Recent progress and challenges of organometal halide perovskite solar cells

Progress of 2D Perovskite Solar Cells: Structure and Performance

2D perovskite has demonstrated great potential for application in photovoltaic devices due to the tunable energy bands, suppressed ion migration, and high stability. However, 2D perovskite solar cells (PSCs) display suboptimal efficiency in comparison to 3D perovskite solar cells, which can be attributed to the quantum confinement and dielectric confinement effects resulting from the intercalation of organic spacer cations into the perovskite lattice. This review starts with the fundamental structural characteristics, optoelectronic properties, and carrier transport dynamics of 2D PSCs, followed by the discussion of approaches to improve the photovoltaic performance of 2D PSCs, including the manipulation of crystal orientation, phase distribution, pure phase, organic layer, and device engineering. Then the advancements in the structural, humidity, thermal, and maximum power point tracking stability of 2D PSCs are summarized. Afterward, the applications of 2D perovskite in 2/3D PSCs to improve efficiency and stability are discussed. This review provides a comprehensive understanding of the relationship between 2D perovskite structure and the performance of the resulting 2D PSCs, as well as offers insights for constructing efficient and stable 2/3D PSCs by integrating 2D and 3D perovskites.

Vapor-Solid Reaction Techniques for the Growth of Organic-Inorganic Hybrid Perovskite Thin Films

Perovskite solar cells are considered next-generation photovoltaic technology due to their remarkable advancements in power conversion efficiency. To transition this technology from the lab to industry, the method for preparing perovskite thin films must support mass production. Currently, the solution-based slot-die technique is the primary method for depositing large-area perovskite thin films. However, solution-based methods are not standard in the semiconductor industry, where vapor-based techniques are favored for their high controllability and reproducibility. The cost of vacuum facilities and the complexity of these processes hinder many researchers, resulting in vapor-based technique development lagging behind solution-based methods in device efficiency and scale. This review focuses on the progress in growing perovskite thin films using vapor-solid reaction techniques, which are believed to offer the most direct path to commercialization. By examining the crystallization and growth mechanisms of perovskite films and discussing specific optimization strategies for vapor-solid reactions, insights into future developments and challenges in fabricating perovskite solar cells using fully vacuum processes are concluded.

Large-Scale, Stretchable, Self-Protective, and Multifunctional Perovskite Luminescent Filament with Ultra-High Stability

Lead halide perovskites possess great application potential in flexible displays and wearable optoelectronics owing to their prominent optoelectronic properties. However, the intrinsic instability upon moisture, heat, and ultraviolet (UV) light irradiation hinders their development and application. In this work, an ultra-stable CsPbX3 (X = Cl, Br, I) perovskite luminescent filament (PLF) with high stretchability (≈2400%) and luminescence performance (photoluminescence quantum yield (PLQY) of 24.5%, tunable emission spectrum, and high color purity) is introduced by a facile environmental-friendly wet-spinning technology via solvent extraction. Benefiting from the in situ encapsulation of the hydrophobic thermoplastic polyurethane (TPU) and the chelation of Lewis base CO in TPU with Lewis acid Pb2+, the CsPbBr3 PLF demonstrates ultra-high photoluminescence (PL) stability when stored in ambient air and high humidity circumstance, annealed at 50 °C, and dipped in water for 30 days, illuminated under ultraviolet light for 300 min, and immersed in organic solvents and solutions with pH of 1-13 for 5 min, respectively. Impressively, it retains 80% of its initial PL after being recycled five times. Overall, the CsPbX3 PLF demonstrates promising prospects in multifunctional applications, including organic dyes and tensile strain sensing, flexible pattern displays, secondary anti-counterfeiting, and hazard warning systems.

Polymer-Doped SnO2 as an Electron Transport Layer for Highly Efficient and Stable Perovskite Solar Cells

To produce highly efficient and repeatable perovskite solar cells (PSCs), comprehending interfacial loss and developing approaches to ameliorate interfacial features is essential. Nonradiative recombination at the SnO2-perovskite interface in SnO2-based perovskite solar cells (PSCs) leads to significant potential loss and variability in device performance. To improve the quality of the SnO2 electron transport layer, a novel polymer-doped SnO2 matrix, specifically using polyacrylic acid, was developed. This matrix is formed by spin-coating a SnO2 colloidal solution that includes polymers. The polymer aids in dispersing nanoparticles within the substrate and is evenly distributed in the SnO2 solution. As a result of the polymer addition, the density and wetting properties of the SnO2 layer substantially improved. Subsequently, perovskite-based photovoltaic devices comprising SnO2 and Spiro-OMeTAD layers and using (FAPbI3)0.97(MAPbBr3)0.03 perovskite are constructed. These optimized devices exhibited an increased efficiency of 17.2% when compared to the 15.7% power conversion efficiency of the control device. The incorporation of polymers in the electron transport layer potentially enables even better performance in planar perovskite solar cells.

Stability of perovskite solar cells: issues and prospects

Even though power conversion efficiency has already reached 25.8%, poor stability is one of the major challenges hindering the commercialization of perovskite solar cells (PSCs). Several initiatives, such as structural modification and fabrication techniques by numerous ways, have been employed by researchers around the world to achieve the desired level of stability. The goal of this review is to address the recent improvements in PSCs in terms of structural modification and fabrication procedures. Perovskite films are used to provide a broad range of stability and to lose up to 20% of their initial performance. A thorough comprehension of the effect of the fabrication process on the device's stability is considered to be crucial in order to provide the foundation for future attempts. We summarize several commonly used fabrication techniques - spin coating, doctor blade, sequential deposition, hybrid chemical vapor, and alternating layer-by-layer. The evolution of device structure from regular to inverted, HTL free, and ETL including the changes in material utilization from organic to inorganic, as well as the perovskite material are presented in a systematic manner. We also aimed to gain insight into the functioning stability of PSCs, as well as practical information on how to increase their operational longevity through sensible device fabrication and materials processing, to promote PSC commercialization at the end.

Durable Defect Passivation of the Grain Surface in Perovskite Solar Cells with π-Conjugated Sulfamic Acid Additives

Defect passivation has shown an essential role in improving the efficiency and stability of perovskite solar cells (PSCs). Herein, an efficient and low-cost π-conjugated sulfamic acid additive, 4-aminobenzenesulfonic acid (4-ABSA), is used to realize durable defect passivation of PSCs. The incorporation of 4-ABSA not only constructs a compact and smooth perovskite film but is also capable of passivating both negative- and positive-charged defects derived from under-coordinated lead and halogen ions. Besides, the π-conjugated system in 4-ABSA can induce preferred perovskite crystal orientation and stabilize the coordination effect between 4-ABSA and perovskite grains. As a result, the inverted planar PSC incorporated with 4-ABSA additives demonstrates an improved power conversion efficiency (PCE) from 18.25 to 20.32%. Moreover, this 4-ABSA passivation agent also enhances the stability of devices, which retains 83.5% of its initial efficiency under ambient condition at 60 °C after 27 days. This work provides a π-conjugated sulfamic acid for durable defect passivation of perovskite optoelectronic devices.

Influence of Additives on the In Situ Crystallization Dynamics of Methyl Ammonium Lead Halide Perovskites

Understanding the kinetics of the crystallization process for organometal halide perovskite formation is critical in determining the crystalline, nanoscale morphology and therefore the electronic properties of the films produced during thin film formation from solution. In this work, in situ grazing incidence small-angle X-ray scattering (GISAXS) and optical microscopy measurements are used to investigate the processes of nucleation and growth of pristine mixed halide perovskite (MAPbI3-x Cl x ) crystalline films deposited by bar coating at 60 °C, with and without additives in the solution. A small amount of 1,8-diiodooctane (DIO) and hydriodic acid (HI) added to MAPbI3-x Cl x is shown to increase the numbers of nucleation centers promoting heterogeneous nucleation and accelerate and modify the size of nuclei during nucleation and growth. A generalized formation mechanism is derived from the overlapping parameters obtained from real-time GISAXS and optical microscopy, which revealed that during nucleation, perovskite precursors cluster before becoming the nuclei that function as elemental units for subsequent formation of perovskite crystals. Additive-free MAPbI3-x Cl x follows reaction-controlled growth, in contrast with when DIO and HI are present, and it is highly possible that the growth then follows a hindered diffusion-controlled mechanism. These results provide important details of the crystallization mechanisms occurring and will help to develop greater control over perovskite films produced.

A Review on Lead-Free Hybrid Halide Perovskites as Light Absorbers for Photovoltaic Applications Based on Their Structural, Optical, and Morphological Properties

Despite the advancement made by the scientific community in the evolving photovoltaic technologies, including the achievement of a 29.1% power conversion efficiency of perovskite solar cells over the past two decades, there are still numerous challenges facing the advancement of lead-based halide perovskite absorbers for perovskite photovoltaic applications. Among the numerous challenges, the major concern is centered around the toxicity of the emerging lead-based halide perovskite absorbers, thereby leading to drawbacks for their pragmatic application and commercialization. Hence, the replacement of lead in the perovskite material with non-hazardous metal has become the central focus for the actualization of hybrid perovskite technology. This review focuses on lead-free hybrid halide perovskites as light absorbers with emphasis on how their chemical compositions influence optical properties, morphological properties, and to a certain extent, the stability of these perovskite materials.

Layered perovskite materials: key solutions for highly efficient and stable perovskite solar cells

Metal halide perovskites having three-dimensional crystal structures are being applied successfully in various optoelectronic applications. To address their most challenging issues-instability and toxicity-without losing efficiency, lower-dimensional perovskites appear to be promising alternatives. Recently, two-dimensional (2D) perovskite solar cells have been developed exhibiting excellent photostability and moisture-stability, together with moderate device efficiency. This review summarizes the photophysical properties and operating mechanisms of 2D perovskites as well as recent advances in their applications in solar cell devices. Also presented is an agenda for the next-stage development of stable perovskite materials for solar cell applications, highlighting the issues of stability and toxicity that require further study to ensure commercialization.

Efficient Nanorod Array Perovskite Solar Cells: A Suitable Structure for High Strontium Substitution in Nature

Organic-inorganic hybrid perovskite solar cells (PSCs) have become a research hotspot because of their excellent power conversion efficiency (PCE), but the presence of toxic lead (Pb) in perovskite films has significantly limited their commercial applications. In this study, using a TiO₂ nanorod array (TNRA) as the electron transport layer, strontium chloride (SrCl₂) was chosen to fabricate lead-less PSCs in air (relative humidity, RH = 50%) by a simple two-step spin-coating method. The influence of introduced strontium (Sr) on the perovskite films and cell properties was systematically investigated by various characterization methods. With an increasing Sr substitution amount from 0 to 15 mol %, the formed perovskite films with a compact structure and large crystalline size essentially remained invariable, while the amount of residual PbI₂ was reduced, which is beneficial for the cell performance. An optimal PCE of 16.08% (average PCE = 15.37%) was obtained for the 5 mol % Sr-substituted PSCs because of the enhanced charge extraction from the perovskite films to the TNRAs and the suppressed charge recombination in the PSCs. Both the humidity and thermal stability of the Sr-substituted PSCs were improved. More importantly, the 15 mol % Sr-substituted PSCs still exhibited a PCE of 15.09% in air (RH = 50%), maintaining 99% of the cell efficiency of the pristine (0 mol % Sr) PSC (15.27%), suggesting that the structure of TNRAs is suitable for the synthesis of high-performance Sr-substituted lead-less PSCs.

Strong Cathodoluminescence and Fast Photoresponse from Embedded CH3NH3PbBr3 Nanoparticles Exhibiting High Ambient Stability

This study presents a comprehensive analysis of the strong cathodoluminescence (CL), photoluminescence (PL), and photoresponse characteristics of CH₃NH₃PbBr₃ nanoparticles (NPs) embedded in a mesoporous nanowire (NW) template. Our study revealed a direct correlation between the CL and PL emissions from the perovskite NPs (Per NPs), for the first time. Per NPs are fabricated by a simple spin-coating of a perovskite precursor on the surface of metal-assisted chemically etched mesoporous Si NW arrays. The Per NPs confined in the mesopores show blue-shifted and enhanced CL emission as compared to the bare perovskite film, while the PL intensity of Per NPs is dramatically high compared to that of their bulk counterpart. A systematic analysis of the CL/PL spectra reveals that the quantum confinement effect and ultralow defects in Per NPs are mainly responsible for the enhanced CL and PL emissions. Low-temperature PL and time-resolved PL analysis confirm the high exciton binding energy and radiative recombination in Per NPs. The room temperature PL quantum yield of the Per NP film on the NW template was found to be 40.5%, while that of Per film was 2.8%. The Per NPs show improved ambient air stability than the bare film due to the protection provided by the dense NW array, since a dense NW array can slow down the lateral diffusion of oxygen and water molecules in Per NPs. Interestingly, the Si NW/Per NP junction shows superior visible light photodetection and the prototype photodetector shows a high responsivity (0.223 A/W) with response speeds of 0.32 and 0.28 s of growth and decay in photocurrent, respectively, at 2 V applied bias, which is significantly better than the reported photodetectors based on CH₃NH₃PbBr₃ nanostructures. This work demonstrates a low-cost fabrication of CH₃NH₃PbBr₃ NPs on a novel porous NW template, which shows excellent photophysical and optoelectronic properties with superior ambient stability.

Electronic and Optical Properties of Ultrasmall ABX3 (A = Cs, CH3NH3/B = Ge, Pb, Sn, Ca, Sr/X = Cl, Br, I) Perovskite Quantum Dots

Perovskite quantum dots (QDs) constitute a novel and rapidly developing field of nanotechnology with promising potential for optoelectronic applications. However, few perovskite materials for QDs and other nanostructures have been theoretically explored. In this study, we present a wide spectrum of different hybrid halide perovskite cuboid-like QDs with the general formula of ABX₃ with varying sizes well below the Bohr exciton radius. Density functional theory (DFT) and time-dependent DFT calculations were employed to determine their structural, electronic, and optical properties. Our calculations include both stoichiometric and nonstoichiometric QDs, and our results reveal several materials with high optical absorption and application-suitable electronic and optical gaps. Our study highlights the potential as well as the challenges and issues regarding nanostructured halide perovskite materials, laying the background for future theoretical and experimental work.

SnO2 -in-Polymer Matrix for High-Efficiency Perovskite Solar Cells with Improved Reproducibility and Stability

Understanding interfacial loss and the ways to improving interfacial property is critical to fabricate highly efficient and reproducible perovskite solar cells (PSCs). In SnO₂ -based PSCs, nonradiative recombination sites at the SnO₂ -perovskite interface lead to a large potential loss and performance variation in the resulting photovoltaic devices. Here, a novel SnO₂ -in-polymer matrix (i.e., polyethylene glycol) is devised as the electron transporting layer to improve the film quality of the SnO₂ electron transporting layer. The SnO₂ -in-polymer matrix is fabricated through spin-coating a polymer-incorporated SnO₂ colloidal ink. The polymer is uniformly dispersed in SnO₂ colloidal ink and promotes the nanoparticle disaggregation in the ink. Owing to polymer incorporation, the compactness and wetting property of SnO₂ layer is significantly ameliorated. Finally, photovoltaic devices based on Cs_0.05 FA_0.81 MA_0.14 PbI_2.55 Br_0.45 perovskite sandwiched between SnO₂ and Spiro-OMeTAD layer are fabricated. Compared with the averaging power conversion efficiency of 16.2% with 1.2% deviation for control devices, the optimized devices exhibit an improved averaging efficiency of 19.5% with 0.25% deviation. The conception of polymer incorporation in the electron transporting layer paves a way to further increase the performance of planar perovskite solar cells.

Fractional deviations in precursor stoichiometry dictate the properties, performance and stability of perovskite photovoltaic devices

The last five years have witnessed remarkable progress in the field of lead halide perovskite materials and devices. Examining the existing body of literature reveals staggering inconsistencies in the reported results among different research groups with a particularly wide spread in the photovoltaic performance and stability of devices. In this work we demonstrate that fractional, quite possibly unintentional, deviations in the precursor solution stoichiometry can cause significant changes in the properties of the perovskite layer as well as in the performance and stability of perovskite photovoltaic devices. We show that while the absorbance and morphology of the layers remain largely unaffected, the surface composition and energetics, crystallinity, emission efficiency, energetic disorder and storage stability are all very sensitive to the precise stoichiometry of the precursor solution. Our results elucidate the origin of the irreproducibility and inconsistencies of reported results among different groups as well as the wide spread in device performance even within individual studies. Finally, we propose a simple experimental method to identify the exact stoichiometry of the perovskite layer that researchers can employ to confirm their experiments are performed consistently without unintentional variations in precursor stoichiometry.

Eliminating Light-Soaking Instability in Planar Heterojunction Perovskite Solar Cells by Interfacial Modifications

Stability remains as a challenge of perovskite solar cells although encouraging progress has been made toward their maximum achievable power conversion efficiency in recent years. Light-soaking issue, where the device performance improves upon continuous light illumination and reduces upon storage in dark, is widely observed and marked as the early-stage instability during device operation. In this work, we have employed conjugated polymer PCDTBT as the dopant-free hole-transport layer to fabricate devices without hysteresis but with reversible light-soaking instability. The introduction of n-type molecules, either organic molecule PDI2 or fullerene derivative PC₆₁BM, as the interfacial layer between TiO₂ and perovskite layers can effectively reduce or eliminate this instability owing to the efficient charge transport and defect passivation at the electron-transport layer interface, accompanied with an efficiency of 15.7 and 17.7%, respectively. We conclude that the light-soaking instability of these perovskite solar cells is mainly originated from the charge accumulation at the TiO₂/perovskite interface and can be eliminated once the interfacial charge can be suppressed by interfacial modifications to improve charge transport at the interface.

Inverted (p-i-n) perovskite solar cells using a low temperature processed TiO x interlayer

In this article, we present the improvement in device performance and stability as well as reduction in hysteresis of inverted mixed-cation-mixed-halide perovskite solar cells (PSCs) using a low temperature, solution processed titanium oxide (TiO x ) interlayer between [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) and an Al electrode. Upon applying a TiO x interlayer, device resistance was reduced compared to that of the control devices, which results in improved rectification of the characteristic current density-voltage (J-V) curve and improved overall performance of the device. PSCs with the TiO x interlayer show an open-circuit voltage (V oc) of around 1.1 V, current density (J sc) of around 21 mA cm-2, fill factor (FF) of around 72% and enhanced power conversion efficiency (PCE) of 16% under AM1.5 solar spectrum. Moreover, devices with the TiO x interlayer show improved stability compared to devices without the TiO x interlayer. This finding reveals the dual role of the TiO x interlayer in improving device performance and stability.

Advancement on Lead-Free Organic-Inorganic Halide Perovskite Solar Cells: A Review

Remarkable attention has been committed to the recently discovered cost effective and solution processable lead-free organic-inorganic halide perovskite solar cells. Recent studies have reported that, within five years, the reported efficiency has reached 9.0%, which makes them an extremely promising and fast developing candidate to compete with conventional lead-based perovskite solar cells. The major challenge associated with the conventional perovskite solar cells is the toxic nature of lead (Pb) used in the active layer of perovskite material. If lead continues to be used in fabricating solar cells, negative health impacts will result in the environment due to the toxicity of lead. Alternatively, lead free perovskite solar cells could give a safe way by substituting low-cost, abundant and non toxic material. This review focuses on formability of lead-free organic-inorganic halide perovskite, alternative metal cations candidates to replace lead (Pb), and possible substitutions of organic cations, as well as halide anions in the lead-free organic-inorganic halide perovskite architecture. Furthermore, the review gives highlights on the impact of organic cations, metal cations and inorganic anions on stability and the overall performance of lead free perovskite solar cells.

Revealing the Self-Degradation Mechanisms in Methylammonium Lead Iodide Perovskites in Dark and Vacuum

Organic-inorganic lead halide perovskite phases segregate (and their structures degrade) under illumination, exhibiting a poor stability with hysteresis and producing halide accumulation at the surface.In this work, we observed structural and interfacial dissociation in methylammonium lead iodide (CH₃ NH₃ PbI₃ ) perovskites even under dark and vacuum conditions. Here, we investigate the origin and consequences of self-degradation in CH₃ NH₃ PbI₃ perovskites stored in the dark under vacuum. Diffraction and photoelectron spectroscopic studies reveal the structural dissociation of perovskites into PbI₂ , which further dissociates into metallic lead (Pb⁰ ) and I₂^- ions, collectively degrading the perovskite stability. Using TOF-SIMS analysis, AuI₂^- formation was directly observed, and it was found that an interplay between CH₃ NH₃⁺ , I₃^- , and mobile I^- ions continuously regenerates more I₂^- ions, which diffuse to the surface even in the absence of light. Besides, halide diffusion causes a concentration gradient between Pb⁰ and I₂^- and creates other ionic traps (PbI₂^- , PbI^- ) that segregate as clusters at the perovskite/gold interface. A shift of the onset of the absorption band edge towards shorter wavelengths was also observed by absorption spectroscopy, indicating the formation of defect species upon aging in the dark under vacuum.

Perovskite-based photodetectors: materials and devices

While the field of perovskite-based optoelectronics has mostly been dominated by photovoltaics, light-emitting diodes, and transistors, semiconducting properties peculiar to perovskites make them interesting candidates for innovative and disruptive applications in light signal detection. Perovskites combine effective light absorption in the broadband range with good photo-generation yield and high charge carrier mobility, a combination that provides promising potential for exploiting sensitive and fast photodetectors that are targeted for image sensing, optical communication, environmental monitoring or chemical/biological detection. Currently, organic-inorganic hybrid and all-inorganic halide perovskites with controlled morphologies of polycrystalline thin films, nano-particles/wires/sheets, and bulk single crystals have shown key figure-of-merit features in terms of their responsivity, detectivity, noise equivalent power, linear dynamic range, and response speed. The sensing region has been covered from ultraviolet-visible-near infrared (UV-Vis-NIR) to gamma photons based on two- or three-terminal device architectures. Diverse photoactive materials and devices with superior optoelectronic performances have stimulated attention from researchers in multidisciplinary areas. In this review, we provide a comprehensive overview of the recent progress of perovskite-based photodetectors focusing on versatile compositions, structures, and morphologies of constituent materials, and diverse device architectures toward the superior performance metrics. Combining the advantages of both organic semiconductors (facile solution processability) and inorganic semiconductors (high charge carrier mobility), perovskites are expected to replace commercial silicon for future photodetection applications.

The 3 D Structure of Twisted Benzo[ghi]perylene-Triimide Dimer as a Non-Fullerene Acceptor for Inverted Perovskite Solar Cells

Here, we introduced benzo[ghi]perylenetriimide (BPTI) derivatives including monomer and twisted dimer (t-BPTI) as an alternative electron-transport layer (ETL) material to replace the commonly used PC₆₁ BM in inverted planar heterojunction perovskite solar cells (PSCs). Moreover, the double ETL was applied in our PSCs with structure of glass/ITO/PEDOT:PSS/perovskite/BPTI/C₆₀ or PDI-C4/BCP/Al. The use of a double ETL structure can effectively eliminate the leakage current. The devices with the t-BPTI/C₆₀ double ETL yield an average power conversion efficiency of 10.73 % and a maximum efficiency of 11.63 %. The device based on the complete non-fullerene electron acceptors of t-BPTI/PDI-C4 as double ETL achieved maximum efficiency of 10.0 %. Moreover, it was found that the utilization of alloy t-BPTI+BPTI as ETL can effectively reduce the hysteresis effect of PSCs. The results suggest that BPTI-based electron-transport materials are potential alternatives for widely used fullerene acceptors in PSCs.

Mechanism and effect of γ-butyrolactone solvent vapor post-annealing on the performance of a mesoporous perovskite solar cell

In this paper, γ-butyrolactone (GBL) solvent vapor post-annealing (SVPA) on CH3NH3PbI3 thin films is reported, aiming to improve the complete transformation of PbI2 and increase the grain size of the CH3NH3PbI3 crystal, thus boosting the performance of mesoporous CH3NH3PbI3 perovskite solar cells (PSCs). The influence of GBL SVPA on the microstructure of perovskite layers and performance of PSCs was studied. The short circuit current density (J sc) of the devices significantly increased, yielding a high efficiency of 16.58%, which was 27.05% higher than that of thermally annealed films. A model was derived to explain the effect of GBL SVPA on PSCs. The perovskite films prepared by this method present several advantages such as complete transformation of PbI2 to CH3NH3PbI3, high crystallinity, large grain size, and fewer grain boundaries than those prepared without GBL SVPA. This improvement is beneficial for charge dissociation and transport in hybrid photovoltaic devices.

A Short Progress Report on High-Efficiency Perovskite Solar Cells

Faced with the increasingly serious energy and environmental crisis in the world nowadays, the development of renewable energy has attracted increasingly more attention of all countries. Solar energy as an abundant and cheap energy is one of the most promising renewable energy sources. While high-performance solar cells have been well developed in the last couple of decades, the high module cost largely hinders wide deployment of photovoltaic devices. In the last 10 years, this urgent demand for cost-effective solar cells greatly facilitates the research of solar cells. This paper reviews the recent development of cost-effective and high-efficient solar cell technologies. This report paper covers low-cost and high-efficiency perovskite solar cells. The development and the state-of-the-art results of perovskite solar cell technologies are also introduced.

Understanding the Effect of Delay Time of Solvent Washing on the Performances of Perovskite Solar Cells

Uniform and dense perovskite films were realized by the one-step solution-processing method combined with toluene washing. The influence of the delay time applied for toluene washing on the film quality of CH₃NH₃PbI₃ (MAPbI₃) was investigated in a comprehensive manner. The optimal delay time was experimentally observed at the critical point when the color of the film changes from transparent to hazy. A detailed X-ray diffraction study suggested that such a color change was caused by the emergence of the MAPbI₃ crystal nucleus. This finding provides a convenient method to determine the optimal time accurately. With the optimal delay time, the most uniformly distributed MAPbI₃ grains with the largest average grain size and the smoothest surface were obtained. Owing to the realization of homogeneous MAPbI₃ films combined with full coverage of perovskite on the substrate achieved by toluene washing at the critical point, open-circuit voltage, short-circuit current, fill factor, and power conversion efficiency of 1.11 V, 18.24 mA/cm², 77.47, and 15.54% were obtained.

Imaging the Anomalous Charge Distribution Inside CsPbBr3 Perovskite Quantum Dots Sensitized Solar Cells

Highly luminescent CsPbBr₃ perovskite quantum dots (QDs) have gained huge attention in research due to their various applications in optoelectronics, including as a light absorber in photovoltaic solar cells. To improve the performances of such devices, it requires a deeper knowledge on the charge transport dynamics inside the solar cell, which are related to its power-conversion efficiency. Here, we report the successful fabrication of an all-inorganic CsPbBr₃ perovskite QD sensitized solar cell and the imaging of anomalous electrical potential distribution across the layers of the cell under different illuminations using Kelvin probe force microscopy. Carrier generation, separation, and transport capacity inside the cells are dependent on the light illumination. Large differences in surface potential between electron and hole transport layers with unbalanced carrier separation at the junction have been observed under white light (full solar spectrum) illumination. However, under monochromatic light (single wavelength of solar spectrum) illumination, poor charge transport occurred across the junction as a consequence of less difference in surface potential between the active layers. The outcome of this study provides a clear idea on the carrier dynamic processes inside the cells and corresponding surface potential across the layers under the illumination of different wavelengths of light to understand the functioning of the solar cells and ultimately for the improvement of their photovoltaic performances.

Spray-cast multilayer perovskite solar cells with an active-area of 1.5 cm2

We utilise spray-coating under ambient conditions to sequentially deposit compact-TiO₂, mesoporous-TiO₂, CH₃NH₃PbI_(3−x)Cl_x perovskite and doped spiro-OMeTAD layers, creating a mesoporous standard architecture perovskite solar cell (PSC). The devices created had an average power conversion efficiency (PCE) of 9.2% and a peak PCE of 10.2%; values that compare favourably with control-devices fabricated by spin-casting that had an average efficiency of 11.4%. We show that our process can be used to create devices having an active-area of 1.5 cm² having an independently verified efficiency of 6.6%. This work demonstrates the versatility of spray-coating as well as its potential as a method of manufacturing low-cost, large-area, efficient perovskite devices.

Amino-Acid-Induced Preferential Orientation of Perovskite Crystals for Enhancing Interfacial Charge Transfer and Photovoltaic Performance

Interfacial engineering of perovskite solar cells (PSCs) is attracting intensive attention owing to the charge transfer efficiency at an interface, which greatly influences the photovoltaic performance. This study demonstrates the modification of a TiO₂ electron-transporting layer with various amino acids, which affects charge transfer efficiency at the TiO₂ /CH₃ NH₃ PbI₃ interface in PSC, among which the l-alanine-modified cell exhibits the best power conversion efficiency with 30% enhancement. This study also shows that the (110) plane of perovskite crystallites tends to align in the direction perpendicular to the amino-acid-modified TiO₂ as observed in grazing-incidence wide-angle X-ray scattering of thin CH₃ NH₃ PbI₃ perovskite film. Electrochemical impedance spectroscopy reveals less charge transfer resistance at the TiO₂ /CH₃ NH₃ PbI₃ interface after being modified with amino acids, which is also supported by the lower intensity of steady-state photoluminescence (PL) and the reduced PL lifetime of perovskite. In addition, based on the PL measurement with excitation from different side of the sample, amino-acid-modified samples show less surface trapping effect compared to the sample without modification, which may also facilitate charge transfer efficiency at the interface. The results suggest that appropriate orientation of perovskite crystallites at the interface and trap-passivation are the niche for better photovoltaic performance.

Elucidating the effect of the lead iodide complexation degree behind the morphology and performance of perovskite solar cells

The inclusion of iodide additives in hybrid perovskite precursor solutions has been successfully exploited to improve the solar cell efficiency but their impact on perovskite formation, morphology and photovoltaic performance is still not clear. Here an extensive analysis of the effect of iodide additives in the solution-phase and during the perovskite film formation, as well as their effect on device performance is provided. The results demonstrate that in the solution-phase the additives promote the formation of lead poly-iodide species resulting in the disaggregation of the inorganic lead iodide framework and in the formation of smaller nuclei inducing the growth of uniform and smooth perovskite films. Most importantly, the complexation capability of different iodide additives does not only directly affect film morphology but also influences the density of defect states by varying the stoichiometry of precursors. These findings demonstrate that the fine control of the interactions of the chemical species in the solution-phase is essential for the precise control of the morphology at the nanoscale and the growth of the perovskite films with a reduced density of defect states. Therefore, the in-depth understanding of all the processes involved in the solution-phase is the first step for the development of a facile and reproducible approach for the fabrication of hybrid perovskite solar cells with enhanced photovoltaic performance.

Influence of Hybrid Perovskite Fabrication Methods on Film Formation, Electronic Structure, and Solar Cell Performance

Hybrid organic/inorganic halide perovskites have lately been a topic of great interest in the field of solar cell applications, with the potential to achieve device efficiencies exceeding other thin film device technologies. Yet, large variations in device efficiency and basic physical properties are reported. This is due to unintentional variations during film processing, which have not been sufficiently investigated so far. We therefore conducted an extensive study of the morphology and electronic structure of a large number of CH₃NH₃PbI₃ perovskite where we show how the preparation method as well as the mixing ratio of educts methylammonium iodide and lead(II) iodide impact properties like film formation, crystal structure, density of states, energy levels, and ultimately the solar cell performance.

CH3NH3PbI3 films prepared by combining 1- and 2-step deposition: how crystal growth conditions affect properties

The growth of CH₃NH₃PbI₃ films is transformed using a new combined one-step and two-step deposition method which results in controlled morphology and property changes.

Influence of Perovskite Morphology on Slow and Fast Charge Transport and Hysteresis in the Perovskite Solar Cells

We have investigated the influence of perovskite morphology on slow and fast charge transport in the perovskite solar cells. Solar cells with different perovskite cuboid sizes (50-300 nm) have been fabricated using various methylammonium iodide concentrations. Both the low-frequency capacitance and hysteresis are maximum for the cell with the largest perovskite grains (300 nm). The low-frequency capacitance is about three orders of magnitude greater than the intermediate frequency capacitance, indicating the great role of ions on the slow responses and hysteresis. The measurement of open-circuit voltage decay indicates that for the large grains of 300 nm up to 70% of V_oc remains across the cell, even after passing ∼40 s. Such a long time V_oc decay demonstrates the large accumulation of the ions at the perovskite interfaces with electron and hole transport layers, which conduct slow redistribution of the charges after the light is turned off.

Functional Scanning Probe Imaging of Nanostructured Solar Energy Materials

From hybrid perovskites to semiconducting polymer/fullerene blends for organic photovoltaics, many new materials being explored for energy harvesting and storage exhibit performance characteristics that depend sensitively on their nanoscale morphology. At the same time, rapid advances in the capability and accessibility of scanning probe microscopy methods over the past decade have made it possible to study processing/structure/function relationships ranging from photocurrent collection to photocarrier lifetimes with resolutions on the scale of tens of nanometers or better. Importantly, such scanning probe methods offer the potential to combine measurements of local structure with local function, and they can be implemented to study materials in situ or devices in operando to better understand how materials evolve in time in response to an external stimulus or environmental perturbation. This Account highlights recent advances in the development and application of scanning probe microscopy methods that can help address such questions while filling key gaps between the capabilities of conventional electron microscopy and newer super-resolution optical methods. Focusing on semiconductor materials for solar energy applications, we highlight a range of electrical and optoelectronic scanning probe microscopy methods that exploit the local dynamics of an atomic force microscope tip to probe key properties of the solar cell material or device structure. We discuss how it is possible to extract relevant device properties using noncontact scanning probe methods as well as how these properties guide materials development. Specifically, we discuss intensity-modulated scanning Kelvin probe microscopy (IM-SKPM), time-resolved electrostatic force microscopy (trEFM), frequency-modulated electrostatic force microscopy (FM-EFM), and cantilever ringdown imaging. We explain these developments in the context of classic atomic force microscopy (AFM) methods that exploit the physics of cantilever motion and photocarrier generation to provide robust, nanoscale measurements of materials physics that are correlated with device operation. We predict that the multidimensional data sets made possible by these types of methods will become increasingly important as advances in data science expand capabilities and opportunities for image correlation and discovery.