Updated Progresses in Perovskite Solar Cells

Zihan Qu(瞿子涵)$^{1,2}$, Fei Ma(马飞)$^{1,2}$, Yang Zhao(赵洋)$^{1,2}$, Xinbo Chu(楚新波)$^{1,2}$,\\ Shiqi Yu(余诗琪)$^{1,2}$, and Jingbi You(游经碧)$^{1,2\hyperlink{s}{*}}$

doi:10.1088/0256-307X/38/10/107801

⇦Chinese Physics Letters, 2021, Vol. 38, No. 10, Article code 107801Review⇨ Updated Progresses in Perovskite Solar Cells Zihan Qu (瞿子涵)1,2, Fei Ma (马飞)1,2, Yang Zhao (赵洋)1,2, Xinbo Chu (楚新波)1,2, Shiqi Yu (余诗琪)1,2, and Jingbi You (游经碧)1,2* Affiliations 1Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China 2Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China Received 19 August 2021; accepted 14 September 2021; published online 26 September 2021 Supported by the National Key R&D Program of China (Grant No. 2020YFB1506400), and the China National Funds for Distinguished Young Scientists (Grant No. 61925405).
^*Corresponding author. Email: jyou@semi.ac.cn Citation Text: Qu Z H, Ma F, Zhao Y, Chu X B, and Yu S Q et al. 2021 Chin. Phys. Lett. 38 107801 Abstract In the last decade, perovskite solar cells (PSCs) have greatly drawn researchers' attention, with the power conversion efficiency surging from 3.8% to 25.5%. PSCs possess the merits of low cost, simple fabrication process and high performance, which could be one of the most promising photovoltaic technologies in the future. In this review, we focus on the summary of the updated progresses in single junction PSCs including efficiency, stability and large area module. Then, the important progresses in tandem solar cells are briefly discussed. A prospect into the future of the field is also included. DOI:10.1088/0256-307X/38/10/107801 © 2021 Chinese Physics Society Article Text 1. Introduction. Since the first perovskite solar cells (PSCs) were developed by Miyasaka et al. in 2009, the power conversion efficiency (PCE) of PSCs was improved dramatically from 3.8% to 25.5% through merely a decade, which makes PSCs the most attractive photovoltaic technology in recent years.[1–14] The light absorbing material was named as perovskite due to its similar crystal structure to CaTiO$_{3}$,[15] with a chemical formula written as ABX$_{3}$. Generally, organic cations such as methylammonium (MA$^{+}$, CH$_{3}$NH$_{3}^{+}$),[16] formamidinium (FA$^{+}$, NH$_{2}$CH = NH$_{2}^{+}$)[17] and inorganic Cs$^{+}$[18] locate at A sites, while B sites are occupied by metal cations such as Pb$^{2+}$ or Sn$^{2+}$[19] and X site contains halogen anions including I$^{-}$ and Br$^{-}$,[7] etc., see Fig. 1(a). Thanks to the unique crystal structure, perovskite materials own a series of merits such as high light absorption coefficient[20,21] [Fig. 1(b)], long charge carrier diffusion length[22] [Figs. 1(c) and 1(d)] and tolerant to defects[23] [Fig. 1(e)]. The rapid development process of PSCs should be attributed to those unique advantages of perovskite layers.

Fig. 1. (a) Schematic diagram of perovskite crystal structure. (b) Curves of absorption coefficient versus photon energy in CH$_{3}$NH$_{3}$PbI$_{3}$, CsSnI$_{3}$ and GaAs film.[21] Reproduced with permission from Ref. [21]. (c) Diffusion length of electron and hole in CH$_{3}$NH$_{3}$PbI$_{3}$ film[20] and (d) TRPL spectrum of CH$_{3}$NH$_{3}$PbI$_{3-x}$Cl$_{x}$ film on PMMA, Spiro-OMeTAD and PCBM.[22] Reproduced with permission from Refs. [20,22]. (e) Defect energy levels distributed in perovskite energy band structure.[23] Reproduced with permission from Ref. [23].

A PSC contains the perovskite light absorbing layer, charge transport layers (CTLs) together with electrodes. When the solar light shines on the device, free electrons and holes are generated, then extracted by electron transport layer (ETL) and hole transport layer (HTL) respectively and finally collected by the electrodes to output power. Three typical device structures have been proposed including mesoporous structure, regular planar structure and inverted planar structure. At the very beginning, mesoporous structure dominated the field, due to its origin related to dye-sensitized solar cell.[1] Later, Snaith et al. discovered the bipolar property of perovskite and the planar structure emerged.[4] The typical device structure of PSC and its working principle are summarized in Fig. 2(a). To evaluate the commercialization potential of a solar cell, PCE, lifetime and cost should be considered simultaneously, which constitute the golden triangle. Thanks to the efforts of researchers, a certificated PCE as high as 25.5% has already been achieved on the single junction PSC, which comes very close to the state-of-the-art 26.7% PCE of silicon solar cells.[14] The lifetime of PSCs has also been extended from merely several seconds[1] to nearly one year,[24] although still lags far behind the 25 years lifetime of commercial silicon solar cells.[25] The cost of PSC was estimated to be less than half the traditional silicon solar cell, which could be as low as only 0.15 US\$/W,[26] making it a very promising PV technology in the future [Fig. 2(b)].

Fig. 2. (a) Schematic illustration of a typical PSC device structure and its working mechanism. (b) The comparison of PCE, lifetime and cost between silicon and perovskite solar cell.

Fig. 3. Important milestones in the PCE improvement process of PSC (published results).

In this review, we mainly focus on the recent progresses of PSCs in PCE, stability and large area. In addition, we also discuss recent progresses in perovskite based tandem solar cells. 2. Typical Strategies towards Highly Efficient PSCs. PCE is one of the most critical parameters of device performance which directly demonstrates the ability to convert light into electricity. The PCE is numerically defined as the ratio of the maximum output power density to the standard solar light power density (100 mW/cm$^{2}$, AM 1.5 G). In a more common format, PCE equals the product of short circuit current density ($J_{\rm SC}$), open circuit voltage ($V_{\rm OC}$) and fill factor (FF),[27] which means that a high PCE requires large $J_{\rm SC}$, $V_{\rm OC}$ and FF simultaneously. The $J_{\rm SC}$ is mainly affected by the absorption coefficient of perovskite and the recombination rate during the charge carrier transport process in the device. When the light is absorbed strongly, more charge carriers can be generated and outputted with a low recombination rate, thus induce a large $J_{\rm SC}$.[28,29] The $V_{\rm OC}$ origins from the split of the quasi-Fermi level between HTL and ETL, the defects in the perovskite itself or at the interface will lead to the extra energy level in the bandgap, which may decrease the $V_{\rm OC}$ dramatically.[30,31] As for FF, things are even more complicated because it is a function of both $J_{\rm SC}$ and $V_{\rm OC}$. FF should be discussed thoroughly through analyzing charge carrier dynamics, i.e., the generation of free charge carriers, the recombination process, the CTL extraction and transport ability of the charge carriers all hold strong impacts on FF.[32] 2.1. Ideal Bandgap for Higher $J_{\rm SC}.$ The Shockley–Queisser (S-Q) limit is quite well-known in the field of solar cells, which describes theoretical PCE limit for single junction solar cells with different bandgaps. Based on the S-Q theory, the semiconductor materials with a bandgap between 1.3 eV and 1.4 eV correspond to the best PCE limit of over 30% in a single junction solar cell[33,34] [Fig. 4(a)]. Through optimizing the composition, one can regulate the bandgap of perovskite[35] closer to the ideal value, which should lead to a higher PCE. Earlier researches on PSC are primarily based on MAPbI$_{3}$ with a bandgap of approximately 1.55 eV,[36–38] which limits the device PCE. Pure FAPbI$_{3}$ owns a bandgap of 1.47 eV,[39] which could be more promising for higher PCE. Unfortunately, the photo-active $\alpha$-phase FAPbI$_{3}$ is apt to be converted into the photo-inactive $\delta$-phase at room temperature.[40] Later, researchers discovered that FA$_{1-x}$MA$_{x}$PbI$_{3}$ could narrow the bandgap, enable the cutoff band-edge to be red shifted compared with MAPbI$_{3}$ and the light within near infrared region could be absorbed better accordingly[41] [Fig. 4(b)].

Fig. 4. (a) The relationship between bandgap of absorption materials in solar cells and their S-Q limit.[34] Reproduced with permission from Ref. [34]. (b) Light absorption spectrum of FA$_{x}$MA$_{1-x}$PbI$_{3}$ with different $x$ values.[41] Reproduced with permission from Ref. [41]. (c) The UV-vis absorption spectrum and PL spectrum of perovskite films with different molar ratios of MDACl$_{2}$ addition and (d) $J$–$V$ curves of PSC with and without MDACl$_{2}$ addition.[29] Reproduced with permission from Ref. [29]. (e) Lattice strain of perovskite with different molar ratios of MDA$^{+}$ and Cs$^{+}$ addition and (f) $J$–$V$ curves of PSCs with or without MDA$^{+}$ and Cs$^{+}$ doping.[42] Reproduced with permission from Ref. [42].

In recent years, the highest PCEs are reported based on FA-perovskite. For example, Seok et al. introduced additional iodide ions into the FA-based system and the deep-level defects were strongly suppressed. The capture cross-section values of different types of defects were calculated by them and the decreased values in the target film confirmed a lower defect density. A certificated PCE of 22.1% was achieved on a small area device, owning a $J_{\rm SC}$ value of 25.0 mA/cm$^2$.[10] In 2019, You et al. demonstrated that FA$_{0.92}$MA$_{0.08}$PbI$_{3}$ combined with surface passivation could show a 23.3% certificated device PCE with a $J_{\rm SC}$ of 25.2 mA/cm$^2$.[12] In the same year, Seok et al. utilized MDACl$_{2}$ to stabilize the $\alpha$-phase of FAPbI$_{3}$ rather than MAPbBr$_{3}$ and successfully maintained the inherent bandgap of FAPbI$_{3}$[29] [Fig. 4(c)]. The PCE was certificated as 23.7% with a $J_{\rm SC}$ over 26 mA/cm$^{2}$ [Fig. 4(d)]. In 2020, the same group doped MDA$^{+}$ and Cs$^{+}$ together to release the lattice strain [Fig. 4(e)] and improved the structure stability of $\alpha$-phase FAPbI$_{3}$. MDA$^{+}$ ion with a larger radius or a Cs$^{+}$ ion with a smaller radius alone may distort the lattice and the two ions adopted together could balance the strain. Their device reached an unprecedentedly certificated PCE of 24.4% and the $J_{\rm SC}$ value was 26.17 mA/cm$^2$[42] [Fig. 4(f)]. Nearly simultaneously, Grätzel et al. used MASCN vapor to treat the $\delta$-phase FAPbI$_{3}$ and the yellow phase could be converted to black $\alpha$-phase thanks to the MA incorporation into the framework together with the SCN stabilization function.[43] Recently, Huang et al. applied ionic liquid methylamine formate as precursor solvent and achieved a PCE as high as 24.1% ($J_{\rm SC}=25.34$ mA/cm$^{2}$). They believed that the nanometer-scale ion channels formed by the use of ionic liquid solvent could facilitate the diffusion of FAI into PbI$_{2}$ and a stable $\alpha$-phase FAPbI$_{3}$ was synthesized.[44] Pure FAPbI$_{3}$ shows the bandgap of 1.47 eV, which is close to the ideal bandgap for S-Q limit, while after stabilized by other ions, the bandgap has been shifted to around 1.5 eV, deviating from the idea case. It has been reported that by partially substituting Pb by Sn, the bandgap of perovskite could be narrowed. However, too many defects are formed inside perovskite while the Sn content is less than 20%,[45] leading to poor performance of devices, in which researchers should put more efforts to address. 2.2. Defects Passivation for Higher $V_{\rm OC}.$ It is generally accepted that the defect energy levels are shallow in the perovskite layer, even though, the defects could act as traps and recombination centers, which limits the further improvement of device performance[46] [Fig. 5(a)].

Fig. 5. (a) Common defect types distributed in perovskite.[46] Reproduced with permission from Ref. [46]. (b) Schematic diagram of choline chloride as passivator and the corresponding device $J$–$V$ curves.[47] Reproduced with permission from Ref. [47]. (c) Crystal model of PEAI as passivator, (d) TRPL of perovskite film with or without PEAI passivation and (e) the certificated $J$–$V$ curve with PEAI passivation.[12] Reproduced with permission from Ref. [12]. (f) SEM image of perovskite film with moderate residual PbI$_{2}$ and (h) the $J$–$V$ curve of PSC with excessive PbI$_{2}$.[48] Reproduced with permission from Ref. [48]. (g) Schematic figure of type-I band structure.[49] Reproduced with permission from Ref. [49]. (i) Comparison of binding affinity towards iodide vacancy site of different anions and (j) the comparison of $J$–$V$ curves between PSCs with and without pseudo-halide anion passivation.[50] Reproduced with permission from Ref. [50].

The defects are normally rich at the boundaries of crystal grains in the perovskite film and the interfaces of charge transfer layer/perovskite in the device. Yang et al. first realized the importance of interface modification to passivate the defects, they modified the ITO electrode with a layer of polyethyleneimine ethoxylated and doped Yb into the TiO$_{2}$ layer, which boosted the device PCE to 19.3% ($V_{\rm OC}=1.13$ V).[8] In 2017, Huang et al. used choline chloride, a type of quaternary ammonium halide, to passivate the interface between perovskite and ETL, thanks to the zwitterion structure, choline chloride passivated both the positively charged defects and negatively charged defects[47] [Fig. 5(b)], a small $V_{\rm OC}$ deficit of 0.39 V and as high as 21.04% PCE was achieved. Later in 2019, You et al. deposited a layer of phenethylammonium iodide (PEAI) between perovskite and HTL to passivate surface defects of perovskite layer [Fig. 5(c)]. For the PEAI-passivated film, the charge carrier lifetime obviously elongated due to the decrease of defect state density [Fig. 5(d)], the $V_{\rm OC}$ was thus dramatically improved from 1.12 V to 1.18 V and a certificated 23.32% PCE was achieved, which was the highest PCE at that time[12] [Fig. 5(e)]. Besides the interface, the defects in the perovskite film are also harmful. You et al. systematically investigated the performances of PSCs with different PbI$_{2}$ contents. They found out that a moderate residual of PbI$_{2}$ in the perovskite film could lead to a higher PCE without hysteresis [Fig. 5(h)] while the performance started to drop with excessive PbI$_{2}$. Deep study revealed that the residual PbI$_{2}$ located at the grain boundaries[48] [Fig. 5(f)] and the residual PbI$_{2}$ could form a type-I band structure and passivate the perovskite[49] [Fig. 5(g)]. Yang et al. doped caffeine into the perovskite to form a “molecular lock” with Pb$^{2+}$ and retarded the crystal growth rate, they formed high quality perovskite crystals with reduced defects and the PCE reached 20.25% with a $V_{\rm OC}$ of 1.14 V.[51] Recently, Grätzel et al. compared the binding affinity towards iodide vacancy site of different anions [Fig. 5(i)], they introduced a pseudo-halide anion HCOO$^{-}$ which owned the largest binding affinity to passivate the defects caused by iodide vacancies at the grain boundaries and the film quality was improved as well. Their device was certificated a PCE as high as 25.2% and the $V_{\rm OC}$ reached 1.17 V, which is among the best results up to date[50] [Fig. 5(j)]. 2.3. Efficient Charge Transport Layers for Higher FF. After charge generation process, free carriers will be extracted and transported to the electrodes. The suitable CTLs should match a good energy band alignment with the perovskite, and also own high charge mobilities.

Fig. 6. (a) Comparison of the energy band structures of TiO$_{2}$ and SnO$_{2}$.[52] Reproduced with permission from Ref. [52]. (b) The logarithm curves of $J$–$V$ to calculate and compare the electron mobility in TiO$_{2}$ and SnO$_{2}$.[53] Reproduced with permission from Ref. [53]. (c) SEM images of a SnCl$_{2}\cdot$2H$_{2}$O precursor deposited SnO$_{2}$ film with different resolutions and (d) forward and backward $J$–$V$ sweep curves of PSC with a SnCl$_{2}\cdot$2H$_{2}$O precursor deposited SnO$_{2}$ film.[54] Reproduced with permission from Ref. [54]. (e) SEM image of SnO$_{2}$ nanoparticle film with high-resolution TEM image inset and (f) The $J$–$V$ curve of nanoparticle SnO$_{2}$-based PSC with barely no hysteresis.[53] Reproduced with permission from Ref. [53]. (g) Schematic illustration of SnO$_{2}$ film coverage condition on substrate deposited by a chemical bath method and (h) the certificated $J$–$V$ curve of PSC with a chemical bath deposited SnO$_{2}$ film as ETL.[13] Reproduced with permission from Ref. [13].

TiO$_{2}$ is a commonly used ETL, however high temperature annealing is needed and also TiO$_{2}$ based devices usually showed large hysteresis.[55–57] Compared with TiO$_{2}$, SnO$_{2}$ can be deposited through a low temperature process, and more importantly, SnO$_{2}$ owns a deeper conduction band, which could form a better band alignment with a perovskite layer[52] [Fig. 6(a)]. In addition, the mobility of SnO$_{2}$ is two orders higher than that of TiO$_{2}$ and the electrons could be transported more efficiently, thus considered to be the most suitable ETL in PSCs[53] [Fig. 6(b)]. Fang et al. introduced precursor to deposit nanocrystalline SnO$_{2}$ film by sol-gel process of SnCl$_{2}\cdot$2H$_{2}$O and employed it as ETL to first obtain an average PCE of over 15% with an FF of 65%[54] [Figs. 6(c) and 6(d)]. Later, Hagfeldt and coworkers applied a low temperature atomic layer deposition method to grow SnO$_{2}$ film as ETL and an FF of 75% could be achieved with a hysteresis-free PCE over 18%.[52] In 2016, You et al. adopted highly crystallized SnO$_{2}$ nanoparticles as the ETL [Fig. 6(e)], they were convinced that the SnO$_{2}$ layer could enhance the electron extraction ability and reduce the charge accumulation at the interface. The charge accumulation at the interfaces was widely regarded as the origin of hysteresis in the PCE measuring process of PSCs. As a result, they obtained a certificated PCE of 19.9% (FF = 76.6%) with almost free hysteresis[53] [Fig. 6(f)]. Liu et al. doped the SnO$_{2}$ with EDTA to form the EDTA-complexed SnO$_{2}$, they figured out the electron mobility of modified SnO$_{2}$ with three times improvement, which facilitated the transport of electrons, and a certificated PCE of 21.52% was achieved.[58] Recently, Seo et al. improved chemical bath deposition method to deposit SnO$_{2}$ and successfully formed a complete and conformal growth of SnO$_{2}$ [Fig. 6(g)]. As a result, they obtained a 25.2% certificated PCE and FF reached an unprecedentedly 84.8%, which is the highest value reported by literature to date[13] [Fig. 6(h)]. Spiro-OMeTAD is the most common HTL in PSCs, which originated from solid state dye-sensitized solar cells.[59] Kim et al.[3] first introduced Spiro-OMeTAD into PSC and the PCE was 9.7%. To improve the conductivity of Spiro-OMeTAD, Li-TFSI[60] and FK209[61] were doped into the Spiro-OMeTAD solution. In addition, 4-tert-butylpyridine (tBP) was added to improve the solubility of lithium salt.[62] The as-fabricated device should be stored in an oxygen atmosphere and Spiro-OMeTAD would be oxidized to increase its hole mobility. Recently, the mechanism behind the oxidization process has been revealed from an electrochemical view, to believe that with the help of water, oxygen was reduced to form OH$^{-}$ at the cathode while Spiro-OMeTAD$^{+}$ was formed at the anode.[63] The diffusion of Li$^{+}$ completed the electrochemical cycle to increase the hole conductivity [Fig. 7(a)]. Recently, Kim et al. synthesized fluorinated isomeric analogues of Spiro-OMeTAD named Spiro-mF and the device based on it achieved a certificated PCE of 24.64% with a $V_{\rm OC}$ loss of only 0.3 V and an FF of 79.6%[64] [Fig. 7(b)]. The PSCs based on other HTLs such as poly(triaryl amine) (PTAA)[65] and poly(3-hexylthiophene) (P3HT)[11,66] also proved to be promising with PCE $> 24$%, whereas the performances still lag behind the Spiro-OMeTAD counterpart.

Fig. 7. (a) Schematic illustration of oxidation process in Spiro-OMeTAD.[63] Reproduced with permission from Ref. [63]. (b) Forward and backward $J$–$V$ scan for device with Spiro-$m$F (molecular structure inset) as HTL.[64] Reproduced with permission from Ref. [64].

3. Stability of PSCs. The stability issue is another crucial issue of PSCs practical application, which is usually characterized by the time when PCE drops to some ratio of the initial value.[67] The operational process of PSC is quite sensitive, which is strongly affected by external factors, such as moisture, oxygen, heat and electrical bias. Mechanical stress and even the indispensable solar light may damage the device as well in the real situation.[68] 3.1. The Measurement Protocols of PSC Stability. The stability of PSCs can be divided into two categories, i.e., shelf stability and operational stability, depending on the measuring condition. Recently, more and more researches began to focus on the operational stability, which reflects the practical situation. A maximum power point tracking (MPPT) method is usually applied in that case.[69] MPP is the point on the $I$–$V$ curve where the product of both parameters reaches the peak value. The applied voltage on PSC is changed through the tracking process to keep the device working at the MPP point with the help of a feedback mechanism and a PCE-time curve is thus generated. In a simpler way, a constant voltage is applied according to the voltage value at the beginning MPP. Earlier in 2020, researchers in the field jointly reported a consensus statement to call for the measurement standards on the stability of PSC, which is an important milestone to make the stability results of different laboratories comparable[70] (Fig. 8). In the future, how the time to failure in an accelerated stress test related with that in a reference stress test should be demonstrated clearly. In the following, we summarize recent main progresses in PSC stability. 3.2. Phase Stable Engineering. The formation energy of perovskite is quite small, thanks to the relatively weak chemical bonds such as ionic bond, van der Waals bond and hydrogen bond between the atoms, thus the crystal structure can be formed easily at relatively low temperature. However, the lattice structure of perovskite can deform with even the minimum perturbation of external conditions.[71] In that case, when applying some stimuli to the device, the atoms/ions are prone to escape from the original sites and thus ionic migration phenomenon takes place [Fig. 9(a)], which is harmful for the device long-term stability.[72] Therefore, fixing the atoms/ions to their initial sites and preventing the decomposition process becomes crucial to elongate the lifetime of PSCs.

Fig. 8. Existing ISOS protocols and additional testing conditions specifically for PSCs.[70] Reproduced with permission from Ref. [70].

Additive engineering has been proved to be useful by strengthening the chemical bond and fixing the atoms at initial sites. For example, Chen and coworkers added a novel hydrophobic and semiconductive molecular F-PDI into perovskite together with antisolvent dripping. They argued that the chelation function between the carbonyl of F-PDI and uncoordinated lead atoms could restrain the migration of lead. The strong hydrogen bond between F and N also immobilized the MA$^{+}$ cation when the temperature rose [Fig. 9(b)]. Their device could withstand 100 ℃ and 50% RH condition for quite a long time.[73] Zhou et al. mixed a little amount of NaF into the PbI$_{2}$ solution and fabricated PSC using the conventional two-step method. They believed that NaF could form strong hydrogen bonds with the organic cations and strong ionic bonds with the lead atoms simultaneously [Fig. 9(c)], which was beneficial for the fixing of ions on the lattice sites and the stability of perovskite was dramatically improved. Based on the NaF doping, their device could remain 90% of its initial PCE after 1000 h of continuous illumination at MPP[74] [Fig. 9(d)]. They also incorporated acetylacetone salt into the perovskite, where they found that the europium (Eu) ion could dramatically enhance the stability of PSC. They explained that Eu$^{3+}$ could oxidize the metal lead while itself being reduced to Eu$^{2+}$ simultaneously. Then the iodide defects in the perovskite could oxidize Eu$^{2+}$ back to Eu$^{3+}$ [Fig. 9(e)]. They named the mechanism a redox shuttle, which could work periodically.[75] As a result, their device could maintain 91% of its initial PCE value after 500 h MPPT operation [Fig. 9(f)]. In 2019, Snaith et al. added a low concentration of ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate into perovskite. They found out that ionic migration in the device was effectively suppressed [Fig. 10(a)] and the PCE only degraded 5% under continuous full-spectrum illumination for over 1800 h at 70–75 ℃ temperature when encapsulated[76] [Fig. 10(b)]. Later, a similar ionic liquid was used as an additive and the PCE retained 95% of the initial value at 85 ℃ after 1200 h illumination.[77]

Fig. 9. (a) Schematic illustration of ion migration phenomenon under external stimuli in perovskite.[72] Reproduced with permission from Ref. [72]. (b) F-PDI improved the thermal and moisture stability of PSC.[73] Reproduced with permission from Ref. [73]. (c) Fluorine in NaF formed hydrogen bond with hydrogen in organic cation and ionic bond with lead atom thus (d) the device could remain 90% of its initial PCE after 1000 h of continuous illumination at MPP.[74] Reproduced with permission from Ref. [74]. (e) Diagram of the mechanism behind the europium ion induced redox shuttle and (f) the degradation curve during MPPT operation for 500 h.[75] Reproduced with permission from Ref. [75].

The composition of perovskite, specifically the radii of compositional ions, influenced the material stability through modulating the tolerance factor. Normally, when the tolerance factor falls into the scale between 0.78 and 1.05, a stable perovskite structure can be formed spontaneously[78] [Fig. 10(c)]. Saliba et al. embedded small rubidium cation into the perovskite to stabilize the photo active $\alpha$-phase of FA-base perovskite by driving the tolerance factor into a more stable scale. After 500 h full spectrum illumination at MPP with 85 ℃ temperature, 95% of initial PCE remained[79] [Fig. 10(d)]. Besides rubidium content modification, Matsui et al. also partly replaced iodide with bromide at X site and the device retained 92% of its starting PCE after 1000 h of 85 ℃/85% RH condition with an absolute value exceeding 20% remained.[80] Recently, inorganic PSC with cesium cation occupying the A site mainly has shown excellent thermal and illumination stability compared with the organic/inorganic hybrid counterpart. However, the phase instability problem remains to be fully solved.[81–84] 3.3. Surface Engineering for Stability Improvement. In the above section, we have mentioned that the defects in the bulk of perovskite or at the interfaces can act as recombination centers, which lowers $V_{\rm OC}$ and hinders the further improvement of device performance. In another aspect, we stress that the defects may also play a role in the stability related issue. For example, defects could form the pathway of ion migration and accelerate the degradation of device. In that case, passivating the defects at the interfaces and grain boundaries proves to be an effective method to improve the stability of PSC.

Fig. 10. (a) PL image of perovskite film with or without ionic liquid adding under 10 V bias and (b) degradation curves of PSC with BMIMBF$_{4}$ addition.[76] Reproduced with permission from Ref. [76]. (c) Relationship between tolerance factor and A site cation size and (d) operation stability of Rb-doped PSC.[79] Reproduced with permission from Ref. [79].

To obtain a better performance, researchers have developed hundreds of materials to passivate the grain boundaries and interfaces of PSC. However, most of them are organic, which makes the passivation function unstable due to the relatively weak forces with the perovskite, thus inorganic passivators with better interaction with perovskite emerge. Huang et al. introduced sulfate ions into the surface of perovskite, and the surface was converted to water-insoluble lead sulphate through chemical reaction. They were confident that the capped wide bandgap lead oxysalt could passivate the uncoordinated lead atoms at the surface and suppress the recombination process of charge carriers [Fig. 11(a)]. Their encapsulated device could retain 96.8% of their initial PCE after working at MPP for 1200 h under simulated AM 1.5 G solar irradiation and 65 ℃ temperature[85] [Fig. 11(b)]. Han et al. inserted a chlorinated graphene oxide layer between perovskite and HTL to passivate the interface [Fig. 11(c)]. They proved that strong Pb–Cl bonds and Pb–O bonds could be formed between perovskite and passivation layer with the lead atoms fixing at the initial sites of perovskite instead of migrating to other layers. The chlorinated graphene oxide could also act as a block layer to prevent the ion migration. After 1000 h of MPP operation at 60 ℃ under AM1.5 G solar spectrum, their PSC could retain 90% of its beginning PCE[86] [Fig. 11(d)].

Fig. 11. (a) Schematic figure of lead sulfate passivation function and (b) the PCE degradation curve with or without lead sulfate passivation under MPPT condition.[85] Reproduced with permission from Ref. [85]. (c) Cross-sectional SEM image of PSC with Cl-GO passivation and (d) MPPT operational stability comparison with or without Cl-GO passivation.[86] Reproduced with permission from Ref. [86].

Low-dimensional (LD) perovskite owns a better material stability, which has been proved by many researchers.[87–89] Moreover, the wide-bandgap of LD perovskite is quite suitable for passivation. Through constructing the LD/3D heterostructure, the defects at the 3D perovskite surface can be passivated and the device stability can be enhanced [Fig. 12(a)]. Mixing the LD perovskite with the 3D perovskite was proved to be another effective approach to enhance the PSC stability[90] [Fig. 12(b)]. In 2017, Nazeeruddin et al. engineered a 2D-(HOOC(CH$_{2})_{4}$NH$_{3})_{2}$PbI$_{4}$/3D-CH$_{3}$NH$_{3}$PbI$_{3}$ heterostructure junction. Thanks to the ultra-stable gradually organized multi-dimensional interface, their device hit the lifetime of more than 10000 h, which is the best result ever even till today[24] [Fig. 12(c)]. Snaith et al. doped n-butylammonium cations into the 3D perovskite and discovered that 2D perovskite platelets were formed at the boundaries of 3D perovskite grains. The 2D perovskite functioned as a passivator and the non-radiation recombination of charge carriers was therefore remarkably impeded [Fig. 12(d)]. Their cells could maintain 80% of the post burn-in PCE after nearly 4000 h when encapsulated under simulated sunlight illumination[91] [Fig. 12(e)]. Wang et al. in situ grew a 2D PEA$_{2}$PbI$_{4}$ layer on the top of 3D perovskite and their devices could be stored at an ambient condition with an RH of 60% for 1000 h while keeping 90% of initial PCE[92] [Figs. 12(f) and 12(g)].

Fig. 12. Schematic illustration of (a) 2D/3D mixed structure and (b) 2D/3D layered structure.[90] Reproduced with permission from Ref. [90]. (c) PCE degradation curve of PSC with over 10000 h lifetime.[24] Reproduced with permission from Ref. [24]. (d) Energy band structure of the 2D/3D heterojunction and (e) PCE degradation curve with or without the 2D platelet.[91] Reproduced with permission from Ref. [91]. (f) The 2D PEA$_{2}$PbI$_{4}$ crystal structure and the 2D/3D heterojunction device architecture with (g) comparison of PCE degradation curve at different solution concentrations.[92] Reproduced with permission from Ref. [92].

3.4. Stable CTLs for Device Stability Improvement. When analyzing the stability of PSC, we should consider the device as a whole, not only the perovskite itself affects the device stability, but also the CTLs matter in the issue. Therefore, developing highly stable CTL should also be paid more attention. Most HTLs used in PSCs need further doping with additives to improve the hole mobility. For example, Spiro-OMeTAD, the most frequently used HTL, requires doping of Li-TFSI, tBP and FK209. Unfortunately, Li-TFSI is prone to adsorbing water, which hinders the moisture stability dramatically.[93] Moreover, when the temperature increases, the additives may volatilize and destroy the thermal stability of device and Spiro-OMeTAD itself would crystallize as well.[94] Poly (triaryl amine) (PTAA) and poly(3-hexylthiophene) (P3HT) are also frequently applied as HTL, which show better stability than Spiro-OMeTAD. Seo et al. inserted a layer of 4-dimethylaminopyridine (DMAP) between perovskite and PTAA, and they proposed that DMAP could enhance the adhesion strength between the two layers while passivating the interface in the meantime [Fig. 13(a)]. Their device could maintain 90% of initial 22.4% PCE under the double 85 condition after 530 h[65] [Fig. 13(b)]. Noh et al. doped hydrophobic Ga(acac)$_{3}$ into the aligned P3HT and the PCE reached 24.6% unprecedently [Fig. 13(c)]. Without encapsulation, their device held the full PCE under 85%RH for 2000 h[66] [Fig. 13(d)]. Park et al. devised a novel molecular HL38 as HTL of PSC. Interestingly, although Li-TFSI and tBP were doped, HL38 device was still much more stable than Spiro-OMeTAD device. The authors emphasized that Li$^{+}$ owned a stronger complexation with HL38 than Spiro-OMeTAD, which suppressed the mobilization of Li$^{+}$ and improved the device thermal stability. Stored under an 85 ℃ condition for over 1000 h, HL38 devices could retain 85.9% of initial PCE while Spiro-OMeTAD almost lost all performances.[95] Many researchers have developed novel dopant-free HTLs, which induced better device stability.[96] Loo et al. designed a new organic material named as YZ22 [Fig. 13(e)], they claimed that YZ22 could act as HTL and passivate the surface at the same time. Without extra doping, YZ22 owned a hole mobility of 8.8$\times$10$^{-4}$ cm$^{2}\cdot$V$^{-1}\cdot$S$^{-1}$, which was close to the doped Spiro-OMeTAD and one magnitude higher than pristine Spiro-OMeTAD. The encapsulated PSC with YZ22 as HTL could retain 80% of its beginning performance after 1000 h MPP operation, which is among the best results obtained by dopant-free HTL[97] [Fig. 13(f)]. Unlike the HTLs, ETLs themselves are much more stable. The most common ETLs are metal oxides such as TiO$_{2}$ and SnO$_{2}$. The unstable factors are normally attributed to the interfaces and grain boundaries. Sargent et al. used chlorine-capped TiO$_{2}$ colloidal nanocrystal film as the ETL and successfully passivated the surface dangling bonds. Their device performed an initial PCE of more than 20% and maintained 90% of it after 500 h MPP operation under the AM1.5 G spectrum.[53]

Fig. 13. (a) Diagram showing how DMAP strengthened the interaction between perovskite and PTAA and (b) the comparison of double 85 degradation trend between DMAP-modified device and control.[65] Reproduced with permission from Ref. [65]. (c) Schematic illustration of how Ga(acac)$_{3}$ behaved on interface between perovskite and P3HT together with (d) degradation curves of Ga-doped device and control.[66] Reproduced with permission from Ref. [66]. (e) Molecular structure of YZ22 and (f) the PCE degradation curve of device based on YZ22 or Spiro-MeOTAD as HTL.[97] Reproduced with permission from Ref. [97].

4. Large Area PSCs and Modules. When the device area becomes larger, forming a continuous and uniform perovskite film tends to be a great challenge. Accordingly, while increasing the device area, the performance showed large efficiency loss. To overcome the obstacle, researchers have developed a series of novel technologies specifically for depositing high quality large area perovskite films. For instance, Huang et al. applied blade coating method to fabricate scaling up PSCs [Fig. 14(a)]. They incorporated bilateral alkylamine into precursor ink and obtained a 20.0% PCE on an aperture area of 1.1 cm$^{2}$[98] [Fig. 14(b)]. Zhu et al. spray coated the TiO$_{2}$ layer and blade coated the perovskite layer to fabricate large area device [Fig. 14(c)], they obtained a stabilized PCE of 15.6% with the active area of 10.36 cm$^{2}$[99] [Fig. 14(d)]. Song et al. improved the inkjet printing method using NMP and DMF as the ink solvent to retard the ink crystallization process, the film exhibited a high homogeneity with large grain size and the defects were strongly suppressed and a PCE of 17.9% was obtained on a 1.01 cm$^{2}$ device.[100] Galagan et al. developed the slot die coating process to deposit large area perovskite film and Spiro-OMeTAD HTL combined with the laser ablation processes for monolithic cell interconnection, they fabricated a module with PCE over 10%.[101] When considering practical applications, perovskite sub-cells should be connected in series into a module to obtain a commercial level active area and power output. Qi et al. employed a holistic interface stabilization strategy to fabricate the perovskite module [Fig. 14(e)]. By introducing EDTAK, EAI and P3HT, they obtained a module as large as 22.4 cm$^{2}$ with a PCE of 16.6%[102] [Fig. 14(f)]. Recently, Huang et al. fabricated a module of 35.8 cm$^{2}$ with a PCE of 18.2%.[103] It must be noted that several companies have shown significant progresses on large area PSC modules, the largest module (804 cm$^{2}$) and highest PCE (17.9%) was reported by the Panasonic in Japan.[104]

Fig. 14. (a) Schematic diagram of blade coating process and (b) the device performance with the sample photograph inset.[98] Reproduced with permission from Ref. [98]. (c) Key procedures to spray coat TiO$_{2}$ and blade coat perovskite layer and (d) forward sweep, backward sweep and stable output of the device.[99] Reproduced with permission from Ref. [99]. (e) The architecture of perovskite module fabricated in a holistic interface stabilization strategy and (f) corresponding $J$–$V$ curves with the sample size inset.[102] Reproduced with permission from Ref. [102].

Fig. 15. (a) Schematic illustration of complementary light absorption in sub-cells of a tandem solar cell.[105] Reproduced with permission from Ref. [105]. (b) Device structure of a perovskite/silicon tandem solar cell with surface micro-pyramid structures on silicon sub-cell and its (c) certificated performance.[106] Reproduced with permission from Ref. [106]. (d) Device structure of a perovskite/silicon tandem solar cell with a self-assembled methyl substituted carbazole monolayer as the HTL of perovskite sub-cell and its (e) certificated performance.[107] Reproduced with permission from Ref. [107]. (f) Device structure of a perovskite/perovskite tandem PSC with (g) certificated report of the performance.[108] Reproduced with permission from Ref. [108].

5. Tandem Structures. The AM 1.5G solar spectrum owns a wide range of light wavelength from 300 nm to over 1000 nm, covering the scale from ultraviolet to infrared.[27] For most single junction solar cells, only part of the spectrum can be utilized effectively due to the bandgap restriction, which limits the PCE. Tandem solar cell is combined by several single junction solar cells, each with a complementary absorption range, thus the solar spectrum can be absorbed more efficiently[105,109] [Fig. 15(a)]. The bandgap of perovskite can be easily tuned by composition engineering, which makes PSC very suitable for tandem solar cells, either by itself or with other semiconductor solar cells. Three typical perovskite tandem structures including perovskite/silicon, perovskite/CIGS and perovskite/perovskite have been reported so far. Among them, perovskite/silicon is regarded as the most promising. In recent years, there are a lot of significant progresses in perovskite/silicon tandem solar cells. For example, McGehee et al. deposited SnO$_{2}$ as a buffer layer with negligible parasitic absorption, they realized a 23.6% PCE on a two-terminal perovskite/silicon tandem solar cell in 2017.[110] Later, Sahli et al. developed a conformal growth method to fabricate perovskite top-cell on textured silicon bottom-cell, which increased the $J_{\rm SC}$ due to the light-trapped effect of surface micro-pyramid structures on silicon sub-cell [Fig. 15(b)], the monolithic tandem device held a certificated PCE of 25.2%[106] [Fig. 15(c)], which was the first perovskite/silicon tandem solar cells with PCE above 25%. More encouragingly, Albrecht et al. designed a self-assembled methyl substituted carbazole monolayer as the HTL of perovskite sub-cell [Fig. 15(d)], the novel HTL accelerated the extraction of holes and pushed the FF of perovskite sub-cell towards 84%. The monolithic tandem device performed a certificated PCE of 29.15% with a $V_{\rm OC}$ of 1.92 V[107] [Fig. 15(e)]. Recently, perovskite/silicon monolithic tandem solar cell reached a PCE as high as 29.5% by Oxford PV.[14] In addition to progresses in the perovskite/silicon tandem solar cells, amazing achievements have also been obtained from perovskite/perovskite and perovskite/CIGS tandem solar cells. In 2019, Tan et al. reduced the Sn vacancies in mixed Pb-Sn narrow bandgap perovskite and achieved a monolithic perovskite/perovskite tandem solar cell with PCE of 24.8% on a 0.049 cm$^{2}$ active area[111] and later 24.2%[108] with an active area of 1 cm$^{2}$ [Figs. 15(f) and 15(g)]. Till now, the perovskite/CIGS tandem device also reached a PCE of 24.2%.[14] 6. Conclusion and Future Outlook. Due to the excellent optoelectronic properties of perovskite materials and simple fabrication process of devices, PSC becomes the hot topic of scientific research in the recent ten years and may play the role of “game changer” in the energy revolution. The certificated PCE of PSC in laboratory has already reached 25.5% and will overpass 30% in the near future based on tandem structure. However, the lifetime of PSC still lags far behind commercial solar cells, e.g., silicon solar cell, which constrains its competitiveness in market. In the recent several years, many strategies have been developed to improve the PSC long-term stability including additive engineering, passivation and design of novel CTLs. To fully utilize the solar spectrum and break the S-Q limit law, tandem structures of perovskite and other PV technologies, especially perovskite/silicon tandem cells, are developed. In reality, solar cells are part of the power system to generate electricity in the form of modules, which requires large size devices. Depositing uniform films on large area substrates is thus critical and many novel technologies have been invented to fabricate PSCs with large active area. References Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells6.5% efficient perovskite quantum-dot-sensitized solar cellLead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide PerovskitesSequential deposition as a route to high-performance perovskite-sensitized solar cellsEfficient planar heterojunction perovskite solar cells by vapour depositionSolvent engineering for high-performance inorganic–organic hybrid perovskite solar cellsInterface engineering of highly efficient perovskite solar cellsHigh-performance photovoltaic perovskite layers fabricated through intramolecular exchangeIodide management in formamidinium-lead-halide–based perovskite layers for efficient solar cellsEfficient, stable and scalable perovskite solar cells using poly(3-hexylthiophene)Surface passivation of perovskite film for efficient solar cellsEfficient perovskite solar cells via improved carrier managementThe emergence of perovskite solar cellsMethylamine-assisted growth of uniaxial-oriented perovskite thin films with millimeter-sized grainsCompositional engineering of perovskite materials for high-performance solar cellsCesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiencyIllumination Durability and High-Efficiency Sn-Based Perovskite Solar Cell under Coordinated Control of Phenylhydrazine and Halogen IonsLong-Range Balanced Electron- and Hole-Transport Lengths in Organic-Inorganic CH3NH3PbI3Unique Properties of Halide Perovskites as Possible Origins of the Superior Solar Cell PerformanceElectron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite AbsorberUnusual defect physics in CH ₃ NH ₃ PbI ₃ perovskite solar cell absorberOne-Year stable perovskite solar cells by 2D/3D interface engineeringDegradations of silicon photovoltaic modules: A literature reviewAddressing the stability issue of perovskite solar cells for commercial applicationsReliable Measurement of Perovskite Solar CellsInsight into the Origins of Figures of Merit and Design Strategies for Organic/Inorganic Lead‐Halide Perovskite Solar CellsEfficient, stable solar cells by using inherent bandgap of α-phase formamidinium lead iodideVisualization and suppression of interfacial recombination for high-efficiency large-area pin perovskite solar cellsOn the Relation between the Open‐Circuit Voltage and Quasi‐Fermi Level Splitting in Efficient Perovskite Solar CellsA Realistic Methodology for 30% Efficient Perovskite Solar CellsDetailed Balance Limit of Efficiency of p‐n Junction Solar CellsResearch Direction toward Theoretical Efficiency in Perovskite Solar CellsFormamidinium lead trihalide: a broadly tunable perovskite for efficient planar heterojunction solar cellsA hole-conductor-free, fully printable mesoscopic perovskite solar cell with high stabilityHysteresis-less inverted CH ₃ NH ₃ PbI ₃ planar perovskite hybrid solar cells with 18.1% power conversion efficiencyThermally Stable MAPbI ₃ Perovskite Solar Cells with Efficiency of 19.19% and Area over 1 cm ² achieved by Additive EngineeringSingle Crystal Formamidinium Lead Iodide (FAPbI ₃ ): Insight into the Structural, Optical, and Electrical PropertiesStabilization of the Trigonal High-Temperature Phase of Formamidinium Lead IodideProgress on Perovskite Materials and Solar Cells with Mixed Cations and Halide AnionsImpact of strain relaxation on performance of α-formamidinium lead iodide perovskite solar cellsVapor-assisted deposition of highly efficient, stable black-phase FAPbI ₃ perovskite solar cellsStabilizing black-phase formamidinium perovskite formation at room temperature and high humidityIdeal Bandgap Organic–Inorganic Hybrid Perovskite Solar CellsRecent Progresses on Defect Passivation toward Efficient Perovskite Solar CellsDefect passivation in hybrid perovskite solar cells using quaternary ammonium halide anions and cationsPlanar‐Structure Perovskite Solar Cells with Efficiency beyond 21%Controllable Self-Induced Passivation of Hybrid Lead Iodide Perovskites toward High Performance Solar CellsPseudo-halide anion engineering for α-FAPbI3 perovskite solar cellsCaffeine Improves the Performance and Thermal Stability of Perovskite Solar CellsHighly efficient planar perovskite solar cells through band alignment engineeringEnhanced electron extraction using SnO2 for high-efficiency planar-structure HC(NH2)2PbI3-based perovskite solar cellsLow-Temperature Solution-Processed Tin Oxide as an Alternative Electron Transporting Layer for Efficient Perovskite Solar CellsA fluorene-terminated hole-transporting material for highly efficient and stable perovskite solar cellsEfficient and stable solution-processed planar perovskite solar cells via contact passivationNanoscale localized contacts for high fill factors in polymer-passivated perovskite solar cellsHigh efficiency planar-type perovskite solar cells with negligible hysteresis using EDTA-complexed SnO2Solid-state dye-sensitized mesoporous TiO2 solar cells with high photon-to-electron conversion efficienciesAdvances in Liquid-Electrolyte and Solid-State Dye-Sensitized Solar CellsNanostructured TiO2/CH3NH3PbI3 heterojunction solar cells employing spiro-OMeTAD/Co-complex as hole-transporting materialUnveiling the Role of tBP–LiTFSI Complexes in Perovskite Solar CellsSynergetic effects of electrochemical oxidation of Spiro-OMeTAD and Li ⁺ ion migration for improving the performance of n–i–p type perovskite solar cellsStable perovskite solar cells with efficiency exceeding 24.8% and 0.3-V voltage lossSelective Defect Passivation and Topographical Control of 4‐Dimethylaminopyridine at Grain Boundary for Efficient and Stable Planar Perovskite Solar CellsSpontaneous interface engineering for dopant-free poly(3-hexylthiophene) perovskite solar cells with efficiency over 24%Towards commercialization: the operational stability of perovskite solar cellsMaterial and Device Stability in Perovskite Solar CellsHill climbing hysteresis of perovskite-based solar cells: a maximum power point tracking investigationConsensus statement for stability assessment and reporting for perovskite photovoltaics based on ISOS proceduresIonization Energy as a Stability Criterion for Halide PerovskitesIonic transport in hybrid lead iodide perovskite solar cellsHigh‐Performance Perovskite Solar Cells with Excellent Humidity and Thermo‐Stability via Fluorinated PerylenediimideCation and anion immobilization through chemical bonding enhancement with fluorides for stable halide perovskite solar cellsA Eu ³⁺ -Eu ²⁺ ion redox shuttle imparts operational durability to Pb-I perovskite solar cellsPlanar perovskite solar cells with long-term stability using ionic liquid additivesA piperidinium salt stabilizes efficient metal-halide perovskite solar cellsStabilizing Perovskite Structures by Tuning Tolerance Factor: Formation of Formamidinium and Cesium Lead Iodide Solid-State AlloysIncorporation of rubidium cations into perovskite solar cells improves photovoltaic performanceCompositional Engineering for Thermally Stable, Highly Efficient Perovskite Solar Cells Exceeding 20% Power Conversion Efficiency with 85 °C/85% 1000 h StabilityCesium Lead Inorganic Solar Cell with Efficiency beyond 18% via Reduced Charge RecombinationInorganic Ammonium Halide Additive Strategy for Highly Efficient and Stable CsPbI ₃ Perovskite Solar CellsSurface Engineering of Ambient-Air-Processed Cesium Lead Triiodide Layers for Efficient Solar CellsThermodynamically stabilized β-CsPbI ₃ –based perovskite solar cells with efficiencies >18%Stabilizing halide perovskite surfaces for solar cell operation with wide-bandgap lead oxysaltsStabilizing heterostructures of soft perovskite semiconductorsOrigin of the stability of two-dimensional perovskites: a first-principles studyTwo-dimensional perovskite materials: From synthesis to energy-related applicationsTwo‐Dimensional Metal‐Halide Perovskite‐based Optoelectronics: Synthesis, Structure, Properties and ApplicationsRecent Progress in 2D/3D Multidimensional Metal Halide Perovskites Solar CellsEfficient ambient-air-stable solar cells with 2D–3D heterostructured butylammonium-caesium-formamidinium lead halide perovskitesIn Situ Growth of 2D Perovskite Capping Layer for Stable and Efficient Perovskite Solar CellsDoping strategies for small molecule organic hole-transport materials: impacts on perovskite solar cell performance and stabilityRole of spiro-OMeTAD in performance deterioration of perovskite solar cells at high temperature and reuse of the perovskite films to avoid Pb-wasteCapturing Mobile Lithium Ions in a Molecular Hole Transporter Enhances the Thermal Stability of Perovskite Solar CellsToward ideal hole transport materials: a review on recent progress in dopant-free hole transport materials for fabricating efficient and stable perovskite solar cellsA hole-transport material that also passivates perovskite surface defects for solar cells with improved efficiency and stabilityBilateral alkylamine for suppressing charge recombination and improving stability in blade-coated perovskite solar cellsHighly Efficient Perovskite Solar Modules by Scalable Fabrication and Interconnection OptimizationInk Engineering of Inkjet Printing PerovskiteUp-scalable sheet-to-sheet production of high efficiency perovskite module and solar cells on 6-in. substrate using slot die coatingA holistic approach to interface stabilization for efficient perovskite solar modules with over 2,000-hour operational stabilityIodine reduction for reproducible and high-performance perovskite solar cells and modulesSolar cell efficiency tables (version 56)Metal halide perovskite tandem and multiple-junction photovoltaicsFully textured monolithic perovskite/silicon tandem solar cells with 25.2% power conversion efficiencyMonolithic perovskite/silicon tandem solar cell with >29% efficiency by enhanced hole extractionAll-perovskite tandem solar cells with 24.2% certified efficiency and area over 1 cm2 using surface-anchoring zwitterionic antioxidantOpportunities and challenges for tandem solar cells using metal halide perovskite semiconductors23.6%-efficient monolithic perovskite/silicon tandem solar cells with improved stabilityMonolithic all-perovskite tandem solar cells with 24.8% efficiency exploiting comproportionation to suppress Sn(ii) oxidation in precursor ink

[1]	Kojima A, Teshima K, Shirai Y, and Miyasaka T 2009 J. Am. Chem. Soc. 131 6050
[2]	Im J H, Lee C R, Lee J W, Park S W, and Park N G 2011 Nanoscale 3 4088
[3]	Kim H S, Lee C R, Im J H, Lee K B, Moehl T, Marchioro A, Moon S J, Humphry-Baker R, Yum J H, Moser J E, Gratzel M, and Park N G 2012 Sci. Rep. 2 591
[4]	Lee M M, Teuscher J, Miyasaka T, Murakami T N, and Snaith H J 2012 Science 338 643
[5]	Burschka J, Pellet N, Moon S J, Humphry-Baker R, Gao P, Nazeeruddin M K, and Gratzel M 2013 Nature 499 316
[6]	Liu M Z, Johnston M B, and Snaith H J 2013 Nature 501 395
[7]	Jeon N J, Noh J H, Kim Y C, Yang W S, Ryu S, and Seok S I 2014 Nat. Mater. 13 897
[8]	Zhou H, Chen Q, Li G, Luo S, Song T B, Duan H S, Hong Z, You J, Liu Y, and Yang Y 2014 Science 345 542
[9]	Yang W S, Noh J H, Jeon N J, Kim Y C, Ryu S, Seo J, and Seok S I 2015 Science 348 1234
[10]	Yang W S, Park B W, Jung E H, Jeon N J, Kim Y C, Lee D U, Shin S S, Seo J, Kim E K, Noh J H, and Seok S I 2017 Science 356 1376
[11]	Jung E H, Jeon N J, Park E Y, Moon C S, Shin T J, Yang T Y, Noh J H, and Seo J 2019 Nature 567 511
[12]	Jiang Q, Zhao Y, Zhang X W, Yang X L, Chen Y, Chu Z M, Ye Q F, Li X X, Yin Z G, and You J B 2019 Nat. Photon. 13 460
[13]	Yoo J J, Seo G, Chua M R, Park T G, Lu Y, Rotermund F, Kim Y K, Moon C S, Jeon N J, Correa-Baena J P, Bulovic V, Shin S S, Bawendi M G, and Seo J 2021 Nature 590 587
[14]	https://www.nrel.gov/pv/assets/pdfs/best-research-cell-efficiencies.20200104.pdf
[15]	Green M A, Ho-Baillie A, and Snaith H J 2014 Nat. Photon. 8 506
[16]	Fan H, Li F, Wang P, Gu Z, Huang J H, Jiang K J, Guan B, Yang L M, Zhou X, and Song Y 2020 Nat. Commun. 11 5402
[17]	Jeon N J, Noh J H, Yang W S, Kim Y C, Ryu S, Seo J, and Seok S I 2015 Nature 517 476
[18]	Saliba M, Matsui T, Seo J Y, Domanski K, Correa-Baena J P, Nazeeruddin M K, Zakeeruddin S M, Tress W, Abate A, Hagfeldt A, and Gratzel M 2016 Energy & Environ. Sci. 9 1989
[19]	Wang C, Zhang Y, Gu F, Zhao Z, Li H, Jiang H, Bian Z, and Liu Z 2021 Matter 4 709
[20]	Xing G, Mathews N, Sun S, Lim S S, Lam Y M, Gratzel M, Mhaisalkar S, and Sum T C 2013 Science 342 344
[21]	Yin W J, Shi T, and Yan Y 2014 Adv. Mater. 26 4653
[22]	Stranks S D, Eperon G E, Grancini G, Menelaou C, Alcocer M J, Leijtens T, Herz L M, Petrozza A, and Snaith H J 2013 Science 342 341
[23]	Yin W J, Shi T T, and Yan Y F 2014 Appl. Phys. Lett. 104 063903
[24]	Grancini G, Roldan-Carmona C, Zimmermann I, Mosconi E, Lee X, Martineau D, Narbey S, Oswald F, De Angelis F, Graetzel M, and Nazeeruddin M K 2017 Nat. Commun. 8 15684
[25]	Ndiaye A, Charki A, Kobi A, Kebe C M F, Ndiaye P A, and Sambou V 2013 Sol. Energy 96 140
[26]	Meng L, You J B, and Yang Y 2018 Nat. Commun. 9 5265
[27]	Wang Y, Liu X, Zhou Z, Ru P, Chen H, Yang X, and Han L 2019 Adv. Mater. 31 e1803231
[28]	Qiu F, Chu J Y, Liu Z R, Xiang J Z, Yang J, and Wang C 2020 Sol. RRL 4 2000452
[29]	Min H, Kim M, Lee S U, Kim H, Kim G, Choi K, Lee J H, and Seok S I 2019 Science 366 749
[30]	Stolterfoht M, Wolff C M, Márquez J A, Zhang S, Hages C J, Rothhardt D, Albrecht S, Burn P L, Meredith P, Unold T, and Neher D 2018 Nat. Energy 3 847
[31]	Caprioglio P, Stolterfoht M, Wolff C M, Unold T, Rech B, Albrecht S, and Neher D 2019 Adv. Energy Mater. 9 1901631
[32]	Ma C and Park N G 2020 Chem 6 1254
[33]	Shockley W and Queisser H J 1961 J. Appl. Phys. 32 510
[34]	Park N G and Segawa H 2018 ACS Photon. 5 2970
[35]	Eperon G E, Stranks S D, Menelaou C, Johnston M B, Herz L M, and Snaith H J 2014 Energy & Environ. Sci. 7 982
[36]	Mei A Y, Li X, Liu L F, Ku Z L, Liu T F, Rong Y G, Xu M, Hu M, Chen J Z, Yang Y, Gratzel M, and Han H W 2014 Science 345 295
[37]	Heo J H, Han H J, Kim D, Ahn T K, and Im S H 2015 Energy & Environ. Sci. 8 1602
[38]	Wu Y, Xie F, Chen H, Yang X, Su H, Cai M, Zhou Z, Noda T, and Han L 2017 Adv. Mater. 29 1701073
[39]	Han Q, Bae S H, Sun P, Hsieh Y T, Yang Y M, Rim Y S, Zhao H, Chen Q, Shi W, Li G, and Yang Y 2016 Adv. Mater. 28 2253
[40]	Binek A, Hanusch F C, Docampo P, and Bein T 2015 J. Phys. Chem. Lett. 6 1249
[41]	Ono L K, Juarez-Perez E J, and Qi Y 2017 ACS Appl. Mater. & Interfaces 9 30197
[42]	Kim G, Min H, Lee K S, Lee D Y, Yoon S M, and Seok S I 2020 Science 370 108
[43]	Lu H Z, Liu Y H, Ahlawat P, Mishra A, Tress W R, Eickemeyer F T, Yang Y G, Fu F, Wang Z W, Avalos C E, Carlsen B I, Agarwalla A, Zhang X, Li X G, Zhan Y Q, Zakeeruddin S M, Emsley L, Rothlisberger U, Zheng L R, Hagfeldt A, and Gratzel M 2020 Science 370 eabb8985
[44]	Hui W, Chao L, Lu H, Xia F, Wei Q, Su Z, Niu T, Tao L, Du B, Li D, Wang Y, Dong H, Zuo S, Li B, Shi W, Ran X, Li P, Zhang H, Wu Z, Ran C, Song L, Xing G, Gao X, Zhang J, Xia Y, Chen Y, and Huang W 2021 Science 371 1359
[45]	Yang Z, Rajagopal A, and Jen A K 2017 Adv. Mater. 29 1704418
[46]	Gao F, Zhao Y, Zhang X W, and You J B 2020 Adv. Energy Mater. 10 1902650
[47]	Zheng X, Chen B, Dai J, Fang Y, Bai Y, Lin Y, Wei H, Zeng X C, and Huang J 2017 Nat. Energy 2 17102
[48]	Jiang Q, Chu Z N, Wang P Y, Yang X L, Liu H, Wang Y, Yin Z G, Wu J L, Zhang X W, and You J B 2017 Adv. Mater. 29 1703852
[49]	Chen Q, Zhou H, Song T B, Luo S, Hong Z, Duan H S, Dou L, Liu Y, and Yang Y 2014 Nano Lett. 14 4158
[50]	Jeong J, Kim M, Seo J, Lu H, Ahlawat P, Mishra A, Yang Y, Hope M A, Eickemeyer F T, Kim M, Yoon Y J, Choi I W, Darwich B P, Choi S J, Jo Y, Lee J H, Walker B, Zakeeruddin S M, Emsley L, Rothlisberger U, Hagfeldt A, Kim D S, Gratzel M, and Kim J Y 2021 Nature 592 381
[51]	Wang R, Xue J, Meng L, Lee J W, Zhao Z, Sun P, Cai L, Huang T, Wang Z, Wang Z K, Duan Y, Yang J L, Tan S, Yuan Y, Huang Y, and Yang Y 2019 Joule 3 1464
[52]	Correa B J P, Steier L, Tress W, Saliba M, Neutzner S, Matsui T, Giordano F, Jacobsson T J, Srimath K A R, Zakeeruddin S M, Petrozza A, Abate A, Nazeeruddin M K, Grätzel M, and Hagfeldt A 2015 Energy & Environ. Sci. 8 2928
[53]	Jiang Q, Zhang L Q, Wang H L, Yang X L, Meng J H, Liu H, Yin Z G, Wu J L, Zhang X W, and You J B 2017 Nat. Energy 2 16177
[54]	Ke W, Fang G, Liu Q, Xiong L, Qin P, Tao H, Wang J, Lei H, Li B, Wan J, Yang G, and Yan Y 2015 J. Am. Chem. Soc. 137 6730
[55]	Jeon N J, Na H, Jung E H, Yang T Y, Lee Y G, Kim G, Shin H W, Seok S I, Lee J, and Seo J 2018 Nat. Energy 3 682
[56]	Tan H R, Jain A, Voznyy O, Lan X Z, De Arquer F P G, Fan J Z, Quintero-Bermudez R, Yuan M J, Zhang B, Zhao Y C, Fan F J, Li P C, Quan L N, Zhao Y B, Lu Z H, Yang Z Y, Hoogland S, and Sargent E H 2017 Science 355 722
[57]	Peng J, Walter D, Ren Y, Tebyetekerwa M, Wu Y, Duong T, Lin Q, Li J, Lu T, Mahmud M A, Lem O L C, Zhao S, Liu W, Liu Y, Shen H, Li L, Kremer F, Nguyen H T, Choi D Y, Weber K J, Catchpole K R, and White T P 2021 Science 371 390
[58]	Yang D, Yang R, Wang K, Wu C, Zhu X, Feng J, Ren X, Fang G, Priya S, and Liu S F 2018 Nat. Commun. 9 3239
[59]	Bach U, Lupo D, Comte P, Moser J E, Weissortel F, Salbeck J, Spreitzer H, and Gratzel M 1998 Nature 395 583
[60]	Snaith H J and Schmidt-Mende L 2007 Adv. Mater. 19 3187
[61]	Noh J H, Jeon N J, Choi Y C, Nazeeruddin M K, Grätzel M, and Seok S I 2013 J. Mater. Chem. A 1 11842
[62]	Wang S, Huang Z, Wang X, Li Y, Gunther M, Valenzuela S, Parikh P, Cabreros A, Xiong W, and Meng Y S 2018 J. Am. Chem. Soc. 140 16720
[63]	Ding C, Huang R, Ahläng C, Lin J, Zhang L, Zhang D, Luo Q, Li F, Österbacka R, and Ma C Q 2021 J. Mater. Chem. A 9 7575
[64]	Jeong M, Choi I W, Go E M, Cho Y, Kim M, Lee B, Jeong S, Jo Y, Choi H W, Lee J, Bae J H, Kwak S K, Kim D S, and Yang C 2020 Science 369 1615
[65]	Song S, Park E Y, Ma B S, Kim D J, Park H H, Kim Y Y, Shin S S, Jeon N J, Kim T S, and Seo J 2021 Adv. Energy Mater. 11 2003382
[66]	Jeong M J, Yeom K M, Kim S J, Jung E H, and Noh J H 2021 Energy & Environ. Sci. 14 2419
[67]	Li N, Niu X, Chen Q, and Zhou H 2020 Chem. Soc. Rev. 49 8235
[68]	Kim H S, Seo J Y, and Park N G 2016 ChemSusChem 9 2528
[69]	Pellet N, Giordano F, Dar M I, Gregori G, Zakeeruddin S M, Maier J, and Gratzel M 2017 Prog. Photovoltaics 25 942
[70]	Khenkin M V, Katz E A, Abate A, Bardizza G, Berry J J, Brabec C, Brunetti F, Bulović V, Burlingame Q, Di Carlo A, Cheacharoen R, Cheng Y B, Colsmann A, Cros S, Domanski K, Dusza M, Fell C J, Forrest S R, Galagan Y, Di Girolamo D, Grätzel M, Hagfeldt A, Von Hauff E, Hoppe H, Kettle J, Köbler H, Leite M S, Liu S, Loo Y L, Luther J M, Ma C Q, Madsen M, Manceau M, Matheron M, Mcgehee M, Meitzner R, Nazeeruddin M K, Nogueira A F, Odabaşı O A, Park N G, Reese M O, De Rossi F, Saliba M, Schubert U S, Snaith H J, Stranks S D, Tress W, Troshin P A, Turkovic V, Veenstra S, Visoly-Fisher I, Walsh A, Watson T, Xie H, YıL R M R, Zakeeruddin S M, Zhu K, and Lira-Cantu M 2020 Nat. Energy 5 35
[71]	Zheng C and Rubel O 2017 J. Phys. Chem. C 121 11977
[72]	Eames C, Frost J M, Barnes P R F, O'regan B C, Walsh A, and Islam M S 2015 Nat. Commun. 6 7497
[73]	Yang J, Liu C, Cai C, Hu X, Huang Z, Duan X, Meng X, Yuan Z, Tan L, and Chen Y 2019 Adv. Energy Mater. 9 1900198
[74]	Li N, Tao S, Chen Y, Niu X, Onwudinanti C K, Hu C, Qiu Z, Xu Z, Zheng G, Wang L, Zhang Y, Li L, Liu H, Lun Y, Hong J, Wang X, Liu Y, Xie H, Gao Y, Bai Y, Yang S, Brocks G, Chen Q, and Zhou H 2019 Nat. Energy 4 408
[75]	Wang L G, Zhou H P, Hu J N, Huang B L, Sun M Z, Dong B W, Zheng G H J, Huang Y, Chen Y H, Li L, Xu Z Q, Li N X, Liu Z, Chen Q, Sun L D, and Yan C H 2019 Science 363 265
[76]	Bai S, Da P, Li C, Wang Z, Yuan Z, Fu F, Kawecki M, Liu X, Sakai N, Wang J T W, Huettner S, Buecheler S, Fahlman M, Gao F, and Snaith H J 2019 Nature 571 245
[77]	Lin Y H, Sakai N, Da P, Wu J Y, Sansom H C, Ramadan A J, Mahesh S, Liu J L, Oliver R D J, Lim J, Aspitarte L, Sharma K, Madhu P K, Morales-Vilches A B, Nayak P K, Bai S, Gao F, Grovenor C R M, Johnston M B, Labram J G, Durrant J R, Ball J M, Wenger B, Stannowski B, and Snaith H J 2020 Science 369 96
[78]	Li Z, Yang M J, Park J S, Wei S H, Berry J J, and Zhu K 2016 Chem. Mater. 28 284
[79]	Saliba M, Matsui T, Domanski K, Seo J Y, Ummadisingu A, Zakeeruddin S M, Correa-Baena J P, Tress W R, Abate A, Hagfeldt A, and Gratzel M 2016 Science 354 206
[80]	Matsui T, Yamamoto T, Nishihara T, Morisawa R, Yokoyama T, Sekiguchi T, and Negami T 2019 Adv. Mater. 31 1806823
[81]	Ye Q F, Zhao Y, Mu S Q, Ma F, Gao F, Chu Z M, Yin Z G, Gao P Q, Zhang X W, and You J B 2019 Adv. Mater. 31 1905143
[82]	Tan S, Shi J, Yu B, Zhao W, Li Y, Li Y, Wu H, Luo Y, Li D, and Meng Q 2021 Adv. Funct. Mater. 31 2010813
[83]	Yoon S M, Min H, Kim J B, Kim G, Lee K S, and Seok S I 2021 Joule 5 183
[84]	Wang Y, Dar M I, Ono L K, Zhang T Y, Kan M, Li Y W, Zhang L J, Wang X T, Yang Y G, Gao X Y, Qi Y B, Gratzel M, and Zhao Y X 2019 Science 365 591
[85]	Yang S, Chen S S, Mosconi E, Fang Y J, Xiao X, Wang C C, Zhou Y, Yu Z H, Zhao J J, Gao Y L, De Angelis F, and Huang J S 2019 Science 365 473
[86]	Wang Y B, Wu T H, Barbaud J, Kong W Y, Cui D Y, Chen H, Yang X D, and Han L Y 2019 Science 365 687
[87]	Yang Y, Gao F, Gao S W, and Wei S H 2018 J. Mater. Chem. A 6 14949
[88]	Lan C Y, Zhou Z Y, Wei R J, and Ho J C 2019 Mater. Today Energy 11 61
[89]	Li H D, Luo T Y, Zhang S F, Sun Z J, He X, Zhang W F, and Chang H X 2021 Energy Environ. Mater. 4 46
[90]	Ge C, Xue Y Z B, Li L, Tang B, and Hu H 2020 Front. Mater. 7 601179
[91]	Wang Z, Lin Q, Chmiel F P, Sakai N, Herz L M, and Snaith H J 2017 Nat. Energy 2 17135
[92]	Chen P, Bai Y, Wang S, Lyu M, Yun J H, and Wang L 2018 Adv. Funct. Mater. 28 1706923
[93]	Schloemer T H, Christians J A, Luther J M, and Sellinger A 2019 Chem. Sci. 10 1904
[94]	Jena A K, Numata Y, Ikegami M, and Miyasaka T 2018 J. Mater. Chem. A 6 2219
[95]	Kim S G, Le T H, De Monfreid T, Goubard F, Bui T T, and Park N G 2021 Adv. Mater. 33 2007431
[96]	Yin X, Song Z, Li Z, and Tang W 2020 Energy & Environ. Sci. 13 4057
[97]	Zhao B X, Yao C, Gu K, Liu T, Xia Y, and Loo Y L 2020 Energy & Environ. Sci. 13 4334
[98]	Wu W Q, Yang Z B, Rudd P N, Shao Y C, Dai X Z, Wei H T, Zhao J J, Fang Y J, Wang Q, Liu Y, Deng Y H, Xiao X, Feng Y X, and Huang J S 2019 Sci. Adv. 5 eaav8925
[99]	Yang M, Kim D H, Klein T R, Li Z, Reese M O, De Tremolet V B J, Berry J J, Van Hest M F A M, and Zhu K 2018 ACS Energy Lett. 3 322
[100]	Li Z, Li P, Chen G, Cheng Y, Pi X, Yu X, Yang D, Han L, Zhang Y, and Song Y 2020 ACS Appl. Mater. & Interfaces 12 39082
[101]	Di Giacomo F, Shanmugam S, Fledderus H, Bruijnaers B J, Verhees W J H, Dorenkamper M S, Veenstra S C, Qiu W M, Gehlhaar R, Merckx T, Aernouts T, Andriessen R, and Galagan Y 2018 Sol. Energy Mater. Sol. Cells 181 53
[102]	Liu Z, Qiu L, Ono L K, He S, Hu Z, Jiang M, Tong G, Wu Z, Jiang Y, Son D Y, Dang Y, Kazaoui S, and Qi Y 2020 Nat. Energy 5 596
[103]	Chen S S, Xiao X, Gu H Y, and Huang J S 2021 Sci. Adv. 7 eabe8130
[104]	Green M A, Dunlop E D, Hohl-Ebinger J, Yoshita M, Kopidakis N, and Hao X 2020 Prog. Photovoltaics 28 629
[105]	Eperon G E, Hörantner M T, and Snaith H J 2017 Nat. Rev. Chem. 1 0095
[106]	Sahli F, Werner J, Kamino B A, Brauninger M, Monnard R, Paviet-Salomon B, Barraud L, Ding L, Diaz L J J, Sacchetto D, Cattaneo G, Despeisse M, Boccard M, Nicolay S, Jeangros Q, Niesen B, and Ballif C 2018 Nat. Mater. 17 820
[107]	Al-Ashouri A, Kohnen E, Li B, Magomedov A, Hempel H, Caprioglio P, Marquez J A, Vilches A B M, Kasparavicius E, Smith J A, Phung N, Menzel D, Grischek M, Kegelmann L, Skroblin D, Gollwitzer C, Malinauskas T, Jost M, Matic G, Rech B, Schlatmann R, Topic M, Korte L, Abate A, Stannowski B, Neher D, Stolterfoht M, Unold T, Getautis V, and Albrecht S 2020 Science 370 1300
[108]	Xiao K, Lin R, Han Q, Hou Y, Qin Z, Nguyen H T, Wen J, Wei M, Yeddu V, Saidaminov M I, Gao Y, Luo X, Wang Y, Gao H, Zhang C, Xu J, Zhu J, Sargent E H, and Tan H 2020 Nat. Energy 5 870
[109]	Leijtens T, Bush K A, Prasanna R, and Mcgehee M D 2018 Nat. Energy 3 828
[110]	Bush K A, Palmstrom A F, Yu Z S J, Boccard M, Cheacharoen R, Mailoa J P, Mcmeekin D P, Hoye R L Z, Bailie C D, Leijtens T, Peters I M, Minichetti M C, Rolston N, Prasanna R, Sofia S, Harwood D, Ma W, Moghadam F, Snaith H J, Buonassisi T, Holman Z C, Bent S F, and Mcgehee M D 2017 Nat. Energy 2 17009
[111]	Lin R, Xiao K, Qin Z, Han Q, Zhang C, Wei M, Saidaminov M I, Gao Y, Xu J, Xiao M, Li A, Zhu J, Sargent E H, and Tan H 2019 Nat. Energy 4 864