Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session T4: Invited Session: Physics and Applications of Transparent Conducting Oxides |
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Sponsoring Units: FIAP Chair: Chris Van de Walle, University of California, Santa Barbara Room: Ballroom IV |
Thursday, March 21, 2013 8:00AM - 8:36AM |
T4.00001: Transparent Conducting Oxides as Potential Thermoelectrics Invited Speaker: Thomas Mason Transparent conducting oxides (TCOs) in their less-doped semiconducting states have potential as thermoelectric oxides or TEOs. They are attractive as TEOs owing to: 1) their good thermochemical stability, 2) their n-type character (to complement existing p-type TEOs), and 3) their high electronic mobilities. The numerator of the TE figure of merit (Z), also known as the ``power factor'' (PF), is the product of the electronic conductivity and the square of the Seebeck coefficient. An experimental procedure named after its developer, ``Jonker'' analysis plots Seebeck coefficient vs. the natural logarithm of the electronic conductivity. Data for bulk ceramic specimens just prior to the onset of degeneracy tend to fall on a line of slope, k/e (k$=$Boltzmann constant, e$=$charge of the electron). From this line, the doping composition corresponding to the highest power factor can be determined and the PF optimized, based upon data from a few carefully chosen compositions. Subsequently, following a procedure originally derived by Ioffe, the zero-thermopower intercept of these Jonker lines can be directly related to the maximum achievable power factor for a given TEO. So-called ``Ioffe'' plots allow for meaningful comparisons between candidate TEO materials, and also indicate the minimum thermal conductivity required to achieve a target ZT value at the temperature of measurement. Results for TCO-based TEOs will be discussed for both simple and compound (including layered) materials. [Preview Abstract] |
Thursday, March 21, 2013 8:36AM - 9:12AM |
T4.00002: Developing New TCOs for Renewable Applications Invited Speaker: David Ginley Transparent conducting oxides are enabling for a broad range of optoelectronic technologies. Not only are conductivity and transparency critical but many other factors are critical including: carrier type, processing conditions, work function, chemical stability, and interface properties. The historical set of materials cannot meet all these needs. This has driven a renaissance in new materials development and approaches to transparent contacts. We will discuss these new developments in general and in the context of photovoltaics specifically. We will present results on new materials and also the development bilayer structrues that enable charge selective contacts. Materials set includes amorphous materials for hybrid solar cells like InZnO and ZnSnO, it includes Nb and Ta doped TiO2 as a high refractive index TCO and it includes the use of thin n- and p-type oxides as electron and hole selective contacts such as has been demonstrated for organic photovotaics.\\[4pt] This work is supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Contract No. DE-AC36-08GO28308 to NREL as a part of the DOE Energy Frontier Research Center ``Center for Inverse Design'' and through the US Department of Energy under Contract no. DOE-AC36-08GO28308 through the National Center for Photovoltaics. [Preview Abstract] |
Thursday, March 21, 2013 9:12AM - 9:48AM |
T4.00003: Fundamental limits on transparency: first-principles calculations of absorption Invited Speaker: Hartwin Peelaers Transparent conducting oxides (TCOs) are a technologically important class of materials with applications ranging from solar cells, displays, smart windows, and touch screens to light-emitting diodes. TCOs combine high conductivity, provided by a high concentration of electrons in the conduction band, with transparency in the visible region of the spectrum. The requirement of transparency is usually tied to the band gap being sufficiently large to prevent absorption of visible photons. This is a necessary but not sufficient condition: indeed, the high concentration of free carriers can also lead to optical absorption by excitation of electrons to higher conduction-band states. A fundamental understanding of the factors that limit transparency in TCOs is essential for further progress in materials and applications. The Drude theory is widely used, but it is phenomenological in nature and tends to work poorly at shorter wavelengths, where band-structure effects are important. First-principles calculations have been performed, but were limited to direct transitions; as we show in the present work, indirect transitions assisted by phonons or defects actually dominate. Our calculations are the first to address indirect free-carrier absorption in a TCO completely from first principles. We present results for SnO$_2$ [1], but the methodology is general and is also being applied to ZnO and In$_2$O$_3$. The calculations provide not just quantitative results but also deeper insights in the mechanisms that govern absorption processes in different wavelength regimes, which is essential for engineering improved materials to be used in more efficient devices. For SnO$_2$, we find that absorption is modest in the visible, and much stronger in the ultraviolet and infrared. \\[4pt] [1] H. Peelaers, E. Kioupakis, and C.G. Van de Walle, Appl. Phys. Lett. {\bf 100}, 011914 (2012). [Preview Abstract] |
Thursday, March 21, 2013 9:48AM - 10:24AM |
T4.00004: Surface electron accumulation layers in oxide semiconductors Invited Speaker: Tim Veal In contrast to the electron depletion at the surface of almost all n-type semiconductors, electron accumulation has long been known to be observable at ZnO surfaces. It has recently been found to be a characteric of several other oxide semiconductors, including CdO [1,2], In$_2$O$_3$ [3] and SnO$_2$. They all have a significant size and electronegativity mismatch between their cation and anion. As a result, they have a particularly low $\Gamma$-point conduction band minimum which is ultimately responsible for the propensity for electron accumulation. In addition to the mere existence of an electron-rich surface layer, it has been found, using angle-resolved photoemission spectroscopy (ARPES), to be quantized into two dimensional subbands [1]. Moreover, the conventional one-electron picture of surface space-charge in semiconductors is shown to be inconsistent with the electronic structure that we observe directly from ARPES, indicating that many-body interactions play a large role in the surface electronic properties of these semiconductors. Such interactions lead to a depth-dependent shrinkage of the semiconductor band gap, resulting in a surface band gap which differs from the bulk value [1]. The most recent studies have focussed on the influence of depositing alkali metals onto the surface of these semiconductors. Many collaborators are acknowledged for samples and ARPES expertise.\\[4pt] [1] P. D. C. King, T. D. Veal et al., Phys. Rev. Lett. 104, 256803 (2010)\\[0pt] [2] P. D. C. King, T. D. Veal et al., Phys. Rev. B 79, 035203 (2009)\\[0pt] [3] P. D. C. King, T. D. Veal et al., Phys. Rev. Lett. 101, 116808 (2008) [Preview Abstract] |
Thursday, March 21, 2013 10:24AM - 11:00AM |
T4.00005: Low temperature growth of conformal, transparent conducting oxides Invited Speaker: Roy Gordon Transparent conductors (TC) are essential components of many widely-used technologies, including energy conserving low-E windows, electronic displays and solar cells. Currently, TC films are made by chemical vapor deposition (CVD) or by sputtering or evaporation (PVD). CVD has generally required high temperatures (greater than 500 C), so that is not applicable to plastic substrates and some solar cells. PVD makes films with low step coverage, so textured substrates, such as those with narrow holes, cannot be coated uniformly. The most effective PVD films are based on indium, a rare and expensive element. Recently, atomic layer deposition (ALD) processes have been developed that overcome all of these limitations, allowing highly uniform and conformal coating of substrates with very narrow holes even at substrate temperatures below 100 C. The metals used in these ALD TCs are tin and/or zinc, which are abundant and inexpensive elements. In this talk, we will review these ALD processes, along with the optical, structural and electrical properties of the TCs that they produce. Applications of these low-temperature, conformal TCs will also be discussed. Record-breaking solar cells made entirely from Earth-abundant elements were enabled by these ALD processes. Transparent transistors with excellent characteristics can now be made at low temperature even on rough or textured plastic surfaces. Micro-channel plate array detectors are being produced for use in highly sensitive imaging applications. [Preview Abstract] |
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