Material design for high-performance organic solar cells

Abstract

Organic solar cells have attracted tremendous research attention in the past two decades. It is a promising renewable energy technology because of its material variability, high-throughput roll-to-roll production, and the possibility of producing light-weight, flexible, and low-cost devices. A detailed description and analysis of this technology are included in Chapter I. Developing organic semiconductors for OSC active layer materials is critically important for improving the device performance as well as understanding the structure-performance relationship. As organic solar cells compose of two active layer materials, developing donor and acceptor materials are both important. In the following chapters, three types of active layer materials are discussed. Chapter II describes the development of fullerene materials as acceptors for organic solar cells. They are combined with a donor polymer to form high-performance organic solar cells. Among them the best C₆₀-based fullerene exhibits a power conversion efficiency of 10.4%, which is the best C₆₀-based organic solar cell to date. Chapter III presents a study of fullerene-free organic solar cells. A reported but rarely used small molecular acceptor was selected because of its high energy levels and high mobility. On the other hand, a difluorobenzothiadiazole-based donor polymer was introduced instead of traditionally used donor polymers. This material matching strategy yielded efficient non-fullerene organic solar cells with efficiencies up to 6.3%, which was the best value achievable at the time. Comparison study between related materials was conducted to highlight the importance of energy levels and morphology. Chapter IV develops two novel molecular acceptors for fullerene-free organic solar cells. The molecular geometries were controlled by a simple difference on the spacer between two perylene diimide units. The electronic structures, morphology and ultimate device performance were therefore affected. These findings contribute to understanding the structure-performance relationship of small molecular acceptor materials in OSCs. Chapter V introduces a new donor polymer. A novel building block, difluorobenzoxiadiazole, was designed and synthesized. It is a promising candidate for constructing conjugated polymers considering the limited number of units available. A polymer with typical temperature-dependent aggregation was developed. Studies were conducted to understand the differences between this polymer and the state-of-the-art polymer based on difluorobenzothiadiazole. A high efficiency up to 9.4% was achieved with thick active layers, which is among the best to date especially for thick film devices.

Date of Award	2015
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology