CPM and PDS - A Critical Interpretation of Experimental Results

1996 ◽  
Vol 420 ◽  
Author(s):  
Helmut Stiebig ◽  
Frank Siebke ◽  
Reinhard Carius
Keyword(s):  
Defect Density ◽  
Optical Transitions ◽  
Defect States ◽  
Defect Distribution ◽  
Urbach Tail ◽  
Yield Information ◽  
Pool Model ◽  
Neutral Defect ◽  
Critical Interpretation ◽  
Good Agreement

AbstractCPM and PDS spectra of a-Si:H yield identical shape of the Urbach tail, while the defect absorption measured by PDS differs significantly from CPM. In this work an analysis of CPM and PDS spectra of annealed and degraded films is presented. Numerical simulations of CPM and PDS data, taking into account optical transitions, capture and emission processes as well as the Fermi level, yield information on the energy distribution and the charge state of the defects. The simulations reveal the coexistence of defects in the D−, D+ and D0 states. The defect distribution is dominated by charged states as predicted by the defect-pool model. Good agreement between measured and simulated PDS and CPM spectra can be obtained in the case of a homogeneous defect density. It is shown that differences between CPM and PDS are due to different sensitivities of the techniques to charged and neutral defect states. Microscopic inhomogeneities may cause significant additional differences.

Examination of the Defect Pool Model by An Improved Analysis of the Constant Photocurrent Method

1995 ◽  
Vol 377 ◽  
Author(s):  
Helmut Stiebig ◽  
Frank Siebke
Keyword(s):  
Valence Band ◽  
Defect Density ◽  
Localized States ◽  
Optical Transitions ◽  
Defect States ◽  
Band Tail ◽  
Defect Distribution ◽  
Density Of Localized States ◽  
Pool Model ◽  
Extended States

ABSTRACTWe have developed an improved analysis of constant photocurrent method (CPM) data. It is based on a numerical simulation of CPM spectra taking into account the full set of optical transitions between localized and extended states, capture and emission processes as well as the position of the Fermi level. Comparing measured and simulated CPM spectra provides information about the density of localized states in a-Si:H, i.e. the valence band tail, the integrated defect density, the energy distribution and the charge state of defect states. Based on these results we examine the predictions of the defect-pool model. The defect distribution in undoped and doped a-Si:H can be described by the defect-pool model taking into account the doping level dependence of principal parameters including the valence band tail, the equilibration temperature, and the width of the defect-pool.

Defect Distributions in Doped and Undoped A-SiGe:H Alloys

1998 ◽  
Vol 507 ◽  
Author(s):  
Helmut Stiebig ◽  
Frank Siebke ◽  
Reinhard Carius ◽  
Josef Klomfaβ
Keyword(s):  
Absorption Spectra ◽  
Defect Density ◽  
Charged Defect ◽  
Defect States ◽  
Charge Neutrality ◽  
Photothermal Deflection Spectroscopy ◽  
Neutral Defect ◽  
Related Defect ◽  
Gap States ◽  
Good Agreement

ABSTRACTIn this work, gap states in doped and undoped a-SiGe:H alloys are examined by numerical simulations of sub-bandgap absorption spectra measured by the constant photocurrent method and photothermal deflection spectroscopy. Deconvolution methods, neglecting the condition of charge neutrality, can be used for a rough estimate of the defect density value but not for ob- taining detailed information on the distribution of gap states in undoped samples. Our numerical analysis uses adapted occupation statistics and takes into account the condition of charge neutrality. Good agreement between measured and simulated PDS and CPM spectra is obtained. For a certain composition, i.e. a certain bandgap, the investigation of doped films yields infor- mation on the density and the position of charged defect states in the bandgap. In addition, the density of neutral defect states can be derived from a comparison of CPM and PDS spectra. The results reveal the coexistence of charged and neutral defects. In doped as well as in undoped films, charged defect states dominate the defect density. In the investigated range of compo- sitions the defect distribution of a-SiGe:H is similar to those found in a-Si:H. The width of the defect distributions does not decrease with decreasing bandgap. No evidence for a different be- havior of Si- and Ge-related defect states can be found in sub-bandgap absorption spectra.

The Interpretation of the Constant Photocurrent Method in Terms of Deep Defect Density of States in A-SI.H

1994 ◽  
Vol 336 ◽  
Author(s):  
Frank Siebke ◽  
Helmut Stiebig
Keyword(s):  
Electron Spin Resonance ◽  
Photoelectron Spectroscopy ◽  
Defect Density ◽  
Total Yield ◽  
Calibration Factor ◽  
Optical Transitions ◽  
Absolute Value ◽  
Spin Resonance ◽  
Pool Model

ABSTRACTThe constant photocurrent Method (CPM) is often used to measure the sub-bandgap absorption for the determination of the defect density. However, the absolute value of the derived defect density depends on the method of data analysis and the calibration factor. Normally the calibration factor is obtained from electron spin resonance (ESR) but the defect pool model gives rise to doubt whether ESR detects the same defects as CPm. Therefore, we propose combined total-yield photoelectron spectroscopy (TYPES) and CPM Measurements on n-type a-Si:H to determine the calibration factor. Furthermore, we calculate CPM spectra by extending an approach to simulate photoconductivity, taking into account the full set of optical transitions, and compare the results with standard evaluation Methods.

Application of the Defect Pool Model in Modelling of a-Si:H Solar Cells

1995 ◽  
Vol 377 ◽  
Author(s):  
Miroslav Zeman ◽  
Guoqiao Tao ◽  
Marcel Trijssenaar ◽  
Joost Willemen ◽  
Wim Metselaar ◽  
...  
Keyword(s):  
Solar Cells ◽  
Amorphous Semiconductor ◽  
Defect States ◽  
Conventional Model ◽  
University Of Technology ◽  
Pool Model ◽  
Analysis Computer ◽  
Hydrogenated Amorphous ◽  
Analysis Computer Program ◽  
Good Agreement

ABSTRACTThe increasing complexity of hydrogenated amorphous silicon (a-Si:H) based solar cells requires continuous extending and testing of the computer models which are used for their simulation. The calculation of the defect states density in the bandgap of a-Si:H based on the defect pool model (DPM) has been rally implemented in the ASA (Amorphous Semiconductor Analysis) computer program developed at Delft University of Technology. We used the technique of inverse modelling to verify the DPM and to calibrate it by fitting the dark and illuminated J-V characteristics of a single-junction a-Si:H solar cell fabricated at Utrecht University. The DPM was also used in simulating the absorption coefficient of a-Si:H layers. Using the DPM for the defect states distribution, a good agreement between simulated and measured data was obtained. The values of the material parameters needed for obtaining the best fits are more realistic than using a conventional model for the defect states distribution.

I-V CHARACTERISTICS OF a-Si:H p-i-n Diodes with Uniform and Non-Uniform Defect Distributions

2000 ◽  
Vol 609 ◽  
Author(s):  
M.A. Kroon ◽  
R.A.C.M.M. van Swaaij ◽  
J.W. Metselaar
Keyword(s):  
Space Charge ◽  
Uniform Distribution ◽  
Charge Distribution ◽  
Defect States ◽  
Defect Distribution ◽  
Intrinsic Region ◽  
Current Voltage ◽  
Space Charge Distribution ◽  
Pool Model ◽  
Current Voltage Characteristics

ABSTRACTThis paper compares a-Si:H p-i-n diodes having a spatially uniform distribution of defect states with diodes in which the defect distribution is non-uniform, i.e. equilibrated according to the Defect-Pool model. Diodes with a uniform defect distribution exhibit a clear dependence of the current-voltage characteristics on the width of the intrinsic region, whereas in equilibrated diodes, this dependence is absent. This difference is explained by comparing the space-charge distribution and the recombination profile of the intrinsic region in both types of diodes.

Investigations of Tail and Defect States in a-Si0.6Ge0.4:H Alloys by PDS and ESR - Implications for the Interaction of Weak and Dangling Bonds

1995 ◽  
Vol 377 ◽  
Author(s):  
Tilo P. Drüsedau ◽  
Andreas N. Panckow ◽  
Bernd Schröder
Keyword(s):  
Silicon Germanium ◽  
Defect Density ◽  
State Density ◽  
Model Parameters ◽  
Defect States ◽  
Conversion Model ◽  
Order Of Magnitude ◽  
Underlying Mechanisms ◽  
Amorphous Silicon Germanium ◽  
Excess Absorption

ABSTRACTInvestigations on the gap state density were performed on a variety of samples of hydrogenated amorphous silicon germanium alloys (Ge fraction around 40 at%) containing different amounts of hydrogen. From subgap absorption measurements the values of the “integrated excess absorption” and the “defect absorption” were determined. Using a calibration constant, which is well established for the determination of the defect density from the integrated excess absorption of a-Si:H and a-Ge:H, it was found that the defect density is underestimated by nearly one order of magnitude. The underlying mechanisms for this discrepancy are discussed. The calibration constants for the present alloys are determined to 8.3×1016 eV−1 cnr2 and 1.7×1016 cm−2 for the excess and defect absorption, respectively. The defect density of the films was found to depend on the Urbach energy according to the law derived from Stutzmann's dangling bond - weak bond conversion model for a-Si:H. However, the model parameters - the density of states at the onset of the exponential tails N*=27×1020 eV−1 cm−3 and the position of the demarcation energy Edb-E*=0.1 eV are considerably smaller than in a-Si:H.

Electronic Defects and Device Performance in CuGaSe2 Solar Cells

2007 ◽  
Vol 1012 ◽  
Author(s):  
J. Jedediah Rembold ◽  
Todd W. Curtis ◽  
Jennifer T. Heath ◽  
David L. Young ◽  
Steve W. Johnston ◽  
...  
Keyword(s):  
Solar Cells ◽  
Defect Density ◽  
Wide Bandgap ◽  
Device Performance ◽  
Defect States ◽  
Materials Properties ◽  
Interface Recombination ◽  
Interface Defect ◽  
Limited Supply ◽  
Electronic Defects

AbstractThe electronic and materials properties of two series of wide-bandgap solar cells with Cu-poor CuGaSe2 (CGS) absorbers have been studied, to better understand limitations on the device performance. One series of samples displayed distinct lateral non-uniformities in Cu/Ga ratio, Na content, and thickness, likely due to a limited supply of Se during CGS growth. The second series of samples appeared uniform. The most prominent electronic difference was the presence of a distinct band of near-interface defect states in the more non-uniform set of samples. The device performance did not appear to be limited by defects in the bulk CGS film until the defect density was larger than 2×1016 cm-3. Instead, interface recombination appears to be a significant factor limiting Voc in both sets of samples.

Charge transport through localized states in sputtered amorphous silicon suboxides

2001 ◽  
Vol 666 ◽  
Author(s):  
J.J. van Hapert ◽  
N. Tomozeiu ◽  
E.E. van Faassen ◽  
A.M. Vredenberg ◽  
F.H.P.M. Habraken
Keyword(s):  
Amorphous Silicon ◽  
Defect Density ◽  
Rf Magnetron Sputtering ◽  
Quantitative Information ◽  
Thin Layers ◽  
Localized States ◽  
Variable Range Hopping ◽  
Defect States ◽  
Variable Range ◽  
Spin Resonance

ABSTRACTUsing an RF magnetron sputtering technique, thin layers (∼500 nm) of amorphous silicon suboxides (a-SiOx) were deposited, with oxygen/silicon ratios x ranging from 0 to 1.8. These layers contain a large density (1020−1021 cm−3) of, mostly silicon dangling bond, defect states. The level of conduction decreases several orders of magnitude with increasing x. The temperature dependence of the DC conductivity showed that the variable range hopping conduction mechanism is dominant for all x, over the temperature range 30- 330 K. In this mechanism the extent of localization and density of states around the Fermi level determine the conductance. We conclude that the decrease in conductance with increasing oxygen content must, for a large part, be due to a variation in the localization, since Electron Spin Resonance (ESR) measurements showed no decrease in defect density with increasing x. We performed DC conduction measurements at both low and high electric field strengths, showing phenomena, which are consistently desribed within the variable range hopping (VRH) model. These measurements allow the extraction of quantitative information, concerning both the localization and the density of the states involved in the hopping process.

Thermally-Induced Change of Conductivity and Defect Density in Amorphous Silicon-Germanium Alloys

1990 ◽  
Vol 192 ◽  
Author(s):  
Tatsuo Shimizu ◽  
Xixiang Xu ◽  
Hiroyuki Sasaki ◽  
Hui Yan ◽  
Akiharu Morimoto ◽  
...  
Keyword(s):  
Amorphous Silicon ◽  
Silicon Germanium ◽  
Defect Density ◽  
Equilibrium Temperature ◽  
Exponential Form ◽  
Thermally Induced ◽  
Neutral Defect ◽  
Fast Cooling ◽  
Amorphous Silicon Germanium ◽  
Silicon Germanium Alloys

ABSTRACTThermally-induced metastable phenomena in amorphous silicon-germanium alloys were studied by conductivity and ESR measurements. Fast cooling from 250 °C reduced both dark- and photo-conductivities by a factor of 3–4 while the neutral defect density remained unchanged. Thermally-induced change in conductivity relaxed towards equilibrium with a stretched exponential form. The thermal equilibrium temperature was found to be roughly proportional to the optical gap for a–Si:H, a–Sii−xCx:H, a–Si1−xNx:H and a–Si1−xGex:H:F.

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