Microstructural modification of polycarbazole-based polymeric solar cells by thermal annealing

Blended thin films were thermal-annealed at various temperatures (T_A) to serve as active layers in polymeric solar cells. The films were composed of poly[N-9´-heptadecanyl-2,7-carbazole-alt-5,5-(4´,7´-di-2-thienyl-2´,1´,3´-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl-C₇₁-butyric acid methyl ester and their microstructural features were examined through joint spectroscopy and microscopy. After they were thermal-annealed at a T_A below but close to the glass transition temperature of PCDTBT, their various properties improved, including absorbance, molecular ordering, coherence length of PCDTBT, homogeneity of PCDTBT chain conformations, and two-phase pathways. Efficient charge transport and suppressed charge recombination occurred in the modified thin films, resulting in an enhancement of the power conversion efficiency of the corresponding devices.