The required characteristics of an ideal photoconverter of solar radiation to electrical or chemical energy are summarized. The four unavoidable loss mechanisms inherent in single-junction photoconverters - lack of absorption of sub-bandgap photons, thermalization of ultra-bandgap photons, the difference between the available energy and internal energy of the thermalized excited states, and the small loss of excited states by radiative decay - are quantified. As the Gibbs energy of the product of an irreversible photochemical reaction falls below the ideal limiting value attained by reversible reactions, the maximum energy stored falls rapidly. This is shown in diagrammatic form for three examples: the isomerization of norbornadiene to quadricyclane, and the splitting of water by a 2-electron, 2-photon and a 2-electron, 4-photon process. The radiative lifetime of the excited states in molecular chromophores and semiconductors is linked to the absorption spectra by the appropriate broadband form of the Einstein relation between absorption and emission probability. For molecules, this is the Förster or the similar Strickler-Berg relation, and for semiconductors it is the van Roosbroeck-Shockley relation. The radiative lifetimes predicted by these relations are compared with those required for ideal performance in four molecular and semiconductor systems. We show that in both cases the Einstein relation may reduce the radiative lifetime by a factor of ∼100 below its ideal value, which causes a moderate decrease in the output voltage or chemical potential. We discuss some of the reasons for the poorer performance of photoconverters based on molecular chromophores as compared with those based on semiconductors, which arise from differences between the two broadband Einstein relations, and between the absorption spectra and transport properties typical of each system. © 1990 American Chemical Society.
