Solar Cells

Photoluminescence around 670 nm (red) of the GaInP top cell excited by a 405 nm (blue) external light source. Several dark spots are visible over the solar cell area caused by local defects.
Luminescence Imaging of Triple-junction Solar Cells

Today's high efficiency solar cells are based on multijunction concepts from different III-V semiconductor materials.

GaInP/Ga(In)As/Ge triple-junction cells are already commercially used in concentrator systems for terrestrial energy generation and as relatively large area (8x4 cm2) devices for the power supply of satellites in space application.

These devices are grown by metal organic vapour phase epitaxy (MOVPE) that can result in spatially inhomogeneous solar cell parameters.

Together with the solar array center of Astrium GmbH in Ottobrunn we are working with a camera based luminescence setup to gain locally.

Today's high efficiency solar cells are based on multijunction concepts from different III-V semiconductor materials.

GaInP/Ga(In)As/Ge triple-junction cells are already commercially used in concentrator systems for terrestrial energy generation and as relatively large area (8x4 cm2) devices for the power supply of satellites in space application.

Different approaches to the fabrication of multilayered organic solar cells with flat, diffused and nanostructured interfaces. The anodic membrane mold used for the structuring is shown in (d) and the actual patterned P3HT interface in (e).
Organic and Hybrid Solar Cells

The field of photovoltaics has received great impulse recently due to the urgent need to find alternative renewable sources to oil and coal for energy production.

While silicon solar cells are dominating the market, their price and relatively low efficiency is limiting their economical impact.

Triple junction solar cells have a much higher efficiency but the costs are too high for terrestrial applications.

One possible solution is provided by organic solar cells which rely on low-cost, large-area fabrication techniques.

Mixing organic materials with nanostructures (e.g. semiconducting quantum dots or graphene) could help improving the efficiency of such solar cells, which is still very low.

In addition, structuring the various layers of such cells via nanoimprinting can be used to optimize light management and in turn further enhance the performance.


Wiedemann W. et al., Nanostructured interfaces in polymer solar cells. Applied Physics Letters 96, 263109 (2010). doi: 

Nesswetter, H. et al. Electroluminescence and Photoluminescence Characterization of Multijunction Solar Cells. IEEE Journal of Photovoltaics (Volume:3 , Issue: 1 ). doi:


Nesswetter, H.

Precision Positioning Systems for Nanoimprint

Attocube FPS3010 interferometric displacement sensor with three independent axes and different all-optical sensor heads. The position signal, generated in the sensor head placed opposite to a high reflective target, is remotely collected by means of an optical fiber, thus allowing noninvasive operation even in the harshest environments because all electronic parts are separated from the sensor head.

The latest progress in nanoimprint and nanotransfer technology seems to be promising for replacing conventional lithographic processes one day.

Nevertheless, to make it applicable for industry applications, novel alignment methodologies have to be developed which impose particularly high demands on the metrological system which enables multi-step printing techniques by providing accurate position information.

Such a system must be small, non-invasive, and insensitive to environmental conditions like vacuum and low temperatures.

To meet all requirements, the attocube research team, in which we contribute, developed a cost efficient, ultra-compact and all fiber-optic based interferometric displacement sensor capable of measuring displacements with sub-nanometer sensitivity and nanometer accuracy.

The technology is based on an attocube systems technique invented earlier.

At the heart of the system is a tunable distributed feedback laser (DFB) operating at telecom wavelength which supplies the interferometric axes.

The interference takes place at the polished flat end face of a single mode fiber and this between the laser beams reflected off the fiber and off the displacing target.

For position detection, a quadrature signal is generated from this interference signal using a wavelength modulation technique.

The system uses a sensor head with improved alignment tolerance which makes the interferometer easy to use.

For further improvement and miniaturization of the attocube sensor head technology, our group works on developing and implementing methods to predict and test the optical behavior of new sensor head designs with respect to interferometric signal quality and tolerances.

This considerably eases the development and manufacturing process and reduces the testing time.


Thurner,K.; Karrai,K.

1D Photonic Band Gap Structures for Thermo-Optic Imagers

The proposed thermo-optic imaging sensor featuring: a) an OLED on the ITOcoated side of the Glass substrate, b) integration of the OLED-1DPC system with a CCD camera and c) visualization of the heat distribution on the surface of the OLED-1DPC ensemble. The BSs were made of i) TiO2 and SiO2 nanoparticle-based layers and ii) dense TiO2 and mesoporous SiO2 layers.

Infrared (IR) sensors are of interest due to their ability to visualize the invisible parts of the spectrum.

Possible applications can be found in Automotive sensors, medical, security and surveillance applications. Our approach to detect IR radiation employs the change of optical transmission of a 1D photonic crystal (1DPC) as a result of the temperature change upon IR absorption.

The 1DPC is back lighted by an organic light emitting diode (OLED) so that transmission changes of the 1DPC can be observed as intensity changes of the OLED-1DPC device.

The 1DPC is made up of a multilayer structure which alternates materials with different thermooptic coefficients.

Such Bragg stack can be fabricated by sputtering techniques. Alternatively, mesoporous TiO2 and SiO2 layers can be sequentially spincoated from a colloidal nanoparticle solution.

The OLED is fabricated on the backside of same substrate as the 1DPC by spin-coating a polymer solution on the indium tinn oxyde (ITO) electrode situated on the glass substrate.

The OLED anode is fabricated by thermal evaporation of calcium and silver on top of the polymer.

Due to the thermal tunability of the transmission spectrum of the optical filter, the intensity of light passing through the filter is modulated by temperature.

The proposed principle features a simple and cheap fabrication which can trigger the emergence of a new class of thermal detectors.

In addition, due to the porous nature of the spin coated films, the 1DPC properties change in the presence of gases, thus opening the path to sensing applications.


Pavlichenko, I. et al. Humidity-Enhanced Thermally Tunable TiO2/SiO2 Bragg Stacks. J. Phys. Chem. C, 116, 298–305 (2012). doi:

Regoliosi, P. et al. Thermal tunability of monolithic polymer microcavities. Applied Physics Letters 92, 253310-2533103 (2008). doi: 


Exner, A.