π-Conjugated polymers with thermocleavable substituents for use as active layers in organic photovoltaics

Abstract

The incorporation of tetrahydropyranyl (THP) groups into poly(3-alkylthiophene) offers an opportunity to prepare soluble π-conjugated polymers (πCPs) which, upon thermal annealing, may be rendered insoluble. Recent studies have shown that πCPs that may be rendered insoluble following thin film fabrication have potential use in photovoltaic (PV) applications. Furthermore, inclusion of 3,4-ethylenedioxythiophene (EDOT) into the polymer backbone, which lowers the band gap of the πCPs, provides a system with increased absorption over the solar spectrum. In that regard, the synthesis, absorption profiles, thin film X-ray diffraction and PV properties of poly(3-(6-(2-tetrahydropyranyloxy)hexyl)thiophene), poly(3-(8-(2-tetrahydropyranyloxy)octyl)thiophene) and poly[3,4-ethylenedioxy-3′-(6-(2-tetrahydropyranyloxy)hexyl)-2,2′-bithiophene] are reported herein. In addition to absorption profiles similar to that of poly(3-hexylthiophene) (P3HT), a large degree of order was observed, especially for the octyl derivative; however, upon device fabrication these systems proved to be inferior to that of P3HT:PCBM the current standard for polymeric PVs. This is attributed to a reduction in crystallinity of the conjugated polymer upon: (i) deprotection to the insoluble thin film and (ii) blending with PCBM.