Surprisingly High Conductivity and Efficient Exciton Blocking in Fullerene/Wide-Energy-Gap Small Molecule Mixtures

We find that mixtures of C-60 with the wide energy gap, small molecular Weight semiconductor bathophenanthroline (BPhen) exhibit a combination of surprisingly high electron Conductivity and efficient exciton blocking when employed as buffer layers in Organic photovoltaic cells. Photoluminescence quenching measurements show that a 1:1 BPhen/C-60 mixed layer has an exciton blocking efficiency Of 84 +/- 5% compared to that of 100% for a neat BPhen layer. This high blocking efficiency accompanied by a 100-fold increase in electron conductivity compared with neat BPhen, Transient photocurrent measurements show that charge transport through a Peat BPhen buffer is' dispersive, in contrast to nondispersive transport in the compound buffer. Interestingly, although the conductivity is high, there is no clearly defined insulating-to-conducting phase transition with increased insulating BPhen fraction. Thus, we infer that C-60 undergoes nanoscale (<10 nm domain size) phase segregation even at very high (>80%) BPhen fractions,