THERMAL APPROACHES

An alternative to specifically engineering multiple energy thresholds in a device or devices is to allow the photons to generate a thermal population of some sort in an absorber. The photon spectrum incident on the cell is essentially a thermal one, generated by the thermal emission from the surface of the sun at an approximate temperature of 6000 K, with an emissivity very close to that of a black body. If this energy is transferred to particles in the absorber this thermal distribution can also be transferred, with the ‘excess energy’ of the multiple energy levels of the incident photons maintained in the thermal distribution of these particles.

This thermal excess energy can be maintained in a number of different ‘particle popu­lations’: the incident photons themselves; the carriers in the absorber (either with separate temperatures for electrons and holes or at a common temperature with electron-hole scat­tering); optical phonons together with electron-hole pairs; or acoustic phonons with fully thermalised electron-hole pairs; or acoustic phonons only with no energised electrons or holes. (The penultimate example is that for a normal solar cell, and the last example that for a fully thermalised material, probably with a zero bandgap, i. e. a metal.) The degree of irreversibility, and hence the entropy production, increases progressively through these examples. This is because for a thermal population described by a single Fermi temperature,

it is not possible to collect the energy of each photon at its optimum chemical potential. And as the number of particles between which the energy is shared increases (an increase in the accessible microstates) the entropy and irreversibility increase.

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