A conjugated polymer photosensitizer for energy conversion applications

Abstract The conjugated polymer, poly(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b’]dithiophene), was evaluated as a near-infrared photosensitizer on TiO 2 electrodes for solar energy conversion. It was characterized through steady-state absorption, photoluminescence, spectro-electrochemical studies, and cyclic voltammetry. Cation generation and excited state decay dynamics with 560-nm excitation in solution and on TiO 2 films with different electrolyte concentrations were also studied using femtosecond transient/pump-probe spectroscopy. A significant difference in the relaxation processes was observed when in solution versus adsorbed onto TiO 2 -coated quartz substrates. A slow ground state bleach (∼20 ps) and slower recoveries were exhibited in solution, whereas a quick ground state bleach and quick recovery along with excited state absorption peaks were observed on TiO 2 films, confirming electron injection. Charge transfer from the excited polymer to TiO 2 ’s conduction band was also suggested from DFT calculations, and was estimated by time-correlated single photon counting (TCSPC) measurements. Although devices exhibited modest efficiency and photocurrent, the TCSPC results indicated an increase in the electron injection efficiency in the presence of electrolytes, which suggests that the careful selection of electrolytes plays a role in the energetics of solar cell fabrication. Modest experimental efficiencies were also observed from increasing localized charge distributions on molecular orbitals of the polymer molecules with increasing length.