Thin microcrystalline silicon-germanium films (μ-Sil.xGex:H) prepared by PECVD at 95 MHz have been investigated. The optical absorption of these films increases in the infrared spectral region with increasing germanium content. In addition to the shift of the indirect gap an increase of the absorption coefficient above the band edge is observed. The material shows high crystallinity and exhibits good structural quality similar to pure μ-Si:H films. The films are homogeneous on a macroscopic to a microscopic scale as confirmed by Raman spectroscopy and Electron Microscopy methods. p-i-n solar cells with pc-Sil-xGex:H i-layers have been prepared for the first time. An efficiency of η = 3.1 % under AM1.5 has been obtained for a cell with 150 nm thin i-layer.

Hydrogenated amorphous silicon based nipiin three color detectors with a bias voltage controlled spectral response have been fabricated. These band-gap and mobility-lifetime product engineered structures employed as two terminal devices exhibit a dynamic range above 95 dB. The maximum of the spectral response shifts by variation of the applied voltage. Three linearly independent spectral response curves can be extracted to generate a red-green-blue signal. Conventional spatial color separation with optical filters is transferred into a voltage multiplexed read out sequence. Bias voltage switching under different monochromatic illumination and illumination switching-on transients for different bias voltages are carried out to investigate the time dependent behavior of the photocurrent. Based on these results optimization criteria to accelerate the transient behavior and to determine the maximum frame rate for color detection are presented.

Amorphous hydrogenated silicon-carbide films (a-SiC:H) were deposited in a standard rf glow discharge. Various gases as carbon sources, in addition to silane (SiH4), were used, namely methane (CH4), disilylmethane (DSM), trisilylmethane (TSM), and tetrasilylmethane (TetraSM). All our films show low hydrogen contents (NH≤2.5×1022 cm−3 for EG≤2.4 eV, determined by elastic recoil detection analysis) and low Urbach energies (E0≤70 meV for EG≤2.0 eV, determined by photothermal deflection spectroscopy). Infrared spectroscopy reveals that the more Si—C bonds are offered in the gas phase by using different carbon sources as feedstock, the more Si—C bonds are incorporated into the a-SiC:H solid. Furthermore, the number of CH3 groups decreases with the increasing number of Si—C bonds. The ratio of the hydrogen to the carbon content suggests less CH3 groups to be present in our films, compared to films of other authors.

To prepare hydrogenated amorphous silicon-germanium alloys as low gap material for multi-junction solar cells in plasma enhanced chemical vapour deposition, the well established concept of strong dilution of the process gases with hydrogen has been used. Two different regimes of alloying were found: for low Ge content (x < 0.40) we observe material with low defect density, small Urbach energy and high values of the ambipolar diffusion length. In the regime of high Ge content (x > 0.40) the defect densities and Urbach energies are high and the values of the ambipolar diffusion length low. The transition is accompanied by the appearance of a low-temperature peak in hydrogen effusion experiments indicating a void rich film structure. Material from just above and below the transition zone is used in pin solar cells leading to a much enhanced red response compared with a-Si:H cells. The differences seen in the material quality are mirrored in the solar cell properties. By carefully adjusting the active layer thickness material with low diffusion length shows also reasonable solar cell performance.

Dedicated to Professor Dr. K. W. BOER on the occasion of his 70th birthday Two examples of recent advances in the field of thin-film, amorphous hydrogenated silicon (a-Si : H) pin solar cells are described: the improved understanding and control of the p/i interface, and the improvement of wide-bandgap a-Si : H material deposited at low substrate temperature as absorber layer for cells with high stabilized open-circuit voltage. Stacked a-Si : H/a-Si: H cells incorporating these concepts exhibit less than 10% (relative) efficiency degradation and show stabilized efficiencies as high as 9 to 10% (modules 8 to 9%). The use of low-gap a-Si:H and its alloys like a-SiGe:H as bottom cell absorber materials in multi-bandgap stacked cells offers additional possibilities. The combination of a-Si : H based top cells with thin-film crystalline silicon-based bottom cells appears as a promising new trend. It offers the perspective to pass significantly beyond the present landmark of 10% module efficiency reached by the technology utilizing exclusively amorphous siliconbased absorber layers, while keeping its advantages of potentially low-cost production.42