Optimal bandgap combinations — Does material quality matter?

Abstract

The balance of photogeneration and recombination gives rise to an optimum band-gap for any solar cell. The radiative limit represents the lowest permissible level of recombination in a solar cell and therefore places an upper limit on the voltage that can be attained. Introducing additional, non-radiative recombination results in a loss in voltage that can only be compensated for by moving to higher band-gaps. Consequently the optimal band-gap for solar energy conversion will rise with increasing non-radiative recombination rate. This balance was recognized by Shockley and Queisser for single junction solar cells and is here extended to multi-junction solar cells. A rise in optimal band-gaps has been observed in simulated single, double and triple junction devices as non-radiative recombination increases. Optimal band gaps between excellent poor diode ‘quality’ devices are shown to differ by 100s of meV under 1-Sun AM0 illumination, but exhibit no significant change under a 500-Sun AM1.5D spectrum.