Rationalizing Phase Transitions with Thermal Annealing Temperatures for P3HT:PCBM Organic Photovoltaic Devices

摘要

We have studied a range of organic photovoltaic devices (OPVs) based on a thin-film blend of P3HT and PCBM. Comparing device studies with a characterization of the thermal transitions of the blend, we can provide a mechanistic description of the optimum annealing temperatures necessary to improve the operational efficiency of a device. For as-cast P3HT:PCBM blend thin-films we evidence two glass transition temperatures, tentatively corresponding to the existence of two compositionally different amorphous states. We demonstrate that an improvement in device efficiency only occurs once the film has been heated above the upper apparent glass transition temperature of the blend. If annealing is performed above the optimum temperature, excessive phase-separation and a partial reduction in film optical density leads to a general decrease in device efficiency. Both of these characteristic temperatures are dependent upon the composition of the blend.