Alejandro L. Briseno, University of Massachusetts
Aram Amassian, King Abdullah University of Science and Technology (KAUST)
Iain McCulloch, Imperial College London
Özlem Usluer, Konya Necmettin Erbakan University
ACS Applied Materials amp
Interfaces | American Chemical Society
Aldrich Materials Science
MD5.1: Theory, Computation and Transport
Tuesday PM, March 29, 2016
PCC West, 100 Level, Room 102 AB
2:30 PM - *MD5.1.01
Impact of Polymer/Fullerene Intermolecular Interactions on the Performance of Organic Solar Cells
Jean-Luc Bredas 1
1 KAUST Thuwal Saudi Arabia,Show Abstract
In this presentation, we seek to provide a rationalization of the impact that intermolecular arrangements and interactions at the polymer/fullerene interfaces have on the performance of bulk-heterojunction solar cells. We discuss the results of combined electronic-structure calculations and molecular-dynamics simulations both for representative systems reported in the literature and new systems synthesized in our Center. In particular, we examine:
(i) the propensity of the fullerene molecules to dock preferentially on top of the electron-poor moiety or electron-rich moiety of the polymer, as a function of the nature and location of the polymer side chains; and
(ii) the impact that the packing arrangements have on the energetic distribution of the charge-transfer interfacial electronic states and their localization/delocalization characteristics.
This work is supported by King Abdullah University of Science and Technology, in the framework of its Solar & Photovoltaics Engineering Research Center (SPERC) and its Collaborative Research Grant Program (Award CRG3-62140391), and by ONR-Global (Award N62909-15-1-2003).
3:00 PM - MD5.1.02
Developing Chemical Insight as to How Molecular Structure Drives the Solid-State Packing of Organic Semiconductor
Chad Risko 1
1 University of Kentucky Lexington United States,Show Abstract
While improved materials, processing protocols, and device designs have ushered organic electronic devices onto the commercial landscape, there remains a need to establish a thorough understanding of the intimate relationships among chemical and molecular structure, processing, solid-state packing, and the underlying physical processes that determine material performance. Through the development and application of multiscale, theoretical materials chemistry approaches, we seek to develop the chemical insight behind these relationships that is necessary to refine and offer novel design pathways for next generation organic semiconducting active layers. In this presentation, we will focus on how such models reveal the influence of seemingly modest changes in chemical structure on the processing and solid-state packing of organic semiconducting active layers.
3:15 PM - MD5.1.03
Ab Initio Investigations on the Donor-Acceptor Interface in Organic Photovoltaics
Hossein Hashemi 1,Michael Waters 1,John Kieffer 1
1 Univ of Michigan Ann Arbor United States,Show Abstract
The structure and electronic properties of a series of donor-acceptor organic molecules are explored using ab initio calculations to understand the behavior of polaron pairs at the interface of the donor-acceptor junction. Results suggest that one may be able to control polaron pair behavior based on the asymmetry of the donor molecule. This allows for control of thermodynamic losses as well as open circuit voltage. These calculations also revealed the probability distribution of the formation of different types of polaron pairs, especially, as it relates to deposition order (i.e. donor on top of acceptor versus acceptor on top of donor). The energetics of crystalline substrates with different surface terminations are mapped out using a single molecule of the partnering species. Accordingly, the interfacial structure and properties are different depending on whether the substrate is a donor or acceptor due to the incongruency between lattices and the disorder that develops in the contact layers of donor and acceptor.
3:30 PM - MD5.1.04
Energy Level Control in Organic Salts for Efficient, Deep Near-Infrared Organic and Transparent Photovoltaics
John Suddard-Bangsund 1,Margaret Young 1,Tyler Patrick 1,Christopher Traverse 1,Natalia Pajares Chamorro 3,Richard Lunt 2
1 Department of Chemical Engineering and Materials Science Michigan State University East Lansing United States,1 Department of Chemical Engineering and Materials Science Michigan State University East Lansing United States,3 Universidad Politécnica de Madrid Madrid Spain1 Department of Chemical Engineering and Materials Science Michigan State University East Lansing United States,2 Department of Physics and Astronomy Michigan State University East Lansing United StatesShow Abstract
Extending photoresponse into the near-infrared (NIR) is one clear route to improving performance of both panchromatic and transparent organic photovoltaics (OPVs). However, current demonstrations of NIR active OPVs have been limited by low open-circuit voltages (VOC) and low external quantum efficiencies (EQE). One reason for this is the challenge of optimizing energy level alignment within the tightened tolerance of a small bandgap. In this work, we show that VOC and EQE can be simultaneously enhanced in organic salt-based OPVs via single-step anion exchanges. We demonstrate that VOC gains are due to improved energy level alignment and that energy levels can be finely tuned by alloying various anions. These effects can be exploited to optimize energy level alignment for abitrary donor-acceptor pairings with novel low bandgap organic salts, and we extend this method to several new molecules with unprecedented VOC (for their spectral range) and photoresponse from 950 nm to as far as 1500 nm. This work bypasses the VOC bottleneck which previously hindered small molecule-based photovoltaics, and presents an exciting path forward for the design of efficient multi- and single-junction transparent photovoltaics.
4:15 PM - MD5.1.05
Organic Donor-Acceptor Charge-Transfer Semiconductors: A Theoretical Characterization of the Microscopic Parameters
Veaceslav Coropceanu 1
1 School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics Georgia Institute of Technology Atlanta United States,Show Abstract
We used Density Functional Theory (DFT) calculations to study the electronic structure of organic mixed-stack charge-transfer crystals. We investigate the impact that the amount of nonlocal Hartree-Fock exchange (HFE) included in a hybrid density functional has on the geometry, the normal vibrational modes, electronic coupling and electron-vibrational couplings in these systems. The crystal geometry and the frequencies of the phonons are found to be only modestly affected by the amount of HFE. In contrast, the electronic couplings and electron-vibration couplings show a strong dependence on this value. We compare the DFT results with results obtained within the G0W0 approximation as a way of benchmarking the optimal amount of HFE needed in a given functional. The electronic structures of a series of donor-acceptor crystals have been investigated in detail. The charge-transport, ferroelectric and non-linear optical properties of several systems that were investigated will be discussed.
4:30 PM -
4:45 PM - MD5.1.07
Experimental and Modeling Studies of Charge Transport and Recombination Mechanisms in Fullerene-Based Organic Solar Cells
Liang Xu 1,Jian Wang 1,Yun-Ju Lee 1,Julia Hsu 1
1 Univ of Texas-Dallas Richardson United States,Show Abstract
The fullerene-based organic solar cells (OSC) with a very minute amount of polymer “donor” have recently attracted intensive research interest due to their simple structure and unique electrical performance especially large Voc. However, charge transport mechanism in such devices with “donor” concentration much lower than the percolation threshold is still unclear. Some studies propose the high Voc is the Schottky barrier height between the anode electrode and the fullerene LUMO. On the other hand, other authors argue that Voc is enhanced because CT states, the major pathways for bimolecular recombination in bulk-heterojunction OSC devices, are significantly reduced in these active layers. Studies based primarily on current density-voltage (J-V) measurements are unable to differentiate different mechanisms. Here we apply impedance spectroscopy (IS), low-energy external quantum efficiency (EQE) spectroscopy, and photoluminescence (PL) dynamics in addition to J-V to study PCBM-based organic solar cells with varying P3HT concentrations. The experimental results are compared to 1D drift-diffusion modeling using SCAPS software. Strong frequency-dependent capacitance behaviors are observed in devices with very low donor concentrations, indicating significant charge accumulation due to space charge limited as well as trap limited transport process. In addition, the role of exciton-polaron annihilation due to poor exciton dissociation and polaron collection will be probed by PL dynamic studies. Finally, impacts of p-type doping of active layer in the fullerene-based OSCs on hole mobility and internal field improvements will be discussed.
 M. Burgelman, P. Nollet, S. Degrave, Modelling polycrystalline semiconductor solar cells, Thin Solid Films. 361 (2000) 527–532.
This project is sponsored by National Science Foundation DMR-1305893
5:00 PM - MD5.1.08
2-Dimensional Series Resistance Modeling of Thin-Film Solar Cells and Modules: Influence on the Geometry-Dependent Efficiency
Marco Seeland 2,Roland Roesch 1,Harald Hoppe 1,Felix Herrmann-Westendorf 1
2 TU Ilmenau Ilmenau Germany,1 Friedrich-Schiller-University Jena Jena GermanyShow Abstract
Limited lateral conductivities of the photo-active materials used in organic thin-film solar cells necessitate the use of semitransparent electrodes for current collection and lateral current transport. Due to the tradeoff between electrical conductivity and optical transmission, which should not underrun 80%, typical values for the sheet resistances of semitransparent electrodes deposited on glass amount to 10–20 Ω/sq. The power loss due to Joule heating and the accompanying voltage drop caused by this sheet resistance increases with cell length in current transport direction and thus defines an upper limit for practical solar cell lengths. In the several approaches existing for calculation of the power loss, the semitransparent electrode layer is either modeled as one lumped resistance in series to the solar cell or as distributed resistance across the whole length of the solar cell in current transport direction. We present a quantitative comparison between these two concepts to investigate the direct influence on the optimal solar cell geometry and to discuss the capabilities as well as limitations of each model. Furthermore the impact of the results on the serial interconnection of a monolithic thin film organic solar cell module in terms of optimal cell lengths and cell interconnection distance is evaluated. The computational study presented here is based on the material system PCDTBT:PC70BM for the photo-active layer material as an example and commonly used semitransparent conductive electrodes: ITO deposited on glass as well as on PET foil and highly doped PEDOT:PSS named PH1000 on PET foil.
5:15 PM - MD5.1.09
How the Energetic Landscape in the Mixed Phase of Organic Bulk Heterojunction Solar Cells Evolves with Fullerene Content
Rohit Prasanna 1,Sean Sweetnam 1,Tim Burke 1,Jonathan Bartelt 1,Michael McGehee 1
1 Stanford Univ Stanford United States,Show Abstract
Energy levels in the mixed polymer-fullerene phase of bulk heterojunction solar cells are significantly shifted from their values in the pure materials . These shifts are important for solar cell performance: they create energy cascades between the mixed phase and pure donor and acceptor phases, which have been shown to improve geminate splitting and suppress bimolecular recombination. This work investigates the origin of these energy level shifts and explains their effect on the charge transfer (CT) state energy and open circuit voltage (Voc).
We use regiorandom P3HT:PCBM as our model system. Regiorandom P3HT is amorphous, and when blended with PCBM, forms only one amorphous mixed phase, allowing us to study the energetics of the mixed phase without effects of polymer crystallites. We measure the polymer ionization potential (IP) and fullerene electron affinity (EA) as a function of blend composition using cyclic voltammetry (CV). The polymer IP monotonically increases in magnitude by around 400 meV and the fullerene EA by around 200 meV as the fullerene content in the blend is increased from 29% to 71%. The effective band gap measured by CV increases by around 300 meV.
Computational modelling studies using molecular dynamics  and classical microelectrostatics  have suggested that electrostatic interactions give rise to dipoles at the molecular interface between polymer and fullerene. We hypothesize that the electrostatic potential created by these interfacial dipoles shifts the energy levels of individual molecules in the blend. To test this hypothesis, we design a simplified electrostatic model to compute the effects of interfacial dipoles on polymer and fullerene energy levels. We fit the energy level shifts predicted by this model to experimentally measured values, using the magnitude of the interfacial dipole as the only fitting parameter. With this model, we show that the energy level shifts can be quantitatively accounted for by interfacial dipoles of comparable magnitude to permanent dipole moments in the molecules.
Despite large changes in the effective band gap, the measured CT state energy and Voc shift by only small amounts and show no trend. We show that energetic disorder in the mixed phase results in broadening of all the densities of states. During normal solar cell operation, only the low-energy tail of the CT density of states is filled, and effectively sets Voc. While changes in blend composition produce large changes in the centres of the energy levels, their low-energy tails are only slightly affected, resulting in Voc not varying by much.
In conclusion, this work shows how energy levels in the mixed phase are shifted by dipoles at the polymer-fullerene interfaces. Tuning these energy level shifts is likely to be an important part of future strategies aimed at improving the efficiencies of organic solar cells.
 J. Am. Chem. Soc. 2014, 40, 14078
 Adv. Mater. 2013, 25, 878
 J. Phys. Chem. C 2013, 117, 12981
5:30 PM - *MD5.1.10
Modifying the Optoelectronic Properties of Rubrene by Strain
Sahar Sharifzadeh 1,Ashwin Ramasubramaniam 2
1 Department of Electrical and Computer Engineering Boston University Boston United States,2 Department of Mechanical amp; Industrial Engineering University of Massachusetts Amherst Boston United StatesShow Abstract
Rubrene is a promising material for organic electronics and optoelectronics; it forms crystalline films with high hole mobility and efficient electroluminescence. Recent studies have shown that the electronic properties of rubrene films can be tuned by substrate-induced strain, suggesting a new route towards the design of more efficient devices. Here, we present a first-principles analysis of the strain-induced changes to the mechanical, electronic, and optical properties of rubrene crystals. Density functional theory and many-body perturbation theory studies predict changes in hole motilities in excellent agreement with electrical conductivity measurements when a strain consistent with the experiment is applied. Furthermore, we predict that the optical absorption and nature of low-energy excitons within the crystal can be tuned by an applied strain as low as 1%. This work utilized resources at the Center for Nanoscale Materials, supported by the U.S. Department of Energy under Contract No. DE-AC02-06CH11357.
MD5.2: Poster Session I
Tuesday PM, March 29, 2016
Sheraton, Third Level, Phoenix Ballroom
9:00 PM - MD5.2.01
Lacunary Polyoxometalates as Effective Electron Conducting Layers for Improving Efficiency in Organic Optoelectronics
Yasemin Topal 2,Marinos Tountas 1,Maria Vasilopoulou 1,Mahmut Kus 2,Mustafa Ersoz 2
2 Selcuk University Advanced Technology Research and Application Center Konya Turkey,1 Institute of Nanoscience and Nanotechnology (INN), National Center for Scientific Athens GreeceShow Abstract
Organicoptoelectronics, such as organicphotovoltaics (OPVs) and organic light emitting diodes (OLEDs), offer the promise of low-cost flexible solar cells, displays, and light sources that have the potential to be manufactured on large-area plastic substrates. One of the key elements for improving efficiencies in organic optoelectronics is finding suitable cathode electrode materials to replace the reactive low work function metals, such as calcium or magnesium, that are typically used to either inject electrons into or extract electrons from the lowest unoccupied molecular orbital (LUMO) of a given organic semiconductor.Polyoxometalates (POMs) are a well-known large group of clusters with frameworks built from transition metal oxo anions linked by shared oxide ions, first reported by Jöns Jacob Berzelius in 1826.One of the most intriguing properties of POM clusters is their high ability to accept a large number of electrons with minimal structural modifications, a property that could enable them to play an important role as excellent electron conductors in electronic devices.
We report here on the preparation of efficient electron conducting layers consisting of lacunaryPOM clusters spin coated from a water/methanol solution between the organic semico