M. Vanecek

Absorption loss at nanorough silver back reflector of thin-film silicon solar cells

Absorption losses at a nanorough silver back reflector of a solar cell were measured with high accuracy by photothermal deflection spectroscopy. Roughness was characterized by atomic force microscopy. The observed increase of absorption, compared to the smooth silver, was explained by the surface plasmon absorption. Two series of silver back reflectors (one covered with thin ZnO layer) were investigated and their absorption related to surface morphology.

Fourier-transform photocurrent spectroscopy of microcrystalline silicon for solar cells

The spectral dependence of the optical absorption coefficient in thin films of hydrogenated microcrystalline silicon is measured over nine orders of magnitude in the subgap, defect-connected region, and in the above-the-band gap region. Transmittance, reflectance, and constant photocurrent method measurements are combined with Fourier-transform photocurrent spectroscopy (FTPS). Results are analyzed and interpreted as due to electron transitions from defects or interband electron transitions, all having direct relevance to the thin-film microcrystalline silicon solar cell performance. FTPS is a fast and sensitive quantitative method for quality assessment of microcrystalline silicon absorber in solar cells and can be used for quality monitoring in solar cell production.

Improved three-dimensional optical model for thin-film silicon solar cells

We present an optical model for thin-film silicon solar cells (both single and multijunction) with nanorough surfaces/interfaces. For these cells, the optical absorptance within each layer and the total reflectance are computed taking into account roughness, angular distribution of scattered light, thicknesses, and optical constants of all layers. In the model, we combine coherent approach, scattering theory, and Monte Carlo tracing method. Results of the model are shown to be in good agreement with the experimentally measured spectral response and the total reflectance of solar cells. Some predictions of the ultimate solar cell performance based on the model are presented as well.

Nanostructured three-dimensional thin film silicon solar cells with very high efficiency potential

We report on the experimental realization of amorphous/microcrystalline silicon tandem solar cells (Micromorph) based on our three-dimensional design. An enhancement is reached in the short-circuit current by 40%, with an excellent open-circuit voltage of 1.41V and a fill factor of 72%. We have used nanoholes or microholes dry etched into the ZnO front contact layer. Monte Carlo optical modeling shows that stable efficiency of amorphous silicon p-i-n solar cells in over 12% range is possible. For the Micromorph cells, efficiency over 15% with the thickness of amorphous Si below 200 nm and of microcrystalline Si around 500 nm is possible.

Intrinsic microcrystalline silicon (μc-Si:H) deposited by VHF-GD (very high frequency-glow discharge): a new material for photovoltaics and optoelectronics

The development of mc-Si:H technology and the introduction of intrinsic eie mc-Si:H as photovotaically active material is retraced. Special emphasis is laid on the use of very high frequency glow discharge as a particularly propitious deposition method for mc-Si:H. Thereby, the use of a gas purifier to reduce oxygen content and obtain intrinsic layers with ‘midgap’ character is described. Recent results obtained with single-junction mc-Si:H solar cells and a-Si:H:mc-Si:H tandem solar cells are given. The analysis of carrier collection in single-junction mc-Si:H solar cells is undertaken with the variable intensity measurements method. It yields effective mobilitylifetime (mt)eff products for the i-layer in p‐i‐n and n‐i‐p solar cells in the range 10 7 ‐10 6 cm 2 V 1 . Similar values have been found for mt-products in individual layers based on photoconductivity and ambipolar diffusion length measurements. Transmission electron microscopy images for mc-Si:H layers are given. They display a complex microstructure not suspected before. On the other hand, atomic force microscopy data reveal a pronounced surface roughness that correlates well with the optical light scattering and with the pronounced enhancement of the apparent optical absorption coefficient, in the 1 ‐2 eV region, as already observed before.

Optical and Electrical Properties of Undoped Microcrystalline Silicon Deposited by the VHF-GD with Different Dilutions of Silane in Hydrogen

Note: IMT-NE Number: 237 Reference PV-LAB-CONF-1997-001 Record created on 2009-02-10, modified on 2017-05-10

Inelastic mean-free path of electrons at nanocrystalline diamond surfaces

Surprisingly large difference between inelastic mean-free path (IMFP) values of signal electrons calculated from optical data and from the predictive TPP-2M formulae for diamond or graphite has motivated us to verify both sources of IMFP by measuring these values using different method. Thin, perfectly transparent nanocrystalline diamond films were characterized by x-ray induced photoelectron and Auger electron spectroscopy. Elastic peak electron spectroscopy measurements and the Monte Carlo calculations of electron transport were applied to determine the IMFPs for medium electron energy. The surface energy losses were also considered when evaluating these IMFPs.

Fourier transform photocurrent spectroscopy of dopants and defects in CVD diamond

Fourier-transform photocurrent spectroscopy (FTPS) was used as a very sensitive spectroscopic method to detect shallow and deep impurities (dopants) in CVD diamond layers. Detailed study of experimental conditions (temperature, frequency, electric field, bias light, surface conditions) was performed. Residual boron contamination was detected in many samples, phosphorus spectra were measured in P doped epitaxial layers. Anomalous (opposite) temperature dependence of the defect level with a threshold approximately 0.9 eV was detected and possible explanation of this effect was discussed.

Nanostructuring of diamond films using self-assembled nanoparticles

We report the use of gold, nickel and diamond nanoparticles as a masking material for realization of diamond nano-structures by applying the dry plasma etching process. Applying low power plasma (100 W) in a gas mixture of CF4/O2 for 5 minutes results in a formation of three different types of diamond nanostructures, depending on the mask type material and particle size. Using of the Ni mask results in realization of diamond nano-rods, applying of the Au mask brings cauliflower-like structures, and using the diamond powder allows the production of irregular nano-structures. The main advance of the presented etching procedure is use of a self-assembly strategy where no lithographic steps are implemented.

Surface and bulk light scattering in microcrystalline silicon for solar cells

We present here an overview of the light scattering influence on the absorption coefficient spectra α(E) measured by constant photocurrent method (CPM) and/or by photothermal deflection spectroscopy (PDS). We compare results of numerical simulation with the measured CPM and PDS spectra and verify our theory by polishing the as-grown nanotextured layers. We show that the experimentally observed apparent absorption coefficient can overestimate the true α(E) by a factor of 10.