A. Poruba

Optical absorption and light scattering in microcrystalline silicon thin films and solar cells

Optical characterization methods were applied to a series of microcrystalline silicon thin films and solar cells deposited by the very high frequency glow discharge technique. Bulk and surface light scattering effects were analyzed. A detailed theory for evaluation of the optical absorption coefficient α from transmittance, reflectance and absorptance (with the help of constant photocurrent method) measurements in a broad spectral region is presented for the case of surface and bulk light scattering. The spectral dependence of α is interpreted in terms of defect density, disorder, crystalline/amorphous fraction and material morphology. The enhanced light absorption in microcrystalline silicon films and solar cells is mainly due to a longer optical path as the result of an efficient diffuse light scattering at the textured film surface. This light scattering effect is a key characteristic of efficient thin-film-silicon solar cells.

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.

Observation of the subgap optical absorption in polymer-fullerene blend solar cells

This letter reports on highly sensitive optical absorption measurements on organic donor-acceptor solar cells, using Fourier-transform photocurrent spectroscopy (FTPS). The spectra cover an unprecedented dynamic range of eight to nine orders of magnitude making it possible to detect defect and disorder related sub-band gap transitions. Direct measurements on fully encapsulated solar cells with an active layer of poly[2-methoxy-5-(3′,7′-dimethyl-octyloxy)]-p-phenylene-vinylene:(6,6)-phenyl-C61-butyric-acid (1:4 weight ratio) enabled a study of the intrinsic defect generation due to UV illumination. Solar cell temperature annealing effects in poly(3-hexylthiophene):PCBM (1:2 weight ratio) cells and the induced morphological changes are related to the changes in the absorption spectrum, as determined with FTPS.

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.

Direct measurement of the deep defect density in thin amorphous silicon films with the ‘‘absolute’’ constant photocurrent method

Direct measurement of the deep defect density in thin amorphous silicon films with the help of the ‘‘absolute’’ constant photocurrent method is demonstrated here. We describe in detail how the optical (photocurrent) absorption spectrum can be measured directly in absolute units (cm−1) without additional calibration and undisturbed by interference fringes. Computer simulation was performed to demonstrate absolute precision of the measurement and to explain residual interferences which are sometimes observed. The residual interferences are shown to be direct fingerprints of an inhomogeneous defect distribution.

Optical properties of microcrystalline materials

Note: IMT-NE Number: 269 Reference PV-LAB-ARTICLE-1998-012doi:10.1016/S0022-3093(98)00202-6 Record created on 2009-02-10, modified on 2017-05-10

Electron spin resonance and optical characterization of defects in microcrystalline silicon

Electron spin resonance (ESR), constant photocurrent method (CPM), photothermal deflection spectroscopy (PDS), Raman and IR spectroscopy have been used to measure microcrystalline silicon films. Besides standard defects with a g-value of 2.0055, new defects with a g-value ∼2.0030 have been created during annealing this material. Proportionality between the subgap optical absorption and ESR spin density has been observed.

Fourier transform infrared photocurrent spectroscopy in microcrystalline silicon

A fast and sensitive method for measurement of spectral dependence of the optical absorption coefficient α(E) in thin films of photosensitive materials is introduced. A Fourier transform infrared (FTIR) spectrometer is used with a photoconductive sample as an external detector. Experimental conditions and procedures to obtain α(E) from normalized FTIR signal are described and results are compared to the standard measurements of transmittance and reflectance, and the constant photocurrent method. The FTIR method is applied to thin microcrystalline silicon layers and the resulting optical data are discussed in terms of optical absorption connected with defects and disorder. The measurement of α(E) is extended down to very low photon energy and reveals the threshold energy for the dangling bond optical absorption in microcrystalline silicon. Photoionization cross section of the silicon dangling bonds is measured over several orders of magnitude and full dynamical range of α(E) exceeds nine orders of magnitude.