The transient photocurrent response measured on a-Si:H layers and solar cells after step-like switch-on of illumination reveals a variety of structures. By changing experimental parameters, e.g. sample temperature, irradiance, preceding dark time interval, under some conditions only a current deficiency (retarded approach to steady state photoconductivity) is observed. This effect is associated with a relatively slow filling of former empty traps. Other conditions lead to overshoot phenomena in the transients. Model calculations show that the dangling bond (DB) states have to be charged significantly before recombination by charged DBs becomes dominant. If effective recombination occurs relatively late compared to the trap filling process, an overshoot over the steady state value is expected. Generation of additional recombination centers by light soaking should therefore influence the overshoot strength and its transient appearance. This is confirmed by transient measurements on degraded samples.

Simple unoptimized planar junction amorphous silicon-based solar cells with TiO(2) based diffuse rear reflectors can obtain i-layer absorption similar to that of solar cells prepared on high quality commercially available textured TCO substrates. Also, the naturally occuring surface roughness in amorphous and microcrystalline materials can be exploited to fabricate very effective diffuse rear reflectors. Accurate determination of surface roughness and film optical properties are required to fully probe the potential of diffuse reflection from naturally occuring surface roughness. However, the coupling of bulk optical parameters with surface roughness artifacts poses difficulties for accurate determination of film optical properties. A new technique was developed and implemented to characterize surface scattering.

Optical optimization of planar junction amorphous and microcrystalline silicon-based thin-film solar cells offers improved conversion efficiency. Planar junction solar cells with diffuse and selective angle rear reflectors were examined in detail. The refractive index of the diffuser material needs to be as close to that of silicon as possible. Insulating diffuse rear reflectors must be used in conjunction with conductive rear-window layers of the appropriate band gap, conductivity and refractive index. The refractive index of the aear window layer should be at least as large as that of the diffuser material. Rear windows comprised of ZnS, CdS and ZnSe (as well as other II–VI semiconductors and semiconductor alloys) used with a TiO2 diffuse reflector could increase the current of optically thin silicon solar cells beyond that possible using rough TCO/glass substrates. Selective angle reflectors must also be comprised of either the solar cell semiconductor itself (not possible in many cases) or comprised of a material having properties similar to those of the conducting rear windows used with the diffuse reflectors.

Improvements in amorphous silicon single junction solar cell stabilized performance can be realized by developing more stable materials as well as more efficient light trapping. The optical losses of thin solar cells are investigated both numerically and experimentally. New approaches involving planar junctions and diffuse rear reflectors are developed. It is theoretically possible to achieve short-circuit currents over 17 mA/cm/sup 2/ with a 150 nm i-layer using a diffuse rear reflector when an appropriate rear window material is employed.

A variation of the temperature T of a p-i-n solar cell shifts the Fermi level and thereby changes the occupation of the defect densities within the device according to the defect model used. Thus the T dependence of the quantum efficiency (QE) promises to give a clearer insight into the defect distribution and recombination within the device. With decreasing T both the red and the blue response of an a-Si:H p-i-n solar cell decrease, the loss in the red being due to the i layer gap shift. The drop on the short wavelength side with decreasing T is attributed to a change of charge state of the D+ in the nonequilibrium case within the first 100 nm of the i-layer. This negative space charge diminishes the field spike near the p/i interface and reduces the hole concentration there by recombination

An optical model calculating the generated carrier profile in a-Si:H solar cells deposited on hazy and nonhazy transparent conducting oxide (TCO) has been integrated into a numerical transport and recombination program. A comparison of simulated and measured reflectances of pin structures opens up a possibility to check the prepared layer thicknesses and the optical device properties. Furthermore, the examination of the current gain employing a ZnO/metal back contact reveals the importance of using accurate light absorption profiles for spectral response modelling and device characterisation of pin solar cells.

Highly conductive n-type microcrystalline (/spl mu/c) silicon layers were deposited by conventional 13.56 MHz PECVD in a commercial multichamber deposition system for amorphous silicon solar cells. A conductivity up to 80 (/spl Omega/ cm)/sup -1/ was obtained. In addition to optical and Raman characterization, the film structure was analyzed by TEM and electron diffraction, revealing a tight assembly of near-spherical clusters of /spl ap/100 nm size, which are built up from smaller crystallites. Information on the process recipe to be used in stacked cells was obtained from the use of /spl mu/c n-layers in pin cells, and by depositing thin /spl mu/c films on thick amorphous layers, which permits to determine the conductivity for device-relevant conditions. A strongly reduced n-p contact resistance was found in comparison to amorphous n-p contact systems. The prepared pinpin stacked cells employing the /spl mu/c films exhibit improved fill factors of up to 75% and an improved short-circuit current as compared to the ones with amorphous n-layers in the inner n-p contact.< >

Electroluminescence spectra were measured on a-Si:H pin-devices at various temperatures and forward-bias current densities. By comparison with optical calculations, information about the depth at which radiative recombination occurs was obtained. It is found that the radiative recombination takes place in a narrow region at the p/i-interface. The results were confirmed by photoluminescence measurements and are in line with other calculations.

Porous Si layers formed on heavily p-type doped wafers are investigated by Raman spectroscopy, infrared spectroscopy and ellipsometry. The influence of different porosities and the changes caused by the partial oxidation of the porous layers are studied. Shifts in the phonon frequencies are related to the formation of microcrystals. The infrared spectra are fitted by using the sophisticated Bergman formalism to include topology effects. By infrared spectroscopy two phonon processes are only observed in non-preoxidized samples. Spectroscopical ellipsometry yields a modification of the dielectric function. The E1 and E2 gaps are shifted towards lower energies and the imaginary part shows a broadening of the structures.

Silver powder samples were prepared by evaporation of silver in an inert gas atmosphere. They were investigated by dc-conductivity-, reflectance- and photoacoustic (PAS) measurements. Model spectra based on the Bergman ansatz for the effective dielectric function εeff, the effective thermal and electrical conductivities λeff, and σeff, were fitted to the experiments. Good agreement has been found assuming a spongily packed, only weakly percolating topology and an increased damping of the free carriers in the small particles (classical size effect).

GaAs layers with thicknesses from 1 to 8 μm were grown by molecular-beam epitaxy onto Si(100) substrates. These epitaxial layers were lightly doped with Si (ND≂2×1016 cm−3). The determination of accurate numbers for the carrier concentrations and mobilities in the GaAs is complicated by the low doping and the dimensions of the films. However, a new approach in IR spectroscopy that combines a conventional reflectance measurement from 50 to 500 cm−1 with a transmittance measurement in the very far-infrared range from 12 to 62 cm−1 is demonstrated to provide precise information on both carrier concentrations and mobilities. A comparison of the results obtained at room temperature from IR and Hall measurements reveals that the nondestructive IR technique is an easy to perform and excellent characterization tool for the Ga As layers.

Thin low doped GaAs layers were deposited on Si(100) substrates using molecular beam epitaxy (MBE). The samples were investigated by nondestructive Far Infrared (FIR) Fourier Transform Spectroscopy. A new evaluation method was employed in order to obtain the phonon parameters the layer thicknesses as well as the densities and mobilities of the carriers. This method was successfully applied for layer thicknesses as low as O. lprn and carrier concentrations of about 2 . 1O''6cm3.

We report on the lattice-matched semiconductor combination InSb/CdTe. Raman scattering by vibrational modes was used to identify chemical phases (In2Te3, Sb) at the interface formed in the molecular beam epitaxial growth of InSb/CdTe heterostructures. Far-IR spectroscopy, in particular, revealed the presence of a free electron gas at the interface and allowed a quantitative description of its properties. As a consequence coupled longitudinal optical phonon-plasmon modes were also observed by Raman scattering. It was thus possible to probe the chemical nature as well as the electronic properties at the semiconductor interface as a function of growth conditions by a combination of both Raman and far-IR spectroscopic techniques.

Boron doped diamond films were deposited onto (100) orientated Si substrates using a thermal filament CVD method. Boron trioxide was used as a doping source and samples with boron to carbon (B/C) ratios of 0, 10, 100, and 1000 ppm were prepared for the optical measurements. The infrared (IR) results reveal the formation of an ultrathin SiC layer at the interface between Si and diamond. Furthermore, the IR data confirm the resistivity data obtained from electrical measurements. Raman spectroscopy was used to probe the quality and homogeneity of the diamond films. Upon increasing B/C ratio the diamond phonon line shifts to lower frequency and is also broadened revealing a softening of the diamond. In addition, the optical absorption was found to increase strongly with increasing B/c ratio.

Diffuse reflectance (DRIFT) spectra in the mid-IR region, especially of inorganic powders, often reveal inverted or derivative-like structures ("reststrahlen" bands) which make their straightforward interpretation in terms of the Kubelka-Munk theory difficult. A simulation technique based on a combination of single scattering (Mie theory) and radiative transfer (with Kubelka-Munk as a special case) is reported that allows measured DRIFT spectra to be described without use of any free "fit" parameters. The results for silica glass microspheres and silicon carbide powder show that not specular reflectance but prominent structures in the single particle scattering cross-section are responsible for the reststrahlen features which cannot be suppressed even by diluting the powder in an excess of KBr. The efficiency of a "blocker" device in DRIFTS is also discussed.

Boron-doped diamond films were deposited onto (100) orientated Si substrates using a thermal filament CVD method. Boron trioxide was used as a doping source and samples with boron to carbon ( B C ) ratios of 0, 10, 100 and 1000 ppm were prepared for the Raman and infrared (ir) spectroscopy studies. The ir results s show that thin layers of SiC are formed at the interface. Furthermore, the ir data confirm the resistivity data obtained from electrical measurements. Raman spectroscopy was used to probe the quality and homogeneity of the diamond films. In addition, the optical absorption was found to increase strongly with increasing B C ratio.

CdTe layers were prepared by molecular beam epitaxy on (100) orientated substrates under various growth conditions involving different substrate preparation methods, substrate temperatures, and Cd/Te flux ratios. Using Raman spectroscopy the crystal structure of the substrate and the overlayer as well as the interfacial chemistry are investigated. Our Raman spectra reveal that interfacial layers consisting of In2Te3 and liberated Sb are formed when CdTe is deposited from a single source. While the thickness of the interfacial layers depends on growth temperature, the formation was independent of the substrate preparation. The application of a Cd overpressure during the growth of the CdTe layers effectively suppresses the chemical reaction at the interface and leads to vastly improved interfaces. Using the combination of Raman and infrared spectroscopies thin layers containing a high density of free carriers are found to be present in the top layer of the InSb substrate.

GaAs layers were deposited onto Si(100) substrates using molecular beam epitaxy (MBE). The substrates were prepared by various methods before the growth of 0.1μm thick GaAs buffer layers. Active GaAs layers were thereafter grown with thicknesses from 1 to 4μm. Infrared spectra were recorded in the spectral ranges from 12 to 65cm-4 and 50 to 500cm-1 of the thin buffer layers as well as the thick active layers. The infrared active phonon of GaAs already shows up in the spectra for the 0.1μm buffer layers. The phonon parameters were derived from a harmonic oscillator fit to the experimental data. The phonon damping constant is correlated with the bulk quality of the GaAs layers. These results are compared with those obtained from Raman and spectroscopic ellipsometry measurements.