Tobias Sontheimer

Microstructure and photovoltaic performance of polycrystalline silicon thin films on temperature-stable ZnO:Al layers

Polycrystalline silicon (poly-Si) thin films have been prepared by electron-beam evaporation and thermal annealing for the development of thin-film solar cells on glass coated with ZnO:Al as a transparent, conductive layer. The poly-Si microstructure and photovoltaic performance were investigated as functions of the deposition temperature by Raman spectroscopy, scanning and transmission electron microscopies including defect analysis, x-ray diffraction, external quantum efficiency, and open circuit measurements. It is found that two temperature regimes can be distinguished: Poly-Si films fabricated by deposition at low temperatures (Tdep 400 °C) directly in crystalline phase reveal columnar, up to 300 nm big crystals with a strong ⟨110⟩ orientation and much better solar cell parameters. It ...

Crystallization kinetics in electron-beam evaporated amorphous silicon on ZnO:Al-coated glass for thin film solar cells

To systematically study the crystallization process of electron-beam evaporated amorphous silicon on ZnO:Al-coated glass for polycrystalline silicon thin film solar cells, transmission electron microscopy and optical microscopy were employed. A time and temperature dependent analysis allowed the individual investigation of the growth and nucleation processes. The growth velocities of Si-crystals on ZnO:Al and SiN-coated glass were found to be identical within the investigated temperature regime of 500–600 °C. However, with a high steady state nucleation rate and a low activation energy, the nucleation process of Si on ZnO:Al-coated glass has shown to differ significantly from nucleation on glass.

Influence of deep defects on device performance of thin-film polycrystalline silicon solar cells

Employing quantitative electron-paramagnetic resonance analysis and numerical simulations, we investigate the performance of thin-film polycrystalline silicon solar cells as a function of defect density. We find that the open-circuit voltage is correlated to the density of defects, which we assign to coordination defects at grain boundaries and in dislocation cores. Numerical device simulations confirm the observed correlation and indicate that the device performance is limited by deep defects in the absorber bulk. Analyzing the defect density as a function of grain size indicates a high concentration of intra-grain defects. For large grains (>2 μm), we find that intra-grain defects dominate over grain boundary defects and limit the solar cell performance.

Characterization and control of crystal nucleation in amorphous electron beam evaporated silicon for thin film solar cells

The kinetics of crystal nucleation in high-rate electron beam evaporated amorphous Si for polycrystalline thin film solar cells was systematically studied on SiN and selected ZnO:Al-coated glass substrates with dissimilar surface topographies by employing Raman spectroscopy, transmission electron microscopy, and optical microscopy. The influence of the surface topography of the substrate and the disorder of the deposited amorphous Si could be correlated to the respective characteristics of the transient and steady state regime of the nucleation rate. The steady state nucleation rate Iss, its corresponding activation energy EIss, and consequently the size of the grains in the crystallized Si were found to be governed by the interplay between the surface roughness and the deposition temperature. The steady state nucleation rate Iss increased gradually upon increasing the substrate roughness, while lowering the deposition temperature of the amorphous Si on rough textures resulted in a decline of Iss. The tim...

Nanophotonic light trapping in 3-dimensional thin-film silicon architectures

Emerging low cost and large area periodic texturing methods promote the fabrication of complex absorber structures for thin film silicon solar cells. We present a comprehensive numerical analysis of a 2 μm square periodic polycrystalline silicon absorber architecture designed in our laboratories. Simulations are performed on the basis of a precise finite element reconstruction of the experimentally realized silicon structure. In contrast to many other publications, superstrate light trapping effects are included in our model. Excellent agreement to measured absorptance spectra is obtained. For the inclusion of the absorber into a standard single junction cell layout, we show that light trapping close to the Yablonovitch limit can be realized, but is usually strongly damped by parasitic absorption.

Large-area 2D periodic crystalline silicon nanodome arrays on nanoimprinted glass exhibiting photonic band structure effects

Two-dimensional silicon nanodome arrays are prepared on large areas up to 50 cm² exhibiting photonic band structure effects in the near-infrared and visible wavelength region by downscaling a recently developed fabrication method based on nanoimprint-patterned glass, high-rate electron-beam evaporation of silicon, self-organized solid phase crystallization and wet-chemical etching. The silicon nanodomes, arranged in square lattice geometry with 300 nm lattice constant, are optically characterized by angular resolved reflection measurements, allowing the partial determination of the photonic band structure. This experimentally determined band structure agrees well with the outcome of three-dimensional optical finite-element simulations. A 16% photonic bandgap is predicted for an optimized geometry of the silicon nanodome arrays. By variation of the duration of the selective etching step, the geometry as well as the optical properties of the periodic silicon nanodome arrays can be controlled systematically.

Challenges and opportunities of electron beam evaporation in the preparation of poly-Si thin film solar cells

Electron-beam (e-beam) evaporation provides both exciting opportunities and challenges for the preparation of poly-crystalline silicon (poly-Si) thin film solar cells. A conversion efficiency of 6.7% was recently achieved for solid phase crystallized poly-Si mini-modules on planar SiN-coated glass deposited at a deposition rate of 600 nm/min, demonstrating the excellent electronic quality of e-beam evaporated silicon. Even at significantly increased background pressures of 5×10−6 mbar, the photovoltaic performance of the mini-modules was considerably high, showing a decline in open circuit voltage of 17 mV per cell. The implementation of light trapping structures into the device led to an efficiency increase of 1.1%, yielding module efficiencies of 7.8%. By systematically studying the implementation of ZnO:Al as a front contact layer into the poly-Si solar cell device structure, we unraveled novel features that prove the supreme suitability of ZnO:Al for poly-Si thin film solar cells. Not only can etched ZnO:Al be utilized as a front side texture, but its electrical properties can also improve during the crystallization process of the Si layer, showing a record charge carrier mobility of 67 cm2/Vs after thermal annealing. In addition, ZnO:Al drastically modifies the crystallization kinetics of the Si on ZnO:Al, enabling us to control the crystallization process by adjusting the deposition temperature. The nucleation process of Si on ZnO:Al was found to be influenced by a variation of the deposition temperature of the amorphous Si in a critical temperature regime of 200 °C to 300 °C. The nucleation rate decreased significantly with decreasing deposition temperature, while the activation energy for nucleation increased from 2.9 eV at a deposition temperature of 300 °C to 5.1 eV at 200 °C, resulting in poly-Si which comprised grains with features sizes of several µm.

Solution-processed amorphous silicon surface passivation layers

Amorphous silicon thin films, fabricated by thermal conversion of neopentasilane, were used to passivate crystalline silicon surfaces. The conversion is investigated using X-ray and constant-final-state-yield photoelectron spectroscopy, and minority charge carrier lifetime spectroscopy. Liquid processed amorphous silicon exhibits high Urbach energies from 90 to 120 meV and 200 meV lower optical band gaps than material prepared by plasma enhanced chemical vapor deposition. Applying a hydrogen plasma treatment, a minority charge carrier lifetime of 1.37 ms at an injection level of 1015/cm3 enabling an implied open circuit voltage of 724 mV was achieved, demonstrating excellent silicon surface passivation.

A novel light trapping concept for liquid phase crystallized poly-Si thin-film solar cells on periodically nanoimprinted glass substrates

Large grained polycrystalline silicon (poly-Si) absorbers were realized by electron beam induced liquid phase crystallization on 2 μm periodically patterned glass substrates and processed into a-Si:H/poly-Si heterojunction thin-film solar cells. The substrates were structured by nanoimprint lithography using a UV curable hybrid polymer sol-gel resist, resulting in a glassy high-temperature stable micro-structured surface. Structural analysis yielded high quality poly-Si material with grain sizes up to several hundred micrometers. An increase of absorption and an enhancement of the external quantum efficiency in the NIR as a consequence of light trapping due to the micro-structured poly-Si/substrate interface were observed. Up to now, only moderate solar cell parameters, a maximum open-circuit voltage of 413 mV and a short-circuit current density of 8 mA cm-2, were measured being significantly lower to what can be achieved with liquid phase crystallized poly-Si thin-film solar cells on planar glass substrates indicating that the substrate texture has impact on the electrical material quality. By reduction of the SiC interlayer thickness at the micro-structured poly- Si/substrate interface defect-related parasitic absorption was considerably minimized. This encourages the implementation of nanoimprinted tailored substrate textures for light trapping in liquid phase crystallized poly-Si thinfilm solar cells.

Micro-contacting of single and periodically arrayed columnar silicon structures by focused ion beam techniques

Micron-sized, periodic crystalline Silicon columns on glass substrate were electrically contacted with a transparent conductive oxide front contact and a focused ion beam processed local back contact. Individual column contacts as well as arrays of >100 contacted columns were processed. Current-voltage characteristics of the devices were determined. By comparison with characteristics obtained from adapted device simulation, the absorber defect density was reconstructed. The contacting scheme allows the fabrication of testing devices in order to evaluate the electronic potential of promising semiconductor microstructures.