Hot carrier dynamics in InGaAs/GaAsP quantum well solar cells

Abstract

A hot carrier solar cell is a device with a steady-state carrier population which is described by a higher temperature than the surrounding lattice. Thermalisation loss is reduced in such a device, offering the potential for substantial efficiency advantages over single junction solar cells. Despite clear efficiency benefits no real world device has ever been developed, partly because of the difficulty of developing a suitable absorber material with sufficiently limited interaction between excited carriers and lattice phonons. This study evaluates the suitability of strain balanced InGaAs/GaAsP quantum well structures as hot carrier absorbers. Ultrafast time resolved photoluminescence (TRPL) spectroscopy measurements are presented which demonstrate hot carrier populations beyond 2ns after excitation in a deep well sample. Continuous wave photoluminescence (CWPL) spectroscopy was used to compare steady-state carrier populations in deep and shallow well samples. In both cases hot distributions were observed under photon flux density greater than 10,000 Suns equivalent. Increasing incident photon flux density was shown to increase carrier distribution temperature, suggesting that the hot carrier effect might be enhanced in a multiple QW structure with better well region absorption. It was also found that the deep well sample achieved significantly higher carrier distribution temperatures than the shallow well sample, demonstrating that increasing quantum confinement further inhibits thermalisation pathways. This study provides a guide to the development of hot carrier solar cells as it indicates deep multiple quantum well samples might exhibit an enhanced hot carrier effect. Strain Balanced InGaAs/GaAsP is a particularly suitable material system for growing this type of structure, making it an exciting prospect for the development of a hot carrier absorber.