Quantum Dot Solar Cells. Harvesting Light Energy with CdSe Nanocrystals Molecularly Linked to Mesoscopic TiO2 Films

Abstract

By using bifunctional surface modifiers (SH−R−COOH), CdSe quantum dots (QDs) have been assembled onto mesoscopic TiO₂ films. Upon visible light excitation, CdSe QDs inject electrons into TiO₂ nanocrystallites. Femtosecond transient absorption as well as emission quenching experiments confirm the injection from the excited state of CdSe QDs into TiO₂ nanoparticles. Electron transfer from the thermally relaxed s-state occurs over a wide range of rate constant values between 7.3 × 10⁹ and 1.95 × 10¹¹ s^-1. The injected charge carriers in a CdSe-modified TiO₂ film can be collected at a conducting electrode to generate a photocurrent. The TiO₂−CdSe composite, when employed as a photoanode in a photoelectrochemical cell, exhibits a photon-to-charge carrier generation efficiency of 12%. Significant loss of electrons occurs due to scattering as well as charge recombination at TiO₂/CdSe interfaces and internal TiO₂ grain boundaries.