Planar heterojunction organometal halide perovskite solar cells: roles of interfacial layers

Abstract

Organometal halide perovskites are promising photo-absorption materials in solar cells due to their high extinction coefficient, broad light absorption range and excellent semiconducting properties. The highest power conversion efficiency (PCE) of perovskite solar cells (PrSCs) is now 20.1%. However, a high-temperature processed mesoscopic metal oxide (e.g., TiO₂) must be removed to realize flexible PrSCs on plastic substrates using low temperature processes. Although the planar heterojunction (PHJ) structure can be considered as the most appropriate structure for flexible PrSCs, they have shown lower PCEs than those with a mesoscopic metal oxide layer. Therefore, development of interfacial layers is essential for achieving highly efficient PHJ PrSCs, and necessary in fabrication of flexible PrSCs. This review article gives an overview of progress in PHJ PrSCs and the roles of interfacial layers in the device, and suggests a practical strategy to fabricate highly efficient and flexible PHJ PrSCs. We conclude with our technical suggestion and outlook for further research direction.

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Hobeom Kim

Hobeom Kim is currently a PhD candidate in the Department of Materials Science and Engineering at Pohang University of Science and Technology (POSTECH), Korea. He received his BS in the School of Advanced Materials Science and Engineering from Sungkyunkwan University (SKKU), Korea in 2011. His current research activity is focused on perovskite and flexible optoelectronics.

Kyung-Geun Lim

Kyung-Geun Lim received his BS in the Department of the Nanoscience and the Nanotechnology from Pusan National University, Korea in 2009, MS and PhD in the Department of Materials Science and Engineering from Pohang University of Science and Technology (POSTECH), Korea in 2011 and 2015 respectively, under supervision of Prof Tae-Woo Lee. He has been performing research on the improvement of device efficiency and lifetime of organic solar cells (OSCs) and perovskite solar cells (PrSCs). His current research interests include understanding various mechanisms of charge extraction layers in OSCs and PrSCs.

Tae-Woo Lee

Tae-Woo Lee is an associate professor in the Department of the Materials Science and Engineering at Pohang University of Science and Technology (POSTECH), Korea. He received his PhD in Chemical Engineering from KAIST, Korea in 2002. Then, he joined Bell Laboratories, USA as a postdoctoral researcher. He worked in Samsung Advanced Institute of Technology as a member of research staff (2003–2008). He received a prestigious Korea Young Scientist Award from the President of Korea in 2008 and The Scientist of the Month Award from the ministry of science, ICT and future planning in 2013. His research focuses on printed electronics and optoelectronics based on organic, inorganic, and organic–inorganic hybrid perovskite materials for flexible electronics, displays, solid-state lightings, and solar energy conversion devices.

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1. Introduction

Organometal metal halide perovskites are promising materials for various optoelectronic and photonic devices including solar cells,^1–4 light-emitting diodes,^5–7 and photodiodes.^8–10 Furthermore, low threshold lasing has been reported from these materials.^11–13 Active research based on organic–inorganic hybrid perovskites has focused on perovskite solar cells (PrSCs) because of the recent rapid achievements in increasing their power conversion efficiency (PCE): the first liquid-junction PrSCs were reported in 2009¹⁴ and PCEs of solid-state PrSCs reached 20.1% in 2015.⁴

This rapid increase in PCE has been achieved mainly by improving the device architecture and fabrication techniques. In addition to high PCE, PrSCs have two great advantages. (1) Organic–inorganic hybrid perovskites have a high absorption coefficient, so a thin (<500 nm) film of perovskite light absorber is sufficient as a photo-active layer in PrSCs. Therefore, the devices can be thin and light. (2) With these features, flexible, large-area PrSCs can be fabricated at low cost. Because of these advantages and high PCE, PrSCs have the potential to replace conventional Si solar cells, and to provide portable, mobile, and wearable power sources.

PrSCs have inherited the architecture of dye-sensitized solar cells (DSSCs),^15–17 most of which include a mesoscopic metal oxide layer (e.g., TiO₂), so the structure of early-stage PrSCs was similar to those of DSSCs,^14,18 and many studies still present mesoscopic PrSCs with very high PCEs. However, this type of PrSC cannot be developed as a flexible device because the formation of the mesoporous structure requires high temperature T > 450 °C, which damages most flexible plastic substrates. Also, the high-T processes for development of mesoscopic PrSCs make it difficult to complete the tandem structure which is a promising configuration for increasing PCE of solar cells, but need low-T processable interconnecting layers between bottom and upper cells to avoid destruction of layers underneath it.

Instead, the use of planar heterojunction (PHJ) PrSCs that include low-T processable interlayers without the mesoscopic structure can provide a method to fabricate flexible PrSCs and tandem PrSCs. Furthermore, nonuse of the mesoscopic structure simplifies the structure of PrSCs and reduces the cost and time for fabrication. Theoretical and experimental research on PHJ PrSCs has led to PCE > 19%.¹⁹ Many studies on PHJ PrSCs to date have focused on engineering of the perovskite film as a photo-active layer to make it favourable for solar cells,^20–46 but studies on interlayer engineering of PrSCs are relatively lacking. PHJ PrSCs with high open circuit voltage V_oc, high short circuit current J_sc, and high fill factor FF can be achieved by using appropriate interfacial layers between the electrodes and perovskite light absorber. Basically, to maximize the built-in potential of the device and to facilitate charge transfer from perovskite to interfacial layers, the PHJ PrSCs should use a hole extraction layer (HEL) and an electron extraction layer (EEL) with well-matching ionization energy (IE) and electron affinity (EA), respectively, to those of perovskites.^47–49 Also, the use of interfacial layers with high electrical conductivity is favourable to make efficient charge transport and extraction to each electrode.

The defect chemistry of perovskites is another topic of study. Traps in the perovskite film and at the interface between perovskite and interfacial layers have a strong influence on the photophysical and photoelectrical properties of devices and thereby on device performance. High density of the trap states results in non-radiative recombination which is an undesirable physical process in PrSCs that leads to hysteresis behaviour, depending on photocurrent measurement conditions.^50–54 Due to these reasons, to improve the performance of PHJ PrSCs, the interfacial layers between the electrodes and perovskite absorber must be engineered appropriately.

In this review, we focus on the development of PHJ PrSCs which have a strong potential to be applied to flexible devices, and especially focus on the work on interfacial layer engineering. Sections 2 and 3 provide basic explanations of structural and physical properties of perovskites and on progress in PrSCs, particularly those of the PHJ type. Section 4 reviews the functions of interfacial layer in PHJ PrSCs. Section 5 reviews research on flexible PHJ PrSCs. Section 6 presents conclusions.

2. Principles of organic–inorganic perovskite materials

Perovskites are crystal structures that have originated from the calcium titanium oxide mineral (CaTiO₃). Generally, perovskites have an ABX₃ formula, where cation A is located at the corner positions (0,0,0), cation B is at the centre (1/2,1/2,1/2) and monovalent anion X is at the centre of the six planes (1/2,1/2,0) assuming them as the idealized cubic unit cell (Fig. 1). The most widely-used component materials of perovskites for PrSCs are organic molecule cations based on amine at A sites (e.g., C_nH_2n+1NH₃⁺, HC(NH₂)₂⁺), metal cations (e.g., Pb²⁺, Sn²⁺, Cu²⁺) at B sites, and halide anions (Cl⁻, Br⁻, I⁻) at X sites. Various kinds of organic–inorganic hybrid perovskites are based on the configuration that Mitzi et al. formulated by incorporating an organic modulation layer between metal halide sheets.⁵⁵ The formation of perovskite structures can be estimated from the tolerance factor⁵⁶
and octahedral factor

μ = R_B/R_X,

where R_A, R_B, and R_X are the effective ionic radii of A, B, and X, respectively. Perovskite structures are stable when 0.813 ≤ t ≤ 1.107 and 0.442 ≤ μ ≤ 0.895.⁵⁷

Fig. 1 (a) Crystal structure of an organic–inorganic perovskite. (b) The generated species, free carriers or excitons after photo-excitation of the perovskite.

Some perovskites (e.g., MAPbI₃, MAPbI_xCl_3−x and FAPbI₃ [MA: methylammonium CH₃NH₃, FA: formamidinium HC(NH₂)₂]) have broad absorption spectra with a suitable band gap ∼1.5–1.6 eV, and therefore are used as light absorbers in PrSCs. In addition to absorption spectra, perovskites that are suitable as light harvesters in PrSCs should have an appropriate dielectric constant ε, exciton binding energy E_b, Bohr radius, and diffusion length of charge carriers. These factors have a strong relationship with photophysical and photoelectrical properties and solar cell device performance. The photo-generated electron–hole pairs in these organic–inorganic perovskites can exist as free charge carriers or excitons with E_b, and the excitons can be classified as Frenkel-type or Wannier-type depending on the perovskite E_b. Many researchers have reported that the E_b of perovskite materials (29–50 meV for MAPbI₃^58–61 and 35–98 meV for MAPbCl_3−xI_x^62,63) comparable to thermal energy (∼25 meV) at room temperature facilitates exciton dissociation. Recently, there has been a report that MAPbI₃ has an E_b of 2 meV which is very lower compared to inorganic semiconductors (e.g., Si, 15.0 meV; GaAs, 4.2 meV; CdTe, 10.5 meV).⁶⁴ Although which species are generated under illumination and what is the make-up of the species in perovskite materials are still under debate, free carriers easily dissociated from excitons contribute to ambipolar charge transport in the materials.^65,66 Based on these excellent capabilities of light absorption and photophysical/photoelectrical properties, perovskites are considered as promising materials for solar cell devices.

3. Progress in perovskite solar cells

3.1. Mesoscopic vs. Planar

Miyasaka et al. (2009) developed the first PrSCs; the devices had a perovskite photosensitizer but otherwise had a structural configuration nearly identical to conventional DSSCs.¹⁴ The structure of devices was TiCl₄ treated fluorine-doped tin oxide (FTO) (anode)/mesoporous TiO₂/perovskite sensitizer/Pt-coated FTO (cathode); the gap between the electrodes was filled with liquid electrolyte. With this structure, the PCEs of devices using MAPbBr₃ and MAPbI₃ were 3.13 and 3.81%, respectively. Subsequent research by Park et al. (2011) used the initial liquid-junction device configuration and achieved an improved PCE = 6.5%, but the liquid electrolyte dissolved the perovskite crystal, so stability of the device could not be guaranteed.¹⁸ This problem was solved by using a solid-state hole conductor, spiro-OMeTAD, on top of the perovskite layer instead of using the liquid electrolyte;¹ the resulting device showed long-term stability for over 500 h and PCE > 9% (Fig. 2a–e). Grätzel et al. (2013) introduced a sequential deposition method in which PbI₂ was first deposited on a mesoporous metal oxide and transformed into a perovskite by exposing methylammonium iodide (MAI).³ Using this method, the devices had greatly increased reproducibility and PCE ≈ 15% (Fig. 2f and g). Seok et al. (2014) achieved a PCE = 16.2% by depositing a uniform and dense perovskite layer by using a mixed solvent of γ-butyrolactone and dimethylsulphoxide (DMSO), and then drop-casting toluene.⁶⁷ Very recently, Seok et al. (2015) achieved a record PCE > 20% by exploiting the intramolecular exchange process between formamidinium iodide (FAI) and DMSO molecules intercalated in PbI₂ (Fig. 2h and i).⁴ This remarkably rapid progress in development of PrSCs and several advantages such as thinness, light weight, and inexpensive processability have made PrSCs appropriate for various solar cell applications.

Fig. 2 Device configurations and J–V characteristics of mesoscopic PrSCs. (a) A real device of the first solid-state mesoscopic perovskite solar cells. (b) A schematic image of the device cross-section. (c) The SEM image of the device cross-section. (d) The magnified cross-sectional SEM image of the FTO/underlayer/active layer region. (e) J–V characteristics of the device at one sun condition. Reproduced from ref. 1 with permission from Nature Publishing Group. (f) The cross-sectional SEM image of another mesoscopic perovskite solar cell. (g) J–V characteristics of the device under simulated AM1.5G solar irradiation (solid line) and in the dark (dashed line) (reproduced from ref. 3 with permission from Nature Publishing Group). (h) The cross-sectional FESEM image of the device with PCE over 20%. (i) J–V characteristics of the device measured in reverse and forward directions under standard AM 1.5G illumination. Reproduced from ref. 4 with permission from Science (AAAS).

Many electronic devices including PrSCs are being developed toward a flexible configuration for future electronics. However, mesoscopic PrSCs cannot easily be made flexible because generally the mesoscopic structure should be sintered at T > 450 °C to function as an EEL in a device,^1–4 but most available flexible substrates (e.g., PET, PEN) have a glass transition temperature that is lower than the sintering temperature, so the polymer substrates deform under high sintering temperature. Therefore, to establish flexible PrSCs, a new device structure that can be fabricated at low T is required.

PHJ PrSCs that do not use the mesoscopic structure are practical options for flexible PrSCs owing to their low-T processability. PHJ PrSCs are divided into two groups according to the layer stacking sequence: i.e., the conventional n–i–p structure in which the EEL is deposited on the bottom cathode first, the HEL is underneath the top anode layer, and the perovskite is sandwiched between EEL and HEL; and the p–i–n structure, which is the inverse of the n–i–p type.

The n–i–p configuration of PHJ devices was introduced in 2013 as a modification of meso-superstructured (MSSC) PrSCs that incorporated a mesoporous insulating scaffold of Al₂O₃ on a compact TiO₂ layer.⁶⁸ The device had a PCE = 4.9% without a mesostructure which is the PHJ type; the authors described that the MAPbI_3−xCl_x perovskite absorber can fulfill both light absorption and efficient charge transport with minimal recombination in the film. Based on these results, they questioned whether a mesostructure is essential in PrSCs. Although the device included a high-T (>500 °C) processed compact TiO₂ layer, many researchers then sought to develop n–i–p type PHJ PrSCs that could be fabricated using low-T processing.^19,69–82

The p–i–n type PHJ PrSC was first developed by Guo et al. in 2013.⁸³ The researchers noticed that the concept of donor/acceptor could be applicable to the device and introduced the structure after referring to a previous work by Etgar et al., who reported that the MAPbI₃ perovskite acts as both the light absorber and hole conductor.⁸⁴ The first p–i–n PHJ PrSC employed poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as a p-type layer for hole extraction, and fullerene-based molecules as the acceptor (n-type) that were deposited on the perovskite (donor) layer. In addition to PEDOT:PSS, various kinds of efficient p-type layers such as NiO_x,⁸⁵ CuSCN,⁸⁶ graphene oxide (GO)⁸⁷ and reduced graphene oxide (rGO)⁸⁸ for p–i–n type PHJ PrSCs have been reported. Most p–i–n type devices can be fabricated at T < 150 °C, so this configuration is regarded as the most suitable for flexible devices.

3.2. Progress in low-temperature processed PHJ PrSCs

In this section, we present some examples and progress of solution processed n–i–p and p–i–n type PHJ PrSCs, and vacuum processed PHJ devices.

Solution processed n–i–p (normal) structure. After n–i–p type PHJ PrSCs were shown to be operational, many researchers tried to develop and improve the PCE of the devices. However, most studies on this type of PrSC have described use of a TiO₂ compact layer processed at T > 450 °C, which is not compatible with fabrication of flexible devices.^2,89–104 Therefore, studies on low-T processable EELs have been required.

Grätzel et al. improved the suitability of a TiO₂ layer on FTO for electron extraction by immersing the substrate in TiCl₄ solution at T = 70 °C;⁷⁸ the resultant TiO₂ was a rutile nanoparticle that could form an intimate nanocrystalline junction with a perovskite layer (Fig. 3a). A device with the FTO/TiO₂/MAPbI₃/Spiro-OMeTAD/Au structure achieved a PCE = 13.7% (Fig. 3b). Boyen et al. used the TiO₂ layer as an ETL based on titanium isopropoxide processed at a low T = 135 °C.⁷⁰ The device had the ITO/TiO₂/MAPbI_3−xCl_x/P3HT/Ag structure and a PCE = 13.6%; the authors stated that the chemical composition and conditions of thermal treatment on the TiO₂ film have critical effects on the device performance. Yang et al. achieved PCE up to 19.3% (average 16.6%) which is so far the highest value in an n–i–p type PHJ PrSC (Fig. 3c and d).¹⁹ The authors introduced an enhanced reconstruction process for the perovskite film by annealing the as-deposited film in humidity-controlled air. For efficient electron transport, they chose poly-(ethyleneimine ethoxylated) (PEIE) processed at 100 °C and Yttrium-doped TiO₂ processed at 150 °C. From transient photovoltage and photocurrent measurements, they found that an extended electron carrier lifetime and reduced trap densities in the perovskite film and faster photocurrent decay of the device using modified TiO₂ contributed superior device performance. In addition to the TiO₂-based EEL, researchers have developed other low-T processable EELs such as ZnO nanoparticles, CdSe nanocrystals, SnO₂, WO_x, and phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM).^{67,69,71–73,75,77}

Fig. 3 (a) Cross-sectional SEM image of a perovskite film deposited on the nanocrystalline TiO₂ film. (b) J–V curves of the n–i–p PHJ PrSC device that used rutile TiO₂. The inset shows a schematic image of the device and SEM images of perovskite and rutile TiO₂ layers. Reproduced from ref. 78 with permission from the American Chemical Society. (c) The cross-sectional SEM image of the n–i–p PHJ PrSC device. (d) J–V curves of the device. Reproduced from ref. 19 with permission from Science (AAAS).

Solution processed p–i–n (inverted) structure. The first reported p–i–n type PHJ PrSCs (Section 3.1) achieved a PCE = 3.9% with the ITO/PEDOT:PSS/MAPbI₃/PCBM/BCP/Al structure (Fig. 4a and b).⁸³ Although the PCE was much lower compared to mesoscopic PrSCs at that time, this work had significant meaning in developing a new design for PrSCs that could be processed at T < 150 °C. Subsequently, Snaith et al. demonstrated devices with the FTO/PEDOT:PSS/MAPbI_3−xCl_x/PCBM/TiO_x/Al structure, which achieved a PCE = 9.8%,¹⁰⁵ and Yang et al. demonstrated a device with ITO/PEDOT:PSS/MAPbI_3−xCl_x/PCBM/Al that achieved a PCE = 11.5%.¹⁰⁶

Fig. 4 (a) Device structure of the first p–i–n PHJ PrSCs. (b) J–V characteristics of the devices with different acceptor materials. (Reproduced from ref. 83 with permission from Wiley-VCH) (c) The device structure of p–i–n PHJ PrSCs. (d) A processing scheme for formation of a perovskite film by the hot-casting method and optical microscopic images illustrating grain formation under various conditions. (Reproduced from ref. 46 with permission from Science (AAAS).)

Based on these reports of inverted PHJ PrSCs with reasonably high PCEs, many groups have been working on this type of PrSC and have reported increasing PCE. Huang et al. used an interdiffusion method which involves stacking layers of PbI₂ and MAI in a sequence by spin-coating.²¹ The MAPbI₃ layer obtained by the method uniformly formed without pin-holes. Controlling the thickness of the perovskite layer, the concentration of MAI solution, and the duration of heating treatment on the perovskite yielded a device that had a PCE = 15.3%.

Some research studies have focused on increasing the grain size and crystallinity of perovskite light absorbers. One method is the solvent-annealing method in which the MAPbI₃ film is exposed to dimethylformamide (DMF) during thermal-annealing.²⁵ The grain size of the solvent-annealed perovskite increased to 1 μm, which was even larger than only thermal-annealed perovskite grain (∼260 nm). The lower density of grain boundaries in the solvent-annealed perovskite film led to a lower density of traps and an increased diffusion length of charge carriers. As a result, the device achieved a high PCE = 15.6%. Mohite et al. introduced a solution-based hot-casting technique to deposit MAPbI_3−xCl_x.⁴⁶ The technique used a hot (70 °C) mixture of PbI₂ and MACl solution, and the solution was cast onto a hot (180 °C) substrate. This method yielded millimeter-scale crystalline grains of MAPbI_3−xCl_x. The film had reduced bulk defects and improved charge carrier mobility of the perovskite so that a device with the perovskite showed a very high PCE approaching 18% with the ITO/PEDOT:PSS/MAPbI_3−xCl_x/PCBM/Al structure (Fig. 4c and d).

In addition to the explained examples above, numerous other low-T and solution-processed PHJ PrSCs have been reported. We give their summaries including their device structures and photovoltaic parameters (Table 1 for n–i–p PHJ PrSCs and Table 2 for p–i–n PHJ PrSCs). Most research studies on solution-processed PHJ PrSCs have considered various approaches to engineering of the perovskite layer (e.g., thermal annealing,^{24,28,30,70,74,107} solvent-assisted processes,^{25,29,43,71,108,109} additive treatment^{20,23,26,33,34,38,44,72,110} and variation of component materials^{22,28,31,36,42,111,112}). Several other studies have considered interfacial layers of PHJ PrSCs.

Table 1 Summary of information of solution processed n–i–p PHJ PrSCs at low-T

Device architecture	V oc (V)	J sc (mA cm^−2)	FF (%)	PCE (%)	Remarks	Ref.
a P: perovskite layer engineering. b E: EEL engineering. c H: HEL engineering. d TA: thermal annealing. e SA: solvent annealing. f SAM: self-assembling monolayer. g WS-C60: water-soluble fullerene derivative. h MWNT: multi-walled carbon nanotube. i P3HT: poly(3-hexylthiophene-2,5-diyl). j PEI: polyethyleneimine. k PTAA: poly(triarylamine). l PEIE: poly-(ethyleneimine ethoxylated). m C3-SAM: 3-aminopropanioc acid. n spiro-OMeTAD: 2,20,7,70-tetrakis(N,N0-di-p-methoxyphenylamine)-9,90-spirobifluorene.
ITO/ZnO NP/MAPbI3/P3HT^i/Ag	0.94	17	62	11.8	P^a: control of morphology (thickness)	69
ITO/TiO2/MAPbI3−xClx/P3HT/Ag	0.936	21.0	69.1	13.6	P: control of morphology (TA^d time)	70
ITO/SnO2/MAPbI3/spiro-OMeTAD^n/Ag	1.08	19.5	61.6	13.0	P: control of morphology (SA^e time)	71
FTO/TiO2/MAPbI3−xClx/spiro-OMeTAD/Au	0.94	19.53	70.35	13.2	P: additive (poly(ethylene glycol))	72
ITO/ZnO nanoparticles/MAPbI3/spiro-OMeTAD/Ag	1.03	20.4	74.9	15.7	E^b: control of thickness	73
ITO/CdSe nanocrystal/MAPbI3/spiro-OMeTAD/Ag	0.99	17.4	67.9	11.7	E: control of thickness and TA temperature	74
FTO/PEI^j/PC61BM/MAPbI3/PTAA^k/Au	0.98	21.8	72	15.3	E: control of concentration	75
ITO/Zr-TiO2/MAPbI3−xClx/spiro-OMeTAD/Au	1.021	20.3	76.6	15.7	E: additive (ZrAc4)	76
ITO/PEIE^l/Y:TiO2/MAPbI3−xClx/spiro-OMeTAD/Au	1.13	22.75	75.01	19.3	E: doping (Yttrium)	19
ITO/ZnO/C3-SAM^m/MAPbI3/spiro-OMeTAD/MoO3/Ag	1.07	22.51	65	15.67	E: SAM^f deposition (3-aminopropanoic acid)	82
FTO/TiO2/MAPbI3/spiro-OMeTAD/Au	1.05	19.8	64	13.7	E: use of rutile TiO2	78
FTO/WOx/MAPbI3−xClx/spiro-OMeTAD/Ag	0.71	21.77	58	8.99	E: use of WOx	79
ITO/TiOx/PC61BM/WS-C60/MAPbI3/P3HT/MoOx/Al	0.95	27.4	56.3	14.6	E: comparison of TiO2, TiO2/PC61BM and TiO2/PC61BM/WS-C60^g	131
FTO/TiO2/MAPbI3/spiro-OMeTAD/spiro-OMeTAD:MWNTs/Au	—	—	—	15.1	H^c: additive (MWNTs^h) and use of hierarchical structure	80
ITO/TiO2/MAPbI3−xClx/DOR3T-TBDT/MoO3/Ag	0.97	20.7	74	14.9	H: dopant-free	81
FTO/ZnO nanoparticles/MAPbI3/carbon	0.77	18.56	56	8.07	Use of carbon electrode	77

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Table 2 Summary of information of solution processed p–i–n PHJ PrSCs at low-T

Device architecture	V oc (V)	J sc (mA cm^−2)	FF (%)	PCE (%)	Remarks	Ref.
a P: perovskite layer engineering. b E: EEL engineering. c H: HEL engineering. d MSSC: meso-superstructured. e SA: solvent annealing. f TA: thermal annealing. g rGO: reduced graphene oxide. h GO: graphene oxide. i PEIE: poly-(ethyleneimine ethoxylated). j P3TMAHT: poly[3-(6-trimethylammoniumhexyl)thiophene]. k PN4N: amino-functionalized polymer. l IPA: isopropanol. m PFN: poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]. n N2200: poly{[N,N′-bis(2-octyldodecyl)-1,4,5,8-naphthalene diimide-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)}. o PNVT-8: poly{[N,N′-bis(alkyl)-1,4,5,8-naphthalene diimide-2,6-diyl-alt-5,5′-di(thiophen-2-yl)-2,2′-(E)-2-(2-(thiophen-2-yl)vinyl)thiophene]}. p PDINO: perylene-diimide. q PCBC: [6,6]-phenyl-C61-butyric acid (18-crown-6)-2-yl methyl ester. r TIPD: titanium (diisopropoxide) bis(2,4-pentanedionate). s TPPI: tetraphenylphosphonium iodide. t SOHEL: self-organized hole extraction layer. u PC61BM: [6,6]-phenyl C61 butyric acid methyl ester. v IC60BA: indene-C60 bisadduct.
FTO/PEDOT:PSS/MAPbI3−xClx/PC61BM/TiOx/Al	0.94	15.8	66	9.8	Comparison between p–i–n and MSSC^d devices	105
ITO/PEDOT:PSS/MAPbI3−xClx/PC61BM/Al	0.87	18.5	72	11.5	Demonstration of low-T processability	106
ITO/PEDOT:PSS/MAPb0.85Sn0.15I3−xClx/Bis-C60/PC61BM/Ag	0.76	19.1	66	10.1	P^a: substitution of Sn for Pb	111
ITO/PEDOT:PSS/MAPbI3−xClx/PC61BM/Bis-C60/Ag	0.92	17.5	73	11.8	P: additive (1,8-diiodooctane)	20
ITO/PEDOT:PSS/MAPbI3/PC61BM/C60/BCP/Al	0.96	21.0	76	15.6	P: control of morphology (thickness and SA^e)	25
ITO/PEDOT:PSS/MAPbI3−xClx/PC61BM/PFN/Al	1.05	20.3	80.2	17.1	P: control of morphology (TA^f environment)	107
ITO/PEDOT:PSS/MAPbI3−xClx/PC61BM/C60/Ag	0.91	18.3	70	11.65	P: control of morphology (precursor ratio)	36
ITO/PEDOT:PSS/MAPbI3−xClx/PC61BM/Ca/Al	0.92	16.8	72	11.1	P: control of thickness and morphology (substrate heating), and use of spray coating method	132
ITO/PEDOT:PSS/MAPbI3/PC61BM/C60/BCP/Al	0.99	19.6	79.3	15.4	P: control of thickness and concentration of MAI	21
ITO/PEDOT:PSS/MAPbI3/IC60BA/C60/BCP/Al	0.97	15.7	80.1	12.2	P: control of morphology (precursor ratio) and thickness, E^b: comparison of PC61BM^u/C60, IC60BA^v/C60 and C60	22
ITO/PEDOT:PSS/MAPbI3/PC61BM/Al	0.91	10.8	76	7.4	P: control of thickness	133
ITO/PEDOT:PSS/MAPbI3−xClx/PC61BM/Bis-C60/Ag	0.94	18.47	75	13.09	P: additive (1,4-diiodobutane)	23
ITO/PEDOT:PSS/MAPbI3/PC71BM/Ca/Al	1.05	19.98	78	16.31	P: control of concentration of MAI, E: comparison between PC61BM and PC71BM, and control of SA time	108
ITO/PEDOT:PSS/MAPbI3/PC61BM/C60/BCP/Al	∼0.9	19.0	78	13.4	P: control of morphology (TA time)	24
ITO/PEDOT:PSS/MAPbI3/PC71BM/Ag	0.87	18.66	75	12.22	P: control of morphology (precursor solvent), E: comparison between PC61BM and PC71BM	134
ITO/PEDOT:PSS/MAPbI3−xClx/PC61BM/Al	0.91	17.27	75	11.80	P: control of morphology (solvent evaporation rate)	37
ITO/PEDOT:PSS/Cs0.1MA0.9PbI3/PC61BM/Al	1.05	10.10	73	7.68	P: substitution of Cs for MA	112
ITO/PEDOT:PSS/MAPbI3/PC61BM/Al	0.88	14.08	80.11	9.93	P: additive (NH4Cl)	26
ITO/PEDOT:PSS/MAPbI3/PC61BM/Al	0.92	8.74	76	6.16	P: control of morphology (precursor solvent)	27
ITO/PEDOT:PSS/MAPbI3−xClx/PC61BM/Al	0.86	14.3	60.9	7.5	P: control of morphology (precursor ratio, TA temperature and time)	28
ITO/PEDOT:PSS/MAPbI3/PC61BM/C60/BCP/Al	0.96	21.0	76	15.6	E: trap passivation (TA time)	50
ITO/PEDOT:PSS/MAPbI3/PC61BM/Bis-C60/Ag	0.91	19.3	80	14.1	P: control of morphology (solvent washing)	29
ITO/PEDOT:PSS/MAPbI3/PC61BM/PFN-Br/Ag	1.03	18.36	78	14.75	P: control of morphology (vacuum-assisted TA)	30
ITO/PEDOT:PSS/MAPbI3/PC61BM/Bis-C60/Ag	0.91	18.38	73	12.21	P: use of blade-coating method	135
ITO/PEDOT:PSS/MAPbIxCl3−x/PC61BM/Bis-C60/Ag	0.89	16.0	74	10.5	P: substitution of Br for IxCl3−x	31
ITO/PEDOT:PSS/MAPbI3/PC61BM/C60/BCP/Al	0.93	17.3	63	10.2	P: use of spray-coating method	136
ITO/PEDOT:PSS/MAPbI3−xClx/PC61BM/Al	0.854	16.7	63.4	8.97	P: additive (1-chloronaphthalene)	33
ITO/PEDOT:PSS/MAPbI3/PC61BM/Ca/Al	0.91	13.11	78.11	9.32	P: additive (NH4Cl)	38
ITO/PEDOT:PSS/MAPbI3/PC61BM/Ca/Au	1.1	20.9	79	18.2	Comparison between p–i–n and n–i–p structure	137
ITO/PEDOT:PSS/MAPbI3−xClx/PC61BM/ZnO/Al	1.027	22.0	74.2	16.8	P: control of morphology (growth rate)	39
ITO/PEDOT:PSS/MAPbI3−xClx/PC61BM/Al	0.86	16.8	70.4	10.9	P: additive (ethylammonium iodide)	34
ITO/PEDOT:PSS/MAPbI3−xClx/PC61BM/Ag	0.91	17.76	73	11.82	P: comparison between (MAPbI3−xClx and MAPbI3)	138
ITO/PEDOT:PSS/MAPbI3/PC61BM/C60/BCP/Al	0.90	20.00	75.1	13.60	P: control of morphology (vapour atmosphere)	40
ITO/PEDOT:PSS/MAPbI3/PC61BM/C60/BCP/Al	0.91	21.06	60.84	11.66	P: control of morphology (liquid droplet assisted)	139
ITO/PEDOT:PSS/MAPbI3/PC61BM/BCP/Al	0.83	19.86	59	9.75	P: control of morphology (solvent-assisted deposition)	109
ITO/PEDOT:PSS/MAPbI3−xClx/PC61BM/Al	0.95	19.79	63.2	11.88	P: control of morphology (thickness)	41
ITO/PEDOT:PSS/MAPbI3/PC61BM/Al	0.99	19.99	63	13.49	P: control of morphology (precursor ratio, thickness)	42
ITO/PEDOT:PSS/MAPbI3−xClx/PC61BM/C60/Al	0.934	19.48	72.07	13.11	P: control of morphology (IPA treatment)	43
ITO/PEDOT:PSS/MAPbI3−xClx/PC61BM/LiF/Al	0.979	18.79	68.8	12.73	P: additive (1,8-diiodooctane)	44
ITO/PEDOT:PSS/MAPbI3/PC61BM/Al	1.04	8.85	65.2	6.16	P: additive (poly(2-ethyl-2-oxazoline))	110
FTO/PEDOT:PSS/MAPbI3−xClx/PC61BM/Al	0.94	22.4	83	17.5	P: control of morphology (hot-casting method)	46
ITO/PEDOT:PSS/MAPbI3/PC61BM/Al	0.86	17.31	77.2	11.45	P: control of morphology (precursor solvent)	45
ITO/PEDOT:PSS/MAPbI3−xClx/PC61BM/Al	0.92	15.4	68.2	9.6	H^c: additive (Ag nanoplates)	140
ITO/SOHEL^t/MAPbI3/PC61BM/Al	0.982	16.7	70.5	11.7	H: additive (perfluorinated ionomer) into PEDOT:PSS, tuning WF	141
ITO/polythiophene/MAPbI3/C60/BCP/Ag	1.03	16.2	70.7	11.8	H: control of thickness	142
ITO/Poly-TPD/MAPbI3/PC61BM/C60/BCP/Ag	1.1	22.0	69.7	15.3	H: use of Poly-TPD	143
ITO/MoO3/PEDOT:PSS/MAPbI3/C60/Bphen/Ag	0.96	20.06	67	12.78	H: insertion of MoO3 and control of solution MoO3 concentration	144
ITO/rGO/MAPbI3/PC61BM/BCP/Ag	0.95	14.81	71.13	10.80	H: use of rGO^g	88
ITO/PEDOT:PSS-AgNPs/MAPbI3−xClx/PC61BM/Bphen/Ag	0.93	21.51	79	15.75	H: additive (Ag nanoparticles), E: control of concentration of Bphen solution	145
ITO/GO/MAPbI3−xClx/PC61BM/ZnO/Al	1.00	17.46	71	12.4	H: use of GO^h and control of its concentration	87
ITO/PEDOT:PSS/MAPbI3/PC61BM/BCP/Al	0.6	10.32	63	3.9	E: comparison of C60, PC61BM and IC60BA	83
ITO/PEDOT:PSS/MAPbI3−xClx/PC61BM/PEIE/Ag	0.899	17.32	77.1	12.01	E: comparison of methanol, PEIE^i and P3TMAHT^j	146
ITO/PEDOT:PSS/MAPbI3/PC61BM/LiF/Al	0.846	20.2	76.7	13.1	E: insertion of LiF, control of PC61BM thickness	147
ITO/PEDOT:PSS/MAPbI3−xClx/PC61BM/PN4N/Ag	1.00	20.61	72.5	15.0	E: comparison of PN4N^k, IPA^l, PFN^m and methanol	148
ITO/PEDOT:PSS/MAPbI3−xClx/PC61BM/ZnO/Al	0.83	14.65	70	9.47	E: comparison of PC61BM, N2200^n, PNVT-8^o and PNDI2OD-TT	149
ITO/PEDOT:PSS/MAPbI3−xClx/PC61BM/DMAPA-C60/Ag	0.97	17.9	77	13.4	E: insertion of DMAPA-C60	150
ITO/PEDOT:PSS/MAPbI3/C60/Bis-C60/Ag	0.92	21.07	80	15.44	E: comparison of IC60BA, PC61BM and C60	151
ITO/PEDOT:PSS/MAPbI3/PC61BM/PC61BM/Ag	0.99	18.11	68	12.2	E: use of double PC61BM layer	152
ITO/PEDOT:PSS/MAPbI3−xClx/PC61BM/PDINO/Ag	0.95	18.8	78.5	14.0	E: comparison between ZnO and PDINO^p	153
ITO/PEDOT:PSS/MAPbI3−xClx/PC61BM/PCBC/Al	0.98	22.08	69.7	15.08	E: comparison between LiF and PCBC^q	154
ITO/PEDOT:PSS/MAPbI3−xClx/PC61BM:graphdiyne/C60/Al	0.969	23.4	65.4	14.8	E: doping (graphdiyne)	155
ITO/PEDOT:PSS/MAPbI3−xClx/PC61BM/Rhodamine101/LiF/Al	1.01	17.72	73	13.2	E: insertion of Rhodamine 101	156
FTO/PEDOT:PSS/MAPbI3/PC61BM/TIPD/Al	0.89	22.57	64.5	12.95	E: insertion of TIPD^r	157
ITO/PEDOT:PSS/MAPbI3−xClx/PC61BM/TPPI/Al	0.90	19.7	73	13.0	E: insertion of TPPI, P: additve (TPPI)^s	158

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Vacuum-processed PHJ PrSCs. Exposure to moisture can strongly affect the material properties of perovskite light absorbers.^113–116 Moreover, fabrication of reproducible perovskite films by solution processing is a difficult task due to its high reaction rates of forming perovskites. In this regard, the vacuum evaporation process is attractive due to its advantages in fabrication of PrSCs, including guaranteed material purity, ease of controlling experimental parameters (e.g., deposition rate, thickness and stoichiometry) and high reproducibility of a device.

Development of vacuum-processed PrSCs was initiated in response to scepticism that the mesostructure is necessary in PrSCs.¹¹⁷ The vacuum-processed PrSCs had the FTO/compact TiO₂/MAPbI_3−xCl_x/Spiro-OMeTAD/Ag structure (n–i–p) without a mesoscopic layer. MAI and PbCl₂ were simultaneously deposited from separate sources under vacuum for formation of the perovskite film, whereas each interfacial layer was deposited by spin-coating. Compared to the solution-processed perovskite film, a thermally-deposited perovskite layer showed superior uniformity. Because of this uniformity, the device showed a higher PCE = 15.4% than the solution-processed device (8.6%). However, because the device contained a compact TiO₂ layer processed at high T (500 °C), it lost compatibility with flexible PrSCs. Bolink et al. reported development of a p–i–n type PHJ PrSC with the ITO/PEDOT:PSS/PolyTPD/MAPbI₃/PCBM/Au structure (PCE ∼ 12%).¹¹⁸ The perovskite layer was formed by simultaneous deposition of MAI and PbI₂ using separate crucibles under vacuum, and interfacial layers were deposited by a low-T solution process. Hence, the device has a potential to be a flexible PrSCs.

Leo et al. showed p–i–n type PHJ PrSCs fabricated using a full vacuum process.¹¹⁹ Fully vacuum-processed devices have the merit of not being exposed to air or solvent, so the devices can have a longer lifetime than devices fabricated with some exposure to air or solvent.¹²⁰ In their work, a series of HELs in a family of triarylamine derivatives were used in devices. The authors presented the influence of energy level offsets between HELs and MAPbI_3−xCl_x on device performance. Another fully vacuum-processed p–i–n type PHJ PrSC with the ITO/MoO₃/NPB/MAPbI₃/C₆₀/BCP/Al structure was reported by Kim et al.¹²¹ The device showed a high PCE = 13.7%, which was attributed to efficient hole extraction of MoO₃/NPB, and energy level alignment between the HOMO level of NPB and the valence band maximum (VBM) level of the perovskite.

We summarized progress in PCEs of mesoscopic and low-T processed PHJ (n–i–p and p–i–n) PrSCs. Also, we suggest the desirable direction of development of PrSCs towards flexible architecture in Fig. 5.

Fig. 5 The green arrow represents improvement in the device architecture of PrSCs and the desirable direction of development of PrSCs we suggest. The graph shows progress in PCEs of mesoscopic PrSCs and low-T processed PHJ (n–i–p and p–i–n) PrSCs.

4. Roles of interfacial layers in planar heterojunction perovskite solar cells

Although fewer studies have considered interfacial layer engineering than perovskite layer engineering for optimum morphology, this difference does not mean that interfacial layer engineering is trivial. To increase the PCE of photovoltaic devices, interfacial layers should be understood in depth and properly used in PHJ PrSCs.^122,123 In this section, we review the studies of the interfacial layer engineering of PHJ PrSCs, and give an insight into their roles. Among several functions of interfacial layers in PHJ PrSCs, we will discuss their contributions to energy level alignment, electrical conductivity, trap passivation and the effect on device long-term stability in special.

4.1. Energy-level tailoring

Energy-level tailoring is one of the most effective ways to directly increase device performance. Appropriate energy-level tailoring can increase V_oc and facilitate charge transfer and extraction, which contribute to increasing J_sc and FF.¹²⁴ In particular, because V_oc of PrSCs is determined by the quasi-Fermi level difference between HELs and EELs, reducing the energy level offset between the VBM of the perovskite and IE of HELs, and between the conduction band minimum (CBM) of the perovskite and EA of EELs is crucial to increase the quasi-Fermi level difference.^125–130 Here, we present examples of energy-level tailoring of HELs and EELs.

Hole extraction layers. Interfacial layers in PHJ PrSCs should be carefully selected considering the energy level of intimate layers. However, the most widely-used perovskite light absorbers MAPbI₃, MAPb_3−xCl_x, MAPbBr₃, and FAPbI₃ have relatively high IEs (5.4, 5.4, 5.9 and 5.4 eV, respectively) compared to commonly-used HELs such as PEDOT:PSS, spiro-OMeTAD, and poly(triarylamine) (PTAA) (4.9–5.2 eV). The difference between the energy levels of the perovskite and HEL causes loss in built-in potential of a device; this loss can lead to decrease in all photovoltaic parameters such as V_oc, J_sc and FF. Therefore, to increase PCE of a PHJ PrSCs, energy-level tailoring of an interfacial layer at the junction between the perovskite layer and metal contact is essential.

To reduce energy level offset between PEDOT:PSS and MAPbI₃ in p–i–n PHJ PrSCs, Lee et al. developed a HEL with a high work function (WF); this HEL is composed of PEDOT:PSS and a perfluorinated ionomer (PFI) which they called a self-organized hole extraction layer (SOHEL).¹⁴¹ The WF of this SOHEL could be tuned by adjusting the concentration of PFI in PEDOT:PSS because the surface energy difference between PEDOT:PSS and PFI causes the PFI which has a high WF to become enriched on the film surface (Fig. 6a). This approach is useful to adjust the energy level of a HEL to align with the VBM of diverse perovskite materials. A PFI/PEDOT : PSS weight ratio ∼0.2 (SOHEL2) yielded a HEL with a higher WF (5.39 eV) than pristine PEDOT:PSS (4.86 eV). The increased WF was well aligned with the VBM of MAPbI₃. As a result, the device with SOHEL2 achieved a PCE = 11.7% that exceeded that of the device with pristine PEDOT:PSS (PCE = 8.1%) (Fig. 6b).

Fig. 6 Energy level tailoring of HELs for PHJ PrSCs (a) the work function change of SOHEL series as a function of the PFI/PEDOT:PSS weight ratio. (b) J–V characteristics of the PHJ PrSCs with pristine PEDOT:PSS or optimal SOHEL as a HEL. Reproduced from ref. 141 with permission from Wiley-VCH. J–V characteristics of the PHJ PrSCs with various HELs with different IEs (c) PCDTBT, DPP-DTT, P3HT and PCPDTBT (Reproduced from ref. 64 with permission from Nature Publishing Group); (d) PEDOT:PSS and Poly-TPD (Reproduced from ref. 143 with permission from Wiley-VCH).

Burn et al. introduced various kinds of p-type polymeric organic semiconductors e.g., DPP-DTT (poly(2,5-(2-di(thien-2-yl)thieno[3,2-b]thiophene))), PCDTBT (poly(N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di(thien-2-yl)-2′,1′,3′-benzothiadiazole))), P3HT (poly(3-n-hexylthiophene)) and PCPDTBT (poly(2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene-alt-4′,7′-2′,1′,3′-benzothiadiazole))) as hole interfacial layers in a p–i–n PHJ PrSCs.⁶⁴ The polymers modified the WF of ITO/PEDOT:PSS and thereby reduced the potential barrier at the junction between PEDOT:PSS and the perovskite, MAPbI₃. IEs of the polymers were 5.0 eV for P3HT, 5.0 eV for PCPDTBT, 5.2 eV for DPP-DTT, and 5.3 eV for PCDTBT. When the polymers were introduced onto PEDOT:PSS, the devices with polymers of higher IEs showed higher V_oc values in general. The average V_oc of devices with the PCDTBT interlayer was 1.03 V higher than those of devices with P3HT (0.7 V) (Fig. 6c). The significant increase in V_oc arose from reduction in the energy level difference between the VBM of MAPbI₃ and IE of the interfacial polymer layer, thereby the increase in the quasi-Fermi level difference between the EEL and HEL. In a similar way, poly-TPD can be used to tailor the energy level as a HEL.^118,143,159 Because Poly-TPD has a higher IE (5.4 eV) than PEDOT:PSS (5.2 eV), the energy offset with MAPbI₃ was decreased. Therefore, the device with Poly-TPD showed a higher V_oc (1.1 V) than the device with PEDOT:PSS (0.8 V) (Fig. 6d).

Small-molecule HELs can also be used to tune energy levels. Leo et al. explained the relation between the IE of small-molecule HELs and V_oc of fully vacuum-processed p–i–n PHJ PrSCs. For comparison, six HELs with different IEs Spiro-MeO-TAD (5.0 eV), MeO-TPD (5.1 eV), Spiro-MeO-TPD (5.1 eV), Spiro-TTB (5.3 eV), Sprio-TAD (5.4 eV) and BPAPF (5.6 eV) were chosen from a family of triarylamine derivatives.¹¹⁹ Devices that used HELs with IEs up to 5.3 eV which is close to the VBM of MAPbI_x−3Cl_x (5.4 eV) showed a gradual increase in V_oc of devices from 0.82 V to 0.97 V; the increase in V_oc was due to reduction of the energy level difference between the HEL and perovskite, which increased the quasi-Fermi level difference in the devices.

Overall changes in V_oc of devices described above were linearly proportional to the energy level difference between perovskites and various HELs (Fig. 7). As the energy level offset approaches zero, V_oc tends to be maximized. This trend strongly indicates that to increase PCE of a device, the HEL must be chosen appropriately, considering its energy level.

Fig. 7 Change in V_oc of PHJ PrSCs as a function of the energy level offset between HELs and perovskite.

Electron extraction layers. Energy level tailoring of the electron extraction side can be explained in a similar manner as in the HEL section. An EEL is required to form good energy-level matching at the junction between the cathode and perovskite layer. Otherwise, loss in built-in potential and V_oc, and associated decrease in PCE can arise; the commonly-used EELs, TiO₂ and ZnO have a CBM around 4.0–4.2 eV which is quite different from the CBM of perovskites: MAPbI₃, MAPb_3−xCl_x, MAPbBr₃, and FAPbI₃ generally have a CBM in the range of 3.6–3.9 eV.

The use of self-assembling monolayers (SAM) is an effective way to tailor the energy level of a layer; the effect results from the formation of a permanent dipole in the SAM. The deposition of 3-aminopropanoic acid SAM (C₃-SAM) onto sol–gel ZnO successfully decreased the WF of ZnO from 4.17 to 3.52 eV, and thereby improved its energy level alignment with the CBM of the MAPbI₃ perovskite (3.75 eV).⁸² As a result, n–i–p PHJ PrSCs with SAM treatment showed a higher V_oc (1.07 V) than the reference device (0.99 V) (Fig. 8a). The energy level of sol–gel ZnO was also tuned by deposition of a thin organic surface modifier, PCBM onto it. The deposition of PCBM changed the electronic structure of the ZnO film into a more favourable form.¹⁶⁰ In particular, the use of a ZnO/PCBM EEL decreased the energy level offset with the CBM of MAPbI₃ and as a result increased the difference in quasi-Fermi levels in a device compared to that in a device with ZnO alone. Therefore, the device with the ZnO/PCBM EEL had a higher V_oc (1.02 V) than a device with a ZnO-only EEL (0.78 V) (Fig. 8b). Also, efficient charge transfer and extraction by the favourable energy level alignment increased the J_sc and FF in the device with ITO/ZnO/PCBM/MAPbI₃/PTB7-Th/MoO₃/Ag; as a result it achieved a PCE = 12.2%.

Fig. 8 J–V characteristics of the PHJ PrSCs with energy level tailored EELs (a) C₃-SAM (Reproduced from ref. 82 with permission from the American Chemical Society) (b) PC₆₁BM (Reproduced from ref. 160 with permission from The Royal Society of Chemistry) (c) PEIE and P3TMAHT (Reproduced from ref. 146 with permission from the American Chemical Society) (d) LiF (Reproduced from ref. 147 with permission from The Royal Society of Chemistry).

PC₆₁BM is a representative material as an EEL in p–i–n PHJ PrSCs because it has excellent electron accepting properties.

However, the difference between the lowest unoccupied molecular orbital (LUMO) of PC₆₁BM and WF of the metal contact causes a decrease in the quasi-Fermi level difference and a lower built-in voltage which lead to low device performance. To form a quasi-Ohmic contact between PC₆₁BM and the Ag cathode, PEIE or poly[3-(6-trimethylammoniumhexyl)thiophene] (P3TMAHT) were inserted between the layers (Fig. 8c).¹⁴⁶ Both polymers formed interface dipoles with a negative charge on Ag and decreased its WF from 4.7 eV to 3.97 eV (PEIE) and to 4.13 eV (P3TMAHT). The modifying interfacial layers minimized the energy difference between PC₆₁BM and the cathode and maximized the quasi-Fermi level difference in the devices. Hence, the V_oc increased from 0.85 V to 0.9 V, and the J_sc and FF were also improved by using the interfacial layers. The device with ITO/PEDOT:PSS/MAPbI_3−xCl_x/PC₆₁BM/PEIE/Ag achieved a PCE = 12.0%, which was higher than the device with a PCE = 8.5% that did not have the interfacial layers. Similarly, a chemisorbed dipolar interface layer, DMAPA-C₆₀ based on an amine functionalized fullerene derivative was used to make a quasi-Ohmic contact between PCBM and the Ag cathode in p–i–n PHJ PrSCs.¹⁵⁰ The device structure was ITO/PEDOT:PSS/MAPbI_3−xCl_x/PC₆₁BM/DMAPA-C₆₀/Ag. Owing to the decreased WF of Ag (3.97 eV) with the interfacial layer, the device achieved a PCE = 13.4% with an increased V_oc = 0.97 V compared to the device without DMAPA-C₆₀ which had a PCE = 9.4% and V_oc = 0.9 V.

A thin LiF layer was also used as an EEL placed between PC₆₁BM and Al in p–i–n PHJ PrSCs.¹⁴⁷ The function of the thin metal fluoride layer in this position has been explained in the field of organic solar cells: (1) formation of a dipole to reduce the WF of the metal; (2) reaction with the metal contact to facilitate charge extraction; and (3) decrease in metal-bonding induced damage to PC₆₁BM.^161–164 Based on the effects, devices with LiF/Al showed a superior PCE = 13.1% to the device without LiF (11.5%) (Fig. 8d).

In short, V_oc strongly depends on the energy level offset at the electrode interfaces. V_oc of devices can be increased linearly by using energy-level-tailored HELs and EELs at the interface of PHJ PrSCs. The maximum V_oc is achieved when the energy levels of HELs and EELs are pinned to the VBM and CBM of perovskites, respectively. We expect that this review of how V_oc depends on the energy-level tailoring of interfacial layers will be helpful to provide an insight into the ways to maximize V_oc of devices with various kinds of perovskite light absorbers. More specific information of the HELs and EELs including their characteristics in terms of the energy level and device performance is summarized (Tables 3 and 4). Also, we summarized energy level diagram of the HELs and EELs described in this section (Fig. 9).

Table 3 HELs for PrSCs and their corresponding IE and device performance*

HEL	IE of HEL (eV)	Perovskite	VBM of perovskite (eV)	Offset (eV)	V oc (V)	J sc (mA cm^−2)	FF (%)	PCE (%)	Device architecture	Ref.
*We surveyed relevant articles that compared HELs in PHJ PrSCs and exhibited energy levels of HELs, perovskites and the corresponding device performance.a PCDTBT: (poly(N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di(thien-2-yl)-2′,1′,3′-benzothiadiazole))).b DPP-DTT: poly(2,5-(2-di(thien-2-yl)thieno[3,2-b]thiophene)).c PCPDTBT: poly(2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4′,7′-(2′,1′,3′-benzothiadiazole).d P3HT: poly(3-n-hexylthiophene)).e SOHEL: self-organized hole extraction layer.f PEDOT:PSS: poly(3,4-ethylenedioxythiophene) polystyrene sulfonate.g Spiro-TTB:.h Spiro-MeO-TPD:.i MeO-TPD:.j Sprio-MeO-TAD: 2,20,7,70-tetrakis(N,N0-di-p-methoxyphenylamine)-9,90-spirobifluorene poly[3-(6-trimethylammoniumhexyl)thiophene].k Poly-TPD: poly(N,N′-bis(4-butylphenyl-N,N′-bis(phenyl)benzidine)).l PC61BM: [6,6]-phenyl C61 butyric acid methyl ester.m BCP: bathocuproine.
PCDTBT^a	5.3	MAPbI3	5.4	0.1	1.03	15.9	66	10.9	ITO/PEDOT:PSS/HEL/MAPbI3/PCBM^l/LiF/Ag	64
DPP-DTT^b	5.2			0.2	1.00	13.3	74	9.8
PCPDTBT^c	5.0			0.4	0.88	13.0	69	7.8
P3HT^d	5.0			0.4	0.7	14.2	78	8.5
SOHEL^e	5.39	MAPbI3	5.43	0.04	0.982	16.7	70.5	11.7	ITO/HEL/MAPbI3/PC61BM/Al	141
PEDOT:PSS^f	4.9			0.53	0.835	14.1	68.5	8.1
Spiro-TTB^g	5.3	MAPbI3−xClx	5.4	0.1	0.970	14.9	63	9.1	ITO/HEL/MAPbI3−xClx/C60/Ag	119
Sprio-MeO-TPD^h	5.1			0.3	0.940	15.5	66	9.6
MeO-TPD^i	5.1			0.3	0.863	14.5	63	7.8
Sprio-MeO-TAD^j	5.0			0.4	0.820	14.1	63	7.2
Poly-TPD^k	5.4	MAPbI3	5.4	0	0.99	20.01	69.55	13.78	ITO/HEL/MAPbI3/PC61BM/C60/BCP^m/Ag	143
PEDOT:PSS	5.2			0.2	0.80	9.41	61.80	4.63

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Table 4 EELs for PrSCs and their corresponding LUMO level and device performance

EEL	LUMO of EEL (eV)	Perovskite	CBM of perovskite (eV)	Offset (eV)	V oc (V)	J sc (mA cm^−2)	FF (%)	PCE (%)	Device architecture	Ref.
*We surveyed relevant articles that compared EELs in PHJ PrSCs and exhibited energy levels of EELs and perovskites and the corresponding device performance. **Regarding EELs used on the PC61BM layer, the energy level offset was investigated between PC61BM and the electrode.a PC61BM: [6,6]-phenyl C61 butyric acid methyl ester.b IC60BA: indene-C60 bisadduct.c BCP: bathocuproine.d PEIE: poly-(ethyleneimine ethoxylated).e P3TMAHT: poly[3-(6-trimethylammoniumhexyl)thiophene].f DMAPA: dimethylaminopropylamine.g PDINO: perylene-diimide.h TIPD: titanium (diisopropoxide) bis(2,4-pentanedionate).i PTB7-Th: poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo][1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl.
ZnO/PC61BM^a	3.9	MAPbI3	3.7	0.2	1.02	14.73	73	10.87	ITO/EEL/MAPbI3/PTB7-Th^i/MoO3/Ag	160
ZnO	4.2			0.5	0.78	14.54	68	7.65
IC60BA^b/C60/BCP^c	3.7	MAPbI3	3.7	0	0.98	11.3	80	8.83	ITO/PEDOT:PSS/MAPbI3/EEL/Al	22
PC61BM/C60/BCP	3.9			0.2	0.82	12.4	74.1	7.53
C60/BCP	4.5			0.8	0.53	12.2	33.1	2.14
PEIE^d	3.97	MAPbI3−xClx	4.2**	0	0.899	17.32	77.10	12.01	ITO/PEDOT:PSS/MAPbI3/PC61BM/EEL/Ag	146
P3TMAHT^e	4.13			0.07	0.899	17.10	74.10	11.28
Ag only	4.7			0.5	0.849	16.00	60.29	8.53
DMAPA^f-C60	3.97	MAPbI3−xClx	3.95**	0.02	0.97	17.9	77	13.4	ITO/PEDOT:PSS/MAPbI3−xClx/PC61BM/EEL/Ag	150
Ag only	4.74			0.79	0.90	17.1	61	9.4
PDINO^g	3.63	MAPbI3−xClx	3.9**	0	0.95	18.8	78.5	14.0	ITO/PEDOT:PSS/MAPbI3−xClx/PC61BM/EEL/Ag	153
ZnO	4.4			0.5	0.95	16.0	74.5	11.3
Ag only	4.6			0.7	0.85	17.5	67.6	10.0
TIPD^h	3.9	MAPbI3	3.9**	0	0.89	22.57	64.5	12.95	ITO/PEDOT:PSS/MAPbI3/PC61BM/EEL/Al	157
Al only	4.3			0.4	0.83	15.42	67.7	8.66

---

Fig. 9 Energy level diagram of HELs (left) and EELs (right) used in PHJ PrSCs and perovskite light absorbers (middle). The unit for each value is [eV] but omitted. The values of IEs of HELs and EAs of EELs were extracted from the references which are provided in Section 4.1. The energy levels of MAPbI₃, MAPbI_3−xCl_x and MAPbBr₃ were extracted from ref. 47. The energy level of FAPbI₃ was extracted from ref. 196.

4.2. Electrical conductivity of interfacial layers

Electrical conductivity of interfacial layers in PHJ PrSCs is also an important factor determining device performance because charge transport occurs through the interfacial layers from perovskite absorbers. In general, high electrical conductivity of an interfacial layer is favourable so that charges can be efficiently collected to a desired electrode with minimized recombination as long as charge balance between electrons and holes is stable in a device. Although spiro-OMeTAD and PTAA are typically used as hole interfacial layers in n–i–p PHJ PrSCs, they have a low conductivity and low hole mobility in the absence of dopants such as lithium bis(trifluoromethylsulfonyl)-imide (Li-TFSi) and tert-butylpyridine (tBP).^165–167 Moreover, the difference in electrical conductivity between the perovskite light absorber and an interfacial layer can cause accumulation of charges at the interface and formation of space charges that can impede efficient charge transfer and transport. Therefore, devices with an undoped HEL showed low PCE.^168–172

To overcome this limitation, a dopant-free donor–acceptor conjugated small molecule, DOR3T-TBDT, was introduced as a HEL in a n–i–p PHJ device with the ITO/TiO₂/MAPbI_3−xCl_x/DOR3T-TBDT/MoO₃/Ag structure.⁸¹ The bulk electrical conductivity of the HEL measured by a transmission line model at room temperature (RT) was ∼4 × 10⁻⁴ S cm⁻¹ and the hole mobility extracted from organic field-effect transistors (OFETs) was 0.26 cm² V⁻¹ s⁻¹; both are higher than those of pristine spiro-OMeTAD (conductivity ∼10⁻⁵ S cm⁻²; hole mobility ∼10⁻⁴ cm² V⁻¹ s⁻¹). Electrochemical impedance spectroscopy (EIS) measurements of devices also showed the lower resistance of the DOR3T-TBDT based device than the device with dopant-free spiro-OMeTAD. Moreover, a significant photoluminescence (PL) quenching and shortened PL lifetime of the DOR3T-TBDT interfaced MAPbI_3−xCl_x layer indicated efficient hole extraction. Therefore, the device with DOR3T-TBDT which achieved a higher PCE = 14.9% than the devices with dopant-free spiro-OMeTAD (3.5%).

Another method to increase the low electrical conductivity of pure spiro-OMeTAD is to introduce multi-walled carbon nanotubes (MWNTs) into it.⁸⁰ Owing to the high carrier mobility (∼220 cm² V⁻¹ s⁻¹), electrical properties of spiro-OMeTAD that incorporated 0.5 wt% of MWNTs had carrier mobility more than two orders of magnitude larger, and conductivity three orders larger than pristine spiro-OMeTAD. Here, however, the lower WF of MWNTs (4.6 eV) provided a path for undesired electron transfer.^173–175 Therefore, a hierarchical structure composed of pure spiro-OMeTAD/spiro-OMeTAD:MWNTs was introduced to block back-electron transfer. The first deposited pure spiro-OMeTAD efficiently blocked back-electron transfer due to its lower LUMO (2.3 eV) than the CBM of MAPbI₃ (3.9 eV). As a result of increased electrical conductivity of the MWNT embedded spiro-OMeTAD and efficient blocking of back-electron transfer by pristine spiro-OMeTAD, the device with the FTO/TiO₂/MAPbI₃/spiro-OMeTAD/spiro-OMeTAD:MWNTs/Au structure achieved a PCE = 15.1% which exceeded the PCE = 12.8% of the device to which MWNTs were not added.

Studies on EELs of p–i–n PHJ PrSCs have also been reported. PCBM, the popular EEL, has been doped with graphdiyne, a novel two-dimensional carbon material.¹⁵⁵ While the electron mobility of an unipolar device with pure PCBM was 2.98 × 10⁻⁴ cm² V⁻¹ s⁻¹, it was increased to 5.32 × 10⁻⁴ cm² V⁻¹ s⁻¹ in the device that used graphdiyne-doped PC₆₁BM. The improvement in electron mobility resulted from good electrical characteristics of graphdiyne owing to its carbon network structure with delocalized π-systems. In addition, the device with PC₆₁BM:graphdiyne exhibited less charge recombination because of the better coverage of the EEL and the interfacial contact with the perovskite surface than undoped PC₆₁BM. Due to the increased electrical conductivity of the EEL, the device achieved a PCE = 14.8%, which was higher than PCE of the device with pristine PC₆₁BM (13.6%).

The importance of roles of a fullerene-based n-type layer in a p–i–n type PHJ cell was highlighted particularly with respect to electrical conductivity of the materials;¹⁵¹ the authors presented a clear correlation between the charge-transporting properties of EELs (IC₆₀BA, PC₆₁BM and C₆₀) and device performance. Electron mobilities of the fullerene derivatives were obtained from FETs based on each fullerene derivative. The gradual increase in electron mobility from IC₆₀BA (6.9 × 10⁻³ cm² V⁻¹ s⁻¹), to PC₆₁BM (6.1 × 10⁻² cm² V⁻¹ s⁻¹), to C₆₀ (1.6 cm² V⁻¹ s⁻¹) was attributed to increased conjugation and dense packing of C₆₀ owing to the lack of bulky side-chains. Due to this reason, the device with C₆₀ as an ETL achieved the highest J_sc = 21.07 mA cm⁻² and PCE = 15.44%; the device with PC₆₁BM had a J_sc = 18.85 mA cm⁻² and PCE = 13.37%, and the device with IC₆₀BA had a J_sc = 11.27 mA cm⁻² and PCE = 8.06%. These results indicate that an EEL with good electrical conductivity promotes efficient charge transport in a device and increase its PCE.

4.3. Passivation of traps in perovskites

The presence of a large density of traps in perovskite absorbers has been identified as the main origin of large photocurrent hysteresis of PrSCs which hampers accurate evaluation of device performance and degrades device reliability.^{22,50,51,137,151,176–180} To be specific, the hysteresis comes from the difference in rates of charge trapping and de-trapping depending on the voltage sweep direction and sweep rate. Therefore, decreasing trap density of states (tDOS) is required to eliminate the hysteresis of a device. One way of decreasing tDOS is to fill or passivate the traps by applying an intimate interfacial layer with the perovskite; hysteresis-less PHJ PrSCs have been realized using this approach.^22,50,151

Huang et al. applied a double fullerene layer (PC₆₁BM/C₆₀) onto an MAPbI₃ film.²² Thermal admittance spectroscopy (TAS) analysis confirmed the decrease in tDOS after interfacial layer passivation on the perovskite film. While the device without the interfacial layers had a large tDOS in the range from 1 × 10¹⁷ to 1 × 10¹⁹ m⁻³ eV⁻¹, the device with the interfacial layers had tDOS about two orders of magnitude less than this. The tDOS was also significantly mitigated by appropriate thermal annealing on PC₆₁BM (100 °C for 45 min) (Fig. 10a).⁵⁰ The schematic image in Fig. 10b shows that PC₆₁BM passivated the trap states and reduced the surface recombination. The increased surface passivation effect of PC₆₁BM on MAPbI₃ was revealed from the blue-shift in PL of the films and the surface passivation effect led to reduced interface recombination, an increased carrier lifetime and increased mobility, thereby yielding a high PCE of around 15%. The excellence of fullerene derivatives in trap passivation of perovskites in PHJ PrSCs has been being reported elsewhere to date.¹⁵¹

Fig. 10 (a) Trap density of states (tDOS) obtained by thermal admittance spectroscopy. (b) A schematic image of the surface recombination reduction by passivating the trap states. Reproduced from ref. 50 with permission from Nature Publishing Group.

4.4. Enhancement in long-term stability of devices

Long-term stability of a device is an essential requirement for commercialization. Although PHJ PrSCs have achieved high PCE near 20%, they have poor long-term stability mainly due to the inherent vulnerability of perovskite light absorbers to moisture and heat. Degradation of perovskites after exposure to moisture and heat can be easily traced in various ways. For example, the color change in perovskite films usually from dark brown to yellow, the change in the XRD peaks of perovskite crystal structures, and decreased intensity of light absorption are observable.^181–183

The sensitivity of perovskites to moisture imparts an additional function to interfacial layers. An appropriate choice of interfacial layers to sandwich perovskites can protect them from exposure to undesirable environments, and thereby extend the stability of a device.

However, the most popular solution-processable conducting polymer HEL in p–i–n PHJ PrSCs, PEDOT:PSS has disadvantages of hygroscopicity and acidity; these properties can degrade the long-term stability of the devices. Therefore, researchers are seeking a way to improve device stability. Chu et al. introduced solution-processable MoO₃ between ITO and PEDOT:PSS, and developed a device that had the ITO/MoO₃/PEDOT:PSS/MAPbI₃/C₆₀/Bphen/Ag structure.¹⁴⁴ The device showed better long-term stability than the device with only PEDOT:PSS. Metal ions released from ITO due to damage by acidic PEDOT:PSS can diffuse into the inner layer of a device and degrade it. However, insertion of MoO₃ seems to effectively prevent corrosion of ITO by PEDOT:PSS and consequent generation of undesirable species. After 10 days under ambient conditions, the PCE of the device with MoO₃ degraded only 7%, whereas the device with only PEDOT:PSS failed completely. Furthermore, owing to increased hole collection efficiency, the device with MoO₃ showed a higher PCE = 12.78% than the device with only PEDOT:PSS HEL (PCE = 9.81%). In another approach, PEDOT:PSS was simply replaced with reduced graphene oxide (rGO) nanosheets as HELs; the replacement increased device stability and PCE at the same time.⁸⁸ The basic structure of the devices was ITO/HEL/MAPbI₃/PC₆₁BM/BCP/Ag. About three times longer half-lifetime of rGO device (∼150 h) than the device with PEDOT:PSS (∼50 h) under ambient conditions was attributed to rGO with nearly neutral properties, unlike PEDOT:PSS, which is acidic. With few surface oxygen functionalities, rGO has inherent passivation ability against moisture and oxygen. Also, the device with rGO achieved a PCE = 10.8%, which was higher than a PCE = 9.14% of the device with PEDOT:PSS. These results were ascribed to the superior charge transport ability due to higher conductivity of rGO than PEDOT:PSS and better-aligned energy levels between rGO and the anode.

5. Flexible perovskite solar cells

PrSCs are promising candidates as power sources. Nevertheless, processability at low T is an essential requirement for the realization of flexible PrSCs; therefore, PHJ PrSCs are suitable for this purpose.

Snaith et al. demonstrated flexible p–i–n PHJ PrSCs on PET/ITO substrates (ITO/PEDOT:PSS/MAPbI_3−xCl_x/PCBM/TiO_x/Al);¹⁰⁵ the devices had a PCE = 6.3% (Fig. 11a and b). Around the same time, Kelly et al. reported flexible n–i–p PHJ PrSCs.⁷³ The devices were completed on PET/ITO substrates as well with the ITO/ZnO/MAPbI₃/spiro-OMeTAD/Ag structure. The PCE of the flexible devices was 10.2%. The demonstrations presented development possibility of flexible PHJ PrSCs (Fig. 11c and d).

Fig. 11 (a) Picture of flexible p–i–n PHJ PrSCs. (b) J–V charateristics of the flexible device and the device on a glass substrate. Reproduced from ref. 105 with permission from Nature Publishing Group. (c) A picture of flexible n–i–p PHJ PrSCs. (d) J–V charateristics measured under illumination (red line) and in the dark (black line). Reproduced from ref. 73 with permission from Nature Publishing Group.

Jung et al. demonstrated bendable n–i–p PHJ PrSCs based on PEN/ITO substrates.¹⁸⁴ The device structure was PEN/ITO/MAPbI_3−xCl_x/Sprio-OMeTAD/Ag. The authors investigated bending stability of the devices under various bending conditions. The flexible PrSCs showed constant PCE as a function of the bending radius r_B from 400 to 4 mm. At r_B of 400 and 40 mm the devices remained stable, but at r_B of 4 mm the devices showed an abrupt decrease in PCE after several tens of bending cycles. The authors attributed this decrease to brittleness of ITO. Here, the researchers examined the fracture behavior of perovskite films. One perovskite film was deposited on PEN/ITO and the other directly on the PEN substrate. The former had cracks from ITO, but the latter did not have any cracks after bending (Fig. 12a and b). The authors attributed these results to the excellent mechanical properties of perovskite films and their compatibility with flexible devices. On the other hand, Kelly et al. reported that repeated bending of the device at a low radius (4 mm) of curvature caused small cracks in the perovskite films. (Fig. 12c and d).¹⁸⁵ Nevertheless, the authors concluded that perovskites can tolerate moderate conditions such as roll-to-roll manufacturing. Based on these conflicting conclusions, it is worth studying in-depth and further debating on the flexibility and mechanical durability of perovskite films.

Fig. 12 SEM images of (a) PEN/ITO/TiO_x/ and (b) PEN/TiO_x/perovskites showing perovskites and their junction parts after 300 bending cycles. Scale bar represents 100 μm. Low-magnification and high-magnification (inset) SEM images of (c) PET/highly conductive PEDOT and (d) PET/highly conductive PEDOT/semiconducting PEDOT/perovskite after 2000 bending cycles. (a) and (b) are reproduced from ref. 184 and (c) and (d) are reproduced from ref. 185 with permission from The Royal Society of Chemistry.

The brittleness of FTO and ITO limits the development of flexible PrSCs. Therefore, finding a way to replace the conventional FTO and ITO electrodes with a transparent flexible electrode is a major requirement for development of flexible PrSCs. Although several papers have reported PrSCs that use flexible transparent conducting electrodes (e.g., aluminum-doped ZnO (AZO)/Ag/AZO and PEDOT:PSS) instead of FTO or ITO, PCEs of the flexible devices were lower than rigid devices due to inferior electrical and optical properties of flexible electrodes.^185–187 Therefore, various kinds of flexible transparent conducting electrodes (e.g., PEDOT:PSS, graphene,^188,189 carbon nanotubes¹⁸⁹ and silver nanowires¹⁹⁰) should be vigorously studied with the goal of enhancing compatibility with flexible PrSCs to replace conventional metal oxide brittle electrodes.

6. Conclusions and outlook

We have discussed and pointed out progress in PHJ PrSCs and several strategies for providing insights into interfacial layer engineering to improve photovoltaic performance and stability of PHJ PrSCs.

Importantly, improvement of PCE and stability of PHJ PrSCs requires the use of appropriate interfacial layers because the interfacial layers have crucial roles in PHJ PrSCs; (1) proper energy-level tailoring can reduce the energy offset between intimate layers, and thereby increase the built-in potential, and facilitate charge transfer and extraction; (2) electrical conductivity of the interfacial layers strongly affects charge transport properties and efficiency of charge collection; (3) passivation of electronic trap sites in perovskite films can eliminate photocurrent hysteresis; (4) protection of perovskites from moisture improves long-term stability. On the basis of various functions and optimization with comprehensive understanding of interfacial layers, PHJ PrSCs are expected to be further improved in PCE and long-term stability. Obviously, to develop highly efficient flexible PHJ PrSCs, we also need to understand the failure mechanism of the devices against mechanical stress, and adopt a transparent conducting flexible electrode that has excellent electrical, optical, and mechanical properties.

Because one of the ultimate goals of research on PrSCs is to make them ubiquitous in everyday life, manufacturing cost in addition to flexibility should be considered. To meet these requirements, we suggest that p–i–n type PHJ PrSCs are more promising than the n–i–p type, because the structure of, p–i–n type PHJ PrSCs can be further simplified. Specifically, PEDOT:PSS, a representative HEL for p–i–n type PHJ PrSCs, can be modified to have great electrical conductivity (∼4000 S cm⁻¹)¹⁹³ and a wide range of WF (4.8–5.9 eV)^6,141,194 by appropriate chemical treatment. With this adjustability, PEDOT:PSS can concurrently perform both charge extraction and collection in a simple integrated single layer in p–i–n type PHJ PrSCs. Secondly, all-solution-processed flexible PrSCs are achievable because PEDOT:PSS is solution processable and a film of PEDOT:PSS is flexible, whereas conventional metal oxide electrodes (e.g., ITO and FTO) are fabricated by vacuum processing, and are brittle. Therefore, flexible p–i–n type PHJ PrSCs can be fabricated by all-solution roll-to-roll processing. In summary, we expect that p–i–n type PHJ PrSCs can have advantages of the structure and processing over the n–i–p type, and that these advantages will lead to cost-effective fabrication and wide use. However, the stability of PEDOT:PSS is a critical limitation. We believe that this problem can be solved by using various types of fluorinated conducting polymers.^{6,141,194,195}

Acknowledgements

This work was supported by the Center for Advanced Soft-Electronics funded by the Ministry of Science, ICT and Future Planning as Global Frontier Project (CASE-2014M3A6A5060947). This work was also supported by POSCO.

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Footnote

† These authors contributed equally to this work.

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