Interfacial adsorption and aggregation of 4H-pyranylidene dyes on TiO2: experimental–computational structure–function study for indoor dye-sensitized solar cells

A series of D-π-A organic dyes incorporating a 4 H -pyranylidene donor, a thiophene π–spacer, and a cyanoacrylic acid anchoring group were developed to investigate substituent-controlled interfacial organization on TiO 2 photoanodes. The influence of bulky triisopropylsilyl (TIPS) and hydroxymethyl groups on adsorption strength, aggregation behavior, and charge-transfer dynamics was analyzed through combined experimental characterization and multiscale modeling. Periodic DFT calculations on anatase (101) and (0 0 1) surfaces reveal stronger adsorption and enhanced surface packing for hydroxymethyl derivatives, together with facet-dependent variations in adsorption energetics and surface relaxation, while molecular dynamics simulations correlate aggregate compactness with device performance. In dye-sensitized solar cells, TIPS-substituted dyes exhibit higher open-circuit voltages due to reduced recombination, whereas hydroxymethyl analogues provide higher photocurrents. Power conversion efficiencies reach 7.4% under AM 1.5G illumination and up to 14.9% under indoor lighting, with devices retaining their efficiency for over 1000 h of operation, demonstrating excellent long-term stability. A clear relationship between interfacial aggregation, recombination resistance, and photovoltaic response is established, highlighting the role of adsorption geometry and collective organization in governing interfacial charge-transfer processes. These results establish substituent engineering on the π-spacer as an effective strategy to control interfacial organization and stability in next-generation DSSCs for both outdoor and indoor applications.