(The) Impact of backbone modification in conjugated polymers on photocurrent in all-polymer solar cells

Abstract

Conducting polymers are a key material in organic electronic devices such as organic solar cells (OSCs), organic field-effect transistors (OFETs), and organic light-emitting devices (OLEDs) because of their potential applications in large area, lightweight, flexible photovoltaic devices through low-cost solution-processing techniques. Among then, all-polymer solar cells (all-PSCs), composed of polymer donor and polymer acceptor, are attracting attention recently because they have high light absorption ability in visible region and excellent stability against mechanical, thermal, photo stresses compared with conventional fullerene-based polymer solar cells. However, all-PSCs still have lower power conversion efficiency (PCE) than small molecule-based OSCs, so it needs to be continuously developed. Optimization of device structure and device processing condition is important for improving the photovoltaic performance of all-PSCs. Fundamentally, it is essential to develop the high performance materials. Therefore, it is very important to understand and investigate the effect of chemical structures on the photovoltaic performance of the all-PSCs because the intrinsic properties of the conducting polymer are determined by the chemical structure. In this thesis, the effects of optical, structural, morphological and electrochemical properties controlled by the backbone modification of conducting polymer on the photocurrent and photovoltaic performance of the all-PSCs have been studied. Furthermore, the design of the molecule demonstrated to be a very effective strategy for improving the performance of all-PSCs. Thus, in this thesis, (1) polymer donors, alternating copolymers, were designed as random copolymers to finely tune the light absorption in the visible region. It has been demonstrated that the complementary light absorption of the designed polymer donor and polymer acceptor improves photocurrent. (2) The effect of sequence of three different monomers in the polymer backbone on the polymer properties and their device performance was investigated. In particular, the polymer packing structure in thin films was observed by X-ray technique, and the correlation between the structural and electrical properties of the polymer thin film was observed. (3) By introducing nitrogen atoms at different positions in the polymer backbone, the effect of the finely tuned electrochemical properties on the charge generation efficiency was investigated. Finally, the correlation between the backbone design of conducing polymer and the photocurrent generation of the all-PSCs covered in this dissertation provides important guidelines for the development of high performance conducting polymers for OSCs.