Reducing Shockley–Read–Hall recombination losses in the depletion region of a solar cell by using a wide-gap emitter layer
Tetsuya Nakamura, Warakorn Yanwachirakul, Mitsuru Imaizumi, Masakazu Sugiyama, Hidefumi Akiyama, Yoshitaka Okada
Abstract
The conversion efficiency of a solar cell depends on the degree of non-radiative recombination, and thus efficiency improvements can also be achieved by reducing Shockley–Read–Hall (SRH) recombination losses. This type of loss depends not only on the crystal quality, but also on the device structure. A clear separation of the contributions of these factors would improve our understanding of the control of non-radiative recombination. In this work, we discuss the reduction of the integrated SRH recombination rate in the depletion region (USRHdep) by changing the emitter material, instead of fabricating a base layer with a higher crystal quality. First, we theoretically show that, by employing a suitable n-InGaP/p-GaAs heterojunction structure instead of a GaAs p–n homojunction, the integrated USRHdep can be reduced, because a significant part of the depletion region in the heterojunction is located in the wide-gap emitter material, which has a lower intrinsic carrier density. Then, the effective SRH recombination coefficient in the depletion region (Adep¯) is obtained from experiments, and the effect of the structural modification on USRHdep is analyzed. We are able to clearly assess the effect of the heterojunction structure on the non-radiative recombination because the grown samples exhibit the same radiative recombination loss. The analysis reveals that, in suitable heterojunction solar cells with an emitter layer containing a low intrinsic carrier concentration, Adep¯ (and thus also USRHdep) is effectively reduced.