It has been advocated that nanocrystalline quantum-dot (QD) systems will form the basis of a new generation of solar cells with an efficiency that would exceed the limit of 31 % of a conventional mono-gap solar cell. We believe that QD systems hold promise for the spectral up- and down-conversion of solar light, and perhaps in a full quantum-dot solar cell. For each of these applications, the electronic properties of the individual QD building blocks, and the electronic interactions between them have to be understood and under control.
We have studied the electrostatic and electronic interactions between quantum dots with cryo-TEM, advanced optical spectroscopy and electrical scanning tunneling spectroscopy. Here, we present experimental results which include (1) dipole-dipole interactions between nanocrystals in the ground-state, (2) exciton energy transfer between weakly coupled quantum dots in a QD-molecule, (3) strongly coupled hetero-nanocrystals with charge-separated excitons and (4) variable quantum mechanical coupling between PbSe QDS assembled in a QD solid. The relevance of these results for applications in third-generation cells will be highlighted.

(1) Dipolar Structures in Colloidal Dispersions of PbSe and CdSe quantum dots. Nano Letters 7, 2931 (2007).
(2) Electronic coupling and exciton energy transfer in CdTe quantum-dot molecules. JACS 128, 10436-10441 (2006).
(3) Highly luminescent CdSe/CdTe colloidal hetero-nanocrystals with temperature-dependent emission color. P. Chin et al., JACS 2007.
(4) Variable orbital coupling in a two-dimensional quantum-dot solid probed on a local scale. Physical Review Letters 97, 096803 (2006).