Fabrication and characterization of co-sensitized dye solar cells incorporating cyclodextrin functional layers with energy transfer from spiropyran derivatives to SQ2 dye

Abstract

We developed dye-sensitized solar cells (DSSCs) using the sensitizing dye 1,5-carboxy-2-[[3-[(2,3-dihydro-1,1-dimethyl-3-ethyl-1H-benzo[e]indol-2-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]-3,3-dimethyl-1-octyl-3H-indolium (SQ2) and the photochromic molecule 1,3,3-trimethyl indolino-6′-nitrobenzopyrylospiran (SPNO₂). DSSCs incorporating photochromic molecules can regulate photoelectric conversion efficiency (η) through external light irradiation. To enhance η, cyclodextrins (CDs) (α-CD, β-CD, γ-CD) were introduced. Among them, γ-CD improved η from 1.2 % to 1.5 % by suppressing SQ2 aggregation. The color of the SQ2/CDs/SPNO₂-containing photoelectrode changed from blue to purple under UV irradiation and from purple to green under visible light. The η of DSSCs increased after UV irradiation and decreased under visible light. However, direct electron injection from the photochromic molecule appeared to be challenging, suggesting that the efficiency enhancement was primarily due to Förster resonance energy transfer (FRET) from SPNO₂ to SQ2. FRET efficiency calculations showed that β-CD exhibited the highest efficiency among CDs. These findings indicate that the co-sensitization of DSSCs by SPNO₂-SQ2 and SQ2 alone, facilitated by their respective photoabsorption properties, enables externally tunable photoelectric conversion. This study provides insights into high-performance DSSCs with tunable efficiency through CD inclusion effects and photochromic molecular energy transfer, contributing to solar energy advancements.