Enhancing the photocurrent in diketopyrrolopyrrole-based polymer solar cells via energy level control.

A series of diketopyrrolopyrrole (DPP)-based small band gap polymers has been designed and synthesized by Suzuki or Stille polymerization for use in polymer solar cells. The new polymers contain extended aromatic π-conjugated segments alternating with the DPP units and are designed to increase the free energy for charge generation to overcome current limitations in photocurrent generation of DPP-based polymers. In optimized solar cells with [6,6]phenyl-C(71)-butyric acid methyl ester ([70]PCBM) as acceptor, the new DPP-polymers provide significantly enhanced external and internal quantum efficiencies for conversion of photons into collected electrons. This provides short-circuit current densities in excess of 16 mA cm(-2), higher than obtained so far, with power conversion efficiencies of 5.8% in simulated solar light. We analyze external and internal photon to collected electron quantum efficiencies for the new polymers as a function of the photon energy loss, defined as the offset between optical band gap and open circuit voltage, and compare the results to those of some of the best DPP-based polymers solar cells reported in the literature. We find that for the best solar cells there is an empirical relation between quantum efficiency and photon energy loss that presently limits the power conversion efficiency in these devices.