This FWF project, led by Prof. (FH) Dr. Bettina Friedel from 01/2015 to 03/2019, was dedicated to a new inorganic semiconductor material for use in organic-inorganic hybrid solar cells: silicon carbide. The main question was whether the doping and surface properties of the silicon carbide nanocrystals could be adjusted in such a way that the typical loss mechanisms for hybrid solar cells would not occur or only at a reduced rate.
In hybrid photovoltaics, the photoactive donor/acceptor layer of the cell consists of a nanocrystalline inorganic semiconductor embedded within an organic semiconductor matrix. Organic semiconductors, known from OLED-based TV displays, are mechanically flexible and can be processed from solution. Classical inorganic semiconductors, on the other hand, show better charge transport, but are fragile and expensive. Hybrid PV uses the advantages of both. Unfortunately, the efficiency of such solar cells is often limited by charge trapping losses at the grain boundaries, which occur mainly at surface defects of the inorganic crystals. In this project, silicon carbide (SiC) nanocrystals were investigated for the first time as inorganic acceptors in hybrid solar cells. For this purpose, differently doped nanocrystalline SiC was chemically synthesized. The introduced not only set the electronic defect state of the semiconductor, but also affected the grown SiC polytype and its surface termination (oxide vs. H/graphene). The latter was essential for the mechanisms at the organic/inorganic interface. When comparing the photovoltaic behavior of SiC:N, SiC:Al or SiC:Ga in hybrid matrices with the semiconducting polymer P3HT, the graphene terminated species showed more efficient photoinduced charge transfer and reduced trapping. Reasons are the better compatibility of the polymer with the carbon surface and the deactivation of SiC dangling bonds by hydrogen. A less effective but similar effect could be achieved with oxide-terminated SiC species by the subsequent addition of fullerenes. These results show that silicon carbide is suitable for hybrid PV and that the typical hybrid losses can be reduced with appropriate interface engineering.
FWF The Austrian Science Fund, grant number P26968
Graz University of Technology, Institute of Solid State Physics, Ao. Prof. Dr. Robert Schennach
O. Kettner, S. Šimić, B. Kunert, R. Schennach, R. Resel, T. Grießer, B. Friedel, Characterization of Surface and Structure of In Situ Doped Sol‐Gel‐Derived Silicon Carbide; Advanced Engineering Materials, 20 (2018) 1701067, DOI: 10.1002/adem.201701067 https://graz.pure.elsevier.com/de/publications/
O. Kettner, A. Pein, G. Trimmel, P. Christian, C. Röthel, I. Salzmann, R. Resel, G. Lakhwani, F. Lombeck, M. Sommer,B. Friedel, Alternating Side-Chain Geometries for Aggregation Control of Poly(fluorene-alt-bithiophene) and their Effects on Photophysics and Charge Transport, Synthetic Metals, 220 (2016) 162-173, DOI: 10.1016/j.synthmet.2016.06.010 https://www.tugraz.at/institutes/ictm/research/trimmel-group/publications/