Session Y49: Focus Session: Organic Electronics and Photonics - Small Molecules and General Advances

Charge transport calculations of organic semiconductors by the time-dependent wave-packet diffusion method

Organic materials form crystals by relatively weak Van der Waals attraction between molecules, and thus differ fundamentally from covalently bonded semiconductors. Carriers in the organic semiconductors induce the drastic lattice deformation, which is called as polaron state. The polaron effect on the transport is a serious problem. Exactly what conduction mechanism applies to organic semiconductors has not been established. Therefore, we have investigated the transport properties using the Time-Dependent Wave-Packet Diffusion (TD-WPD) method [1]. To consider the polaron effect on the transport, in the methodology, we combine the wave-packet dynamics based on the quantum mechanics theory with the molecular dynamics. As the results, we can describe the electron motion modified by (electron-phonon mediated) time-dependent structural change. We investigate the transport property from an atomistic viewpoint and evaluate the mobility of organic semiconductors. We clarify the temperature dependence of mobility from the thermal activated behavior to the power law behavior. I will talk about these results in my presentation. [1] H. Ishii, N. Kobayashi, K. Hirose, Phys. Rev. B, 82 085435 (2010).   

Investigation of Interfaces between Sub-Phthalocyanine and C60 using First-Principles Calculations

The structure and electronic properties of a boron subphthalocyanine (SubPc) adsorbed on buckminsterfullerene (C60) and of C60 on SubPc surfaces, mimicking reverse orders of deposition, have been studied using density-functional theory (DFT) including long-van der Waals. Total-energy calculations are used to elucidate the initial adsorption of SubPc on C60 low index surfaces and also C60 on SubPc surfaces. The energetics of crystalline substrates with different surface terminations were mapped out using a single molecule of the partnering species. Accordingly, the interfacial structure and properties are different depending on whether the substrate is SubPc or C60, due to the incongruency between lattices and the disorder that develops in the contact layers of C60 and SubPc, respectively. The dependence of the charge transfer energies on the interface morphology is studied using range separated hybrid functionals. The stabilization of charge transfer states to below the absorbing state, needed to optimize the fill factor, also depends on the order of layer deposition. These results are discussed in the context of experiments performed on organic solar cells, showing trade-offs in the short circuit current and open circuit voltage with varying deposition order of the organic layers.   

First principles study of the electrical and optical properties of SubPc single crystal

We studied the electrical and optical properties of the single crystal of boron subphthalocyanine chloride (SubPc), a popular donor material used in organic photovoltaic (OPV) devices within the framework of density-functional theory (DFT) with added van der Waals long range interactions to deepen our understanding of its performance in light absorption and charge transport. We calculated the frequency-dependent dielectric response, refractive index, extinction coefficient, and intrinsic charge mobility. The complex dielectric constant was computed using first-order perturbation theory, using the electronic wave functions and eigenvalues obtained from supercell DFT calculations. This was done for wave propagation in the (001), (010), (100) directions of optimized SubPc crystal with DFT calculations, revealing significant anisotropy in both electrical and optical properties. Comparison with experimental results allows us to draw conclusions regarding the structural organization of SubPc molecules deposited on a variety of substrates, as well as the conditions for thin film growth and property optimization.   

Density-functional Theory and Beyond for Donor-Acceptor Complexes: The Example of TTF/TCNQ

We study the performance of density-functional theory (DFT) with various exchange-correlation (XC) functionals in describing electronic and structural properties of the prototypical donor/acceptor complex TTF/TCNQ. We find that the binding energetics and the amount of electron transfer between TTF and TCNQ depends strongly on the functional. In particular, all semilocal functionals give rise to significant, aritificial electron transfer due to a wrong ordering of Kohn-Sham (KS) levels. We consider the HSE [1] ``family'' of XC functionals using the fraction of exact exchange ($\alpha$) as adjustable parameter. The optimum XC functional is then identified as that for which the $G_0W_0$ quasiparticle correction to the energy gap of the KS LUMO of the acceptor and the HOMO of the donor is minimized. We obtain $\alpha \sim 0.8$ which gives an electronic level alignment that is consistent with experiment and free from spurious asymptotic charge transfer. We conclude that the proposed scheme improves the KS spectrum, and that the investigated TTF-TCNQ dimer exhibits intra-molecular electron-density rearrangement rather than electron transfer. \\[4pt] [1] A.V. Krukau, et al., J. Chem. Phys. {\bf 125}, 224106 (2006)   

Understanding the high device efficiency of a class of solution-processed small-molecule solar cells

We perform a first principles study of light absorption, exciton and charge carrier transport for two recently synthesized molecular crystals which in bulk heterojunction solar cells with PC70BM acceptor show up to 6.7\% power conversion efficiency. Our results distinguish the following factors important for this high efficiency. The large conjugation length facilitates strong light absorption with low-energy absorption edge. The crystalline ordering of properly oriented molecules leads to exciton delocalization over the typical size of the crystallites. The tightly packed pi-stacks allow for fast disorder-resistant hole transport along these stacks. Nevertheless the microscopic characteristics of the considered crystals are typical and comparable with other materials used in photovoltaics. Therefore we conclude that the main factors of the high device efficiency should be searched at the mesoscale including interfaces and grain boundaries.   

Relating Molecular-Scale Structure to Spectroscopy in Pentacene-Perfluoropentacene Donor-Acceptor Assemblies from First-Principles

Using van der Waals-corrected density functional theory and many-body perturbation theory, we compute the spectroscopic properties of the archetypal organic semiconductors pentacene (PEN), perfluoropentacene (PFP), and their composite donor-acceptor blends. Band structures, bulk crystal densities of states, and low-energy optical excitations are computed for the isolated bulk crystals and their composite assemblies. For the individual crystals, transport and optical gaps are in good agreement with experiment, and the nature and orientation of the excitonic wavefunctions is found to be sensitive to the degree of co-facial packing. For the PEN-PFP systems, different molecular arrangements and compositions are considered in an effort to connect to thin film measurements. The relationship between packing in these structures, the transport gap, and the nature and binding energies of low-lying excitons are explored. We acknowledge DOE-BES, NSF, and US-Israel BSF for support, and NERSC for computational resources.   

Quantitative structural analysis of organic thin film deposition: a real time synchrotron X-ray scattering study

Direct writing gives us the ability to deposit films from solution with controlled thickness, grain structure and orientation. We have investigated TIPS-Pentacene films deposited from toluene solution at various speeds via a combination of real time synchrotron x-ray scattering and polarized-light video microscopy. Through video microscopy we observe a well-defined crystallization front that becomes less defined as the writing speed is increased. In synchrotron x-ray scattering we observe that the ordering process is an order of magnitude slower than what is seen under the optical microscope. This discrepancy in the apparent crystallization rate raises questions such as, which part of the film actually rotates the polarized light and becomes visible under crossed polarizers? Observation with varying speeds and substrate temperatures suggest that the crystallization first occurs near the top surface of the drying film, while subsurface regions remain in a disordered state for up to several seconds.   

Diindenoperylene as donor and acceptor for organic photovoltaic cells

In organic photovoltaic cells (OPVCs) typically two organic materials with electron acceptor and donor character are sandwiched between anode and cathode, forming heterojunctions where charge separation occurs. To improve the efficiency of charge separation, understanding the mechanisms of the energy level alignment at these heterojunctions is crucial. We report on ultraviolet photoelectron spectroscopy (UPS) measurements on three different organic-diindenoperylene (DIP) heterojunctions formed on PEDT:PSS electrodes. The measurement reveal that the energy level alignment of C$_{60}$ on DIP/PEDT:PSS corresponds to a type II heterojunction, with DIP acting as donor. The offset between the highest occupied molecular orbital (HOMO) of DIP and the lowest unoccupied molecular orbital (LUMO) of the acceptor C$_{60}$, an estimate for the maximum achievable open circuit voltage, is 1.35 eV. In contrast, the energy level alignment of DIP on sexithiophene (6T) and poly(3-hexylthiophene) (P3HT) is of type II as well, but DIP acting as acceptor. The offset between the HOMO of the donors 6T and P3HT and the LUMO of DIP is found to be 1.75 eV and 1.6 eV, respectively.   

Gradual thickness change of CuPc on MoOx on Oxygen Plasma Treated ITO

The thickness dependence of copper phthalocyanine (CuPc) interlayer on molybdenum trioxide (MoO$_3$) and conducting indium tin oxide (ITO) has been investigated with ultraviolet photoemission spectroscopy (UPS). We also investigated the air exposure effect on the CuPc/MoO$_3$/ITO interlayers. It was found that the MoO$_3$ interlayer substantially increased the substrate work function (WF). With the deposition of CuPc the WF decreased and saturated at the thickness of 80 {\AA}. We also found that 3x10$^6$ Langmuir (L) air exposure decreased both the MoO$_3$ WF and the interface dipole between CuPc/MoO$_3$ interface.   

Voltage dependence of the electric double layer structure at an ionic liquid/Au interface

Ionic liquids (ILs) have been studied extensively because of their unique characteristics. One of the utilization of them is applying a strong electric field to solids through the electric double layer. Using this field, one can reduce the gate voltage for an organic field effect transistors (FETs) to operate [1]. In order to clarify the microscopic structure of such IL-gated organic FETs, we have performed synchrotron x-ray scattering experiments at BL-13XU of the SPring-8, Japan. While the electric double layer structures at IL-solid interfaces have been studied by x-ray reflectometry [2], the electric double layer is stabilized by the natural polarity of the surface of the solid. In order to observe the electric field effect, we measured an electric double layer formed at the interface between an IL and a Au(111) single crystal under external electric field. The reflectivity profile was found to depend on the applied electric field, which reflects the formation of the electric double layer. \\[4pt] [1] T.~Uemura {\it et al.}, Appl. Phys. Lett., {\bf 95}, 103301 (2009).\\[0pt] [2] M.~Mezger {\it et al.}, Science {\bf 322}, 424 (2008).   

A critical assessment of thiolate self-assembled monolayers (SAMs) on platinum.

Thiolate SAMs have been successfully anchored on metal surfaces, but a critical assessment of the impact on the structural and electronic properties of the metal surfaces has remained elusive. CH$_{3}$S and CF$_{3}$S were selected as model systems in this work, because of their simple structures which can provide insights about how the composition and electronegativity of SAMs affect the properties of metal-SAM systems. Density functional theory calculations have been used in this work to study the adsorption of CH$_{3}$S and CF$_{3}$S molecules on the Pt (111) surface at different coverage (1/3, 1/4, 1/6, 1/9 and 1/12) and adsorption sites (fcc and hcp). The geometry, adsorption energy and the work function of the Pt-SAM systems have been determined. Several interesting observations could be made: (1) the optimized SAM is tilted with respect to the Pt surface and the tilted angle decreases with the molecular coverage on the Pt surface; (2) the adsorption energy of both systems are almost always lower at the fcc site compared to the hcp one and shows a coverage-dependence; (3) the work function of Pt-SAM also shows a dependence on coverage and hence controlling the molecular coverage is probably an effective technique to tune the work function.   

Nonlinear light propagation in photopolymers: from self-trapped beams to 3-D optical lattices

While liquid crystals, surfactants and colloidal crystal systems assemble into ordered phases to attain free energy minima, strikingly complex patterns can also emerge when condensed matter systems are perturbed away from equilibria. This talk will be an overview of research in our group into the dynamics of light beams that propagate while simultaneously initiating free-radical polymerisation in photopolymers. The consequent nonlinear and reciprocal interactions between the optical field and self-induced refractive index changes in the medium elicit a rich assortment of three-dimensional spatial patterns. These include self-trapping bright and dark beams, beam filamentation due to modulation instability, diffraction rings due to self-phase modulation and the formation of 2-D and 3-D bright and dark optical lattices. The potential of these optical phenomena to spontaneously inscribe complex 3-D polymer architectures that are inaccessible through conventional lithographic techniques and that would have advanced optical applications such as nonlinear photonic crystals will also be described.   

Enhancing sensing of nitroaromatic vapors by thiophene-based polymer films

Sensing of nitroaromatic-based explosives is important for homeland security and for the detection of landmines in war zones. These compounds are detected using fluorescence quenching of poly(phenyleneethynylene) (PPE). When compared with PPE, polythiophenes have several features that make them excellent candidates for sensing. However, due to strong $\pi -\pi $ aggregation in polythiophenes, the permeation of the analyte in the thin film is poor. Therefore polythiophenes have poor quenching efficiencies. We have addressed this problem by tuning the side chains on these polymers to disrupt the polymer aggregation, thereby enhancing the analyte diffusion into the polymer thin film. We have now developed a materials platform for next generation sensory materials based on polythiophenes. This talk will discuss our approach and the studies that showed enhanced sensing of nitroaromatics in polythiophene thin films.   

Giant Electrocaloric Effect in Ferroelectric Polymers with Great Impact on Energy and Environment

Refrigeration and air conditioning overall consume around 20{\%} of the energy budget in developed countries which necessitates a search for new approaches to increase the energy efficiency of these cooling technologies. Cooling technologies based on the electrocaloric effect (ECE) hold great potential and promise in realizing these goals. The electrocaloric effect (ECE) refers to the change in temperature and/or entropy of a dielectric material by an applied voltage. Recently, a class of P(VDF-TrFE) based ferroelectric polymers have been discovered that provide a giant electrocaloric effect with an adiabatic temperature change of $\Delta $T $\sim $ 20 K and an isothermal entropy change $\Delta $S $>$ 90 J/kgK at room temperature. This talk will review the earlier works in the ECE, as well as present the basic materials considerations and experimental results of the ECE in both normal and relaxor ferroelectric polymers. It will be shown he relaxor ferroelectric polymer displays a nearly flat ECE response over a broad temperature range, which is very attractive for practical cooling device applications Furthermore, we will present our recent investigation, exploiting the giant ECE in these polymers for cooling devices with compact size, high cooling power and efficiency.