# Efficiency Enhancement of GaAs Single-Junction Solar Cell by Nanotextured Window Layer
## 2 Materials and Methods  
The inverted GaAs 1J SCs were grown on Si-doped (100) GaAs substrates oriented 2 \({}^{\circ}\)C off [111], through a metalorganic chemical vapor deposition (AX200/4 RF, AIXRON Inc., Cambridge, UK). Trimethylgallium, trimethylindium, and trimethylaluminum were employed as a group III source. Arsine and phosphine were employed as a group V source. Diethylzinc and disilane were employed as the p-dopant and n-dopant, respectively. The area of the fabricated SCs was 0.27 cm\({}^{2}\). Commonly, the GaAs 1J SC was composed of a n\({}^{+}\)-GaAs Ohmic layer, n-InAlP window, n-GaAs emitter, p-pressure of 30 mTorr, and radio frequency power of 250 W for etching SiO\({}_{2}\). The GaAs contact layer and InAlP window layer was etched using HBr and Ar gas by an inductive co GaAs base, and p-InGaP back surface field (BSF) layer as shown in Figure 1. The doping concentration and thickness of each layer were grown under the following conditions: a 0.3 \(\upmu\)m p-type GaAs contact layer with a hole concentration of 1.49 \(\times\) 10\({}^{19}\) cm\({}^{-3}\) and a GaAs bottom cell consisting of a 0.3 \(\upmu\)m p-type GaAs contact layer with a hole concentration of 1.49 \(\times\) 10\({}^{19}\) cm\({}^{-3}\), a 100 nm thick 3.94 \(\times\) 10\({}^{18}\) cm\({}^{-3}\) p-type In\({}_{0.51}\)Ga\({}_{0.49}\)P back surface field (BSF) layer, a 3.3 \(\upmu\)m thick 1.23 \(\times\) 10\({}^{17}\) cm\({}^{-3}\) p-type GaAs base layer, a 100 nm thick 2.00E18 cm\({}^{-3}\) n-type GaAs emitter layer, a 4.15E17 cm\({}^{-3}\) n-type InAlP window layer, and a 0.3 \(\upmu\)m thick 8.5 \(\times\) 10\({}^{19}\) cm\({}^{-3}\) n\({}^{+}\)-type GaAs contact layer in succession. The doping concentrations of each layer were investigated by Hall measurement. The InAlP window layer with a thickness of ~500 nm was grown. Then, 150 nm of SiO\({}_{2}\) was deposited on GaAs 1J SC by a plasma-enhanced chemical vapor deposition (PECVD) system (SLR-730, Oerlikon Corp., Freienbach, Switzerland). A monolayer of PS beads that floats on DI-water (diameter of ~400 nm, 4% \(w/v\) in DI-water, purchased from Thermo Fisher Scientific Corp., Waltham, MA, USA) was scooped by SiO\({}_{2}\)-deposited GaAs 1J SC and dried naturally. The diameter of dried PS beads was adjusted to ~300 nm by a reactive ion etching (RIE) system (FABstarRIE, Titiel Corp., London, UK) using O\({}_{2}\) gas. The etched PS beads monolayer transferred the pattern to the SiO\({}_{2}\) layer by etching SiO\({}_{2}\) using CF\({}_{4}\) gas by RIE, which is utilized as a mask layer. PS beads were eliminated by RIE using O\({}_{2}\) gas after carrying out fabricating the SiO\({}_{2}\) mask layer by RIE. RIE was operated at the working pressure of 60 mTorr and the radio frequency power of 80 W for etching PS beads and the working coupled plasma (ICP) system (SLR-770, Oerlikon Corp.). ICP was operated at the working pressure of 10 mTorr and the radio frequency power of 150 W for etching the GaAs/InAlP layer. The mask layer was removed by immersing SC in BOE for ~5 s. For exposing the NT InAlP window layer, the GaAs contact layer was etched by dipping in citric acid solution for ~60 s. The rough surface of NT InAlP layer was polished by dipping in the solution of HCl:H\({}_{2}\)O = 1:5 for 1 s. Then, 105 nm of SiN\({}_{\rm x}\) was deposited on the polished NT InAlP widow layer as a passivation layer. The fabricated sequence of texturing is illustrated in Figure 2, and the schematic structure of GaAs 1J NT SC is shown in Figure 1.  
In addition, in order to compare performances, two types of surface conditions of the GaAs 1J SCs were prepared with the \(\sim\)30 nm of the InAlP window layer. One is an GaAs 1J SC with a bare surface, i.e., the InAlP window layer of the SC was exposed.