Ambient Degradation of Perylene Diimide Based Organic Transistors

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C: Energy Conversion and Storage; Energy and Charge Transport

Ambient Degradation of Perylene Diimide Based Organic Transistors: Hidden Role of Ozone and External Electric Field Xiaoling Zhan, Weicong Huang, Hu Shi, and Hongguang Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12280 • Publication Date (Web): 14 Mar 2018 Downloaded from http://pubs.acs.org on March 14, 2018

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The Journal of Physical Chemistry

Ambient Degradation of Perylene Diimide Based Organic Transistors: Hidden Role of Ozone and External Electric Field Xiaoling Zhan,†,1 Weicong Huang,†,1 Hu Shi,‡ and Hongguang Liu†,* †

College of Chemistry and Materials Science, Jinan University, 601 Huang-Pu Avenue West, Guangzhou 510632,

China ‡

1

College of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, China

Contributed equally to this work

*Email: [email protected]

Abstract A thorough interpretation on the mechanisms that control degradation of the electrical performance of organic thin-film transistors (OTFTs) during exposure to ambient environments is still developing. This is particularly true for n-type OTFTs. By performing density functional theory (DFT) calculations, we have proposed a different degradation pathway of perylene diimide (PDI) in ambient air. Compared to the most common ambient oxidant, O3 though seldom considered, can easily react with >C=C< in the π-conjugated charge-transfer center forming stable ozonides, which could be the underlying cause for relevant device failures. Noteworthy, external electric fields which are ubiquitous while often overlooked in electronic devices, can either accelerate or hamper the degradation process depending on the field direction. This finding underlines, in a rigid device configuration where electrodes are largely fixed, the way the molecules align on the substrate is pivotal to their ambient stability. Among the tested substituents, cyanation at the periphery of the perylene core resists O3/O2 attack, and favors electron transport

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by lowering the internal reorganization energy. This work constitutes the first step on understanding the interplay of interfacial oxidations and molecular charge-transport properties toward modeling the bulk electrical performance.

Introduction Recent advances in p-type organic semiconductors have fulfilled many of the requirements for use in diverse applications;1-6 however, n-type materials, needed for diodes, complementary circuits and solar cells, continue to present challenges.7-11 One of the major hurdles remaining is the vulnerability of n-type charge carriers to ambient conditions. So far, the mechanisms by which the electron mobility of organic thin-film transistors (OTFTs) degrades during long-term storage in air (or in other environments that contain oxygen) are still not clear.12,13 de Leeuw et al.14 first discussed this issue and ascribed the drop in mobility to electron trapping by the most common reactive species in an ambient atmosphere, H2O and O2. Later, Frisbie et al.15 shed light on the mechanism of device failures by increasing the O2 partial pressure in the operating atmosphere for OTFTs fabricated with perylene diimide (PDI) derivatives. The positive shift in threshold voltage was assigned to the creation of metastable PDI/O2 trap states. Although which atmospheric gas plays the dominant role has not been determined, a consensus has been preliminarily reached that oxygen exposure is adverse to n-type OTFTs performance.12,16,17

Compared to O2, the impacts of its allotrope, O3 (ozone), on n-channel materials have gone unexplored, possibly due to its presumably 'low concentration' in air. In fact, as a highly oxidizing pollutant, tropospheric O3 has become a worldwide environmental problem, marked by the


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presence of NO2 and NO in the lower atmosphere (Scheme 1). Today, tropospheric O3 concentrations are monitored to evaluate the extent of air pollution in urban areas, where emissions from transportation and industrial plants are substantial.18 O3 can also be generated in laboratory areas where people are using ultraviolet lights, high-voltage electronics, laser printers, etc.19-22 Considering its high chemical potential and abundance, O3 when it is introduced to n-channel materials may overwhelm O2 in degrading OTFTs mobilities.

Scheme 1. Reaction path from the photolytic dissociation of ambient NO2 to O3

In the gas phase, O3 can readily initiate reactions with vinyl-containing organics and polycyclic aromatics via cycloaddition across double bonds to form ozonides.22-24 The oxidative reaction of unsaturated compounds with ozone was first studied in detail by Harries.25 Later, Criegee26 proposed a reaction mechanism for alkenes+ozone regarding ozonolysis, claiming the likely formation

of

the

primary

ozonide

(1,2,3-trioxolanes)

and

the

secondary

ozonide

(1,2,4-trioxolanes), both of which have been experimentally confirmed by using matrix isolations.22 As a key product state, the formation of primary ozonide is not an individual event, already studied on sizable aromatics such as anthracene, pyrene, fullerene and carbon nanotubes.23,24,27,28 We speculate analogous chemistry may also occur on organic semiconductors, because these materials are usually polycyclic aromatics contain various C=C bonds.29-31 However, compare to O2, scientists are yet to decipher how chemisorption of O3 influences the transport properties of organic semiconductors, and how local structure and external electric fields (ubiquitous in electronic devices) will affect the adsorption process. Analyzing the ease of



ozonization on π-conjugated surface and factors that promote the adsorption process may assist to understand relevant device degradations in air, and contribute to the searching of effective precautions.

Computational Details We computed the internal electron reorganization energy (λ) for the self-exchange electron-transfer process via the Nelsen's four-point method.32,33 It is described as the vertical ionization of a neutral molecule followed by geometric relaxation, and then vertical neutralization of a charged molecule followed by geometric relaxation = + = − + − where E is energy, Q is geometry, and the subscripts 0 and - denote neutral and anionic states, respectively. Among the tested hybrid meta exchange-correlation functionals, M06-2X provides the best results for the combination of main-group thermochemistry, kinetics, and noncovalent interactions.34 We have shown in previous works35,36 that this level of method can predict the conjugated surface reaction energetics with sufficient accuracy which is in good agreement with the experimental data. All energies and structures unless otherwise specified were obtained by density functional theory (DFT) at the M06-2X/6-31+g(d) level34,37 with the Gaussian 09 package.38 Integral=ultrafine option was used to minimize the integration grid errors that may arise from using an inadequate grid in the current M06 suite of functionals.39 All structure optimizations have no additional constraints. All open-shell calculations were performed using unrestricted methods and spin contamination for the radical cation species was found to be negligible (the value of differs from the exact expectation value 0.75 by less than 10%).


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Frequency calculations were carried out at the same level of theory to identify all of the stationary points as minima (zero imaginary frequencies) or first-order saddle points (one imaginary frequency). IRC calculation was performed to verify that the optimized geometry to be used is in fact a transition state. Mulliken charge distribution was derived from the single point calculation at the same level of theory but without diffusion functions at the M06-2X/6-31+g(d) optimized structure so as to avoid the likely erroneous guess of atomic charges caused by diffusion functions. Fukui function of the bare PDI was plotted using Multiwfn Software.40 To estimate the impact of environmental H2O on the proposed reaction mechanisms, we performed full optimizations using the polarizable continuum model (PCM)41 implemented in Gaussian 09. To mimic the external electric field conditions, a built-in electric dipole field was applied to the molecule along each axial direction by invoking the Field keyword in the same package. Field strengths were chosen as referenced by molecular systems reported elsewhere.42,43

Results and Discussion With the above objective, we select a prototypical n-type building block, PDI as the test molecule. By performing DFT calculations, the relative energetics for O3/O2 cycloadding to vinyl sites at the perylene core are compared. Figure 1a clearly shows, O3 possesses a lower reaction barrier than does the O2 at all considered active sites. B2 and B4 are positions most favorable to accommodate ozone, with a barrier at least two times smaller than that of the other positions, forming thermodynamically stable ozonides as indicated by the computed binding energies. Peripheral positions (B2-4) except B1 are seen easier to bind O3/O2. Conformably, for both O3 and O2, B2 requires the lowest energy (10.3 and 26.5 kcal/mol) to initiate the electrophilic addition, while the



same reaction at a single benzene molecule (14.9 and 39.3 kcal/mol) or the neighboring bridge site B1 of PDI (54.4 and 57.3 kcal/mol) is much more difficult. The addition selectivity is thought due primarily to the electronic nature of the host molecule, where negative charges are preferentially distributed on the outermost carbons of the perylene core (Figure 1c). Once the electrophilic reaction is triggered (one C-O bond is formed), the binding of the other C (positively charged or nearly neutral)…O (negatively charged) at B2 or B4 is electronically favorable, as revealed by the mulliken charge analysis. Fukui function plot further confirms B2 and B4 are the nucleophilic regions at the perylene core. The cycloaddition reactions considered here manifest a one-step process. Once the ozonide or peroxide is formed, the reverse reaction is much harder.


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Figure 1. (a) Seven selected vinyl sites (Bn, n=1-7) on PDI for O3/O2 cycloaddition and calculated energetics. (b) Representative energy profiles of reactant state (RT), transition state (TS) and product state (PT) in the cycloaddition process at B2. (c) Mulliken charge distribution and



nucleophilic Fukui function plot (isovalue=0.002) of the bare PDI. Positive and negative charges are represented respectively by red and blue circles, the size of which denotes the amount of a given charge.

Figure 2. Electron reorganization energies of primary ozonide (POZ), secondary ozonide (SOZ) epoxide (EPO) and peroxide (PER) formed at different vinyl sites on PDI. Dash line is the reference representing the electron reorganization energy (0.371 eV) of the bare PDI.

Chemisorption of O3/O2 at the PDI charge-transfer center, is bound to influence the electron mobility due to the changes of extended π-conjugation and overall molecular rigidity.44-46 In the single molecule dimension, a descriptor most appropriate to characterize these changes and enters directly into the computation of the carrier mobility, is the internal electron reorganization energy (λ).47-49 Since large λ is believed adverse to achieving high carrier mobility,46,47 calculation of λ should provide a door to understand the mobility loss caused by ambient oxidation. To this end, four possible product states (POZ, SOZ, EPO and PER) are investigated according to


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literature22-24,26-28,50 and the computed λ of each are plotted in Figure 2. SOZ is assumed here, because a similar 1,2,4-trioxolanes structure has been derived via the Criegee mechanism26 from POZ analogues,22,27 while in another reported reaction path POZ can transform to an EPO structure after releasing an oxygen molecule.24,28 With nearly no exception, O3/O2 oxidations result in elevated λ compared to that of the bare PDI. As λ measures the strength of the local electron-phonon coupling,51,52 or simply the extent of geometrical change upon electron transfer (i.e. less structural alteration induces a smaller λ),46 the enhancement of λ indicates oxidation affords PDI with extra flexibilities, which can be utilized to accommodate and localize mobile electrons.53 The lessened molecular rigidity might be attributed to: i) strain received from structure deplanarization,35,36 and ii) chemisorption induced loss of conjugation, the latter of which could also impede charge hopping by reducing the effective intermolecular π-orbital overlap.54 Except B1, although peripheral positions (B2-4) are seen easier to bind O3/O2 (Figure 1a), oxidation at the inner positions (B5-7) once occurred can considerably pull up λ. We should note the surprisingly small λ found for SOZ@B1 is quite unexpected. Despite small λ is desirable for efficient electron transfer, yet to reach that a substantial barrier (54.4 kcal/mol) should be overcome firstly to form POZ@B1 prior to SOZ@B1.



Figure 3. Electron reorganization energies of multi-site POZ formed on one or both sides of PDI. The number on the PDI backbone has two meanings: i) the sequence of adding O3 and ii) the quantity of POZ. For both-side adsorptions POZ formed on the opposite side of the molecular plane are differentiated in colors.

Since B2 and B4 are both active sites to accommodate O3 and there is no warranted experimental basis for assuming an single oxidative site on PDI, multiposition O3 adsorptions are further inspected. Considering the kinetic barrier of doping the second O3 does not notably increase (12.1 and 11.7 kcal/mol for the same and opposite side adsorption, respectively), and neither SOZ nor EPO (>30 kcal/mol) is kinetically easier to attain, hence we postulate a possible saturated adsorption scheme based on gradual accumulation of POZ. For cases considered in Figure 3, ozonizations all lift up λ compared to that of the bare PDI. As the extent of oxidation grows, λ increases notably, indicating excessive adsorption further frustrates charge hopping. Meantime, the formation of ozonides tend to i) expand the molecular space, ii) induce bending (for one-side adsorption) and ripple (for both-side adsorption) to the monomer configuration, both of which are


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considered detrimental to overall material conductivity owing to a likely disruption of intermolecular π-stacking or unit lift-off from the bulk.55

Figure 4. Barrier change as a function of applied axial electric field (F±x, F±y, F±z) respectively for O3 (a) and O2 (b) cycloadding to PDI at B2. (c)Impact of F±z on the inner C=C (B6) reactivity to



O3/O2. The positive direction of F+z is set vertical on the paper plane toward readers (pointing to the incoming O3/O2). Note that we failed to locate a proper transition state for the formation of PERy@B2 under F+y = 0.75 V/Å, hence the data is not provided here.

How to make PDI O3/O2 resistant is thus of great importance to maintain its original conductivity in air. We have previously reported surface reactions on graphene nanodots can be facilitated or suppressed by a proper electric field.35 Despite its ubiquity in OTFTs, few papers focus on the ambient oxidation of the material triggered by the field. This area appears topical not only because it is controversially discussed on whether the field is playing in this process,12,13,15 but also due to a lack of molecular-level characterization on the degradation mechanism. As B2 is susceptible to O3 and O2 attack, we first place our focus on this active site. Shown in Figure 4b, the adsorption barriers for O2 all decrease with increasing positive axial fields. Although F-x, F-y, F-z seem to suppress the reaction, yet due to symmetry they are capable as well to activate the same reaction at the mirror sites of B2. Therefore, the selected axial fields no matter applied in the positive or negative directions all can be used to facilitate O2 addition on PDI. This isotropical character has not been spotted so far, but could be pivotal in elucidating the origin of device failure in an atmosphere contains O2. Remarkably, F+z lowers the barrier by 36% (26.5→16.9 kcal/mol) as the field intensity increases from 0 to 0.75 V/Å, implying by the assistance of electric field seemingly difficult O2 cycloaddition on PDI could be triggered at room temperature.24 Similar role of F+z is also identified in the case for O3 (Figure 4a). F+z buckles the PDI molecular plane, and makes O3 easier to bind on the concave side, which could be dominated by the increased electrostatic interactions between O3 and the underlying C=C (see Figure S1 in the Supporting Information).


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When F+z increases to 0.75 V/Å, the calculated reaction barrier is only 4.4 kcal/mol. Such a small value reveals at strong F+z the formation of POZ@B2 is kinetically very facile. In contrast to O2, although chemisorption of O3 is noticeably suppressed by F±x and F±y, the energy barriers at tested field strengths are still below the minimum value (16.9 kcal/mol) found for O2 in Figure 4b. Based on the above, we can therefore envision if PDI molecules are loosely packed so that O3/O2 may partially penetrate into the aggregate, F±z considered here will enhance oxygen capture at least on one face of PDI because O3/O2 can then interact with both sides of the molecule; however, for densely π-stacked PDI molecules in which case O3/O2 may only allow to interact with the outermost surface molecules, if we assume -z is the stacking direction, F-z will act as a suppressing force to block interfacial oxidation from deteriorating the material. Further tests (Figure 4c) of F±z on the inner C=C (e.g. B6) reactivity to O3/O2 qualitatively support the presented idea, but the computed energy barriers are numerically much larger than those at B2.



Figure 5. (a) Dual-substitution scheme and corresponding oxidation barrier for the formation of POZ@B2 and PER@B2. (b) Barrier change for the formation of POZ@B2 as the number of -CN increases. Inset is the frontier orbital energy variation for bare cyanated PDI without O3. (c) Reorganization energy evolution of POZ@B2 and bare cyanated PDI as -CN increases.

In addition to electric fields, introducing electron withdrawing groups (e.g. -CN or -F) on the periphery of the perylene core has been reported useful in improving PDI air stability.17,56,57 Such improvement is usually ascribed to a reduced LUMO energy driven by the electron withdrawing group which presumably could stabilize the charge carriers,16 but few literature has directly correlated PDI air stability with the resistance to ambient oxidants. With this objective, we start by inspecting the reactivity of B2 to O3/O2 when substitution effects come into play. As shown in Figure 5a, at the same position (R1) closest to B2, electron withdrawing groups such as -F, -CF3 and -CN lead to a discernable larger barrier than that of the others to bind O3. But -F in this case is only helpful to resist O3, while -CF3 and -CN performs similarly well in inhibiting both O3 and O2 from chemisorption. Together with that the computed reorganization energies go into the order of PDI-CN(0.337eV)

Ambient Degradation of Perylene Diimide Based Organic Transistors

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