Effect of the quantum well thickness on the performance of InGaN photovoltaic cells

International audienceWe report on the influence of the quantum well thickness on the effective band gap and conversion efficiency of In0.12Ga0.88N/GaN multiple quantum well solar cells. The band-to-band transition can be redshifted from 395 to 474 nm by increasing the well thickness from 1.3 to 5.4 nm, as demonstrated by cathodoluminescence measurements. However, the redshift of the absorption edge is much less pronounced in absorption: in thicker wells, transitions to higher energy levels dominate. Besides, partial strain relaxation in thicker wells leads to the formation of defects, hence degrading the overall solar cell performance. InGaN alloys are considered as promising candidates for high-efficiency photovoltaic devices [1-4] since their band gap spans almost the whole solar spectrum from 0.7 eV (InN) to 3.4 eV (GaN). This makes theoretically possible the development of all-InGaN multijunction solar cells with a freely customizable number of junctions to enhance the overall efficiency. However, the large lattice mismatch between GaN and InN has led several groups to study the possibility of hybrid integration, combining an InGaN cell in a tandem device with silicon [5,6] or other non-III-nitride [7] photovoltaic cells. The difficulty of growing high-quality InGaN layers increases with the In content. Reports of InGaN-based junctions with an In mole fraction exceeding 0.3 are rare [1]; the best external quantum efficiencies (EQEs) exceeding 0.7 are obtained at around 400 nm and quickly drop for longer wavelengths [8-10]. The main challenges are the large dislocation density and In-clustering, caused by the strong tendency to phase separation during growth. Absorbing layers in the form of a multiple quantum well (MQW) structure are often used to delay strain relaxation. Furthermore, the quantum confined Stark effect (QCSE) associated to the strong piezoelectric fields in the InGaN/GaN system [11] offers the possibility to tune the effective band gap of the structure by adjusting the quantum well (QW) and barrier thickness (tQW and tB, respectively). The effect of tuning tB in InGaN/GaN MQW photovoltaic devices has been studied by Wierer et al. [12] and Watanabe et al. [13]. According to their results, the absorption cutoff of the solar cells redshifts with decreasing tB. However, this does not always translate in enhanced overall cell efficiency, since the short circuit current density (Jsc) and open circuit voltage (Voc) also depend on tB. In this paper, we focus on the influence of the QW thickness on the effective band gap of the junction and its impact on the overall cell efficiency. We experimentally demonstrate that the band-to-band transition in InGaN QWs can be significantly redshifted in larger QWs. However, this redshift appears linked to a dramatic enlargement of the Stokes shift, so that increasing the tQW above a few nm is n