Characterization of Luminescent Down-Shifting Spectral Conversion Effects on Silicon Solar Cells with Various Combinations of Eu-Doped Phosphors.

Luminescent down-shifting (LDS) spectral conversion is a feasible approach to enhancing the short-wavelength response of single junction solar cells. This paper presents the optical and electrical characteristics of LDS spectral conversion layers containing a single species or two species of Eu-doped phosphors applied to the front surface of silicon solar cells via spin-on coating. The chemical composition, surface morphology, and fluorescence emission of the LDS layers were respectively characterized using energy-dispersive X-ray analysis, optical imaging, and photoluminescence measurements. We also examined the LDS effects of various phosphors on silicon solar cells in terms of optical reflectance and external quantum efficiency. Finally, we examined the LDS effects of the phosphors on photovoltaic performance by measuring photovoltaic current density-voltage characteristics using an air-mass 1.5 global solar simulator. Compared to the control cell, the application of a single phosphor enhanced efficiency by 17.39% (from 11.14% to 13.07%), whereas the application of two different phosphors enhanced efficiency by 31.63% (from 11.14% to 14.66%).

Characterization of Plasmonic Scattering, Luminescent Down-Shifting, and Metal-Enhanced Fluorescence and Applications on Silicon Solar Cells.

This paper studied characterized the plasmonic effects of silver nanoparticles (Ag-NPs), the luminescent down-shifting of Eu-doped phosphor particles, and the metal-enhanced fluorescence (MEF) achieved by combining the two processes to enhance the conversion efficiency of silicon solar cells. We obtained measurements of photoluminescence (PL) and external quantum efficiency (EQE) at room temperature to determine whether the fluorescence emissions intensity of Eu-doped phosphor was enhanced or quenched by excitation induced via surface plasmon resonance (SPR). Overall, fluorescence intensity was enhanced when the fluorescence emission band was strongly coupled to the SPR band of Ag-NPs and the two particles were separated by a suitable distance. We observed a 1.125× increase in PL fluorescence intensity at a wavelength of 514 nm and a 7.05% improvement in EQE (from 57.96% to 62.05%) attributable to MEF effects. The combined effects led to a 26.02% increase in conversion efficiency (from 10.23% to 12.89%) in the cell with spacer/NPs/SOG-phosphors and a 22.09% increase (from 10.23% to 12.48%) in the cell with spacer/SOG-phosphors, compared to the bare solar cell. This corresponds to an impressive 0.85% increase in absolute efficiency (from 12.04% to 12.89%), compared to the cell with only spacer/SOG.

Enhancing Photovoltaic Performance of GaAs Single-Junction Solar Cells by Applying a Spectral Conversion Layer Containing Eu-Doped and Yb/Er-Doped Phosphors.

In this study, we examined efforts to increase the photovoltaic performance of GaAs single-junction solar cells using spectral conversion layers, respectively, composed of europium-doped (Eu-doped) phosphors, ytterbium/erbium-doped (Yb/Er-doped) phosphors, and a combination of Eu-doped and Yb/Er-doped phosphors. Spin-on film deposition was used to apply the conversion layers, all of which had a total phosphor concentration of 3 wt%. The chemical compositions of the phosphors were examined by energy-dispersive X-ray spectroscopy. The fluorescence emissions of the phosphors were confirmed by using photoluminescence measurements. Under laser diode excitation at 405 nm, we observed green luminescent downshift (LDS) emissions by Eu-doped phosphors at wavelengths of 479 nm to 557 nm, and under excitation at 980 nm, we observed red up-conversion (UC) emissions by Yb/Er-doped phosphors at wavelengths of 647 nm to 672 nm. The spectral conversion layers were characterized in terms of optical reflectance, external quantum efficiency, and photovoltaic current and voltage under AM 1.5 G simulations. The conversion efficiency of the cell combining Eu-doped and Yb/Er-doped phosphors (23.84%) exceeded that of the cell coated with Yb/Er-doped phosphors (23.72%), the cell coated with Eu-doped phosphors (23.19%), and the cell coated without phosphors (22.91%).

The Fabrication and Characterization of InAlAs/InGaAs APDs Based on a Mesa-Structure with Polyimide Passivation.

This paper presents a novel front-illuminated InAlAs/InGaAs separate absorption, grading, field-control and multiplication (SAGFM) avalanche photodiodes (APDs) with a mesa-structure for high speed response. The electric fields in the InAlAs-multiplication layer and InGaAs-absorption layer enable high multiplication gain and high-speed response thanks to the thickness and concentration of the field-control and multiplication layers. A mesa active region of 45 micrometers was defined using a bromine-based isotropic wet etching solution. The side walls of the mesa were subjected to sulfur treatment before being coated with a thick polyimide layer to reduce current leakage, while lowering capacitance and increasing response speeds. The breakdown voltage (VBR) of the proposed SAGFM APDs was approximately 32 V. Under reverse bias of 0.9 VBR at room temperature, the proposed device achieved dark current of 31.4 nA, capacitance of 0.19 pF and multiplication gain of 9.8. The 3-dB frequency response was 8.97 GHz and the gain-bandwidth product was 88 GHz. A rise time of 42.0 ps was derived from eye-diagrams at 0.9 VBR. There was notable absence of intersymbol-interference and the signals remained error-free at data-rates of up to 12.5 Gbps.

Enhancing Photovoltaic Performance of Plasmonic Silicon Solar Cells with ITO Nanoparticles Dispersed in SiO2 Anti-Reflective Layer.

In this study, we sought to enhance the photovoltaic performance of silicon solar cells by coating them (via the spin-on film technique) with a layer of SiO2 containing plasmonic indium-tin-oxide nanoparticles (ITO-NPs) of various concentrations. We demonstrated that the surface plasmon resonance absorption, surface morphology, and transmittance of the ITO-NPs dispersed in SiO2 layer at various concentrations (1-7 wt%). We also assessed the plasmonic scattering effects of ITO-NPs within a layer of SiO2 with and without a sub-layer of ITO in terms of optical reflectance, external quantum efficiency, and photovoltaic current-voltage under air mass (AM) 1.5G solar simulation. Compared to an uncoated reference silicon solar cell, applying a layer of SiO2 containing 3 wt% ITO-NPs improved efficiency by 17.90%, whereas applying the same layer over a sub-layer of ITO improved efficiency by 33.27%, due to the combined effects of anti-reflection and plasmonic scattering.

Enhancing Output Power of Textured Silicon Solar Cells by Embedding Indium Plasmonic Nanoparticles in Layers within Antireflective Coating.

In this study, we sought to enhance the output power and conversion efficiency of textured silicon solar cells by layering two-dimensional indium nanoparticles (In NPs) within a double-layer (SiNx/SiO2) antireflective coating (ARC) to induce plasmonic forward scattering. The plasmonic effects were characterized using Raman scattering, absorbance spectra, optical reflectance, and external quantum efficiency. We compared the optical and electrical performance of cells with and without single layers and double layers of In NPs. The conversion efficiency of the cell with a double layer of In NPs (16.97%) was higher than that of the cell with a single layer of In NPs (16.61%) and greatly exceeded that of the cell without In NPs (16.16%). We also conducted a comprehensive study on the light-trapping performance of the textured silicon solar cells with and without layers of In NPs within the double layer of ARC at angles from 0° to 75°. The total electrical output power of cells under air mass (AM) 1.5 G illumination was calculated. The application of a double layer of In NPs enabled an impressive 53.42% improvement in electrical output power (compared to the cell without NPs) thanks to the effects of plasmonic forward scattering.

Fabrication and Characterization of Planar-Type Top-Illuminated InP-Based Avalanche Photodetector on Conductive Substrate with Operating Speeds Exceeding 10 Gbps.

This paper presents a high-speed top-illuminated InP-based avalanche photodetector (APD) fabricated on conductive InP-wafer using planar processes. The proposed device was then evaluated in terms of DC and dynamic performance characteristics. The design is based on a separate absorption, grading, charge, and multiplication (SAGCM) epitaxial-structure. An electric field-profile of the SAGCM layers was derived from the epitaxial structure. The punch-through voltage of the SAGCM APD was controlled to within 16⁻17 V, whereas the breakdown voltage (VBR) was controlled to within 28⁻29 V. We obtained dark current of 2.99 nA, capacitance of 0.226 pF, and multiplication gain of 12, when the APD was biased at 0.9 VBR at room temperature. The frequency-response was characterized by comparing the calculated 3-dB cut-off modulation-frequency (f3-dB) and f3-dB values measured under various multiplication gains and modulated incident powers. The time-response of the APD was evaluated by deriving eye-diagrams at 0.9 VBR using pseudorandom non-return to zero codes with a length of 231-1 at 10⁻12.5 Gbps. There was a notable absence of intersymbol-interference, and the signals remained error-free at data-rates of up to 12.5 Gbps. The correlation between the rise-time and modulated-bandwidth demonstrate the suitability of the proposed SAGCM-APD chip for applications involving an optical-receiver at data-rates of >10 Gbps.

Photovoltaic Performance Enhancement of Silicon Solar Cells Based on Combined Ratios of Three Species of Europium-Doped Phosphors.

This paper presents a scheme for the enhancement of silicon solar cells in terms of luminescent emission band and photovoltaic performance. The proposed devices are coated with an luminescent down-shifting (LDS) layer comprising three species of europium (Eu)-doped phosphors mixed within a silicate film (SiO2) using a spin-on film deposition. The three species of phosphor were mixed at ratios of 0.5:1:1.5, 1:1:1, or 1.5:1:0.5 in weight percentage (wt %). The total quantity of Eu-doped phosphors in the silicate solution was fixed at 3 wt %. The emission wavelengths of the Eu-doped phosphors were as follows: 518 nm (specie-A), 551 nm (specie-B), and 609 nm (specie-C). We examined the extended luminescent emission bands via photoluminescence measurements at room temperature. Closely matching the luminescent emission band to the high responsivity band of the silicon semiconductor resulted in good photovoltaic performance. Impressive improvements in efficiency were observed in all three samples: 0.5:1:1.5 (20.43%), 1:1:1 (19.67%), 1.5:1:0.5 (16.81%), compared to the control with a layer of pure SiO2 (13.80%).

Enhancing Photovoltaic Performance Using Broadband Luminescent Down-Shifting by Combining Multiple Species of Eu-Doped Silicate Phosphors.

This paper demonstrates the application of a broadband luminescent downshifting (LDS) layer with multiple species of europium (Eu)-doped silicate phosphors using spin-on film technique to enhance the photovoltaic efficiency of crystalline silicon solar cells. The surface morphology of the deposited layer was examined using a scanning electron microscope (SEM). The chemical composition of the Eu-doped silicate phosphors was analyzed using energy-dispersive X-ray spectroscopy (EDS). The fluorescence emission of the Eu-doped silicate phosphors was characterized using photoluminescence (PL) measurements at room temperature. We also compared the optical reflectance and external quantum efficiency (EQE) response of cells with combinations of various Eu-doped phosphors species. The cell coated with two species of Eu-doped phosphors achieved a conversion efficiency enhancement (∆η) of 19.39%, far exceeding the ∆η = 15.08% of the cell with one species of Eu-doped phosphors and the ∆η = 8.51% of the reference cell with the same silicate layer without Eu-doped phosphors.

Photovoltaic Performance Characterization of Textured Silicon Solar Cells Using Luminescent Down-Shifting Eu-Doped Phosphor Particles of Various Dimensions.

This paper reports on efforts to enhance the photovoltaic performance of textured silicon solar cells through the application of a layer of Eu-doped silicate phosphor with particles of various dimensions using the spin-on film technique. We examined the surface profile and dimensions of the Eu-doped phosphors in the silicate layer using optical microscopy with J-image software. Optical reflectance, photoluminescence, and external quantum efficiency were used to characterize the luminescent downshifting (LDS) and light scattering of the Eu-doped silicate phosphor layer. Current density-voltage curves under AM 1.5G simulation were used to confirm the contribution of LDS and light scattering produced by phosphor particles of various dimensions. Experiment results reveal that smaller phosphor particles have a more pronounced effect on LDS and a slight shading of incident light. The application of small Eu-doped phosphor particles increased the conversion efficiency by 9.2% (from 12.56% to 13.86%), far exceeding the 5.6% improvement (from 12.54% to 13.32%) achieved by applying a 250 nm layer of SiO2 and the 4.5% improvement (from 12.37% to 12.98%) observed in cells with large Eu-doped phosphor particles.

Electrical and Optical Characterization of Sputtered Silicon Dioxide, Indium Tin Oxide, and Silicon Dioxide/Indium Tin Oxide Antireflection Coating on Single-Junction GaAs Solar Cells.

This study characterized the electrical and optical properties of single-junction GaAs solar cells coated with antireflective layers of silicon dioxide (SiO2), indium tin oxide (ITO), and a hybrid layer of SiO2/ITO applied using Radio frequency (RF) sputtering. The conductivity and transparency of the ITO film were characterized prior to application on GaAs cells. Reverse saturation-current and ideality factor were used to evaluate the passivation performance of the various coatings on GaAs solar cells. Optical reflectance and external quantum efficiency response were used to evaluate the antireflective performance of the coatings. Photovoltaic current-voltage measurements were used to confirm the efficiency enhancement obtained by the presence of the anti-reflective coatings. The conversion efficiency of the GaAs cells with an ITO antireflective coating (23.52%) exceeded that of cells with a SiO2 antireflective coating (21.92%). Due to lower series resistance and higher short-circuit current-density, the carrier collection of the GaAs cell with ITO coating exceeded that of the cell with a SiO2/ITO coating.

Plasmonic Light Scattering in Textured Silicon Solar Cells with Indium Nanoparticles from Normal to Non-Normal Light Incidence.

In this study, we sought to improve the light trapping of textured silicon solar cells using the plasmonic light scattering of indium nanoparticles (In NPs) of various dimensions. The light trapping modes of textured-silicon surfaces with and without In NPs were investigated at an angle of incidence (AOI) ranging from 0° to 75°. The optical reflectance, external quantum efficiency (EQE), and photovoltaic performance were first characterized under an AOI of 0°. We then compared the EQE and photovoltaic current density-voltage (J-V) as a function of AOI in textured silicon solar cells with and without In NPs. We observed a reduction in optical reflectance and an increase in EQE when the cells textured with pyramidal structures were coated with In NPs. We also observed an impressive increase in the average weighted external quantum efficiency (∆EQEw) and short-circuit current-density (∆Jsc) in cells with In NPs when illuminated under a higher AOI. The ∆EQEw values of cells with In NPs were 0.37% higher than those without In NPs under an AOI of 0°, and 3.48% higher under an AOI of 75°. The ∆Jsc values of cells with In NPs were 0.50% higher than those without In NPs under an AOI of 0°, and 4.57% higher under an AOI of 75°. The application of In NPs clearly improved the light trapping effects. This can be attributed to the effects of plasmonic light-scattering over the entire wavelength range as well as an expanded angle of incident light.

Electrical and optical performance of plasmonic silicon solar cells based on light scattering of silver and indium nanoparticles in matrix-combination.

This study demonstrates the efficacy of combining a matrix of silver nanoparticles (Ag-NPs) with indium nanoparticles (In-NPs) to improve the electric and optical performance of plasmonic silicon solar cells. We examined the excitation of localized surface plasmons of Ag-NPs and In-NPs using surface enhanced Raman scattering measurements. Optical reflectance and external quantum efficiency (EQE) measurements demonstrate that the light scattering of Ag-NPs at short wavelengths can be improved by surrounding them with In-NPs. This also leads to high EQE band matching in the high energy band of the AM1.5G solar energy spectrum. Impressive improvements in optical reflectance and EQE response were also observed at short wavelengths. Cells with a matrix of Ag-NPs (20% surface coverage) surrounded by In-NPs (80% surface coverage) increased the overall efficiency of the cell by 31.83%, as confirmed by photovoltaic current density-voltage characterization under AM 1.5 G illumination.

Optical and Electrical Performance of MOS-Structure Silicon Solar Cells with Antireflective Transparent ITO and Plasmonic Indium Nanoparticles under Applied Bias Voltage.

This paper reports impressive improvements in the optical and electrical performance of metal-oxide-semiconductor (MOS)-structure silicon solar cells through the incorporation of plasmonic indium nanoparticles (In-NPs) and an indium-tin-oxide (ITO) electrode with periodic holes (perforations) under applied bias voltage. Samples were prepared using a plain ITO electrode or perforated ITO electrode with and without In-NPs. The samples were characterized according to optical reflectance, dark current voltage, induced capacitance voltage, external quantum efficiency, and photovoltaic current voltage. Our results indicate that induced capacitance voltage and photovoltaic current voltage both depend on bias voltage, regardless of the type of ITO electrode. Under a bias voltage of 4.0 V, MOS cells with perforated ITO and plain ITO, respectively, presented conversion efficiencies of 17.53% and 15.80%. Under a bias voltage of 4.0 V, the inclusion of In-NPs increased the efficiency of cells with perforated ITO and plain ITO to 17.80% and 16.87%, respectively.

20-Gbps optical LiFi transport system.

A 20-Gbps optical light-based WiFi (LiFi) transport system employing vertical-cavity surface-emitting laser (VCSEL) and external light injection technique with 16-quadrature amplitude modulation (QAM)-orthogonal frequency-division multiplexing (OFDM) modulating signal is proposed. Good bit error rate (BER) performance and clear constellation map are achieved in our proposed optical LiFi transport systems. An optical LiFi transport system, delivering 16-QAM-OFDM signal over a 6-m free-space link, with a data rate of 20 Gbps, is successfully demonstrated. Such a 20-Gbps optical LiFi transport system provides the advantage of a free-space communication link for high data rates, which can accelerate the visible laser light communication (VLLC) deployment.

Performance-Enhanced Textured Silicon Solar Cells Based on Plasmonic Light Scattering Using Silver and Indium Nanoparticles.

Performances of textured crystalline-silicon (c-Si) solar cells enhanced by silver nanoparticles (Ag-NPs) and indium nanoparticles (In-NPs) plasmonic effects are experimentally demonstrated and compared. Plasmonic nanoparticles integrated into textured c-Si solar cells can further increase the absorption and enhance the short-circuit current density (Jsc) of the solar cell. To examine the profile of the proposed metallic particles, the average diameter and coverage of the In-NPs (Ag-NPs) at 17.7 nm (19.07 nm) and 30.5% (35.1%), respectively, were obtained using scanning electron microscopy. Optical reflectance and external quantum efficiency response were used to measure plasmonic light scattering at various wavelengths. Compared to a bare reference cell, the application of In-NPs increased the Jsc of the cells by 8.64% (from 30.32 to 32.94 mA/cm²), whereas the application of Ag-NPs led to an increase of 4.71% (from 30.32 to 31.75 mA/cm²). The conversion efficiency of cells with embedded In-NPs (14.85%) exceeded that of cells with embedded Ag-NPs (14.32%), which can be attributed to the broadband plasmonic light scattering of the In-NPs.

External quantum efficiency response of thin silicon solar cell based on plasmonic scattering of indium and silver nanoparticles.

This study characterized the plasmonic scattering effects of indium nanoparticles (In NPs) on the front surface and silver nanoparticles (Ag NPs) on the rear surface of a thin silicon solar cell according to external quantum efficiency (EQE) and photovoltaic current-voltage. The EQE response indicates that, at wavelengths of 300 to 800 nm, the ratio of the number of photo-carriers collected to the number of incident photons shining on a thin Si solar cell was enhanced by the In NPs, and at wavelengths of 1,000 to 1,200 nm, by the Ag NPs. These results demonstrate the effectiveness of combining the broadband plasmonic scattering of two metals in enhancing the overall photovoltaic performance of a thin silicon solar cell. Short-circuit current was increased by 31.88% (from 2.98 to 3.93 mA) and conversion efficiency was increased by 32.72% (from 9.81% to 13.02%), compared to bare thin Si solar cells.

Performance enhancement of ITO/oxide/semiconductor MOS-structure silicon solar cells with voltage biasing.

In this study, we demonstrate the photovoltaic performance enhancement of a p-n junction silicon solar cell using a transparent-antireflective ITO/oxide film deposited on the spacing of the front-side finger electrodes and with a DC voltage applied on the ITO-electrode. The depletion width of the p-n junction under the ITO-electrode was induced and extended while the absorbed volume and built-in electric field were also increased when the biasing voltage was increased. The photocurrent and conversion efficiency were increased because more photo-carriers are generated in a larger absorbed volume and because the carriers transported and collected more effectively due to higher biasing voltage effects. Compared to a reference solar cell (which was biased at 0 V), a conversion efficiency enhancement of 26.57% (from 12.42% to 15.72%) and short-circuit current density enhancement of 42.43% (from 29.51 to 42.03 mA/cm(2)) were obtained as the proposed MOS-structure solar cell biased at 2.5 V. In addition, the capacitance-volt (C-V) measurement was also used to examine the mechanism of photovoltaic performance enhancement due to the depletion width being enlarged by applying a DC voltage on an ITO-electrode.

Novel ROF/FTTX/CATV hybrid three-band transport system.

A novel cost-effective radio-over-fiber (ROF)/fiber-to-the-X (FTTX)/CATV hybrid three-band transport system based on direct modulation of a distributed feedback laser diode (DFB LD) with multi-wavelength output characteristic is proposed and experimentally demonstrated. Radio-frequency (RF) (1.25 Gbps/6 GHz) signal with direct modulation, as well as baseband (BB) (622 Mbps) and CATV (channels 2-78) signals with external remodulation are successfully transmitted simultaneously. Low bit error rate (BER) and clear eye diagram were achieved for ROF and FTTX applications; as well as good performances of carrier-to-noise ratio (CNR), composite second-order (CSO) and composite triple beat (CTB) were obtained for CATV signals over an 80-km single-mode fiber (SMF) transport.