New alkoxylphenyl substituted benzo[1,2-b:4,5-b′] dithiophene-based polymers: synthesis and application in solar cells

Abstract

Two new alkoxylphenyl substituted benzo[1,2-b:4,5-b′]dithiophene (BDTPO)-based polymers (PBDTPO-DTBO and PBDTPO-DTBT) were synthesized. Their structures were verified by NMR spectroscopy, the molecular weights were determined by gel permeation chromatography (GPC), and the thermal properties were investigated by thermogravimetric analysis (TGA). UV-Vis absorption spectra of the polymers show broad and strong absorption bands from 300–750 nm both in CHCl₃ solutions and films. The resulting copolymers exhibit relatively deep HOMO energy levels (−5.56 and −5.46 eV) and surprisingly high hole mobilities (2.2 × 10⁻¹ and 3.3 × 10⁻² cm² V⁻¹ s⁻¹) for PBDTPO-DTBO and PBDTPO-DTBT, respectively. Preliminary photovoltaic properties of the copolymers blended with [6,6]-phenyl-C_{71 (or 61)}-butyric acid methyl ester (PCBM) as an electron acceptor were investigated. The polymer solar cell (PSC) based on the single layer device structure of ITO/PEDOT:PSS/PBDTPO-DTBO:PC₇₁BM (1 : 1.5, w/w)/Ca/Al demonstrates a high power conversion efficiency of 6.2% under the illumination of AM 1.5G, 100 mW cm⁻².