Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session L24: Focus Session: NanoPV Novel Photophysics and Transport I |
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Sponsoring Units: GERA Chair: Richard Wiener, Research Corporation for Science Advancement Room: 504 |
Wednesday, March 5, 2014 8:00AM - 8:36AM |
L24.00001: Mapping Energy Flow with Ultrafast Optical Spectroscopy Invited Speaker: Vanessa Huxter Ultrafast electronic spectroscopy connects the spatial, temporal and dynamic landscapes of complex systems. These connections are essential to our understanding of structure-function relationships and energy transport through materials. Using two-dimensional electronic spectroscopy (2DES), we can generate correlation maps that relate the initial absorptive interaction with the signal emission, allowing us to follow the flow of energy through a system via the vibrational and electronic coherences and populations. Using 2DES to study synthetic and natural photosynthetic pigments provides insight into the advantages of particular molecular architectures that are ubiquitous in nature, optimizing efficient energy transfer and demonstrating the importance of static disorder and vibrational coupling. The critical role of vibrations in these pigments is mirrored in the response of the nitrogen vacancy centers in diamond (NV-diamond) quantum material system. 2DES studies of NV-diamond reveal an array of coherent nuclear vibrations coupled to the electronic state. The effect of the vibrational coherences on the dynamics of the NV-diamond system may provide a route to increased efficiency of energy transport in nanostructured solar cells. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L24.00002: Energy level modification in lead sulfide quantum dot photovoltaics through ligand exchange Patrick Brown, Donghun Kim, Richard Lunt, Moungi Bawendi, Jeffrey Grossman, Vladimir Bulovic The electronic properties of lead sulfide colloidal quantum dots (PbS QDs) can be controlled through modification of QD size and surface chemistry. Novel surface passivation techniques involving organic or inorganic ligands have contributed to a rapid rise in the efficiency of QD photovoltaics, yet the influence of ligand-induced surface dipoles on PbS QD energy levels and photovoltaic device operation is not yet completely understood. Here, the valence band energies of PbS QDs treated with twelve different ligands are measured using ultraviolet photoelectron spectroscopy (UPS), and a valence band shift of up to 0.75 eV is observed between different ligand treatments. Atomistic simulations of ligand binding to pristine PbS(100) and PbS(111) slabs qualitatively reproduce the measured energy level shifts. 1,2-benzenedithiol and 1,3-benzendithiol treatments, which result in valence band energies differing by $\sim$ 0.2 eV, are employed for PbS QDs in three different solar cell architectures, and changes in device performance are correlated with the measured energy level shift. These findings complement the known bandgap-tunability of colloidal QDs and highlight an additional level of control over the electronic properties of PbS QDs. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L24.00003: Light Harvesting in Functionalized SiQD Assemblies via Spatially Separated Excitons Huashan Li, Zhigang Wu, Tianlei Zhou, Alan Sellinger, Mark Lusk Silicon quantum dots (SiQDs) with diameters less than 5 nm are particularly attractive for photovoltaic applications [1,2], but their optical gap is too large to match the solar spectrum. Although recent progress on solution processing techniques provides more opportunities for functionalizing SiQD, nontrivial absorption under 3 eV has yet to be achieved [3]. The absorption of photons through the direct generation of spatially separated excitons at dot-ligand interfaces may be a promising strategy for overcoming this challenge. We consider the idea computationally and show that it is indeed possible to capture photons of much lower energy using very small SiQD. The key is to establish a type-II energy level alignment in conjunction with strong electronic coupling between the dot and ligand. Our analysis indicates that conjugated vinyl bonds to common organic ligands satisfy both of these conditions. In principle, this allows the optical gap of SiQD to be tuned to arbitrarily small values independent of their size. For the prototype system of 2.6 nm SiQDs, we predict that triphenylamine (TPA) termination will result in a 0.47 eV redshift of the optical gap along with a boost of absorption intensity near the band edge, a result consistent with our experimental realization of the system. We will also discuss the results of a computational analysis of the robustness of the absorption spectrum against oxidation and extra alkyl ligands within this new paradigm. [1]Lin, Z. et al., ACS Nano 6, 4029, 2012. [2] Li, H. et al., ACS Nano 6, 9690, 2012. [3] Dung, M. X. et al., Chem. Asian J. 8, 653, 2013. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L24.00004: Electronic and Optical Properties of Novel Phases of Silicon and Silicon-Based Derivatives Chin Shen Ong, Sangkook Choi, Steven Louie The vast majority of solar cells in the market today are made from crystalline silicon in the diamond-cubic phase. Nonetheless, diamond-cubic Si has an intrinsic disadvantage: it has an indirect band gap with a large energy difference between the direct gap and the indirect gap. In this work, we perform a careful study of the electronic and optical properties of a newly discovered cubic-Si$_{20} $ phase of Si that is found to sport a direct band gap. In addition, other silicon-based derivatives have also been discovered and found to be thermodynamically metastable. We carry out \textit{ab initio} GW and GW-BSE calculations for the quasiparticle excitations and optical spectra, respectively, of these new phases of silicon and silicon-based derivatives. This work was supported by NSF grant No. DMR10-1006184 and U.S. DOE under Contract No. DE-AC02-05CH11231. Computational resources have been provided by DOE at Lawrence Berkeley National Laboratory's NERSC facility and the NSF through XSEDE resources at NICS. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L24.00005: ``New'' energy states lead to phonon-less optoelectronic properties in nanostructured silicon Vivek Singh, Yixuan Yu, Brian Korgel, Prashant Nagpal Silicon is arguably one of the most important technological material for electronic applications. However, indirect bandgap of silicon semiconductor has prevented optoelectronic applications due to phonon assistance required for photon light absorption/emission. Here we show, that previously unexplored surface states in nanostructured silicon can couple with quantum-confined energy levels, leading to phonon-less exciton-recombination and photoluminescence. We demonstrate size dependence (2.4 - 8.3 nm) of this coupling observed in small uniform silicon nanocrystallites, or quantum-dots, by direct measurements of their electronic density of states and low temperature measurements. To enhance the optical absorption of the these silicon quantum-dots, we utilize generation of resonant surface plasmon polariton waves, which leads to several fold increase in observed spectrally-resolved photocurrent near the quantum-confined bandedge states. Therefore, these enhanced light emission and absorption enhancement can have important implications for applications of nanostructured silicon for optoelectronic applications in photovoltaics and LEDs. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L24.00006: Quantum confined nanocrystalline silicon Tianyuan Guan, Chito Kendrick, San Theingi, Luigi Bagolini, Kory Riskey, Lauren Vitti, Grant Klafehn, Craig Taylor, Mark Lusk, Brain Gorman, Reuben Collins, Jeremy Fields, Pauls Stradins Quantum confined (QC) semiconductors have drawn much attention in photovoltaics due to their tunable optoelectronic properties and potential for efficiency improvements. Here, we report a study of nanocrystalline silicon (nc-Si:H), consisting of silicon nano-particles (SiNPs) embedded in hydrogenated amorphous silicon (a-Si:H) matrix. Films were grown by depositing the SiNPs and a-Si:H sequentially from separate plasma reactors in a common deposition chamber. Several characterizations were used to ensure the material had low defect density and that the SiNPs were highly crystalline and well within the QC regime. Optical properties of hybrid SiNP/a-Si:H films were explored using visible to near infrared photoluminescence (PL). At low temperature, PL revealed two primary emission features, one from conventional a-Si:H $\sim$ 1.3 eV and a second peak which can be attributed to recombination in SiNPs. The energy of this peak is higher than the bulk c-Si bandgap ($\sim$ 1.2 eV), and with decreasing SiNP size, it increases to $\sim$ 1.7 eV. This quantum confinement effect agrees with Density Functional Theory predictions. In addition, we also see that the PL peak for SiNPs surrounded by a-Si:H shifts to lower energy relative to the isolated SiNPs. This shift is also consistent with the modeling results which show that surrounding SiNPs with a-Si:H leads to a softening of the confinement barrier and a redshift in the optical gap. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L24.00007: PECVD Environmental Effects on Silicon Nanoparticle Size and Quality Grant Klafehn, Chito Kendrick, Tianyuan Guan, San Theingi, Kory Riskey, Lauren Vitti, Luigi Bagolini, Mark Lusk, Brian Gorman, Craig Taylor, Reuben Collins, Jeremy Fields, Paul Stradins Silicon based nanoparticles (SiNPs) have recently been of great interest to the PV community because of their unique properties compared to their bulk constituents. By decreasing a nanoparticle's (NP) size below its exciton Bohr radius, its band gap can be increased relative to the bulk. This talk will discuss fundamental variables involved in defining and controlling plasma-grown SiNP size and quality. A quartz tube with a RF electrode ring is used to create a plasma in an argon-silane mixture to grow the SiNPs. Their quality and size can be changed by varying the reactor pressure, gas flow, and thus the resulting residence time. They are then characterized by Raman, PL, ESR, XRD, and TEM, and then mapped to a phase diagram with respect to pressure and flow. Higher residence times of 10 ms resulted in highly crystalline, 7 nm SiNPs. Residence times of 2 ms create 4 nm particles, while below 2 ms will result in highly defective material, even though the PL exhibits peaks at 1.6 eV. These parameters will be discussed, including how each variable affects the resultant SiNP size, quality. Also included will be a discussion about additive gasses and their additional effects on SiNP characteristics. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L24.00008: Study of band-edge optical absorption of silicon nanoparticles using photothermal deflection spectroscopy San Theingi, Chito Kendrick, Tianyuan Guan, Lauren Vitti, Grant Klafehn, Luigi Bagolini, Mark Lusk, Brian Gorman, Paul Stradins, Craig Taylor, Reuben Collins Silicon nanoparticles (SiNPs) are a promising optoelectronic material with unique properties such as a size tunable bandgap, sensitivity to surface termination, and efficient optical emission. Here, we present an optical absorption study of size varied, free standing SiNPs films using photothermal deflection spectroscopy (PDS). In general, it is difficult to directly observe the absorption threshold in SiNPs because of silicon's low absorption coefficient. PDS, which directly measures the optical absorption of materials through the generated heat, is known for its extremely high sensitivity. The SiNPs are grown using a plasma process and deposited as films on quartz substrates. Different amounts of SF$_{6}$ gas are introduced into the process gas to control the size of these SiNPs. Photoluminescence measurements show a strong blue shift in emission with increased SF$_{6}$ flow. PDS measurements allow a corresponding blue shift in the band edge absorption which is attributable to quantum confinement to be observed. In addition, PDS measurements also allow us to probe the defect level of our material, and the size distribution of SiNPs in our sample. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L24.00009: The influence of Auger recombination on the performance of quantum-dot light-emitting diodes Jeffrey Pietryga, Wan Ki Bae, Young-Shin Park, Istvan Robel, Victor Klimov Colloidal quantum dots are the subject of intense research as fluorophores for light-emitting diodes (LEDs) due to properties such as spectrally narrow, tunable emission and facile processibility via solution-based methods. Continued improvement of LEDs based on quantum dots is restricted by an incomplete understanding of the physics underlying current performance limitations. More specifically, little is known about the influence of multi-carrier processes on overall LED efficiency, and on the reduction of efficiency at high currents (known as efficiency roll-off, or droop). Here, we present an investigation of this issue involving studies that correlate the excited state dynamics of structurally engineered quantum dots with their emissive performance within LEDs. We find that because of significant charging of quantum dots with extra electrons, multi-carrier Auger recombination greatly impacts both LED efficiency and the onset of efficiency roll-off at high currents. We conclude by examining two specific approaches for mitigating this problem using heterostructured quantum dots that either suppress Auger recombination, or that directly address the problem of charge-injection imbalance. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L24.00010: Nanoscale engineering of efficient photovoltaic conversion in quantum dot media Sergeev Andrei, Li Yanshu, Vagidov Nizami, Mitin Vladimir, Sablon Kimberly, Oktyabrsky Serge, Yakimov Michael The main problem of photovoltaic nanomaterials for high efficiency conversion is enhanced recombination of photocarriers. Selective doping of quantum dot (QD) media allows for control of three-dimensional potential profile and adds more functionality and scalability to photovoltaic materials and structures. Optimization of the nanoscale barriers and reduction of wetting layer in a QD medium substantially suppress recombination processes and enhance ittersubband transitions, which provide electron extraction from QDs. We report that the optimized 1-$\mu $m InAs/GaAs QD media placed in 3-$\mu $m base GaAs p-n junction increases the short circuit current from 22.0 mA/cm$^{\mathrm{2}}$ to 28 mA/cm$^{\mathrm{2}}$. Spectral analysis of conversion processes shows that the IR sub-bangap photons and hot electrons created by high energy photons provide comparable contributions to photovoltaic conversion via charged QDs. The reduction of the wetting layer, which otherwise accumulates electrons, increases extraction of electrons from QDs due to interaction with hot electrons created by high energy photons. Nanoscale engineering of electron processes by charging of QDs provides wide possibilities for further suppression of recombination and thermalization losses in QD photovoltaic devices. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L24.00011: Exciton shelves for charge and energy transport in third-generation quantum-dot devices Samuel Goodman, Vivek Singh, Hyunwoo Noh, Josep Casamada, Anushree Chatterjee, Jennifer Cha, Prashant Nagpal Quantum dots are semiconductor nanocrystallites with size-dependent quantum-confined energy levels. While they have been intensively investigated to utilize hot-carriers for photovoltaic applications, to bridge the mismatch between incident solar photons and finite bandgap of semiconductor photocells, efficient charge or exciton transport in quantum-dot films has proven challenging. Here we show development of new coupled conjugated molecular wires with ``exciton shelves'', or different energy levels, matched with the multiple energy levels of quantum dots. Using single nanoparticle and ensemble device measurements we show successful extraction and transport of both bandedge and high-energy charge carriers, and energy transport of excitons. We demonstrate using measurements of electronic density of states, that careful matching of energy states of quantum-dot with molecular wires is important, and any mismatch can generate midgap states leading to charge recombination and reduced efficiency. Therefore, these exciton-shelves and quantum dots can lead to development of next-generation photovoltaic and photodetection devices using simultaneous transport of bandedge and hot-carriers or energy transport of excitons in these nanostructured solution-processed films. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L24.00012: Silicon nanowire arrays with passivated axial p-i-n junctions for photovoltaic applications Peng Zhang, Pei Liu, Alexander Zaslavsky, Domenico Pacifici, Jong-Yoon Ha, Sergiy Krylyuk, Albert Davydov Metal catalyst-assisted vapor-liquid-solid mechanism can be used to grow large areas of nanowires (NWs) with compositional and doping control in either axial or core-shell geometries. Here, we report on vertical arrays of Si axial $p$-$i$-$n$ oxide-passivated NWs that were 12 microns long with a 4 micron intrinsic section. The NW arrays were planarized using SU-8 photoresist, followed by reactive ion etching to expose the NW tips. Top $n$-contact was realized by sputter deposition of a 200 nm IZO layer. The $p$-contact was made by backside metallization of the $p$-Si substrate. Under AM 1.5 illumination, unpassivated NW arrays exhibited an open-circuit voltage, $V_{\mathrm{OC}}$ of 170 mV, a short-circuit current density $J_{\mathrm{SC}}$ \textgreater 3.7 mA/cm$^{2}$ (with uncertainty due to the unknown fraction of properly contacted NWs), and a fill factor of 28.9{\%}. After the passivation, $V_{\mathrm{OC}}$, $J_{\mathrm{SC}}$ and FF increased to 250 mV, \textgreater 9.2 mA/cm$^{2}$ and 35.7{\%}, respectively. The measured normal reflectance was around 6{\%} over the 400--1000 nm spectral range, whereas the diffuse reflectance was around 20{\%} over the same range, indicating strong light scattering and absorption by the NWs. The photovoltaic performance of passivated single NWs and NW arrays were compared using a 532 nm laser with a power density of about 10 W/cm$^{\mathrm{2}}$. Higher values of $V_{\mathrm{OC}}$ and FF obtained for the latter are explained by light trapping in the NW arrays. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L24.00013: Multiple Exciton Generation in Silicon QD arrays Andrei Kryjevski, Dmitri Kilin We use Density Functional Theory (DFT) combined with the many body perturbation theory to calculate multiple exciton generation (MEG) in several semiconductor nanosystems. Hydrogen-passivated $Si_{29}H_{36}$ quantum dots (QDs) with crystalline and amorphous core structures, the quasi one dimensional (1-D) arrays constructed from these QDs, as well as crystalline and amorphous Si nanowires have been studied. Quantum efficiency, the average number of excitons created by a single photon, has been calculated in these nanoparticles to the leading order in the screened Coulomb interaction. Amorphous nanostructures are predicted to have more effective carrier multiplication. [Preview Abstract] |
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