Quantum-dot-sensitized
solar cells are emerging as a promising
development of dye-sensitized solar cells, where photostable semiconductor
quantum dots replace molecular dyes. Upon photoexcitation of a quantum
dot, an electron is transferred to a high-band-gap metal oxide. Swift
electron transfer is crucial to ensure a high overall efficiency of
the solar cell. Using femtosecond time-resolved spectroscopy, we find
the rate of electron transfer to be surprisingly sensitive to the
chemical structure of the linker molecules that attach the quantum
dots to the metal oxide. A rectangular barrier model is unable to
capture the observed variation. Applying bridge-mediated electron-transfer
theory, we find that the electron-transfer rates depend on the topology
of the frontier orbital of the molecular linker. This promises the
capability of fine tuning the electron-transfer rates by rational
design of the linker molecules