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

Abstract

New unsymmetrical low band-gap push-pull squaraine chromophores, [TPA-bisHT-HP]-X-Ind [X: SQ, diCNSQ], which are composed of triphenylamine electron donating, pi-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/C-71-PCBM film exhibited a PCE of 2.05% (+/- 0.07) with short-circuit current (J(sc))=9.05 mA/cm(2), fill factor (FF)=0.33, and open-circuit voltage (V-oc)=0.69 V, which is similar to 3 times higher efficiency compared to [TPA-bisHT-HP]-SQ-Ind/C-71-PCBM BHJ films. (C) 2011 Elsevier B.V. All rights reserved.