# Efficiency Enhancement of GaAs Single-Junction Solar Cell by Nanotextured Window Layer
## 1 Introduction  
Flexible III-V semiconductor solar cells (SCs) are advantageous materials with rare heat loss compared to Si SCs. This is because GaAs SCs have the features of a physically direct bandgap and lattice match properties, which means that carriers pass SCs without recombination. Therefore, III-V semiconductor SCs show a high efficiency even with a thin film. In solar cell applications, the photocurrent conversion efficiency (\(\eta\)) of GaAs SCs predicted by theoretical calculations or demonstrated by actual fabrication has been found to be higher than that of silicon [1; 2; 3; 4; 5; 6]. A higher \(\eta\) with thin thickness is an advantageous factor for flexible SCs applied to a wearable or portable device [7; 8; 9; 10; 11; 12]. The issue of a flexible SC attached on a curved surface is \(\eta\) degradation compared with the planar SC because of the tilted incident angle of the light source. In order to alleviate the \(\eta\) loss, various techniques have been developed to improve the performance such as depositing the planar anti-reflection coated (ARC) film and three-dimensional surface texturing, such as texturing the window surface of the SC and texturing the oxide layer covering the SC [13; 14; 15].  
When a light source is irradiated on the nanoscale textured surface, light in a wide range of \(\lambda\) is evenly scattered on the nanotextured (NT) layer, where the refractive index is gradually changed by the moth-eye effect [16; 17]. It leads to higher light transmittance and longer optical path length, which improves the low short-circuit current density (J\({}_{\text{SC}}\)) and \(\eta\)[18; 19; 20; 21]. The micro-hole structured InAlP window surface InGaP single-junction (I)) SC was designed by Millennium Communication Co., Ltd. [14]. This textured SC improved the external quantum efficiency (EQE), J\({}_{\text{SC}}\), and \(\eta\) rather than an SC with a bare surface. However, \(\eta\) with varying incident angle (\(\theta\)) of light source was not investigated, so it is not possible to find the omnidirectional effect of textured SCs. The ZnO nanoneedle layer covering the InGaP/GaAs/Ge triple-junction SC was designed by KAUST [15]. The AZO nanoneedle-shaped SC reduce the reflectance and enhance the EQE, J\({}_{\text{SC}}\), and \(\eta\) such as a previous study. In addition, the result of \(\eta\) with varying \(\theta\) was also measured, whichshowed that textured SCs have the feature of the omnidirectional effect compared to the planar SC with a bare surface. However, the comparison of performance between NT SC and ARC SC was not provided, although ARC SCs actually have been utilized more in commercials and industries than SCs with the bare surface. According to the studying result for solar cell with a nanotextured window layer in the other group [14,15], these papers have only shown the superiority of nanostructures by comparing solar cells with nanostructures and the bare solar cells excluding ARC. This study is different from comparing the solar cell with the ARC thin film layer used as a practical fabric with the solar cell with the nanostructure of the InAlP window layer. Additionally, it is refreshing to investigate the efficiency change with respect to the angle of incidence.  
In this study, the NT window layer of inverted epi-grown GaAs 1J SC was fabricated by using polystyrene (PS) beads. The measured performance of fabricated NT SC was compared to that of the planar ARC SC. Furthermore, \(\eta\) with various \(\theta\)s of the light source was examined and compared.