Ashish Dubey

Bio: Ashish Dubey is an academic researcher from South Dakota State University. The author has contributed to research in topics: Perovskite (structure) & Polymer solar cell. The author has an hindex of 23, co-authored 46 publications receiving 1656 citations.

Strategic review of secondary phases, defects and defect-complexes in kesterite CZTS–Se solar cells

Abstract: Earth abundant kesterite copper-zinc-tin-sulfide–selenide (CZTS–Se) is considered as cost-effective material for next generation solar cells. However, current CZTS–Se solar cells have much lower efficiency than CIGS solar cells. Rapid progress in achieving the target efficiency in CZTS–Se solar cells is hindered by the narrow phase stability of the quaternary phase, Cu2ZnSn(SxSe1−x)4, and the existence of other competitive and complex secondary phases and defects. This resulted in structural inhomogeneity, local fluctuation of open circuit voltage and high carrier recombination that finally lead to poor device performance and repeatability issues. The higher performance of off-stoichiometric CZTS materials, copper-poor and zinc-rich, and their inherent association with secondary phases and defects force the scientific community to investigate them together. This work aims to provide a comprehensive review for optimum growth conditions to achieve efficient kesterite CZTS–Se material under different conditions, complementary characterization techniques to detect unwanted phases, defects and defect-complexes and various approaches to reduce the secondary phases, defects and defect-complexes for higher performance in CZTS–Se solar cells. Understanding and addressing the structural inhomogeneity, control growth and material characterization are expected to yield closer performance parity between CZTS–Se and CIGS solar cells.

A strategic review on processing routes towards highly efficient perovskite solar cells

Abstract: An organic–inorganic perovskite is comprised of an organic cation (CH3NH3+, FAI, or Cs), a metal cation (Pb2+ or Sn2+) and a halide (I−, Cl−, or Br−) molecule. Precursor salts containing these cations, molecules and halide ions dissolved in solvents are used to prepare perovskite films. Perovskite film formation takes place through the reaction of precursor elements, which is assisted by various processing conditions such as thermal annealing, moisture and solvent treatment. This review focuses on various perovskite formation and crystallization routes with respect to processing parameters including the precursor solvent, solvent mixture, temperature, time, formation of solvent led-intermediate complex species, doping and humidity. Adding water as the dopant to the precursor solvent and exposure to moisture from atmospheric humidity to improve perovskite film quality are also discussed. Processing conditions and crystallization processes are described in correlation with the perovskite film morphology, crystallinity, defects, charge transport and device performance. This article will aim to highlighting recent findings in the selection of solvents in the crystallization of perovskite films, solvent induced intermediate phases, and effects of water in assisting perovskite crystallization for improved film quality and device performance. The review will also present various structural and nanoscale characterization techniques that have been used to probe solvent based intermediate species transformation processes to the perovskite phase and understand the effects in correlation with device performance.

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

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

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

Abstract: We report effects of an interface between TiO2-perovskite and grain-grain boundaries of perovskite films prepared by single step and sequential deposited technique using different annealing times at optimum temperature. Nanoscale kelvin probe force microscopy (KPFM) measurement shows that charge transport in a perovskite solar cell critically depends upon the annealing conditions. The KPFM results of single step and sequential deposited films show that the increase in potential barrier suppresses the back-recombination between electrons in TiO2 and holes in perovskite. Spatial mapping of the surface potential within perovskite film exhibits higher positive potential at grain boundaries compared to the surface of the grains. The average grain boundary potential of 300-400 mV is obtained upon annealing for sequentially deposited films. X-ray diffraction (XRD) spectra indicate the formation of a PbI2 phase upon annealing which suppresses the recombination. Transient analysis exhibits that the optimum device has higher carrier lifetime and short carrier transport time among all devices. An optimum grain boundary potential and proper band alignment between the TiO2 electron transport layer (ETL) and the perovskite absorber layer help to increase the overall device performance.

Halide Perovskite Photovoltaics: Background, Status, and Future Prospects

Abstract: The photovoltaics of organic–inorganic lead halide perovskite materials have shown rapid improvements in solar cell performance, surpassing the top efficiency of semiconductor compounds such as CdTe and CIGS (copper indium gallium selenide) used in solar cells in just about a decade. Perovskite preparation via simple and inexpensive solution processes demonstrates the immense potential of this thin-film solar cell technology to become a low-cost alternative to the presently commercially available photovoltaic technologies. Significant developments in almost all aspects of perovskite solar cells and discoveries of some fascinating properties of such hybrid perovskites have been made recently. This Review describes the fundamentals, recent research progress, present status, and our views on future prospects of perovskite-based photovoltaics, with discussions focused on strategies to improve both intrinsic and extrinsic (environmental) stabilities of high-efficiency devices. Strategies and challenges regardi...

Efficient, stable solar cells by using inherent bandgap of α-phase formamidinium lead iodide

Abstract: In general, mixed cations and anions containing formamidinium (FA), methylammonium (MA), caesium, iodine, and bromine ions are used to stabilize the black α-phase of the FA-based lead triiodide (FAPbI3) in perovskite solar cells. However, additives such as MA, caesium, and bromine widen its bandgap and reduce the thermal stability. We stabilized the α-FAPbI3 phase by doping with methylenediammonium dichloride (MDACl2) and achieved a certified short-circuit current density of between 26.1 and 26.7 milliamperes per square centimeter. With certified power conversion efficiencies (PCEs) of 23.7%, more than 90% of the initial efficiency was maintained after 600 hours of operation with maximum power point tracking under full sunlight illumination in ambient conditions including ultraviolet light. Unencapsulated devices retained more than 90% of their initial PCE even after annealing for 20 hours at 150°C in air and exhibited superior thermal and humidity stability over a control device in which FAPbI3 was stabilized by MAPbBr3.

Hole‐Transport Materials for Perovskite Solar Cells

Abstract: The pressure to move towards renewable energy has inspired researchers to look for ideas in photovoltaics that may lead to a major breakthrough. Recently the use of perovskites as a light harvester has lead to stunning progress. The power conversion efficiency of perovskite solar cells is now approaching parity (>22 %) with that of the established technology which took decades to reach this level of performance. The use of a hole transport material (HTM) remains indispensable in perovskite solar cells. Perovskites can conduct holes, but they are present at low levels, and for efficient charge extraction a HTM layer is a prerequisite. Herein we provide an overview of the diverse types of HTM available, from organic to inorganic, in the hope of encouraging further research and the optimization of these materials.

Compound Copper Chalcogenide Nanocrystals

Abstract: This review captures the synthesis, assembly, properties, and applications of copper chalcogenide NCs, which have achieved significant research interest in the last decade due to their compositional and structural versatility. The outstanding functional properties of these materials stems from the relationship between their band structure and defect concentration, including charge carrier concentration and electronic conductivity character, which consequently affects their optoelectronic, optical, and plasmonic properties. This, combined with several metastable crystal phases and stoichiometries and the low energy of formation of defects, makes the reproducible synthesis of these materials, with tunable parameters, remarkable. Further to this, the review captures the progress of the hierarchical assembly of these NCs, which bridges the link between their discrete and collective properties. Their ubiquitous application set has cross-cut energy conversion (photovoltaics, photocatalysis, thermoelectrics), en...

PEDOT:PSS for Flexible and Stretchable Electronics: Modifications, Strategies, and Applications

Abstract: Substantial effort has been devoted to both scientific and technological developments of wearable, flexible, semitransparent, and sensing electronics (e.g., organic/perovskite photovoltaics, organic thin-film transistors, and medical sensors) in the past decade. The key to realizing those functionalities is essentially the fabrication of conductive electrodes with desirable mechanical properties. Conductive polymers (CPs) of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) have emerged to be the most promising flexible electrode materials over rigid metallic oxides and play a critical role in these unprecedented devices as transparent electrodes, hole transport layers, interconnectors, electroactive layers, or motion-sensing conductors. Here, the current status of research on PEDOT:PSS is summarized including various approaches to boosting the electrical conductivity and mechanical compliance and stability, directly linked to the underlying mechanism of the performance enhancements. Along with the basic principles, the most cutting edge-progresses in devices with PEDOT:PSS are highlighted. Meanwhile, the advantages and plausible problems of the CPs and as-fabricated devices are pointed out. Finally, new perspectives are given for CP modifications and device fabrications. This work stresses the importance of developing CP films and reveals their critical role in the evolution of these next-generation devices featuring wearable, deformable, printable, ultrathin, and see-through characteristics.