Compositional Engineering and Post-treatment of Caesium based Halide Perovskite Materials for Perovskite Solar Cells

Abstract

Organic-inorganic hybrid perovskite solar cells (PSCs) are considered an excellent replacement for high-cost conventional silicon solar cells. The unique optical and electrical properties of the perovskite materials, low-cost fabrication techniques and potential for solution processing make PSCs economically viable and scalable for large scale production. However, the instability of organic cations in organic-inorganic hybrid perovskite materials presents a challenge that needs to be addressed for the commercialization of PSCs. To overcome this, caesium (Cs) based all-inorganic perovskite (CsPbX3 where X = Cl, Br, and I) has emerged as a more suitable alternative due to its excellent stability under various environmental conditions and excellent optoelectronic properties. However, various imperfections in perovskite absorber material and its interfaces greatly reduce the efficiency and stability of perovskite solar cells (PSCs) and retard the practical applicability of inorganic halide perovskite. This thesis offers strategies to overcome these issues. Two different methodologies were adopted which include the compositional engineering and surface or post-treatment of CsPbI2Br perovskite. The structural analysis of modified perovskite material was done by XRD and photophysical properties were investigated by UV-Visible and photoluminescence (PL) spectroscopy. Photoelectron spectroscopy in air (PESA) measurements were used to analyze the valence band structure of the materials. Morphological analysis of perovskite material was done by SEM and AFM images. XPS measurements were performed to study the details of changes at an atomic level and bonding information. Further, the modified perovskite material was also applied to PSC devices to see the effects of different modifications on the photovoltaic performance of fabricated devices. Current density voltage (J-V) measurements were performed to acquire data for the characterization of the devices. The compositional engineering of CsPbI2Br perovskite was done by altering the stoichiometry of CsPbI2Br referred as non-stoichiometric perovskite. The non stoichiometric perovskite material demonstrated improved stability and better photovoltaic characteristics compared to its stoichiometric counterpart. Afterward, the B-site doping of non-stoichiometric perovskite was done using CuBr2. The structural, photophysical and morphological analysis confirmed the presence of Cu2+ and xxvi Abstract demonstrated that an optimal quantity of Cu2+ can significantly improve the optoelectronic properties of the perovskites. The PSC devices were prepared which showed 15% improvement in the performance. For post-treatment two different strategies were used, one was the surface treatment of the CsPbI2Br perovskite film with isopropanol (IPA) while another was the defect passivation via Lewis acid base post-passivation method. The IPA treated CsPbI2Br absorber layer showed reduced defect density and showed energetically more favorable band alignment with the electron transport layer (ETL). The resultant PSC led to a 30% improvement in the photovoltaic performance with reduced hysteresis. In case of Lewis acid base passivation, the simplest amine ethylene diamine (EDA) was used. This passivation reduced the surface trap states and prolonged the charge carrier lifetime within the device. Consequently, the PSC fabricated with EDA passivation exhibited significantly improved PCE (9.4%) as compared to the reference device (7.3%) under 100 mWcm-2 illumination. In addition, the concept of a hybrid energy harvester (HEH) device was also demonstrated to collectively harvest outdoor solar and mechanical energies by a single device. For that, perovskite solar cell with normal architecture was chosen, wherein the perovskite absorber layer converts solar energy, while the piezoelectric properties of ZnO nanorods served as a unit for harvesting mechanical energy. The active response of HEH to both solar and mechanical energies demonstrates strong potential of the proposed device and as future ubiquitous energy harvester