Enhanced Charge Collection with Passivation Layers in Perovskite Solar Cells

The Role of Optimal Electron Transfer Layers for Highly Efficient Perovskite Solar Cells-A Systematic Review

Perovskite solar cells (PSCs), which are constructed using organic-inorganic combination resources, represent an upcoming technology that offers a competitor to silicon-based solar cells. Electron transport materials (ETMs), which are essential to PSCs, are attracting a lot of interest. In this section, we begin by discussing the development of the PSC framework, which would form the foundation for the requirements of the ETM. Because of their exceptional electronic characteristics and low manufacturing costs, perovskite solar cells (PSCs) have emerged as a promising proposal for future generations of thin-film solar energy. However, PSCs with a compact layer (CL) exhibit subpar long-term reliability and efficacy. The quality of the substrate beneath a layer of perovskite has a major impact on how quickly it grows. Therefore, there has been interest in substrate modification using electron transfer layers to create very stable and efficient PSCs. This paper examines the systemic alteration of electron transport layers (ETLs) based on electron transfer layers that are employed in PSCs. Also covered are the functions of ETLs in the creation of reliable and efficient PSCs. Achieving larger-sized particles, greater crystallization, and a more homogenous morphology within perovskite films, all of which are correlated with a more stable PSC performance, will be guided by this review when they are developed further. To increase PSCs' sustainability and enable them to produce clean energy at levels previously unheard of, the difficulties and potential paths for future research with compact ETLs are also discussed.

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.

Advances on the Application of Wide Band-Gap Insulating Materials in Perovskite Solar Cells

In recent years, the development of perovskite solar cells (PSCs) is advancing rapidly with their recorded photoelectric conversion efficiency reaching 25.8%. However, for the commercialization of PSCs, it is also necessary to solve their stability issue. In order to improve the device performance, various additives and interface modification strategies have been proposed. While, in many cases, they can guarantee a significant increase in efficiency, but not ensure improved stability. Therefore, materials that improve the device efficiency and stability simultaneously are urgently needed. Some wide band-gap insulating materials with stable physical and chemical properties are promising alternative materials. In this review, the application of wide band-gap insulating materials in PSCs, including their preparation methods, working roles, and mechanisms are described, which will promote the commercial application of PSCs.

Tunable resonance of a graphene-perovskite terahertz metasurface

The combination of graphene and perovskite has received extensive research attention because its photoelectric properties are excellent for the dynamic manipulation of light-matter interactions. Combining graphene and perovskite with a metasurface is expected to effectively improve the metasurface device's performance. Here, we report a terahertz graphene-perovskite metasurface with a tunable resonance. Under 780 nm laser excitation, the device's THz transmission is significantly reduced, and the Fano resonance mode can be manipulated in multiple dimensions. We verify the experimental results using a finite-difference time-domain (FDTD) simulation. Graphene and perovskite interact strongly with the metasurface, resulting in a short-circuit effect, which significantly weakens the resonance intensity of the Fano mode. The photoinduced conductivity enhancement intensifies the short-circuit effect, reducing the THz transmission and resonance intensity of the Fano mode and causing the resonance frequency to redshift. Finally, we provide a reference value for applications of hybrid metasurface-based optical devices in a real environment by investigating the effect of moisture on device performance.

Constructing Chromium Multioxide Hole-Selective Heterojunction for High-Performance Perovskite Solar Cells

Perovskite solar cells (PSCs) suffer from significant nonradiative recombination at perovskite/charge transport layer heterojunction, seriously limiting their power conversion efficiencies. Herein, solution-processed chromium multioxide (CrOx ) is judiciously selected to construct a MAPbI3 /CrOx /Spiro-OMeTAD hole-selective heterojunction. It is demonstrated that the inserted CrOx not only effectively reduces defect sites via redox shuttle at perovskite contact, but also decreases valence band maximum (VBM)-HOMO offset between perovskite and Spiro-OMeTAD. This will diminish thermionic losses for collecting holes and thus promote charge transport across the heterojunction, suppressing both defect-assisted recombination and interface carrier recombination. As a result, a remarkable improvement of 21.21% efficiency with excellent device stability is achieved compared to 18.46% of the control device, which is among the highest efficiencies for polycrystalline MAPbI3 based n-i-p planar PSCs reported to date. These findings of this work provide new insights into novel charge-selective heterojunctions for further enhancing efficiency and stability of PSCs.

Enhanced Thermal Stability of Planar Perovskite Solar Cells Through Triphenylphosphine Interface Passivation

While extensive research has driven the rapid efficiency trajectory noted to date for organic-inorganic perovskite solar cells (PSCs), their thermal stability remains one of the key issues hindering their commercialization. Herein, a significant reduction in surface defects (a precursor to perovskite instability) could be attained by introducing triphenylphosphine (TPP), an effective Lewis base passivator, to the vulnerable perovskite/spiro-OMeTAD interface. Not only did TPP passivation enable a high power conversion efficiency (PCE) of 20.22 % to be achieved, these devices also exhibited superior ambient and thermal stability. Unlike the pristine device, which exhibited a sharp descend to 16 % of its initial PCE on storing in relative humidity of 10 %, at 85 °C for more than 720 h, the TPP-passivated devices retained 71 % of its initial PCE. Hence, this study presents a facile yet excellent approach to attain high-performing yet thermally stable PSCs.

Multifunctional Heterocyclic-Based Spacer Cation for Efficient and Stable 2D/3D Perovskite Solar Cells

Two-dimensional/three-dimensional (2D/3D) Ruddlesden-Popper perovskite materials have shown the enormous potential to achieve both efficient and stable photovoltaic devices for commercial applications. Unfortunately, the single function of spacer cations limits their further improvements in efficiency to reach values as high as those of 3D perovskites. Herein, we developed a new-type multifunctional heterocyclic-based spacer cation of 2-(methylthio)-4,5-dihydro-1H-imidazole (MTIm⁺) to achieve a synchronous improvement of efficiency and stability for 2D/3D perovskite solar cells (PSCs). Owing to the presence of special chemical groups (imidazole and methylthio), strong interactions have been found between MTIm⁺ and the 3D perovskite component, leading to an excellent passivation effect. More important, at the initial stage of crystallization, uniform nucleation distribution would be generated around the spacer cation, which is helpful for improved crystallinity and reduced growth defects. The smaller layer space compared to that of cations based on aromatic hydrocarbons caused effective carrier transfer between inorganic layers in 2D/3D perovskites. As a result, the 2D/3D (n = 30) PSCs based on MTIm exhibit a champion PCE up to 21.25% with a high V_oc of 1.14 V. Besides, the 2D/3D perovskite devices have realized dramatically enhanced humidity and thermal stability, maintaining 94% of the starting PCE enduring aging at about 50% RH for 2880 h and at 85 °C for 360 h, respectively. We believe that it would provide a significant strategy to further promote the photovoltaic performances and the long-term stability of 2D/3D perovskite devices toward future practical applications.

Surface Passivation Using 2D Perovskites toward Efficient and Stable Perovskite Solar Cells

3D perovskite solar cells (PSCs) have shown great promise for use in next-generation photovoltaic devices. However, some challenges need to be addressed before their commercial production, such as enormous defects formed on the surface, which result in severe SRH recombination, and inadequate material interplay between the composition, leading to thermal-, moisture-, and light-induced degradation. 2D perovskites, in which the organic layer functions as a protective barrier to block the erosion of moisture or ions, have recently emerged and attracted increasing attention because they exhibit significant robustness. Inspired by this, surface passivation by employing 2D perovskites deposited on the top of 3D counterparts has triggered a new wave of research to simultaneously achieve higher efficiency and stability. Herein, we exploited a vast amount of literature to comprehensively summarize the recent progress on 2D/3D heterostructure PSCs using surface passivation. The review begins with an introduction of the crystal structure, followed by the advantages of the combination of 2D and 3D perovskites. Then, the surface passivation strategies, optoelectronic properties, enhanced stability, and photovoltaic performance of 2D/3D PSCs are systematically discussed. Finally, the perspectives of passivation techniques using 2D perovskites to offer insight into further improved photovoltaic performance in the future are proposed.

Highly-stable PEN as a gas-barrier substrate for flexible displays via atomic layer infiltration

Polymer substrates with superior barrier properties are of great importance for the development of highly-stable flexible displays. The atomic layer infiltration (ALI) method has been utilized to integrate nanoscale inorganic materials in the subsurface of commercial PEN substrates, and the in-suit quartz crystal microbalance (QCM) is employed to study the growth behaviour as the process parameters vary, in which the nucleation and infiltration stages have been demonstrated. The O₂ plasma pre-treatment prior to Al₂O₃ infiltration was used to determine its effect on the water vapor transmission rate (WVTR), and significantly improved barrier properties were observed compared to those of the ones without the O₂ plasma pre-treatment via the electrical Ca tests, which was attributed to the surface clean and improved film adhesion. The lowest WVTR value measured was 1.28 × 10^-5 g m^-2 day^-1 for the O₂ plasma pre-treated PEN substrate coated with 100 ALI cycles, which improved 3-4 orders of magnitude compared to that of the pristine ones. Besides, the infiltrated PEN substrate with O₂ plasma pre-treatment exhibited good mechanical stability, with only a slight increase of the WVTR value which was observed after the bending fatigue test with a radius of 5 mm. Furthermore, when applied to the encapsulation of organic light-emitting diodes (OLEDs), the normalized luminance remained above 94% after storage for 800 hours.

Surface passivation of organometal halide perovskites by atomic layer deposition: an investigation of the mechanism of efficient inverted planar solar cells

Interface passivation plays a pivotal role in achieving high-efficiency organic metal halide perovskite solar cells (PSCs). It has been recently revealed that atomic layer deposition (ALD) of wide-band gap oxides shows great potential to effectively passivate defects at the interface, and ALD is also of great technological promise for industrial upscaling. However, the conflicting observations of ALD passivation are often reported in the literature, even with very similar ALD conditions. To unveil the involved crucial mechanism, this work carefully investigates the evolution of a representative MAPbI₃ perovskite surface during the ALD of Al₂O₃, by employing the technique of in situ X-ray photoelectron spectroscopy. The ALD at 125 °C was found to cause significant degradation of the perovskite; lowering the deposition temperature can largely minimize the degradation, and 75 °C was found to be the best ALD temperature. Following this conclusion, inverted planar perovskite solar cells were prepared in ambient conditions with ALD Al₂O₃ interlayers. Indeed, cells with the interlayer deposited at 75 °C exhibited a significantly enhanced power conversion efficiency from 18.8% (champion 19.2%) to 20.0% (champion 20.4%). Photoluminescence measurements further evidence that the ALD layer can effectively passivate defect states at the perovskite surface. Considering the great representativeness and broad applicability of MAPbI₃ and ALD Al₂O₃, the mechanism and strategy reported herein should be of significant value for the perovskite interface engineering in general.

Colorful Efficient Moiré-Perovskite Solar Cells

Light harvesting is crucial for thin-film solar cells. To substantially reduce optical loss in perovskite solar cells (PSCs), hierarchical light-trapping nano-architectures enable absorption enhancement to exceed the conventional upper limit and have great potential for achieving state-of-the art optoelectronic performances. However, it remains a great challenge to design and fabricate a superior hierarchical light-trapping nano-architecture, which exhibits extraordinary light-harvesting ability and simultaneously avoids deteriorating the electrical performance of PSCs. Herein, colorful efficient moiré-PSCs are designed and fabricated incorporating moiré interference structures by the imprinting method with the aid of a commercial DVD disc. It is experimentally and theoretically demonstrated that the light harvesting ability of the moiré interference structure can be well manipulated through changing the rotation angle (0°-90°). The boosted short-circuit current is credited to augment light diffraction channels, leading to elongated optical paths, and fold sunlight into the perovskite layer. Moreover, the imprinting process suppresses the trap sites and voids at the active-layer interfaces with eliminated hysteresis. The moiré-PSC with an optimized 30° rotation angle achieves the best enhancement of light harvesting (28.5% higher than the pristine), resulting in efficiencies over 20.17% (MAPbI₃ ) and 21.76% ((FAPbI₃ )_{1- x} (MAPbBr₃ )_x ).

Gentle Materials Need Gentle Fabrication: Encapsulation of Perovskites by Gas-Phase Alumina Deposition

Metal halide perovskites have attracted tremendous attention as promising materials for future-generation optoelectronic devices. Despite their outstanding optical and transport properties, the lack of environmental and operational stability remains a major practical challenge. One of the promising stabilization avenues is metal oxide encapsulation via atomic layer deposition (ALD); however, the unavoidable reaction of metal precursors with the perovskite surface in conventional ALD leads to degradation and restructuring of the perovskites' surfaces. This Perspective highlights the development of a modified gas-phase ALD technique for alumina encapsulation that not only prevents perovskites' degradation but also significantly improves their optical properties and air stability. The correlation between precise atomic interactions at the perovskite-metal oxide interface with the dramatically enhanced optical properties is supported by density functional theory calculations, which also underlines the widespread applicability of this gentle technique for a variety of perovskite nanostructures unbarring potential opportunities offered by combination of these approaches.

Terahertz Analysis of CH3NH3PbI3 Perovskites Associated with Graphene and Silver Nanowire Electrodes

In order to investigate the thermal and chemical (in)stabilities of MAPbI₃ incorporated with graphene and silver nanowire (AgNW) electrodes, we employed the terahertz (THz) time-domain spectroscopy, which has a unique ability to deliver the information of electrical properties and the intermolecular bonding and crystalline nature of materials. In in situ THz spectroscopy of MAPbI₃, we observed a slight blue-shift in frequency of the 2 THz phonon mode as temperatures increase across the tetragonal-cubic structural phase transition. For MAPbI₃ with the graphene top electrode, no noticeable frequency shift is observed until the temperature reaches the maximum operating temperature of solar cells (85 °C). Phonon frequency shift is sensitive to the strain-induced tilt of PbI₆ octahedra and our results indicate that graphene forms a stable interface with MAPbI₃ and is also effective in suppression of the undesirable phase transition. Meanwhile, for MAPbI₃ coupled with the AgNW bottom electrode, the THz conductivity was found to be as low as that of the MAPbI₃ single layer, attributed to the chemical reaction between Ag atoms and iodide ions. The THz conductivity is greatly increased when an ultrathin Al₂O₃ interlayer is introduced to cover the AgNW network via the atomic layer deposition (ALD) method. ALD of Al₂O₃ on the AgNW surfaces at low temperature guarantees a conformal coating, which strongly affects the ohmic contacts between the NWs. Our results demonstrate the advantage of THz spectroscopy for the comprehensive analysis of thermal and chemical stabilities of perovskites associated with the electrode materials.

A facile new modified method for the preparation of a new cerium-doped lanthanium cuperate perovskite energy storage system using nanotechnology

New ferroelectric perovskite sample: excellent dielectric, negligible dielectric loss for energy storage systems such as solar cells, solar ponds, and thermal collectors has been prepared at low cost using nanotechnology.

Establishing Multifunctional Interface Layer of Perovskite Ligand Modified Lead Sulfide Quantum Dots for Improving the Performance and Stability of Perovskite Solar Cells

While organic-inorganic halide perovskite solar cells (PSCs) show great potential for realizing low-cost and easily fabricated photovoltaics, the unexpected defects and long-term stability against moisture are the main issues hindering their practical applications. Herein, a strategy is demonstrated to address the main issues by introducing lead sulfide quantum dots (QDs) on the perovskite surface as the multifunctional interface layer on perovskite film through establishing perovskite as the ligand on PbS QDs. Meanwhile, the multifunctions are featured in three aspects including the strong interactions of PbS QDs with perovskites particularly at the grain boundaries favoring good QDs coverage on perovskites for ultimate smooth morphology; an inhibition of iodide ions mobilization by the strong interaction between iodide and the incorporated QDs; and the reduction of the dangling bonds of Pb²⁺ by the sulfur atoms of PbS QDs. Finally, the device performances are highly improved due to the reduced defects and non-radiative recombination. The results show that both open-circuit voltage and fill factor are significantly improved to the high values of 1.13 V and 80%, respectively in CH₃ NH₃ PbI₃ -based PSCs, offering a high efficiency of 20.64%. The QDs incorporation also enhances PSCs' stability benefitting from the induced hydrophobic surface and suppressed iodide mobilization.

Recent progress on nanostructured carbon-based counter/back electrodes for high-performance dye-sensitized and perovskite solar cells

Dye-sensitized solar cells (DSSCs) and perovskite solar cells (PSCs) favor minimal environmental impact and low processing costs, factors that have prompted intensive research and development. In both cases, rare, expensive, and less stable metals (Pt and Au) are used as counter/back electrodes; this design increases the overall fabrication cost of commercial DSSC and PSC devices. Therefore, significant attempts have been made to identify possible substitutes. Carbon-based materials seem to be a favorable candidate for DSSCs and PSCs due to their excellent catalytic ability, easy scalability, low cost, and long-term stability. However, different carbon materials, including carbon black, graphene, and carbon nanotubes, among others, have distinct properties, which have a significant role in device efficiency. Herein, we summarize the recent advancement of carbon-based materials and review their synthetic approaches, structure-function relationship, surface modification, heteroatoms/metal/metal oxide incorporation, fabrication process of counter/back electrodes, and their effects on photovoltaic efficiency, based on previous studies. Finally, we highlight the advantages, disadvantages, and design criteria of carbon materials and fabrication challenges that inspire researchers to find low cost, efficient and stable counter/back electrodes for DSSCs and PSCs.

A surface modifier enhances the performance of the all-inorganic CsPbI2Br perovskite solar cells with efficiencies approaching 15

All-inorganic perovskite solar cells (PSCs) are attracting considerable attention due to their promising thermal stability, but their inferior power-conversion efficiencies (PCE) hinder their realistic application. Here, we propose an approach through surface modification based on methyl ammonium halide (MAX) for inorganic CsPbI2Br solar cells processed at a low temperature. The combined benefits of the introduced MAX modifier enable the boosting of the power conversion efficiency to 14.8% with an impressive FF of 82.2% in CsPbI2Br PSCs. Our experimental analyses coupled with optical modeling indicate that the incorporated MAX leads to an effective passivation of the surface traps originating from Pb2+ and I- ions in CsPbI2Br and simultaneously mediates the crystallization of CsPbI2Br with slightly enlarged grains and reduced numbers of structural defects and pinhole. As a result, the interfacial trap-assisted recombination is suppressed and the charge extraction is promoted. Mechanistically, we show that in the presence of MAX, the deep-level traps in the perovskites are passivated, leaving the energy of the trapping centers to become shallower. In this situation, the negative impacts of the traps on carrier transport and recombination are mitigated.

Easy Strategy to Enhance Thermal Stability of Planar PSCs by Perovskite Defect Passivation and Low-Temperature Carbon-Based Electrode

Organic-inorganic lead halide perovskite has recently emerged as an efficient absorber material for solution process photovoltaic (PV) technology, with certified efficiency exceeding 25%. The development of low-temperature (LT) processing is a challenging topic for decreasing the energy payback time of perovskite solar cell (PSC) technology. In this context, the LT planar n-i-p architecture meets all the requirements in terms of efficiency, scalability, and processability. However, the long-term stability of the LT planar PSC under heat and moisture stress conditions has not been carefully assessed. Here, a detailed study on thermal and moisture stability of large-area (1 cm²) LT planar PSCs is presented. In particular, the key role in thermal stability of potassium iodide (KI) insertion in the perovskite composition is demonstrated. It is found that defect passivation of triple-cation perovskite by KI doping inhibits the halide migration induced by thermal stress at 85 °C and delays the formation of degradation subproducts. T80, defined as the time when the cell retains 80% of initial efficiency, is evaluated both for reference undoped devices and KI-doped ones. The results show that T80 increases 3 times when KI doping is used. Moreover, an HTL-free architecture where the Au top electrode is replaced with low-T screen-printable carbon paste is proposed. The combination of the carbon-based HTL-free architecture and KI-doped perovskite permits T80 to increase from 40 to 414 h in unsealed devices.

Double-Mesoscopic Hole-Transport-Material-Free Perovskite Solar Cells: Overcoming Charge-Transport Limitation by Sputtered Ultrathin Al2O3 Isolating Layer

The electrically insulating space layer takes a fundamental role in monolithic carbon-graphite based perovskite solar cells (PSCs) and it has been established to prevent the charge recombination of electrons at the mp-TiO₂/carbon-graphite (CG) interface. Thick 1 μm printed layers are commonly used for this purpose in the established triple-mesoscopic structures to avoid ohmic shunts and to achieve a high open circuit voltage. In this work, we have developed a reproducible large-area procedure to replace this thick space layer with an ultra-thin dense 40 nm sputtered Al₂O₃ which acts as a highly electrically insulating layer preventing ohmic shunts. Herewith, transport limitations related so far to the hole diffusion path length inside the thick mesoporous space layer have been omitted by concept. This will pave the way toward the development of next generation double-mesoscopic carbon-graphite-based PSCs with highest efficiencies. Scanning electron microscope, energy dispersive X-ray analysis, and atomic force microscopy measurements show the presence of a fully oxidized sputtered Al₂O₃ layer forming a pseudo-porous covering of the underlying mesoporous layer. The thickness has been finely tuned to achieve both electrical isolation and optimal infiltration of the perovskite solution allowing full percolation and crystallization. Photo voltage decay, light-dependent, and time-dependent photoluminescence measurements showed that the optimal 40 nm thick Al₂O₃ not only prevents ohmic shunts but also efficiently reduces the charge recombination at the mp-TiO₂/CG interface and, at the same time, allows efficient hole diffusion through the perovskite crystals embedded in its pseudo-pores. Thus, a stable V _OC of 1 V using CH₃NH₃PbI₃ perovskite has been achieved under full sun AM 1.5 G with a stabilized device performance of 12.1%.

Atomic Layer Deposition of an Effective Interface Layer of TiN for Efficient and Hysteresis-Free Mesoscopic Perovskite Solar Cells

Perovskite solar cells (PSCs) have experienced outstanding advances in power conversion efficiencies (PCEs) by employing new electron transport layers (ETLs), interface engineering, optimizing perovskite morphology, and improving charge collection efficiency. In this work, we study the role of a new ultrathin interface layer of titanium nitride (TiN) conformally deposited on a mesoporous TiO₂ (mp-TiO₂) scaffold using the atomic layer deposition method. Our characterization results revealed that the presence of TiN at the ETL/perovskite interface improves the charge collection as well as reduces the interface recombination. We find that the morphology (grain size) and optical properties of the perovskite film deposited on the optimized mp-TiO₂/TiN ETL are improved drastically, leading to devices with a maximum PCE of 19.38% and a high open-circuit voltage (V_oc) of 1.148 V with negligible hysteresis and improved environmental (∼40% RH) and thermal (80 °C) stabilities.

High-Performance and Hysteresis-Free Perovskite Solar Cells Based on Rare-Earth-Doped SnO2 Mesoporous Scaffold

Tin oxide (SnO₂), as electron transport material to substitute titanium oxide (TiO₂) in perovskite solar cells (PSCs), has aroused wide interests. However, the performance of the PSCs based on SnO₂ is still hard to compete with the TiO₂-based devices. Herein, a novel strategy is designed to enhance the photovoltaic performance and long-term stability of PSCs by integrating rare-earth ions Ln³⁺ (Sc³⁺, Y³⁺, La³⁺) with SnO₂ nanospheres as mesoporous scaffold. The doping of Ln promotes the formation of dense and large-sized perovskite crystals, which facilitate interfacial contact of electron transport layer/perovskite layer and improve charge transport dynamics. Ln dopant optimizes the energy level of perovskite layer, reduces the charge transport resistance, and mitigates the trap state density. As a result, the optimized mesoporous PSC achieves a champion power conversion efficiency (PCE) of 20.63% without hysteresis, while the undoped PSC obtains an efficiency of 19.01%. The investigation demonstrates that the rare-earth doping is low-cost and effective method to improve the photovoltaic performance of SnO₂-based PSCs.

Dopant-Free Squaraine-Based Polymeric Hole-Transporting Materials with Comprehensive Passivation Effects for Efficient All-Inorganic Perovskite Solar Cells

Development of high-performance dopant-free hole-transporting materials (HTMs) with comprehensive passivation effects is highly desirable for all-inorganic perovskite solar cells (PVSCs). Squaraines (SQs) could be a candidate for dopant-free HTMs as they are natural passivators for perovskites. One major limitation of SQs is their relatively low hole mobility. Herein we demonstrate that polymerizing SQs into pseudo two dimensional (2D) p-π conjugated polymers could overcome this problem. By rationally using N,N-diarylanilinosquaraines as the comonomers, the resulting polysquaraine HTMs not only exhibit suitable energy levels and efficient passivation effects, but also achieve very high hole mobility close to 0.01 cm^-2  V^-1  s^-1 . Thus as dopant-free HTMs for α-CsPbI₂ Br-based all-inorganic PVSCs, the best PCE reached is 15.5 %, outperforming those of the doped-Spiro-OMeTAD (14.4 %) based control devices and among the best for all-inorganic PVSCs.

Interfacial Engineering for High-Efficiency Nanorod Array-Structured Perovskite Solar Cells

TiO₂ nanorod (NR) array for perovskite solar cells (PSCs) has attained great importance due to its superb power conversion efficiency (PCE) compared to that of the traditional mesoporous TiO₂ film. A TiO₂ compact layer for the growth of TiO₂ NR array via spin-coating cannot meet the requirements for efficient NR-based PSCs. Herein, we have developed and demonstrated the insertion of a bifunctional extrathin TiO₂ interlayer (5 nm) by atomic layer deposition (ALD) at the interface of the fluorine-doped tin oxide (FTO)/TiO₂ compact layer to achieve alleviated electron exchange and a reduced energetic barrier. Thus, an accelerated extraction of electrons from TiO₂ NR arrays via the compact layer and their transfer to the FTO substrate can improve the PSC efficiency. The thickness of the spin-coated TiO₂ compact layer on the ALD-deposited TiO₂ layer is spontaneously optimized. Finally, an outstanding efficiency of 20.28% has been achieved from a champion PSC with negligible hysteresis and high reliability. To the best of our knowledge, this is the first study demonstrating the superiority of TiO₂-NR-based PSCs withstanding the dry heat and thermal cycling tests. The results are of great importance for the preparation of efficient and durable PSCs for real-world applications.

Nonsiliceous Mesoporous Materials: Design and Applications in Energy Conversion and Storage

In this work, the progress in the design of nonsiliceous mesoporous materials (nonSiMPMs) over the last five years from the perspectives of the chemical composition, morphology, loading, and surface modification is summarized. Carbon, metal, and metal oxide are in focus, which are the most promising compositions. Then, representative applications of nonSiMPMs are demonstrated in energy conversion and storage, including recent technical advances in dye-sensitized solar cells, perovskite solar cells, photocatalysts, electrocatalysts, fuel cells, storage batteries, supercapacitors, and hydrogen storage systems. Finally, the requirements and challenges of the design and application of nonSiMPMs are outlined.

Defect and Contact Passivation for Perovskite Solar Cells

Metal-halide perovskites are rapidly emerging as an important class of photovoltaic absorbers that may enable high-performance solar cells at affordable cost. Thanks to the appealing optoelectronic properties of these materials, tremendous progress has been reported in the last few years in terms of power conversion efficiencies (PCE) of perovskite solar cells (PSCs), now with record values in excess of 24%. Nevertheless, the crystalline lattice of perovskites often includes defects, such as interstitials, vacancies, and impurities; at the grain boundaries and surfaces, dangling bonds can also be present, which all contribute to nonradiative recombination of photo-carriers. On device level, such recombination undesirably inflates the open-circuit voltage deficit, acting thus as a significant roadblock toward the theoretical efficiency limit of 30%. Herein, the focus is on the origin of the various voltage-limiting mechanisms in PSCs, and possible mitigation strategies are discussed. Contact passivation schemes and the effect of such methods on the reduction of hysteresis are described. Furthermore, several strategies that demonstrate how passivating contacts can increase the stability of PSCs are elucidated. Finally, the remaining key challenges in contact design are prioritized and an outlook on how passivating contacts will contribute to further the progress toward market readiness of high-efficiency PSCs is presented.

Model for the Prediction of the Lifetime and Energy Yield of Methyl Ammonium Lead Iodide Perovskite Solar Cells at Elevated Temperatures

With the realization of highly efficient perovskite solar cells, the long-term stability of these devices is the key challenge hindering their commercialization. In this work, we study the temperature-dependent stability of perovskite solar cells and develop a model capable of predicting the lifetime and energy yield of perovskite solar cells outdoors. This model results from the measurement of the kinetics governing the degradation of perovskite solar cells at elevated temperatures. The individual analysis of all key current-voltage parameters enables the prediction of device performance under thermal stress with high precision. An extrapolation of the device lifetime at various European locations based on historical weather data illustrates the relation between the laboratory data and real-world applications. Finally, the understanding of the degradation mechanisms affecting perovskite solar cells allows the definition and implementation of strategies to enhance the thermal stability of perovskite solar cells.

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.

Diboron-Assisted Interfacial Defect Control Strategy for Highly Efficient Planar Perovskite Solar Cells

Metal halide perovskite films are endowed with the nature of ions and polycrystallinity. Formamidinium iodide (FAI)-based perovskite films, which include large cations (FA) incorporated into the crystal lattice, are most likely to induce local defects due to the presence of the unreacted FAI species. Here, a diboron-assisted strategy is demonstrated to control the defects induced by the unreacted FAI both inside the grain boundaries and at the surface regions. The diboron compound (C₁₂ H₁₀ B₂ O₄ ) can selectively react with unreacted FAI, leading to reduced defect densities. Nonradiative recombination between a perovskite film and a hole-extraction layer is mitigated considerably after the introduction of the proposed approach and charge-carrier extraction is improved as well. A champion power conversion efficiency of 21.11% is therefore obtained with a stabilized power output of 20.83% at the maximum power point for planar perovskite solar cells. The optimized device also delivers negligible hysteresis effect under various scanning conditions. This approach paves a new way for mitigating defects and improving device performance.

Enhanced Performance of Perovskite Solar Cells by Using Ultrathin BaTiO3 Interface Modification

Efficiency promotion has been severely constrained by charge recombination in perovskite solar cells (PSCs). Interface modification has been proved to be an effective way to reduce the interfacial charge recombination. In this work, a mesoporous TiO₂ (mp-TiO₂) layer was modified by an ultrathin BaTiO₃ layer to suppress charge recombination in PSCs. The ultrathin BaTiO₃ modification layer was prepared by the spin coating method using a barium salt solution. The concentration of the barium salt solution was optimized, and the effect of the BaTiO₃ modification layer on the performance of the cells was also investigated. The modification layer can not only successfully retard charge recombination but also effectively boost the rate of electron extraction at the interface, resulting in enhanced open-circuit voltage ( V_oc), short circuit current density ( J_sc), and fill factor. Furthermore, the hysteresis of the PSCs was also significantly reduced after the modification. By optimizing and employing the BaTiO₃ modification layer, the power conversion efficiency of the cells was increased from 16.13 to 17.87%.

Low-Temperature Plasma-Assisted Atomic-Layer-Deposited SnO2 as an Electron Transport Layer in Planar Perovskite Solar Cells

In this work, we present an extensive characterization of plasma-assisted atomic-layer-deposited SnO₂ layers, with the aim of identifying key material properties of SnO₂ to serve as an efficient electron transport layer in perovskite solar cells (PSCs). Electrically resistive SnO₂ films are fabricated at 50 °C, while a SnO₂ film with a low electrical resistivity of 1.8 × 10^-3 Ω cm, a carrier density of 9.6 × 10¹⁹ cm^-3, and a high mobility of 36.0 cm²/V s is deposited at 200 °C. Ultraviolet photoelectron spectroscopy indicates a conduction band offset of ∼0.69 eV at the 50 °C SnO₂/Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_2.7Br_0.3) interface. In contrast, a negligible conduction band offset is found between the 200 °C SnO₂ and the perovskite. Surprisingly, comparable initial power conversion efficiencies (PCEs) of 17.5 and 17.8% are demonstrated for the champion cells using 15 nm thick SnO₂ deposited at 50 and 200 °C, respectively. The latter gains in fill factor but loses in open-circuit voltage. Markedly, PSCs using the 200 °C compact SnO₂ retain their initial performance at the maximum power point over 16 h under continuous one-sun illumination in inert atmosphere. Instead, the cell with the 50 °C SnO₂ shows a decrease in PCE of approximately 50%.

Effect of Ammonium Halide Additives on the Performance of Methyl Amine Based Perovskite Solar Cells

CH₃NH₃PbI3-xClx species were fabricated as light-absorbing layers for perovskite solar cells (PSCs), by employing NH₄I, NH₄Br, and NH₄Cl as additives via annealing at 100 °C for different times. Solutions containing CH₃NH₃I, PbI₂, and PbCl₂ (4:1:1 molar ratio) in N,N-dimethylformamide were used to prepare perovskites with NH₄I, NH₄Br, and NH₄Cl as additives, at concentrations of 0.1 M and 0.3 M. The additives helped increase the grain size and reduce pinholes in the perovskite films, as confirmed by field-emission scanning electron microscopy. The X-ray diffraction profiles of CH₃NH₃PbI3-xClx clearly showed peaks at 14° and 28° for the samples with additives, indicative of crystallinity. The best PSC performance with a power conversion efficiency of 9.13%, was achieved using 0.1 M NH₄I by annealing for 5 min, whereas the power conversion efficiency of the perovskite solar cells without additives was 5.40%.

Toward Perovskite Solar Cell Commercialization: A Perspective and Research Roadmap Based on Interfacial Engineering

High-efficiency and low-cost perovskite solar cells (PVKSCs) are an ideal candidate for addressing the scalability challenge of solar-based renewable energy. The dynamically evolving research field of PVKSCs has made immense progress in solving inherent challenges and capitalizing on their unique structure-property-processing-performance traits. This review offers a unique outlook on the paths toward commercialization of PVKSCs from the interfacial engineering perspective, relevant to both specialists and nonspecialists in the field through a brief introduction of the background of the field, current state-of-the-art evolution, and future research prospects. The multifaceted role of interfaces in facilitating PVKSC development is explained. Beneficial impacts of diverse charge-transporting materials and interfacial modifications are summarized. In addition, the role of interfaces in improving efficiency and stability for all emerging areas of PVKSC design are also evaluated. The authors' integral contributions in this area are highlighted on all fronts. Finally, future research opportunities for interfacial material development and applications along with scalability-durability-sustainability considerations pivotal for facilitating laboratory to industry translation are presented.

A Novel Conductive Mesoporous Layer with a Dynamic Two-Step Deposition Strategy Boosts Efficiency of Perovskite Solar Cells to 20

Lead halide perovskite solar cells (PSCs) with the high power conversion efficiency (PCE) typically use mesoporous metal oxide nanoparticles as the scaffold and electron-transport layers. However, the traditional mesoporous layer suffers from low electron conductivity and severe carrier recombination. Here, antimony-doped tin oxide nanorod arrays are proposed as novel transparent conductive mesoporous layers in PSCs. Such a mesoporous layer improves the electron transport as well as light utilization. To resolve the common problem of uneven growth of perovskite on rough surface, the dynamic two-step spin coating strategy is proposed to prepare highly smooth, dense, and crystallized perovskite films with micrometer-scale grains, largely reducing the carrier recombination ratio. The conductive mesoporous layer and high-quality perovskite film eventually render the PSC with a remarkable PCE of 20.1% with excellent reproducibility. These findings provide a new avenue to further design high-efficiency PSCs from the aspect of carrier transport and recombination.

Efficient Perovskite Solar Cells with Reduced Photocurrent Hysteresis through Tuned Crystallinity of Hybrid Perovskite Thin Films

Hybrid perovskite materials used for realization of efficient perovskite solar cells have drawn great attention in both academic and industrial sectors. It was reported that the crystallinity of hybrid thin-film perovskite materials plays an important role in device performance. In this study, we report a novel and simple method to tune the crystallinity of CH₃NH₃PbI₃ thin film for device performance of perovskite solar cells. By employing tetraphenylphosphonium chloride on the top of PbI₂ thin layer in the two-step perovskite deposition processes, the crystallinity of the resultant CH₃NH₃PbI₃ thin film was tuned. As a result, perovskite solar cells by the CH₃NH₃PbI₃ thin film with tuned crystallinity exhibit an enlarged open-circuit voltage and enhanced short-circuit current, thus boosted efficiency as well as reduced photocurrent hysteresis compared to pristine CH₃NH₃PbI₃ thin film. These results indicate that our study provides a new simple way to boost device performance of perovskite solar cells through tuning the crystallinity of CH₃NH₃PbI₃ thin film.

Efficient Planar Perovskite Solar Cells Using Passivated Tin Oxide as an Electron Transport Layer

Planar perovskite solar cells using low-temperature atomic layer deposition (ALD) of the SnO2 electron transporting layer (ETL), with excellent electron extraction and hole-blocking ability, offer significant advantages compared with high-temperature deposition methods. The optical, chemical, and electrical properties of the ALD SnO2 layer and its influence on the device performance are investigated. It is found that surface passivation of SnO2 is essential to reduce charge recombination at the perovskite and ETL interface and show that the fabricated planar perovskite solar cells exhibit high reproducibility, stability, and power conversion efficiency of 20%.

Elucidating the Impact of Thin Film Texture on Charge Transport and Collection in Perovskite Solar Cells

Organic-inorganic halide perovskites have emerged as one of the most promising materials for photovoltaic applications. Because of the polycrystalline nature of perovskite thin films, it is crucial to investigate the impact of microstructures on device performance. In this study, we employ ramp-annealing to tailor the texture of perovskite thin films via controlling the nucleation of perovskite grains. Electrochemical impedance spectroscopy studies further suggest that the thin film texture impacts not only the charge collection at the contact but also the carrier transport in the bulk perovskite layer. The combination of the two effects leads to enhanced performance in devices constructed of preferentially oriented perovskite thin films.

Spinel Co3O4 nanomaterials for efficient and stable large area carbon-based printed perovskite solar cells

Carbon based perovskite solar cells (PSCs) are fabricated through easily scalable screen printing techniques, using abundant and cheap carbon to replace the hole transport material (HTM) and the gold electrode further reduces costs, and carbon acts as a moisture repellent that helps in maintaining the stability of the underlying perovskite active layer. An inorganic interlayer of spinel cobaltite oxides (Co₃O₄) can greatly enhance the carbon based PSC performance by suppressing charge recombination and extracting holes efficiently. The main focus of this research work is to investigate the effectiveness of Co₃O₄ spinel oxide as the hole transporting interlayer for carbon based perovskite solar cells (PSCs). In these types of PSCs, the power conversion efficiency (PCE) is restricted by the charge carrier transport and recombination processes at the carbon-perovskite interface. The spinel Co₃O₄ nanoparticles are synthesized using the chemical precipitation method, and characterized by X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and UV-Vis spectroscopy. A screen printed thin layer of p-type inorganic spinel Co₃O₄ in carbon PSCs provides a better-energy level matching, superior efficiency, and stability. Compared to standard carbon PSCs (PCE of 11.25%) an improved PCE of 13.27% with long-term stability, up to 2500 hours under ambient conditions, is achieved. Finally, the fabrication of a monolithic perovskite module is demonstrated, having an active area of 70 cm² and showing a power conversion efficiency of >11% with virtually no hysteresis. This indicates that Co₃O₄ is a promising interlayer for efficient and stable large area carbon PSCs.

An optical dynamic study of MAPbBr3 single crystals passivated with MAPbCl3/I3-MAPbBr3 heterojunctions

Recently, perovskite based solar cells have attracted lots of research interest, some of which is in the passivation of perovskite surfaces, particularly the heterojunction based surface passivation. In this study, the optical dynamics of MAPbBr₃ single crystals with and without heterojunction passivation were studied systematically by means of a time-resolved spectroscopic technique for the first time. The emission lifetime of MAPbBr₃ single crystals under two-photon (1064 nm) excitation is a few orders of magnitude longer than that measured under one-photon (355 nm or 532 nm) excitation. Interestingly, with surface passivation, the lifetime measured at 355 nm excitations could be tuned significantly, whereas the lifetime change under 1064 nm excitations was considerably less. Our results give a direct evidence of surface quench by comparing the lifetimes before and after surface passivation. Furthermore, the results demonstrate that proper MAPbCl₃-MAPbBr₃ heterojunctions can dramatically reduce the recombination channels in the surface region, which can be potentially useful for perovskite based solar cells, light emitting diodes (LED), and sensitive detectors.

Efficient Yttrium(III) Chloride-Treated TiO2 Electron Transfer Layers for Performance-Improved and Hysteresis-Less Perovskite Solar Cells

Hybrid organic-inorganic metal halide perovskite solar cells have attracted widespread attention, owing to their high performance, and have undergone rapid development. In perovskite solar cells, the charge transfer layer plays an important role for separating and transferring photogenerated carriers. In this work, an efficient YCl₃ -treated TiO₂ electron transfer layer (ETL) is used to fabricate perovskite solar cells with enhanced photovoltaic performance and less hysteresis. The YCl₃ -treated TiO₂ layers bring about an upward shift of the conduction band minimum (E_CBM ), which results in a better energy level alignment for photogenerated electron transfer and extraction from the perovskite into the TiO₂ layer. After optimization, perovskite solar cells based on the YCl₃ -treated TiO₂ layers achieve a maximum power conversion efficiency of about 19.99 % (19.29 % at forward scan) and a steady-state power output of about 19.6 %. Steady-state and time-resolved photoluminescence measurements and impedance spectroscopy are carried out to investigate the charge transfer and recombination dynamics between the perovskite and the TiO₂ electron transfer layer interface. The improved perovskite/TiO₂ ETL interface with YCl₃ treatment is found to separate and extract photogenerated charge rapidly and suppress recombination effectively, which leads to the improved performance.

The optimum titanium precursor of fabricating TiO2 compact layer for perovskite solar cells

Perovskite solar cells (PSCs) have attracted tremendous attentions due to its high performance and rapid efficiency promotion. Compact layer plays a crucial role in transferring electrons and blocking charge recombination between the perovskite layer and fluorine-doped tin oxide (FTO) in PSCs. In this study, compact TiO₂ layers were synthesized by spin-coating method with three different titanium precursors, titanium diisopropoxide bis (acetylacetonate) (c-TTDB), titanium isopropoxide (c-TTIP), and tetrabutyl titanate (c-TBOT), respectively. Compared with the PSCs based on the widely used c-TTDB and c-TTIP, the device based on c-TBOT has significantly enhanced performance, including open-circuit voltage, short-circuit current density, fill factor, and hysteresis. The significant enhancement is ascribed to its excellent morphology, high conductivity and optical properties, fast charge transfer, and large recombination resistance. Thus, a power conversion efficiency (PCE) of 17.03% has been achieved for the solar cells based on c-TBOT.

High Efficiency MAPbI3 Perovskite Solar Cell Using a Pure Thin Film of Polyoxometalate as Scaffold Layer

Here, we successfully used a pure layer of [SiW₁₁ O₃₉ ]^8- polyoxomethalate (POM) structure as a thin-film scaffold layer for CH₃ NH₃ PbI₃ -based perovskite solar cells (PSCs). A smooth nanoporous surface of POM causes outstanding improvement of the photocurrent density, external quantum efficiency (EQE), and overall efficiency of the PSCs compared to mesoporous TiO₂ (mp-TiO₂ ) as scaffold layer. Average power conversion efficiency (PCE) values of 15.5 % with the champion device showing 16.3 % could be achieved by using POM and a sequential deposition method with the perovskite layer. Furthermore, modified and defect-free POM/perovskite interface led to elimination of the anomalous hysteresis in the current-voltage curves. The open-circuit voltage decay study shows promising decrease of the electron recombination in the POM-based PSCs, which is also related to the modification of the POM/ perovskite interface and higher electron transport inside the POM layer.

Interface Engineering of Perovskite Solar Cells with Air Plasma Treatment for Improved Performance

For high-efficiency perovskite solar cells (PSCs), interface engineering becomes critical for carrier collection from the active perovskite material to the transport layer. To enhance the power conversion efficiency (PCE), herein we demonstrate a novel method named surface plasma treatment on a mesoporous TiO₂ electron-transport layer (ETL) to improve electron extraction and transport properties at the perovskite/TiO₂ interface. According to the XPS results, the plasma treatment induced a partial reduction of Ti⁴⁺ to Ti³⁺ within the TiO₂ lattice and increased the concentration of oxygen vacancies on the TiO₂ surface. Ultraviolet photoelectron spectra (UPS) show that the Fermi level of TiO₂ upshifts about 0.2 eV which may effectively promote carrier separation and transfer at the perovskite/TiO₂ interface. In addition, these created donor levels of Ti³⁺ and oxygen vacancies donate extra electrons, increasing the conductivities of TiO₂ films and which could further promote transport. The time-resolved photoluminescence spectra (TRPL) confirm that the decay time decreases dramatically from 656 ns to 235 ns after 90 s plasma treatment, which indicates a more efficient electron-transfer process. Based on all the above-mentioned results, a remarkable enhancement in cell efficiency was obtained, such that the average efficiency was improved from 11.5 % to 14.3 % under AM 1.5G irradiation (100 mW cm^-2 ).