Explanation of Light-Enhanced Annealing of Defects in Amorphous Silicon

1994 ◽  
Vol 336 ◽  
Author(s):  
David Redfield ◽  
Richard Bube
Keyword(s):  
Light Intensity ◽  
Amorphous Silicon ◽  
Rate Equation ◽  
Experimental Test ◽  
Stretched Exponential ◽  
Defect Generation ◽  
Recovery Processes ◽  
Usual Rate ◽  
Stretched Exponentials

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.

Thermal and Optical Stretched Exponentials in Defect Kinetics in a-Si:H

1993 ◽  
Vol 297 ◽  
Author(s):  
David Redfield ◽  
Richard Bube
Keyword(s):  
Solar Cell ◽  
Rate Equation ◽  
Defect Density ◽  
Dispersion Parameter ◽  
Thermally Induced ◽  
Defect Generation ◽  
Weak Light ◽  
New Equation ◽  
Stretched Exponentials ◽  
Dispersive Character

Dispersive description of defect generation in a-Si:H that leads to stretched-exponential transients is extended by relaxing the assumption that light-induced processes and thermally induced processes have the same dispersive character. This is done by separating the rate equation for the defect density into two parts, one thermal and one optical, each with its own dispersion parameter. The solutions of this new equation — which must be obtained numerically — generally have two distinct parts: there may be a two-part rise or a peak, depending on the relative values of the two stretch parameters. Using this formulation we have readily simulated the recently observed peak in relaxation of a previously heavily degraded solar cell while exposed to a weak light. We find no way to explain other reports in similar two-part experiments that relaxation is faster under weak excitation than without.

Comprehensive Kinetic of Defects in a-SI:H

1991 ◽  
Vol 219 ◽  
Author(s):  
David Redfield ◽  
Richard H. Bube
Keyword(s):  
Solar Cells ◽  
Steady State ◽  
Light Intensity ◽  
Rate Equation ◽  
Defect Density ◽  
Microscopic Models ◽  
Dx Center ◽  
Stretched Exponentials ◽  
Kinetics Of ◽  
Insight Into

ABSTRACTLThe introduction of several new principles into the analysis of transition kinetics of metastable defects in a-Si:H has produced substantially improved rate equation for the density of defects as functions of time, light intensity, and temperature. The solution of this equation is stretched exponential (SE) having properties that explain in unifying way many observations of defect properties, including generation and anneal of the defect density in homogeneous films and degradation and anneal of solar cells. Major consequences are found for both the steady-state and transient properties of the defect density and for interpretations of microscopic models of the defects. These properties are also shown to be analogous to those of metastable centers in other materials, particularly the metastable DX center in AlGaAs which offers rare insight into the microscopic origins of stretched exponentials that can be applied to a-Si:H in ways that provide new perspectives on effects of alloying and hydrogen on stability.

Saturation behavior of the Light-Induced Defect Density in Hydrogenated Amorphous Silicon

1990 ◽  
Vol 192 ◽  
Author(s):  
H. R. Park ◽  
J. Z. Liu ◽  
P. Roca i Cabarrocas ◽  
A. Maruyama ◽  
M. Isomura ◽  
...  
Keyword(s):  
Amorphous Silicon ◽  
Room Temperature ◽  
Urbach Energy ◽  
Defect Density ◽  
Generation Rate ◽  
Carrier Generation ◽  
Defect Generation ◽  
Ion Laser ◽  
Hydrogenated Amorphous ◽  
Defect Densities

ABSTRACTUsing a Kr ion laser (λ = 647.1 nm) to produce a carrier generation rate G of 3 × 1020 cm−3s−1, we have saturated the light-induced defect generation in hydrogenated (and fluorinated) amorphous silicon (a-Si:H(F)), within a few hours near room temperature. While the defect generation rate scales roughly with 1/G2, the saturation defect densities Ns,sat are essentially independent of G. The saturation is not due to thermal annealing. We have further measured Ns,sat m 37 a-Si:H(F) films grown in six different reactors under different conditions. The results show that Ns,sat lies between 5 × 1016 and 2 × 1017 cm−3, that Ns,sat drops with decreasing optical gap and hydrogen content, and that Ns,sat is not correlated with the initial defect density or with the Urbach energy.

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

1993 ◽  
Vol 297 ◽  
Author(s):  
Jong-Hwan Yoon
Keyword(s):  
Amorphous Silicon ◽  
Defect Density ◽  
Hydrogenated Amorphous Silicon ◽  
Generation Rate ◽  
Defect Generation ◽  
Saturated State ◽  
Microscopic Models ◽  
Hydrogenated Amorphous ◽  
Substrate Temperatures

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.

scholarly journals Model for Staebler-Wronski Degradation Deduced from Long-Term, Controlled Light-Soaking Experiments

2000 ◽  
Vol 609 ◽  
Author(s):  
Bolko von Roedern ◽  
Joseph A. del Cueto
Keyword(s):  
Light Intensity ◽  
Amorphous Silicon ◽  
Operating Conditions ◽  
Degradation Mechanisms ◽  
Time Constants ◽  
Photovoltaic Modules ◽  
History Of ◽  
Exposure History ◽  
Operating History

ABSTRACTLong-term light-soaking experiments of amorphous silicon photovoltaic modules have now established that stabilization of the degradation occurs at levels that depend significantly on the operating conditions, as well as on the operating history of the modules. We suggest that stabilization occurs because of the introduction of degradation mechanisms with different time constants and annealing activation energies, depending on the exposure conditions. Stabilization will occur once a sufficient accumulation of different degradation mechanisms occurs. We find that operating module temperature during light-soaking is the most important parameter for determining stabilized performance. Next in importance is the exposure history of the device. The precise value of the light intensity seems least important in determining the stabilized efficiency, as long as its level is a significant fraction of 1-sun.

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