Role of an Auxiliary Donor and Ethynyl π-Spacer in a New Phenothiazine Derivative as a Dye for Dye-Sensitized Solar Cells

A new donor-donor-π-acceptor 2-cyano-3-(7-(2-(4-methoxyphenyl)ethynyl)-10-ethyl-10H-phenothiazin-3-yl)acrylic acid (PEDCA) has been synthesized in good yield and was characterized using nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR) spectroscopies. The obtained phenothiazine (PTZ) derivative was explored in dye-sensitized and tandem dye-sensitized solar cells (TDSCs). For reference purposes, the devices based on commercial organometallic dye N719 and a similar PTZ-based dye but without auxiliary donor and ethynyl π-spacer, that is, 2-cyano-3-(10-ethyl-10H-phenothiazin-3-yl)acrylic acid (PEC) were used. The experimental results had also been supported by theoretical calculations performed by employing density functional theory. It was observed that the presence of an auxiliary donor and an ethynyl π-spacer in PTZ unexpectedly decreased the molar absorption coefficient. However, this modification positively influenced the dye loading on the photoanode, which led to a 37% increase in the power conversion efficiency (PCE) of the dye-sensitized solar cell (DSSC) compared to the device sensitized with PEC. Further modifications to the device architecture allowed to obtain DSSC with efficiency above 7%. Despite the fact that the molar absorption coefficients of the PEC solution were higher compared to the PEDCA solution, the key issue is the number of dye molecules, which determines the photocurrent obtained, as shown in the following studies.