Synthesis of Poly(fluorene-alt-thieno[3,4-b]thiophene) for Photovoltaic Devices

Abstract

Polymer-based bulk heterojunction solar cells have been fabricated using blended films composed of an electrondonating conjugated polymer and an electron-accepting material such as a fullerene derivative; they are promising low-cost, flexible renewable energy sources. A solar cell’s power conversion efficiency (PCE) is the product of the short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF) divided by the incident light power density. The Voc of polymer solar cells is generally proportional to the energy difference between the highest occupied molecular orbital (HOMO) of the electron donor and the lowest unoccupied molecular orbital (LUMO) of the electron acceptor, suggesting that polymers with deep HOMO levels are advantageous if conditions are otherwise similar. Photovoltaic cells have had PCEs of ca. 5% reported when a mixture of poly(3-hexylthiophene) (P3HT) and a fullerene derivative were respectively used as the electron donor and acceptor. Interesting low band gap polymers consisting of alternating ester or ketone-substituted thieno[3,4-b]thiophene and alkoxybenzo[1,2-b:4,5-b']dithiophene units have been developed. Among them, a polymer consisting of a fluorinated ketone-substituted thieno[3,4-b]thiophene and alkoxybenzo[1,2-b:4,5-b']dithiophene units, whose band gap and HOMO level were 1.77 eV and −5.22 eV, respectively, exhibited a maximum PCE of 7.73%. We have also synthesized conjugated polymers for photovoltaic applications. Especially, a polymer consisting of alternating carbazole and ester group-substituted thieno[3,4b]thiophene units (PCTT1) showed a band gap of 2.1 eV and HOMO level of −5.52 eV. This polymer, blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), when used as the photoactive layer of solar cells, yielded a maximum PCE of 2.53%. Polyfluorenes have advantages such as thermal and oxidation stabilities. They can also form films easily and have good hole-transporting properties. The band gaps and HOMO levels of fluorene-containing polymers have been reported to be similar to those of carbazole-containing counterpart polymers, though different PCEs were shown by resulting polymer-based solar cells. Thus poly(fluorenealt-thieno[3,4-b]thiophene polymer) (PFTT) was synthesized as a counterpart of PCTT1. This work describes the synthesis and characterization of the polymer along with preliminary tests of resulting polymer-based photovoltaic devices. Experimental Section