Wei-Lun Chang

Biomimetic nanostructured antireflection coating and its application on crystalline silicon solar cells

In this report, we demonstrate the implementation of biomimetic nanostructured antireflection coatings with polymethyl methacrylate (PMMA) layer on the micro-textured surface of silicon crystalline solar cells. To reduce cost, the process combines colloidal lithography, cast molding method, and reversal nanoimprint lithography. The technique is simple, low cost, and does not cause damage to the thin and brittle conventional crystalline solar cells. The antireflection properties of this biomimetic nanostructure coating are considered as effective as those of a conventional single-layer SiNx thin film. The resultant structures alone could reduce the average reflectance of solar cell from 13.2% to 7.8% and enhance power conversion efficiency from 12.85% to 14.2%.

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.

Efficiency enhancement of silicon solar cells using a nano-scale honeycomb broadband anti-reflection structure

This experiment demonstrates the process for manufacturing a ZnO honeycomb sub-wavelength structure using nanosphere lithography technology exhibiting excellent anti-reflection properties from the UV to NIR wavelength regions. This honeycomb nanostructure, combined with commercially available crystalline Si solar cells, show substantially improved conversion efficiency from 15.6% to 16.6% using optimized honeycomb sizes and precursor concentrations of ZnO. The present work develops an unsophisticated and economical technique suitable for industrial applications in producing a uniform and low-reflective texture.

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.

Characterization of selective-emitter solar cells consists of laser opened window and subsequently screen-printed electrodes

Selective emitter technique is still an interesting research subject and attracts many attentions in photovoltaic industry. A selective emitter is a doping layer that is heavily doped underneath the electrode while lightly doped in between the electrode grids. It offers good short-wavelength response due to low surface doping concentration and in the mean time maintains low contact resistance. To successfully incorporate the selective-emitter technique into production, one of the requirements for a cost-effective selective emitter is that the efficiency should be increased significantly compare to those conventional solar cells with uniform doped emitter. However, the developments of commercial Ag-pastes, which are suitable for high sheet-resistance silicon, make uniform lightly-doped solar cell possible. In other words, the successfully developed Ag-pastes may limit the demand of selective-emitter techniques. In this study, we try to demonstrate that the selective-emitter is still an attractive technique even in light/light sheet-resistance combination. In comparison to solar cells with uniform lightly-doped 65 Ohm/sq emitter, the results show an efficiency improvement of more than 0.5% absolute can be achieved for selective-emitter solar cells. The process sequence of the selective emitters in this work includes a laser opening through the oxide mask. It was followed by conventional POCl3 diffusion and a subsequent electrode screen printing.

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.

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%.

Efficiency enhancement of the thin-silicon photovoltaics using indium-tin-oxide nanowhiskers

Thin wafer-based solar cells have the potential to significantly decrease the cost of photovoltaics. Light trapping is particularly critical in such thin-wafer crystalline silicon solar cells in order to increase light absorption and hence cell efficiency. In this article we investigate the indium-tin-oxide nanowhisker on textured silicon surface for enhancing the near-infrared absorbance of silicon photovoltaics. 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 quantum efficiencies enhancement for a broad wavelength range of 900 to 1100 nm, which is crucial to the development of advanced thin-substrate silicon solar cells.

Compound surface textures for enhanced near-infrared light havesting of crystallin silicon solar cells

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. Compared to the reflectance of the conventional silicon solar cell, the combined textures structure provided broadband high absorption, especially in the near infrared region. As a result, the solar cell with combined textures achieves over 90% external quantum efficiencies for a broad wavelength range of 460 to 980 nm, which is crucial to the development of advanced thin-substrate silicon solar cells. Due to the high photocurrent contributed to the performance, the compound textured solar cell increased the 1.1% absolute power conversion efficiency, from 16.1% to 17.2%.