Substituted 2,1,3-benzothiadiazole- and thiophene-based polymers for solar cells - introducing a new thermocleavable precursor

Alkoxysubstituted and unsubstituted 2,1,3-benzothiadiazoles were prepared and copolymerized with substituted and unsubstituted thiophenes using both Stille and Yamamoto cross-coupling reactions. One class of the materials bore thermally labile ester groups. The materials were all found to have a reduced band gap in the range of 1.69-1.75 eV and were explored in polymer photovoltaic devices as mixtures with the soluble fullerene PCBM. High open circuit voltages of up to 0.93 V and power conversion efficiencies (PCE) of up to 2.22% was observed for materials without the thermally labile groups. The thermocleavable materials have the advantage that they are insoluble after a thermal treatment, enabling a larger degree of processing freedom when preparing multilayer devices and they provide a better operational stability for the devices. So far the process of thermocleavage has led to poorer device performance than for the soluble precursor polymers; however, we found processing conditions that lead to a higher performance for the thermocleaved product, where open circuit voltages of up to 0.9 V could be obtained with power conversion efficiencies of up to 0.42%, representing a doubling as compared to the soluble precursor polymer.