Enhanced photoresponse in solid-state excitonic solar cells via resonant energy transfer and cascaded charge transfer from a secondary absorber.

We present a spiro-linked molecule 2,2',7,7'-tetrakis(3-hexyl-5-(7-(4-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)thiophen-2-yl)-9,9'-spirobifluorene which acts as a secondary absorber in solid-state excitonic solar cells. Blending with a hole-transporting material 2,2'7,7'-tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9'-spirobifluorene and used in conjunction with a near-infrared dye (termed TT1) results in an extended spectral response which yields a notable increase in short-circuit current and power conversion efficiency. This enhancement is due to both exciton energy transfer and also nanoscale charge generation in the blend via the formation of an excited state spiro-complex with charge transfer character.