Interfacial Study To Suppress Charge Carrier Recombination for High Efficiency Perovskite Solar Cells

Influence of Donor Skeleton on Intramolecular Electron Transfer Amount for Efficient Perovskite Solar Cells

The passivation of the defects derived from rapid-crystallization with electron-donating molecules is always a prerequisite to obtain desirable perovskite films for efficient and stable solar cells, thus, the in-depth understanding on the correlations between molecular structure and passivation capacity is of great importance for screening passivators. Here, we introduce the double-ended amide molecule into perovskite precursor solution to modulate crystallization process and passivate defects. By regulating the intermediate bridging skeletons with alkyl, alkenyl and benzene groups, the results show the passivation strength highly depends on the spin-state electronic structure that serves as an intrinsic descriptor to determine the intramolecular charge distribution by controlling orbital electron transfer from the donor segment to acceptor segment. Upon careful optimization, the benzene-bridged amide molecule demonstrates superior efficacy on improving perovskite film quality. As a physical proof-of-concept, the carbon-based, all-inorganic CsPbI2Br solar cell delivers a significantly increased efficiency of 15.51 % with a remarkably improved stability. Based on the same principle, a champion efficiency of 24.20 % is further obtained on the inverted (Cs0.05MA0.05FA0.9)Pb(I0.93Br0.07)3 solar cell. These findings provide new fundamental insights into the influence of spin-state modulation on effective perovskite solar cells.

Emerging Trends in Electron Transport Layer Development for Stable and Efficient Perovskite Solar Cells

Perovskite solar cells (PSCs) stand at the forefront of photovoltaic research, with current efficiencies surpassing 26.1%. This review critically examines the role of electron transport materials (ETMs) in enhancing the performance and longevity of PSCs. It presents an integrated overview of recent advancements in ETMs, like TiO2, ZnO, SnO2, fullerenes, non-fullerene polymers, and small molecules. Critical challenges are regulated grain structure, defect passivation techniques, energy level alignment, and interfacial engineering. Furthermore, the review highlights innovative materials that promise to redefine charge transport in PSCs. A detailed comparison of state-of-the-art ETMs elucidates their effectiveness in different perovskite systems. This review endeavors to inform the strategic enhancement and development of n-type electron transport layers (ETLs), delineating a pathway toward the realization of PSCs with superior efficiency and stability for potential commercial deployment.

Eco-Friendly Boost for Perovskite Photovoltaics: Harnessing Cellulose-Modified SnO2 as a High-Performance Electron Transporting Material

In this study, a passivated tin oxide (SnO2) film is successfully obtained through the implementation of sodium carboxymethyl cellulose (Na-CMC) modifier agent and used as the electron transporting layer (ETL) within the assembly of perovskite solar cells (PSCs). The strategic incorporation of the Na-CMC modifier agent yields discernible enhancements in the optoelectronic properties of the ETL. Among the fabricated cells, the champion cell based on Na-CMC-complexed SnO2 ETL achieves a conversion efficiency of 22.2% with an open-circuit voltage (VOC) of 1.12 V, short-circuit current density (JSC) of 24.57 mA/cm2, and fill factor (FF) of 80.6%. On the other hand, these values are measured for the pristine SnO2 ETL-based control cell as VOC = 1.11 V, JSC = 23.59 mA/cm2, and FF = 76.7% with an efficiency of 20.1%. This improvement can be ascribed to the high charge extraction ability, higher optical transmittance, better conductivity, and decrease in the trap state density associated with the passivated ETL structure. In addition, the cells employing Na-CMC-complexed SnO2 ETL exhibit prolonged stability under ambient conditions during 2000 h. Based on the preliminary results, this study also presents a set of findings that could have substantial implications for the potential use of the Na-CMC molecule in both large-scale perovskite cells and perovskite/Si tandem configuration.

Characterizing Spatial and Energetic Distributions of Trap States Toward Highly Efficient Perovskite Photovoltaics

Due to their greater opt electric performance, perovskite photovoltaics (PVs) present huge potential to be commercialized. Perovskite PV's high theoretical efficiency expands the available development area. The passivation of defects in perovskite films is crucial for approaching the theoretical limit. In addition to creating efficient passivation techniques, it is essential to direct the passivation approach by getting precise and real-time information on the trap states through measurements. Therefore, it is necessary to establish quantitative characterization methods for the trap states in energy and 3D spaces. The authors cover the characterization of the spatial and energy distributions of trap states in this article with an eye toward high-efficiency perovskite photovoltaics. After going over the strategies that have been created for characterizing and evaluating trap states, the authors will concentrate on how to direct the creative development of characterization techniques for trap states assessment and highlight the opportunities and challenges of future development. The 3D space and energy distribution mappings of trap states are anticipated to be realized. The review will give key guiding importance for further approaching the theoretical efficiency of perovskite photovoltaics, offering some future research direction and technological assistance for the development of appropriate targeted passivation technologies.

Cs2TiI6 (Cs2TiIxBr6-x) Halide Perovskite Solar Cell and Its Point Defect Analysis

This work presents a comprehensive numerical study for designing a lead-free, all-inorganic, and high-performance solar cell based on Cs₂TiI₆ halide perovskite with all-inorganic carrier transport layers. A rigorous ab initio density-functional theory (DFT) calculation is performed to identify the electronic and optical properties of Cs₂TiI₆ and, upon extraction of the existing experimental data of the material, the cell is designed and optimized to the degree of practical feasibility. Consequently, a theoretical power conversion efficiency (PCE) of 21.17% is reported with inorganic TiO₂ and CuI as carrier transport layers. The calculated absorption coefficient of Cs₂TiI₆ reveals its enormous potential as an alternative low-bandgap material for different solar cell applications. Furthermore, the role of different point defects and the corresponding defect densities on cell performance are investigated. It is found that the possible point defects in Cs₂TiI₆ can form both the shallow and deep defect states, with deep defect states having a prominent effect on cell performance. For both defect states, the cell performance deteriorates significantly as the defect density increases, which signifies the importance of high-quality material processing for the success of Cs₂TiI₆-based perovskite solar cell technology.

Morphology-Dependent Charge Carrier Dynamics and Ion Migration Behavior of CsPbBr3 Halide Perovskite Quantum Dot Films

Exceptional electronic, optoelectronic, and sensing properties of inorganic Cs-based perovskites are significantly influenced by the defect chemistry of the material. Although organic halide perovskites that have a polycrystalline structure are heavily studied, understanding of the defect properties at the grain boundaries (GB) of inorganic Cs-based perovskite quantum dots (QDs) is still limited. Here, morphology-dependent charge carrier dynamics of CsPbBr₃ quantum dots at the nanoscale by performing scanning probe microscopy of thermally treated samples are investigated. The grain boundaries of defect-engineered samples show higher surface potential than the grain interiors under light illumination, suggesting an effective role of GBs as charge collection and transport channels. The lower density of crystallographic defects and lower trap density at GBs specifically of heat-treated samples cause insignificant dark current, lower local current hysteresis, and higher photocurrent, than the control samples. It is also shown that the decay rate of surface photovoltage of the heated sample is quicker than the control sample, which implies a considerable impact of ion migration on the relaxation dynamic of photogenerated charge carriers. These findings reveal that the annealing process is an effective strategy to control not only the morphology but also the optoelectrical properties of GB defects, and the dynamic of ion migration. Understanding the origin of photoelectric activity in this material allows for designing and engineering optoelectronic QD devices with enhanced functionality.

Photoelectrochemical Application and Charge Transport Dynamics of a Water-Stable Organic-Inorganic Halide (C6H4NH2CuCl2I) Film in Aqueous Solution

A water-stable thin film composed of C₆H₄NH₂CuCl₂I was fabricated using spin-coating precursor solutions that dissolved equimolar amounts of C₆H₄NH₂I and CuCl₂ in N,N-dimethylformamide. Photoelectrochemical characteristics show that the C₆H₄NH₂CuCl₂I film demonstrated a stable photocurrent (∼1 μA/cm²) in an aqueous solution under white light (11.5 mW/cm²) even after 3000 s, while exhibiting a photon-to-current efficiency of 0.093% under AM1.5 (100 mW/cm²) illumination. However, these values were significantly lower than those of the CH₃NH₃PbX₃ (X = I, Cl) film in solid devices. The electron diffusion length L(e^-) (373 nm) and hole diffusion length L(h⁺) (177 nm) in the C₆H₄NH₂CuCl₂I photoelectrode were significantly lower than those of CH₃NH₃PbX₃, limiting the photoelectrochemical and photocatalysis performances. Moreover, L(h⁺) was shorter than L(e^-) in the C₆H₄NH₂CuCl₂I photoelectrode, resulting in the hole-collecting efficiency [η_c(h⁺)] being lower than the electron-collecting efficiency [η_c(e^-)]. A CuO interlayer was introduced as a hole transport layer for the C₆H₄NH₂CuCl₂I photoelectrode, which improved L(h⁺) and η_c(h⁺).

Hot Electrons in TiO2-Noble Metal Nano-Heterojunctions: Fundamental Science and Applications in Photocatalysis

Plasmonic photocatalysis enables innovation by harnessing photonic energy across a broad swathe of the solar spectrum to drive chemical reactions. This review provides a comprehensive summary of the latest developments and issues for advanced research in plasmonic hot electron driven photocatalytic technologies focusing on TiO2-noble metal nanoparticle heterojunctions. In-depth discussions on fundamental hot electron phenomena in plasmonic photocatalysis is the focal point of this review. We summarize hot electron dynamics, elaborate on techniques to probe and measure said phenomena, and provide perspective on potential applications-photocatalytic degradation of organic pollutants, CO2 photoreduction, and photoelectrochemical water splitting-that benefit from this technology. A contentious and hitherto unexplained phenomenon is the wavelength dependence of plasmonic photocatalysis. Many published reports on noble metal-metal oxide nanostructures show action spectra where quantum yields closely follow the absorption corresponding to higher energy interband transitions, while an equal number also show quantum efficiencies that follow the optical response corresponding to the localized surface plasmon resonance (LSPR). We have provided a working hypothesis for the first time to reconcile these contradictory results and explain why photocatalytic action in certain plasmonic systems is mediated by interband transitions and in others by hot electrons produced by the decay of particle plasmons.

Highly Efficient and Stable Perovskite Solar Cells Enabled by Low-Cost Industrial Organic Pigment Coating

Surface passivation of perovskite solar cells (PSCs) using a low-cost industrial organic pigment quinacridone (QA) is presented. The procedure involves solution processing a soluble derivative of QA, N,N-bis(tert-butyloxycarbonyl)-quinacridone (TBOC-QA), followed by thermal annealing to convert TBOC-QA into insoluble QA. With halide perovskite thin films coated by QA, PSCs based on methylammonium lead iodide (MAPbI₃ ) showed significantly improved performance with remarkable stability. A PCE of 21.1 % was achieved, which is much higher than 18.9 % recorded for the unmodified devices. The QA coating with exceptional insolubility and hydrophobicity also led to greatly enhanced contact angle from 35.6° for the pristine MAPbI₃ thin films to 77.2° for QA coated MAPbI₃ thin films. The stability of QA passivated MAPbI₃ perovskite thin films and PSCs were significantly enhanced, retaining about 90 % of the initial efficiencies after more than 1000 hours storage under ambient conditions.

Molecular-Level Insight into Correlation between Surface Defects and Stability of Methylammonium Lead Halide Perovskite Under Controlled Humidity

Perovskite-based photovoltaics (PVs) have garnered tremendous interest, enabling power conversion efficiencies exceeding 25%. Although much of this success is credited to the exploration of new compositions, defects passivation and process optimization, environmental stability remains an important bottleneck to be solved. The underlying mechanisms of thermal and humidity-induced degradation are still far from a clear understanding, which poses a severe limitation to overcome the stability issues. Herein, in situ X-ray diffraction (XRD), in operando liquid-cell transmission electron microscopy (TEM) and ex situ solid-state (ss)NMR spectroscopy are combined with time-resolved spectroscopies to reveal new insights about the degradation mechanisms of methylammonium lead halide (MAPbI₃ ) under 85% relative humidity (RH) at different length scales. Liquid-cell TEM enables the live visualizations from meso-to-nanoscale transformation between the perovskite particles and water molecules, which are corroborated by the changes in local structures at sub-nanometer distances by ssNMR and longer range by XRD. This work clarifies the role of surface defects and the significance of their passivation to prevent hydration and decomposition reactions.

Perovskite Solar Cells Based on Compact, Smooth FA0.1MA0.9PbI3 Film with Efficiency Exceeding 22

The utilization of mixed cations is beneficial for taking the advantages of cations and achieving highly efficient perovskite solar cells (PSCs). Herein, the precisely small incorporation of CH(NH₂)₂ (FA) cations in methyl ammonium lead iodide (MAPbI₃) enables the formation of compact, smooth perovskite film with high crystallinity. Consequently, the short-circuit current and the fill factor of the PSCs based on FA_xMA_1-xPbI₃ perovskite are greatly improved, leading to the enhanced device efficiency. The champion PSC based on FA_0.1MA_0.9PbI₃ exhibits a remarkably high efficiency of 22.02%. Furthermore, the PSCs based on FA_0.1MA_0.9PbI₃ perovskite also show improved device stability. This work provides a simple approach to fabricate high-quality perovskite films and high-performance PSCs with better stability.

Interfacial and structural modifications in perovskite solar cells

The rapid and continuous progress made in perovskite solar cell (PSC) technology has drawn considerable attention from the photovoltaic research community, and the application of perovskites in other electronic devices (such as photodetectors, light-emitting diodes, and batteries) has become imminent. Because of the diversity in device configurations, optimization of film deposition, and exploration of material systems, the power conversion efficiency (PCE) of PSCs has been certified to be as high as 25.2%, making this type of solar cells the fastest advancing technology until now. As demonstrated by researchers worldwide, controlling the morphology and defects in perovskite films is essential for attaining high-performance PSCs. In this regard, interface engineering has proven to be a very efficient way to address these issues, obtaining better charge collection efficiency, and reducing recombination losses. In this review, the interfacial modification between perovskite films and charge-transport layers (CTLs) as well as CTLs and electrodes of PSCs has been widely summarized. Grain boundary (GB) engineering and stress engineering are also included since they are closely related to the improvement in device performance and stability.

Effects of Illumination Direction on the Surface Potential of CH3NH3PbI3 Perovskite Films Probed by Kelvin Probe Force Microscopy

The development of organic-inorganic hybrid perovskite solar cells requires critical understanding in the charge-carrier behaviors in the perovskite light absorbers and devices. Kelvin probe force microscopy (KPFM) has been applied as a powerful tool to probe the electrical potential distribution of perovskite films and devices, providing fundamental insights into their charge-carrier properties. When measuring the material photoresponses, various approaches have been employed to illuminate the samples. Here, we measured the surface potential of the layer in the regular mesoporous structure (CH₃NH₃PbI₃/m-TiO₂/c-TiO₂/FTO) and inverted planar structure (CH₃NH₃PbI₃/NiO/FTO) devices via KPFM. Effects of two representative illumination methods are compared-illumination from top, and from underneath through the transparent glass substrate. By comparing the variation in surface potential under two illumination methods, the surface potential of the perovskite-absorbing layer in a regular structure is higher than that in the inverted structure. The potential difference in two structures implies that the photogenerated charge carriers are injected to the TiO₂ electron-transport layer and NiO hole-transport layer, resulting in positive charges and negative charges accumulated in the perovskite-absorbing layer. We demonstrated that the illumination direction has an impact on the surface potential measurement. For the CH₃NH₃PbI₃/TiO₂ structure, illumination from underneath facilitates a larger potential change. While for the CH₃NH₃PbI₃/NiO structure with insensitive photoresponse in potential change, the illumination direction has a minor effect.

Surface Recombination in Ultra-Fast Carrier Dynamics of Perovskite Oxide La0.7Sr0.3MnO3 Thin Films

Aspects of the optoelectronic performance of thin-film ferromagnetic materials are evaluated for application in ultrafast devices. Dynamics of photocarriers and their associated spin polarization are measured using transient reflectivity (TR) measurements in cross linear and circular polarization configurations for La_0.7Sr_0.3MnO₃ films with a range of thicknesses. Three spin-related recombination mechanisms have been observed for thicker films (thickness of d ≥ 20 nm) at different time regimes (τ), which are attributed to the electron-phonon recombination (τ < 1 ps), phonon-assisted spin-lattice recombination (τ ∼ 100 ps), and thermal diffusion and radiative recombination (τ > 1 ns). Density functional theory (DFT+U) based first-principles calculations provide information about the nature of the optical transitions and their probabilities for the majority and the minority spin channels. These transitions are partly responsible for the aforementioned recombination mechanisms, identified through the comparison of linear and circular TR measurements. The same sets of measurements for thinner films (4.4 nm ≤ d < 20 nm) revealed an additional relaxation dynamic (τ ∼ 10 ps), which is attributed to the enhanced surface recombination of charge carriers. Our DFT+U calculations further corroborate this observation, indicating an increase in the surface density of states with decreasing film thickness which results in higher amplitude and smaller time constant for surface recombination as the film thickness decreases.

Enhanced Performance and Stability of Perovskite Solar Cells Using NH4I Interfacial Modifier

Despite organic-inorganic hybrid perovskite solar cells have rapid advances in power conversion efficiency in recent years, the serious instability of the device under practical working conditions is the current main challenge for commercialization. In this study, we have successfully inserted NH₄I as an interfacial modifier between the TiO₂ electron transport layer and perovskite layer. The result shows that it can significantly improve the quality of the perovskite films and electron extraction efficiency between the perovskite and electron transport layer. The devices with NH₄I are obtained an improved power conversion efficiency of 18.31% under AM 1.5G illumination (100 mW cm^-2). More importantly, the humidity and UV light stability of the devices are greatly improved after adding NH₄I layer. The uncoated devices only decrease by less than 15% of its original efficiency during 700-h stability tests in a humidity chamber (with a relative humidity of 80%) and the efficiency almost maintains 70% of its initial value over 20 h under UV light stress tests. This work provides a potential way by interfacial modification to significantly improve photovoltaic performance and stability of perovskite solar cells.

Environmentally Friendly Plasma-Treated PEDOT:PSS as Electrodes for ITO-Free Perovskite Solar Cells

Solution processed poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) transparent electrodes (TEs) offer great potential as a low cost alternative to expensive indium tin oxide (ITO). However, strong acids are typically used for enhancing the conductivity of PEDOT:PSS TEs, which produce processing complexity and environmental issues. This work presents an environmentally friendly acid free approach to enhance the conductivity of PEDOT:PSS using a light oxygen plasma treatment, in addition to solvent blend additives and post treatments. The plasma treatment was found to significantly reduce the sheet resistance of PEDOT:PSS TEs from 85 to as low as 15 Ω sq^-1, which translates to the highest reported conductivity of 5012 S/cm for PEDOT:PSS TEs. The plasma treated PEDOT:PSS TE resulted in an ITO-free perovskite solar cell efficiency of 10.5%, which is the highest reported efficiency for ITO-free perovskite solar cells with a PEDOT:PSS electrode that excludes the use of acid treatments. This research presents the first demonstration of this technology. Moreover, the PEDOT:PSS TEs enabled better charge extraction from the perovskite solar cells and reduced hysteresis in the current density-voltage (J-V) curves.

Improved Reproducibility for Perovskite Solar Cells with 1 cm2 Active Area by a Modified Two-Step Process

With rapid progress in recent years, organohalide perovskite solar cells (PSC) are promising candidates for a new generation of highly efficient thin-film photovoltaic technologies, for which up-scaling is an essential step toward commercialization. In this work, we propose a modified two-step method to deposit the CH₃NH₃PbI₃ (MAPbI₃) perovskite film that improves the uniformity, photovoltaic performance, and repeatability of large-area perovskite solar cells. This method is based on the commonly used two-step method, with one additional process involving treating the perovskite film with concentrated methylammonium iodide (MAI) solution. This additional treatment is proved to be helpful for tailoring the residual PbI₂ level to an optimal range that is favorable for both optical absorption and inhibition of recombination. Scanning electron microscopy and photoluminescence image analysis further reveal that, compared to the standard two-step and one-step methods, this method is very robust for achieving uniform and pinhole-free large-area films. This is validated by the photovoltaic performance of the prototype devices with an active area of 1 cm², where we achieved the champion efficiency of ∼14.5% and an average efficiency of ∼13.5%, with excellent reproducibility.

Highly Efficient All-Inorganic Planar Heterojunction Perovskite Solar Cells Produced by Thermal Coevaporation of CsI and PbI2

We report here all inorganic CsPbI₃ planar junction perovskite solar cells fabricated by thermal coevaporation of CsI and PbI₂ precursors. The best devices delivered power conversion efficiency (PCE) of 9.3 to 10.5%, thus coming close to the reference MAPbI₃-based devices (PCE ≈ 12%). These results emphasize that all inorganic lead halide perovskites can successfully compete in terms of photovoltaic performance with the most widely used hybrid materials such as MAPbI₃.

Surface and Interface Aspects of Organometal Halide Perovskite Materials and Solar Cells

The current challenges (e.g., stability, hysteresis, etc.) in organometal halide perovskite solar cell research are closely correlated with surfaces and interfaces. For instance, efficient generation of charges, extraction, and transport with minimum recombination through interlayer interfaces is crucial to attain high-efficiency solar cell devices. Furthermore, intralayer interfaces may be present in the form of grain boundaries within a film composed of the same material, for example, a polycrystalline perovskite layer. The adjacent grains may assume different crystal orientations and/or have different chemical compositions, which impacts charge excitation and dynamics and thereby the overall solar cell performance. In this Perspective, we present case studies to demonstrate (1) how surfaces and interfaces can impact material properties and device performance and (2) how these issues can be investigated by surface science techniques, such as scanning probe microscopy, photoelectron spectroscopy, and so forth. We end this Perspective by outlining the future research directions based on the reported results as well as the new trends in the field.

Carrier Transport Improvement of CH3NH3PbI3 Film by Methylamine Gas Treatment

Recently, perovskite solar cells with high photovoltaic performance based on methylammonium lead halide have attracted great interest due to the superior physical properties of the perovskite optical absorption layer. Here, we investigate the interface carrier transport properties of CH₃NH₃PbI₃ film by applying the reported treatment with methylamine gas, to reveal the possible mechanism of high performance perovskite-sensitized solar cell results. It is found that the crystal structure and surface morphology are effectively improved by the room-temperature repair of methylamine atmosphere. The preferred 110 orientation results in a slightly larger band gap, which may contribute to the better energy level matching and carrier transport. Further investigations on relaxation time and electron mobility confirm the significantly enhanced carrier diffusion length, revealing the important role of optimized crystallization on charge transport properties, which may be helpful to seek high-powered perovskite solar cells by optimizing the perovskite synthetic process.

Photo-Carrier Multi-Dynamical Imaging at the Nanometer Scale in Organic and Inorganic Solar Cells

Investigating the photocarrier dynamics in nanostructured and heterogeneous energy materials is of crucial importance from both fundamental and technological points of view. Here, we demonstrate how noncontact atomic force microscopy combined with Kelvin probe force microscopy under frequency-modulated illumination can be used to simultaneously image the surface photopotential dynamics at different time scales with a sub-10 nm lateral resolution. The basic principle of the method consists in the acquisition of spectroscopic curves of the surface potential as a function of the illumination frequency modulation on a two-dimensional grid. We show how this frequency-spectroscopy can be used to probe simultaneously the charging rate and several decay processes involving short-lived and long-lived carriers. With this approach, dynamical images of the trap-filling, trap-delayed recombination and nongeminate recombination processes have been acquired in nanophase segregated organic donor-acceptor bulk heterojunction thin films. Furthermore, the spatial variation of the minority carrier lifetime has been imaged in polycrystalline silicon thin films. These results establish two-dimensional multidynamical photovoltage imaging as a universal tool for local investigations of the photocarrier dynamics in photoactive materials and devices.

Observation of Space Charge Dynamics Inside an All Oxide Based Solar Cell

The charge transfer dynamics at interfaces are fundamental to know the mechanism of photovoltaic processes. The internal potential in solar cell devices depends on the basic processes of photovoltaic effect such as charge carrier generation, separation, transport, recombination, etc. Here we report the direct observation of the surface potential depth profile over the cross-section of the ZnO nanorods/Cu2O based solar cell for two different layer thicknesses at different wavelengths of light using Kelvin probe force microscopy. The topography and phase images across the cross-section of the solar cell are also observed, where the interfaces are well-defined on the nanoscale. The potential profiling results demonstrate that under white light illumination, the photoinduced electrons in Cu2O inject into ZnO due to the interfacial electric field, which results in the large difference in surface potential between two active layers. However, under a single wavelength illumination, the charge carrier generation, separation, and transport processes between two active layers are limited, which affect the surface potential images and corresponding potential depth profile. Because of changes in the active layer thicknesses, small variations have been observed in the charge carrier transport mechanism inside the device. These results provide the clear idea about the charge carrier distribution inside the solar cell in different conditions and show the perfect illumination condition for large carrier transport in a high performance solar cell.

Mapping the Photoresponse of CH3NH3PbI3 Hybrid Perovskite Thin Films at the Nanoscale

Perovskite solar cells (PSCs) based on thin films of organolead trihalide perovskites (OTPs) hold unprecedented promise for low-cost, high-efficiency photovoltaics (PVs) of the future. While PV performance parameters of PSCs, such as short circuit current, open circuit voltage, and maximum power, are always measured at the macroscopic scale, it is necessary to probe such photoresponses at the nanoscale to gain key insights into the fundamental PV mechanisms and their localized dependence on the OTP thin-film microstructure. Here we use photoconductive atomic force microscopy spectroscopy to map for the first time variations of PV performance at the nanoscale for planar PSCs based on hole-transport-layer free methylammonium lead triiodide (CH3NH3PbI3 or MAPbI3) thin films. These results reveal substantial variations in the photoresponse that correlate with thin-film microstructural features such as intragrain planar defects, grains, grain boundaries, and notably also grain-aggregates. The insights gained into such microstructure-localized PV mechanisms are essential for guiding microstructural tailoring of OTP films for improved PV performance in future PSCs.

Crystallization of a perovskite film for higher performance solar cells by controlling water concentration in methyl ammonium iodide precursor solution

An optimal small amount of water added into methyl ammonium iodide (MAI) solution in isopropyl alcohol (IPA) helps perovskite crystallization and leads to larger grain size from sequential deposition of perovskite films. The concentration of water was varied from 1% to 7% (vol% of IPA) in MAI solution and optical absorption, crystallization, morphology of perovskite films and their photovoltaic performance were studied in perovskite solar cells. 5% by volume was found to lead to preferential crystallization in the (110) plane with grain size about three times that of perovskite films prepared without adding water into the MAI solution. The optimal water concentration of 5% by volume in the MAI solution led to average perovskite grain size of ∼600 nm and solar cell efficiency of 12.42% at forward scan with a rate of 0.5 V s(-1). Device performance decreases after increasing water concentration beyond 5% in the MAI solution due to formation of the PbI2 phase. Transient photocurrent and photovoltage measurements show the shortest charge transport time at 0.99 μs and the longest charge carrier life time at 13.6 μs for perovskite films prepared from 5% water in MAI solution, which improved perovskite solar cell efficiency from 9.04% to 12.42%.

Strategic improvement of the long-term stability of perovskite materials and perovskite solar cells

This review provides a comprehensive overview of the recent strategies aimed at enhancing the long-term stability of perovskite materials and perovskite solar cells (PSCs). It also extensively discusses the stability problem of perovskite materials and PSCs from perspectives of experimental tests and theoretical calculations.

Formation and evolution of the unexpected PbI2 phase at the interface during the growth of evaporated perovskite films

A PbI₂ phase is found to be inevitably formed at the interface in dual-source evaporated perovskite films that will block carrier transport.