A review on the classification of organic/inorganic/carbonaceous hole transporting materials for perovskite solar cell application

What Should be Considered While Designing Hole-Transporting Material for Perovskite Solar Cells? A Special Attention to Thiophene-Based Hole-Transporting Materials

The molecular design and conformations of hole-transporting materials (HTM) have unravelled a strategy to enhance the performance of environmentally sustainable perovskite solar cells (PSC). Several attempts have been made and several are underway for improving the efficiency of PSCs by designing an efficient HTM, which is crucial to preventing corrosion, facilitating effective hole transportation, and preventing charge recombination. There is a need for a potential alternative to the current market-dominating HTM due to its high cost of production, dopant requirements, moisture sensitivity, and low stability. Among several proposed HTMs, molecules derived from thiophene exhibit unique behaviour, such as the interaction with under-coordinated Pb2+, thereby facilitating the passivation of surface defects in the perovskite layer. In addition, coupling a suitable side chain imparts a hydrophobic character, eventually leading to the development of a moisture-sensitive and highly stable PSC. Furthermore, thiophene-backboned polymers with ionic pendants have been employed as an interfacial layer between PSC layers, with the backbone facilitating efficient charge transfer. This perspective article comprehensively presents the design strategy, characterization, and function of HTMs associated with thiophene-derived molecules. Hence, it is observed that thiophene-formulated HTMs have an enhanced passivation effect, good performance in an open-circuit environment, longevity, humidity resistance, thermostability, good hole extraction, and mobility in a dopant-free condition. For a better understanding, the article provides a comparative description of the activity and function of thiophene-based small molecules and polymers and their effect on device performance.

Tuning ZnO-based piezoelectric nanogenerator efficiency through n-ZnO/p-NiO bulk interfacing

ZnO based piezoelectric nanogenerators (PENG) hold immense potential for harvesting ambient vibrational mechanical energy into electrical energy, offering sustainable solutions in the field of self-powered sensors, wearable electronics, human-machine interactions etc. In this study, we have developed flexible ZnO-based PENGs by incorporating ZnO microparticles into PDMS matrix, with ZnO concentration ranging from 5 to 25 wt%. Among these, the PENG containing 15 wt% ZnO exhibited the best performance with an open-circuit output voltage/short-circuit current of ~ 42.4 V/2.4 µA. To further enhance the output performance of PENG, p-type NiO was interfaced with ZnO in a bulk hetero-junction geometry. The concentration of NiO was varied from 5 to 20 wt% with respect to ZnO and incorporated into the PDMS matrix to fabricate the PENGs. The PENG containing 10 wt% NiO exhibits the best performance with an open-circuit output voltage/short-circuit current of ~ 65 V/4.1 µA under loading conditions of 30 N and 4 Hz. The PENG exhibiting the best performance demonstrates a maximum instantaneous output power density ~ 37.9 µW/cm2 across a load resistance of 20 MΩ under loading conditions of 30 N and 4 Hz, with a power density per unit force and Hertz of about ~ 0.32 µW/cm2·N·Hz. The enhanced output performance of the PENG is attributed to the reduction in free electron concentration, which suppresses the internal screening effect of the piezopotential. To assess the practical utility of the optimized PENG, we tested the powering capability by charging various commercial capacitors and used the stored energy to illuminate 10 LEDs and to power a stopwatch displays. This work not only presents a straightforward, cost-effective, and scalable technique for enhancing the output performance of ZnO-based PENGs but also sheds light on its underlying mechanism.

Critical Review of Cu-Based Hole Transport Materials for Perovskite Solar Cells: From Theoretical Insights to Experimental Validation

Despite the remarkable efficiency of perovskite solar cells (PSCs), long-term stability remains the primary barrier to their commercialization. The prospect of enhancing stability by substituting organic transport layers with suitable inorganic compounds, particularly Cu-based inorganic hole-transport materials (HTMs), holds promise due to their high valence band maximum (VBM) aligning with perovskite characteristics. This review assesses the advantages and disadvantages of these five types of Cu-based HTMs. Although Cu-based binary oxides and chalcogenides face narrow bandgap issues, the "chemical modulation of the valence band" (CMVB) strategy has successfully broadened the bandgap for Cu-based ternary oxides and chalcogenides. However, Cu-based ternary oxides encounter challenges with low mobility, and Cu-based ternary chalcogenides face mismatches in VBM alignment with perovskites. Cu-based binary halides, especially CuI, exhibit excellent properties such as wider bandgap, high mobility, and defect tolerance, but their stability remains a concern. These limitations of single anion compounds are insightfully discussed, offering solutions from the perspective of practical application. Future research can focus on Cu-based composite anion compounds, which merge the advantages of single anion compounds. Additionally, mixed-cation chalcogenides such as CuxM1-xS enable the customization of HTM properties by selecting and adjusting the proportions of cation M.

Performance analysis of un-doped and doped titania (TiO 2 ) as an electron transport layer (ETL) for perovskite solar cells

CONTEXT

Density functional theory (DFT) calculations are carried out on pure and doped rutile TiO2 . The bandgap (Eg ) for pristine, S-doped, Fe-doped, and Fe/S co-doped materials is direct, with values of 2.98 eV, 2.18 eV, 1.58 eV, and 1.40 eV. The effective mass of charge carriers (m*) and ratio of effective masses of holes to effective masses of electrons (R) are also investigated, and it is discovered that Fe/S co-doped materials have the lowest charge carrier recombination rate. The Fe/S co-doped material has the highest ε ( ω ) . α ( ω ) of doped materials shifted into the visible range. Due to the high dopant concentration in Fe and Fe/S-doped cases, the Eg is lowered to a relatively small value; hence, only pristine and S-doped materials are verified as electron transport layer (ETL). A solar cell device analysis employing pure and S-doped rutile TiO2 as ETL is completed using DFT-derived parameters in SCAPS-1D modeling software for the first time. For the optimized solar cells, current-voltage (IV) characteristics, quantum efficiency (QE), capacitance-voltage (CV) characteristics, and capacitance-frequency (Cf) characteristics are provided. The aim of the present study is to improve efficiency of perovskite solar cell by doping as well as to improve accuracy of simulation by applying DFT extracted parameters as input. From the analysis, improvement is found in efficiency of doped TiO2 compared to un-doped TiO2 . The efficiency of the PSC with S-doped ETL is 1.418% higher than the PSC with un-doped ETL.

METHOD

Quantumwise Automistic Tool Kit (ATK) is used to extract DFT parameters. Using these DFT parameters as input in SCAPS-1D (Solar Cell Capacitance Simulator), solar cells for doped and un-doped material are simulated. The density functional theory (DFT)-based orthogonalized linear combination of atomic orbital (OLCAO) technique is used. Structural optimization is done using the LBFGS (Limited-memory Broyden-Fletcher-Goldfarb-Shanno). PBESol-GGA (Perdew-Burke-Ernzerhof solid-generalized gradient approximation) is applied as exchange correlation for calculating structural parameters, while MGGA-TB09 (meta-generalized gradient approximation-Tran and Blaha) is applied as exchange correlation for calculating optical and electronic properties.

Air-Stable and Eco-Friendly Symmetrical Imine with Thiadiazole Moieties in Neutral and Protonated form for Perovskite Photovoltaics

This paper proposes molecular and supramolecular concepts for potential application in perovskite solar cells. New air-stable symmetrical imine, with thiadiazole moieties PPL2: (5E,6E)-N2,N5-bis(4-(diphenylamino)benzylidene)-1,3,4-thiadiazole-2,5-diamine), as a hole-transporting material was synthesised in a single-step reaction, starting with commercially available and relatively inexpensive reagents, resulting in a reduction in the cost of the final product compared to Spiro-OMeTAD. Moreover, camphorsulfonic acid (CSA) in both enantiomeric forms was used to change the HOMO-LUMO levels and electric properties of the investigated imine-forming complexes. Electric, optical, thermal, and structural studies of the imine and its complexes with CSA were carried out to characterise the new material. Imine and imine/CSA complexes were also characterised in depth by the proton Nuclear Magnetic Resonance 1H NMR method. The position of nitrogen in the thidiazole ring influences the basicity of donor centres, which results in protonation in the imine bond. Simple devices of ITO/imine (with or without CSA(-) or CSA(+))/Ag/ITO architecture were constructed, and a thermographic camera was used to find the defects in the created devices. Electric behaviour was also studied to demonstrate conductivity properties under the forward current. Finally, the electrical properties of imine and its protonated form with CSA were compared with Spiro-OMeTAD. In general, the analysis of thermal images showed a very similar response of the samples to the applied potential in terms of the homogeneity of the formed organic layer. The TGA analysis showed that the investigated imine exhibits good thermal stability in air and argon atmospheres.

Recent Advances in Graphene-Enabled Materials for Photovoltaic Applications: A Comprehensive Review

Graphene's two-dimensional structural arrangement has sparked a revolutionary transformation in the domain of conductive transparent devices, presenting a unique opportunity in the renewable energy sector. This comprehensive Review critically evaluates the most recent advances in graphene production and its employment in solar cells, focusing on dye-sensitized, organic, and perovskite devices for bulk heterojunction (BHJ) designs. This comprehensive investigation discovered the following captivating results: graphene integration resulted in a notable 20.3% improvement in energy conversion rates in graphene-perovskite photovoltaic cells. In comparison, BHJ cells saw a laudable 10% boost. Notably, graphene's 2D internal architecture emerges as a protector for photovoltaic devices, guaranteeing long-term stability against various environmental challenges. It acts as a transportation facilitator and charge extractor to the electrodes in photovoltaic cells. Additionally, this Review investigates current research highlighting the role of graphene derivatives and their products in solar PV systems, illuminating the way forward. The study elaborates on the complexities, challenges, and promising prospects underlying the use of graphene, revealing its reflective implications for the future of solar photovoltaic applications.

Advances in Smart Photovoltaic Textiles

Energy harvesting textiles have emerged as a promising solution to sustainably power wearable electronics. Textile-based solar cells (SCs) interconnected with on-body electronics have emerged to meet such needs. These technologies are lightweight, flexible, and easy to transport while leveraging the abundant natural sunlight in an eco-friendly way. In this Review, we comprehensively explore the working mechanisms, diverse types, and advanced fabrication strategies of photovoltaic textiles. Furthermore, we provide a detailed analysis of the recent progress made in various types of photovoltaic textiles, emphasizing their electrochemical performance. The focal point of this review centers on smart photovoltaic textiles for wearable electronic applications. Finally, we offer insights and perspectives on potential solutions to overcome the existing limitations of textile-based photovoltaics to promote their industrial commercialization.

New bithiophene-based molecules as hole transporting materials for perovskite solar cells and or as donor for organic solar cells

DFT and TDDFT approaches were used to design three (T16,17,18) molecules based on 4,4'-dimethoxy-2,2'-bithiophene core to explore the influence of substitution of triphenylamine (TPA) fragment by methoxy groups, and introduction of azomethine π-bridges on the optoelectronic properties of hole transporting materials for perovskite solar cells (PSCs) or as donor for organic solar cells (OSCs). To shed light on the efficiency, stability, and solubility several physicochemical parameters were computed in dichloromethane solvent. All designed molecules show appropriate frontier molecular orbital levels, which facilitates effective hole transfer from the perovskite materials to the HTMs in the hole-transporting layer in PSC devices. They all show good efficiency and pore-fillings and are stable and soluble in dichloromethane. Electron-hole pairs can easily dissociate into free charge carriers, especially for T16 and T17; consequently, improve short-circuit current densities and facilitate hole transport. It is also advised to use T18 which includes azomethine bridges as a donor with a non-fullerene Y6 acceptor to create effective OSCs because it exhibits high open circuit voltage, fill factor and low gap energy.

Evaluating the influence of novel charge transport materials on the photovoltaic properties of MASnI3 solar cells through SCAPS-1D modelling

In recent decades, substantial advancements have been made in photovoltaic technologies, leading to impressive power conversion efficiencies (PCE) exceeding 25% in perovskite solar cells (PSCs). Tin-based perovskite materials, characterized by their low band gap (1.3 eV), exceptional optical absorption and high carrier mobility, have emerged as promising absorber layers in PSCs. Achieving high performance and stability in PSCs critically depends on the careful selection of suitable charge transport layers (CTLs). This research investigates the effects of five copper-based hole transport materials and two carbon-based electron transport materials in combination with methyl ammonium tin iodide (MASnI3) through numerical modelling in SCAPS-1D. The carbon-based CTLs exhibit excellent thermal conductivity and mechanical strength, while the copper-based CTLs demonstrate high electrical conductivity. The study comprehensively analyses the influence of these CTLs on PSC performance, including band alignment, quantum efficiency, thickness, doping concentration, defects and thermal stability. Furthermore, a comparative analysis is conducted on PSC structures employing both p-i-n and n-i-p configurations. The highest-performing PSCs are observed in the inverted structures of CuSCN/MASnI3/C60 and CuAlO2/MASnI3/C60, achieving PCE of 23.48% and 25.18%, respectively. Notably, the planar structures of Cu2O/MASnI3/C60 and CuSbS2/MASnI3/C60 also exhibit substantial PCE, reaching 20.67% and 20.70%, respectively.

Synthesizing and formulating metal oxide nanoparticle inks for perovskite solar cells

The perovskite solar cell has commercial potential due to the low-cost of materials and manufacturing processes with cell efficiencies on par with traditional technologies. Nanomaterials have many properties that make them attractive for the perovskite devices, including low-cost inks, low temperature processing, stable material properties and good charge transport. In this feature article, the use of nanomaterials in the hole transport and electron transport layers are reviewed. Specifically, SnO2 and NiOx are the leading materials with the most promise for translation to large scale applications. The review includes a discussion of the synthesis, formulation, and processing of these nanoparticles and provides insights for their further deployment towards commercially viable perovskite solar cells.

Designing of phenothiazine-based hole-transport materials with excellent photovoltaic properties for high-efficiency perovskite solar cells (PSCs)

In this study, a series of highly efficient organic hole-transporting materials (HTMs) were designed using Schiff base chemistry by modifying a phenothiazine-based core with triphenylamine through end-capped acceptor engineering via thiophene linkers. The designed HTMs (AZO1-AZO5) exhibited superior planarity and greater attractive forces, making them ideal for accelerated hole mobility. They also showed deeper HOMO energy levels (-5.41 eV to -5.28 eV) and smaller energy band gaps (2.22 eV to 2.72 eV), which improved charge transport behavior, open-circuit current, fill factor, and power conversion efficiency of perovskite solar cells (PSCs). The dipole moments and solvation energies of the HTMs revealed their high solubility, making them suitable for the fabrication of multilayered films. The designed HTMs showed tremendous enhancements in power conversion efficiency (26.19 % to 28.76 %) and open-circuit voltage (1.43 V to 1.56 V), with higher absorption wavelength than the reference molecule (14.43 %). Overall, the Schiff base chemistry-driven design of thiophene-bridged end-capped acceptor HTMs is highly effective in enhancing the optical and electronic properties of perovskite solar cells.

Selection of a compatible electron transport layer and hole transport layer for the mixed perovskite FA0.85Cs0.15Pb (I0.85Br0.15)3, towards achieving novel structure and high-efficiency perovskite solar cells: a detailed numerical study by SCAPS-1D

The first and foremost intent of our present study is to design a perovskite solar cell favorable for realistic applications with excellent efficiency by utilizing SCAPS-1D. To ensure this motive, the detection of a compatible electron transport layer (ETL) and hole transport layer (HTL) for the suggested mixed perovskite layer entitled FA_0.85Cs_0.15Pb (I_0.85Br_0.15)₃ (MPL) was carried out, employing diver ETLs such as SnO_2, PCBM, TiO₂, ZnO, CdS, WO₃ and WS₂, and HTLs such as Spiro-OMeTAD, P3HT, CuO, Cu₂O, CuI, and MoO_3. The attained simulated results, especially for FTO/SnO₂/FA_0.85Cs_0.15Pb (I_0.85Br_0.15)₃/Spiro-OMeTAD/Au, have been authenticated by the theoretical and experimental data, which endorse our simulation process. From the detailed numerical analysis, WS₂ and MoO₃ were chosen as ETL and HTL, respectively, for designing the proposed novel structure of FA_0.85Cs_0.15Pb (I_0.85Br_0.15)₃-based perovskite solar cells. With the inspection of several parameters such as variation of the thickness of FA_0.85Cs_0.15Pb (I_0.85Br_0.15)₃, WS_2, and MoO₃ including different defect densities, the novel proposed structure has been optimized, and a noteworthy efficiency of 23.39% was achieved with the photovoltaic parameters of V_OC = 1.07 V, J_SC = 21.83 mA cm^-2, and FF = 73.41%. The dark J-V analysis unraveled the reasons for the excellent photovoltaic parameters of our optimized structure. Furthermore, the scrutinizing of QE, C-V, Mott-Schottky plot, and the impact of the hysteresis of the optimized structure was executed for further investigation. Our overall investigation disclosed the fact that the proposed novel structure (FTO/WS₂/FA_0.85Cs_0.15Pb (I_0.85Br_0.15)₃/MoO₃/Au) can be attested as a supreme structure for perovskite solar cells with greater efficiency as well as admissible for practical purposes.

Improvement Strategies for Stability and Efficiency of Perovskite Solar Cells

Recently, perovskites have garnered great attention owing to their outstanding characteristics, such as tunable bandgap, rapid absorption reaction, low cost and solution-based processing, leading to the development of high-quality and low-cost photovoltaic devices. However, the key challenges, such as stability, large-area processing, and toxicity, hinder the commercialization of perovskite solar cells (PSCs). In recent years, several studies have been carried out to overcome these issues and realize the commercialization of PSCs. Herein, the stability and photovoltaic efficiency improvement strategies of perovskite solar cells are briefly summarized from several directions, such as precursor doping, selection of hole/electron transport layer, tandem solar cell structure, and graphene-based PSCs. According to reference and analysis, we present our perspective on the future research directions and challenges of PSCs.

Roles of Inorganic Oxide Based HTMs towards Highly Efficient and Long-Term Stable PSC-A Review

In just a few years, the efficiency of perovskite-based solar cells (PSCs) has risen to 25.8%, making them competitive with current commercial technology. Due to the inherent advantage of perovskite thin films that can be fabricated using simple solution techniques at low temperatures, PSCs are regarded as one of the most important low-cost and mass-production prospects. The lack of stability, on the other hand, is one of the major barriers to PSC commercialization. The goal of this review is to highlight the most important aspects of recent improvements in PSCs, such as structural modification and fabrication procedures, which have resulted in increased device stability. The role of different types of hole transport layers (HTL) and the evolution of inorganic HTL including their fabrication techniques have been reviewed in detail in this review. We eloquently emphasized the variables that are critical for the successful commercialization of perovskite devices in the final section. To enhance perovskite solar cell commercialization, we also aimed to obtain insight into the operational stability of PSCs, as well as practical information on how to increase their stability through rational materials and device fabrication.

Data-Driven Analysis of Hole-Transporting Materials for Perovskite Solar Cells Performance

We have created a dataset of 269 perovskite solar cells, containing information about their perovskite family, cell architecture, and multiple hole-transporting materials features, including fingerprints, additives, and structural and electronic features. We propose a predictive machine learning model that is trained on these data and can be used to screen possible candidate hole-transporting materials. Our approach allows us to predict the performance of perovskite solar cells with reasonable accuracy and is able to successfully identify most of the top-performing and lowest-performing hole-transporting materials in the dataset. We discuss the effect of data biases on the distribution of perovskite families/architectures on the model's accuracy and offer an analysis with a subset of the data to accurately study the effect of the hole-transporting material on the solar cell performance. Finally, we discuss some chemical fragments, like arylamine and aryloxy groups, which present a relatively large positive correlation with the efficiency of the cell, whereas other groups, like thiophene groups, display a negative correlation with power conversion efficiency (PCE).

Improved Stability of MAPbI3 Perovskite Solar Cells Using Two-Dimensional Transition-Metal Dichalcogenide Interlayers

Perovskite solar cells (PSCs) have been receiving considerable attention as next-generation solar cells. However, their short lifetime is a major obstacle to their commercialization. In addition to the properties of the materials used in PSCs, their interfaces play an important role in device stability by maintaining their initial design. In this study, we developed a transition-metal dichalcogenide (TMD) as a stable and efficient interlayer. MoS₂ and WSe₂ were applied to both the hole and electron transport sides of the PSCs with general FTO/TiO₂/MAPbI₃/Spiro-OMeTAD/Au structures, respectively. Owing to efficient charge transfer by TMD interlayers, our PSCs achieved a 19.24% efficiency, which is higher than the efficiency of the control devices (18.22%). Furthermore, the device stability was markedly improved by the passivation and strain-release effects of the TMD interlayers. Thus, the PSCs with TMD interlayers demonstrated a stable performance over 1000 h under damp heat (85 °C and 85% relative humidity) conditions.

Design of new hole transport materials based on triphenylamine derivatives using different π-linkers for the application in perovskite solar cells. A theoretical study

New organic molecules containing five different compounds, commonly called p-linkers, located between the triphenylamine units, were theoretically designed and analyzed in order to be proposed as new hole transport materials (HTMs) in perovskite solar cells, in total ten new molecules were analyzed. The electronic, optical and hole transport properties were determined, similarly, the relationship of these properties with their molecular structure was also investigated by Density Functional Theory (DFT) and Density Functional Tight Binding (DFTB) calculations. Eight of the ten analyzed compounds exhibited the main absorption band out of the visible region; therefore these compounds did not present an overlap with the absorption spectra of the typical methylammonium lead iodide (MAPI) hybrid-perovskite. The results showed that the Highest occupied molecular orbital (HOMO) levels of the compounds are higher than the perovskite HOMO level, and in some cases these are even higher than the Spiro-OMeTAD HOMO. The calculated electronic couplings and the reorganization energy values provided useful information in order to determine if the systems were hole or electron transport materials.

Green Synthesis of Lead Sulphide Nanoparticles for High-Efficiency Perovskite Solar Cell Applications

In this study, lead sulfide (PbS) nanoparticles were synthesized by the chemical precipitation method using Aloe Vera extract with PbCl₂ and Thiourea (H₂N-CS-NH₂). The synthesized nanoparticles have been investigated using x-ray diffraction (XRD), UV-Vis, energy-dispersive x-ray spectroscopy (EDX), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). XRD and TEM results confirm that the films are in the cubic phase. The crystallite size, lattice constant, micro-strain, dislocation density, optical bandgap, etc. have been determined using XRD and UV-Vis for investigating the quality of prepared nanoparticles. The possible application of these synthesized nanoparticles in the solar cells was investigated by fabricating the thin films on an FTO-coated and bare glass substrate. The properties of nanoparticles were found to be nearly retained in the film state as well. The experimentally found properties of thin films have been implemented for perovskite solar cell simulation and current-voltage and capacitance-voltage characteristics have been investigated. The simulation results showed that PbS nanoparticles could be a potential hole transport layer for high-efficiency perovskite solar cell applications.

0D/2D Mixed Dimensional Lead-Free Caesium Bismuth Iodide Perovskite for Solar Cell Application

Bismuth-based perovskites are potentially a promising alternative for lead-free perovskites. During bond formation, however, trivalent ions on Cs3Bi2I9 with CsI/BiI3 ratio of 1.5/1 form 0D-neutral charged compounds with higher bandgap (>2.0 eV) and poor absorption capacity. Mixed 0/2-dimensional structures are potentially suitable substitutes due to their low bandgap. So far, the reported CsI/BiI3 ratios for 0D/2D structures are 1:1, 1:2 and 1:3. Herein, a new ratio of 1/1.5 is reported. Caesium bismuth iodide at a ratio of CsI/BiI3 of 1/1.5 was synthesised using a one-step processing method with/without solvent vapour annealing. During solvent annealing, a 1/4 (v/v) mixture of DMF/methanol was used as a solvent. The crystal structure formed at a ratio of 1/1.5 is more similar to 1.5/1 than to 1/3. The XRD pattern revealed additional characteristics peaks at 009, 012, 209 and 300, indicating the growth of another phase. The formed heterogeneous mixed 0D/2D structure has an extended light absorption capacity greater than 720 nm. Solvent vapour annealing improved film morphology by enhancing grain size and packing density. When cells with and without solvent vapour annealing are compared, the power conversion efficiency of caesium bismuth iodide increases from 0.26% without solvent annealing to 0.98% with solvent vapour annealing. This study establishes a new route for future research on crystal configuration, nomenclature, film and morphology, quality tailoring and applications toward the goal of lead-free perovskite solar cells.

A Review of Recent Developments in Preparation Methods for Large-Area Perovskite Solar Cells

The recent rapid development in perovskite solar cells (PSCs) has led to significant research interest due to their notable photovoltaic performance, currently exceeding 25% power conversion efficiency for small-area PSCs. The materials used to fabricate PSCs dominate the current photovoltaic market, especially with the rapid increase in efficiency and performance. The present work reviews recent developments in PSCs’ preparation and fabrication methods, the associated advantages and disadvantages, and methods for improving the efficiency of large-area perovskite films for commercial application. The work is structured in three parts. First is a brief overview of large-area PSCs, followed by a discussion of the preparation methods and methods to improve PSC efficiency, quality, and stability. Envisioned future perspectives on the synthesis and commercialization of large-area PSCs are discussed last. Most of the growth in commercial PSC applications is likely to be in building integrated photovoltaics and electric vehicle battery charging solutions. This review concludes that blade coating, slot-die coating, and ink-jet printing carry the highest potential for the scalable manufacture of large-area PSCs with moderate-to-high PCEs. More research and development are key to improving PSC stability and, in the long-term, closing the chasm in lifespan between PSCs and conventional photovoltaic cells.

Suppressing PEDOT:PSS Doping-Induced Interfacial Recombination Loss in Perovskite Solar Cells

PSS is widely used as a hole transport layer (HTL) in perovskite solar cells (PSCs) due to its facile processability, industrial scalability, and commercialization potential. However, PSCs utilizing PEDOT:PSS suffer from strong recombination losses compared to other organic HTLs. This results in lower open-circuit voltage (V _OC) and power conversion efficiency (PCE). Most studies focus on doping PEDOT:PSS to improve charge extraction, but it has been suggested that a high doping level can cause strong recombination losses. Herein, we systematically dedope PEDOT:PSS with aqueous NaOH, raising its Fermi level by up to 500 meV, and optimize its layer thickness in p-i-n devices. A significant reduction of recombination losses at the dedoped PEDOT:PSS/perovskite interface is evidenced by a longer photoluminescence lifetime and higher magnitude of surface photovoltage, leading to an increased device V _OC, fill factor, and PCE. These results provide insights into the relationship between doping level of HTLs and interfacial charge carrier recombination losses.

Bithieno Thiophene-Based Small Molecules for Application as Donor Materials for Organic Solar Cells and Hole Transport Materials for Perovskite Solar Cells

This quantum mechanical study focuses on the designing of twelve (MPAM1-MPAM12) bithieno thiophene (BTTI) central core-based small molecules to explore optoelectronic properties as donor candidates for organic solar cells (OSCs) and hole transport materials (HTMs) accompanied by enhanced charge mobility for perovskite solar cells (PSCs). MPAM1-MPAM6 have been designed by the substitution of thiophene-bridged end-capped acceptors on both side terminals of reference (MPAR). MPAM7-MPAM12 are tailored by adopting the same tactic on one side terminal only. MPW1PW91/6-311G (d,p) has been employed for all computational simulations. MPAM12 revealed the highest λmax at 639 nm in dichloromethane (DCM) solvent with the lowest E g of 1.78 eV and dipole moment (20.74 D) in the solvent phase, showing excellent miscibility as compared to the reference. All designed chromophores (MPAM1-MPAM12) demonstrated higher estimated V OC and power conversion efficiency (PCE) when compared to MPAR, suggesting their prominent operational efficiency. Among all, MPAM4 manifested the highest PCE (47.86%). MPAM2 portrayed the highest electron mobility (0.0041573 eV) and MPAM3 exhibited the highest hole mobility (0.0047178 eV). The outcomes highlight the adequacy of the planned strategies, paving a new route for the development of small-molecule HTMs for PSCs and donor contributors for OSCs.

Supercritical carbon dioxide assisted formation of crystalline materials for various energetic applications

This highlight gives an overview of recent advances in production of crystalline materials for high energy density applications for rechargeable batteries and solar cells or energetic compounds in supercritical carbon dioxide medium.

On the efficiency of perovskite solar cells with a back reflector: effect of a hole transport material

Organometal halide perovskites are promising, high-performance absorbers in solar cells. However, the light-harvesting performance of these devices is still limited by excessive charge carrier recombination. Charge carrier management can be improved, taking into account the transport properties of layers surrounding the absorber. In particular, the choice of an appropriate hole-transport material (HTM) could provide a path towards increasing the device performance of perovskite solar cells (PSCs). The Lambertian reflection on the cell's back-surface reflector could increase the power conversion efficiency (PCE) of PSCs as well. Taking into account these facts, we analyse the absorptance and the PCE of a perovskite thin-film solar cell with the Lambertian reflection on the cell's back-surface reflector for various organic and inorganic HTMs. The analysis is done by means of the Monte-Carlo ray tracing simulations complemented by the transfer-matrix method to account for the interference phenomenon in the local generation rate G of carriers in a thin-film multilayer system. This function is employed further in the transport equations to calculate the current-voltage characteristics of the cell. We show that wide band gap HTMs, that possess negligible absorption, increase the photocurrent in the perovskite, passing reflected photons from the back reflector. In contrast, at the same perovskite thickness the PSC gains less photocurrent with narrow band gap HTMs, where an excessive non-radiative recombination takes place. Our analysis demonstrates that the optimal thickness of the solar cell with the typical absorber CH₃NH₃PbI₃ is ∼300 nm, providing the maximal efficiency ∼18.8% for the wide band gap HTM (CuSCN) at the moderate absorber purity (the diffusion length _D ∼ 1 μm).

Solvation of NiOxfor hole transport layer deposition in perovskite solar cells

A series of nickel oxide (NiO_x) inks, in the perovskite antisolvent chlorobenzene (CB) containing 15% ethanol, were prepared for the fabrication of p-i-n perovskite solar cells by blade coating. The inks included triethylamine (Et₃N) and alkyl xanthate salts as ligands to disperse NiO_xparticle aggregates and stabilize suspension. A total of four inks were evaluated: 0X (Et₃N with no alkyl xanthate), 4X (Et₃N + potassiumn-butyl xanthate), 12X (Et₃N + potassiumn-dodecyl xanthate), and 18X (Et₃N + potassiumn-octadecyl xanthate). The inks were characterized by UV-visible spectroscopy and FT-IR spectroscopy and the resulting films analyzed by thermogravimetry and scanning electron microscopy. Devices prepared using the 0X ink resulted in a peak power conversion efficiency (PCE) of 14.47% (0.25 cm²) and 9.96% (1 cm²). The 0X devices showed no significant loss of PCE after 100 days in a nitrogen flow box. Devices prepared with inks containing alkyl xanthate ligand had lower PCE that decreased with decreasing chain length, 18X > 12X > 4X.

Design and Modelling of Eco-Friendly CH3NH3SnI3-Based Perovskite Solar Cells with Suitable Transport Layers

An ideal n-i-p perovskite solar cell employing a Pb free CH3NH3SnI3 absorber layer was suggested and modelled. A comparative study for different electron transport materials has been performed for three devices keeping CuO hole transport material (HTL) constant. SCAPS-1D numerical simulator is used to quantify the effects of amphoteric defect based on CH3NH3SnI3 absorber layer and the interface characteristics of both the electron transport layer (ETL) and hole transport layer (HTL). The study demonstrates that amphoteric defects in the absorber layer impact device performance significantly more than interface defects (IDL). The cell performed best at room temperature. Due to a reduction in Voc, PCE decreases with temperature. Defect tolerance limit for IL1 is 1013 cm−3, 1016 cm−3 and 1012 cm−3 for structures 1, 2 and 3 respectively. The defect tolerance limit for IL2 is 1014 cm−3. With the proposed device structure FTO/PCBM/CH3NH3SnI3/CuO shows the maximum efficiency of 25.45% (Voc = 0.97 V, Jsc = 35.19 mA/cm2, FF = 74.38%), for the structure FTO/TiO2/CH3NH3SnI3/CuO the best PCE is obtained 26.92% (Voc = 0.99 V, Jsc = 36.81 mA/cm2, FF = 73.80%) and device structure of FTO/WO3/CH3NH3SnI3/CuO gives the maximum efficiency 24.57% (Voc = 0.90 V, Jsc = 36.73 mA/cm2, FF = 74.93%) under optimum conditions. Compared to others, the FTO/TiO2/CH3NH3SnI3/CuO system provides better performance and better defect tolerance capacity.

Combination of Metal Oxide and Polytriarylamine: A Design Principle to Improve the Stability of Perovskite Solar Cells

In the last decade, perovskite photovoltaics gained popularity as a potential rival for crystalline silicon solar cells, which provide comparable efficiency for lower fabrication costs. However, insufficient stability is still a bottleneck for technology commercialization. One of the key aspects for improving the stability of perovskite solar cells (PSCs) is encapsulating the photoactive material with the hole-transport layer (HTL) with low gas permeability. Recently, it was shown that the double HTL comprising organic and inorganic parts can perform the protective function. Herein, a systematic investigation and comparison of four double HTLs incorporating polytriarylamine and thermally evaporated transition metal oxides in the highest oxidation state are presented. In particular, it was shown that MoOx, WOx, and VOx-based double HTLs provided stable performance of PSCs for 1250 h, while devices with NbOx lost 30% of their initial efficiency after 1000 h. Additionally, the encapsulating properties of all four double HTLs were studied in trilayer stacks with HTL covering perovskite, and insignificant changes in the absorber composition were registered after 1000 h under illumination. Finally, it was demonstrated using ToF-SIMS that the double HTL prevented the migration of perovskite volatile components within the structure. Our findings pave the way towards improved PSC design that ensures their long-term operational stability.

Methods of Stability Control of Perovskite Solar Cells for High Efficiency

The increasing demand for renewable energy devices over the past decade has motivated researchers to develop new and improve the existing fabrication techniques. One of the promising candidates for renewable energy technology is metal halide perovskite, owning to its high power conversion efficiency and low processing cost. This work analyzes the relationship between the structure of metal halide perovskites and their properties along with the effect of alloying and other factors on device stability, as well as causes and mechanisms of material degradation. The present work discusses the existing approaches for enhancing the stability of PSC devices through modifying functional layers. The advantages and disadvantages of different methods in boosting device efficiency and reducing fabrication cost are highlighted. In addition, the paper presents recommendations for the enhancement of interfaces in PSC structures.

High Mobility Reactive Sputtered CuxO Thin Film for Highly Efficient and Stable Perovskite Solar Cells

Copper oxide (CuxO) films are considered to be an attractive hole-transporting material (HTM) in the inverted planar heterojunction perovskite solar cells due to their unique optoelectronic properties, including intrinsic p-type conductivity, high mobility, low-thermal emittance, and energy band level matching with the perovskite (PS) material. In this study, the potential of reactive sputtered CuxO thin films with a thickness of around 100 nm has been extensively investigated as a promising HTM for effective and stable perovskite solar cells. The as-deposited and annealed films have been characterized by using X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Photoluminescence (PL), UV-Vis spectroscopy, and Hall-effect measurement techniques. The significant change in structural and optoelectronic properties has been observed as an impact of the thermal annealing process. The phase conversion from Cu2O to CuO, including grain size increment, was observed upon thermal annealing. The transmittance and optical bandgap were found to vary with the films’ crystallographic transformation. The predominant p-type conductivity and optimum annealing time for higher mobility have been confirmed from the Hall measurement. Films’ optoelectrical properties were implemented in the complete perovskite solar cell for numerical analysis. The simulation results show that a 40 min annealed CuxO film yields the highest efficiency of 22.56% with a maximum open-circuit voltage of 1.06 V.

Crystal Engineering Approach for Fabrication of Inverted Perovskite Solar Cell in Ambient Conditions

In this paper, we demonstrate the high potentialities of pristine single-cation and mixed cation/anion perovskite solar cells (PSC) fabricated by sequential method deposition in p-i-n planar architecture (ITO/NiOX/Perovskite/PCBM/BCP/Ag) in ambient conditions. We applied the crystal engineering approach for perovskite deposition to control the quality and crystallinity of the light-harvesting film. The formation of a full converted and uniform perovskite absorber layer from poriferous pre-film on a planar hole transporting layer (HTL) is one of the crucial factors for the fabrication of high-performance PSCs. We show that the in-air sequential deposited MAPbI3-based PSCs on planar nickel oxide (NiOX) permitted to obtain a Power Conversion Efficiency (PCE) exceeding 14% while the (FA,MA,Cs)Pb(I,Br)3-based PSC achieved 15.6%. In this paper we also compared the influence of transporting layers on the cell performance by testing material depositions quantity and thickness (for hole transporting layer), and conditions of deposition processes (for electron transporting layer). Moreover, we optimized second step of perovskite deposition by varying the dipping time of substrates into the MA(I,Br) solution. We have shown that the layer by layer deposition of the NiOx is the key point to improve the efficiency for inverted perovskite solar cell out of glove-box using sequential deposition method, increasing the relative efficiency of +26% with respect to reference cells.

Hexaarylbenzene based high-performance p-channel molecules for electronic applications

Hexaarylbenzene-based molecules find potential applications in organic electronics due to wider energy gap, high HOMO level, higher photoconductivity, electron-rich nature, and high hole-transporting property. Due to the unique propeller structure, these molecules show low susceptibility towards self-aggregation. This property can be tailored by proper molecular engineering by the incorporation of appropriate groups. Therefore, hexaarylbenzene chromophores are widely used as the materials for high-efficiency light-emitting materials, charge transport materials, host materials, redox materials, photochemical switches, and molecular receptors. This review highlights the diverse structural modification techniques used for the synthesis of symmetrical and unsymmetrical structures. Also, the potential applications of these molecules in organic light-emitting diodes, organic field-effect transistors, organic photovoltaics, organic memory devices, and logic circuits are discussed.

Influence of Fluorinated Components on Perovskite Solar Cells Performance and Stability

Several valuable scientific investigations have been conducted these last few years in materials design and device engineering for perovskite solar cells (PSCs) to make them competitive compared to traditional silicon-based photovoltaic technologies. Consequently, high power conversion efficiency beyond 25% is nowadays reported. However, their long-term stability remains a significant challenge to overcome. Herein, the influence of fluorinated compounds on each layer of PSCs devices and their impact on the resulted device performances and stability is spotlighted. The fluorinated compounds exhibit attractive properties due to their very high electronegativity attributed to the fluorine atom, and their strong hydrophobicity. Thus, the introduction of these compounds is found to be a successful strategy to positively suppress the surface trap states, enhancing charge collection and reducing interfacial charge recombination. Besides, a better film quality and better energy level alignment is obtained, resulting in the improvement of device photovoltaic parameters such as the open-circuit voltage (V_oc ), short-circuit current (J_sc ), and fill factor (FF), and then, the device's overall power conversion efficiency (PCE). Their long-term stability is also found to further be improved.

Hysteresis and Instability Predicted in Moisture Degradation of Perovskite Solar Cells

The degradation of the perovskite solar cell structure was expected recently to be reversible, which opened a new gate to the enhancement of the device lifetime by reversing the process. However, the kinetic details of the structural collapse and recovery are still missing, without which the perovskite reversibility cannot be further explored. Due to the experimental difficulty, a purposeful numerical model was conducted in this report, to simulate the water diffusion process in the perovskite structure in both directions. It was found that the moisture diffusion needs to be initiated by a certain level of structural imperfection and is non-Fickian, as assisted by the collapse of the perovskite into the 1D chains. The reversibility was verified by the back diffusion, but accompanied by hysteresis, stagnancy, and even surprising instability, which initiated the water flow under initial equilibrium, due possibly to the imbalance during the reconstruction of the perovskite lattice. These observations offer new insights to form strategies of improvement, for example, via the possible self-healing perovskite devices.

CuCrO2 Nanoparticles Incorporated into PTAA as a Hole Transport Layer for 85 °C and Light Stabilities in Perovskite Solar Cells

High-mobility inorganic CuCrO2 nanoparticles are co-utilized with conventional poly(bis(4-phenyl)(2,5,6-trimethylphenyl)amine) (PTAA) as a hole transport layer (HTL) for perovskite solar cells to improve device performance and long-term stability. Even though CuCrO2 nanoparticles can be readily synthesized by hydrothermal reaction, it is difficult to form a uniform HTL with CuCrO2 alone due to the severe agglomeration of nanoparticles. Herein, both CuCrO2 nanoparticles and PTAA are sequentially deposited on perovskite by a simple spin-coating process, forming uniform HTL with excellent coverage. Due to the presence of high-mobility CuCrO2 nanoparticles, CuCrO2/PTAA HTL demonstrates better carrier extraction and transport. A reduction in trap density is also observed by trap-filled limited voltages and capacitance analyses. Incorporation of stable CuCrO2 also contributes to the improved device stability under heat and light. Encapsulated perovskite solar cells with CuCrO2/PTAA HTL retain their efficiency over 90% after ~900-h storage in 85 °C/85% relative humidity and under continuous 1-sun illumination at maximum-power point.

Selected Electrochemical Properties of 4,4'-((1E,1'E)-((1,2,4-Thiadiazole-3,5-diyl)bis(azaneylylidene))bis(methaneylylidene))bis(N,N-di-p-tolylaniline) towards Perovskite Solar Cells with 14.4% Efficiency

Planar perovskite solar cells were fabricated on F-doped SnO₂ (FTO) coated glass substrates, with 4,4’-((1E,1’E)-((1,2,4-thiadiazole-3,5-diyl)bis(azaneylylidene))bis(methaneylylidene))bis(N,N-di-p-tolylaniline) (bTAThDaz) as hole transport material. This imine was synthesized in one step reaction, starting from commercially available and relatively inexpensive reagents. Electrochemical, optical, electrical, thermal and structural studies including thermal images and current-voltage measurements of the full solar cell devices characterize the imine in details. HOMO-LUMO of bTAThDaz were investigated by cyclic voltammetry (CV) and energy-resolved electrochemical impedance spectroscopy (ER-EIS) and were found at −5.19 eV and −2.52 eV (CV) and at −5.5 eV and −2.3 eV (ER-EIS). The imine exhibited 5% weight loss at 156 °C. The electrical behavior and photovoltaic performance of the perovskite solar cell was examined for FTO/TiO₂/perovskite/bTAThDaz/Ag device architecture. Constructed devices exhibited good time and air stability together with quite small effect of hysteresis. The observed solar conversion efficiency was 14.4%.

Interfacing green synthesized flake like-ZnO with TiO2 for bilayer electron extraction in perovskite solar cells

To improve the performance of PSCs, interfacing green synthesized flake like-ZnO nanostructure have been introduced between ETL/perovskite layer which reduces the recombination losses at the interface.

Efficiency vs. stability: dopant-free hole transporting materials towards stabilized perovskite solar cells

Doping of hole transporting materials typically increases the efficiency of perovskite solar cells but remains questionable for overall device stability.

In the last decade, perovskite solar cells have been considered a promising and burgeoning technology for solar energy conversion with a power conversion efficiency currently exceeding 24%. However, although perovskite solar cells have achieved high power conversion efficiency, there are still several challenges limiting their industrial realization. The actual bottleneck for real uptake in the market still remains the cost-ineffective components and instability, to which doping-induced degradation of charge selective layers may contribute significantly. This article overviews the highest performance molecular and polymeric doped and dopant-free HTMs, showing how small changes in the molecular structure such as different atoms and different functional groups and changes in substitution positions or the length of the π-conjugated systems can affect photovoltaic performance and long-term stability of perovskite solar cells.

Doping strategies for small molecule organic hole-transport materials: impacts on perovskite solar cell performance and stability

Hybrid organic/inorganic perovskite solar cells (PSCs) have dramatically changed the landscape of the solar research community over the past decade, but >25 year stability is likely required if they are to make the same impact in commercial photovoltaics and power generation more broadly. While every layer of a PSC has been shown to impact its durability in power output, the hole-transport layer (HTL) is critical for several reasons: (1) it is in direct contact with the perovskite layer, (2) it often contains mobile ions, like Li+ - which in this case are hygroscopic, and (3) it usually has the lowest thermal stability of all layers in the stack. Therefore, HTL engineering is one method with a high return on investment for PSC stability and lifetime. Research has progressed in understanding design rules for small organic molecule hole-transport materials, yet, when implemented into devices, the same dopants, bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) and tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(iii) tri[bis(trifluoromethane)sulfonimide] (FK209), are nearly always required for improved charge-transport properties (e.g., increased hole mobility and conductivity). The dopants are notable because they too have been shown to negatively impact PSC stability and lifetime. In response, new research has targeted alternative dopants to bypass these negative effects and provide greater functionality. In this review, we focus on dopant fundamentals, alternative doping strategies for organic small molecule HTL in PSC, and imminent research needs with regard to dopant development for the realization of reliable, long-lasting electricity generation via PSCs.

Facile NiOx Sol-Gel Synthesis Depending on Chain Length of Various Solvents without Catalyst for Efficient Hole Charge Transfer in Perovskite Solar Cells

Nickel oxide (NiO_x)–based perovskite solar cells (PSCs) have recently gained considerable interest, and exhibit above 20% photovoltaic efficiency. However, the reported syntheses of NiO_x sol-gel used toxic chemicals for the catalysts during synthesis, which resulted in a high-temperature annealing requirement to remove the organic catalysts (ligands). Herein, we report a facile “NiO_x sol-gel depending on the chain length of various solvents” method that eschews toxic catalysts, to confirm the effect of different types of organic solvents on NiO_x synthesis. The optimized conditions of the method resulted in better morphology and an increase in the crystallinity of the perovskite layer. Furthermore, the use of the optimized organic solvent improved the absorbance of the photoactive layer in the PSC device. To compare the electrical properties, a PSC was prepared with a p-i-n structure, and the optimized divalent alcohol-based NiO_x as the hole transport layer. This improved the charge transport compared with that for the typical 1,2-ethanediol (ethylene glycol) used in earlier studies. Finally, the optimized solvent-based NiO_x enhanced device performance by increasing the short-circuit current density (J_sc), open-circuit voltage (V_oc), and fill factor (FF), compared with those of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)–based devices.