We 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.
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Calculations considered the whole 30-period MQW structure sandwiched between the p- and n-type GaN regions. An InN band gap of 0.69 eV and an InGaN band gap bowing parameter of 1.4 eV were assumed. The residual doping level in the MQW was set to 1015 cm−3.
Relaxation is defined as R = (aMQW – aGaN)/(aMQW0 – aGaN), where aMQW is the measured in-plane average lattice parameter of the MQW, aMQW0 is the average lattice parameter of the relaxed superlattice, and aGaN is the lattice parameter of the GaN buffer layer. For the sample design under study, the relaxation error bars were estimated to be ±40% in sample A and ±20% in sample B, due to the relatively low average In content in a MQW period. It is reasonable to assume fully strained QWs in sample A, due to the fact that the wells are very thin in comparison to the barriers. However, the QWs of sample B may be partially relaxed in view of the defect structure observed by HR-TEM. Thus, for the calculations in Fig. 1(d) and in the inset of Fig. 5(a) (stars), we have assumed 10% relaxation in sample B.
