The effect of damaging radiation (<Emphasis Type="Italic">p,e, γ</Emphasis>) on photovoltaic and tunneling GaAs and GaSb <Emphasis Type="Italic">p-n</Emphasis> junctions

The properties of multiple-junction solar cells depend on the properties of the constituent photovoltaic and tunneling p-n junctions. In this study, the properties of the space-charge region for photovoltaic and tunneling p-n junctions were examined using the dark current-voltage characteristics for two semiconductors: GaSb (a narrow-gap semiconductor) and GaAs (a wide-gap semiconductor). The effects of irradiation with protons (the energy of 6.78 MeV and the maximum fluence of 3 × 10¹² cm^?2), electrons (the energy of 1 MeV and the maximum fluence of 3 × 10¹⁶ cm^?2), and γ-ray photons (the energy of 1.17–1.33 MeV and the maximum dose of 17 Mrad) on the lifetime of charge carriers in the space-charge region of photovoltaic p-n junctions and on the peak current of connecting tunneling p-n junctions were studied. The coefficients of the damage for the inverse lifetime are determined for photovoltaic p-n junctions. The coefficients of equivalence between the used types of radiation are determined; these coefficients are found to be almost independent, on the order of magnitude, of the type and material of the p-n-junction (and nearly equal for photovoltaic GaAs p-n junctions and tunneling GaAs and GaSb p-n junctions).     

Nitrogen Plasma Doping of Single-Crystal ZnTe and CdZnTe on Si by MBE

We have investigated in situ p-type doping of ZnTe and CdZnTe on Si(211) by molecular beam epitaxy using a radiofrequency (RF)-nitrogen plasma source for application to multijunction II–VI-based solar cells. CdZnTe would be used as a wide-gap top cell in a monolithic multijunction device, and ZnTe or CdZnTe could be used for the p-side of tunnel junctions. Highly p-type material is required for producing the high-quality tunnel junctions crucial for maintaining current flow, and p-doping of order 10¹⁷ cm^?3 is required for the generation of a large built-in potential in the absorber region of solar cells. Our uniformly doped films exhibited good Hall characteristics, especially considering the large lattice mismatch between Si and either ZnTe or CdZnTe. Crystal quality was examined by x-ray diffraction. Nitrogen incorporation was examined as a function of the source-gas dilution with argon. A sample with layers of CdZnTe doped using 1% to 100% nitrogen was grown on nominally undoped CdZnTe and analyzed using secondary-ion mass spectrometry. The nitrogen incorporation differed by only a factor of 10, despite the factor of 100 difference in the nitrogen concentration in the plasma, indicating a saturation effect.     

Current flow and potential efficiency of solar cells based on GaAs and GaSb p-n junctions

Dependence of the efficiency of single-junction and multijunction solar cells on the mechanisms of current flow in photoactive p-n junctions, specifically on the form of the dark current-voltage characteristic J-V, has been studied. The resistanceless J-V _j characteristic (with the series resistance disregarded) of a multijunction solar cell has the same shape as the characteristic of a single-junction cell: both feature a set of exponential portions. This made it possible to develop a unified analytical method for calculating the efficiency of singlejunction and multijunction solar cells. The equation relating the efficiency to the photogenerated current at each portion of the J-V _j characteristic is derived. For p-n junctions in GaAs and GaSb, the following characteristics were measured: the dark J-V characteristic, the dependence of the open-circuit voltage on the illumination intensity P-V _OC, and the dependence of the luminescence intensity on the forward current L-J. Calculated dependences of potential efficiency (under idealized condition for equality to unity of external quantum yield) on the photogenerated current for single-junction GaAs and GaSb solar cells and a GaAs/GaSb tandem are plotted. The form of these dependences corresponds to the shape of J-V _j characteristics: there are the diffusion- and recombination-related portions; in some cases, the tunneling-trapping portion is also observed. At low degrees of concentration of solar radiation (C < 10), an appreciable contribution to photogenerated current is made by recombination component. It is an increase in this component in the case of irradiation with 6.78-MeV protons or 1-MeV electrons that brings about a decrease in the efficiency of conversion of unconcentrated solar radiation.     

Characteristic features of silicon multijunction solar cells with vertical p-n junctions

A relatively simple technology (without photolithography) based on diffusion welding and ion-plasma deposition of an insulating coating has been developed for fabricating multijunction silicon solar cells with vertical p-n junctions. The effective collection factor for such structures is independent of the wavelength of the incident light in the wavelength range λ=340–1080 nm. Fiz. Tekh. Poluprovodn. 31, 855–857 (July 1997)     

Features of physical differentiation with respect to light absorbance in junction photovoltage spectra

Junction-photovoltage pleochroism is studied in crystalline silicon under the conditions of a conductivity anisotropy induced by a uniaxial compressive strain. Polarization modulation of light has been used: the samples are excited by linearly polarized light, with the polarizations periodically alternating with respect to the optical axis. The spectral characteristics obtained in such a way represent the polarization difference of the photovoltages, which depends on the light absorbance. A heavy dependence of the spectrum shape on the type of p-n junctions, which differ in relation to base parameters and emitter technologies, is detected. An analysis of the spectra shows that the condition of physical differentiation with respect to absorbance is satisfied only in p-n junctions with a negligible space-charge thickness.     

High-efficiency GaSb photocells

High-current solar cells based on gallium antimonide and intended for use in solar modules and systems with solar-spectrum splitting at large solar light concentration ratios, in thermophotovoltaic generators with a high-temperature emitter, and in laser energy converters have been designed and fabricated by the diffusion of zinc from the gas phase. The influence exerted by the thickness of the p ⁺ diffusion layer on the basic characteristics of the solar cell has been studied. The optimal doping profile and the p-n-junction depth providing a high photovoltaic conversion efficiency at photocurrent densities of up to 100 A cm^?2 have been determined.     

Phase transformations during the Ag-In plating and bonding of vertical diode elements of multijunction solar cells

The conditions of the bonding of silicon multijunction solar cells with vertical p-n junctions using Ag-In solder are studied. The compositions of electrodeposited indium films on silicon wafers silver plated by screen printing and silver and indium films fabricated by layer-by-layer electrochemical deposition onto the surface of silicon vertical diode cells silver plated in vacuum are studied. Studying the electrochemical-deposition conditions, structure, and surface morphology of the grown layers showed that guaranteed bonding is provided by 8-min heat treatment at 400°C under the pressure of a stack of metallized silicon wafers; however, the ratio of the indium and silver layer thicknesses should not exceed 1: 3. As this condition is satisfied, the solder after wafer bonding has the InAg₃ structure (or InAg₃ with an Ag phase admixture), due to which the junction melting point exceeds 700°C, which guarantees the functioning of such solar cells under concentrated illumination.     

Germanium subcells for multijunction GaInP/GaInAs/Ge solar cells

Photovoltaic converters based on n-GaInP/n-p-Ge heterostructures grown by the OMVPE under different conditions of formation of the p-n junction are studied. The heterostructures are intended for use as narrow-gap subcells of the GaInP/GaInAs/Ge three-junction solar cells. It is shown that, in Ge p-tn junctions, along with the diffusion mechanism, the tunneling mechanism of the current flow exists; therefore, the two-diode electrical equivalent circuit of the Ge p-n junction is used. The diode parameters are determined for both mechanisms from the analysis of both dark and “light” current-voltage dependences. It is shown that the elimination of the component of the tunneling current allows one to increase the efficiency of the Ge subcell by ∼1% with conversion of nonconcentrated solar radiation. The influence of the tunneling current on the efficiency of the Ge-based devices can be in practice reduced to zero at photogenerated current density of ∼1.5 A/cm² due to the use of the concentrated solar radiation.     

The dislocation origin and model of excess tunnel current in GaP p-n structures

An excess tunnel current in GaP epitaxial nondegenerate p-n junctions on GaP and Si substrates was studied. An important experimental result is that the slope of exponential current-voltage (I–V) characteristic (in lnI–V coordinates) is independent of the width of the space-charge region, i.e., on n-and p-region doping levels. This fact is unexplained by existing models. A dislocation shunt model based on multihop tunneling through a dislocation line, which may be considered as a chain of parabolic potential barriers, is proposed. The density of dislocations predicted by this model is in agreement with the transmission electron microscopy (TEM) observations.     

Thermophotovoltaic cells based on In0.53Ga0.47As/InP heterostructures

Reflection of infrared radiation from n-InP substrates with a rear MgF₂/Au mirror is investigated in the wavelength range 1000–2200 nm. It is found that the reflectance weakly depends on substrate thickness and free-carrier concentration in the (0.1–6) × 10¹⁸ cm^?3 range. Thermophotovoltaic cells based on the InP/In_0.53Ga_0.47As lattice-matched heterostructure of p-n and n-p are fabricated by liquid-phase epitaxy and Zn and P diffusion from a gas phase. The characteristics of p-n and n-p thermophotovoltaic cells with an identical configuration of the contacts of 1 cm² area are determined. These characteristics are the open-circuit voltage U _oc = 0.465 V, the filling factor FF = 64% at the current density of 1 A/cm², and the reflectance R = 76–80% for wavelengths longer than 1.86 μm.     

Design of multijunction GaPNAs/Si heterostructure solar cells by computer simulation

The designs of two- and three-junction solar cells based on GaPNAs/Si lattice-matched hetero-structures are calculated. It is shown that the efficiency of two-junction solar cells constituted by a junction based on a GaPNAs solid solution with a band gap E _g of 1.78 eV and a junction based on Si may reach a value of 30.3% under AM1.5 D, 100 mW/cm², and 35.4% under AM1.5D, 20 W/cm². The maximum values of the efficiency of the three-junction solar cell constituted by top and middle junctions based on GaPNAs with E _g of 2 and 1.5 eV, respectively, and a Si-based bottom junction are 39.2% under AM1.5 D, 100 mW/cm², and 44.5% under AM1.5D, 20 W/cm². It is shown that the thickness and minority carrier lifetime of the photoactive layers affect the efficiency of solar-light conversion by the heterostructures being developed.     

High-efficiency dual-junction GaInP/GaAs tandem solar cells obtained by the method of MOCVD

Monolithic dual-junction GaInP/GaAs solar cells grown by the MOCVD method were studied. The conditions of the growth of ternary Ga _x In_1?x P and Al _x In_1?x P alloys lattice-matched to GaAs are optimized. Technology for fabrication of a tunneling diode with a high peak current density of 207 A/cm² on the basis of heavily doped n ⁺⁺-GaAs:Si and p ⁺⁺-AlGaAs:C layers is developed. Cascade GaInP/GaAs solar cells obtained as a result of relevant studies featuring a good efficiency of the solar-energy conversion both for space and terrestrial applications. The maximum value of the GaInP/GaAs solar-cell efficiency was 30.03% (at AM1.5D, 40 suns).     

Electroluminescence in the region of interband transitions in a high-efficiency silicon light-emitting diode with a small area of the rectifying contact

Electroluminescence in the region of interband transitions is studied in a Si diode at room temperature with the densities of current (J) as high as 36 kA/cm² through the p-n junction with an area of 0.008 mm². The kinetics of the electroluminescence decay is represented by exponential function with the time constant τ_d ≈ 35 μs in the linear portion of the dependence of the electroluminescence intensity on current. At J > 2.5 kA/cm², the current dependence of the electroluminescence intensity becomes sublinear, while the decay kinetics is described by the exponential function with the same τ_d, however, only after the initial portion of a more rapid decay of electroluminescence. The specific features observed at J > 2.5 kA/cm² can be related to an appreciable contribution of the Auger recombination (in addition to the Shockley-Read-Hall mechanism) to nonradiative recombination. It is shown that it is feasible to fabricate Si-based light-emitting diodes with the emission power as high as ～50 mW at the emitting-surface area of about 6 mm².     

Generation of microwave oscillations in a no-base diode

The GHz-frequency microwave oscillations of voltage in a no-base p ⁺-p-n ⁺ silicon diode driven by reverse current with a pulse duration of ～300 ns and a current density of several kA/cm² were experimentally observed for the first time. The mechanism of initiation of these oscillations was theoretically considered. The frequency and the modulation percentage of the microwave oscillations were shown to depend on the current density and dopant-concentration gradient in the p-n-junction plane.     

On the theory of anomalous photovoltage in multilayer structures with p-n junctions

Steady-and nonsteady-state photovoltages appearing in a multilayer structure with p-n junctions under the conditions of nonuniform illumination are considered for an arbitrary ratio between the diffusion length L and the sizes d of the p-and n-regions. It is shown that, for , the photovoltage is substantially lower (by d ²/12L ² times) than in the case of owing to the mutual influence of neighboring p-n junctions. Relaxation of the photovoltage is controlled by recharging of the barrier capacitances of p-n junctions, and the relaxation time exceeds by several orders of magnitude the lifetime of nonequilibrium charge carriers in the p-and n-regions. The results obtained make it possible to explain the special features of the effect of anomalous photovoltage in polycrystalline films.     

Effect of bremsstrahlung γ-ray photons and neutrons on the parameters of indium-selenium photoconverters

The effect of Bremsstrahlung γ-ray photons and neutrons (effective energy E _eff = 8 MeV) with fluences of 10¹²–10¹³ neutron/cm² on the electrical and photoelectric parameters of layered p-n-InSe is studied for the first time. Even at the highest radiation fluence, an improvement in the current-voltage characteristics and an increase in the open-circuit voltage at an insignificant decrease in the short-circuit current are observed. Significant variations in the spectral curve of the photoresponse are not detected in general. At the same time, even the initial stage of irradiation gives rise to a pronounced degradation of the parameters of the test silicon solar cells. This fact makes it possible to recommend the photodiodes under study for use as radiation-resistant photodetectors.     

Highly conductive p + + ‐AlGaAs/n + + ‐GaInP tunnel junctions for ultra‐high concentrator solar cells

Tunnel junctions are key for developing multijunction solar cells (MJSC) for ultra‐high concentration applications. We have developed a highly conductive, high bandgap p ^{+ +} ‐AlGaAs/n ^{+ +} ‐GaInP tunnel junction with a peak tunneling current density for as‐grown and thermal annealed devices of 996 A/cm ² and 235 A/cm ², respectively. The J–V characteristics of the tunnel junction after thermal annealing, together with its behavior at MJSCs typical operation temperatures, indicate that this tunnel junction is a suitable candidate for ultra‐high concentrator MJSC designs. The benefits of the optical transparency are also assessed for a lattice‐matched GaInP/GaInAs/Ge triple junction solar cell, yielding a current density increase in the middle cell of 0.506 mA/cm ² with respect to previous designs. Copyright © 2014 John Wiley & Sons, Ltd.     

Tunneling recombination in silicon avalanche diodes

Distribution of the tunneling-recombination current over the space-charge region in a p-n junction was simulated mathematically. It is shown that the recombination rate saturates if the probability of tunneling is low. An expression for current-voltage characteristics of the p-n junction in the case of tunneling recombination is derived. The current-voltage characteristics of silicon avalanche diodes containing dislocations were studied experimentally. The results of numerical calculations based on the tunneling-recombination model are consistent with experimental data.     

Interface states characterization in heterojunction solar cells from CV-GV measurements and modeling

A new method is proposed to extract interface states density D_it at the hydrogenated amorphous/crystalline silicon interfaces (aSi:H/cSi) of heterojunction solar cells - HET. This technique based on CV and GV measurements consists in adapting standard electrical D_it models for MOS structures to the specific case of HET solar cells. In particular, a parasitic conductance is introduced to account for the high leakage current of the diode in the forward regime. The relevance and accuracy of such an analytical model is then demonstrated by comparison with experimental results and with more complex numerical approaches. Finally, this technique enables us to demonstrate the high quality of the interface of HET solar cells which exhibit D_it levels below 10¹¹ defects per cm².     

CdTe/ZnTe/GaAs Heterostructures for Single-Crystal CdTe Solar Cells

Amorphous CdS/single-crystal CdTe solar cells were grown on GaAs substrates by metalorganic chemical vapor deposition. The structures of the films and the electrical properties of the devices were characterized. Highly conducting arsenic-doped ZnTe was grown on GaAs(100) substrates as the buffer layer for CdTe growth. By use of a ～30-nm ZnTe buffer layer, a p-CdTe film with a doping level of ～5×10¹⁶ cm^?3 was achieved. The hole concentration of p-CdTe increased with increasing VI/II ratio under a high As concentration during growth. From temperature-dependent Hall transport measurements, the ionization energy of the As acceptor in the p-CdTe was estimated to be approximately 88 meV. Ohmic behavior of the junctions between CdTe/ZnTe and ZnTe/GaAs was also confirmed. The solar cell performance of this structure, for example an open circuit voltage of 0.63 V, could be improved if the crystal quality of the CdTe film is optimized and the dislocation density of the CdTe film is minimized.