A review is given of the fabrication technology and applications of a new type of semi-transparent solar cell having the structure p(a-SiC)-i(a- SiC)-n(μ-Si) heterojunction. A series of systematic investigations has been made on the effect of i-layer thickness and the optical energy gap on the photovoltaic performance of p(a-SiC)-i(a-SiC)-n(μ-Si) heterojunction solar cells. The results show that there is an optimum zone in the i-layer properties. For purposes of making cells with sufficient visible light transmission, we adopted an a-SiC active layer whose optical energy gap is in the range from 1.9 to 2.0 eV. In this case, a conversion efficiency of 6.3% with visible light transmittance of more than 25% has been obtained. The rate of light induced degradation has also been checked on this new type of solar cell. This newly developed semi-transparent solar cell, named “SOLAR GLASS,” might open new areas in a wide variety of applications for modern life such as home electronics, automobiles, medical electronics, etc. As an example, an automobile application is demonstrated and discussed.

It is now well-recognized that multijunction, multi-gap amorphous silicon-based alloy solar cells offer the attractive advantage of obtaining high efficiency with good long-term stability. In this paper we review the progress made in this field. Research directions to further improve the efficiency are discussed.

A high-efficiency integrated-type a-Si solar cell submodule with a size of 10cm × 10cm has been fabricated and a total area efficiency of 9.6% is obtained by using a high-quality p-layer doped with B(CH3)3 We have developed an advanced direct photo-CVD method. High-quallt” a-SI films with low tail characteristic energy and low light-induced degradation is prepared by this method. We have also studied the role of Si-H2 bonds on the light-induced effect. The result implies that Si-H bonds stabilize the defect states, resulting in a large light-induced degradation.

Direct measurements of the electron and hole mobility-lifetime products, μτ, on a 10μm thick hydrogenated amorphous silicon (a-Si:H) pi- n solar cell are presented. The μτ products, determined from charge collection using strongly absorbed light are μτ|h = 2.2 × 10−8cm2V−1 and μτ|e = 3.0 × 10−7cm2V−1, for holes,and electrons, respectively. Measurements of the drift length, ld = μτ|e + μτ|h, using uniformly absorbed light and analyzed using the uniform field model,1 give ld = 2.9 × 10−7 cm2 V−1 s−1. These results are the first experimental evidence that the carrier with the larger, μτ product determines the photovoltaic behavior. Evidence for space charge limited transport of photogenerated holes is also be presented.

We report on the development of the first large area semi-transparent amorphous silicon solar modules on pre-shaped compound curved substrates. To date, we have achieved an active area (approximately 1800 cm2) efficiency of about 7%. It is shown that, because of enhanced internal reflection, a metal/dielectric back contact is a good alternative to one consisting of merely a transparent conducting oxide (TCO). The dielectric serves as both a passivating layer and an anti-reflection coating.

We have studied the spectral dependence of various types of amorphous silicon-germanium (a-Si:Ge) alloy p-i-n solar cells in which the band gap of the intrinsic (i) layer is profiled between 1.4 and 1.7 eV. It is observed that the cell performance depends critically on the shape of the profile, especially for red-light illumination where the device output is found to vary by more than a factor of two. We have correlated the experimental data with optical absorption and dynamic internal collection efficiency (DICE) measurements. We have also fabricated two-cell tandem and three-cell triple devices by incorporating a-Si:Ge alloy with multiple band-gap profiles in the bottom cell and achieved 13.0% and 13.7% conversion efficiencies, respectively. These are the highest efficiency amorphous silicon-based alloy solar cells reported to date.

Amorphous SiC p-layers doped with trimethylboron (B(CH3) 3) were prepared with optical and electrical properties superior to those prepared with B2H6. Devices were prepared with efficiencies as high as 11.4% using trimethyl boron. The improved properties of B(CH3)3-doped a-SiC result from the fact that trimethylboron is a more effective doping agent than B2H6 and produces p-layers with a higher bandgap.

We report on the influence of the discharge excitation frequency on the material properties of intrinsic a-Si:H films deposited from disilane. A monotonic increase in the deposition rate with increasing frequency is observed in the range 13.56–110 MHz. Comparison of the properties of materials deposited at ˜1.5 nm/s using 13.56 and 110 MHz discharges in disilane indicates that there are some advantages to using 110 MHz. The performance of p-i-n devices fabricated from disilane at ˜2 nm/s at these two frequencies support this conclusion. We have fabricated a device from disilane at 1.8 nm/s using a frequency of 110 MHz with an AMl.5 efficiency of 9.7%.

The Solar Energy Research Institute (SERI) has been conducting field tests on amorphous silicon modules and analyzing the results in terms of both stability and performance. Both single p-i-n and tandem modules have been under evaluation for the past few years. This paper presents performance test results for several of these modules. Analytical models for efficiency and fill factor are presented. These models are used to project design lifetime performance. Mean square error curve fitting techniques are used to overcome data scatter. The models bracket best and worst case performance. A phenomena based, double exponential model is included to project end of life performance. This model primarily focuses on degradation due to light induced defects.

By combining the steady-state and transient behaviors of a recently generalized analysis of the kinetics of metastable defects in good amorphous Si:H, it is shown that no treatment can remove all metastable defects. There is always a significant remnant that is proposed to be the observed built-in defects, which are then not a separate species, distinct from the metastable defects. This remnant is due to vibrational breaking of weak bonds in latent defect centers, and cannot be due to recombination of thermally excited carriers. If the stability can be improved by reducing the number of these latent defect centers, this will also reduce the density of built-in defects that control the initial efficiency. Furthermore, there are good reasons to believe that the source of these defects is extrinsic to a-Si:H, so that improvement in both properties may be achievable.

We use measurements of subbandgap photocurrent spectra on films, barrier structures and pin cells of different thicknesses to estimate the valence band tailing and to characterize the dangling bond defect region. Large differences in signal sizes in the structures are observed and particularly for thick and thin pin-diodes, the dependence of the signal on voltage bias is discussed. In pin-diodes, these effects can be explained by a change of defect occupation in the i-layer.

We have studied the effects of a hydrogen-plasma reactive flush between the pand i-layer depositions in fabricating solar cells of glass/TCO/p(a-SiC:H)-i(a-Si:H)-n(a-Si:H)/metal in a single-chamber, glow-discharge deposition system. Spectral response, photoluminescence, and photovoltaic conversion efficiency measurements show that the hydrogen-plasma reactive flush is effective in reducing the p-i interface recombination of charge carriers and in improving solar cell performance.

Amorphous silicon (a-Si) based solar cells with a double-stacked tandem structure have been studied with a view to reduce light-induced degradation in photovoltaic performance without lowering the initial conversion efficiency. We have attained a conversion efficiency of 11.3% in a 1cm2 a-Si/a-Si tandem cell and 10.8% in a 1cm2 a-SiC/a-Si tandem cell. Using an a-Si/a-Si tandem cell with an initial efficiency of more than 10%, we have carried out a light soaking test, and found that the degradation in conversion efficiency was about 10% under open-circuit condition after 600 hours exposure.

Optical and transport studies of both cb- and vb-tail states in a-Si1−xGex:H such as subband absorption (PDS), instationary photocurrent experiments (TOF, PTS) for electrons and holes, Modulated Photocurrent Spectroscopy (MPS), and Raman scattering have been performed. The main consequences of Ge-alloying into the a-Si:H network are i) an increase in cb-tail state density at the conduction band edge and in the exponential cb- tail even for small x (O<x<0.3), accompanied by ii) a rise in deep cb-tail and midgap states which to some extent can be reduced by appropriate deposition methods; iii) at the valence band side up to x≈0.3 the tail seems not to be affected at all and for 0.3<x<0.9 the vb-tail obviously can be kept similar to that of a-Si:H (Evo≈(50–60) meV). Halfwidths of Raman TO-like modes point to the existence of a rigid Si-network in O<x<0.3 in which Ge is incorporated and to a transition at x>0.35 into a Si-Ge compound structure with maximum disorder at x≈0.5.

Raman spectroscopy is used to characterize the microscopic bonding structure of a-SiGe:H alloys. Excess Ge-Ge bonding over what is predicted for random alloys is observed. Correlation between the degree of Ge-Ge clustering and the transport properties of the alloys is demonstrated. Implications of the experimental findings regarding improvement in the quality of the alloy material are discussed.

1H NMR has been employed to study the local environments of bonded hydrogen and trapped molecular hydrogen (H2) in a series of a-Si1−xGex:H alloys. There is a monotonic decrease of bonded hydrogen with increasing x from ≈ 10 at. % at x = 0 (a-Si:H) to ≈ 1 at. % at x = 1 (a-Ge:H). The amplitude of the broad 1H NMR line, which is attributed to clustered bonded hydrogen, decreases continuously across the system. The amplitude of the narrow 1H NMR line, which is attributed to bonded hydrogen essentially randomly distributed in the films, decreases as x increases from 0 to ≈ 0.2. From x = 0.2 to x ≈ 0.6 the amplitude of the narrow 1H NMR line is essentially constant, and for x ≥ 0.6 the amplitude decreases once again. The existence of trapped H2 molecules is inferred indirectly by their influence on the temperature dependence of the spin-lattice relaxation times, T1. Through T1, measurements it is determined that the trapped H2 concentration drops precipitously between x = 0.1 and x = 0.2, but is fairly constant for 0.2 ≤ x ≤ 0.6. For a-Si:H (x = 0) the H2 concentration is ≈ 0.1 at. %, while for x ≥ 0.2 the concentration of H2 is ≤ 0.02 at. %.

We present a study of the temperature and intensity dependence of the (μτ)p for Schottky barriers on an a-Si:H and a 1.3eV aSi,Ge:H,F alloy. Measured values for the activation energy of (μτ)p, , and the generation rate exponent of the (μτ)p, φ, show good agreement with calculated values. We also relate the (μτ)p with the mid-gap defect density, Na.

Results are presented on the photoluminescence of plasma deposited amorphous carbon (a-C:H) films. Emission and exci tation spectra anti-Stokes emission, PL decay and fatigue, T-dependence of PL and photobleaching were mesuared. The data are discussed in terms of radiative recombination of the strongly localized electron-hole pairs.

As recently shown, in amorphous hydrogenated carbon films (a-C:H) powerful talls of localized states occur in the mobility gap that are due to a high degree of structural disorder [1] while a radiative recombination in this material is highly efficient [2, 3]. These properties, in our view, make a-C:H suitable for verification of models of recombination kinetics of photoexcited charge carriers in highly disordered systems with the use of photoluminescence (PL).

A reaction engineering model of the mercury sensitized photochemical vapor deposition of amorphous silicon-germanium thin films is used in conjunction with experimental results to describe the role of hydrogen radical reactions in determining film composition, hydrogen bonding and microcrystallinity. Selective removal of silicon from the film by hydrogen radicals is shown to increase with decreasing temperature, raising the germanium content of films in hydrogen diluted depositions below 2 torr. Etching of a-Ge:H by hydrogen radicals is determined to be nearly two orders of magnitude slower than that of a-Si:H. Hydrogen radicals are also shown to promote microcrystallinity by selectively removing amorphous phase silicon and bonded hydrogen.

The optoelectronic properties of the new semiconducting alloy a-CSiGe:H have been studied with particular regard to the dependence upon deposition and annealing temperature. Infrared and electrical results are interpreted in terms of hydrogen bonding.