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[<i>N</i>-9′-heptadecanyl-2,7-carbazole-<i>alt</i>-5,5-{4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole}]/[6,6]-phenyl-C₇₁-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 <i>n</i>- and <i>p</i>-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