Thermocleavable Low Band Gap Polymers and Solar Cells Therefrom with Remarkable Stability toward Oxygen

Thermocleavable esters of low band gap monomers and polymers based on diphenyldithienylthienopyrazine were prepared by incorporating carboxylic acid functionalities into the system. A series of different ester groups were prepared and the temperature of elimination of the ester group was studied. The lowest temperatures of elimination obtained were in the range 220-240 degrees C for tertiary esters giving the free acid. The highest temperatures of elimination were found for primary esters that also lead to decomposition of the molecule. Only the tertiary esters offer a good degree of control over the chemistry in the thermocleaved product. The photovoltaic performance of the polymers prepared was tested under simulated sunlight (1000 W m(-2), AM1.5G, 72 degrees C) and the best power conversion efficiency that could be reached for devices with an active area of 3 cm(2) was up to 0.4% in an ITO/PEDOT/polymer-PCBM/aluminum device geometry. The best performing polymer material was subjected to lifetime studies in four different atmospheres (dry nitrogen, dry oxygen, humid nitrogen and the ambient atmosphere). The best stability was observed in nitrogen while the devices showed nearly the same degree of stability in dry oxygen. In both the ambient atmosphere and the humid nitrogen atmospheres the devices degraded quickly. Finally the stability was compared with two other polymer systems that are known to give stable devices, poly(3-hexylthiophene) (P3HT) and native polythiophene (PT) obtained from the thermocleavable poly(3-(2-methylhexyloxycarbonyl)dithiophene) (P3MHOCT). The performance of the materials reported here was inferior to the performance of P3HT and PT in terms of power conversion efficiency (PCE). The photovoltaic parameters as studied under continuous illumination were however much more stable than those of the reference compounds.