NIR-Absorbing Merocyanine Dyes for BHJ Solar Cells

Abstract

We have synthesized a series of new, polymethine chain extended merocyanine dyes <b>1</b>–<b>4</b> bearing varied acceptor units and an aminothiophene donor moiety. The optical and electronic properties of these new merocyanines have been studied in comparison with their corresponding lower homologues <b>5</b>–<b>8</b>, which contain two methine groups less, by UV–vis and electro-optical absorption (EOA) spectroscopy and cyclic voltammetry. The absorption spectra of π-extended merocyanines are markedly red-shifted, and their extinction coefficients are significantly increased compared to those of their lower homologues. The photovoltaic characteristics of these dyes have been explored in devices using them as donor and PC₆₁BM fullerene as acceptor materials. Our detailed studies reveal that, despite more favorable absorption properties, the π-extended merocyanines exhibit lower short-circuit current densities (<i>J</i>_SC) as well as decreased open-circuit voltages (<i>V</i>_OC) and power conversion efficiencies (<i>PCE</i>) compared with those of their respective lower homologues. The unexpected decreased <i>J</i>_SC values could be explained in terms of looser packing features of π-extended chromophores in the solid state as revealed by single-crystal X-ray analysis of two pairs (<b>1</b>/<b>5</b> and <b>4</b>/<b>8</b>) of these dyes. By optimization of device setup <i>PCE</i> of 2.3% has been achieved with the π-extended donor material <b>4</b>