Novel unsymmetrical push–pull squaraine chromophores for solution processed small molecule bulk heterojunction solar cells

New unsymmetrical low band-gap push–pull squaraine chromophores, [TPA-bisHT-HP]-X-Ind [X: SQ, diCNSQ], which are composed of triphenylamine electron donating, π-conjugated dithiophene-pyrrole bridging, and squaraine-indol electron withdrawing moieties, for their application in solution processed BHJ solar cell. The [TPA-bisHT-HP]-diCNSQ-Ind having the stronger electron withdrawing moiety exhibits significantly the red-shifted absorption spectrum due to its strong intramolecular charge transfer in solution as well as in solid-state. The bulk-heterojunction photovoltaic devices fabricated with [TPA-bisHT-HP]-diCNSQ-Ind/C71-PCBM film exhibited a PCE of 2.05% (±0.07) with short-circuit current (Jsc)=9.05 mA/cm2, fill factor (FF)=0.33, and open-circuit voltage (Voc)=0.69 V, which is ∼3 times higher efficiency compared to [TPA-bisHT-HP]-SQ-Ind/C71-PCBM BHJ films.