Back Surface Reflector Research Articles

Commercial white paint as back surface reflector for thin-film solar cells

In this work, commercially available white paint is applied as a pigmented diffuse reflector (PDR) on the rear surface of thin-film crystalline silicon (c-Si) solar cells with a silicon thickness in the 1–2 μm range. We show that white paint increases the short-circuit current density of the solar cells enormously, with a boost of 41% observed for very thin planar solar cells illuminated with the global AM1.5 solar spectrum. We also show that white paint is a better back surface reflector (BSR) than aluminium, air, a transparent conductive oxide (TCO)/aluminium stack, and even a detached aluminium mirror. While previous studies have investigated the influence of PDRs on silicon solar cells with thicknesses of over 27 μm, this work closes the gap that has existed for much thinner cells.

Wafer bonding and epitaxial transfer of GaSb-based epitaxy to GaAs for monolithic interconnection of thermophotovoltaic devices

GaInAsSb/AlGaAsSb/InAsSb/GaSb epitaxial layers were bonded to semi-insulating (SI) GaAs handle wafers with SiOx/Ti/Au as the adhesion layer for monolithic interconnection of thermophotovolatic (TPV) devices. Epitaxial transfer was completed by removal of the GaSb substrate, GaSb buffer, and InAsSb etch-stop layer by selective chemical etching. The SiOx/Ti/Au provides not only electrical isolation, but also high reflectivity and is used as an internal backsurface reflector (BSR). Characterization of wafer-bonded (WB) epitaxy by high-resolution x-ray diffraction (HRXRD) and time-decay photoluminescence (PL) indicates minimal residual stress and enhancement in optical quality. The 0.54-eV GaInAsSb cells were fabricated and monolithically interconnected in series. A ten-junction device exhibited linear voltage building with an opencircuit voltage of 1.8 V.

Monolithically series-interconnected GaInAsSb/AlGaAsSb/GaSb thermophotovoltaic devices with an internal backsurface reflector formed by wafer bonding

GaInAsSb/AlGaAsSb/GaSb thermophotovoltaic (TPV) cells were monolithically interconnected in series to build open-circuit voltage Voc. GaInAsSb epitaxial layers were transferred to GaAs by wafer bonding with SiOx/Ti/Au, which provides electrical isolation of individual cells and forms an internal backsurface reflector. This configuration is compatible with monolithic series interconnection of TPV cells; can mitigate the requirements of filters used for front-surface spectral control; and has the potential to improve TPV device performance. Wafer-bonded GaInAsSb TPV cells exhibit nearly linear voltage building. At a short-circuit current density of 0.4 A/cm2, Voc of a single TPV cell is 0.2 V, compared to 0.37 and 1.8 V for 2- and 10-junction series-interconnected TPV cells, respectively.

Fabrication and Characterization of Wafer-Bonded GaInAsSb Epitaxy for Monolithically Interconnected Thermophotovoltaic Devices

AbstractThe fabrication, characterization, and performance of wafer-bonded (WB) GaInAsSb thermophotovoltaic (TPV) devices for monolithically series-interconnected cells are reported. TPV epilayers were bonded to GaAs handle wafers with SiOx/Ti/Au. This dielectric/metal layer is multi-functional in that it is used as the adhesive to bond the epilayers; it provides electrical isolation; and it is an internal back surface reflector. Considerations were made to minimize residual stress and provide high reflectivity. Excellent structural and optical properties of WB TPV structures are observed. The external quantum efficiency of WB and unbonded TPV cells is comparable. TPV cells were monolithically series interconnected, and a 10-junction device exhibited linear voltage building with an open-circuit voltage of 1.8 V.

Multi-junction solar cells and novel structures for solar cell applications

The present status of R&D program for super-high efficiency III–V compound multi-junction solar cells in the New Sunshine Project in Japan is presented. As a result of InGaP top cell material quality improvement, development of optically and electrically low-loss double-heterostructure InGaP tunnel junction, photon and carrier confinements, and lattice matching between active cell layers and substrate, InGaP/InGaAs/Ge monolithic cascade 3-junction cells with an efficiency of 31.7% at 1-sun AM1.5 and InGaP/GaAs//InGaAs mechanically stacked 3-junction cells with the highest (world-record) efficiency of 33.3% at 1-sun AM1.5 have been realized. As an approach for low-cost and high-efficiency cells, better radiation resistance of GaAs thin-film solar cells with novel structures fabricated on Si substrates has also been demonstrated. Novel structures such as Bragg reflector and super-lattice structures are found to show a better initial cell performance and radiation resistance since those layers act as buffer layers to reduce dislocations, and act as a back-surface field and back-surface reflector layers.

Influence of wafer thickness on the performance of multicrystalline Si solar cells: an experimental study

AbstractThe influence of the thickness of silicon solar cells has been investigated using neighbouring multicrystalline silicon wafers with thickness ranging from 150 to 325 μm. For silicon solar cell structures with a high minority‐carrier diffusion length one expects that Jsc would decrease as the wafer becomes thinner due to a shorter optical path length. It was found experimentally that Jsc is nearly independent of the thickness of the solar cell, even when the minority‐carrier diffusion length is about 300 μm. This indicates that the Al rear metallisation acts as a good back surface reflector. A decrease in Jsc is observed only if the wafer thickness becomes less than about 200 μm. The observed trend in Voc as a function of the wafer thickness has been explained with PC1D modelling by a minority‐carrier diffusion length in the Al‐oped BSF which is small in relation to the thickness of the BSF. This effectively increases the recombination velocity at the rear of the cell. We have shown that the efficiency of solar cells made with standard industrial processing is hardly reduced by reducing the wafer thickness. Solar cell efficiencies might be increased by better rear surface passivation. Copyright © 2002 John Wiley & Sons, Ltd.

Radiation resistant AlGaAs/GaAs concentrator solar cells with internal Bragg reflector

The problem of increasing efficiency, reliability and radiation resistance of solar cells based on AlGaAs/GaAs heterostructures can be solved by using an internal Bragg reflector. The Bragg reflector as a back surface reflector and as a back surface potential barrier which allows to conserve the high photosensitivity in the long- and middle-wavelength parts of the spectrum after electron and proton irradiation. The effect of base doping and base thickness on the radiation resistance of AlGaAs/GaAs solar cells with the internal Bragg reflector has been investigated. Concentrator solar cells efficiency and related parameters before and after 3 MeV electron irradiation at the fluence up to 3×10 15 cm −2 are represented. A base doping level of 1×10 15 cm −3 and base thickness in the range 1.1–1.6 μm give an EOL AM0 efficiency of 15.8% (BOL–22%) at 30 Suns concentration after exposure to 1×10 15 cm −2 electron fluence. This EOL efficiency is among the highest reported for GaAs single-junction concentrator cells under AM0 conditions. Making the base doping level lower and the base thinner allows retaining a j EOL/ j BOL ratio of 0.96 upon exposure up to 3×10 15e/cm 2 3 MeV electron fluence. These results are additionally supported by the modeling calculations of the relative damage coefficient.

Light-trapping and back surface structures for polycrystalline silicon solar cells

Light-trapping in polycrystalline silicon solar cells is usually considered to be more difficult to implement than that in single crystal silicon solar cells due to the random crystallographic orientations in various grains. Furthermore, if minority carrier diffusion length is on the order of or less than solar cell thickness, which is the case of most cost-effective polycrystalline silicon, the translation of optical gain, achieved from light-trapping, into electrical gain will be rather limited, even with a perfect back surface passivation. In this work, geometrical light-trapping structures are demonstrated using a simplified isotropic etching at polycrystalline silicon surfaces. Combined with a back surface reflector (BSR), an enhanced absorption in the long wavelength region is measured with a low parasitic absorption. Different light-trapping structures are experimentally compared. To further examine the electrical gain from light-trapping, a three-terminal solar cell structure is used. This structure allows three different back surface configurations to be realized in a single device: unpassivated, passivated with a floating junction, and enhanced with a collecting junction. Results indicate that even with a relatively short minority-carrier diffusion length the current collection in the long wavelength region can be significantly improved and the light-trapping effect is enhanced as well. Copyright © 1999 John Wiley & Sons, Ltd.

Thermophotovoltaic and photovoltaic conversion at high-flux densities

We first discuss the similarities between generation of electricity using thermophotovoltaic (TPV) and high-optical-concentration solar photovoltaic (PV) devices. Following this, we consider power losses due to above- and below-bandgap photons, and we estimate the ideal bandgap by minimizing the sum of these, for a 6000 K black-body spectrum. The ideal bandgap, based on this approach, is less than that previously predicted, which could have a significant influence on the performance of devices and systems. To reduce the losses, we show that the low-energy photons may be removed from both types of cells and consider the specific case of a back-surface reflector. This approach to the management of waste heat may offer a useful additional tool with which to facilitate the design of high-photon-flux solar cells. In the case of the high-energy photons and the associated problem of thermalization of hot electrons, however, the heat must be removed by other means, and we consider the applicability of microchannel cooling systems. These appear to have the potential to handle thermal loads at least several times those generated by 1000 times concentrators, or by black-body TPV radiators at a temperature of far greater than 1500 K. We go on to consider the management of the very high currents generated in both concentrator TPV and PV systems and discuss the concept of the monolithically integrated minimodule.

A simultaneously diffused, textured, in situ oxide AR-coated solar cell process (STAR process) for high-efficiency silicon solar cells

A novel device fabrication process called the STAR process is presented, which incorporates a Simultaneously diffused emitter and Back Surface Field (BSF), on a textured silicon wafer, with an in situ thermal oxide for surface passivation and Anti-Reflection (AR) coating. In a single high-temperature step, the STAR process provides four important quality-enhancement features: (1) emitter oxide passivation, (2) back surface passivation via a boron back surface field, (3) a low reflectance (SiO/sub 2/) single layer AR coating, and (4) a back surface reflector (BSR) for light trapping. The STAR process is implemented using a novel diffusion technique which can simultaneously form boron and phosphorus diffusions and grow an in situ thermal oxide in a conventional diffusion furnace, without the deleterious effects of cross doping. Conversion efficiencies as high as 20.1% have been obtained for this structure on 2.0 /spl Omega//spl middot/cm float zone silicon. This paper presents a detailed characterization of the impact of each of the above quality enhancement features, using a combination of an extended IQE analysis, minority carrier lifetime measurements, and measurements of the emitter saturation current density J/sub oe/. Computer simulations are used to improve the understanding of STAR cells and show that the STAR process is capable of producing device efficiencies over 19% on thin, relatively modest quality, solar grade silicon materials.

Multijunction InGaAs Thermophotovoltaic Devices

AbstractA monolithic interconnected module (MIM) structure has been developed for thermophotovoltaic (TPV) applications. The MIM consists of many individual InGaAs cells series-connected on a single semi-insulating (S.I.) InP substrate. An infrared (IR) back surface reflector (BSR), placed on the rear surface of the substrate, returns the unused portion of the TPV radiator output spectrum back to the radiator for recuperation, thereby providing for high system efficiencies. MIMs were fabricated with an active area of 0.9×l cm, and with 15 cells monolithically connected in series. Both lattice-matched and lattice-mismatched InGaAs/InP devices were fabricated, with bandgaps of 0.74 and 0.55 eV, respectively. The 0.74 eV MIMs demonstrated an open-circuit voltage (Voc) of 6.16 V and a fill factor of 74.2% at a short-circuit current (Jsc) of 0.84 A/cm2, under flashlamp testing. The 0.55 eV MIMs demonstrated a Voc of 4.85 V and a fill factor of 57.87percnt; at a Jsc of 3.87 A/cm2 Electrical performance results for these MIMs are presented.

A Thin-Film Silicon Solar Cell: Design and Processing Approach

AbstractWe present a new device structure for a high efficiency, thin-film, silicon solar cell. A preliminary design and an approach for fabrication of such a cell are discussed. The cell structure uses interface texturing and a back surface reflector for effective light trapping. A theoretical analysis is applied to determine the major parameters of the cell. These analyses indicate that a cell efficiency of about 18% is attainable with a Si film thickness of 10–15 μm, and grain size of about 50 μm. A method for making a large-grain thin cell is proposed.

Measurement of solar cell parameters using impedance spectroscopy

Measurement of solar cell parameters is important for the design of satellite power systems. These parameters can be measured using impedance spectroscopy and an equivalent circuit model developed. In this study parameters of a Back Surface Reflector Field solar cell (BSFR) have been measured using impedance spectroscopy. The results show high diffusion capacitance of BSFR cells and their exponential relation to the operating voltage. Cell dynamic resistance, diode factor, transition capacitance, and series resistance could also be measured. The minority carrier life time also has been calculated.

Characterization of photon recycling in thin crystalline GaAs light emitting diodes

Gallium arsenide light emitting diodes (LEDs) were fabricated using molecular beam epitaxial films on GaAs substrates and removed by epitaxial lift-off (ELO). Lifted off devices were then mounted on a Si wafer using a Pd/Au/Cr contact layer, which also served as a back surface reflector. Devices were characterized by electrical and optical measurements, and the results for devices on the GaAs substrate were compared to those for ELO devices. ELO LEDs coated with a ZnS/MgF2 antireflection coating exhibited an optical output that was up to six times that of LEDs on GaAs substrates. At the same time, the measured current-voltage characteristics of the ELO devices displayed a lower n=1 current component. ELO LEDs with efficiencies up to 12.5% were realized. We attribute these results to photon recycling enhanced by the back-surface reflector in the ELO LEDs. The luminescence versus current and current versus voltage characteristics of the LEDs were analyzed to obtain the nonradiative minority carrier lifetimes and the photon recycling factors. The results demonstrate that the measured characteristics are well described by photon recycling theory. ELO LEDs may prove useful for characterizing recombination processes in LEDs, and thin-crystalline structures could provide substantial efficiency enhancements for LEDs and solar cells.

A new texturing geometry for producing high efficiency solar cells with no antireflection coatings

Abstract Various schemes to trap weakly absorbed light into solar cells have been proposed. These schemes include texturing the cell, texturing the cover glass and geometric arrangements of the individual cells. The perpendicular slats geometry is considered to be the best cell texturing design for light trapping. In this paper a new cell surface texturing design is proposed which, without the use of anti-reflection coatings, can outperform the perpendicular slats geometry with a double layer anti-reflection coating by virtue of efficient internal light trapping and a decrease in the front surface reflectance. The single sided texture uses three perpeendicular planes on the front surface and a planar back surface. The three perpendicular planes provide a triple bounce for the incoming light and efficient confinement for light which has entered the cell. TEXTURE, a raytracing program for textured cells, was used to predict the performance of this new design. A quantitative comparison with other texturing schemes is also provided. It is shown that for a cell without an anti-reflection coating on the front and a 98% effective back surface reflector, the new design produces a maximum short circuit current density of 40.99 mA/cm2 as compared to 41.46 mA/cm2 and 35.16 mA/cm2 for the perpendicular slats geometry and flat surfaces, respectively, with a conventional single layer AR coating on the front. Effects of different front surface reflection coefficients are examined to show that as the front reflectance is decreased by improved antireflection coatings, the importance of the triple bounce is reduced and most promising surface texturing schemes approach the same value of maximum current.

Optimizing the radiation resistance of InP solar cells: Effect of dopant density and cell thickness

The effect of 1 MeV electron and 10 MeV proton irradiations on the electrical properties of high-efficiency InP solar cells grown by metalorganic chemical-vapor deposition has been measured and analyzed. The base dopant levels ranged from 3×1015 to 2×1017 cm−3, and the effect of the dopant concentration on the radiation response is presented and analyzed. The radiation-induced degradation of the solar cells was measured through both dark and 1 sun air mass zero current-voltage measurements and through spectral response measurements. The radiation-induced defects in the solar-cell junction were monitored by deep-level transient spectroscopy. Thermal and photoinjection annealing experiments were performed on the irradiated cells. The highest postirradiation efficiencies measured after a fluence of 1016, 1 MeV electrons cm−2 were 11.6% which is higher than has been reported for any other solar-cell material. Although all of the cells showed high radiation resistance, the cells with a lightly doped base (essentially an n-i-p structure after irradiation) degraded more in the open-circuit voltage and less in the short-circuit current than the more heavily doped cells. Modeling indicates that the thickness of the junction and base layers is very important in determining the end of life efficiency of the cell. Using the measured data, efficiencies up to 14.1% are predicted with a thin base and a back surface reflector, following irradiation with 1016 1 MeV electrons cm−2.

Influence of surface texturization on the light trapping and spectral response of silicon solar cells

A quantitative model that explains the spectral response, internal quantum efficiency, total short-circuit current, open-circuit voltage, and efficiency of high-efficiency solar cells with textured front surface and Lambertian back-surface reflectors is presented. A comparison of the textured cell characteristics is made with those of planar cells, and the separate roles of the front surface reflection coefficient and internal quantum efficiency in enhancing the short-circuit current have been investigated. It is shown that, in the case of large diffusion lengths, almost all the contribution to the increase of spectral response on texturization is due to the reduced reflection coefficient whereas, for small diffusion lengths, there is a significant increase in internal quantum efficiency on texturization, especially in the region of higher wavelengths. However, there is a small decrease in open-circuit voltage for large diffusion lengths, whereas no significant change is observed for small diffusion lengths on texturization. Nevertheless, there is a net gain in power conversion efficiency which is larger for smaller diffusion lengths. >

The space performance of silicon vertical junction solar cells on the LIPS-III satellite

The authors present the preliminary analysis and results of a space experiment to evaluate the performance of a new generation of silicon vertical junction solar cells and three adhesives for attaching coverglass to the solar cells. Two of the adhesives are used for the first time in coverglass applications in the space environment. The SOLAREX vertical junction solar cells, which are of 10- Omega -cm silicon with back surface fields (BSFs) and back surface reflectors (BSRs) are compared to SOLAREX planar junction cells, also of 10- Omega -cm, BSF, BSR silicon. The results up to 386 days in space indicate that the two types of solar cell show about the same degradation rate in power output. There are no significant differences at this point in the performances of the three different adhesives.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Textured back-surface reflectors for thin-film solar cells

A textured interface between α-Si:H and silver is modeled as a periodic corrugation by using the exact extinction theorem. Light is reflected from such a surface into grating modes, which may be either propagating or evanescent. Deeper textures place the reflected light preferentially into modes of higher-order number. The higher-order propagating modes are absorbed with increasing strength by the α-Si:H. The evanescent modes have effective extinction coefficients that may be many orders of magnitude greater than the ‘‘bulk’’ value for α-Si:H. Numerical results suggest the texture dimensions that would most increase the long-wavelength solar cell efficiency. The surface-plasmon polariton dispersion relation is calculated for the interface between α-Si:H and silver. This is used to interpret published photoconductivity data for α-Si:H backed by textured silver.

Testing of materials for solar power space applications

This paper summarizes the results of a program initiated at the Naval Research Laboratory to test conventional and state-of-the-art solar power space systems by flying them aboard satellites. The program (approximately nine years in duration) confirmed the practicality of improvements in advanced silicon solar cells such as textured surfaces, shallow junctions, back surface field and back surface reflector techniques, as well as novel methods of bonding coverslips to conventional cells. In addition, the performance of gallium aluminum arsenide solar cells first tested in a space environment and demonstrated to be satisfactory. Finally, advanced silicon cells such as lithium-diffused and vertical junction cells, which were reported to be radiation resistant on the basis of measurements in the laboratory, were found unsuitable for extended space application.