This Literature Review discusses the degradation and annealing tests which have been conducted on amorphous silicon. The types of tests and material properties are the primary focus. Amorphous silicon is a useful material for making solar photovoltaic devices. The degradation is known as the Staebler Wronski Effect or SWE.
Related Appopedia Projects[edit | edit source]
Amorphous Silicon Cell Degradation Tests[edit | edit source]
RECENT DEVELOPMENTS IN AMORPHOUS SILICON SOLAR CELLS 1980[edit | edit source]
Abstract: This article reviews recent advances in the development of amorphous silicon solar cells. Both the glow-discharge deposition conditions and the solar-cell structures are discussed in some detail. The performance characteristics of present amorphous silicon cells are described, and the loss mechanisms that limit performance are considered. An effort has been made to point out those areas where further research is needed. Recently, amorphous silicon p-i-n cells with areas of 1.19 cm 2 have been fabricated with conversion efficiencies as high as 6.1%.
D. E. CARLSON. RECENT DEVELOPMENTS IN AMORPHOUS SILICON SOLAR CELLS. Solar Energy Materials 3 (1980) 503-518
Stability of n-i-p amorphous silicon solar cells 1981[edit | edit source]
Abstract: Unencapsulated, amorphous silicon indium tin oxide/n-i-p/stainless-steel solar cells were tested for stability. All cells have excellent shelflife. Changes occur during exposure to light, but can be controlled by the deposition conditions of the amorphous silicon. The changes are due to trapping and recombination of optically generated carriers in the i layer, and are reversibly annealed out above 175 'c. Preliminary life tests on two relatively stable cells showed a small initial drop to 5%, followed by a weak logarithmic decay that predicts only - 20% further decrease in efficiency after 20 years in sunlight. Work is continuing on improving the efficiency and stability of these cells.
D. L. Staebler, A. S. Crandall, and A. Williams. Stability of n-i-p amorphous silicon solar cells. Appl. Phys. Lett. 39(9). 1 November 1981
Photoconductivity, trapping, and recombination in discharge-produced, hydrogenated amorphous silicon 1981[edit | edit source]
Abstract: Photoconductivity, trapping, and recombination have been studied in undoped, hydrogenated amorphous silicon (a-SiHx) films prepared by the discharge decomposition of silane. In this study the effects of the photoinduced, reversible conductivity changes have been taken into account in the characterization of the different types of electron trapping and recombination kinetics. These kinetics, which over the temperature range of ∼350 to 120 K are found to be consistent with free-carrier transport, are correlated with the densities, energies, and free-carrier capture cross sections of the states in the gap. The electron lifetimes, between ∼10-6 and 10-3 s, are shown to be dependent on two types of recombination centers located at or below midgap with one of these centers having an electron capture cross section, Sn, of ∼10-19 cm2. The electron lifetimes are found to be sensitive to these centers even though their densities are ≲10-4 that of the hydrogen present in the films. The electron trapping is determined by the states above midgap, which have densities of ∼1017 cm-3 eV-1 over the energy range of ∼0.6 to 0.35 eV from the free electron band and for energies within ∼0.2 eV, densities of ∼1019 cm-3. No evidence is found for a large peak in the densities of states at ∼0.4 eV from Ec, a peak which has been extensively reported for a-SiHx films.
Christopher R. Wronski and Ronald E. Daniel. Photoconductivity, trapping, and recombination in discharge-produced, hydrogenated amorphous silicon. Phys. Rev. B 23, 794–804 (1981) 10.1103/PhysRevB.23.794
EFFECTS OF PROLONGED ILLUMINATION ON THE PROPERTIES OF HYDROGENATED AMORPHOUS SILICON 1982[edit | edit source]
This paper reviews various models which have been proposed for metastable effects and looks a experimental observations.
- Photoconductivity affected by deg
- no degradation of absorption coefficient.
- eff decreases with deg
Two models by SW
- "(1) localized defects that undergo a bond reorientation or atomic displacement, and "
- "(2) centers that can trap free carriers but that are in poor communication with the extended states."
More models now
- "photogenerated excitons can self-trap leading to intimate charge-transfer defects (ICTD's). These ICTD's are oppositely charged dangling bonds that are stabilized by the Coulomb interactions. These defects are thought to occur near weak bonds, and the large hydrogen content of a-Si:H may be responsible for a 'softening' of the network, thus allowing the necessary relaxations"
- "has proposed a similar model where these intimate pairs naturally occur in a-Si:H; i.e., 2T3°-~T3 + +T3-, where T3 ° is a neutral dangling bond site, and the Coulomb attraction leads to a negative effective correlation energy. The Staebter-Wronski effect would then be due to preferential trapping of electrons or holes by T 3 ~ or T 3 sites, respectively, depending on the position of the Fermi level."
No model perfect, can't explain Crandall's observations - "an electron trap in a-Si:H and that the capture and emission process have comparable activation energies"
D.E. CARLSON. EFFECTS OF PROLONGED ILLUMINATION ON THE PROPERTIES OF HYDROGENATED AMORPHOUS SILICON. Solar Energy Materials 8 (1982) 129-140
LIGHT-INDUCED INSTABILITY OF AMORPHOUS SILICON PHOTOVOLTAIC CELLS 1983[edit | edit source]
Abstract:Changes in the characteristics of amorphous silicon (a-Si) solar cells caused by light exposure were studied. The degradation ratio of the conversion efficiency of p-i-n a-Si solar cells caused by light exposure depends on the thickness of the i layer. A decrease in the fill factor was commonly observed, and in such cases the diode quality factor and shunt current density increased, which suggested a change in junction properties. It was shown that additional doping of the i layer with a small amount of boron prevents the decrease in conversion efficiency with light exposure. In a 1 year experiment on a 2 kW a-Si power generating system, a 10% decrease in conversion efficiency was observed (without additional boron doping).
S. TSUDA, N. NAKAMURA, K. WATANABE, T. TAKAHAMA, H. NISHIWAKI, M. OHNISHI and Y. KUWANO. LIGHT-INDUCED INSTABILITY OF AMORPHOUS SILICON PHOTOVOLTAIC CELLS. Solar Cells, 9 (1983) 25 - 36
LIGHT-INDUCED EFFECTS IN AMORPHOUS SILICON MATERIAL AND DEVICES 1983[edit | edit source]
Abstract: We have studied the stability of hydrogenated amorphous silicon (a-Si:H) using several new techniques such as diffusion length measurements, photovoltage profiling and IR absorption via multiple internal reflections. We find that prolonged illumination generally causes a decrease in the diffusion length and an increase in the space charge density of undoped a-Si:H films.
D. E, CARLSON, A. R. MOORE, D. J. SZOSTAK, B. GOLDSTEIN, R. W. SMITH, P. J. ZANZUCCHI and W. R. FRENCHU. LIGHT-INDUCED EFFECTS IN AMORPHOUS SILICON MATERIAL AND DEVICES. Solar Cells, 9 (1983) 19 - 23 19
Light-induced metastable defects in hyrogenated amorphous silicon: A systematic study 1984[edit | edit source]
Abstract: We study the magnitude of metastable light-induced changes in undoped hydrogenated amorphous silicon (the Staebler-Wronski effect) with electron-spin-resonance and photoconductivity measurements. The influence of the following parameters is investigated in a systematic way: sample thickness, impurity content, illumination time, light intensity, photon energy, and illumination and annealing temperatures. The experimental results can be explained quantitatively by a model based on the nonradiative recombination of photoexcited carriers as the defect-creating step. In the framework of this model, the Staebler-Wronski effect is an intrinsic, self-limiting bulk process, characterized by a strongly sublinear dependence on the total light exposure of a sample. The experimental results suggest that the metastable changes are caused by recombination-induced breaking of weak Si–Si bonds, rather than by trapping of excess carriers in already existing defects. Hydrogen could be involved in the microscopic mechanism as a stabilizing element. The main metastable defect created by prolonged illumination is the silicon dangling bond. An analysis of the annealing behavior shows that a broad distribution of metastable dangling bonds exists, characterized by a variation of the energy barrier separating the metastable state from the stable ground state between 0.9 and 1.3 eV.
M. Stutzmann, W. B. Jackson, and C. C. Tsai. Light-induced metastable defects in hydrogenated amorphous silicon: A systematic study. Phys. Rev. B 32, 23–47 (1985)
Annealing of Metastable Defects in Hydrogenated Amorphous Silicon 1985[edit | edit source]
This paper uses electron spin resonance to understand how the metastable properties of amorphous silicon work. It studied the before and after affects on the structure of the cells. It found that impurities greatly effects the annealing process.
M. Stutzmann, W.B. Jackson and C.C. Tsai. Annealing of metastable defects in hydrogenated amorphous silicon. Physical Review B, Vol 34 num 1 july 1, 1986.
Intrinsic dangling-bond density in hydrogenated amorphous silicon 1985[edit | edit source]
This paper discusses the various models wrt dangling bonds. It also discusses "freeze-in" dangling bond densities which are caused by typical cooling rates of the cells. The paper talks about spin and annealing kinetics and spin densities with the various equations associated with it.
Z. E. Smith and S. Wagner. Intrinsic dangling-bond density in hydrogenated amorphous silicon. Phys. Rev. B 32, 5510–5513 (1985)
Carrier lifetime model for the optical degradation of amorphous silicon solar cells 1985[edit | edit source]
Abstract: The light‐induced performance degradation of amorphous silicon solar cells is described well by a model in which the carrier lifetimes are determined by the dangling bond density. Degradation will be slower in solar cells operating at lower excess carrier concentrations. This is documented with a comparison of degradation data for cells at open circuit versus load, and for single versus cascade cells. At sufficiently long times, the efficiency will decrease at approximately the same rate for all cases, with an offset in time between the individual cases which can be calculated.
Smith, Z E.; Wagner, S.; Faughnan, B. W.;, "Carrier lifetime model for the optical degradation of amorphous silicon solar cells," Applied Physics Letters, vol.46, no.11, pp.1078-1080, Jun 1985 doi: 10.1063/1.95767
Hydrogenated Microvoids and Light-Induced Degradation of Amorphous-Silicon Solar Cells 1986[edit | edit source]
This paper shows the degradation with respect to time and discuss the properties that influence a-Si:H. It's a good read to get the basics of a-Si:H and is useful in explaining what happens during a degradation.
D. E. Carlson. Hydrogenated Microvoids and Light-Induced Degradation of Amorphous-Silicon Solar Cells. Appl. Phys. A 41,305-309 (1986)
Defects in Amorphous Silicon: A New Perspective 1986[edit | edit source]
This paper focuses on the atomic structure of amorphous silicon to discuss how the defects works. It discusses concepts of self-interstitials and vacancies found in c-Si as the framework to describe a-Si defects. Talks about bonding mechanisms.
Sokrates T. Pantelides. Defects in Amorphous Silicon: A New Perspective. Physical Review Letters. Vol 57, num 23. dec 8, 1986.
Thermal equilibrium in doped amorphous silicon 1986[edit | edit source]
Abstract: The structure of doped amorphous silicon is shown to be in metastable thermal equilibrium above 130deC, having temperature-dependent densities of dangling bonds and donors. The time to reach equilibrium is thermally activated, so that cooling establishes a slowly relaxing nonequilibrium state resembling a glass. The results are interpreted in terms of the recent defect-compensation model of doping.
Street, R. A.; Kakalios, J.; Hayes, T. M. Thermal equilibration in doped amorphous silicon. Physical Review B (Condensed Matter), Volume 34, Issue 4, August 15, 1986, pp.3030-3033
Thermal-equilibrium processes in amorphous silicon 1987[edit | edit source]
Abstract: Data are presented which show that a major part of the localized electronic state distribution in hydrogenated amorphous silicon is in thermal equilibrium at elevated temperatures. Measurements of electronic transport are reported, with particular emphasis on the effects of annealing and cooling the samples. Two regimes of behavior are observed. When samples are cooled below a temperature TE, the electronic and atomic structures slowly relax with a temperature-dependent time constant. In n-type samples the relaxation time is several weeks at room temperature, and TE is ∼130 °C. In p-type samples the time constant is a few hours and TE is ∼80 °C. The second regime above TE corresponds to a relaxation time short compared to experimental times, and the structure attains a metastable thermal equilibrium. We show that the defect-compensation model of doping provides an accurate phenomenological description of the results. Furthermore, a quantitative fit to the data is obtained using the known density-of-states distribution. The bonding rearrangements that enable changes in the localized-state structure are discussed. We propose that the motion of bonded hydrogen is important, and that it can be considered to form a separate substructure that has properties similar to a glass. In this model the equilibration temperature TE is identified with the glass transition temperature. New measurements of hydrogen diffusion are presented to support the model.
R. A. Street, J. Kakalios, C. C. Tsai, and T. M. Hayes. Thermal-equilibrium processes in amorphous silicon. Phys. Rev. B 35, 1316–1333 (1987)
Kinetic studies of the annealing behavior of a‐Si:H p‐i‐n solar cells 1987[edit | edit source]
Abstract: Results of measurements of the annealing of photodegraded amorphous silicon p‐i‐n solar cells are presented. The annealing process can be characterized by a relatively fast initial stage followed by a much slower second stage. Although the annealing behavior cannot be described by simple exponential kinetics, it can be characterized by a single activation energy of 1.2 eV. The degradation temperature and the duration of the light soaking do not affect this activation energy, although the ratio of the fast to the slow portions of the recovery change as these experimental parameters are varied. We have explained these phenomena in terms of hydrogen motion in the material.
Bennett, M. S.; Newton, J. L.; Rajan, K.; Rothwarf, A.;, "Kinetic studies of the annealing behavior of a‐Si:H p‐i‐n solar cells," Journal of Applied Physics, vol.62, no.9, pp.3968-3975, Nov 1987 doi: 10.1063/1.339195
Evidence of hydrogen motion in annealing of light-induced metastable defects in hydrogenated amorphous silicon 1988[edit | edit source]
- motion of other defects such as three/five fold coord Si unlikely
- discusses hydrogen diffusion through a-Si
- annealing of LID bonds has time decay like a stretched exp and similar to decays of excess band-tail carriers.
W.B. Jackson and J. Kakalois. Evidence of hydrogen motion in annealing of light-induced metastable defects in hydrogenated amorphous silicon. Physical Review B, Vol 37, num 2. Jan 15, 1988.
Amorphous Silicon Solar Cells 1980's [Book][edit | edit source]
This is a chapter in a book which talks about the history of amorphous silicon cells, how they are made and their properties and characteristics. It's a good read to get all the basics summed up, however it is a little dated.
By D. E. Carlson and C. R. Wronski. Chapter 10...
Reinterpretation of degradation kinetics of amorphous silicon 1988[edit | edit source]
Abstract: Generation oflight-induced metastable defects in amorphous Si:H(a-Si:H) is shown to follow the same stretched exponential (SE) that describes relaxation ofthermaHy induced metastability at room temperature for a simple case. Apparent power laws derived from the central part of the SE are (time)Ol and (intensity)O.h, agreeing well with the (time) 1/3 and (intensity)2!3 dependences often reported in the mid range of defect density, thus providing an alternative description of defect generation. The SE link between light-induced and thermally induced instabilities suggests that the thermal effects are also due to defect processes, and offers an alternative defect-based explanation to a macroscopic "stru.ctural relaxation" or "glass transition."
David Redfield and Richard H. Bube. Reinterpretation of degradation kinetics of amorphous silicon. AppL Phys. Lett. 54 (11), 13 March 1989
Role of band-tail carriers in metastable defect formation and annealing in hydrogenated amorphous silicon 1990[edit | edit source]
Abstract: This paper presents results on annealing of carrier-induced metastable defects in hydrogenated amorphous silicon (a-Si:H) and on the dependence of the defect kinetics on carrier density. The metastable defects were studied by measuring the threshold-voltage shifts on capacitors as a function of time, temperature, and bias. The defect generation and annealing exhibit stretched-exponential-like behavior where the characteristic time for defect generation is a function of carrier density. The ratio of carrier density to defects in equilibrium are determined to be approximately 0.1, the same ratio found in doped a-Si:H. The results are consistent with dispersive hydrogen motion through an exponential distribution of barrier heights. The hopping rate and the final-state energy depend on the carrier density. This dependence on carrier density explains the carrier-, light-, and doping-induced defect formation in a-Si:H. The increase of the hopping rate due to carriers accounts for the increase in the hydrogen-diffusion rate in doped material. While much of the data are consistent with a single-hop model, the lack of correlation between generation and annealing rates indicates that defect formation occurs by multiple hopping.
W. B. Jackson. Role of band-tail carriers in metastable defect formation and annealing in hydrogenated amorphous silicon. Phys. Rev. B 41, 1059–1075 (1990)
Excitons and light-induced degradation of amorphous hydrogenated silicon 1991[edit | edit source]
- Observed excitonic states in photoconductivy by looking at changes in density during deg
- Uses ESR determine results / discusses how it works
Martin S. Brandt and Martin Stutzmann. Excitons and light-induced degradation of amorphous hydrogenated silicon. Appl. Phys. Lett. 58 (15). 15 April 1991
Light-Enhanced Hydrogen Motion in a-Si:H 1991[edit | edit source]
The cells thicknesses were 100, 200, 500 nm. Deutrerium was added with silane (20% vol.) The paper shows that hydrogen-induced metastable defects are caused by recombination energy moving hydrogen which then results in dangling bonds. The paper looked at temperatures ranging 75-300 C and measured the photoconductivity of the cells.
P. V. Santos, N. M. Johnson, and R. A. Street. Light-enhanced hydrogen motion in a-Si:H. Phys. Rev. Lett. 67, 2686–2689 (1991)
Sub-bandgap absorption in a-Si:H pin cells illuminated with infrared light 1991[edit | edit source]
Abstract: We discuss some aspects of the Constant Photocurrent Method (CPM) when applied to a-Si:H pin diodes. One can get valuable information on midgap defect states when the cell is reverse biased. A direct correlation is established between a device characterizing quantity, the fill factor (FF), and the concentration of defects which is related to the sub-bandgap optical absorption constant (αD). Thus changes in the i-layer of pin cells, due to the creation of metastable defects by light-soaking or forward-current injection, are observed in changes of both αD and FF.
Herbert Rübel, Walter Frammelsberger, Peter Lechner and Norbert Kniffler. Sub-bandgap absorption in a-Si:H pin cells illuminated with infrared light. Volumes 137-138, Part 2, 1991, Pages 1169-1172. Journal of Non-Crystalline Solids. doi:10.1016/S0022-3093(05)80331-X
Defect relaxation in amorphous silicon: Stretched exponentials, the Meyer-Neldel rule, and the SWE 1991[edit | edit source]
This is one of the many papers that explains the phenomenons that occur during annealing with the use of various models. The stretch-exponential time depends on the defect relaxation and the Meyer-Neldel model depends on the relaxation-time constant.
Richard S. Crandall. Defect relaxation in amorphous silicon: Stretched exponentials, the Meyer-Neldel rule and the SWE. Physical Review B. Vol 43, Num 5. Feb 15, 1991.
Thickness dependence of light induced degradation in a-Si:H solar cells[edit | edit source]
Abstract: The long term stability of a-Si:H solar cells as a function of i-layer thickness was investigated using the accelerated degradation test. It is demonstrated that stabilized efficiency improves substantially as the i-layer thickness decreases. The thickness dependences of the stabilized efficiency under the actual cell operating conditions were estimated using the kinetic model for the solar cell degradation.
Liyou Yang and Liang-fan Chen. Thickness dependence of light induced degradation in a-Si:H solar cells. Volumes 137-138, Part 2, 1991, Pages 1189-1192. Journal of Non-Crystalline Solids. doi:10.1016/S0022-3093(05)80336-9
Effect of microvoids on initial and light-degraded efficiencies of hydrogenated amorphous silicon alloy solar cells 1992[edit | edit source]
- voids higher with poorer material
- larger degradation with larger void fraction
- deposition process affects voids
S. Guha and J. Yang,Scott J. Jones, Yan Chen, and D. L. Williamson. Effect of microvoids on initial and light-degraded efficiencies of hydrogenated amorphous silicon alloy solar cells. Appl. Phys. Lett. 61 (12), 21 September 1992 0003-6951/92/371444-03$03.00
Amorphous silicon based solar cells deposited from H2-diluted SiH4 at low temperatures 1993[edit | edit source]
Abstract: Amorphous silicon based solar cells have been developed which have both improved initial conversion efficiency and greater resistance to light induced degradation. The improved initial efficiency arises from higher open circuit voltage which is a result of depositing the i-layer at lower temperatures from SiH4 diluted in H2. The improvement in open circuit voltage is substantially greater than would be expected from the small increase in optical bandgap that is observed as deposition temperature is lowered and there are indications that charge transport across the cell might change as the deposition temperature is lowered. By optimizing the deposition parameters the authors were able not only to reduce the total light-induced degradation, but to affect a qualitative change in the nature of the functional dependence of the conversion efficiency on light soaking time, so that after a few hundred hours of light soaking time the efficiency asymptotically approaches a limiting value ("saturation")
Bennett, M.; Rajan, K.; Kritikson, K.;, "Amorphous silicon based solar cells deposited from H2-diluted SiH4 at low temperatures," Photovoltaic Specialists Conference, 1993., Conference Record of the Twenty Third IEEE, vol., no., pp.845-849, 10-14 May 1993 doi: 10.1109/PVSC.1993.347111
Shortfall of defect models for amorphous silicon solar cell performance 1993[edit | edit source]
Abstract: It is suggested that the prevailing models for the Staebler–Wronski effect are incorrect because they ignore the effects of charged dangling bonds. The degradation behavior of material parameters such as photoconductivity or midgap defect densities does not allow us to predict either the magnitude or the kinetic behavior of solar cell degradation.
von Roedern, Bolko;, "Shortfall of defect models for amorphous silicon solar cell performance," Applied Physics Letters, vol.62, no.12, pp.1368-1369, Mar 1993 doi: 10.1063/1.108681
On the lack of correlation between film properties and solar cell performance of amorphous silicon‐germanium alloys 1993[edit | edit source]
Abstract: We have studied the performance of amorphous silicon‐germanium alloy single‐junction solar cells both before and after light soaking. The intrinsic layers of the cells have different germanium contents. Films were grown on glass with parameters nominally identical to those for the intrinsic layer of the cells and the defect density was measured using the constant photocurrent method. We do not find good correlation between cell performance and the measured defect density for these high quality materials.
Xu, X.; Yang, J.; Guha, S.;, "On the lack of correlation between film properties and solar cell performance of amorphous silicon‐germanium alloys," Applied Physics Letters, vol.62, no.12, pp.1399-1401, Mar 1993 doi: 10.1063/1.108692
"Fast" and "slow"' metastable defects in hydrogenated amorphous silicon 1993[edit | edit source]
This paper degrades the cell at 50 sun for 5 min and 1 sun for 100 hr and what was found was this produces different effects. One is that the annealing time for the 50 sun was significantly faster even though in both cases the cells degradation eff was the same. Therefore depending on the treatment of the cell will affect the cell's defect density.
- this might affect annealing times for various conditions of testing...
L. Yang and L. Chen. "Fast" and "slow"' metastable defects in hydrogenated amorphous silicon. Solarex Corporation, Thin Film Division Newtown, Pennsylvania 18940
Thermal annealing recovery and saturation of light-induced degradation of amorphous silicon alloy solar cells with different microvoid density 1993[edit | edit source]
Abstract: We have studied the light-induced degradation and thermal annealing recovery of amorphous silicon alloy solar cells with different microvoid density in the intrinsic layer. The microvoid density was changed by altering the deposition rate. The experiments show that cells with higher microvoid density need longer annealing time to recover after prolonged light-soaking. As a consequence, cells with high density of microvoids do not seem to saturate even after long duration of light exposure. The cells with high microvoid density also show much lower degraded efficiency. A careful comparison between degradations caused by accelerated and one-sun light soaking and subsequent annealing recovery indicates that the defects created in the two cases have different nature.
Xu, X | Yang, J | Guha, S. Thermal annealing recovery and saturation of light-induced degradation of amorphous silicon alloy solar cells with different microvoid density. the MRS Spring Meeting; San Francisco, CA; USA; 13-16 Apr. 1993. pp. 649-654. 1993
have a hard copy.
Light-induced degradation on porous silicon 1993[edit | edit source]
Abstract: A study of photoluminescence degradation profiles in porous silicon as a function of time, temperature, and excitation intensity is reported. It is found that the degradation of the photoluminescence follows a stretched exponential function with the stretching parameter and the relaxation-time constant independent of the measured temperatures. The intensity of the saturated photoluminescence is linearly proportional to the excitation intensity, which indicates that the number of saturated nonradiative recombination centers does not depend on the excitation intensity. These results can be understood on the basis of the following mechanism: the defects in porous silicon are distributed exponentially in energy, and energy liberated upon carrier capture by the defects is localized in the vicinity of the defect and can be utilized to promote defect reactions and create nonradiative recombination centers. In addition, the striking results of the slowdown of the degradation rate and the enhancement of the photoluminescence intensity by illumination with an additional He-Ne laser can also be explained using the same mechanism.
I. M. Chang, S. C. Pan, and Y. F. Chen. Light-induced degradation on porous silicon. Phys. Rev. B 48, 8747–8750 (1993)
Accelerated stability test for amorphous silicon solar cells 1993[edit | edit source]
Abstrac: Fast light‐induced degradation of amorphous silicon p‐i‐n solar cells has been investigated by replacing cw illumination by light pulses of the same average intensity. This method allows us to evaluate the long‐term device performance with exposure times of the order of minutes and avoids complication due to cell temperature increase.
Rossi, M. C.; Brandt, M. S.; Stutzmann, M.;, "Accelerated stability test for amorphous silicon solar cells," Applied Physics Letters, vol.60, no.14, pp.1709-1711, Apr 1992 doi: 10.1063/1.107193
Extraction of amorphous silicon solar cell parameters by inverse modelling 1994[edit | edit source]
Abstract: A novel and unique numerical method of extraction of physical parameters from the measured characteristics was applied for the first time to single junction p-i-n amorphous silicon solar cells to determine several important input parameters used for their modelling. A set of realistic parameters, which describe the solar cells, has been determined from the fits of simulated behaviour to the measured one. The single junction p-i-n solar cells were deposited at the Utrecht University. A set of input parameters that closely describes the solar ceils behaviour is an important step for their further optimisation.
M. Zeman, J.A. Willemen, S. Solntsev, J.W. Metselaar. Extraction of amorphous silicon solar cell parameters by inverse modelling. Solar Energy Materials and Solar Cells 34 (1994) 557-563
PHOTO-INDUCED STRUCTURAL CHANGES ASSOCIATED WITH THE STAEBLER-WRONSKI EFFECT IN HYDROGENATED AMORPHOUS SILICON 1995[edit | edit source]
This paper discusses the various properties which reduce the photoconductivity. One interesting point was exposing the sample to low temeperatures (78 K) lowered the photoconductivity the same amount as at 300 K.
H. Fritzsche. PHOTO-INDUCED STRUCTURAL CHANGES ASSOCIATED WITH THE STAEBLER-WRONSKI EFFECT IN HYDROGENATED AMORPHOUS SILICON. Solid State Communications, Vol. 94, No. 11, pp. 953-955, 1995
Kinetics of light induced degradation in a-Si:H solar cells 1991[edit | edit source]
Abstract:The kinetic model proposed by Redfield and Bube1 for the light induced degradation was found to be self-consistent in explaining the extensive degradation data on solar cells. The model predicts that the effect of thermal annealing under the normal cell operating condition (50°C, AM1.5) is significant which stabilizes the cell at a higher efficiency than that attained under stronger illumination.
Liang-fan Chen and Liyou Yang. Kinetics of light induced degradation in a-Si:H solar cells. Journal of Non-Crystalline Solids. Volumes 137-138, Part 2, 1991, Pages 1185-1188. doi:10.1016/S0022-3093(05)80335-7
Irreversible light-enhanced degradation in amorphous silicon solar cells at elevated temperatures 1995[edit | edit source]
The tests were done at 50 suns and the temperature of the cells were 130 C. Has a plot of FF,EFF,Voc, Jsc and QE.
"the irreversible light-enhanced degradation of amorphous silicon solar cells at elevated temperatures appears to be associated with the light-enhanced diffusion of hydrogen. At lower temperatures, the reversible light induced degradation may also be associated with hydrogen motion, but since the metastable defects can be created at low temperatures by the trapping or recombination of free carriers, the hydrogen motion in this case is probably limited to localized bond switching on the internal surfaces of microvoids"
D. E. Carlson and K. Rajan. Irreversible light-enhanced degradation in amorphous silicon solar cells at elevated temperatures. Appl. Phys. Lett. 68 (1), 1 January 1996
The Future of Amorphous Silicon Photovoltaic Technology NREL 1995[edit | edit source]
Abstract: Amorphous silicon modules are commercially available. They are the first truly commercial thin-film photovoltaic (PV) devices. Well-defined production processes over very large areas (>1 m2) have been implemented. There are few environmental issues during manufacturing, deployment in the field, or with the eventual disposal of the modules. Manufacturing safety issues are well characterized and controllable. The highest measured initial efficiency to date is 13.7% for a small triple-stacked cell and the highest stabilized module efficiency is 10%. There is a consensus among researchers, that in order to achieve a 15% stabilized efficiency, a triple-junction amorphous silicon structure is required. Fundamental improvements in alloys are needed for higher efficiencies. This is being pursued through the DOE/NFEL Thin-Film Partnership Program. Cost reductions through improved manufacturing processes are being pursued under the National Renewable Energy Laboratory/U.S. Department of Energy (NRELD0E)-sponsored research in manufacturing technology (PVMaT). Much of the work in designing a-Si devices is a result of trying to compensate for the Staebler-Wronski effect. Some new deposition techniques hold promise because they have produced materials with lower stabilized defect densities. However, none has yet produced a high efficiency device and shown it to be more stable than those from standard glow discharge deposited material.
R. Crandall, W. Luft. The Future of Amorphous Silicon Photovoltaic Technology. NRELLlT-411-8019 UC Category: 1262 DE95009235
ESTIMATION OF THE DEGRADATION OF AMORPHOUS SILICON SOLAR CELLS 1996[edit | edit source]
The paper degraded 5 cells and averaged the results and temperatures were 10, 30, 60 and 80 C at 0.13, 0.25, 0.5 and 1 kW/m^2 irradiance. It was found that the high temp produced the best results. The paper then models these results to predict degradation.
TAKESHI YANAGISAWA.ESTIMATION OF THE DEGRADATION OF AMORPHOUS SILICON SOLAR CELLS. Microelectron. Reliab., Vol. 37, No. 4, pp. 549-554, 1997
Stability of a-Si:H solar cells and corresponding intrinsic materials fabricated using hydrogen diluted silane 1996[edit | edit source]
Abstract: We report on a study in which properties of p(a-SiC:H)/i(a-Si:H)/n(μc-Si) a-Si:H solar cells and their i-materials prepared with hydrogen dilution are investigated and compared with films and cells prepared without hydrogen dilution. The cells and the corresponding intrinsic films were fabricated in a multi-chamber PECVD system with pure silane (SiH4) and silane diluted with hydrogen in the ratio [H2]/[SiH4]=10. The initial performance of both types of cells (~4000 Å thick) fabricated without optical enhancement are quite similar but the diluted cells are significantly more stable. Despite the reported importance of the interface regions in determining their solar cell characteristics, a direct correlation between the degradation of the diluted solar cells and their intrinsic films is observed in this study. Both diluted cells and films reach a steady state of degradation under AM1 illumination within 100 hours. Distinctly different kinetics from the undiluted materials and cells and the ability to reach steady state degradation in less than 100 hours offer a new probe for improving our understanding of the mechanisms limiting cell performance
Lee, Y.-H.; Jiao, L.-H.; Liu, H.-Y.; Lu, Z.; Collins, R.; Wronski, C.R.;, "Stability of a-Si:H solar cells and corresponding intrinsic materials fabricated using hydrogen diluted silane," Photovoltaic Specialists Conference, 1996., Conference Record of the Twenty Fifth IEEE, vol., no., pp.1165-1168, 13-17 May 1996 doi: 10.1109/PVSC.1996.564339
Distribution of charged defects in a:Si---H n-i Schottky barrier solar cells 1996[edit | edit source]
Abstract: Nickel n-i Schottky barriers solar cell structures with i-layer thicknesses between 1.0 and 2 μm were characterized by dark I–V's at different temperatures and internal quantum efficiencies (QE). These characteristics were modeled using AMPS (analysis of microelectronic and photonic structures) with charged defect distributions which were derived from results on corresponding i-layer films. This self-consistency was obtained with distributions of gap states which consist of charged defects where dangling bond states are represented by three Gaussians: positively charged D+ above mid-gap; neutral D0 around mid-gap; and negatively charged D− below mid-gap. Excellent fits to the results on Schottky barriers and films can be obtained with these bulk distributions of gap states which is not true for the commonly used two Gaussian D−, D0 gap state distributions.
Hongyue Liu, L. Jiao, S. Semoushkina and C. R. Wronski. Distribution of charged defects in a:Si---H n-i Schottky barrier solar cells. Journal of Non-Crystalline Solids. Volumes 198-200, Part 2, 2 May 1996, Pages 1168-1171