Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Q14: Focus Session: Dopants and Defects in Hybrid Perovskite and Other Materials |
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Sponsoring Units: DMP FIAP Chair: Yong Zhang, University of North Carolina, Charlotte Room: 008A |
Wednesday, March 4, 2015 2:30PM - 3:06PM |
Q14.00001: The Photophysics of Perovskite Solar Cells Invited Speaker: Tze-Chien Sum Solution processed organic-inorganic lead halide perovskite solar cells, with power conversion efficiencies approaching 20{\%}, are presently the forerunner amongst the next generation photovoltaic technologies. These remarkable performances can be attributed to their large absorption coefficients, long charge carrier diffusion lengths and low non-radiative recombination rates. In addition, these materials also possess excellent light emission and optical gain properties. In this talk, I will review the developmental milestones in this field and distil the recent findings on the photophysical mechanisms of this remarkable material. I will also highlight some of our latest charge dynamics studies and other investigations on the novel properties of this amazing material system. [Preview Abstract] |
Wednesday, March 4, 2015 3:06PM - 3:18PM |
Q14.00002: Elucidating the degradation of methylammonium lead iodide perovskite (CH$_3$NH$_3$PbI$_3$) at high temperatures and humid conditions via molecular dynamics simulations with ab initio force fields Omololu Akin-Ojo In recent times, the efficiencies of lead halide perovskite solar cells have soared to more than 15\% and there is a potential for further increase. However, these solar cells degrade quickly when exposed to heat and moisture. This degradation limits the large scale production of the device. In this work we use molecular dynamics simulations to study CH$_3$NH$_3$PbI$_3$ when exposed to high temperatures ($30 - 80^\circ$C) and water. The force fields used in the simulations were determined from ab initio calculations on the system. The structural changes that occur in methylammonium lead halide perovskite under conditions of high temperature and humidity will be inferred from the simulations. [Preview Abstract] |
Wednesday, March 4, 2015 3:18PM - 3:30PM |
Q14.00003: Influence of Defects on the Photovoltaic Properties of Perovskite Semiconductor CsSnI$_{3}$ Peng Xu, Shiyou Chen, Hongjun Xiang, Xin-Gao Gong, Su-Huai Wei CsSnI$_{3}$ is a prototype inorganic halide perovskite that has recently been proposed as a photovoltaic material. Through first-principles calculations, we show that the concentration control of intrinsic defects is critical for optimizing the photovoltaic properties of CsSnI$_{3}$. Under a Sn-poor condition, high concentration of acceptor defects such as Sn or Cs vacancies can form easily and produce a high p-type conductivity, and deep level defects that can become electron-hole recombination centers, all have high energy. This condition is optimal for growing CsSnI$_{3}$ as hole-transport material in solar cells. In contrast, when Sn becomes richer, the concentration of acceptor defects decreases, so the p-type conductivity may drop to a moderate level, which can increase the shunt resistance and thus the efficiency of the solar cells with CsSnI$_{3}$ as the light absorber material (LAM). However, under the Sn-rich condition, the concentration of a deep-level donor defect Sn$_{\mathrm{I}}$ will increase, causing electron traping and non-radiative electron-hole recombination. Therefore, we propose that a moderately Sn-rich condition is optimal when CsSnI$_{3}$ is used as LAM. [Preview Abstract] |
Wednesday, March 4, 2015 3:30PM - 3:42PM |
Q14.00004: Magnetic Field Effects in Hybrid Perovskite Devices C. Zhang, D. Sun, C.-X. Sheng, Y. Zhai, K. Mielczarek, A. Zakhidov, Z.V. Vardeny Solar cells based on the organic-inorganic perovskites (CH$_{3}$NH$_{3}$PbX$_{3}$, X$=$halogen) have reached a remarkable power conversion efficiency approaching 20{\%}, which calls for research studies of the photophysics behind this high device performance. We measured significant magneto-photocurrent (MPC) response in CH$_{3}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3-x}}$Cl$_{\mathrm{x}}$ photovoltaic cells, in the form of Lorentzian up to field $B=$1T. We attribute the MPC(B) response to spin mixing of loosely-bound photogenerated e-h pairs having different $g$-factor (dubbed ``$\Delta g$ mechanism''). We verified this mechanism by measuring $\Delta g$ directly, using the field induced circularly polarized photoluminescence emission at low temperature, along with the photocarriers' lifetime measured by picosecond pump-probe spectroscopy. We conclude that MPC of spin 1/2 e-h pairs provides a promising method for investigating the spin-related properties of photoexcitations in the novel hybrid perovskites. [Preview Abstract] |
Wednesday, March 4, 2015 3:42PM - 3:54PM |
Q14.00005: New GeSi doping strategies based on P(SiH$_{3})_{3}$ for next-generation CMOS technologies Andrew Chizmeshya, Chi Xu, James Gallagher, Patrick Sims, David Smith, Jose Menendez, John Kouvetakis GeSi $n$-type films are synthesized using the specially designed hydrides P(SiH$_{3})_{3}$, Ge$_{3}$H$_{8}$ and Ge$_{4}$H$_{10}$ for applications in next-generation CMOS technologies. The films are grown on Ge-buffered Si(100) at 340 $^{\circ}$C using two methods. The first employs a gas-source molecular epitaxy approach and Ge$_{4}$H$_{10}$ to yield films with P doping densities up to 3.5 x 10$^{19}$/cm$^{3}$. The amount of Si incorporated equals or exceeds the 3:1 ratio in the P(SiH$_{3}$)$_{3}$ compound. The second approach applies an ultra-high vacuum chemical vapor deposition technique and Ge$_{3}$H$_{8}$ in place of Ge$_{4}$H$_{10}$ to achieve higher carrier concentrations up to 6 x 10$^{19}$/cm$^{3}$. The Si:P ratio in this case is well below the 3:1 value expected from the precursor. The electron mobilities for both types of samples are significantly higher than state-of-the-art prototypes, probably due to superior microstructure and dearth of inactive donors. The relative stability of Si-P and Ge-P bonds in a Ge matrix is studied with \textit{ab initio} methods. $P-I-N $diodes fabricated using P(SiH$_{3}$)$_{3}$ show excellent $I$-$V$ characteristics that are virtually undistinguishable from similar diodes doped with the P(GeH$_{3}$)$_{3}$ precursor. These results confirm P(SiH$_{3}$)$_{3}$ as a viable doping source that is practical from a process standpoint and therefore attractive for industrial scale-up. [Preview Abstract] |
Wednesday, March 4, 2015 3:54PM - 4:06PM |
Q14.00006: Theoretical study of defect properties in thermoelectric (GeTe)$_x$(AgSbTe$_2$)$_{1-x}$ Hikari Shinya, Akira Masago, Tetsuya Fukushima, Hiroki Funashima, Hiroshi Katayama-Yoshida We investigate the structural stability of a pseudo-binary alloy (GeTe)$_x$(AgSbTe$_2$)$_{1-x}$ called TAGS by the density functional theory. TAGS shows intermittent reductions of the thermal conductivity without change of the electric conductivity. However, the mechanism of the drastic change of the thermal conductivity has yet to be understood, and even the crystal structures are still under discussion. In this presentation, we will discuss these problems from a viewpoint of the structural stability. To clarify the stable structure, we estimate the formation energies of the point and the complex defects. As a result, a chain structure of Ag-Te-Sb in GeTe host crystal has a lower formation energy as compared to the homogeneous distribution. Moreover, the system becomes more stable by an assemblage of the chain structure. Furthermore, the calculated mixing energy shows that the system is favorable to the phase separation. In the phase separation, the grain boundary must play an important role in the large phonon scattering; therefore, it can lead to the thermal conductivity reduction. These calculations were done with Vienna ab initio Simulation Package and MACHIKANEYAMA2002 program package. [1] H. Shinya, et al., Jpn. J. Appl. Phys. 53, 111201(2014). [Preview Abstract] |
Wednesday, March 4, 2015 4:06PM - 4:18PM |
Q14.00007: A first-principles study of co-doping in lanthanum bromide Daniel Aberg, Babak Sadigh, Andre Schleife, Paul Erhart It was recently shown that the energy resolution of Ce-doped LaBr$_3$ scintillator radiation detectors can be crucially improved by co-doping with Sr, Ca, or Ba. Here we outline a mechanism for this enhancement on the basis of electronic structure calculations. We show that Sr dopants create and bind to Br vacancies, resulting in stable neutral complexes. The association with Sr causes the deep vacancy level to move toward the conduction band edge. This is essential for reducing the effective carrier density available for Auger quenching during thermalization of hot carriers. Subsequent de-trapping of electrons from the complexes can activate Ce dopants that have previously captured a hole leading to luminescence. This mechanism implies an overall reduction of Auger quenching of free carriers, which is expected to improve the linearity of the photon light yield with respect to the energy of incident electron or photon. Optical properties of the Ce-Sr-vacancy triple complex are discussed and compared to experiment. Prepared by LLNL under Contract DE-AC52-07NA27344. [Preview Abstract] |
Wednesday, March 4, 2015 4:18PM - 4:30PM |
Q14.00008: Formation and function of vacancies in Si/Ge Clathrates: The importance of broken symmetries Amrita Bhattacharya, Christian Carbogno, Matthias Scheffler One promising material class for improved thermoelectrics are the clathrates,~i.e.,~semiconducting host lattices encapsulating guest atom. Even in simple clathrates, such as, Si$_{46}$ and Ge$_{46}$, the introduction of guests can result in important but not yet understood effects: In Si hosts, the addition of K~(or~Ba) results in defect-free K$_8$Si$_{46}$~(Ba$_8$Si$_{46}$) phases. In spite of their structural and electronic similitude, Ge hosts behave fundamentally different upon filling, where, the spontaneously formed framework vacancies completely~(or partially) balance the electron donated by K~(or Ba) guests leading to K$_8$Ge$_{44}$~(or Ba$_8$Ge$_{43}$) clathrates. In this work, we use density-functional theory, carefully validating the exchange correlation functional, to compute the formation energies of vacancies and vacancy complexes in Si- and Ge-hosts as function of the filling of guests. By taking into account of the structural disorder, geometric relaxations, and vibrational entropies, we verify the experimentally found vacancy concentration and the thermodynamic stabilities of these compounds. We can trace back the contrasting behaviour of Si/Ge clathrates upon filling to a curious, charged vacancy induced break in symmetry that occurs in Si but not in Ge hosts. [Preview Abstract] |
Wednesday, March 4, 2015 4:30PM - 4:42PM |
Q14.00009: Cation-vacancy and electron-hole relaxation in single-walled aluminosilicate nanotubes: a linear-scaling Density Functional Theory study Emiliano Poli, Gilberto Teobaldi We report a linear-scaling Density Functional Theory (DFT) study of cation-vacancy related defects in single-walled aluminosilicate nanotubes (AlSi NTs), based on the structures derived from solid-state Nuclear Magnetic Resonance.\footnote{G. Yucelen et al.,\textbf{J. Phys. Chem. C.},116, 17149, 2012} Defect geometry optimization leads to water condensation and modifications to the AlSi NT hydrogen network around the defect sites, leaving no dangling bond. Electronic structure analysis indicates that defect-states are highly localized in real-space and energy, with appearance of shallow and deep occupied defect states above the valence band (VB) edge of the pristine-NT. Electrostatic alignment of the defect states suggests energetically favourable separation of photo-generated electrons and holes on different defects, which may promote defect-centred photochemistry. The peculiar energy alignment of the defect-states is found to be qualitative unaffected by protonation of the defect-sites. These results should be a useful complement to ongoing experimental research in the potential of (alumino)silicate-based nano-porous materials for photocatalysis.\footnote{F. Sastre et al.,\textbf{J. Am. Chem. Soc.},133, 17257, 2011}. [Preview Abstract] |
Wednesday, March 4, 2015 4:42PM - 4:54PM |
Q14.00010: ABSTRACT MOVED TO P1.00345 |
Wednesday, March 4, 2015 4:54PM - 5:06PM |
Q14.00011: Electronic structure of vitamin B$_{\bf 12}$ within the framework of the Haldane-Anderson impurity model Zafer Kandemir, Selma Mayda, Nejat Bulut We study the electronic structure of vitamin B$_{12}$ (cyanocobalamine C$_{63}$H$_{88}$CoN$_{14}$O$_{14}$P) by using the framework of the multi-orbital single-impurity Haldane-Anderson model of a transition-metal impurity in a semiconductor host. Here, our purpose is to understand the many-body effects originating from the transition-metal impurity. In this approach, the cobalt $3d$ orbitals are treated as the impurity states placed in a semiconductor host which consists of the rest of the molecule. The parameters of the resulting effective Haldane-Anderson model are obtained within the Hartree-Fock approximation for the electronic structure of the molecule. The quantum Monte Carlo technique is then used to calculate the one-electron and magnetic correlation functions of this effective Haldane-Anderson model for vitamin B$_{12}$. We find that new states form inside the semiconductor gap due to the on-site Coulomb interaction at the impurity $3d$ orbitals and that these states become the highest occupied molecular orbitals. In addition, we present results on the charge distribution and spin correlations around the Co atom. We compare the results of this approach with those obtained by the density-functional theory calculations. [Preview Abstract] |
Wednesday, March 4, 2015 5:06PM - 5:18PM |
Q14.00012: Characterization of structural defects in GST based nano-PCM devices through resistance drift measurements Ibrahim Cinar, Egecan Cogulu, Aisha Gokce, Barry Stipe, Jordan Katine, Gulen Aktas, Ozhan Ozatay Phase change memory (PCM) is a promising nonvolatile data storage technology with its high signal to noise ratio and superior scalability. Resistance drift in amorphous phase of the phase change material poses a crucial reliability problem, especially in multiple-bit-per cell PCM devices. The resistance of the amorphous phase uncontrollably increases with time after a reset operation which alters the read/write conditions of the device. Structural relaxation (SR) through a defect annihilation process is considered to be the underlying physical mechanism for resistance drift. Here, we report on our measurements of the resistance drift in a phase change memory device with a single layer Ge2Sb2Te5 (GST) material not only in the amorphous state but also in the intermediate resistance state in devices with square top contact geometry which enables us to assess the reliability of multiple-bit per cell PCM memory devices. Through an analysis of electrical measurements as a function of time and temperature for increasing annealing times, we estimate a rate of change in trap density for both amorphous and mixed phases of the GST material after a switching operation. Our study allows engineering the phase change materials and optimizing programing conditions for future PCM applications. [Preview Abstract] |
Wednesday, March 4, 2015 5:18PM - 5:30PM |
Q14.00013: Bulk photovoltaic effect in CH$_{3}$NH$_{3}$PbI$_{3}$ and CH$_{3}$NH$_{3}$PbI$_{3-x}$Cl$_{x}$ Nathan Z. Koocher, Fan Zheng, Hiroyuki Takenaka, Fenggong Wang, Andrew M. Rappe The power conversion efficiency of methylammonium lead iodide (MAPbI$_{3})$-based organometal halide perovskites has increased to nearly 20{\%}, fueling interest in understanding the mechanism of its photovoltaic effect. High open-circuit photovoltages and $I$/$V$ hysteresis curves have been experimentally measured for these materials, which suggest that the bulk photovoltaic effect (BPVE) could be in operation. Shift current is a main mechanism of the BPVE in ferroelectric perovskite oxides, and thus in our work, we calculate the shift current response of MAPbI$_{3}$ and MAPbI$_{3-x}$Cl$_{x}$. We find that MAPbI$_{3}$ and MAPbI$_{3-x}$Cl$_{x}$ have shift current responses about three times larger than that of BiFeO$_{3}$. Specifically, the shift current response is enhanced when the molecular dipoles from the methylammonium molecules are aligned in the same direction and when Cl is substituted into the lattice. Because of the large shift current response, the BPVE may play a role in enhancing the performance of the solar cells. [Preview Abstract] |
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