Transparent and colorless DSSCs featuring thienyl-pyrrolopyrrole cyanine sensitizers

Fully transparent and colorless dye sensitized solar cells have recently gained much attention due to their unconventional properties. Following the known strategy to redshift the absorption band of diketopyrrolopyrrole chromophores that consists in substituting phenyl rings by thiophene, we propose herein to extend that procedure to pyrrolopyrrole cyanine (PPcy) in order to prepare new thienyl-PPcy dyes. These new dyes were investigated by UV-vis absorption and emission spectroscopy and by electrochemistry, and their electronic properties were rationalized by DT-DFT calculations. A significant bathochromic shift (30 nm) was observed compared to previous PPcys allowing a better average visible transmittance of 67% to be reached. A detailed photophysical study demonstrates that the lower performances are due to a stronger aggregation tendency and a slower electron injection rate connected with a lower driving force inducing a higher quenching degree by energy transfer towards the aggregates. This study reveals the great potential of this type of dyes for TPVs because these drawbacks could be circumvented by molecular engineering of new PPcys-th.