understanding heterogeneous nucleation in binary, .understanding heterogeneous nucleation in binary,
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Understanding Heterogeneous Nucleation in Binary, Solution-Processed, Organic Semiconductor Thin FilmsStephanie S. Lee, Srevatsan Muralidharan, Arthur R. Woll, Marsha A. Loth, Zhong Li,
John E. Anthony, Mikko Haataja, and Yueh-Lin Loo*,
Department of Chemical and Biological Engineering and Department of Mechanical and Aerospace Engineering, PrincetonUniversity, Princeton, New Jersey 08544, United StatesCornell High Energy Synchrotron Source, Cornell University, Ithaca, New York 14853, United StatesDepartment of Chemistry, University of Kentucky, Lexington, Kentucky 40506, United States
*S Supporting Information
ABSTRACT: Heterogeneous nucleation is often the precursor to crystallizationin solution-processed organic semiconductor thin films. Here, we study theefficacy of a series of nine small-molecule organic semiconductor additives inseeding the crystallization of solution-processable triethylsilylethynyl anthradi-thiophene (TES ADT). By systematically varying the concentrations of theadditives in TES ADT thin films, we found the tendency of the additives tocrystallize, their solubility in the casting solvent, and their similarity in chemicalstructure to TES ADT, to determine the nucleation and resulting density ofnuclei. Tracking the crystallization process further yields information about themechanism of nucleation. While pure TES ADT nucleates instantaneously at theonset of crystallization, nucleation transitions to a distributed process occurringthroughout crystallization with the incorporation of increasing amounts ofadditives.
KEYWORDS: heterogeneous nucleation, organic semiconductor, Avrami kinetics, dopant, solution processing
INTRODUCTIONBlending two organic semiconductors to form thin films is apromising strategy to bring about unique or enhancedelectronic properties in the active layers of organic electronicdevices.1 Bulk-heterojunction organic solar cells (OSCs), forexample, utilize blends of two organic semiconductors withdifferent energy levels to efficiently dissociate excitons forharvesting light.2 Doping an organic semiconductor host withfractional amounts of another organic semiconductor, orguest, has also been successfully employed to improve themobility of organic thin-film transistors (OTFTs),3,4 theconductivity of charge transport layers in organic light-emittingdiodes (OLEDs),5,6 and the luminescence of light-emittinglayers in OLEDs.79 In choosing organic semiconductor pairsfor such blends or guest-host systems, it is important toconsider the respective electronic properties of the constituentcomponents. Equally imperative is a comprehensive under-standing of the morphological development of such systems.10
The presence of an additive, for example, can inducecrystallization and improve the crystallinity of the host organicsemiconductor in the active layers of OTFTs, improving devicemobility.4,11 Aggregation of emissive dopants at high loadinglevels in guest-host OLEDs, on the other hand, can decreasethe overall luminescence of the light-emitting layer.8,9 In lightof the importance of the active layer morphology indetermining overall device performance, it is critical that we
understand how the physical parameters of one compound inbinary, organic semiconductor thin films influences the finalfilm morphology.In single-component, organic semiconductor systems,
structural development occurs through nucleation and growth.Heterogeneous nucleation on foreign objects, like defects anddust particles on the substrate surface,12 determines thenucleation density and thus the final size of crystalline domains,with boundaries between these domains acting as barriers tocharge transport.1315 In binary organic semiconductorsystems, structural development is more complicated, withboth phase separation and constituent crystallization occurringsimultaneously. Depending on the chemical compatibility of thetwo organic semiconductors and their respective interactionswith the underlying substrates, phase separation can occur bothlaterally and vertically in these thin films.10,1619 Furthercomplicating the structural development of two-componentsystems is the fact that the organic semiconductor constituentscan individually crystallize and one can aid in nucleating4 orsuppressing20 crystallization of the other. Cho and co-workers,for example, found the incorporation of a small-moleculedopant to poly(3-hexyl thiophene) (P3HT) thin filmsin
Received: April 6, 2012Revised: July 5, 2012Published: July 6, 2012
2012 American Chemical Society 2920 dx.doi.org/10.1021/cm3010858 | Chem. Mater. 2012, 24, 29202928
addition to increasing the carrier concentrationto seed P3HTcrystallization.4 Both the presence of the dopant and thisimproved crystallinity are reported to increase device mobilityby 30-fold compared to OTFTs comprising pristine P3HT asactive layers. At high blend ratios of P3HT and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), on the other hand, thecrystallization of both P3HT and PCBM is hindered by thepresence of the other component.20 Postprocessing steps, suchas thermal annealing21 and solvent-vapor annealing,16 or theincorporation of solvent additives to the cosolution prior todeposition,22 have thus been employed to enhance crystal-lization and to improve the efficiencies of OSCs comprisingblends of P3HT and PCBM in the active layers. For maximumdevice performance, it is commonly accepted that suchcrystallization should result in domains on the order of tensof nanometers, a length scale that is commensurate with thecharacteristic exciton diffusion length.23
In this manuscript, we examine how the presence of small-molecule additives affects the structural development of thehost organic semiconductor by studying their effectiveness inheterogeneously nucleating triethylsilylethynyl anthradithio-phene (TES ADT).24 We chose TES ADT as the host organicsemiconductor for several reasons. When spun cast ontosubstrates, TES ADT forms largely amorphous films, allowingus to controllably induce crystallization with subsequentsolvent-vapor annealing.25 During exposure to 1,2-dichloro-ethane (DCE) solvent vapors, TES ADT crystallizes throughnucleation and growth of spherulites. Because each spheruliteresults from a single nucleus, tracking spherulite formationallows us to infer information about nucleation events.Furthermore, the diameters of TES ADT spherulites rangefrom 30 m to 3 mm, and as such are easily identifiable viaoptical microscopy. By measuring the average spherulitediameter, L, in TES ADT thin films after crystallization iscomplete, we can calculate the average number density ofspherulites, S, and thus extract the average number of nucleiin TES ADT thin films using the following equation:
by approximating spherulites as circles.We have chosen nine different small-molecule organic
semiconductors as additives in TES ADT thin films. We haveselected the additives to be electrically active and usedconcentrations ranging from 0 to 10 mol % relative to TESADT so the findings of our study would be relevant to guest-host OTFT and OLED systems.36 Establishing structure-function relationships between the morphology in the activelayer and overall device performance in organic electronics firstrequires a fundamental understanding of structural develop-ment during processing. Central to this manuscript is thus howthe presence of additives affects the overall structure of TESADT thin films. The electrical characterization of these two-component, organic semiconductor thin films is currentlyunderway and will be reported subsequently. By comparing theextent with which these additives can heterogeneously nucleateTES ADT during solvent-vapor annealing, we can determinethe key attributes of the additives that influence their ability toact as nuclei. By tracking the kinetics of crystallization duringDCE solventvapor annealing, we further elucidate how thepresence of additives alters the mechanism with which TESADT thin films are nucleated.
EXPERIMENTAL METHODSFilm Formation. The 300 nm thermally grown SiO2 on doped-Si
wafers purchased from Process Specialties were used as substrates forTES ADT thin films. The wafers were rinsed sequentially with acetone,isopropyl alcohol, and deionized water and then dried with housenitrogen prior to spin coating TES ADT solutions. TES ADT,24
fluorinated 5,11-bis(triethylsilylethynyl) anthradithiophene (F-TESADT),26 chlorinated 5,11-bis(triethylsilylethynyl) anthradithiophene(Cl-TES ADT),27 brominated 5,11-bis(triethylsilylethynyl) anthradi-th iophene (Br-TES ADT), 27 and iod inated 5 ,11-b i s -(triethylsilylethynyl) anthradithiophene (I-TES ADT),27 diethyl-5,11-bis(triethylsilylethynyl) anthradithiophene (ethyl-TES ADT),28 triiso-propylsilylethynyl pentacene (TIPS pen),29 and triisobutylsilylethynylpentacene (TIBS pen)30 were synthesized according to previouslypublished procedures. The procedure to synthesize a fullerenederivative that is functionalized with triethylsilylethynyl tetracene(TES Tet Fu) is included in the Supporting Information. PCBM waspurchased from Nano-C. To form TES ADT thin films, TES ADT wasfirst dissolved in toluene at a concentration of 2 wt %. In a separatevial, a small-molecule additive was dissolved in toluene atconcentrations between 1 2 wt %. The appropriate volume of theadditive solution was then added to TES ADT solutions in order toachieve the desired additive molar concentration relative to TES ADT.Given that TES ADT photobleaches easily, the solution was onlyallowed to sit for
three samples for each additive to estimate the error associated withthe measurements.
RESULTS AND DISCUSSIONTo compare the effectiveness of different small-moleculeadditives as nucleating agents in TES ADT thin films, wesystematically