Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session M24: Focus Session: NanoPV Novel Photophysics and Transport II |
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Sponsoring Units: GERA Chair: Mark Lusk, Colorado School of Mines Room: 504 |
Wednesday, March 5, 2014 11:15AM - 11:51AM |
M24.00001: Modeling Charge Mobility in Nanoparticle Solar Cells Invited Speaker: Gergely Zimanyi Nanoparticle (NP) solar cells show the promise to enhance the efficiency of solar cells over the Shockley-Queisser limit due to quantum confinement enhanced charge multiplication processes [1]. A fundamental challenge of NP solar cells, however, is that the very reason that leads to enhanced charge generation also tends to hinder charge transport. To address this challenge, we outline a multi-scale transport modeling scheme based on our previous calculations [2] that involves determining NP parameters from ab-initio and semi-empirical calculations, such as energy level structures, charging energies. This is then embedded in a Kinetic Monte Carlo hopping transport framework to calculate electron and hole mobilities in NP devices as a function composition, disorder, temperature. As a first demonstration, we apply our method to PbSe NP Schottky devices.\\[4pt] Work done in collaboration with Ian Carbone, Physics Department, University of California, Santa Cruz and Marton Voros, Physics Department, University of California, Davis.\\[4pt] [1] Matthew C. Beard et al., Acc. Chem. Res. 46, 1252 (2013).\\[0pt] [2] Ian Carbone, S.A. Carter, G.T. Zimanyi, accepted in J. of Appl. Phys. [Preview Abstract] |
Wednesday, March 5, 2014 11:51AM - 12:03PM |
M24.00002: Ultrafast spectroscopy of CuInSeS colloidal quantum dots: Auger recombination, carrier multiplication, and electron transfer Nikolay Makarov, Hunter McDaniel, Istvan Robel, Victor Klimov We perform systematic transient absorption and time-resolved photoluminescence measurements on CuInSe$_{\mathrm{x}}$S$_{\mathrm{2-x}}$ quantum dots (QDs), with sizes of 3-5 nm, that have recently been utilized in sensitized solar cells achieving certified efficiencies above 5{\%}. We study QD volume and composition dependence of various excited charge carrier processes, including biexciton Auger recombination, carrier multiplication (CM), and electron transfer (ET) to TiO$_{\mathrm{2}}$. Biexciton decay is similar to that of CdSe QDs of the same volume, which supports the previously reported generality of Auger lifetimes in QDs. CM quantum yields approach 20{\%} indicating that this material could enable photovoltaic efficiencies exceeding the Shockley-Queisser limit. Size-dependent ET (20-40 ns) is fairly slow, which highlights the need for efficient suppression of competing nonradiative processes that can be associated, for example, with the surfaces of poorly passivated QDs. We also demonstrate the importance of having a redox electrolyte (used in sensitized solar cells for hole extraction) present during ET studies in order to prevent charge build-up. Our measurements are critical for understanding the photophysical properties of this new material, and they also suggest general pathways towards improving QD solar cells. [Preview Abstract] |
Wednesday, March 5, 2014 12:03PM - 12:15PM |
M24.00003: High-pressure Phase Ge nanoparticles and Si-ZnS nanocomposites: New Paradigms to Improve the Efficiency of MEG Solar Cells Stefan Wippermann, Marton Voros, Balint Somogyi, Adam Gali, Dario Rocca, Francois Gygi, Gergely Zimanyi, Giulia Galli The efficiency of nanoparticle (NP) solar cells may substantially exceed the Shockley-Queisser limit by exploiting multi-exciton generation. However, (i) quantum confinement tends to increase the electronic gap and thus the MEG threshold beyond the solar spectrum and (ii) charge extraction through NP networks may be hindered by facile recombination. Using \emph{ab initio} calculations we found that (i) Ge NPs with exotic core structures such as BC8 exhibit significantly lower gaps and MEG thresholds than particles with diamond cores, and an order of magnitude higher MEG rates. (ii) We also investigated Si NPs embedded in a ZnS host matrix and observed complementary charge transport networks, where electron transport occurs by hopping between NPs and hole transport through the ZnS-matrix. Such complementary pathways may substantially reduce recombination, as was indeed observed in recent experiments. We employed several levels of theory, including DFT with hybrid functionals and GW calculations. [Preview Abstract] |
Wednesday, March 5, 2014 12:15PM - 12:27PM |
M24.00004: InAs quantum wells with AlAs$_{0.16}$Sb$_{0.84}$ barriers and GaAs$_{0.09}$Sb$_{0.91}$ absorber for hot carrier and multicarrier generation solar cell Vincent R. Whiteside, Sangeetha Vijeyaragunathan, Tetsuya D. Mishima, Michael B. Santos, Ian R. Sellers, Tobias Zederbauer, Gottfried Strasser We present an investigation of a series of InAs/AlAs$_{0.16}$Sb$_{0.84}$ superlattice structures tuned to 0.7 eV to facilitate the study of carrier multiplication and hot carrier effects in the narrow gap material. The alloy composition of the barrier materials is designed such that photons of over three times the well energy gap are absorbed in the InAs wells. Three distinct structures are studied: 1) a superlattice composed of multiple wells and barrier material, 2) a hybrid structure composed of a GaAs$_{0.09}$Sb$_{0.91}$ bulk absorber with a superlattice structure, and 3) a bulk heterostructure of GaAs$_{0.09}$Sb$_{0.91}$ with an energy matched to the superlattice, as reference. Power and temperature dependent photoluminescence measurements will be presented to describe the relative (hot) carrier temperatures, and their potential for next generation solar cells. With this in mind, the performance of solar cells based on these designs will also be presented. [Preview Abstract] |
Wednesday, March 5, 2014 12:27PM - 12:39PM |
M24.00005: InAs quantum dots in a GaAs$_{1-x}$Sb$_{x}$ matrix for intermediate band solar cell Yang Cheng, Mukul Debnath, Vincent R. Whiteside, Tetsuya Mishima, Michael B. Santos, Ian R. Sellers, Lucas Phinney, Khalid Hossain Self-assembled InAs quantum dots (QDs) were grown by the migration-enhanced epitaxy (MEE) technique in a GaAs$_{1-x}$Sb$_{x}$ matrix material on a GaAs substrate for application as intermediate band single junction solar cells. Initially, a series of InAs QDs structures were studied with a nominal deposition of 1.75 -- 3.5 ML and Sb concentration of $x = 0.13$. The areal density measured by atomic force microscopy was observed to increase with total deposition to a maximum of $\sim$ 4.0x10$^{11}$/cm$^{2}$ after $\sim$ 3 MLs. A high QD density is required to facilitate the formation of an intermediate band (IB) within the band gap of the matrix material. With increasing QD density a simultaneous increase in the optical emission is also observed. The promise in this system is the potential to form a degenerate valence band offset, while forming an IB in the conduction band. As such, a second series of QDs was investigated in which the concentration of Sb in the matrix varied from $x = 0.10$ to $x = 0.18$. The transition from type-I band alignment to type-II is observed. Temperature and power dependent photoluminescence, along with 8 band $k \cdot p$ calculations of the band structure will also be presented. [Preview Abstract] |
Wednesday, March 5, 2014 12:39PM - 12:51PM |
M24.00006: Asymmetric tunneling rates for electrons and holes at CdSe quantum dot/carbon nanotube interfaces Sohrab Ismail-Beigi, Jie Jiang Decorating carbon nanotubes with CdSe quantum dots (QDs) is one potential approach for creating high efficiency photovoltaics. Our collaborators at Yale recently produced a ligand-free covalent attachment of CdSe QDs to carbon nanotubes through an organic ligand exchange mechanism. Our prior first principles work described the energetics of the various binding processes and rationalized the experimental growth methodology. After a brief review of the system, we will describe our intriguing finding that excited electrons and holes tunnel with different rates out of the QD and into the carbon nanotubes. The asymmetric tunneling rate itself can, in principle, boost the separation of photo-excited charge at the interface even if there are insufficient band energy differences across the interface. We describe our results for the tunneling rates computed using (i) a brute force approach with increasing simulation cell size to remove periodic effects, and (ii) a Green's function method that directly connects the QD to a thermodynamically large electron reservoir (e.g., a very long pristine nanotube). [Preview Abstract] |
Wednesday, March 5, 2014 12:51PM - 1:03PM |
M24.00007: Quantum Dot TiO$_2$-Ge Solar Cells Carena Church, Elayaraja Muthuswamy, Susan Kauzlarich, Sue Carter Colloidal germanium (Ge) quantum dots (CQDs) are attractive solar materials due to their low toxicity compared to Pb- or Cd- based nanocrystals (NC), low cost, and optimal, tunable bandgap for both increased IR response and potential power conversion efficiency ($\eta$) boosts from Multiple Exciton Generation (MEG). We report on the successful fabrication and characterization of spun-cast donor/acceptor type TiO$_2$-Ge CQD solar cells utilizing Ge colloidal quantum dots (CQD) synthesized via a facile microwave method as the active layer. We find that our Ge QD size performance-related trends are similar to other QD systems studied. Additionally, our best heterojunction devices achieved short circuit currents (J$_{SC}$) of 450 $\mu$A and open circuit voltages (V$_{OC}$) of 0.335 V, resulting in $\eta$ = 0.022$\%$. While this represents significant increases over previous Ge CQD PV (85$\%$ over hybrid Ge-P3HT PV, $\>$ 350$\%$ over Ge NC PV), our photocurrents are still much lower than other NC systems. Analysis of intensity-dependent J-V characteristics reveal that our currents are limited by a space-charge region that forms leading to unbalanced charge extraction. We conclude by discussing a variety of film treatments and device structures we have tested to increase J$_{SC}$. [Preview Abstract] |
Wednesday, March 5, 2014 1:03PM - 1:15PM |
M24.00008: Investigation on Photoelectric Behavior of Metal-Insulator-Semiconductor Structure Based on Titania Nanotubes Arrays Lili Wang, Eugen Panaitescu, Christiaan Richter, Latika Menon Titanium dioxide (TiO$_{2})$ has attracted great interest as an inexpensive, earth-abundant and environment-friendly anode material for next generation photovoltaic devices and the metal-insulator-semiconductor (MIS) concept is one of the most promising approaches for improving solar cell cost effectiveness (in {\$}/W). We investigated hybrid MIS structures of semiconducting ordered titania nanotube arrays integrated with insulating iron oxide or copper oxide layers and metallic copper. The morphological and structural properties of the samples were analyzed by scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy with elemental mapping, and X-ray diffraction. The nanotubular morphology represents a step change from the current thin film approach, providing significantly larger surface area while facilitating the charge separation and electron transport. Photoelectric behavior of the new structures was estimated by transient response, quantum efficiency and spectral response, and a solar simulator was used for recording the photovoltaic response. [Preview Abstract] |
Wednesday, March 5, 2014 1:15PM - 1:27PM |
M24.00009: Size Effects in Dye-Sensitized TiO$_{2}$ Clusters Noa Marom, Thomas K\"{o}rzd\"{o}rfer, Xinguo Ren, Alexandre Tkatchenko, James Chelikowsky The development of solar cells is driven by the need for clean and sustainable energy. Organic and dye sensitized cells are considered as promising technologies, particularly for large area, low cost applications. However, the efficiency of such cells is still far from the theoretical limit. \textit{Ab initio} simulations may be used for computer-aided design of new materials and nano-structures for more efficient solar cells. It is essential to obtain an accurate description of the electronic structure, including the fundamental gaps and energy level alignment at the interfaces in the device active region. This requires going beyond ground-state DFT to the GW approximation. A recently developed GW method [PRB 86, 041110R (2012)] is applied to dye-sensitized TiO$_{2}$ clusters [PRB 84, 245115 (2011)]. The effect of cluster size on the energy level alignment at the dye-TiO$_{2}$ interface is discussed. With the increase in the TiO$_{2}$ cluster size its gap narrows. The gap of the molecule attached to the cluster subsequently narrows due to screening. As a result, the energy level alignment at the interface changes in an unexpected way [Marom, K\"{o}rzd\"{o}rfer, Ren, Tkatchenko, Chelikowsky, \textit{to be published}]. [Preview Abstract] |
Wednesday, March 5, 2014 1:27PM - 1:39PM |
M24.00010: An integrated approach to realizing high-performance liquid-junction quantum dot sensitized solar cells Hunter McDaniel, Nobuhiro Fuke, Nikolay Makarov, Jeffrey Pietryga, Victor Klimov Solution processed semiconductor quantum dot solar cells offer a path towards both reduced fabrication cost and higher efficiency enabled by novel processes such as hot-electron extraction and carrier multiplication. Here we use a new class of low-cost, low-toxicity CuInSe$_{x}$S$_{\mathrm{2-x}}$ quantum dots to demonstrate sensitized solar cells with certified efficiencies exceeding 5{\%}. Among other material and device design improvements studied, use of a methanol-based polysulfide electrolyte results in a particularly dramatic enhancement in photocurrent and reduced series resistance. Despite the high vapor pressure of methanol, the solar cells are stable for months under ambient conditions, which is much longer than any previously reported quantum dot sensitized solar cell. A study of electron transfer QD/TiO2 interface reveals the process to be surprisingly slow and confirms that methanol does not act as a sacrificial donor. This study demonstrates the large potential of CuInSe$_{x}$S$_{\mathrm{2-x}}$ quantum dots as active materials for the realization of low-cost, robust, and efficient photovoltaics as well as a platform for investigating various advanced concepts derived from the unique physics of the nanoscale size regime. This work was just accepted to Nature Communications. [Preview Abstract] |
Wednesday, March 5, 2014 1:39PM - 1:51PM |
M24.00011: Transient lateral photovoltaic effect in patterned metal-oxide-semiconductor films. Juan Pedro Cascales, Isidoro Martinez, David Diaz, Jose Rodrigo, Arkadi Levanyuk, Chris van Haesendonk, Farkhad Aliev Time dependent transient lateral photovoltaic effect (T-LPE) has been studied in lithographically patterned thin Co films grown over naturally passivated p-type Si (100) substrates. Investigation has been done at room temperature in 21 nm thick, 5 and 10 microns wide and 700 microns long Co films as a function of the position of the laser focused spot with respect to the contacts, pulse frequency (in kHz range) and up to few mW (at wavelength 405 nm or 487 nm) laser power with the spot diameter ranging between 1 and 10 microns. The observed abrupt (faster than in 5 microsecond) change in sign of the T-LPE after the laser is switched off was qualitatively explained by the model which considers redistribution of the life time of non-equilibrium carriers in the electric field due to charged local centres formed during the previous illumination. Exponential relaxation in the inverted T-LPE allows the characterization of the relaxation process as a function of the spot position with respect to the contacts. Numerical simulations satisfactory reproduce the observed unusual time dependence of the T-LPE. [Preview Abstract] |
Wednesday, March 5, 2014 1:51PM - 2:03PM |
M24.00012: Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber Samuel Stranks, Giles Eperon, Giulia Grancini, Christopher Menelaou, Marcelo Alcocer, Tomas Leijtens, Laura Herz, Annamaria Petrozza, Henry Snaith Organic-inorganic perovskites have shown promise as high-performance absorbers in solar cells, first as a coating on a mesoporous metal oxide scaffold and more recently as a solid layer in planar heterojunction architectures. Here, we report transient absorption and photoluminescence-quenching measurements to determine the electron-hole diffusion lengths, diffusion constants, and lifetimes in mixed halide (CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3-x}}$Cl$_{\mathrm{x}})$ and triiodide (CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}})$ perovskite absorbers. We found that the diffusion lengths are greater than 1 micrometer in the mixed halide perovskite, which is an order of magnitude greater than the absorption depth. In contrast, the triiodide absorber has electron-hole diffusion lengths of order 100 nanometers. Finally, we fabricated solution-processed thin-film planar heterojunction devices, achieving power conversion efficiencies of over 12{\%} using the mixed halide absorber but only 4{\%} with the triiodide perovskite. Our results show that the long diffusion lengths justify the high efficiency of planar heterojunction perovskite solar cells, and identify a critical parameter to optimize for future perovskite absorber development. [Preview Abstract] |
Wednesday, March 5, 2014 2:03PM - 2:15PM |
M24.00013: The role of the methylammonium cation in the structural and electronic properties of 3D organic-inorganic perovskite halides: a DFT analysis including Spin Orbit Coupling Giacomo Giorgi, Jun-Ichi Fujisawa, Hiroshi Segawa, Koichi Yamashita Many papers have been recently published reporting the enhanced photoconversion efficiency (PCE) up to 15{\%} [Nature 501, 395 (2013)] for organic-inorganic solar cells containing sandwiches of perovskite compounds (the light harvester), mesoporous TiO$_{2}$, and a polymeric hole conductor. The usage of these 3D MAPbX$_{3}$ (MA$=$CH$_{3}$NH$_{3}^{+}$;X$=$Cl$^{-}$,Br,I$^{-})$ perovskites, stems by their chemical stability and good transport characteristics in the device. Anyway, these materials with so high applicability in PV and with many undisclosed features still find scarce attention in the theoretical community. Here, two aspects stimulated our work: the Spin Orbit Coupling (SOC) impact previously always speculated but ignored in predicting the electronic properties of these compounds, and the overlooked role played by the organic part. We focused on the electronic properties of MAPbI$_{3}$, on the impact played by SOC, on how hybrid functionals improve the bandgap prediction, and on the role ascribed to MA cation. [Preview Abstract] |
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