Boosting Infrared Light Harvesting by Molecular Functionalization of Metal Oxide/Polymer Interfaces in Efficient Hybrid Solar Cells

Hybrid solar cells based on light absorbing semiconducting polymers infiltrated in nanocrystalline TiO2 electrodes, have emerged as an attractive concept, combining benefits of both low material and processing costs with well controlled nano-scale morphology. However, after over ten years of research effort, power conversion efficiencies remain around 0.5%. Here, a spectroscopic and device based investigation is presented, which leads to a new optimization route where by functionalization of the TiO2 surface with a molecular electron acceptor promotes photoinduced electron transfer from a low-band gap polymer(poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b0]dithiophene)-alt-4,7-(2,1,3-benzothiadia-zole)] (PCPDTBT) to the metal oxide. This boosts the infrared response and the power conversion efficiency to over 1%. As a further step, by co-functionalizing the TiO2 surface with the electron acceptor and an organic dye-sensitizer, panchromatic spectral photoresponse is achieved in the visible to near-IR region. This novel architecture at the heterojunction opens new material design possibilities and represents an exciting route forward for hybrid photovoltaics.