Charge Transport and Trapping in Bulk-Heterojunction Solar Cells

: Charge carrier transport and trapping was investigated in organic solar cell structures consisting of poly-3-hexylthiophene blended with the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester in 6:5 weight ratio. The analysed devices having solar efficiency of 3.7 per cent were produced in the inverted layer sequence. The fill factor of the IV characteristics was as high as 68 per cent. It was demonstrated that despite of such relatively high fill factor carrier trapping is effectively involved in the charge transport phenomena. The density of the trapping states was evaluated to be up to 10(20) division by 7 x 10(21) cm(-3) and their activation energy was about 0.18 eV. At such high densities these states may probably act as transport states, limiting carrier mobility. The results were analyzed by taking into account carrier thermal generation from traps as well their mobility variation according to the Gaussian disorder model. The mobility parameters obtained by both methods demonstrated good coincidence.