Radial p-n Junction Solar Cells by Core-Shell Silicon Nanowire Arrays
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Radial p-n Junction Solar Cells by Core-Shell Silicon Nanowire Arrays
Tai-Yuan Huang1 and Ta-Jen Yen1,2 1
2
Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan, R.O.C. National Nano Device Laboratories, Science-based Industrial Park, Hsinchu 300, Taiwan, R.O.C.
ABSTRACT We first fabricated a p-type single-crystalline SiNW array as the core by statistic electroless metal deposition (SEMD) method[1]. This structure exhibits per excellent absorption efficiency without increasing the diffusion path, indicating 1.75 times greater performance than Si-based planar solar cells under the same condition[2]. Next, we employed a method of spin-on dopant (SOD) to fabricate an n-type layer as an external thin shell, which benefits to decouple the absorption of light from charge transport by allowing lateral diffusion of minority carriers to the p-n junction rather than many microns away as in Si bulk solar cells, and is suitable for our SiNW array with a hydrophilic surface. Finally, our SiNW-based solar cell possesses strong broadband absorption and low reflection from visible light to near IR, in which the highly light trapping mechanism stems from the effective medium theory (EMT) to demonstrate only less than 3% of total reflectance in the range of 500-1100 nm. It also shows conversion efficiency improvement of 20% compared with the planar single-crystalline Si solar cell by the same fabrication processes. The proposed novel photovoltaic device by our core-shell SiNW array revolutionizes the current architecture of solar cells, promising niche points of (1) better absorption, (2) self-antireflection, and (3) low-cost process. INTRODUCTION Recently there have been increasing interests in the research of using SiNW on photovoltaic applications [3-5]. Due to its cylindrical geometry, SiNW allows to decouple the requirements for light absorption and carrier extraction into orthogonal spatial directions[2]. This structure would curtail the length of minor carriers diffuse to the p-n junction, which decreases carrier recombination rate before collected in quasi-neutral region. Moreover, unlike conventional Si-based planar solar cells that have to be covered by an antireflection layer to increase the absorption, SiNW array has strong broadband absorption and low reflection from visible light to near IR[3] . As a result, the absorption efficiency would be enhanced for a broader range of photons in solar cells without further thickening the entire device, in order to absorb the long wavelength photon of greater intensity in the conventional planar solar cells.
However, most fabricated methods of SiNWs have some practical issues for real applications. For example, SiNWs fabricated by the catalytic processes inevitably contain saturated dopants from the catalyst [6-10] and typically the process temperature is above 500 ℃[7-9, 11] , radically impeding their feasibility in the PV industry. Moreover, those growth methods are not easy enough to control the dimensions and orientation of S
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