5–20 MeV proton irradiation effects on GaAs/Ge solar cells for space use

This paper reports the high-energy proton irradiation effects on GaAs/Ge space solar cells. The solar cells were irradiated by protons with energy of 5–20 MeV at a fluence ranging from 1×10⁹ to 7×10¹³ cm⁻², and then their electric parameters were measured at AM0. It was shown that the I_sc, V_oc and P_max degrade as the fluence increases, respectively, but the degradation rates of I_sc, V_oc and P_max decrease as the proton energy increases, and the degradation is relative to proton irradiation-induced defect Ec−0.41 eV in irradiated GaAs/Ge cells.     

Low-energy proton irradiation effects on GaAs/Ge solar cells

This paper reports the low-energy proton irradiation effects on GaAs/Ge solar cells for space use. The proton irradiation experiments were performed with a fluence of 1.2×10¹³ cm⁻², energies ranging from 0.1 to 3.0 MeV. The results obtained demonstrate that the irradiation with a proton energy of 0.3 MeV gives rise to the most degradation rates of I_sc, V_oc and P_max of the solar cells with no coverglass, which is related to the proton irradiation-induced vacancies near the pn junction in GaAs/Ge cells. The degradation rates of I_sc, V_oc and P_max of the solar cells with coverglass increase as the proton energy increases due to the cascade ions induced by collision processes. It is found that the coverglass has an obvious protection effect against the irradiation with the proton energy below 0.5 MeV.     

Epi-n-IZO thin films/1 0 0 Si, GaAs and InP by L-MBE––a novel feasibility study for SIS type solar cells

High quality epitaxial indium zinc oxide (heavily indium oxide doped) (epi-n-IZO) thin films were optimized by laser-molecular beam epitaxy (L-MBE) i.e., pulsed laser deposition (PLD) technique for fabricating novel iso- and hetero-semiconductor–insulator–semiconductor (SIS) type solar cells using Johnson Matthey “specpure”- grade 90% In₂O₃ mixed 10% ZnO (as commercial indium tin oxide (ITO) composition) pellets. The effects of substrate temperatures, substrates and heavy indium oxide incorporation on IZO thin film growth, opto-electronic properties with 1 0 0 silicon (Si), gallium arsenide (GaAs) and indium phosphide (InP) wafers were studied. As well as the feasibility of developing some novel models of iso- and hetero-SIS type solar cells using epi-IZO thin films as transparent conducting oxides (TCOs) and 1 0 0 oriented Si, GaAs and InP wafers as base substrates was also studied simultaneously. The optimized films were highly oriented, uniform, single crystalline approachment, nano-crystalline, anti-reflective (AR) and epitaxially lattice matched with 1 0 0 Si, GaAs and InP wafers without any buffer layers. The optical transmission T (max) 95% is broader and absolute rivals that of other TCOs such as ITO. The highest conductivity observed is σ=0.47×10³ Ω⁻¹ cm⁻¹ (n-type), carrier density n=0.168×10²⁰ cm⁻³ and mobility μ=123 cm²/V s. From opto-electronic characterizations, the solar cell characteristics and feasibilities of fabricating respective epi-n-TCO/1 0 0 wafer SIS type solar cells were confirmed. Also, the essential parameters of these cells were calculated and tabulated. We hope that these data be helpful either as a scientific or technical basis in semiconductor processing.     

Effect of copper diffusion on photovoltaic characteristics of CuGaSe2–GaAs cells

CuGaSe₂–GaAs heterojunctions were fabricated by fast evaporation of polycrystalline CuGaSe₂ from a single source on n-type GaAs substrates. The best CuGaSe₂–GaAs photocell (without an antireflective coating) exhibited an efficiency of 11.5%, J_sc=32 mA/cm², V_oc=610 mV and FF=0.60. The spectral distribution of photosensitivity of CuGaSe₂–GaAs junctions extends from 400 to 900 nm. The CuGaSe₂ films were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. XRD analysis indicated that the thin films were strongly oriented along the (1 1 2) plane. SEM studies of CuGaSe₂ films showed nearly stoichiometric composition with grain size about 1–2 μm. The energy dispersive X-ray spectroscopy (EDX) analysis of Cu concentration distribution in n-type GaAs showed that Cu diffused from the film into n-type GaAs during the growth process resulting in formation of the latent p–n homojunction in substrate. The diffusion coefficient of Cu in GaAs at growth temperature (520°C) estimated from EDX measurements was 6×10⁻⁸ cm²/s.     

Electrosynthesis and characterization of GaAs in acid solutions by potentiostatic method

Gallium arsenide (GaAs) is one of the important materials used for the fabrication of light emitting diodes, solar cells, microwave devices, etc. In the present work, electrodeposition of GaAs was successfully carried out potentiostatically from an aqueous solution mixture of gallium chloride (GaCl₃) and arsenic oxide (As₂O₃). The optimum deposition potential, pH and bath temperature to synthesize GaAs thin films are found to be −0.8 V versus SCE, 2.0±0.1 and 60 ^°C, respectively. The effects of solution pH, bath temperature and deposition potential on the gallium content of GaAs films are studied. Photoelectrochemical (PEC) solar cells using n-GaAs photo-anode in a polysulphide electrolyte is constructed and I–V, C–V studies are carried out. Various semiconductor parameters such as, flat-band potential, band bending, donor density, depletion layer width are evaluated and the results are discussed.     

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 3×10¹⁵ cm⁻² are represented. A base doping level of 1×10¹⁵ 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 1×10¹⁵ 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 3×10¹⁵e/cm² 3 MeV electron fluence. These results are additionally supported by the modeling calculations of the relative damage coefficient.     

Thin film photodetectors grown epitaxially on silicon

Epitaxial germanium films of thickness d≈4 μm, grown on silicon (001) by a low-temperature MBE process, constituted the base material of a silicon-integrated infrared detector. Characterization of the fully relaxed films was performed mainly by ex situ techniques, such as X-ray diffraction, Rutherford backscattering spectrometry and channeling, room temperature Hall effect and defect etching. Mesa diodes, fabricated from the originally p-type Ge films after pn-junction formation by thermal diffusion of antimony (Sb), showed quantum yields above 40% at wavelengths between 1200 nm and 1500 nm without an antireflection coating. The rise time of the photodiode signal in response to a picosecond laser pulse (τ = 300 ps) at wavelength of 1300 nm was 530 ps. Forward current-voltage characteristics of the devices were described by an ideality factor n = 1.25, while excess current under reverse bias was attributed to leakage caused by threading dislocations in the active layers.     

Dependence of the recombination in thin-film Si solar cells grown by ion-assisted deposition on the crystallographic orientation of the substrate

One promising strategy for achieving high-quality polycrystalline silicon thin-film solar cells on glass is based on low-temperature ion-assisted deposition for epitaxial thickening of a thin, large-grained seeding layer on glass. The crystal growth on the seeding layer is influenced by various factors, amongst which the crystal orientation of the grains plays a substantial role. In this paper we investigate how the electronic properties of solar cells grown on “ideal” seeding layers (Si wafers) are influenced by the crystallographic orientation of the substrate. The Si wafers are heavily doped p-type, ensuring that their contribution to the photogenerated current is small. The films grown on (1 0 0)-oriented Si substrates have a very low density of structural defects, while the films grown on (1 1 1)-oriented Si substrates display a high density of twin defects. The electronic properties of the thin-film solar cells were investigated by means of open-circuit voltage measurements as a function of the incident light intensity. The (1 0 0)-oriented diodes consistently exhibit a higher V_oc than the (1 1 1)-oriented diodes throughout the entire illumination range from 10⁻³ to 10³ Suns. We determine 7 μm as the bulk minority carrier diffusion length of the as-grown (1 0 0)-oriented Si film. A lower bound of 3 μm was found for the bulk minority carrier diffusion length in the as-grown (1 1 1)-oriented Si film. The performances of both types of solar cells were improved by hydrogenation in an ammonia plasma. At voltages around the 1-Sun maximum power point the improvement is due to a reduction of non-ideal current mechanisms. The diffusion length of the (1 0 0) diode remains unaffected by hydrogenation while the lower bound of the diffusion length of the (1 1 1) diode improves to 10 μm.     

Investigation of ohmic contact to P-type CdTe:P using ac and dc techniques

The effect of ohmic contact to P-type CdTe:P single crystal has been studied for different back contacts: Au and graphite doped with Cu, Te and CdCl₂. The ac electrical measurements were performed at room temperature in the frequency range 10⁻¹ Hz to 10⁶ Hz. The results were compared with dc measurements at the same temperature. Al/CdTe:P Schottky diodes were fabricated using thermal evaporation technique. The same applies for Au back contacts while for the others, Cu, Te and CdCl₂, powder forms were mixed in fixed portions with graphite. In ac and dc measurements, it was found that Cu electrodes give the lowest contact resistance which is nearly frequency independent in a frequency range 10⁻¹ Hz to about 10³ Hz. However, the plot of the imaginary component (Z″) versus the real component (Z′) of the complex ac impedance yields two semi-circles in the case Cu and Te back contacts with less defined two semi-circles in the case of CdCl₂. For Au back contact a single semi-circle was constructed with a single relaxation time in the order of 0.04 ms. The two semi-circles correspond to the bulk material (imperfections) at high frequency region, and the electrode interface between CdTe:P and the contact electrode. This may indicate that a P⁺ layer has been formed in the interface between the material and the back-contact layer. However, a good correlation was found between the ac and dc measurements for all back contacts.     

Substrate to substrate aluminized bonding of 10 cm round silicon substrates for application in solar photovoltaics

Large area silicon solar cells with screen printed contacts have been realized for the first time on 10 cm diameter, p-type, Cz silicon wafers which were bonded to silicon substrates by alloying of a suitably thick screen printed layer of Al on them. In cells made on 300 μm thick wafers without texturization, antireflection coating and passivation of the front surface, the values of the open-circuit voltage (V_oc), the short-circuit current density (J_sc), curve factor (CF) and the efficiency (η) were found to be in the range 572–579 mV, 16–19.2 mA cm⁻², 0.53–0.61 and 5.5–5.89%, respectively, under simulated tungsten halogen light of 100 mW cm⁻² intensity. Using thinner wafers and having optical confinement, surface passivation and effective back surface field, the cell performance would be substantially improved. In fact, an efficiency close to 18% (AM1.5) would be realizable with this approach. Another attractive feature of this approach is that a low-cost silicon substrate could be used at the bottom that would act as support for the thin top surface without disadvantage to the cell performance. In this paper only the principle has been demonstrated experimentally. Possible improvements have been shown by computer simulation.     

Recent developments in high-efficiency Ga0.5In0.5P/GaAs/Ge dual- and triple-junction solar cells: steps to next-generation PV cells

Dual-junction Ga_0.5In_0.5P/GaAs solar cells on Ge substrates have rapidly gone from small, high-efficiency laboratory cells, to large-area, high-efficiency cells manufactured at Spectrolab in high volume. Over 500,000 of these dual-junction (DJ) cells with 27-cm² area have been produced, with average AM0 load point efficiency of 21.4%. The next step in the evolution of this type of multijunction solar cell has been taken, with the development of triple-junction (TJ) Ga_0.5In_0.5P/GaAs/Ge cells. The addition of the germanium third junction, plus several significant improvements in the device structure, have led to a measured efficiency of 27.0% (AM0, 28°C) at Spectrolab on large-area (>30 cm²) TJ cells. The TJ cell is now in production at Spectrolab. Ga_0.5In_0.5P/GaAs/Ge cells are viable not only for non-concentrating space applications, but also for terrestrial and space concentrator systems. Efficiencies up to 32.3% at 47 suns under the terrestrial AM1.5D spectrum have been achieved.     

Electron irradiation and thermal annealing effect on GaAs solar cells

This paper describes the effect of electron irradiation and thermal annealing on LPE AlGaAs/GaAs heterojunction solar cells with various p/n junction depths. The electron irradiation experiments were performed with energy of 3 MeV, fluences ranging from 1×10¹⁴ to 5×10¹⁵ e/cm². The results obtained demonstrate that the irradiation-induced degradation of performances of the cells is mainly in the short circuit current and could be mostly recovered by annealing at 260°C for 30 min. Four electron traps, E_c−0.24 eV, E_c−0.41 eV, E_c−0.51 eV, E_c−0.59 eV, were found by DLTS analysis, only two shallow levels of which could be removed by the annealing.     

AlGaAs/GaAs/InGaAs composite MQW structures for photovoltaic applications

AlGaAs/GaAs/InGaAs composite MQW structures were theoretically studied and simulated. The computer simulation indicated that an appropriate composite MQW, both with symmetrical and non-symmetrical structures, could keep |ψ|² of quantized carriers at proper locations in electric-field-tilted quantum wells, so the efficient transition by photon absorption would be possible and applicable for photovoltaic cells which have the composite MQW as the active region. The AlGaAs/GaAs MQW and GaAs/InGaAs (4x) SQW structures were separately prepared by MBE and were evaluated for their spectral responses. The AlGaAs/GaAs MQW has a high response at a short wavelengths (peak at 685 nm) due to the quantized states in GaAs wells, while the GaAs/InGaAs SQW has a broader spectral response covering longer wavelengths (600–850 nm) because of the strong absorption in the GaAs barrier and substrate. However, (4x) photoluminescence peaks at 900–1100 nm that werefound from GaAs/InGaAs strained quantum wells at room temperature are promising evidence for the longer wavelength spectral response. The AlGaAs/GaAs/InGaAs composite SQW and MQW samples were experimentally prepared by MBE techniques and tested for their optical properties. The broader photoluminescence peak was observed and reflected the nature of the composite structure. The study on the photospectral response of composite MQW structures has been conducted which provides the basic information for high performance solar cell design.     

Porous alumina templates and nanostructured CdS for thin film solar cell applications

Nanostructured CdS was grown by electrodeposition of cadmium sulfide inside a porous alumina template. Uniform pore size and spacing in the template was achieved when the starting material for the template was aluminum foil. Typical pore size was 45 nm. Nanostructured CdS was also deposited by electrodeposition on indium tin oxide (ITO)-coated glass and by solution growth on ITO-coated glass. Schottky diodes were formed on nanocrystalline CdS and the analysis of their current–voltage characteristics yielded a diode ideality factor (n) of 2.6 and a reverse saturation current density (J_S) of 1.00×10⁻⁵ A/cm². Corresponding values for the Schottky diode on polycrystalline CdS were 3.4 and 1.93×10⁻⁶ A/cm².     

Organic ultraviolet photovoltaic diodes based on copper phthalocyanine as an electron acceptor

Organic ultraviolet (UV) light-sensitive photovoltaic (PV) diodes, based on 4, 4′, 4″-tris-(2-methylphenyl phenylamino) triphenylamine (m-MTDATA) as an electron donor and copper phthalocyanine (CuPc) as acceptor, have been fabricated. The PV diode exhibits high open-circuit voltage (V_OC) of 1.05 V under illumination of 365 nm UV light with 1.7 mW/cm², although the CuPc was generally used as electron donor in other PV diodes. And the short-circuit current (I_SC) of 54.6 μA/cm², fill factor (FF) of 0.304 and power conversion efficiency (η_e) of 1.03% are respectively achieved. This diode can accurately detect the UV radiation according to photo-generated voltage signal.     

Density of interface states, excess capacitance and series resistance in the metal–insulator–semiconductor (MIS) solar cells

Dark and illuminatied current–voltage (I–V) characteristics of Al/SiO_x/p-Si metal–insulator–semiconductor (MIS) solar cells were measured at room temperature. In addition to capacitance–voltage (C–V) and conductance–voltage (G–V), characteristics are studied at a wide frequency range of 1 kHz–10 MHz. The dark I–V characteristics showed non-ideal behavior with an ideal factor of 3.2. The density of interface states distribution profiles as a function of (E_ss−E_v) deduced from the I–V measurements at room temperature for the MIS solar cells on the order of 10¹³ cm⁻² eV⁻¹. These interface states were responsible for the non-ideal behavior of I–V, C–V and G–V characteristics. Frequency dispersion in capacitance for MIS solar cells can be interpreted only in terms of interface states. The interface states can follow the a.c. signal and yield an excess capacitance, which depends on the relaxation time of interface states and the frequency of the a.c. signal. It was observed that the excess capacitance C_o caused by an interface state decreases with an increase of frequency. The capacitances characteristics of MIS solar cells are affected not only in interface states but also series resistance. Analysis of this data indicated that the high interface states and series resistance leads to lower values of open-circuit voltage, short-circuit current density, and fill factor. Experimental results show that the location of interface states and series resistance have a significant effect on I–V, C–V and G–V characteristics.     

Preparation and characterisation of electrodeposited n-CdS/p-CdTe thin film solar cells

Cadmium sulphide and cadmium telluride films have been electrodeposited for n-CdS/p-CdTe solar cells. Cell efficiency varied considerably from 9.5% to 11.5% for each deposition set. The reverse saturation currents of 9.5% and 11.5% cells at 298 K were 25 and 6.7 nA cm⁻², respectively. The cells with higher efficiency has a lower number of interface states than the less efficient cells. The 11.5% cell had interface states (N_IS) of 3× 10¹⁰ cm⁻² eV⁻¹ at zero volt bias in dark and when it was illuminated with 35 mW cm⁻² light at zero volt bias N_IS increased by two orders to 1.2×10¹² cm⁻² eV⁻¹. At higher frequency the large voltage intercept of the Mott-Schottky plot indicates the existence of the near intrinsic layer of the polycrystalline heterojunction.     

The study on high efficient AlxGa1−xAs/GaAs solar cells

The investigation of Al_xGa_1−xAs/GaAs solar cells is carried out by means of both metalorganic chemical vapor deposition (MOCVD) and liquid-phase epitaxial (LPE) technique. The measurements of illuminated I–V characteristics, dark I–V characteristics and quantum efficiencies were performed for the GaAs solar cells made in author's laboratory. The measuring results revealed that the quality of materials in GaAs solar cell's structures is the key factor for getting high-efficient GaAs solar cells, but the effect of post-growth technology on the performances of GaAs solar cells is also very strong. The 21.95% (AM0, 2×27 cm², 25°C) high conversion efficiency in a typical GaAs solar cell has been achieved owing to improving the quality of materials as well as optimizing the post-growth technology of devices.     

Multi-junction III–V solar cells: current status and future potential

Our recent R&D activities of III–V compound multi-junction (MJ) solar cells are presented. Conversion efficiency of InGaP/InGaAs/Ge has been improved up to 31–32% (AM1.5) as a result of technologies development such as double hetero-wide band-gap tunnel junction, InGaP–Ge hetero-face structure bottom cell, and precise lattice-matching of InGaAs middle cell to Ge substrate by adding indium into the conventional GaAs layer. For concentrator applications, grid structure has been designed in order to reduce the energy loss due to series resistance, and world-record efficiency InGaP/InGaAs/Ge 3-junction concentrator solar cell with an efficiency of 37.4% (AM1.5G, 200-suns) has been fabricated. In addition, we have also demonstrated high-efficiency and large-area (7000 cm²) concentrator InGaP/InGaAs/Ge 3-junction solar cell modules of an outdoor efficiency of 27% as a result of developing high-efficiency InGaP/InGaAs/Ge 3-junction cells, low optical loss Fresnel lens and homogenizers, and designing high thermal conductivity modules.Future prospects are also presented. We have proposed concentrator III–V compound MJ solar cells as the 3rd generation solar cells in addition to 1st generation crystalline Si solar cells and 2nd generation thin-film solar cells. We are now developing low-cost and high output power concentrator MJ solar cell modules with an output power of 400 W/m² for terrestrial applications.     

Multicrystalline silicon wafers prepared from upgraded metallurgical feedstock

A solution to the problem of the shortage of silicon feedstock used to grow multicrystalline ingots can be the production of a feedstock obtained by the direct purification of upgraded metallurgical silicon by means of a plasma torch. It is found that the dopant concentrations in the material manufactured following this metallurgical route are in the 10¹⁷ cm⁻³ range. Minority carrier diffusion lengths L_n are close to 35 μm in the raw wafers and increases up to 120 μm after the wafers go through the standard processing steps needed to make solar cells: phosphorus diffusion, aluminium–silicon alloying and hydrogenation by deposition of a hydrogen-rich silicon nitride layer followed by an annealing. L_n values are limited by the presence of residual metallic impurities, mainly slow diffusers like aluminium, and also by the high doping level.