Effect of back reflectors on light trapping in μc-Si:H cells has been investigated with self-ordered Al substrates obtained by anodic oxidation. With increasing the period of the patterned substrates from 0 to 1.1 μm, 1-μm-thick μc-Si:H cells on the patterned substrates have shown a significant enhancement of spectral response in the near infrared region, giving an increment of the short circuit current density from 18 to 24 mA/cm2. This enhanced light trapping effect are attributed to the improved reflectivity of the rear side and effective light scattering at the front side, as well as light scattering at the rear side.

We report our recent progress on nc-Si:H single-junction and a-Si:H/nc-Si:H/nc-Si:H triple-junction cells made by a modified very-high-frequency (MVHF) technique at deposition rates of 10-15 Å/s. First, we studied the effect of substrate texture on the nc-Si:H single-junction solar cell performance. We found that nc-Si:H single-junction cells made on bare stainless steel (SS) have a good fill factor (FF) of ˜0.73, while it decreased to ˜0.65 when the cells were deposited on textured Ag/ZnO back reflectors. The open-circuit voltage (Voc) also decreased. We used dark current-voltage (J-V), Raman, and X-ray diffraction (XRD) measurements to characterize the material properties. The dark J-V measurement showed that the reverse saturated current was increased by a factor of ˜30 when a textured Ag/ZnO back reflector was used. Raman results revealed that the nc-Si:H intrinsic layers in the two solar cells have similar crystallinity. However, they showed a different crystallographic orientation as indicated in XRD patterns. The material grown on Ag/ZnO has more random orientation than that on SS. These experimental results suggested that the deterioration of FF in nc-Si:H solar cells on textured Ag/ZnO was caused by poor nc-Si:H quality. Based on this study, we have improved our Ag/ZnO back reflector and the quality of nc-Si:H component cells and achieved an initial and stable active-area efficiencies of 13.4% and 12.1%, respectively, in an a-Si:H/nc-Si:H/nc-Si:H triple-junction cell.

Microcrystalline silicon (μc-Si:H) thin-film transistors (TFTs) have lately gained much attention due to their high charge carrier mobilities. We report on top-gate μc-Si:H TFTs fabricated by plasma-enhanced chemical vapor deposition at process temperatures below 180 °C with high electron and hole charge carrier mobilities exceeding 50 cm2/Vs and 12 cm2/Vs, respectively. Based on the μc-Si:H TFTs different thin-film inverters were realized including ambipolar and complimentary metal-oxide-semiconductor (CMOS) inverters. Microcrystalline CMOS inverters exhibit high voltage gains exceeding 22, whereas ambipolar inverters show reduced voltage gains of 10 at low operating voltages. The electrical characteristics of the μc-Si:H CMOS and ambipolar thin-film inverters will be discussed in terms of the voltage transfer curve, the voltage gain and the power dissipation.

Noise spectra in plasma deposited SixGeyBz:H thermo-sensing films for micro-bolometers have been studied. The samples were characterized by SIMS (composition) and conductivity (room temperature conductivity, activation energy) measurements. The noise spectra were measured in the temperature range from T= 300 K to T=400 K and in the frequency range from f=2 Hz to f=2×104 Hz. The noise spectra SI(f) for the samples Si0.11Ge0.88:H and Si0.04Ge0.71B0.23 can be described by SI(f) ˜ f– β with β = 1 and β = 0.4, respectively. For the sample Si0.06Ge0.67B0.26 two slopes were observed: in low frequency region f≤ 103 Hz β1= 0.7 and at higher frequencies f>103 Hz β2= 0.13. Increasing temperature resulted in an increase of noise magnitude and a change of β values. The latter depended on film composition. The correlation observed between noise and conductivity activation energies suggests that noise is due to bulk rather than interface processes. Noise spectrum of the thermo-sensing film Si0.11Ge0.88:H was compared with that for micro-bolometer structure with the same thermo-sensing film. The micro-bolometer structure showed higher noise value in entire frequency range that assumed additional processes inducing noise.

A microfabricated amorphous silicon photodiode is used to detect chemiluminescent and colorimetric horseradish peroxidase (HRP) enzymatic reactions. Detections limits of 1 nM and 1 pM of HRP are obtained for chemiluminescent and colorimetric measurements, respectively, with the reactions carried out in solution volume of 50 μL in polystyrene microwells. Surface-adsorbed HRP can be detected with a limit of 1 fmol.cm-2 by both detection methods. Immunoassays were performed using HRP-labeled antibodies and the detection of specific antibody-antigen molecular recognition is demonstrated both in the plastic well and inside a microfluidic channel. The application of the a-Si:H/HRP system is extended by coupling HRP with oxidase enzyme systems for glucose detection and a sensitivity of 0.1 mmol/L was achieved.

High rate deposition of hydrogenated microcrystalline silicon (μc-Si:H) films and solar cells were prepared by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) process in a high power and high pressure regime. The experiment results demonstrate that in high-rate deposited μc-Si:H films, the structural evolution is much more dramatic than that in low-rate deposited μc-Si:H films. A novel VHF power profiling technique, which was designed by dynamically decreasing the VHF power step by step during the deposition of μc-Si:H intrinsic layers, has been developed to control the structural evolution along the growth direction. Another advantage of this VHF power profiling technique is the reduced ion bombardments on growth surface because of decreasing the VHF power. Using this method, a significant improvement in the solar cell performance has been achieved. A high conversion efficiency of 9.36% (Voc=542mV, Jsc=25.4mA/cm2, FF=68%) was obtained for a single junction μc-Si:H p-i-n solar cell with i-layer deposited at deposition rate over 10 �/s.

We carried out extensive first-principles modeling of microscopic structural, vibrational, electronic properties and chemical bonding in hydrogenated amorphous silicon (a-Si:H) in a wide range of hydrogen concentration and preparation conditions. The theory has been compared with experimental results to comprehensively characterize this semiconductor material. The computer modeling includes ab-initio Molecular Dynamics (MD), atomic structure optimization, advanced signal processing and computer visualization of dynamics. We extracted parameters of hydrogen and silicon bonding, electron charge density and calculated electron density of states (EDOS) and hydrogen diffusion. A good agreement of the theory with various experiments allowed us to correlate microscopic processes at the atomic level with macroscopic properties. Here we focus on correlation of the amorphous structure of the material, atom dynamics and electronic properties. These results are of increasing interest due to extensive application of a-Si:H in modern research and technology and to the significance of detailed understanding of the material structure, bonding, disordering mechanisms and stability.

Light trapping effect in hydrogenated amorphous silicon-germanium alloy (a-SiGe:H) and nano-crystalline silicon (nc-Si:H) thin film solar cells deposited on stainless steel substrates with various back reflectors is reviewed. Structural and optical properties of the Ag/ZnO back reflectors are systematically characterized and correlated to solar cell performance, especially the enhancement in photocurrent. The light trapping method used in our current production lines employing an a-Si:H/a-SiGe:H/a-SiGe:H triple-junction structure consists of a bi-layer of Al/ZnO back reflector with relatively thin Al and ZnO layers. Such Al/ZnO back reflectors enhance the short-circuit current density, Jsc, by ˜20% compared to bare stainless steel. In the laboratory, we use Ag/ZnO back reflector for higher Jsc and efficiency. The gain in Jsc is about ˜30% for an a-SiGe:H single-junction cell used in the bottom cell of a multi-junction structure. In recent years, we have also worked on the optimization of Ag/ZnO back reflectors for nano-crystalline silicon (nc-Si:H) solar cells. We have carried out a systematic study on the effect of texture for Ag and ZnO. We found that for a thin ZnO layer, a textured Ag layer is necessary to increase Jsc, even though the parasitic loss is higher at the Ag and ZnO interface due to the textured Ag. However, a flat Ag can be used for a thick ZnO to reduce the parasitic loss, while the light scattering is provided by the textured ZnO. The gain in Jsc for nc-Si:H solar cells on Ag/ZnO back reflectors is in the range of ˜60-75% compared to cells deposited on bare stainless steel, which is much larger than the enhancement observed for a-SiGe:H cells. The highest total current density achieved in an a-Si:H/a-SiGe:H/nc-Si:H triple-junction structure on Ag/ZnO back reflector is 28.6 mA/cm2, while it is 26.9 mA/cm2 for a high efficiency a-Si:H/a-SiGe:H/a-SiGe:H triple-junction cell.

The aim of this paper is to find a parameter space for deposition of amorphous silicon films at low substrate temperature by VHF PECVD process for application in solar cell fabrication on cheap plastics. Our studies show that at lower substrate temperature, keeping the pressure constant, the ion energy flux reaching the growth surface decreases, which we partly attribute to increasing gas phase collisions arising from an increase in gas density. The role of hydrogen is two fold: (1) higher hydrogen dilution increases the ion energy and restores it to its required value at low temperatures; (2) a normal to dusty plasma transition occurs at lower hydrogen to silane flow ratio and this transition regime shifts to higher dilution ratios for lower substrate temperatures. Thus the role of high hydrogen dilution at low temperature is to avoid the dusty regime. Thus the role of high hydrogen dilution at low temperature is to avoid the dusty regime. The ion energy flux at low substrate temperature can also be restored to the value obtained at high substrate temperature, without increasing hydrogen dilution, by simply lowering the chamber pressure or increasing the delivered plasma power, though the IEDFs in these cases differ substantially from the IEDF at high temperature conditions. We propose that a low pressure or high power in combination with a modest hydrogen dilution (high enough to avoid dusty regime) will deliver silicon films at low temperature without sacrificing deposition rate.

We fabricated nc-Si TFTs in order to investigate the effect of the active-layer thickness on the characteristic of the nc-Si TFT. Bottom gate nc-Si TFTs were fabricated at 350°C using ICP-CVD. The thicknesses of the nc-Si layer were remained to 700, 1200 and 1700 Å. As the active-layer thickness increases, the mobility and the on-current level were not altered. However, the off-current level increased considerably and on/off ratio decreased. It may be attributed to highly doped characteristic of thick nc-Si film. As the nc-Si film thicker, the conductivity increases considerably and the Fermi level approaches to the conduction band minimum, which indicates the increases of doping level. The oxygen concentration shows high level of unintentional doping. Also, columnar growth of nc-Si film makes that the crystallinity of top region is much higher than that of bottom region. So, the conductivity of thick nc-Si film becomes high compared to that of thin nc-Si film. The structure of the nc-Si TFT with thick nc-Si film can be similar to the serial connection of N+, N- and N+ resistance, so that it suffers difficulty to suppress the off current and to secure high on/off ratio. Therefore, the off current can be suppressed by thinning of the high conducting active nc-Si layer and nc-Si TFT with channel thickness of 700 Å shows good on/off characteristic. It is deduced that bottom gate nc-Si TFT is necessary to have intrinsic channel layer as well as thin channel layer to reduce the leakage current.

Series of Raman spectra were measured for microcrystalline silicon thin film with variable crystallinity. Five sets of Raman spectra (corresponding to excitations at 325 nm, 442 nm, 514.5 nm, 632.8 nm and 785 nm wavelengths) were subjected to factor analysis which showed that each set of spectra consisted of just two independent spectral components. Decomposition of the measured Raman spectra into the amorphous and the microcrystalline components is illustrated for 514.5 nm and 632.8 nm excitations. Effect of the light scattering on absolute intensity of Raman spectra was identified even for excitation wavelength highly absorbed in the mixed phase silicon layers.

It is presented in this work a p(a-SiC:H)/i(a-SiC:H)/i((a-Si:H)/ n(a-Si:H) single junction for application in color sensing domain produced with the PECVD technique. The interest of this device resides in its simplicity of realization and utilization, as it takes advantage from the well known properties of the a-Si:H p-i-n junctions together with the possibility of color filtering by tailoring the optical gap of a-Si:H with the introduction of a small percentage of Carbon. The thicknesses of the i(200 nm) and i (1000 nm) layers are optimized for light absorption in the blue and red ranges, respectively. Measurements of the spectral response under forward and reverse polarization show a dependence of the wavelength of the maximum absorption on the intensity of the applied bias. A comparison of the photocurrent reading with and without a 650 nm background DC optical bias permits a complete separation of blue and red color under reverse and forward applied bias, respectively. The application of the LSP technique (AC regime of the optical bias) permits a complete RGB reading of the incoming light. Simulation results obtained with the program ASCA will support and explain the measurements about spectral response and photocurrent reading under DC and AC regimes.

Hole drift mobilities in hydrogenated amorphous silicon (a-Si:H) and nanocrystalline silicon (nc-Si:H) are in the range of 10-3 to 1 cm2/Vs at room-temperature. These low drift mobilities establish corresponding hole mobility limits to the power generation and useful thicknesses of the solar cells. The properties of as-deposited a-Si:H nip solar cells are close to their hole mobility limit, but the corresponding limit has not been examined for nc-Si:H solar cells. We explore the predictions for nc-Si:H solar cells based on parameters and values estimated from hole drift-mobility and related measurements. The indicate that the hole mobility limit for nc-Si:H cells corresponds to an optimum intrinsic-layer thickness of 2-3 μm, whereas the best nc-Si:H solar cells (10% conversion efficiency) have thicknesses around 2 μm.

The transparent conducting oxide (TCO) ZnO:Al is often used as the window layer and a source of light trapping in thin-film silicon solar cells. Light scattering in sputtered zinc oxide is achieved by wet chemical etching, which results in craters distributed randomly over the ZnO surface. To gain a better understanding of the etching process on ZnO thin films, a method for atomic force microscope (AFM) realignment between etching steps is developed. Using this method, the evolution of the HCl etch on a polycrystalline ZnO thin-film is observed. Results showed that this observation method did not modify the etching behavior, nor did stopping and restarting the etching change the points of attack, indicating that the points of HCl attack are built into the films as they are grown. Additionally, we investigated the evolution of the HCl etch on a ZnO surface previously etched in KOH, and found that the etch sites for both the acidic and basic solution are identical. We conclude that “peculiar” defects, which induce accelerated etching, are built into the film during growth, and that these defects can extend part or all the way though the thin-film in a similar way as screw dislocations in single crystalline ZnO.

Photonic crystal based back-reflectors are an attractive solution for light management and enhancing optical absorption in thin film solar cells, without undesirable losses. We have fabricated prototype photonic crystal back-reflectors using photolithographic methods and reactive-ion etching. The photonic crystal back-reflector has a triangular lattice symmetry, a thickness of 250 nm, and a pitch of 765 nm. Scanning electron microscopy images demonstrate high quality long range periodicity. An a-Si:H solar cell device was grown on this back-reflector using standard PECVD techniques. Measurements demonstrate strong diffraction of light and high diffuse reflectance by the photonic crystal back-reflector. The photonic crystal back-reflector increases the average photon collection by ˜9% in terms of normalized external quantum efficiency, relative to a reference device on a stainless steel substrate with an Ag coated back surface.

We have investigated the local electron transport in polycrystalline silicon (pc-Si) thin-films by atomic force microscopy (AFM)-based measurements of the electron-beam-induced current (EBIC). EVA solar cells are produced at UNSW by <i>EVAporation</i> of a-Si and subsequent <i>solid-phase crystallization</i>–a potentially cost-effective approach to the production of pc-Si photovoltaics. A fundamental understanding of the electron transport in these pc-Si thin films is of prime importance to address the factors limiting the efficiency of EVA solar cells. EBIC measurements performed in combination with an AFM integrated inside an electron microscope can resolve the electron transport across individual grain boundaries. AFM-EBIC reveals that most grain boundaries present a high energy barrier to the transport of electrons for both p-type and n-type EVA thin-films. Furthermore, for p-type EVA pc-Si, in contrast with n-type, charged grain boundaries are seen. Recombination at grain boundaries seems to be the dominant factor limiting the efficiency of these pc-Si solar cells.

Single crystalline Si thin films on insulating substrates (SOI) have a variety of potential applications to such as high mobility TFT and to high efficiency and low cost solar cells. Since the SOI is limited to a thin layer, it is needed to develop a low temperature epitaxial growth technology to form active layers thicker than several micorns at low temperatures. The purpose of this study is to develop a deposition technique of single crystalline Si thin films by a reactive CVD method [1] at temperatures less than 600○C utilizing gas-phase reaction (SiH4, F2). Deposition of Si films was performed on a single crystalline Si (100) wafer. Substrate-temperature was varied between 100 and 700○C, reaction-pressure 1 and 500mTorr, flow-rate between SiH4/F2 = 1/1 and 1/3, and the geometry of the substrate and the gas-outlet were optimized. First, it was found that deposition rate was sensitive to the distance between the gas-outlet and the substrate and to the total pressure. For four different combinations of pressures, 250 and 500 mTorr and distances, 50 and 150 mm. The deposition took place only for the combination of 500 mTorr and 50 mm, and otherwise the deposition rate was significantly lower or etching of Si wafer was observed. The deposition rate for gas flow ratio, SiH4/F2 of 1/1 was 1.7 nm/s at a substrate-temperature of 400○C, while for higher F2 flow rate ratio, SiH4/F2 = 1/2 and 1/3, the deposition rates were 8.3×10-3 nm/s and etching, respectively. Raman measurements show that crystallinity depends on the substrate-temperature; broad amorphous signal appears at 300, microcrystalline signal at 300 and 500○C and sharp crystalline at 400○C. RHEED observation shows a halo-pattern of amorphous-Si at 200○C, a mixed pattern of streak and spot without 2×1 superstructure at 300○C, a 2×1 streak-pattern at 400○C and a spot-pattern at 500○C. The reason of the narrow temperature window for epitaxial layer is a characteristic feature of low temperature epitaxy as reported before [2]. It is noteworthy the deposition rate of epitaxy obtained in this work is quite high, 1.7 nm/s even at 400○C. These observations are ascribed to the gas phase reaction between SiH4 and F2 and successive surface reactions. The SiH4 and F2 cause an exothermic reaction in the gaseous phases to generate radicals such as SiHx, H and F. The SiHx acts as a film precursor and others act as etchant. Under the conditions which radical density ratio SiHx/F increases, therefore, the deposition rate decreases or etching occurs. The material properties also will be discussed in relation to the growth mechanism. [1]J. Hanna et al., J. Non-Crst. Solids 114 (1989) 172-174 [2]T. Kitagawa, M. Kondo et al, Appl. Surf. Sci. 159-160 (2000) 30-34

This work is a step towards a viable process for poly-SiGe MEMS structural layers deposited at substrate temperatures below 250°C. Laser annealing was used for post-deposition layer treatment to realize poly-SiGe structural layers with the desired electrical and mechanical properties at low substrate temperatures. The technique uses a pulsed excimer laser beam for the local thermal treatment of a SiGe layer deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) at 210°C. By tuning the laser treatment and the film deposition conditions, 1-1.8 μm thick films having an electrical resistivity as low as 14.1 mΩ∙cm and optimal strain gradient in the range of -4.3×10-6 to +6.8×10-6 μm-1 were realized.

X-ray diffraction (XRD) analysis of thin silicon films was carried out using both the symmetric Bragg-Brentano and asymmetric thin-film attachment geometries. The asymmetric configuration allows quantitative phase analysis of the films and reveals that amorphous silicon films deposited from silane diluted with hydrogen have the strongest peak in the XRD patterns located around 27.5 degrees. This peak corresponds to the signal from ordered domains of tetragonal silicon hydride and not from cubic silicon crystallites. The full width at half maximum (FWHM) of this peak narrows from 5.1 to 4.8 degrees as the ratio of hydrogen to silane flow (R) increases to 20 and does not change significantly for higher hydrogen dilutions. The amorphous silicon films fabricated at different hydrogen dilution were applied as absorber layers in single-junction solar cells. Degradation experiments confirm a substantial reduction of the degradation when the dilution ratio is increased from R=0 to R=20. The light induced degradation of solar cells with absorber layers prepared at R > 20 is not further reduced by increasing R.

We fabricated highly transparent and high haze ZnO:Al film for front TCO of amorphous and microcrystalline silicon solar cells. We have sputtered ZnO:Al film of 1.3 μm on the thin seed layer of about 60nm which was previously sputtered on the glass substrate by using 4% dilution of oxygen to argon gas. The ZnO:Al film grown on the seed layer had much higher crystalline phase than one without any seed layer. Our bi-layer ZnO:Al film showed low resistivity of 2.66×10-4 Ω•cm and sheet resistance of 2.08 Ω/⇐ while conventional ZnO:Al film showed resistivity of 3.24×10-4 Ω•cm and sheet resistance of 2.46 Ω/⇐. After surface texturing by 0.5% HCl wet-chemical etching, the transmittance of ZnO:Al film was increased from 83.7% to 88.1% at wavelength of 550nm through the seed layer. Also the transmittance at 800nm was increased from 82.3% to 88.9%. Especially, haze values of the ZnO:Al film were drastically increased from 58.7% to 90.6% at wavelength of 550nm by employing the seed layer. Also haze values at 800nm were increased from 22.1% to 68.1%. It is expected that the seed layer method to improve the quality of ZnO:Al film will contribute to an increase of solar cell efficiency due to the high capability of light trapping and low electrical resistivity.