Backside Reflector Research Articles

Efficiency enhancement in Si solar cells by textured photonic crystal back reflector

An efficient light-trapping scheme is developed for solar cells that can enhance the optical path length by several orders of magnitude using a textured photonic crystal as a backside reflector. It comprises a reflection grating etched on the backside of the substrate and a one-dimensional photonic crystal deposited on the grating. Top-contacted crystalline Si solar cells integrated with the textured photonic crystal back reflector were designed and fabricated. External quantum efficiency was significantly improved between the wavelengths of 1000 and 1200nm (enhancement up to 135 times), and the overall power conversion efficiency was considerably increased.

Optimization of Textured Photonic Crystal Backside Reflector for Si Thin Film Solar Cells

ABSTRACTA new backside reflector, textured photonic crystal, is introduced into Si thin film solar cells. Scattering matrix method is used to systematically optimize all the parameters of the two components of the backside reflector, grating and distributed Bragg reflector, to achieve the highest power conversion efficiency for a given solar cell thickness. Experimentally, Si-on-insulator solar cells are being fabricated to verify the tremendous efficiency enhancement and optimal design. It is found that while the optimal period and etch depth of the grating, the Bragg wavelength of the distributed Bragg reflector, as well as the antireflection coating thickness all decrease as the cell becomes thinner, the optimum duty cycle of the grating remains almost constant at 0.5. For a 2 μm thick cell, the relative efficiency enhancement can be as high as 52% using the optimized design.

Nitride based Power Chip with Indium-Tin-Oxide p-Contact and Al Back-Side Reflector

Nitride-based large size (i.e. 1 mm×1 mm) indium-tin-oxide (ITO) light emitting diodes (LEDs) were successfully fabricated. In order to enhance the output intensity of power chips, Al reflector was deposited by e-beam evaporator on the chip backside. It was found that the 350 mA output power was 84.8 mW (W-P-E=7.2%) at 460 nm for the power chip with ITO as p-contacts and Al as back-side reflector. It was also found that ITO power chip with Al reflector was more reliable.

New Light Trapping in Thin Film Solar Cells Using Textured Photonic Crystals

AbstractA novel light trapping scheme is developed to enhance the optical path length in solar cells by using a photonic structure as the backside reflector. This structure combines a reflection grating on the substrate with an over-deposited distributed Bragg reflector (DBR). With this structure, the optical path length can be enhanced by more than 104 times with very little reflection loss. In turn, solar cell efficiency is predicted to be enhanced enormously.

New Solar Cells with Novel Light Trapping via Textured Photonic Crystal Back Reflector

ABSTRACTWe have successfully developed a new light-trapping scheme for solar cells that can enhance the optical path length by more than 104 times using a textured photonic crystal structure as a backside reflector. Top-contacted crystalline Si solar cells integrated with the new back reflector were designed, fabricated and characterized. Both external quantum efficiency and power conversion efficiency of the cells have shown significant improvement due to the path length enhancement furnished by the new back reflector despite of the 675 um thick wafers and relatively short minority carrier diffusion length.

Crystalline silicon thin films with porous Si backside reflector

Thin film crystalline Si solar cells on cheap Si-based substrates have a large potential in PV technology. Optical light confinement is a very crucial point of such thin film structures. Porous Si (PS) as a perfect light diffuser could be used as a backside reflector if its multi-layer structure would be preserved during the deposition of a thin Si film. That is why low-energy plasma enhanced chemical vapor deposition (LEPECVD) was chosen to deposit a thin Si film on a PS multilayer structure at low temperature and high deposition rate. This technique allows one to deposit a Si film with epitaxial quality on the top of PS without destroying its multi-layer structure as revealed by high-resolution X-ray diffraction and cross-sectional transmission electron microscopy (TEM). The epi-layers of 10 μm are grown at very high deposition rates (approx. 3 nm/s) at 590°C. TEM-analysis reveals that during the deposition a high density of defects forms at the PS/epi-Si interface and spreads through the whole epi-layer. The defect density is decreased when the deposition temperature is increased to 645°C. LEPECVD appears to be an appropriate deposition technique to grow thin Si films on cheap Si based substrates with PS reflector.

Crystalline silicon thin films with porous Si backside reflector (abstract only)

Thin film crystalline Si solar cells on cheap Si-based substrates have a large potential in PV technology. Optical light confinement is a very crucial point of such thin film structures. Porous Si (PS) as a perfect light diffuser could be used as a backside reflector if its multi-layer structure would be preserved during the deposition of a thin Si film. That is why low-energy plasma enhanced chemical vapor deposition (LEPECVD) is chosen to deposit a thin Si film on a PS multilayer structure at low temperature and a high deposition rate. This technique allows one to deposit a Si film with an epitaxial quality on the top of PS without destroying its multilayer structure as revealed by high-resolution X-ray diffraction and cross-sectional transmission electron microscopy (TEM). The epilayers of 10 μm are grown at very high deposition rate (around 3 nm/s) at 590°C. TEM-analysis reveals that during the deposition a high density of defects forms at the interface PS/epi-Si and spreads through the whole epilayer. The defect density is decreased when the deposition temperature is increased to 645°C. LEPECVD appears to be an appropriate deposition technique to grow thin Si films on cheap Si based substrates with PS reflector.

A Novel Processing Technology for High‐Efficiency Silicon Solar Cells

A novel simultaneous boron and phosphorus diffusion technique is presented to produce simple, high‐efficiency silicon solar cells in one thermal cycle. This technique uses boron and phosphorus spin‐on dopant films to fabricate limited solid doping sources out of dummy silicon wafers. This approach results in the delivery of a fixed dose of or to the diffused sample. The resulting diffusion glass is extremely thin (∼60 Å), which allows for the in situ growth of a passivating thermal oxide without increasing the solar cell reflectance. Reverse saturation current density measurements show that the in situ oxide passivation for a light boron and phosphorus diffusion provides excellent passivation properties, resulting in values in the range. Measurements of the bulk minority carrier lifetime show that by fabricating separate boron solid sources, trace impurities in the spin‐on dopant film are not transported to the diffused sample. This filtering action is shown to result in bulk lifetimes in excess of 1 ms for silicon doped indirectly from the source wafers but gives much lower lifetimes (∼6 μs) for the wafers on which the boron spin‐on film was directly applied. This process was validated by fabricating in situ oxide passivated, solar cells in one high‐temperature cycle incorporating several high‐efficiency features including surface texturing and a back side reflector, resulting in confirmed efficiencies in the 19–20% range. © 1999 The Electrochemical Society. All rights reserved.

High-efficiency and high-speed silicon metal–semiconductor–metal photodetectors operating in the infrared

A silicon metal–semiconductor–metal photodetector with high-efficiency and high-speed in the infrared is reported. The high performance is achieved by using a Si-on-insulator substrate with a patterned nanometer-scale scattering reflector buried underneath a 170-nm-thick Si active layer. This scattering reflector causes light to be trapped inside the thin Si active layer, resulting in a fast and efficient carrier-collection by the electrodes. The impulse response of the photodetector, measured by electro-optic sampling at 780 nm wavelength, has a full width at half-maximum of 5.4 ps, corresponding to a 3-dB bandwidth of 82 GHz. At both 633 and 850 nm wavelengths, the responsivities of the photodetector with the buried backside reflector are at least an order of magnitude larger than those without the reflector.

Extension of longwavelength IR photovoltaic detector operation to near room-temperatures

Practical realization of near room temperature (230–300 K) longwavelength (5–12 μm) photovoltaic detectors is reported. The devices are epitaxial n +ip p photodiodes operated at ambient temperature or with a simple, two-stage thermoelectric cooling. The performance of the photodiodes has been improved by the use optimized composition and doping profile structures. The tunnel currents were minimized by interfacing the n + and p-type layers with a thin (0.5 μm) lightly doped i-region. The quantum efficiency has been increased by the use of backside reflector. Further improvement of performance was achieved by the use of monolithic optical immersion. Large area devices with useful performance were obtained by the use of small close-spaced elements connected in series. The near room temperature photovoltaic detectors are of particular significance for very low and very high frequency applications.