Performance comparison of different methyl group positioning on salicylic acid sensitizers for photoelectrochromic device

Photoelectrochromic device (PECD) has become an important prospect technology for the deployment as an energy saving smart window in commercial and residential building sector. In this study, the effects of the position of the methyl group in salicylic acid was investigated. Three different types of salicylic acid derivatives namely 3-methylsalicylic acid (3-MSA), 4-methylsalicylic acid (4-MSA) and 5-methylsalicylic acid (5-MSA) were used to sensitize TiO2 layer in PEC device. The experimental results confirmed that the device impregnated with 3-MSA had better coloration depth (Delta T) of 35.79% at 550nm achieved within 2min. The same device experienced fast bleaching speed with 97.25% of the initial transmittance recovered within 2.5hrs. Additionally, a theoretical simulation was performed for 3-MSA, 4-MSA and 5-MSA ligands. The properties of molecular structure and electronic excitation was computed using density functional theory/time dependent density functional theory (DFT/TDDFT) calculation. The obtained results confirmed that electronic excitation and the energy gap between highest occupied molecular orbital and lowest unoccupied molecular orbital (HOMO-LUMO) of the ligands contributes to the coloration and bleaching kinetics. The comprehensive discussion on the theoretical simulation and experimental results contributes to our understanding of the PECD smart window.