We present the progress made in attaining high-efficiency large-area nc-Si:H based multi-junction solar cells using Modified Very High Frequency technology. We focused our effort on improving the spatial uniformity and homogeneity of nc-Si:H film growth and cell performance. We also conducted both indoor and outdoor light soaking studies and achieved 11.2% stabilized efficiency on large-area (≥400 cm2) encapsulated a-Si:H/nc-Si:H/nc-Si:H triple-junction cells.

The influence of the location of the embedded a-Si:H layer in the gate dielectric film of the floating-gate a-Si:H TFT on the charge trapping and detrapping mechanisms has been investigated. The thin channel-contact SiNx gate dielectric layer favors both hole and electron trappings under the proper gate voltage condition. The sweep gate voltage affect the locations and shapes of forward and backward transfer characteristics curves, which determines the memory function. In order to achieve a large memory window, both the location of the embedded a-Si:H layer and the gate voltage sweep range need to be optimized.

The microstructure and crystallization kinetics of electron beam evaporated Si on ZnO:Al coated glass for polycrystalline solar cells was studied by electron backscatter diffraction and optical microscopy at various deposition temperatures. A time dependent analysis of the dynamics of the crystallization allowed for the individual determination of growth and nucleation processes. The nucleation process of Si on ZnO:Al was found to be influenced by a variation of the deposition temperature of the amorphous Si in a critical temperature regime of 200 ˚C to 300 ˚C. The nucleation rate decreased significantly with decreasing deposition temperature, while the activation energy for nucleation increased from 2.9 eV at a deposition temperature of 300 ˚C to 5.1 eV at 200 ˚C, resulting in poly-Si which comprised grains with features sizes of several μm.

Hydrogenated amorphous silicon (a-Si:H) thin films were deposited on pre-oxidized Si wafers by electron cyclotron resonance chemical vapor deposition (ECRCVD). The rapid thermal annealing (RTA) treatments were applied to the as-grown samples in nitrogen atmosphere, and the temperature range for the RTA process is from 450 to 950 °C. The crystallization and grain growth behaviors of the annealed films were investigated by Raman spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM). The onset temperature for the crystallization and grain growth is around 625 ∼ 650°C. The crystalline fraction of annealed a-Si:H films can reach ∼80%, and a grain size up to 17 nm could be obtained from the RTA treatment at 700 °C. We found that the crystallization continues when the grain growth has stopped.

Quantum efficiency measurements in (nc-Si:H) solar cells deposited onto textured substrates indicate that these cells are close to the “stochastic light-trapping limit“ proposed by Yablonovitch in the 1980s. An interesting alternative to texturing is “plasmonic“ light-trapping based on non-textured cells and using an overlayer of metallic nanoparticles to produce light-trapping. While this type of light-trapping has not yet been demonstrated for nc-Si:H solar cells, significant photocurrent enhancements have been reported on silicon-on-insulator devices with similar optical properties to nc-Si:H. Here we report our work on the measurerements of quantum efficiencies in nc-Si:H solar cells and normalized photoconductance spectra in SOI photdetectors with and without silver nanoparticle layers. As was done previously, the silver nanoparticles were created by thermal annealing of evaporated silver thin films. We observed enhancement in the normalized photoconductance spectra of SOI photodetectors at longer wavelengths with the silver nanoparticles. For nc-Si:H solar cells, we have not yet observed significant improvement of the quantum efficiency with the addition of annealed silver films.

This work reports the first results of a new generation plasma-enhanced chemical vapor deposition (PECVD) reactor manufactured by Roth and Rau. This large area parallel plate reactor has been especially designed for the manufacturing of silicon heterojunction solar cells which are made of very thin amorphous silicon films over monocrystalline silicon substrates. Layer thickness uniformity below ± 3 % is reported for both intrinsic and doped layer over a 400 × 400 mm2 area. Moreover, it is shown that the passivation quality is excellent with life-times up to 4.15 ms on n-type FZ silicon substrates. A ± 0.6 % uniformity in open circuit voltage (mean value of 701.4 mV) is achieved over 32 devices having a 4 cm2 area and an average conversion efficiency of 19.5 %.

A new type of micro-channel plate detector based on hydrogenated amorphous silicon is proposed which overcomes the fabrication and performance issues of glass or bulk silicon ones. This new type of detectors consists in 80-100 μm thick layers of amorphous silicon which are micro-machined by deep reactive ion etching to form the channels. This paper focuses on the structure and fabrication process and presents first results obtained with test devices on electron detection which demonstrate amplification effects. Fabrication and performance issues are also discussed.

Studies have shown that wide bandgap material is required for high efficiency multi-junction solar cell applications. Here, we address proper deposition condition for high quality a-SiC:H films. In high power high pressure regime, we observed that the defect density get much lowered to the similar defect level of a-Si:H film with high H2 dilution. Single junction solar cells fabricated with the optimized condition show high open circuit voltage and low LID effect. The degradation after the LID test was only 13 % reduction of the efficiency indicating that a-SiC:H could be promising material for multi-junction solar cells.

A new shift register using p-type poly-Si thin-film transistors (TFTs) for active matrix display is proposed. It utilizes only p-type TFTs to simplify the fabrication process, and provides time-shifted output signals with a voltage swing from VSS to VDD without signal-level loss. In the proposed shift register, output is structurally separated from carry and therefore has a high immunity to output signal distortion caused by output load capacitance. We also propose a new light emitting control method using this shift register for high image quality active-matrix organic light emitting diode (AMOLED) displays. The proposed shift register was verified by simulation and measurement.

The crystallinity of the hydrogenated microcrystalline silicon (μc-Si:H) film was known to influence the solar cell efficiency greatly. Also hydrogen was found to play a critical role in controlling the crystallinity. Instead of employing conventional plasma deposition techniques, this work focused on using catalytic chemical vapor deposition (Cat-CVD) to study the effect of hydrogen dilution and the filament-to-substrate distance on the crystallinity, deposition rate, microstructure factor and electrical property of the μc-Si:H film. We found that the substrate material and structure can affect the crystallinity of the μc-Si:H film and the incubation effect. Comparing bare glass, TCO-coated glass, a-Si:H-coated glass and μc-Si:H-coated glass, the microcrystalline phase grows the fastest onto μc-Si:H surface, but the slowest onto a-Si:H surface. Surprisingly, the template effect lasted for more than a thousand atomic layers of silicon.

Protocrystalline silicon, which is a material that has enhanced medium range order (MRO), can be prepared by using high hydrogen dilution in PECVD, or, alternatively, using high atomic H production from pure silane in HWCVD. We show that this material can accommodate percentage-level concentrations of oxygen without deleterious effects. The advantage of protocrystalline SiO:H for application in multijunction solar cells is not only that it has an increased band gap, providing a better match with the solar spectrum, but also that the solar cells incorporating this material have a reduced temperature coefficient. Further, protocrystalline materials have a reduced susceptibility to light-induced defect creation. We present the unique result in the PV field that these oxygenated protocrystalline silicon solar cells have an efficiency temperature coefficient (TCE) that is virtually zero (TCE is between -0.08%/°C and 0.0/°C). It is thus beneficial to make this cell the current limiting cell in multibandgap cells, which will lead to improved annual energy yield.

The present work concerns novel approaches to fabricate silicon-based electrodes. In this study silicon nano-particles are synthesized via two aerosol routes: Laser assisted Chemical Vapour Pyrolysis (LaCVP) and Spark Discharge Generation (SDG). These two techniques allow the generation of uniformly sized particles, with a good control over the composition and the range of sizes. Herein, particles with size ranging from 2 to 70 nm were obtained. Starting from these nano-particles, nano-structured porous thin films are produced either by electrospraying a polymeric solution in which LaCVP-produced particles were previously dispersed, or by inertial impacting the SDG-produced particles directly from gas phase onto a substrate. The Electro- Spray (ES) is used to produce composite films for Li-ion battery applications, whereas the Inertial Impaction (II) is used to produce pure silicon films for other purposes (i.e.: sensors).

The grain size distribution allows characterizing quantitatively the microstructure at different stages of crystallization of an amorphous solid. We propose a generalization of the theory we established for spherical grains, to the case of grains with ellipsoidal shape. We discuss different anisotropic growth mechanisms of the grains in thin films. An analytical expression of the grain size distribution is obtained for the case where grains grow through a change of volume while keeping their shape invariant. The resulting normalized grain size distribution is shown to be affected by anisotropy through the time-decay of the effective growth rate.

This paper discusses the accuracy of the distribution of the fatigue lifetime of polysilicon thin films predicted from their strength distribution. On the basis of the authors' previous studies, where the fatigue process determining the lifetime was formulated using the well-known fatigue crack extension Paris' law, prediction error ranges for polysilicon specimens with different levels of strength are determined. The errors of the predicted fatigue lifetime in the logarithmic scale, defined as △logN = |log10Nexp-log10Npred| where Nexp and Npred were the experimental and predicted number of cycle, were found to be less than 1 in the range of the cumulative fracture probability F between 0.1 and 0.9. Therefore, based on the measured Paris' law parameters of polysilicon, the fatigue lifetimes of different polysilicon thin film structures can be predicted from their strength distributions with errors of roughly 10% in the logarithmic scale, which was average of percentages of △logN to log10N of experimental data.

Photonic crystal back reflectors offer enhanced optical absorption in thin-film solar cells, without undesirable losses. Rigorous simulations of photonic crystal back reflectors predicted maximized light absorption in amorphous silicon solar cells for a pitch of 700-800 nm. Simulations also predict that for typical 250 nm i-layer cells, the periodic photonic crystal back reflector can improve absorption over the ideal randomly roughened back reflector (or the ‘4n2 classical limit') at wavelengths near the band edge. The PC back reflector provides even higher enhancement than roughened back reflectors for cells with even thinner i-layers. Using these simulated designs, we fabricated metallic photonic crystal back reflectors with different etch depths and i-layer thicknesses. The photonic crystals had a pitch of 760 nm and triangular lattice symmetry. The average light absorption increased with the PC back reflectors, but the greatest improvement (7-8%) in short circuit current was found for thinner i-layers. We have studied the dependence of cell performance on the etch depth of the photonic crystal. The photonic crystal back reflector strongly diffracts light and increases optical path lengths of solar photons.

The characterization of void-related microstructure in amorphous and microcrystalline Ge:H films is reported. Various methods are applied including effusion measurements of hydrogen and of implanted helium and neon, measurements of the infrared absorption of C-H bonds due to in-diffusion of contaminants and of the stretching modes of bonded hydrogen. Several microstructure effects like interconnected voids and isolated voids and a quite different material homogeneity are detected and are found to depend on the preparation conditions. Amorphous Ge:H can be prepared with a (largely) homogeneous structure while microcrystalline Ge:H tends to consist of compact grains surrounded by more or less open voids. Enhanced substrate temperatures (Ts ≈ 250°C) favour the growth of more compact material.

In the optimization of high-growth rate hydrogenated microcrystalline silicon (μc-Si:H) for photovoltaics, the observation of the infrared (IR) absorption peaks around 2000 cm-1 - which originate from silicon hydrogen (Si-HX) bonds in the film bulk - can provide useful information about the properties of the film. If the films are deposited on IR transparent substrates, Fourier Transform infrared (FTIR) spectroscopy is the most commonly used technique to measure the absorption in this regime. The rich spectrum of Si-HX peaks that can be observed in this wavenumber region describes the distribution of the bonded hydrogen amongst the various possible SiHX configurations, and as well, the presence of a pair of narrow, twin peaks at 2085 and 2100 cm-1 is indicative of material that is porous and prone to oxidation upon exposure to air. Many of these high wavenumber peaks are also observable using Raman scattering spectroscopy, allowing one to observe films on substrates that are not IR transparent. As well, the different absorption/scattering cross sections of the two techniques provide a useful tool to understand the origins of the various peaks, and in particular, those of the twin narrow peaks. In this work, we examine the dynamics upon exposure to atmosphere of these twin peaks in μc-Si:H grown by Matrix Distributed Electron Cyclotron Resonance (MD-ECR) PECVD. This deposition technique is an extremely promising one for growing μc-Si:H thin films at high deposition rates, and has demonstrated μc-Si:H film deposition at up to 28Å/s. By using both FTIR and Raman spectroscopy to characterize these films, we examine the differences in the dynamics of each of the twin peaks according to the technique used. In addition, secondary ion mass spectrometry (SIMS) measurements on the films show the physical distribution of oxygen in the films after five months of air exposure, and XRD measurements correlate the presence of these twin peaks with a [111] preferential surface orientation. Using these observations, we assign these absorption and scattering features to specific configurations of Si-HX within the film, and explain both the dynamics of the peaks over time and the similarity of the peaks for a wide range of samples.

We have developed a computer program based on the Finite Difference Time Domain (FDTD) algorithm able to simulate the propagation of electromagnetic waves with wavelengths in the range of the visible spectrum within a-Si:H p-i-n structures. Understanding of light transmission, reflection and propagation inside semiconductor structures is crucial for development of photovoltaic devices. Permitting 1D analysis of light propagation over time evolution, our software produces results in well agreement with experimental values of the absorption coefficient. It shows the light absorption process together with light reflection effects at the incident surface as well as at the semiconductor interfaces. While the effects of surface reflections are easily taken into account by the algorithm, light absorption represents a more critical point, because of its non-linear dependence from conductivity. Doping density, density of states and photoconductivity calculation are therefore crucial parameters for a correct description of the light absorption-transmission phenomena through a light propagation model.

The results presented in this paper demonstrate that is possible to describe the effect of the light-semiconductor interaction through the application of the FDTD model to a a-Si:H solar cell. A more general application of the model to 2D geometries will permit the analysis of the influence of surface and interface roughness on the device photovoltaic efficiency.

Hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) are widely used in many areas and the most important application is in active matrix liquid crystal display. However, the instability of the a-Si:H TFTs constrains their usability. These TFTs have been annealed at higher temperatures in hope of improving their electrical performance. But, higher anneal temperatures become a constraint when the TFTs are grown on polymer-based flexible substrates. This study investigates the effect of anneal time on the performance of the a-Si:H TFTs on PEN. Thin-film transistors are annealed at different anneal times (4 h, 24 h, and 48 h) and were stressed under different bias conditions. Sub-threshold slope and the off-current improved with anneal time. Off-current was reduced by two orders of magnitude for 48 hours annealed TFT and sub-threshold slope became steeper with longer annealing. At positive gate-bias-stress (20 V), threshold voltage shift (∆Vt) values are positive and exhibit a power-law time dependence. High temperature measurements indicate that longer annealed TFTs show improved performance and stability compared to unannealed TFTs. This improvement is due to reduction of interface trap density and good a-Si:H/insulator interface quality with anneal time.

We report on the growth and properties of novel amorphous Silicon (a-Si:H) p-i-n devices prepared using chemical annealing with argon gas. The i layer in the p-i-n devices was grown using a layer by layer approach, where the growth of a very thin a-Si:H layer (7-30 angstroms) grown using a silane:argon mixture was followed by chemical anneal by argon ions. Repeated cycling of such growth/anneal cycles was used to produce the desired total thickness of the i layer. The thickness of the a-Si layer, and duration of the anneal time, were varied systematically. Pressure and power of the plasma discharge were also systematically varied. It was found that a thin a-Si layer, <10 angstroms, and low pressures which led to relatively high ion flux on the surface, gave rise to a significantly smaller bandgap in the device, as indicated by a significant lateral shift in the quantum efficiency vs. photon energy curve to lower energies. The smallest Tauc gap observed was in the range of 1.62 eV. Corresponding to this smaller bandgap, the current in the solar cell increased, and the voltage decreased. The Urbach energies of the valence band tail were also measured in the device, using the quantum efficiency vs. energy curve, and found to be in the range of45 meV, indicating high quality devices. Too much ion bombardment led to an increase in Urbach energy, and an increase in defect density in the material. Raman spectra of the device i layer indicated an amorphous structure. When hydrogen was added to argon during the annealing cycle, some materials turned microcrystalline, as indicated by the Raman spectrum, and confirmed using x-ray diffraction.