Efficiency improvement of InGaP/GaAs/Ge solar cells by hydrothermal-deposited ZnO nanotube structure
© Chung et al.; licensee Springer. 2014
Received: 10 February 2014
Accepted: 19 June 2014
Published: 5 July 2014
In this paper, a zinc oxide (ZnO) nanotube, fabricated by the hydrothermal growth method on triple-junction (T-J) solar cell devices to enhance efficiency, is investigated. Compared to those of bare T-J solar cells (without antireflection (AR) coating) and solar cells with Si3N4 AR coatings, the experimental results show that the T-J solar cells, which use a ZnO nanotube as an AR coating, have the lowest reflectance in the short wavelength spectrum. The ZnO nanotube has the lowest light reflection among all experimental samples, especially in the range of 350 to 500 nm from ultraviolet (UV) to visible light. It was found that a ZnO nanotube can enhance the conversion efficiency by 4.9%, compared with a conventional T-J solar cell. The Si3N4 AR coatings also enhance the conversion efficiency by 3.2%.The results show that a cell with ZnO nanotube coating could greatly improve solar cell performances.
In solar power technologies, the III-V solar cell is so far the commercial solar cell with the highest efficiency. It is expected that III-V solar cell will play an important role in the future high-efficiency and low-cost photovoltaic cell industry . Triple-junction (T-J) solar cells composed of three subcells, namely,InGaP (top cell, band gap energy Eg = 1.9 eV), GaAs (middle cell, Eg = 1.42 eV), and Ge (bottom cell, Eg = 0.67 eV), are GaAs-based solar cells which achieved conversion efficiencies of over 40% and have been applied extensively to space and terrestrial use [2, 3]. For high-performance multi-junction solar cells, the antireflection plays an important role because it can reduce about 30% of the light absorption due to the reflection between the interface of the air and top cell. The excellent antireflection (AR) performance benefits from the rough interfaces between air/zinc oxide (ZnO) nanotube layers and the ZnO nanotube/solar cell; the decreased nanotube densities provide the gradient of effective indices. The nanostructures have been applied to photovoltaic devices to reduce reflectance. The AR nanostructure is also described elsewhere [4–7]. Nanostructure arrays, like the subwavelength structures, exhibit very low specular or total reflectance compared to film layers. The low reflectance is due to a combination of AR coating and light tapping structures, demonstrating the nanostructure can potentially be applied a PV [8, 9]. ZnO has been recognized as a very promising material for optoelectronic application in the UV region; so, there is an increasing interest in high-quality ZnO film. ZnO has a wide bandgap of 3.37 eV and has a large exciton binding energy (60 meV) at room temperature [10, 11]. Compared with the planar thin-film devices, nanostructure devices are expected to have a greater response to light, especially for the spectrum from ultraviolet (UV) to green light in the solar spectrum [12, 13], which can increase light absorption in the top cell for short wavelengths. For solar cells, ZnO thin film acts as a transparent conductive oxide (TCO) and AR layer (refractive index of 2.0). There is a great deal of information on fabricating one-dimensional (1D) ZnO nanotubes using chemical vapour deposition for high-quality transistor devices, which requires a high-temperature process, ranging from 400 to 1,050°C [14, 15]. However, the high temperatures required for the CVD process degrade the characteristics of the solar cells. The ZnO nanotube with interest stemming from the facile synthesis with aligned and uniform ZnO nanotube arrays by using low-temperature (below 100°C) hydrothermal methods was also tried on the solar cells, without degrading the properties of the solar cells . In this study, the growth of a ZnO nanotube on T-J solar cells via the hydrothermal growth method is investigated. The main motivation behind this study is the fact that nanostructures will act as a second ARC layer with an effective refractive index so that the refractive index of the total structure will perform as a double-layer AR coating layer. The optical and electrical properties ofthe III-V solar cells with the above-proposed double-layer AR coating in this study are measured and compared.
Results and discussion
Measured illuminated electrical properties of bare triple (T-J) solar cell and T-J solar cell with SiN 4 and ZnO nanotube coating, respectively
Bare T-J solar cell
With SiNx AR coating
With ZnO NW AR coating
where J sc (λ) is the total photogenerated short-circuit current density at a given wavelength λ, ϕ(λ) is the photon flux of the corresponding incident light, and q is the elementary charge . We measured the spectral response of the external quantum efficiency (EQE), in which a xenon lamp and a halogens lamp were used as the illumination source sources. The EQE of the T-J solar cell device with SiN4 and ZnO nanotube coating, respectively, are presented in Figure 5b. Physically, EQE means the ability to generate electron-hole pairs caused by the incident photon . The cell with ZnO nanotube coating shows an enhanced EQE in a range from of 350 to 1800 nm. The average EQE enhancements (△EQE) of the top and middle cells were 2.5 and 6.6%, respectively. This is due to the low reflection between the wavelength 350 to 500 nm, in respect to the solar cell coated with a ZnO nanotube. The photocurrent generated in cell is in proportion to values of EQE and will optimize the III-V solar cell. The T-J solar cell is built by three series subcells, in which each subcell provides a short circuit current (Jsc 1, Jsc 2, Jsc 3) and open circuit voltage (Voc 1, Voc 2, Voc 3). The total Voc is the sum of three subcells and Jsc is limited by the smallest one. The short circuit limits of the current density of the top and middle cell can be calculated by ref. .
A ZnO nanotube grown on triple-junction (T-J) solar cell devices by the hydrothermal growth method to enhance efficiency is investigated. The reflectance spectra and I-V characteristics indicate that the ZnO nanotube solar cell had the lowest reflectance, especially in the range of 350 to 500 nm from ultraviolet to visible light. Solar cells with a ZnO nanotube exhibited a conversion efficiency increase of 4.9% compared with a bare T-J solar cell, whereas T-J solar cells with SiNx AR coating had only a 3.2% increase. After encapsulation, the results also suggested that the cell with ZnO nanotube coating could provide the best solar cell performances.
The authors would like to give special thanks to the NCTU-UCB I-RiCE program, National Science Council of Taiwan, for sponsorship under Grant No. NSC102-2911-I-009-302. We also are thankful for the support from the Green Energy & Environment Research Labs (GEL) and Industrial Technology Research Institute (ITRI) of Taiwan.
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