Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session L34: Thin Film Photovoltaics (Perovskites, etc) |
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Sponsoring Units: GERA Room: 210A |
Wednesday, March 4, 2015 8:00AM - 8:12AM |
L34.00001: Hybrid Perovskite Solar Cells with Copper Iodide as Hole Transportlayer Ross Haroldson, Zane Olds, Alexander B Cook, Anvar Zakhidov Hybrid organo-metallic solar cells based on perovskite-structured nanocrystals have had steadily improving power conversion efficiencies over the past several years, and within this short period of time are capable of achieving efficiencies over 19{\%}. In our work we show that dopantsa thin layer of Copper Iodide (CuI) on top of a hole transport layer such as PEDOT:PSS increases the open circuit voltage, of the devices. CuI is a p-type hole conducting material with a large band gap that has been used before for hole transport layers by itself. We demonstrate that CuI as the working hole transport layer increases the Voc about 10{\%} increase. [Preview Abstract] |
Wednesday, March 4, 2015 8:12AM - 8:24AM |
L34.00002: Morphological Optimization of Perovskite Thin Films via Dynamic Zone Annealing Yan Sun, Kai Wang, Xiong Gong, Alamgir Karim Organolead Halide Perovskites have been proved to be excellent candidates for application in low-cost high-efficient solar cells owing to their superior desired optical and electrical properties, as well as compatibility with low-temperature solution-processed manufacturing. However, most perovskites applications in photovoltaics require high quality perovskite films. Although tremendous works on tuning perovskite film morphology have been reported previously, it is still a challenge to realize high quality perovskite film with controllable film uniformity and surface coverage, neither the mechanisms in the formation of perovskite. To address the issues above, here we demonstrate the effect of Dynamic Zone Annealing (DZA) on perovskite morphologies, which is proved as an efficient method to control the structure and morphology in crystalline polymer and block copolymers. Via applying the DZA method, the mechanism in perovskite film formation is studied. Furthermore, by optimizing DZA parameter such as maximum temperature, temperature gradient and zone velocity to control dendritic morphology and the grain growth, enhanced device performance was realized eventually. [Preview Abstract] |
Wednesday, March 4, 2015 8:24AM - 8:36AM |
L34.00003: Photovoltaic properties of low band gap ferroelectric perovskite oxides Xin Huang, Tula Paudel, Shuai Dong, Evgeny Tsymbal Low band gap ferroelectric perovskite oxides are promising for photovoltaic applications due to their high absorption in the visible optical spectrum and a possibility of having large open circuit voltage. Additionally, an intrinsic electric field present in these materials provides a bias for electron-hole separation without requiring p-n junctions as in conventional solar cells. High quality thin films of these compounds can be grown with atomic layer precision allowing control over surface and defect properties. Initial screening based on the electronic band gap and the energy dependent absorption coefficient calculated within density functional theory shows that hexagonal rare-earth manganites and ferrites are promising as photovoltaic absorbers. As a model, we consider hexagonal TbMnO$_3$. This compound has almost ideal band gap of about 1.4 eV, very high ferroelectric Curie temperature, and can be grown epitaxially. Additionally hexagonal TbMnO$_3$ offers possibility of coherent structure with transparent conductor ZnO. We find that the absorption is sufficiently high and dominated by interband transitions between the Mn d-bands. We will present the theoretically calculated photovoltaic efficiency of hexagonal TbMnO$_3$ and explore other ferroelectric perovskite oxides. [Preview Abstract] |
Wednesday, March 4, 2015 8:36AM - 8:48AM |
L34.00004: Infrared spectrum and normal-mode assignment in methyl-ammonium lead halide perovskite CH$_3$NH$_3$PbI$_3$ Miguel Angel Perez Osorio, Marina Filip, Callum Docherty, Laura Herz, Michael Johnston, Feliciano Giustino Solar cells based on MAPbI$_3$ (MA=CH$_3$NH$_3$) have attracted enormous attention during the past two years owing to their high energy-conversion efficiency, reaching up to 19.3$\%$ in record devices. A detailed understanding of the structure/property relations of this compound may help us explain its extraordinary performance. Here, we investigate the vibrational modes and infrared (IR) absorption spectrum of MAPbI$_3$ by combining first-principles calculations and experiment. Our calculations indicate that the normal modes at high frequency, 400-3100 cm$^{-1}$, correspond to internal vibrations of the MA cations, whereas those at low frequency, up to 180 cm$^{-1}$, can be assigned either to vibrations of the PbI nework or to the libration and spinning of the cations. Using a factor group analysis we establish the symmetry of the normal modes and predict which mode will be IR or Raman active. In order to confirm these assignments we explicitly calculate the IR spectrum of the MAPbI$_3$. The calculated spectrum is in good agreement with experiment, therefore we now have a complete characterization of the vibrational properties of MAPbI$_{3}$. This work will serve as a solid reference for future structural and characterization studies of hybrid organic-inorganic perovskites. [Preview Abstract] |
Wednesday, March 4, 2015 8:48AM - 9:00AM |
L34.00005: Nanoscale charge localization induced by random orientations of organic molecules in hybrid perovskite CH3NH3PbI3 Jie Ma, Lin-Wang Wang Perovskite-based solar cells have achieved high solar-energy conversion efficiencies and attracted wide attentions nowadays. Despite the rapid progress in solar-cell devices, many fundamental issues of the hybrid perovskites have not been fully understood. Experimentally, it is well known that in CH3NH3PbI3, the organic molecules CH3NH3 are randomly orientated at the room temperature, but the impact of the random molecular orientation has not been investigated. Using linear-scaling~\textit{ab-initio~}methods, we have calculated the electronic structures of the tetragonal phase of CH3NH3PbI3 with randomly orientated organic molecules in large supercells up to $\sim$ 20,000 atoms. Due to the dipole moment of the organic molecule, the random orientation creates a novel system with long-range potential fluctuations unlike alloys or other conventional disordered systems. We find that the charge densities of the conduction-band minimum and the valence-band maximum are localized separately in nanoscales due to the potential fluctuations. The charge localization causes electron-hole separation and reduces carrier recombination rates, which may contribute to the long carrier lifetime observed in experiments. We have also proposed a model to explain the charge localization. [Preview Abstract] |
Wednesday, March 4, 2015 9:00AM - 9:12AM |
L34.00006: Dynamics of CH3NH3PbI3 from first principles simulations Ali Kachmar, Marcelo Carignano We address the dynamical and optical properties of CH3NH3PbI3 using molecular dynamics simulations based on forces calculated with density functional theory. We have studied the three stable phases of CH3NH3PbI3 but most of the effort was dedicated to the intermediate tetragonal phase, which is stable at standard ambient conditions. In this case, two different system sizes have been considered, one with 8 unit cells (384 atoms) and a larger one with 27 unit cells (1296 atoms). The total simulated time reached 40 ps. Our findings reveal the interplay between the thermal energy of the system and the electronic degrees of freedom. For example, the organic molecule undergoes relatively fast rotations and the energy band gap, approximated by the LUMO-HOMO energy difference, fluctuates around the equilibrium value of $\sim$1.5 eV with a width of 0.2 eV. The rotation of the CH3NH3 molecule is not isotropic, and more importantly, it is quite sensitive to the size of the simulation box. Our study also provides a quantitative measure for the finite size effects affecting the calculated properties and provides a contextual scenario on which to analyze the more typical density functional theory studies based on static calculations on optimal structures. [Preview Abstract] |
Wednesday, March 4, 2015 9:12AM - 9:24AM |
L34.00007: Bandstructure, optical spectra, and mean free paths in the room-temperature structure of CH$_{3}$NH$_{3}$PbI$_{3}$ from many-body perturbation theory Derek Vigil-Fowler, Marco Bernardi, Steven G. Louie The organometallic halide pervoskites have generated enormous interest due to the rapidly increasing efficiency of solar cells fabricated from these materials. Most research on the organometallic halide pervoskites has been experimental due to the challenges posed by these materials to theoretical study, including the size of the unit cell, the presence of many defects, the orientational disorder in of the methyammonium (MA) cation, and the heavy atoms involved with the corresponding large spin-orbit coupling (SOC). We study the room-temperature tetragonal structure of CH$_{3}$NH$_{3}$PbI$_{3}$ using density functional theory (DFT) and a many-body Green's functions approach. We use DFT to study the effect of the dependence of the bandstructure on the orientation of the MA cation, while we perform GW and GW plus Bethe-Salpeter equation (GW-BSE) calculations to study the quasiparticle bandstructure and optical spectra, respectively, paying close attention to convergence and the effect of SOC. We particularly investigate the existence of a proposed charge-transfer state in this material. We also briefly discuss the mean free paths due to electron-phonon and electron-electron scattering in the ideal structure. [Preview Abstract] |
Wednesday, March 4, 2015 9:24AM - 9:36AM |
L34.00008: Exciton Binding energies and effective masses in Organo-lead Tri-Halide Perovskites Oliver Portugall, Atsuhiko Miyata, Anatol Mitioglu, Paulina Plochocka, Jacob Tse-Wei Wang, Samuel Stranks, Henry Snaith, Robin Nicholas Solid-state perovskite-based solar cells have made a dramatic impact on emerging PV research with efficiencies of over 17\% already achieved. However, to date the basic electronic properties of the perovskites such as the electron and hole effective masses and the exciton binding energy are not well known. We have measured both for methyl ammonium lead tri-iodide using magneto absorption in very high magnetic fields up to 150T showing that the exciton binding energy at low temperatures is only 16 meV, a value three times smaller than previously thought and sufficiently small to completely transform the way in which the devices must operate. Landau level spectroscopy shows that the reduced effective mass of 0.104 me is also smaller than previously thought. In addition by using a fast pulse 150T magnet we measure the band structure change due to the structural phase transition that occurs in this system at around 160K. We also observe Landau levels in the high temperature phase as used for device production, which has a very similar effective mass and the analysis suggests an exciton binding energy which is even smaller than in the low temperature phase. [Preview Abstract] |
Wednesday, March 4, 2015 9:36AM - 9:48AM |
L34.00009: Ferritin-based nanocrystals for solar energy harvesting John Colton, Stephen Erickson, Cameron Olsen, Jacob Embley, Trevor Smith, Richard Watt Ferritin is a 12 nm diameter hollow protein with an 8 nm cavity that can be filled with a variety of nanocrystals (ferrihydrite being native). We report on several experiments with ferritin-based nanocrystals designed to utilize ferritin for solar energy harvesting. First, we have shown that the native band gap can be altered by controlling nanocrystal size, by replacing the native iron oxide core with other metal oxides, and by depositing halides and oxo-anions with the iron oxide core. This gives available band gaps of 1.6 to 2.3 eV. Theoretical efficiency calculations based on these band gaps show that the efficiency of a multi-junction solar cell based on layered structures of ferritin can be as high as 44.9$\%$, and up to 63.1$\%$ if a ferritin-based material with band gap of ~1.1 eV can be developed. For the latter case, the efficiencies remain quite high even in a current-matched configuration, namely 50.0$\%$. We have also demonstrated that photo-excitation of these materials can produce charge separation and give rise to usable electrons; we have used photo-excited electrons to reduce gold in solution and thereby produce gold nanoparticles on the surface of the ferritin. This technique can potentially be extended to platinum, whose nanoparticles catalyze water splitting. [Preview Abstract] |
Wednesday, March 4, 2015 9:48AM - 10:00AM |
L34.00010: Tuning Fermi Level Beyond the Intrinsic Equilibrium Doping Limit through Quenching: the Case in CdTe Ji-Hui Yang, Ji-Sang Park, Joongoo Kang, Wyatt Metzger, Teresa Barnes, Su-Huai Wei The ability to tune the Fermi levels is of great importance for many electronic device applications. However, the Fermi level is often limited to a certain range in the band gap due to the existence of certain intrinsic compensating defects. Here, we demonstrate that quenching can be used as an effective way to overcome this limit and tune the Fermi levels in a much wider range. Taking a photovoltaic material, CdTe, as a prototype example, we analyzed the physical origin behind the Fermi level pinning and explain why growing the sample at high temperature and then rapidly quenching it to room temperature can overcome the self-compensation limit. We show that for CdTe, quenching can enlarge the Fermi level range from only about 0.6 eV to 1.1 eV, which has a great potential in improving CdTe solar cell performance. Our proposed strategy of tuning Fermi level positions beyond intrinsic equilibrium doping limit is general and can be applied to other semiconductor systems. [Preview Abstract] |
Wednesday, March 4, 2015 10:00AM - 10:12AM |
L34.00011: Photovoltaic efficiency of an indirect bandgap material Michelle Tomasik, Niall Mangan, Jeffrey Grossman Photovoltaic materials with direct band gap transitions absorb light more readily than those with indirect gaps, allowing for thinner devices. However, direct bands also suffer faster rates of radiative recombination than indirect bandgap materials. Some novel photovoltaic absorber materials, such as tin sulfide, have both direct and indirect gaps. Such materials raise the question of whether the multiple energy states benefit or harm device efficiency. We develop a model for current in a device with direct and indirect band gaps using detailed balance, similar to the Shockley-Quiesser model for direct band photovoltaics. We explore the effects of the following on device performance: transition probability of carriers between the direct and indirect state, and relative transport rate in each band. [Preview Abstract] |
Wednesday, March 4, 2015 10:12AM - 10:24AM |
L34.00012: Ab initio modeling of the optical properties in organometallic halide perovskites for photovoltaic applications Amanda Neukirch, Wanyi Nei, Laurent Pedesseau, Jacky Even, Claudine Katan, Aditya Mohite, Segrei Tretiak The need for an inexpensive, clean, and plentiful source of energy has generated large amounts of research in an assortment of solution processed organic and hybrid organic-inorganic solar cells. A relative newcomer to the field of solution processed photovoltaics is the lead halide perovskite solar cell. In the past 5 years, the efficiencies of devices made from this material have increased from 3.5{\%} to nearly 20{\%}. Despite the rapid development of organic-inorganic perovskite solar cells, a thorough understanding of the fundamental photophysical processes driving the high performance of these devices is not well understood. I am using state-of-the-art ab initio computational techniques in order to characterize the properties at the interface of perovskite devices in order to aide in materials design and device engineering. I will present an in-depth analysis of the electronic and optical properties of bulk and surface states of pure and mixed halide systems. The high-level static quantum mechanical calculations, including spin-orbit-coupling and the many body GW approach, identify the key electronic states involved in photoinduced dynamics. This knowledge provides important information on how the optical properties change with variations to the system. [Preview Abstract] |
(Author Not Attending)
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L34.00013: Band-Gap Tuning in Perovskite-type Ferroelectric ZnSnO$_{3}$ by Doping and Core-Shell Approach for Solar Cell Applications Corisa Kons, Anuja Datta, Pritish Mukherjee Ferroelectric (FE) perovskite materials are an emerging class of potential absorbers in next generation solar cells due to their spontaneous polarization which facilitates electron-hole separation and drive charge carriers at opposite ends. With a large remnant polarization of $\approx $ 59 $\mu $C/cm$^{2}$, perovskite-type LiNbO$_{3}$ (LN)-ZnSnO$_{3}$, containing earth abundant elements is of much interest as a high performance solar absorber. However, the wide band-gap in ZnSnO$_{3}$ ($\sim$ 3.7 eV) is unsuitable to absorb the broad solar range, which can be overcome by band-gap engineering. Here, we discuss band-gap tuning through substitutional doping (Sb, Cu, Ca, Ba) in LN-type ZnSnO$_{3}$ nanorods, synthesized by a facile solvothermal process. A band-gap as low as 2.5 eV was obtained in 5 at.{\%} Ca doped ZnSnO$_{3}$ nanorods showing superior FE properties. The current-voltage ($I-V)$ measurements under light revealed multiple orders of enhancement as compared to the dark. The band-gap in ZnSnO$_{3}$ is also found to be a strong function of the lattice constant which is tuned by introducing a slight strain through lattice mismatching using a core shell approach. A detailed structural, optical, and FE analyses are provided to predict the future of this technologically important material in next generation FE photovoltaics. [Preview Abstract] |
Wednesday, March 4, 2015 10:36AM - 10:48AM |
L34.00014: Nanoscale optimization of quantum dot solar sells Yanshu Li, Andrei Sergeev, Nizami Vagidov, Vladimir Mitin, Kimberly Sablon Quantum dots (QDs) offer possibilities for nanoscale control of photoelectron processes via engineering the band structure and potential profile. Nanoscale potential profile (potential barriers) and nanoscale band engineering (AlGaAs atomically thin barriers) effectively suppress the photoelectron capture to QDs. QDs also increase conversion efficiency of the above-bandgap photons due to extraction of electrons from QDs via Coulomb interaction with hot electrons that excited by high-energy photons. To study the effects of the band structure engineering and nanoscale potential barriers on the photovoltaic performance we fabricated 3-$\mu $m base GaAs devices with various InAs quantum dot media and selective doping. All quantum dot devices show improvement in conversion efficiency compared with the reference cell. Quantum efficiency measurements allow us to associate the spectral characteristics of photoresponse enhancement with nanoscale structure of QD media. The dark current analysis provides valuable information about recombination in QD solar cells. The two-diode model well fit the scope of data and recovers the measured open circuit voltage. [Preview Abstract] |
Wednesday, March 4, 2015 10:48AM - 11:00AM |
L34.00015: Theoretical study on single-phase stability and intrinsic defects in different Cu$_{2}$ZnSn(Se$_{1-x}$S$_{x})_{4}$ alloys Pranab Sarker, Muhammad N. Huda Cu$_{2}$ZnSn(Se$_{1-x}$S$_{x})_{4}$ (CZTSSe) alloy has been emerged as a potential next generation commercialized photovoltaic cell because of its higher solar-to-current efficiency (12.6 {\%}) over parent compounds Cu$_{2}$ZnSnS$_{4}$ (CZTS) and Cu$_{2}$ZnSnSe$_{4}$ (CZTSe). However, the values of composition x in higher efficient CZTSSe (\textgreater 11{\%}) are not known yet. It has been inferred from the recent theoretical and experimental evidences that 0.375 $\le $ x $\le $ 0.625 (x $=$ alloy ratio per unit cell) could be the range that poses to ensure higher PV efficiency in CZTSSe. The crystal structure of CZTSSe at those x values were determined using density functional theory. In addition, the probability of forming different intrinsic defects in those different CZTSSe alloys were evaluated at various growth conditions determined from chemical potential landscape analysis for the first time. Chemical potential landscape analysis further reveals that CZTSSe alloys have higher single phase stability than that of their parent structures. [Preview Abstract] |
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