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    Effect of 1,3-Disubstituted Urea Derivatives as Additives on the Efficiency and Stability of Perovskite Solar Cells
    (Amer Chemical Soc, 2022) Kruszynska, Joanna; Sadegh, Faranak; Patel, Manushi J.; Akman, Erdi; Yadav, Pankaj; Tavakoli, Mohammad Mahdi; Gupta, Sanjeev K.
    Additive engineering in perovskites precursor solution is one of the most effective methods to fabricate high-quality perovskite films. Finding proper additives for morphology improvement and passivation of the perovskite defects is critical to fabricate highly efficient and stable perovskite solar cells (PSCs). In this work, 1,3-disubstituted urea additives are employed to study the effect of different substituents at -NH moiety on the quality of the perovskite layer and device performance. By adding 1,3-diphenyl urea (Ph-urea) or 1,3-di(tert-butyl)urea (tBu-urea) into the precursors, the crystallization process leads to the formation of perovskite films with larger grains and lower defect densities as compared to the nonsubstituted urea additive. Using density functional theory (DFT) calculations and experimental spectro-scopic measurements, we found that the selected 1,3-disubstituted ureas are prone to form stronger coordination interaction with undercoordinated Pb2+ ions than the urea. Applying this additive engineering to the devices reduced the current density-voltage (J-V) hysteresis and improved the photovoltaic performance, resulting in maximum power conversion efficiencies of 21.7 and 21.2% for the Ph-urea and tBu-urea modified devices, respectively. In addition, the device with Ph-urea enhanced long-term stability, where it remains at 90% of its initial efficiency, while the device with tBu-urea degrades fast reaching 20% of its initial efficiency after aging for 90 days due to the high moisture permeability of tBu-urea.
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    The effect of B-site doping in all-inorganic CsPbIxBr3-x absorbers on the performance and stability of perovskite photovoltaics
    (Royal Soc Chemistry, 2023) Akman, Erdi; Ozturk, Teoman; Xiang, Wanchun; Sadegh, Faranak; Prochowicz, Daniel; Tavakoli, Mohammad Mahdi; Yadav, Pankaj
    Despite the impressive efficiency of perovskite solar cells (PSCs), their operational stability is still hindered by the thermodynamic instability of the hybrid organic-inorganic absorber layer with ABX(3) structure (A: organic/inorganic cation, B: metal cation, X: halogen anion and mixtures thereof). Due to the hygroscopic and volatile nature of the organic cations, i.e., methylammonium (MA(+)), they show very poor stability not only against thermal stress but also moisture. Therefore, a photoactive material free from organic components could offer great opportunities to prolong the operational stability of devices. In this context, all inorganic CsPbIxBr3-x perovskites are meticulously developed in terms of their structural/thermal stability and have triggered increasing research interest due to great prospects in the commercialization of perovskite technology. However, besides relatively low performance, the poor phase stability of inorganic perovskites associated with lattice strain and vacancies still requires a thorough understanding and permanent solutions for tackling these problems. In this comprehensive review, the recently reported B-site doping strategy in inorganic CsPbIxBr3-x perovskite thin films, which has been elucidated to passivate the defects, tune the grain orientation, and enhance the lifetime of charge-carriers, is presented based on different B-site elements belonging to group IIIA, IVA and VA, alkaline-earth, transition, and lanthanide metals. Solutions for confronting these current problems are elaborated and an outlook on further strategies is given.
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    Facile NaF Treatment Achieves 20% Efficient ETL-Free Perovskite Solar Cells
    (Amer Chemical Soc, 2022) Sadegh, Faranak; Akman, Erdi; Prochowicz, Daniel; Tavakoli, Mohammad Mahdi; Yadav, Pankaj; Akin, Seckin
    Electron transporting layer (ETL)-free perovskite solar cells (PSCs) exhibit promising progress in photovoltaic devices due to the elimination of the complex and energy-/timeconsuming preparation route of ETLs. However, the performance of ETL-free devices still lags behind conventional devices because of mismatched energy levels and undesired interfacial charge recombination. In this study, we introduce sodium fluoride (NaF) as an interface layer in ETL-free PSCs to align the energy level between the perovskite and the FTO electrode. KPFM measurements clearly show that the NaF layer covers the surface of rough underlying FTO very well. This interface modification reduces the work function of FTO by forming an interfacial dipole layer, leading to band bending at the FTO/perovskite interface, which facilitates an effective electron carrier collection. Besides, the part of Na+ ions is found to be able to migrate into the absorber layer, facilitating enlarged grains and spontaneous passivation of the perovskite layer. As a result, the efficiency of the NaF-treated cell reaches 20%, comparable to those of state-of-the-art ETL-based cells. Moreover, this strategy effectively enhances the operational stability of devices by preserving 94% of the initial efficiency after storage for 500 h under continuous light soaking at 55 degrees C. Overall, these improvements in photovoltaic properties are clear indicators of enhanced interface passivation by NaF-based interface engineering.