Rational Design of High Performance Conjugated Polymers for Organic Solar Cells

Abstract

The research on the polymer based solar cells (PSCs) has attracted increasing amount of attention in recent years and great progresses have been made in the field of bulk heterojunction (BHJ) polymer solar cells since its inception in 1995. The power conversion efficiency (PCE) has increased from 1% in the 1990s to over 9% just recently. These great advances are mainly fueled by the development of conjugated polymers used as the electron-donating materials in BHJ solar cells. My research was focused on the rational design of those conjugated polymers. And first we investigated the positioning effect of alkyl side chains on the properties of conjugated polymers and find the best attaching position where high molecular weight and good solubility of conjugated polymers can be achieved without introducing large steric hindrance. Then, we proposed and demonstrated a weak donor-strong acceptor designing strategy to construct donor-acceptor type polymers with controlled energy levels. With those strategy to increase molecular weight, solubility and to control energy levels, we designed and synthesized several weak donor based conjugated polymers with solubilizing chains. Those polymers exhibited low HOMO levels, good solubility and molecular weight, thus high Voc and high PCE over 5% were obtained. Since most of the conjugated polymers developed so far have high-lying LUMO levels than desired LUMO, we developed some strong acceptor units to decrease the LUMO level of conjugated polymers. Combined with the methods we developed, conjugated polymers with high molecular weight, good solubility and near ideal energy levels were synthesized and they exhibit excellent PCE over 6%. In addition, by introducing fluorine atoms into conjugated backbone, we successfully created polymers with both low-lying HOMO and LUMO levels. Although band gap of this polymer is not ideal, large short circuit current (Jsc), open circuit voltage (Voc) and fill factor (FF) can still be obtained. Amazing PCE over 7% was demonstrated, which is among the best performances for polymer solar cells.