Advanced photovoltaic concepts and new routes to other electronic

6 Advanced photovoltaic concepts and new routes to other electronic devices 

Xi02 (b) Excitation scheme for two-step photon absorption in the PhS/Ti02 heterostructure, involving excitation from the Xi02 valence hand to Xi02 surface states, to PbS quantum-dot states, to PbS excited states and finally to Xi02 conduction-hand states. 

The process with the lowest excitation probability is the first excitation, from the Ti02 valence band to its surface state. Using an intricate electrochemical method for a determination of the qnantnm efficiency, we were able to show that this excitation occnrs at a probability of 10 (Bayon and Konenkamp, unpublished). While this efficiency is indeed qnite low, it should be borne in mind that these were only first experiments. Considerably more work is needed to establish and understand the feasibility of this suggestion. 

Concepts for tandem arrangements have been suggested for dye-sensitised solar cells using dye sensitisation at both electrodes and an electrolyte that provides the coupling between the two dyes (He et al, 2000). A similar concept appears feasible also for solid-state devices, althongh it has not been explored yet in the context of ETA cells. 

With sufficiendy thin absorber layers, particnlarly with quantum-dot absorbers, it also appears possible to harvest hot carriers and reduce the thermalisation loss in solar cells. This idea was pursned at an early time by Cooper et al. (1983). Hot electron transfer has been demonstrated in an electrochemical arrangement, and the possibility of hot-carrier transfer across solid interfaces is starting to be realised. Furthermore, the associated phenomenon of mnltiple exciton generation from a single hot electron-hole pair created by a single photon within a semicondnctor qnantnm dot has now been realised, as Art Nozik discnsses in Chapter 3. 

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