Statistical thermodynamic foundation for photovoltaic and photothermal conversion. IV. Solar cells with larger-than-unity quantum efficiency revisited

A detailed balance solar energy conversion model offering a single treatmen t of both photovoltaic and photothermal conversion is expounded. It include s a heat rejection mechanism. The effect of multiple impact ionizations on the solar cell efficiency is reconsidered by including the constraints dict ated by the first law of thermodynamics (which already exist in the model) and it improves of course the solar cell efficiency. However the upper boun d efficiencies previously derived are too optimistic as they do not take in to consideration the necessary increase in solar cell temperature. The cell efficiency operating under unconcentrated radiation is a few percent lower than in the ideal case (i.e., with perfect cooling). Wider band gap materi als are recommended for those applications where the cell cooling is not ef fective. The best operation of naturally ventilated cells is under unconcen trated or slightly concentrated solar radiation. Increasing the (forced) ve ntilation rate allows an increase of the optimum concentration ratio. Addit ional effects such as the radiation reflectance and radiative pair recombin ation efficiency are also considered. A sort of threshold minimum band gap depending on the last effect is emphasized: materials with band gaps narrow er than this threshold are characterized by very low cell efficiency. (C) 2 001 American Institute of Physics.