On Realizing Higher Efficiency Polymer Solar Cells Using a Textured Substrate Platform

and emergence of new conjugated polymers with tailored energy levels. [ 4–6 ] Power conversion effi ciency (PCE) exceeding 7% has recently been achieved. [ 4 ] The state-of-the-art devices are so called bulk-heterojunction (BHJ) type in which the PV activelayer is coated from a blend of donor and acceptor species. The nanoscale nature of phase separation between the donors and acceptors in a BHJ active-layer alleviates the mismatch between exciton diffusion length ( ∼ 10 nm) and optical absorption length ( > 100 nm). However, there still exists a mismatch between optical absorption length and charge transport scale. BHJ activelayers tend to suffer from cul-de-sacs in the charge transport pathways, and hole mobilities in conjugated polymers remain low. Both of these factors lead to recombination losses, higher series resistances and lower fi ll-factors. [ 7 ] Thus, it is imperative to develop fabrication methodologies that can enable effi cient optical absorption in fi lms thinner than optical absorption length. The most desirable methodology would be one which can also substantially improve absorption at the band edge of conjugated polymers, which usually lies in the red/near-infrared region, and where signifi cant amount of solar fl ux is also located. It is more so important because the charge carriers photoexcited at the band edge were found to have a higher dissociation effi ciency than the ones excited at higher energies. [ 8 ]