Lessons Learnt from Spatially Resolved Electro‐ and Photoluminescence Imaging: Interfacial Delamination in CH3NH3PbI3 Planar Perovskite Solar Cells upon Illumination

The influence of illumination on the long‐term performance of planar structured perovskite solar cells (PSCs) is investigated using fast and spatially resolved luminescence imaging. The authors analyze the effect of illuminated current density–voltage (J–V) and light‐soaking measurements on pristine PSCs by providing visual evidence for the spatial inhomogeneous evolution of device performance. Regions that are exposed to light initially produce stronger electroluminescence signals than surrounding unilluminated regions, mainly due to a lower contact resistance and, possibly, higher charge collection efficiency. Over a period of several days, however, these initially illuminated regions appear to degrade more quickly despite the device being stored in a dark, moisture‐ and oxygen‐free environment. Using transmission electron microscopy, this accelerated degradation is attributed to delamination between the perovskite and the titanium dioxide (TiO₂) layer. An ion migration mechanism is proposed for this delamination process, which is in accordance with previous current–voltage hysteresis observations. These results provide evidence for the intrinsic instability of CH₃NH₃PbI₃‐based devices under illumination and have major implications for the design of PSCs from the standpoint of long‐term performance and stability.     

Solvent Engineering Boosts the Efficiency of Paintable Carbon‐Based Perovskite Solar Cells to Beyond 14%

Carbon‐based hole transport material (HTM)‐free perovskite solar cells (PSCs) have shown much promise for practical applications because of their high stability and low cost. However, the efficiencies of this kind of PSCs are still relatively low, especially for the simplest paintable carbon‐based PSCs, in comparison with the organic HTM‐based PSCs. This can be imputed to the perovskite deposition methods that are not very suitable for this kind of devices. A solvent engineering strategy based on two‐step sequential method is exploited to prepare a high‐quality perovskite layer for the paintable carbon‐based PSCs in which the solvent for CH₃NH₃I (MAI) solution at the second step is changed from isopropanol (IPA) to a mixed solvent of IPA/Cyclohexane (CYHEX). This mixed solvent not only accelerates the conversion of PbI₂ to CH₃NH₃PbI₃ but also suppresses the Ostwald ripening process resulting in a high‐quality perovskite layer, e.g., pure phase, even surface, and compact capping layer. The paintable carbon‐based PSCs fabricated from IPA/CYHEX solvent exhibits a considerable enhancement in photovoltaic performance and performance reproducibility in comparison with that from pure IPA, especially on fill factor (FF), owing mainly to the better contact of perovskite/carbon interface, lower trap density in perovskite, higher light absorption ability, and faster charge transport of perovskite layer. As a result, the highest power conversion efficiency (PCE) of 14.38% is obtained, which is a record value for carbon‐based HTM‐free PSCs. Furthermore, a PCE of as high as 10% is achieved for the large area device (1 cm²), also the highest of its kind.     

Extrinsic Movable Ions in MAPbI3 Modulate Energy Band Alignment in Perovskite Solar Cells

Ionic movement is considered awful in perovskite solar cells (PSCs) for relating with the hysteresis and long‐term instability. However, the positive role of ions to enhance the energy band bending for high performance PSC is always overlooked, let alone reducing the hysteresis. In this work, LiI is doped in CH₃NH₃PbI₃. It is observed that the aggregation of Li⁺/I^? tunes the energy level of the perovskite and induces n/p doping in CH₃NH₃PbI₃, which makes charge extraction quite efficient from perovskite to both NiO and [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) layer. Therefore, in NiO/LiI doped perovskite/PCBM solar cells, Li⁺ and I^? modulate the interface energy band alignment to facilitate the electron/hole transport and reduce the interface energy loss. On the other hand, n/p doping enlarges Fermi energy level splitting of the PSCs to improve the photovoltage. The performance of LiI doped PSCs is much higher with reduced hysteresis compared to the undoped solar cells. This work highlights the positive effect of selective ionic doping, which is promisingly important to design the stable and efficient PSCs.     

Efficient and Stable Chemical Passivation on Perovskite Surface via Bidentate Anchoring

Chemical passivation is an effective approach to suppress the grain surface dominated charge recombination in perovskite solar cells (PSCs). However, the passivation effect is usually labile on perovskite crystal surface since most passivating agents are weakly anchored. Here, the use of a bidentate molecule, 2‐mercaptopyridine (2‐MP), to increase anchoring strength for improving the passivation efficacy and stability synchronously is demonstrated. Compared to monodentate counterparts of pyridine and p‐toluenethiol, 2‐MP passivation on CH₃NH₃PbI₃ film results in twofold improvement of photoluminescence lifetime and remarkably enhanced tolerance to chlorobenzene washing and vacuum heating, which improve the power conversion efficiency of n–i–p planar structured PSCs from 18.35% to 20.28%, with open‐circuit voltage approaching 1.18 V. Moreover, the CH₃NH₃PbI₃ films passivated with 2‐MP exhibit unprecedented humid‐stability that they can be exposed to saturated humidity for at least 5 h, mainly due to the passivation induced surface deactivation, which renders the unencapsulated devices retaining 93% of the initial efficiency after 60 days aging in air with relative humidity of 60–70%.     

Perovskite Solar Cell Stability in Humid Air: Partially Reversible Phase Transitions in the PbI2‐CH3NH3I‐H2O System

After rapid progress over the past five years, organic–inorganic perovskite solar cells (PSCs) currently exhibit photoconversion efficiencies comparable to the best commercially available photovoltaic technologies. However, instabilities in the materials and devices, primarily due to reactions with water, have kept PSCs from entering the marketplace. Here, laser beam induced current imaging is used to investigate the spatial and temporal evolution of the quantum efficiency of perovskite solar cells under controlled humidity conditions. Several interesting mechanistic aspects are revealed as the degradation proceeds along a four‐stage process. Three of the four stages can be reversed, while the fourth stage leads to irreversible decomposition of the photoactive perovskite material. A series of reactions in the PbI₂‐CH₃NH₃I‐H₂O system explains the interplay between the interactions with water and the overall stability. Understanding of the degradation mechanisms of PSCs on a microscopic level gives insight into improving the long‐term stability.     

A Smooth CH3NH3PbI3 Film via a New Approach for Forming the PbI2 Nanostructure Together with Strategically High CH3NH3I Concentration for High Efficient Planar‐Heterojunction Solar Cells

The photovoltaic performance of perovskite solar cells (PVSCs) is extremely dependent on the morphology and crystallization of the perovskite film, which is affected by the deposition method. In this work, a new approach is demonstrated for forming the PbI₂ nanostructure and the use of high CH₃NH₃I concentration which are adopted to form high‐quality (smooth and PbI₂ residue‐free) perovskite film with better photovoltaic performances. On the one hand, self‐assembled porous PbI₂ is formed by incorporating small amount of rationally chosen additives into the PbI₂ precursor solutions, which significantly facilitate the conversion of perovskite without any PbI₂ residue. On the other hand, by employing a relatively high CH₃NH₃I concentration, a firmly crystallized and uniform CH₃NH₃PbI₃ film is formed. As a result, a promising power conversion efficiency of 16.21% is achieved in planar‐heterojunction PVSCs. Furthermore, it is experimentally demonstrated that the PbI₂ residue in perovskite film has a negative effect on the long‐term stability of devices.     

Efficient and Highly Air Stable Planar Inverted Perovskite Solar Cells with Reduced Graphene Oxide Doped PCBM Electron Transporting Layer

Reduced graphene oxide (rGO) is added in the [6,6]‐Phenyl‐C61‐butyric acid methyl ester (PCBM) electron transport layer (ETL) of planar inverted perovskite solar cells (PSCs), resulting in a power conversion efficiency (PCE) improvement of ≈12%, with a hysteresis‐free PCE of 14.5%, compared to 12.9% for the pristine PCBM based device. The universality of the method is demonstrated in PSCs based on CH₃NH₃PbI_3?x Cl_x and CH₃NH₃PbI₃ perovskites, deposited through one step and two step spin coating process, respectively. After a comprehensive spectroscopic characterization of both devices, it is clear that the introduction of rGO in PCBM ETL results in an important increase of the ETL conductivity, together with reduced series resistance and surface roughness. As a result, a significant photoluminescence quenching of such perovskite/ETL is observed, confirming the increased measured short circuit current density. Transient absorption measurements reveal that in the rGO‐based device, the relaxation process of the excited electrons is significantly faster compared to the reference, which implies that the charge injection rate is significantly faster for the first. Furthermore, the light soaking effect is significantly reduced. Finally, aging measurements reveal that the rGO stabilizes the ELT/perovskite interface, which results in the stabilization of perovskite crystal structure after prolonged illumination.     

Halogen Engineering for Operationally Stable Perovskite Solar Cells via Sequential Deposition

The performance of perovskite solar cells (PSCs) relies on the synthesis method and chemical composition of the perovskite materials. So far, PSCs that have adopted two‐step sequential deposited perovskite with the state‐of‐art composition (FAPbI₃)_1?x(MAPbBr₃)_x (x < 0.05) have achieved record power conversion efficiency (PCE), while their one‐step antisolvent dripping counterparts with typical composition Cs_0.05FA_0.81MA_0.14Pb(I_0.85Br_0.15)₃ with more bromine have exhibited much better long‐term operational stability. Thus, halogen engineering that aims to elevate bromine content in sequential deposited perovskite film would push operational stability of PSCs toward that of antisolvent dripping deposited perovskite materials. Here, a Br‐rich seeding growth method is devised and perovskite seed solution with high bromine content is introduced into a PbI₂ precursor, leading to bromine incorporation in the resulting perovskite film. Photovoltaic devices fabricated by Br‐rich seeding growth method exhibit a PCE of 21.5%, similar to 21.6% for PSCs having lower bromine content. Whereas, the operational stability of PSCs with higher bromine content is significantly enhanced, with over 80% of initial PCE retained after 500 h tracking at maximum power point under 1‐sun illumination. This work highlights the vital importance of halogen composition for the operational stability of PSCs, and introduces an effective way to incorporate bromine into mixed‐cation‐halide perovskite film via sequential deposition method.     

Few‐Layer MoS2 Flakes as Active Buffer Layer for Stable Perovskite Solar Cells

Solution‐processed few‐layer MoS₂ flakes are exploited as an active buffer layer in hybrid lead–halide perovskite solar cells (PSCs). Glass/FTO/compact‐TiO₂/mesoporous‐TiO₂/CH₃NH₃PbI₃/MoS₂/Spiro‐OMeTAD/Au solar cells are realized with the MoS₂ flakes having a twofold function, acting both as a protective layer, by preventing the formation of shunt contacts between the perovskite and the Au electrode, and as a hole transport layer from the perovskite to the Spiro‐OMeTAD. As prepared PSC demonstrates a power conversion efficiency (η) of 13.3%, along with a higher lifetime stability over 550 h with respect to reference PSC without MoS₂ (Δη/η = ?7% vs. Δη/η = ?34%). Large‐area PSCs (1.05 cm² active area) are also fabricated to demonstrate the scalability of this approach, achieving η of 11.5%. Our results pave the way toward the implementation of MoS₂ as a material able to boost the shelf life of large‐area perovskite solar cells in view of their commercialization.     

Stable Inverted Planar Perovskite Solar Cells with Low‐Temperature‐Processed Hole‐Transport Bilayer

Low‐temperature‐processed perovskite solar cells (PSCs), which can be fabricated on rigid or flexible substrates, are attracting increasing attention because they have a wide range of potential applications. In this study, the stability of reduced graphene oxide and the ability of a poly(triarylamine) underlayer to improve the quality of overlying perovskite films to construct hole‐transport bilayer by means of a low‐temperature method are taken advantage of. The bilayer is used in both flexible and rigid inverted planar PSCs with the following configuration: substrate/indium tin oxide/reduced graphene oxide/polytriarylamine/CH₃NH₃PbI₃/PCBM/bathocuproine/Ag (PCBM = [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester). The flexible and rigid PSCs show power conversion efficiencies of 15.7 and 17.2%, respectively, for the aperture area of 1.02 cm². Moreover, the PSC based the bilayer shows outstanding light‐soaking stability, retaining ≈90% of its original efficiency after continuous illumination for 500 h at 100 mW cm^?2.     

The Synergism of DMSO and Diethyl Ether for Highly Reproducible and Efficient MA0.5FA0.5PbI3 Perovskite Solar Cells

Composition and film quality of perovskite are crucial for the further improvement of perovskite solar cells (PSCs), including efficiency, reproducibility, and stability. Here, it is demonstrated that by simply mixing 50% of formamidinium (FA⁺) into methylammonium lead iodide (MAPbI₃), a highly crystalline, stable phase, and compact, polycrystalline grain morphology perovskite is formed by using a solvent‐mediated phase transformation process via the synergism of dimethyl sulfoxide and diethyl ether, which shows long carrier lifetime, low trap state density, and a record certified 21.8% power conversion efficiency (PCE) in pure‐iodide, alkaline‐metal‐free MA_0.5FA_0.5PbI₃ perovskite‐based PSCs. These PSCs show very high operational stability, with 85% PCE retention upon 1000 h 1 Sun intensity illumination. A 17.33% PCE module (6.5 × 7 cm²) is also demonstrated, attesting to the scalability of such devices.     

On the Use of Luminescence Intensity Images for Quantified Characterization of Perovskite Solar Cells: Spatial Distribution of Series Resistance

Perovskite solar cells (PSCs) have made rapid advances in efficiency when fabricated as small‐area devices. A key challenge is to increase the active area while retaining high performance, which requires fast and reliable measurement techniques to spatially resolve cell properties. Luminescence imaging‐based techniques are one attractive possibility. A thermodynamic treatment of the luminescence radiation from MAPbI₃ and related perovskite semiconductors predicts that the intensity of luminescence emission is proportional to the electrochemical potential in the perovskite absorber, bringing with it numerous experimental advantages. However, concerns arise about the impact of the often‐observed hysteretic behavior on the interpretation of luminescence‐based measurements. This study demonstrates that despite their hysteretic phenomena, at steady‐state perovskite solar cells are amenable to quantitative analysis of luminescence images. This is demonstrated by calculating the spatial distribution of series resistance from steady‐state photoluminescence images. This study observes good consistency between the magnitude, voltage‐dependence, and spatial distribution of series resistance calculated from luminescence images and from cell‐level current–voltage curves and uncalibrated luminescence images, respectively. This method has significant value for the development of PSC process control, design and material selection, and illustrates the possibilities for large‐area, spatially resolved, quantitative luminescence imaging‐based characterization of PSCs.     

Charge Accumulation and Hysteresis in Perovskite‐Based Solar Cells: An Electro‐Optical Analysis

Organic–inorganic hybrid perovskite solar cells based on CH₃NH₃PbI₃ have achieved great success with efficiencies exceeding 20%. However, there are increasing concerns over some reported efficiencies as the cells are susceptible to current–voltage (I–V) hysteresis effects. It is therefore essential that the origins and mechanisms of the I–V hysteresis can clearly be understood to minimize or eradicate these hysteresis effects completely for reliable quantification. Here, a detailed electro‐optical study is presented that indicates the hysteresis originates from lingering processes persisting from sub‐second to tens of seconds. Photocurrent transients, photoluminescence, electroluminescence, quasi‐steady state photoinduced absorption processes, and X‐ray diffraction in the perovskite solar cell configuration have been monitored. The slow processes originate from the structural response of the CH₃NH₃PbI₃ upon E‐field application and/or charge accumulation, possibly involving methylammonium ions rotation/displacement and lattice distortion. The charge accumulation can arise from inefficient charge transfer at the perovskite interfaces, where it plays a pivotal role in the hysteresis. These findings underpin the significance of efficient charge transfer in reducing the hysteresis effects. Further improvements of CH₃NH₃PbI₃‐based perovskite solar cells are possible through careful surface engineering of existing TiO₂ or through a judicious choice of alternative interfacial layers.     

High‐Performance Perovskite Solar Cells with Enhanced Environmental Stability Based on a (p‐FC6H4C2H4NH3)2[PbI4] Capping Layer

Supported by the density functional theory (DFT) calculations, for the first time, a fluorinated aromatic cation, 2‐(4‐fluorophenyl)ethyl ammonium iodide (FPEAI), is introduced to grow in situ a low dimensional perovskite layer atop 3D perovskite film with excess PbI₂. The resulted (p‐FC₆H₄C₂H₄NH₃)₂[PbI₄] perovskite functions as a protective capping layer to protect the 3D perovskite from moisture. In the meantime, the thin layer facilitates charge transfer at the interfaces, thereby reducing the nonradiative recombination pathways. Laser scanning confocal microscopy unveils visually the distribution of the 2D perovskite layer on top of the 3D perovskite. When employing the 3D–2D perovskite as the absorbing layer in the photovoltaic cells, a high power conversion efficiency of 20.54% is realized. Superior device performance and moisture stability are observed with the modified perovskite over the whole stability test period.     

Highly Efficient and Stable Perovskite Solar Cells Based on Monolithically Grained CH3NH3PbI3 Film

The synthesis and growth of perovskite films with controlled crystallinity and microstructure for highly efficient and stable solar cells is a critical issue. In this work, thiourea is introduced into the CH₃NH₃PbI₃ precursor with two‐step sequential ethyl acetate (EA) interfacial processing. This is shown for the first time to grow compact microsized and monolithically grained perovskite films. X‐ray diffraction patterns and infrared spectroscopy are used to prove that thiourea significantly impacts the perovskite crystallinity and morphology by forming the intermediate phase MAI·PbI₂·S?C(NH₂)₂. Afterward, the residual thiourea which coursed charge recombination is completely extracted by the sequential EA processing. The product has improved light harvesting, suppressed defect state, and enhanced charge separation and transport. The sequentially EA processed perovskite solar cells offer an impressive 18.46% power conversion efficiency and excellent stability in ambient air. More importantly, the EA postprocessed perovskite solar cells also have excellent voltage response under ultraweak light (0.05% sun) with promising utility in photodetectors and photoelectric sensors.     

Precursor Engineering for Ambient‐Compatible Antisolvent‐Free Fabrication of High‐Efficiency CsPbI2Br Perovskite Solar Cells

High temperature stable inorganic CsPbX₃ (X: I, Br, or mixed halides) perovskites with their bandgap tailored by tuning the halide composition offer promising opportunities in the design of ideal top cells for high‐efficiency tandem solar cells. Unfortunately, the current high‐efficiency CsPbX₃ perovskite solar cells (PSCs) are prepared in vacuum, a moisture‐free glovebox or other low‐humidity conditions due to their poor moisture stability. Herein, a new precursor system (HCOOCs, HPbI₃, and HPbBr₃) is developed to replace the traditional precursors (CsI, PbI₂, and PbBr₂) commonly used for solar cells of this type. Both the experiments and calculations reveal that a new complex (HCOOH?Cs⁺) is generated in this precursor system. The new complex is not only stable against aging in humid air ambient at 91% relative humidity, but also effectively slows the perovskite crystallization, making it possible to eliminate the popular antisolvent used in the perovskite CsPbI₂Br film deposition. The CsPbI₂Br PSCs based on the new precursor system achieve a champion efficiency of 16.14%, the highest for inorganic PSCs prepared in ambient air conditions. Meanwhile, high air stability is demonstrated for an unencapsulated CsPbI₂Br PSC with 92% of the original efficiency remaining after more than 800 h aging in ambient air.     

Lattice‐Matching Structurally‐Stable 1D@3D Perovskites toward Highly Efficient and Stable Solar Cells

The stability of perovskite solar cells (PSCs) has been identified to be the bottleneck toward their industrialization. With the aim of tackling this challenge, a 1D PbI₂‐bipyridine (BPy)(II) perovskite is fabricated, which is shown to be capable of in situ assembly of a 1D@3D perovskite that is promoted by a PbI₂‐dimethyl sulfoxide complex with a skeletal linear chain structure. The as‐prepared 1D@3D perovskite is observed to demonstrate extremely high stability under external large electric fields in humid environments by means of an in situ characterization technique. This stability is associated with its well lattice‐matching heterojunction structure between 1D and 3D heterojunction domains. Importantly, ion migration is alleviated through blocking of the ion‐migration channels. Accordingly, the 1D@3D hybrid PSC shows a power conversion efficiency of 21.18% maintaining remarkably high long‐term stability in the presence of water, illumination, and external electric fields. This rational design and microstructure study of 1D@3D perovskites provides a new paradigm that may enable higher efficiency and stability of PSCs.     

Efficient and Hysteresis‐Free Perovskite Solar Cells Based on a Solution Processable Polar Fullerene Electron Transport Layer

Fullerene derivatives, which possess extraordinary geometric shapes and high electron affinity, have attracted significant attention for thin film technologies. This study demonstrates an important photovoltaic application using carboxyl‐functionalized carbon buckyballs, C60 pyrrolidine tris‐acid (CPTA), to fabricate electron transport layers (ETLs) that replace traditional metal oxide‐based ETLs in efficient and stable n‐i‐p‐structured planar perovskite solar cells (PSCs). The uniform CPTA film is covalently anchored onto the surface of indium tin oxide (ITO), significantly suppressing hysteresis and enhancing the flexural strength in the CPTA‐modified PSCs. Moreover, solution‐processable CPTA‐based ETLs also enable the fabrication of lightweight flexible PSCs. The maximum‐performing device structures composed of ITO/CPTA/CH₃NH₃PbI₃/2,2′,7,7′‐tetrakis‐(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene (spiro‐OMeTAD)/Au yield power conversion efficiencies of more than 18% on glass substrates and up to 17% on flexible substrates. These results indicate that the CPTA layers provide new opportunities for solution‐processed organic ETLs by substantially simplifying the procedure for fabricating PSCs for portable applications.     

Aerosol‐Jet‐Assisted Thin‐Film Growth of CH3NH3PbI3 Perovskites—A Means to Achieve High Quality,Defect‐Free Films for Efficient Solar Cells

A high level of automation is desirable to facilitate the lab‐to‐fab process transfer of the emerging perovskite‐based solar technology. Here, an automated aerosol‐jet printing technique is introduced for precisely controlling the thin‐film perovskite growth in a planar heterojunction p–i–n solar cell device structure. The roles of some of the user defined parameters from a computer‐aided design file are studied for the reproducible fabrication of pure CH₃NH₃PbI₃ thin films under near ambient conditions. Preliminary power conversion efficiencies up to 15.4% are achieved when such films are incorporated in a poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate‐perovskite‐phenyl‐C71‐butyric acid methyl ester type device format. It is further shown that the deposition of atomized materials in the form of a gaseous mist helps to form a highly uniform and PbI₂ residue‐free CH₃NH₃PbI₃ film and offers advantages over the conventional two‐step solution approach by avoiding the detrimental solid–liquid interface induced perovskite crystallization. Ultimately, by integrating full 3D motion control, the fabrication of perovskite layers directly on a 3D curved surface becomes possible. This work suggests that 3D automation with aerosol‐jet printing, once fully optimized, could form a universal platform for the lab‐to‐fab process transfer of solution‐based perovskite photovoltaics and steer development of new design strategies for numerous embedded structural power applications.     

Diammonium and Monoammonium Mixed‐Organic‐Cation Perovskites for High Performance Solar Cells with Improved Stability

Remarkable power conversion efficiencies (PCE) of metal–halide perovskite solar cells (PSCs) are overshadowed by concerns about their ultimate stability, which is arguably the prime obstacle to commercialization of this promising technology. Herein, the problem is addressed by introducing ethane‐1,2‐diammonium (⁺NH₃(CH₂)₂NH₃⁺, EDA²⁺) cations into the methyl ammonium (CH₃NH₃⁺, MA⁺) lead iodide perovskite, which enables, inter alia, systematic tuning of the morphology, electronic structure, light absorption, and photoluminescence properties of the perovskite films. Incorporation of <5 mol% EDA²⁺ induces strain in the perovskite crystal structure with no new phase formed. With 0.8 mol% EDA²⁺, PCE of the MAPbI₃‐based PSCs (aperture of 0.16 cm²) improves from 16.7% ± 0.6% to 17.9% ± 0.4% under 1 sun irradiation, and fabrication of larger area devices (aperture 1.04 cm²) with a certified PCE of 15.2% ± 0.5% is demonstrated. Most importantly, EDA²⁺/MA⁺‐based solar cells retain 75% of the initial performance after 72 h of continuous operation at 50% relative humidity and 50 °C under 1 sun illumination, whereas the MAPbI₃ devices degrade by approximately 90% within only 15 h. This substantial improvement in stability is attributed to the steric and coulombic interactions of embedded EDA²⁺ in the perovskite structure.