In this paper we present a study of ac the Constant Photocurrent Measurement (CPM) with the aim to assess the precision of this method compared to the de CPM. In particular, we introduce a small signal theory of the ac photoconductivity, we relate the sub-bandgap absorption to the density of defects, and we assume a combination of a constant plus a sinusoidal generation rate. The method allows to calculate the variation occurring in defect occupation as a function of the wavelength and intensity of the light, and of the ac test frequency. We show that, in ac condition, carriers lifetime is a complicated function of the chosen parameter, and it is affected by the delay due to charge accumulation of free and trapped carrier densities. Large influence on the lifetime is played not only by recombination, which takes place all along the gap defects included between the two quasi-Fermi levels, but also by the process of carrier trapping and emission by defects located close to the quasi-Fermi levels. Both processes introduce large phase delay. Further information can thus be obtained from CPM by evaluating the phase behavior. Our analysis leads to a new method of evaluation of the dangling bond capture cross section and of their charged-to-neutral capture cross section ratio.

To prepare hydrogenated amorphous silicon-germanium alloys as low gap material for multi-junction solar cells in plasma enhanced chemical vapour deposition, the well established concept of strong dilution of the process gases with hydrogen has been used. Two different regimes of alloying were found: for low Ge content (x < 0.40) we observe material with low defect density, small Urbach energy and high values of the ambipolar diffusion length. In the regime of high Ge content (x > 0.40) the defect densities and Urbach energies are high and the values of the ambipolar diffusion length low. The transition is accompanied by the appearance of a low-temperature peak in hydrogen effusion experiments indicating a void rich film structure. Material from just above and below the transition zone is used in pin solar cells leading to a much enhanced red response compared with a-Si:H cells. The differences seen in the material quality are mirrored in the solar cell properties. By carefully adjusting the active layer thickness material with low diffusion length shows also reasonable solar cell performance.

Thin semiconducting films of hydrogenated amorphous silicon (a-Si:H) and its carbon alloy (a-Si:C:H) were applied to gas microstrip detectors in order to control gain instabilities due to charges on the substrate. Thin (∼100 nm) layers of a-Si:H or p-doped a-Si:C:H were placed either over or under the electrodes using the plasma enhanced chemical vapor deposition (PECVD) technique to provide the substrate with a suitable surface conductivity. By changing the carbon content and boron doping density, the sheet resistance of the a-Si:C:H coating could be successfully controlled in the range of 1012 ∼ 1017 μ/□, and the light sensitivity, which causes the resistivity to vary with ambient light conditions, was minimized. An avalanche gain of 5000 and energy resolution of 20% FWHM were achieved and the gain remained constant over a week of operation. A-Si:C:H film is an attractive alternative to ion-implanted or semiconducting glass due to the wide range of resistivities possible and the feasibility of making deposits over a large area at low cost.

We describe the synthesis and use of the novel molecular precursors C (SiH3) 4, CH3GeH3and SiH3GeH3 to generate silicon-carbon-germanium materials by ultrahigh vacuum chemical vapor deposition. By using these precursors in reactions with S1H4 and GeH4 between 470°C and 650°C we obtained: 1) heteroepitaxial Si1-x-y. GexCy (y=0.04–0.06) alloys with C (SiH3) 4; (2) polycrystalline alloys with carbon compositions ranging from 2–14 at.% with CH3GeH3; (3) mixtures of diamond cubic nanocrystals (Ge, Si1-xGex) and amorphous SiC with SiH3CH2GeH3. The effect of the precursor chemistry on composition, crystallinity, and microstructure of the materials as characterized by Rutherford backscattering spectroscopy (RBS), secondary ion mass spectrometry (SIMS) and transmission electron microscopy (TEM) is discussed.

The properties of amorphous Si1-xPx alloy thin films with 20–44 at.% P were studied. The results showed that these alloys have a wide bandgap Eo ranging from 1.5 to 2.15 eV, where the alloy films with 20 at.% P have the widest bandgap (1.75–1.82 eV) at annealing temperatures ≤600 °C. Conductivity measurements showed that two electron conduction processes mainly exist: hopping conduction in the band tail at low temperatures and extended-state conduction in the conduction band at high temperatures. Crystallization studies showed that the alloys are thermally stable and crystallize at temperatures between 850 and 1100 °C. A new phosphide, Si7P3 was formed by annealing the alloys with 30–44 at.% P at temperatures ≥950 °C depending on the P concentration.

The effects of thermal history and light-soaking are systematically studied on boron-doped a-Si:H and a-SiC:H. Light-soaking increases dark conductivity at a low carbon content but decreases it at a high carbon content. This reaction is reversible, and the dark conductivity recovers to the initial value with thermal annealing. The effect of thermal history is also influenced by carbon content in samples deposited at a relatively low temperature (110°C). The increase in dark conductivity occurs with thermal annealing at a higher temperature than the deposition temperature at a high carbon content. But this effect is not significant at a low carbon content. These phenomena can be explained by the hydrogen passivation of four-fold boron, and the hydrogen motion and trapping in the network. The present model suggests that a microstructure containing deep traps of mobile hydrogen contributes to improvement in the stability of a-Si:H and its alloys.

We have characterized the deep defect densities and their energy distributions for a series of a-Si1-xGex:H alloys with large Ge content (0.57< × < 1.00) prepared by the cathode deposited glow discharge method. Our results indicate markedly superior properties for these samples. A small Urbach tail slope (about 45meV) was found for all samples in this alloy range. The defect densities were either obtained directly from the drive-level capacitance profiling or deduced from the sub-band-gap optical spectra. Both are substantially lower than the trend line determined from previous studies of a-Si1-xGex:H samples produced by conventional glow discharge and by Photo-CVD methods. However, the relation between the total defect densities and the optical spectra in the cathodic samples obeys the same defect formation model that has been used to successfully predict the defect densities in other types of a-Si1-xGex:H material.

Amorphous silicon germanium alloys with improved properties for photovoltaic applications are investigated by electron spin resonance. Spin densities in this material are strongly reduced with optimized deposition conditions. Still the Ge dangling bond remains the dominating deep defect and the ratio of Ge over Si dangling bond defects is not much changed. The spin resonance spectra of these samples show distinctive differences from conventional material. The superposition signal can not be deconvoluted into modified Si- and Ge- dangling bond resonances as done previously. Instead a lineshape asymmetry or additional resonances are needed to deconvolute the spectra. The development of g-values with alloy composition indicate a more homogeneous chemical ordering in optimized material in agreement with other experimental data.

The dependence of the properties of hydrogenated amorphous silicon-carbon alloys deposited with a 1:10 silane (SiH4) to methane (CH4) or ethylene (C2H4) gas flow ratios with the hydrogen dilution, deposition pressure and power is studied. In methane-based films the carbon content shows a decrease (from ≈0.75 to 0.55) with increasing hydrogen dilution from 0 to 98%, while the ethylene-based films show no dependence of the carbon fraction (≈0.9) on hydrogen dilution. The photoconductivity shows an increase for hydrogen dilutions above 90= for both methane and ethylene-based films. While this increase is attributed, in the case of methane, to the observed decrease in carbon content, no corresponding decrease in carbon content is observed in the ethylene-based films, suggesting a decrease in the density of recombination centers with hydrogen dilution. The Urbach tail and the room-temperature photoluminescence peak correlate with carbon content independent of the carbon source-gas and deposition conditions used.

Investigations 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.

Thermopower, conductivity and 1/f noise measurements have been performed on a series of n-type doped hydrogenated amorphous silicon carbon films that are prepared with varying gas phase concentrations of CH4. The increased disorder at the mobility edge associated with alloying is characterized by the Q-function, which is obtained by combining thermopower and conductivity measurements, and is also reflected in the noise power spectra and noise statistics.

We report a study of the mobility-lifetime products of the two charge carriers in a-SiC:H alloys. The measurements were carried out as a function of the carbon concentration and the temperature. An analysis, relying on the interpretation of the corresponding light intensity exponents, indicates that neutral dangling bonds control the electrons' lifetime while another recombination center controls the holes' lifetime.

Self-powered “smart” windows utilize an electro-optic transmittance modulator based on electrochromic (EC) thin films that exhibit reversible and controlled changes in optical properties with an applied voltage between 0.7 and 2.0 V. Existing window designs require an external electrical connection, which may be economically unfeasible. This problem is solved by the tandem photovoltaic-electrochromic device, in which a wide-gap amorphous silicon-based alloy photovoltaic device is deposited together with an electrochromic optical transmittance modulator in a monolithic device on a single substrate. In this paper, we discuss our proposed monolithic photovoltaic-electrochromic device.

We also present studies of transparent, wide-gap (1.8 to 2.2 eV) amorphous silicon-carbon thin films and p-i-n devices designed for use in the photovoltaic-electrochromic device. These photovoltaic cells can operate at low current (<1 mA/cm2) because a total injected charge of only 60 μC/cm2 will darken the EC layer to a visible transmission of 5%, but they need a high open-circuit voltage (>1.0 V) and high transparency (≈70%). We present our progress toward these design targetxxss.

A cross sectional TME study has been conducted into the structure and morphology of p- and i-type a-Si:H layers of device quality deposited on textured TCO on glass. The layer thickness over peaks is shown to be equivalent to that for flat regions, while defective regions are found in narrow valleys, initiating from the pit of the valleys. These regions may act as regions of excessive recombination and/or shunting regions, thus leading to a reduced Voc and fill factor in thin solar cells. A cosine relationship was found between the deposited thickness and the facet angles of the surface TCO crystals. It is concluded that for best performance of the deposited layer, the deposition has to be completely isotropie, and that the preferred surface morphology of textured TCO be sharp peaks with wider valleys.

We have applied real time spectroscopie ellipsometry (RTSE) to monitor the successive growth of p-type a-Si1-xCx:H and i-type a-Si:H on specular SnO2:F (i.e., the superstrate solar cell configuration) in a single-chamber deposition system. Both the microstructural evolution, which includes the surface roughness and bulk layer thicknesses versus time and bulk layer void volume fraction, as well as the optical properties, which include the dielectric function (1.5–4.0 eV) and optical gap of the individual layers, were determined from RTSE data collected during growth. The accuracy of our approach is demonstrated by correlating structural parameters obtained both by RTSE and atomic force microscopy. Based on prior information deduced by RTSE, the TCO/p/i structure was fabricated with optimized procedures that have sought to minimize TCO/'p and p/i interfacial problems. These studies illustrate that RTSE can be a valuable tool for identifying problems in the fabrication of a-Si:H solar cells and ultimately improving cell performance.

An amorphous silicon solar cell deposited on plastic film substrate has been studied. The solar cell has a new monolithic series-connected structure called SCAF, and will be fabricated by a newly developed fabrication process based on “Stepping Roll” film deposition system.

The process was compared with the conventional glass solar cell process. Preliminary results for small and large area SCAF solar cells were presented and discussed.

The optimum deposition conditions for growth of amorphous silicon (a-Si:H) and silicon-germanium (a-SiGe:H) alloys deposited at high rates using microwave glow-discharge are found to be quite different from those for radio-frequency glow-discharge material deposited at low rates. High substrate temperature (350 to 500 °C), low pressure (1–5 mtorr) and positive ion bombardment are found to be desirable for optimum growth conditions at high deposition rates. We have achieved an active-area efficiency of 11.44% for a double-junction structure in which the bottom cell incorporates a-SiGe:H alloy deposited at 100 Å/sec using microwave glow-discharge.

Structures were carried out on the degradation of heterojunction and homojunction a-Si:H solar cells, using Xenon arc and tungsten halogen light sources. The degradations were carried out with both white and red light with the intensities being calibrated using the saturated reverse biased photocurrents. Because of the different light intensities, I, involved, comparisons between the different illumination were made after exposure times, t, where I1.8t = constant. The degraded states of the cells were characterized with light I-V, dark I-V, and quantum efficiency measurements. The same degraded states, as indicated by the all three characteristics, are obtained with either white and red light providing the intensities are calibrated using the actual p-i-n cells. It is also found that the empirical relation for obtaining the same degradation found by L. Yang holds for these illuminations being about 1 and 10 suns. Analysis of the dark I-V's using AMPS with a charged defect distributions of defects indicates that with the exposures used, the light induced defects have a close to uniform distribution throughout the i-layer.

We have measured the intensity of electroluminescence (EL) and its energy spectrum in a-Si:H solar cells having an initial energy conversion efficiency from 5.75 to 9.8 %, and open-circuit voltages (Voc) between 0.799 and 0.952 V. We found that (a) at room temperature, EL efficiency is proportional to the initial solar cell energy conversion efficiency; (b) the defect energy distribution in the i-layer can be detected by EL energy spectrum at room temperature; and (c) Voc is simply related to the quasi-Fermi level splitting obtainable in the i-layer and that the buffer layer serves to increase this splitting.

The 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.