Time-dependent photocurrent response by the monochromatic square-pulsed excitation light of different wavelenths has been studied on the PIN a-Si:H solar cells. The experiments show that the shape of the photocurrent transient is distinctly changed by varying the photon energy of the excitation light and the applied biases. Our results can be explained by the relaxation of space charge distribution in the i-layer.

The turn-off character in a-Si:H solar cells has been studied by rectangular monochromatic light pulses. It is found that the turn-off time increases obviously with the decrease of the excitation light wavelengths when the turn-off steady photovoltages are the same. This phenomenon can be explained by the wavelength dependence of the photocapacitance and the time constant products RC in the a-Si:H solar cells.

We examine the morphology and optical properties of fluorine doped tin oxide films prepared by the chemical vapor deposition of tetramethyltin and tin tetrachloride and the influence of these properties on light trapping in amorphous silicon solar cells having the structure glass/SnO2 /P (a-Si1-xCx:H) -i-n/metal. Measured tin oxide properties include total and diffuse transmission as a function of wavelength, angular dependence of the scattered light, surface topography, and sheet resistance. Experimental quantum efficiency plots as a function of wavelength are presented and compared to computer calculations based on multilayer optics programs and measured optical constants. Using granular tin oxide, p-i-n cells 1 cm2 in area have been prepared with efficiencies exceeding 10%.

The use of transparent conducting oxides (TCO) as electrical contacts in a-Si:H solar cells has stimulated interest in the multitude of effects that these layers have on a-Si:H solar cell performance. The study of a-Si:H p-i-n junctions using a TCO contact involves many factors such as, interdiffusion, transmission, reflection, and resistivity. In this paper, we attempt to distinguish between these factors through the role they play in determining the solar cell device performance. Devices were characterized via dark and illuminated current vs. voltage (I-V) measurements, and spectral response. It was found that the properties of the TCO have an important role in influencing FF and Jsc in the devices.

The results obtained upon reactively RF sputtering indium - tin oxide (ITO) films are presented. It is found that while the amount of oxygen in the chamber is very critical in determining the properties of the films, it is easy to deposit reproducible films even at low oxygen concentrations when using an oxide target. At concentrations of oxygen below 2 vol% the films deposited exhibit almost metallic conductivities, while retaining an average transparency in the visible range of over 85%. The resistivity of these films increases with the oxygen concentration in the chamber, but the transition is not as sharp as that observed upon sputtering a metal target. These films are found to have very good “figures of merit” and are promising in their use as transparent conductors.

The present state of a-Si:H solar cells and studies on the light-induced effects in a-Si:H films and a-Si:H solar cells are presented. The conversion efficiencies attained are 11.5%, 10.1% and 7.5% (8.5% in active area) for 1.0cm2 singlejunction, multijunction cells and 1200cm2 submodule, respectively. The density of light-induced defects, ANs, changes with deposition temperature and annealing temperature. This suggests a strong correlation between the ANs and the network structure in a-Si:H films. Field testing results indicate that the degradation of a-Si:H modules can be suppressed within 10% of the original efficiency after 2 years operation. It is also indicated that the multijunction cells are more stable to light exposure than single p-i-n cells.

Basic technologies for solving problems in thermal and light-induced degradation have been developed. The tandem type a-SiC/a-Si solar cell with blocking layers exhibits excellent stability for both thermal and sun light conditions. An observable light-induced degradation is not seen in the cell performance after light exposure test of 2000 hours. Two instability modes, that is, thermal and light-induced degradation have been investigated. For thermal degradation, a blocking layer for preventing diffusion has been inserted between the back side metal electrode and a n-layer and another blocking layer has been introduced between the np tunnel junction. To prevent light-induced degradation, p-type a-SiC layer of the pin structure on the side of a glass substrate/SnO2 has been deposited at the temperature of 70°C. The highest efficiency is 9.0% at the present stage, but it is expected to be improved to more than 10%.

Variation of environmental conditions during light soaking of thin film silicon:hydrogen alloy (TFS) solar cells can have measurable effect on durability. Such sensitivity must be considered in optimizing processes for maximizing long-term performance. This paper will describe the effects of operating temperature, light intensity, operating bias, and duration of light soaking on the stability of TFS p-i-n solar cells.

Theoretical evaluation of stability and efficiency of two-junction two-terminal amorphous tandem solar cells is performed using an analytical model which takes into account not only optical characteristics and transport properties but also photodegradation effects due to dangling bond formation under light exposure.

Requirements for material quality and device design are particularly investigated.

Recently considerable interest has been generated in the application of thin film transistor (TFT) technology in electronic input and output devices. Amorphous silicon (a-Si:H) has made a major impact in these applications because of the relative ease with which the material can be economically manufactured over large areas. This paper will review in particular the current status of a-Si:H TFTs in liquid crystal display and printer applications, in addition to sensor/TFT arrays for page width input scanner document readers. There are a number of materials issues which limit the performance of these devices. In addition, there are some technological issues which influence the yield of the processes used to fabricate these electronic devices over large areas. Some novel new devices have been recently developed which dramatically increase the operating voltage of a- Si:H TFTs. Other new structures have been proposed and demonstrated for increasing the current and speed of these devices.

Thin film semiconductor devices have been investigated over the past twenty years for application in large area flat panel displays. The development of thin film transistors and diodes based on amorphous silicon (a-Si) alloy materials has made the application of these devices, to display technologies, very attractive. More recently, manufacturing techniques to produce high quality large area films of amorphous silicon alloys have been demonstrated for photovoltaic applications.

Most of the current research and development effort on active matrix liquid crystal displays (LCDs) has concentrated on a-Si alloy TFTs. The success of TFT based displays for large area flat panel displays has been limited so far, mainly due to the difficulty of obtaining a high quality gate dielectric by plasma deposition and due to the presence of crossing conductors on the same substrate, both increasing the probability of defects in the display. When a two terminal sandwich device is used, on the other hand, no gate dielectric is required, hence, a higher yield can be expected. Metal-insulator-metal and hydrogenated amorphous silicon alloy devices have been proposed for incorporation in LCDs. Performance requirements for a useful active matrix switching element and a comparison among the different a-Si alloy thin film devices used for this purpose will be reviewed.

A number of new amorphous silicon alloy microelectronic devices, including LCD active matrix displays, linear image sensors, and thin film multilayer computer memories, have been developed in our company. These applications rely heavily on the quality of the intrinsic semiconductor as well as its ability to withstand the many processing steps used in a modern photolithographic process. In this paper, we present electrical data on amorphous silicon alloy p-i-n diodes after such a process. These devices have an active area of 20μm × 20μm defined using standard photolithographic techniques and etched using a dry etch process. These diodes are characterized by ideality factors (n) of 1.4 and extrapolated reverse saturation current densities of 1013A/cm2h. The diodes exhibit nearly 10 orders of magnitude rectification at ± 3V and the reverse bias current density remains below 10-8 A/cm2 for reverse bias voltages of -15V. In pulsed forward bias, these diodes can be operated at current densities greater than 300A/cm2. Thin film amorphous silicon diodes moreover have the advantage that varying the thickness of the intrinsic layer allows the optimization of parameters such as the capacitance per unit area, the reverse bias current density and the forward bias conductance per unit area. We find that these devices are fully compatible with state of the art VLSI processing techniques and are suitable for applications in integrated circuit structures, for example rectification devices in microelectronic arrays and isolation devices in display matrices.

We propose a new general principle of operation for solid state devices, and demonstrate a novel transistor which we call a Double Injection Field Effect Transistor, based on this principle. We have fabricated amorphous silicon alloy double injection transistors, operating on the modulation of a double injection current by a gate field covering the complete path of the current channel.

Using these amorphous silicon alloy double injection transistors, we have achieved currents over fourteen times those theoretically possible for conventional amorphous silicon field effect transistors operating under similar conditions. This new principle, applicable to both thin film amorphous and crystalline devices, offers the potential of high current, high speed field effect transistors.

There is a rapid growth of interest in the application of amorphous silicon alloy thin film devices to large area microelectronic circuits. Increased current levels are a constant goal since gains in device current result in proportional gains in power and speed. Mobility limitations in amorphous silicon thin film transistors have directed interest toward short conduction channel devices to achieve higher current levels; furthermore, compatibility with large area processing makes photolithography with 10μm feature size very attractive. Consequently, innovative techniques, which define channel lengths by processing parameters rather than by mask feature size are necessary. Previous work has applied such techniques to vertical structure TFT's which define channel lengths by the vertical height of deposited layers. Here, we report on a technique which achieves short channel lengths in planar structures using etching parameters to define short channel lengths. Amorphous silicon alloy TFTs have been fabricated with channel lengths of ≈2μm which reach currents of lma. These techniques broaden the range of application of amorphous silicon alloy TFTs by providing devices capable of operating at higher currents and higher speeds.

A new novel thin film high voltage transistor in a-Si:H is described in this paper. This new structure extends the operation of a-Si:H TFT to 500 volts or more. The fabrication process of this new high voltage transistor is simple and compatible with that of the conventional low voltage TFT. The high voltage TFT can be switched by low voltage signals and is demonstrated to be capable of switching several hundreds of volts.

Two-step deposition method for fabricating ultra-thin a-Si FETs has been investigated theoretically and experimentally. Theoretical relation between field-effect mobility and a-Si layer thickness agreed qualitatively with the experimental results. The mobility of the 15nm-thick FET was drastically improved by an application of the two-step deposition method.

This paper describes the preparation and electrical characteristics of a-Si:H p-i-n-i-p and n-i-p-i-n thin film diodes suitable for driving monochrome liquid crystal displays with more than 500 lines. The symmetrical current-voltage curves in the reverse breakdown regime can ge described by i=i exp (E/Eo) where E ≃9×104 V/cm. In the range 20–125°C, the current is thermally activated with an energy of 0.25eV. The response time to applied voltage pulses is ≤ 10 us. The stability of the electrical characteristics is adequate for at least 104 hours of operation in an liquid crystal display.

Electrical characteristics indicate that the reverse breakdown current is a tunneling current injected into the i-layer and that electrons are likely to be dominant.

For negative charging electrophotographic applications a three layer heterostructure (Al/n+-a-Si:H/a-Si:H/a-SiC:H) has been investigated. To some extent a separation of the function of charge storage by a high band gap amorphous silicon carbon alloy and of that of photogeneration of carriers and of sufficient photosensitivity in an intrinsic amorphous silicon film has been achieved. Due to a low injection of electrons from the surface into the a-SiC:H and a small thermal generation rate of carriers within the a- Si:H film, the dark decay of the negative surface potential is dominated by space charge limited currents of holes. Thus a considerable difference in surface potential for negative and positive charging occurs, which is related to the difference in electron and hole mobility in a-Si:H. Since there is no remarkable barrier at the heterointerface a-Si:H/a-SiC:H an excellent injection of photogenerated holes into the a-SiC:H can be provided.

A thin multi-layered photoreceptor drum has been fabricated, using glow-discharge decomposition of disilane. The photoreceptor is thin(10 μm), but it is charged up sufficiently. The photoreceptor has three layers; the p-type a-Si:H (blocking layer), the intrinsic B-doped a-Si:H (photoconductive layer) and a-SiC:H (passivation layer). Disilane is highly reactive. The intrinsic B-doped a-Si:H was prepared from a slightly gaseous mixture of B2H6 (3 ppm). The amorphous-SiC:H passivation layer deposited from Si2H6 and C3H8 was developed to achieve sufficient surface potential and to improve the environmental characteristics.

High photosensitivity and fast photoresponse have proved a-Si:H to be a suitable thin-film photoconductor for large-size linear image sensors. Besides the a-Si:H bulk properties, the a-Si:H/electrode interfaces are of major influence on the sensor performance. In view of this performance, the readout circuit has to be designed carefully with respect to the desired dynamic range. Care has to be taken to avoid electrostatic hazards when mounting the sensor, as the breakdown voltage of a-Si:H sensor elements was found to be about 60 volts.