Annealing treatment improves the morphology and performance of photovoltaic devices prepared from thieno[3,4-c]pyrrole-4,6-dione-based donor/acceptor conjugated polymers and CdSe nanostructures

We have prepared photovoltaic devices based on blend films of CdSe tetrapods and the donor/acceptor conjugated polymer PDTTTPD, which comprises 2,5-di(thiophen-2-yl) thieno[3,2-b]thiophene and thieno[3,4-c]pyrrole-4,6-dione units. The AM1.5 power conversion efficiency (PCE) of a photovoltaic device containing a PDTTTPD/CdSe tetrapod blend (1:9, w/w) that had experienced thermal annealing (130 degrees C, 20 min) was three times greater than that of the corresponding device incorporating the as-prepared PDTTTPD/CdSe tetrapod blend (2.9% vs. 1.0%). Synchrotron X-ray reflectivity revealed that annealing (i.e., removal of pyridine ligands from the surfaces of the CdSe tetrapods) caused the thickness of the PDTTTPD/CdSe tetrapod blend film to decrease (and its average density to increase) relative to that of the as-prepared blend film. Transmission electron microscopy and atomic force microscopy revealed that thermal annealing enhanced the degree of aggregation of the CdSe tetrapods and induced denser morphologies, leading to substantially increased charge transport, which enhanced the PCE of the device.