Efficiency improvement of silicon solar cells enabled by ZnO nanowhisker array coating
© Yu et al.; licensee Springer. 2012
Received: 6 April 2012
Accepted: 1 June 2012
Published: 15 June 2012
An efficient antireflection coating is critical for the improvement of silicon solar cell performance via increased light coupling. Here, we have grown well-aligned ZnO nanowhisker (NW) arrays on Czochralski silicon solar cells by a seeding-growth two-step process. It is found that the ZnO NWs have a great effect on the macroscopic antireflection effect and, therefore, improves the solar cell performance. The ZnO NW array-coated solar cells display a broadband reflection suppression from 500 to 1,100 nm, and the minimum reflectance smaller than 3% can easily be achieved. By optimizing the time of ZnO NW growth, it has been confirmed that an increase of 3% relatively in the solar cell efficiency can be obtained. These results are quite interesting for the application of ZnO nanostructure in the fabrication of high-efficiency silicon solar cells.
Antireflection (AR) technology plays an important role in the fabrication of high-efficiency solar cells by increasing light coupling into the active region of devices. Generally, a relatively low-light reflectivity can be obtained for crystalline silicon solar cells by combining surface texturization with SixNy-thin-film deposition [1, 2]. However, in order to further reduce the light reflectivity for the improvement of solar cell efficiency, a variety of nanostructures with size comparable to the wavelength of m:most of the utilized solar spectrum have been proposed, such as photonic crystals , porous structures , plasm:monics [5–8], etc. The subwavelength structures can be nanowires , nanocones [10, 11], frustum nanorods , biomimetic nanostructures [13–16], dielectric nanoparticles [17, 18] etc. All these nanostructures should be based on the wide bandgap materials which, themselves, cannot absorb sunlight.
The ZnO material has a wide bandgap, which has been popularly used in the fields of photocatalysis  and optoelectronic nanodevices . For photovoltaic application, the ZnO nanostructures as antireflective layers could be fabricated by various approaches like pulsed laser deposition , chemical vapor deposition , vapor–liquid-solid growth , hydrothermal method , and electrochemical deposition technique . Am:mong them, a seeding and growth two-step process in zinc salt and amide mixed solutions seems quite acceptable for photovoltaics due to its low cost and good potential for scale-up [26, 27]. Experiments have shown that ZnO nanorod arrays can exhibit better antireflective properties than other oxide compounds . A weighted global reflectance of 6.6% has been achieved in the broadband range from 400 to 1,200 nm . However, m:most work on ZnO nanorod arrays as antireflective layers is performed on the naked silicon wafer by now [29, 30]. The application of ZnO nanostructures in the practical silicon solar cells is seldom reported.
In this work, we have grown ZnO nanowhiskers (NWs) with different lengths by the seeding and growth two-step process, and meanwhile have integrated the ZnO NW technique into the fabrication of Czochralski (CZ) silicon solar cells based on the current standard celling technology. The density and the length of ZnO NWs have been optimized for the improvement of solar cell performances by controlling the time of growth. The result shows that the efficiency of CZ silicon solar cell can be improved by a value of 3% relatively using the ZnO NWs as the AR layer in practice.
The starting samples are p-type CZ silicon solar cells fabricated by a standard process including surface texturization, phosphorus diffusion, SixNy film deposition, screen printing, and metallization. The size of solar cells is 4 × 4 cm2. The seed layer of ZnO was first deposited on the front surface of solar cells by a magnetron sputtering system. The deposition temperature is 150°C, and the seed layer thickness is about 10 nm. After protecting the parts without the ZnO seed film with waterproof tapes, the samples were immersed in the solution of zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and (CH2)6 N4 mixture for 30 to 60 min for the growth of ZnO NW. The ZnO NW growth temperature is 90°C.
For characterization, a scanning electron microscope (SEM; Hitachi S-4800, Hitachi, Ltd., Chiyoda, Tokyo, Japan) was used to observe the ZnO NWs on the solar cells. The reflection spectra of the samples were measured by a UV–vis spectrophotometer (Hitachi U-4100) in the wavelength range of 300 to 1,100 nm. The external quantum efficiencies (EQEs) of the final solar cells were obtained from the spectral responses. The current–voltage characteristics of the cells under the dark and illumination circumstances were measured. For illumination, one sun (100 mW/cm2) with AM 1.5 Global spectrum was used.
Results and discussion
Zn2+ is provided by Zn(NO3)2·6H2O through its ionization in the solution. The decomposition of hexamethylenetetramine produces amm:monia that can supply the OH− needed for the ZnO NW formation reaction and acts as a pH buffer to regulate the pH value of the solution . The pH value of the solution should be 6 or 7 for the whole reaction process. During the process, the ZnO nanocrystals existing in the seed layer serve as the nucleation centers where the growth of ZnO NWs takes place. Due to its growth habit, c-axis is the m:most preferred growth orientation , which leads to the formation of ZnO NWs.
Performance parameters of c-Si solar cells with and without ZnO NWs
ZnO growth time (min)
In summary, around 100 to 300 nm ZnO NWs are grown on the surface of Czochralski Si solar cells based on a low cost aqueous solution method. ZnO NWs grown for different times, such as 30, 45, and 60 min, can reduce the reflectivity of the solar cells effectively (R < 3%) in a large spectral region and improve the external quantum efficiency in the corresponding range. Furtherm:more, ZnO NWs with 45-min growth time can improve the efficiency of the solar cells by 3%, corresponding to their increment of short-circuit current density and fill factor. Finally, long-time immersion of solar cells in aqueous solution during the ZnO NW growth process can deteriorate the solar cell performance drastically.
Deren Yang and Xuegong Yu are both professors and doctorate degree holders in the State Key Laboratory of Silicon Materials, Department of Materials Science & Engineering, Zhejiang University. Dong Wang, Dong Lei, and Genhu Li are masters degree holder candidates of the same university.
This work is supported by the National Natural Science Foundation of China (grant nos. 60906002 and 50832006), National Key Technology R&D Program (2011BAE03B13), Key Laboratory of Optoelectronic Materials Chemistry and Physics, CAS (2008DP173016), and Innovation Team Project of Zhejiang Province (2009R50005).
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