Relaxation of Photogenerated Carriers in P3HT:PCBM Organic Blends

Relaxing in the sunlight . Long time‐transient decays of photogenerated carriers in P3HT:PCBM blends for organic solar cells are interpreted in terms of the relaxation of hole carriers in a broad density of states.The after‐pulse time‐resolved microwave conductivity (TRMC) decays observed in P3HT:PCBM blends display a dependence on time close to t − β , independent of excitation intensity, in the 10 ns–1 μs range. This is explained in terms of the relaxation of carriers in a Gaussian density of states (DOS). The model is based on a demarcation level that moves with time by thermal release and retrapping of initially trapped carriers. The model shows that when the disorder is large the after‐pulse decay of the type t −β is obtained, while at low disorder and large temperature the carrier distribution becomes independent of time. In the measurements different β values were observed depending on the solvent used for spin‐coating: 0.4–0.6 for chlorobenzene and 0.3–0.4 for toluene. The model was applied to extract the shape of the DOS from the TRMC decays, giving a dispersion parameter of about 120 meV for blends with high P3HT content.