Solution processable thin film organic photovoltaic cells based on far red sensitive soluble squaraine dyes

Symmetrical squaraine (SQ) dyes with varying alkyl chain lengths have been synthesized towards their utilization as sensitizer for enhancing the photon harvesting in far-red region of solar spectrum. It has been observed that energy level of the SQ-dyes can be systematically tuned only by alkyl chain substitution. It has also been shown that SQ-dyes behave like p-type donors forming a p–n heterojunction with soluble fullerene derivative phenyl-C61-butyric acid methyl ester (PCBM) as n-type organic electron acceptor. Thin film organic photovoltaic p–n heterojunction devices have been fabricated using soluble SQ-dyes with PCBM without using any thermal evaporation technique for the active layers. A perusal of the photo-action spectra clearly corroborates that at least 0.25 eV energy barrier is necessary for the facile electron transfer from donor moiety to the acceptor after the photo-excitation.

► Squaraine dyes with systematically varying energy levels have been synthesized.
► They exhibit intense light absorption in far-red region with narrow absorption window.
► Completely solution processable organic solar cells were fabricated.
► Solar cells were made using squaraine dyes with a fullerene derivative.
► Need of about 0.25 eV energy barriers for facile electron transfer was demonstrated.