The unique electronic features of highly mismatched alloys such as III-V GaNAs are suitable for the intermediate band solar cell (IBSC) application, in which an intermediate band (IB) acts as a stepping stone to generate additional photocarriers across the host semiconductor bandgap through sequential two-step below-bandgap photon absorption (TSPA). However, the collection of photocarriers in a realistic GaNAs IBSC is much lower and often accompanies S-shape kink features in the current–voltage (J–V) curves under illumination for which a coherent picture is lacking. Based on the solar cell characterization of GaNAs IBSC devices grown with and without barriers, with and without antimony, and with and without indium using molecular beam epitaxy, and also with the photocarrier collection analysis using equivalent circuit models, it was identified that the TSPA and the S-shape J–V of this system depend on two critical factors: (1) high carrier recombination currents (I0CI) across the GaNAs sub-gap between the conduction- and intermediate bands (EgCI) and (2) the counterdiode effect of the AlGaAs IB electron barrier. Dramatic improvements in the S-shape J–V feature of the solar cell characteristics were achieved when lattice-strain was compensated in GaInNAsSb epitaxial layers.
The luminescent coupling effect in a multijunction solar cell is known to help achieve current matching among subcells through carrier redistribution. We demonstrate the carrier redistribution in III-V multijunction solar cell devices using a moisture-resistant, all-inorganic perovskite quantum dot (PQD) film. This hydrophobic PQD film was applied on a full III-V multijunction solar cell device. This successfully demonstrated current redistribution vertically, shown by the increased current collection in the lower bandgap subcells, and laterally, as observed from improved current collection homogeneity in the lower bandgap subcell adjacent to the higher bandgap subcell where the luminescence originated.
Strong luminescent coupling (LC) effect reduces current mismatch in III-V-based multijunction solar cells and consequently improves their power conversion efficiency. The LC current production in the Si bottom cell of InGaP/AlGaAs//Si triple junction solar cells having different sizes was investigated to determine the cell area dependence of LC effect. This was probed through laser beam-induced current (LBIC) mapping. The areal mapping was then assessed by obtaining the absolute difference between the quantum efficiencies calculated from the fitted LBIC map and simulated LC current considering uniform distribution using a quasi-two-dimensional electro-optical prediction model. At 9.17 suns concentration difference between the AlGaAs middle cell and the current-limiting Si bottom cell, the absolute LC quantum efficiency differences ranged between 0.16% and 1.68% in various cell sizes, which indicate potential current production increase if the LC current is made uniform.
Strong luminescent coupling (LC) effect in a multijunction solar cell (MJSC) allows better current balance among its subcells. The temperature dependence of wafer-bonded III-V on silicon (Si) MJSCs with LC effect was investigated. Experimentally, this was explored through light current density–voltage (J – V) characteristic measurements, external quantum efficiency measurements, and laser beam-induced current mapping of InGaP/AlGaAs//Si triple junction solar cells at different cell temperatures. Measurement results were analyzed using a quasi-two-dimensional electro-optical prediction model for LC current production and bandgap temperature dependence models of various semiconductors. It was revealed that at lower temperatures, LC current production in the limiting Si bottom cell resulted in a larger absolute LC quantum efficiency difference. At 15°C, the absolute LC quantum efficiency difference calculated was 0.17%, which indicates potential current production improvement if vertical LC current is made uniform.
Optimizing the luminescence coupling (LC) effect can reduce the current mismatch in multijunction solar cells (MJSCs), thereby enhancing their performance. In a previous work, the LC current collection was measured at room temperature. The temperature dependence of LC effect in InGaP/GaAs/Ge triple junction solar cells was observed by light J–V characteristics measurements and by laser beam-induced current mapping at operating temperatures between 18°C and 72°C. Results were analyzed using quasi-two-dimensional electro-optical model. At low operating temperatures, the photocurrent collection by LC effect has been found to be more dominant than the current collection due to bandgap reduction and thermal excitation of carriers. At 18°C and 72°C, when Ge bottom cell was made current limiting, the conversion efficiencies calculated were 6.68% and 0.01%, respectively. These findings show that the LC effect can contribute best to the MJSC performance when operated at the lowest temperature possible.
The impact of nonuniform spatial distribution of the luminescence coupling (LC) effect to the limiting cell conversion efficiency of multijunction solar cells (MJSCs) has been investigated. For this purpose, the laser beam induced current distribution maps of the limiting bottom cell have been acquired experimentally under varying middle-to-bottom cell LC efficiencies. The minimum and the maximum LC efficiencies demonstrated were 8.5% and 69%, respectively. To further analyze the measurement results, a quasi-two-dimensional simulation model considering the spatially nonuniform nature of the LC effect has been developed. A good agreement between the simulation and the measurement results suggests that the nonuniform LC current distribution is induced by optical phenomena such as photon escape and internal reflection. This nonuniformity then causes the absolute conversion efficiency of the limiting cell to be reduced by 1.35% at maximum LC efficiency. This reduction, when suppressed, can yield higher limiting cell conversion efficiency, which in turn may improve the overall MJSC conversion efficiency.
The subband features E‒ and E+ for the conduction band of III-V dilute nitride alloys make them promising for intermediate band solar cell application. However, presence of bandgap states can limit the two-step photon absorption activity, a necessary requirement for IBSC functionality. A model analysis is performed to characterize the density of states. The sub-band tails states are characterized using a temperature-dependent map of photo-modulated reflectance spectroscopy for GaNAs thin films grown on GaAs substrates using molecular beam epitaxy. The effect of indium and antimony incorporation on the subband features were investigated. Marked improvements in the thin films were observed both for the lower (E‒) and the upper (E+) conduction bands (CB) when In was introduced with marginal enhancement by Sb. These improvements are associated with suppression of tail states below both the E‒ and E+ bands. Sb rather mainly plays a surfactant role improving the abruptness of the GaNAs/GaAs hetero-interface.
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