Reduced bimolecular recombination in polymer solar cells

Polymer solar cells with a thick active layer can generate large photocurrent because they absorb many more photons. In most cases, however, the active layer is as thin as ~100 nm under the optimized condition. In this work, we have studied bimolecular charge recombination in highly efficient polymer solar cells with a thick active layer by transient photovoltage and photocurrent measurements. As a result, we found that the bimolecular recombination rate is smaller by three orders of magnitude than that predicted from the diffusion-limited Langevin recombination. We therefore conclude that such reduced bimolecular recombination enables efficient charge collection even in the solar cell with a thick active layer