Charge transport in hybrid silicon solar cells

Sara Jäckle has demonstrated the formation of a pn-heterojunction at the interface between an organic contact and n-doped silicon. Photo

Sara Jäckle has demonstrated the formation of a pn-heterojunction at the interface between an organic contact and n-doped silicon. Photo © Björn Hoffmann

Systematic measurement of  characteristic curves using silicon wafers with different doping concentrations lead to the discovery. Photo

Systematic measurement of characteristic curves using silicon wafers with different doping concentrations lead to the discovery. Photo © Björn Hoffmann

An HZB team headed by Prof. Silke Christiansen has made a surprising discovery about hybrid organic/inorganic solar cells. Contrary to expectations, a diode composed of the conductive organic PEDOT:PSS and an n-type silicon absorber material behaves more like a pn junction between two semiconductors than like a metal-semiconductor contact (Schottky diode). Their results have now been published in the Nature journal Scientific Reports and could point the way toward improvements in hybrid solar cells.

The system they investigated is based on conventional n-type silicon wafers coated with the highly conductive polymer mixture PEDOT:PSS and displays a power conversion efficiency of about 14 %. This combination of materials is currently extensively investigated by many teams in the research community.

“We systematically surveyed the characteristic curves, the dark current as well as the capacitance of such devices using silicon wafers with different doping concentrations” explains Sara Jäckle, lead author of the article and Ph.D. student in Prof. Silke Christiansen’s team (HZB Institute of Nano-architectures for Energy Conversion and research group leader at the Max Planck Institute for the Science of Light MPL in Erlangen).

Doping concentration matters

“We learned that the characteristic curves in the dark as well as the open-circuit voltage of the solar cells are dependent on the doping concentration of the silicon wafer. This behaviour and the order of magnitude of the measured values do not correspond at all to those of a typical Schottky junction.”

The result is surprising because the n-type silicon is a typical semiconductor, while PEDOT:PSS is usually described as a metallic conductor. Up to now it has therefore been assumed that a typical metal-semiconductor junction would exist between these two materials, one that can be described by the Schottky equation.

Heterojunction behaviour observed

Supported by measurements also in collaboration with the research team of Prof. Klaus Lips (Energy Materials In-Situ Laboratory Berlin (EMIL), Institute for Nanospectroscopy)  the data and a comparison with theoretical models demonstrate otherwise. In a junction with n-type silicon, the conductive organic layer behaves like a p-type semiconductor rather than a metal. “The data shows a dependence on the degree of doping in the n-material just like a heterojunction between a p- and n-type semiconductors does”, says Sara Jäckle.

Results might be valid for other hybrid systems for optoelectronics

“This work deals with a very important aspect of these kinds of hybrid  systems, namely the behaviour at the interface”, says Silke Christiansen. “The results are probably also valid for other hybrid systems, important for photovoltaics or other optoelectronic applications such as perovskite solar cells. They suggest new ways for optimising devices by tuning the interface properties”.

Note: The Collaborative Research Centre (SFB 951) Hybrid Inorganic/Organic Systems for Opto-Electronics (HIOS) has just entered its second funding period. Prof. Christiansen and her team will be continuing their research on hybrid interfaces and devices in a subproject of this SFB.

arö

  • Copy link

You might also be interested in

  • Green hydrogen: MXenes shows talent as catalyst for oxygen evolution
    Science Highlight
    09.09.2024
    Green hydrogen: MXenes shows talent as catalyst for oxygen evolution
    The MXene class of materials has many talents. An international team led by HZB chemist Michelle Browne has now demonstrated that MXenes, properly functionalised, are excellent catalysts for the oxygen evolution reaction in electrolytic water splitting. They are more stable and efficient than the best metal oxide catalysts currently available. The team is now extensively characterising these MXene catalysts for water splitting at the Berlin X-ray source BESSY II and Soleil Synchrotron in France.
  • SpinMagIC: 'EPR on a chip' ensures quality of olive oil and beer
    News
    04.09.2024
    SpinMagIC: 'EPR on a chip' ensures quality of olive oil and beer
    The first sign of spoilage in many food products is the formation of free radicals, which reduces the shelf-life and the overall quality of the food. Until now, the detection of these molecules has been very costly for the food companies. Researchers at HZB and the University of Stuttgart have developed a portable, small and inexpensive 'EPR on a chip' sensor that can detect free radicals even at very low concentrations. They are now working to set up a spin-off company, supported by the EXIST research transfer programme of the German Federal Ministry of Economics and Climate Protection. The EPRoC sensor will initially be used in the production of olive oil and beer to ensure the quality of these products.
  • Review on ocular particle therapy (OPT) by international experts
    Science Highlight
    03.09.2024
    Review on ocular particle therapy (OPT) by international experts
    A team of leading experts in medical physics, physics and radiotherapy, including HZB physicist Prof. Andrea Denker and Charité medical physicist Dr Jens Heufelder, has published a review article on ocular particle therapy. The article appeared in the Red Journal, one of the most prestigious journals in the field. It outlines the special features of this form of eye therapy, explains the state of the art and current research priorities, provides recommendations for the delivery of radiotherapy and gives an outlook on future developments.