Recently the authors have demonstrated that compensated or “midgap” intrinsic hydrogenated microcrystalline silicon (μc-Si:H), as deposited by the Very High Frequency Glow Discharge (VHF-GD) technique, can be used as active layer in p-i-n solar cells. Compared to amorphous silicon (a-Si:H), μc-Si:H was found to have a significantly lower energy bandgap of around 1 eV. The combination of both materials (two absorbers with different gap energies) leads to a “real” tandem cell structure, which was called the “micromorph” cell. Micromorph cells can make better use of the sun's spectrum in contrast to conventional double-stacked a-Si:H / a-Si:H tandems.

The present study will show that the compensation technique (involving boron “microdoping”) used sofar for obtaining midgap μc-Si:H can be replaced by the application of a gas purifier. The use of this gas purifier has a beneficial influence on the transport properties of undoped intrinsic μc-Si:H. By this procedure, increased cell efficiencies in both, single microcrystalline silicon p-i-n as well as micromorph cells could be obtained. In the first case 7.7 % stable, and in the second case 13.1% initial efficiency could be achieved under AM1.5 conditions. Preliminary light-soaking experiments performed on the tandem cells indicate that microcrystalline silicon could contribute to an enhancement of the stable efficiency performance. Micromorph cell manufacturing is fully compatible to a-Si:H technology; however, its deposition rate is still too low. With further increase of the rate, a similar cost reduction potential like in a-Si:H technology can be extrapolated.

In order to realize large area integrated a-Si modules using a commercial, single chamber, PECVD reactor, an accurate optimization of deposition process for p-i-n solar cell has been performed leading to a 1 cm2 device efficiency value of 10.3 %.

Besides the efficiency improvements achieved by the insertion of a graded layer at p/i interface and by the introduction of SnO2 Asahi type U substrate, an “interface cleaning procedure”, based on NF3 flushing step, was the key for the cells Voc and fill factor increase.

Microcrystalline n+ layer, ZnO/Ag back contact and device thermal annealing gave further contributions to the cell efficiency.

Utilizing this technology, a large area p-i-n modules (900 cm2) with an initial efficiency of 8.5% has been manufactured.

A-Si:H p+-i-n+ solar cells have been made employing plasma enhanced chemical vapour deposition at frequencies between 30–80 MHz. Here, only the i-layer was fabricated at these very high frequencies (VHF). Both the p+- and n+-layer were made using 13.56 MHz. A previous study has shown the material quality to depend on mainly the applied rf-power, and only slightly on the frequency. It should be noted that for homogeneity reasons a certain optimized pressure is required for each frequency. There is a clear correlation between material quality and solar cell parameters. An initial efficiency of 10 % has been obtained for cells deposited at 65 MHz using a low power density, while the deposition rate still is 2–3 times higher than the one at 13.56 MHz. Light-soaking reveals stabilisation at 6 % for the best cell, which compares well to conventional 13.56 MHz cells.

In this paper we focus our attention on compensated materials with μ-doping concentration in order to obtain a stable intrinsic layer with initial high photoconductivity suitable for p-i-n solar cells. Films were grown from a mixture of undiluted silane, hydrogen diluted phosphine and hydrogen diluted diborane. Values of dark conductivity around 10-11 Ω-1cm-1 and photosensitivity ratio under AM 1.5 of 6 orders of magnitude have been obtained for phosphine/diborane ratio around 102. The difference between the two dopant concentrations is in agreement with the difference in doping efficiency of the two gases found in the characterization of single μ-doped films.

We compared the degradation behavior of compensated and intrinsic materials with the same initial dark and light conductivity. After about 20 h the photoconductivity of the compensated and the intrinsic material decreased of 33% and 70%, respectively. The space of investigable deposition parameters has been limited by the stress induced by the simultaneous presence of phosphine and diborane which leads to a macroscopic, periodic and regular damage of the film.

We achieved a stabilized efficiency of 9.2 % after only 8 % relative degradation for an a-Si:H/a-Si:H stacked cell with the top-cell i-layer prepared at 140 °C using a high hydrogen dilution of the silane process gas. From a comprehensive characterization of p-i-n cells and the corresponding i-layer material prepared at 140 °C and 190 °C substrate temperature with different hydrogen dilutions, we conclude that the performance of these pin cells strongly correlates with the material properties of the corresponding i-layers. High fill factors after light soaking are reflected in a good microstructure, high photo-conductivity, and relatively low defect density. Whereas the initial Voc is limited by interface recombination, volume recombination dominates the forward-dark current after light soaking. The stabilized Voc as well as the short-circuit current densities correlate with the optical bandgap of the i-layer.

We report on the preparation and properties of a-Si:H devices prepared at high temperatures on stainless steel substrates using low pressure ECR plasma deposition techniques. The devices were prepared using either Hydrogen or Helium as the plasma diluent gas. The use of He as the plasma gas led to films having significantly lower H concentration(4–5%) and a lower bandgap than comparable films made using hydrogen dilution. We find that we can make excellent devices, with good fill factors(over 70%) and voltages(over 0.86 V) using either H or He dilution. The use of He led to devices having smaller bandgap, by about 35–40 meV compared to devices made using H dilution. Detailed quantum efficiency measurements show that the hole collection in both types of devices is excellent, and that Urbach energies of tail states in each case is in the range of 43–45 meV.

a-Si:H / c-Si heterojunction diodes were produced by PECVD with varying amorphous silicon layer thickness and hydrogen dilution of the gas phase. An accurate determination of the growth rate also in the initial stages of the deposition was made possible by an original chemical method based on the dissolution of the films followed by spectroscopical analysis of the obtained solution.

The electrical characterization of the diodes confirms the generation - recombination - multitunneling nature of the transport. Although H2 dilution is important, however, beyond a certain level it is detrimental for the junction quality, probably due to the transition to a microcrystalline phase deposition. Solar cells were also produced, the best results being an open circuit voltage of 610 mV and an intrinsic efficiency of 14.2%.

Hydrogen dilution and buffer layers, as two ways to obtain higher Voc values in a-Si:H p-i-n solar cells, are directly compared in the present study. Special emphasis is laid on stability against light soaking. H2-dilution in combination with lower substrate temperature yields higher Voc values and better stability as compared to buffer layers. However, light absorption is decreased due to the increased gap in H2-diluted cells. The stability of buffer layer cells can remarkably be ameliorated by boron doping and H2-dilution of the a-SiC:H buffer layer. However, stabilized efficiency is higher for optimized diluted cells than for cells with a buffer layer. An a-Si/a-Si stacked cell with a graded dilution for both cells yielded 10% initial efficiency with 17% relative degradation. Diluted a-Si:H cells at lower temperature become specially interesting in combination with a microcrystalline bottom cell. For such a “micromorph” tandem cell we obtained 11.4% initial efficiency.

The effect of ion bombardment on the plasma enhanced chemical vapor deposition of a-Si:H and on TCO/a-Si:H interface reactions has been studied. An external DC-bias voltage is applied to the deposition plasma in order to change the ion flux and ion energy. The deposition rate increases with the applied bias-voltage i.e. with the plasma potential. Hydrogenated amorphous silicon is grown on natively rough transparent conductive oxide. With cross-sectional transmission electron microscopy lower density regions can be observed in the a-Si:H in all sharp valleys of the TCO. The appearance of the lower density regions changes under influence of the ion bombardment. The observed changes are in agreement with the changes observed when no ion bombardment is present at all, like in a hot wire chemical vapor deposition process. The increased ion bombardment did not give rise to observable chemical reactions at the TCO/a-Si:H interface.

It is known that the a-Si:H solar cell with H-diluted i-layer (p-iH-n) exhibits a better stability under AMI light exposure than the non-diluted solar cell (p-i-n). Recently, Wang et al. found that the a-Si:H solar cell when inserting a thin H-diluted i-layer between the p- and the i-layer (p-iH-i-n) shows the same stability as a p-iH-n solar cell after 600 hours light soaking. It was suggested that the stabilization of a H-diluted a-Si:H solar cell is through controlling its p/i interface. In this work, we report the electroluminescence (EL) measurements of a-Si:H solar cells with p-iH-n, p-iH-i-n and p-i-n structures made in identical conditions except the difference in H-dilution processing. EL spectra in a wide range of temperature indicate that the defect-band emission can be controlled through a thin H-diluted i-layer near the p/i interface region, but the main-band emission depends more on the diluted i-layer thickness. According to the temperature dependence of the EL efficiency, the valence band-tail width of the active layer in the p-iH-i-n cell is estimated to be about 13% broader than that in the p-iH-n cell. From the total EL intensity as a function of the forward current density, the recombination processes at room temperature are suggested to be a monomolecular type for p-iH-wn, p-iH-i-n, and p-i-n structures.

We have applied real time spectroellipsometry (RTSE) to study hydrogenated amorphous silicon (a-Si:H) solar cells fabricated in the Cr/n-i-p configuration using plasma-enhanced chemical vapor deposition (PECVD) in a single-chamber system. The microstructural evolution of the n-, i-, and p-layers of the devices has been determined, including the thicknesses of the bulk, interface, and surface roughness layers versus time. The optical properties of the individual layers, including the dielectric functions and optical gaps, have also been obtained in the same analysis. In this study, we have focused on i/p interface formation and, in particular, on the nucleation process for differently-prepared a-Si:C:H and mixed-phase μc-Si:H/a-Si1-xCx:H p-layers on the a-Si:H i-layer. From the thickness dependence of the p-layer void volume fraction, we can obtain an estimate of the thickness at which nuclei make contact to form a continuous film. For the mixed-phase p-layers, the nuclei contact thickness can be reduced by exposing the i-layer to a H2-plasma prior to p-layer deposition. We have found that for similarly-prepared p-layers this reduction in contact thickness leads to an increase in open-circuit voltage of the solar cell

The current status and next-generation technologies for liquid-crystal displays based on amorphous silicon TFTs are described. Panel size, resolution, design rule, and viewing angle are discussed from the standpoint of panel performance, productivity, and product application. The material properties for future TFT/LCDs are specified based on the discussions of design rule.

High deposition rates and good quality electrical properties and thickness uniformities over large areas are required for all three films (SiNx, a-Si:H and n+a-Si:H) composing the thin film transistors (TFTs) for the AMLCD industry, while maintaining high tool up-time and low particle formation. Generally these conditions have been achieved for most single-panel multichamber PECVD systems; however, it has become increasingly apparent that a compromise is drawn between the TFT mobility and the deposition rate of the a-Si:H layer. Thus it becomes essential to clearly assess the industry requirements for both deposition rates as well as TFT performance for the different device structures used for AMLCDs, and to discover and control these underlying material properties.

The TEL VHF (40/60 MHz) PECVD system produces high quality, low defect density a- Si:H at deposition rates exceeding 1500 Å/min when analyzed by FTIR, CPM, photo and dark conductivity. Even though the low deposition rate a-Si:H exhibits very similar bulk properties, higher mobility TFTs are produced with a-Si:H deposited at lower RF power. Having both a high ion flux and low ion energy in the SiH4 discharge are likely the most critical conditions for controlling the a-Si:H quality and thus the TFT mobility. Increasing the RF frequency enhances both of these effects, as well as provides a higher deposition rate for a given power density and a higher power threshold for particle/powder formation. For these reasons it is likely a 40/60 MHz plasma will produce better performing TFTs for a given deposition rate when compared with a conventional 13.56 MIHz system. Other process conditions such as diluting the SiH4 in H2 or Ar also seem to play an important role in the optoelectronic properties of the a-Si:H film and ultimately the TFT performance.

Conventional high voltage thin-film transistors (HVTFTs) suffer from performance limitations such as low on-current, Vx. shift and large curvature in the linear region of the output characteristics. These limitations are associated with the highly resistive dead region in conventional HVTFT structures. In this paper, we present a novel TFT structure which has a high on-current, improved output characteristics in the linear region, and no Vx, shift. The higher on-current and significant improvement in output characteristics allows faster switching. Elimination of the Vx shift leads to more reliable circuit operation. The new structure is based on the conventional low voltage TFT (LVTFT) structure except that it does not suffer from low-voltage breakdown. The low-voltage breakdown of the gate nitride in conventional LVTFTs is perceived to be due to spiking of the drain metallization into the underlying layers which creates regions of very high electric field. In our novel structure, a higher breakdown is achieved by locating the metal contacts away from the gate edge while keeping the necessary drain to gate overlap through a heavily doped microcrystalline layer. Therefore, the new TFT extends the same performance as LVTFTs to high voltage operation. Furthermore, this structure also enhances the yield and reliability by minimizing the common faults in TFTs such as short circuits between gate, source and drain.

The surface morphology of 40-MHz PECVD SiNx films is investigated. We report on the correlation between the deposition conditions, bulk properties, and surface roughness of these TFT insulators. The roughness is measured with atomic-force microscopy, AFM. A link will be presented between the AFM properties and the effects of hydrogen dilution during deposition: gas composition and rf-power-density (P) dependences will be discussed. An increase of the surface roughness to 3.7 nm is observed upon H2 dilution and P increase, ascribed to enhanced ion bombardment of the surface during growth.

The laser annealing effects on the TEOS (Tetra-Ethyl-Ortho-Silicate) oxide of MOS (AI/TEOS/n+ Silicon ) structures was investigated with different initial oxide conditions, such as breakdown field. The breakdown field increased upto the 170 mJ/cm2 with increasing laser energy density and decreased at 220 mJ/cm2. It is considered that the increase of breakdown field is originated from the restore of strains which exist mainly at the metal/oxide interface.

We present the first thin film transistors (TFTs) incorporating a low hydrogen content (5 - 9 at.-%) amorphous silicon (a-Si:H) layer deposited by the Hot-Wire Chemical Vapor Deposition (HWCVD) technique. This demonstrates the possibility of utilizing this material in devices. The deposition rate by Hot-Wire CVD is an order of magnitude higher than by Plasma Enhanced CVD. The switching ratio for TFTs based on HWCVD a-Si:H is better than 5 orders of magnitude. The field-effect mobility as determined from the saturation regime of the transfer characteristics is still quite poor. The interface with the gate dielectric needs further optimization. Current crowding effects, however, could be completely eliminated by a H2 plasma treatment of the HW-deposited intrinsic layer. In contrast to the PECVD reference device, the HWCVD device appears to be almost unsensitive to bias voltage stressing. This shows that HW-deposited material might be an approach to much more stable devices.

Laser interference structuring with a pulsed Nd:YAG laser set-up is introduced and typical geometric dimensions and limitations are discussed. Intensity modulated laser beams are used to recrystallize amorphous silicon layers periodically at the intensity maxima. Stripe or tip structures of multi-crystalline (mc) silicon with sub-micrometer dimensions are shown. Structural, electronic and optical properties of the mc-Si structures are discussed. A hybrid thin film p-i-n solar cell architecture is described where the textured TCO is replaced by a flat TCO and a laterally structured boron doped mc-Si window layer. Light trapping, light diffraction and higher internal electric fields are proposed to be the main advantages of such a solar cell.

Large area color image scanners are currently being widely investigated. The a-Si:H/a-SiC:H Adjustable Threshold Color Detector (ATCD) recently presented by the authors is suitable for greatly simplifying the structure of the arrays since ATCDs have just two external electrical connections. This makes possible to use the row-column addressing of pixels.

In this work, the transient operation of ATCDs in the charge integration regime is discussed. Charge integration is operated by photocurrent discharging the three series connected capacitances which compose the device structure. The penetration depth of the incident radiation depends on the wavelength and determines which of the three capacitances is discharged. The applied bias voltage determines the polarization of the three junctions and the three capacitance actually sensed. Correct blue, green and red light detection is shown in the read-out transients and the read-out and saturation times are presented. Finally design rules of the device and read-out system are given in order to achieve a correct detection of the three colors.

A novel two terminal thin film photo diode for color detection has been developed. The device structure which is based on standard amorphous silicon nipin multilayers exhibits three or even more linearly independent spectral sensitivity peaks and provides linearity over a wide range of illumination levels. Band gap engineering and electric field tailoring allow a precise voltage controlled shift of the collection region of photo generated carriers. The steady-state as well as the transient device characteristics have been studied in detail. Emphasis was put on optimization of the spectral sensitivity of the color diodes. Furthermore, an electronic color correction algorithm is presented which results in an improved color separation.