Wen-Ching Sun

Combined micro- and nano-scale surface textures for enhanced near-infrared light harvesting in silicon photovoltaics

As silicon photovoltaics evolve towards thin-wafer technologies, efficient optical absorption for the near-infrared wavelengths has become particularly challenging. In this work, we present a solution that employs combined micro- and nano-scale surface textures to increase light harvesting in the near-infrared for crystalline silicon photovoltaics, and discuss the associated antireflection and scattering mechanisms. The surface textures are achieved by uniformly depositing a layer of indium-tin-oxide nanowhiskers on micro-grooved silicon substrates using electron-beam evaporation. The nanowhiskers facilitate optical transmission in the near-infrared by functioning as impedance matching layers with effective refractive indices gradually varying from 1 to 1.3. Materials with such unique refractive index characteristics are not readily available in nature. As a result, the solar cell with combined textures achieves over 90% external quantum efficiencies for a broad wavelength range of 460-980 nm, which is crucial to the development of advanced thin-substrate silicon solar cells.

Enhanced angular characteristics of indium tin oxide nanowhisker-coated silicon solar cells.

Omnidirectional and broadband light harvesting is critical to photovoltaics due to the suns movement and its wide spectral range of radiation. In this work, we demonstrate distinctive indium-tin-oxide nanowhiskers that achieve superior angular and spectral characteristics for crystalline silicon solar cells using angle-resolved reflectance spectroscopy. The solar-spectrum weighted reflectance is well below 6% for incident angles of up to 70° and for the wavelength range between 400nm and 1000nm. As a result, the nanowhisker coated solar cell exhibits broadband quantum efficiency characteristics and enhanced short-circuit currents for large angles of incidence.

High Efficiency Silicon Solar Cells with Bilayer Passivation Structure

The effects of passivation on monocrystalline silicon solar cells were investigated. Al 2 O 3 and SiO 2 films, prepared by atomic layer deposition and thermal oxidation, respectively, were used as passivation layers. Passivation using a monolayer (SiO 2 ) yielded a cell efficiency of 17.5%. However, the Al 2 O 3 /SiO 2 bilayer structure drastically increased cell efficiency to 20.1%. Quantum efficiency results revealed a significant improvement in the IR range, suggesting that bilayer passivation was effective on the back of a solar cell in providing a high energy conversion efficiency.

TiO 2 -Based Metal–Semiconductor–Metal Ultraviolet Photodetectors Deposited by Ultrasonic Spray Pyrolysis Technique

This paper uses nonvacuum ultrasonic spray pyrolysis deposition method to grow TiO₂ for ultraviolet (UV) detection. The analyses of the materials like X-ray photoelectron spectroscopy, X-ray diffraction, and photoluminescence were investigated. The 600 °C annealing temperature is the optimum condition to obtain the anatase TiO₂. The metal-semiconductor- metal (MSM) photodetectors (PDs) with 3-, 5-, 7-, 10-μm finger spacing were fabricated and the performance was investigated. The PD with 10-μm finger spacing has the lowest dark current of 2.92 × 10^-11 A and the highest UV-to-visible rejection ratio (RUV/VIS) of 2.1 × 10⁵ at 5 V. The PD with 5-μm finger spacing has the lowest noise equivalent power of 2.57 × 10^-9 W and the highest detectivity (D*) of 5.46 × 108 cmHz^0.5W^-1. The rising time and the falling time of the PD are 5 and 12 s. In addition, the TiO₂-based MSM PD in this paper operated normally at 450 K; however, the performance is slightly degraded. The mechanisms causing degradation at high temperature were investigated.

Characterization of TiO 2 -Based MISIM Ultraviolet Photodetectors by Ultrasonic Spray Pyrolysis

In this letter, non-vacuum ultrasonic spray pyrolysis deposition was used to grow Al₂O₃/TiO₂ thin film as the metal-insulator-semiconductor-insulator-metal ultraviolet photodetector (MISIM UV PD). The anatase TiO₂ with 400 °C annealing was used as the active layer of the UV PD. X-ray diffraction and Raman spectra were used to characterize the crystal phase of the TiO₂. The Al₂O₃/TiO₂ MISIM UV PD has lower dark current (~57.2 pA), higher photoresponse (~24.48 A/W), and higher detectivity (~8.25 × 10¹³ Jones), all of which were better than the TiO₂ MSM UV PD. Similar device performance was obtained from the SiO₂/TiO₂ MISIM UV PD. The external quantum efficiency of the Al₂O₃/TiO₂ and SiO₂/TiO₂ MISIM UV PDs was 8204% and 6840%. Such high external quantum efficiency results from the internal photoconductive gain and the interfacial trap controlled charge injection.

Indium-tin-oxide nanowhiskers crystalline silicon photovoltaics combining micro- and nano-scale surface textures

In this work, we present a solution that employs combined micro- and nano-scale surface textures to increase light harvesting in the near infrared for crystalline silicon photovoltaics, and discuss the associated antireflection and scattering mechanisms. The combined surface textures are achieved by uniformly depositing a layer of indium-tin-oxide nanowhiskers on passivated, micro-grooved silicon solar cells using electron-beam evaporation. The nanowhiskers facilitate optical transmission in the near-infrared, which is optically equivalent to a stack of two dielectric thin-films with step- and graded- refractive index profiles. The ITO nanowhiskers provide broadband anti-reflective properties (R<5%) in the wavelength range of 350-1100nm. In comparison with conventional Si solar cell, the combined surface texture solar cell shows higher external quantum efficiency (EQE) in the range of 700-1100nm. Moreover, the ITO nano-whisker coating Si solar cell shows a high total efficiency increase of 1.1% (from 16.08% to17.18%). Furthermore, the nano-whiskers also provide strong forward scattering for ultraviolet and visible light, favorable in thin-wafer silicon photovoltaics to increase the optical absorption path.

Growth mechanism of an aluminium-induced solid phase epitaxial (AI-SPE) Si0.5Ge0.5 layer using in situ heating transmission electron microscopy

The mechanism of growth of an epitaxial Si0.5Ge0.5 layer on a single crystalline (sc) Si (100) substrate by aluminum-induced solid phase epitaxy (AI-SPE) at a relatively low temperature (450 °C) has been revealed using in situ heating transmission electron microscopy (TEM). The analysis of the thermodynamics exactly supports the finding from in situ TEM. It evidences that the Si0.5Ge0.5 prefers to nucleate at the interface of the Al layer and the sc-Si (100) substrate due to the lowest critical thickness for nucleation. Based on the results from in situ TEM and thermodynamic analysis, the germanium (Ge) virtual substrate of the compositional gradient can be successfully prepared via a multi-run AI-SPE process at low-temperature.

Enhanced angular response of power conversion efficiency for silicon solar cells utilizing a uniformly distributed nano-whisker medium

In the research of photovoltaic devices, eliminating Fresnel reflection loss is a critical issue on the way to pursue higher efficiency. To maximize the power conversion efficiency, dielectric antireflective coating shows a cost-effective approach, but not enough to absorb broadband solar radiation effectively. Recently, the functional nanostructure shows high potential to be an omnidirectional antireflective coating for the photovoltaic devices. Here we demonstrate Indium-Tin-Oxide (ITO) nano-whiskers, grown by the self-catalyst vapor-liquid-solid (VLS) mechanisms on the textured Si substrate. The ITO nano-whiskers can provide broadband anti-reflective properties (R<5%) in the wavelength range of 350–1100nm. In comparison with conventional Si solar cell, the ITO nano-whiskers coating solar cell shows higher external quantum efficiency (EQE) in the range of 700–1100nm. Moreover, the ITO nano-whisker coating Si solar cell shows a high total efficiency increase of 1.1% (from 16.08% to17.18%). The angular response of the conversion efficiency also increases from 7% at the normal incidence to more than 15% for incident angles over 70°.

Novel Indium-Tin-Oxide nano-whiskers for enhanced transmission of surface-textured silicon photovoltaic cells

Conductive Indium-Tin-Oxide (ITO) nano-whiskers were deposited on surface-textured Si solar cells using glancing-angle electron-beam deposition. With different deposited time, the ITO nano-structured layer exhibit tunable thickness, which can be related to the surface reflectance. The optimized nano-whisker surface demonstrates a broadband anti-reflective properties (R≪5%), better than the traditional Si3N4 antireflection coating. Current-voltage and quantum efficiency analyses with the measured reflectivity data show enhanced optical transmission in the long wavelength range from 700nm to 1000nm, corresponding to a conversion efficiency improvement from 13.93% to 14.37%.

Excellent passivation structure of high efficiency multicrystalline silicon solar cells

The passivation effects for multicrystalline silicon solar cell with different configurations are investigated. Al2O3 and SiO2 films are used as the passivation layer in this work. They are prepared by atomic layer deposition and thermal oxidation, respectively. Using monolayer (Al2O3 or SiO2) as the passivation layer, the cell efficiency is 16.33% and 17.41%, respectively. The excellent passivation structure shows the drastic enhancement in cell efficiency of 19.09%. The quantum efficiency results show the improvement in the IR range, which explains the high energy conversion efficiency for the excellent structure.