Aurangzeb Khan

High-radiation-resistant InGaP, InGaAsP, and InGaAs solar cells for multijuction solar cells

The radiation response of 3 MeV proton-irradiated InGaP, InGaAsP and InGaAs solar cells was measured and analyzed in comparison with those of InP and GaAs. The degradation of the minority-carrier diffusion length was estimated from the spectral response data. The damage coefficient KL for the 3 MeV proton-irradiated InGaP, InGaAsP and InGaAs was also determined. The radiation resistance increases with an increase in the fraction of In–P bonds in InGaP, InGaAsP and InGaAs. Differences in the radiation resistance of InGaP, InGaAs and InGaAs materials are discussed. Minority-carrier injection under forward bias is found to cause partial recovery of the degradation on irradiated InGaP and InGaAsP cells.

Thermal annealing study of 1 MeV electron-irradiation-induced defects in n+p InGaP diodes and solar cells

The study presents detailed isothermal and isochronal annealing recovery of photovoltaic parameters in n+/p InGaP solar cells after 1 MeV electron irradiation. Correlation of the solar cells characteristics with changes in the deep level transient spectroscopy data observed in irradiated and annealed n+/p InGaP diodes and solar cells shows that the H2 (Ev+0.50 eV) and H3 (Ev+0.76 eV) defects have a dominant role in governing the minority-carrier lifetime as well as carrier removal. However, capacitance–voltage measurements indicate that other defects must also play a role in the carrier removal process. In addition, the concentration of the H2 defect is found to decay significantly as a result of room temperature storage for 40 days, suggesting that InGaP-based solar cells will display superior radiation tolerance in space. Finally, the deep donor-like-defect H2 is tentatively identified as a phosphorus Frenkel pair.

Role of the impurities in production rates of radiation-induced defects in silicon materials and solar cells

The present extensive systematic study of defect introduction rates as a function of boron, gallium, oxygen, and carbon concentrations by means of deep level transient spectroscopy has drawn a quite complete picture towards the identification of the dominant radiation-induced defects in Si. The radiation-induced defect EV+0.36 eV has been identified as Ci–Oi complexes. The absence of an EC−0.18 eV complex center in gallium-doped samples and the linear dependence of its introduction rates on both the boron and oxygen content fixed its identification as the Bi–Oi complex in boron-doped Si. One of the technologically important results of present study is that the gallium appears to strongly suppress the radiation induced defects, especially hole level EV+0.36 eV (Ci–Oi), which is thought to act as a recombination center as well as the dominant compensating center at EC−0.18 eV (Bi–Oi). As a result, the effects of lifetime degradation and carrier removal could be partially offset to higher radiation fluences ...

DEEP LEVEL ANALYSIS OF RADIATION-INDUCED DEFECTS IN SI CRYSTALS AND SOLAR CELLS

Deep level transient spectroscopy (DLTS) analysis of radiation-induced defects in p-type Si crystals and solar cells has been carried out to clarify the mechanism of the anomalous degradation of Si n+–p–p+ structure space cells induced by high-energy, high-fluence electron/proton irradiations. A large concentration of a minority-carrier trap with an activation energy of about 0.18 eV has been observed in irradiated p-Si using DLTS measurements, as well as the majority-carrier traps at around Ev+0.18 eV and Ev+0.36 eV, Correlations between DLTS data and solar-cell properties for irradiated and annealed Si diodes and solar cells have shown that type conversion of p-Si base layer from p-type to n-type is found to be mainly caused by introduction of the 0.18 eV minority-carrier trap center, that is, this center acts as a deep-donor center. The Ev+0.36 eV majority-carrier trap center is thought to also act as a recombination center that decreases minority-carrier lifetime (diffusion length). Moreover, origins ...

A detailed model to improve the radiation-resistance of Si space solar cells

An accurate radiation degradation model, based on measured radiation damage to devices and physical principles on radiation-induced defects in Si, has been established to improve the radiation-resistance of the Czochralski (CZ)-grown and floating-zone (FZ)-grown single-crystal Si space solar cells. We have successfully carried out the optimization of radiation-resistant Si space solar cells by taking into account the effective base carrier concentration dependence of the most important analytical parameters, damage coefficient K/sub L/, for the minority-carrier diffusion length and carrier removal rate R/sub c/ for majority-carriers. The model can be used to adequately predict the radiation degradation of the Si solar cells irradiated with a complete spectrum of electron fluence. It has been established that the radiation-resistance of the silicon solar cell is very dependent on effective carrier concentration in the high fluence range and irradiation tolerance can be improved further by varying the base carrier concentration upon irradiation.

Correlation of nitrogen related traps in InGaAsN with solar cell properties

The thermal annealing of nitrogen related traps in p-type InGaAsN and GaAsN is investigated by deep level transient spectroscopy (DLTS). Upon annealing, an apparent recovery of the photovoltaic properties correlates with changes in the DLTS data observed for InGaAsN and GaAsN diodes and solar cells, revealing that a nitrogen related E1 (EC−0.20eV) center has an important role in governing the solar cell performance. The large electron capture cross section (∼8.9×10−15cm2) of this center indicates that this defect may act as an efficient recombination center. Therefore, its complete removal by annealing or by some other process is essential for the high performance of GaInAsN solar cells. The internal quantum efficiency data were modeled to quantify the change in material properties associated with this improvement upon annealing. Annealed cells with indium impurity (InGaAsN) show a slightly higher photoresponse, which could be due to low scattering caused by In–N pair formation after annealing.

Recombination enhanced defect reactions in 1 MeV electron irradiated p InGaP

Direct recombination enhanced annealing of the radiation-induced defect H2 in p InGaP has been observed by deep level transient spectroscopy (DLTS). Detailed analysis of the annealing data at zero and reverse bias shows that annealing rates are independent of the defect charge state or this defect interacts with the two bands, i.e., is a recombination center trapping alternatively an electron, then a hole. An experiment based on minority carrier capture on a majority trap by the double carrier pulse DLTS technique further supports the evidence that H2 has a large minority carrier capture cross section and is an efficient nonradiative recombination center. Recombination-enhanced defect annealing rates obeys a simple Arrhenius law with an activation enthalpy of 0.51±0.09 eV, in contrast to athermal processes observed in GaP. Detailed analysis of results reveals that the mechanism involved in the minority carrier injection annealing of the H2 defect is energy release mechanism in which enhancement is induced...

Room-temperature minority-carrier injection-enhanced recovery of radiation-induced defects in p-InGaP and solar cells

We present here the direct observation of minority-carrier injection-enhanced annealing of radiation-induced defects in metalorganic chemical vapor deposition grown p-InGaP at room temperature, and the consequent recovery of radiation damage in InGaP n+-p junction solar cells. Deep level transient spectroscopy analysis shows that the main defect H2 (Ev+0.55 eV) in p-InGaP exhibits minority-carrier injection-enhanced annealing characterized by an activation energy (ΔE=0.51 eV) close to the activation energy for the recovery (ΔE=0.54 eV) of the defect responsible for diffusion length degradation in n+-p solar cells. The marked recovery of radiation damage in InGaP solar cells induced by minority-carrier injection is found to be correlated with the annihilation of the H2 defect.

Effects of proton irradiation on n+p InGaP solar cells

3 MeV proton irradiation effects on In0.5Ga0.5P single junction and In0.5Ga0.5P/GaAs tandem solar cells have been investigated for the fluence range from 1×1011 to 1×1013 cm−2. The overall radiation degradation of In0.5Ga0.5P/GaAs tandem cells was higher than In0.5Ga0.5P single junction cells. It was observed that the spectral response of the GaAs bottom cell degrades more than the InGaP top cell. Proton irradiation decreases the longer wavelength spectral response more significantly than the shorter wavelength in both In0.5Ga0.5P and In0.5Ga0.5P/GaAs cells. The difference in the degradation properties of n+p and p+n polarity InGaP solar cells is discussed. The radiation response of a tandem n+p InGaP/GaAs cell is very nearly that which is predicted from the information of these two cells independently. The minority-carrier diffusion length in the base layer was determined from the spectral response data. The minority-carrier diffusion length damage coefficient KL was analyzed for In0.5Ga0.5P and GaAs cel...

Processing and structure of carbon nanofiber paper

A unique concept of making nanocomposites from carbon nanofiber paper was explored in this study. The essential element of this method was to design and manufacture carbon nanofiber paper with well-controlled and optimized network structure of carbon nanofibers. In this study, carbon nanofiber paper was prepared under various processing conditions, including different types of carbon nanofibers, solvents, dispersants, and acid treatment. The morphologies of carbon nanofibers within the nanofiber paper were characterized with scanning electron microscopy (SEM). In addition, the bulk densities of carbon nanofiber papers were measured. It was found that the densities and network structures of carbon nanofiber paper correlated to the dispersion quality of carbon nanofibers within the paper, which was significantly affected by papermaking process conditions.