Everyman´s guide to third generation efficiencies

A number of ´third generation´ mechanisms that have been suggested to improve the solar cell efficiency include impact ionisation, hot electrons, impurities, intermediate bands or quantum wells. This paper shows how some of these efficiencies might be estimated using general thermodynamic values for the maximum free energy which can be generated by a quantum solar converter. The aim is to obtain ´rule of thumb´ estimates which are valid for generic groups of devices rather than for specific structures. The principal novel element is a simple expression for the open circuit voltage which is obtained by generalising an approximation which follows from the detailed balance theory for the conventional one photon /spl rarr/ one electron-hole pair scenario, to more general situations. Bv modelling the device with the use of an equivalent circuit optimal the usual laws which govern voltages and currents in electrical circuits can be re-interpreted in terms of thermodynamic quantities.