When Aggregation-Induced Emission Meets Perovskites: Efficient Defect-Passivation and Charge-Transfer for Ambient Fabrication of Perovskite Solar Cells

The intrinsic defects in perovskite film can serve as non-radiative recombination center to limit the performance and stability of metal halide perovskite solar cells (PSCs). The additive engineering in perovskite film is always applied to produce high-efficiency PSCs in recent years. Here, a typical donor-acceptor (D−A) structured aggregation-induced emission (AIE) molecule tetraphenylethene-2-dicyano-methylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (TPE-TCF) was introduced into perovskite film. The D−A structure of TPE-TCF molecule provided additional charge transfer channels, contributing to transporting electron of TPE-TCF-based device. The cyano (C≡N) of TPE-TCF can interact with the uncoordinated Pb to from a relatively stable intermediate, PbI₂⋅TPE-TCF, resulting in the slower crystal growth, reduced the defects at the grain boundaries and suppressed carrier recombination. As a consequence, the power conversion efficiency (PCE) of TPE-TCF-modified PSCs achieved a remarkably enhanced from 15.63 to 19.66 % with negligible hysteresis, which was prominent in methylammonium lead iodide-based devices fabricated under ambient condition. Furthermore, the PSCs modified by AIE molecule possessed an outstanding stability and maintain about 86 % of the initial PCE after 300 h storage in air at 25–35 °C with a high relative humidity (RH) of ≈85 %. This work suggests that incorporating AIE molecule into perovskite is a promising strategy for facilitating high-performance PSCs commercialization in ambient environment without glovebox.     

Molecular Bridge on Buried Interface for Efficient and Stable Perovskite Solar Cells

The interface of perovskite solar cells (PSCs) is significantly important for charge transfer and device stability, while the buried interface with the impact on perovskite film growth has been paid less attention. Herein, we use a molecular modifier, glycocyamine (GDA) to build a molecular bridge on the buried interface of SnO₂/perovskite, resulting in superior interfacial contact. This is achieved through the strongly interaction between GDA and SnO₂, which also appreciably modulates the energy level. Moreover, GDA can regulate the perovskite crystal growth, yielding perovskite film with enlarged grain size and absence of pinholes, exhibiting substantially reduced defect density. Consequently, PSCs with GDA modification demonstrate significant improvement of open circuit voltage (close to 1.2 V) and fill factor, leading to an improved power conversion efficiency from 22.60 % to 24.70 %. Additionally, stabilities of GDA devices under maximum power point and 85 °C heat both perform better than the control devices.     

Ambient Air Temperature Assisted Crystallization for Inorganic CsPbI2Br Perovskite Solar Cells

Inorganic cesium lead halide perovskites, as alternative light absorbers for organic–inorganic hybrid perovskite solar cells, have attracted more and more attention due to their superb thermal stability for photovoltaic applications. However, the humid air instability of CsPbI₂Br perovskite solar cells (PSCs) hinders their further development. The optoelectronic properties of CsPbI₂Br films are closely related to the quality of films, so preparing high-quality perovskite films is crucial for fabricating high-performance PSCs. For the first time, we demonstrate that the regulation of ambient temperature of the dry air in the glovebox is able to control the growth of CsPbI₂Br crystals and further optimize the morphology of CsPbI₂Br film. Through controlling the ambient air temperature assisted crystallization, high-quality CsPbI₂Br films are obtained, with advantages such as larger crystalline grains, negligible crystal boundaries, absence of pinholes, lower defect density, and faster carrier mobility. Accordingly, the PSCs based on as-prepared CsPbI₂Br film achieve a power conversion efficiency of 15.5% (the maximum stabilized power output of 15.02%). Moreover, the optimized CsPbI₂Br films show excellent robustness against moisture and oxygen and maintain the photovoltaic dark phase after 3 h aging in an air atmosphere at room temperature and 35% relative humidity (R.H.). In comparison, the pristine films are completely converted to the yellow phase in 1.5 h.     

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 CH₃NH₃PbI₃ 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 CH₃NH₃PbI₃/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 CH₃NH₃PbI₃/C₆₀ interfacial structure from the perspective of the atomic layer and insight into the double-edged sword effect of PbI₂ as an additive.     

Fluorine-Containing Passivation Layer via Surface Chelation for Inorganic Perovskite Solar Cells

Minimizing surface defect is vital to further improve power conversion efficiency (PCE) and stability of inorganic perovskite solar cells (PSCs). Herein, we designed a passivator trifluoroacetamidine (TFA) to suppress CsPbI_3−xBr_x film defects. The amidine group of TFA can strongly chelate onto the perovskite surface to suppress the iodide vacancy, strengthened by additional hydrogen bonds. Moreover, three fluorine atoms allow strong intermolecular connection via intermolecular hydrogen bonds, thus constructing a robust shield against moisture. The TFA-treated PSCs exhibit remarkably suppressed recombination, yielding the record PCEs of 21.35 % and 17.21 % for 0.09 cm² and 1.0 cm² device areas, both of which are the highest for all-inorganic PSCs so far. The device also achieves a PCE of 39.78 % under indoor illumination, the highest for all-inorganic indoor photovoltaic devices. Furthermore, TFA greatly improves device ambient stability by preserving 93 % of the initial PCE after 960 h.     

Low-Cost Hydroxyacid Potassium Synergists as an Efficient In Situ Defect Passivator for High Performance Tin-Oxide-Based Perovskite Solar Cells

Perovskite solar cells (PSCs) based on SnO₂ electron transport layers have attracted extensive research due to their compelling photovoltaic performance. Herein, we presented an in situ passivation of SnO₂ with low-cost hydroxyacid potassium synergist during deposition to optimize the interface carrier extraction and transport for high power conversion efficiency (PCE) and stabilities of PSCs. The orbital overlap of the carboxyl oxygen with the Sn atom alongwith the homogenous nano-particle deposition effectively suppresses the interfacial defects and releases the internal residual strains in the perovskite. Accordingly, a PCE of 24.91 % with a fill factor (FF) up to 0.852 is obtained for in situ passivated devices, which is one of the highest values for SnO₂-based PSCs. Moreover, the unencapsulated device maintained 80 % of its initial PCE at 80 °C over 600 h, 100 % PCE at ambient conditions for 1300 h, and 98 % after one week maximum power point tracking (MPPT) under continuous AM1.5G illumination.     

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.     

Limiting Perovskite Solar Cell Performance by Heterogeneous Carrier Extraction

Although the power conversion efficiency of perovskite solar cells has improved rapidly, a rational path for further improvement remains unclear. The effect of large morphological heterogeneity of polycrystalline perovskite films on their device performance by photoluminescence (PL) microscopy has now been studied. Contrary to the common belief on the deleterious effect of morphological heterogeneity on carrier lifetimes and diffusivities, in neat CH₃NH₃PbI₃(Cl) polycrystalline perovskite films, the local (intra‐grain) carrier diffusivities in different grains are all surprisingly high (1.5 to 3.3 cm² s^?1; comparable to bulk single‐crystals), and the local carrier lifetimes are long (ca. 200 ns) and surprisingly homogenous among grains, and uniform across grain boundary and interior. However, there is a large heterogeneity of carrier extraction efficiency at the perovskite grain–electrode interface. Improving homogeneity at perovskite grain–electrode contacts is thus a promising direction for improving the performance of perovskite thin‐film solar cells.     

Antioxidant induced bulk passivation for efficient and stable hole transport layer-free carbon electrode perovskite solar cells

Defect passivation is one of the important strategies to improve the efficiency and stability of perovskite solar cells. In this work, 2,6-di‑tert‑butyl‑4-methylphenol (BHT) as antioxidant was introduced into the perovskite precursor solution to improve the quality of the prepared perovskite films, so that these films performed a larger and uniform grain size. Moreover, the −OH functional group in BHT interacts with I⁻, thus reducing the density of defect states and inhibiting the non-radiative recombination. The presence of hydrophobic groups in BHT protects the film from moisture erosion and improves the long-term stability of PSCs devices. The maximum photoelectric conversion efficiency of the constructed ITO/SnO₂/BHT-MAPbI₃/Carbon device is 16.88%, and the unpackaged cell maintains the initial efficiency of 99.3% after 698 h of storage under the environmental condition of 30% humidity. This work provides an efficient approach to improve the performance of printable hole transport layer-free carbon electrode perovskite solar cells.     

Tetra‐ammonium Zinc Phthalocyanine to Construct a Graded 2D–3D Perovskite Interface for Efficient and Stable Solar Cells

The crystallographic defects inevitably incur during the solution processed organic‐inorganic hybrid perovskite film, especially at surface and the grain boundaries (GBs) of perovskite film, which can further result in the reduced cell performance and stability of perovskite solar cells (PSCs). Here, a simple defect passivation method was employed by treating perovskite precursor film with a hydrophobic tetra‐ammonium zinc phthalocyanine (ZnPc). The results demonstrated that a 2D‐3D graded perovskite interface with a capping layer of 2D (ZnPc)_0.5MA_n ? 1Pb_nI_3n + 1 perovskite together with 3D MAPbI₃ perovskite was successfully constructed on the top of 3D perovskite layer. This situation realized the efficient GBs passivation, thus reducing the defects in GBs. As expected, the corresponding PSCs with modified perovskite revealed an improved cell performance. The best efficiency reached 19.6%. Especially, the significantly enhanced long‐term stability of the responding PSCs against humidity and heating was remarkably achieved. Such a strategy in this work affords an efficient method to improve the stability of PSCs and thus probably brings the PSCs closer to practical commercialization.     

Managing Excess Lead Iodide with Functionalized Oxo-Graphene Nanosheets for Stable Perovskite Solar Cells

Stability issues could prevent lead halide perovskite solar cells (PSCs) from commercialization despite it having a comparable power conversion efficiency (PCE) to silicon solar cells. Overcoming drawbacks affecting their long-term stability is gaining incremental importance. Excess lead iodide (PbI₂) causes perovskite degradation, although it aids in crystal growth and defect passivation. Herein, we synthesized functionalized oxo-graphene nanosheets (Dec-oxoG NSs) to effectively manage the excess PbI₂. Dec-oxoG NSs provide anchoring sites to bind the excess PbI₂ and passivate perovskite grain boundaries, thereby reducing charge recombination loss and significantly boosting the extraction of free electrons. The inclusion of Dec-oxoG NSs leads to a PCE of 23.7 % in inverted (p-i-n) PSCs. The devices retain 93.8 % of their initial efficiency after 1,000 hours of tracking at maximum power points under continuous one-sun illumination and exhibit high stability under thermal and ambient conditions.     

Dual-Interface Engineering in Perovskite Solar Cells with 2D Carbides

Passivating the interfaces between the perovskite and charge transport layers is crucial for enhancing the power conversion efficiency (PCE) and stability in perovskite solar cells (PSCs). Here we report a dual-interface engineering approach to improving the performance of FA_0.85MA_0.15Pb(I_0.95Br_0.05)₃-based PSCs by incorporating Ti₃C₂Cl_x Nano-MXene and o-TB-GDY nanographdiyne (NanoGDY) into the electron transport layer (ETL)/perovskite and perovskite/ hole transport layer (HTL) interfaces, respectively. The dual-interface passivation simultaneously suppresses non-radiative recombination and promotes carrier extraction by forming the Pb−Cl chemical bond and strong coordination of π-electron conjugation with undercoordinated Pb defects. The resulting perovskite film has an ultralong carrier lifetime exceeding 10 μs and an enlarged crystal size exceeding 2.5 μm. A maximum PCE of 24.86 % is realized, with an open-circuit voltage of 1.20 V. Unencapsulated cells retain 92 % of their initial efficiency after 1464 hours in ambient air and 80 % after 1002 hours of thermal stability test at 85 °C.     

A Multifunctional Liquid Crystal as Hole Transport Layer Additive Enhances Efficiency and Stability of Perovskite Solar Cells

Lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) has been identified as the most used and effective p-dopant for hole transport layer (HTL) in perovskite solar cells (PSCs). However, the migration and agglomeration of Li-TFSI in HTL negatively impact PSCs performance and stability. Herein, we report an effective strategy for adding a liquid crystal organic small molecule (LQ) into Li-TFSI doped (2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′- spirobifluorene (Spiro-OMeTAD) HTL. It was found that the introduction of LQ into Spiro-OMeTAD HTL can efficiently enhance the charge carrier extraction and transportation in device, which can strongly retard the charge carrier recombination in device. Consequently, the PSCs efficiency is significantly enhanced to 24.42 % (Spiro-OMeTAD+LQ) from 21.03 % (Spiro-OMeTAD). The chemical coordination between LQ and Li-TFSI can strongly confine Li⁺ ions migration and agglomeration of Li-TFSI, thus, achieving the enhanced device stability. Only a 9 % efficiency degradation is observed for un-encapsulated device prepared with Spiro-OMeTAD and LQ after 1700 h under air environment, while the efficiency drops by 30 % for the reference device. This work provides an effective strategy for improving the efficiency and stability of PSCs, and gives some important insights for understanding intrinsic hot carriers dynamics for perovskite-based optoelectronic devices.     

Bulk heterojunction perovskite solar cells incorporated with solution-processed TiOx nanoparticles as the electron acceptors

In the past ten years, perovskite solar cells were rapidly developed, but the intrinsic unbalanced charge carrier diffusion lengths within perovskite materials were not fully addressed by either a planar heterojunction or meso-superstructured perovskite solar cells. In this study, we report bulk heterojunction perovskite solar cells, where perovskite materials CH₃NH₃PbI₃ is blended with solution-processed n-type TiO_x nanoparticles as the photoactive layer. Studies indicate that one-step solution-processed CH₃NH₃PbI₃:TiO_x bulk-heterojunction thin film possesses enhanced and balanced charge carrier mobilities, superior film morphology with enlarged crystal sizes, and suppressed trap-induced charge recombination. Thus, bulk heterojunction perovskite solar cells by CH₃NH₃PbI₃ mixed with 5 wt% of TiO_x, which is processed by one-step method rather than typical two-step method, show a short-circuit current density of 20.93 mA/cm², an open-circuit voltage of 0.90 V, a fill factor of 80% and with a corresponding power conversion efficiency of 14.91%, which is more than 30% enhancement as compared with that of perovskite solar cells with a planar heterojunction device structure. Moreover, bulk heterojunction perovskite solar cells possess enhanced device stability. All these results demonstrate that perovskite solar cells with a bulk heterojunction device structure are one of apparent approaches to boost device performance.     

Hole‐Boosted Cu(Cr,M)O2 Nanocrystals for All‐Inorganic CsPbBr3 Perovskite Solar Cells

The all‐inorganic CsPbBr₃ perovskite solar cell (PSC) is a promising solution to balance the high efficiency and poor stability of state‐of‐the‐art organic–inorganic PSCs. Setting inorganic hole‐transporting layers at the perovskite/electrode interface decreases charge carrier recombination without sacrificing superiority in air. Now, M‐substituted, p‐type inorganic Cu(Cr,M)O₂ (M=Ba²⁺, Ca²⁺, or Ni²⁺) nanocrystals with enhanced hole‐transporting characteristics by increasing interstitial oxygen effectively extract holes from perovskite. The all‐inorganic CsPbBr₃ PSC with a device structure of FTO/c‐TiO₂/m‐TiO₂/CsPbBr₃/Cu(Cr,M)O₂/carbon achieves an efficiency up to 10.18 % and it increases to 10.79 % by doping Sm³⁺ ions into perovskite halide, which is much higher than 7.39 % for the hole‐free device. The unencapsulated Cu(Cr,Ba)O₂‐based PSC presents a remarkable stability in air in either 80 % humidity over 60 days or 80 °C conditions over 40 days or light illumination for 7 days.     

Carbon‐Electrode‐Tailored All‐Inorganic Perovskite Solar Cells To Harvest Solar and Water‐Vapor Energy

Moisture is the worst enemy for state‐of‐the‐art perovskite solar cells (PSCs). However, the flowing water vapor within nanoporous carbonaceous materials can create potentials. Therefore, it is a challenge to integrate water vapor and solar energies into a single PSC device. We demonstrate herein all‐inorganic cesium lead bromide (CsPbBr₃) solar cells tailored with carbon electrodes to simultaneously harvest solar and water‐vapor energy. Upon interfacial modification and plasma treatment, the bifunctional PSCs yield a maximum power conversion efficiency up to 9.43 % under one sun irradiation according to photoelectric conversion principle and a power output of 0.158 μW with voltage of 0.35 V and current of 0.45 μA in 80 % relative humidity through the flowing potentials at the carbon/water interface. The initial efficiency is only reduced by 2 % on exposing the inorganic PSC with 80 % humidity over 40 days. The successful realization of physical proof‐of‐concept multi‐energy integrated solar cells provides new opportunities of maximizing overall power output.     

Enhanced Lifetime and Photostability with Low‐Temperature Mesoporous ZnTiO3/Compact SnO2 Electrodes in Perovskite Solar Cells

Perovskite solar cells (PSCs) with power conversion efficiencies (PCEs) of 25 % mainly have SnO₂ or TiO₂ as electron‐transporting layers (ETLs). Now, zinc titanate (ZnTiO₃, ZTO) is proposed as mesoporous ETLs owing to its weak photo‐effect, excellent carrier extraction, and transfer properties. Uniform mesoporous films were obtained by spinning coating the ZTO ink and annealed below 150 °C. Photovoltaic devices based on Cs_0.05FA_0.81MA_0.14PbI_2.55B_r0.45 perovskite sandwiched between SnO₂‐mesorporous ZTO electrode and Spiro‐OMeTAD layer achieved the PCE of 20.5 %. The PSCs retained more than 95 % of their original efficiency after 100 days lifetime test without being encapsulated. Additionally, the PSCs retained over 95 % of the initial performance when subjected at the maximum power point voltage for 120 h under AM 1.5 G illumination (100 mW cm^?2), demonstrating superior working stability. The application of ZTO provides a better choice for ETLs of PSCs.     

Stability improvement under high efficiency—next stage development of perovskite solar cells

With efficiency of perovskite solar cells(PSCs) overpassing 23%, to realize their commercialization, the biggest challenge now is to boost the stability to the same level as conventional solar cells. Thus, tremendous effort has been directed over the past few years toward improving the stability of these cells. Various methods were used to improve the stability of bulk perovskites,including compositional engineering, interface adjustment, dimensional manipulation, crystal engineering, and grain boundary decoration. Diverse device configurations, carrier transporting layers, and counter electrodes are investigated. To compare the stability of PSCs and clarify the degradation mechanism, diverse characterization methods were developed. Overall stability of PSCs has become one central topic for the development of PSCs. In this review, we summarize the state-of-the-art progress on the improvement of device stability and discuss the directions for future research, hoping it provides an overview of the current status of the research on the stability of PSCs and guidelines for future research.     

Anti-Solvent-Free Preparation for Efficient and Photostable Pure-Iodide Wide-Bandgap Perovskite Solar Cells

The perovskite/silicon tandem solar cell (TSC) has attracted tremendous attention due to its potential to breakthrough the theoretical efficiency set for single-junction solar cells. However, the perovskite solar cell (PSC) designed as its top component cell suffers from severe photo-induced halide segregation owing to its mixed-halide strategy for achieving desirable wide-bandgap (1.68 eV). Developing pure-iodide wide-bandgap perovskites is a promising route to fabricate photostable perovskite/silicon TSCs. Here, we report efficient and photostable pure-iodide wide-bandgap PSCs made from an anti-solvent-free (ASF) technique. The ASF process is achieved by mixing two precursor solutions, both of which are capable of depositing corresponding perovskite films without involving anti-solvent. The mixed solution finally forms Cs_0.3DMA_0.2MA_0.5PbI₃ perovskite film with a bandgap of 1.68 eV. Furthermore, methylammonium chloride additive is applied to enhance the crystallinity and reduce the trap density of perovskite films. As a result, the pure-iodide wide-bandgap PSC delivers efficiency as high as 21.30 % with excellent photostability, the highest for this type of solar cells. The ASF method significantly improves the device reproducibility as compared with devices made from other anti-solvent methods. Our findings provide a novel recipe to prepare efficient and photostable wide-bandgap PSCs.     

Antioxidant Grain Passivation for Air‐Stable Tin‐Based Perovskite Solar Cells

Tin‐based perovskites with excellent optoelectronic properties and suitable band gaps are promising candidates for the preparation of efficient lead‐free perovskite solar cells (PSCs). However, it is challenging to prepare highly stable and efficient tin‐based PSCs because Sn²⁺ in perovskites can be easily oxidized to Sn⁴⁺ upon air exposure. Here we report the fabrication of air‐stable FASnI₃ solar cells by introducing hydroxybenzene sulfonic acid or its salt as an antioxidant additive into the perovskite precursor solution along with excess SnCl₂. The interaction between the sulfonate group and the Sn²⁺ ion enables the in situ encapsulation of the perovskite grains with a SnCl₂–additive complex layer, which results in greatly enhanced oxidation stability of the perovskite film. The corresponding PSCs are able to maintain 80 % of the efficiency over 500 h upon air exposure without encapsulation, which is over ten times longer than the best result reported previously. Our results suggest a possible strategy for the future design of efficient and stable tin‐based PSCs.