Interface Engineering in Emerging Thin Film Photovoltaics
Authors:   , Tulus Tulus 
Published: July 2023

This thesis explores interface engineering in emerging photovoltaics with the aim of designing selective contact layer to reduce recombination losses. I present the motivation and basic concepts of this work in Chapters 1 and 2. The primary focus of this work is on the development and characterization of novel solar cell architectures based on metal halide perovskites and reaction center - light harvesting 1 (RC-LH1) absorber layer, with specific attention given to designing, optimizing, and characterizing selective contacts. Next, I present the impedance analysis complemented with optical, structural and electrical characterization to elucidate the physicochemical properties of device interfaces in various emerging photovoltaic structures.
Chapter 3 specifically focuses on quantifying electrochemical losses in perovskite solar cell (PSC) based on methylammonium lead triiodide (MAPbI3) films using impedance analysis. I explicate the underlying physical origin of the negative capacitance by applying a generalized equivalent circuit model (ECM) for PSCs. The ECM takes into account for fast electrical dynamics resulting in high frequency signatures due to electronic processes, and much slower electrochemical dynamics that result in low frequency (LF) signatures in the spectra. Analysis of the LF dynamics provides detailed insight into loss processes in the PSC, particularly ionic dynamics, which can be attributed to the migration of MA+ and I- ions in the MAPbI3 absorber layers.
Chapter 4 focuses on quantifying the impact of C60-passivation at the ZnO/perovskite interface. In this study, I apply the C60/ZnO as electron transport layer (ETL) in solar cells based on mixed-cation and mixed-halide lead perovskite (Cs0.15FA0.85PbI2.75Br0.25). The incorporation of the C60 interlayer results in an improvement in the power conversion efficiency (PCE), due to an increase in the open-circuit voltage (Voc) and in the fill factor (FF). However, there is a decrease in the short-circuit current density (Jsc) compared to the device without a C60 interlayer. I apply a combination of impedance spectroscopy, photoluminescence (PL) spectroscopy, and X-ray diffraction to identify the origin for the increase in PCE and operational stability for solar cells fabricated with C60/ZnO ETL versus reference cells with a ZnO ETL. Solar cells with C60/ZnO ETL demonstrate less pronounced and slower electrochemical dynamics in the impedance spectra than solar cells with ZnO ETL. I conclude that C60 leads to the formation of PbI2-rich and Br-rich domains in the perovskite absorber layer, resulting in reduced recombination losses and improved operational stability.
Chapter 5 aims to investigate the defect surface which was controlled using vacuum deposited Au nanoparticles on the surface of hydrothermally synthesized ZnO nanorod arrays (ZnO-NR). With a combination of PL, X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy, it is observed that the presence of Au nanoparticles decreases the density of oxygen vacancies and OH-related surface defects in the ZnO-NR. I discuss these results in terms of Au-induced band bending in surface-near regions of the ZnO-NR. As a proof of concept, the Au-decorated ZnO nanorods as ETLs in Cs0.15FA0.85PbI2.75Br0.25 PSCs was employed and resulting the PCE increase by surface control, where it lead to an increase in Voc, FF, and PCE, yet no increase in the Jsc.
Chapter 6 presents the experiment of a novel solid state bio-solar cell which incorporates RC-LH1 as the active layer. Here, the C60 layer integrated into ZnO as ETLs enables a high Voc (maximally 0.3 V) in these solid state biophotovoltaic devices. This work represents the largest voltage reported for RC-LH1 based solid state biophotovoltaic device to date.
As a closing remark, the combination of impedance analysis, optical, structural, and electrical characterization proves to be a powerful tool for elucidating the physical and chemical processes occurring at device interfaces in the new architecture of emerging photovoltaics.