Abstract
Multi-junction solar cells constitute the main source of power for space applications. However, exposure of solar cells to the space radiation environment significantly degrades their performance across the mission lifetime. Here, we seek to improve the radiation hardness of the triple junction solar cell, GaInP/Ga(In)As/Ge, by decreasing the thickness of the more sensitive middle junction. Thin junctions facilitate the collection of minority carriers and show slower degradation due to defects. However, thinning the junction decreases the absorption, and consequently, the expected photocurrent. To compensate for this loss, we examined two bioinspired surface patterns that exhibit anti-reflective and light-trapping properties: (a) the moth-eye structure which enables vision in poorly illuminated environments and (b) the patterns of the hard cell of a unicellular photosynthetic micro-alga, the diatoms. We parametrize and optimize the biomimetic structures, aiming to maximize the absorbed light by the solar cell while achieving significant reduction in the middle junction thickness. The density of the radiation-induced defects is independent of the junction thickness, as we demonstrate using Monte Carlo simulations, allowing the direct comparison of different combinations of middle junction thicknesses and light trapping structures. We incorporate the radiation effects into the solar cell model as a decrease in minority carrier lifetime and an increase in surface recombination velocity, and we quantify the gain in efficiency for different combinations of junction thickness and the light-trapping structure at equal radiation damage. Solar cells with thin junctions compensated by the light-trapping structures offer a promising approach to improve solar cell radiation hardness and robustness, with up to 2% higher end-of-life efficiency than the commonly used configuration at high radiation exposure.
Highlights
Photovoltaic solar panels are the main power source for Earth-orbiting satellites and have proven their capabilities up to the orbit of Jupiter, with Juno [1] being the solar-powered mission that operated the furthest from the Sun to date
The current paper examines an alternative application for the light trapping structure (LTS); as a way to design thin junction solar cells with enhanced radiation hardness, with minimal sacrifice in efficiency
The selected designs are marked in figure 2(a) by the points and the associated absorption profiles are contrasted in figure 2(b) for the wavelengths of interest of the middle junction
Summary
Photovoltaic solar panels are the main power source for Earth-orbiting satellites and have proven their capabilities up to the orbit of Jupiter, with Juno [1] being the solar-powered mission that operated the furthest from the Sun to date. Solar cell efficiency and mass are of great interest for space applications, because improvements to either result in significant reduction in in-orbit solar array costs [2]. Multijunction cells, achieving superior efficiency than their single-junction counterparts, are the norm for powering space vehicles and satellites [3]. The high efficiency of the multi-junction cells as produced (beginning-of-life, BOL) is half of the story; the end-of-life (EOL) efficiency has to be considered, which takes into account the performance degradation during the lifespan of the mission. The space radiation environment constitutes a significant contributor to the aforementioned degradation. Radiation hardness is an additional and decisive characteristic when selecting solar cells for space operations [4]
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