectively improved the infiltration of polymer and fullerene blends into the space of ZnO NRs.
Chou et al. reported that the device performance of inverted PSCs with ZnO NRs could be improved by increasing the polymer solidification time. 218 They suggested that the increase of the polymer solidification time made the polymer chains have enough time to self-organize, leading to the enhanced crystallinity of polymers and improved the infiltration of the photoactive layer into ZnO NP space. For the inverted device with the active layer up to 400 nm in thickness, the PCE of the device improved to 3.58% as a result of enhanced FF (58%). 218 Thitima et al. reported that the performance of inverted PSCs could also be improved by modifying the surface of the ZnO nanostructure with the N719 dye. 223 The dye interlayer covered on the surface of ZnO was thought to serve as a mediator to improve the electron transportation and injection from the polymer to ZnO. Huang et al. reported that the solution-processed PCBM interlayer could efficiently modify ZnO NRs and accordingly enhance the PCE from 2.35% to 3.2%. 226 They demonstrated that the PCBM interlayer enhanced the infiltration of the photoactive layer into the intervals between ZnO NRs (Fig. 16b) and may facilitate the self-organization of polymers.
Ajuria et al. investigated the functionalization of the ZnO nanowire (NW) surface with ZnO nanoparticles (NPs) with the consideration to improve the contact between ZnO and the P3HT:PCBM active layer. 232 It was found that, by modifying the ZnO NW array with a thin layer of ZnO NPs by immersing in a ZnO colloidal suspension, improved P3HT:PCBM blend penetration was achieved (Fig. 16d and e). A PCE as high as 4.1% was obtained for the inverted device based on ZnO NP-coated NWs and commercial P3HT:PCBM blend active layers. The ZnO NP modification was found to contribute to enhance the exciton dissociation and increase the charge lifetime by reducing charge recombination. More recently, Ho et al. fabricated ZnO NRs with controlled density and length by using a two-growth hydrothermal method (as shown in Fig. 15b). 238 Compared to the devices made with planar ZnO films, the PCEs of devices with the two-growth ZnO NRs are enhanced from 5.40% to 7.80% and from 7.24% to 8.01% for the PBDTTT-C-T:PC 71 BM and PTB7:PC 71 BM active layer, respectively. The improved device performances were ascribed to the morphology of two-growth ZnO NRs, which helps in achieving deeper and superior infiltration of the organic active layer. 238 From these results, we can conclude that, for 1D ZnO nanostructures applied in inverted PSCs, their length, density, surface morphology, and surface modification are key parameters that will significantly affect the penetration of polymers and fullerene blends and thereby affect the device performance.

Surface modification of ZnO CBLs
The performance of polymer solar cells is critically dependent on the properties of each layer and the interfacial contacts. For ZnO CBLs, without interface engineering, the inorganic ZnO CBL and surface defects may lead to a poor interfacial contact with the polymer based active layer and, thus, result in poor electron extraction, which is one of the main reasons leading to a relatively high charge recombination and series resistance (R S ). 240,241 Especially, in the case of ZnO CBLs prepared with widely used sol-gel processing or colloidal nanocrystals, the low-temperature and solution process often lead to high densities of surface defects, such as dangling bonds, surface groups and charged oxygen molecules. 41,128,195,242 Such surface defects can act as recombination centres for photogenerated charge carriers, decreasing the photocurrent and power conversion efficiency as well as the device stability. It has been demonstrated that the deteriorated electrical properties at the interface between ZnO CBLs and organic active layers are caused by incompatible chemical interfaces and the formation of electron trapping states on the surface of ZnO layers. 243 A phenomenon commonly observed on the J-V curve of inverted PSCs when employing ZnO CBLs is the appearance of an inflection point, i.e. the so-called ''S-shape'' kink feature. It is believed to be due to the accumulation of charges at the ZnO/active layer interface. 144,148,197,201 Therefore, appropriate electrical contacts at interfaces between ZnO CBLs and BHJ active layers are highly desirable and essential to achieve a high power conversion efficiency and good stability of inverted PSCs. In this regard, many studies have focused on engineering the surface of ZnO CBLs to reduce the R S of solar cells through improving the interfacial electrical properties, better aligning the energy-level and controlling the surface energy. There have been several efficient surface engineering methods to modify the surface of ZnO CBLs, such as ultraviolet (UV) 41,133,144,148,182,197,201 or UV-ozone treatment, 144,172 thermal and vacuum treatment, 167 and incorporation of the surface modification interlayer, such as the self-assembled monolayers (SAMs). 169,244-247

UV illumination treatment of ZnO CBLs
UV or UV-containing light illumination treatment, also known as ''light-soaking'', is a common way to improve the electrical characteristics and optimize optimum photovoltaic performance of inverted devices with ZnO CBLs. 41,133,144,148,182,197,201 UV illumination can passivate the defect states, such as the negatively charged oxygen molecules trapped at the grain boundaries and the surface, and thus remove the electron extraction barrier and reduce carrier recombination at the ZnO/photoactive layer interface. 41,128,195 The metal-oxide/organic active layer interface is identified as the reason of the S-shaped J-V curve due to an electron extraction barrier. 181,182,197 Under dark conditions, electrons in the conduction band of the ZnO layer are trapped by the chemisorption of O 2 molecules (Fig. 17a, step (1)), and therefore form a depletion layer and band bending next to the surface (Fig. 17a, step (2)). 181,182 It has been suggested that both the depletion layer and the band bending can result in the formation of a barrier for the electron injection and extraction at the ZnO/BHJ active layer interface and are more pronounced in the case of small ZnO nanocrystals with a high surface-areato volume ratio. Upon UV illumination, the electron-hole pairs are generated in ZnO, leading to neutralization of O 2 species and desorption of oxygen from the ZnO surface, as shown in Fig. 17a, steps (3)-(5). 181 The desorption of O 2 from the ZnO surface reduces the electron traps, and thus increases the concentration of mobile electrons. 182 As a result, the inflection point problem (S-shaped) on the J-V curve of the inverted devices incorporating ZnO CBLs could be solved by exposing the cells to UV illumination, leading to the device performance improvement (Fig. 17b). 41 The UV-ozone (UVO) treatment has also been investigated to modify ZnO CBLs. 144,172 Chen et al. found that the UVO treatment of ZnO NP films could effectively passivate the defect states leading to longer carrier life time of devices compared to the devices treated with UV light soaking. 144 By reducing the interfacial recombination via UVO treatment and employing a high performance PDTG-TPD:PC 71 BM active layer, the inverted PSCs with PCEs exceeding 8% were achieved. According to the PL measurement, it was proposed that the passivation mechanism under UVO treatment of ZnO films is correlated with the reduction of oxygen vacancies (V O ) in both the surface and bulk due to the penetration of oxygen in the porous NP films. As a result, the defect passivation in ZnO NP layers can help to reduce the carrier recombination at the ZnO/active layer interface, and thus enhance the device performances. 144 However, the UV exposure approach is limited in the case when the devices are based on substrates with low UV transmittance, UV blocking filters or UV to VIS downconversion concepts. 197 Many active organic donor polymers must be protected from being exposed to UV light so as to achieve a reasonable device stability. 55,197,248 In order to alleviate the requirement for UV illumination, surface modification

Fullerene based interlayer modification of ZnO CBLs
Modifying ZnO CBLs with a functional interlayer derived from fullerene derivatives (C60-SAM), 50,169,202,244,246,247,249 conjugated polyelectrolytes (CPEs) 250,251 and poly(ethyleneimine) (PEI) 87 is another representative method to enhance the electrical properties of the ZnO/BHJ active layer interface as well as the performance of inverted PSCs. Among these interfacial modification materials, fullerene derivatives (C60-SAM), which are very effective in acting as an interlayer for the modification of ZnO CBLs in inverted PSCs, are the most extensively investigated interfacial modification materials. Fig. 18 shows several cross-section schematics of the inverted devices with fullerene derivative modified ZnO CBLs. The improvement of photovoltaic parameters of inverted devices using surface modified ZnO CBLs with fullerene derivatives (C60-SAM) is summarized in Table 4. C60-SAMs can be easily processed to form a modification layer on the surface of ZnO CBLs through either a solution immersion technique or a solution-based spin-coating method. The Jen group developed a series of C60-based SAM modifiers containing different anchoring groups (catechol, carboxylic acid, and phosphonic acid), linkage location, and functionalization (Fig. 18a). 169,244 They found that these C60-SAM functional layers could help in reducing the charge recombination at the ZnO/ active layer interface, improve the morphology of the active layer, and enhance the charge selectivity, leading to high FF and J sc . For instance, an average PCE of 4.5% (with the highest PCE of 4.9%) was achieved for the devices with a C60-SAM (SAMs [1], shown in Fig. 18a) modified ZnO CBL surface; such an efficiency is over 20% higher than those unmodified devices. Cheng et al. synthesized a PCBM-based and cross-linked n-type interlayer (C-PCBSD) (Fig. 18b) for the modification of ZnO CBLs. 246,249 This C-PCBSD interlayer was considered to have several positive impacts on the interfaces of the active layer, including improved exciton dissociation efficiency, reduced charge recombination, decreased interface contact resistance, and introduced vertical phase separation to reduce the bulk resistance of the active layer as well as passivating the local shunts of the ZnO CBL interface. 246,249 As shown in Fig. 18b, the authors suggested that, first, the C-PCBSD interlayer provided an extra P3HT/C-PCBSD interface area for ultrafast exciton dissociation; second, the LUMO energy level of C-PCBSD (3.8 eV) was located between the LUMO of P3HT (3.3 eV) and the conduction band of ZnO (4.4 eV), which makes the electrons to efficiently transport from the active layer to ZnO through C-PCBSD as an energetically favorable downhill pathway. 246 As a result, the inverted devices based on C-PCBSD modified ZnO CBL with a P3HT:PCBM blend active layer gained an improvement in J sc from 11.6 mA cm À2 to 12.8 mA cm À2 , and exhibited a PCE as high as 4.4%, which is much higher than 3.5% for a reference device with unmodified ZnO CBL. 246 An impressive high PCE of 6.2% has been achieved through introducing C-PCBSD modified ZnO CBLs into inverted devices based on the ICBA:P3HT blend active layer, leading to a 29% enhancement compared to the device with un-modified ZnO CBLs. 249 The improved device photovoltaic performance in terms of J sc and FF of the devices with C-PCBSD modified ZnO CBLs was attributed to a high shunt resistance (R SH ) and a low series resistance (R S ) due to the decreased contact resistance and increased electron collection efficiency at the ZnO/C-PCBSD interface. Besides high PCE, the devices with C-PCBSD modified ZnO CBLs also demonstrated good device stability in air without encapsulation. In their results, the PCE of the unencapsulated devices with P3HT:PCBM active layers and based on bare ZnO CBLs retained 80% of their original value after being stored under ambient conditions for 35 days. In contrast, the device based on C-PCBSD modified ZnO CBLs exhibited a better longterm stability, retaining above 85% of its original value under the same conditions. 246 In sharp contrast, the device with a conventional structure (ITO/PEDOT:PSS/PCBM:P3HT/Ca/Al) and without encapsulation suffer a rapid decay in just two days. It was suggested that the coverage of a three-dimensional C-PCBSD network on the ZnO buffer layer might have passivated the hot spots generated in ZnO CBLs to suppress the leakage current, and thus improved the device lifetime. 246,249 Stubhan et al. investigated the modification of the AZO buffer layer with phosphonic acid anchored SAMs (as illustrated in Fig. 18c). 202 They demonstrated that the phosphonic acid anchored SAM decorated AZO could increase R SH and decrease the R S of the device due to the improved charge transfer from the PCBM to AZO. The average PCE had been improved by approximately 15% from 2.9% to 3.3%. More recently, Cho et al. demonstrated that the PCBM itself could serve as an interfacial modification material (Fig. 18d). 247 They suggested that the PCBM layer between ZnO and the BHJ active layer can effectively quench the electron-hole recombination by reducing trapped charges at the ZnO surface, which resulted in a 16% increase in the PCE (up to 7.7%) compared to devices without an additional PCBM layer. The C60-based modifiers with weaker acids have been also explored to modify ZnO CBLs. 50,169,244 It has been found that the modifiers with ester units exhibit much weaker acidity and better affinity for the ZnO surface. 169 Li et al. developed a C60 pyrrolidine tris-acid ethylester (PyC60) with three ester groups to modify ZnO CBLs (Fig. 18e). 50 By using this PyC60 upper layer modified ZnO NP films as the CBL, high efficiency (average PCE of 8.62%) inverted PSCs based on the photoactive layer of the PTB7:PC 71 BM blend were achieved. The authors demonstrated that the PyC60 improves the morphology quality of ZnO/PyC60 films by filling in the voids of the ZnO layer and partly passivating the defects of ZnO NPs. It was indicated that the PyC60 could also improve the interfacial contact between the ZnO and the active layer, lowering the electron injection barrier. As a result, the ZnO/PyC60 CBL effectively decreased R S and interfacial charge recombination, and increased electron injection and collection efficiency in the inverted PSCs. 50
Bulliard et al. reported a mixed SAM to control the surface energy of the ZnO buffer layer. 241 They found that, by changing the surface composition of two different SAMs (Fig. 19a), the surface energies of the ZnO CBL were tuned over a wide range of values (between 40 mN m À1 and 70 mN m À1 ) with negligible changes in its work function. 241 They suggested that a surface treatment to obtain an intermediate surface energy above the neutral range favors the competition between lateral and vertical phase separation, resulting in morphology with desirable domain size. By tuning the surface energy of ZnO CBLs in a given range, the PCE increased from 3.27% to 3.70%. Yoo et al. developed Table 4 Non-exhaustive survey of inverted PSCs including modified ZnO CBLs with fullerene derivatives (C60-SAM). The photovoltaic parameters are compared to those measured for a reference cell (values in parentheses), when available, made with a pristine ZnO CBL several SAMs derived from benzoic acid or benzoyl chloride to tune the work function and surface properties of ZnO CBLs. 254,255 It was demonstrated that the benzoic acid interlayer could form a favorable interface dipole at the interface between the ZnO and the active layer, thus to passivate the ZnO surface traps and accordingly reduce the work function and facilitate electron transport. Conjugated polyelectrolytes (CPEs) have been reported to serve as a cathode buffer layer between the cathodic electrode and the active layer in inverted PSCs to improve device performance by changing the electronic and orbital interactions at the interface. 45,51,88 CPEs have also been employed as efficient interlayer materials to modify ZnO CBLs. 205 Fig. 19b, on ZnO CBLs to reengineer the interface between ZnO and PBDT-DTNT:PC 71 BM. 251 In their study, the inverted PSCs with a device architecture of ITO/ ZnO/PFN-Br/PBDT-DTNT:PC 71 BM/MoO 3 /Ag showed a J sc of 17.4 mA cm À2 , a V oc of 0.75 V, a FF of 61% and an average PCE up to 8.4% was attained. The conduction band edge of ZnO was tuned from À4.46 eV to À4.08 eV (ZnO/PFN-Br) by using a PFN-Br thin layer, leading to an enhancement of the V oc of the device. The enhanced J sc and FF in the inverted PSCs with the PFN-Br interfacial layer were ascribed to the improved contact between the PFN-Br interfacial layer and the active layer, which was better than the interfacial layer formed between the naked ZnO layer and the active layer. As a result, over 37% enhancement in the PCE was obtained. Another conjugated polyelectrolyte (CPE), poly(9,9 0 -bis(6 00 -N,N,N-trimethylammoniumhexyl)fluoreneco-alt-phenylene), with bromide counterions (FPQ-Br, as shown in Fig. 19c) has also been successfully employed as an interlayer between ZnO and the active layer to improve the electron transport and interfacial contact. 250 Compared with the device based on a bare ZnO layer, J sc increased by 11% and the PCE increased from 6.37% to 7.04% for the device with ZnO/CPE layers.
Woo et al. report an appreciable PCE enhancement in inverted PSCs by using an electron-rich polymer nano-layer (poly(ethyleneimine) (PEI)) modified ZnO CBL (Fig. 20a). 87 The thickness of the PEI nano-layer was controlled to be only 2 nm in view of its high insulting character. It has been demonstrated that the enhanced PCEs are due to the lowered conduction band energy of ZnO via the formation of an interfacial dipole layer at the interface between the ZnO layer and the PEI nano-layer (Fig. 20b). They also found that the PEI nano-layer could increase the surface roughness of ZnO, which noticeably decreases the series resistance of the device. As a result, the inverted PSCs with the structure of ITO/ZnO-PEI/ PTB7:PC 71 BM/MoO 3 /Ag resulted in a PCE of up to 8.9%, while the devices using only the ZnO or PEI buffer layer show relatively low PCEs, 6.99% and 7.49%, respectively. The enhanced PCE mainly resulting from the increased J sc and FF for devices with ZnO/PEI layers was attributed to the fast electron transport.
Ionic liquids (ILs) have been investigated to serve as a CBL material and as a surface modification layer to modify the ZnO buffer for inverted PSCs. Yu   fabricated with solution-processing at low temperature (Fig. 21a). 242 They demonstrated that the covalent bonding of EDT molecules onto ZnO nanocrystals could effectively remove various surface defects of ZnO nanocrystal layers by forming zinc ethanedithiolates during the EDT treatment. Due to the chemical changes through EDT-passivation, the intragap states in pristine ZnO nanocrystals were modulated and a new intragap band was introduced ( Fig. 21b and c), resulting in enhanced electron selectivity and improved electron transport properties to EDTtreated ZnO CBLs. As a result, the inverted devices with EDTtreated ZnO CBLs exhibited reduced charge recombination and enhanced charge extraction properties. The EDT treatment was also found to result in significant improvement of the ambient stability of the inverted PSCs. For the unencapsulated P3HT:PC61BM devices with EDT-treated ZnO CBLs, the PCE retained 90% of its original value after being exposed under dark and ambient conditions for 30 days. In contrast, the PCE of the control device with pure ZnO CBLs degraded to around 60% of the original value under the same conditions. The improved device stability was ascribed to the well-passivated EDT-treated ZnO films, which are less susceptible to oxygen and water molecules (as shown in Fig. 21d). Furthermore, it has been demonstrated that the EDT-passivation method is suitable for both colloidal nanocrystals and sol-gel derived ZnO films. 242 Several kinds of metal oxides and salts, such as TiO 2 , 136 Cs 2 CO 3 , 252 hafnium dioxide 149 151 They found that a 1-2 nm ALD-ZnO layer can effectively quench the electron-hole recombination caused by surface defects of ZnO ripples, and thus enhance the J sc and PCE of inverted devices. Modifying the ZnO CBL surface with a 0.6 nm ALD HfO 2 layer has also been reported to enhance the electron injection and hole-blocking, leading to the PCE increasing from 4.1% to 4.5%. 149 The R SH of the devices increased significantly with the addition of the HfO 2 layer, while the R S decreased. It was explained that the enhancement in the electron transport arose from the HfO 2 layer which passivated the ZnO CBL and reduced the defect states on the ZnO surface, thereby suppressing the loss of electrons caused by charge recombination at the defect sites. It should be noted that the deteriorated performance of devices with an HfO 2 layer thicker than 0.6 nm indicated that the enhancement in electron transport is reduced due to the insulating effect of the HfO 2 layer, which became dominant in the case of thick HfO 2 .
Besides the modification of the interface between ZnO and the active layer, the modification of the interface between ZnO and the ITO electrode has also been investigated to achieve more efficient collection and transport of electrons from the ZnO  View Article Online layer to ITO. 256 Yoon et al. reported that inserting a nanoscopic copper hexadecafluorophthalocyanine (F16CuPc) nanowire film between the ZnO layer and the ITO electrode of inverted PSCs can increase the performance of inverted PSCs by enhancing interfacial electron transport. 256 6. ZnO-based nanocomposite CBLs

ZnO/polymer nanocomposite CBLs
For low-temperature solution-processed ZnO CBLs, the general presence of high density defects, such as dangling bonds and surface groups, and poor spatial distribution of nanoparticles over a large area restrict their contribution to enhance the performance of inverted PSCs. 172,173 The development of uniform ZnO NP films with low density defects is essential to achieve high performance of inverted PSCs. Recently, polymer modified ZnO films, known as the so-called ZnO/polymer hybrids or nanocomposites, have been studied to meet such challenges. 173,174,[257][258][259][260] For example, the ZnO/polymer composites, such as ZnO/poly(ethylene oxide) (PEO), 174 ZnO/poly(ethylene glycol) PEG, 258 ZnO/fullerene-endcapped poly(ethylene glycol) (C60-PEG), 261 ZnO/polyethylenimine (PEI), 260 ZnO/polyacrylic acid (PAA), 262 ZnO/poly(zinc diacrylate) (pZA), 257 ZnO/poly(diallyldimethylammonium chloride) (PDAD-MAC) 259 and ZnO/polyvinylpyrrolidone (PVP), 173,259 have been studied to serve as the efficient CBL in inverted PSCs to further improve the device performance. The improvement of photovoltaic parameters of inverted devices using ZnO/polymer hybrids or nanocomposite CBLs is summarized in Table 6. Shao et al. reported that the poly(ethylene oxide) (PEO) modification to the ZnO NP surface could effectively passivate the surface traps, suppress the recombination loss of carriers, reduce the R S as well as improve the electrical coupling of the ZnO/ active layer. 174 For the inverted devices composed of TQ1:PCBM and PCDTBT:PC 71 BM active layers, the PCE of the devices using 0.05% PEO-modified ZnO NP (PEO/ZnO wt%) layer increased to 5.4% and 6.59% from 4.5% and 5.39% for the reference devices based on unmodified ZnO. However, when the PEO content was increased to 1 wt%, the device performance decreased due to the very rough surface of ZnO-PEO films. To effectively hybridize ZnO with a polymer, both the amount and molecular weight of the polymer should be tailored to achieve the desired interface and electrical properties of ZnO/polymer nanocomposite CBLs, as well as the device performance. 174,258 Hu et al. fabricated ZnO/poly-(ethylene glycol) PEG hybrids as CBLs to improve the performance of inverted PSCs, as shown in Fig. 22a. 258 They demonstrated that the PEG can passivate the surface traps of ZnO NPs and thus suppressing the interfacial charge recombination, decreasing the work function and improving the energy level alignment between the PC 61 BM and ZnO. They suggested that the PEG chains wrapped around the ZnO surface could share their lone electron pairs of oxygen in the backbone with ZnO NPs, and thus passivate the shallow surface traps of ZnO. In their study, the effect of the molecular weight of PEG on the properties of ZnO-PEG CBLs and the performance of inverted PSCs has been investigated. It has been found that, compared to the devices with ZnO/PEG400 CBLs and ZnO/PEG20000 CBLs (with an average molecular weight of 400 and 20 000, respectively), the devices using ZnO/PEG6000 (with an average molecular weight of 6000) hybrids exhibited the best performance (PCE = 3.3%). This result was ascribed to the short backbone of PEG (e.g., M W = 400) containing less oxygen which could not effectively passivate ZnO surface traps. But the PEG with the long backbone (e.g., M W = 20 000) could lead to the formation of a charge transport barrier because of the insulating nature of PEG with the long backbone. 258 Recently, by in situ grown ZnO from amphiphilic fullerene-end-capped poly(ethylene glycol) (C60-PEG) at relatively low temperatures, Hu et al. employed this amphiphilic fullerene/ZnO hybrids (ZnO@C60-PEG) as CBLs to improve the charge selectivity of inverted PSCs. 261 They demonstrated that the C60-PEG could act as an n-dopant to the ZnO while the oxygen-rich PEG side chain of C60-PEG could passivate the defects of the ZnO. In addition, it was believed that the amphiphilic C60-PEG could facilitate the compatibility of the inorganic ZnO CBL and the organic active layer. As a result, compared to the pure ZnO, they demonstrated that the ZnO@C60-PEG possessed a decreased work function, less defects, and higher electron mobility, which could effectively reduce the recombination of carriers and enhance electron extraction. Consequently, the PCE of the inverted PSC based on the ZnO@C60-PEG CBL and with the PTB7: PC 71 BM active layer reached 8.0%. Due to the modification of C60-PEG to ZnO with fewer defects, the inverted PSCs with

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ZnO@C60-PEG hybrid CBLs also exhibited better stability than the devices with bare ZnO CBLs. Poly(zinc diacrylate) (pZA) was used to passivate the defects in ZnO CBLs, as shown in Fig. 22b. 257 By in situ cross-linking poly(zinc diacrylate) (pZA) on ZnO films with a 3D network, Wu et al. demonstrated that the incorporation of pZA into the ZnO NP layer could decrease the trap-assisted recombination rather than giving better energy alignment between the active layer and the ZnO interlayer. 257 Based on the pZA cross-linked ZnO CBL derived from ZnO NP solution containing 0.5 wt% zinc diacrylate (ZDA), the inverted PSCs composed of respective P3HT:PCBM and PTB7:PC 71 BM active layers achieved a PCE of 3.26% and 7.37%, respectively, while the PCE for the reference devices without pZA were 2.58% and 6.67% for P3HT:PCBM and PTB7:PC 71 BM active layers, respectively. The improvement in the device performance was attributed to the enhanced charge generation and transport. Polyvinylpyrrolidone (PVP) is yet another polymer studied for the preparation of ZnO/polymer nanocomposite CBLs for highly efficient inverted PSCs. 173 nanocomposite CBLs on the device performance. 259 They found that the combination of ZnO with PVP and PDADMAC gave the highest improvement in the PCEs. Besides serving as a matrix for suppressing ZnO aggregation and passivating the surface traps in ZnO films, it has also been found that the ZnO/polymer composite exhibited adjustable energy bands. 260 Chen et al. reported solution processed ZnO/polyethylenimine (PEI) nanocomposite electron transport layers for facilitating electron extraction in inverted PSCs. 260 They found that the energy bands of ZnO/PEI composites could be tuned considerably by varying the content of PEI, and thus forming a good electron transport layer. In addition, by adjusting the content of PEI, the structural order of ZnO in the ZnO/PEI films was adjusted to align perpendicularly to the ITO electrode, facilitating the electron transport vertically in ZnO/PEI nanocomposite films. 260 Fig. 22 (a) Schematic illustration of the inverted PSC with the ZnO/PEG hybrid film as the CBL. The inset is the AFM images and schematic diagrams of ZnO aggregates and PEG modified ZnO nano-clusters. 258 Adapted with permission from ref. 258 Copyright 2013 American Chemical Society. (b) Schematic diagram of an inverted PSC with ZnO/pZA CBL (left) and a brief processes in the fabrication of a pZA network (inset), and the AFM images of ZnO/pZA hybrid films with 0% and 0.5% pZA weight ratio to ZnO NPs (right), the scan size is 2 mm Â 2 mm. 257

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It is worth noting that, a crucial issue for ZnO-polymer composites or organic-stabilized ZnO NP CBLs is the removal of polymers or organic surfactants so as to improve the electrical properties of ZnO CBLs as well as the device performance. This is because the insulating polymer or the organic surfactant will aggravate the charge transport by introducing additional resistance. 172,173 Ultraviolet-ozone (UVO) treatment is regarded as a promising approach to remove the polymer from the ZnO surface and to ensure a good electrical coupling between ZnO and the active layer in view of the act that UVO treatment did not alter the size, shape or distribution of ZnO NPs. Cho et al. suggested that UVO treatment could effectively remove the residual organic stabilizer molecules on the surface of ZnO NP films through an UV induced decomposition mechanism. 172 Small et al. investigated the UVO treatment duration on the performance of inverted solar cells with the ZnO-PVP composite film as the CBL and found that ZnO-PVP nanocomposite films with UVO treated for 10 min led to the highest enhancement in the device performance compared to the devices without UVO treatment. 173 It has been found that the UVO treatment removed the top PVP enabling the ZnO NPs to expose at the film surface, which was proven by both AFM (Fig. 23a-d) and X-ray photoelectron spectroscopy (XPS) characterization techniques. The enhanced device performance with UVO treated ZNO-PVP nanocomposite films was ascribed to the modified surface composition promoting charge collection. The UVO treatment time is a crucial factor that determines the morphology and electrical properties of ZNO-PVP nanocomposite films. It has been demonstrated that a too short time treatment results in an incomplete removal of the PVP from the top surface of ZnO-PVP composite films, while a too long time treatment may induces excess oxygen onto the ZnO surface, which will reduce the electron extraction efficiency. 173 Over all, polymer modified ZnO NP/polymer hybrids or composite films are more uniform and with low defects than the unmodified ZnO NP films. By choosing a suitable polymer or an organic surfactant material, adjusting the amount and molecular weight of the polymer, and employing an optimized UVO treatment process, one can expect to enhance the performance of ZnO/polymer composite CBLs as well as the device performance.

ZnO and oxide or titanate composite CBLs
The composites of ZnO and other oxides or titanates have also been studied as CBLs of inverted PSCs. Lan et al. studied the Ta 2 O 5 -ZnO composite films with varied compositions fabricated by sol-gel processing as the CBL for inverted polymer solar cells, and demonstrated enhanced PCE with excellent stability. 263 It was found that the CBLs incorporated with Ta 2 O 5 exert two competing impacts on the solar cell performances. On one hand, the presence of Ta 2 O 5 is likely to induce more positive charges around the Zn atom and form Ta-O-Zn bonding; it can reduce the surface charge recombination between the active layer and the Ta 2 O 5 -ZnO composite CBL, and result in high power conversion efficiency; however, on the other hand an excessive amount of Ta 2 O 5 would block the pathways of charge transport and lead to a drastic reduction in power conversion efficiency. In their results, the integration of 12-15% Ta 2 O 5 into ZnO may result in the efficiency enhanced maximally for different active layers; the addition of excessive Ta 2 O 5 would however lead to a dramatic decrease in the device performance, as per the photovoltaic parameters shown in Table 7. In addition, the inverted PSCs with Ta 2 O 5 -ZnO nanocomposite CBLs exhibited excellent longterm stability in air. In their work, the inverted PSCs with ZnO or Ta 2 O 5 -ZnO nanocomposite CBLs retained almost 100% of their original PCEs after being exposed under ambient conditions for 42 days without encapsulation.
In another work also done by Lan et al., dual phase SrTiO 3 : ZnO nanocomposite films with varied composition ratios were fabricated by sol-gel processing and applied as CBLs in inverted PSCs, and demonstrated enhanced device performance. 98 In their work, when the SrTiO 3 : ZnO nanocomposite CBLs were assembled into the solar cell, the device properties including incident photon-to-current conversion efficiency (IPCE), power conversion efficiency, and electron mobility were investigated systematically. SrTiO 3 in CBLs was found to be amorphous or quasi-amorphous. Although more detailed experiments are needed, SrTiO 3 is more likely to have some local ordering structure with aligned TiO 6 octohedra, i.e., a quasi-amorphous phase, and thus possesses spontaneous polarization. It has been demonstrated that the localized polar molecules SrTiO 3 in the nanocomposite CBL are in random orientations under the dark conditions or open circuit voltage conditions (Fig. 24a), while a self-built electric field on the interface between the active layer and CBL was generated by electron and hole transfer to the opposite direction and there is a net interface dipole directed away from the cathodic buffer layer under illumination (Fig. 24b). It was suggested that such a spontaneous polarization is likely to induce a self-built electric field to prevent electron recombination on the interface of the active layer and the CBL, which shows an enhancement of FF of device performance. As a result, an enhancement in PCE from 3.58% to 4.1% was achieved for the device with SrTiO 3 : ZnO of 90 : 10 CBL and P3HT:PCBM active layer. 98 Fig. 24c shows the I-V curves of inverted devices with various SrTiO 3 : ZnO nanocomposite CBLs. It was suggested that a further improvement of the solar cell performance could be expected by improving the crystallinity of both SrTiO 3 and ZnO.

Concluding remarks
PSCs are complex devices, so far high performance can only be achieved when all the following requirements are met: (1) a good p-type polymer donor and n-type fullerene derivative acceptor with the desired electronic structure, (2) a bulk heterojunction with the desired micro and nanostructures, and (3) interfaces between different layers for efficient charge separation and transport. Inverted PSCs introduce a cathode buffer layer of electron transport but hole blocking material, and thus demand extra efforts to understand and engineer both the interface and the material in terms of chemistry, crystallinity, morphology, surface roughness and contacts, chemical and physical stability, and electrical and electronic properties.
To achieve high power conversion efficiency, long-term device stability and compatible with the low-cost roll-to-roll manufacturing, much more efforts in the aspect of developing new materials, improving chemical and physical properties of interfaces, and low cost scalable processing are required. Further fundamental understanding and innovation and engineering of CBLs are an  important aspect for the achievement of highly efficient inverted PSCs. ZnO is the most extensively investigated material serving as CBLs in inverted PSCs due to its relatively high electron mobility, high transparency, ease of fabrication by solution-based processing at low temperature, good chemical stability and low cost. The role of ZnO CBLs in an inverted PSC is to collect and transport electrons and block holes. The contact between ZnO CBLs and the polymer active layer can significantly affect the performance of inverted polymer solar cells. The contact quality relies on both the preparation method and the post-treatment, which can influence the properties of ZnO CBLs in terms of morphology, thickness and transmittance, and the surface state. Doping and surface modification are effective ways to tailor the electronic, optical and chemical properties of the ZnO CBL and thus to improve the photovoltaic performance of the inverted PSCs. To further improve the efficiency of inverted PSCs, some specific considerations with regard to the ZnO-based CBL are listed as follows: (1) Development of ZnO-based CBLs with dense packing, a large surface area and high transmittance. A dense and homogenous surface of ZnO CBLs favors the formation of an intimate contact between ZnO CBLs and the BHJ photoactive layer, and thus lowers the contact resistance and decreases the series resistance (R S ) of the device. In addition, the dense surface also makes the ZnO layer having better electrical properties and higher transmittance. The large surface area of ZnO CBLs can enlarge the ZnO/BHJ active layer contact area, thus providing larger electron extraction interface and enabling more electrodes to penetrate into the BHJ active layer so as to reduce the charge transport distance and increase the J sc of devices.
(2) Doping ZnO for higher conductivity and appropriate energy level. Appropriate doping can enhance the electrical conductivity of ZnO and thus improve the photovoltaic performance of inverted PSCs.
(3) Development of interfacial materials for the surface modification of ZnO-based CBLs, for instance the functional fullerene based SAMs, with appropriate surface energy and improved interfacial properties. The surface modification of ZnO-based CBL helps to reduce the charge recombination and improve the charge transport at the interface by passivating the surface defects, tuning the surface energy and work function, and may thus lead to improved photovoltaic performance of the inverted devices.