Upscaling of Indium Tin Oxide (ITO)-Free Polymer Solar Cells : Performance, Scalability, Stability, and Flexibility

Abstract

Polymer solar cells (PSCs) aim to produce clean energy that is cost-competitive to energy produced by fossil fuel-based conventional energy sources. From an environmental perspective, PSCs already compares favorably to other solar cell technologies in terms of fewer emissions of greenhouse gases during production. The costcompetitiveness of PSCs is envisioned achievable by the use of inexpensive materials and high throughput roll-to-roll (R2R) printing and coating techniques. The state-ofthe-art of the laboratory PSCs is, however, far removed from the vision of the widely disseminated low-cost solar cells as the laboratory solar cells are mostly focused on increasing the power conversion efficiency through materials design with little emphasis on the choice of materials, operational stability and large-scale processing. Indium-tinoxide (ITO), the commonly used transparent conductor, represents majority of the share of cost and energy footprint in terms of materials and processing in a conventional PSC module. Furthermore, the scarcity of indium is feared to create bottleneck in the dawning PSC industry and its brittle nature is an obstacle for fast processing of PSCs on flexible substrates as well as for applications in flexible end products. Thus, the replacement of ITO with low-cost alternatives is crucial for the commercial feasibility of PSCs. Encompassing these concerns, my PhD study has contributed to the development and evaluation of alternatives to ITO in laboratory cells, upscaling of ITO-free concepts from laboratory cells to R2R produced large-area modules, and integration of these module in demonstrator consumer applications. Accordingly, this dissertation is organized into nine chapters. Chapter 1 is aimed at contextualizing PSCs on the world energy map. It aims to address the question: why should PSCs be pursued? Chapter 2 attempts to provide a concise yet encompassing introduction to PSCs; and the problem with ITO and possible solutions. It also lays out specific targets that were set before the beginning of PhD study which provides a frame-of-reference for the later chapters. A holistic evaluation of several ITO-free concepts was carried out to determine low-cost upscaling compatibility of these concepts (Chapter 3). The results highlighted three architectures that represented different competencies with regards to photovoltaic performance, stability, and low-cost processing. These three architectures were upscaled (Chapter 5-7) using R2R techniques described in detail in Chapter 4. One of the three upscaled architecture (Chapter 7) represented an efficient alternative to ITO in terms of photovoltaic performance and were further investigated for stability and flexibility. These modules were then integrated in a credit-card size laser pointer for demonstration purposes. A colleague, Nieves Espinosa, has conducted life-cycle analyses (LCA) on all the three upscaled ITO-free architectures. Drawing upon the data from her published work, Chapter 8 provides concise and comparative LCA of the three upscaled ITO-free architectures in order to determine which technology can be pursued further among the three architectures. LCA results of the ITO-free architectures are also compared against ITO-based upscaled PSCs as well as against other photovoltaic technologies. Finally, the last chapter (Chapter 9) puts everything in the nutshell and identifies future challenges.