A. Shah

Improved equivalent circuit and analytical model for amorphous silicon solar cells and modules

An improved equivalent circuit for hydrogenated amorphous silicon (a-Si:H) solar cells and modules is presented. It is based on the classic combination of a diode with an exponential current-voltage characteristic, of a photocurrent source plus a new term representing additional recombination losses in the i-layer of the device. This model/equivalent circuit matches the I(V) curves of a-Si:H cells over an illumination range of six orders of magnitude. The model clearly separates effects related to the technology of the device (series and parallel resistance) and effects related to the physics of the p-i-n junction (recombination losses). It also allows an effective /spl mu//spl tau/ product in the i-layer of the device to be determined, characterizing its state of degradation.

Effects of dangling bonds on the recombination function in amorphous semiconductors

Abstract A closed-form expression for the recombination function in steady-state illuminated hydrogenated amorphous silicon (a-Si: H) is given for the case that recombination occurs mainly via the dangling-bond states. Based on the three charge conditions (positive, neutral or negative) for the dangling bonds, the three corresponding occupation functions are derived; an expression for the recombination function R DB is thereby obtained. The latter differs considerably from the commonly used Shockley-Read and Hall function RSRH. As an illustration, the limiting carrier in an a-Si: H p-i-n solar cell under reverse voltage is shown to be either one with a longer drift length, using KSRH, or one with a shorter drift length, using R DB. Therefore one can conclude that the use of the proper recombination function is critical for a discussion of the relevant physical parameters involved in the description of p-i-n devices. Interpretation for ambipolar diffusion length and photoconductivity are also mentioned as ...

VHF Plasma Deposition: A Comparative Overview

The use of plasma excitation frequencies f in the VHF band (30–300 MHz), and particularly of f=70 MHz, for the high-rate deposition of amorphous hydrogenated silicon (a-Si:H) is described. Deposition rates, using monosilane (SiH 4 ) as source gas, are thereby increased roughly five fold to over 10 A/s as compared with the conventional case of RF plasma enhanced chemical vapour deposition with f=13.56 MHz. This may possibly be attributed to an enhancement in the high-energy tail of the electron energy distribution function (EEDF) of the plasma. Thereby, no noticeable deterioration in film properties is found. Characteristics of VHF-deposited a-Si:H films are extensively reported, including properties like microstructure, hydrogen effusion behaviour and its low internal mechanical stress. High quality hydrogenated microcrystalline silicon (μc-Si:H) can be deposited at low substrate temperature and low plasma power densities thanks to VHF glow discharge. This can be linked to a reduction in sheath potential and to the energy of the ions arriving at the growing surface. Thereafter, use of VHF plasma in applications such as 100 μm thick a-Si:H layer for particle detectors and powder-free deposition of solar cells with efficiencies over 8% are reported.

Effect of the dangling-bond charge on the ambipolar diffusion length in a-Si: H

Abstract The light intensity dependence of the photoconductivity (σph), of the ambipolar diffusion length (L amb) and of the ratio of the effective mobilities (b = μn∗/μp∗) has been measured on undoped a-Si: H. L amb and b have been measured using the steady-state photocarrier grating (SSPG) method. The kinetics of the light-induced degradation (Staebler-Wronski effect) of a typical undoped hydrogenated amorphous silicon (a-Si: H) film is presented: it illustrates that the measured value of L amb varies in a quite different manner with deep defect density that σph. All these results can be explained by incorporating the effect of the dangling-bond charge into the model (used hitherto), that included only the effect of the charge trapped in the bandtails.

Characteristic lengths for steady-state transport in illuminated, intrinsic a-Si:H

The steady-state transport equations are solved for a general small-signal case. This solution leads to the spatial distribution of the carriers in the bulk of undoped hydrogenated amorphous silicon (a-Si:H) under illumination. The general small-signal differential equations for electrons and holes, including a local and/or external electric field, are first established. Subsequently, the analysis is restricted to the case with a negligible external electric field. Two characteristic lengths appear in the analytical solution. They are interpreted for the lifetime regime as the ambipolar diffusion length and the dielectric relaxation length. Depending on the material parameters, one or the other of these lengths dominates the transport. Illustrative numerical examples for typical solar-cell-quality a-Si:H are given. >

Assessment of Free Electron to Free Hole Ratios in a-Si:H By Ambipolar Diffusion and Photoconductivity Measurements

The authors report on systematic photoconductivity σ ph and ambipolar diffusion length L amb measurements carried out on a series of lightly p- and n-doped samples and monitored throughout the entire light-induced degradation process. The results are interpreted based on a transport model involving free carriers (nf and pf), localized charge on bandtails, as well as dangling bond and dopant charges.

X-Ray Detectors Based on “Thick” a-Si:H Layers Deposited by the VHF-GD Process

In spite of its low absorption coefficient for X-rays, amorphous Silicon can be an interesting alternative approach for X-ray detection because of its low cost, its potential for large-area deposition and the possibility to deposit on a curved surface. For this application, basically two approaches have been proposed up to now: either a thick solar cell type n-i-p structure (the i-layer as to be sufficiently thick i.e typically 50 μm or more), or a normal solar cell type n-i-p structure (with a relatively thin i-layer, i.e (typically 1 to 2 μm) together with a fluorescent layer emitting visible light composed e.g. of CsJ. In this paper, we present first results of a X-ray detectors with thick i-layers (15 to 100 μm) prepared by the high deposition rate VHF-GD technique introduced at our laboratory. Detectors with low leakage currents ( 2 ) under high reverse bias voltages (about 100 V) could be fabricated at rates as high as 22 A/s. As substrates, aluminium as well as TCO-coated glass substrates were used. The detectors have a n-i-p structure, where highly conductive (100 S/cm) n-doped μc-Si:H was first deposited. For the substrate preparation, a high energy Ar plasma was applied before the first deposition step; in this way excellent sticking conditions could be achieved, although in the thicker detectors considerable curvature due to the internal mechanical stress could be observed. A medical X-ray radiation source was used, where the detector was exposed to a continuous X-ray spectrum at acceleration voltages between 80 kV and 240 kV. The paper presents measurements on the linearity of the detector, as well as on the value of the reverse current in the dark, which must be as small as possible to have the best signal to noise ratio.

STM on doped and undoped hydrogenated amorphous and microcrystalline silicon films

Abstract STM was used to investigate the nucleation and growth mechanism of hydrogenated microcrystalline and amorphous silicon films. Silicon films of different thickness were prepared by glow discharge of SiH4 on HOPG and silicon wafers. A growth mechanism was found leading to hemispherically shaped structures which were resolved down to the atomic scale.

a-Si:H films deposited at high rates in a 'VHF' silane plasma: potential for low-cost solar cells

The very-high-frequency glow discharge (VHF-GD) is a high-rate deposition method for amorphous silicon based on the use of plasma excitation frequencies in the range 30-150 MHz. Thereby the high-energy tail of the electron energy density function is enhanced, increasing the deposition rate R, without a corresponding increase in electric field and ion bombardment. An extensive set of optoelectronic properties ( sigma /sub dark/, E/sub a/, sigma p/sub h/, CPM, PDS, TOF, SSPG, steady-state Hecht plot) is presented for samples prepared by VHF-GD for the above frequency range and R approximately=12-15 AA/s. Emphasized are hole transport properties. With values of ( mu /sup D/ tau /sup t/)/sub h/ by TOF (time of flight) around 3*10/sup -10/ but up to approximately=5*10/sup -9/ cm/sup 2//V, VHF-GD is judged to be adequate for solar-cell applications.<<ETX>>