Photovoltaic Performance of Polymers Based on Dithienylthienopyrazines Bearing Thermocleavable Benzoate Esters

Thermocleavable low-band-gap polymers based on dithienylthienopyrazines were prepared and copolymerized with different donor units like dialkoxybenzene, fluorene, thiophene, and cyclopentadithiophene (CPDT) using both Stille and Suzuki cross-coupling reactions. In the solid state the band gaps are in the range of 1.17−1.37 eV.

The polymers were explored as donor materials in bulk heterojunction solar cells together with PCBM as the acceptor material where they were shown to exhibit a photoresponse in the full absorption range up to 900 nm and power conversion efficiencies of up to 1.21% under 1 sun irradiation. A red shift of the absorption edge on going from solution to the solid film was observed for all the polymers.

Thermogravimetric analysis of the polymers in the temperature range from 25 to 500 °C showed a weight loss at just above 200 °C, corresponding to loss of the tertiary ester groups, and a second weight loss above 400 °C, corresponding to loss of CO2 and decomposition. Upon thermocleavage the power conversion efficiency decreased for all the polymers while the polymer films became insoluble which was desired in the context of multilayer film processing.

Thermocleavable low-band-gap materials can potentially offer better light harvesting, better operational stability, and a higher level of permissible processing conditions due to the insolubility of thermocleaved films in all solvents.