Thermophotovoltaics involves the photovoltaic conversion by a receiver cell of radiation from an emitter, which could be heated by various sources including sunlight. A prime difference from normal solar photovoltaics is that emitted energy unable to be used by the receiver can, in principle, be recycled allowing high conversion efficiency. Thermophotonics is a recent development of this concept where the emitter is "active", namely a heated diode, increasing the rate of energy transfer for a given emitter temperature and concentrating emission in an energy range more suited for conversion by the receiver. This paper evaluates thin semiconducting layers as emitters for thermophotovoltaics and thermophotonics. It is shown that thermophotonics avoids a major challenge for thermophotovoltaics: the sensitive dependence of system efficiency on the recycling of below bandgap radiation. Possible ways of achieving the high external quantum efficiency light-emitting diode required for thermophotonics are discussed. (C) 2003 Elsevier Ltd. All rights reserved.