Y-shaped organic dyes with D2–π–A configuration as efficient co-sensitizers for ruthenium-based dye sensitized solar cells

Abstract Three new Y-shaped organic dyes, coded CD1–3, have been synthesized for dye-sensitized solar cells (DSSCs). They have a D2–π–A configuration and been molecularly engineered to modulate the spectral properties and energy levels. For comparison, a D–π–A type sensitizer with one donor group (CD4) is synthesized as well. We have performed a combined experimental and theoretical investigation of their optical, electrochemical, photovoltaic, geometrical and electronic characteristics. These new Y-shaped dyes display intensive UV–vis absorption with high molar extinction coefficients. So their roles as co-sensitizers for ruthenium-based DSSCs are further evaluated. As a consequence, the co-sensitized devices exhibit improved power conversion efficiencies (PCEs) in the range of 7.24–7.96% due to the enhanced short-circuit photocurrents (Jsc) and open-circuit voltages (Voc), which are higher than the DSSCs sensitized with N719 (PCE = 6.75%) or Z907 (PCE = 6.55%) alone. In addition, the mechanisms of these improvements have been elucidated by incident photon to current efficiency (IPCE) and electrochemical impedance spectroscopy (EIS) studies. The Jsc is improved because CD1–3 can compensate for the photocurrent loss by I−/I3−; while the Voc is enhanced because the electron recombination process is retarded by these Y-shaped dyes upon co-sensitization.