Large-area perovskite solar cells – a review of recent progress and issues

In recent years, perovskite solar cells (PSCs) have attracted great attention in the photovoltaic research field, because of their high-efficiency (certified 22.1%) and low-cost. In this review paper, we briefly introduce the history of efficiency development for PSCs, and discuss some of the major problems for large-area (≥1 cm2) PSC devices. In addition, we summarize the recent progress in the aspects of fabrication methods for large-area perovskite films, and improving the efficiency and stability of the large-area PSC devices. Finally, we give a short summary and outlook of large-area PSC devices. This article is mainly organized into three parts. The first part focuses on the main fabricating technologies for large-area perovskite films. The second section discusses some methods that are used to improve the efficiency of PSCs. In the last part, different approaches are used to improve the stability of PSCs.


Introduction
Due to the growing population, the global energy demand is increasing year by year. Moreover, the global energy demand is predicted to double by 2050. 1,2 Thus, the development of renewable energy becomes an imminent requirement, such as water energy, wind energy, and solar energy. The photovoltaic power generation capacity is installed to be 303 GW and increased 75 GW in 2016. In 2016, photovoltaic power generation accounted for only 1.5% of the world's total electricity generation. So high performance, long-term stability, low cost and environmental friendly solar cells become the focus of current energy research.
In recent years, hybrid metal halide perovskite materials have revolutionized the eld of photovoltaics materials research, due to the power conversion efficiency (PCE) of PSC devices having been rapidly improved, from the point 3. Miyasaka et al. 6 has creatively made CH 3 NH 3 PbBr 3 /TiO 2 -based and CH 3 NH 3 PbI 3 /TiO 2 -based DSCs, the PCE of the cells is 3.13% and 3.81%, respectively. The PSCs attracted researchers' attention then happened in 2012, when M. Grätzel and N. G. Park et al. 27 made PSCs device using perovskite lms as the photoactive absorber layer, the mp-TiO 2 and spiro-MeOTAD were used as the electron transport layer (ETL) and hole transport layer (HTL), respectively ( Fig. 1), achieving the PCE of 9.7%. In 2013, M. Z. Liu, M. B. Johnston and H. J. Snaith 8 fabricated planar heterojunction PSCs via vapor deposition, and the efficiency of the PSCs device is up to 15.4%. The yttrium (Y) doping the TiO 2 (ETL) improves the electron transport channel in the PSCs device, and increase its carrier concentration and modify the ITO electrode to reduce its work function. These changes achieved a PCE of 19.3%. 28 In 2015, S. I. Seok et al. 29 attained an efficiency of PSCs up to 20.1%. In 2016, A. Zettl et al. 30 made an architecture of GaN/CH 3 NH 3 SnI 3 /monolayer h-BN/CH 3 NH 3 -PbI 3Àx Br x /HTL and graphene aerogel/Au (Fig. 2). The graded bandgap PSCs demonstrated with PCE averaging 18.4%, with a best of 21.7%. Other researchers, E. H. Sargent et al. 31 (2017) achieved the certied efficiencies of 20.1% via contactpassivation strategy, retaining 90% (97% aer dark recovery) of their initial PCE aer 500 hours of continuous roomtemperature. Meanwhile, E. K. Kim, J. H. Noh, and S. I. Seok et al. 3 reported that the introduction of additional iodide ions into the organic cation solution, that was used to form the perovskite layers through an intramolecular exchanging process and decrease the concentration of deep-level defects. The certied PCE of PSCs attained 22.1%. 3 In addition, high efficiency PSCs devices include not only small devices, but also larger cells. A PSCs device with area of large-area ($1 cm 2 ) and maximum PCE of 20.5% (certied 19.7%) has been reported. 3 Table 1 shows some results for largearea PSCs have been reported in the literatures.
But, for large-area PSCs device, it still has some issues need to be solved, namely fabrication, stability, hysteresis, fabrication cost and environmental concerns. Such as, the continuous fabrication of cracks-free and pinholes-free the perovskite and the selective carrier extraction layers lms is difficulty with large-area PSCs devices. The dilemma with optimizing such charge carrier extraction layers in solar cells is that the lm should be thin to minimize resistive losses, while at the same time, it should cover the entire collector area in a contiguous and uniform manner. 48 In the large-area PSCs device, surfaces, bulk defects and interfaces introduce recombination centers that lead to fast nonradiative losses, 49 and interface losses, which lead to the V oc , J sc and ll factor (FF) decrease. Meanwhile, the perovskite material is easily thermal decomposition and hydrodecomposition, that leads to the lack of stability for PSCs device. The poor stability of the perovskite materials and devices is a big challenge, which hinder the PSCs device could be transferred from the laboratory to industry and outdoor applications. Thus, for large-area PSCs device, the major challenges relate to the improving efficiency and keeping the stability of the device. In this review paper, giving an update of the PSCs eld, briey, introducing the history of PSCs and then focus on the key progress of the fabrication, improving the efficiency and the stability of the large-area PSCs device.

Perovskite structure and characteristics
Perovskite was discovered in 1839, which originally referred to a kind of ceramic oxides with the general molecular formula ABX 3 . 1 Recently, PSCs absorber layer is mainly organic-inorganic perovskite layer, the general molecular formula is also ABX 3 (Fig. 3 Ge 2+ ) and X is halogen anion (i.e. F À , Cl À , Br À , I À ), are the most relevant ones for PSCs. The perovskite arrangement is approximated on its geometric tolerance factor (t), where r A , r B and r X are the efficient ionic radius for A, B and X ions, respectively. When the t ¼ 1.0, the perovskite is a perfect cubic perovskite. 50 However, octahedral distortion is assessed when t < 1, which inuences electronic characteristics. 51 For alkali metal halide perovskite, formability is anticipated for 0.813 < t < 1.107. 50,51 In Table 2, the r A in APbX 3 (X ¼ Cl, Br, I) perovskite has been calculated for t ¼ 0.8 and t ¼ 1 based on effective ionic radii. 51 As the tolerance of CH 3 NH 3 PbI 3 (MAPbI 3 ) is 0.83, in this manner, the deviation from a perfect cubic structure is likely to happen. 50,51 In the visible range, for the MAPbI 3 , the effective absorption coefficient is around 1.0 Â 10 5 (mol L À1 ) À1 cm À1 at 550 nm, 4,52 when the thickness of perovskite lms range is 500-600 nm, it can absorb complete light in lms. Meanwhile, organic-inorganic perovskite exhibits better charge transfer characteristics. H. J. Snaith et al. 5 reported the diffusion lengths (L D ) of the electrons and holes in MAPbI 3 and MAPbI 3Àx Cl x , the L D of MAPbI 3 is 130 nm (electrons) and 100 nm (holes) and this of MAPbI 3Àx Cl x is 1100 nm (electrons) and 1200 nm (holes), respectively. 5 So, the organic-inorganic perovskite is an ideal absorber layer material for solar cells.

Typical PSCs structure
Some of the typical structures of PSCs are shown in Fig. 4. The typical PSCs structures include the mesoporous structure ( Fig. 4(a)), the planar heterojunction structure ( Fig. 4(b)) and the inverted planar heterojunction structure (Fig. 4(c)). PSCs with regular conguration is transparent conductive oxide (TCO)/blocking layer (electron transport layer (ETL))/perovskite absorber layer/hole transport layer (HTL) material/gold (Au). The widely accepted a simplied operation principle of PSCs is presented as: perovskite absorber layer absorbs light and generates charges while the light on the PSCs. The electrons and holes pairs are created by the thermal energy, which diffuse and get separate through electron and hole selective contacts, respectively ( Fig. 4(d)). 53 Once electrons and holes are present at the cathode and anode, respectively, external load can be powered by connecting a circuit through it. TiO 2 is the most common ETL material, 3 [87][88][89][90] have been used as electrode. 53 Fig. 5 shows the energy levels for some commonly used ETL materials, HTL materials and absorbers materials.
3. Large-area ($1 cm 2 ) perovskite films fabricating technologies The continuous fabrication of cracks-and pinholes-free the perovskite lms and the selective carrier extraction layers lms is difficulty for the large-area PSCs devices. So, some researchers have reported many fabrication methods to improve the quality of the large-area perovskite lms.

Spin-coating and vacuum ash-assisted solution process (VASP)
Spin-coating has been widely used to fabricate the large-area perovskite lms. 16,35,[39][40][41]45,91 The main advantage of the spincoating method is to deposit thin lms with well-dened the composition of chemical elements and the lm thicknesses. Spin-coating includes one step spin-coating and two step spin-coating. One step spin-coating, briey, methyl ammonium iodide (MAI) and lead iodide (PbI 2 ) powders are mixed and dissolved in N,N-dimethylformamide (DMF) or dimethyl sulfoxide (DMSO), the mixed solution is spun on a TCO substrate and then annealed, attaining the perovskite lms ( Fig. 6). In 2015, M. Grätzel and L. Y. Han et al. 41 prepared perovskite absorber lms via one step spin-coating, they achieved largearea PSCs with an active area 1.02 cm 2 that had a PCE > 15% (certied 15%). In 2016, W. Qiu and P. Heremans et al. 39 achieved large-area PSCs with 4 cm 2 aperture area and an active area of 1 cm 2 , that had a PCE of 13.6%. Aiming at uncovered pinhole areas derive from large perovskite grains, M. J. Kim and G. H. Kim et al. 35 also developed one step spin-coating, and using high-temperature short-time annealing (HTSA) process ( Fig. 7(a)), achieving the perovskite grains with sizes more than 1 mm without pinhole ( Fig. 7(d, e, h, i)). In addition, the VASP was used to fabricate perovskite lm ( Fig. 8(a)), the sizes of perovskite grains were between 400 and 1000 nm (Fig. 8(c)), which covered the TiO 2 layer. 37 Two-step spin coating, briey, MAI and PbI 2 powders are dissolved in DMF or DMSO, respectively. 16 First, the PbI 2 solution is spun coating on a TCO substrate and then annealing,   achieving the PbI 2 lms. Second, the MAI solution is spun coating on PbI 2 lms and then annealing, achieving the perovskite lms (Fig. 9). In 2016, C. Chang et al. 45 prepared perovskite absorber lms with two step spin-coating, they achieved large-area PSCs with an active area 1.2 cm 2 that had a PCE of 16.2%. In 2017, E. K. Kim, J. H. Noh and S. I. Seok et al. 3 achieved large-area PSCs with an active area 1 cm 2 that had a certied PCE of 19.7%. In 2017, X. W. Zhang and J. B. You et al. 38 have adopted two-step spin-coating method to fabricate the (FAPbI 3 ) 1Àx (MAPbBr 3 ) x lms and congure n-i-p planar structure PSCs with an active area 1 cm 2 that has a PCE of 20.1%.

Vapor deposition
Comparing to the fabrication of the PSCs device with the spincoating technology, vapor deposition technology offers a very superior device and superior performance ( Fig. 10(a)). The vapor deposition includes dual-source evaporation technology, 8 vapor-solid reaction, 32 and vapor-assisted method, 50 etc. For dual-source co-evaporation technology, it is that PbI 2 powders and MAI powders are made as target source, and pre-heated to 116 C and 325 C, respectively, which has achieved the PSCs yield an PCE of 15.4%. 8 This method fabricates high quality and uniformity of the perovskite lms, subsequently resulting in good performance. But this method is very dependent on high temperature and high vacuum conditions. Alternate methods research in the literature 32 is vapor-solid reaction (VSR), depositing the perovskite lm with low temperature (Fig. 10(b)). First, the PbI 2 lm was spin-coated onto the ETL, and then baking on a 70 C hot plate in air for 10 min. Second, MAI powders were dissolved in ethanol. Then the solution was homogeneously sprayed onto the bottom surface of the top plate that had been keeping at 80 C. Finally, inside vacuum desiccator, two parallel hot plates (PHP) were putted together to synthesize perovskite thin lms. 32 H. Zhou and S. Yin et al. 32 used this method to achieve the 8 Â 8 cm 2 PSCs module, the average PCE was 6.0% with the active area of 1.5 cm 2 .

Gas-induced method
For the organic-inorganic halide perovskites (OIHPs) materials, gas-induce formation/transformation (GIFT) reveal surprising properties, such as gas-induced phase/morphology transformation. 92 Z. Zhou, S. Pang, G. Cui et al. 93 reported that the discovery of CH 3 NH 2 (MA) induced phase/morphology transformation of the MAPbI 3 . As show in Fig. 11, MA gas is introduced at room temperature (RT), aer 120 min, two MAPbI 3 single-crystals become liqueed (MAPbI 3 $xCH 3 NH 2 ), eventually, merge into one liquid sphere. 93 Then MA gas is removed, aer 120 min, perovskite back-conversion completed. Fig. 12(b) shows a poor quality of MAPbI 3 thin lm (incomplete coverage, rough), then the MA gas treatment has been introduced to create smooth, uniform and full coverage MAPbI 3 thin lms ( Fig. 12(c)). 93 In 2017, M. Grätzel and L. Han et al. 36 achieved 8 Â 8 cm 2 perovskite lms via GIFT, briey, at atmospheric environment, dried CH 3 NH 2 gas (0.5 l min À1 ) was passed into a bottle that contained 2 mmol CH 3 NH 3 I or PbI 2 powders (Fig. 13). Aer 30 min, the CH 3 NH 3 I powders changed into transparent colorless liquid (CH 3 NH 2 I$3CH 3 NH 2 ), and the PbI 2 powders  changed into a pale-yellow paste (PbI 2 $CH 3 NH 2 , Fig. 13). 36 For the synthesis of perovskite precursor, CH 3 NH 2 I$3CH 3 NH 2 and PbI 2 $CH 3 NH 2 were blended stoichiometrically and ultrasonicated for 15 min (Fig. 13). 36 The perovskite precursor (200 ml) was dropped on a 8 Â 8 cm 2 substrate and then the precursor was covered by the polyimide (PI) lm. 36 A pressure of 120 bar was loaded via a pneumatically driven squeezing board which spread the liquid precursor under the PI lm. The pressure was held for 60 s and then unloaded. The thin liquid lm covered with the PI lm was heated at 50 C for 2 min before peeling off the PI lm. Aer peeling the PI lm (50 mm s À1 ), a dense and uniform perovskite lm was formed (Fig. 14(b)). 36 They achieved the PSCs with the device area 36 cm 2 (Fig. 14(c)) that had a certied PCE of 12.1%. 36

Other approaches
In addition, a blade coating technology is also frequently used, the schematic shows in Fig. 15. 22 The advantage of the blade Fig. 11 In situ optical microscopy of the morphology evolution of two touching MAPbI 3 perovskite crystals (same magnification) upon exposure to CH 3 NH 2 gas and CH 3 NH 2 degassing. 93  94 reported an approach to fabricate ultra-long nanowires array and highly oriented CH 3 -NH 3 PbI 3 thin lms in ambient environments, briey, this approach included large-scale roll-to-roll micro-gravure printing and doctor blading (Fig. 16), which produced perovskite nanowires lengths as long as 15 mm. 94 For the large-area PSCs device, improving the PCE, the rst method is to change the chemical composition of perovskite, adjusting its band gap and increasing the charge generation. 3,29,31,35,38,46 The second approach is to increase the grain size of perovskite, decreasing the cracks and pinholes, that reduces the bulk defect recombination and electric leakage, and increase V oc . [35][36][37][38] The third approach is interface modication, which reduces interface contact resistance, and reduce interface and surface recombination, and increase J sc . 31,41,45,95 For the large-area PSCs device, with the increasing of cell size, the series resistance (R s ) increase among the charge transfer layers, the absorber layer and the electrode layers. At the same time, the number of the crack and the pinholes increase, that from the shunt resistance (R sh ) and the value of R sh decrease. Incorporating these resistances into the circuit  Fig. 17. 96 The increasing of R s and the decreasing of R sh increase the interface losses of the large-area PSCs device, that is the major reason of the lower efficiency for the large-area PSCs device. 95 The current expression in the circuit can be written as eqn (2). 96 where I SC0 is the short-circuit current when there are no parasitic resistances (R s and R sh ). The effect of these parasitic resistances on the I-V characteristic is shown in Fig. 18. Form the eqn (2), the series resistance, R s increase, has no effect on the open-circuit voltage, but reduces the short-circuit current (J sc ) and ll factor (FF) (Fig. 18(a)). Conversely, the shunt resistance, R sh decrease, has no effect on the short circuit current, but reduces the open-circuit voltage (V oc ) and FF ( Fig. 18(b)).

Improving preparation technology
The high quality (cracks-and pinholes-free) large-area perovskite lm is precondition for the achieving high PCE of PSCs. Because the cracks and pinholes can form electric leakage (forming the R sh ), which lead to the decreasing the V oc and FF, and reduce the PCE of PSCs. So, M. J. Kim and G. H. Kim et al. 35 developed one step spin-coating, and using high-temperature short-time annealing process ( Fig. 7(a)), achieving the perovskite grains with sizes more than 1 mm without pinhole (HTSA-400, Fig. 7(d, e, h and i)). They fabricated PSCs device with 1 cm 2 , which achieved the PCE of 18.32% with HTSA-400 ( Fig. 19(d)), but the PCE is only 13.82% with HTSA-100 ( Fig. 19(c)). 35 X. Li and M. Grätzel et al. 37 used the vacuum ash-assisted solution processing (VASP) to fabricate perovskite lm ( Fig. 8(a)), the sizes of perovskite grains were between 400 and 1000 nm (Fig. 8(c)). They fabricated the PSCs device with an aperture area exceeding 1 cm 2 , the certied PCE of 19.6%. 37 In 2015, Z. Zhou, S. Pang, G. Cui et al. 93 reported that the MA gas treatment has been introduced to create smooth, uniform and full coverage MAPbI 3 thin lms (Fig. 12(c)). 93 This MAPbI 3 was used to fabricate the PSCs device, the PCE increased from 5.7% to 15.1%, was observed, which was clearly the result of the improving lm morphology. 93

Interface engineering
Interface engineering can optimize interface contact, mitigate carrier recombination and increase carrier collection, which is extremely important to achieve high-performance and high- Doping for the charge transport layers, that can improve their electrical performance, such as improving carrier concentration and mobility. For Li-Mg co-doped NiO lms, the conductivity is 2.32 Â 10 À3 S cm À1 , $12 times greater than that of the pure Mg x Ni 1Àx O. 41 The conductivity of Nb 5+ doped TiO 2 lms is $10 4 S cm À1 , $100 to 1000 times greater than that of the pure TiO 2 . 41 In 2015, M. Grätzel and L. Y. Han et al. 41 have used Mg-Li co-doped NiO as HTL and Nb doped TiO x as ETL material in inverted planar PSCs to achieve very rapid carrier extraction, increasing the cell FF from 0.64 to 0.827. Meanwhile, they fabricated a large-area (>1 cm 2 ) PSCs ( Fig. 20(a)) with a certied efficiency of 15%. 41 The contact-passivation can mitigate interfacial recombination and improve interface binding in lowtemperature planar PSCs. H. R. Tan and E. H. Sargent et al. 31 reported a contact-passivation strategy using chlorine-capped TiO 2 (Cl-TiO 2 ) colloidal nanocrystal lm as ETL, the chargerecombination lifetime increased from 64 ms to 145 ms compare with pure TiO 2 lm. 31 They fabricated the planar PSCs for active areas of 1.1 cm 2 , that achieved a certied efficiency of 19.5% without hysteresis. 31 Interlayers are thin layers or monolayers of organic molecules that modify a specic interface in the solar cell. 97 In 2016, C.Y. Chang and Y. C. Chang et al. 45 reported an approach for the modication of interface layer via introducing thiol-functionalized self-assembled monolayers (SAMs, Fig. 21(b)), which decreased interface charge recombination and increased the value of J sc (19.43 mA cm À2 to 21.68 mA cm À2 ) and FF (0.67 to 0.72). They fabricated a large-area (1.2 cm 2 ) PSCs with the PCE up to 15.98%. 45 Y. Wu and X. Yang et al. 42 reported a perovskite-fullerene graded heterojunction structure, which improved the photoelectron collection and reduced recombination loss. They fabricated the PSCs of 1.022 cm 2 , that had a certied PCE of 18.21%. 42

Stability of large-area ($1 cm 2 ) perovskite solar cells
In recent years, the certicated PCE of the large-area (1 cm 2 ) PSCs has achieved 20.1%. 38 However, the major issue of largearea PSCs for commercial applications is the poor long-term device stability. For the stability of the perovskite materials and devices, it is necessary to consider the effects of temperature, illumination and ambient (oxygen, moisture) exposure. Many papers have reported about this important issue. 1,51,53,97-107

Degradation mechanisms
The degradation of the PSCs device includes the degradation of the active layer, the degradation of charge transport layers, and the degradation of electrodes. 104 The MAPI 3 lms are frequently used as absorber layer lm. But the major problem with MAPbI 3 is that has thermal decomposition (exceeding 85 C) 108,109 and water decomposition. 1,48,110 Some researchers have reported the decomposition process of MAPbI 3 . B. Philippe and H. Rensmo et al. 109 exposed the MAPbI 3 and MAPbI 3Àx Cl x to various environments. From the photoelectron spectroscopy results with the different environments, the perovskite has decomposed into PbI 2 , but this degradation seems to occur already at 100 C and is not only related to large humidity ( Fig. 22(a)). Meanwhile, they observed a slow degradation occurs even when stored in an inert atmosphere such as argon. 109 L. D. Wang et al. 48 veried that oxygen, together with moisture, could lead to the irreversible degradation of MAPbI 3 . They exposed TiO 2 /CH 3 NH 3 PbI 3 lm to air with a humidity of 60% at 35 C for 18 h, and then, the absorption between 530 and 800 nm greatly decreased ( Fig. 22(b)), the MAPbI 3 decomposed into PbI 2 and I 2 (Fig. 21(c)). 48 The degradation mechanism of MAPbI 3 upon exposure to moisture in absence of illumination involves the formation of hydrate form, which can be reversible. 11,48,111,112 However, continuing exposure to moisture and/or exposure to illumination leads to the irreversible degradation to PbI 2 . 111 For ETL material, TiO 2 is especially sensitive to ultraviolet light, in the ultraviolet light, Ti 4+ adsorb O 2 and convert into Ti 3+ , increasing the charge recombination. 113 Meanwhile, the lithium salt in spiro-MeOTAD is easy to absorb moisture and decrease the PSCs device stability.

Methods of improving stability
In recent years, many methods have been researched to improve the PSCs device stability. Due to the poor stability of MAPI 3 , the rst method is to modify the chemical constituents or structure of the perovskite. For example, 2D perovskites, compared with 3D perovskites, 2D perovskites have the higher carrier mobility while maintaining good ambient stability. 114,115 The 2D Ruddlesden-Popper layered perovskites ((BA) 2 (MA) 2 Pb 3 I 10 and (BA) 2 (MA) 3 Pb 4 I 13 ) have been studied (Fig. 23(a)). 115 The (BA) 2 (MA) 3 Pb 4 I 13 lm color gets darker with increasing temperature (Fig. 23(b)). 115 H. Tsai and W. Nie et al. 115 have achieved a PCE of 12.51% with 2D (BA) 2 (MA) 3 Pb 4 I 13 PSCs device. Under the constant light illumination, aer 2500 h, the 2D perovskite devices is retaining 70% of its original PCE without encapsulated and 98% with encapsulated. The 3D perovskite devices have degraded < 10% of its original PCE aer 2500 h (Fig. 24(a and c)). Fig. 24(b) shows the PCE of the unencapsulated 2D and 3D devices, that shows degradation aer 60 h, under 65% relative humidity. 115 With simple encapsulation, aer 2500 h, the 2D devices retained 80% of its original PCE under 65% relative humidity, but the 3D devices had been degraded (Fig. 24(d)). 115 K. Yao et al. 46 used the polyethylenimine (PEI) cations to fabricate the 2D perovskite compounds (PEI) 2 (MA) nÀ1 Pb n I 3n+1 (n ¼ 3, 5, 7), which was used as absorber layer to fabricate PSCs with an aperture area of 2.32 cm 2 under ambient humidity that have a PCE up to 8.77%. Aer 500 h, the PCE of the 2D large-area PSCs device only decreased by $5%. 46 Furthermore, the alkali metal cation is introduced into the perovskite material, which can improve the stability of the PSCs device. 31,68,116 E. H. Sargent et al. 31 added cesium cation to fabricate a triple-cation perovskite compositions lms (Cs 0.05 -FA 0.81 MA 0.14 PbI 2.55 Br 0.45 ), that was made the large-area (1.1 cm 2 ) PSCs with a PCE up to 20.3% (Fig. 25(b and c)). Aer 90 days, the PSCs devices retained 96% of its initial PCE (Fig. 25(a)). 31 Rubidium (Rb) cations can stabilize the black illumination, the device has retained 95% of its initial PCE (Fig. 26(d)). 116 The second method for improving the PSCs device stability is to modify the charge transport layer (ETL and HTL), or use the new type charge transport material. 61 Because TiO 2 is especially sensitive to ultraviolet light, 113 some new ETL materials have been reported. A. D. Carlo et al. 117 reported an additional lithium-neutralized graphene oxide (GO-Li) layer as interface layer was inserted between TiO 2 ETL and perovskite layer, that improved the stability of PSCs devices. 117 A. Hagfeldt et al. 118 has used ZnO nanorod arrays as ETL replace the TiO 2 , achieving the PSCs device, it has been exposed in atmospheric environment without encapsulation, and maintaining 90% of the original efficiency. X. W. Zhang and J. B. You et al. 57 have used SnO 2 as ETL for planar-structure PSCs, it is found that the devices can maintain almost their original efficiency when store in dry air conditions for 40 days. aer 1000 hours, whereas the TiO 2 cells had completely degraded within 500 hours.
For the HTL materials, spiro-OMeTAD is the most commonly used HTL material, 35,57,67 the certied PCE of 22.1% in small cell. 3 But the lithium salt in spiro-MeOTAD is easy to absorb moisture and reduce the PSCs device stability. So inorganic and hydrophobic hole transport material are used to improve the PSCs device stability. 41,54,74,78,79 79 has fabricated inverted planar heterojunction structure for NiObased PSCs device (p-i-n), achieving more than 85% of its original PCE has been kept aer 150 days. Z. B. He et al. 78 used NiO x nanocrystal as HTL in planar PSCs device. Aer 1000 h, the PCE of PSCs device maintained 87% of its initial value. N. Arora and M. Grätzel et al. 74 used one new HTL material CuSCN. They achieved the PSCs with PCE > 20%, aer 1000 hours at 60 C, the PSCs devices retained >95% of their initial efficiency. CuGaO 2 as HTL in n-i-p conguration PSCs, exposing it directly to the ambient environment without encapsulation. Aer 30 days, it maintains 87% its initial PCE. 54 Other methods for improving the PSCs device stability include the PSCs structure optimization, interface optimization, encapsulation, etc. 81,82,119 A hole-conductor-free structure of the PSCs can achieve long-term stability. Exposing the PSCs device (c-TiO 2 /m-TiO 2 /ZrO 2 /carbon) under full AM 1.5 simulated sunlight over 1008 hours, the PCE maintains 100% of its initial value. 82 To improve the stability of the device, the insulation material encapsulate the PSCs device is frequently used. M. Grätzel and L. Y. Han et al. 36 encapsulated the largearea PSCs device (36.1 cm 2 , TiO 2 ETL, Fig. 27(a and b)) by the insulation material, the module retained 90% of its initial performance aer 500 h (Fig. 27(c)).

Cost analysis
For conventional solar PV technology, it need high energy and vacuum to process solar cells. Thus, these PSCs can turn-out to be a promising solution in replacing the conventional PV technology. In this section, we briey analyze the cost for various raw materials of a 1 m 2 PSCs module. Conventional PSCs device architecture is shown in Fig. 4(a), that include glass substrate, TCO (FTO), ETL (TiO 2 ), perovskite absorber layer (MAPbI 3 ), HTL (spiro-OMeTAD) and metal electrode (Au). For 1 m 2 conventional PSCs module, raw material utilization for cleaning, deposition of various layers and encapsulation of the module were extracted from various available literature sources and their corresponding data are included in Table 3. 120,121 From the data (Table 3), it is clear that about 43% of the total raw material cost is from FTO substrate, about 34% from the HTL material (spiro-OMeTAD), and 18% from metal electrode (Au). 120,121 These data suggest the need for replacement of conventional FTO substrate, HTL material and Au electrode. The efficiency of the PSCs device on ITO-free analogues achieved 11%. 122 81 Although, the PCE of spiro-OMeTAD-free PSCs device is little lower than the conventional PSCs, with small sacrice in efficiency, low-cost and highly stable carbon based HTM-free PSCs can be fabricated.

Environmental issuesthe presence of lead
Environmental issues are a well-recognized issue for PSCs. 104,123 Like CdTe, a toxic heavy metal exists in the PSCs devices. But, the CdTe is very chemically stable, organolead halide perovskites are not stable and upon ambient exposure they can degrade into products that are readily leached into the environment. 104,123 In the life cycle assessments (LCA), the hazards of Pb for environmental impacts exist in all stages, which include raw material extraction, synthesis of starting products, fabrication, use and decommissioning. 120 Thus, ideally PSCs should be subject to even more stringent safety standards and any commercial products should have clear plans for end-of-life disposal and/or recycling. 104,123 To address the concerns about lead, lead-free perovskite materials have attracted the attention of many researchers, which include tin-based perovskite materials and other perovskites materials (lead-free and tin-free perovskites, such as MA 2 CuCl x Br 4Àx , 124 CsGeI 3 , MAGeI 3 , and FAGeI 3 , 125 A 3 Sb 2 I 9 (A ¼ Cs, Rb), 126 Cs 2 BiAgCl 6 , 127 (N-methylpyrrolidinium) 3 Sb 2 Br 9 , 128 etc.). But, compare with lead-based perovskites, the efficiencies of tin-based PSCs commonly well below 10%, 129,130 the PCE values for other perovskites have been below 1%. [124][125][126] Thus, improving encapsulation technologies, it could limit the Pb leakage during the cell operation. Researching the leadfree perovskite materials, achieving high performance lead-free PSCs device, which could to replace the lead-based PSCs device.

Conclusions
In this article, we briey summarized the studies on large-area PSCs in recent years. Progress has been made in manufacturing larger area cells as well as modules, which is the interesting for commercialization of the technology. Approaches for fabricating the lager-area perovskite lm layer are described such as spin-coating, vapor deposition, gas-induced and blade coating etc. It is demonstrated that these processes are useful to realize more uniform perovskite layer with larger grain sized and better surface coverage, which strongly affect consequent photovoltaic performance of devices.
Going forward, PSCs will have to reduce non-radiative recombination and improve charge transport in order to achieve the highest possible V oc values and ll factors. For the largearea PSCs device, improving the PCE, the rst method is to change the chemical composition of perovskite, adjusting its band gap and increasing the charge generation. The second approach is to increase the grain size of perovskite, decreasing the cracks and pinholes, that reduces the bulk defect recombination and electric leakage, and increase V oc . The third approach is interface modication, which reduces interface contact resistance, and reduce interface and surface recombination, and increase J sc . Meanwhile, one key issue of the largearea PSCs is the long-term poor stability. To the improving of the stability of PSCs, which requires interdisciplinary research to nd new stable materials, the choice of electrodes, barrier layers, charge transport layers and encapsulation strategies. Undoubtedly, in the near future, halide perovskite materials have emerged as an attractive alternative to conventional silicon solar cells.

Conflicts of interest
The authors declare that there is no conict of interests regarding the publication of this paper.