Computational analysis of ligand design for Ru half-sandwich sensitizers in bulk heterojunction (BHJ) solar cells: Exploring the role of –NO2 group position and π-conjugation in optimizing efficiency

Abstract

Bulk Heterojunction (BHJ) solar cells offer cost-effective photovoltaic solutions, flexible manufacturing, and improved efficiency with reduced environmental impact. Organometallic half-sandwich complexes, specifically a series of monometallic ruthenium complexes (Ru(1)–Ru(6)), have emerged as promising candidates for BHJ solar cells due to their outstanding charge transport properties. This study utilised Density Functional Theory (DFT) calculations with the B3LYP functional and SDD basis set augmented with 6-31 + G(d,p) to assess these Ru complexes with various N,O-chelating ligands, featuring distinct π-conjugated systems and positions of nitro groups, for BHJ solar cell applications. Our investigation focused on elucidating their electronic structures, identifying key electrophilic and nucleophilic sites, and exploring their optical properties. Our findings indicate that these complexes have maximum absorption wavelengths (λmax) ranging from 702 to 1083 nm, highlighting their strong photon absorption capabilities. Specifically, Ru(6), which includes the NO2 group, exhibits the narrowest energy gap but shows relatively lower values for oscillator strengths (f) and light harvesting efficiencies (LHE), indicating efficient light absorption but reduced energy conversion efficiency. In contrast, Ru(3), lacking the NO2 group but featuring enhanced π conjugation through cyclisation, demonstrates the widest energy gap yet boasts the highest values for f and LHE, suggesting superior efficiency in converting absorbed energy into electronic energy. These results emphasise the crucial role of ligand π-conjugation in enhancing energy conversion efficiency, while the strategic positioning of the NO2 group aids effective light absorption. Our study offers valuable insights into the design principles for optimizing organometallic ruthenium complexes in BHJ solar cell applications.