Detrimental Effect of Unreacted PbI2 on the Long-Term Stability of Perovskite Solar Cells

Danger in the Dark: Stability of Perovskite Solar Cells with Varied Stoichiometries and Morphologies Stressed at Various Conditions

The long-term stability of perovskite solar cells (PSCs) remains a bottleneck for commercialization. While studies on the stoichiometry and morphology of PSCs with regard to performance are prevalent, understanding the influence of these factors on their long-term stability is lacking. In this work, we evaluate the impact of stoichiometry and morphology on the long-term stability of cesium formamidinium-based PSCs. We demonstrate that the lead iodide (PbI2) to formamidinium iodide (FAI) ratio influences stability under various stress factors (elevated temperature and light). A high molar ratio (PbI2/FAI > 1.1) in the perovskite precursor displays drastic degradation under ISOS-L1 (100 mW/cm2, 25 °C, maximum power point tracking) conditions. However, postdegradation analysis contradicts these results. Devices with PbI2/FAI ≤ 1.1 are stable under light, but intermittent current density-voltage characterizations indicate that device performance decreases during storage in the dark. Migration of iodide (I-) ions to the electron-transport layer (ETL) and iodine vacancies (VI-+) to the hole-transport layer (HTL) forms localized shunts in the absorber layer. Pinhole formation, surrounded by FA+-rich regions, explains the extent of damage in comparably aged films. In summary, this work emphasizes the importance of reporting stability under different stress conditions, coupled with postdegradation and dark recovery analyses of PSCs to better understand the complexities of perovskite instability under real-life conditions such as expected during outdoor operation.

Solvent-free preparation and thermocompression self-assembly: an exploration of performance improvement strategies for perovskite solar cells

Perovskite solar cells (PSCs) exhibit sufficient technological efficiency and economic competitiveness. However, their poor stability and scalability are crucial factors limiting their rapid development. Therefore, achieving both high efficiency and good stability is an urgent challenge. In addition, the preparation methods for PSCs are currently limited to laboratory-scale methods, so their commercialization requires further research. Effective packaging technology is essential to protect the PSCs from degradation by external environmental factors and ensure their long-term stability. The industrialization of PSCs is also inseparable from the preparation technology of perovskite thin films. This review discusses the solvent-free preparation of PSCs, shedding light on the factors that affect PSC performance and strategies for performance enhancement. Furthermore, this review analyzes the existing simulation techniques that have contributed to a better understanding of the interfacial evolution of PSCs during the packaging process. Finally, the current challenges and possible solutions are highlighted, providing insights to facilitate the development of highly efficient and stable PSC modules to promote their widespread application.

Control of positive and negative photo- and thermal-responses in a single PbI2@CH3NH3PbI3 micro/nanowire-based device for real-time sensing, nonvolatile memory, and logic operation

CH3NH3PbI3 has shown great potential for photodetectors and photovoltaic devices due to its excellent positive response to visible light. However, its real-time response characteristics hinder its application in optical memory and logic operation; moreover, the presence of excessive PbI2 is a double-edged sword. Herein, we constructed a dual-terminal device using a single CH3NH3PbI3 micro/nanowire with two Ag electrodes, and then in situ introduced PbI2 quantum dots (QDs) as hole trap centres by thermal decomposition at 160 °C. An anomalous negative photoconductivity (NPC) effect for sub-bandgap light below the PbI2 bandgap is obtained. Importantly, an electrically erasable nonvolatile photomemory can be realized. Furthermore, the device also exhibits an abnormal positive thermal resistance (PTR)-related thermomemory effect, and the thermal-induced high-resistance state (HRS) can be erased by a large bias or an illumination of 365 nm super-bandgap UV light. Additionally, logical "OR" gate operations are achieved through a combination of 650 nm sub-bandgap light and a 70 °C temperature-induced HRS, as well as a large bias and 365 nm super-bandgap light-triggered low-resistance state. These effects are attributed to the excitation and injection of holes in QDs and structural defect traps. This multifunctional device, integrating real-time sensing, nonvolatile memory, and logical operation, holds significant potential for novel electronic and optoelectronic applications.

Ethanol purification enables high-quality α-phase FAPbI3 perovskite microcrystals for commercial photovoltaic applications

Reliable quality and sustainable processes must be developed for commodities to enter the commercial stage. For next-generation photovoltaic applications such as perovskite solar cells, it is essential to manufacture high-quality photoactive perovskites via eco-friendly processes. We demonstrate that ethanol, an ideal green solvent, can be applied to yield efficient alpha-phase FAPbI3 perovskite microcrystals.

A Universal Perovskite/C60 Interface Modification via Atomic Layer Deposited Aluminum Oxide for Perovskite Solar Cells and Perovskite-Silicon Tandems

The primary performance limitation in inverted perovskite-based solar cells is the interface between the fullerene-based electron transport layers and the perovskite. Atomic layer deposited thin aluminum oxide (AlOX) interlayers that reduce nonradiative recombination at the perovskite/C60 interface are developed, resulting in >60 millivolts improvement in open-circuit voltage and 1% absolute improvement in power conversion efficiency. Surface-sensitive characterizations indicate the presence of a thin, conformally deposited AlOx layer, functioning as a passivating contact. These interlayers work universally using different lead-halide-based absorbers with different compositions where the 1.55 electron volts bandgap single junction devices reach >23% power conversion efficiency. A reduction of metallic Pb0 is found and the compact layer prevents in- and egress of volatile species, synergistically improving the stability. AlOX-modified wide-bandgap perovskite absorbers as a top cell in a monolithic perovskite-silicon tandem enable a certified power conversion efficiency of 29.9% and open-circuit voltages above 1.92 volts for 1.17 square centimeters device area.

Multi-Functional Spirobifluorene Phosphonate Based Exciplex Interface Enables Voc Reaching 95% of Theoretical Limit for Perovskite Solar Cells

Metal halide perovskite solar cells (PSCs) show significant advancements in power conversion efficiency (PCE). However, the open-circuit voltage (VOC) of PSCs is limited by interfacial factors such as defect-induced recombination, energy band mismatch, and non-intimate interface contact. Here, an exciplex interface is first developed based on the strategically designed and synthesized two spirobifluorene phosphonate molecules to mitigate VOC loss in PSCs. The exciplex interface constructed by the intimate contact between the multi-functional molecules and hole transport layer takes the roles to promote the hole extraction by donor-acceptor interaction, passivate coordination-unsaturated Pb2+ defects by equipped phosphonate groups, and optimize the energy level alignment. As a result, a record VOC of 1.26 V with a perovskite bandgap of 1.61 eV is achieved, representing over 95% of theoretical limit. This advancement leads to an increase in PCE from 21.29% to 24.12% and improved stability. The exciplex interface paves the way for addressing the long-standing challenge of VOC loss and promotes the wider application of PSCs.

Triisocyanate Derived Interlayer and High-Melting-Point Doping Promoter Boost Operational Stability of Perovskite Solar Cells

Formamidinium lead triiodide serves as the optimal light-absorbing layer in single-junction perovskite solar cells. However, achieving operational stability of high-efficiency n-i-p type devices at elevated temperatures remains challenging. In this work, we implemented effective surface modifications on microcrystalline perovskite films. This involved the nucleophilic addition of formamidinium cations and coordination of residual PbI2 with triphenylmethane triisocyanate as well as subsequent polymerization. The in situ growth of a cross-linking network chemically anchored on the perovskite film in this approach effectively reduced trap densities, favorably altered surface work function, suppressing interface charge recombination and thus enhancing cell efficiency. Coupled with a high-melting-point air-doping promoter, we fabricated n-i-p type perovskite solar cells surpassing 25 % efficiency, demonstrating excellent operational stability at 65 °C.

Regulating Charge Transport Dynamics at the Buried Interface and Bulk of Perovskites by Tailored-phase Two-dimensional Crystal Seed Layer

Targeting the trap-assisted non-radiative recombination losses and photochemical degradation occurring at the interface and bulk of perovskite, especially the overlooked buried bottom interface, a strategy of tailored-phase two-dimensional (TP-2D) crystal seed layer has been developed to improve the charge transport dynamics at the buried interface and bulk of perovskite films. Using this approach, TP-2D layer constructed by TP-2D crystal seeds at the buried interface can induce the formation of homogeneous interface electric field, which effectively suppress the accumulation of charge carriers at the buried interface. Additionally, the presence of TP-2D crystal seed has a positive effect on the crystallization process of the upper perovskite film, leading to optimized crystal quality and thus promoted charge transport inside bulk perovskites. Ultimately, the best performing PSCs based on TP-2D layer deliver a power conversion efficiency of 24.58 %. The devices exhibit an improved photostability with 88.4 % of their initial PCEs being retained after aging under continuous 0.8-sun illumination for 2000 h in air. Our findings reveal how to regulate the charge transport dynamics of perovskite bulk and interface by introducing homogeneous components.

Two-Step Perovskite Solar Cells with > 25% Efficiency: Unveiling the Hidden Bottom Surface of Perovskite Layer

While significant efforts in surface engineering have been devoted to the conversion process of lead iodide (PbI2) into perovskite and top surface engineering of perovskite layer with remarkable progress, the exploration of residual PbI2 clusters and the hidden bottom surface on perovskite layer have been limited. In this work, a new strategy involving 1-butyl-3-methylimidazolium acetate (BMIMAc) ionic liquid (IL) additives is developed and it is found that both the cations and the anions in ILs can interact with the perovskite components, thereby regulating the crystallization process and diminishing the residue PbI2 clusters as well as filling vacancies. The introduction of BMIMAc ILs induces the formation of a uniform porous PbI2 film, facilitating better penetration of the second-step organic salt and fostering a more extensive interaction between PbI2 and the organic salt. Surprisingly, the oversized residual PbI2 clusters at the bottom surface of the perovskite layer completely diminish. In addition, advanced depth analysis techniques including depth-resolved grazing-incidence wide-angle X-ray scattering (GIWAXS) and bottom thinning technology are employed for a comprehensive understanding of the reduction in residual PbI2. Leveraging effective PbI2 management and regulation of the perovskite crystallization process, the champion devices achieve a power conversion efficiency (PCE) of 25.06% with long-term stability.

Amine-releasable Mediator In situ Repair Perovskites for Efficient and Stable Perovskite Solar Cells

Residual lead iodide (PbI2) is deemed to a double-edged sword in perovskite film as small amounts of PbI2 are beneficial to the photovoltaic performance, but excessive will cause degradation of photovoltaic performance and stability. Herein, an in situ repair strategy has been developed by introducing amine-releasable mediator (methylammonium pyridine-2-carboxylic, MAPyA) to eliminate over-residual PbI2 and regulate the crystal quality of perovskite film. Notably, MAPyA can be partially decomposed into methylamine (MA) gas and pyridine-2-carboxylic (PyA) during high temperature annealing. The released MA can locally form liquid intermediate phase, facilitating the reconstruction of perovskite microcrystals and residual PbI2. Moreover, the leftover PyA is confirmed to effectively passivate the uncoordinated lead ions in final perovskite film. Based upon this, superior perovskite film with optimized crystal structure and holistic negligible PbI2 is acquired. The assembled device realizes outstanding efficiency of 24.06 %, and exhibits a remarkable operational stability that maintaining 87 % of its origin efficiency after continuous illumination for 1480 h. And the unencapsulated MAPyA-treated devices present significant uplift in humidity stability (maintaining ~93 % of the initial efficiency over 1500 h, 50-60 % relative humidity). Furthermore, the further optimization of this strategy with nanoimprint technology proves its superiority in the amplificative preparation for perovskite films.

A Self-Assembled 3D/0D Quasi-Core-Shell Structure as Internal Encapsulation Layer for Stable and Efficient FAPbI3 Perovskite Solar Cells and Modules

FAPbI3 perovskites have garnered considerable interest owing to their outstanding thermal stability, along with near-theoretical bandgap and efficiency. However, their inherent phase instability presents a substantial challenge to the long-term stability of devices. Herein, this issue through a dual-strategy of self-assembly 3D/0D quasi-core-shell structure is tackled as an internal encapsulation layer, and in situ introduction of excess PbI2 for surface and grain boundary defects passivating, therefore preventing moisture intrusion into FAPbI3 perovskite films. By utilizing this method alone, not only enhances the stability of the FAPbI3 film but also effectively passivates defects and minimizes non-radiative recombination, ultimately yielding a champion device efficiency of 23.23%. Furthermore, the devices own better moisture resistance, exhibiting a T80 lifetime exceeding 3500 h at 40% relative humidity (RH). Meanwhile, a 19.51% PCE of mini-module (5 × 5 cm2) is demonstrated. This research offers valuable insights and directions for the advancement of stable and highly efficient FAPbI3 perovskite solar cells.

Kornblum Oxidation Reaction-Induced Collective Transformation of Lead Polyhalides for Stable Perovskite Photovoltaics

The iodide vacancy defects generated during the perovskite crystallization process are a common issue that limits the efficiency and stability of perovskite solar cells (PSCs). Although excessive ionic iodides have been used to compensate for these vacancies, they are not effective in reducing defects through modulating the perovskite crystallization. Moreover, these iodide ions present in the perovskite films can act as interstitial defects, which are detrimental to the stability of the perovskite. Here, an effective approach to suppress the formation of vacancy defects by manipulating the coordination chemistry of lead polyhalides during perovskite crystallization is demonstrated. To achieve this suppression, an α-iodo ketone is introduced to undergo a process of Kornblum oxidation reaction that releases halide ions. This process induces a rapid collective transformation of lead polyhalides during the nucleation process and significantly reduces iodide vacancy defects. As a result, the ion mobility is decreased by one order of magnitude in perovskite film and the PSC achieves significantly improved thermal stability, maintaining 82% of its initial power conversion efficiency at 85 °C for 2800 h. These findings highlight the potential of halide ions released by the Kornblum oxidation reaction, which can be widely used for achieving high-performance perovskite optoelectronics.

Suppressing Ion Migration by Synergistic Engineering of Anion and Cation toward High-Performance Inverted Perovskite Solar Cells and Modules

Ion migration-induced intrinsic instability and large-area fabrication pose a tough challenge for the commercial deployment of perovskite photovoltaics. Herein, an interface heterojunction and metal electrode stabilization strategy is developed by suppressing ion migration via managing lead-based imperfections. After screening a series of cations and nonhalide anions, the ideal organic salt molecule dimethylammonium trifluoroacetate (DMATFA) consisting of dimethylammonium (DMA+) cation and trifluoroacetate (TFA-) anion is selected to manipulate the surface of perovskite films. DMA+ enables the conversion of active excess and/or unreacted PbI2 into stable new phase DMAPbI3, inhibiting photodecomposition of PbI2 and ion migration. Meanwhile, TFA- can suppress iodide ion migration through passivating undercoordinated Pb2+ and/or iodide vacancies. DMA+ and TFA- synergistically stabilize the heterojunction interface and silver electrode. The DMATFA-treated inverted perovskite solar cells and modules achieve a maximum efficiency of 25.03% (certified 24.65%, 0.1 cm2) and 20.58% (63.74 cm2), respectively, which is the record efficiency ever reported for the devices based on vacuum flash evaporation technology. The DMATFA modification results in outstanding operational stability, as evidenced by maintaining 91% of its original efficiency after 1520 h of maximum power point continuous tracking.

Nuclear Quantum Effects Accelerate Charge Recombination but Boost the Stability of Inorganic Perovskites in Mild Humidity

Experiments have demonstrated that mild humidity can enhance the stability of the CsPbBr3 perovskite, though the underlying mechanism remains unclear. Utilizing ab initio molecular dynamics, ring polymer molecular dynamics, and non-adiabatic molecular dynamics, our study reveals that nuclear quantum effects (NQEs) play a crucial role in stabilizing the lattice rigidity of the perovskite while simultaneously shortening the charge carrier lifetime. NQEs reduce the extent of geometric disorder and the number of atomic fluctuations, diminish the extent of hole localization, and thereby improve the electron-hole overlap and non-adiabatic coupling. Concurrently, these effects significantly suppress phonon modes and slow decoherence. As a result, these factors collectively accelerate charge recombination by a factor of 1.42 compared to that in scenarios excluding NQEs. The resulting sub-10 ns recombination time scales align remarkably well with experimental findings. This research offers novel insight into how moisture resistance impacts the stability and charge carrier lifetime in all-inorganic perovskites.

RbPbI3 Seed Embedding in PbI2 Substrate Tailors the Facet Orientation and Crystallization Kinetics of Perovskites

High power conversion efficiencies (PCEs) in perovskite solar cells (PSCs) have always been awe-inspiring, but perovskite films scalability is an exacting precondition for PSCs commercial deployment, generally unachievable through the antisolvent technique. On the contrary, in the two-step sequential method, the perovskite's uncontrolled crystallization and unnecessary PbI2 residue impede the device's performance. These two issues motivated to empower the PbI2 substrate with orthorhombic RbPbI3 crystal seeds, which act as grown nuclei and develop orientated perovskites lattice stacks, improving the perovskite films morphologically and reducing the PbI2 content in eventual perovskite films. Thence, achieving a PCE of 24.17% with suppressed voltage losses and an impressive life span of 1140 h in the open air.

Multifunctional Surface Treatment against Imperfections and Halide Segregation in Wide-Band Gap Perovskite Solar Cells

Mixed-halide wide-band gap perovskites (WBPs) still suffer from losses due to imperfections within the absorber and the segregation of halide ions under external stimuli. Herein, we design a multifunctional passivator (MFP) by mixing bromide salt, formamidinium bromide (FABr) with a p-type self-assembled monolayer (SAM) to target the nonradiative recombination pathways. Photoluminescence measurement shows considerable suppression of nonradiative recombination rates after treatment with FABr. However, WBPs still remained susceptible to halide segregation for which the addition of 25% p-type SAM was effective to decelerate segregation. It is observed that FABr can act as a passivating agent of the donor impurities, shifting the Fermi-level (Ef) toward the mid-band gap, while p-type SAM could cause an overweight of Ef toward the valence band. Favorable band bending at the interface could prevent the funneling of carriers toward I-rich clusters. Instead, charge carriers funnel toward an integrated SAM, preventing the accumulation of polaron-induced strain on the lattice. Consequently, n-i-p structured devices with an optimal MFP treatment show an average open-circuit voltage (VOC) increase of about 20 mV and fill factor (FF) increase by 4% compared with the control samples. The unencapsulated devices retained 95% of their initial performance when stored at room temperature under 40% relative humidity for 2800 h.

Ligand-Induced In Situ Epitaxial Growth of PbI2 Nanosheets/MAPbI3 Heterojunction Realizes High-Performance HTM-Free Carbon-Based MAPbI3 Solar Cells

Hole-transporting layer-free carbon-based perovskite solar cells (HTL-free C-PSCs) hold great promise for photovoltaic applications due to their low cost and outstanding stability. However, the low power conversion efficiency (PCE) of HTL-free C-PSCs mainly results from grain boundaries (GBs). Here, epitaxial growth is proposed to rationally design a hybrid nanostructure of PbI2 nanosheets/perovskite with the desired photovoltaic properties. A post-treatment technique using tri(2,2,2-trifluoromethyl) phosphate (TFEP) to induce in situ epitaxial growth of PbI2 nanosheets at the GBs of perovskite films realizes high-performance HTL-free C-PSCs. The structure model and high-resolution transmission electron microscope unravel the epitaxial growth mechanism. The epitaxial growth of oriented PbI2 nanosheets generates the PbI2 /perovskite heterojunction, which not only passivates defects but forms type-I band alignment, avoiding carrier loss. Additionally, Fourier-transform infrared spectroscopy, 31 P NMR, and 1 H NMR spectra reveal the passivation effect and hydrogen bonding interaction between TFEP and perovskite. As a result, the VOC is remarkably boosted from 1.04 to 1.10 V, leading to a substantial gain in PCE from 14.97% to 17.78%. In addition, the unencapsulated PSC maintains the initial PCE of 80.1% for 1440 h under air ambient of 40% RH. The work offers a fresh perspective on the rational design of high-performance HTL-free C-PSCs.

Enhancing perovskite solar cells efficiency through cesium fluoride mediated surface lead iodide modulation

The presence of an excessive amount of lead iodide on the surface of perovskite solar cells (PSCs) is a significant contributing factor that adversely affects the stability of these devices when exposed to continuous light. To address this issue, we developed an effective strategy involving polishing PbI2 on a perovskite surface using CsF. In this study, we investigated the effects of CsF post-treatment on perovskite films and their photovoltaic properties. The results of the time-resolved photoluminescence and ultraviolet photoelectron spectroscopy tests reveal the significant positive impact of our passivation method based on CsF, which reduces the valence band offset between the perovskite and hole transport layers while simultaneously enhancing the carrier interface transport. PSCs treated with CsF exhibited a photoelectric conversion efficiency (PCE) of 24.25% and an increased fill factor (FF) of 81.72%, which surpassed those of the original PSCs (PCE = 22.12% and FF = 77.40%). Furthermore, after aging for over 2500 h at room temperature and in 30 ± 10% humidity, the PCE of the unpacked PSCs reduced to only 42% of the initial value. Furthermore, the devices treated with CsF maintained their impressive performance, with the PCE maintaining optimal levels at 91% of the initial efficiency.

Aspartate all-in-one doping strategy enables efficient all-perovskite tandems

All-perovskite tandem solar cells hold great promise in surpassing the Shockley-Queisser limit for single-junction solar cells1-3. However, the practical use of these cells is currently hampered by the subpar performance and stability issues associated with mixed tin-lead (Sn-Pb) narrow-bandgap perovskite subcells in all-perovskite tandems4-7. In this study, we focus on the narrow-bandgap subcells and develop an all-in-one doping strategy for them. We introduce aspartate hydrochloride (AspCl) into both the bottom poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate) and bulk perovskite layers, followed by another AspCl posttreatment. We show that a single AspCl additive can effectively passivate defects, reduce Sn4+ impurities and shift the Fermi energy level. Additionally, the strong molecular bonding of AspCl-Sn/Pb iodide and AspCl-AspCl can strengthen the structure and thereby improve the stability of Sn-Pb perovskites. Ultimately, the implementation of AspCl doping in Sn-Pb perovskite solar cells yielded power conversion efficiencies of 22.46% for single-junction cells and 27.84% (27.62% stabilized and 27.34% certified) for tandems with 95% retention after being stored in an N2-filled glovebox for 2,000 h. These results suggest that all-in-one AspCl doping is a favourable strategy for enhancing the efficiency and stability of single-junction Sn-Pb perovskite solar cells and their tandems.

Correlation between Interfacial Structures and Device Performance: The Double-Edged Sword Effect of Lead Iodide in Perovskite Solar Cells

The interfacial electronic structure of perovskite layers and transport layers is critical for the performance and stability of perovskite solar cells (PSCs). The device performance of PSCs can generally be improved by adding a slight excess of lead iodide (PbI2 ) to the precursor solution. However, its underlying working mechanism is controversial. Here, we performed a comprehensive study of the electronic structures at the interface between CH3 NH3 PbI3 and C60 with and without the modification of PbI2 using in situ photoemission spectroscopy measurements. The correlation between the interfacial structures and the device performance was explored based on performance and stability tests. We found that there is an interfacial dipole reversal, and the downward band bending is larger at the CH3 NH3 PbI3 /C60 interface with the modification of PbI2 as compared to that without PbI2 . Therefore, PSCs with PbI2 modification exhibit faster charge carrier transport and slower carrier recombination. Nevertheless, the modification of PbI2 undermines the device stability due to aggravated iodide migration. Our findings provide a fundamental understanding of the CH3 NH3 PbI3 /C60 interfacial structure from the perspective of the atomic layer and insight into the double-edged sword effect of PbI2 as an additive.

Suppressing Excess Lead Iodide Aggregation and Reducing N-Type Doping at Perovskite/HTL Interface for Efficient Perovskite Solar Cells

Excess lead iodide (PbI2 ) aggregation at the charge carrier transport interface leads to energy loss and acts as unstable origins in perovskite solar cells (PSCs). Here, a strategy is reported to modulate the interfacial excess PbI2 by introducing π-conjugated small-molecule semiconductors 4,4'-cyclohexylbis[N,N-bis(4-methylphenyl)aniline] (TAPC) into perovskite films through an antisolvent addition method. The coordination of TAPC to PbI units through the electron-donating triphenylamine groups and π-Pb2+ interactions allows for a compact perovskite film with reduced excess PbI2 aggregates. Besides, preferred energy level alignment is achieved due to the suppressed n-type doping effect at the hole transport layer (HTL) interfaces. As a result, the TAPC-modified PSC based on Cs0.05 (FA0.85 MA0.15 )0.95 Pb(I0.85 Br0.15 )3 triple-cation perovskite achieved an improved PCE from 18.37% to 20.68% and retained ≈90% of the initial efficiency after 30 days of aging under ambient conditions. Moreover, the TAPC-modified device based on FA0.95 MA0.05 PbI2.85 Br0.15 perovskite produced an improved efficiency of 23.15% compared to the control (21.19%). These results provide an effective strategy for improving the performance of PbI2 -rich PSCs.

Managing Excess Lead Iodide with Ordered Distribution and Reduced Photoactivity via Chelating Ligands for Stable Inverted Perovskite Solar Cells

Excess lead iodide (PbI2) aggregates distributed in perovskite photoreactive absorbers will perturb carrier collection and become a key source of instability in PSCs. Herein, a multisite heterocyclic ligand of 2-mercaptonicotinic acid (2-MNA) is introduced as a chelating agent to manage excess PbI2 in inverted PSCs. The chelating coordination of 2-MNA to Pb2+ ions through the carbonyl, sulfhydryl, and pyridinyl groups enables a high-quality perovskite film with reduced PbI2 aggregates and the formation of an ordered distribution at grain boundaries. Moreover, the coordination of 2-MNA with the [PbX6]4- octahedron effectively inhibits the photodecomposition of PbI2-rich perovskites, thus preventing the generation of metallic lead (Pb0) and iodine (I2) species in response to environmental stimuli. As a result, the inverted PSC based on a 2-MNA modified triple cation perovskite photoactive layer achieves a PCE of 21.27% and a fill factor of 82.07%, accompanied by improved thermal and photostability.

Relationship between the Annealing Temperature and the Presence of PbI2 Platelets at the Surfaces of Slot-Die-Coated Triple-Halide Perovskite Thin Films

We investigated triple-halide perovskite (THP) absorber layers with 5 mol % MAPbCl3 added to the double-halide perovskite (Cs0.22FA0.78)Pb(I0.85Br0.15)3. As a deposition method, a highly scalable printing technique, slot-die coating, with a subsequent annealing step was used. We found a strong power conversion efficiency (PCE) dependence of the corresponding solar cells on the annealing temperature. The device performance deteriorated when increasing the annealing temperature from 125 to 170 °C, mainly via losses in the open-circuit voltage (Voc) and in the fill factor (FF). To understand the mechanisms behind this performance loss, extensive characterizations were performed on both, the THP thin films and the completed solar-cell stacks, as a function of annealing temperature. Correlative scanning electron microscopy analyses, i.e., electron backscatter diffraction, energy-dispersive X-ray spectroscopy, and cathodoluminescence, in addition to X-ray diffraction and photoluminescence, confirmed the presence of PbI2 platelets on the surface of the THP thin films. Moreover, the area fraction of the PbI2 platelets on the film surface increased with increasing annealing temperature. The deteriorated device performance when the annealing temperature is increased from 125 to 170 °C is explained by the increased series resistance and increased interface recombination caused by the PbI2 platelets, leading to decreased Voc and FF values of the solar-cell devices. Thus, the correlative analyses provided insight into microscopic origins of the efficiency losses.

Tailoring passivators for highly efficient and stable perovskite solar cells

There is an ongoing global effort to advance emerging perovskite solar cells (PSCs), and many of these endeavours are focused on developing new compositions, processing methods and passivation strategies. In particular, the use of passivators to reduce the defects in perovskite materials has been demonstrated to be an effective approach for enhancing the photovoltaic performance and long-term stability of PSCs. Organic passivators have received increasing attention since the late 2010s as their structures and properties can readily be modified. First, this Review discusses the main types of defect in perovskite materials and reviews their properties. We examine the deleterious impact of defects on device efficiency and stability and highlight how defects facilitate extrinsic degradation pathways. Second, the proven use of different passivator designs to mitigate these negative effects is discussed, and possible defect passivation mechanisms are presented. Finally, we propose four specific directions for future research, which, in our opinion, will be crucial for unlocking the full potential of PSCs using the concept of defect passivation.

Pure-Phase, Large-Grained Wide-Band-Gap Perovskite Films for High-Efficiency, Four-Terminal Perovskite/Silicon Tandem Solar Cells

High-quality, stable perovskite films with a wide band gap between 1.65 and 1.80 eV are highly suitable for efficient and cost-competitive silicon-based tandem solar cells. Herein, we demonstrate that the combined strategies of the Pb(SCN)2 additive and air annealing can enable the Cs0.22FA0.78Pb(I0.85Br0.15)3 films with a wide band gap of 1.65 eV and favored properties including pure composition, high crystallinity, micro-sized grains, and reduced defects. With these desired films, the average efficiencies of semitransparent perovskite solar cells (PSCs) are boosted from (18.13 ± 0.31) to (20.35 ± 0.28)%. Further, the semitransparent PSC is used to assemble the four-terminal perovskite/TOPCon tandem solar cell. Benefiting from its excellent performance and preferred optical properties, the obtained tandem solar cell yields a milestone efficiency of 30.32%.

Self-Assembled 1D/3D Perovskite Heterostructure for Stable All-Air-Processed Perovskite Solar Cells with Improved Open-Circuit Voltage

Environmental instability and photovoltage loss induced by defects are inevitable obstacles in the development of all-air-processed perovskite solar cells (PSCs). In this study, the ionic liquid 1-ethyl-3-methylimidazolium iodide ([EMIM]I) is introduced into the hole transport layer/three-dimensional (3D) perovskite interface to form a self-assembled 1D/3D perovskite heterostructure, which significantly reduces iodine vacancy defects and modulates band energy alignment, resulting in pronouncedly improved open-circuit voltage (Voc ). As a result, the corresponding device exhibits a high power conversion efficiency with negligible hysteresis and a high Voc of 1.14 V. Most importantly, together with the high stability of the 1D perovskite, remarkable high environmental and thermal stabilities of the 1D/3D PSC devices are achieved, which maintain 89 % of unencapsulated device initial efficiency after 1320 h in air and retain 85 % of the initial efficiency when heated at 85 °C for 22 h. This study affords an effective strategy to fabricate high-performance all-air-processed PSCs with outstanding stability.

Multifunctional Metal-Organic Frameworks Capsules Modulate Reactivity of Lead Iodide toward Efficient Perovskite Solar Cells with UV Resistance

The two-step sequential deposition process is demonstrated as a reliable technology for the fabrication of efficient perovskite solar cells (PVSCs). However, the complete conversion of dense PbI2 to perovskite in planar PVSCs is tough without mesoporous titanium dioxide as support. Herein, multifunctional capsules consisting of zeolitic imidazolate framework-8 (ZIF-8) encapsulant and formamidinium iodide (FAI) are introduced between tin oxide (SnO2 ) and lead iodide (PbI2 ) layer. Intriguingly, the capsule dopant interlayer benefits the formation of porous PbI2 film due to the porous nanostructure of ZIF-8 that is favorable for the subsequent intercalation reaction. Furthermore, the constituent of the perovskite precursor in ZIF-8 pores can convert into the crystal nuclei of perovskite by reacting with PbI2 first, thereby promoting further perovskite crystallization. Significantly, the incorporation of ZIF-8 can enhance the resistance of perovskite against UV illumination due to down-conversion effect. Consequently, the modified device achieves a champion power conversion efficiency (PCE) of 24.08% and displays enhanced UV stability, which can sustain 83% of its original PCE under 365 nm UV illumination for 300 h. Moreover, the unencapsulated device maintains 90% of initial PCE after 1500 h storage in dark ambient conditions with a relative humidity range of 50%-70%.

Fabrication of Highly Luminescent Quasi Two-Dimensional CsPbBr3 Perovskite Films in High Humidity Air for Light-Emitting Diodes

Perovskite light-emitting diodes (LEDs) have attracted extensive attention in recent years due to their outstanding performance and promise in lighting and display applications. However, the fabrication of perovskite LEDs usually requires a low-humidity atmosphere, which is unfavorable for industrial production. Herein, we report an effective strategy to fabricate highly luminescent quasi two-dimensional CsPbBr₃ perovskite films in an ambient atmosphere with a humidity up to 60%. We reveal that the hole transport layer (HTL) plays a significant role in the morphology and optical properties of the perovskite films. Using hydrophobic self-assembled monolayer materials as HTLs can remarkably improve the quality of the perovskite films processed in high humidity air. The resultant perovskite LEDs show reduced leakage current and significantly enhanced performance. Furthermore, surface treatment is conducted to prevent water invasion and promote radiative recombination in perovskite films and LEDs. Eventually, the perovskite LEDs exhibit bright green emission with an external quantum efficiency of 4.87%. The present work provides a feasible pathway to overcome the humidity limitation for obtaining bright perovskite films and LEDs, which would contribute to further reducing the fabrication cost of perovskite LEDs and promoting their applications.

Iodide/triiodide redox shuttle-based additives for high-performance perovskite solar cells by simultaneously passivating the cation and anion defects

Halide perovskite solar cells (PSCs) have received remarkably increasing interests due to their facile fabrication procedures, use of cost-effective raw materials, and high power conversion efficiencies (PCEs) during the past 10 years. Nevertheless, the state-of-the-art organic-inorganic PSCs suffer from high defect concentration and inferior humid/thermal stability, significantly restricting the widespread applications of PSCs. More specifically, point defects including metallic lead (Pb⁰) and halide iodine (I⁰) are easily generated in Pb/I-based PSCs during fabrication processes and operational conditions due to the inferior interaction between the anions and cations in halide perovskites and promote detrimental carrier recombination and ion migration, leading to inferior PCEs and durability of the PSCs. Herein, to tackle the above-mentioned issues, iodide/triiodide (I^-/I₃^-) redox shuttles as a new additive were introduced to simultaneously passivate the cation and anion defects of methylammonium lead iodide (MAPbI₃)-based PSCs. In particular, I^-/I₃^- redox shuttles play a vital role in regenerating the cation (Pb⁰) and anion (I⁰) defects through the redox cycles of Pb⁰/Pb²⁺ and I⁰/I^-. Consequently, the cell with an optimized amount of I^-/I₃^- additive generated a superior PCE of 20.4%, which was 12% higher than the pristine device (18.2%). Furthermore, the introduction of the I^-/I₃^- additive remarkably improved the humid and thermal stability of MAPbI₃-based PSCs. This work manifests the importance of the design of redox shuttle-based additives to boost the efficiency and durability of organic-inorganic PSCs.

Rational Design of Ferroelectric 2D Perovskite for Improving the Efficiency of Flexible Perovskite Solar Cells Over 23

Despite the great progress of flexible perovskite solar cells (f-PSCs), it still faces several challenges during the homogeneous fabrication of high-quality perovskite thin films, and overcoming the insufficient exciton dissociation. To the ends, we rationally design the ferroelectric two-dimensional (2D) perovskite based on pyridine heterocyclic ring as the organic interlayer. We uncover that incorporation of the ferroelectric 2D material into 3D perovskite induces an increased built-in electric field (BEF), which enhances the exciton dissociation efficiency in the device. Moreover, the 2D seeds could assist the 3D crystallization by forming more homogeneous and highly-oriented perovskite crystals. As a result, an impressive power conversion efficiency (PCE) over 23 % has been achieved by the f-PSCs with outstanding ambient stability. Moreover, the piezo/ferroelectric 2D perovskite intrigues a decreased hole transport barriers at the ITO/perovskite interface under tensile stress, which opens new possibilities for developing highly-efficient f-PSCs.

Managing Secondary Phase Lead Iodide in Hybrid Perovskites via Surface Reconstruction for High-Performance Perovskite Solar Cells with Robust Environmental Stability

Rationally managing the secondary-phase excess lead iodide (PbI₂ ) in hybrid perovskite is of significance for pursuing high performance perovskite solar cells (PSCs), while the challenge remains on its conversion to a homogeneous layer that is robust stable against environmental stimuli. We herein demonstrate an effective strategy of surface reconstruction that converts the excess PbI₂ into a gradient lead sulfate-silica bi-layer, which substantially stabilizes the perovskite film and reduces interfacial charge transfer barrier in the PSCs device. The perovskite films with such bi-layer could bear harsh conditions such as soaking in water, light illumination at 70 % relative humidity, and the damp-thermal (85 °C and 30 % humidity) environment. The resulted PSCs deliver a champion efficiency up to 24.09 %, as well as remarkable environmental stability, e.g., retaining 78 % of their initial efficiency after 5500 h of shelf storage, and 82 % after 1000 h of operational stability testing.

FA cation replenishment-induced second growth of printed MA-free perovskites for efficient solar cells and modules

Printing is one industrially compatible scalable method for the preparation of perovskite thin films but suffers from a low nucleation rate that causes an inferior crystal quality. In this work, we applied a slot-die coating method to deposit a MA-free perovskite thin film with a subsequently introduced FA⁺ replenishment to induce a second growth and prepare a FA-rich film. As a result, the inverted perovskite mini-module reached an efficiency of 17.56% with an aperture area of 60.84 cm².

Highly Efficient and Stable Self-Powered Mixed Tin-Lead Perovskite Photodetector Used in Remote Wearable Health Monitoring Technology

Realization of remote wearable health monitoring (RWHM) technology for the flexible photodiodes is highly desirable in remote-sensing healthcare systems used in space stations, oceans, and forecasting warning, which demands high external quantum efficiency (EQE) and detectivity in NIR region. Traditional inorganic photodetectors (PDs) are mechanically rigid and expensive while the widely reported solution-processed mixed tin-lead (MSP) perovskite photodetectors (PPDs) exhibit a trade-off between EQE and detectivity in the NIR region. Herein, a novel functional passivating antioxidant (FPA) strategy has been introduced for the first time to simultaneously improve crystallization, restrain Sn²⁺ oxidization, and reduce defects in MSP perovskite films by multiple interactions between thiophene-2-carbohydrazide (TAH) molecules and cations/anions in MSP perovskite. The resultant solution-processed rigid mixed Sn-Pb PPD simultaneously achieves high EQE (75.4% at 840 nm), detectivity (1.8 × 10¹² Jones at 840 nm), ultrafast response time (t_rise /t_fall = 94 ns/97 ns), and improved stability. This work also highlights the demonstration of the first flexible photodiode using MSP perovskite and FPA strategy with remarkably high EQE (75% at 840 nm), and operational stability. Most importantly, the RWHM is implemented for the first time in the PIN MSP perovskite photodiodes to remotely monitor the heart rate of humans at rest and after-run conditions.

Precursor engineering for efficient and stable perovskite solar cells

The perovskite film prepared by the two-step spin coating method is widely used in photovoltaic devices due to its good film morphology and great reproducibility. However, there usually exists excessive lead iodide (PbI₂) in the perovskite film for this method, which is believed to passivate the grain boundaries (GBs) to increase the efficiency of the perovskite solar cells. Nevertheless, the excessive PbI₂at the GBs of perovskite is believed to induce the decomposition of the perovskite film and undermine the long-term stability of devices. In this study, we utilize precursor engineering to realize the preparation of perovskite solar cells with high efficiency and stability. The concentration of organic salts (AX: A = MA⁺, FA⁺; X = I^-, Cl^-) in the precursor solution for the second step of the two-step spin coating method is adjusted to optimize the perovskite light-absorbing layer so that the excessive PbI₂is converted into perovskite to obtain a smooth and pinhole-free perovskite film with high performance. Our results indicate that by adjusting the concentration of AX in the precursor solution, PbI₂in the film could be completely converted into perovskite without excessive AX residue. Both the efficiency and stability of the perovskite solar cells without excessive PbI₂have been significantly improved. A planar perovskite solar cell with the highest power conversion efficiency (PCE) of 21.26% was achieved, maintaining about 90% of the initial PCE after 300 h of storage in a dry air environment and in the dark, about 76% of the initial PCE after 300 h of continuous illumination of 1 Sun.

Hot-Casting-Assisted Liquid Additive Engineering for Efficient and Stable Perovskite Solar Cells

High-performance inorganic-organic lead halide perovskite solar cells (PSCs) are often fabricated with a liquid additive such as dimethyl sulfoxide (DMSO), which retards crystallization and reduces roughness and pinholes in the perovskite layers. However, DMSO can be trapped during perovskite film formation and induce voids and undesired reaction byproducts upon later processing steps. Here, it is shown that the amount of residual DMSO can be reduced in as-spin-coated films significantly through use of preheated substrates, or a so-called hot-casting method. Hot casting increases the perovskite film thickness given the same concentration of solutions, which allows for reducing the perovskite solution concentration. By reducing the amount of DMSO in proportion to the concentration of perovskite precursors and using hot casting, it is possible to fabricate perovskite layers with improved perovskite-substrate interfaces by suppressing the formation of byproducts, which increase trap density and accelerate degradation of the perovskite layers. The best-performing PSCs exhibit a power conversion efficiency (PCE) of 23.4% (23.0% stabilized efficiency) under simulated solar illumination. Furthermore, encapsulated devices show considerably reduced post-burn-in decay, retaining 75% and 90% of their initial and post-burn-in efficiencies after 3000 h of operation with maximum power point tracking (MPPT) under high power of ultraviolet (UV)-containing continuous light exposure.

2,2'-Dihydroxy-4,4'-dimethoxy-benzophenon as Bifunctional Additives for Passivated Defects and Improved Photostability of Efficient Perovskite Photovoltaics

Organic-inorganic hybrid perovskite solar cells (PSCs) have developed rapidly in the past decade, but their commercial applications are restricted by further improvement in their photovoltaic performance and stability. Herein, we propose a facile and effective method employing 2,2'-dihydroxy-4,4'-dimethoxy-benzophenon (BP6) as bifunctional additive to construct efficient and photostable PSCs. BP6, as an additive, improves the crystallization quality of perovskite absorbers and further inhibits defect-mediated non-radiative recombination through interaction between the C═O group and defects; as a UV absorber, BP6 protects the PSCs from UV degradation by effectively absorbing UV light through molecular tautomerism under continuous strong UV irradiation. Eventually, the champion PSC demonstrates an efficiency of 22.85% with enhanced UV stability after addition of 0.024 wt % BP6. These results reveal that addition of UV absorbers (such as BP6 in this study) is a simple and effective strategy to fabricate efficient and photostable PSCs.

Zwitterion-Functionalized SnO2 Substrate Induced Sequential Deposition of Black-Phase FAPbI3 with Rearranged PbI2 Residue

Black-phase formamidinium lead iodide (FAPbI₃ ) with narrow bandgap and high thermal stability has emerged as the most promising candidate for highly efficient and stable perovskite photovoltaics. In order to overcome the intrinsic difficulty of black-phase crystallization and to eliminate the lead iodide (PbI₂ ) residue, most sequential deposition methods of FAPbI₃ -based perovskite will introduce external ions like methylammonium (MA⁺ ), cesium (Cs⁺ ), and bromide (Br^- ) ions to the perovskite structure. Here a zwitterion-functionalized tin(IV) oxide (SnO₂ ) is introduced as the electron-transport layer (ETL) to induce the crystallization of high-quality black-phase FAPbI₃ . The SnO₂ ETL treated with the zwitterion of formamidine sulfinic acid (FSA) can help rearrange the stack direction, orientation, and distribution of residual PbI₂ in the perovskite layer, which reduces the side effect of the residual PbI₂ . Besides, the FSA functionalization also modifies SnO₂ ETL to suppress deep-level defects at the perovskite/SnO₂ interface. As a result, the FSA-FAPbI₃ -based perovskite solar cells (PSCs) exhibit an excellent power conversion efficiency of up to 24.1% with 1000 h long-term operational stability. These findings provide a new interface engineering strategy on the sequential fabrication of black-phase FAPbI₃ PSCs with improved optoelectronic performance.

Dual Functional Dopant-Free Contacts with Titanium Protecting Layer: Boosting Stability while Balancing Electron Transport and Recombination Losses

Combining electron- and hole-selective materials in one crystalline silicon (Si) solar cell, thereby avoiding any dopants, is not considered for application to photovoltaic industry until only comparable efficiency and stable performance are achievable. Here, it is demonstrated how a conventionally unstable electron-selective contact (ESC) is optimized with huge boost in stability as well as improved electron transport. With the introduction of a Ti thin film between a-Si:H(i)/LiF and Al electrode, high-level passivation (S_eff  = 4.6 cm s^-1 ) from a-Si:H(i) and preferential band alignment (ρ_C  = 7.9 mΩ cm² ) from low work function stack of LiF/Ti/Al are both stably retained in the newly constructed n-Si/a-Si:H(i)/LiF/Ti/Al ESC. A detailed interfacial elements analysis reveals that the efficiently blocked inward diffusion of Al from electrode by the Ti protecting layer balances transport and recombination losses in general. This excellent electron-selective properties in combination with large process tolerance that enable remarkable device performance, particularly high efficiencies of 22.12% and 23.61%, respectively, are successfully approached by heterojunction solar cells with dopant-free ESC and dopant-free contacts for both polarities.

Surface Re-Engineering of Perovskites with Buckybowls to Boost the Inverted-Type Photovoltaics

Despite their multifaceted advantages, inverted perovskite solar cells (PSCs) still suffer from lower power conversion efficiencies (PCEs) than their regular counterparts, which is largely due to recombination energy losses (E_loss) that arise from the chemical, physical, and energy level mismatches, especially at the interfaces between perovskites and fullerene electron transport layers (ETLs). To address this problem, we herein introduce an aminium iodide derivative of a buckybowl (aminocorannulene) that is molecularly layered at the perovskite-ETL interface. Strikingly, besides passivating the PbI₂-rich perovskite surface, the aminocorannulene enforces a vertical dipole and enhances the surface n-type character that is more compatible with the ETL, thus boosting the electron extraction and transport dynamics and suppressing interfacial E_loss. As a result, the champion PSC achieves an excellent PCE of over 22%, which is superior compared to that of the control device (∼20%). Furthermore, the device stability is significantly enhanced, owing to a lock-and-key-like grip on the mobile iodides by the buckybowls and the resultant increase of the interfacial ion-migration barrier. This work highlights the potential of buckybowls for the multifunctional surface engineering of perovskite toward high-performance and stable PSCs.

Local Nanoscale Phase Impurities are Degradation Sites in Halide Perovskites

Understanding the nanoscopic chemical and structural changes that drive instabilities in emerging energy materials is essential for mitigating device degradation. The power conversion efficiency of halide perovskite photovoltaic devices has reached 25.7% in single junction and 29.8% in tandem perovskite/silicon cells^1,2, yet retaining such performance under continuous operation has remained elusive³. Here, we develop a multimodal microscopy toolkit to reveal that in leading formamidinium-rich perovskite absorbers, nanoscale phase impurities including hexagonal polytype and lead iodide inclusions are not only traps for photo-excited carriers which themselves reduce performance^4,5, but via the same trapping process are sites at which photochemical degradation of the absorber layer is seeded. We visualise illumination-induced structural changes at phase impurities associated with trap clusters, revealing that even trace amounts of these phases, otherwise undetected with bulk measurements, compromise device longevity. The type and distribution of these unwanted phase inclusions depends on film composition and processing, with the presence of polytypes being most detrimental for film photo-stability. Importantly, we reveal that performance losses and intrinsic degradation processes can both be mitigated by modulating these defective phase impurities, and demonstrate that this requires careful tuning of local structural and chemical properties. This multimodal workflow to correlate the nanoscopic landscape of beam sensitive energy materials will be applicable to a wide range of semiconductors for which a local picture of performance and operational stability has yet to be established.

Revealing the Dynamics of the Thermal Reaction between Copper and Mixed Halide Perovskite Solar Cells

Copper (Cu) is present not only in the electrode for inverted-structure halide perovskite solar cells (PSCs) but also in transport layers such as copper iodide (CuI), copper thiocyanate (CuSCN), and copper phthalocyanine (CuPc) alternatives to spiro-OMeTAD due to their improved thermal stability. While Cu or Cu-incorporated materials have been effectively utilized in halide perovskites, there is a lack of thorough investigation on the direct reaction between Cu and a perovskite under thermal stress. In this study, we investigated the thermal reaction between Cu and a perovskite as well as the degradation mechanism by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Kelvin probe force microscopy (KPFM). The results show that high temperatures of 100 °C induce Cu to be incorporated into the perovskite lattice by forming "Cu-rich yet organic A-site-poor" perovskites, (Cu_xA_1-x)PbX₃, near the grain boundaries, which result in device performance degradation.

Controllable Acceleration and Deceleration of Charge Carrier Transport in Metal-Halide Perovskite Single-Crystal by Cs-Cation Induced Bandgap Engineering

Charge carrier transport in materials is of essential importance for photovoltaic and photonic applications. Here, the authors demonstrate a controllable acceleration or deceleration of charge carrier transport in specially structured metal-alloy perovskite (MACs)PbI₃ (MA= CH₃ NH₃ ) single-crystals with a gradient composition of CsPbI₃ /(MA_{1- x} Cs_x )PbI₃ /MAPbI₃ . Depending on the Cs-cation distribution in the structure and therefore the energy band alignment, two different effects are demonstrated: i) significant acceleration of electron transport across the depth driven by the gradient band alignment and suppression of electron-hole recombination, benefiting for photovoltaic and detector applications; and ii) decelerated electron transport and thus improved radiative carrier recombination and emission efficiency, highly beneficial for light and display applications. At the same time, the top Cs-layer results in hole localization in the top layer and surface passivation. This controllable acceleration and deceleration of electron transport is critical for various applications in which efficient electron-hole separation and suppressed nonradiative electron-hole recombination is demanded.

Two-Dimensional Cs3Sb2I9-xClx Film with (201) Preferred Orientation for Efficient Perovskite Solar Cells

All-inorganic Sb-perovskite has become a promising material for solar cell applications owing to its air stability and nontoxic lead-free constitution. However, the poor morphology and unexpected (001) orientation of Sb-based perovskite films strongly hinder the improvement of efficiency. In this work, two-dimensional Cs₃Sb₂Cl_xI_9-x with (201) preferred orientation has been successfully fabricated by introducing thiourea (TU) to the precursor solution. The presence of the C=S functional group in TU regulates the crystallization dynamics of Cs₃Sb₂I_9-xCl_x films and generates the (201) preferred orientation of Cs₃Sb₂Cl_xI_9-x films, which could effectively improve the carrier transport and film morphology. As a result, the Cs₃Sb₂I_9-xCl_x perovskite solar cells (PSCs) delivered a power conversion efficiency (PCE) of 2.22%. Moreover, after being stored in nitrogen at room temperature for 60 days, the devices retained above 87.69% of their original efficiency. This work demonstrates a potential pathway to achieve high-efficiency Sb-based PSCs.

High-Polarizability Organic Ferroelectric Materials Doping for Enhancing the Built-In Electric Field of Perovskite Solar Cells Realizing Efficiency over 24

The built-in electric field (BEF) intensity of silicon heterojunction solar cells can be easily enhanced by selective doping to obtain high power conversion efficiencies (PCEs), while it is challenging for perovskite solar cells (pero-SCs) because of the difficulty in doping perovskites in a controllable way. Herein, an effective method is reported to enhance the BEF of FA_0.92 MA_0.08 PbI₃ perovskite by doping an organic ferroelectric material, poly(vinylidene fluoride):dabcoHReO₄ (PVDF:DH) with high polarizability, that can be driven even by the BEF of the device itself. The polarization of PVDF:DH produces an additional electric field, which is maintained permanently, in a direction consistent with that of the BEF of the pero-SC. The BEF superposition can more sufficiently drive the charge-carrier transport and extraction, thus suppressing the nonradiative recombination occurring in the pero-SCs. Moreover, the PVDF:DH dopant benefits the formation of a mesoporous PbI₂ film, via a typical two-step processing method, thereby promoting perovskite growth with high crystallinity and a few defects. The resulting pero-SC shows a promising PCE of 24.23% for a 0.062 cm² device (certified PCE of 23.45%), and a remarkable PCE of 22.69% for a 1 cm² device, along with significantly improved moisture resistances and operational stabilities.

Suppressing Residual Lead Iodide and Defects in Sequential-Deposited Perovskite Solar Cell via Bidentate Potassium Dichloroacetate Ligand

In sequential-deposited polycrystalline perovskite solar cells, the unreacted lead iodide due to incomplete conversion of lead iodide to perovskite phase, can contribute to ionic defects, such as residual lead ions (Pb²⁺ ). At present, passivation of interfacial and grain boundary defects has become an effective strategy to suppress charge recombination. Here, we introduced potassium acetate (KAc) and potassium dichloroacetate (KAcCl₂ ) as additives in the sequential deposition of polycrystalline perovskite thin films and found that acetate ions (Ac^- ) can effectively reduce the residual lead iodide. Compared with acetate (Ac), dichloroacetate (AcCl₂ ) can form Pb-Cl and Pb-O bonding as "dual anchoring" bonds with residual Pb²⁺ , resulting in strong binding force and effective passivation of residual Pb²⁺ defects. Furthermore, K⁺ can enlarge grain size and restrain ion migration at the grain boundaries. Consequently, perovskite solar cells with KAcCl₂ additive show power conversion efficiencies (PCE) from 19.67 % to 22.12 %, with the open-circuit voltage increasing from 1.06 V to 1.14 V. The unencapsulated device can maintain 82 % of the initial PCE under a humidity of 30±5 % for 1200 h. This work provides a new approach for the regulation of ionic defects and grain boundaries at the same time to develop high-performance planar perovskite solar cells.

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 (V_OC) 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%.

Homologous Bromides Treatment for Improving the Open-Circuit Voltage of Perovskite Solar Cells

The power conversion efficiency (PCE) of solution-processed organic-inorganic mixed halide perovskite solar cells has achieved rapid improvement. However, it is imperative to minimize the voltage deficit (W_oc  = E_g /q - V_oc ) for their PCE to approach the theoretical limit. Herein, the strategy of depositing homologous bromide salts on the perovskite surface to achieve a surface and bulk passivation for the fabrication of solar cells with high open-circuit voltage is reported. Distinct from the conclusions given by previous works, that homologous bromides such as FABr only react with PbI₂ to form a large-bandgap perovskite layer on top of the original perovskite, this work shows that the bromide also penetrates the perovskite film and passivates the perovskite in the bulk. This is confirmed by the small-bandgap enlargement observed by absorbance and photoluminescence, and the bromide element ratio increasing in the bulk by time-of-flight secondary-ion mass spectrometry and depth-resolved X-ray photoelectron spectroscopy. Furthermore, a clear suppression of non-radiative recombination is confirmed by a variety of characterization methods. This work provides a simple and universal way to reduce the W_oc of single-junction perovskite solar cells and it will also shed light on developing other high-performance optoelectronic devices, including perovskite-based tandems and light-emitting diodes.