Relating Nongeminate Recombination to Charge-Transfer States in Bulk Heterojunction Organic Photovoltaic Devices

Abstract

The importance of nongeminate recombination of free photogenerated carriers through charge-transfer (CT) states compared with at the electrode interfaces in bulk heterojunction (BHJ) organic photovoltaic (OPV) devices was investigated using impedance spectroscopy (IS) and low-energy external quantum efficiency (EQE) measurement. Thin (∼60 nm) poly(di(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene-co-octylthieno[3,4-c] pyrrole-4,6-dione):[6,6]-phenyl-C70-butyric acid methyl ester (PBDT-TPD:PC71BM) OPVs processed with varying concentrations of solvent additive 1-chloronaphthalene (CN) provide a well-controlled system in which only the donor–acceptor interfacial area, hence CT concentration, is changed. We found that additive inclusion resulted in increased CT state concentration, measured by low-energy EQE spectroscopy, and higher nongeminate recombination, determined from IS. In contrast, the energetic disorder in the BHJ, as determined from the dependence of carrier density on open-circuit voltage, did not show a dependence on CN concentration, suggesting that it is not related to changes in donor–acceptor morphology. The correlated relationship between nongeminate recombination strength and CT state concentrations presents unambiguous evidence of CT states as the major channels in nongeminate recombination loss in these BHJ OPV devices.