Effect of Ozone on the Stability of Solution-Processed

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Effect of Ozone on the Stability of Solution-Processed Anthradithiophene-based Organic Field-Effect Transistors Iyad Nasrallah, Kulbinder K Banger, Yana Vaynzof, Marcia M Payne, Patrick Too, Jan Jongman, John E Anthony, and Henning Sirringhaus Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 11 Jun 2014 Downloaded from http://pubs.acs.org on June 19, 2014

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Chemistry of Materials

Effect of Ozone on the Stability of Solution-Processed Anthradithiophene-based Organic Field-Effect Transistors Iyad Nasrallah†, Kulbinder K. Banger†, Yana Vaynzof†§, Marcia M. Payne‡, Patrick Too||, Jan Jongman||, John E. Anthony‡, Henning Sirringhaus†* †

Cavendish Laboratory, University of Cambridge, J J Thomson Avenue, Cambridge CB3 0HE, United Kingdom

‡

Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506, USA

||

Plastic Logic Ltd, 34 Cambridge Science Park, Cambridge CB4 0FX, United Kingdom

We have investigated the degradation effects of ozone exposure on Organic Field-Effect Transistors (OFETs) based on 2,8-Difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TES ADT) as the organic semiconducting channel layer, as well as on thin films of this widely used, high mobility, small molecule semiconductor. Electrical I-V measurements showed a loss of transistor characteristic behavior. We present 1H Nuclear Magnetic Resonance (NMR) spectroscopy results as well as X-ray Photoemission Spectroscopy (XPS) and Fourier Transform Infrared (FTIR) spectroscopy measurements showing the oxidation of the parent molecule, from which we suggest various possible reaction paths.

INTRODUCTION The interest in organic transistors has seen consistent growth over the past decade due to the new technological applications which these devices can enable, but also the scientific insight they provide about the charge transport physics of organic semiconductors. OFETs have enabled flexible display backplanes on plastic substrates and are also being developed for a wide range of sensing, imaging and logic functionalities embedded into flexible substrates.1 As these technologies are beginning to become available commercially, one of the main concerns for organic electronics is device stability. Many studies have been conducted aiming to improve the stability and performance of OFETs.2-4 Investigations are often carried out in ambient environments or ones with high humidity or oxygen content. Whilst these studies are vital, there have been very limited reports characterizing the effect of trace gases, such as ozone, which is known to readily degrade organic materials, in particular molecules with sp and sp2 carbon centers. Chabinyc et al.5 showed that ozone could be a major cause of changes in the characteristics of thiophene-based conjugated polymers in ambient air. Furthermore, not only is ozone present in the atmosphere, but it is also a natural product of several fabrication processes, especially those which require UV illumination in an air environment. This study has been conducted to characterize the effect of sizeable concentrations of ozone on diF-TES

ADT based techniques.

OFETs

through

various

measurement

diF-TES ADT is a p-type small molecule (chemical structure shown in Figure 1). It has proven to show good crystallinity, and high mobilities of >1 cm2/Vs in solutionprocessed OFETs.6 Hence, it is a widely used material for the fabrication of OFETs.

EXPERIMENTAL SECTION Top-gate, bottom-contact OFET structures were used throughout the experiments (architecture shown in Figure 1). All transistor measurements were performed on devices with 20µm channel length and 1mm channel width. Electrical characterization was performed on 8 pristine transistors, as well as 8 transistors for each ozone exposure time. A 50nm Polyimide layer was spun on glass substrates from a solution in 1-Methyl-2-pyrrolidone (NMP), and cured inside a nitrogen glovebox for 1 hour at 160˚C then 3 hours at 300˚C. This provides a planarization layer, thus mimicking a plastic substrate and improving the film forming properties of diF-TES ADT. Photolithography and thermal evaporation were used to pattern 2 nm/25 nm of Chromium/Gold as source and drain electrodes. The electrodes were treated with pentafluorobenzenethiol (PFBT) in Isopropyl alcohol (IPA) solution in order to improve injection into the semiconductor. This also helps to improve the crystal growth into the device channel region.7

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Figure 1. a) and b) show the change in the transfer characteristics due to low (0.15 ppm) and high (5-6.5 ppm) concentration ozone exposures, respectively. c) shows the change in output characteristics due to low and high ozone exposures compared to a pristine OFET. c) also shows the molecular structure of diF-TES ADT, as well as the device architecture of the top-gate bottom-contact transistors used.


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In all the experiments, diF-TES ADT solutions were spun inside a nitrogen glovebox using a constant spincoating recipe for consistency. Solutions were spun at 1000 rpm for 1 minute. The films were then annealed at 100°C for 2 minutes to evaporate excess solvent. Unless otherwise stated in the text, the solvent used was Mesitylene (C9H12, Fluka, >99% purity). In OFET characterization, the ozone exposure was carried out on the diF-TES ADT thin film before spinning the dielectric layer. It has been shown that the dielectric may act as an encapsulation layer, hence protecting the semiconductor.8 CYTOP was used as a gate dielectric, and was spun inside a nitrogen glovebox to provide a thickness of approximately 500 nm. Finally, a 25 nm Aluminium gate electrode was thermally evaporated. The OFETs were measured using a probe station housed inside a nitrogenfilled glovebox. Ozone exposure was carried out using a commercially available ozone generator bought from Heaven Fresh (Model: HF 10). The generator and the samples to be exposed were placed in a diecast box with the ozone flow output placed approximately 4 cm away from the samples. An ozone concentration meter (Ecosensors Model: A-21ZX) was also placed at a similar distance to the samples to measure the concentration over time. The exposure was done for short intervals at 0.15 ppm, as well as in half hour intervals at an average ozone concentration of 5.0-6.5 ppm inside the diecast box. The concentrations of ozone that are present in ambient air are typically 0.08 ppm. Smog conditions in cities provide ozone concentrations of typically 0.1-1 ppm. The high exposure concentrations of 5-6.5 ppm were used to accelerate the degradation effects and saturate all chemical reaction paths within a practically observable time scale of a few hours. Absorption spectra were performed on 30 nm films of diF-TES ADT spun on Spectrosil substrates. The measurements were taken in a vacuum in the order of 10-6 mbar, using a Varian Cary 6000i Spectrophotometer with a spectral range from 175 nm to 1800 nm. The samples were exposed to 5-6.5 ppm of ozone. For X-Ray Photoemission Spectroscopy (XPS) measurements, 30 nm layers of diF-TES ADT were spun on silicon substrates coated with a 50 nm layer of thermally evaporated gold to assist with charge compensation. The measurements were done using a Thermo Scientific Escalab 250Xi XPS/UPS system. Both low (0.15ppm) and high (5-6.5 ppm) concentration ozone exposure was carried out. Thin films of diF-TES ADT with 100 nm thickness were spun on glass substrates for Proton (1H) Nuclear Magnetic Resonance (NMR) measurements. Toluene (C7H8, ROMIL, Hi-Dry anhydrous) was used as a solvent for spinning. Pristine films were dissolved off using anhydrous deuterated Chloroform (CDCl3, Sigma-Aldrich,

99.8% purity anhydrous) in air, immediately after spinning. The remaining thin films were exposed to 5-6.5 ppm of ozone for a total of 6 hours. These were then dissolved off in a similar manner to the pristine films. For Fourier Transform Infrared (FTIR) spectroscopy 100 nm thick diF-TES ADT films were spun on Potassium Bromide (KBr) pellets. 5-6.5 ppm ozone exposure was carried out for 7 hours. The IR spectrum of several pristine and ozone exposed films were taken to confirm the change induced in the spectrum due to the exposure.

RESULTS & DISCUSSION Figure 1a shows that upon ozone exposure at 0.15 ppm for 10 to 20 minutes, a general reduction in p-type mobility from an average of 0.7 cm2/Vs in the pristine state to 0.1 cm2/Vs is seen, with the ON current reducing by an order of magnitude. We also see evidence for pdoping in this initial phase of exposure, which manifests itself as a more prominent shoulder in the positive gate voltage range. However, the dominant effect even at this low concentration is the dramatic drop in ON current, the significant shift of the turn-on voltage to more positive gate voltages, and the appearance of significant hysteresis. After 80 minutes of ozone exposure, we see that the transistor performance is degraded further, with larger reduction in mobility and even more apparent hysteresis. The sign of the hysteresis is such that the current is higher on the forward scan from OFF-to-ON conditions than on the reverse scan. In Figure 1b we see that when we use higher ozone concentration a very similar degradation behavior is observed and a similar nonworking state of the device is already reached after half an hour of exposure. This behavior can be explained by the ozone exposure degrading the molecule, and creating species that can trap electrons. During the positive gate voltage part of the forward transfer characteristics electrons are injected from the source-drain electrodes, but then quickly form negatively charged trap species inside the semiconducting layer without leading to a measurable mobile electron current. This negative space charge then facilitates the formation of hole accumulation already at positive gate voltages giving rise to a positive turn-on-voltage. While operating the device in the ON state the injected holes then recombine with these negative trapped electrons leading to a negative shift of turn-on voltage during the ON-part of the transfer measurement and giving rise to the observed hysteresis. In this regime the onset voltage shifts to more positive values as the transfer characteristic is started at more positive voltages (shown for 80 minutes in Figure 1a by the dashed blue line, and 0.5 hours in Figure 1b by red lines of varied style). The dramatic decrease in the ON current upon ozone is ascribed to the fact that any molecular species formed by a chemical oxidation reaction with ozone are likely to have a larger band gap and hinder efficient hole transport. After prolonged exposure more and more of the film is oxidized



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and after 1-1.5 hours the device is no longer operational. A similar degradation also appears in the output

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characteristics upon ozone exposure.

Figure 2. a) and b) show absorption spectra of diF-TES ADT with increased ozone exposure (at 5-6.5 ppm), with a) showing the change in the vibronic progression. c) shows the change in color of the thin film due to ozone exposure. d) shows the loss of crystallinity in the film with increased ozone exposure. The ideal output characteristic shape is lost after 80 minutes of low concentration ozone (Figure 1c). The current ceases to saturate whilst the overall current magnitude decreases severely with increased exposure. Hysteresis also becomes evident. The sign of the observed hysteresis in the output characteristics may be explained by assuming that under conditions where |Vd|>|Vg-VT|, i.e. in saturation, negative electrons are injected and trapped near the drain electrode in ozone generated species leading to a dynamic shift of the turn-on voltage towards positive gate voltage and a temporary rise in current on the reverse scans in the output characteristics.

polycrystalline morphology of the film; after 1.5 hrs of exposure the films appeared near amorphous (Figure 2d). The FTIR spectra for pristine and ozone treated films are shown in Figure 3. The fingerprint features at around 2300 and 2900 wavenumbers correspond to CO2 and C-H bonds respectively. Comparing the two spectra, we can see the emergence of two new peaks at 1750 and 3500 wavenumbers after ozone exposure. These are in the correct regions assignable to the C=O bond stretch and the O-H bond stretch, respectively.9

Figure 2b shows that the reduction of absorption strength associated with the higher lying π−π* transition around at 330-340 nm indicates that the conjugation length of the chromophore is being shortened due to the exposure. This is accompanied by new absorption peaks at shorter wavelengths in the ultraviolet (UV) region (below 280 nm). This shows that ozone leads to the oxidation of the molecules and creates molecular species with a higher energy gap. Figure 2c shows the discoloring of the film after 7 hours of exposure at 5-6.5 ppm. Furthermore, using an optical microscope with crosspolarizers, it was possible to observe a loss of the typical


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Chemistry of Materials Figure 3 FTIR results confirm the formation of C=O and O-H bonds due to ozone exposure.

Figure 4. The results of XPS on pristine and ozone exposed thin films of diF-TES ADT. The elements shown are oxygen (O1s), silicon (Si2p), fluorine (F1s), sulphur (S2p) and carbon (C1s). The 20 minute and 80 minute exposures are performed at low ozone concentrations.

The rise in the overall background signal below 1000 wavenumbers after ozone exposure can be attributed to the formation of O-O bonds belonging to endoperoxide groups.10 The breadth of these newly formed peaks could collectively raise the background signal when superimposed on the already existing signals. XPS (Figure 4) was used to characterize the change in the chemical composition of the films with increased exposure. The scans at 20 minutes and 80 minutes were performed at low ozone concentration exposure (0.15 ppm). The trends seen in the data are initiated even at short exposure times, and low concentrations. Whilst the oxygen (O1s) peak for the pristine film (shown in black) shows no features, a signal at approximately 532.5 eV begins to emerge with increased exposure time. The emerging O1s peak is very broad, which indicates that there are several oxidized moieties contributing to this

signal. For the remainder of the constituents (silicon (Si2p), fluorine (F1s), sulphur (S2p) and carbon (C1s)) we observe a reduction in signal and a broadening of the respective peaks. This is clearly seen in the case of S2p, for example, for which the initial characteristic doublet peak shape of sulphur becomes significantly broadened and decreases in intensity. Due to the high electronegativity of oxygen, oxidized moieties are seen in the XPS spectra for the ozone damaged films at higher binding energies than their respective pristine reference peaks. This is clearly seen in the cases of S2p, where a new broad peak begins to appear at a higher binding energy (168 eV). In the case of C1s, the peak at around 287 eV corresponding to the carbon-fluorine bond broadens whilst maintaining a significant intensity. This is also the



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correct region for carbon-oxygen bonds (single and double bonds), hence the broadening is due to new contributing signals from oxidized carbon. A new C1s peak at 288.8 eV begins to emerge with increased exposure time, even at low ozone concentrations. This peak is 4.3 eV away from the main carbon peak located at 284.5 eV. Cross-referencing with XPS libraries and previous publications, this peak can be assigned to carboxylic acid groups.11-14 To obtain further insight regarding the change in molecular structure of diF-TES ADT upon ozone treatment, NMR analysis was conducted. The 1H NMR of pristine diF-TES ADT was also recorded to serve as a reference, (Supporting Information–S1). The spectrum of the pristine material shows chemical environments which can be fully assigned, and agree with spectra reported previously in literature.15 The full 1H NMR for the ozone treated diF-TES ADT is also shown in Supporting Information-S1. Figure 5a clearly shows that the chemical environments located at 1.2 ppm and 0.9 ppm in the pristine film (black line in Figure 5a) are no longer evident after ozone exposure (red line). The NMR data on the ozone treated film also suggests a change of proton environment just below 9 ppm (Figure 5b, and Supporting Information–S2), corresponding to both ends of the anthracene core (C-C=C-H region) in the vicinity of the thiophene rings. The complexity of the reaction of ozone with organic compounds has been previously conveyed in literature.16 The strong reactivity of ozone allows for many possible oxidation pathways to take place, and hence several products to be formed. The FTIR data has revealed the formation of C=O bonds, which attach to the chromophore forming quinones (or semiquinones).17 This supports the change in proton environment along the chromophore seen in NMR (Figure 5b, Supporting Information–S2), and the oxidation of sulphur seen in XPS. Quinones are the strongest contenders to explain the hysteresis seen in the OFET electrical characteristics as they have high electron affinities.

Figure 5. 1H NMR results showing the change of chemical environment at a) the side chains (0.9ppm to 1.2ppm), and b) the chromophore.

The new peak in the C1s XPS data at 288.8 eV is most likely derived from the reaction occurring to the side chains. Criegee and Lederer18 showed that the reaction of ozone with alkynes in the presence of water produces carboxylic acid derivatives,16 as shown in Figure 6.

R

R'

+

-

O O

O

O

R

R'

+

O O O R O R'

ozone water

O O R OH HO R'

Figure 6. A highlighted reaction path in the Criegee mechanism which best describes the reaction taking place at the side chains.

This is further corroborated by the appearance of an OH bond peak in the FTIR data, a constituent of carboxylic acid groups. The XPS data agrees very closely with the path highlighted in Figure 6, due to the humidity present in air during the exposure. Furthermore, the disappearance of the NMR peaks at 1.2 ppm and 0.9 ppm suggests that the chemical environment experienced by the alkyl side chains is now dissimilar to when attached to the anthracene core for


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pristine diF-TES ADT. It has previously been shown by Fudickar et al.19 that the triple bond in the alkyne side chains of acenes act to stabilize the molecule by preventing the molecule from decomposing as a result of oxidation. Endoperoxides (also shown in our FTIR data) on the central ring are formed as a result, with the reaction being reversible to the pristine molecule by thermolysis. The method of oxidation described in the above-mentioned study involves photooxidation as a result of reacting with singlet oxygen. The reaction with ozone in our investigation is expected to be much more damaging, resulting in permanent degradation of the molecule.

CONCLUSIONS

encapsulation provided, ozone may also be an important contributing factor to long term device degradation during storage and operation.

ASSOSCIATED CONTENT Supporting Information S1 – Pristine and Ozone treated diFTES ADT NMR Spectra Supporting Information S2 - Change of Proton Environment around the Anthracene Core

This material is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION

We have shown that ozone exposure has a detrimental non-reversible effect on the performance of OFETs based on diF-TES ADT as the semiconducting layer. OFET devices show a marked decline from their pristine working state to a completely redundant form after ozone exposure in air. We have examined the oxidized films in comparison with their pristine analogues using UV/VIS, NMR, XPS and FTIR. Our study points us to suggest that diF-TES ADT decomposes through several oxidation pathways. We are able to link our experimental results to established reactions in literature, such as the formation of quinones, and the formation of a carboxylic acid compounds. The quinones introduce strong hysteresis in the OFET electrical characteristics due to their high electron affinities. Furthermore, the destruction of the molecule is the reason for the loss of ideal charge transport. Our primary objective in this work has not been to conclusively identify the final reaction product (or products) due to the exposure of diF-TES ADT to ozone. This is beyond the scope of our investigation and might be addressed by future High Performance Liquid Chromatography / Mass Spectrometry (HPLC/MS). Our aim here has been to understand the relationship between device performance and ozone exposure. These reactions occur throughout the bulk of the film and are responsible for the dramatic degradation of the device characteristics upon ozone exposure. We would like to emphasize that the observed reactivity of diF-TES ADT towards ozone is not a consequence of the relatively high exposure concentration used here. We have also observed similar degradation effects at lower concentration albeit over longer timescales. Our results have important implications for the fabrication and operation of organic FETs based on diF-TES ADT. During device fabrication care needs to be taken to minimize the exposure of the films to ozone, particularly during the critical stages of manufacture where the semiconductor film is not yet protected by the gate dielectric layer. Ozone is often present in cleanroom environments where UV cleaning and other UV operated equipment is used. We anticipate that depending on the level of air

Corresponding Author *E-mail: [email protected]

Present Addresses Dr. Yana Vaynzof has relocated to the following address: § Centre for Advanced Materials, Universität Heidelberg, Im Neuenheimer Feld 227, 69120 Heidelberg, Germany

ACKNOWLEDGEMENTS The authors would like to thank Plastic Logic Ltd. for initiating and funding this project.

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(17) Kwamena, N.-O. A.; Earp, M. E.; Young, C. J.; Abbatt, J. P. D. J. Phys. Chem. A 2006, 110, 3638. (18) Criegee, R.; Lederer, M. Liebigs Ann. Chem. 1953, 583, 29. (19) Fudickar, W.; Linker, T. J. Am. Chem. Soc. 2012, 134, 15071.

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Effect of Ozone on the Stability of Solution-Processed

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