Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session N38: Novel Photophysics and Transport in NanoPV I |
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Sponsoring Units: GERA DPOLY DCOMP Chair: Sean Shaheen, University of Denver Room: 347 |
Wednesday, March 20, 2013 11:15AM - 11:51AM |
N38.00001: Tuning charge transport in organic devices: From in silico to carbon to device Invited Speaker: Alan Aspuru-Guzik I will describe our work towards first-principles design of organic semiconducting materials. In particular, I will describe our efforts towards the rational design of high hole-mobility organic crystals. I will describe a case study where the in silico prediction of a material led to the synthesis and characterization of it by the Bao and Toney groups. I will also discuss other related research directions. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N38.00002: Conformational Disorder in Energy Transfer: Beyond Forster Theory Tammie Nelson, Sebastian Fernandez-Alberti, Adrian Roitberg, Sergei Tretiak Energy transfer in donor/acceptor chromophore pairs, where the absorption of each species is well separated, can be understood through a F\"{o}rster resonance energy transfer model. The picture is more complex for organic conjugated polymers, where the total absorption spectrum can be described as a sum of the individual contributions from each subunit, whose absorption is not well separated. Although excitations in these systems tend to be well localized, traditional {\it donors} and {\it acceptors} cannot be defined and energy transfer can occur through various pathways. In addition, fast torsional motions between individual monomers can break conjugation and lead to reordering of excited state energy levels. Energy transfer in these systems can be induced by both electronic transitions and by fast torsional fluctuations where both mechanisms occur simultaneously. We use non-adiabatic excited state molecular dynamics (NA-ESMD) to simulate energy transfer between two poly-phenylene vinylene (PPV) segments composed of 3-rings and 4-rings separated by varying distances. The transition density decay represents the transfer rate, and can be decomposed into contributions from various transfer pathways. [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N38.00003: Phonon-assisted nonradiative energy transfer in quantum dot-silicon nanostructures Pedro Ludwig Hernandez Martinez, Aydan Yeltik, Burak Guzelturk, Alexander O. Govorov, Hilmi Volkan Demir Silicon is one of the most dominant materials in photovoltaics and understanding the processes of energy transfer is of great importance. In this work, we study the phonon-assisted nonradiative energy transfer (NRET) in quantum dot (QD)-silicon hybrid nanostructures. Here, the NRET dynamics is investigated as a function of temperature for distinct separation thicknesses between the donor QDs and the acceptor silicon plane. We propose a theoretical model based on the phonon-assisted energy transfer process. We estimate the energy transfer rate using the Fermi's Golden rule where the matrix elements are derived for the phonon-assisted energy transfer process. To support our findings the temperature-dependent fluorescence lifetimes in QD-silicon nanostructures are analyzed. The experimental data analyses agree with the resulting theoretical model. The results indicate that phonons play an important role in NRET to Si as an indirect bandgap semiconductor. [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N38.00004: Directed Energy Transfer through Size-Gradient Nanocrystal Layers into Si Substrates Michael Nimmo, Louis Caillard, Will deBenedetti, Hue Nguyen, Yves Chabal, Yuri Gartstein, Anton Malko Nanostructured materials attract great interest as candidates for next generation of photoelectronic devices. Presently, the majority of hybrid devices are based on charge transfer in which exciton break-up occurs at the interface between dissimilar materials. Poor interface quality and carrier transport are issues that result in a conversion efficiencies lower than in the inorganic crystalline devices. An alternative approach is based on hybrid structures, which combine strongly absorbing components such as nanocrystal quantum dots (NQDs) and adjacent high-mobility semiconductor layers coupled via proximal energy transfer. Building on our previous work,\footnote{H. M. Nguyen et al., \textit{APL} \textbf{98}, 161904 (2011)} we examine non-radiative energy transfer (NRET) between NQDs grafted on a hydrogenated Si surface via amine modified carboxy-alkyl chain linkers. A macroscopically thick, size--gradient NQD film is prepared on top of crystalline Si layer to explore directed energy tranfer into the substrate. Steady-state and time-resolved photoluminescence studies show effective energy transfer between adjacent layers and into the Si substrate with the transfer efficiency exceeding 90{\%} among layers. This demonstrates the viability of NQD-Si hybrid structures for photovoltaic devices. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 1:03PM |
N38.00005: Quantum coherence and noise in open quantum systems Invited Speaker: Ahsan Nazir Recent experiments demonstrating signatures of quantum coherence in the excitonic energy transfer dynamics of a variety of systems have sparked renewed interest in the theoretical modelling of energy transfer processes within a dissipative environment. A major challenge remains the development of techniques that allow one to probe the diverse parameter regimes relevant to such systems. Master equation methods provide useful tools with which to efficiently analyse energy transfer dynamics in open quantum systems. However, they are often valid only in rather restrictive parameter regimes, limiting their applicability in the present context. Here, I shall present a versatile variational master equation approach to the non-equilibrium dynamics of dissipative excitonic quantum systems, which allows for the exploration of a wide range of parameter regimes within a single formalism. Derived through the combination of a variationally optimised unitary transformation and the time-local projection operator technique, the master equation can be applied to a range of bath spectral densities, temperatures, and system-bath coupling strengths, and accounts for both non-Markovian and non-equilibrium environmental effects. Applying the formalism in the case of excitonic energy transfer, I shall show that while it correctly reproduces Redfield, polaron, and Foerster dynamics in the appropriate limits, it can also be used in intermediate regimes where none of these theories may be applicable. I shall also discuss the extension of the theory to many-site energy transfer systems Variational master equations thus represent a promising avenue for the exploration of (essentially non-perturbative) dissipative dynamics in a variety of physical systems. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N38.00006: Quantum Relaxation in Singlet Fission Paul Teichen, Joel Eaves Singlet fission is a multielectron process in organic chromophores, where an initially excited singlet state decays into two independent triplets. First observed in organic semiconductors almost 40 years ago, the phenomenon may be a promising route for increasing yields in next-generation photovoltaics. Early theories that ignored quantum coherence between excited states were capable of explaining the fission process on nanosecond timescales, but recent observations of fission on sub picosecond timescales call several tenants of those theories into question. We present a theory of optical dephasing and decoherence in singlet fission, drawing on ideas from quantum information theory to establish conditions for decoherence and disentanglement between the relevant quantum states on the picosecond timescale. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N38.00007: Double Super-Exchange in Silicon Quantum Dots Connected by Short-Bridged Networks Huashan Li, Zhigang Wu, Mark Lusk Silicon quantum dots (QDs) with diameters in the range of 1-2 nm are attractive for photovoltaic applications. They absorb photons more readily, transport excitons with greater efficiency, and show greater promise in multiple-exciton generation and hot carrier collection paradigms. However, their high excitonic binding energy makes it difficult to dissociate excitons into separate charge carriers. One possible remedy is to create dot assemblies in which a second material creates a Type-II heterojunction with the dot so that exciton dissociation occurs locally. This talk will focus on such a Type-II heterojunction paradigm in which QDs are connected via covalently bonded, short-bridge molecules. For such interpenetrating networks of dots and molecules, our first principles computational investigation shows that it is possible to rapidly and efficiently separate electrons to QDs and holes to bridge units. The bridge network serves as an efficient mediator of electron superexchange between QDs while the dots themselves play the complimentary role of efficient hole superexchange mediators. Dissociation, photoluminescence and carrier transport rates will be presented for bridge networks of silicon QDs that exhibit such double superexchange. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N38.00008: Highly Efficient Charge Transfer in Nanocrystalline Si:H Reuben Collins, Matthew Bergren, Brian Simonds, Jeremy Fields, Craig Taylor, Thomas Furtak, Kristin Kiriluk, Guozhen Yue, Baojie Yan, Jeff Yang, Tining Su, Subhendu Guha, Matthew Beard We demonstrate that in films of silicon nanocrystals imbedded in a hydrogenated amorphous silicon matrix, carriers generated in the amorphous region are efficiently transported to the nanocrystals prior to thermalization into band tail states of the amorphous phase. This transfer causes electron paramagnetic resonance and photoluminescence signals from the amorphous phase to be rapidly quenched as the volume fraction of Si nanocrystals exceeds about 30 percent. Ultrafast carrier dynamics, probed using time-resolved terahertz spectroscopy (TRTS), confirm rapid transport between phases before complete relaxation. TRTS results are consistent with a model where electrons excited in the amorphous material are first trapped at interface states at the amorphous/nanocrystal boundary prior to being thermally emitted into the crystalline phase. These results, which indicate nanocrystalline Si:H is effectively a type I bulk heterojunction material, help explain the enhanced photo stability of this material compared to amorphous silicon by itself. They also suggest routes to using similar structures to increase the efficiency of thin film silicon solar cells. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N38.00009: Ultrafast carrier dynamics of CdSe quantum dots prepared by pulse laser deposition for photovoltaic applications Meg Mahat, Baichhabi Yakami, Qilin Qilin Dai, Jinke Tang, Jon Pikal Quantum-dot sensitized solar cells are a promising alternative to existing photovoltaic technology. Over the last decade solution based colloidal quantum dots (QDs) have been extensively studied. Here we have carried out ultrafast transient absorption measurements on CdSe QDs fabricated using pulse laser deposition (PLD) in order to understand the carrier relaxation dynamics in these nanostructures. The differential transmission measurements show that the PLD QDs have a very fast decay process resulting in a recovery time of less than 10 picoseconds. This is in stark contrast to the colloidal QDs that show a decay process of more than 4 nanoseconds. We also find that the fast decay process observed in the PLD QDs is a function of the carriers density generated in CdSe QDs. To understand these carrier relaxation processes and improve the optical properties of the QDs we perform transient absorption measurements on PLD QDs prepared in different media (e.g. water, methanol, ethanol), under different growth conditions, and with and without ligand. We present a comparison study of the carrier relaxation dynamics in these PLD grown QDs to provide insight into the competing relaxation effects and guide their use in Quantum-dot sensitized solar cells. [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N38.00010: Spin-Dependent Light-Harvesting in Colloidal Nanocrystals by Controlling Electronic Trap States with Optically Detected Magnetic Resonance K.J. van Schooten, J. Huang, D.V. Talapin, C. Boehme, J.M. Lupton Colloidal synthesis of semiconductor nanocrystals offers high levels of control over both particle size and geometry, leading to the development of novel optoelectronic device architectures (e.g. CdSe/CdS tetrapods). Unfortunately, realization of such devices is forestalled due to the ubiquitous existence of energetic ``trap'' states which compete with quantum-confined band-edge excitonic states and drive down device efficiencies. Although the existence of such states is readily confirmed via observation of single particle photoluminescence blinking and delayed photoluminescence decay dynamics, little detail is actually known as to the characteristics of these trap states due to difficulties in directly accessing them experimentally. We use pulsed optically detected magnetic resonance spectroscopy in order to begin to probe the chemical and electronic nature of these long-lived states, shedding light on their relation to band-edge states. Ultimately, it is found that spin coherence extends up to $T_{2}=328\pm22$~ns at 3.5~K, allowing for the coherent control of light harvesting in heterostructured nano-tetrapods which permits remote readout of spin information. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N38.00011: Charge transfer between a CdSe/CdS quantum rod and an attached ferrocene molecule: a first principle study Kartick Tarafder, Lin-Wang Wang Semiconductor quantum dot (QD) shows interesting opto-electrical properties, very different from bulk semiconductors. However, one major challenge for opto-electrical application is to get the charge carrier out of the QD. One approach is to use an attached molecule to extract the photon generated carrier from the QD. Ferrocene has a potential to change its electron transition level either by adjusting the Ferrocene and Ferrocene$+$ ratio in a solvent, or by adding other functional groups. However, proper understanding of the interactions between QD and molecule is limited, which is extremely useful for further design of such system. One of the main difficult is that there are thousands of atoms contained in the system, a first principle study of which is beyond the limit of existing computational power using direct density functional theory method. In this work we used a novel technique called charge-patching method [1], and combined that with Marcus model to study the electron and hole transfer between ferrocene and CdS/CdSe core-shell quantum dot. This study allows us to gain insights into the molecule dot interactions and underlying photoluminescence quenching process.\\[4pt] [1] L-W Wang, Phys. Rev. B 65, 153410(2002) [Preview Abstract] |
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