Spectroscopic ellipsometry characterization of vacuum-deposited organic films for the application in organic solar cells

Wynands, David; Erber, Michael; Rentenberger, Rosina; Levichkova, Marieta; Walzer, Karsten; Eichhorn, Klaus‐Jochen; Stamm, Manfred

doi:10.1016/j.orgel.2012.01.036

Cited by 29 publications

(36 citation statements)

References 39 publications

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“…This liquid-crystalline ordering is simply the effect of the rod-like architecture of the molecules, under the artificially imposed constraint that the smectic layers have to line up periodically. Even though our analysis correctly indicates a face-on orientation of the DCV4T on C 60 as the preferred configuration [62], dependable insights into the properties of the experimental system are beyond the scope of the approach. Nevertheless, the obtained non-equilibrium structures prove useful to understand the electronic properties of partially-ordered systems, which is indeed how we will make use of them later on in Chapters 4 and 5.…”

Section: Molecular Landscapesmentioning

confidence: 83%

“…In fact, anisotropic dielectric properties and, consequently, nematic or smectic ordering are frequently observed in thin organic films [167] and are sufficient to influence the level offset between donor and acceptor domains via uncompensated interface-active quadrupole moments Q 20 = Q zz and Q 22c ∼ Q xx − Q yy . Structural coherence in the active layer of bulk heterojunctions will differ from the planar setup, but the nematic (uniaxial) order is retained [62] with the effect that the same energetic mechanisms as for bilayers also apply here. This is supported by the common experimental finding that open-circuit voltages are only slightly larger in the planar layout.…”

Section: Summary: Interface Energeticsmentioning

confidence: 99%

“…This energetic similarity of bulk and planar heterojunctions remains enigmatic; to identify its origin, more details would have to be known about the microscopic structure of these junctions. In particular, the "centro-symmetric" layout of bulk morphologies may have to be reassessed, as in-situ growth studies [107] and ellipsometric characterizations [62] indicate that donor-acceptor mixtures are not only well phase-separated, but in fact macroscopically ordered with respect to the substrate. In view of this residual preferential ordering, future computational studies will hopefully resolve to which degree mesoscopic interactions persist in both planar and bulk heterojunctions.…”

Section: Vice and Virtue Of Low-energy Configurationsmentioning

confidence: 99%

“…3.1c) appears to converge faster, when in fact the individual contributions of the CT-hole and CT-electron exhibit the same slow convergence that typifies the thin-film scenario from Fig. 3.1b.As long-range, crystalline and liquid-crystalline (and hence at least uniaxial) ordering prevails in most organic materials and devices, even including bulk heterojunctions [62,107], an account of the ensuing long-range effects on the energy landscape is indispensable. In the following, we will detail the theoretical foundation of the embedding technique developed as part of this thesis and implemented in the VOTCA suite [50].…”

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confidence: 99%

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The (Non-)Local Density of States of Electronic Excitations in Organic Semiconductors

Poelking

2018

Springer Theses

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The (Non-)Local Density of States of Electronic Excitations in Organic Semiconductors The rational design of organic semiconductors for optoelectronic devices relies on a detailed understanding of how their molecular and morphological structure condition the energetics and dynamics of charged and excitonic states. Investigating the role of molecular architecture, conformation, orientation and packing, this work reveals mechanisms that shape the spatially resolved densities of states in organic, small-molecular and polymeric heterostructures and mesophases. The underlying computational framework combines multiscale simulations of the material morphology at atomistic and coarse-grained resolution with a long-range-polarized embedding technique to resolve the electronic structure of the molecular solid. We show that longrange electrostatic interactions tie the energetics of microscopic states to the mesoscopic structure, with a qualitative and quantitative impact on charge-carrier level profiles across organic interfaces. The computational approach provides quantitative access to the charge-density-dependent opencircuit voltage of planar heterojunctions. The derived and experimentally verified relationships between molecular orientation, architecture, level profiles and open-circuit voltage rationalize the acceptor-donor-acceptor pattern for donor materials in high-performing solar cells. Proposing a pathway for barrier-less dissociation of charge transfer states, we highlight how mesoscale fields generate charge splitting and detrapping forces in systems with finite interface roughness. The associated design rules reflect the dominant role played by lowest-energy configurations at the interface. Acknowledgements 121 Die (nicht-)lokale Zustandsdichte elektronischer Anregungen in organischen Halbleitern List of Publications 123Bibliography 125 Chapter 1 Organic Electronics in a NutshellThe discovery of conductive polymers in the 1970s [1,2] -rewarded with the Nobel prize in chemistry in the year 2000 -and subsequent development of the first polymeric electronic devices in the 1980s [3-5] have marked the beginnings of a vast interdisciplinary research field referred to as organic electronics. Drawing from both polymeric and small-molecular semiconductors, organic thin-film transistors, solar cells, light-emitting diodes, photodetectors and sensors name just a few out of many applications that make use of the supreme chemical versatility and mechanical flexibility of the underlying "soft" molecular materials. Furthermore enabling low-temperature solution-based processing, printing and spray coating, organic electronics is set to complement the conventional silicon-based electronics in diverse waysadding functionality and improving sustainability. This introductory chapter will provide a short review of the materials and device physics, as well as of new trends and challenges that emerged in the field over the past decade. MaterialsOrganic semiconductors are molecular materials that exist at the interface between orga...

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Section: Molecular Landscapesmentioning

confidence: 83%

Section: Summary: Interface Energeticsmentioning

confidence: 99%

Section: Vice and Virtue Of Low-energy Configurationsmentioning

confidence: 99%

mentioning

confidence: 99%

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The (Non-)Local Density of States of Electronic Excitations in Organic Semiconductors

Poelking

2018

Springer Theses

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“…Such an approach was used in ref. [53] for anisotropic polyfluorene thin films, and more recently by Wynands et al [54] for organic films relevant for OPVs. This last reference gives a nice overview about the care one normally should take in evaluating and modelling the data.…”

Section: Error Sources For Determination Of Molecular Orientationmentioning

confidence: 96%

Small Organic Molecules

Gordan

Zahn

2014

Ellipsometry of Functional Organic Surfaces and Films

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In order to improve devices based on organic thin films like organic light emitting diodes (OLEDs) and organic photo voltaic (OPV) cells, the molecular orientation has to be determined and optimized to increase the carrier mobility and the light emission and absorption within the layers. As many of the organic molecules possess an intrinsic molecular anisotropy, molecular ordering will induce optical and electrical anisotropy in the films.The optical anisotropy can be used to determine the average molecular orientation by modeling the anisotropic dielectric function using ellipsometric measurements. An overview of the procedure, valid for planar molecules, will be given in the first part of this chapter, with the main focus on the Phthalocyanine molecular class. The second part of the chapter focuses on vacuum ultra violet (VUV) ellipsometric measurements and the sensitivity gain at ultra-low coverages. Here the discussion will be restricted to optically isotropic films. IntroductionOrganic molecules, which in solid form exhibit semiconducting properties, were long hailed as candidates for cheap, roll to roll electronics. As of today, organic electronics is not anymore a faraway promise, as the huge success and growth of the smartphones market is powered by innovations like organic light emitting diodes (OLEDs). The recent launch of OLED TVs indicates that the OLED technology already reached a maturation state which ensures the long life time needed in television displays. Even in organic photo voltaic (OPV) applications, higher efficiencies than the limit predicted by the exciton dissociation energy were reached. Plastic electronics using organic field effect transistors (OFETs) is another field where huge progress was made in the past decade, and probably soon we will see commercial organic based radio frequency identification tags (ORFIDs). Compared to OPVs, OFETs, and especially to the already commercial OLEDs, molecular spintronics

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