Dynamics and Efficiency of Photovoltaic Processes in Supramolecular Structures and Semiconductors

Abstract

Light harvesting and electron-hole pair separation was studied with picosecond time resolution in supramolecular structures and in semiconductors. Slow singlet exciton diffusion requires an absorption coefficient >10⁵ cm⁻¹ throughout the solar specturm for efficient light harvesting in the supra-molecular antennae system or the assembly of several such less efficient systems in series to facilitate complete light harvesting. The at least 100 times faster minority carrier diffusion in semiconductors imposes a much weaker requirement on the absorption coefficient for complete light harvesting in semiconductors. Optimized electron-hole pair separation along a molecular chain requires a very fast initial hop (ps) and a standard free energy difference smaller than 0.25 eV. Thus the reorganisation energy must be kept small with rigid molecules incorporated into a rigid environment with not too different values for the optical and static dielectric constant. A desirable fuel producing solar energy converter could consist of an efficient photovoltaic semiconductor device protected against corrosion and coupled directly to molecular fuel producing catalyzers.