Photovoltaic applications of micro- and nano-crystalline silicon carbide
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Photovoltaic applications of micro- and nano-crystalline silicon carbide A. Konopka, S. Greulich-Weber, E. Rauls, W.G. Schmidt, and U. Gerstmann Department of Physics, University of Paderborn, Paderborn, Germany. ABSTRACT One of the challenges on the way to optimized solar cells is to make the thickness of the individual layers smaller than the diffusion length of the charge carriers. In this work, we propose 3C-SiC microcrystals grown by a sol-gel based process as a promising acceptor material for photovoltaic applications. The μc-SiC samples were characterized by optical spectroscopy and electron paramagnetic resonance (EPR). The experimental data is analyzed with the help of ab-inito calculations in the framework of density functional theory (DFT) resulting in electronic band structures and g-tensors. Based on this, a possible scenario for the observed acceptor process is discussed. INTRODUCTION Microcrystalline silicon carbide (μc-SiC) has become an attractive new class of advanced microstructured materials for heterojunction photovoltaic (PV) devices and light emitting diodes. Its wide bandgap yields lower absorption in the visible region and by controlled doping a high conductivity can be achieved. Wide-bandgap microcrystals are also of interest as effective charge carrier collectors in organic solar cells. In the absence of light, there are no charge carriers if two organic semiconductors are brought into contact. When a photon is absorbed by an organic photoactive material, an exciton, i.e., a bound state of an electron and a hole, is created. This neutral quasi-particle has a – compared to inorganic semiconductors – notably short lifetime of several tens of nanoseconds. The most important design criterion for solar cells is to make the thickness of the individual layers smaller than the diffusion length of the exciton, in order to keep the collection efficiency close to unity [1, 2]. One approach to achieve an efficient charge carrier generation in polymer-based light absorbers is to blend them with suited acceptors. By this and upon photoexcitation, an ultrafast electron transfer between a donor and a proximate acceptor can take place. One of the most extensively studied device concepts, so far, is based on the bulk heterojunction approach [3]. Usually, fullerene molecules are dispersed in a polymer matrix. The thin photoactive film is then sandwiched between two electrodes with asymmetric work functions forming ohmic contacts to the respective p- and n-type semiconductors (PEDOT, respectively ITO and Al, see also Figure 1). One approach to increase the efficiency of this kind of solar cells is to use alternative acceptor materials with a suitable position of the LUMO (lowest unoccupied molecular orbital) level. By adjusting the latter the open circuit voltage of the solar cell can be increased at the expense of the energy lost by the electrons connected with charge transfer from donor to acceptor material. Instead of using rather expensive fullerenes, currently wide bandgap micro- and nano-crystals as acc
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