Oxygen Vacancy Mediation in SnO2 Electron Transport Layers Enables Efficient, Stable, and Scalable Perovskite Solar Cells.

Previous findings have suggested a close association between oxygen vacancies in SnO2 and charge carrier recombination as well as perovskite decomposition at the perovskite/SnO2 interface. Underlying the fundamental mechanism holds great significance in achieving a more favorable balance between the efficiency and stability. In this study, we prepared three SnO2 samples with different oxygen vacancy concentrations and observed that a low oxygen vacancy concentration is conducive to long-term device stability. Iodide ions were observed to easily diffuse into regions with high oxygen vacancies, thereby speeding up the deprotonation of FAI, as made evident by the detection of the decomposition product formamide. In contrast, a high oxygen vacancy concentration in SnO2 could prevent hole injection, leading to a decrease in interfacial recombination losses. To suppress this decomposition reaction and address the trade-off, we designed a bilayer SnO2 structure to ensure highly efficient carrier transport still while maintaining a chemically inert surface. As a result, an enhanced efficiency of 25.06% (certified at 24.55% with an active area of 0.09 cm2 under fast scan) was achieved, and the extended operational stability maintained 90% of their original efficiency (24.52%) after continuous operation for nearly 2000 h. Additionally, perovskite submodules with an active area of 14 cm2 were successfully assembled with a PCE of up to 22.96% (20.09% with an aperture area).

Anion Binding Interaction Enhances the Robustness of Iodide for High-Performance Perovskite Solar Cells.

Owing to the ionic bond nature of the Pb-I bond, the iodide at the interface of perovskite polycrystalline films was easily lost during the preparation process, resulting in the formation of a large number of iodine vacancy defects. The presence of iodine vacancy defects can cause nonradiative recombination, provide a pathway for iodide migration, and be harmful to the power conversion efficiency (PCE) and stability of organic-inorganic hybrid perovskite solar cells (HPSCs). Here, in order to increase the robustness of iodides at the interface, a strategy to introduce anion binding effects was developed to stabilize the perovskite films. It was demonstrated that the N,N'-diphenylurea (DPU), characterized by high anionic binding constants and a Y-shaped structure, provides a relatively strong hydrogen bond donor site to effectively reduce the iodine loss during film preparation and inhibits iodide migration in the device working condition. As expected, the reduced iodine loss considerably improves the quality of the perovskite films and suppresses nonradiative recombination. The performance of the device after DPU modification was significantly increased, with the PCE rising from 23.65 to 25.01% with huge stability enhancement as well.

Chemical Reaction of FA Cations Enables Efficient and Stable Perovskite Solar Cells.

Organometal halide perovskite solar cells (PSCs) have received great attention owing to a rapid increase in power conversion efficiency (PCE) over the last decade. However, the deficit of long-term stability is a major obstacle to the implementation of PSCs in commercialization. The defects in perovskite films are considered as one of the primary causes. To address this issue, isocyanic acid (HNCO) is introduced as an additive into the perovskite film, in which the added molecules form covalent bonds with FA cations via a chemical reaction. This chemical reaction gives rise to an efficient passivation on the perovskite film, resulting in an improved film quality, a suppressed non-radiation recombination, a facilitated carrier transport, and optimization of energy band levels. As a result, the HNCO-based PSCs achieve a high PCE of 24.41% with excellent storage stability both in an inert atmosphere and in air. Different from conventional passivation methods based on coordination effects, this work presents an alternative chemical reaction for defect passivation, which opens an avenue toward defect-mitigated PSCs showing enhanced performance and stability.

Intragrain impurity annihilation for highly efficient and stable perovskite solar cells.

Intragrain impurities can impart detrimental effects on the efficiency and stability of perovskite solar cells, but they are indiscernible to conventional characterizations and thus remain unexplored. Using in situ scanning transmission electron microscopy, we reveal that intragrain impurity nano-clusters inherited from either the solution synthesis or post-synthesis storage can revert to perovskites upon irradiation stimuli, leading to the counterintuitive amendment of crystalline grains. In conjunction with computational modelling, we atomically resolve crystallographic transformation modes for the annihilation of intragrain impurity nano-clusters and probe their impacts on optoelectronic properties. Such critical fundamental findings are translated for the device advancement. Adopting a scanning laser stimulus proven to heal intragrain impurity nano-clusters, we simultaneously boost the efficiency and stability of formamidinium-cesium perovskite solar cells, by virtual of improved optoelectronic properties and relaxed intra-crystal strain, respectively. This device engineering, inspired and guided by atomic-scale in situ microscopic imaging, presents a new prototype for solar cell advancement.

Suppressing Element Inhomogeneity Enables 14.9% Efficiency CZTSSe Solar Cells.

Kesterites, Cu2ZnSn(SxSe1- x)4 (CZTSSe), solar cells suffer from severe open-circuit voltage (VOC) loss due to the numerous secondary phases and defects. The prevailing notion attributes this issue to Sn-loss during the selenization. However, this work unveils that, instead of Sn-loss, elemental inhomogeneity caused by Cu-directional diffusion toward Mo(S,Se)2 layer is the critical factor in the formation of secondary phases and defects. This diffusion decreases the Cu/(Zn+Sn) ratio to 53% at the bottom fine-grain layer, increasing the Sn-/Zn-related bulk defects. By suppressing the Cu-directional diffusion with a blocking layer, the crystal quality is effectively improved and the defect density is reduced, leading to a remarkable photovoltaic coversion efficiency (PCE) of 14.9% with a VOC of 576 mV and a certified efficiency of 14.6%. The findings provide insights into element inhomogeneity, holding significant potential to advance the development of CZTSSe solar cells.

Enhanced Quasi-Fermi Level Splitting of Perovskite Solar Cells by Universal Dual-Functional Polymer.

Perovskite solar cells (PSCs) have attracted extensive attention due to their higher power conversion efficiency (PCE) and simple fabrication process. However, the open-circuit voltage (VOC) loss remains a significant impediment to enhance device performance. Here, a facile strategy to boost the VOC to 95.5% of the Shockley-Queisser (S-Q) limit through the introduction of a universal multifunctional polymer additive is demonstrated. This additive effectively passivates the cation and anion defects simultaneously, thereby leading to the transformation from the strong n-type to weak n-type of perovskite films. Benefitting from the energy level alignment and the suppression of bulk non-radiative recombination, the quasi-Fermi level splitting (QFLS) is enhanced.  Consequently, the champion devices with 1.59 eV-based perovskite reach the highest VOC value of 1.24 V and a PCE of 23.86%. Furthermore, this strategy boosts the VOC by at least 0.07 V across five different perovskite systems, a PCE of 25.04% is achieved for 1.57 eV-based PSCs, and the corresponding module (14 cm2) also obtained a high PCE of 21.95%. This work provides an effective and universal strategy to promote the VOC approach to the detailed balance theoretical limit.

Interfacial Rivet to Fill Structural Defects: A Spacer Engineering Gift for 3D Solar Cells.

The combination of 2D and 3D perovskites to passivate surfaces or interfaces with a high concentration of defects shows great promise for improving the efficiency of perovskite solar cells (PSCs). Constructing high-quality perovskite film systems by precisely modulating 2D perovskites with good morphologies and growth sites on 3D perovskite films remains a formidable challenge due to the complexity of spacer-engineered surface reactions. In this study, phase-pure 2D (HA)2(MA)n-1PbnI3n+1 perovskites with a controlled number of layers (n) are separated on a large scale and exploited as interface rivets to optimize 3D perovskite films, resulting in tunable film structural defects and grain boundaries. The optimized PSCs system benefits from a reduction in non-radiative recombination, resulting in improved optical performance, higher mobility, and lower trap density. The corresponding device achieves a champion power conversion efficiency (PCE) of more than 25%, especially for voltage (VOC) and fill factor (FF). The quality and uniformity of the perovskite films are further confirmed using large-area devices with an active area of 14 cm2, which exhibits a PCE of more than 21.24%. The high-quality thin-film system based on the 2D perovskites presented herein provides a new perspective for improving the efficiency and stability of PSCs.

Regulating Hetero-Nucleation Enabling Over 14% Efficient Kesterite Solar Cells.

Developing well-crystallized light-absorbing layers remains a formidable challenge in the progression of kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. A critical aspect of optimizing CZTSSe lies in accurately governing the high-temperature selenization reaction. This process is intricate and demanding, with underlying mechanisms requiring further comprehension. This study introduces a precursor microstructure-guided hetero-nucleation regulation strategy for high-quality CZTSSe absorbers and well-performing solar cells. The alcoholysis of 2-methoxyethanol (MOE) and the generation of high gas-producing micelles by adding hydrogen chloride (HCl) as a proton additive into the precursor solution are successfully suppressed. This tailored modification of solution components reduces the emission of volatiles during baking, yielding a compact and dense precursor microstructure. The reduced-roughness surface nurtures the formation of larger CZTSSe nuclei, accelerating the ensuing Ostwald ripening process. Ultimately, CZTSSe absorbers with enhanced crystallinity and diminished defects are fabricated, attaining an impressive 14.01% active-area power conversion efficiency. The findings elucidate the influence of precursor microstructure on the selenization reaction process, paving a route for fabricating high-quality kesterite CZTSSe films and high-efficiency solar cells.

Understanding the Role of Fluorine Groups in Passivating Defects for Perovskite Solar Cells.

Introducing fluorine (F) groups into a passivator plays an important role in enhancing the defect passivation effect for the perovskite film, which is usually attributed to the direct interaction of F and defect states. However, the interaction between electronegative F and electron-rich passivation groups in the same molecule, which may influence the passivation effect, is ignored. We herein report that such interactions can vary the electron cloud distribution around the passivation groups and thus changing their coordination with defect sites. By comparing two fluorinated molecules, heptafluorobutylamine (HFBM) and heptafluorobutyric acid (HFBA), we find that the F/-NH2 interaction in HFBM is stronger than the F/-COOH one in HFBA, inducing weaker passivation ability of HFBM than HFBA. Accordingly, HFBA-based perovskite solar cells (PSCs) provide an efficiency of 24.70 % with excellent long-term stability. Moreover, the efficiency of a large-area perovskite module (14.0 cm2 ) based on HFBA reaches 21.13 %. Our work offers an insight into understanding an unaware role of the F group in impacting the passivation effect for the perovskite film.

Thermally Crosslinked F-rich Polymer to Inhibit Lead Leakage for Sustainable Perovskite Solar Cells and Modules.

High-performance perovskite solar cells have demonstrated commercial viability, but still face the risk of contamination from lead leakage and long-term stability problems caused by defects. Here, an organic small molecule (octafluoro-1,6-hexanediol diacrylate) is introduced into the perovskite film to form a polymer through in situ thermal crosslinking, of which the carbonyl group anchors the uncoordinated Pb2+ of perovskite and reduces the leakage of lead, along with the -CF2 - hydrophobic group protecting the Pb2+ from water invasion. Additionally, the polymer passivates varieties of Pb-related and I-related defects through coordination and hydrogen bonding interactions, regulating the crystallization of perovskite film with reduced trap density, releasing lattice strain, and promoting carrier transport and extraction. The optimal efficiencies of polymer-incorporated devices are 24.76 % (0.09 cm2 ) and 20.66 % (14 cm2 ). More importantly, the storage stability, thermal stability, and operational stability have been significantly improved.

Polymerization Strategies to Construct a 3D Polymer Passivation Network toward High Performance Perovskite Solar Cells.

The spontaneously formed uncoordinated Pb2+ defects usually make the perovskite films demonstrate strong n-type with relatively lower carrier diffusion length and serious non-radiative recombination energy loss. In this work, we adopt different polymerization strategies to construct three-dimensional passivation frameworks in the perovskite layer. Thanks to the strong C≡N⋅⋅⋅Pb coordination bonding and the penetrating passivation structure, the defect state density is obviously reduced, accompanied by a significant increase in the carrier diffusion length. Additionally, the reduction of iodine vacancies also changed the Fermi level of the perovskite layer from strong n-type to weak n-type, which substantially promotes the energy level alignment and carrier injection efficiency. As a result, the optimized device achieved an efficiency exceeded 24 % (the certified efficiency is 24.16 %) with a high open-circuit voltage of 1.194 V, and the corresponding module achieved an efficiency of 21.55 %.

Molecular Dipole Engineering of Carbonyl Additives for Efficient and Stable Perovskite Solar Cells.

Carbonyl functional materials as additives are extensively applied to reduce the defects density of the perovskite film. However, there is still a lack of comprehensive understanding for the effect of carbonyl additives to improve device performance. In this work, we systematically study the effect of carbonyl additive molecules on the passivation of defects in perovskite films. After a comprehensive investigation, the results confirm the importance of molecular dipole in amplifying the passivation effect of additive molecules. The additive with strong molecular dipole possesses the advantages of enhancing the efficiency and stability of perovskite solar cells (PSCs). After optimization, the companion efficiency of PSCs is 23.20 %, and it can maintain long-term stability under harsh conditions. Additionally, a large-area solar cell module-modified DLBA was 20.18 % (14 cm2 ). This work provides an important reference for the selection and designing of efficient carbonyl additives.

Unveiling the Intrinsic Structure and Intragrain Defects of Organic-Inorganic Hybrid Perovskites by Ultralow Dose Transmission Electron Microscopy.

Transmission electron microscopy (TEM) is a powerful tool for unveiling the structural, compositional, and electronic properties of organic-inorganic hybrid perovskites (OIHPs) at the atomic to micrometer length scales. However, the structural and compositional instability of OIHPs under electron beam radiation results in misunderstandings of the microscopic structure-property-performance relationship in OIHP devices. Here, ultralow dose TEM is utilized to identify the mechanism of the electron-beam-induced changes in OHIPs and clarify the cumulative electron dose thresholds (critical dose) of different commercially interesting state-of-the-art OIHPs, including methylammonium lead iodide (MAPbI3 ), formamidinium lead iodide (FAPbI3 ), FA0.83 Cs0.17 PbI3 , FA0.15 Cs0.85 PbI3 , and MAPb0.5 Sn0.5 I3 . The critical dose is related to the composition of the OIHPs, with FA0.15 Cs0.85 PbI3 having the highest critical dose of ≈84 e Å-2 and FA0.83 Cs0.17 PbI3 having the lowest critical dose of ≈4.2 e Å-2 . The electron beam irradiation results in the formation of a superstructure with ordered I and FA vacancies along <110>c , as identified from the three major crystal axes in cubic FAPbI3 , <100>c , <110>c , and <111>c . The intragrain planar defects in FAPbI3 are stable, while an obvious modification is observed in FA0.83 Cs0.17 PbI3 under continuous electron beam exposure. This information can serve as a guide for ensuring a reliable understanding of the microstructure of OIHP optoelectronic devices by TEM.

Thermally Crosslinked Hole Conductor Enables Stable Inverted Perovskite Solar Cells with 23.9% Efficiency.

Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) represents the state-of-the-art hole transport material (HTM) in inverted perovskite solar cells (PSCs). However, unsatisfied surface properties of PTAA and high energy disorder in the bulk film hinder the further enhancement of device performance. Herein, a simple small molecule 10-(4-(3,6-dimethoxy-9H-carbazol-9-yl)phenyl)-3,7-bis(4-vinylphenyl)-10H-phenoxazine (MCz-VPOZ) is strategically developed for in situ fabrication of polymer hole conductor (CL-MCz) via a facile and low-temperature cross-linking technology. The resulting polymer CL-MCz offers high energy ordering and improved electrical conductivity, as well as appropriate energy-level alignment, enabling efficient charge carrier collection in the devices. Meanwhile, CL-MCz synchronously provides satisfied surface wettability and interfacial functionalization, facilitating the formation of high-quality perovskite films with fewer bulk iodine vacancies and suppressed carrier recombination. Significantly, the device with CL-MCz yields a champion efficiency of 23.9% along with an extremely low energy loss down to 0.41 eV, which represents the highest reported efficiency for non-PTAA-based polymer HTMs in inverted PSCs. Furthermore, the corresponding unencapsulated devices exhibit competitive shelf-life stability under various operational stressors up to 2500 h, reflecting high promises of CL-MCz in the scalable PSC application. This work underscores the promising potential of the cross-linking approach in preparing low-cost, stable, and efficient polymer HTMs toward reliable PSCs.

A Multifunctional Polymer as an Interfacial Layer for Efficient and Stable Perovskite Solar Cells.

Metal-cation defects and halogen-anion defects in perovskite films are critical to the efficiency and stability of perovskite solar cells (PSCs). In this work, a random polymer, poly(methyl methacrylate-co-acrylamide) (PMMA-AM), was synthesized to serve as an interfacial passivation layer for synergistically passivating the under-coordinated Pb2+ and anchor the I- of the [PbI6 ]4- octahedron. Additionally, the interfacial PMMA-AM passivation layer cannot be destroyed during the hole transport layer deposition because of its low solubility in chlorobenzene. This passivation leads to an enhancement in the open-circuit voltage from 1.12 to 1.22 V and improved stability in solar cell devices, with the device maintaining 95 % of the initial power conversion efficiency (PCE) over 1000 h of maximum power point tracking. Additionally, a large-area solar cell module was fabricated using this approach, achieving a PCE of 20.64 %.

Ammonia for post-healing of formamidinium-based Perovskite films.

Solvents employed for perovskite film fabrication not only play important roles in dissolving the precursors but also participate in crystallization process. High boiling point aprotic solvents with O-donor ligands have been extensively studied, but the formation of a highly uniform halide perovskite film still requires the participation of additives or an additional step to accelerate the nucleation rate. The volatile aliphatic methylamine with both coordinating ligands and hydrogen protons as solvent or post-healing gas facilitates the process of methylamine-based perovskite films with high crystallinity, few defects, and easy large-scale fabrication as well. However, the attempt in formamidinium-containing perovskites is challenged heretofore. Here, we reveal that the degradation of formamidinium-containing perovskites in aliphatic amines environment results from the transimination reaction of formamidinium cation and aliphatic amines along with the formation of ammonia. Based on this mechanism, ammonia is selected as a post-healing gas for a highly uniform, compact formamidinium-based perovskite films. In particular, low temperature is proved to be crucial to enable formamidinium-based perovskite materials to absorb enough ammonia molecules and form a liquid intermediate state which is the key to eliminating voids in raw films. As a result, the champion perovskite solar cell based on ammonia post-healing achieves a power conversion efficiency of 23.21% with excellent reproducibility. Especially the module power conversion efficiency with 14 cm2 active area is over 20%. This ammonia post-healing treatment potentially makes it easier to upscale fabrication of highly efficient formamidinium-based devices.

Pressure-Assisted Space-Confinement Strategy to Eliminate PbI2 in Perovskite Layers toward Improved Operational Stability.

The existence of the PbI2 phase in the perovskite film is normally inevitable because of the easy sublimation of the organic component during the crystallization process under a relatively high annealing temperature. However, excess PbI2 will cause significant degradation on open current voltage (VOC) and fill factor (FF) under continuous illumination. Here, we developed a pressure-assisted space-confinement (PASC) method to enhance the phase purity of the perovskite film fabricated by the two-step spin-coating method. It was found that high pressure is more conductive to lower the sublimation rate of the organic units, and the space confinement is more favorable for the Ostwald ripening. The combination of them can easily fabricate high-quality perovskite films with large crystal grains and eliminated PbI2 remnants. As expected, the efficiency of the solar cell was improved from 20.38 to 22.26%; more importantly, the operational stability of the corresponding device had a pronounced improvement, which remains over 85% of its initial efficiency after 500 h maximum power point tracking measurement. Based on this PASC method, a prototype PSC module (PSM) with an active area of 14 cm2 was also fabricated reaching an efficiency over 17%.

Atomically Resolved Electrically Active Intragrain Interfaces in Perovskite Semiconductors.

Deciphering the atomic and electronic structures of interfaces is key to developing state-of-the-art perovskite semiconductors. However, conventional characterization techniques have limited previous studies mainly to grain-boundary interfaces, whereas the intragrain-interface microstructures and their electronic properties have been much less revealed. Herein using scanning transmission electron microscopy, we resolved the atomic-scale structural information on three prototypical intragrain interfaces, unraveling intriguing features clearly different from those from previous observations based on standalone films or nanomaterial samples. These intragrain interfaces include composition boundaries formed by heterogeneous ion distribution, stacking faults resulted from wrongly stacked crystal planes, and symmetrical twinning boundaries. The atomic-scale imaging of these intragrain interfaces enables us to build unequivocal models for the ab initio calculation of electronic properties. Our results suggest that these structure interfaces are generally electronically benign, whereas their dynamic interaction with point defects can still evoke detrimental effects. This work paves the way toward a more complete fundamental understanding of the microscopic structure-property-performance relationship in metal halide perovskites.

Polyacrylonitrile-Coordinated Perovskite Solar Cell with Open-Circuit Voltage Exceeding 1.23†V.

In solution-processed organic-inorganic halide perovskite films, halide-anion related defects including halide vacancies and interstitial defects can easily form at the surfaces and grain boundaries. The uncoordinated lead cations produce defect levels within the band gap, and the excess iodides disturb the interfacial carrier transport. Thus these defects lead to severe nonradiative recombination, hysteresis, and large energy loss in the device. Herein, polyacrylonitrile (PAN) was introduced to passivate the uncoordinated lead cations in the perovskite films. The coordinating ability of cyano group was found to be stronger than that of the normally used carbonyl groups, and the strong coordination could reduce the I/Pb ratio at the film surface. With the PAN perovskite film, the device efficiency improved from 21.58 % to 23.71 % and the open-circuit voltage from 1.12 V to 1.23 V, the ion migration activation energy increased, and operational stability improved.

Carrier Diffusion and Recombination Anisotropy in the MAPbI3 Single Crystal.

MAPbI3, one of the archetypical metal halide perovskites, is an exciting semiconductor for a variety of optoelectronic applications. The photoexcited charge-carrier diffusion and recombination are important metrics in optoelectronic devices. Defects in grain interiors and boundaries of MAPbI3 films cause significant nonradiative recombination energy losses. Besides defect impact, carrier diffusion and recombination anisotropy introduced by structural and electronic discrepancies related to the crystal orientation are vital topics. Here, large-sized MAPbI3 single crystals (SCs) were grown, with the (110), (112), (100), and (001) crystal planes simultaneously exposed through the adjusting ratios of PbI2 to methylammonium iodide (MAI). Such MAPbI3 SCs exhibit a weak n-type semiconductor character, and the Fermi levels of these planes were slightly different, causing a homophylic p-n junction at crystal ledges. Utilizing MAPbI3 SCs, the photoexcited carrier diffusion and recombination within the crystal planes and around the crystal ledges were investigated through time-resolved fluorescence microscope. It is revealed that both the (110) and (001) planes were facilitated to be exposed with more MAI in the growth solutions, and the photoluminescence (PL) of these planes manifesting a red-shift, longer carrier lifetime, and diffusion length compared with the (100) and (112) planes. A longer carrier diffusion length promoted photorecycling. However, excessive MAI-assisted grown MAPbI3 SCs could increase the radiative recombination. In addition, it revealed that the carrier excited within the (001) and (112) planes was inclined to diffuse toward each other and was favorable to be extracted out of the grain boundaries or crystal ledges.