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We demonstrated the efficiency improvement of GaAs single-junction (SJ) solar cells with the single-material zinc sulfide (ZnS) bi-layer based on the porous/dense film structure, which was fabricated by the glancing angle deposition (GLAD) method, as an antireflection (AR) coating layer. The porous ZnS film with a low refractive index was formed at a high incident vapor flux angle of 80° in the GLAD. Each optimum thickness of ZnS bi-layer was determined by achieving the lowest solar weighted reflectance (SWR) using a rigorous coupled-wave analysis method in the wavelength region of 350-900 nm, extracting the thicknesses of 20 and 50 nm for dense and porous films, respectively. The ZnS bi-layer with a low SWR of ~5.8% considerably increased the short circuit current density (Jsc) of the GaAs SJ solar cell to 25.57 mA/cm2, which leads to a larger conversion efficiency (η) of 20.61% compared to the conventional one without AR layer (i.e., SWR~31%, Jsc = 18.81 mA/cm2, and η = 14.82%). Furthermore, after the encapsulation, its Jsc and η values were slightly increased to 25.67 mA/cm2 and 20.71%, respectively. For the fabricated solar cells, angle-dependent reflectance properties and external quantum efficiency were also studied.
©2013 Optical Society of America
1. Introduction
Recently, efficiency enhancement of solar cells has been getting more important due to the energy crisis and environmental issues. To improve the light harvesting in solar cells, it is crucial to minimize undesirable Fresnel surface reflection losses at the interface between air and the top layer of solar cells for the entire range of the solar spectrum. Although conventional λ/4 thickness stacked multilayers consisting of different materials with high and low refractive indices have been mainly employed as antireflection coatings (ARCs), there exist some disadvantages such as thermal expansion mismatch, diffusion of one material into another, and material selection [1]. Over the few past years, to enhance the light harvesting in the wide solar spectrum range, there have been many studies on the optimization of subwavelength nanostructures (e.g., nanogratings [2], nanodomes [3], nanowires [4], and nanorods [5,6]) for eliminating the abrupt optical interface between air and semiconductor and hierarchical micro-/nanostructures (e.g., microgrooves/nanorods [7] and microdomes/nanorods [8]) as efficient broadband and wide-angle ARCs for solar cell applications. To fabricate these micro- or nanoscale structures, however, high-cost and complicated processes which include either various lithography or etching techniques can be required. Furthermore, dry plasma etching process can damage the surface of devices, which may degrade the cell efficiency [9]. Therefore, it is necessary to develop efficient ARCs in an effective way including low-cost, simple, and low-damaged processes.
In this view, glancing angle deposition (GLAD) can be one of techniques to fabricate the ARCs and optical components [10–12]. Due to the nucleation formation and self-shadowing effect, the GLAD enables the control of the volume fraction ratio of air within the film by introducing porosity, which affects its refractive index [13]. Meanwhile, there is also a study on the optically anisotropic nanostructures (e.g., single crystalline zinc oxide nanowire arrays) fabricated by the GLAD method [12]. These nanostructures with highly oblique angles of 75-85° not only allow important technological applications in passive photonic components but also benefit the development of the optoelectronic devices in polarized light sensing and emission. Using the GLAD method, several reports on the AR multilayer consisted of the same materials with a step graded index profile have been demonstrated for photovoltaic applications [14–16]. However, there is very little or no report on the use of the zinc sulfide (ZnS) bi-layer (porous/dense films) as an ARC in solar cells using the GLAD method. In this work, we fabricated the single-material ZnS bi-layer consisting of porous/dense films using the GLAD method. The effect of the single-material ZnS bi-layer as an AR layer on the device characteristics of GaAs single-junction (SJ) solar cell with/without an encapsulation was investigated. For optical reflectance properties, a theoretical analysis was also performed using the rigorous coupled-wave analysis (RCWA) method.
2. Experimental and simulation modeling details
The epilayer structures of GaAs SJ solar cell were grown on n-type GaAs (100) substrates with 2° off-orientation toward (111) by using an AIXTRON multi-wafer metal-organic chemical vapor deposition system (AIX2600 G3 IC). The GaAs SJ solar cells with a chip aperture area, including grid, of 0.55 × 0.55 cm2 were fabricated by conventional photolithography, metal evaporation, lift-off, and etch processes. Further details on the fabrication of the GaAs SJ solar cell can be found in our previous work [17]. Figure 1 shows the schematic diagram for depositing the ZnS films by the GLAD method. The dense and porous ZnS films were deposited at different incident vapor flux angles (θα) of 0 and 80°, respectively, by using an electron-beam (e-beam) evaporation system without substrate rotation at deposition rate of 0.2 nm/s. The thickness of deposited ZnS films was ~100 nm. For an ARC, the ZnS single-layer deposited at θα = 0° and the bi-layer consisting of the porous film deposited at θα = 80° on the top of the dense film deposited at θα = 0° (i.e., porous/dense films (θα = 80° /0°)) were prepared on the GaAs SJ solar cells, respectively. And then, the cells were encapsulated by a coverglass (Corning Eagle 2000) which was attached on the top of an ultraviolet (UV)-curable polymer (NOA 89, Norland Products Inc.) and subsequently exposed to an UV source for 20 min to cure the polymer.
The deposited profiles of ZnS films were observed from a scanning electron microscope (SEM; LEO SUPRA 55, Carl Zeiss). To evaluate the crystallinity and orientation, X-ray diffraction (XRD; M18XHFSRA, Mac Science) measurements with the monochromated CuKα line (λ = 0.154178 nm) in the scan range of 2θ between 20 and 60° were performed. The optical reflectance was explored by using a UV-vis-NIR spectrophotometer (Cary 5000, Varian) at near normal incidence of ~8°. Spectroscopic ellipsometry (V-VASE, J. A. Woollam Co. Inc.) was used to determine the refractive index (n) and extinction coefficient (k) of ZnS films and to measure the angle-dependent reflectance. The current-voltage (J-V) characteristics were measured by using a solar simulator (WXS-220S-L2, Wacom) under 1-sun air mass 1.5 global (AM1.5G) illumination. The external quantum efficiency (EQE) was characterized by using a QEX7 system (PV Measurements, Inc.). For a theoretical optical analysis, RCWA calculations were performed using a commercial software (DiffractMOD 3.1, Rsoft Design Group). The n and k values of the constituent materials in the solar cell used in this calculation were referred [18].
3. Results and discussion
Figure 2 shows the (a) XRD patterns and (b) measured n and k of the deposited ZnS films on GaAs substrates at θα = 0 and 80°. The cross-sectional and top-view SEM images of the corresponding ZnS films are also shown in the inset of Fig. 2(a). At θα = 80°, the inclined columnar nanostructures were clearly created owing to the self-shadowing mechanism, as shown in the SEM images of Fig. 2(a). From the XRD patterns in Fig. 2(a), the diffraction peaks from (111) and (220) plane orientations at 2θ = 28.8 and 47.6°, respectively, were observed in the ZnS film at θα = 0°, indicating the preferred orientation of (220). This is attributed to the cubic ZnS phase (JCPDS Card No. 77-2100). However, for the ZnS film with inclined nanocolumnar structures at θα = 80°, the (220) diffraction peak was not almost shown while the (111) one was observed. This means that the degree of crystallinity in the ZnS film is decreased at a large θα value because the diffusion of deposited atoms is disturbed due to the shadowing effect during the GLAD, thus forming the porous film [13,15]. Moreover, these micro- or nanostructures with inclined columns will have a significant influence on the optical properties (i.e., n and k) of thin films [19]. As can be seen in Fig. 2(b), for the ZnS film at θα = 80°, the n was decreased compared to one at θα = 0° over a wide wavelength region of 350-900 nm. At a wavelength of 550 nm, the n value of ZnS films was changed from 2.38 at θα = 0° to 1.95 at θα = 80°. This is attributed to the increased porosity within the ZnS film with a slanted columnar nanostructure caused by the GLAD at the high θα value, as mentioned above. The porosity within the film can be evaluated by the Bruggeman effective medium approximation [20]. The porosity of the ZnS film at θα = 80° was estimated to be ~30%. In this calculation, we assumed that the dense ZnS film at θα = 0° as the reference has zero porosity. Also, the k was somewhat decreased.
Fig. 2 (a) XRD patterns and (b) measured n and k of the deposited ZnS films on GaAs substrates at θα = 0 and 80°. The cross-sectional and top-view SEM images of the corresponding ZnS films are also shown in the inset of (a).
Figure 3 shows the (a) schematic diagram of the GaAs SJ solar cell epilayer structure with the ZnS bi-layer (θα = 80°/0°) and (b) calculated solar weighted reflectance (SWR) as functions of ZnS film thicknesses at θα = 0 and 80°. The ZnS bi-layer as an ARC was employed on the GaAs SJ solar cell. To obtain an efficient AR layer at wavelengths of 350-900 nm, it is necessary to optimize each film thickness of ZnS bi-layer. For this, we carried out the reflectance calculations of the ZnS bi-layer on the GaAs SJ solar cell using the RCWA method in the film thickness ranges of 0-60 nm at θα = 0° and 40-70 nm at θα = 80°, respectively. And then, the effect of ZnS bi-layer on the solar cell performance was investigated using a SWR. The SWR, which is the ratio of the usable photons reflected to the total usable photons, can be estimated by normalizing the reflectance spectra with the solar spectral photon flux integrated over a wavelength range of 350-900 nm [21]. The SWR formula is given by
where S(λ) is the spectral photon flux (i.e., AM1.5G) and R(λ) is the surface reflectance. As shown in Fig. 3(b), the lowest SWR value of ~5.2% (i.e., dark blue part) could be obtained at the film thicknesses of 20 and 50 nm for θα = 0 and 80°, respectively.Fig. 3 (a) Schematic diagram of the GaAs SJ solar cell epilayer structure with the ZnS bi-layer (θα = 80°/0°) and (b) calculated SWR as functions of ZnS film thicknesses at θα = 0 and 80°.
Figure 4 shows the calculated (dashed lines) and measured (solid lines) (a) reflectance spectra and (b) SWR as a function of incident light angle (θi) for the GaAs SJ solar cells without AR layer and with the ZnS single- and bi-layer. The photograph of the corresponding samples and cross-sectional low- and high-magnification SEM images of the fabricated ZnS bi-layer on the GaAs SJ solar cell are also shown in the inset of Fig. 4(a). It can be observed that the ZnS bi-layer consisting of porous (50 nm)/dense (20 nm) films was well deposited on the GaAs SJ solar cell by the GLAD method, as shown in the SEM images of Fig. 4(a). Also, the characters in LCD monitor were strongly reflected on the surface of the GaAs SJ solar cell without AR layer. For the solar cell with the ZnS bi-layer, on the contrary, its surface exhibited a dark black without the reflected characters. This can be confirmed by its efficient antireflection property. For comparison, the ZnS single-layer with a λ/4 thickness of ~55 nm (λ = 550 nm) was also prepared on the GaAs SJ solar cell. As can be seen in Fig. 4(a), the reflectance spectrum of the GaAs SJ solar cell without AR layer was higher than ~30% over a wide wavelength region of 350-900 nm, exhibiting the SWR of ~31%. However, the GaAs SJ solar cell with the ZnS bi-layer had a lower reflectance spectrum as well as a lower SWR (i.e., ~5.8%) compared to ones without AR layer and with the ZnS single-layer (i.e., SWR of ~10.2%). This is attributed to the gradient refractive index profile from air to the window layer of the GaAs SJ solar cell via the ZnS bi-layer. In RCWA simulations, the calculated reflectance results were similar to the experimentally measured spectra, indicating the SWR values of ~30.3, 9.8, and 5.2% for GaAs SJ solar cells without AR layer and with ZnS single- and bi-layer, respectively. The angle-dependent antireflection property is also an important parameter to achieve reasonable light absorption during the day. To investigate the angle-dependent reflectance characteristics of all the AR layers on solar cells, the SWR was estimated at wavelengths of 350-900 nm for θi = 15-70° under un-polarized light. As the θi was increased, the SWR values of all the samples were generally increased. However, the SWR of the GaAs SJ solar cell with the ZnS bi-layer was less dependent on the incident angle of light compared to ones without AR layer and with ZnS single-layer. Its SWR values of < 10% were maintained up to θi = 50°, which were lower compared to those without AR layer and with ZnS single-layer at θi = 8-70° though the value was rapidly increased to ~20% at θi = 70°. In the RCWA calculations, the GaAs SJ solar cell with the ZnS bi-layer also exhibits the less angle-dependent reflectance characteristics compared to other solar cells in wide ranges of wavelengths and incident angles. From these results, the ZnS bi-layer can lead to the efficiency enhancement of solar cell due to the relatively increased light trapping in the absorption layer.
Fig. 4 Calculated (dashed lines) and measured (solid lines) (a) reflectance spectra and (b) SWR as a function of θi for the GaAs SJ solar cells without AR layer and with the ZnS single- and bi-layer. The photograph of the corresponding samples and cross-sectional low- and high-magnification SEM images of the fabricated ZnS bi-layer on the GaAs SJ solar cell are shown in the inset of (a).
Figure 5 shows the measured J-V curves of the (a) unencapsulated and (b) encapsulated GaAs SJ solar cells without AR layer and with the ZnS single- and bi-layer. The EQE spectra of the corresponding cells are also shown in the insets. 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 Table 1. As shown in Fig. 5(a), in device characteristics, the results of the comparison indicate that the Jsc and η of the GaAs SJ solar cell incorporated with the ZnS single-layer were effectively increased to 24.48 mA/cm2 and 19.49%, respectively, compared to one without AR layer (i.e., Jsc = 18.81 mA/cm2 and η = 14.82%). The use of ZnS bi-layer yielded an additionally improved Jsc value of 25.57 mA/cm2 and η value of 20.61% due to its superior AR ability, exhibiting the enhancement percentages by ~35.9% in Jsc and ~39.1% in η, compared to one without AR layer. This explains that the efficient ARC can improve the light absorption of solar cells, and thus produce the larger photocurrent. The Voc and FF were kept at similar values. As shown in the inset of Fig. 5(a), for both the cells with the ZnS single- and bi-layer, their EQE spectra were considerably improved compared to one without AR layer over a wide wavelength region of 370-870 nm. This observation reverberates with the reflectance spectra in Fig. 4(a) which shows the broadband antireflection property of the ZnS single- and bi-layer on the GaAs SJ solar cell at near normal incidence. However, for both cells with the ZnS single- and bi-layer, the EQE spectra in Fig. 5(a) give a very slight difference at wavelengths below than 600 nm while the reflectance of one with the ZnS single-layer is higher than that of one with the ZnS bi-layer, as can be seen in Fig. 4(a). This is probably attributed to the absorption of the ZnS film, which is related to the extinction coefficient and is dependent on the film thickness, at the corresponding wavelengths, as shown in Fig. 2(b). Therefore, the ZnS bi-layer with a total film thickness of ~70 nm may absorb more lights compared to the 55 nm-thick ZnS single-layer, leading to the decrease of EQE spectrum. Thus, to obtain the efficient multilayer ARCs for photovoltaic applications, the absorption and thickness of the constituent layers should be considered [22]. On the other hand, in the wavelength range of 700-870 nm, the EQE values of cell with the ZnS bi-layer were further enhanced compared to one with the ZnS single-layer due to its lower reflectance. Previously, efficiency enhancement of GaAs photovoltaics employing highly-oriented indium tin oxide (ITO) nanocolumns, prepared by the GLAD technique using e-beam evaporation, as an ARC was reported [10]. The ITO nanocolumns exhibited superior antireflective characteristics in wide ranges of wavelength and incident light angle. To fabricate these ITO nanocolumns, however, it requires an additional post annealing process at 350 °C for 25 min to improve their transmission. Furthermore, the total length of grown ITO nanocolumns is relatively long, measuring ~1.2 μm. In contrast, for the single-material ZnS bi-layer with a graded refractive index profile in our work, the total film thickness of ~70 nm (i.e., porous/dense ZnS films with the thicknesses of 50/20 nm at θα = 80°/0°, respectively, as shown in the SEM images of Fig. 3(a)) is not only much lower than the total length of ITO nanocolumns but also it needs no additional post process, leading to the higher enhancement percentages by ~35.9% in Jsc and ~39.1% in η of GaAs-based SJ solar cells than by ~18% in Jsc and ~28% in η for ITO nanocolumns.
Fig. 5 Measured J-V curves of the (a) unencapsulated and (b) encapsulated GaAs SJ solar cells without AR layer and with the ZnS single- and bi-layer. The EQE spectra of corresponding cells are shown in the insets of (a) and (b), respectively.
Table 1. Measured device characteristics of the GaAs SJ solar cells with different AR layers before and after the encapsulation.
For practical applications, all the GaAs SJ solar cells were also encapsulated by using UV-curable polymer (NOA 89) and coverglass. Compared with the unencapsulated GaAs SJ solar cells, it was found that the Jsc value of the encapsulated cell without AR layer was increased by 1.15 mA/cm2. Similarly, the Jsc values of the encapsulated cells with the ZnS single- and bi-layer were slightly increased by 0.42 and 0.1 mA/cm2, respectively, exhibiting the enhanced η values of 19.79 and 20.71% for the ZnS single- and bi-layer, respectively. This may be attributed to the decreased reflectance caused by the more efficiently graded refractive index profile between air and the InGaP window layer via the coverglass (n~1.49)/polymer (n~1.51)/ZnS single- or bi-layer (n = 1.95/2.38 for θα = 80°/0°) structure. As can be seen in the inset of Fig. 5(b), the EQE spectra of the corresponding cells also exhibited the slightly increased results due to the increased Jsc compared to the unencapsulated cells. Similarly, in the case of both the encapsulated cells with ZnS single- and bi-layer, the difference in EQE spectra may be also the same reason, as mentioned above, but there exists an influence by the polymer and coverglass. Thus, it is worthwhile to mention that the single-material ZnS bi-layer can be applicable to many other optimized photovoltaic cells because it slightly increases the Jsc of solar cell after the encapsulation as well as induces the relatively low damages to the internal device structures compared to the dry etching process for AR nanostructures. Also, the GLAD method using e-beam evaporation is a simple, fast, and low-cost process.
4. Conclusion
The single-material ZnS bi-layer consisting of porous/dense film structure was fabricated on the GaAs SJ solar cell by the GLAD method via e-beam evaporation and its antireflective properties were investigated experimentally and theoretically in the wavelength region of 350-900 nm. The optimized thickness of each ZnS film was obtained by estimating the lowest SWR, determining the 20 and 50 nm at θα = 0 and 80°, respectively. For the GaAs SJ solar cells, the ZnS bi-layer with a graded index profile between air and the window layer exhibited a SWR of ~5.8%, which is much lower than that (i.e., ~31%) of one without AR layer. By incorporating the ZnS bi-layer as an ARC into the GaAs SJ solar cell, the Jsc was not only increased to 25.57 mA/cm2 but also the EQE spectrum was enhanced at wavelengths of 370-870 nm, thus leading to the significantly enhanced efficiency of η = 20.61% (i.e., Jsc = 18.81 mA/cm2 and η = 14.82% for one without AR layer). Additionally, for the encapsulated GaAs SJ solar cell with the ZnS bi-layer, the Jsc and η were slightly improved to 25.67 mA/cm2 and 20.71%. These results can provide deep insight into the single-material ZnS bi-layer for omnidirectional broadband ARCs in various photovoltaic applications.
Acknowledgments
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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