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Abstract
The growth of high-quality compound semiconductor materials on silicon substrates has long been studied to overcome the high price of compound semiconductor substrates. In this study, we successfully fabricated nanowire solar cells by utilizing high-quality hetero p-n junctions formed by growing n-type III-V nanowires on p-silicon substrates. The n-InAs0.75P0.25 nanowire array was grown by the Volmer–Weber mechanism, a three-dimensional island growth mode arising from a lattice mismatch between III-V and silicon. For the surface passivation of n-InAs0.75P0.25 core nanowires, a wide bandgap InP shell was formed. The nanowire solar cell was fabricated by benzocyclobutene (BCB) filling, exposure of nanowire tips by reactive-ion etching, electron-beam deposition of ITO window layer, and finally metal grid electrode process. In particular, the ITO window layer plays a key role in reducing light reflection as well as electrically connecting nanowires that are electrically separated from each other. The deposition angle was adjusted for conformal coating of ITO on the nanowire surface, and as a result, the lowest light reflectance and excellent electrical connectivity between the nanowires were confirmed at an oblique deposition angle of 40°. The solar cell based on the heterojunction between the n-InAs0.75P0.25/InP core-shell nanowire and p-Si exhibited a very high photoelectric conversion efficiency of 9.19% with a current density of 27.10 mA/cm2, an open-circuit voltage of 484 mV, and a fill factor of 70.1%.
Highlights
The high cost associated with their raw materials such as gallium and arsenide and wafer manufacturing are a setback for expanding the compound semiconductors industry
A 400-nm-thick indium tin oxide (ITO) thin film as a window layer is deposited on the exposed NW tips by using an e-beam evaporator at an oblique angle deposition (OAD) configuration (Figure 1c)
As shown in the dark J-V curves of photovoltaic devices with 0◦ and 40◦ angled deposited ITO layer (Figure 4d), this is in good agreement with the enhancement of photovoltaic parameters after angled deposition
Summary
Heterostructures including nanocomposites or heterojunctions have been highly attractive as building blocks for optoelectronic devices [1–5]. III-V semiconductors and Si have a huge material dissimilarity including a large lattice mismatch and difference of thermal expansion coefficient which causes a high density of various defects during the growth of III-V on Si, deteriorating device performance [14–16]. Catalyst-free-, direct growth- NW array has been focused on due to their easy processing and prevention of the formation of the deep level defects by Au catalyst, enabling a superior level of hetero-interface which can be possibly utilized as high-performance electronic devices [20–24]. The ITO nanocolumns antireflective coating has been reported to achieve an enhancement factor of 42% for photocurrents generated at wavelengths transparent to the window layer, demonstrating a viable efficiency-boosting strategy for GaAs solar cells [31]. We successfully demonstrated n-InAsP/InP NW- p-Si heterojunction solar cells by catalyst-free and direct-growth of n-type III-V nanowires on p-Si substrates. 484 mV and fil factor (FF) of 70.1%, resulting in a high PCE of 9.19%
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