# Efficiency Enhancement of GaAs Single-Junction Solar Cell by Nanotextured Window Layer
## 3 Results  
The monolayer of naturally dried PS beads covered the planar surface as shown in Figure 3a. To make a SiO\({}_{2}\) layer with a diameter of about 300 nm, PS beads were dry etched  
Figure 1: Schematic structure of the NT InAlP window layer GaAs 1J SC. Figure 2: Schematic illustration of sequence of the nanotexturing process. by RIE to have a suitable diameter. Figure 3b shows the shape in which the diameter of the 400 nm PS bead was changed to about 300 nm by RIE etching after carrying out fabricating the SiO\({}_{2}\) mask layer by RIE. The GaAs contact layer and InAlP window layer were etched using HBr and Ar gas by ICP. Figure 3c,d shows the SEM cross-sectional image and 45 \({}^{\circ}\)C tilted surface of the NT InAlP window layer, respectively. The NT InAlP window layer was applied to the GaAs 1J SC. Figure 4 shows the spectra of specular reflectance measured on GaAs substrates with different surface conditions: the bare GaAs surface, the planar ARC surface, and the three-dimensional NT InAlP surface. Both ARC and NT surfaces exhibited suppressed reflectance compared to the bare surface. The planar ARC layer suppressed the reflectance through destructive interference effects caused by out-of-phase reflected waves of the light from interfaces between the bilayers of ARC and SC. The NT layer reduced the reflectance by gradually changing the refractive index by the moth-eye effect [16; 17]. The noticeable distinction of the plotted reflectance\(\uplambda\) curves between the ARC and NT surfaces is the difference of \(\uplambda\) range, where the reflectance is effectively suppressed compared with the specular, reflected, bare GaAs surface. The NT surface suppressed the reflection at a wide \(\uplambda\) range from 550 nm to 1200 nm evenly, which covers the whole range of GaAs 1J SCs enough. The suppressed reflectance by the NT surface mainly originates in the nanoscale three-dimensional layer. Since the geometric features of the factor NT surface exhibit the most index grading from air to the device, a remarkably low reflectance is achieved. Whereas the planar ARC surface suppresses the reflectance at a partial \(\uplambda\) range, approximately from \(\sim\)750 nm to \(\sim\)1000 nm, and consequently reveals a limitation of the planar ARC layer. As a result, the broad \(\uplambda\) range of reflectance is more considerably reduced by the NT surface than the planar ARC surface. Figure 4: Surface reflectance for the bare surface SC, SC with SiN\({}_{x}\) ARC, and InAlP NT surface. Figure 3: SEM images: Top view of (**a**) PS beads monolayer on GaAs substrate and (**b**) PS beads, whose diameter of about 350 nm was adjusted by RIE using O\({}_{2}\) gas, (**c**) a profile, and (**d**) tilted images of the NT InAlP window layer. The effects of the surfaces with different conditions on the nanostructure are mostly smaller than the studied wavelengths; \(\lambda\) of the incident light regards the nanostructure as an effective medium, whose effective refractive index falls off the InAIP towards the bottom, significantly reducing the undesired reflectance through destructive interferences among the beams reflected from different depths into the nanostructure [17; 19; 22]. As the effective medium provides a graded refractive index between the air and InAIP because of the increased space filling, the photocurrent generation by each SC was examined from the EQE spectra, as shown in Figure 5. The EQE spectra reveal that both ARC and NT surfaces benefit the carrier generation of GaAs 1J SC and elongate the diffusion length [23]. However, there is no significant difference of the EQE spectra between the ARC and the NT SC as plotted in Figure 5, whereas the reflectance spectra between the ARC and NT surfaces, as shown in Figure 4, present a distinct difference.