Influence of perimeter recombination on high-efficiency GaAs p/nheteroface solar cells

Perimeter recombination currents have been characterized for 0.5-cm-square and 2-cm-square p/n GaAs solar cells. Measurements show that perimeter recombination dominates the n=2 dark current component of these high-efficiency solar cells. The results also suggest that perimeter recombination will be substantial even in much-larger-area solar cells. Although little influence on open-circuit voltage is expected, perimeter recombination may adversely affect the cell's one-sun fill factor. Because of its importance to one-sun applications, recombination at the junction perimeter must be suppressed before GaAs solar cells approach their limiting conversion efficiencies     

Ultrathin, high-efficiency solar cells made from GaAs films prepared by the CLEFT Process

Solar cells with conversion efficiencies as high as 17% at AM1 have been fabricated from single-crystal 10-µm-thick GaAs films prepared by the CLEFT process. These cells are the first devices to employ CLEFT films. In making a cell, a GaAs film with an n⁺/p/p⁺shallow-homojunction structure is grown by vapor-phase epitaxy on a specially masked single-crystal GaAs substrate, then transferred to a glass substrate that serves as the cell cover glass. The GaAs substrate can be reused repeatedly for preparing additional CLEFT films.     

Emitter design for high-efficiency silicon solar cells. Part 2: Space cells

Over the last decade there have been substantial improvements in the performance of silicon solar cells. This second part of a two-part paper examines the implications for use in space. High-efficiency cells, including cells of above 20% Air Mass 0 efficiency were exposed to 1 MeV of electron irradiation. Although the relative performance loss was higher, the cells gave higher performance than conventional silicon space cells even after 5 × 10¹⁵ cm⁻² 1-MeV electron radiation damage. However, spectral response measurement shows that the rapid degradation mainly came from damage on the emitter surface. This can be accommodated simply by shallow emitter designs. the efficiency of an optimized silicon space cell is expected to be over 14% after 1 × 10¹⁵ cm⁻² 1 MeV electron radiation.     

Emitter design for high-efficiency silicon solar cells. Part I: Terrestrial cells

Emitter diffusions for conventional high-efficiency silicon solar cells have commonly been designed to be very shallow to improve the short-wavelength response of the cells. This first part of a two-part paper has a substantial tutorial content and analyses the effect of the emitter design on cell performance with different cell surface passivation conditions. the analysis shows that shallow emitters are only necessary for cells with poor surface passivation. In contrast, high-efficiency cells with good surface passivation do not necessarily need shallow emitters. the application of these design insights into recent generations of high-efficiency passivated emitter solar cells is also discussed.     

Analysis of high-efficiency silicon solar cells

Performance data on high-efficiency concentrator (>18 percent at 25 suns) silicon solar cells are compared to results from an exact numerical model in which all parameters for the calculation are taken from the existing literature on bulk silicon. The numerical solution of the transport equations includes the effects of Fermi-Dirac statistics, bandgap narrowing, and Auger recombination. Cell performances as a function of sunlight concentration are predicted with reasonable accuracy using this model. Evidence for the existence of bandgap-narrowing effects is found by comparing experimental data to calculated values of spectral quantum efficiencies and open-circuit voltages under a variety of lifetime assumptions. The validity of using superposition with simple diode equations to approximate the behavior of silicon solar cells also is examined.     

GaAs太阳电池辐照效应的研究现状

文章介绍了ＧａＡｓ太阳电池辐照效应的研究状况，和间述了研究中常用的方法及重要的结论。现有的研究结果表明，ＧａＡｓ太阳电池性能的退化与辐照粒子的能量、剂量及入射方式相关，不同种类的粒子引起的损损伤很不一样。通过退火处理辐照损伤后的太阳电池，其性能可得到部分恢复。     

Mixed-mode simulation of DX trap-induced slow transient effects onAlGaAs/GaAs HEMT inverters

A mixed level device and circuit simulation was performed to analyze DX trap-induced slow transient effects on the performance degradation of AlGaAs/GaAs high electron mobility transistor (HEMT) circuits. The variation of the output pulsewidth and the hysteretic characteristics of the input-output voltage transfer function in direct-coupled FET logic (DCFL) HEMT inverters have been simulated. In the model, a DX trap rate equation is calculated in the AlGaAs layer. The self-consistent Schrodinger and Poisson equations are solved numerically at each cross section of a device. A two-region Grebene-Ghandhi model is used to derive the I-V characteristics. The simulation confirms that the output pulse broadening and narrowing effects in string cascaded DCFL HEMT inverters are a consequence of the inverter voltage transfer function shift caused by deep traps     

Theoretical effects of surface diffused region lifetime models on silicon solar cells

A computer simulation of silicon solar cells has indicated that the combination of band gap reduction due to heavy doping and certain spatial forms of lifetime dependence can combine to form severe limitations to the open circuit voltage of silicon solar cells. The interaction of these effects tends to shift the active region of the diffused surface layer away from the injecting junction resulting in an increase in the current density injected into the surface region. Reductions in open circuit voltage as great as 10% over models which do not include these effects can be seen.     

High-efficiency GaAs solar cells

An updated review of the state of the art in the development of GaAs solar cells is provided, with emphasis on AlGaAs-GaAs cells suitable for space applications. A set of theoretically derived characteristics is given for this type of solar cell. Comparison of measured performance with theory shows excellent agreement. Data on the effects of radiation damage (high-energy electrons, protons, and neutrons) is also integrated into a form useful for evaluation purposes. Techniques for fabricating (AlGa)As-GaAs solar cells in quantities large enough for practical applications are discussed and are shown to have been demonstrated. The possibility of extending these techniques to the fabrication of very thin low-weight cells for space applications is also considered. Finally, the results obtained to date in the development of GaAs solar cells for applications requiring concentrated sunlight are reviewed, for terrestrial as well as for space applications. As a milestone toward the practical application of AlGaAs-GaAs solar cells in space systems, a brief account is provided on the development status of small experimental AlGaAs-GaAs solar-cell panels for specific space flights.     

A MBE-grown high-efficiency GaAs solar cell with a directlydeposited aluminum front contact

A novel metallization scheme for GaAs p-n solar cells grown by molecular beam epitaxy (MBE) is described. A p⁺-GaAs contact layer was grown on top of the AlGaAs window layer, followed by a pure aluminum layer. This MBE-grown aluminum layer serves both as metallization and as self-aligned mask for selective etching down to the AlGaAs window. The solar cell showed an efficiency of about 20%; the I-V characteristics revealed that negligible series resistance was present in the structure. To test the novel p-contact a second sample was grown. Using a transmission-line model (TLM) structure, a metal-semiconductor contact resistance of 1.5×10^-2 Ω-cm² was found     

Simulation of the real efficiencies of high-efficiency silicon solar cells

The temperature dependences of the efficiency η of high-efficiency solar cells based on silicon are calculated. It is shown that the temperature coefficient of decreasing η with increasing temperature decreases as the surface recombination rate decreases. The photoconversion efficiency of high-efficiency silicon-based solar cells operating under natural (field) conditions is simulated. Their operating temperature is determined self-consistently by simultaneously solving the photocurrent, photovoltage, and energy-balance equations. Radiative and convective cooling mechanisms are taken into account. It is shown that the operating temperature of solar cells is higher than the ambient temperature even at very high convection coefficients (~300 W/m² K). Accordingly, the photoconversion efficiency in this case is lower than when the temperature of the solar cells is equal to the ambient temperature. The calculated dependences for the open-circuit voltage and the photoconversion efficiency of high-quality silicon solar cells under concentrated illumination are discussed taking into account the actual temperature of the solar cells.     

GaAs Shallow-homojunction solar cells on Ge-coated Si substrates

Solar cells with conversion efficiencies of 12% (AM1) have been fabricated from single-crystal GaAs epilayers grown by CVD on Ge-coated Si substrates. The cells utilize an n⁺/p/p⁺shallow-homo junction GaAs structure on a thin (<0.2 µm) epitaxial Ge layer. These solar cells are the first reported GaAs devices fabricated on Si substrates.     

Proton damage in GaAs solar cells

A simplified model for the short-circuit current reduction caused by proton-induced radiation damage is described. The model accounts for the nonuniformity of defect production within heteroface GaAs shallow junction solar cells. The results from the model show agreement with the strong energy dependence observed in proton radiation damage experiments.     

The effects of a ground shield on the characteristics andperformance of spiral inductors

The frequency dependence of the model parameters of patterned ground shield (PGS) inductors in large part is explained as a consequence of modeling a distributed system with a lumped model. The effects of PGS shape and material on inductor characteristics have been examined and explained. There is an optimum area for a PGS to maximize Q. Using an n⁺ buried/n-well PGS, the peak Q is improved by ~25% from that of an inductor without a PGS while only slightly changing L and C_p in comparison to inductors with other PGSs. Having a PGS does not significantly improve isolation between adjacent inductors when isolation is limited by magnetic coupling since a PGS is specifically designed to limit termination of magnetic fields     

Theory of transient photovoltaic effects used for measurement of lifetime of carriers in solar cells

Transient photovoltaic effects are frequently used for measurement of lifetime of the carriers in solar cells. A detailed theoretical analysis of transient photocurrent, photovoltage and photoconductivity of a solar cell is presented, considering the actual absorption coefficient of light and a finite surface recombination velocity. Application of the theory to the measurement of lifetime of the carriers in discussed. Decay time of photocurrent for a very thin cell comes out to be independent of the lifetime of the carriers but is found to depend on its thickness. This is in agreement with the experimental observation of earlier workers.     

Probability of large solar proton events and their effect on the efficiency of GaAs solar cells

Using a recently updated database that includes the last four complete solar cycles, the probability of occurrence of large solar proton events has been analyzed for time periods of up to seven active solar years using extreme value statistics. Then an approach is presented for evaluating solar cell efficiency degradation caused by the large event. These results enable spacecraft designers to evaluate precisely the risk of exposure of solar arrays to large solar proton events over the course of a mission. As an example, the efficiency degradation of n/p GaAs/Ge solar cells caused by an event with a >10-MeV fluence of 1×10¹⁰cm⁻² is shown. This is compared to the degradation resulting from all events in the last solar cycle, and to the electron environment in geostationary orbit over the same period of time. Copyright © 1998 John Wiley & Sons, Ltd.     

Modelling of perimeter recombination in GaAs solar cells

To investigate perimeter recombination current in heteroface GaAs solar cells, two models were proposed; the first concerned the analysis of recombination at the surface that intersects the space-charge layer and the second dealt with recombination at the quasi-neutral region. Recombination at the depleted layer surface has a 2kT character and was treated in a similar way to that of the bulk, using the model of Sah, Noyce and Shockley. The electric field at the surface due to Fermi level pinning is different from that of the bulk. We suggested a simple model to obtain an analytical form of the perimeter current at the space-charge region surface that yielded values of the product of the characteristic length by the surface recombination velocity (L_sS₀) that agreed well with experimental values. The recombination current outside the space-region is of two dimensional nature and has a kT behaviour, the model adopted consisted mainly of solving numerically the bidimensional continuity equation. An effective recombination velocity was introduced to account for bend bending caused by the charged surface states. As the ratio of perimeter to area (P/A) is increased the perimeter current acquired significant proportions, thus the expected 2kT current due to bulk deep levels existing in the depletion layer is two to three orders of magnitude too small to account for.     

GaAs solar cells grown on Si substrates for space use

GaAs single‐junction and InGaP/GaAs multi‐junction thin‐film solar cells fabricated on Si substrates have great potential for high‐efficiency, low‐cost, lightweight and large‐area space solar cells. Heteroepitaxy of GaAs thin films on Si substrates has been examined and high‐efficiency GaAs thin‐film solar cells with total‐area efficiencies of 18·3% at AM0 and 20·0% at AM 1·5 on Si substrates (GaAs‐on‐Si solar cells) have been fabricated. In addition, 1‐MeV electron irradiation damage to GaAs‐on‐Si cells has been studied. The GaAs‐on‐Si cells are found to show higher end‐of‐life efficiency than the conventional GaAs cells fabricated on GaAs substrates (GaAs‐ on‐GaAs cells) under high‐fluence 1‐MeV electron irradiation of more than 1 × 10¹⁵ cm⁻². The first space flight to make use of them has been carried out. Forty‐eight 2 × 2 cm GaAs‐on‐Si cells with an average AM0 total‐area efficiency of 16·9% have been evaluated in the Engineering Test Satellite No.6 (ETS‐VI). The GaAs‐on‐Si cells have been demonstrated to be more radiation‐resistant in space than GaAs‐on‐GaAs cells and 50, 100 and 200‐μm‐thick Si cells. These results show that the GaAs‐on‐Si single‐junction and InGaP/GaAs‐on‐Si multi‐junction cells have great potential for space applications. Copyright © 2001 John Wiley & Sons, Ltd.     

GaAs/InGaAsN heterostructures for multi-junction solar cells

Solar-cell heterostructures based on GaAs/InGaAsN materials with an InAs/GaAsN superlattice, grown by molecular beam epitaxy, are studied. A p-GaAs/i-(InAs/GaAsN)/n-GaAs p–i–n test solar cell with a 0.9-μm-thick InGaAsN layer has an open-circuit voltage of 0.4 V (1 sun, AM1.5G) and a quantum efficiency of >0.75 at a wavelength of 940 nm (at zero reflection loss), which corresponds to a short-circuit current of 26.58 mA/cm² (AM1.5G, 100 mW/cm²). The high open-circuit voltage demonstrates that InGaAsN can be used as a material with a band gap of 1 eV in four-cascade solar cells.     

Comprehensive models for the analysis of high-efficiency GaAs IMPATT's

High-efficiency GaAs IMPATT's have several distinctive characteristics which have been the subject of considerable theoretical investigation. In general, the approach has been to isolate a particular physical mechanism and consider its effect on the device. In this paper, the interaction between all these effects is discussed in terms of comprehensive models. Accurate numerical results are derived from a full computer simulation of the device. These results provide a number of new insights into device operation, especially the importance of thermal effects. An overall assessment of the numerical results leads to the development of a graphical construction which incorporates the most important physical effects in one simple diagram. This enables a rapid and accurate estimate of the effects of structure, temperature, current density, operating frequency, and RF voltage on the operation of the device. Finally, the models described are used to derive a design procedure for a high-efficiency structure.