Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance

Enhanced charge carrier transport and defects mitigation of passivation layer for efficient perovskite solar cells

Surface passivation has been developed as an effective strategy to reduce trap-state density and suppress non-radiation recombination process in perovskite solar cells. However, passivation agents usually own poor conductivity and hold negative impact on the charge carrier transport in device. Here, we report a binary and synergistical post-treatment method by blending 4-tert-butyl-benzylammonium iodide with phenylpropylammonium iodide and spin-coating on perovskite surface to form passivation layer. The binary and synergistical post-treated films show enhanced crystallinity and improved molecular packing as well as better energy band alignment, benefiting for the hole extraction and transfer. Moreover, the surface defects are further passivated compared with unary passivation. Based on the strategy, a record-certified quasi-steady power conversion efficiency of 26.0% perovskite solar cells is achieved. The devices could maintain 81% of initial efficiency after 450 h maximum power point tracking.

Investigating the potential of triclinic ABSe3 (A = Li, Na, K, Rb, Cs; B = Si, Ge, Sn) perovskites as a new class of lead-free photovoltaic materials

In recent years, chalcogenide perovskites have emerged as promising candidates with favorable structural, electrical, and optical properties for photovoltaic applications. This paper explores the structural, electronic, and optical characteristics of ABSe3 perovskites (where A = Li, Na, K, Rb, Cs; B = Si, Ge, Sn) in their triclinic crystallographic phases using density functional theory. The stability of these materials is ensured by calculating formation energies, tolerance factors (Tf), and phonon dispersion. The Eform values of all ABSe3 are negative, suggesting favorable thermodynamic stability. The Tf values range between 0.82 and 1.1, which is consistent with stable perovskites. The phonon dispersion analysis of the chalcogenide perovskites revealed no imaginary frequencies in any of the vibrational modes, confirming their stability. The electronic band structures and corresponding density of states are computed to unveil the semiconducting nature of the studied compounds. These perovskites are promising for high-performance solar cells due to their indirect bandgaps (Eg, 1.10-2.33 eV) and a small difference between these indirect and direct gaps (0.149-0.493 eV). The Eg values increase as the ionic radii of A-site elements increase (Li < Na < K < Rb < Cs). At the B-site, Si-based chalcogenides have the largest Eg values, followed by Sn-based and then Ge-based materials. Furthermore, optical properties such as the real part and imaginary part of the dielectric function, refractive index extinction coefficient, optical conductivity, absorption coefficient, reflectivity, and energy loss are predicted within the energy range of 0-50 eV. Several ABSe3 materials, particularly LiGeSe3 and NaGeSe3, demonstrated optical properties comparable to both traditional and emerging materials, suggesting their potential for effective use in solar cells.

Ultra-uniform perovskite crystals formed in the presence of tetrabutylammonium bistriflimide afford efficient and stable perovskite solar cells

Compositional engineering of organic-inorganic metal halide perovskite allows for improved optoelectrical properties, however, phase segregation occurs during crystal nucleation and limits perovskite solar cell device performance. Herein, we show that by applying tetrabutylammonium bistriflimide as an additive in the perovskite precursor solution, ultra-uniform perovskite crystals are obtained, which effectively increases device performance. As a result, power conversion efficiencies (PCEs) of 24.5% in a cell and 21.2% in a module are achieved, together with high stability under illumination, humidity and elevated thermal conditions.

Spontaneous Heterointerface Modulation by a Methylammonium Tetrafluoroborate Additive for a Narrow-Bandgap FAPbI3 Photoabsorber in Perovskite Solar Cells

Over the past decade, the photovoltaic (PV) performance of perovskite solar cells (PSCs) has been considerably improved with the development of perovskite photoabsorbers. Among these, formamidinium-lead-iodide (FAPbI3) is a promising photoabsorber owing to its narrow bandgap and is mainly used in n-i-p-structured PSCs. The property modulation of FAPbI3 photoabsorbers while retaining their narrow bandgap is imperative for further development of PSCs. Molecular tetrafluoroborate anion (BF4-)-based materials can be used as additives in perovskite layers to prevent bandgap widening, while facilitating perovskite crystal growth; thus, they are suitable for FAPbI3 photoabsorbers in principle. However, BF4--based additives for narrow-bandgap FAPbI3 photoabsorbers have not been developed. This is presumably because of the higher temperatures required for FAPbI3 formation than that for other wide-bandgap perovskites, which likely changes the effects of BF4-based additives from those for wide-bandgap perovskites. In this study, we verified the applicability of methylammonium tetrafluoroborate (MABF4) as an additive in narrow-bandgap FAPbI3 photoabsorbers for improving their PV performance primarily via the spontaneous modulation of the heterointerfaces between FAPbI3 and carrier-transport materials, rather than the bulk quality improvement of FAPbI3 perovskite. At the interface of the hole-transport material and FAPbI3, MABF4 addition effectively eliminates the surface defects in all FAPbI3 components, even in the absence of BF4- over the heated FAPbI3 surface, suggesting a defect-suppression mechanism that differs from that observed in conventional ones. Moreover, at the interface of FAPbI3 and the TiO2 electron-transport material, the BF4-derived species, which likely includes decomposed BF4- owing to the high-temperature heating, spontaneously segregates upon deposition, thereby modulating the heterointerface. Furthermore, in addition to the carrier mobility ratio in FAPbI3 (e-:h+ ≈ 7:3), a time-resolved microwave conductivity measurement revealed that MABF4 addition eliminates carrier traps at the heterointerfaces. Our findings provide insights into promising FAPbI3-based PSCs, offering a valuable tool for their further development.

Interface Engineering by Unsubstituted Pristine Nickel Phthalocyanine as Hole Transport Material for Efficient and Stable Perovskite Solar Cells

Lead halide perovskite solar cells (PSCs) have been rapidly developed in the past decade. With the development of a PSC, interface engineering plays an increasingly important role in maximizing device performance and long-term stability. We report a simple and effective interface engineering method for achieving improvement of PSCs up to 20% by employing unsubstituted pristine nickel phthalocyanine (NiPc). Thermal annealing of NiPc improves the interface between NiPc and perovskite because of the incorporation of NiPc molecules into the perovskite grain boundaries, which creates improvements in hole extraction from the perovskite absorber layer, as evidenced by time-resolved photoluminescence measurements. This significantly improves the charge transfer and collection efficiency, which are closely related to the improvement of the interface between perovskite and NiPc.

Enhancing Thermal Stability of Perovskite Solar Cells through Thermal Transition and Thin Film Crystallization Engineering of Polymeric Hole Transport Layers

Organic hole transport layers (HTLs) have been known to be susceptible to thermal stress, leading to poor long-term stability in perovskite solar cells (PSCs). We synthesized three 2,5-dialkoxy-substituted, 1,4-bis(2-thienyl)phenylene (TPT)-based conjugated polymers (CPs) linked with thiophene-based (thiophene (T) and thienothiophene (TT)) comonomers and evaluated them as HTLs in n-i-p PSCs. TPT-T (MB/C6), which has branched 2-methylbutyl and linear hexyl (MB/C6) side chains, emerged as a promising HTL candidate, enabling power conversion efficiencies (PCEs) greater than 15%. In addition, PSCs with this HTL showed an improvement in long-term stability at elevated temperatures of 65 °C when compared to those with the state-of-art HTL, 2,2',7,7'-tetrakis(N,N-p-dimethoxyphenylamino)-9,9'-spirobifluorene (spiro-OMeTAD). This improvement is ascribed to the lack of thermal transitions within the operational temperature range of PSCs for TPT-T (MB/C6), which is attributed to the relatively short branched side chains of this polymer. We propose that the elimination of thermal transitions below 200 °C leads to HTLs without cracking as-deposited and after conducting a stress test at 65 °C, which can serve as a new design guideline for HTL development.

Strategies for Improving Efficiency and Stability of Inverted Perovskite Solar Cells

Perovskite solar cells (PSCs) have attracted widespread research and commercialization attention because of their high power conversion efficiency (PCE) and low fabrication cost. The long-term stability of PSCs should satisfy industrial requirements for photovoltaic devices. Inverted PSCs with a p-i-n architecture exhibit considerable advantages because of their excellent stability and competitive efficiency. The continuously broken-through PCE of inverted PSCs shows huge application potential. This review summarizes the developments and outlines the characteristics of inverted PSCs including charge transport layers (CTLs), perovskite compositions, and interfacial regulation strategies. The latest effective CTLs, interfacial modification, and stability promotion strategies especially under light, thermal, and bias conditions are emphatically analyzed. Furthermore, the applications of the inverted structure in high-efficiency and stable tandem, flexible photovoltaic devices, and modules and their main obstacles are systematically introduced. Finally, the remaining challenges faced by inverted devices are discussed, and several directions for advancing inverted PSCs are proposed according to their development status and industrialization requirements.

Spontaneous Compositional-Interfacial Co-Modification Engineering via Ion Exchange Reaction Between Perovskite and Electron-Transporting Layer for Exceptionally Long-Term Stability of Photovoltaics

The long-term stability of perovskite solar cells (PSCs) is still challenging for commercialization and mainly linked to the life span of perovskite films. Herein, a spontaneous compositional-interfacial co-modification strategy is developed based on the ion exchange reaction by introducing ammonium hexafluorophosphate (NH4PF6) into antisolvent to form gradient structures through a simple one-step solvent engineering. With the assistance of the ion exchange reaction, NH4PF6 forms a multifunctional structure to protect perovskite films from both internal and external factors for the exceptionally long-term stability of photovoltaics. The reason for this is linked to the high hydrophobicity of NH4PF6 for preventing H2O invasion, suppressing ion migration by forming hydrogen bonding, and reducing perovskite defects. The resulting unencapsulated devices show exceptionally long-term stability under standardized the International Summit on Organic Photovoltaic Stability (ISOS) protocols, with over 94%, 81%, and 83% retained power conversion efficiencies after aging tests under N2 (ISOS-D-1I), ambient air (ISOS-D-1), and 85 °C (ISOS-D-2I) for 14016, 2500, and 1248 h, respectively. These performances compare well with the state-of-the-art stability of inverted PSCs. Further investigations are conducted to study the evolution of macroscopic morphology and microscopic crystal structure in aged perovskite films, aiming to provide evidence supporting the aforementioned improvements in stability.

The Promise and Challenges of Inverted Perovskite Solar Cells

Recently, there has been an extensive focus on inverted perovskite solar cells (PSCs) with a p-i-n architecture due to their attractive advantages, such as exceptional stability, high efficiency, low cost, low-temperature processing, and compatibility with tandem architectures, leading to a surge in their development. Single-junction and perovskite-silicon tandem solar cells (TSCs) with an inverted architecture have achieved certified PCEs of 26.15% and 33.9% respectively, showing great promise for commercial applications. To expedite real-world applications, it is crucial to investigate the key challenges for further performance enhancement. We first introduce representative methods, such as composition engineering, additive engineering, solvent engineering, processing engineering, innovation of charge transporting layers, and interface engineering, for fabricating high-efficiency and stable inverted PSCs. We then delve into the reasons behind the excellent stability of inverted PSCs. Subsequently, we review recent advances in TSCs with inverted PSCs, including perovskite-Si TSCs, all-perovskite TSCs, and perovskite-organic TSCs. To achieve final commercial deployment, we present efforts related to scaling up, harvesting indoor light, economic assessment, and reducing environmental impacts. Lastly, we discuss the potential and challenges of inverted PSCs in the future.

Assembling the 2D-3D-2D Heterostructure of Quasi-2D Perovskites for High-Performance Solar Cells

Quasi-two-dimensional (quasi-2D) layered perovskites with mixed dimensions offer a promising avenue for stable and efficient solar cells. However, randomly distributed three-dimensional (3D) perovskites near the film surface limit the device performance of quasi-2D perovskites due to increased nonradiative recombination and ion migration. Herein, we construct a 2D (n = 4 top)-3D-2D (n = 2 bottom) heterostructure of quasi-2D perovskites by using 3-chlorobenzylamine iodine, which can effectively reduce defect density and restrain ion migration. A champion efficiency of 22.22% for quasi-2D perovskite solar cells is achieved due to remarkably reduced nonradiative voltage loss and increased electron extraction. Additionally, the 2D-3D-2D perovskite solar cells also exhibit excellent thermal and humidity stabilities, retaining over 90 and 85% of the initial efficiencies after 2000 h under a heat stress of 65 °C and at air ambient of ∼50% humidity, respectively. Our results provide a general approach to tune perovskite films for suppressing ion migration and achieving high-performance perovskite solar cells.

What Matters for the Charge Transport of 2D Perovskites?

Compared to 3D perovskites, 2D perovskites exhibit excellent stability, structural diversity, and tunable bandgaps, making them highly promising for applications in solar cells, light-emitting diodes, and photodetectors. However, the trade-off for worse charge transport is a critical issue that needs to be addressed. This comprehensive review first discusses the structure of 3D and 2D metal halide perovskites, then summarizes the significant factors influencing charge transport in detail and provides a brief overview of the testing methods. Subsequently, various strategies to improve the charge transport are presented, including tuning A'-site organic spacer cations, A-site cations, B-site metal cations, and X-site halide ions. Finally, an outlook on the future development of improving the 2D perovskites' charge transport is discussed.

Unveiling the Effect of Cooling Rate on Grown-in Defects Concentration in Polycrystalline Perovskite Films for Solar Cells with Improved Stability

Numerous efforts are devoted to reducing the defects at perovskite surface and/or grain boundary; however, the grown-in defects inside grain is rarely studied. Here, the influence of cooling rate on the point defects concentration in polycrystalline perovskite film during heat treatment processing is investigated. With the combination of theoretical and experimental studies, this work reveals that the supersaturated point defects in perovskite films generate during the cooling process and its concentration improves as the cooling rate increases. The supersaturated point defects can be minimized through slowing the cooling rate. As a result, the optimized FAPbI3 polycrystalline films achieve a superior carrier lifetime of up to 12.6 µs and improved stability. The champion device delivers a 25.47% PCE (certified 24.7%) and retain 90% of their initial value after >1100 h of operation at the maximum power point. These results provide a fundamental understanding of the mechanisms of grown-in defects formation in polycrystalline perovskite film.

Sustainable Mixed-Halide Perovskite Resistive Switching Memories Using Self-Assembled Monolayers as the Bottom Contact

The complex ionic-electronic conduction in mixed halide perovskites enables their use beyond von Neumann architectures implemented in resistive switching memory devices. Although device fabrication based on perovskite compounds involves solution-processing at low temperatures, reducing further fabrication costs by eliminating expensive materials can improve their compatibility with upscalable deposition techniques. Notably, the substrate on which the perovskite active layer is developed has been reported to severely affect its quality and thus the overall device performance. Hereby, we demonstrate the sustainable manufacturing of memristive perovskite solar cells by replacing the expensive poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) that serves as a hole transporting layer (HTL) with a self-assembled monolayer (SAM), namely [2-(3,6-dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid (MeO-2PACz). Multiple sequential memristive current-voltage characteristics of single devices are reported, and average data of multiple reference and targeted devices are compared. Resistive switching memory devices based on SAM exhibit improved performance having reduced average SET voltage values and narrower statistical variation compared to reference devices with PTAA. It is shown that both PTAA and SAM based devices exhibit high ON/OFF ratio of about 103 operating at low switching electric fields. Replacing an expensive polymer-based HTL with this approach reduces fabrication costs compared to PTAA.

High-entropy alloy screening for halide perovskites

As the concept of high-entropy alloying (HEA) extends beyond metals, new materials screening methods are needed. Halide perovskites (HP) are a prime case study because greater stability is needed for photovoltaics applications, and there are 322 experimentally observed HP end-members, which leads to more than 1057 potential alloys. We screen HEAHP by first calculating the configurational entropy of 106 equimolar alloys with experimentally observed end-members. To estimate enthalpy at low computational cost, we turn to the delta-lattice parameter approach, a well-known method for predicting III-V alloy miscibility. To generalize the approach for non-cubic crystals, we introduce the parameter of unit cell volume coefficient of variation (UCV), which does a good job of predicting the experimental HP miscibility data. We use plots of entropy stabilization versus UCV to screen promising alloys and identify 102 HEAHP of interest.

CO2 Laser Crystallization in Ambient for Highly Efficient FAPbI3 Perovskite Solar Cells

Metal halide perovskite solar cells have achieved tremendous progress and have attracted enormous research and development efforts since the first report of demonstration in 2009. Due to fabrication versatility, many heat treatment methods can be utilized to achieve perovskite film crystallization. Herein, 10.6 µm carbon dioxide laser process is successfully developed for the first time for perovskite film crystallization. In addition, this is the first time formamidinium lead triiodide solar cells by laser annealing under ambient are demonstrated. The champion cell produces a power conversion efficiency of 21.8%, the highest for laser-annealed perovskite cells. And this is achieved without any additive, passivation, or post-treatment.

High-Entropy Design for 2D Halide Perovskite

Hybrid halide perovskites are good candidates for a range of functional materials such as optical electronic and photovoltaic devices due to their tunable band gaps, long carrier diffusion lengths, and solution processability. However, the instability in moisture/air, the toxicity of lead, and rigorous reaction setup or complex postprocessing have long been the bottlenecks for practical application. Herein, we present a simultaneous configurational entropy design at A-sites, B-sites, and X-sites in the typical (CHA)2PbBr4 two-dimensional (2D) hybrid perovskite. Our results demonstrate that the high-entropy effect favors the stabilization of the hybrid perovskite phase and facilitates a simple crystallization process without precise control of the cooling rate to prepare regular crystals. Moreover, high-entropy 2D perovskite crystals exhibit tunable energy band gaps, broadband emission, and a long carrier lifetime. Meanwhile, the high-entropy composition almost maintains the initial crystal structure in deionized water for 18 h while the original (CHA)2PbBr4 crystal mostly decomposes, suggesting obviously improved humidity stability. This work offers a facile approach to synthesize humidity-stable hybrid perovskites under mild conditions, accelerating relevant preparation of optoelectronics and light-emitting devices and facilitating the ultimate commercialization of halide perovskite.

Synergetic Optimization of Upper and Lower Surfaces of the SnO2 Electron Transport Layer for High-Performance n-i-p Perovskite Solar Cells

The SnO2 electron transport layer (ETL) has been recognized as one of the most effective protocols for achieving high-efficiency perovskite solar cells (PSCs). To date, most research has primarily focused on the modification of the upper surface of SnO2 ETL films. The lower surface of the SnO2 film, which directly influences the film formation of solution-processed SnO2, is equally important but receives relatively less attention. Herein, we present a synergetic optimization approach involving the deposition of aluminum oxide (AlOx) via atomic layer deposition (ALD) as a buffer layer and the incorporation of rubidium acetate (RbAc) as an upper surface passivation additive. This process leads to a conformal coating of SnO2 nanoparticles, improved electrical performance, and higher-quality perovskite crystals. As a result, with this composite ETL film, the power conversion efficiency (PCE) reached 22.41 from 20.77%. Further modification with p-butyl iodide (BAI) on the perovskite upper surface increased the champion PCE to 23.32%, with a voltage loss of 0.41 V, ranking among the lowest values for the triple-cation mixed-halide perovskite absorber (1.58 eV). Importantly, the perovskite solar cells remained 87.30% of its initial performance after 14 days of aging and exhibited photostability under long-term UV (254 nm) illumination.

Recent progress of metal halide perovskite materials in heterogeneous photocatalytic organic reactions

Photocatalytic technology is widely regarded as an important way to utilize solar energy and achieve carbon neutrality, which has attracted considerable attentions in various fields over the past decades. Metal halide perovskites (MHPs) are recognized as "superstar" materials due to their exceptional photoelectric properties, readily accessible and tunable structure, which made them intensively studied in solar cells, light-emitting diodes, and solar energy conversion fields. Since 2018, increased attention has been focused on applying the MHPs as a heterogeneous visible light photocatalyst in catalyzing organic synthesis reactions. In this review, we present an overview of photocatalytic technology and principles of heterogeneous photocatalysis before delving into the structural characteristics, stability, and classifications of MHPs. We then focus on recent developments of MHPs in photocatalyzing various organic synthesis reactions, such as oxidation, cyclization, C-C coupling etc., based on their classifications and reported reaction types. Finally, we discuss the main limitations and prospects regarding the application of metal halide perovskites in organic synthesis.

Perovskite Colloidal Nanocrystal Solar Cells: Current Advances, Challenges, and Future Perspectives

The power conversion efficiencies (PCEs) of polycrystalline perovskite (PVK) solar cells (SCs) (PC-PeSCs) have rapidly increased. However, PC-PeSCs are intrinsically unstable without encapsulation, and their efficiency drops during large-scale production; these problems hinder the commercial viability of PeSCs. Stability can be increased by using colloidal PVK nanocrystals (c-PeNCs), which have high surface strains, low defect density, and exceptional crystal quality. The use of c-PeNCs separates the crystallization process from the film formation process, which is preponderant in large-scale fabrication. Consequently, the use of c-PeNCs has substantial potential to overcome challenges encountered when fabricating PC-PeSCs. Research on colloidal nanocrystal-based PVK SCs (NC-PeSCs) has increased their PCEs to a level greater than those of other quantum-dot SCs, but has not reached the PCEs of PC-PeSCs; this inferiority significantly impedes widespread application of NC-PeSCs. This review first introduces the distinctive properties of c-PeNCs, then the strategies that have been used to achieve high-efficiency NC-PeSCs. Then it discusses in detail the persisting challenges in this domain. Specifically, the major challenges and solutions for NC-PeSCs related to low short-circuit current density Jsc are covered. Last, the article presents a perspective on future research directions and potential applications in the realm of NC-PeSCs.

Navigating challenges and solutions for metal-halide and carbon-based electrodes in perovskite solar cells (NCS-MCEPSC): An environmental approach

The urgent need to shift to renewable energy is highlighted by rising global energy use and environmental issues like global warming from fossil fuel dependency. Perovskite solar cells (PSCs) stand out as a promising option, providing high efficiency and potential for cost-effective production. This study delves into the environmental concerns and viable solutions linked with metal-halide PSCs (M-PSCs) and carbon-based electrode PCSs (C-PSCs). It showcases the swift progress in PSC technology, highlighting its potential to deliver efficient and economical renewable energy options. Yet, the environmental implications of these technologies, especially the utilization of toxic lead (Pb) in M-PSCs and the issues of stability and degradation in C-PSCs, represent considerable hurdles for their broad application and sustainability. The paper details the recent advances in PSCs, focusing on enhancements in device efficiency and stability through innovative material combinations and device designs. Nonetheless, the environmental hazards linked to the dispersal of toxic substances from compromised or deteriorating PSCs into the ecosystem raise significant concerns. In particular, the risk of Pb from M-PSCs contaminating soil and aquatic ecosystems is a pressing issue for human and environmental health, spurring investigations into alternative materials and methods to diminish these impacts. The authors examine several strategies, including the introduction of Pb-free perovskites, encapsulation methods to block the escape of hazardous substances, and the recycling of PSC elements. The study stresses the necessity of aligning technological innovations with considerations for the environment and health, calling for ongoing research into PSC technologies that are sustainable and safe. This review highlights the need for detailed assessments of PSC technologies, focusing on their renewable energy contributions, environmental impacts, and strategies to mitigate these effects. The authors call for a cohesive strategy to develop PSCs that are efficient, cost-effective, eco-friendly, and safe for widespread use.

Polaronic Mass Enhancement and Polaronic Excitons in Metal Halide Perovskites

In metal halide perovskites, the complex dielectric screening together with low energy of phonon modes leads to non-negligible Fröhlich coupling. While this feature of perovskites has already been used to explain some of the puzzling aspects of carrier transport in these materials, the possible impact of polaronic effects on the optical response, especially excitonic properties, is much less explored. Here, with the use of magneto-optical spectroscopy, we revealed the non-hydrogenic character of the excitons in metal halide perovskites, resulting from the pronounced Fröhlich coupling. Our results can be well described by the polaronic-exciton picture where electron and hole interactions are no longer described by a Coulomb potential. Furthermore, we show experimental evidence that the carrier-phonon interaction leads to the enhancement of the carrier's effective mass. Notably, our measurements reveal a pronounced temperature dependence of the carrier's effective mass, which we attribute to a band structure renormalization induced by the population of low-energy phonon modes. This interpretation finds support in our first-principles calculations.

Top-Down Induced Crystallization Orientation toward Highly Efficient p-i-n Perovskite Solar Cells

Crystallization orientation plays a crucial role in determining the performance and stability of perovskite solar cells (PVSCs), whereas effective strategies for realizing oriented perovskite crystallization is still lacking. Herein, a facile and efficient top-down strategy is reported to manipulate the crystallization orientation via treating perovskite wet film with propylamine chloride (PACl) before annealing. The PA+ ions tend to be adsorbed on the (001) facet of the perovskite surface, resulting in the reduced cleavage energy to induce (001) orientation-dominated growth of perovskite film and then reduce the temperature of phase transition, meanwhile, the penetrating Cl ions further regulate the crystallization process. As-prepared (001)-dominant perovskite films exhibit the ameliorative film homogeneity in terms of vertical and horizontal scale, leading to alleviated lattice mismatch and lowered defect density. The resultant PVSC devices deliver a champion power conversion efficiency (PCE) of 25.07% with enhanced stability, and the unencapsulated PVSC device maintains 95% of its initial PCE after 1000 h of operation at the maximum power point under simulated AM 1.5G illumination.

Instability of Formamidinium Lead Iodide (FAPI) Deposited on a Copper Oxide Hole Transporting Layer (HTL)

Copper oxide appears to be a promising candidate for a hole transport layer (HTL) in emerging perovskite solar cells. Reasons for this are its good optical and electrical properties, cost-effectiveness, and high stability. However, is this really the case? In this study, we demonstrate that copper oxide, synthesized by a spray-coating method, is unstable in contact with formamidinium lead triiodide (FAPI) perovskite, leading to its decomposition. Using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and ultraviolet-visible (UV-vis) spectrophotometry, we find that the entire copper oxide diffuses into and reacts with the FAPI film completely. The reaction products are an inactive yellow δ-FAPI phase, copper iodide (CuI), and an additional new phase of copper formate hydroxide (CH2CuO3) that has not been reported previously in the literature.

High-throughput calculation for the screening of formamidinium halide perovskite for solar cells

128 organic halide perovskites are systematically investigated using high-throughput first principles calculations where Ge and Sn-based materials are searched. The results revealed that all calculated materials exhibited exothermic reactions. Notably, a correlation between the heat of formation and X-site ions is identified. Six specific compounds, namely FA-Ge-I-I-I, FA-Sn-F-I-I, FA-Sn-Cl-I-I, FA-Sn-Br-Br-I, FA-Sn-Br-I-I, and FA-Sn-I-I-I, where FA stands for formamidinium, are found to have a bandgap ranging from 1.0 to 2.0 eV, characterized by a direct bandgap in their band structure. Electronic structure analysis indicated that the CBM (conduction band minimum) is influenced by the B-site p-orbital, while the VBM (valence band maximum) is influenced by the X-site p-orbitals. This study underscores the capability of high-throughput calculations to unveil hidden trends in perovskite materials, offering an effective approach for the exploration of promising perovskite materials.

Defects and Defect Passivation in Perovskite Solar Cells

Perovskite solar cells have made significant strides in recent years. However, there are still challenges in terms of photoelectric conversion efficiency and long-term stability associated with perovskite solar cells. The presence of defects in perovskite materials is one of the important influencing factors leading to subpar film quality. Adopting additives to passivate defects within perovskite materials is an effective approach. Therefore, we first discuss the types of defects that occur in perovskite materials and the mechanisms of their effect on performance. Then, several types of additives used in perovskite solar cells are discussed, including ionic compounds, organic molecules, polymers, etc. This review provides guidance for the future development of more sustainable and effective additives to improve the performance of solar cells.

Mechanical Durability and Flexibility in Perovskite Photovoltaics: Advancements and Applications

The remarkable progress in perovskite solar cell (PSC) technology has witnessed a remarkable leap in efficiency within the past decade. As this technology continues to mature, flexible PSCs (F-PSCs) are emerging as pivotal components for a wide array of applications, spanning from powering portable electronics and wearable devices to integrating seamlessly into electronic textiles and large-scale industrial roofing. F-PSCs characterized by their lightweight, mechanical flexibility, and adaptability for cost-effective roll-to-roll manufacturing, hold immense commercial potential. However, the persistent concerns regarding the overall stability and mechanical robustness of these devices loom large. This comprehensive review delves into recent strides made in enhancing the mechanical stability of F-PSCs. It covers a spectrum of crucial aspects, encompassing perovskite material optimization, precise crystal grain regulation, film quality enhancement, strategic interface engineering, innovational developed flexible transparent electrodes, judicious substrate selection, and the integration of various functional layers. By collating and analyzing these dedicated research endeavors, this review illuminates the current landscape of progress in addressing the challenges surrounding mechanical stability. Furthermore, it provides valuable insights into the persistent obstacles and bottlenecks that demand attention and innovative solutions in the field of F-PSCs.

Crystallization Kinetics of Hybrid Perovskite Solar Cells

Metal halide perovskites (MHPs) are considered ideal photovoltaic materials due to their variable crystal material composition and excellent photoelectric properties. However, this variability in composition leads to complex crystallization processes in the manufacturing of Metal halide perovskite (MHP) thin films, resulting in reduced crystallinity and subsequent performance loss in the final device. Thus, understanding and controlling the crystallization dynamics of perovskite materials are essential for improving the stability and performance of PSCs (Perovskite Solar Cells). To investigate the impact of crystallization characteristics on the properties of MHP films and identify corresponding modulation strategies, we primarily discuss the relevant aspects of MHP crystallization kinetics, systematically summarize theoretical methods, and outline modulation techniques for MHP crystallization, including solution engineering, additive engineering, and component engineering, which helps highlight the prospects and current challenges in perovskite crystallization kinetics.

Achieving Well-Oriented FAPbI3 Perovskite Photovoltaics by Cyclohexane Modification

It is essential and challenging to develop green and cost-effective solar cells to meet the energy demands. Solar cells with a perovskite light-harvesting layer are the most promising technology to propel the world toward next-generation solar energy. Formamidinium lead tri-iodide (FAPbI3)-based perovskite solar cells (F-PSCs), with their considerable performance, offer cost-effective solar cells. One of the major issues that the PSC community is now experiencing is the stability of α-FAPbI3 at relatively low temperatures. In this study, we fabricated FAPbI3-PSCs using cyclohexane (CHX) material via a two-step deposition method. For this purpose, CHX is added to the formamidinium iodide:methylammonium chloride (FAI:MACl) solution as an additive and used to form a better FAPbI3 layer by controlling the reaction between FAI and lead iodide (PbI2). The CHX additive induces the reaction of undercoordinated Pb2+ with FAI material and produces an α-FAPbI3 layer with low charge traps and large domains. In addition, the CHX-containing FAPbI3 layers show higher carrier lifetimes and facilitate carrier transfer in F-PSCs. The CHX-modified F-PSCs yield a high champion efficiency of 22.84% with improved ambient and thermal stability behavior. This breakthrough provides valuable findings regarding the formation of a desirable FAPbI3 layer for photovoltaic applications and holds promise for the industrialization of F-PSCs.

Co-Self-Assembled Monolayers Modified NiOx for Stable Inverted Perovskite Solar Cells

[4-(3,6-dimethyl-9H-carbazol-9yl)butyl]phosphonic acid (Me-4PACz) self-assembled molecules (SAM) are an effective method to solve the problem of the buried interface of NiOx in inverted perovskite solar cells (PSCs). However, the Me-4PACz end group (carbazole core) cannot forcefully passivate defects at the bottom of the perovskite film. Here, a Co-SAM strategy is employed to modify the buried interface of PSCs. Me-4PACz is doped with phosphorylcholine chloride (PC) to form a Co-SAM to improve the monolayer coverage and reduce leakage current. The phosphate group and chloride ions (Cl-) in PC can inhibit NiOx surface defects. Meantime, the quaternary ammonium ions and Cl- in PC can fill organic cations and halogen vacancies in the perovskite film to enable defects passivation. Moreover, Co-SAM can promote the growth of perovskite crystals, collaboratively solve the problem of buried defects, suppress nonradiative recombination, accelerate carrier transmission, and relieve the residual stress of the perovskite film. Consequently, the Co-SAM modified devices show power conversion efficiencies as high as 25.09% as well as excellent device stability with 93% initial efficiency after 1000 h of operation under one-sun illumination. This work demonstrates the novel approach for enhancing the performance and stability of PSCs by modifying Co-SAM on NiOx.

Phase Transitions and Dynamics in Mixed Three- and Low-Dimensional Lead Halide Perovskites

Lead halide perovskites are extensively investigated as efficient solution-processable materials for photovoltaic applications. The greatest stability and performance of these compounds are achieved by mixing different ions at all three sites of the APbX3 structure. Despite the extensive use of mixed lead halide perovskites in photovoltaic devices, a detailed and systematic understanding of the mixing-induced effects on the structural and dynamic aspects of these materials is still lacking. The goal of this review is to summarize the current state of knowledge on mixing effects on the structural phase transitions, crystal symmetry, cation and lattice dynamics, and phase diagrams of three- and low-dimensional lead halide perovskites. This review analyzes different mixing recipes and ingredients providing a comprehensive picture of mixing effects and their relation to the attractive properties of these materials.

Synergetic interfacial conductivity modulation dictating hysteresis evolution in perovskite solar cells under operation

In this work, the configuration of compact TiO2 coating (c-TiO2) interface as electron transport layer (ETL) in giving rise to loss and gain of fill factor (FF) and therefore modulation of hysteresis behavior in perovskite solar cells (PSCs) is investigated. For this purpose, PSCs based on planar compact TiO2 (c-TiO2) as well as a scaffold-based architecture are studied. In the latter case c-TiO2 coats a hydrothermally grown titania nanorod scaffold. The results demonstrate that when c-TiO2 is used in planar configuration, FF considerably improves with prolonged light soaking which is in sharp contrast to what is observed for scaffold-based PSCs. Moreover, higher thickness of planar c-TiO2 is shown to be beneficial for sustaining FF in forward scan. Finally, through studying the intricate interfacial dynamics utilizing electrochemical impedance spectroscopy (EIS), it was concluded that for a PSC under operation, the cumulative effect of conductivity modulation at the perovskite with transport layer interfaces, for their respective charge carriers, determines the loss and gain in performance depending on scan rate, applied bias and prolonged light soaking. This work points towards multiple factors affecting PSC output, which could work either in confluence or against one another depending on the interfacial configuration of transport layers.

Performance Enhancement of Tin-Based Perovskite Photodetectors through Bifunctional Cesium Fluoride Engineering

Tin halide perovskites are rising as promising candidates for next-generation optoelectronic materials due to their good optoelectronic properties and relatively low toxicity. However, the high defect density and the easy oxidation of Sn2+ have limited their optoelectronic performance. Herein, we report the treatment of the FASnI3 (formamidinium tin, FA) perovskite film by a bifunctional cesium fluoride (CsF) additive, which improves the film quality and significantly enhances the photoelectric performance. The responsivity of the perovskite-based photodetector (PD) with an optimal CsF concentration of 15% is over 60 times larger than that of the PD without CsF. It indicates that both the Cs substitution and the fluoride anion additive from CsF inhibit the oxidation of Sn2+, optimize the crystal growth, and passivate the defects, demonstrating the dual roles of the CsF additive in improving the photoelectric performance. This work offers valuable insights into the additive selection for developing high-quality tin-based perovskite films and devices.

First-Principles Investigation into the Interaction of H2O with α-CsPbI3 and the Intrinsic Defects within It

CsPbI3 possesses three photoactive black phases (α, β, and γ) with perovskite structures and a non-photoactive yellow phase (δ) without a perovskite structure. Among these, α-CsPbI3 exhibits the best performance. However, it only exists at high temperatures and it tends to transform into the δ phase at room temperature, especially in humid environments. Therefore, the phase stability of CsPbI3, especially in humid environments, is the main obstacle to its further development. In this study, we studied the interaction of H2O with α-CsPbI3 and the intrinsic defects within it. It was found that the adsorption energy in the bulk is higher than that on the surface (-1.26 eV in the bulk in comparison with -0.60 eV on the surface); thus, H2O is expected to have a tendency to diffuse into the bulk once it adsorbs on the surface. Moreover, the intrinsic vacancy of VPb0 in the bulk phase can greatly promote H2O insertion due to the rearrangement of two I atoms in the two PbI6 octahedrons nearest to VPb0 and the resultant breaking of the Pb-I bond, which could promote the phase transition of α-CsPbI3 in a humid environment. Moreover, H2O adsorption onto VI+1 contributes to a further distortion in the vicinity of VI+1, which is expected to enhance the effect of VI+1 on the phase transition of α-CsPbI3. Clarifying the interaction of H2O with α-CsPbI3 and the intrinsic defects within it may provide guidance for further improvements in the stability of α-CsPbI3, especially in humid environments.

Toward stabilization of formamidinium lead iodide perovskites by defect control and composition engineering

Phase instability poses a serious challenge to the commercialization of formamidinium lead iodide (FAPbI3)-based solar cells and optoelectronic devices. Here, we combine density functional theory and machine learning molecular dynamics simulations, to investigate the mechanism driving the undesired α-δ phase transition of FAPbI3. Prevalent iodine vacancies and interstitials can significantly expedite the structural transition kinetics by inducing robust covalency during transition states. Extrinsically, the detrimental roles of atmospheric moisture and oxygen in degrading the FAPbI3 perovskite phase are also rationalized. Significantly, we discover the compositional design principles by categorizing that A-site engineering primarily governs thermodynamics, whereas B-site doping can effectively manipulate the kinetics of the phase transition in FAPbI3, highlighting lanthanide ions as promising B-site substitutes. A-B mixed doping emerges as an efficient strategy to synergistically stabilize α-FAPbI3, as experimentally demonstrated by substantially higher initial optoelectronic characteristics and significantly enhanced phase stability in Cs-Eu doped FAPbI3 as compared to its Cs-doped counterpart. This study provides scientific guidance for the design and optimization of long-term stable FAPbI3-based solar cells and other optoelectronic devices through defect control and synergetic composition engineering.

Polymeric Metal Halides with Bright Luminescence and Versatile Processability

Most of current metal halide materials, including all inorganic and organic-inorganic hybrids, are crystalline materials with poor workability and plasticity that limit their application scope. Here, we develop a novel class of materials termed polymeric metal halides (PMHs) through introducing polycations into antimony-based metal halide materials as A-site cations. A series of PMHs with orange-yellow broadband emission and large Stokes shift originating from inorganic self-trapped excitons are successfully prepared, which meanwhile exhibit the excellent processability and formability of polymers. The versatility of these PMHs is manifested as the broad choices of polycations, the ready extension to manganese- and copper-based halides, and the tolerance to molar ratios between polycations and metal halides in the formation of PMHs. The merger of polymer chemistry and inorganic chemistry thus provides a novel generic platform for the development of metal halide functional materials.

Strain Regulation and Defect Passivation of FA-Based Perovskite Materials for Highly Efficient Solar Cells

Formamidine lead triiodide (FAPbI3 ) perovskites have attracted increasing interest for photovoltaics attributed to the optimal bandgap, high thermal stability, and the record power conversion efficiency (PCE). However, the materials still face several key challenges, such as phase transition, lattice defects, and ion migration. Therefore, external ions (e.g., cesium ions (Cs+ )) are usually introduced to promote the crystallization and enhance the phase stability. Nevertheless, the doping of Cs+ into the A-site easily leads to lattice compressive strain and the formation of pinholes. Herein, trioctylphosphine oxide (TOPO) is introduced into the precursor to provide tensile strain outside the perovskite lattice through intermolecular forces. The special strain compensation strategy further improves the crystallization of perovskite and inhibits the ion migration. Moreover, the TOPO molecule significantly passivates grain boundaries and undercoordinated Pb2+ defects via the forming of P═O─Pb bond. As a result, the target solar cell devices with the synergistic effect of Cs+ and TOPO additives have achieved a significantly improved PCE of 22.71% and a high open-circuit voltage of 1.16 V (voltage deficit of 0.36 V), with superior stability under light exposure, heat, or humidity conditions.

Unraveling the Mechanism of Alkali Metal Fluoride Post-Treatment of SnO2 for Efficient Planar Perovskite Solar Cells

The facile synthesis and beneficial properties of tin oxide have driven the development of efficient planar perovskite solar cells (PSCs). To increase the PSC performance, alkali salts are used to treat the SnO2 surface to minimize the defect states. However, the underlying mechanism of alkali cations' role in the PSCs needs further exploration. Herein the effect of alkali fluoride salts (KF, RbF, and CsF) on the properties of SnO2 and PSC performance is investigated. The results show different alkali have significant roles depending on their nature. Larger cations Cs+ preferably locate at the SnO2 film surface to passivate surface defects and enhance conductivity, while smaller cations like Rb+ or K+ cations tend to diffuse into the perovskite layer to reduce trap density of the material. The former effect leads to enhanced fill factor while the latter effect increases the open circuit voltage of the device. It is then demonstrated that a dual cation post-treatment of the SnO2 layer with RbF and CsF achieves PSC with a significantly higher power conversion efficiency (PCE) of 21.66% compared to pristine PSC with a PCE of 19.71%. This highlights the significance of defect engineering of SnO2 using selective multiple alkali treatment to improve PSC performance.

Metal complex as p-type dopant-based organic spiro-OMeTAD hole-transporting material for free-Li-TFSI perovskite solar cells

Lithium bis(fluorosulfonyl)imide (Li-TFSI) is an efficient p-dopant that has been used to enhance the conductivity of perovskite solar cells (PSCs). However, the performance of the corresponding devices is still not satisfactory due to the impact of Li-TFSI on the fill factor and the short-circuit current density of these PSCs. Herein, a new Mn complex [(Mn(Me-tpen)(ClO4)2-)]2+ was introduced as a p-type dopant into spiro-OMeTAD and was successfully applied as a hole transport material (HTM) for PSCs. Analytical studies used for device characterization included scanning electron microscopy, UV-Vis spectroscopy, current-voltage (IV) characteristics, incident photon to current efficiency, power conversion efficiency (PCE), and electrochemical impedance spectroscopy. The UV-Vis spectra displayed oxidation in the HTM by the addition of a dopant. Moreover, the movement of electrons from the higher orbital of the spiro-OMeTAD to the dopant stimulates the generation of the hole carriers in the HTM, enhancing its conductivity with outstanding long-term stability under mild conditions in a humid (RH ∼ 30%) environment. The incorporation of the Mn complex into the composite improved the material's properties and the stability of the fabricated devices. The Mn complex as a p-type dopant for spiro-OMeTAD exhibits a perceptible PCE of 16.39% with an enhanced conductivity of 98.13%. This finding may pave a rational way for developing efficient and stable PSCs in real environments.

Effects of MgF2anti-reflection coating on optical losses in metal halide perovskite solar cells

The importance of light management for perovskite solar cells (PSCs) has recently been emphasized because their power conversion efficiency approaches their theoretical thermodynamic limits. Among optical strategies, anti-reflection (AR) coating is the most widely used method to reduce reflectance loss and thus increase light-harvesting efficiency. Monolayer MgF2is a well-known AR material because of its optimal refractive index, simple fabrication process, and physical and chemical durabilities. Nevertheless, quantitative estimates of the improvement achieved by the MgF2AR layer are lacking. In this study, we conducted theoretical and experimental evaluations to assess the AR effect of MgF2on the performance of formamidinium lead-triiodide PSCs. A sinusoidal tendency to enhance the short-circuit current density (JSC) was observed depending on the thickness, which was attributed to the interference of the incident light. A transfer matrix method-based simulation was conducted to calculate the optical losses, demonstrating the critical impact of reflectance loss on theJSCimprovement. The predictedJSCs values, depending on the perovskite thickness and the incident angle, are also presented. The combined use of experimental and theoretical approaches offers notable advantages, including accurate interpretation of photocurrent generation, detailed optical analysis of the experimental results, and device performance predictions under unexplored conditions.

Computational Modulation in Electronic Structures of Halide Perovskites via Element/Dopant/Phase

This study employs computational chemistry to investigate the electronic properties of halide perovskite materials, focusing on structural frameworks, elemental composition, surface engineering, and defect engineering. The tetragonal phase generally exhibits higher band gaps than the cubic phase due to conduction band differences, with LiPbCl3 showing the greatest band gap difference. The ionic radius of the A element influences band gaps for both phases, with Cs having the highest impact. Surface engineering significantly affects the electronic properties, and surface direction and composition play vital roles in determining band gaps. Defect engineering induces semiconducting-to-metallic transitions, impacting band gaps. Understanding these core variables is crucial for tailoring the electronic properties of halide perovskites for photovoltaic and optoelectronic applications.

Towards Long-Term Stable Perovskite Solar Cells: Degradation Mechanisms and Stabilization Techniques

It is certain that perovskite materials must be a game-changer in the solar industry as long as their stability reaches a level comparable with the lifetime of a commercialized Si photovoltaic. However, the operational stability of perovskite solar cells and modules still remains unresolved, especially when devices operate in practical energy-harvesting modes represented by maximum power point tracking under 1 sun illumination at ambient conditions. This review article covers from fundamental aspects of perovskite instability including chemical decomposition pathways under light soaking and electrical bias, to recent advances and techniques that effectively prevent such degradation of perovskite solar cells and modules. In particular, fundamental causes for permanent degradation due to ion migration and trapped charges are overviewed and explain their interplay between ions and charges. Based on the degradation mechanism, recent advances on the strategies are discussed to slow down the degradation during operation for a practical use of perovskite-based solar devices.

Suppressed Ion Migration in FA-Rich Perovskite Photovoltaics through Enhanced Nucleation of Encapsulation Interface

With excellent homogeneity, compactness and controllable thickness, atomic layer deposition (ALD) technology is widely used in perovskite solar cells (PSCs). However, residual organic sources and undesired reactions pose serious challenges to device performance as well as stability. Here, ester groups of poly(ethylene-co-vinyl acetate) are introduced as a reaction medium to promote the nucleation and complete conversion of tetrakis(dimethylamino)tin(IV) (TDMA-Sn). Through simulations and experiments, it is verified that ester groups as Lewis bases can coordinate with TDMA-Sn to facilitate homogeneous deposition of ALD-SnOx , which acts as self-encapsulated interface with blocking properties against external moisture as well as internal ion migration. Meanwhile, a comprehensive evaluation of the self-encapsulated interface reveals that the energy level alignment is optimized to improve the carrier transport. Finally, the self-encapsulated device obtains a champion photovoltaic conversion efficiency (PCE) of 22.06% and retains 85% of the initial PCE after being stored at 85 °C with relative humidity of 85% for more than 800 h.

Phase Separation of Br-Doped CsPbI3: A Combined Cluster Expansion, Monte Carlo, and DFT Study

Cluster expansion, which is a method that describes the concentration-dependent thermodynamic properties of materials while maintaining density functional theory accuracy, was used to predict new (CsPbIxBr1-x) structures. The cluster-expansion method generated 42 new stable (CsPb)xIyBrz (where x = 1 to 3 and y and z = 1 to 8) structures and these were ranked as meta-stable structures based on their enthalpies of formation. Monte Carlo calculations showed that CsPbI0.5Br0.5 composition separates into different phases at 300 K, but changes to a homogeneous phase at 700 K, suggesting that a different phase of CsPbI3 may exist at higher temperatures. Among the 42 predicted structures, randomly selected structures around iodide-rich, 50:50, and bromine-rich sites were studied further by determining their electronic, optical, mechanical, and thermodynamic properties using first-principle density functional theory. The materials possess similar properties as cubic Br-doped CsPbI3 perovskites. The mechanical properties of these compounds revealed that they are ductile in nature and mechanically stable. This work suggests that the introduction of impurities into CsPbI3 perovskite materials, as well as compositional engineering, can alter the electronic and optical properties, making them potential candidates for solar cell applications.

Enhancing the Efficiency and Stability of Perovskite Solar Cells through Defect Passivation and Controlled Crystal Growth Using Allantoin

In this study, we present a robust approach that concurrently manages crystal growth and defect passivation within the perovskite layer through the introduction of a small molecule additive─allantoin. The precise regulation of crystal growth in the presence of allantoin yields perovskite films characterized by enhanced morphology, larger grain size, and improved grain orientation. Notably, the carbonyl and amino groups present in allantoin passivate under-coordinated Pb2+ and I- defects, respectively, through molecular interactions. Trap density in the perovskite layer is measured, and it is 0.39 × 1016 cm-3 for the allantoin-incorporated device and 0.83 × 1016 cm-3 for the pristine device. This reduction in defects leads to reduced trap-assisted nonradiative recombination, as confirmed by the photoluminescence, transient photo voltage, and impedance measurements. As a result, when these allantoin-incorporated perovskite films are implemented as the active layer in solar cells, a noteworthy efficiency enhancement to 20.63% is attained, surpassing the 18.04% of their pristine counterparts. Furthermore, devices with allantoin exhibit remarkable operational stability, maintaining 80% of their efficiency even after 500 h of continuous illumination, whereas the pristine device degraded to 65% of its initial efficiency in 400 h. Also, allantoin-incorporated devices exhibited exceptional stability against high humidity and elevated temperatures.

Boosting photodetection performance of Cs2AgBiBr6 through A-site Rb substitution and interfacial engineering

Bismuth-based double perovskite Cs2AgBiBr6 shows promise as a photodetection material. However, its detection performance and application are limited by high-exciton binding energy and poor carrier mobility. In this study, we address these limitations by delicately designing a solution-based method for incorporating A-site Rubidium (Rb) substitution into Cs2AgBiBr6 double perovskite films. The introduction of Rb resulted in a significant decrease in trap defect density and an improvement in film quality. The trap-filled limit voltage (VTFL) of pure and Rb-doped CABB film is determined to be 1.71 V and 0.48 V, respectively. Subsequently, by introducing an ultrathin atomic-layer-deposited (ALD) TiO2 films, the fabricated CABB photodetectors exhibit significantly improved photoresponse performance. The response speed and -3dB bandwidth are boosted from ∼93 ms to ∼350 μs and broadened from 1.4 kHz to 17 kHz, respectively. Density Functional Theory (DFT) calculations indicate Rb-substitution shortens the bond length and weaken exciton binding energy. Furthermore, we demonstrate a wireless near ultraviolet (UV) light communication system using CABB photodetectors as light receivers. Our findings provide an efficient approach to utilize A-site cation substitution as a tuning parameter for photodetection in high-exciton binding energy perovskite materials, thereby extending the potential applications of other functional perovskites.

Influence of an Organic Salt-Based Stabilizing Additive on Charge Carrier Dynamics in Triple Cation Perovskite Solar Cells

Besides further improvement in the power conversion efficiency (PCE) of perovskite solar cells (PSC), their long-term stability must also be ensured. Additives such as organic cations with halide counter anions are considered promising candidates to address this challenge, conferring both higher performance and increased stability to perovskite-based devices. Here, a stabilizing additive (N,N-dimethylmethyleneiminium chloride, [Dmmim]Cl) is identified, and its effect on charge carrier mobility and lifetime under thermal stress in triple cation perovskite (Cs0.05 MA0.05 FA0.90 PbI3 ) thin films is investigated. To explore the fundamental mechanisms limiting charge carrier mobility, temperature-dependent microwave conductivity measurements are performed. Different mobility behaviors across two temperature regions are revealed, following the power law Tm , indicating two different dominant scattering mechanisms. The low-temperature region is assigned to charge carrier scattering with polar optical phonons, while a strong decrease in mobility at high temperatures is due to dynamic disorder. The results obtained rationalize the improved stability of the [Dmmim]Cl-doped films and devices compared to the undoped reference samples, by limiting temperature-activated mobile ions and retarding degradation of the perovskite film.

Predicting Distortion Magnitudes in Prussian Blue Analogues

Based on simple electrostatic and harmonic potential considerations, we derive a straightforward expression linking the composition of a Prussian blue analogue (PBA) to its propensity to undergo collective structural distortions. We demonstrate the existence of a threshold value, below which PBAs are undistorted and above which PBAs distort by a degree that is controlled by a geometric tolerance factor. Our analysis rationalizes the presence, absence, and magnitude of distortions in a wide range of PBAs and distinguishes their structural chemistry from that of other hybrid perovskites.

Raman Spectroscopy of Formamidinium-Based Lead Mixed-Halide Perovskite Bulk Crystals

In recent years, there has been an impressively fast technological progress in the development of highly efficient lead halide perovskite solar cells. Nonetheless, the stability of perovskite films and associated solar cells remains a source of uncertainty and necessitates sophisticated characterization techniques. Here, we report low- to mid-frequency resonant Raman spectra of formamidinium-based lead mixed-halide perovskites. The assignment of the different Raman lines in the measured spectra is assisted by DFT simulations of the Raman spectra of suitable periodic model systems. An important result of this work is that both experiment and theory point to an increase of the stability of the perovskite structure with increasing chloride doping concentration. In the Raman spectra, this is reflected by the appearance of new lines due to the formation of hydrogen bonds. Thus, higher chloride doping results in less torsional motion and lower asymmetric bending contributing to higher stability. This study yields a solid basis for the interpretation of the Raman spectra of formamidinium-based mixed-halide perovskites, furthering the understanding of the properties of these materials, which is essential for their full exploitation in solar cells.

Stable and High-Efficiency Perovskite Solar Cells Using Effective Additive Ytterbium Fluoride

With better light utilization, larger tolerance factor, and higher power conversion efficiency (PCE), the HC(NH2 )2 + (FA)-based perovskite is proven superior to the popular CH3 NH3 + (MA)- and Cs-based halide perovskites in solar cell applications. Unfortunately, limited by intrinsic defects within the FA-based perovskite films, the perovskite films can be easily transformed into a yellow δ-phase at room temperature in the fabrication process, a troublesome challenge for its further development. Here, ytterbium fluoride (YbF3 ) is introduced into the perovskite precursor for three objectives. First of all, the partial substitution of Yb3+ for Pb2+ in the perovskite lattice increases the tolerance factor of the perovskite lattice and facilitates the formation of the α phase. Second, YbF3 and DMSO in the solvent form a Lewis acid complex YbF3 ·DMSO, which can passivate the perovskite film, reduce defects, and improve device stability. Consequently, the YbF3 modified Perovskite solar cell exhibits a champion conversion efficiency of 24.53% and still maintains 90% of its initial efficiency after 60 days of air exposure under 30% relative humidity.

Extrinsic Interstitial Ions in Metal Halide Perovskites: A Review

Perovskite solar cells have rapidly developed as a promising technology for the next generation of low-cost photovoltaics, receiving enormous attention from researchers and industries. Compared to traditional semiconducting materials, metal halide perovskite exhibits outstanding tolerance to extrinsic ions. At a certain range of doping concentration, the interstitial occupancy of extrinsic ions provides appealing benefits to the perovskite films, contributing to higher performance and stability of the devices. This review summarizes the research progress of interstitial ions for metal halide perovskite, providing insights into the mechanism and identification of interstitial doping of extrinsic ions, covering the benefits of interstitial ions in regulating crystal growth, inhibiting ion migration, and reducing defect density. Finally, based on the latest progress and findings, further topics and directions of research on interstitial ions in metal halide perovskite are proposed to advance the understanding of interstitial ions in perovskite and promote the development of perovskite photovoltaic technology.