Size dependent electron transfer from CdTe quantum dots linked to TiO2 thin films in quantum dot sensitized solar cells

In this present study, we demonstrate the size dependent charge transfer from CdTe quantum dots (QDs) into TiO₂ substrate and relate this charge transfer to the actual behavior of a CdTe sensitized solar cell. CdTe QDs was synthesized using mercaptopropionic acid as the capping agent. The conduction band offset for TiO₂ and CdTe QDs indicates thermodynamically favorable band edge positions for smaller QDs for the electron-transfer at the QD–TiO₂ interface. Time-resolved emission studies were carried out for CdTe QD on glass and CdTe QD on TiO₂ substrates. Results on the quenching of QD luminescence, which relates to the transfer kinetics of electrons from the QD to the TiO₂ film, showed that at the smaller QD sizes the transfer kinetics are much more rapid than at the larger sizes. I–V characteristics of quantum dot sensitized solar cells (QDSSC) with different sized QDs were also investigated indicating higher current densities at smaller QD sizes consistent with the charge transfer results. The maximum injection rate constant and photocurrent were obtained for 2.5 nm CdTe QDs. We have been able to construct a solar cell with reasonable characteristics (V_oc = 0.8 V, J_sc = 1 mA cm⁻², FF = 60%, η = 0.5%).