Carrier traps as transport states in bulk-heterojunction P3HT:PCBM structures for solar photovoltaics

We report the investigation of carrier transport as affected by trapping states in blends of poly-3-hexylthiophene (P3HT) with the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) that are promising materials for the organic solar cell development. Devices have been fabricated from P3HT:PCBM blends with 6:5 wt. ratio in the inverted layer configuration. Their solar efficiency is 3.7% and the fill factor of the current-voltage dependences is up to 68%. We demonstrate that, despite such relatively good macroscopic parameters, the carrier trapping is effectively involved in transport phenomena. The results have been analysed numerically by taking into account the carrier thermal generation from traps as well as the mobility variation according to the Gaussian disorder model. Mobility parameters obtained by both methods demonstrate good coincidence. The evaluated density of trapping states is up to 10(20)-7.10(21) cm(-3) and their activation energy is about 0.18 eV. At such a high density these relatively shallow states could presumably act as transport states, limiting the carrier mobility.