Kevin Füchsel

Black silicon for solar cell applications

We present experimental results and rigorous numerical simulations on the optical properties of Black Silicon surfaces and their implications for solar cell applications. The Black Silicon is fabricated by reactive ion etching of crystalline silicon with SF6 and O2. This produces a surface consisting of sharp randomly distributed needle like features with a characteristic lateral spacing of about a few hundreds of nanometers and a wide range of aspect ratios depending on the process parameters. Due to the very low reflectance over a broad spectral range and a pronounced light trapping effect at the silicon absorption edge such Black Silicon surface textures are beneficial for photon management in photovoltaic applications. We demonstrate that those light trapping properties prevail upon functionalization of the Black Silicon with dielectric coatings, necessary to construct a photovoltaic system. The experimental investigations are accompanied by rigorous numerical simulations based on three dimensional models of the Black Silicon structures. Those simulations allow insights into the light trapping mechanism and the influence of the substrate thickness onto the optical performance of the Black Silicon. Finally we use an analytical solar cell model to relate the optical properties of Black Silicon to the maximum photo current and solar cell efficiency in dependence of the solar cell thickness. The results are compared to standard light trapping schemes and implications especially for thin solar cells are discussed.

Low temperature deposition of indium tin oxide films by plasma ion-assisted evaporation

Coatings of transparent conductive oxides, especially indium tin oxide (ITO), are important in different fields. So far, application of these materials has been limited to substrates with high thermal stability. We describe an improved coating process for ITO based on plasma ion-assisted evaporation at a substrate temperature below 100 °C, which is suitable for organic substrates. In characterizing the thin films, we used the classical Drude theory to calculate the resistivity from optical film properties and compared the data with linear four-point measurements. X-ray diffraction spectroscopy was used to determine the structural properties of the thin films.

Heteroepitaxial Ge-on-Si by DC magnetron sputtering

The growth of Ge on Si(100) by DC Magnetron Sputtering at various temperatures is studied by Spectroscopic Ellipsometry and Transmission Electron Microscopy. Smooth heteroepitaxial Ge films are prepared at relatively low temperatures of 380°C. Typical Stransky-Krastanov growth is observed at 410°C. At lower temperatures (320°C), films are essentially amorphous with isolated nanocrystallites at the Si-Ge interface. A minor oxygen contamination at the interface, developing after ex-situ oxide removal, is not seen to hinder epitaxy. Compensation of dislocation-induced acceptors in Ge by sputtering from n-doped targets is proposed.

Black silicon photovoltaics

The challenge of future solar cell technologies is the combination of highly efficient cell concepts and low cost fabrication processes. A promising concept for high efficiencies is the usage of nanostructured silicon, so-called black silicon. Due to its unique surface geometry the optical path of the incoming light through the silicon substrate is enhanced to nearly perfect light trapping. Combined with the semiconductor-insulator-semiconductor (SIS) solar cell concept it is possible to fabricate a low cost device by using conventional sputtering technologies. Therefore, a thin insulator is coated on the nanostructured silicon surface, followed by the deposition of a transparent conductive oxide (TCO), e.g. indium tin oxide (ITO) or aluminum doped zinc oxide (AZO). In such systems the TCO induces a heterojunction, hence, high temperature diffusion processes are not necessary. The optical and geometrical properties of different nanostructured silicon surfaces will be presented. Furthermore, the influence of the used TCO materials will be discussed and the solar cell performance under AM1.5G illumination of unstructured and structured SIS devices is shown.

Induced terahertz emission as a probe for semiconductor devices

A nondestructive and contact free method for the characterization of semiconductor devices is presented using stimulated terahertz (THz) emission. For demonstration purposes, nanostructured semiconductor-insulator-semiconductor solar cells are investigated. These solar cells are based on indium tin oxide (ITO) upon black silicon (BS). During illumination with fs laser pulses, free charge carriers are generated at the junction between ITO and BS yielding the emission of broadband THz radiation. Since the THz field strength depends on the acceleration characteristics of the photoinduced charge carriers, phase sensitive detection of the emitted THz signal reflects the existing electric field distribution at the boundary zone. In contrast to existing methods where the sample is illuminated by an additional THz generator, here, the THz emission itself characterizes the sample. Moreover, only the region of THz generation is probed yielding a depth-resolved measurement setup that can be applied for the investiga...

Structural and electrical properties of low temperature deposited ITO films

Highly transparent thin films of indium tin oxide are important for different kinds of optical and electrical applications. So far, deposition of these materials has been limited to high temperature processes. This study describes a plasma ion-assisted evaporation process with substrate temperatures below 100°C and correlates the structural and electrical properties of the coatings with the process parameters. The influence of gas-mixture, mean ion energy and temperature has been investigated by four-point-measurement, atomic force microscopy, scanning electron microscopy and x-ray spectroscopy. The coatings exhibit mean extinction coefficients of 7•10-3 in the VIS range and specific resistivities in the range of 4.0 μΩm.

Transparent conductive oxides for nano-SIS solar cells

As a reason of their electrical conductivity and transparency in the visible spectral range transparent conductive oxides (TCOs) are well known as electrodes for OLEDs or LCD displays. Another promising application is a semiconductor-insulator-semiconductor (SIS) solar cell, in which the TCO induces the pn junction and realises a low cost solar cell on crystalline silicon. By using nanostructured silicon interfaces broadband antireflection properties with effective light coupling into the silicon can be achieved. Combined with the SIS concept it is possible to fabricate a low cost and high efficient PV device. For the deposition of thin films of indium tin oxide (ITO) and aluminum doped zinc oxide (AZO) pulsed dc magnetron sputtering is used. The paper presents the surface modification of silicon by inductive coupled plasma (ICP) etching technology, discusses the influence of different TCO materials to the device, and analyses the optical and structural properties of the cells. Furthermore, the solar cell performance under AM1.5G illumination will be shown.

ZnO:Al films prepared by inline DC magnetron sputtering

Abstract Aluminum-doped zinc oxide (AZO) is one of the most promising transparent conductive oxide (TCO) materials that can substitute the high-quality but costly indium tin oxide (ITO). To ensure high-quality films as well as moderate production costs, inline DC magnetron sputtering was chosen to deposit thin AZO films. The influence of sputter gas pressure, substrate temperature, and film thickness on the electrical, optical, and structural properties was analyzed. The resistivity reaches a minimum of 1.3×10-5 Ωm at around 1 Pa for a substrate temperature of 90°C. A maximum conductivity was obtained by increasing the substrate temperature to 160°C. An annealing step after deposition led to a further decrease in resistivity to a value of 5.3×10-6 Ωm in a 200 nm thin film. At the same time, the optical performance could be improved. Additionally, simulations of the transmittance and reflectance spectra were carried out to compare carrier concentration and mobility determined by optical techniques with those from Hall measurements.

Passivation of different black silicon surfaces by ALD deposited Al 2 O 3

Optical properties of black silicon (b-Si) can be tailored to minimize reflection losses to less than 0.6 % between 300-1000 nm and to improve the absorption at the silicon band-edge by light-trapping. Recently, metal assisted wet-chemically etched (MACE) b-Si was exploited to fabricate high efficiency (18.2 %) solar cells with surface passivation by thermal SiO2 and recombination velocities (SRV) of ~100 cm/s [1]. We compare surface passivation performance of ALD-Al2O3 on different dry and wet etched nanostructures. SRVs ≤ 8 cm/s on bifacially black 1 Ωcm p-type Si FZ wafers were measured. This technological advance will enable higher efficiencies for various PV-cell concepts.

Opto-electronic properties of different black silicon structures passivated by thermal ALD deposited Al 2 O 3

Black silicon (b-Si) structures offer improved light absorption but require appropiate surface passivation for photovoltaic applications. Here, we compare the opto-electronic performance of different wet and dry etched b-Si structures passivated by thermal ALD deposited Al2O3.