Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session D25: Focus Session: OPV/DSSC Photophysics and Interfaces |
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Sponsoring Units: GERA Chair: Sean Shaheen, University of Colorado at Boulder Room: 503 |
Monday, March 3, 2014 2:30PM - 3:06PM |
D25.00001: Uncovering location-specific ultrafast exciton dynamics in organic semiconducting thin films Invited Speaker: Naomi Ginsberg In solid state semiconducting molecular materials used in electro-optical applications, relatively long exciton diffusion lengths hold the promise to boost device performance by relaxing proximity constraints on the locations for light absorption and interfacial charge separation. The architecture of such materials determines their optical and electronic properties as a result of spacing- and orientation-dependent Coulomb couplings between adjacent molecules. Exciton character and dynamics are generally inferred from bulk optical measurements, which can present a severe limitation on our understanding of these films because their constituent molecules are neither perfectly ordered nor perfectly disordered. Nevertheless, such microstructure can have profound impacts on transport properties. The ultrafast spectroscopy of single domains of polycrystalline films of TIPS-pentacene, a small-molecule organic semiconductor of interest in electronic and photovoltaic applications, is investigated using transient absorption microscopy. Individual domains are distinguished by their different polarization-dependent linear and nonlinear optical responses. As compared to bulk measurements, we show that the nonlinear response within a given domain can be tied more concretely to specific physical processes that transfer exciton populations between specified electronic states. By use of this approach and a simple kinetic model, the signatures of singlet fission as well as vibrational relaxation of the initially excited singlet state are identified. As such, observing exciton dynamics within and comparing exciton dynamics between different TIPS-pentacene domains reveal the relationship between photophysics and film morphology and the potential to resolve unique signatures at interfaces between different regions of the film. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D25.00002: Geminate and non-geminate recombination of triplet excitons formed by singlet fission Sam L. Bayliss, Alexei Chepelianskii, Alessandro Sepe, Bruno Ehrler, Brian J. Walker, Matt J. Bruzek, John E. Anthony, Neil C. Greenham Singlet fission is a promising route to enhance solar cells by harvesting two electron-hole pairs from high-energy photons. Through singlet fission, an optically generated singlet exciton is transformed into two spin-correlated triplet excitons, which serve as a unique signature of the process. We use optically detected magnetic resonance to identify and study triplet excitons created through singlet fission in the solution-processable small molecule TIPS-tetracene (bis(triisopropylsilylethynyl)tetracene). Through changes in photoluminescence under spin resonance, we identify geminate recombination of triplet pairs directly following singlet fission, as well as recombination from bimolecular triplet-triplet annihilation. We show that both processes can be present in spin-coated films, and correlate the two distinct annihilation pathways to film morphology. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D25.00003: Understanding Singlet and Triplet Excitons in Acene Crystals from First Principles Tonatiuh Rangel Gordillo, Sahar Sharifzadeh, Leeor Kronik, Jeffrey Neaton Singlet fission, a process in which two triplet excitons are formed from a singlet exciton, has the potential to increase the solar cell efficiencies above 100\%. Efficient singlet fission has been reported in larger acene crystals, such as tetracene and pentacene, in part attributable to their low-lying triplet energies. In this work, we use many-body perturbation theory within the GW approximation and the Bethe-Salpeter equation approach to compute quasiparticle gaps, low-lying singlet and and triplet excitations, and optical absorption spectra across the entire acene family of crystals, from benzene to hexacene. We closely examine the degree of localization and charge-transfer character of the low-lying singlets and triplets, and their sensitivity to crystal environment, and discuss implications for the efficiency of singlet fission in this systems. This work supported by DOE and computational resources provided by NERSC. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D25.00004: Controllable thin film crystal growth of a novel squaraine molecule in organic solar cells Brad Conrad, Susan Spencer, Cortney Bougher, Jesse Brown, Kyle Kelley, Patrick Heaphy, Victor Murcia, Cameron Gallivan, Amber Monfette, John Andersen, Jeremy Cody, Tonya Coffey, Christopher Collison We will discuss the formation, structures, and properties of squarine and squarine-PCBM blend thin-films using Atomic Force Microscopy, electrical characterization, UV-VIS-NIR, and Thin-film Xray Diffraction. Film properties are inferred from spectroscopic measurements and are correlated with crystallinity as determined by TFXRD and AFM. A comprehensive explanation of DiPSQ(OH)2 structures is provided and related to measured efficiencies up to 4.3. By controlling the blend ratio and other fabrication conditions, crystalline regions of higher mobility can be developed so as to make significant gains in power conversion efficiency, necessary to achieve long term goals for commercially viable NIR-active OPV devices. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D25.00005: Crystal fields of porphyrins and phthalocyanines P.S. Johnson, I. Boukahil, F.J. Himpsel, C. Kennedy, N. Jersett, P.L. Cook, J.M. Garcia-Lastra Polarization-dependent X-ray absorption spectroscopy at the N 1s and metal 2p edges is combined with density functional and atomic multiplet calculations to determine the crystal field parameters 10Dq, Ds, and Dt of transition metal (Mn, Fe, Co, Ni) phthalocyanines and octaethylporphyrins [1]. Octaethyl porphyrins are observed to lie flat on Si with native oxide, while phthalocyanines lie on edge. Strong polarization dependence is found at all edges, which facilitates a unique determination of the crystal field parameters. Crystal field values from PBE density functional calculations provide helpful starting values, which are refined by fitting atomic multiplet calculations to the data. Since the crystal field affects electron-hole separation in solar cells, the systematic set of crystal field parameters obtained here can be useful for optimizing dyes for solar cells.\\[4pt] [1] P. S. Johnson, et al., J. Chem. Phys., to be submitted. See also: P. L. Cook, et al., J. Chem. Phys. 131, 194701 (2009); D. F. Pickup, et al., J. Phys. Chem. C. 117, 4410 (2013). [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D25.00006: Improving Band Line-up: DFT study of interface effects Michelle Tomasik, David Strubbe, Alexie Kolpak, Jeffrey Grossman Solar cells, organic light emitting diodes, and other devices that involve organic molecules require metal contacts to either extract or supply electricity. Unfortunately standard band-line-up diagrams fail to include important interface effects. Using density functional theory (DFT), we studied metal/organic interfaces to probe the different interface effects, including image charge interactions and dipoles arising from various sources at the interface, and predict how they will affect the line-up of the energy levels. Specifically we have looked at two very different organics, Alq3 and anthracene on different metals. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D25.00007: Enhanced stability of ZnO-based inverted organic photovoltaic devices by phosphonic acid modification Bradley MacLeod, Bertrand Tremolet de Villers, Sarah Cowan, Erin Ratcliff, Dana Olson Solution-processed ZnO thin films are now commonly used as $n$-type bottom contacts in inverted-geometry organic photovoltaics (OPVs). The use of ZnO eliminates the need for highly-reactive top-contact (air-interface) electrode material, such as calcium and aluminum which are commonly used in conventional geometries, which enables operational lifetimes of unencapsulated devices to shift from minutes or hours to days. Modification of the ZnO film by self-assembled monolayers (SAMs) has been shown to enhance performance as well as air-stability during storage. We modify ZnO with dipolar phosphonic acids and observe enhanced performance and stability. We show for the first time devices measured under continuous illumination at one-sun intensity which have significantly enhanced stability when utilizing SAM-modified ZnO. These continuous-illumination stability measurements allow us to investigate the degradation mechanisms of these more stable inverted OPV devices. This work was was supported by of the Center for Interface Science: Solar Electric Materials (CISSEM), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001084. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D25.00008: Influence of the hole-collecting interlayer on the stability and lifetime of inverted organic solar cells Bertrand Tremolet de Villers, Bradley MacLeod, Dana Olson In organic photovoltaics (OPVs), interlayers between the photoactive layer and the electrodes are often used to modify the work-function of the electrode, provide charge-blocking selectivity, and improve the lifetime of the solar cell. To date, PEDOT:PSS has been the most commonly-used interlayer; however, due to its acidic and hygroscopic nature, it can facilitate degradation. To improve the stability of the device, molybdenum oxide (MoO$_3$) has emerged as an attractive alternative to PEDOT:PSS, and solar cells utilizing MoO$_3$ have shown significantly enhanced lifetimes. Furthermore, degradation of low work-function cathode metals such as calcium can be eliminated when the typical cell design is inverted. In inverted solar cells, interlayers remain a critical component but we find their role in the degradation of the OPV changes. Contrary to what is observed in a conventional-architecture OPV, degradation studies of inverted solar cells under constant illumination lasting $>$1000 hours reveal solar cells utilizing a MoO$_3$ interlayer degrade faster than those with PEDOT:PSS. Understanding the influence of the charge-collecting interfaces in OPVs provides a pathway to increased reproducibility and longer lifetimes. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D25.00009: Effect of conjugation linkage in molecular dipolar phosphonic acids for modification of zinc oxide in organic photovoltaics Jennifer Braid, Sarah Cowan, Brad MacLeod, Unsal Koldemir, Alan Sellinger, Reuben Collins, Tom Furtak, Dana Olson Self assembled monolayers of small molecules have become popular to modify the physical and electronic properties of metal oxide and conductive contacts for use in organic and hybrid electronics. Here, we discuss the application of phosphonic acid interface modifiers with strong molecular dipole moments that are able to effectively tune the work function of zinc oxide. This effect is exploited in organic photovoltaic devices, where decreasing the work function of the electron transport layer relative to the LUMO level of the active layer electron acceptor translates to an increased open circuit voltage. However, the effect of the molecular dipole moment is only part of the story, as we have found that bond type within the phosphonic acid also plays a role in the work function shift. Zinc oxide modified with 2,6-difluorophenylvinylphosphonic acid and its nonconjugated counterparts was studied via Kelvin probe and IR spectroscopy, as well as in P3HT:ICBA inverted devices, revealing that the conjugated double bond dramatically accentuates the aforementioned change in work function, resulting in improved device performance relative to the unconjugated modified or the un-modified ZnO contact. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D25.00010: Increasing The Work Function of NiO$_{\mathrm{x}}$ Hole Transport Layer Using Triethoxysilane-Based Monolayers Gang Chen, Thomas Brenner, Thomas Furtak, Reuben Collins, Sarah Cowan, Dana Olson Nickel Oxide (NiO$_{\mathrm{x}})$ is an effective hole transport layer in organic solar cells. $^{\mathrm{\thinspace }}$However, the NiO$_{\mathrm{x}}$ /organic interfacial energy level alignment needs to be optimized. Unlike the commonly used O2 plasma treatment, molecular monolayer modification can provide a more stable and controlled work function change for tuning the interface by introducing dipoles that form a molecular layer.$^{\mathrm{\thinspace }}$Previous work has shown the triethoxysilane (TES) chemistry bonds covalently to Zinc Oxide and can effectively tune the work function. In this study, the TES chemistry is transferred to NiO$_{\mathrm{x}}$ in order to tune the energy level alignment at the NiO$_{\mathrm{x}}$ /organic interface using three different TES modifiers. Contact angle (CA) measurements show that TES treated surfaces are much more hydrophobic than the untreated surface, which indicates the successful attachment of these molecules. Infrared spectroscopy shows that the coverage is sub-monolayer, consistent with our previous studies of other metal oxide surfaces. Kelvin probe measurements show that the TES treatment increases the NiO$_{\mathrm{x}}$ work function by as much as 450 meV compared to untreated NiO$_{\mathrm{x}}$. Standard bulk heterojunction devices were fabricated and we find that the open circuit voltage improves with increasing work function of the TES-treated surfaces. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D25.00011: X-ray Characterization of Dye Adsorption in Coadsorbed Dye-Sensitized Solar Cells Mitsunori Honda, Masatoshi Yanagida, Liyuan Han, Kenjiro Miyano We performed X-ray measurements to elucidate the adsorption mode of N719 dye molecules on nanoporous TiO2 with and without coadsorption of D131 dye. Two techniques, X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy, were employed in order to obtain depth profile information about the substrate. In both cases, we found that the isothiocyanate groups of N719 strongly interact with TiO2 via S atoms when the dye is adsorbed from a single-component solution. In contrast, S-substrate interaction is strongly suppressed when D131 is coadsorbed with N719, indicating that the presence of D131 changes the adsorption mode of N719. Based on this finding, we designed a procedure to promote the preferential adsorption of D131, by which we successfully improved the short-circuit current and conversion efficiency. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D25.00012: Spectroscopy of donor-pi-acceptor complexes for solar cells F.J. Himpsel, I. Zegkinoglou, P.S. Johnson, C.D. Pemmaraju, D. Prendergast, M.-E. Ragoussi, G. de la Torre, D.F. Pickup, J.E. Ortega A recent improvement in the design of dye sensitized solar cells has been the combination of light absorbing, electron-donating, and electron-withdrawing groups within the same sensitizer molecule. This dye architecture has contributed to increase the energy conversion efficiency, leading to record efficiency values. Here we investigate a zinc(II)-porphyrin-based complex with triphenylamine donor groups and carboxyl linkers for the attachment to an oxide acceptor. The unoccupied orbitals of these three moieties are probed by element-selective X-ray absorption spectroscopy at the N 1s, C 1s, and Zn 2p edges, complemented by time-dependent density functional theory [1,2]. The attachment of electron-donating groups to the porphyrin ring significantly delocalizes the highest occupied molecular orbital (HOMO) of the molecule. This leads to a spatial separation between the HOMO and the lowest unoccupied molecular orbital (LUMO), reducing the recombination rate of photoinduced electrons and holes.\\[4pt] [1] P. L. Cook, et al., J. Chem. Phys. 134, 204707 (2011).\\[0pt] [2] I. Zegkinoglou, et al., J. Phys. Chem. C 117, 13357 (2013). [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D25.00013: Carboxylic Acid Modification of Etch-Resistant Zn$_{\mathrm{1-x}}$Mg$_{\mathrm{x}}$O for Interface Tuning and Dye Sensitization Thomas Brenner, Erich Meinig, Gang Chen, Thomas Furtak, Reuben Collins, Thomas Flores, Dana Olson The bonding of carboxylic acids to metal oxide surfaces is an important monolayer modification scheme for tuning the properties of these surfaces in organic electronic devices and dye sensitized solar cells (DSSCs). However, the commonly used transparent semiconductor ZnO is very sensitive to acids and employment of carboxylic acids on its surface leads to etching, resulting in a non-ideal layer. This is especially troublesome in ZnO-based DSSCs where the products of etching accumulate on the surface and act as 'photon parasites', reducing device efficiency. We have found that, while the electronic properties are similar, the etch rate of Zn$_{\mathrm{1-x}}$Mg$_{\mathrm{x}}$O (ZnMgO) alloys decreases with Mg content and is up to 10 times smaller (at x$=$0.2) than that of ZnO when exposed to the modifier benzoic acid (BA). IR spectroscopy shows that BA forms a surface-bonded monolayer on ZnMgO after which etch products begin to accumulate on low (x$=$0-0.1) Mg content films. We suggest that ZnMgO may make a good replacement for ZnO where carboxylic acid modifiers are commonly used. In DSSCs we expect the etch resistance of ZnMgO to reduce the accumulation of 'photon parasites.' UV-Vis and photoluminescence measurements of dye-soaked ZnMgO show that the accumulation rate is reduced compared to ZnO. [Preview Abstract] |
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