We have used a femtosecond-resolved spectroscopic technique based on the Stark effect (electromodulated
differential absorption) in order to investigate free charge generation and charge drift in solar cell devices of
neat conjugated polymer pBTTT and in its 1:1 (by weight) blend with PCBM. In the latter, the fullerene
molecules intercalate between the polymer side-chains, yielding a co-crystal phase. Our results show that free
charge generation in both materials is ultrafast and strongly dependent on the applied reverse bias. Charge
drift to the electrodes (under strong reverse bias) occurs with comparable dynamics on the 1.2 ns time scale
for neat pBTTT and the blend, and is probably dominated by hole transport within/between polymer chains.