Advanced Luminescence-Based Characterization Techniques For Perovskite Solar Cells

Abstract

This thesis focuses on the comprehensive characterization of hybrid organic-inorganic perovskite-based solar cells (PSCs). Despite impressive advancements in perovskite-based solar cell research, resulting in certified power conversion efficiencies (PCE) of up to 26% for single-junction cells and 33.7% for monolithic perovskite-silicon tandem solar cells, further efforts are essential for commercializing this technology. A key challenge lies in the non-uniformity of the perovskite layer, which can serve as a degradation center during long-term operation and limit the device's original performance. To address this limitation, a series of novel characterization techniques based on photoluminescence (PL) imaging have been developed to spatially resolve crucial optoelectronic parameters, such as implied open-circuit voltage (iVoc), pseudo-fill factor (pFF), ideality factor (nid), and series resistance (Rs). The correlation between these parameters enables a profound understanding of device performance losses and their origins. In the first part of the thesis, we employ generalized Planck's law to convert the relative detected PL signal to quasi-Fermi level splitting (QFLS), representing the implied open-circuit voltage. We calibrate the halogen lamp with known absolute intensity to accurately calculate the scaling factor (SF) accounting for the detected light fraction. Our approach is validated through various independent techniques, including lifetime-calibrated PL, spectrally resolved PL, and comparison with terminal Voc. We apply this method to spatially resolve iVoc of different devices, encompassing bare perovskite films, perovskite top cells, and silicon bottom cells of tandem perovskite-silicon solar cells. The VOC loss due to non-radiative recombination and energy band alignment is subsequently calculated, providing a vital target for the fabrication process. In the second part of the thesis, illumination intensity and temperature-dependent PL imaging are employed to extract various optoelectronic properties of PSCs, including pFF, nid, and activation energy of recombination (Ea). The pFF parameter, which represents the fill factor of the device without series resistance, is used to evaluate the potential PCE of PSCs, offering the second target for the fabrication process. The PL-based method's contactless advantage enables the study of different layers and fabrication processes' effects on device performance, providing valuable insights. nid and EA help predict major non-radiative recombination mechanisms, offering further understanding of the performance loss origins. Additionally, correlations between these parameters are investigated for devices before and after degradation tests. In the third part of the thesis, we investigate the PL intensity at different terminal voltages, including open-circuit, maximum power point, and short-circuit conditions, to calculate the PCE image and the series resistance (Rs) image. A strong correlation between PL quenching and PCE is observed, highlighting the significance of PL intensities in assessing device performance. However, a weak correlation is found between the PL image intensity under open-circuit conditions and the final PCE, emphasizing the risk of misinterpreting device performance based solely on PL intensities. Furthermore, we demonstrate the impact of voltage-dependent series resistance on device simulation accuracy, underscoring another crucial contribution of luminescence imaging to the perovskite solar cell technology's research and development.