Conventional Perovskite Solar Cells Research Articles

Two Birds with One Stone: Dual‐Functional Difluorinated Modifier Enabling Efficient and Stable Perovskite Solar Cells

The interface between the perovskite layer and charge transport layer (CTL) plays an important role in the photoelectric conversion of perovskite solar cells (PSCs). In essence, the presence of defects and unideal contact at the perovskite/CTL interface induces severe nonradiative charge recombination, thus limiting the open‐circuit voltage and fill factor of PSCs. Herein, a two‐birds‐with‐one‐stone strategy to overcome the earlier challenges is reported, in which a single reagent 3,5‐difluorobenzenesulfonamide (3,5‐DFBS) is applied as an interface modifier between perovskite layer and spiro‐OMeTAD hole transport layer (HTL) in conventional PSCs. The 3,5‐DFBS molecules can passivate the undercoordinated Pb2+‐related surface defects by forming coordination through SO group and simultaneously strengthen the perovskite/HTL interface contact via F–π interactions, thus promoting hole extraction greatly. As a result, a champion efficiency of 23.69% with substantially improved stability is accomplished for 3,5‐DFBS PSCs. This work demonstrates that the two‐birds‐with‐one‐stone strategy is promising for achieving highly efficient and stable PSCs.

Spring‐Like Ammonium Salt Assisting Stress Release for Low‐Temperature Deposited FAPbI3 Films Toward Flexible Photovoltaic Application

AbstractThe substrates of conventional flexible perovskite solar cells (FPSCs) are thermoplastic polymer material polyethylene naphthalate (PEN), which will deform during high temperature annealing process. In addition, lead iodide (PbI2) permanently formed and the substrate undergoes reversible deformation from 20 °C to 200 °C and back to 20 °C. Therefore, to balance the substrate supporting capacity and the crystalline quality of narrow band gap α‐phase formamidinium lead iodide (α‐FAPbI3), an annealing process of 120 °C for 30 minutes is determined. Additionally, there will also be a large number of gaps and lattice strain at the perovskite grain boundaries during the annealing process as the FAPbI3 phase transition is accompanied by much lattice shrinkage. As a result, 1,6‐hexanediammonium diiodide (HADI) is chosen to passivate the defects and release the stress of perovskite film. Therefore, a recorded 1.4% extended stretch rate of the flexible film is attained. Finally, the champion PCE of 21.14% under AM 1.5G and 31.52% under 1062 lux is achieved after HADI treatment, accompanied by a better long‐term and mechanical stability. This study provides annealing process optimization and stress relief strategies for the further development of narrow band gap FPSCs.

Small molecule dopant-free dual hole transporting material for conventional and inverted perovskite solar cells

Small molecule dopant-free HTL with the ability to self-assemble shows a dual performance that leads to very similar efficiencies in p–i–n and n–i–p perovskite solar cells.

Prospect of near-infrared quantum dots in the development of tandem solar cells: Detail charge balance study

The conventional single-junction solar cells (ex: monocrystalline silicon (cSi) and perovskite solar cells) are limited to 30% efficiency, known as the Shockley–Queisser limit. Due to operational constraints, the single-junction solar cells are transparent to a large part of the near-infrared (NIR) solar spectrum. Harvesting the unused NIR photons in tandem solar cells could be a potential way forward. However, the unavailability of suitable NIR bandgap semiconductors is a huge practical challenge in realising tandem solar cells. The advancement of nanotechnology has enabled size dependent tunability of energy bandgap of semiconductors quantum dots (QDs). It is shown here that NIR bandgap QDs can be used to augment the performance of the existing perovskite and cSi solar cells in tandem architecture. Using detailed charge balance calculation, the optimum bandgaps of QDs for the development of perovskite-QD and cSi-QD tandem solar cells have been identified. Further, It is also depicted that the QD solar cells can produce 11.8% and 5.2% efficiency by using the unused transmitted photons which improve the limiting efficiency from perovskite and cSi solar cells, respectively. In tandem solar cells, this may help realise 39.8% and 34.2% efficiency in the case of perovskite-QD and cSi-QD tandem solar cells.

Simultaneously Modifying Hole Transport Material and Perovskite via a Crown Ether‐Based Semiconductor Toward Efficient and Stable Perovskite Solar Cells

In conventional (n‐i‐p) perovskite solar cells (PSCs), spiro‐OMeTAD is the most widely used hole‐transporting material (HTM), which contributes to the current state‐of‐the‐art efficiency. Suffering from the low conductivity, dopants such as LiTFSI (lithium bis(trifluoromethanesulfonyl)imide) and tBP are usually required to achieve excellent hole transport properties in spiro‐OMeTAD. Nevertheless, the hygroscopicity and the migration of Li+ during device operation severely affect the device's stability. To address the aforementioned issue, a 12‐crown‐4‐based organic semiconductor (CDT) is synthesized and applied in PSCs. Notably, CDT is simultaneously doped in spiro‐OMeTAD and perovskite layer through the antisolvent method. In this way, the strong “host‐guest” interaction between crown ether and Li+ effectively inhibits its migration both in the hole transporting layer (HTL) and at the perovskite/HTM interface. Furthermore, the carbazole diphenylamine group in CDT facilitates hole transport, and meanwhile improves the hydrophobicity of the HTL. In addition, CDT added into the perovskite layer is also able to passivate defects by interacting with the undercoordinated Pb2+. In light of the aforementioned advantages, the CDT‐based device shows a high power conversion efficiency approaching 23%, with excellent long‐term stability.

Stabilized Preparation of Fiber-Shaped Perovskite Solar Cells with Ti and Graphene Substrates

Organic–inorganic hybrid metal halide perovskite solar cells (PSCs) have made dramatic progress over the past few years. Fiber-shaped PSCs (FPSCs) extend the application of PSCs to wearable and portable electronics. The one-dimensional flexible structure of the fiber substrate and the high efficiency photoelectric conversion ability of the perovskite materials make FPSCs promising for energy applications. However, FPSCs have limited preparation methods and show poorer optoelectronic performance compared to conventional PSCs. In our work, we present a multiple quantitative coating preparation method, which was optimized by controlling the PbI2 concentration and the cycle number of coating process. We demonstrated the effectiveness of the methods in improving the film performance, with a photoelectric conversion efficiency (PCE) of 5.2% under air. The applicability of the multiple quantitative coating methods was studied by the FPSCs fabricated on reduced graphene fibers (rGFs), which present PCEs of about 1.5%. In addition, by extending the length of rGFs, the device performance was further improved. Thus, we demonstrated a stable fabrication method of FPSCs based on rGFs, which exhibited significant potential for using PSCs as a power source in wearable devices.

Simulation and Investigation of 26% Efficient and Robust Inverted Planar Perovskite Solar Cells Based on GA0.2FA0.78SnI3-1%EDAI2 Films.

A hybrid tin-based perovskite solar cell with p-i-n inverted structure is modeled and simulated using SCAPS. The inverted structure is composed of PEDOT:PSS (as hole transport layer-HTL)/GA0.2FA0.78SnI3-1% EDAI2 (as perovskite absorber layer)/C60-fullerene (as electron transport layer-ETL). Previous experimental studies showed that unlike conventional tin-based perovskite solar cells (PSC), the present hybrid tin-based PSC passes all harsh standard tests and generates a power conversion efficiency of only 8.3%. Despite the high stability that this material exhibits, emphasis on enhancing its power conversion efficiency (PCE) is crucial. To that end, various ETL and HTL materials have been rigorously investigated. The impact of energy level alignment between HTL/absorber and absorber/ETL interfaces have been elucidated. Moreover, the thickness and the doping concentration of all the previously mentioned layers have been varied to inspect their effect on the photovoltaic performance of the PSC. The optimized structure with CuI (copper iodide) as HTL and ZnOS (zinc oxysulphide) as ETL scored a PCE of 26%, which is more than three times greater than the efficiency of the initial structure. The current numerical simulation on GA0.2FA0.78SnI3-1% EDAI2 could greatly increase its chance for commercial development.

Versatile Hole Selective Molecules Containing a Series of Heteroatoms as Self‐Assembled Monolayers for Efficient p‐i‐n Perovskite and Organic Solar Cells

AbstractInverted type perovskite solar cells (PSCs) have recently emerged as a major focus in academic and industrial photovoltaic research. Their multiple advantages over conventional PSCs include easy processing, hysteresis‐free behavior, high stability, and compatibility for tandem applications. However, the maximum power conversion efficiency (PCE) of inverted PSCs still lags behind those of conventional PSCs because suitable charge‐selective materials for inverted PSCs are limited. In this study, excellent hole‐selective materials for inverted PSCs are introduced. A series of tricyclic aromatic rings containing O, S, or Se, respectively, as a core heteroatom, along with a phosphonic acid anchor, form a self‐assembled monolayer (SAM) that directly contacts the perovskite absorber. The influence of heteroatoms in the aromatic structure on the molecular energetics and operating characteristics of the corresponding inverted PSCs is investigated using complementary experimental techniques as well as density functional theory (DFT) calculations. It is found that all of the SAMs formed an energetically well‐aligned interface with the perovskite absorber. The interaction energy between the Se‐containing SAM and perovskite absorber is the strongest among the series and it reduces the interfacial defect density, in turn leading to an extended charge carrier lifetime. As a result, PSCs incorporating the Se‐containing SAM achieves a PCE of 22.73% and retains ≈96% of their initial efficiency after a maximum power point tracking test of 500 h without encapsulation under ambient conditions. All of the SAMs are then employed in organic solar cells (OSCs). Again, the Se‐containing SAM‐based OSCs demonstrates the highest PCE of 17.9% among the three molecular SAM‐based OSCs. This study demonstrates the great potential for precisely engineered SAMs for use in high‐performance solar cells.

An Efficient Modification at the Hole Transporting Layer/Electrode Interface for High‐Performance Ag‐Electrode‐Based Perovskite Solar Cells

Owing to the high power conversion efficiency (PCE) and the solution‐processed fabrication process, perovskite solar cells (PSCs) have attracted wide attention, while most high‐performance PSCs have an n‐i‐p (conventional) configuration that requires the expensive Au electrode. Silver is widely considered as one of the most promising candidates to replace gold as the top electrode material in conventional perovskite solar cells, due to its high conductivity and relatively low price. However, due to the mismatching energy levels between the Ag electrode and the hole transporting layer, Ag electrode‐based PSCs require long‐time aging to achieve their best performance. Herein, an AgI modification layer at the Ag electrode/hole transporting layer interface is employed, which eliminates the s‐shaped bend in current density–voltage (J–V) curves of fresh PSCs with the Ag electrode and helps to achieve high‐performance devices without any posttreatment. Optimized devices show a top PCE of up to 20.45% at the beginning, which is higher than that of the fresh (5.80%) and aged (19.31%) pristine counterparts. In addition, the facile method is also available for other iodides such as NH4I, KI, and CuI. This universal approach enables Ag to be a competitive material for electrode in high‐performance and low‐cost PSCs.

Identifying dominant recombination mechanisms in spiro-based conventional perovskite solar cells: Roles of interface and bulk recombination

Perovskite solar cells (PSCs) have been popular in the photovoltaic (PV) community, owing to their ease of processing and high efficiency. In order to improve their device performance, the interfaces are believed to play a vital role. In this regard, the interfacial energies and the associated physical processes such as charge transport, accumulation and recombination have been the focus of the research community. Despite the rise in power conversion efficiency (PCE) and all development, PSCs still suffer from the charge carrier recombination. As a result, highly efficient PSCs cannot meet the theoretical values of fill factor (FF) and open-circuit voltage ( VOC). Therefore, to reach out to the theoretical limit of performance for PSCs, carrier recombination needs to be controlled both in the bulk and at the corresponding interfaces. Herein, we have systematically studied the recombination in PSCs by establishing the role of different layers of PSCs using electrochemical impedance spectroscopy (EIS). To get insights into the role of different layers in recombination we fabricated PSCs with n-i-p configuration and changed the thickness of TiO 2 electron transport layer (ETL), absorber layer, and spiro-OMeTAD hole transport layer (HTL). Our results suggest that the recombination is reduced in case of devices with thicker HTL and is trap-assisted recombination. These results are in good agreement with impedance spectroscopy analysis. The value of recombination resistance (extracted by fitting of Nyquist response) is higher at VOC, which is consistent with higher VOC in these two cases. In case of the increased ETL thickness, the performance of the device is dropped since the increased interfacial resistance encourages the recombination. Our study presents a simple and effective approach to elucidate the dominating recombination sites in PSCs and improve the device performance.

A Smart Way to Prepare Solution‐Processed and Annealing‐free PCBM Electron Transporting Layer for Perovskite Solar Cells

AbstractAs a state‐of‐the‐art n‐type material, PC61BM ((6,6)‐phenyl C61 butyric‐ methylester) has been largely used as the top electron transporting layer (ETL) in inverted perovskite solar cells (PSCs). Whereas so far, there have been few reports on using pristine PC61BM as the bottom ETL in conventional (n‐i‐p‐type) PSCs, because it will be partially dissolved by the perovskite precursor solution (such as N,N‐Dimethylformamide and Dimethyl sulfoxide). In this work, the successful application of pristine PC61BM as an annealing‐free ETL is reported for conventional PSCs, by ingeniously utilizing the “saturated solution” strategy. Specifically, the perovskite precursor solution is presaturated with PC61BM before deposition, in this way, not only the underlying PC61BM layer can be “reserved”, the added PC61BM in perovskite is also able to passivate defects in perovskite grain boundaries, thus facilitating charge transporting and reducing charge recombination. Encouragingly, the PC61BM ETL can avoid high‐temperature calcination or thermal annealing, thus greatly simplifying the manufacturing processes and reducing production costs. The CH3NH3PbI3–xClx‐based champion device shows the highest efficiency of 20.85% with high reproducibility. The strategy is simple, straightforward, and cost‐effective, when compared to most of the present reported ways for depositing ETLs.

Lead-Tin Laminated All-Perovskite Solar Cells: Verification of Feasibility from the Perspective of Device Simulation

Although conventional lead-based perovskite solar cells have good power conversion efficiency, their toxic drawbacks seriously affect the environment and commercialization. This work introduces lead-tin laminated all-perovskite solar cells, and the addition of a tin-based perovskite layer can reduce this, while effectively compensating for the absorption of light in different wavelength ranges by the lead-based perovskite layer. The SACPS-1D (Solar Cell Capacitance Simulator) was used to study the intrinsic factors affecting each performance parameter of the device and optimize the parameters to maximize the overall performance of the device. We have obtained power conversion efficiency (PCE) of 17.59% and 6.14% lead-based and tin-based single-section perovskite solar cells, which are close to the experimental results. The simulation results show that the n-type doped n-CH3NH3SnI3 layer (electron transport layer ETL) and the p-type doped p-CH3NH3PbI3 layer (hole transport layer HTL) of the lead-tin laminated all-perovskite solar cell have a significant impact on the overall performance of the device. The optimized ETL and HTL are 1000 nm and 200 nm, respectively, which are consistent with the purpose of lead reduction. Accurate control of the n-MASnI3 or p-MAPbI3 to the top region doping concentration is very important process and has a great impact on the open circuit voltage VOC. The optimized ETL and HTL are optimal at 1017 cm−3 and 1014 cm−3, respectively. We also found that in all-perovskite solar cells, the ETL defect density near the light source dominates the overall device effect, and the larger the defect density the worse the device performance. This shows that to improve the performance of laminated all-perovskite solar cells, we need to pay more attention to the quality of the absorber layer close to the light source. Taking these factors into account, we optimized the device performance parameters as VOC = 1.07 V, JSC =19.56 mA cm−2, FF = 70.55% and PCE = 17.29%.

Enhanced efficiency and stability in Sn-based perovskite solar cells by trimethylsilyl halide surface passivation

Lead free tin perovskite solar cells (PKSCs) are the most suitable alternative candidate for conventional lead perovskite solar cells. However, the efficiency and the stability are insufficient, mainly because of the poor film quality and numerous defects. Here we introduce an efficient strategy based on a simple trimethylsilyl halide surface passivation to increase the film quality and reduce the defect density. At the same time, a hydrophobic protective layer on the perovskite surface is formed, which enhanced the PKSCs’ stability. The efficiency of the solar cell after the passivation was enhanced from 10.05 % to 12.22% with the improved open-circuit voltage from 0.57 V to 0.70 V. In addition, after 92 days of storage in N2 filled glovebox, the modified T-PKSCs demonstrated high stability maintaining 80% of its initial efficiency. This work provides a simple and widely used strategy to optimize the surface/interface optoelectronic properties of perovskites for giving more efficient and stable solar cells and other optoelectronic devices.

Carbon-Based CsPbI3 Perovskite Solar Cells

The perovskite solar cells (PSCs) based on carbon electrode (C-PSCs) are expected to address the instability issues faced by conventional PSCs. Recently, inorganic perovskites have been widely used as the light absorber in C-PSCs, which tended to further enhance device stability. Among various inorganic perovskites, CsPbI3 perovskite has been showing the greatest promise due to its suitable band gap (~ 1.7 eV) and high chemical stability. Benefiting from the progresses on phase stability, crystal quality and surface defect passivation, CsPbI3 C-PSCs have achieved the efficiency of over 15% and exhibited considerable enhancement in device stability. In this perspective, the main advances on CsPbI3 C-PSCs will be highlighted and the future research directions will be proposed.

Reveal the large open-circuit voltage deficit of all-inorganic CsPbIBr2 perovskite solar cells

Due to excellent thermal stability and optoelectronic properties, all-inorganic perovskite is one of the promising candidates to solve the thermal decomposition problem of conventional organic-inorganic hybrid perovskite solar cells (PSCs), but the larger voltage loss (V loss) cannot be ignored, especially CsPbIBr2, which limits the improvement of efficiency. To reduce V loss, one promising solution is the modification of the energy level alignment between the perovskite layer and adjacent charge transport layer (CTL), which can facilitate charge extraction and reduce carrier recombination rate at the perovskite/CTL interface. Therefore, the key issues of minimum V loss and high efficiency of CsPbIBr2-based PSCs were studied in terms of the perovskite layer thickness, the effects of band offset of the CTL/perovskite layer, the doping concentration of the CTL, and the electrode work function in this study based on device simulations. The open-circuit voltage (V oc) is increased from 1.37 V to 1.52 V by replacing SnO2 with ZnO as the electron transport layer (ETL) due to more matching conduction band with the CsPbIBr2 layer.

Design and simulation of 3D perovskite solar cells based on titanium dioxide nanowires to achieve high-efficiency

The advantages of perovskite solar cells over other photovoltaic technologies in recent years lead many works to improve the efficiency of these solar cells. The short circuit current density of perovskite solar cells can be improved using 3D structures. The structure that we present is consists of a perovskite layer on a nanowire metal oxide scaffold layer. The morphology of these layers is determined by using a 3D structure algorithm. In this paper, we simulated perovskite solar cells based on the TiO2 nanowire scaffold layer, which acts as an electron transport layer and a 3D scaffold layer, by using FDTD and FE simulation methods. These methods are respectively used for optical and electrical simulations. According to our simulation results and with our proposed and optimized structure, the maximum power conversion efficiency that we have reached is 18.9%. We ignored some practical limitations to achieve this efficiency. The Period and height of nanowires for this structure are 100 nm and 400 nm respectively. All these dimensions are determined by reviewing the practical papers, so these dimensions can be applied in practical cases. Also in this study, when we apply all recombinations and practical limits, we achieved 14.7% PCE that showed 61% improvement than conventional flat structure perovskite solar cells. Both of these power conversion efficiencies that we achieved can compete against conventional silicon solar cells. Because producing these perovskite solar cells with metal oxide nanowires especially TiO2 is much more affordable and convenient than conventional silicon solar cells.

Blending isomers of fluorine-substituted sulfonyldibenzene as hole transport materials to achieve high efficiency beyond 21% in perovskite solar cells

In this article, two fluorine-substituted sulfonyldibenzene compounds are synthesized and characterized (denoted as 3, 6-SFY and 2, 7-SFY), and they are mixed as hole transport material (HTM) in conventional perovskite solar cells (PSCs). 3, 6-SFY and 2, 7-SFY show good compatibility when they are blended due to their highly similar conjugated structures. The mixed HTM film exhibits suitable energy level and uniform surface morphology. Moreover, improved hole mobility and hole extraction ability was detected in the mixed isomers-based HTM compared to the single-component HTM. As a result, conventional perovskite solar cells using the mixed isomers as HTM deliver a maximum power conversion efficiency of 21.41%, significantly higher than that obtained in 3,6-SFY (19.02%) and 2,7-SFY-based devices (20.29%). These results demonstrate that mixing two isomers as HTM is a promising way to improve device performance. As far as we know, this is the first example of blending two isomers in conventional PSCs, and the results in this work also provide a new sight in developing highly efficient hole transport materials.

Ambipolar carrier transport properties of triphenylamine/dibenzofulvene derivative and its application for efficient n-i-p perovskite solar cells

Herein, the carrier transport properties of 9-(4-(diphenylamino)benzylidene)-N′,N′,N′,N′-tetraphenyl-9H-fluorene-2,7-diamine (YC-1) was firstly investigated with time-of-flight method. YC-1 exhibited ambipolar conductive behaviour, with hole and electron mobilities up to 10−4 cm2(Vs)−1. We introduced YC-1 as a hole transporting layer (HTL) in a planar n-i-p perovskite solar cell. A device fabricated using YC-1 exhibited enhanced performance compared to the device fabricated using P3HT, demonstrating 14.91% power conversion efficiency (PCE) and low hysteresis with a hysteresis effect index (HEI) of 0.007. A comprehensive analysis of the J-V characteristics in hole-only devices revealed the improvement of the YC-1 based devices were mainly contributed to the higher conductivity and lower trap density, which were beneficial to the device performance. This work suggests the thermally evaporable triphenylamine/dibenzofulvene can be a good candidate for HTL of conventional perovskite solar cells.

Enhanced Charge Extraction in Metal–Perovskite–Metal Back-Contact Solar Cell Structure Through Electrostatic Doping: A Numerical Study

In conventional perovskite solar cells (PSCs), a thin active layer of perovskite is sandwiched between two charge transport layers (CTLs)-electron transport layer (ETL) and hole transport layer (HTL). CTLs help in extracting and navigating the photogenerated electron-hole (e-h) pairs to the respective electrodes. Although this phenomenon gives high-energy conversion efficiencies but leads to quite a few performances as well as fabrication challenges. The ways to partially overcome these challenges are to have a device without the need of having CTLs altogether and opting for the back-contact (BC) design for PSCs. Dipole fields (DFs) present at the metal perovskite interface may be thought for their possible utilization to have CTL-free BC PSC. However, the performance of such devices is limited by the difference between the metal work functions across the perovskite layer. In this article, we report the results for our studies to establish that an electrostatically doped DF-assisted metal perovskite metal back-contact (ED-DF-MPM) PSC structure has the ability to overcome the limitations of DF-assisted metal perovskite metal back-contact (DF-MPM) PSCs. As a part of the work carried out here, ED p-n-junction and corresponding built-in potential have been combined for DF-assisted extraction of generated carriers within the perovskite layer so as to enhance the collection probability and open-circuit voltage. Quantitively, 32.7%, 10.6%, and 8.6% improvement in short-circuit current density (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SC</sub> ), open-circuit voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> ), and fill factor (FF) are obtained, respectively, which resulted in an observation of 59.4% improvement in power conversion efficiency (PCE) for ED-DF-MPM PSC compared to DF-MPM PSC. Besides that, the reported ED-DF-MPM PSC structure delivers the photovoltage and photocurrent of 659 mV and 14.19 mA ·cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> , respectively. The work reported in this article may pave the way for the development of “transport layer-free” ED scalable and low-cost PSCs in future.

Tin Halide Perovskites Going Forward: Frost Diagrams Offer Hints

Lead (Pb) in conventional perovskite solar cells (PSCs) is toxic and has to be replaced. Situated in one group of the periodic table of elements, tin (Sn) has the same valence electrons’ configuration as Pb (ns2np2), promising analogous chemical properties. Hence, Sn is considered a suitable replacement to Pb. However, because of the lack of lanthanide shrinkage, Sn behaves differently: Pb is stable in Pb2+ form, an oxidation state needed for perovskite structure, while Sn tends to lose all its valence electrons forming Sn4+. As a result, PSCs based on Sn are not efficient. Traces of oxygen have been conventionally discussed as a source of Sn oxidation. But recent findings point to the oxidation of Sn-based perovskites even in the absence of oxygen. This perspective summarizes recently-discovered unconventional oxidation pathways of Sn perovskites, including reaction with solvent molecules and disproportionation. We explain these phenomena by a Frost–Ebsworth diagram and argue that a deeper understanding of this diagram is a key toward stable and efficient Pb-free Sn-based PSCs.