External electric field modulation of a high-performance D–A–π–A sensitizer for dye-sensitized solar cells: a DFT/TD-DFT investigation

Abstract Rational design of metal-free organic sensitizers is crucial for advancing dye-sensitized solar cells. While D–A–π–A dyes such as XY1 show high performance, their behaviour is strongly influenced by interfacial electric fields (EFs). Using DFT/TD-DFT (CAM-B3LYP/6-311+G(d,p)) in a solvent model, we reveal a non-monotonic response of the electronic properties of XY1 to external EFs. The HOMO energy reaches a minimum at –4 × 10−4 a.u., maximizing the HOMO–LUMO gap and inducing a blueshift in λmax to 530 nm; other field strengths cause redshifts. The light-harvesting efficiency peaks (0.997) at +8 × 10−4 a.u., whereas the driving forces for electron injection (ΔGinj) and dye regeneration (ΔGreg) are optimal at the critical negative field. These competing effects define an ideal operational window between –4 and +8 × 10−4 a.u., where the photovoltaic parameters are balanced. This work introduces ‘Stark-tuning’ strategic manipulation of molecular response to local EFs as a key design principle for sensitizers tailored to real device environments.