Each type of quantum solar energy conversion system employs a conversion mechanism that can be radically different than the next. In each, photo-product and energy production can be varied and diverse, yet one optical concept may connect all quantum conversion systems - the time reversal of excitation by light is the production of luminescence. This luminescence characterizes the maximum work extraction and energy production in the converter. For any quantum system described, the absorber functions as a âpumpâ to increase the chemical potential on one side of a barrier. Recombination and "back" reactions (which include luminescence) compete with the work production in an external load. The general quantum-converter scheme is shown in Fig. 5.1. The amount of work done is given by the difference between the chemical potentials of electrons and holes on the acceptor and pump sides of the converter. This chemical potential difference is maintained by the light-driven pump due to the ability of the pump to maintain a difference between the concentration of excited electrons and holes on the acceptor and pump sides of the barrier. The barrier at the interface between the two sides of the device inhibits the back reaction. The concentration of excited electrons on the N-type (electron acceptor) side is n â , and the concentration of excited holes on the P-type side is p â .
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