Accessory monazite-(Ce) with an extraordinarily high proportion of the xenotime component in solid solution of 21–42 mol.% (6.5–14 wt.% Y2O3, 6–11 wt.% HREE 2O3) was discovered in a retrogressed Variscan high-pressure, high-temperature granulite from the southern Bohemian Massif, Austria. The grains with the highest proportion of xenotime (XXno ~0.4) should have had a minimum formation temperature of ~1050°C, according to published monazite-xenotime miscibility gap thermometers. This high temperature is consistent with previous petrological studies on the south Bohemian granulites indicating ~1000°C/16 kbar for the peak metamorphic stage.

Laser and oven annealing effects on hydrogen concentration, hydrogen diffusion and material microstructure in hydrogenated amorphous silicon films deposited on crystalline silicon substrates are compared. For laser annealing, a 6 W green (532 nm) continuous wave laser with 100 µm focus diameter was applied and samples of about 1 cm2 were scanned in ambient with a line distance of 50 µm and at a speed of 1 – 100 mm/s. Hydrogen content and microstructure were measured by infrared spectroscopy, and hydrogen diffusion was investigated by secondary ion mass spectroscopy (SIMS) measurements of depth profiles of deuterium and hydrogen in layered structures of deuterated and hydrogenated material. The results show that in both annealing experiments hydrogen diffuses predominantly in form of atoms although some formation of H2 molecules cannot be excluded. By comparison of laser and oven treatment, an effective temperature describing the laser treated state can be defined. Furthermore, the temperature of the thin silicon film during laser treatment is estimated.

The effect of process parameters on the plasma deposition of μc-Si:H solar cells is reviewed in this article. Several in situ diagnostics are presented, which can be used to study the process stability as an additional parameter in the deposition process. The diagnostics were used to investigate the stability of the substrate temperature during deposition at elevated power and the gas composition during deposition at decreased hydrogen dilution. Based on these investigations, an updated view on the role of the process parameters of plasma power, heater temperature, total gas flow rate, and hydrogen dilution is presented.

The effect of conventional process parameters on the deposition of μc-Si:H solar cells is reviewed. Then, an approach to solar cell optimization is presented in which hidden, internal parameters are adjusted rather than conventional, external process parameters. The investigation focuses on deposition at low H2 dilution ratio and low total gas flow. A hidden parameter is identified through time resolved optical emission spectroscopy on SiH emission: Transient depletion of the SiH4 source gas leads to uncontrolled deposition conditions during the first 90 s after plasma ignition. There hardly is any effect on plasma properties and deposited film properties for the remainder of deposition after the transient depletion phase. As demonstrator a 9.5 % efficient single junction μc-Si:H solar cell was deposited from a pure SiH4 flow. A reinterpretation of the role of H2 dilution is discussed.

Microcrystalline silicon carbide (μc-SiC) was prepared at substrate temperatures between 300°C and 450°C using Hot Wire Chemical Vapour Deposition (HWCVD). The SiC films were deposited from monomethylsilane (MMS) diluted in hydrogen on glass and crystalline silicon substrates. The influence of the hydrogen dilution on the deposition rate and the structural and the optoelectronic properties was investigated. Infrared and Raman spectroscopy and transmission electron microscopy (TEM) were applied to study the structural properties. Highly crystalline material with large columnar grains was obtained at high hydrogen dilutions. The optical absorption below the band gap is high and the dark conductivities are far above the values expected for intrinsic SiC. At lower hydrogen dilution, less crystalline or amorphous Si1-xCx is growing, showing broader IR- and Raman peaks, lower dark conductivity and higher absorption above the band gap energy. An extended nucleation zone with large structural disorder was observed even for highly crystalline material.

We present photocarrier time-of-flight measurements of the hole drift-mobility in microcrystalline silicon samples with a high crystalline volume fraction; typical room-temperature values are about 1 cm2/Vs. Temperature-dependent measurements are consistent with the model of multiple-trapping in an exponential bandtail. While this model has often been applied to amorphous silicon, its success for predominantly crystalline samples is unexpected. The valence bandtail width is 31 meV, which is about 10-20 meV smaller than values reported a-Si:H, and presumably reflects the greater order in the microcrystalline material. The hole band-mobility is about 1 cm2/Vs – essentially the same magnitude as has been reported for electrons and for holes in amorphous silicon, and suggesting that this magnitude is a basic characteristic of mobility-edges, at least in silicon-based materials. The attempt-frequency is about 109 s-1; this value is substantially smaller than the values 1011 - 1012 s-1 typically reported holes in amorphous silicon, but the physical significance of the parameter remains obscure.

A study of the effects of light-soaking and atmospheric adsorption (aging) on the dark- and photo-conductivity of a series of microcrystalline silicon films of varying crystallinity is presented. Light-soaking in vacuum slightly reduces photoconductivity in films close to the amorphous – microcrystalline transition, and there is also a reduction in dark current. Aging increases the dark current, and thus unless due care is taken during light-soaking experiments to eliminate or compensate for aging, the apparent effect of light-soaking may be reduced or even reversed in sign. Transient photocurrent decays confirm the presence of a large density of metastable light-induced defects. A shift in the apparent distribution of defects occurs on prolonged aging, which may be due either to changes in the DOS or a shift in the Fermi level.

Photoluminescence spectroscopy has been applied to investigate localized states in microcrystal-line silicon (μc-Si:H) films and to address the problem of the changes of the electronic properties of this material upon changes of the hydrogen dilution during film growth. By a comparison of photoluminescence and Raman spectra on device grade sample series prepared at different silane concentration in hydrogen (SC) by PE-CVD and HW-CVD a correlation between the micro-structure and the photoluminescence energy is found. It is proposed that the density of band tail states is reduced with increasing SC leading to the increase of the PL energy as well as to the increase of Voc of solar cells. The reason for the tails and their reduction is not clear but strain might play a crucial role and the amorphous hydrogenated phase might be effective for strain reduction.

Electron spin resonance and conductivity measurements were used to study adsorption and oxidation effects on microcrystalline silicon with different structure compositions ranging from porous, highly crystalline to compact, mixed phase amorphous/crystalline. We found a correlation between active surface area and the magnitude of observed meta-stable and irreversible effects.

Electron spin resonance accompanied by conductivity measurements in n-type microcrystalline silicon with different doping concentrations and different structure compositions has been applied for the study of the density of gap states and the influence of these states on charge carrier density. We studied doping concentrations close to the defect density where the doping induced Fermi level (EF) shift is determined by compensation of gap states. We found a correlation between the EF shift, the intrinsic defect density and structural changes.

Thin film microcrystalline silicon solar cells were prepared with intrinsic absorber layers by Hot Wire CVD at various silane concentrations and substrate temperatures. Independently from the substrate temperature, a maximum efficiency is observed close to the transition to amorphous growth, i.e. the best cells already show considerable amorphous volume fractions. A detailed analysis of the thickness dependence of the solar cell parameters in the dark and under illumination indicate a high electronic quality of the i-layer material. Solar cells with very high open circuit voltages Voc up to 600mV in combination with fill factors above 70% and high short circuit current densities jsc of 22mA/cm2 were obtained, yielding efficiencies above 9%. The highest efficiency of 9.4% was achieved in solar cells of 1.4μm and 1.8μm thickness. These cells with high Voc have considerable amorphous volume fractions in the i-layer, leading to a reduced absorption in the infrared wavelength region.

The influence of the preparation conditions in hot wire chemical vapour deposition (HWCVD) on the electronic properties of microcrystalline silicon is investigated in view of application of the material in thin film solar cells. Poor grain boundary passivation, as a result of hydrogen etching at strong hydrogen dilution of the process gas or thermal desorption of hydrogen at high deposition temperatures, is considered a main obstacle for material optimisation. We conclude that optimum μc-Si:H solar cell material, both from HW-CVD and from plasma enhanced CVD, is not necessarily obtained with largest grain sizes and apparent highest crystalline content, but rather by a material prepared under conditions which yield a compact morphology with an effective grain boundary passivation.

The structural properties of nip-µc-Si:H solar cells are investigated by transmission electron microscopy, X-ray diffraction and Raman spectroscopy. Different structural compositions are obtained by variation of the gas mixture during preparation by plasma enhanced chemical vapour deposition. Nucleation and growth of the n-layer onto textured TCO substrate was found to be similar to the growth on glass substrates. The growth of the i-layer follows a local epitaxy. This implies that the structure of the n-layer is of special importance regarding the control of the microstructure in microcrystalline Si nip solar cells.

Microcrystalline silicon solar cells were prepared at various substrate temperatures using a plasma enhanced chemical vapor deposition technique at 95 MHz. Devices in superstrate configuration, i.e. prepared on transparent glass/ZnO substrates with deposition sequence p-i-n, suffer from a reduction of short wavelength response upon increasing substrate temperature. As underlying mechanism adverse effects on the p-i interface region are discussed. For devices in substrate configuration (deposition sequence n-i-p on Ag/ZnO back-reflectors) a pronounced efficiency maximum with a highest value of 8.7 % is observed at substrate temperatures of about 250 °C. Comparing the dark J-V characteristics obtained for different device thicknesses at substrate temperatures of 200 °and 250 °C, respectively, improved i-layer material and transport properties are suggested in the latter case. The results illustrate the sensitivity ofmicrocrystalline silicon devices with respect to the employed substrate temperature by effects on the absorber layer material properties on the one hand and by effects related to the device design, e.g. the specific deposition sequence of the individual layers, on the other hand.

We report on electrically detected magnetic resonance (EDMR) studies in three different types of thin-film silicon solar cells: (i) c-Si absorbers with epitaxially grown silicon thin-film emitters (ii) c-Si absorbers with hydrogenated amorphous silicon (a-Si:H) emitters, and (iii) microcrystalline silicon (µc-Si:H) pin diodes. Although cells of type (i) and (ii) are of a similar structure their EDMR spectra are completely different. We identify surface recombination via Pb0 centers in cells with c-Si emitters but hopping transport through conduction bandtail states in the 30 nm thin a-Si:H emitter layer. No signals related to interface recombination are detected in either cell. The EDMR signals of cell (iii) are identified as hopping among tail states of the conduction band with a subsequent nonradiative tunneling transition to neutral dangling bonds. At elevated temperature recombination is suggested to be dominated by direct capture. The EDMR signal of the dark current can be described with a simple diode model only involving diffusion and recombination currents.

Thin film microcrystalline silicon solar cells with absorber layers of various structural composition have been prepared. The highest conversion efficiency is observed at preparation conditions close to the transition to the amorphous growth regime, i.e. crystalline volume fraction is high but not at its maximum. The optimized material consists of crystalline “fibers” with small diameter which extend through the whole absorber layer. On further approach to the transition regime a set in of amorphous growth can be observed, resulting in decreasing solar cell performance. Surprisingly, material prepared under conditions favoring highly crystalline growth exhibits a less efficient carrier extraction if applied to the solar cell. We discuss increasing bulk recombination as possible cause for this observation. The maximum conversion efficiency obtained was 8.7 % for a 1 νm single junction solar cell. Using our optimized deposition conditions with simultaneously higher discharge powers the deposition rate can be increased up to 4.6 Å/s at the high efficiency of 8.3 %.

Microcrystalline silicon (μc-Si:H) solar cells require an effective light trapping in the near infrared (NIR) to enhance the long wavelength spectral response. For this purpose we investigated back reflectors based on texture-etched ZnO/Ag stacks prepared on glass substrates by magnetron sputtering. With decreasing sputter pressure the resulting surface texture of the glass/Ag/ZnO substrates after etching exhibits a larger feature size and root mean square roughness. The increase in feature size corresponds to an increase of diffuse reflectivity. Applied in microcrystalline solar cells prepared by VHF plasma enhanced chemical vapour deposition (PECVD), the reflectors showing the largest feature size (prepared at the lowest possible sputter pressure) yielded the highest long wavelength spectral response. The μc-Si n-i-p cells prepared on the latter back reflector exhibited efficiencies of 6.9 % (short circuit current density jsc= 18.8 mA/cm2) and 7.5 % (jsc=25 mA/cm2) for an i-layer thickness of 1 μm and 3.5 μm, respectively.

The optical absorption of microcrystalline silicon germanium alloys (μc-Si1-xGex:H) increases in the near infrared region with increasing germanium content. Therefore, these alloys are promising candidates for the application as intrinsic absorber material in thin film solar cells with tandem and triple structure or IR-detectors. The material properties for a germanium content (x) up to 0.6 and the performance of solar cells based on this material were investigated. Graded bandgap structures are used to optimize the cell performance. For cells with x > 0.3 a continuously graded bandgap in the rear 20 nm of the i-layer (at the i/n interface) results in an enhancement of the open circuit voltage by 40-80 mV compared to an abrupt bandgap discontinuity. When the design of the p/i interface region is changed in a similar way no effect on Voc is observed.

Microcrystalline silicon with various crystalline volume fractions was prepared by plasma enhanced chemical vapour deposition. The material was studied by steady state and transient electron spin resonance in the dark and under light illumination. The observed resonances at g-values of 2.01, 2.0052, 2.0043, 1.998 can be attributed to the amorphous and microcrystalline constituents, and their respective intensities change as the ratio of amorphous to crystalline volume is varied. The origin of a fifth resonance at g = 1.995 remains unclear. Smaller crystalline volume fractions lead to lower spin densities and affect the recombination behaviour of photogenerated charge carriers. The recombination behaviour in highly crystalline material is also influenced by moderate Fermi level shifts, where differences show up between n-type (or undoped) and p-type samples. The differences are attributed to trapping of photo-generated holes in deep states within the disordered regions.