Panchromatic Dye-Doped Polymer Solar Cells: From Femtosecond Energy Relays to Enhanced Photo-Response

There has been phenomenal effort synthesizing new low-band gap polymer hole-conductors which absorb into the near-infrared (NIR), leading to >10% efficient all-organic solar cells. However, organic light absorbers have relatively narrow bandwidths, making it challenging to obtain panchromatic absorption in a single organic semiconductor. Here, we demonstrate that (poly[2,6-(4,4-bis-(2-ethylhexyl)-4Hcyclopenta[2,1-b;3,4-b0]clithiophene)-alt-4,7-(2,1,3-benzothiadia-zole)] (PCPDTBT) can be photo-sensitized across the whole visible spectrum by doping with a visible absorbing dye, the (2,2,7,7-tetrakis(3-hexyl-5-(7-(4-hexylthiophen-2-yObenzo[c][1,2,5]thiadiazol-4-yl)thiophen-2-yl)-9,9-spirobifluorene) (spiro-TBT). Through a comprehensive sub-12 femtosecond nanosecond spectroscopic study, we demonstrate that extremely efficient and fast energy transfer occurs from the photoexcited spiro-TBT to the PCPDTBT, and ultrafast charge injection happens when the system is interfaced with ZnO as a prototypal electron-acceptor compound. The visible photosensitization can be effectively exploited and gives panchromatic photoresponse in prototype polymer/oxide bilayer photovoltaic diodes. This concept can be successfully adopted for tuning and photoresponse in a broad range of polymeric and hybrid solar cells.