Abstract-Measurement of charge carrier mobilities in thin films on metal substrates by reflection time resolved terahertz spectroscopy

H. Hempel, T. Unold, and R. Eichberger

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-15-17227&origin=search

We show that charge carrier mobilities can be measured by reflection time resolved THz spectroscopy (R-TRTS) even for thin films on metal contacts, such as polycrystalline Cu2SnZnSe4grown on molybdenum. In the measurement a reduced THz reflection upon photo-excitation is observed in contrast to increased THz reflection commonly observed on insulating substrates, and which excludes standard analytic R-TRTS analyses. Instead, a numerical transfer matrix method is used to model the THz reflection from which we derive carrier mobilities of 100 cm2/Vs consistent with literature. We show that R-TRTS on metal substrates is ~100x less sensitive compared to measurements on insulating substrates. These sensitivity of these R-TRTS measurements can be increased by using lower substrate refractive indices, lower substrate conductivities, thicker sample layers or higher THz probe frequencies.

© 2017 Optical Society of America

Abstract-Direct comparison of time-resolved terahertz spectroscopy and Hall Van der Pauw methods for measurement of carrier conductivity and mobility in bulk semiconductors

Brian G. Alberding, W. Robert Thurber, and Edwin J. Heilweil

https://www.osapublishing.org/josab/abstract.cfm?uri=josab-34-7-1392

Charge carrier conductivity and mobility for various semiconductor wafers and crystals were measured by ultrafast above bandgap, optically excited time-resolved terahertz spectroscopy (TRTS) and Hall Van der Pauw contact methods to directly compare these approaches and validate the use of the non-contact optical approach for future materials and in situ device analyses. Undoped and doped silicon (Si) wafers with resistances varying over 6 orders of magnitude were selected as model systems because contact Hall measurements are reliably made on this material. Conductivity and mobility obtained at room temperature by terahertz transmission and TRTS methods yield the sum of electron and hole mobility which agree very well with either directly measured or literature values for corresponding atomic and photodoping densities. Careful evaluation of the optically generated TRTS frequency-dependent conductivity also shows it is dominated by induced free carrier absorption rather than small probe pulse phase shifts, which is commonly ascribed to changes in the complex conductivity from sample morphology and evaluation of carrier mobility by applying Drude scattering models. Thus, in this work, the real-valued, frequency-averaged conductivity was used to extract sample mobility without application of models. Examinations of germanium (Ge), gallium arsenide (GaAs), gallium phosphide (GaP), and zinc telluride (ZnTe) samples were also made to demonstrate the general applicability of the TRTS method, even for materials that do not reliably make good contacts (e.g., GaAs, GaP, ZnTe). For these cases, values for the sum of the electron and hole mobility also compare very favorably to measured or available published data.

Abstract-Ultrafast Dynamics of Hole Injection and Recombination in Organo Metal Halide Perovskite Using Nickel Oxide as p-Type Contact Elec-Trode

Alice Corani, Ming-Hsien Li, Po-Shen Shen, Peter Chen, Tzung-Fang Guo, Amal El Nahhas, Kaibo Zheng, Arkady Yartsev, Villy Sundstrom, and Carlito S. Ponseca

J. Phys. Chem. Lett., Just Accepted Manuscript

DOI: 10.1021/acs.jpclett.6b00238

Publication Date (Web): March 4, 2016

Copyright © 2016 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/acs.jpclett.6b00238?journalCode=jpclcd

There is a mounting effort to use nickel oxide (NiO) as p-type selective electrode for organo-metal halide perovskite based solar cells. Recently, an overall power conversion efficiency using this hole acceptor has reached 18%. However, ultrafast spectroscopic investigations on the mechanism of charge injection as well as recombination dynamics have yet to be studied and understood. Using time-resolved terahertz spectroscopy, we show that hole transfer is complete on the sub-picosecond time scale, driven by the favorable band alignment between the valence bands of perovskite and NiO nanoparticles (NiO(np)). Recombination between holes injected into NiO(np) and mobile electrons in the perovskite material is shown to be hundreds of ps to few ns. Due to the low conduc-tivity of NiO(np), holes are pinned at the interface and it is electrons that determines the recombination rate. This recom-bination competes with charge collection and therefore must be minimized. Doping NiO that promotes higher mobility of holes is desirable in order to prevent back recombination.