Dynamics of charge generation and transport in polymer-fullerene blends elucidated using a PhotoFET architecture

High efficiency polymer-fullerene bulk heterojunction organic solar cells can generate photocurrent by excitation of the electron donor and acceptor components via Channel I and Channel II processes, respectively. Using a planar Photo-Field-Effect-Transistor (PhotoFET) architecture operated in steady state and quasi-transient modes we have studied the dynamics of charge generation and transport in blends of PCDTBT/70-PCBM (poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-{4',7'-di-2-thienyl-2',1',3'-benzothiadiazole}]/[6,6]-phenyl-C-71-butyric acid methylester). The PhotoFET architecture allows independent measurement of the electron and hole photocurrents and mobilities for charge carriers generated by the Channel I (electron transfer) and Channel II (hole transfer) processes as a function of fullerene content. We find dramatic increases in the photocurrent yield and electron mobility with higher 70-PCBM loading. By analyzing the External Quantum Efficiency (EQE) in n-and p-PhotoFET modes, we estimate that 80-90% of the photocurrent generated in the optimum 1:4 (polymer/fullerene) blend is derived from fullerene absorption and hole transfer, that is, Channel II