Role of π-Spacers and Acceptors in Regulating the Photophysical Properties of BTPA Donor-Based Dyes: First-Principles Approach

Dye-sensitized solar cells (DSCs) have the potential to access sunlight in the near-infrared (NIR) region. A precise understanding of the design and stability of photosensitizers is one of the ways to improve efficiency. Through this computational study, we systematically investigated the effect of π-spacers and acceptor groups on the photophysics of the BTPA-based dye to access photons from the NIR region. We incorporated two distinct acceptor groups, benzoic acid (BA) and cyanoacrylic acid (CA), with 20 different π-spacers such as BBT and iso-BBT fragments. Our calculation found that the asymmetry of the π-spacers is the decisive factor in lowering the energy gap. Thus, the bathochromic shift of the absorption peak is to the NIR region. Our result revealed that the maximum absorption peak was at 969 nm and broadened to 2000 nm. The perfect complementary absorption spectra and light-harvesting efficiency (LHE) curves of the designed dyes and XY1b dyes indicate their feasibility for the application in cosensitized DSCs. In addition, asymmetric π-spacers show a relatively lower intersystem crossing (ISC) rate, which increases the probability of charge transfer (CT) to the semiconductor and a higher excited-state lifetime. Designed dyes with BBT fragments show better intermolecular CT (ICT) properties, although the acceptor group does not affect the ICT. The results proved that the rational molecular design provides a valuable reference for synthesizing dyes with higher efficiency and can be explored further.