3 research outputs found
Plasmon-plasmon interaction and the role of buffer in epitaxial graphene micro-flakes
We investigate the origin of the translational symmetry breaking in
epitaxially grown single-layer graphene. Despite the surface morphology of
homogeneous graphene films influenced by the presence of mutually parallel SiC
surface terraces, the far-infrared magneto-plasmon absorption is almost
independent of the angle between the probing light polarization and the
orientation of terraces. Based on a detailed analysis of the plasmon absorption
lineshape and its behavior in the magnetic field, supported by confocal Raman
mapping and atomic force microscopy, we explain this discrepancy by
spontaneously formed graphene micro flakes. We further support our conclusions
using data collected on artificially created graphene nanoribbons: we recognize
similar plasmon origin in artificial ribbons and naturally formed grains. An
unexpectedly large plasmon resonance redshift was observed in nanoribbons. In a
hydrogen-intercalated sample (which does not contain the buffer), this redshift
is quantitatively taken into account by a plasmon-plasmon interaction. In
non-intercalated samples featuring a buffer layer, this redshift is due to an
interplay between the plasmon-plasmon coupling and Coulomb screening by the
buffer-induced interface states. This model determines the density of interface
states in good agreement with experimentally reported values.Comment: 17 pages, 10 figures, 4 table
Predicting solar cell performance from terahertz and microwave spectroscopy
Mobilities and lifetimes of photogenerated charge carriers are core properties of photovoltaic materials and can both be characterized by contactless terahertz or microwave measurements. Here, the expertise from fifteen laboratories is combined to quantitatively model the current-voltage characteristics of a solar cell from such measurements. To this end, the impact of measurement conditions, alternate interpretations, and experimental inter-laboratory variations are discussed using a (Cs,FA,MA)Pb(I,Br)3 halide perovskite thin-film as a case study. At 1 sun equivalent excitation, neither transport nor recombination is significantly affected by exciton formation or trapping. Terahertz, microwave, and photoluminescence transients for the neat material yield consistent effective lifetimes implying a resistance-free JV-curve with a potential power conversion efficiency of 24.6 %. For grainsizes above â20 nm, intra-grain charge transport is characterized by terahertz sum mobilities of â32 cm2 Vâ1 sâ1. Drift-diffusion simulations indicate that these intra-grain mobilities can slightly reduce the fill factor of perovskite solar cells to 0.82, in accordance with the best-realized devices in the literature. Beyond perovskites, this work can guide a highly predictive characterization of any emerging semiconductor for photovoltaic or photoelectrochemical energy conversion. A best practice for the interpretation of terahertz and microwave measurements on photovoltaic materials is presented
Predicting Solar Cell Performance from Terahertz and Microwave Spectroscopy
Mobilities and lifetimes of photogenerated charge carriers are core properties of photovoltaic materials and can both be characterized by contactless terahertz or microwave measurements. Here, the expertise from fifteen laboratories is combined to quantitatively model the current-voltage characteristics of a solar cell from such measurements. To this end, the impact of measurement conditions, alternate interpretations, and experimental inter-laboratory variations are discussed using a (Cs,FA,MA)Pb(I,Br)3 halide perovskite thin-film as a case study. At 1 sun equivalent excitation, neither transport nor recombination is significantly affected by exciton formation or trapping. Terahertz, microwave, and photoluminescence transients for the neat material yield consistent effective lifetimes implying a resistance-free JV-curve with a potential power conversion efficiency of 24.6Â %. For grainsizes above â20Â nm, intra-grain charge transport is characterized by terahertz sum mobilities of â32Â cm2 Vâ1 sâ1. Drift-diffusion simulations indicate that these intra-grain mobilities can slightly reduce the fill factor of perovskite solar cells to 0.82, in accordance with the best-realized devices in the literature. Beyond perovskites, this work can guide a highly predictive characterization of any emerging semiconductor for photovoltaic or photoelectrochemical energy conversion. A best practice for the interpretation of terahertz and microwave measurements on photovoltaic materials is presented.ChemE/Opto-electronic Material