Energy levels, charge injection, charge recombination and dye regeneration dynamics for donor–acceptor π-conjugated organic dyes in mesoscopic TiO2 sensitized solar cells

TLDR

The study examines how minor structural modifications of donor–acceptor dyes influence interfacial charge transfer and overall solar cell efficiency. Two D–π–A dyes, MP124 and MP‑I‑50, were synthesized and characterized electrochemically and spectroscopically, and their DSSC efficiencies were evaluated under varied sensitization conditions. Optimized MP124 cells achieved 7.45 % efficiency, outperforming MP‑I‑50’s 5.66 %; faster dye regeneration, longer electron lifetimes, and a higher HOMO in MP124 explain its superior performance, while MP‑I‑50’s tendency to form charge‑transfer complexes with I₂ reduces its efficiency.

Abstract

Two new D–π–A type organic sensitizers, MP124 and MP-I-50, were synthesized and their electrochemical and spectroscopic properties studied. Efficiencies of DSSC devices utilizing these dyes were also investigated, where sensitization solvent, sensitization time and additive concentration were all varied. Under standard AM 1.5G simulated solar radiation, optimized MP124 devices show an efficiency of 7.45% (Voc = 0.73 V; Jsc = 14.44 mA cm−2; FF = 70%) while optimized MP-I-50 devices show an efficiency of 5.66% (Voc = 0.68 V; Jsc = 12.06 mA cm−2; FF = 69%). Transient absorption spectroscopy studies show that regeneration of dye cations by the red-ox electrolyte was more efficient in MP124cells which is attributed to its higher HOMO energy leading to greater driving force for the regeneration reaction. Transient photovoltage studies showed that electron lifetimes were longer lived in MP124 explaining the higher Voc for these cells compared to MP-I-50cells. DFT and MP2 calculations indicate that this is due to the greater tendency of MP-I-50 to form charge-transfer complexes with the I2 species in the electrolyte, due to the presence of an additional EDOT in its structure compared to MP124. This work highlights the effect that small changes to the sensitizer structure can have on the interfacial charge transfer reactions and ultimately on the device efficiency.