Relationship between film morphology, optical, and conductive properties of poly(thienothiophene): [6,6]-phenyl C-61-butyric acid methyl ester bulk heterojunctions

The morphology and the optoelectronic properties of films of poly(dialkylthieno[3,2-b]thiophene-co-bithiophene) (pDA2T) and blends of this polymer with [6, 6]-phenyl C-61-butyric acid methyl ester (PCBM) is investigated. Upon spin-coating of the blend a thin film is formed containing both PCBM and the polymer in nanocrystalline form. On annealing, phase separation occurs, leading to formation of the PCBM-rich domains embedded in a polymer-rich matrix. The electrodeless time-resolved microwave conductivity technique is used to study the photogeneration of charge carriers and their decay over time. The photoconductance increases dramatically on adding PCBM to the polymer. Annealing of the blend reduces the photogeneration yield of charge carriers, due to the smaller interfacial area between pDA2T and PCBM. The phase separation of the polymer and PCBM after annealing retards recombination of charge carriers, which is beneficial for charge collection in a solar cell. The magnitude of the photoconductance of the pDA2T:PCBM blend is comparable to that for a P3HT:PCBM blend. The above findings, together with the smaller energy loss involved in electron transfer from pDA2T to PCBM, as compared to blends of P3HT and PCBM, make pDA2T a promising material for photovoltaic applications.