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    Deconvoluting the Impedance Response of Halide Perovskite Single Crystals: The Distribution of Relaxation Time Method
    (Amer Chemical Soc, 2023) Pandey, Siddhi Vinayak; Parikh, Nishi; Mahapatra, Apurba; Kalam, Abul; Akin, Seckin; Satapathi, Soumitra; Prochowicz, Daniel
    Electrochemical impedance spectroscopy (EIS) has beenemergingas a promising tool to study the core mechanisms occurring withinmetal halide perovskites (MHPs). Generally, MHPs show one or two semicirclesin the Nyquist spectra in the probed frequency range. However, inthe presence of external stimuli, often a Warburg diffusion or aninductive loop is observed at low frequencies. In such cases, a comparisonof low-frequency parameters in both cases cannot be drawn becauseof the lack of a unique electrical circuit (EC). To overcome the issueof lack of EC, transformation of the frequency-domain technique tothe time domain is carried out. In this work, we investigated threedifferent cases of MAPbI(3), MAPbBr(3), and surface-passivatedMAPbBr(3) single crystals (SCs), which showed one suppressedsemicircle, two semicircles, and a Warburg-like diffusion, respectively,in the Nyquist response of EIS. Next, we transformed these spectrainto the time domain using the distribution of relaxation times (DRT)technique, a machine-learning-assisted tool. The obtained resultssuggest that in the case of Nyquist spectra with one semicircle (thecase of MAPbI(3) SCs), the observed time constants usingEC and DRT are close enough. However, in the case of MAPbBr(3) SC, three different time constants are obtained, associated withhigh, medium, and low frequencies, although the Nyquist response showedtwo semicircles. At last, in the presence of surface-passivated SCs,the Warburg-like feature changes significantly for different passivationtimes. Interestingly, the DRT spectra showed almost similar time constants,through which reliable information on the low-frequency RC can beextracted. Thus, DRT can pave the way for the easy and reliable interpretationof EIS spectra, which is not possible using EC.
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    Molecular Engineering of Azahomofullerene-based Electron Transporting Materials for Efficient and Stable Perovskite Solar Cells
    (Amer Chemical Soc, 2023) Chavan, Rohit D.; Bonczak, Bartlomiej; Kruszynska, Joanna; Mahapatra, Apurba; Ans, Muhammad; Nawrocki, Jan; Nikiforow, Kostiantyn
    The rational molecular design of fullerene-based molecules with exceptional physical and electrical properties is in high demand to ensure efficient charge transport at the perovskite/electron transport layer interface. In this work, novel azahomofullerene (AHF) is designed, synthesized, and introduced as the interlayer between the SnO2/perovskite interface in planar n-i-p heterojunction perovskite solar cells (PSCs). The AHF molecule (denoted as AHF-4) is proven to enhance charge transfer capability compared to the commonly used fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) due to its superior coordination interaction and electronic coupling with the SnO2 surface. In addition, the AHF-4 interlayer concurrently improves the quality of the perovskite film and reduces charge recombination in PSCs. The resultant AHF-4-based device exhibits a maximum efficiency of 21.43% with lower hysteresis compared to the PCBM device (18.56%). Benefiting from the enhanced stability of the AHF-4 film toward light soaking and elevated temperature, the AHF-4-based devices show improved stability under continuous 1 sun illumination at the maximum power point and 45 ?. Our work opens a new direction to the design of AHF derivatives with favorable physical and electrical properties as an interlayer material to improve both the performance and stability of PSCs.