# Design of Novel Iminocoumarins for D‑π‑A System DSSCs: A (TD)DFT Study

## TL;DR

This paper designs new iminocoumarin dyes for solar cells and uses computational methods to evaluate their performance.

## Contribution

The study introduces novel iminocoumarin dyes with π-linkers and evaluates their photovoltaic potential using DFT and TD-DFT calculations.

## Key findings

- A3, C3, and D3 dyes show strong photon-absorption characteristics with high light-harvesting efficiencies.
- Incorporating π-linkers like 2,2′-bithiophene improves molecular planarity and narrows the HOMO–LUMO gap.
- Transition dipole moment is identified as a key descriptor for designing efficient DSSC sensitizers.

## Abstract

Dye-sensitized solar cells (DSSCs) are third-generation
photovoltaic
devices in which photogenerated electrons are injected from an excited
dye into a mesoporous TiO2 film. The molecular design of
the dye therefore plays a central role in tuning the light absorption
and charge-transfer efficiency. In this work, we investigate the photovoltaic
potential of a new series of 3-benzo­[d]­[1,3]­thiazole-substituted iminocoumarin
dyes by comparing their properties with the benchmark coumarin dye
NKX-2677. Using DFT and TD-DFT calculations, we evaluated Boltzmann-weighted
conformer distributions, absorption/emission profiles, π-electron
delocalization, orbital overlap, and the feasibility of electron injection
and dye regeneration. The incorporation of 2,2′-bithiophene
and benzo­[d]­[1,2,5]­diathiazole π-linkers was found to be essential
for narrowing the HOMO–LUMO gap and enhancing molecular planarity.
Among the designed sensitizers, A3, C3, and D3 display the most favorable
spectral properties, with tunable absorption profiles modulated by
substitution at the 7-coumarin position. Their light-harvesting efficiencies
(LHE), reported here as a spectral proxy derived from the oscillator
strength (f), highlight their potential as DSSC photosensitizers.
Specifically, A3 shows λmax = 427 nm, f = 0.971, LHE = 89.3%; C3 exhibits λmax = 520 nm, f = 0.999, LHE = 90.0%; and D3 presents λmax = 457 nm, f = 1.248, LHE = 94.4%. These λmax/f/LHE data indicate strong photon-absorption
characteristics and suggest competitive charge-separation behavior
relative to that of the reference dye. Furthermore, the visible-light
transition dipole moment (TDM) emerges as a robust descriptor for
guiding the rational in-silico design of future DSSC
sensitizers.