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Institute for Heterogeneous Material Systems

The research activities of the department Heterogeneous Materials Systems are centred around three main topics:

• Chalcopyrite Based Thin Film Solar Cells
• Analytics and Engineering of Interfaces
• New Materials and Solar Cells Structures

The research subjects range from basic studies of the materials properties and interfaces to device-oriented technological problems. This provides a mix of medium and long term goals in the development of photovoltaics, as well as fundamental research in materials science and solid state physics.

Themes

• Compoundsemiconductor
• Transparent Conductive Oxide
• PVD
• Grain boundaries
• CBD
• ILGAR
• CuGaSe2 by CCSVT
• Synchrotron Spectroscopy at BESSY
• Optoelectronic Nanocharacterization
• Eta
• Organic Solar Cell
  
• ZnO Nanodevices

Staff

• Head of Department, Deputy Head of Department, Office, Staff and Guests

Publications

• Lists on 1998, Publication Search

Chalcopyrites

Chalcopyrite-based solar modules are unique in combining the advantages of thin-film technology with the efficiency and stability of conventional crystalline silicon cells. It is therefore believed that chalcopyrite based modules can take up a large part of the PV market growth once true mass production is started. Scientifically, recent results, to which the department has significantly contributed, suggest that chalcopyrites have truly unique properties not found in classical semiconductors. Some of the main research topics in the department are:

• Wide gap absorbers. Particular emphasis is given to the wide band gap absorbers CuIn(Ga)S₂, and CuGaSe₂. These offer higher open circuit voltages, better temperature coefficients and are also developed for the long term goal of a tandem chalcopyrite solar cell. The department holds the current efficiency record for the best sulfide thin film solar cell at 12.3%.
• Window layers and monolithic integration. Special focus is given to the properties of ZnO TCO layers, including stability and scaling issues. Part of this work is the deposition of CdS and ZnO layers in the HZB CuInS₂ and Cu(In,Ga)Se₂ baseline processes which are run together with the department Technology.
• Contact layers/buffer layers. Several Cd-free buffer layer technologies are developed for the full range of chalcopyrite absorbers. These buffer layers help to passivate the absorber surface and provide a selective contact for the flow of electrons. Deposition techniques include CBD, ILGAR, Sputtering and PVD.
• Grain boundaries and point defects. Understanding the role of grain boundaries and point defects in the Cu(In,Ga)(S,Se)₂ system is crucial for the future of the technology. Films with a single grain boundary are grown epitaxially on bicrystal substrates using MBE and MOCVD. Point defects in various materials are studied using photoluminescence.
• Development of new deposition methods suitable for the cost effective production of thin film solar cell layers. These include the Ion Layer Gas Reaction (ILGAR) and chemical closed space vapour transport (CCSVT).

Further activities on chalcopyrite solar cells can be found in the dept. Technology.

Analytics and engineering of interfaces

Success in the field of photovoltaics strongly relies on the ability to understand, prepare and engineer semiconductor interfaces. A wide variety of characterisation tools are used for this purpose and some of the special projects within the department are:

• X-ray spectroscopy methods. The CISSY project has developed a specially designed end-station at the BESSY synchrotron in Berlin to perform XPS/PES and XES analysis. The end station allows chemical treatments and sputter deposition to be performed in-situ and is used for the analysis of model systems and industrially relevant materials.
• KPFM. Kelvin Probe Force Microscopy is an AFM related technique which maps the contact potential of a device on the nanoscale. Work includes both the fundamental development and understanding of the technique as well as its application to characterise solar cell structures. The related techniques of AFM, STM and STS are also performed.
• Transient Surface Photovoltage Spectroscopy (SPS) measures the carrier dynamics which contribute to the photovoltage over many orders of magnitude.

New materials and cells structures

Further areas of research are aimed at materials and devices which could offer extraordinary power-to-cost ratios in the future.

• Organic ultra-thin film solar cells. Focus is given to the use of small organic molecules such as Phthalocyanines and C₆₀ deposited by thermal evaporation techniques under UHV as well as Organic Vapour Phase Deposition (OVPD). New device structures are developed and materials properties are investigated. Organic-inorganic interfaces and molecular crystal structures are engineered.
• Extremely Thin Absorber solar cells. This type of cell was first proposed in the HZB and consists of a highly structured TCO coated with an extremely thin absorber layer. It has the potential to reduce the amount of active material needed in the solar cell and is often cited as an application of nanotechnology in the field of photovoltaics.
• ZnO Nanodevices. Investigations on the use of ZnO nanowires for highly structure solar cells. It is also a spin-off of our photovoltaics research which has lead to the development of a method for producing flexible light emitting diodes based on ZnO nanowires in polymer substrates.