Alkyl chain length dependence of the charge-transfer, recombination and electron diffusion length on the photovoltaic performance in double donor-acceptor-based organic dyes for dye sensitized solar cells

Abstract

A series of double donor-acceptor organic dyes (C4C12) have been synthesized to investigate the influence of the alkyl chain length between the two chromophores on the performance of dye sensitized solar cells (DSSCs). The chain length dependent performance was investigated further using the electrochemical impedance and stepped light induced transient measurement methods. Dye with longer alkyl chain (C6C12) shows a broader and higher IPCE as well as photo-current density (Jsc) with an enhanced photovoltage (Voc) relative to the reference PTZ-S. In contrast, C4 with shorter alkyl chain presents a relatively low IPCE within the whole spectral region, along with low Jsc and Voc, which predominately arising from the short electron diffusion length with significant electron loss during charge transport. The higher Jsc and Voc obtained with the devices incorporating C10 and C12 dyes with long alkyl chain resulted in a higher conversion efficiencies of 4.02% and 4.03%, respectively, (3.14% for single donor-acceptor dye PTZ-S). Density functional theory simulations were also performed to examine the effect of the alkyl chain length on electrochemical and optical properties of the dyes. Adsorption studies of the dyes on TiO2 clusters were carried out to explain the variation in Jsc with the alkyl chain length.