Using Spin-Coated Silver Nanoparticles/Zinc Oxide Thin Films to Improve the Efficiency of GaInP/(In)GaAs/Ge Solar Cells

We synthesized a silver nanoparticle/zinc oxide (Ag NP/ZnO) thin film by using spin-coating technology. The treatment solution for Ag NP/ZnO thin film deposition contained zinc acetate (Zn(CH3COO)2), sodium hydroxide (NaOH), and silver nitrate (AgNO3) aqueous solutions. The crystalline characteristics, surface morphology, content of elements, and reflectivity of the Ag NPs/ZnO thin film at various concentrations of the AgNO3 aqueous solution were investigated using X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, atomic force microscopy, and ultraviolet–visible–near infrared spectrophotometry. The results indicated that the crystalline structure, Ag content, and reflectance of Ag NP/ZnO thin films depended on the AgNO3 concentration. Hybrid antireflection coatings (ARCs) composed of SiNx and Ag NPs/ZnO thin films with various AgNO3 concentrations were deposited on GaInP/(In)GaAs/Ge solar cells. We propose that the optimal ARC consists of SiNx and Ag NP/ZnO thin films prepared using a treatment solution of 0.0008 M AgNO3, 0.007 M Zn(CH3COO)2, and 1 M NaOH, followed by post-annealing at 200 °C. GaInP/(Al)GaAs/Ge solar cells with the optimal hybrid ARC and SiNx ARC exhibit a conversion efficiency of 34.1% and 30.2% with Voc = 2.39 and 2.4 V, Jsc = 16.63 and 15.37 mA/cm2, and fill factor = 86.1% and 78.8%.


Introduction
Multi-junction solar cells (MJSCs) based on III-V compound semiconductors have attracted much attention for space and terrestrial applications because they are composed of inherently tunable and direct bandgap materials, resulting in a high conversion efficiency due to the absorption of a varied solar spectrum [1][2][3]. However, the conversion efficiency of MJSCs depends on not only the absorption but also the intensity of sunlight; consequently, MJSCs exhibit a lack of absorption under one-sun or low-concentration sunlight for terrestrial applications, and this causes low conversion efficiency. To enhance the absorption of the solar spectrum and obtain a high conversion efficiency, several studies have endeavored to optimize the solar cell structures of MJSCs by using various bandgap materials such as graded AlGaInP solar cells, Ga(In)NAs(Sb) solar cells, GaInP/GaInAsP/GaAs triple-junction solar cells, GaP/InGaAs/InGaSb triple-junction solar cells, GaInP-based multiple quantum well solar cells, and wafer-bonded InP-based four junction MJSCs [4][5][6][7][8][9]; in MJSCs, each junction is connected by a tunneling junction (TJ). In conventional III-V compound semiconductor-based MJSCs, the TJ is composed of heavily doped p-GaAs and n-GaAs. Some studies have suggested that conventional GaAs TJs can be replaced with other Al-based materials such as GaInP/AlGaAs, GaAs/AlGaAs, film was observed through a field emission scanning electron microscopy (FE-SEM, JEOL, Tokyo, Japan), and the film's Ag content was determined using energy-dispersive X-ray spectroscopy (EDS) (JSM-7500F, JEOL, Tokyo, Japan). The crystalline characteristics of the Ag NP/ZnO thin film were characterized by X-ray diffraction (XRD) patterns using an advanced diffractometer (Bruker D8, Billerica, MA, USA) equipped with CuKa (λ = 0.154 nm). The root mean square (RMS) roughness of the Ag NP/ZnO film was analyzed using atomic force microscopy (AFM) (D13100, Digital instruments Veeco Metrology Group, Plainview, NY, USA). Finally, the Ag NPs/ZnO thin film was deposited on SiN x -coated GaInP/GaAs/Ge solar cells. The current density versus voltage (JV) characteristics for completed solar cell chips were measured using a solar simulator with an Xe lamp light source calibrated to a one-sun condition. During JV measurements, the ambient temperature was controlled using a temperature control stage (STC200, Instec, Boulder, CO, USA). The Ag NPs/ZnO thin film with SiN x was used as a hybrid ARC to reduce the reflection of sunlight. The conversion efficiency and short current density of GaInP/GaAs/Ge solar cells with the optimal hybrid ARC constituted a substantial improvement over GaInP/GaAs/Ge solar cells with SiN x ARCs.

Results and Discussion
The Zn(CH 3 COO) 2 aqueous solution composed of Zn 2+ and CH 3 COO − ions was mixed with the NaOH aqueous solution containing Na + and OH − ions to form a ZnO treatment solution. Ions in the ZnO treatment solution reacted to form a ZnO thin film on the silicon nitride-coated (SiN x ) glass (SNG) substrate through the following reaction: Zn(OH) 2 2 H + + ZnO 2 2− , AgNO 3 added to a ZnO treatment solution, as an Ag NP/ZnO treatment solution can thermally decompose into Ag NPs, gaseous oxynitride, and oxygen during the post-annealing process at 200 • C, as indicated in the following reaction [28]: Consequently, the Ag NP/ZnO thin film was obtained on the SNG substrate after spin-coating and the 200 • C post-annealing process.  [37].
To study the crystallization of Ag NPs on ZnO thin films, the XRD spectra of the Ag NP/ZnO thin films at various AgNO 3 concentrations were measured, and are plotted in Figure 1. A preferential (002) peak (34.4 • ) alongside a (101) peak (36.2 • ) and (100) peak (31.8 • ) was found in the spin-coated ZnO thin film, thereby indicating a hexagonal wurtzite structure and a polycrystalline nature. Similar XRD patterns were observed in spin-coated Ag NP/ZnO thin films with extra peaks of face-center-cubic crystal Ag, namely a (111) peak (38.1 • ) and (200) peak (44.3 • ). As depicted in Figure 1, the diffraction intensity of the Ag (111) and Ag (200) peaks gradually increased when the AgNO 3 concentrations rose from 0 M to 0.05 M because of the increase in Ag content in the Ag NP/ZnO treatment solution, as indicated in Equation (6). Furthermore, no diffraction peak for Ag oxides such as AgO or Ag 2 O occurred (see Figure 1) because of decomposition of Ag oxide to Ag and O 2 during the 200 • C post-annealing process [32]. Ag NP reduction at an AgNO 3 concentration of 0.005 M is rare in an Ag NP/ZnO treatment solution, because the low AgNO 3 concentration engenders low diffraction intensity and a wide pattern of Ag (111) and (200) peaks. As the AgNO 3 concentration increased from 0.008 M to 0.05 M, a large quantity of Ag NPs formed in the treatment solution; consequently, these small-sized NPs aggregated at the surface or interfaced between ZnO grains, and gathered to a large grain size of Ag NPs during the post-annealing process, resulting in an enhanced diffraction intensity and narrow pattern of Ag (111) and Ag (200) peaks. In addition, the intensity of the ZnO-(100)-indexed diffraction peak increased, whereas that of the ZnO-(002)-indexed diffraction peak decreased with the rising AgNO 3 concentration; this was attributed to the (002)-orientated ZnO thin films being destroyed or bended by the large numbers of Ag NPs at a high AgNO 3 concentration [38]. and O2 during the 200 °C post-annealing process [32]. Ag NP reduction at an AgNO3 concentration of 0.005 M is rare in an Ag NP/ZnO treatment solution, because the low AgNO3 concentration engenders low diffraction intensity and a wide pattern of Ag (111) and (200) peaks. As the AgNO3 concentration increased from 0.008 M to 0.05 M, a large quantity of Ag NPs formed in the treatment solution; consequently, these small-sized NPs aggregated at the surface or interfaced between ZnO grains, and gathered to a large grain size of Ag NPs during the post-annealing process, resulting in an enhanced diffraction intensity and narrow pattern of Ag (111) and Ag (200) peaks. In addition, the intensity of the ZnO-(100)-indexed diffraction peak increased, whereas that of the ZnO-(002)-indexed diffraction peak decreased with the rising AgNO3 concentration; this was attributed to the (002)-orientated ZnO thin films being destroyed or bended by the large numbers of Ag NPs at a high AgNO3 concentration [38].  The surface morphologies of the ZnO and Ag NP/ZnO thin films were examined based on scanning electron microscopy (SEM) images. Figure 2 depicts top-view images of the ZnO and Ag NP/ZnO thin films at various AgNO3 concentrations on SNG substrates. According to Equations (1)-(5), the ZnO thin films without Ag NPs were able to develop, and then formed on the SNG substrate after spin-coating and the post-annealing process; the surface morphology of the ZnO thin film was textured and rough, as depicted in Figure 2a. A textured ZnO thin film can serve as an ARC to reduce reflectivity in the GaInP/GaAs/Ge triple-junction solar cell [39]. Figure 2b depicts the Ag NP/ZnO thin film prepared with an AgNO3 concentration of 0.005 M. A small grain size and rare distribution of Ag NP were observed and are depicted in Figure 2b; the rare distribution was attributable to the small number of Ag NPs in the treatment solution, and the small grain size was attributed to the incomplete gathering of Ag NPs under a post-annealing temperature of 200 °C; a rare distribution of Ag NPs cannot effectively reduce the reflection of sunlight for solar cell applications [27,39,40]. Although the grain size and distribution of Ag NPs on Ag NP/ZnO thin films can be improved by increasing the post-annealing temperature, a high annealing temperature will rise the reflectivity of Ag NP/ZnO/SiNx hybrid thin films, as shown in Figure 2f, possibly due to the larger grain sizes of the Ag NPs. In order to achieve a small interval between two Ag NPs and The surface morphologies of the ZnO and Ag NP/ZnO thin films were examined based on scanning electron microscopy (SEM) images. Figure 2 depicts top-view images of the ZnO and Ag NP/ZnO thin films at various AgNO 3 concentrations on SNG substrates. According to Equations (1)-(5), the ZnO thin films without Ag NPs were able to develop, and then formed on the SNG substrate after spin-coating and the post-annealing process; the surface morphology of the ZnO thin film was textured and rough, as depicted in Figure 2a. A textured ZnO thin film can serve as an ARC to reduce reflectivity in the GaInP/GaAs/Ge triple-junction solar cell [39]. Figure 2b depicts the Ag NP/ZnO thin film prepared with an AgNO 3 concentration of 0.005 M. A small grain size and rare distribution of Ag NP were observed and are depicted in Figure 2b; the rare distribution was attributable to the small number of Ag NPs in the treatment solution, and the small grain size was attributed to the incomplete gathering of Ag NPs under a post-annealing temperature of 200 • C; a rare distribution of Ag NPs cannot effectively reduce the reflection of sunlight for solar cell applications [27,39,40]. Although the grain size and distribution of Ag NPs on Ag NP/ZnO thin films can be improved by increasing the post-annealing temperature, a high annealing temperature will rise the reflectivity of Ag NP/ZnO/SiN x hybrid thin films, as shown in Figure 2f, possibly due to the larger grain sizes of the Ag NPs. In order to achieve a small interval between two Ag NPs and large-grain-sized Ag NP on Ag NP/ZnO thin films without raising the post-annealing temperature, an Ag NP/ZnO treatment solution with a high AgNO 3 concentration can be used to deposit Ag NP/ZnO on thin films.      Figure 4 depicts the reflectivity of the Ag NP/ZnO/SiNx hybrid thin film (named "hybrid ARC") grown at varied AgNO3 concentrations as a function of wavelength over 400-700 nm. The average reflectivity of the hybrid ARCs grown at AgNO3 concentrations of 0.005 M, 0.008 M, 0.02 M, and 0.05 M were 2.99%, 2.67%, 2.53%, 5.99%, and 7.47%, respectively, all of which were lower than the reflectivity of the SiNx ARC (approximately 9.2%) obtained in our previous study [39]. The effective refractive index (neff) of the hybrid ARC is approximately 1.87, and can be calculated using the following equation: where nZnO (1.76) and nSiNx (2.0) are the refractive indices of Ag NP/ZnO and SiNx thin films, respectively. The calculated refractive index of the ARC (nARC) is approximately 1.84, which can be calculated from nARC = (1 × nGaAs) 1/2 , where nGaAs is the refractive index of GaAs. The refractive index of the hybrid ARC (1.87) is closer to 1.84 than is SiNx (2.0), leading to low reflectivity. The amplitude of reflectance (rAPZN) and reflectivity (RAPZN) between air and Ag NP/ZnO thin films are provided in [41] and expressed as follows:  Figure 4 depicts the reflectivity of the Ag NP/ZnO/SiN x hybrid thin film (named "hybrid ARC") grown at varied AgNO 3 concentrations as a function of wavelength over 400-700 nm. The average reflectivity of the hybrid ARCs grown at AgNO 3 concentrations of 0.005 M, 0.008 M, 0.02 M, and 0.05 M were 2.99%, 2.67%, 2.53%, 5.99%, and 7.47%, respectively, all of which were lower than the reflectivity of the SiN x ARC (approximately 9.2%) obtained in our previous study [39]. The effective refractive index (n eff ) of the hybrid ARC is approximately 1.87, and can be calculated using the following equation: where n ZnO (1.76) and n SiNx (2.0) are the refractive indices of Ag NP/ZnO and SiN x thin films, respectively. The calculated refractive index of the ARC (n ARC ) is approximately 1.84, which can be calculated from n ARC = (1 × n GaAs ) 1/2 , where n GaAs is the refractive index of GaAs. The refractive index of the hybrid ARC (1.87) is closer to 1.84 than is SiN x (2.0), leading to low reflectivity. The amplitude of reflectance (r APZN ) and reflectivity (R APZN ) between air and Ag NP/ZnO thin films are provided in [41] and expressed as follows: where σ is the RMS roughness of the Ag NP/ZnO thin film, and λ is the wavelength of light in a vacuum. The reflectivity of the Ag NP/ZnO thin film decreased when the AgNO 3 concentration increased from 0 M to 0.008 M; this was attributable to the increasing RMS roughness depicted in Figure 3. However, the reflectivity of the Ag NP/ZnO thin films increased when the AgNO 3 concentration increased from 0.02 M to 0.05 M; this was attributable to the potent light scattering that resulted from the large grain size of Ag NPs evident in the SEM and AFM images.
where σ is the RMS roughness of the Ag NP/ZnO thin film, and λ is the wavelength of light in a vacuum. The reflectivity of the Ag NP/ZnO thin film decreased when the AgNO3 concentration increased from 0 M to 0.008 M; this was attributable to the increasing RMS roughness depicted in Figure 3. However, the reflectivity of the Ag NP/ZnO thin films increased when the AgNO3 concentration increased from 0.02 M to 0.05 M; this was attributable to the potent light scattering that resulted from the large grain size of Ag NPs evident in the SEM and AFM images.   A high light-trapping effect achieved through a reduction in the surface reflectivity of GaInP/(In)GaAs/Ge solar cells is required to obtain a high short-circuit current density and conversion. Compared with the SiNx ARC, the hybrid ARC with a lower surface reflectivity enhanced the short-circuit current density of the GaInP/(In)GaAs/Ge solar cell because of the high transmitting intensity of sunlight into the solar cell. Moreover, the short-circuit current density of the GaInP/(In)GaAs/Ge solar cell with an Ag NP/ZnO ARC grown at an AgNO3 concentration of 0.008 M noticeably increased; this was attributable to the low surface reflectivity and high light trapping. The short-circuit current densities of the GaInP/(In)GaAs/Ge solar cells with Ag NP/ZnO ARCs grown at AgNO3 concentrations of 0.02 M and 0.05 M decreased to 15.54 mA/cm 2 and 14.79 mA/cm 2 , respectively; these findings were attributable to the high surface reflectivity and light scattering that resulted from the large grain size of the Ag NPs. In this study, the conversion efficiency of a solar cell depended on the short-circuit current density because the open-circuit voltage and fill factor were fairly constant. As depicted in the inset of Figure 5, the GaInP/(In)GaAs/Ge solar cell with an Ag NP/ZnO ARC grown at an AgNO3 concentration of  A high light-trapping effect achieved through a reduction in the surface reflectivity of GaInP/(In)GaAs/Ge solar cells is required to obtain a high short-circuit current density and conversion. Compared with the SiN x ARC, the hybrid ARC with a lower surface reflectivity enhanced the short-circuit current density of the GaInP/(In)GaAs/Ge solar cell because of the high transmitting intensity of sunlight into the solar cell. Moreover, the short-circuit current density of the GaInP/(In)GaAs/Ge solar cell with an Ag NP/ZnO ARC grown at an AgNO 3 concentration of 0.008 M noticeably increased; this was attributable to the low surface reflectivity and high light trapping. The short-circuit current densities of the GaInP/(In)GaAs/Ge solar cells with Ag NP/ZnO ARCs grown at AgNO 3 concentrations of 0.02 M and 0.05 M decreased to 15.54 mA/cm 2 and 14.79 mA/cm 2 , respectively; these findings were attributable to the high surface reflectivity and light scattering that resulted from the large grain size of the Ag NPs. In this study, the conversion efficiency of a solar cell depended on the short-circuit current density because the open-circuit voltage and fill factor were fairly constant. As depicted in the inset of Figure 5, the GaInP/(In)GaAs/Ge solar cell with an Ag NP/ZnO ARC grown at an AgNO 3 concentration of 0.008 M demonstrated a maximum conversion efficiency of 34.17% because of the high short-circuit current density. Spin-coating is a method for synthesizing Ag NP/ZnO thin films through a chemical reaction in an aqueous solution at room temperature. According to the XRD ( Figure 1) and SEM (Figure 2) images, and EDS analysis (Table 1), the Ag content and Ag NP grain size of Ag NP/ZnO/SiNx hybrid ARCs depend on the AgNO3 concentration in treatment solution; Ag NPs in the treatment solution are in the form of clusters on the surface of ZnO thin films or at the interfaces between ZnO grains during the post-annealing process. The RMS roughness of Ag NP/ZnO/SiNx hybrid ARCs related to the grain size of Ag NPs determines the surface reflectivity; the reflectivity of Ag NP/ZnO/SiNx hybrid ARCs can be adjusted by AgNO3 concentration. Table 2 shows the measured short-circuit current density, open-circuit voltage, fill factor, and conversion efficiency of GaInP/(In)GaAs/Ge solar cells with varied ARCs. The highest conversion efficiency of 34.17% can be observed in Table 2 with the textured Ag NP/ZnO/SiNx hybrid ARC proposed by an AgNO3 concentration of 0.008 M. The RMS roughness of an Ag NP/ZnO/SiNx hybrid ARC below 7.2 nm (with an AgNO3 concentration of 0.005 M) cannot effectively reduce the surface reflection, as indicated in Equation (9); the RMS roughness of the Ag NP/ZnO/SiNx hybrid ARC, which is higher than 7.2 nm (with an AgNO3 concentration of 0.02 M or 0.05 M), shows a low reflectivity; the Ag NP with large grain sizes causes high reflection and light scattering.  Spin-coating is a method for synthesizing Ag NP/ZnO thin films through a chemical reaction in an aqueous solution at room temperature. According to the XRD ( Figure 1) and SEM (Figure 2) images, and EDS analysis (Table 1), the Ag content and Ag NP grain size of Ag NP/ZnO/SiN x hybrid ARCs depend on the AgNO 3 concentration in treatment solution; Ag NPs in the treatment solution are in the form of clusters on the surface of ZnO thin films or at the interfaces between ZnO grains during the post-annealing process. The RMS roughness of Ag NP/ZnO/SiN x hybrid ARCs related to the grain size of Ag NPs determines the surface reflectivity; the reflectivity of Ag NP/ZnO/SiN x hybrid ARCs can be adjusted by AgNO 3 concentration. Table 2 shows the measured short-circuit current density, open-circuit voltage, fill factor, and conversion efficiency of GaInP/(In)GaAs/Ge solar cells with varied ARCs. The highest conversion efficiency of 34.17% can be observed in Table 2 with the textured Ag NP/ZnO/SiN x hybrid ARC proposed by an AgNO 3 concentration of 0.008 M. The RMS roughness of an Ag NP/ZnO/SiN x hybrid ARC below 7.2 nm (with an AgNO 3 concentration of 0.005 M) cannot effectively reduce the surface reflection, as indicated in Equation (9); the RMS roughness of the Ag NP/ZnO/SiN x hybrid ARC, which is higher than 7.2 nm (with an AgNO 3 concentration of 0.02 M or 0.05 M), shows a low reflectivity; the Ag NP with large grain sizes causes high reflection and light scattering.

Deposition of the Ag NP/ZnO Thin Film
The Ag NP/ZnO treatment solution consisted of zinc acetate (Zn(CH 3 COO) 2 ), sodium hydroxide (NaOH), and silver nitrate (AgNO 3 ) aqueous solutions. Zn(CH 3 COO) 2 and AgNO 3 were used as the raw materials for preparing zinc oxide (ZnO) and silver NPs (Ag NPs), and NaOH was used as a reductant. Since the solubility levels of Zn(CH 3 COO) 2 , NaOH, and AgNO 3 powder in de-ionized (DI) water differed, these powders were dissolved in DI water individually to form aqueous solutions with 0.007 M, 1 M, and 0.005-0.1 M concentrations under room temperature. These solutions were then mixed and stirred uniformly to form an Ag NP/ZnO treatment solution at room temperature. Before the Ag NP/ZnO treatment solution was spread on the silicon nitride-coated (SiN x ) glass (SNG) substrate and the GaInP/GaAs/Ge solar cell with an SiN x layer, the surfaces of these substrates were treated using an oxygen plasma to obtain a hydrophilic surface. The substrates were baked on a hot plate at 100 • C for three minutes for dewatering after the Ag NP/ZnO treatment solution had been spread on the substrates via the spin-coating process. Finally, the dewatered SNG substrate and GaInP/GaAs/Ge solar cell with an SiN x layer were coated with the Ag NP/ZnO treatment solution and treated through a post-annealing process at 200 • C for 1 h in an N 2 -ambient furnace to form the Ag NP/ZnO thin film.

Fabrication of GaInP/GaAs/Ge Solar Cells with an Ag NP/ZnO Window Layer
GaInP and (In)GaAs were grown on 150-µm thick p-type Ge substrate through metal-organic chemical vapor deposition (MOCVD). The TJs used to link the Ge substrate, InGaAs, and GaInP subcells were heavily doped with n-GaAs/p-GaAs and p-AlGaAs/n-GaInP. An alloy composed of Au/Zn/Ag/Au was coated onto the back of the p-Ge substrate as a p-type contact metal. The wafer was successively patterned using a standard photolithographic process to define the n-contact region through partial exposure of n-InGaAs. An alloy composed of AuGe/Ni/Au was used as an n-type contact metal in the n-InGaAs contact region. Finally, an Si 3 N 4 (n = 2.0) grown through plasma-enhanced chemical vapor deposition and an Ag NP/ZnO thin film were coated onto n-AlInP, which was defined through partially etched n-InGaAs.

Conclusions
We proposed an Ag NP/ZnO thin film on an SNG substrate and a GaInP/(Al)GaAs/Ge solar cell created using spin-coating technology. The grain size and content of Ag on the Ag NP/ZnO thin film depended on the AgNO 3 concentration in the Ag NP/ZnO treatment solution. An optimal hybrid ARC comprising SiN x and Ag NP/ZnO thin film grown with an Ag NP/ZnO treatment solution contained 0.0008 M AgNO 3 , 0.007 M Zn(CH 3 COO) 2 , and 1 M NaOH; following the 200 • C post-annealing process; this ARC exhibited a lower average reflectivity (2.53%) over wavelengths of 400-700 nm compared with the conventional SiN x ARC. This finding was attributable to the textured surface, as determined by the grain size of the Ag NPs, the ZnO surface morphology, and a suitable effective refractive index constructed of SiN x and Ag NP/ZnO. GaInP/(Al)GaAs/Ge solar cells incorporating the optimal hybrid ARC demonstrated a high conversion efficiency rate of 34.1%, with V oc = 2.39 V, J sc = 16.63 mA/cm 2 , and fill factor = 86.1%. Furthermore, well scale-defined and uniform-distributed Ag NPs are required in an Ag NP/ZnO/SiN x hybrid ARC to reduce the reflectivity of GaInP/(Al)GaAs/Ge solar cells for industrial-scale application. An adjusted spinner speed and controlled drop volume of treatment solution would be the method for fulfilling the aforementioned requirements.

Conflicts of Interest:
The authors declare no conflicts of interest.