Material Properties of a-SiGe:H Solar Cells as a Function of Growth Rate

2010 ◽  
Vol 1245 ◽  
Author(s):  
Peter Hugger ◽  
JinWoo Lee ◽  
J. David Cohen ◽  
Guozhen Yue ◽  
Xixiang Xu ◽  
...  
Keyword(s):  
Valence Band ◽  
Material Properties ◽  
Defect Density ◽  
Schottky Contact ◽  
Drive Level ◽  
Measurement Methods ◽  
Device Performance ◽  
Band Tail ◽  
Contact Devices ◽  
Defect Densities

AbstractWe have examined a series of a Si,Ge:H alloy devices deposited using both RF and VHF glow discharge in two configurations: SS/n+/i (a-SiGe:H)/p+/ITO nip devices and SS/n+/i (a-SiGe:H)/Pd Schottky contact devices, over a range of deposition rates. We employed drive-level capacitance profiling (DLCP), modulated photocurrent (MPC), and transient junction photo-current (TPI) measurement methods to characterize the electronic properties in these materials. The DLCP profiles indicated quite low defect densities (mid 1015 cm-3. to low 1016 cm-3 depending on the Ge alloy fraction) for the low rate RF (∼1Å/s) deposited a-SiGe:H materials. In contrast to the RF process, the VHF deposited a-SiGe:H materials did not exhibit nearly as rapid an increase of defect density with the deposition rate, remaining well below 1017 cm-3. up to rates as high as 10Å/s. Simple examination of the TPI spectra on theses devices allowed us to determine valence band-tail widths.. Modulated photocurrent (MPC) obtained for several of these a-SiGe:H devices allowed us to deduce the conduction band-tail widths. In general, the a-Si,Ge:H materials exhibiting narrower valence band-tail widths and lower defect densities correlated with the best device performance.

States in the Gap of Improved a-Ge:H Studied by Photomodulation Spectroscopy

1991 ◽  
Vol 219 ◽  
Author(s):  
L. Chen ◽  
J. Tauc ◽  
D. Pang ◽  
W. A. Turner ◽  
W. Paul
Keyword(s):  
Glow Discharge ◽  
Valence Band ◽  
Density Of States ◽  
Pump Beam ◽  
Defect Density ◽  
Dangling Bond ◽  
Band Tail ◽  
Valence Bands

ABSTRACTThe photomodulation spectra of a-Ge:H of average photoelectronic quality(ημπ = 1 × 10-10cm2/V) and of improved quality (ημπ = 3 × 10-7cm2/V), produced under different plasma conditions in an r.f. diode reactor by glow discharge, were measured at 80K and are analyzed in analogy with earlier studies of a-Si:H. The spectra of the poorer material are dominated by transitions between dangling bond states and the conduction and valence bands. By contrast, the spectra of the better material require contributions of transitions from the band tail states, indicating that the reduced defect density has resulted in pump-beam induced quasi-Fermi levels reaching near the conduction and valence band edges. A very acceptable fit between plausible density-of-states distributions and the experimental spectra has been found.

Capacitance and Transient Photocapacitance Studies of Tetrahedral Amorphous Carbon

2000 ◽  
Vol 621 ◽  
Author(s):  
Kimon C. Palinginis ◽  
A. Ilie ◽  
W.I. Milne ◽  
J. David Cohen
Keyword(s):  
Amorphous Carbon ◽  
Urbach Energy ◽  
Defect Density ◽  
Intrinsic Property ◽  
Band Tail ◽  
Junction Capacitance ◽  
Tetrahedral Amorphous Carbon ◽  
Thermally Activated ◽  
A Value ◽  
Defect Densities

ABSTRACTWe have applied junction capacitance and transient photocapacitance measurements to undoped tetrahedral amorphous carbon (ta-C)/silicon carbide (SiC) heterostructures to deduce defect densities and defect distributions in ta-C. The junction capacitance measurements show two thermally activated processes. One can be related to the activation of carriers out of defects at the ta-C/SiC interface while the other one with an activation energy of 0.36eV is an intrinsic property of the ta-C. The defect density at the ta-C/SiC interface is estimated to be roughly 9 ± 2 × 109 cm−2. The transient photocapacitance measurements have allowed us to observe the broader band tail of ta-C, giving a value (Urbach energy) of 230meV.

An Alternative Model for the Kinetics of Light-Induced Defects in A-Si:H

1991 ◽  
Vol 219 ◽  
Author(s):  
Paulo V. Santos ◽  
W. B. Jackson ◽  
R. A. Street
Keyword(s):  
Time Dependence ◽  
Valence Band ◽  
Defect Formation ◽  
Defect Density ◽  
Generation Rate ◽  
Band Tail ◽  
Defect Generation ◽  
Wide Range ◽  
Induced Defects ◽  
Kinetics Of

ABSTRACTThe kinetics of light-induced defect generation in a-Si:H was investigated over a wide range of illumination intensities and temperatures. The defect density around 1016cm-3 exhibits a power-law time dependence Ns ∼ G2εfε with ε = 0.2 to 0.3, where G is the photo-carrier generation rate. A model for the kinetics of defect generation is proposed based on the existence of an exponential distribution of defect formation energies in the amorphous network, associated with the valence band tail states. The model reproduces the observed time dependence of the defect density with an exponent e determined by the exponential width of the valence band tail. The temperature dependence of the defect generation rate is well-reproduced by the model, which provides a connection between the Stabler-Wronski effect and the weak-bond model.

Reduction of Defects in GaN Epitaxial Films Grown Heteroepitaxially on SiC

2006 ◽  
Vol 527-529 ◽  
pp. 1483-1488 ◽  
Author(s):  
Charles R. Eddy ◽  
N.D. Bassim ◽  
Michael A. Mastro ◽  
R.L. Henry ◽  
Mark E. Twigg ◽  
...  
Keyword(s):  
Defect Density ◽  
Device Performance ◽  
Nucleation Layer ◽  
Lattice Match ◽  
Extended Defect ◽  
Dislocation Densities ◽  
Previous State ◽  
Step Density ◽  
Defect Densities ◽  
Very High

Silicon carbide (SiC) has become the substrate of choice for III-N epilayers applied to electronic devices due to the lack of a native III-N substrate. This is particularly true for high power applications, since the thermal conductivity of the substrate enhances device performance. Although the GaN lattice match is slightly better for SiC than for sapphire, the dislocation densities that result are still very high (generally in the high 108 cm-2 range) and often deleterious to device performance. Screw-component dislocations are especially critical since they serve as leakage paths in vertically conducting III-N devices. In this paper efforts to reduce the extended defect density in III-N films grown on SiC will be reviewed. Details on recent efforts to use step-free SiC mesa surfaces arrayed on commercial 4HSiC substrates will then be highlighted showing dramatic reductions in extended defect densities and the virtual elimination of critical defects for vertically conducting devices. In these experiments, SiC surfaces that are homoepitaxially grown step-free or of very low step density have been used as growth templates for thin (<3 μm) GaN films deposited on a novel 1000 Å AlN nucleation layer characterized by a total dislocation density two orders of magnitude lower than the previous state-of-the-art, and with no evidence of screw-component dislocations.

Effect of C Impurities in a-Si:H as Measured by Drive-Level Capacitance, Photo Current, and Electron Spin Resonance

1993 ◽  
Vol 297 ◽  
Author(s):  
J. Hautala ◽  
T. Unold ◽  
J.D. Cohen
Keyword(s):  
Electron Spin Resonance ◽  
Electron Spin ◽  
Defect Density ◽  
Drive Level ◽  
Level Data ◽  
Definite Effect ◽  
Wide Range ◽  
Spin Resonance ◽  
Defect Densities ◽  
Photo Current

The effect of C impurities in a-Si:H in levels of 0.4 to 2.6 at. % were studied over a wide range of metastable defect densities. Three complimentary experimental techniques [electron spin resonance (ESR), drive-level capacitance (DLC) and photo-current] were employed to track the material's defect density with light soaking and annealing, as well as Urbach energies, midgap absorption and mobility gaps energies as a function of the C content. Our results show C impurities have a definite effect on the initial and saturated defect densities, as well as the midgap absorption and Urbach energies at levels 1 at. % and above. The results indicate that C acts mainly as a center for increased disorder in the material which results in an increase in the bandtail widths, and consequently an increase in intrinsic defects. Comparison to the ESR and drive-level data show an excellent agreement between these two techniques in determining the bulk defect densities in a-Si:H.

The Electronic Properties of a-Si:H Deposited With Hydrogen or Helium Dilution

1996 ◽  
Vol 420 ◽  
Author(s):  
F. Zhong ◽  
W. S. Hong ◽  
V. Perez-Mendez ◽  
C. C. Chen ◽  
J. D. Cohen
Keyword(s):  
Defect Density ◽  
Schottky Diodes ◽  
Growth Conditions ◽  
Drive Level ◽  
Standard Samples ◽  
Level Densities ◽  
Helium Dilution ◽  
Neutral Defect ◽  
Defect Densities ◽  
Deposition Conditions

AbstractWe have applied the Drive-Level Capacitance Profiling (DLCP) method to n-i-p a-Si:H diodes to characterize the mid-gap defect densities in the i layer. Our results show that there are no significant changes in the drive-level densities, Ndl, in n-i-p diodes and p+-i-m as well as n-i-m Schottky diodes, which indicates that DLCP can directly provide reliable energy distribution and spatial distribution of the mid-gap defects in the n-i-p device. We have found that the ratio of Ndl to ND*, the ionized defect density determined by hole onset measurement, is changed with the deposition conditions; it is 3 for standard samples, 2 for helium diluted samples and 6 for hydrogen diluted samples. These results indicate that there may be different defect distributions in these materials, which suggest the ratio of charged (D-) density to the neutral defect (D0) density may be altered when growth conditions are varied.

Correlation of Material Properties and Open-Circuit Voltage of Amorphous Silicon Based Solar Cells

2003 ◽  
Vol 762 ◽  
Author(s):  
Baojie Yan ◽  
Jeffrey Yang ◽  
Guozhen Yue ◽  
Subhendu Guha
Keyword(s):  
Solar Cells ◽  
Solar Cell ◽  
Amorphous Silicon ◽  
Band Gap ◽  
Material Properties ◽  
Open Circuit Voltage ◽  
Defect Density ◽  
Open Circuit ◽  
Band Tail ◽  
Simulation Results

AbstractCorrelation of hydrogenated amorphous silicon (a-Si:H) alloy material properties and solar cell characteristics have been studied experimentally and by computer simulation. Simulation results show that all three solar cell parameters, short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF), decrease with increased defect density. For a given intrinsic layer thickness, a larger band gap (Eg) results in a higher Voc but a lower Jsc. However, FF does not depend on band gap. This allows us to distinguish the effect of change in band gap from that in defect density on the variation in Voc. For solar cells with good interface characteristics, a linear relation FF = βVoc + γ is obtained by light soaking experiments and simulation with different defect densities. The slope β is in the range from 2 to 3 V-1 depending on cell properties and light soaking condition, and the intersect γ depends mainly on the band gap. Comparing cells made with high H2 dilution to no H2 dilution, we find that a 58 mV enhancement in Voc with H2 dilution is due to both widening of band gap and reduced defect density. Simulation results also show that a narrower valence band tail leads to a higher Voc. We did not include this effect in the analysis due to lack of available data for correlation between H2 dilution and band tail narrowing.

Effect Of Increasing Growth Rates on Film Properties and Device Performance for DC Glow Discharge Amorphous Silicon

2002 ◽  
Vol 715 ◽  
Author(s):  
Jennifer T. Heath ◽  
James J. Gutierrez ◽  
J. David Cohen ◽  
Gautam Ganguly
Keyword(s):  
Growth Rate ◽  
Amorphous Silicon ◽  
Film Growth ◽  
Drive Level ◽  
Device Performance ◽  
Dc Glow Discharge ◽  
Film Properties ◽  
Defect Band ◽  
Device Efficiency ◽  
Defect Densities

AbstractWe have examined the electronic properties of intrinsic amorphous silicon films as a function of deposition rate, and compared these with the performance of companion solar cell p-i-n devices. The device efficiency in the light-soaked state was strongly inversely correlated with growth rate. Film properties were evaluated in both the as-grown and light soaked states using drive-level capacitance profiling and transient photocapacitance spectroscopy. Although deep defect densities measured by drive-level capacitance profiling did not vary significantly between samples, the magnitude of the defect band deduced via transient photocapacitance spectroscopy was well correlated with device performance. Possible reasons for this discrepancy are discussed. Urbach energies were also correlated with film growth rate.

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.

scholarly journals An antibonding valence band maximum enables defect-tolerant and stable GeSe photovoltaics

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Shun-Chang Liu ◽  
Chen-Min Dai ◽  
Yimeng Min ◽  
Yi Hou ◽  
Andrew H. Proppe ◽  
...  
Keyword(s):  
Solar Cells ◽  
Valence Band ◽  
Surface Passivation ◽  
Defect Density ◽  
Perovskite Solar Cells ◽  
Valence Band Maximum ◽  
Ambient Conditions ◽  
Band Maximum ◽  
Point Tracking ◽  
Power Point

AbstractIn lead–halide perovskites, antibonding states at the valence band maximum (VBM)—the result of Pb 6s-I 5p coupling—enable defect-tolerant properties; however, questions surrounding stability, and a reliance on lead, remain challenges for perovskite solar cells. Here, we report that binary GeSe has a perovskite-like antibonding VBM arising from Ge 4s-Se 4p coupling; and that it exhibits similarly shallow bulk defects combined with high stability. We find that the deep defect density in bulk GeSe is ~1012 cm−3. We devise therefore a surface passivation strategy, and find that the resulting GeSe solar cells achieve a certified power conversion efficiency of 5.2%, 3.7 times higher than the best previously-reported GeSe photovoltaics. Unencapsulated devices show no efficiency loss after 12 months of storage in ambient conditions; 1100 hours under maximum power point tracking; a total ultraviolet irradiation dosage of 15 kWh m−2; and 60 thermal cycles from −40 to 85 °C.

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