Effect of Morphological Changes on Presence of Trap States in P3HT:PCBM Solar Cells Studied by Cross-Sectional Energy Filtered TEM and Thermally Stimulated Current Measurements

A combination of energy filtered transmission electron microscopy (EFTEM) and thermally stimulated current (TSC) was used in order to investigate the effect of thermal annealing on the performance of an organic solar cell based on P3HT and PCBM as a well-studied reference system. By probing specific elements, EFTEM allowed spectroscopic imaging with enhanced resolution compared to standard TEM techniques. Here, we applied EFTEM to cross-sections of pristine and thermally annealed organic solar cells to probe the sulfur concentration as a measure for the P3HT distribution within the photoactive layer. Thermal annealing for 10 min at 130 degrees C resulted in a reordering of P3HT and PCBM into better defined domains. The effect of the morphological changes on the presence of trap states was studied by TSC measurements. The TSC spectra recorded for the pristine and the thermally annealed solar cells showed three contributions, respectively, that could be assigned to the neat materials P3HT and PCBM as well as the blend. The pristine solar cell revealed a significantly lower density of trap states in the P3HT phase compared to the annealed solar cell. In combination with our EFTEM measurements, we were able to attribute this finding to the increased number of P3HT rich domains present in the annealed device. Annealing of P3HT:PCBM solar cells had a beneficial impact not only on the local molecular order, but in particular on providing percolation paths for both charge carrier types.