Verständnis von Multi-Exzitonen Prozessen und Ladungsextraktion in vollständig anorganischen nanostrukturierten Solarzellen mittels ab initio Simulationen
Zusammenfassung der Projektergebnisse
Third generation solar cells aspire to transcend the Shockley-Queisser limit of 33.7% conversion efficiency by employing path breaking new paradigms. Multiple Exciton Generation (MEG) is one such new paradigm, where one incoming photon creates a high energy exciton, which then decays into multiple low energy excitons. While the efficiency of MEG in bulk semiconductors is small, the enhanced Coulomb interaction in quantum confined nanostructures can drive MEG efficiency to promisingly high values, e. g. in nanoparticles. However, the quantum confinement also increases the electronic gap and can swiftly shift the MEG threshold energy outside the solar spectrum. Presently, no materials are known featuring efficient MEG within the solar spectrum. Silicon is one of the most technologically mature materials and features highly efficient MEG in nanocrystalline form, but the gaps are too large for MEG based solar energy conversion. Here we propose Si nanocrystals with core structures resembling the Si BC8 high pressure phase as a promising material for solar energy conversion. BC8 is a metastable form of Si, much like diamond is a metastable form of carbon, and is essentially a zero gap semiconductor. Using state of the art ab initio calculations we predict that in nanocrystalline BC8 the quantum confinement induced gap increase yields gaps ideally suited for MEG based solar energy conversion in the 4 nm to 8 nm size range. Despite the low gaps, our ab initio calculations of impact ionization rates predict highly efficient MEG in Si BC8 nanocrystals. To the best of our knowledge, this is the first known material to feature both efficient MEG and gaps suitable for solar energy conversion at the same time. Synthesis of the material in question has recently been achieved in several experiments. A synopsis can be found at http://www.sciencedaily.com/releases/2013/01/130128142900.htm. In solar energy conversion devices the nanocrystals are often embedded in a matrix to allow for efficient charge extraction and transport. However, at present very little is known about the influence exerted by the embedding matrix on the electronic and optical properties of the NP, as well as its impact on MEG. For device applications it is necessary to identify quantum confined nanostructures that feature both efficient MEG within the solar spectrum and allow for efficient charge extraction despite quantum confinement. We investigated Si nanocrystals embedded in amorphous ZnS. Our ab initio calculations demonstrate that a-ZnS embedded Si nanocrystals feature very different electronic properties, compared to either hydrogenated Si nanocrystals or bulk amorphous ZnS. The properties of the nanocomposite are dominated by interface effects, specifically in this case the formation of a sulfur shell around the Si nanocrystal. The resulting band alignment at the interface gives rise to charge separated transport channels, with holes conducted through the matrix and electrons tunneling from nanocrystal to nanocrystal. Initial exploratory ab initio calculations of impact ionization rates demonstrate efficient MEG despite the embedding in the host matrix and the band alignment allowing for charge extraction. In addition, both Si and ZnS are earth-abundant and non-toxic. We therefore propose Si (and possibly Ge) nanocrystals embedded in group II-VI sulfides (and possibly selenides) as promising materials for light absorbers in quantum dot solar cells.
Projektbezogene Publikationen (Auswahl)
- High pressure core structures of Si nanoparticles for solar energy conversion, Phys. Rev. Lett. 110, 046804 (2013)
S. Wippermann, M. Vörös, D. Rocca, A. Gali, G. Zimanyi, G. Galli
