We report the efficiency enhancement of III-V InGaP/GaAs/ Ge triple-junction (TJ) solar cells using a novel structure, i.e., vertically-oriented gallium oxide hydroxide (GaOOH) nanopillars (NPs), as an antireflection coating. The optical reflectance properties of rhombus-shaped GaOOH NPs, which were synthesized by a simple, low-cost, and large-scalable electrochemical deposition method, were investigated, together with a theoretical analysis using the rigorous coupled-wave analysis method. For the GaOOH NPs, the solar weighted reflectance of ~8.5% was obtained over a wide wavelength range of 300-1800 nm and their surfaces exhibited a high water contact angle of ~130° (i.e., hydrophobicity). To simply demonstrate the feasibility of device applications, the GaOOH NPs were incorporated into a test-grown InGaP/GaAs/Ge TJ solar cell structure. For the InGaP/GaAs/Ge TJ solar cell with broadband antireflective GaOOH NPs, the conversion efficiency (η) of ~16.47% was obtained, indicating an increased efficiency by 3.47% compared to the bare solar cell (i.e., η~13%).
© 2014 Optical Society of America
Multi-junction solar cells based on crystalline III-V compound semiconductor materials have been steadily developed to enhance the efficiency by covering a larger part of the solar spectrum, demonstrating solar cells with high efficiencies exceeding 40% . Due to the multiple absorption bands, efficient antireflection coatings (ARCs), which can cover a broad wavelength region of the solar spectrum, are required. Conventional ARCs consisting of a quarter-wavelength (λ/4) stack of dielectrics with different refractive indices such as MgF2/ZnS , Al2O3/TiO2 , SiO2/TiO2  have several disadvantages including thermal expansion mismatch, material selection, and narrow bands of wavelengths and incident angles. Glancing angle deposited porous/dense multilayer ARCs (e.g., SiO2/TiO2 ) were previously reported. For this, the electron beam evaporation technique which may be a relatively expensive process with a limit on the size of the substrate has been very commonly used. As an alternative to the multilayer ARCs, on the other hand, subwavelength structures (SWSs) or compound eye structures (CESs) inspired by the moth-eye effect have been widely studied because they can efficiently reduce the surface reflection in the wide range of wavelengths and incident angles [6–8]. But, to form nanopatterned etch masks for the SWSs and CESs, complicated lithography and etching processes are required. Additionally, the dry etching process might cause undesired surface defects which are related to the recombination losses, thus degrading the performance of solar cells. Therefore, it is necessary to develop efficient ARCs in an effective way including low-cost, simple, and low-damaged processes in terms of high-performance device and industry applications.
Recently, there have been many studies on the growth of various antireflective nanostructures such as nanowires and nanorods using zinc oxides, indium tin oxides, gallium oxide hydroxides [9–11]. Particularly, the gallium oxide hydroxide (GaOOH) with a wide bandgap of about 4.75 eV exhibits a high transparency in the wide wavelength region, a high thermal stability, and a relatively lower refractive index than 1.7-1.9 of gallium oxide (Ga2O3) [12,13]. Furthermore, GaOOH nanopillars (NPs) can be easily synthesized by a simple, cost-effective, controllable, rapid, and low-temperature electrochemical deposition (ED) method over a larger area . Although GaOOH NPs have been reported for the enhancement of light extraction efficiency in light-emitting diodes [11,14], there is no work on the solar cells as an ARC. Meanwhile, a hydrophobic surface, which can self-clean dust and other contaminants on the surface of devices, is commonly needed for solar cell applications . Thus, it is very meaningful to analyze the surface reflection and wettability properties of the GaOOH NPs. In this work, we demonstrate a useful approach to enhance the efficiency of the III-V InGaP/GaAs/Ge triple-junction (TJ) solar cells with antireflective GaOOH NPs. The structural, optical, and surface wetting properties of GaOOH NPs, which were grown by the ED method, were investigated. For the analysis of optical reflection, theoretical calculations were also performed by the rigorous coupled-wave analysis (RCWA) method. For device application feasibility, the device characteristics of a test-grown InGaP/GaAs/Ge TJ solar cell with AR GaOOH NPs were studied in comparison with the bare solar cell structure without an ARC.
2. Experimental details
Figure 1 shows the schematic diagram of layer structure of the III-V InGaP/GaAs/Ge TJ solar cell with GaOOH NPs grown by the ED method. An AIXTRON multi-wafer MOCVD reactor was used for the epitaxial growth of InGaP/GaAs/Ge TJ solar cell structure on 6 degree misoriented germanium (Ge) substrate from (100) towards (110) direction. The solar cell chips with a chip aperture area of 0.3025 cm2 including grids were fabricated using conventional photolithography, metal evaporation, and lift-off processes. To reduce the reflection losses at the top surface of solar cells, a novel structure which consists of GaOOH NPs was employed as an ARC. To obtain the uniform vertically-oriented GaOOH NPs, an antimony (Sb)-doped tin oxide (6 wt.% Sb2O3-doped SnO2) (i.e., ATO) thin film was used as a seed layer . The ATO thin films were deposited by using an RF magnetron sputtering system. The thickness of deposited ATO thin films was about 60 nm. After the deposition, the GaOOH NPs were grown on the fabricated InGaP/GaAs/Ge TJ solar cell as well as the Ge substrate with a size of 1.5 × 1.5 cm2 by the ED method. In the ED, the growth was performed with an externally applied cathodic voltage of −2.2 V in an aqueous gallium nitrate solution at a temperature of 80 °C for 2 h. The aqueous solution consists of gallium nitrate (5 mM, Ga(NO3)3·nH2O), ammonium nitrate (5 mM, NH4NO3), and de-ionized water. During the reaction time, the solution was stirred with 80 rpm. For the growth of GaOOH NPs, further details can be found in our previous work . The surface morphology of the grown GaOOH NPs was observed by using a scanning electron microscopy (SEM; LEO SUPRA 55, Carl Zeiss). The optical reflectance was measured by using a UV-Vis-NIR spectrophotometer (Cary 5000, Varian). The water contact angles were measured by using a contact angle measurement system (Phoenix-300, SEO Co., Ltd.). The current density-voltage (J-V) measurements were characterized by using a solar simulator (WXS-220S-L2, Wacom) under 1-sun air mass 1.5 global (AM1.5g) illumination.
3. Results and discussion
Figure 2(a) shows the calculated average reflectance (Ravg) and solar weighted reflectance (SWR) of ATO film on the Ge substrate (i.e., ATO/Ge) as a function of ATO film thickness. The refractive index (n) and extinction coefficient (k) of ATO and Ge used in this calculation are also shown in the inset of Fig. 2(a). The ATO film as a seed layer allows for the growth of vertically-oriented GaOOH NPs. However, the reflection is strongly dependent on the thickness of the film. Thus, the desirable thickness of the ATO film should be chosen to efficiently suppress the surface reflection of solar cells. For the theoretical analysis of optical reflectance, the RCWA simulations were performed using a commercial software (DiffractMOD 3.1, Rsoft Design Group). We assumed that the incident light enters from air into the structure at normal incidence. The Ravg value is decreased from 40.9 to 21.3% with increasing the thickness of ATO film from 10 to 100 nm. These values are much lower than that (i.e., Ravg~42.3%) of Ge substrate due to the graded refractive index profile between air (nair = 1) and the Ge (nGe~4.7) via the ATO (nATO~1.96). For solar cell applications, it is necessary to investigate the SWR , which is the ratio of the usable photons reflected to the total usable photons, of ATO film. The SWR can be evaluated by normalizing the reflectance and the AM1.5g spectrum integrated over a wavelength range of 300-1800 nm. For the ATO film, the estimated SWR value was decreased from 42.9% at 10 nm of film thickness to 19.4% at 70 nm, and then was increased to 21.7% at 100 nm. It is noted that the optical absorption of film is strongly dependent on the film thickness. Therefore, we chose the ATO film thickness of 60 nm, which is about λ/4 thickness at λ~500 nm (near the maximum intensity of the solar spectrum), (i.e., SWR = 20.2%) to suppress the surface reflection and to reduce the optical absorption in the ATO film as much as possible for the efficient antireflection and seed layer. Figure 2(b) shows the top- and side-view SEM images of the GaOOH NPs grown on the ATO/Ge substrate. The GaOOH NPs with a closely rhombus-shaped structure were vertically and uniformly formed on the ATO/Ge by the ED method. Clearly, the ATO film as a seed layer helps the GaOOH NPs with high density to grow along the vertical direction. This can be explained by the fact that the heterogeneous nucleation is enhanced by the ATO under a cathodic voltage in the ED process . The average height and lateral length of the grown GaOOH NPs were approximately 1 μm ± 250 nm and 500 nm ± 150 nm, respectively.
Figure 3(a) shows the measured reflectance spectra of Ge substrate, ATO/Ge, and GaOOH NPs/ATO/Ge and solar spectral irradiance of AM1.5g. For comparison, the measured reflectance spectrum of a typical Al2O3/TiO2 double-layer ARC (DLARC) with λ/4 thicknesses of ~70/55 nm at λ = 500 nm (i.e., nAl2O3 = 1.77, nTiO2 = 2.3) is also shown in Fig. 3(a). For the ATO (60 nm)/Ge, the reflectance was lower than that of the Ge substrate over a wide wavelength range of 300-1800 nm, especially at wavelengths of around 500 nm which are the highest region of AM1.5g solar spectrum. This is ascribed to the graded refractive index profile between air and the Ge substrate via the ATO as wells as the λ/4 thickness (i.e., ~60 nm) of ATO with a refractive index of ~1.96 at λ = 500 nm. The incorporation of vertically-oriented GaOOH NPs into the ATO/Ge led to much lower reflectance compared to the ATO/Ge and Ge substrate at wavelengths of 300-1800 nm though it was slightly higher than that of ATO/Ge at wavelengths of 440-550 nm, exhibiting the SWR value of ~8.5% which is lower than those of the other samples (i.e., SWR~19.3% for ATO/Ge, SWR~43.5% for Ge substrate, and SWR~11.6% for Al2O3/TiO2 DLARC).
To explore the influence of the geometry of GaOOH NPs on the antireflective characteristics, a theoretical optical analysis was carried out using the RCWA simulation. For simplicity, we designed the model of periodic square-shaped GaOOH NPs with an area of 500 × 500 nm2 and height of 1 μm on ATO (60 nm)/Ge (175 μm). The period between GaOOH NPs with a two-dimensional six-fold hexagonal symmetry pattern was kept at 1.1 μm. For the GaOOH, the refractive index was assumed to be 1.75 and the extinction coefficient was not considered at the whole wavelength range. Figure 3(b) shows the calculated reflectance spectra and electric field intensity distributions at λ = 800 nm of (i) Ge substrate, (ii) ATO/Ge, and (iii) GaOOH NPs/ATO/Ge. For the calculated results, the overall trend appears to be similar with the measured data. As expected, the GaOOH NPs on ATO/Ge exhibit a relatively lower reflectivity over a wide wavelength region of 300-1800 nm except for wavelengths of 450-600 nm compared to the other structures. This results from the effective medium effect . The GaOOH pillars with nanoscale dimensions can be regarded as an effective single layer, thus creating the gradient effective refractive index profile between air (nair = 1) and the Ge (nGe~4.7) via the GaOOH NPs (nGaOOH~1.75)/ATO (nATO~1.96). The surface reflection is also suppressed by the destructive interference between the waves with different phases which are reflected at different positions (or depths) from the surface of ATO by the GaOOH NPs. Moreover, the GaOOH NPs help the incident light to propagate across the interfaces of air/ATO and ATO/Ge as well as to diffuse within the Ge substrate by increasing the light scattering at the surface. Thus, the relatively strong electric field intensity transmitted through the GaOOH NPs is observed within the Ge substrate compared to the other structures, as illustrated in Fig. 3(b).
Figure 4 shows the photograph images of a water droplet on the Ge substrate, ATO/Ge, and GaOOH NPs/ATO/Ge. The GaOOH NPs produced the hydrophobic surface with a water contact angle (θc) of ~130° which is higher value than those of the other samples (i.e., θc~69° for Ge substrate, θc~85° for ATO/Ge). This may be attributed to the high surface roughness due to the nanoscale GaOOH pillars though it is also related to the surface energy of materials. This hydrophobic surface with a high water contact angle may self-clean the dust and other contaminants on the surface of solar cells in real environments, which is known as the ‘Lotus Effect’ .
In order to investigate the effect of the antireflective GaOOH NPs on the efficiency of actual solar cells, the GaOOH NPs were synthesized on a test-grown InGaP/GaAs/Ge TJ solar cell structure. Figure 5 shows the (a) low- and high-magnification SEM images of GaOOH NPs grown on the InGaP/GaAs/Ge TJ solar cell with the ATO seed layer and (b) measured J-V characteristics on the test-grown III-V InGaP/GaAs/Ge TJ solar cells integrated with ATO film and GaOOH NPs/ATO structures as an ARC. For a reference, the J-V curve of solar cell with the bare surface is also shown in Fig. 5(b). As shown in Fig. 5(a), the vertically-oriented GaOOH NPs were relatively well grown on ATO seed layer in the InGaP/GaAs/Ge TJ solar cell. The measured device characteristics (i.e., open circuit voltage, Voc; short circuit current density, Jsc; fill factor, FF; conversion efficiency, η) of the corresponding solar cells are summarized in the inset of Fig. 5(b). For the solar cell with the ATO film as a seed layer, the η was increased to 15.3% compared to the bare solar cell (i.e., η = 13%) mainly due to the increase of Jsc from 7.98 to 9.35 mA/cm2 rather than other solar cell characteristics (e.g., Voc and FF). By growing the GaOOH NPs on the ATO seed layer, the higher η value of 16.47% was achieved, exhibiting a further enhanced Jsc value of 10.01 mA/cm2. These Jsc and η values are also slightly higher than those (i.e., Jsc = 9.9 mA/cm2 and η = 16.34%) of the test-grown III-V InGaP/GaAs/Ge TJ solar cell employed with a typical Al2O3/TiO2 DLARC while there are almost no variations on the Voc and FF. This is due to the graded effective refractive index profile between air and the window layer of solar cell via the GaOOH NPs/ATO seed layer and the destructive interference of waves reflected at different positions (or depths) from the surface due to the GaOOH NPs. Besides, the GaOOH NPs help the incident light to propagate across the interfaces of air/ATO/solar cell and to spread within the solar cell by increasing the light scattering on the surface. This phenomenon can effectively trap the light in the active medium of solar cell, which leads to the efficiency enhancement.
Vertically-oriented GaOOH NPs were synthesized on a test-grown III-V InGaP/GaAs/Ge TJ solar cells as an ARC using the ATO seed layer by the ED method. Their antireflective characteristics over a wide wavelength range as well as wetting behavior were investigated, together with the theoretical analysis using the RCWA method. The GaOOH NPs/ATO/Ge structure exhibited much lower reflectivity than that of Ge substrate over a wide wavelength region of 300-1800 nm, indicating a lower SWR value of ~8.5% (i.e., SWR~43.5% for Ge substrate). Also, the hydrophobic surface with a water contact angle of ~130° was formed. For the InGaP/GaAs/Ge TJ solar cell structure integrated with broadband antireflective GaOOH NPs, the η value of ~16.47% was obtained under AM1.5g illumination, indicating an efficiency improvement by ~3.47% compared to the bare solar cell (i.e., η~13%). These results can give a deep understanding of the vertically-oriented GaOOH NPs, which can be easily fabricated by a simple and cost-effective ED method, with broadband antireflective surface as well as self-cleaning function for high-efficiency III-V compound material-based multi-junction solar cell applications.
The work was supported by the International Collaborative R&D Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean government Ministry of Knowledge Economy (No. 20118520010030-11-2-100).
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