View/Open - JUWEL - Forschungszentrum Jülich

Abstract
Textured and rough surfaces are known to increase light trapping in solar cells
significantly. The development and optimization of these nano-structures is
essential to improve the energy conversion efficiency of thin-film solar cells. In the
past, first research approaches covered classical and macroscopic investigations,
e.g. determining the haze or angularly resolved scattering. These methods do not
provide precise explanation for the optical improvement of the devices, because
layer thicknesses and structure sizes in thin-film solar cells are smaller than
the wavelength of visible light. The impact of local nano-structures and their
contribution to the local absorption enhancement is not resolved by macroscopic
measurements. In this thesis, near-field scanning optical microscopy is introduced
as first near-field investigations of nano-structures for photovoltaics. This provides
an insight into local optical effects for relevant surfaces of photovoltaic devices.
Investigating the distribution of the electric fields in layer stacks is crucial to
understand the absorption in solar cells. Evanescent fields, which occur due to
total internal reflection at the interfaces, are measurable by near-field scanning
optical microscopy and yield important information about local light trapping.
Within the framework of this thesis, correlations between local surface structures
and optical near-field effects are shown. In this case structure features of randomly
textured surfaces, which optimize local light trapping, are identified. It paves the
way to connect microscopic optical effects on the surface with the macroscopic
performance of thin-film solar cells. Moreover, the measurement yields a 3D
illustration of the electric field distribution over the sample surface. It is an
important criterion to prove the results of rigorous diffraction theory. An excellent
agreement between experiment and simulation is found. The simulations provide
an insight into the material, which is not accessible by the experiment. Therefore,
impacts of optical improvements such as absorption enhancement due to different
surface textures or properties of selective and diffractive filters can be studied.
ix