Light-induced Defect Generation Research Articles

Behaviour of Light Induced Defect Generation and Carrier Lifetime Degradation in Solar Grade Silicon

Light-induced defect generation seriously reduces the minority-carrier lifetime of crystalline silicon (c-Si) wafers which causes a decrease in solar cell efficiency. In this paper we investigate the impact of boron-oxygen complexes and iron impurities on the light induced minority-carrier lifetime degradation in c-Si, comparing electronic grade and upgraded metallurgical grade materials. For the later, the characteristic of the decay process is shown to be composed of a fast initial decay and a subsequent slow asymptotic decay. We conclude that the dissociation of iron-boron pairs must be taken into account to explain the light-induced lifetime reduction.

Light-induced boron-oxygen defect generation in compensated p-type Czochralski silicon

The concentration of boron-oxygen defects generated in compensated p-type Czochralski silicon has been measured via carrier lifetime measurements taken before and after activating the defect with illumination. The rate of formation of these defects was also measured. Both the concentration and the rate were found to depend on the net doping rather than the total boron concentration. These results imply that the additional compensated boron exists in a form that is not able to bond with the oxygen dimers, thus prohibiting the formation of the defect. This could be explained by the presence of boron-phosphorus complexes, as proposed in previous work. Evidence for reduced carrier mobilities in compensated silicon is also presented, which has implications for photoconductance-based carrier lifetime measurements and solar cell performance.

Metastability in hydrogenated nanocrystalline silicon solar cells

Light-induced metastability in hydrogenated nanocrystalline silicon (nc-Si:H) single-junction solar cells was studied systematically. First, we observed no light-induced degradation when the photon energy was lower than the band gap of the amorphous phase; degradation occurred when the energy was higher than the band gap in the amorphous phase. The light-induced degradation could be annealed away at an elevated temperature. We concluded that the light-induced defect generation occurred mainly in the amorphous phase. Second, forward current injection did not degrade the nc-Si:H cell performance. However, a reverse bias during light soaking enhanced the degradation. Third, the nc-Si:H cells made with an optimized hydrogen dilution profile showed minimal degradation although these cells had a high amorphous volume fraction. This indicated that the amorphous volume fraction was not the only factor determining the degradation. Other factors also played important roles in the nc-Si:H stability.

Metastability in Hydrogenated Nanocrystalline Silicon Solar Cells

AbstractWe have recently reported that hydrogenated nanocrystalline silicon (nc-Si:H) solar cells exhibit metastability behavior that is significantly different from hydrogenated amorphous silicon (a-Si:H) solar cells. First, we studied the light-induced change of nc-Si:H cells under different light spectra, and observed that no light-induced degradation occurs when the photon energy is lower than the bandgap of a-Si:H, while light-induced degradation indeed occurs when the photon energy is higher than the bandgap of a-Si:H. Based on this observation, we conclude that the light-induced defect generation occurs mainly in the amorphous and/or grain boundary regions. Secondly, we found that forward-bias current injection in the dark does not cause any degradation in the performance of nc-Si:H cells, in contrast to observations in a-Si:H cells. This phenomenon can be explained by assuming transport percolation through crystallites, where excess carrier recombination does not cause degradation. Third, we found that a reverse electrical bias does not reduce, but rather enhances the light-induced degradation in the nc-Si:H cell performance. This enhancement increases with the magnitude of the reverse bias. We carried out a systematic study including measurements of quantum efficiency losses and fill factors under different color filters. In view of the heterogeneity of the material structure, we proposed a “back-to-back” diode model which explains most of the experimental results. Finally, we present results of the improvement of stability of nc-Si:H cells made using hydrogen dilution profiling. We find that the nc-Si:H cells with an optimized hydrogen dilution profile show very little light-induced degradation though these cells have a high amorphous volume fraction. This indicates that the amorphous volume fraction is not the only factor determining the degradation. Other factors, such as the structure and distribution of the amorphous phase, as well as the properties of the grain boundaries, apparently play important roles in the overall stability of nc-Si:H cells.

Light-induced metastability in hydrogenated nanocrystalline silicon solar cells

Light-induced metastability in hydrogenated nanocrystalline silicon (nc-Si:H) single-junction solar cells has been studied under different light spectra. The nc-Si:H studied contains a certain fraction of hydrogenated amorphous silicon (a-Si:H). We observe no light-induced degradation when the photon energy used is lower than the bandgap of a-Si:H, while degradation occurs when the photon energy is higher than the bandgap. We conclude that the light-induced defect generation occurs mainly in the amorphous phase. Light soaking experiments on a-Si:H∕a-SiGe:H∕nc-Si:H triple-junction solar cells show no light-induced degradation in the bottom cell, because the a-Si:H top and a a-SiGe:H middle cells absorb most of the high-energy photons.

Buffer-layer Effect on Mixed-Phase Cells Studied by Micro-Raman and Photoluminescence Spectroscopy

ABSTRACTWe use micro-Raman and photoluminescence (PL) spectroscopy to study the effects of an a-Si:H buffer layer at the i/p interface of the mixed-phase silicon solar cells. We find that the signature of the crystalline 520 cm−1 mode still appears on the Raman spectrum for the cells with a 100 Å thick a-Si:H buffer layer; but it completely disappears for cells with a 500 Å thick a-Si:H buffer layer. At 80 K, the PL spectral lineshape reflects the features of the electronic states in the band tails. The characteristics of the PL spectra of the mixed-phase cells are a narrower main band than the standard a-Si:H band and an extra low energy band from the grain boundary region. As the thickness of the a-Si:H buffer layer increases, the PL main band becomes broader, and the low energy band is depressed. We find that, after light soaking, the PL main band is slightly broadened for the cells with no a-Si:H buffer layer, almost no change for the cells with a 100 Å thick buffer layer, and a remarkable decrease in total PL intensity for the cells with a 500 Å thick buffer layer. In addition, the PL intensity of the defect band increases after light soaking for the cells with a 500 Å thick buffer layer, where light-induced defect generation in the a-Si:H buffer layer masks the changes in the mixed-phase intrinsic layer. The Raman and PL results are consistent with previous observations of the effect of an a-Si:H buffer layer on the performance and metastability against light soaking for mixed-phase solar cells.

Light-induced effects on transport in hydrogenated amorphous silicon-sulfur alloys at different temperatures

The effect of light illumination on the electronic transport in hydrogenated amorphous silicon-sulfur alloys (a-SiSx:H) has been investigated at various temperatures. The light-induced effects were monitored using photoconductivity and dark conductivity measurements, including subgap defect density by the constant photocurrent method. The light-induced changes in the dark conductivity, as illumination temperature increases, reveal a transition from normal to inverse Staebler–Wronski (S–W) behavior at a critical temperature (Tc) that decreases with increasing sulfur concentration. On the other hand, the changes in the photoconductivity reveal normal S–W behavior at all the observed temperatures, but those are significantly reduced with increasing temperature and sulfur concentration. Correlation between subgap defect density and photoconductivity indicates that the defects created at temperatures higher than Tc act as inefficient recombination centers as compared with those created at temperatures lower than Tc. Two competing reaction processes, controlled by hydrogen diffusion, for light-induced defect generation are proposed to account for the experimental results.

Theory of Hydrogen Interactions with Amorphous Silicon

AbstractWe present an overview of recent results for hydrogen interactions with amorphous silicon (a-Si), based on first- principles calculations. We review the current understanding regarding molecular hydrogen, and show that H2 molecules are far less inert than previously assumed. We then discuss results for motion of hydrogen through the material, as relating to diffusion and defect formation. We present a microscopic mechanism for hydrogen-hydrogen exchange, and examine the metastable ≠ SiH2 complex formed during the exchange process. We also discuss the enhanced stability of Si-D compared to Si-H bonds, which may provide a means of suppressing light-induced defect generation.

Relaxation processes and metastability in amorphous hydrogenated silicon investigated with differential scanning calorimetry

In this work we investigated the thermodynamics and kinetics of structural relaxation in a-Si:H films using differential scanning calorimetry (DSC). The relation between structural relaxation and light-induced metastability (Staebler–Wronski effect, SWE) has also been investigated. The a-Si:H films were prepared by RF (13.56 MHz) glow discharge decomposition of the mixtures (10% SiH 4+90% H 2) and (5% SiH 4+95% He), and by 55 kHz plasma-enhanced chemical-vapor deposition (PECVD) of pure silane for high deposition rates. Detailed analysis of the peaks on DSC curves shows that the low-temperature effect (LTEP) is caused by the relaxation of weak Si bonds and that the high-temperature effect is caused by hydrogen effusion with subsequent relaxation of the microstructure. A method for the calculation of kinetic parameters of structural relaxation using DSC data is proposed. We found from the joint investigation of DSC and SWE that both the light-induced defect generation and the LTEP on DSC curves have a common nature and are described by the same reaction between weak Si–Si bonds and dangling bonds. We show that the rates of this reaction in both the forward and the reverse directions are controlled by hydrogen microstructure.

A Chemical Bonding Model for Photo-Induced Defects in Hydrogenated Amorphous Silicon (a-Si:H): Intrinsic and Extrinsic Reaction Pathways

ABSTRACTIn device grade a-Si:H photo- or light-induced defect generation is an intrinsic effect for impurity concentrations of oxygen and nitrogen below about 1019 to 1020 cm−3; however, at higher concentrations it increases with increasing impurity content. Charged defect configurations are identified by empirical chemistry and are studied by ab initio calculations. This paper addresses: i) the chemical stability of charged defects, ii) the reaction pathways for defect metastability, and iii) the transition between extrinsic and intrinsic behavior.

The Perspectives Of High-Rate Low Frequency a-Si:H Films Deposition: Solar Cell Application And Stability Control

AbstractThe perspectives for solar cell application of structural inhomogeneous a-Si:H films deposited at high growth rates (∼10–20 Å/s) from 100% SiH4 in low frequency (LF) 55kHz glow discharge plasma have been investigated. In this case the influence of structural inhomogeneity on dark dc and photoconductivities and light-induced defect generation kinetics (Staebler-Wronski effect, SWE) in a-Si:H films have been studied. The microstructure of films was investigated by IR spectroscopy analysis. Microstructural parameter R=[SiH2]/([;SiH]+[SiH2]), was used for the quantitative characterization of structural inhomogeneity in the material bulk.It was found that Fermi level position is fixed by deep defect states and does not depend on microstructure parameter R. The comparative analysis of photoconductivity modeling and ESR measurements have shown that recombination in a-Si:H films is controlled by neutral dangling bonds and doesn't depend on parameter R. Meanwhile it was found that the kinetics of light-induced defect generation was controlled by SiH2 or clustered SiH groups content. Thus, the above results allow to perform an independent control of stability and electronic properties of a-Si:H films deposited in LF glow discharge plasma.

Saturation of the metastable defect density in amorphous silicon semiconductor

We report on a study of the metastable defect creation under pulsed light illumination in undoped hydrogenated amorphous silicon. Defect generation rate, saturation value of defect density, N sat, and characteristic time, τ c , required to reach N sat have been investigated. Results show approximately a t 1/2 time-dependence of the defect generation, different from a t 1/3 time-dependence under continuous wave light illumination. It is observed that the saturation value of the defect density is about one order of magnitude higher than by cw illumination in device-quality films. It is found that the characteristic time decreases with hydrogen content and microstructure fraction, but increases with substrate temperature. Results are discussed within the framework of existing models for the light-induced defect generation.

The Stability Improvement of a-Si:H Films for Photovoltaic Applications

AbstractThe origin of metastability in a-SiH films deposited at different deposition conditions has been investigated by joint analysis of light-induced defect generation kinetics (Staebler-Wronski Effect) and differential scanning calorimetry (DSC) measurements. Light-induced defect generation kinetics were measured at elevated temperatures of 80-150°C_ DSC measurements were performed in a temperature range of 20°C - 570°C. The films microstructure were studied by IR spectroscopy analysis.The simulation of DSC curves has been used to deduce the kinetics of the low-temperature exothermic peak (LTEP) observed at 80-150° C. It was found that kinetic parameters of LTEP and SW effects has the same dependence on microstructure parameters, γ, which suggests their common nature. The analysis of a-SiH stability improvement concerning the influence of films microstructure on the kinetics of relaxation processes is presented.

The Stability Improvement of a-Si:H Films for Photovoltaic Applications

AbstractThe origin of metastability in a-Si:H films deposited at different deposition conditions has been investigated by joint analysis of light-induced defect generation kinetics (Staebler-Wronski Effect) and differential scanning calorimetry (DSC) measurements. Light-induced defect generation kinetics were measured at elevated temperatures of 80–150 °C. DSC measurements were performed in a temperature range of 20 °C – 570°C. The films microstructure were studied by IR spectroscopy analysis.The simulation of DSC curves has been used to deduce the kinetics of the low-temperature exothermic peak (LTEP) observed at 80–150°C. It was found that kinetic parameters of LTEP and SW effects has the same dependence on microstructure parameters, γ, which suggests their common nature. The analysis of a-Si:H stability improvement concerning the influence of films microstructure on the kinetics of relaxation processes is presented.

Multiphoton micrometer-scale photoetching in silicate-based glasses

Silicate-based glasses containing approximately 10% by weight boron or more have been observed to exhibit selective etching in hydrofluoric acid solution after direct exposure to intense visible laser light at 532 nm. The observation of a ring-shaped etch pattern in samples exposed to solid Gaussian beams suggests that selective etching is related to a charge-diffusion process rather than to local light-induced defect generation. The technique has so far resulted in the maskless production of micrometer-scale features with submicrometer depths.

Light-induced defect generation of a-Si: H at below-room-temperature

We report new experimental results on the behavior of the light- induced defect generation of a-Si: H at below-room-temperature. Light-soaking of well characterized hydrogenated (and fluorinated) amorphous silicon was carried out under constant illumination at the temperature range of 305–350 K. Light-induced defects are generated even at far below-room-temperature, although the amount is less. The experimental data on defect density as a function of light-soaking time can be described with the streched exponential by adjusting the values of both dispersion parameter β and time constant τ. Light-induced defect generation has a weak temperature dependence at below-room- temperature. We discuss our results within the framework of existing models for light-induced defects.

Explanation of Light-Enhanced Annealing of Defects in Amorphous Silicon

ABSTRACTSeveral recent measurements have shown that annealing of metastable defects in a- Si:H can be accelerated by the presence of light. This is the opposite of the usual light-induced defect generation, and no existing rate equation explains it while maintaining the necessary symmetry of generation and recovery processes, and consistency with the stretched-exponential transients that best describe observed generation and anneal behavior. This paper shows that this light-enhanced annealing (LEA) can be explained readily by the usual rate equation leading to stretched exponentials with no other terms by allowing a variation of coefficients with temperature or light intensity. This equation then leads to good simulations of observed LEA. Interpretation of these results in terms of distributional changes is presented, and an experimental test is proposed.

Effect of Deposition-Induced Annealable Defects on Light-Induced Defect Generation in a-Si:H

In this paper we present a method to determine the annealable defect density(ΔNann) present in hydrogenated amorphous silicon(a-Si:H). The effects of the annealable defects on the light-induced defect generation rate, saturated defect density (Nsat) and the change of defect density in the light-induced saturated state(ΔNsat) have been studied. Annealable defect density was varied by depositing samples at various substrate temperatures or by post-growth anneals of samples grown at low substrate temperatures. It is found that the generation rate, N satand ΔNsat are well correlated with ΔNann. In particular, the ΔNsat is found to follow a relation ΔNsat ≈ ΔNann. These results suggest that defect-related microscopic models are appropriate for light-induced metastability.

Light-induced defect generation in hydrogenated amorphous silicon under the constant photocurrent conditions

We measured the decay of the steady state photoconductivity in a-Si : H due to light soaking under the constant photocurrent condition, in which the light intensity was increased to keep the photocurrent constant and the photoconductivity was measured at a preset light intensity. The results are discussed in terms of the Stutzmann's model and the model of Redfield and Bube. We have found that Redfield and Bube's equation in a modified form including carrier enhanced dispersion may be used to describe light-induced defect generation.

Isolation of Temperature Effects on the Kinetics of Light Induced Defect Generation in a-Si:H

ABSTRACTData are presented here that show the effects of temperature on the kinetics of metastable defect formation in undoped a-Si:H over the range 45°-110°C. CPM (Constant Photocurrent Method), photoconductivity, and dark conductivity measurements were made and provide independent checks of the defect generation behavior. A stretched exponential description of defect formation as a function of time was used to fit the CPM defect density data. The stretched exponential time constant, τSE, is thermally activated with an apparent activation energy of 1 eV, a value that agrees well with data for defect anneal and solar cell degradation. The data indicate that thermal terms are not negligible for temperatures as low as 45°C, and therefore should be included in any model of the kinetics of defect formation. The role of adistribution of anneal energies and the regimes of dominance of thermal and optical rate terms are discussed in the context of the model.