Optimizing photovoltaic performance of squaraine derivative dyes: a DFT study on different anchoring groups

Abstract

In this study, we designed squaraine-based dyes with a 2-amino pyrrole donor unit and acene groups like anthracene and pentacene. These dyes incorporate three different electron-withdrawing groups – cyanoacrylate (A1), phosphonate (A2) and boronic acid (A3) – as linkers to the TiO₂ semiconductor. The spectroscopic, electronic and photochemical properties of these compounds were investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT) simulations. Compared to the squarylium dye, SQD, the UV-vis data indicate excellent absorption especially for pentacene-based dyes, which extended beyond 920 nm, enhancing the panchromatic effect. The calculated excited-state lifetimes of these dyes were notably longer than SQD, particularly for those containing pentacene and either A1 or A2 withdrawing groups, with lifetimes approximately four times longer. In contrast, boronic acid derivatives had shorter excited-state lifetimes, hindering charge transfer. Simulations suggest all sensitizers can inject electrons into TiO₂ and be efficiently regenerated by electron transfer from the electrolyte. The best results were achieved with pentacene and A1 or A2 as linkers, notably A1 dyes achieve superior short circuit photocurrent, J_sc, and power conversion efficiency, PCE, with over 50% improvement compared to SQD. Phosphonate derivatives exhibited the highest energy adsorption on TiO₂ while still achieving significant open-circuit voltage, V_oc, J_sc, and PCE values. After surface adsorption, all dyes displayed efficient electron recovery, with HOMO levels significantly dropping below −4.8 eV. Our study demonstrates that computational design can significantly enhance experimental work, offering valuable insights to improve dye design and boost the performance of dye-sensitized solar cells.