Axial GaAs/AlGaAs Nanowire Solar Cell on Si with Ultra-High Power-per-Weight Ratio

Epitaxially grown III-V semiconductor nanowire (NW) arrays have become an emerging solar cell architecture with great promise for high-efficiency and low-cost solar energy harvest devices [1]. In NW arrays with dimensions comparable to the wavelength of solar radiation, the light-NW interaction can be controlled by both the NW geometry and the pitch of the NW array in order to enhance the solar cell efficiency beyond the Shockley-Queisser theoretical limit from a single planar junction solar cell [2]. In addition, the nanoscale footprints of NWs can accommodate lattice mismatch efficiently, resulting in a superior quality of the interface for heteroepitaxial integration and allow for high-performance III-V semiconductors on a Si platform in order to <i>e.g.</i> form dual-junction tandem structures monolithically on a Si bottom solar cell. Theoretical simulations indicate that a dual-junction tandem cell, using a III-V top cell on a Si bottom solar cell, can achieve efficiencies higher than 40 % [3]. For such dual tandem cells to be cost-effective and used on a large scale in the future, it is crucial that the III-V top cell is monolithically integrated on the Si cell (i.e., no use of III-V substrate), and that a minimum cost is used for the growth and processing of the III-V top cell, as this is at present the major cost issue holding back this technology [4].<br/>In this work, we demonstrate a very effective use of the III-V photoconversion material leading to an ultra-high power-per-weight ratio by utilizing an axial junction GaAs/AlGaAs NW array grown by molecular beam epitaxy on a Si substrate.<b> </b>By analyzing single NW multi-contact devices, we firstly show that an n-GaAs shell is self-formed radially outside the axial p- and i-core of the GaAs NW during n-core growth, which significantly deteriorate the rectification property of the NWs in the axial direction. When employing a selective-area ex-situ etching process for the n-GaAs shell, a clear rectification of the axial NW p-i-n junction with a high on/off ratio was revealed. Such controlled etching process of the self-formed n-GaAs shell was further introduced to fabricate axial p-i-n junction GaAs NW array solar cells. Employing this method, a GaAs NW array solar cell with only ~ 1.3 % areal coverage of the NWs shows a photoconversion efficiency ~ 7.7 % under 1 sun intensity (AM 1.5G), which is the highest achieved efficiency from any single junction GaAs NW solar cell grown on Si substrate so far [5]. This corresponds to a power-per-weight ratio of the active III-V photoconversion material as high as 560 W/g, showing great promise for future high-efficiency and low-cost III-V/Si tandem solar cells.<br/><br/>References:<br/>1. J. Wallentin et al., InP nanowire array solar cells achieving 13.8% efficiency by exceeding the ray optics limit. <i>Science </i><b>339</b>, 1057-1060 (2013)<br/>2. G. Otnes and M.T. Borgström, Towards high efficiency nanowire solar cells. <i>Nano Today </i><b>12</b>, 31-45 (2017)<br/>3. I. Almansouri et al., Supercharging silicon solar cell performance by means of multijunction concept. IEEE J.<i> </i>Photovolt. 5, 968–976 (2015).<br/>4. S. Essig et al., Raising the one-sun conversion efficiency of III–V/Si solar cells to 32.8% for two junctions and 35.9% for three junctions. <i>Nature Energy</i> <b>2</b>, 1-9 (2017).<br/>5. A. Mukherjee et al., GaAs/AlGaAs nanowire array solar cell grown on Si with ultra-high power-per-weight ratio. <i>ACS Photonics</i> <b>8</b>, 2355-2366 (2021).