Abstract

The generation of free charge carriers and their extraction to the electrodes are key processes in organic solar cells. There is an ongoing debate on how the electric field across the active layer of such devices affects the efficiency of these processes in competition to geminate and non-geminate recombination. Similar to this, contradictory results have been published regarding the role of hot CT-states in the free carrier generation process.

We have used time-delayed extraction experiments to quantify generation and extraction of charge in various polymer-based solar cells. We find that for P3HT :PCBM, the efficiency for free carrier generation is not affected by the electric field in both poorly-performing as-prepared blends as well as in efficient annealed blends, meaning that the solar cell properties of this material combination are entirely determined by the field-driven sweep-out of carriers in competition with non-geminate recombination. Similar conclusions are drawn for polymer-polymer blends based on P3HT with N2200 . These blends were shown to exhibit exceptionally high fill factors. In contrast, blends of the low bandgap polymer PCPDTBT with PCBM exhibit a pronounced field-dependence of charge generation, suggesting efficient geminate recombination. We show that the efficiency of geminate and non-geminate recombination in these blends is correlated to the blend morphology, and that both decay channels are strongly reduced in blends with extensive interchain order. Finally, we address the question whether free charge generation proceeds predominately via hot CT-states. We find that reducing the energy loss upon singlet exciton split-up causes a reduced free carrier formation yield, with increased field-dependence, however, both yield and field-dependence are only very slightly affected when the excess energy is lowered by exclusive excitation of the interfacial charge transfer states. This indicates that the free carrier generation mechanism proceeds via the same CT state manifold, irrespective of the initial photon energy.