Search

This report summarizes the development and evaluation of higher-bandgap absorbers in the CIS alloy system. The major effort focused on exploring suitable absorbers with significant sulfur alloying in collaboration with Shafarman's group at the Institute of Energy Conversion. Three series of samples were examined; first, a series of quaternary CuIn(SeS)2-based devices without Ga; second, a series of devices with pentenary Cu(InGa)(SeS)2 absorbers in which the Se-to-S and In-to-Ga ratios were chosen to keep the bandgap nearly constant, near 1.52 eV. Third, based on the most-promising samples in those two series, we examined a series of devices with pentenary Cu(InGa)(SeS)2 absorbers with roughly 25 at.% S/(Se+S) ratios and varying Ga fractions. We also characterized electronic properties of several wide-bandgap CuGaSe2 devices from both IEC and NREL. The electronic properties of these absorbers were examined using admittance spectroscopy, drive-level capacitance profiling, transient photocapacitance, and transient photocurrent optical spectroscopies. The sample devices whose absorbers had Ga fraction below 40 at.% and S fractions above 20 at.% but below 40% exhibited the best electronic properties and device performance.

A major effort during this subcontract period has been to evaluate the microcrystalline Si material under development at United Solar Ovonics Corporation (USOC). This material is actually a hydrogenated nanocrystalline form of Si and it will be denoted in this report as nc-Si:H. Second, we continued our studies of the BP Solar high-growth samples. Third, we evaluated amorphous silicon-germanium alloys produced by the hot-wire chemical vapor deposition growth process. This method holds some potential for higher deposition rate Ge alloy materials with good electronic properties. In addition to these three major focus areas, we examined a couple of amorphous germanium (a-Ge:H) samples produced by the ECR method at Iowa State University. Our studies of the electron cyclotron resonance a-Ge:H indicated that the Iowa State a Ge:H material had quite superior electronic properties, both in terms of the drive-level capacitance profiling deduced defect densities, and the transient photocapacitance deduced Urbach energies. Also, we characterized several United Solar a Si:H samples deposited very close to the microcrystalline phase transition. These samples exhibited good electronic properties, with midgap defect densities slightly less than 1 x 1016 cm-3 in the fully light-degraded state.

This report describes our experimental studies which have been concentrated in roughly five areas. Specifically: (1) We have examined a?Si:H grown very close to the microcrystalline phase boundary, so-called''edge material,'' to help understand why such material is more stable with respect to light-induced degradation; (2) We have also studied the electronic properties, and degradation characteristics of mixed phase material that is mostly a?Si:H, but which contains a significant microcrystalline component; (3) We have examined the electronic properties of high deposition rate material. These studies have included both moderately high deposition rate material (up to 6/s) produced by the PECVD growth method, and extremely high deposition rate material (up to 130/s) produced by the HWCVD growth method. (4) We have examined series of a-Si,Ge:H alloys from several sources. In one extensive series of studies we examined low Ge fraction alloys in an attempt to learn more about the fundamentals of degradation in general. In a couple other studies we evaluated the properties of a-Si,Ge:H alloys produced by methods we had not previously examined. (5) Finally, for three different types of samples we compared basic material properties with companion cell performance data. This was carried out in each case on series of samples for which one or more specific deposition parameters were varied systematically.

An overriding theme of the work described in this report has been the effect of partial crystallinity, or the approach to partial crystallinity, on the electronic properties of a-Si:H. This includes, of course, how degradation or the relative stability of these films is affected by the approach to, or onset of, microcrystallinity. The authors first discussed the results on a set of samples produced by dc reactive magnetron sputtering, obtained in collaboration with John Abelson's group at the University of Illinois, for which they demonstrated the existence of a small, but significant, microcrystalline component. For these films, the degradation kinetics was found to be quite unusual; however, it could be well accounted for by a model that postulated two phases of degrading material. One was a-Si:H host material of good quality and the other was a more defective component associated with boundary regions near the microcrystallites. The sub-band-gap photocapacitance measurements on these films also indicated the existence of a distinct feature (a ``shoulder'' with a threshold near 1.1 eV) that signaled the presence of the microcrystalline phase. The second set of samples investigated were produced by Uni-Solar, deposited under conditions of high hydrogen dilution, very close to but just below the microcrystalline phase boundary. Here they found that the defect density following light-induced degradation decreased as the film thickness increased. Corroborating their findings with X-ray diffraction results obtained by Don Williamson on sets of similar films, the authors concluded that the films were becoming more ordered and less defective just prior to the onset of a detectable microcrystalline component. Furthermore, they found that at almost exactly the conditions that Williamson found XRD evidence for the onset of microcrystallinity, they found the appearance of the distinctive ``shoulder'' in the sub-band-gap photocapacitance spectra. Third, they investigated two sets of samples where the deposition rate had been varied to include samples grown at moderate to high rates. In one set of samples, produced at ETL, samples deposited under H{sub 2} dilution were found to exhibit extremely low deep defect densities and narrow Urbach tails, indicating films of exceptional quality. The photocapacitance spectra for these films were found to contain evidence for a small degree of microcrystallinity. In another set of samples, produced at UniSolar, they found evidence for increasing defect densities plus somewhat larger Urbach energies for the films deposited at higher rates. This is consistent with the fact that the photovoltaic device performance is significantly poorer for the higher deposition rate material. Finally, they discussed the general issue of deep defect densities in the a-Si,Ge:H alloys. They again demonstrated how well the deep defect densities in such samples from several sources could be fit using the spontaneous bond-breaking model of Martin Stutzmann. This implies that such state-of-the-art alloy films have been optimized in a quantifiable sense. They also found that the increase in deep defect density with small amounts of P and B dopants could also be reproduced reasonably well by modifying the spontaneous bond-breaking model to include the extra energy terms associated with charged defects.

The work carried out by the University of Oregon Under this subcontract focused on the characterization and evaluation of low-gap (a-Si,Ge:H) alloy materials and on issues related to overall stability in the mid-gap (a-SiH) materials. First, researchers characterized an extensive series of Uni-Solar a-Si,Ge:H samples using drive-level capacitance profiling and the analysis of sub-band-gap photocapacitance and photocurrent spectra. Thus, several bands of deep defect transitions were identified. Researchers were able to verify that charged defects are responsible for the different observed defect bands in device-quality a-Si,Ge:H alloy material. Second, they reported results of their measurements on a-Si,Ge:H alloy ''cathodic'' samples produced at Harvard University; these samples were found to exhibit significantly lower defect densities in the high Ge composition range (>50at.% Ge) than alloy samples produced either by conventional glow discharge of photo-chemical vapor deposition. Third, they performed voltage pulse stimulated capacitance transient measurements on a-Si:H/a-Si,Ge:H heterostructure samples to look for carrier trapping states that might be associated with this interface; they found there was a clear signature of trapped hole emission extending over long times associated specifically with the interface itself in concentrations of roughly 10{sup 11} cm{sup -2}. Fourth, researchers reported the results on several hot-wire a-Si:H samples produced with varying hydrogen levels. Their studies indicate that hot-wire-produced a-Si:H, with H levels between 2-5at.%, should lead to mid-gap devices with superior properties. Finally, they discussed some results on glow-discharge material, as well electron-cyclotron-resonance-deposited a-Si:H grown under hydrogen dilution conditions, and confirmed that, in terms of deep-defect creation, such films exhibited improved stability compared to conventional glow-discharge material.

This report describes works to use transient photocapacitance and photocurrent measurements to determine the deep defect distribution and processes in low-band-gap a-Si,Ge:H alloys. Samples for these studies were produced by the photochemical vapor deposition (photo-CVD) growth method and were obtained through a collaboration with researchers at the University of Delaware. This report discusses how a detailed comparison between the photocapacitance and photocurrent spectra can be used to separately examine the majority and minority carrier processes. The results are as follows: (1) The midgap defect densities in the alloy regime near 1.3 eV can be as low as 5 {times} 10{sup 16} cm{sup {minus}3} in such photo-CVD samples. (2) There exists a second defect band roughly 0.4 eV below E{sub c} of a similar magnitude to the midgap defect density that exhibits significant lattice relaxation behavior in its electron trapping dynamics. (3) The hole {mu}{tau} products determined for the lowest defect sample are roughly 5 {times} 10{sup {minus}10} cm{sup 2}/V, comparable with the highest hole {mu}{tau} products reported in sandwich geometry measurements for alloys in this composition range. (4) The hole {mu}{tau} is found to be roughly inversely proportional to the midgap defect density for the samples studied. This is consistentmore » with the fact that the effective minority carrier lifetime for such measurements is limited by the deep state trapping time.« less

The work performed for this contract continued investigations of the origins of metastable effectS in a-Si:H through three kinds of studies: (1) the effect of carbon impurities in a-Si:H samples at low concentrations using drive-level capacitance profiling measurements on samples whose carbon content was intentionally modulated spatially during growth, (2) the characterization of metastable states in n-type doped a Si:H samples caused by quench cooling and by light-soaking with partial annealing, and (3) the use of depletion-width-modulated ESR spectroscopy together with junction capacitance spectroscopy to investigate deep defect states for various metastable states of a 10- and an 80-Vppm PH{sub 3}-doped a-Si:H sample.

This report contains results of Phase II of research to investigate metastable defect creation processes in hydrogenated amorphous silicon. The study relied primarily on junction capacitance methods to monitor the changes in deep defect distribution. Results obtained applying capacitance profiling methods to the study of undoped samples are summarized and indicate a significant role of extrinsic impurities in the degree of light-induced degradation that occurred. Photocapacitance transient measurements were taken to investigate changes in undoped a-Si:H. Studies were also begun of metastable changes in n-type doped a-Si:H samples by using both the drive-level capacitance profiling technique and transient photocapacitance methods, and results are described. 22 refs., 10 figs., 1 tab.