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 SF<sub>6</sub> and O<sub>2</sub>. 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.
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.
The metal-like electrical conductivity in combination with a high visual transmittance is the characteristic property that
opens up a broad spectrum of applications to transparent conductive oxides (TCOs). To fulfill the manifold requirements
in each individual case, especially the optical properties of TCOs have to be adapted.
The transmittance in the near infrared spectral range can be tailored by a modification of the carrier concentration Ν and
mobility μ. The theoretical description for this behavior is based on the well-known Drude theory. Highly conductive
indium tin oxide films (ITO) have been prepared by pulsed DC magnetron sputtering. However, due to its excellent
electrical properties, the plasma resonance of free carriers occurs near the visual spectral range which results in a very
low transmittance in the NIR. In contrast, ITO films with a NIR transmittance of ca. 80% have been prepared by plasma
ion assisted evaporation. The combination of high transmittance and low resistivity of ρ=7.4x10<sup>-6</sup>Ω was achieved
by a decrease of the carrier concentration and a simultaneous enhancement of the electron mobility μ.
Secondary, the transmittance of aluminum doped zinc oxide films (AZO) in the UV spectral range could be adapted by
changing the doping concentration Ν. This is a direct consequence of the Burstein-Moss shift that leads to a band gap
widening dependent on Ν. However, the comparison of the experimental data with theory has shown that the contrary
effect of band gap narrowing is not negligible, too.
Advances in the deposition of metallic thin films are discussed. The ALD growth of ultrathin Ir films is analyzed by
transmission electron microscopy, energy dispersive X-ray spectroscopy, atomic force microscopy, and optical and
electrical measurements. The morphology of iridium metallic layers is assessed based on Ir/ Al<sub>2</sub>O<sub>3</sub> nanolaminate films.
High resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy measurements show sharp
interfaces and pure Ir layers in the nanolaminates. The iridium films as polycrystalline. Excellent thickness control, high
uniformity and low roughness of ALD films are demonstrated. Four point probe measurements of the resistivity of Ir
coatings with various thicknesses have been performed and proved conductive layers with an Ir film thickness of ca. 10
nm. The optical properties of the Ir films deposited by ALD are similar to those of the bulk Ir. Thin iridum layers
deposited on high aspect ratio linear gratings have been successfully used as electrodes in the electrochemical deposition
of gold nanoparticles and gold layers. The gold deposition evolves through the formation of gold islands with ca. 40 nm
diameters that coalesce after ca. 60 seconds deposition. The density of the gold islands within the grating pattern is much
lower than on the flat region of the substrate. The combination of ALD with electrochemical deposition allows the
diversification of conductive layers on complex nanostructured surfaces.
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.
Due to their electrical conductivity and transparency in the visible spectral range, transparent conductive oxides
(TCOs) are suitable as transparent front electrodes for multiple cell concepts. One promising device is a
semiconductor-insulator-semiconductor (SIS) solar cell, in which the TCO induces the pn juntion and acts as a
transparent electrode at the same time. Due to its work function, the thin TCO layer leads to the inversion of the
subsurface region. The high refractive index of transparent conductive oxides enables antireflection coating in a
limited spectral range. One approach to achieve broadband antireflection properties with effective light coupling
into the absorber is a nanostructured silicon interface. For large area modifications of silicon, the inductive
coupled plasma (ICP) etching technology is a possible technique.
The combination of the nanostructured surface and the SIS system leads to a novel solar cell concept with
promising properties and a simple production process. In our study, we used pulsed dc magnetron sputtering
for the deposition of thin ITO films on p-doped unstructured and ICP-structured silicon substrates. Optical
and structural properties have been analyzed. Furthermore, the solar cell performance of the first devices under
AM1.5G illumination will be shown and discussed.
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<sup>-3</sup> in the VIS range and specific resistivities in the range of 4.0 <i>μΩ</i><i>m</i>.