Study on Naphtho[2,1-b:3,4-b']dithiophene-based Copolymers for Bulk Heterojunction Organic Solar Cells

Abstract

Organic semiconductor materials that can be obtained from simple solution processes have attracted the attention of many research groups in recent years because of the widely increased demand for low-cost and flexible electronic products. The potential applications of these organic materials have led to intense researches in the field of organic electronics, especially in the field of organic solar cells (OSCs). OSCs are expected to be used as large scale and flexible electronic devices in various structures or buildings. However, the poor performance and stability are still serious obstacles to their commercial use. The work presented in this thesis is to study on the organic semiconductor materials of the active layer and network formation of blend films with donor polymers and acceptors. Accordingly, different NDT polymer derivatives were used and their properties and performances on OSCs were investigated. Basic background of the organic donor materials and OSCs are introduced in Chapter 1. Two novel polymer materials based on naphtho[2,1-b:3,4-b']dithiophene (NDT), PNDT-TTT and PNDT-TET, have been designed and tested in bulk heterojunction (BHJ) solar cells. Optical and electrochemical properties of these new polymers and OSC performances are studied and discussed in Chapter 2. Two donor polymers, PNDT-TTT and PNDT-TET, have tri-thiophene and bi-thiophene coupled by vinyl group, respectively, which exhibit red-shifted absorption spectra compared to PNDT-T (previously reported). These effects result in high short circuit currents in OSCs. Also, desired morphologies of these polymer:PC71BM blend films show good percolation pathways, which lead to increase device efficiency. The nano-scale network formation of donor polymer and acceptor materials are studied and dealt with in Chapter 3. Among the conjugated polymers, PNDT-T, PNDT-TT and PNDT-TTT are used for donor materials and PC71BM are used as a acceptor material. Three copolymers are NDT derivatives with different thiophene lengths. Through a combination of X-ray diffraction analysis and atomic force microscopy we see three distinct nanoscale morphologies by donor materials with different thiophene lengths. PNDT-TT presents strong intermolecular interaction, which results in the lowest efficiency of devices. However, PNDT-TTT with proper torsion structure exhibits good a mixed morphology, which leads to high OSCs performances.