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Review

Recent Progress in Perovskite Solar Cells: Status and Future

by
Ying Chen
1,2,*,
Man Zhang
3,
Fuqiang Li
1,2 and
Zhenyuan Yang
1,2
1
Hubei Engineering Technology Research Center of Energy Photoelectric Device and System, Hubei University of Technology, Wuhan 430068, China
2
School of Science, Hubei University of Technology, Wuhan 430068, China
3
School of Electrical and Electronic Engineering, Hubei University of Technology, Wuhan 430068, China
*
Author to whom correspondence should be addressed.
Coatings 2023, 13(3), 644; https://doi.org/10.3390/coatings13030644
Submission received: 18 February 2023 / Revised: 10 March 2023 / Accepted: 14 March 2023 / Published: 18 March 2023
(This article belongs to the Special Issue Optical Thin Film and Photovoltaic (PV) Related Technologies)
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Abstract

:
The power conversion efficiency (PCE) of perovskite solar cells (PSCs) has seen effective performance upgrades, showing remarkable academic research and commercial application value. Compared with commercial silicon cells, the PCE gap is narrowing. However, the stability, cost, and large-scale production are still far behind. For scale-up preparing high-efficiency and stable PSCs, there is a variety of related research from each functional layer of perovskite solar cells. This review systematically summarizes the recent research on the functional layers, including the electron transport layer, perovskite layer, hole transport layer, and electrode. The common ETL materials, such as TiO2, SnO2, and ZnO, need doping and a bi-layer ETL to promote their property. Large-scale and low-cost production of perovskite layers with excellent performance and stability has always been the focus. The expensive and instability problems of Spiro-OMeTAD and electrode materials remain to be solved. The main problems and future development direction of them are also discussed.

1. Introduction

The serious consumption of energy and environmental problems have aroused people’s attention to renewable energy. Over the past decade, solar energy systems have proved to be the most compelling of all renewable energy systems [1]. Perovskite is considered a hopeful photovoltaic candidate due to its high optical absorption coefficient, tunable band gaps, long charge carrier life, low cost, and simple preparation process [2,3,4,5,6,7]. The power conversion efficiency (PCE) of perovskite solar cells (PSCs) has jumped from 3.8% to 25.73% (certified) [8]. As shown in Figure 1, ABX3 is the general formula crystal structure of perovskite materials. A is a large radius cation such as CH3NH3+ (MA+), NH2CH=NH2+ (FA+), Cs+, Ca2+, and Sr2+ B is Pb2+, Sn2+, Ti4+, Nb5+, Mn4+, and other small radius cations. X are anions such as O2, F−, Cl−, Br−, and I− [9,10]. The tolerance factor t could be used to calculate the perovskite crystal structure stability. The t = (RA + RX)/ 2 (RB + RX), where RA, RB, and RX refer to the ionic radii of A, B, and X site ions, separately. In general, the tolerance factor of structurally stable, highly symmetrical cubic structured perovskites usually ranges from 0.813 to 1.107 [11,12]. Within this range of tolerance factors, different stabilities and band structures of perovskite materials were obtained by doping or replacing the A, B, and X site ions [13]. For example, adding Sn to Pb-based perovskites could reduce the band gap and effectively broaden the absorption spectrum in the near-infrared [14,15]. With this advantage, perovskite materials can be combined with different forbidden bandwidths of light-absorbing materials, such as Si and quantum dots, to make stacked cells, such as perovskite-Si [16,17], perovskite-chalcogenides [18,19], and perovskite-quantum dots [20].
Perovskite solar cells are primarily divided into mesoporous structures and planar structures. The planar structures contain regular planar structures (n-i-p) and inverted planar structures (p-i-n) [21]. The typical mesoporous structures from bottom to top are a transparent conductive glass substrate (FTO, ITO), an electron transport layer (ETL), a mesoporous layer (mainly TiO2, Al2O3, etc.), a perovskite layer, a hole transport layer (HTL), and metal electrodes (Figure 2a). The ETL plays the role of electron transport and hole blocking. The mesoporous layer is relatively thin. Thus, a perovskite layer could cover it completely to isolate the mesoporous material from HTL [22]. The HTL plays the role of hole transport. This structure avoids interfacial compounding of charges while maximizing charge transfer efficiency, enhancing the open-circuit voltage and charge collection efficiency, and most of the higher-certified efficiency perovskite solar cells use this structure [23]. However, the preparation of the mesoporous layers requires high-temperature processing, which limits its application in flexible devices.
The planar structure is similar to a “sandwich structure”. The perovskite layer is placed between the n-type material and the p-type material, which benefits from the excellent bipolar carrier transport properties of perovskite materials. They have n-i-p and p-i-n types (Figure 2b,c). The n-i-p structure is simplified from the mesoporous structure. The p-i-n structure is developed mainly from the structure and materials of classical organic solar cells. Planar-structured perovskite solar cells do not have a mesoporous layer, which is suitable for the fabrication of large-area flexible devices at low temperatures. Their preparation process is relatively simple too. However, without mesoporous material as a backbone, the perovskite morphology is difficult to control, the repeatability of the devices is poor, and additional auxiliary means are required to improve the film quality.
The key point of PSCs is the generation, separation, and transportation of photogenerated electrons and holes [23]. Rational selection and preparation of the functional layers are the main research priorities in this field, which is crucial for improving performance and reducing costs. In this review, we summarize the recent research status and existing problems of ETLs, perovskite layers, HTLs, and electrodes. Then, the challenges of PSCs and the strategies for improving performance are discussed.

2. Electron Transport Layer (ETL)

The ETL acts as a function of collecting electrons and blocking the transport of holes to the FTO electrode in the PSC. The mesoporous structure of the ETL promotes the crystallization and film formation of perovskite and shortens the migration path of photogenerated electrons. A suitable ETL should have an energy band position that matches the perovskite material. The conduction band position should be slightly below the conduction band minimum of the perovskite layer to facilitate electron injection, while the valence band is at a deeper position to effectively block holes and has a high electron conductivity to ensure electron transport and collection [24].
TiO2, an N-type oxide, is considered the most common choice for ETL material, with a wide band gap (~3 eV), easy-to-tune electronic properties, ability to form both dense and mesoporous layers, and producible easily and at low cost. TiO2 has two thermodynamically stable crystal phases: anatase and rutile. Kim et al. [25] found that the anatase phase showed better performance than the rutile phase. However, Wang et al. [26] showed that a rutile TiO2 ETL had better conductivity and match with the MAPbI3 layer, which significantly enhanced performance. Several new methods to prepare TiO2 for flexible devices or mass production have emerged in recent years, such as ball milling [27], ultrasonic spray [28,29], atomic layer deposition [30], inkjet-print [31,32,33], hydrothermal [34,35,36], sol–gel chemistry [37], low-temperature CO2 plasma [38], and low-temperature microwave [39,40]. The morphology of TiO2 has also attracted much attention. TiCl4 was a good choice. Jarwal et al. [41] obtained TiO2 nanorod arrays by solvothermal etching and/or TiCl4 treatment to improve their surface-to-volume ratio and direct carrier transportation. Shahvaranfard et al. [42] adopted TiCl4 treatment and a PC61BM monolayer to obtain a TiO2 nanorod array ETL. Lu et al. [43] exquisitely tuned TiCl4 precursor solution to construct nanowire arrays and nanoflower composite of TiO2 film. The composite structure has the characteristics of a short direct charge transmission path, small leakage current and charge transmission resistance, slow recombination rate, and large light harvest. The best PCE was more than 20%. Doping is a great method for improving the quality of TiO2. The commonly doped materials include Li [44], Mg [45], Zn [46,47], EuAc3 [48], Nb [49], Ce [50,51], Zr [52,53], Ta [54,55], and graphene quantum dots (QDs) [56]. The performance of related PSCs is shown in Table 1. Bilayer ETLs also work well, such as TiO2/SnO2 [57,58,59,60,61,62,63,64,65,66], TiO2/ZnO [67,68], TiO2/WO3 [69,70], TiO2/graphene [71,72], TiO2/fullerene [73], and TiO2/NiO [74].
SnO2, as an ETL material, has excellent electrical and optical properties. Its disadvantage is that there are surface defects and hysteresis phenomena. Cao et al. [75] used pyrrolidine fullerene C60-substituted phenol (NPC60-OH) to weaken the hysteresis of SnO2. The SnO2/NPC60-OH-based PSCs obtained a PCE of 21.39%. Liang et al. [76] passivated SnO2 with chlorine to improve electron mobility. The open-circuit voltage increased from 1.195 V to 1.135 V. Jiang et al. [77] adopted phosphoric acid to increase the electron collection efficiency of SnO2 by excluding its surface dangling bonds. Wang et al. [78] passivated SnO2 ETLs by fullerene to improve the PCE and reproducibility. Cao et al. [79] modified the SnO2 ETL by sulfur-doped graphite carbon nitride nanosheets to obtain a PCE of 20.33%. Liu et al. [80] introduced nontoxic phytic acid (PA) into the SnO2 to obtain the ETL with fewer defects. PA-SnO2-based PSCs had a high PCE of 21.43%. Lin et al. [81] obtained PCE of 21.87% with a SnO2 ETL by precursor engineering. Li et al. [82] utilized vacuum-assisted annealing to synthesize a SnO2 ETL at 100 °C for flexible solar cells, with a PCE of 20.14%. Liu et al. [83] adopted polydentate phytic acid dipotassium (PAD) to passivate the defects of the SnO2/perovskite interface and obtained a PCE of 19.52%. Xu et al. introduced functional polymers such as polyethylene oxide-polypropylene oxide-polyethylene oxide (P123) [84] and poly(amidoamine) (PM) [85] in SnO2 precursor to construct a uniform and dense SnO2 ETL. The PM-treated PSC had a PCE of up to 22.93%, with negligible hysteresis. Zong et al. used continuous spin coating to fabricate SnO2@K:Cs [86] and SnO2@Na:Cs ETLs [87], respectively. The better PCE was 22.06%, based on a SnO2@Na:Cs ETL. Gu et al. [88] added NaCl to raise the charge exchange between the SnO2 ETL and perovskite and obtained a PCE of 21.2%. Large-scale fabrication methods include vacuum thermal evaporation [89], magnetron sputtering [90], radio frequency [91,92], sputtering deposition [93], spray deposition [94,95], atomic layer deposition [96], hydrothermal [97], and printing [98]. Doping can improve the properties of SnO2. Common doping materials include Ga [99,100], Ta [101], Nb [102,103], Zn [104], Li [105], Zr/F [106], KF [107], Cl [108], NH4Cl [109], and graphene quantum dots [110]. The performance of related PSCs is shown in Table 1. Double ELT is also a valid way to promote the performance of SnO2, such as SnO2/CdS [111], SnO2-Ti3C2 Mxene [112], SnO2/ZnO [113,114,115], SnO2/TiO2 [116,117], SnO2/carbon nanotubes [118], SnO2/KCl [119], SnO2-assisted CdS [120], and NH2-ZnO@SnO2 [121].
ZnO, as another ETL material, has relatively large electron mobility, good light transmittance, proper work function, stability, low cost, and low-temperature preparation. Environmental friendliness, low-temperature, and large area are still hot topics of preparation method research. Zhang et al. [122] promoted the energy-level alignment and charge carrier extraction of ZnO by the sol–gel method. They also deposited ZnO by a simple water-based processing route [123]. Zhao et al. [124] used magnetron sputtering to prepare big-size grains, low defect state density, and a great optical ZnO ETL. They found that the PSCs based on an Ar/O2 ratio of 1:4 treated ZnO had a maximum PCE of 17.22%. However, ZnO shows less chemical compatibility with the perovskite layer. Surface passivation and ion doping are frequently used strategies. Yang et al. [125] optimized the hydrophilicity of the ZnO surface with three amino compounds. They found that the PSCs with isobutylamine (IBA) modification exhibited improved stability, and PCE can reach 18.84%. Eswaramoorthy et al. [126] used plasmonic nanoparticles to modify ZnO for high PCE and stability. Commonly used doping materials are Al [127], Co [128], Mg [129,130], reduced graphene oxide (rGO) sheet/Ag [131], and PbS [132]. The performance of related PSCs is shown in Table 1.
Table
Table 1. Summary of various PSCs performances using doped ETL.
Table 1. Summary of various PSCs performances using doped ETL.
ETLDevice ConfigurationJsc
(mA cm−2)		VOC (V)FFPCE (%)Ref.
TiO2LiFTO/c-TiO2/Li-m-TiO2/MAPbI3/Spiro-OMeTAD/Au22.861.1010.69917.59[44]
MgFTO/Mg-TiO2/Perovskite/Spiro-OMeTAD/Au22.271.080.60914.65[45]
ZnFTO/Zn-TiO2/MAPbI3/Spiro-OMeTAD/Au21.831.100.73417.60[46]
ZnFTO/Zn-TiO2 NAs/(FAPbI3)0.87(MAPbBr3)0.13/CuSCN/Carbon22.250.9560.67914.45[47]
EuAc3FTO/EuAc3-c-TiO2/CsPbI3/P3HT/Au21.201.10.7717.92[48]
NbFTO/Nb-TiO2/FA0.79MA0.16Cs0.05Pb(BrXI1−X)3/Spiro-OMeTAD/Au24.701.120.7821.30[49]
CeFTO/Ce-TiO2/MAPbI3/Spiro-OMeTAD/Ag21.951.070.6916.18[51]
ZrFTO/Zr-TiO2/MAPbI3/Spiro-OMeTAD/Ag23.660.920.56712.35[52]
ZrFTO/Zr-TiO2/MAPbI3/Spiro-OMeTAD/Au23.571.0760.71618.16[53]
TaFTO/Ta-TiO2/Cs0.1(FA0.83MA0.17)0.9Pb(I0.83Br0.17)3/Spiro-OMeTAD/Ag22.451.130.7719.62[54]
GQDsFTO/c-TiO2/GQDs-mTiO2/Cs0.05(FA0.83MA0.17)0.95 Pb(I0.83Br0.17)3/Spiro-OMeTAD/Au21.920.970.6314.36[56]
SnO2GaITO/Ga-SnO2/(FAPbI3)x(MAPbBr3)1-x/Spiro-OMeTAD/Ag23.901.0680.71418.18[99]
GaFTO/Ga-SnOx/CsPbBr3/Carbon7.581.3110.6025.98[100]
TaITO/Ta-SnO2/Perovskite/Spiro-OMeTAD/Au22.791.1610.78620.8[101]
NbFTO/Nb-SnO2/CsPbBr3/Carbon8.921.310.7318.54[103]
ZnFTO/Zn-SnO2/CsPbBr3/CuPc/Carbon23.401.0980.69217.78[104]
LiLi-FTO/SnO2/Al2O3/MAPbI3/Carbon22.180.760.5910.01[105]
Zr/FFTO/Zr/F-SnO2/Perovskite/Spiro-OMeTAD/Au24.391.1050.71219.19[106]
KFITO/KF-SnO2/CsPbI2Br/Spiro-OMeTAD/MoO3/Au14.791.310.79215.39[107]
ClFTO/Cl-SnO2/Perovskite/Spiro-OMeTAD/Au24.251.070.7318.94[108]
NH4ClITO/NH4Cl-L-SnO2/NH4Cl-H-SnO2/Perovskite/PEAI/Spiro-OMeTAD/Au23.601.2080.76221.75[109]
GQDsITO/GQDs-SnO2/MAFAPbI3Cl3-x/Spiro-OMeTAD/Ag24.401.110.7821.10[110]
ZnOCoPET/ITO/Co-ZnO/MAPbI3/Spiro-OMeTAD/Au14.301.040.477.00[128]
MgFTO/Mg-ZnO/MAPbI3/Spiro-OMeTAD/Ag25.060.830.6513.52[130]
rGO/AgFTO/rGO/Ag-ZnO/MAPbI3/Spiro-OMeTAD/Au17.820.900.7211.03[131]
PbSITO/PbS-ZnO/MAPbI3/Spiro-OMeTAD/Ag22.81.140.7920.53[132]

3. Perovskite Layer

The property of the perovskite layer, such as thickness, grain size, grain boundary defects, and surface roughness, affects the capability of the perovskite solar directly. Various preparation methods have been developed to produce higher-quality perovskite films for high PCE and stability [133]. The main preparation methods for the perovskite layer are spin coating [134], blade coating [135], vapor deposition [136], spray coating [137,138], drop casting [139,140,141], dip coating [142,143,144], electrodeposition [145,146], screen printing [147], inkjet printing [148,149]. All the above processes have their limitations and commercial viabilities. To compete with commercial silicon cells, stability and large-scale production are the focus of research. The trend of spin coating and blade coating is towards environmentally friendly solutions. The vapor deposition is mainly on a lab scale. They have advantages in cost, design freedom, optimal use of materials, and scalability, but they still need a rapid transformation from small, laboratory-scale work to large, industrial-scale production. Some of the new mass-production methods, such as spray coating, electrodeposition, screen printing, and inkjet printing, also need to be improved in terms of efficiency and repeatability.

3.1. Spin Coating

Spin coating is the most general method to prepare low-cost and high-efficiency PSCs. It contains “one-step coating” and “two-step coating”. In one-step spin coating, appropriate proportions of BX2 (B = Pb2+, Sn2+, etc.) and AX (A = MA+, FA+, Cs+, etc.; X = I−, Br−, Cl−, etc.) are dissolved in solvents to form precursor solution, and then the precursor solution is spin-coated on the ETL with adding anti-solvent. Solvents, anti-solvent, and the annealing process are the three main points to improve the quality of perovskite. Commonly used solvents are highly toxic and environmentally hazardous, such as N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), and γ-butyrolactone (GBL) [134]. Giuliano et al. [150] employed a green solvent additive, alpha-terpineol, to promote the quality of CH3NH3PbI3-xClx films. The planar n-i-p PSCs showed a PCE of 17.5% with decreased hysteresis and reinforced ambient stability. Kadhim et al. [151] obtained a high-quality Cs(MAFA)Pb(IBr)3) film by using a green EA solvent. A PCE of 18.63% and a fill factor of 79.78% were obtained. Cao et al. [152] proposed a green triethyl phosphate (TEP) solvent for perovskite precursor solution, and a non-polar dibutyl ether (DBE) solvent to be anti-solvent. Then, they obtained the best PCE of 20.13%. Zhu et al. [153] used Pb(SCN)2 to restrain the formation of an unacceptable PbI2·xDMSO-H2O compound for perfect perovskite crystallizes, and used PEASCN to increase moisture tolerance of perovskite films. MAPbI3 and MA0.6FA0.4PbI3 layers were successfully prepared in high-humidity air, and the PSCs based on them had PCEs of 19.83% and 21.18%, respectively. Taylor et al. [154] studied the influence of antisolvent on the quality of perovskite, as shown in Figure 3. They identified that the solubility of the organic precursors in the antisolvent and its miscibility with the host solvent(s) of the perovskite precursor solution combined to influence the quality of the perovskite layer. Sin et al. [155] found that the quality of MAPbI3 depended on the anti-solvent treatment (AST) time, amount, and temperature. If AST time was 9.5 s at 25 °C, the PSCs had the best PCE of 18.9%. The annealing is a necessary and critical process which serves to vaporize the solvent and induce the crystallization of perovskite [156]. Xu et al. [157] used millisecond light pulses to anneal for the crystallization of MAPbI3 and fabricated PSCs rapidly, which was a hopeful method for large-area, high-performance PSC fabricating. Chen et al. [158] adopted a rapid microwave-annealing method to avoid miscellaneous phases. Serafini et al. [159] proposed that flash infrared annealing could promote the optoelectrical property of the perovskite. Turgut et al. [160] proposed a light-assisted annealing (LA) method to form larger and uniform grains of the perovskite layer.
Overall, the advantage of one-step spin coating is the utilization of solution processing in the production of the perovskite layer, which simplifies the production process and reduces costs. However, there are many limitations in the production of perovskite films and stringent testing conditions, which places high demands on the actual conditions and processes used to produce the films.
Two-step spin coating involves spin coating BX2 on the substrate to form BX2 film, then spin coating AX on it to form a perovskite layer. This method could obtain fine perovskite morphology with great reproducibility. The disadvantage is that excess PbI2 can affect the performance of the final perovskite. Therefore, some recent research focuses on converting excess PbI2 into perovskite. Li et al. [161] used the remaining dimethyl sulfoxide (DMSO) to adjust the crystallization of PbI2. They also tried to delay the crystallization of PbI2 by adding dithizone into the PbI2 precursor [162]. Luan et al. [163] adjusted the concentration of organic salts in the precursor solution for the second step to translate all PbI2 to perovskite successfully. Both the PCE (21.26%) and stability of the PSCs without excessive PbI2 have been significantly improved. Jiao et al. [164] replaced PbI2 with FA4Pb3I6Ac4 (Ac−: CH3COO−) to fabricate stable black-phase FAPbI3 and obtain a PCE of 23.65% with high reproductivity, due to avoiding the extra PbI2. Bi et al. [165] fabricated high-phase purity and large grains of the CsPbBr3 perovskite layer by a facile thiourea-assisted two-step spin-coating method. In the first step, thiourea was added into the PbBr2 layer to improve its wettability. In the second step, green high-concentration CsBr/H2O solution provided enough CsBr to deposit. Thus, the CsPbBr3 PSC with a carbon electrode obtained a high PCE of 9.11%. The unencapsulated device showed fine stability. Zhi et al. [166] added N-methyl-2-pyrrolidone (NMP) in the MAI/IPA solution, then annealed in closed steam to intensify the Oswald ripening effect for the high-quality CH3NH3PbI3 layer. In addition, the low solubility of Cs salts is a problem for perovskite quality. Cheng et al. [167] used ethanol and methanol mixed solvent to improve the solubility of CsI and obtained a high-quality FA1-xCsxPbI3.
Chang et al. [168] found that the one-step spin coating obtained more MA+, resulting in a P-type majority carrier-type perovskite layer. The two-step spin coating obtained more Pb2+, resulting in an N-type majority carrier-type perovskite layer. The one-step spin coating had extra PbI2, which was harmful to the interface and the perovskite layer quality. Liu et al. [169] studied the difference between one-step spin coating and two-step spin coating from crystalline growth mode, optical properties, defect types, and carrier transport mechanisms. They found that the two-step spin coating obtained a smaller open-circuit voltage and fill factor, but a bigger short-circuit current.

3.2. Blade Coating

Blade coating has some obvious advantages, such as large-area preparation, high PCE, easy manipulation, and low cost, and includes doctor-blade coating, slot-die coating, and roller coating. Substrate temperature, blade coating speed, and precursor concentration are important factors of blade coating [135]. The most processing challenge was ambient moisture [170]. Wang et al. [171] ensured the perovskite phase transition by the appropriate substrate temperature and delayed perovskite crystallization by the dimethyl sulfoxide (DMSO) in the precursor solution. Thus, the doctor-blade coating prepared a large-grained perovskite layer, helping the PCE to 15.34%. Lee et al. [172] used the optimum substrate temperature and appropriate solvent ratio to n-i-p and p-i-n PSCs. The PCEs were 17.55% and 16.90%. Yu et al. [173] found that butyltrimethylammonium chloride (BTACl) passivation could reduce the influence of ambient humidity on the blade coating process, which produced a PCE of 20.5% with negligible hysteresis. Zendehdel et al. [174] combined blade and spin coating to improve the utilization rate of precursor materials. They fabricated a 10 cm2 active area of PSCs with an 18.8% PCE via blade-spin/blade deposition.
Vijayan et al. [175] adjusted the deposition parameters of the slot-die coating to obtain a dense and highly crystalline perovskite layer, which obtained a PCE of 14.5%. Huang et al. [176] used slot-die coating and near-infrared irradiation heating to make an area of 12 × 12 cm2 PSCs in air rapidly. Bernard et al. [177] enhanced the perovskite quality by adding methylammonium chloride into the ink formulation, then used vacuum solvent extraction to obtain a stabilized PCE of 17.6%. Bisconti et al. [178] synthesized good morphology perovskite layer at 60 °C for flexible solar cells by starch-polymer-assisted slot-die coating. Li et al. [179] found that 12 mol% DMSO as an additive of the slot-die coating ink formulation could block the intermediate phase about MAPbI3 and MAPbI3-2-ME and induce MAPbI3 crystallization, improving the PCE to 20.8% (Figure 4).

3.3. Vapor Deposition

Vapor deposition methods include high-vacuum deposition techniques (such as thermal evaporation) and low-vacuum deposition techniques (such as hybrid chemical vapor deposition) [136]. Vaynzof [180] compared thermal evaporation and solution processing. He reported that the difference in performance was diminishing and that thermal evaporation was more promising for commercial use. Choi et al. [181] fabricated large-area (250 mm2) and scalable (75 × 75 mm2) PSCs by the all-vacuum deposition method. In addition, stability is also important. There is much less research on stability improvement in thermal evaporation than the solution method [182]. Li et al. [183] used mixed-vapor deposition to prepare a PEA2MAn-1PbnI3n, which improved PCE to 18.08%. Lohmann et al. [184] controlled the crystallite size of the CH3NH3PbI3 layer by managing the substrate temperature when co-deposition of PbI2 and organic CH3NH3I. Qiu et al. [185] adopted rapid hybrid chemical vapor deposition (HCVD) to prepare 22.4 cm2 PSCs with a PCE of 12.3%. Qiu et al. [186] presented the recent development of modified CVD. They showed the potential of modified CVD for scalable fabrication of PSCs and modules with low cost. Defect passivation and interface modification are also a few relative reports. The performance of related PSCs is shown in Table 2.

4. Hole Transport Layer (HTL)

The HTL is located between the perovskite layer and the counter electrode, which is very important in perovskite solar cell devices. It plays the role of fast extraction of transported photogenerated holes and blocking electrons, effectively avoiding the interface compounding caused by their direct contact. The energy level of the HTL material must coincide with the valence band maximum of the perovskite material. The difference between these two energy levels ensures hole transport, but too large an energy level difference can lead to energy loss.
The organic small molecule Spiro-OMeTAD (2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene) is the most common HTL material, due to its wonderful hole transport properties. However, it is expensive and unstable. Improving its stability is the key research direction [187,188]. Niu et al. [189] improved the stability of Spiro-OMeTAD by adding a hydrophobic polymeric poly(4-vinylpyridine) (P4VP). Thus, PSCs remained 80% of the original PCE (20.6%) after over 6000 h in ambient air. Zhou et al. [190] added a metal–organic framework-derived 2D graphitic N-rich porous carbon (NPC) in Spiro-OMeTAD to decrease its defects. Du et al. [191] used Sb2S3 nanoparticles to inhibit the Li-TFSI aggregation for a compact spiro-OMeTAD:Sb2S3 HTL which could block the moisture and oxygen on the perovskite layer efficiently. The PCE was 22.13%, with great chemical stability. Cao et al. [192] prepared Co(III)-grafted CN nanosheets to improve the hole extraction and reduce the interfacial recombination. PCE of the doped PSCs reached 23.01%. In addition, other useful materials have recently been reported, such as carbon quantum dots (QDs) [193], Er@C-82 [194], fluorinated Graphene [195], PbSO4(PbO)4 QDs [196], and SnS nanoparticles [197].
PEDOT:PSS (poly(3,4-ethenedioxythiophene):poly(styrene sulfonate)), as a polymer HTL material in inverted PSCs, has brilliant optical transparency. It can be prepared at low temperatures. However, low fitness of work function, conductivity, and polarity mismatch between PEDOT:PSS and perovskite materials limited its development [198]. Modification, bilayer HTLs, and doping are recently effective methods. Elbohy et al. [199] adjusted the morphology, conductivity, and work function of PEDOT:PSS with urea, improving the PCE from 14.4% to 18.8%. Yi et al. [200] employed WO3/PEDOT:PSS to enhance the performance of PSCs. Yang et al. [201] diluted PEDOT:PSS for a less defective state density and a more appropriate work function. Sodium citrate [202], graphene [203], and ethanolamine [204] are the doping materials reported recently.
There is more and more research about NiO as an HTL material for inverted PSCs. It could be prepared in low temperatures for large-scale and flexible devices too. Compared to organic HTL materials, it has inappreciable hysteresis effects and better stability. However, it still has problems with low conductivity and mismatched bands [205]. Doping is an effective method. Lee et al. [206] used near-infrared radiation annealed Co-doped NiOx to reduce Voc loss. Chen et al. [207] found that Rb+-doped NiOx showed higher conductivity and better energy-level alignment. Choi et al. [208] reported cerium and zinc co-doped, where Zn doping improved electrical conductivity and Ce doping optimized surface morphology. Some other inorganic materials also attract attention, such as Cu2ZnSnS4 [209], Cu2ZnGeS4 [210], CuSCN [211,212,213], and Cu-doped Ga2O3 (Ga2O3:Cu) [214]. Compared with the ETL materials, the HTL materials have less research, but they have more research space on preparation and performance. The performance of related PSCs is shown in Table 3.

5. Electrode

PSC electrode materials need to have the appropriate work function, high electrical conductivity, and stability. It is inevitable to explore new low-cost electrode materials to replace traditional noble metal electrodes (Au and Ag) for a higher performance–price ratio [215,216]. Carbon materials are ideal candidates because they have similar functions and fine electrical conductivity to Au and can be prepared by silk-screen printing [217]. Lee et al. [218] reported that trifluoromethanesulfonic acid (TFMS) vapor doping of the free-standing carbon nanotube (CNT) sheet enabled the tuning of conductivity and work function of the CNT electrode. As shown in Figure 5, they found that PSCs based on TFMS 30 second-doped CNT obtained a champion PCE of 17.56%, with enhanced stability. Other carbon materials have been reported, such as mesoporous carbon [219], carbon nanofiber [220], graphite [221], and graphene [222].

6. Summary

The PCE of PSCs has seen effective performance upgrades between 2009 and 2023, showing remarkable academic research and commercial application value. Compared with commercial silicon cells, the PCE gap is narrowing. However, stability, cost, and large-scale production are still far behind. We have summarized some recent research on each functional layer (ETL, perovskite layer, HTL, and electrodes) of PSCs. TiO2, SnO2, and ZnO are the three most commonly used ETL materials. Doping and bilayer ETLs could promote their property. The ETL, which can be obtained at low temperatures, is advantageous for application in flexible PSCs. The perovskite layer is the soul of PSCs. Large-scale and low-cost production of perovskite layers with excellent performance and stability has always been the focus. The problems of lead toxicity and organic solvents are not perfectly solved too. Spiro-OMeTAD is the most common HTL material, but it is expensive and unstable. Additives and structural engineering help it to promote stability. PEDOT:PSS and NiO are often used in inverted PSCs. Their efficiency is also unsatisfactory. Compared with the ETL, the research on the HTL is less. The reliability of the new HTL material needs to be improved. Non-precious metal electrode materials are becoming more and more popular to reduce costs. A variety of carbon materials and other materials are good choices. The stability of PSCs hinders commercialization, even now. The degradation mechanism needs to be deeply researched. Perhaps, simulation experiments can help with stability testing. Two-/three-dimensional PSCs have already shown attractive efficiencies. However, more efforts are needed to make them commercially viable. Of course, improving the performance of each functional layer one by one is not enough. The interaction between layers is also very important. The right materials and overall structure will pave the way for the development of PSCs. The world’s first plant for the production of photovoltaic panels made of perovskites was opened in Poland in the town of Wroclaw two years ago. Perovskite photovoltaics from laboratory to industry is no longer a paper project [223].

Author Contributions

Conceptualization, Y.C. and M.Z.; validation, Y.C., M.Z., F.L. and Z.Y.; writing—original draft preparation, Y.C. and M.Z.; writing—review and editing, Y.C.; visualization, Y.C.; supervision, Y.C.; project administration, Y.C.; funding acquisition, Y.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Natural Science Foundation of China, grant number 21701041, the Open Foundation of Hubei Collaborative Innovation Center for High-Efficiency Utilization of Solar Energy, grant number HBSKFQN2017001, and the Talents of the High-Level Scientific Research Foundation of the Hubei University of Technology, grant number BSQD2017010.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is available within the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Coatings 13 00644 g001 550
Figure 1. Typical crystalline structure of perovskite.
Figure 1. Typical crystalline structure of perovskite.
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Figure 2. Three representative structures of PSCs: (a) mesoporous structures; (b) regular planar (n-i-p) structures, and (c) inverted planar (p-i-n) structures.
Figure 2. Three representative structures of PSCs: (a) mesoporous structures; (b) regular planar (n-i-p) structures, and (c) inverted planar (p-i-n) structures.
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Figure 3. Summary of the processes taking place during perovskite film formation for the three antisolvent types. Top: solubility of MAI in a solution of DMF:DMSO:antisolvent, meant to simulate the perovskite film intermediate phase during the antisolvent step of fabrication. Bottom: summary of the various mechanisms involved in perovskite film formation by the different categories of antisolvents [154].
Figure 3. Summary of the processes taking place during perovskite film formation for the three antisolvent types. Top: solubility of MAI in a solution of DMF:DMSO:antisolvent, meant to simulate the perovskite film intermediate phase during the antisolvent step of fabrication. Bottom: summary of the various mechanisms involved in perovskite film formation by the different categories of antisolvents [154].
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Figure 4. Schematic illustration of slot-die coating. (a) Coating from 2-ME and 2-ME-DMSO inks; (b) Cross-section SEM images (scale bar is 800 nm); (c) SAXS curves of two backgrounds, subtracting scattering curves of MAPbI3 in 2-ME (blue) compared to MAPI in 2ME with 11.77 mol% DMSO (red). Both solutions are in the same concentration; (d) Images of 2-ME DMSO inks (in the duration of 1 to 15 h) stored inside the glovebox (with O2 and H2O levels less than 1 ppm) [179].
Figure 4. Schematic illustration of slot-die coating. (a) Coating from 2-ME and 2-ME-DMSO inks; (b) Cross-section SEM images (scale bar is 800 nm); (c) SAXS curves of two backgrounds, subtracting scattering curves of MAPbI3 in 2-ME (blue) compared to MAPI in 2ME with 11.77 mol% DMSO (red). Both solutions are in the same concentration; (d) Images of 2-ME DMSO inks (in the duration of 1 to 15 h) stored inside the glovebox (with O2 and H2O levels less than 1 ppm) [179].
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Figure 5. (a) Schematic illustration of planar heterojunction PSCs based on TFMS-doped CNT. (b) J−V curves of the highest PCE PSCs, incorporating bare CNT and CNT doped by TFMS 30 and 50 s. (c) EQE spectra and corresponding integrated short-circuit current density (JSC) of the device based on bare CNT and CNT doped by TFMS for 30 s. (d) JSC. (e) open circuit voltage (VOC). (f) fill factor (FF) and (g) PCE of the PSCs incorporating bare CNT and CNT doped by TFMS [218] 2019 American Chemical Society.
Figure 5. (a) Schematic illustration of planar heterojunction PSCs based on TFMS-doped CNT. (b) J−V curves of the highest PCE PSCs, incorporating bare CNT and CNT doped by TFMS 30 and 50 s. (c) EQE spectra and corresponding integrated short-circuit current density (JSC) of the device based on bare CNT and CNT doped by TFMS for 30 s. (d) JSC. (e) open circuit voltage (VOC). (f) fill factor (FF) and (g) PCE of the PSCs incorporating bare CNT and CNT doped by TFMS [218] 2019 American Chemical Society.
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Table
Table 2. Summary of various PSC performances using various preparation methods.
Table 2. Summary of various PSC performances using various preparation methods.
Preparation MethodsDevice ConfigurationJsc
(mA cm−2)		VOC (V)FFPCE (%)Ref.
Drop castingITO/PEDOT:PSS/(C4A)2MA4Pb5I16/PC61BM/PEIE/Ag19.181.120.70215.08[139]
ITO/PEDOT:PSS/MAPbI3/PC61BM/BCP/Ag22.761.080.75618.57[140]
Dip coatingFTO/TiO2/ZnO/MAxFA1-xPbI3/Spiro-OMeTAD/Ag21.200.990.67014.10[142]
FTO/TiO2/ZnO/MAPbI3/Spiro-OMeTAD/MoO3/Ag20.330.950.63012.17[143]
FTO/TiO2/ZnO/MAPbI3-XClX/Spiro-OMeTAD/MoO3/Ag21.311.040.69015.29[144]
Screen printingFTO/TiO2/Perovskite/Spiro-OMeTAD/MoO3/Ag23.121.140.77920.52[147]
Inkjet printingITO/PEDOT:PSS/(BA)2(MA)3Pb4I13/PC61BM/PEIE/Ag18.801.140.69514.90[148]
FTO/TiO2/C60/Cs0.05MA0.14FA0.81PbI2.55Br0.45/Spiro-OMeTAD/Au23.481.1080.76219.60[149]
Spin coatingFTO/TiO2/MAPbI3-XClX/Spiro-OMeTAD/MoO3/Ag23.001.0590.72117.50[150]
FTO/TiO2/Cs(MAFA)Pb(IBr)3/Spiro-OMeTAD/Au22.161.0540.79818.63[151]
FTO/SnO2/(FAPbI3)0.95(MAPbBr3)0.05/Spiro-OMeTAD/Au24.691.090.74820.13[152]
FTO/SnO2/MA0.6FA0.4PbI3/PCBM/Au23.781.120.79521.18[153]
ITO/PTAA/Cs0.05FA0.80MA0.15PbI2.55Br0.45/PCBM/C60/BCP/Ag23.811.160.78521.59[158]
FTO/c-TiO2/m-TiO2/MAPbI3/Spiro-OMeTAD/Au23.901.080.7519.50[159]
ITO/SnO2/Perovskite/Spiro-OMeTAD/Au23.601.140.77120.66[162]
ITO/SnO2/FAPbI3/Spiro-OMeTAD/Ag25.731.150.79823.65[164]
FTO/TiO2/CsPbBr3/C8.811.380.759.11[165]
FTO/TiO2/MAPbI3/Spiro-OMeTAD/Au22.681.000.7016.78[166]
FTO/SnO2/FA1-xCsxPbI3/Spiro-OMeTAD/Ag24.121.1420.76921.17[167]
Blade coatingFTO/NiOx/MAPbI3/PCBM/BCP/Ag20.931.1130.68815.34[171]
FTO/TiO2/MAPbI3/Spiro-OMeTAD/Ag21.941.050.76217.55[172]
FTO/c-TiO2/m-TiO2/3D/2D Perovskite/PTAA/Au22.661.0880.79319.55[174]
FTO/c-TiO2/m-TiO2/Cs0.17FA0.83Pb(I0.83Br0.17)3/Spiro-OMeTAD/Au20.301.150.75517.60[177]
ITO/SAM/MAPbI3/C60/BCP/Cu23.671.1380.77320.80[179]
Vapor depositionITO/Spiro-mF/MAPbI3/PC61BM/ZnO/Ag21.901.070.7918.50[181]
ITO/PTAA/CsPbI3/C60/BCP/Cu17.800.960.7312.50[182]
FTO/TiO2/C60/(PEA)2(MA)n-1PbnI3n/Spiro-OMeTAD/Au23.751.080.70418.08[183]
FTO/C60/MAPbI3/Spiro-OMeTAD/Au21.701.080.77818.30[184]
ITO/SnO2/Cs0.1FA0.9PbI3/Spiro-OMeTAD/Au22.300.990.70215.50[185]
Table
Table 3. Summary of various PSCs performances using different HTLs.
Table 3. Summary of various PSCs performances using different HTLs.
HTLDevice ConfigurationJsc
(mA cm−2)		VOC (V)FFPCE (%)Ref.
Spiro-OMeTADP4VPITO/PEDOT:PSS/Perovskite/Spiro-OMeTAD:P4VP/Au23.491.1290.78720.84[189]
NPCFTO/c-TiO2/Cs0.05FA0.81MA0.14PbI2.55Br0.45/Spiro-OMeTAD:NPC/Au23.511.060.7618.51[190]
Sb2S3ITO/SnO2-KCl/CsFAMA/Spiro-OMeTAD:Sb2S3/Au24.751.1320.7922.13[191]
Co(III)-CNFTO/TiO2/Cs0.05MA0.1FA0.85Pb(I0.97Br0.03)3/Spiro-OMeTAD:CoCN2/Au25.431.1380.79523.01[192]
CQDsFTO/SnO2/(FAPbI3)0.95(MAPbBr3)0.05/CQDs/Spiro-OMeTAD/Ag24.181.0640.79917.92[193]
PbSO4(PbO)4 QDsITO/SnO2/CsFAMA/PbSO4(PbO)4-Spiro-OMeTAD/Au24.801.1420.8022.66[196]
SnSITO/SnO2/Perovskite/SnS- Spiro-OMeTAD/Au24.011.170.80722.59[197]
PEDOT:PSSUreaITO/PEDOT:PSS/MAPbI3/PCBM/Rhodamine/Ag22.571.030.80918.80[199]
WO3ITO/SnO2/FA0.4MA0.6PbI2.8Br0.2/WO3/PEDOT:PSS/MoO3/Ag 22.691.030.64815.10[200]
DilutedITO/D-PEDOT:PSS/MAPbI3−xClx/C60/BCP/Ag20.561.080.80417.85[201]
CitrateITO/SC-PEDOT:PSS/MAPbI3−xClx/PCBM/BCP/Ag21.621.1340.75018.39[202]
NiOCoFTO/NIR-4 Co:NiOx/MAPbI3/PCBM/PEI/Ag20.461.090.79817.77[206]
RbFTO/Rb-NiOx/MAPbI3/PCBM/BCP/Ag23.351.1330.82421.80[207]
Zn-CeITO/NiOx:ZnCe/MAPbI3/PCBM/BCP/Ag22.301.030.6314.47[208]
Cu2ZnGeS4FTO/SnO2/MAPbI3/Cu2ZnGeS4/C23.101.0820.72118.02[210]
CuSCNFTO/c-TiO2/MAPbI3/CuSCN/Au23.770.9810.72416.89[212]
Ga2O3:CuFTO/Ga2O3:Cu/Cs0.175FA0.75MA0.075Pb(I0.88Br0.12)3/PCBM/C60/BCP/Ag22.901.120.7619.5[214]
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Chen, Y.; Zhang, M.; Li, F.; Yang, Z. Recent Progress in Perovskite Solar Cells: Status and Future. Coatings 2023, 13, 644. https://doi.org/10.3390/coatings13030644

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Chen Y, Zhang M, Li F, Yang Z. Recent Progress in Perovskite Solar Cells: Status and Future. Coatings. 2023; 13(3):644. https://doi.org/10.3390/coatings13030644

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Chen, Ying, Man Zhang, Fuqiang Li, and Zhenyuan Yang. 2023. "Recent Progress in Perovskite Solar Cells: Status and Future" Coatings 13, no. 3: 644. https://doi.org/10.3390/coatings13030644

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