Impact of S1→S0 internal conversion in betalain-based dye sensitized solar cells

Abstract

Betalains are natural dyes which can be used in non-toxic environmentally friendly dye-sensitized solar cells (DSSC). In this paper we provide results characterizing the performance of DSSC constructed using betanin, 2,17-bidecarboxybetanin, vulgaxanthin I and betanidin-6-O-(6′,6″-di-O-trans-4-coumaroyl)-β-sophoroside, as well from the red beetroot aqueous extract. The photovoltaic parameters of the cells correlate with the results of time-resolved measurements for fully assembled cells. We reveal that insufficient electron injection quantum yield is mainly responsible for the limited sunlight conversion efficiency. In betacyanin-based DSSC, only about 50% of the excited dye molecules inject electrons, while the rest suffer from a dye S1 fast internal conversion process. In betaxanthin-based DSSC, the quantum yield of electron injection is higher (about 70%), but the total sunlight conversion efficiency is smaller due to the blue-shifted absorption band. A probable explanation of the differences in the electron injection quantum yields in the studied dyes is that of different rates of the competing S1→S0 internal conversion processes.