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A theoretical model, combining trapping/detrapping and recombination mechanisms, is formulated to explain the origin of random current fluctuations in silicon-based solar cells. In this framework, the comparison between dark and photo-induced noise allows the determination of important electronic parameters of the defect states. A detailed analysis of the electric noise, at different temperatures and for different illumination levels, is reported for crystalline silicon-based solar cells, in the pristine form and after artificial degradation with high energy protons. The evolution of the dominating defect properties is studied through noise spectroscopy.

Excess charge carrier transport and relaxation in semiconductor layered structures have been numerically simulated by a finite-difference method.

Here we present new materials obtained using, either fungal or isolated tobacco cells in association with different percentages of carbon nanotubes (CNTs). As a proof of concept, we used eitherCandida albicansor a non-green Tobacco BY-2 cell line combined with multi-walled CNTs. The electrical, mechanical, and conductivity vs temperature properties for some of these materials have been determined.C. albicans-based tissues have high conductivity and are stable at elevated temperatures. By lowering the CNTs content, we obtained a stable, electrically conductive optical transparent film, though with a relatively high sheet resistance. Further, we produced, using tobacco cells, a material that exhibits good electrical as well as mechanical properties.

Contactless in-situ measurements of the microwave detected transient photoconductivity during growth of intrinsic a-Si:H films by plasma enhanced chemical vapour deposition are presented. It is shown, that these measurements can be performed without perturbation of the deposition process. The growth of a-Si:H films at 250° C and 120° C substrate temperature is studied and the information obtained from these measurements is discussed. In-situ characterization during growth of a multilayer structure with films deposited subsequently at 120° C, 250° C and again at 120° C is shown.

Irradiation of unsupported samples of hydrogenated amorphous silicon below 100 K with 360-MeV Xe ions results in macroscopically visible changes in sample dimensions. These changes have two different causes. Formation and growth of cavities lead to an isotropic increase of the specimen dimensions and to a drastic decrease of the mass density. Simultaneously plastic flow occurs, which produces additional but anisotropic changes of the specimen dimensions. The dimensions perpendicular to the beam grow whereas the dimension parallel to the ion beam shrinks. Neither effect saturates in the investigated fluence range (Φt< 1014Xe/cm2) and both are absent in crystalline silicon. The effects are most likely provoked by electronic excitations and/or ionizations in the wake of the fast ions. With respect to plastic flow,a–Si:H behaves like metallic and oxide glasses. Formation of cavities and their growth, however, seem to occur only in tetrahedrally coordinated solids.

Contactless transient photoconductivity measurements in a- Si:H/c-Si heterojunctions are presented. The experimental data are decomposed in the contribution of excess electrons in a-Si:H, the contribution of excess carriers in c-Si and the contribution of excess carriers injected from a-Si:H into c-Si. A value for the electron drift mobility of in a-Si:H of 1cm2 V-1 s-1 is determined. At low excitation densities a high injection efficiency is observed, which is quenched by bias illumination.

The signal of the Transient Microwave Detected Photoconductivity after laser pulse illumination is followed during the PECVD growth of intrinsic amorphous silicon on top of crystalline silicon (c-Si) substrates. Depending on the thickness of the amorphous layer the observed phototransients can be assigned to the excess charge carriers in different regions of the so grown heterojunction. For thin a-Si:H layers the signal amplitude decreases exponentially with deposition time. In the case of thick amorphous silicon the signal reflects only the properties of the amorphous silicon and the signal amplitude is constant. In an intermediate range a large contribution of charge carriers absorbed in the a-Si:H and subsequently transported to the crystalline part of the heterostructure to the measured signal can be observed. Due to their much higher mobility in the crystalline layer these excess charge carriers become visible only after reaching the crystalline silicon. This leads to the observed delay of the TRMC signal amplitude relative to the exciting laser pulse. The experiments show, however, that this charge carrier transfer takes place only in the case of a low defect a-Si:H layer to a measurable extent and is suppressed by the low carrier mobility in highly defective a-Si:H deposited at lower substrate temperatures.

A study of excess carrier kinetics in a-Si:H / crystalline n-Si heterojunctions by transient photoconductivity measurements as a function of the excitation density is presented. It is observed that excess carriers can be injected from the a-Si:H in the c-Si part of the junctions, where the time associated to this injection process can be clearly resolved. The number of injected carriers increases with the number of excess carriers induced in the a-Si:H at low excitation density reaching a limit in the moderate excitation range. The driving force of the injection process is tentatively ascribed to the space charge field at the interface. The injection efficiency is estimated to be about 10%.

The deposition process of multiple layer structures of intrinsic and p-type hydrogenated amorphous silicon was followed by measuring the microwave detected transient photoconductivity (TRMC) during the film growth. In an i-p-i structure we can show that after deposition of an upper layer of about 500 nm, former deposited layers do not influence the TRMC-signal any more. In an i-p+-i-p structure we can clearly distinguish between p-layers of different doping concentration.

Several stationary and transient techniques are used to characterize a large number of intrinsic a-Si:H films prepared in a narrow range of production conditions. Correlation between stationary photoconductivity, the activation energy of the dark conductivity and the long-time range decay of the transient photoconductivity and photoinduced absorption is observed. The applicability of a-Si:H films for solar cells in view of these properties is discussed.