Interfacial Engineering for Efficient and Aesthetic Perovskite Solar Cells

Abstract

Securing eco-friendly renewable energy is one of the essential tasks of modern society. Solar energy has been considered the most promising energy sources, as it is one the most abundant and evenly distributed eco-friendly, renewable energy sources. Solar cells are therefore key energy conversion devices in realizing a new era of eco-friendly renewable energy. Perovskite solar cells (PeSCs) are newly emerged thin-film solar cells that have undergone intensive research over the last 10 years owing to advantages such as the outstanding optoelectronic properties of metal halide perovskite materials and the potential low costs resulting from solution-based manufacturing at low temperatures. These semiconductor devices are composed of several layers, including a perovskite light-absorbing layer, charge extraction layers, electrodes, and substrates. Controlling the conditions in the interfaces between these layers is key to realizing the high-performance of PeSCs. Interfacial engineering is an effective way to solve critical problems such as compatibility issues surrounding the coating, high density of defects at the surface, and unfavorable energy level alignments at the interfaces between layers. The simplicity of semi-transparent PeSCs fabrication with transparent electrodes on top of the cell has led to semi-transparent PeSCs attracting attention for utilization in Building Integrated Photovoltaics (BIPV) and perovskite/Si tandem solar cells. Therefore, the aesthetic aspect of PeSCs is considered important, and the development of interlayers for use in semi-transparent PeSCs is a new requirement. In this study, we deal with the critical problems that can occur at the interfaces between the layers in PeSCs and the introduction of a well-designed interlayer for solving the problems at the interfaces. Different interlayers are newly developed for the three interfaces in PeSCs, allowing the realization of highly efficient and aesthetic semi-transparent PeSCs. Chapter 2 describes the development of conjugate polyelectrolytes (CPEs) that are synthesized based on poly(fluorene-co-phenylene) by varying the ionic density of the side-chains in each repeating unit. The effect of CPEs as an interlayer between the hydrophobic hole extraction layer and the perovskite layer is investigated in p-i-n structure PeSCs. CPEs have both a hydrophobic conjugated polymer backbone and hydrophilic ionic side chains in their structure. Amphiphilic CPEs improve the compatibility between the hydrophobic hole extraction layer and the hydrophilic perovskite solutions. Additionally, ions from the side chains of CPEs efficiently passivate the interfacial defects of the perovskite. The compatibilizing and defect passivating properties of CPE were further improved by optimizing the ionic density of the repeating units. In Chapter 3, polyethyleneimine (PEI) is introduced as an interlayer between the C60 electron extraction layer and the indium zinc oxide (IZO) transparent top electrode in semi-transparent PeSCs. PEI, which contains high number of amines, effectively reduces the surface tension between the C60 and IZO, resulting in the lateral growth of IZO on the C60 and lower interfacial resistance. In addition, PEI induces an interfacial dipole that forms between the C60 and the IZO, changing the energy level alignment so that it is more favorable for electron extraction. In Chapter 4, ethylene-vinyl acetate (EVA) is utilized as anti-reflection (AR) film at the interface between the IZO top transparent electrode and air. Texturing EVA with an inverted pyramid surface reduces the reflection occurring between the highly refractive IZO and the lowly refractive air. In addition, coloration of the semi-transparent PeSCs without the loss of absorption in the visible range and minimized PCE loss is possible by adding UV light downshifting dyes to the textured EVA film.