Numerical Demonstration of Trade-off between Carrier Confinement Effect and Carrier Transport for Multiple-Quantum-Well Based High-efficiency InGaP Solar Cells

To promote InGaP solar cell efficiency toward the theoretical limit, one promising approach is to incorporate multiple quantum wells (MQWs) into the InGaP host and improve its open-circuit voltage by facilitating radiative carrier recombination owing to carrier confinement. In this research, we demonstrate numerically that a strain-balanced (SB) In_1-xGa_xP/In_1-yGa_yP MQW enhances confined carrier density while degrades the effective carrier mobility. However, a smart design of the MQW structure is possible by considering quantitatively the trade-off between carrier confinement effect and carrier transport, and MQW can be advantageous over the InGaP bulk material for boosting photovoltaic efficiency.

I. Introduction

In_0.49Ga_0.51P subcell serving as the top cell of multi-junction solar cells is of great importance toward achieving energy conversion efficiency over 50%. The efficiency of a single junction InGaP cell nevertheless remains lower than the Shockley-Queisser limit and needs to be further improved. Because a good solar cell is also a good LED [1], it has been suggested that multiple quantum wells (MQWs) can replace bulk materials toward higher efficiency, due to the inherent strong radiative efficiency originated from the carrier confinement effect. Despite a few reports seemingly confirming this expectation, a fair comparison of the carrier confinement effect has not yet been carried out between MQW designs and bulk materials owing to the different absorption threshold. On the other hand, MQWs are expected to present poorer carrier transport properties compared to bulk materials. In this research, we propose novel strain-balanced (SB) In_1-xGa_xP/In_1-yGa_yP MQW designs of the consistent effective bandgap energy 1.91 eV with that of the In_0.49Ga_0.51P bulk, as shown in Figure 1. We numerically demonstrate the trade-off between the carrier confinement effect and carrier transport for the first time. Fig. 1.

The schematic of the conventional multiple-quantum-well (MQW) design for solar cells and the novel design in this research. In this research, the effective bandgap energy (the transition energy of 1e-1hh) is designed to be the same as its bulk counterpart so that a fair comparison between MQWs and bulk materials is possible. Here, the bulk material of interest is In_0.49Ga_0.51P, the well material is In_1-xGa_xP, and the barrier material is In_1-yGa_yP. and denote for the thickness of the well material and the barrier material, respectively.