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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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    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.
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    Probing the Low-Frequency Response of Impedance Spectroscopy of Halide Perovskite Single Crystals Using Machine Learning
    (Amer Chemical Soc, 2023) Parikh, Nishi; Akin, Seckin; Kalam, Abul; Prochowicz, Daniel; Yadav, Pankaj
    Electrochemical impedance spectroscopy (EIS) has emergedas a versatiletechnique for characterization and analysis of metal halide perovskitesolar cells (PSCs). The crucial information about ion migration andcarrier accumulation in PSCs can be extracted from the low-frequencyregime of the EIS spectrum. However, lengthy measurement time at lowfrequencies along with material degradation due to prolonged exposureto light and bias motivates the use of machine learning (ML) in predictingthe low-frequency response. Here, we have developed an ML model topredict the low-frequency response of the halide perovskite singlecrystals. We first synthesized high-quality MAPbBr(3) singlecrystals and subsequently recorded the EIS spectra at different appliedbias and illumination intensities to prepare the dataset comprising8741 datapoints. The developed supervised ML model can predict thereal and imaginary parts of the low-frequency EIS response with an R (2) score of 0.981 and a root mean squared error(RMSE) of 0.0196 for the testing set. From the ground truth experimentaldata, it can be observed that negative capacitance prevails at a higherapplied bias. Our developed model can closely predict the real andimaginary parts at a low frequency (50 Hz-300 mHz). Thus, ourmethod makes recording of EIS more accessible and opens a new wayin using the ML techniques for EIS.
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    Robust Nonspiro-Based Hole Conductors for High-Efficiency Perovskite Solar Cells
    (Wiley-V C H Verlag Gmbh, 2022) Akin, Seckin; Bauer, Michael; Hertel, Dirk; Meerholz, Klaus; Zakeeruddin, Shaik M.; Graetzel, Michael; Baeuerle, Peter
    Despite considerable development in performance, both poor operational stability and high costs associated with hole conductors such as 2,2 ',7,7 '-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9 '-spirobifluorene (spiro-OMeTAD) and Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA) of perovskite solar cells (PSCs) need to be addressed by the research community. Here, two nonspiro hole transporting materials (HTMs), namely HTM-1 and HTM-2, are designed and straightforwardly synthesized exhibiting remarkable electrochemical properties and hole mobilities. In particular, the PSC based on the methoxy derivative (HTM-2) exhibits a remarkable efficiency of 21.2% (stabilized efficiency of 20.8%), which is superior to the benchmark HTM spiro-OMeTAD (stabilized efficiency of 20.4%). These results establish that the molecular design is effective in improving the performance of PSCs. Importantly, these two HTMs show admissible long-term stability under different harsh conditions such as thermal stress up to 85 degrees C, high humidity level of 60% +/- 10%, and continuous illumination over 1000 h. These insights allow correlating the impact of molecular design on optoelectronic properties of nonspiro-based hole conductors with the overall device performance.