Photoexcited charge separation occurs at the interface between the two phases. This cell represents a new design in solar cells and is very efficient, despite the fact it is made from inexpensive, poor electrical quality oxide semiconductor nanocrystals presently manufactured in huge quantities for the ceramic and paint industries. Nanocrystals have higher electron affinities than organic polymers such as PPV, which are used as hole conductors in organic displays. The optical and electrical properties of such organized nanocrystal solids could be tuned through nanocrystal size, through the inter-nanocrystal coupling as controlled by surface passivation, and by doping via material in the interstitial spaces. Work over the past decade has lead to an understanding of the evolution from molecules to bulk solids, at least in outline form, for individual nanocrystals, as shown in Fig.
Schematic size regimes for semiconductor nanocrystals.
In the nanocrystal regime, the unit cell is present, yet the electronic states are strongly quantum-confined in three dimensions. The excited electronic states and the band gap variation with size can be quantitatively calculated with advanced theory. In the polariton regime, the interaction with the electromagnetic field becomes strong and can be described by Mie theory. In the absence of a moving charge, nanocrystals are intrinsic dielectric particles. Also, monolayer surface passivation with a higher band gap material can eliminate most nanocrystal surface states, while still allowing efficient electron transfer on and off the particle.