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Highly efficient hybrid multijunction solar cells are constructed with a wide-bandgap amorphous silicon for the front subcell and a low-bandgap polymer for the back subcell. Power conversion efficiencies of 11.6% and 13.2% are achieved in tandem and triple-junction configurations, respectively. The high efficiencies are enabled by deploying effective optical management and by using photoactive materials with complementary absorption.

The sub gap absorption at 1.17 eV in hydrogenated amorphous silicon (a-Si:H) has been measured by means of the ex situ cavity ring down (CRD) absorption technique. The evolution of defects has been studied as a function of film thickness and deposition temperatures. A comparison with the dual beam photoconductivity (DBP) technique shows that the CRD results are systematically higher. Furthermore, it is shown that the CRD technique is sensitive to surface defects and typical surface dangling bond coverages of 2×10−4 up to 6×10−3 are obtained for air-exposed a-Si:H samples depending on the deposition temperature.

Time-of-flight measurements on hydrogenated amorphous silicon deposited with a remote expanding thermal plasma at growth rates up to 12 nm/s have revealed a 7 to 10 times larger hole mobility than for films deposited with conventional rf-PECVD. The electron mobility on the other hand is up to 3 times less. Based on a determination of the density of states by post-transit photo-current analysis we suggest a comparable defect density at mid-gap as for films deposited with rf-PECVD. These material properties have been obtained at a substrate temperature of 400°C, which is needed to obtain solar grade material at these growth rates. Possible causes of these particular material properties, which may have application in thin film solar cells, are discussed. Furthermore we show that the high substrate temperature is still a drawback in solar cell preparation when using the standard p-i-n configuration.

An in situ single wavelength HeNe rotating ellipsometry study of high rate (~80 AA s/sup -1/) hydrogenated amorphous silicon (a-Si:H) deposition using an expanding thermal plasma is presented. An optical growth model is used to simulate the measured ellipsometric trajectories similar to models used for low rate a-Si:H growth in the literature. The in situ growth at high growth rates was studied as function of the substrate temperature. The refractive index n (at 632.8 nm) increases with increasing temperature corresponding to an increase in the density of the films. The in situ extinction coefficient k (at 632.8 nm) increases with increasing substrate temperature due to a smaller optical band gap and due to an increase in indirect absorption. It is shown that the ellipsometry setup in combination with optical modeling enables us to monitor the surface roughness evolution during deposition and to obtain the dynamic scaling exponent beta for postinitial growth

The scaling behavior of the surface morphology of hydrogenated amorphous silicon deposited from a SiH3 dominated plasma has been studied using atomic force microscopy and in situ ellipsometry. The observed substrate temperature dependence of growth exponent β reflects a crossover behavior from random deposition at 100 °C to a surface diffusion controlled smoothening around 250 °C to full surface relaxation around 500 °C. This crossover behavior has been reproduced by Monte Carlo simulations assuming a site dependent surface diffusion process, revealing an activation energy of ∼1.0 eV for the ruling surface smoothening mechanism. The implications for a-Si:H growth are discussed.

We propose a model which describes the frequency shift $(\ensuremath{\Delta}{\ensuremath{\omega}}_{\mathit{SM}})$ of the stretching mode (SM) of $\mathrm{Si}\mathrm{H}$ monohydrides (MH's) when incorporated in hydrogenated amorphous silicon $(a\text{\ensuremath{-}}\mathrm{Si}:\mathrm{H})$ with respect to the unscreened MH SM at $\ensuremath{\sim}2099\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. The model is based on an effective medium approximation of the dielectric using multiple Lorentz-Lorenz dielectrics, corresponding to a host dielectric with MH's embedded in cavities, separately. The $\ensuremath{\Delta}{\ensuremath{\omega}}_{\mathit{SM}}$ as derived in this model correctly predicts all bulk MH-SM positions in $a\text{\ensuremath{-}}\mathrm{Si}:\mathrm{H}$ films and relates it directly to the nanostructure of the MH bulk configurations.

The interchangeability of ion bombardment and deposition temperature during a-Si:H film growth at high deposition rates (10-42 Å/s) by means of the expanding thermal plasma has been studied. The ion bombardment is generated by applying an external rf bias voltage on the substrate. It is shown that the opto-electronic performance of the a-Si:H films improves considerably when a moderate rf substrate bias voltage (∼20-60 V) is applied, i.e. the photo response increases two orders of magnitude up to 106. Furthermore, it is also revealed that the additional energy supplied to the growth surface by the ion bombardment, makes a reduction of the deposition temperature by ∼100 °C possible, while preserving good material properties. On the basis of the results obtained, three effects caused by the rf substrate bias can be distinguished: creation of an additional growth flux, a reduction of the void incorporation, and an increase in the vacancy density.