Organic field-effect devices as tool to characterize the bipolar transport in polymer-fullerene blends: The case of P3HT-PCBM

In organic bulk heterojunction solar cells (oBHJ) the blend morphology in combination with the charge transport properties of the individual components controls the extracted photocurrent. The organic field-effect transistor (OFET) has been proved as a powerful instrument to evaluate the unipolar carrier transport properties in a wide range of cases. In our work we extend the OFET concept to the evaluation of the bipolar transport properties in polymer-fullerenes blends and propose a method to improve the accuracy of the evaluation. The method is based on capacitance-voltage (C-V) measurements on MOS structures prepared on the same blends and delivers complementary information on the bulk heterojunction to the one obtained with FETs. The relevance for photovoltaic applications is investigated through the correlation between the current-voltage behavior of solar cells and the bipolar mobility for composites with varying polymer molecular weight and processed from different solvents. In particular the transport features of solar cells produced from o-Xylene (oX), a non chlorinated solvent more suitable to production requirements, have been compared to the one of devices cast from Chlorobenzene (CB) solution. For the P3HT-PCBM blend a consistent correlation between the mobility and the electrical fill factor and power performance was found. A significant asymmetry in the bipolar carrier mobility, together with low electron mobility dependent on the M-w value, affects the performances of thick o-Xylene cast devices. In the case of devices processed from Chlorobenzene the slower carrier has higher mobility and the small electrical losses detected are eventually more related to the formation of space-charge and eventually to surface recombination. This results in an efficient charge collection that is almost thickness independent. We report a dependence of the slow-carrier type (electrons or holes) and their mobility on the specific combination of molecular weight and solvent. The mobility data and the solar cell performance coherently fit to the prediction of a device model only based on the drift of carriers under the built-in electric field originated in the donor-acceptor oBHJ.