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We investigated chemical sputtering of silicon films by Hy+ ions (with y being 2 and 3) in an asymmetric VHF Plasma Enhanced Chemical Vapor Deposition (PECVD) discharge in detail. In experiments with discharges created with pure H2 inlet flows, we observed that more Si was etched from the powered than from the grounded electrode, and this resulted in a net deposition on the grounded electrode. With experimental input data from a power density series of discharges with pure H2 inlet flows, we were able to model this process with a chemical sputtering mechanism. The obtained chemical sputtering yields were (0.3-0.4) ± 0.1 Si atom per bombarding Hy+ ion at the grounded electrode and at the powered electrode the yield ranged from (0.4 to 0.65) ± 0.1. Subsequently, we investigated the role of chemical sputtering during PECVD deposition with a series of silane fractions SF (SF(%)=[SiH4]/[H2]*100) ranging from SF=0% to 20%. We experimentally observed that the SiHy+ flux is not proportional to SF but decreasing from SF=3.4% to 20%. This counterintuitive SiHy+ flux trend was partly explained by an increasing chemical sputtering rate with decreasing SF and partly by the reaction between H3+ and SiH4 that forms SiH3+. [ABSTRACT FROM AUTHOR]

The behavior of the electrical conductivity in hydrogenated microcrystalline silicon (μc-Si:H) that is frequently observed is explained by considering the statistical shift in the chemical potential as a function of the crystalline fraction (Xc), the dangling bond density (Ndb), and the doping density (Nd). Our model shows that temperature dependent dc conductivity measurements above room temperature can be very well explained by (unintentional) micro doping of μc-Si:H. It is shown that the statistical shift in the chemical potential (μ) is influenced mostly by the ratio between Nd and Ndb. It is concluded that the anomalous dependence of the apparent activation energy (Ea) and the apparent exponential prefactor (σ0) on Xc can be explained by behavior of μ, that can be induced by a change in this ratio between Nd and Ndb. We used an effective medium approximation for the electron density of states (DOS) of μc-Si:H. The DOS is calculated as a weighted sum of the DOS of c-Si and the DOS of a-Si:H, parameterized by Xc, Ndb, and Nd. The conductivity is deduced assuming a single dominant conduction path above the conduction edge of a-Si:H. [ABSTRACT FROM AUTHOR]

The transport mechanisms controlling the forward dark current-voltage characteristic of the silicon micromorph tandem solar cell were investigated with numerical modeling techniques. The dark current-voltage characteristics of the micromorph tandem structure at forward voltages show three regions: two with an exponential dependence and a third where the current grows more slowly with the applied voltage. In the first exponential region the current is entirely controlled by recombination through gap states of the top cell. In the second exponential region the current is controlled by the mixture of recombination through gap states of both the top and bottom cells and by free carrier diffusion along the a-Si:H intrinsic layer. In the third region the onset of the electron space charge limited current on the a-Si:H top cell can be observed along with some other mechanisms that are discussed in the paper. The high forward dark J-V curve of the tandem cell can be used as diagnosis tool to quickly inspect the efficiency of the recombination junction in recombining the electron-hole pairs generated under illumination in the top and bottom cells. The dark current at high forward voltages is highly influenced by the amount of electron-hole pairs thermally generated inside the recombination junction. [ABSTRACT FROM AUTHOR]

We report on picosecond, time-resolved measurements of the vibrational relaxation and decay pathways of the Si–H and Ge–H stretching modes in hydrogenated amorphous silicon-germanium thin films (a-SiGe:H). It is demonstrated that the decay of both modes has a nonexponential shape, attributable to the local environment of the Si–H and Ge–H bonds. Temperature dependent measurements of the ensemble averaged population decay time 1> are used to demonstrate that the stretch modes relax to Si(Ge)-H bending modes and that the excess energy is dissipated into a combination of bulk vibrations. The influence of the mixed character Si-Ge bulk vibrations upon the relaxation dynamics is discussed. [ABSTRACT FROM AUTHOR]

For substrate n-i-p-type cells rough reflecting back contacts are used in order to enhance the short-circuit currents. The roughness at the electrode/silicon interfaces is considered to be the key to efficient light trapping. Root-mean-square (rms) roughness, angular resolved scattering intensity, and haze are normally used to indicate the amount of scattering, but they do not quantitatively correlate with the current enhancement. It is proposed that the lateral dimensions should also be taken into account. Based on fundamental considerations, we have analyzed by atomic force microscopy specific lateral dimensions that are considered to have a high scattering efficiency. Textured back reflectors with widely varying morphologies have been developed by the use of sputtered Ag and Ag:AlOx layers. For these layers we have weighted the rms roughness of the surface with the lateral dimensions of the effective scattering features. A clear correlation is found between the current generation under (infra)red light in microcrystalline (μc-Si:H) n-i-p solar cells and the weighted rms value of the Ag back contacts. Furthermore, the surface plasmon absorption of the rough Ag back contact has been found to be a significant limiting factor for the current enhancement. Using Ag:AlOx layers on glass, deposited at substrate temperatures below 300 °C, a μc-Si:H n-i-p solar cell is obtained with an efficiency of 8.1%. Using textured Ag layers made at a higher substrate temperature on a stainless steel substrate we have developed a hot-wire chemical vapor deposited μc-Si:H n-i-p-type solar cell with 8.5% efficiency. [ABSTRACT FROM AUTHOR]

This paper presents a compact electrical equivalent circuit which describes the dark current-voltage characteristics of nonideal p-n junction solar cells in a wide range of temperatures. The model clearly separates the voltage drop in the junction and bulk regions. It is based on the combination of two exponential mechanisms, shunt and series resistances and space-charge limited current. In order to increase the accuracy of the parameter extraction process, both ln(I-V) and its derivative plots are fitted simultaneously. From the temperature dependence of the extracted parameters, the conduction mechanisms governing the I-V characteristics can be obtained without assuming dominating terms. In addition, the extracted parameters can be related to other electrical magnitudes obtained from such independent measurements as capacitance-voltage measurements (diffusion potential) and illuminated current-voltage characteristics (series resistance and open-circuit voltage). To exemplify the application, a p+ a-SiC:H/n c-Si solar cell is studied and a number of major physical aspects derived from the analysis of the fitting values are discussed. [ABSTRACT FROM AUTHOR]

The implementation of trap-assisted tunneling of charge carriers into numerical simulators ASPIN and D-AMPS is briefly described. Important modeling details are highlighted and compared. In spite of the considerable differences in both approaches, the problems encountered and their solutions are surprisingly similar. Simulation results obtained for several tunneling recombination junctions made of amorphous silicon (a-Si), amorphous silicon carbide (a-SiC), or microcrystalline silicon (μc-Si) are analyzed. Identical conclusions can be drawn using either of the simulators. Realistic performances of a-Si/a-Si tandem solar cells can be reproduced with simulation programs by assuming that extended-state mobility increases exponentially with the electric field. The same field-enhanced mobilities are needed in single tunneling recombination junctions in order to achieve measured current levels. Temperature dependence of the current-voltage characteristics indicates that the activation energy of enhanced mobilities should be determined. Apparent discrepancies between simulation results and measurements are explained and eliminated making use of Gill’s law. For application in tandem and triple solar cell structures, tunneling recombination junctions made of (μc-Si) are the most promising of all examined structures. [ABSTRACT FROM AUTHOR]

Microcrystalline silicon (μc-Si:H) p- and n-type layers have been developed by Layer-by-Layer (LbL) deposition at high temperatures. The LbL deposition consists of alternating boron or phosphorus doped amorphous silicon depositions and hydrogen plasma treatments by Very High Frequency Chemical Vapor Deposition (VHF PECVD). The layers are developed to be resistant to the temperature and hydrogen flux of a micro- of polycrystalline intrinsic layer grown at a high deposition rate in a p-i-n or an n-i-p solar cell device. It is concluded that the LbL method is suitable to produce device quality μc-Si:H p- and n-type doped layers in a temperature range from 250 to 400 °C. This is not possible with standard continuous PECVD employing high hydrogen dilution of silane, where the addition of dopants reduces the crystallinity. An optimum effective thickness per deposition cycle (total thickness divided by the number of cycles) of 1.5 nm/cycle is needed for the crystallization. This optimal effective sub layer thickness is independent of dopants and deposition temperature. However, a minimum thickness of the first layer is needed for a sustaining growth in the LbL process. The doped layers grown by LbL are smoother than reference samples grown by continuous wave (cw). The doping efficiencies in our LbL deposited layers are structurally higher than those in cw deposition (for p layers a doping efficiency of 39% in case of LbL, compared to 1% for cw). The properties of the best high-temperature doped layers are as follows: for LbL p-type μc-Si:H (Ts=350 °C, 29 nm), activation energy=0.11 eV and dark conductivity=0.1 Ω-1 cm-1; for LbL n-type μc-Si:H (Ts=400 °C, 31 nm), activation energy=0.056 eV and dark conductivity=2.7 Ω-1 cm-1. Test solar cells have been deposited using Hot-Wire CVD (HWCVD) and VHF PECVD deposited μc-Si:H i-layers on top of the high-temperature LbL μc-Si:H n-type doped layer in an n-i-p configuration on a stainless steel substrate without a back reflector. A high open circuit voltage of 0.56 V and a fill factor of 0.7 show the high doping efficiency and crystallinity of the n-type doped layer and the resistance to the impinging atomic hydrogen during the HWCVD deposition. The mechanism behind the LbL μc-Si:H growth phenomenon is a controversial subject. We studied the LbL growth and nucleation mechanism as well as the incorporation of dopant atoms in the μc-Si:H layers. Etching, abstraction, and hydrogen diffusion are analyzed and it is concluded that our observations support the nucleation model that is based on hydrogen diffusion, while chemical transport and epitaxial growth are excluded to be the mechanism behind the crystallization.© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Hot-wire chemical vapor deposition (HWCVD) is a promising technique for the deposition of silicon nitride layers (a-SiN[sub x]:H) at low temperatures. In contrast to the commonly used plasma-enhanced chemical vapor deposition, no ion bombardment is present in HWCVD, which makes it particularly attractive for the deposition of passivation layers on structures that are sensitive to the impact of energetic ions. We deposit hot-wire a-SiN[sub x]:H from a mixture of silane and ammonia at substrate temperatures in the range of 300-500 °C. Layers deposited with an ammonia/ silane gas-flow ratio of R=30 are close to stoichiometry (N/Si=1.33) with a hydrogen content around 10 at. %. Such films have been implemented in hot-wire a-Si:H thin-film transistors. Deposition with R>30 did not result in an increase of the N content, but led to more porous films. Infrared spectroscopy revealed that moisture penetrates these layers and that oxygen is incorporated in the network under air exposure. Cross-sectional transmission electron microscopy images show that these layers contain spherical voids with diameters of several nanometers. In contrast, films deposited with a lower gas-flow ratio (R<30) are inert and do not contain these voids. Both types of films show columnar growth. To better understand the deposition process, we used deuterated silane (SiD[sub 4]) as a source gas. We found that no deuterium was incorporated in the films. This gives rise to the assumption that SiH[sub 4](SiD[sub 4]) is cracked effectively at the filaments. We infer that the ammonia species are scarcely dissociated at the filaments but rather in the gas phase by the atomic hydrogen (deuterium) originating from the dissociated silane. The abundance of atomic hydrogen in the gas phase is crucial for the breakup of ammonia and the incorporation of N in the film. [ABSTRACT FROM AUTHOR]

Hot-wire deposited amorphous silicon is an excellent material for incorporation as the absorbing layer in solar cells. We show the beneficial effect of a low deposition temperature of hot-wire deposited intrinsic amorphous silicon for solar cells. The influence of a few specific deposition parameters on the material properties was investigated. It is shown that both the filament history and the deposition pressure are crucial parameters for the material quality. Optimized material, deposited at 250 °C, was incorporated in efficient single- and multi-junction solar cells on flexible stainless steel substrates. The n-i-p structure was used to avoid any influence of TCO- and p-layer degradation, which is otherwise present in p-i-n structures. The cells have a high open-circuit voltage and high fill factor, clearly showing the improved performance of hot-wire deposited amorphous silicon made at moderate temperature. [ABSTRACT FROM AUTHOR]

In this article we discuss basic aspects of single junction a-SiGe:H p–i–n solar cells by coupling computer simulations with experimental characteristics. We are able to fit the dark illuminated current–voltage characteristics and the spectral response curves of a-SiGe:H p–i–n structures in the initial state, modeling the density of dangling bonds in each device layer by using either uniform density profiles or the defect pool model. Although we can fit these experimental curves with any of these two electrical models, band gap profiling in the a-SiGe:H intrinsic layer leads to improvement of the solar cell performance only when the defect pool model is implemented in our simulations. A U-shaped band gap profile is tailored in our samples by a staircase band gap profile composed of (i) several front band gap graded steps, (ii) one lowest band gap region, and (iii) several back band gap graded steps. Only by using the defect pool model are we able to predict an optimum thickness for the front band gap graded steps and for a buffer layer located at the p/i interface. Furthermore, using the defect pool model, the simulation predicts that higher efficiencies in single junction a-SiGe:H p–i–n solar cells can be achieved by depositing nonuniform graded steps, i.e., thicker band gap graded layers besides the lowest band gap a-SiGe:H layer and thinner band gap graded layers besides the p/i and i/n interfaces. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An approach to study the optical behavior of hydrogenated amorphous silicon solar cells with rough interfaces using computer modeling is presented. In this approach the descriptive haze parameters of a light scattering interface are related to the root mean square roughness of the interface. Using this approach we investigated the effect of front window contact roughness and back contact material on the optical properties of a single junction a-Si:H superstrate solar cell. The simulation results for a-Si:H solar cells with SnO[sub 2]:F as a front contact and ideal Ag, ZnO/Ag, and Al/Ag as a back contact are shown. For cells with an absorber layer thickness of 150-600 nm the simulations demonstrate that the gain in photogenerated current density due to the use of a textured superstrate is around 2.3 mA cm[sup -2] in comparison to solar cells with flat interfaces. The effect of the front and back contact roughness on the external quantum efficiency (QE) of the solar cell for different parts of the light spectrum was determined. The choice of the back contact strongly influences the QE and the absorption in the nonactive layers for the wavelengths above 650 nm. A practical Ag back contact can be successfully simulated by introducing a thin buffer layer between the n-type a-Si:H and Ag back contact, which has optical properties similar to Al, indicating that the actual reflection at the n-type a-Si:H/Ag interface is smaller than what is expected from the respective bulk optical parameters. In comparison to the practical Ag contact the QE of the cell can be strongly improved by using a ZnO layer at the Ag back contact or an ideal Ag contact. The photogenerated current densities for a solar cell with a 450 nm thick intrinsic a-Si:H layer with ZnO/Ag and ideal Ag are 16.7 and 17.3 mA cm[sup -2], respectively, compared to 14.4 mA cm[sup -2] for the practical Ag back contact. The effect of increasing the roughness of the contact interfaces was investigated for both... [ABSTRACT FROM AUTHOR]