NEXAFS spectroscopy of conjugated polymers

Accepted Manuscript NEXAFS Spectroscopy of Conjugated Polymers Masrur Morshed Nahid, Eliot Gann, Lars Thomsen, Christopher R. McNeill PII: DOI: Reference:

S0014-3057(16)30017-9 http://dx.doi.org/10.1016/j.eurpolymj.2016.01.017 EPJ 7209

To appear in:

European Polymer Journal

Received Date: Revised Date: Accepted Date:

9 November 2015 17 December 2015 7 January 2016

Please cite this article as: Nahid, M.M., Gann, E., Thomsen, L., McNeill, C.R., NEXAFS Spectroscopy of Conjugated Polymers, European Polymer Journal (2016), doi: http://dx.doi.org/10.1016/j.eurpolymj.2016.01.017

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NEXAFS Spectroscopy of Conjugated Polymers Masrur Morshed Nahid,1 Eliot Gann,1,2 Lars Thomsen,2 Christopher R. McNeill1* 1

Department of Materials Science and Engineering, Monash University, Wellington Road,

Clayton, VIC 3800 Australia. 2Australian Synchrotron, 800 Blackburn Road, Clayton, VIC 3168, Australia *Corresponding author. Email: [email protected] Abstract This feature article provides a general introduction to Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy of conjugated polymers as well as a perspective on the information that NEXAFS spectroscopy is able to provide. While conjugated polymers are of interest for a range of applications including organic light-emitting diodes, polymer solar cells and organic field-effect transistors (OFETs), this feature article will focus on the application of NEXAFS spectroscopy for revealing information about the interfacial orientation and alignment of conjugated polymer chains with respect to substrate surfaces which is of particular relevance for understanding charge transport through the thin accumulation layer in a polymer OFET. The potential for NEXAFS spectroscopy to provide experimental information about dihedral angles in donor-acceptor copolymers is also highlighted. Keywords NEXAFS Spectroscopy; organic field-effect transistors; conjugated polymers; synchrotron radiation

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1. Introduction Conjugated polymers are characterised by having a backbone with alternating single and double bonds that afford extensive electron delocalisation along the polymer chain, see figure 1(a).1 This extensive electron delocalisation imparts conjugated polymers with interesting electronic and optical properties, qualifying them for application in a range of devices including light emitting diodes, 2 solar cells3 and fieldeffect transistors.4 To enable solution processability, conjugated polymers typically have flexible side chains that also influence the way the polymer chains pack in the solid-state. Many conjugated polymers are semicrystalline (though many are also amorphous or weakly aggregating) with polymer chains packing in crystalline lamellae as shown schematically in figure 1(b). The optical and electronic properties of conjugated polymers are highly anisotropic, with polymer chains preferentially absorbing (or emitting) light with electric field vector parallel to the polymer backbone. Change transport is also highly anisotropic, with charge carrier mobility being highest along the polymer backbone direction, followed by along the - stacking direction, and charge transport being very inefficient along the lamellar stacking direction due to the insulating nature of the side chains. For many device applications, the alignment and orientation of polymer chains strongly influences device functionality.5-8

Figure 1. (a) Chemical structure of poly(3-hexylthiophene) (P3HT), a prototypical conjugated polymer. (b) Schematic diagram showing the way conjugated polymers pack to form crystalline lamellae with the lamellar stacking and - stacking directions shown.

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Figure 2. Schematic diagram of the three most common OFET geometries. Organic field-effect transistors (OFETs) employ an organic semiconductor such as a conjugated polymer as the active semiconducting layer,9 see figure 2. The development of new conjugated polymers has enabled continued improvement in charge carrier mobilities with mobilities of the order of 10 cm2/Vs if not higher,8, 10-12 opening the door to applications such as wearable, portable and distributed electronics.4 The current that flows between the source and drain electrodes that contact the organic semiconductor in an OFET is mediated by the voltage applied to the gate electrode. This gate electrode is separated from the organic semiconductor layer by an insulating dielectric layer. Organic semiconductors used in OFETs are not intentionally doped and have low intrinsic free-carrier densities. With no applied gate voltage, the organic semiconductor has low conductivity and very little current flows between source and drain electrodes (the OFET is said to be in the off-state). When a gate voltage is applied, charges are injected from the source/drain electrodes and accumulate at the dielectric/semiconductor interface, establishing a layer of charges that can move under the influence of an electric field (the OFET is then said to be in the on-state). When a voltage difference is then applied between the source and drain electrodes a current flows. Due to the anisotropic electronic properties of conjugated polymers, the packing and molecular orientation of polymer chains in the active layer of an OFET strongly influence the charge carrier mobility in the channel and hence the performance of the device.13 Importantly, since the layer of accumulated charge that forms in an OFET is restricted to only a few nanometres from the semiconductor/dielectric interface, 14 it is the packing and molecular orientation of polymer chains at the surface of the conjugated polymer film that determines OFET operation. While there are a number of characterisation techniques that are able to probe the packing and molecular orientation of conjugated polymer thin films, such as grazing-incidence wide-angle x-ray scattering (GIWAXS),15-18 transmission electron microscopy,19-21 variable angle spectroscopic ellipsometry,22-24 and polarised infrared spectroscopy,23, 25 none of these techniques has the surface sensitivity required to exclusively probe the structure of the first few

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nanometres of a conjugated polymer film commensurate with the thickness of the accumulation layer. By measuring X-ray absorption via the detection of Auger electrons, near edge X-ray absorption fine-structure (NEXAFS) spectroscopy is able to achieve surface sensitivity of  3 nm26, 27 giving it the ability to exclusively probe the molecular orientation of the relevant charge transporting region in an OFET. This feature article serves to provide a general introduction to the fundamentals of NEXAFS spectroscopy, the experimental requirements for a successful NEXAFS experiment and how molecular orientation is determined from angle-resolved NEXAFS experiments. A particular focus is given to the measurement of polymer thin films, providing more background and technical information than is able to be provided in a research paper. This feature article also serves to highlight the unique information that NEXAFS spectroscopy can provide and its relevance to understanding the operation of organic polymer FETs. 2. NEXAFS Spectroscopy 2.0. Historical background Near-Edge X-ray Absorption Fine-Structure (NEXAFS) spectroscopy was developed in the 1980s primarily as a tool for studying the orientation of molecules adsorbed on surfaces. For the study of monolayer or sub-monolayer samples, the surface sensitivity of NEXAFS originates from being able to tune the X-ray beam to the absorption edge specific to an element that is present in the surface layer but not present in the substrate (see section 5.1 in Stöhr26). The application of NEXAFS has been extended to thin films, particularly polymer thin films, where high surface sensitivity can still be achieved using electron yield detection, relying on the limited electron mean-freepath in solids. A full introduction to the technique can be found in the monograph by Joachim Stöhr who pioneered the technique.26 2.1. Basics NEXAFS spectroscopy involves the measurement of the X-ray absorption spectrum of a material close to one of its absorption edges. Each element has different characteristic absorption edges corresponding to the different energies required to ionise different atoms by removing electrons from different core levels. For carbon-based polymers, the carbon K-edge is the most important absorption edge, which occurs at around 285 eV corresponding to the energy required to ionise a core 1s electron. Other important absorption edges for organic polymers include the nitrogen K-edge at around 400 eV and the oxygen K-edge at around 530 eV with N and O being common heteroatoms. Close to these absorption edges there are additional non-ionizing resonant transitions from the relevant core level (for example the carbon 1s level) to unoccupied

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electronic states often associated with (but not limited to) * and * anti-bonding orbitals. For a given X-ray energy, the total X-ray absorption cross section at that energy will have contributions from all shells present with binding energies less than the incident X-ray energy. Close to the absorption edge of an inner shell (such as the carbon K-edge) the contribution from outer shells contributes a smoothly varying non-resonant crosssection, and what is typically reported in NEXAFS spectroscopy is the partial X-ray absorption cross-section with the underlying contribution from out shell transitions subtracted. For transmission NEXAFS measurements, where the absorption spectrum is measured by comparing the intensity of the incident and transmitted X-ray beam, X-ray scattering can in principle contribute to the reduced directly transmitted intensity, however the elastic scattering of X-rays from the electronic potential is generally weak for low Z elements such as carbon. The NEXAFS spectrum of a molecule is generally characterised by resonant transitions from the core state to antibonding molecular orbitals as well as transitions to continuum states above the vacuum level, see figure 3. Excitation to continuum states contributes a step-like contribution, corresponding to ionisation of the relevant core electrons. Transitions from the core state to * antibonding states are generally found below the ionisation potential, while transitions to

* antibonding states are found above the ionisation potential. Just below the step-edge, Rydberg-like states can also be observed, corresponding to transitions to bound H-like states with the excited electron bound to the core hole. For conjugated polymers that contain C-H bonds, pure Rydberg orbitals become mixed with hydrogen-derived valence orbitals and are not as distinct as in simpler systems.26

Figure 3. Schematic energy diagram showing possible electronic transitions from a core state to anti-bonding and continuum states following absorption of an X-ray by a conjugated polymer, and the associated NEXAFS spectrum. For simplicity Rydberg states are not shown. The precise assignment of peaks in the NEXAFS spectrum of a conjugated polymer can be challenging particularly for co-polymers containing multiple chemical

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species. Compared to techniques such as infrared spectroscopy, the peaks observed are rather broad, due in part to the limited experimental energy resolution of ~ 30 meV to 100 meV at the carbon K-edge depending on specifications of the beamline. Similar to Xray photoelectron spectroscopy, shifts in the energy of core electrons due to different chemical bonding environments also result in peak broadening or shifting. Variations in

-bond length due to either different bonding configurations (eg C-C vs C=C) and molecular vibration also lead to broadening of * resonances. Theoretical calculations can assist in associating particular peaks with specific transitions, 28, 29 however such calculations themselves are not straight forward due to strong Coulomb interaction between the excited electron and the core hole. Nevertheless, general distinctions can be made between the position of transitions to * and * anti-bonding orbitals and intuition can be built up through the study of the NEXAFS spectra of simpler molecules. An example of how the different peaks in the NEXAFS spectrum of a conjugated polymer can be related to the constituent parts of a conjugated polymer is given in figure 4. Figure 4 compares the carbon K-edge NEXAFS spectrum of poly(3hexylthiophene) P3HT (thin film) to that of thiophene in the gas phase 30 and that of octadecyltrichlorosilane (OTS) in a self-assembled monolayer (SAM).31 The NEXAFS spectrum of the OTS SAM is characteristic of the long alkyl chain with a peak at ~ 287.5 eV associated with the C 1s  C-H * transition and the peak at ~ 293 eV associated with the C 1s  C-C * transition (see sections 4.2 and 4.3 in Stöhr).26 The thiophene molecule in contrast which has no alkyl side chain has two characteristics peaks at ~ 285.5 eV and 287 eV associated with the C 1s  C=C * transition and the C 1s  C-S

* transition respectively.26 The NEXAFS spectrum of P3HT can then be seen as a combination of the separate NEXAFS spectra of thiophene and OTS, with the NEXAFS spectrum of thiophene accounting for X-ray absorption by carbon atoms of the polymer backbone and the NEXAFS spectrum of OTS reflecting the X-ray absorption by carbon atoms in the alkyl sidechains. The ability to describe the NEXAFS spectrum of more complicated molecules in terms of the NEXAFS spectra of the constituent parts is known as the building block approach (see page 179 in Stöhr26) There are limitations to the validity of this approach, particularly for situations where the assembly of the constituent parts leads to a significant change in electronic structure. However for the example presented here, the relationship between the general features of the NEXAFS spectrum of P3HT and its constituent parts is clear, and provides a good example for the identification of where peaks associated with C 1s  C=C * transitions, C 1s  C-H * are expected.

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Figure 4. Comparison of the NEXAFS spectra of P3HT, thiophene in the gas phase and OTS. The NEXAFS data of thiophene is taken from Ref. 30 while the OTS data is taken from Ref. 31. Further analysis of a NEXAFS spectrum can be provided by peak fitting. In this semi-empirical approach, the NEXAFS spectrum for P3HT is fitted with a series of Gaussian peaks and a step-edge, as shown in figure 5. The Gaussian peaks describe resonant transitions to molecular orbitals, while the step-edge describes excitation of core electrons to the continuum. The details of the fitted Gaussian peaks does not provide direct information about the underlying physical transitions however as there are likely to be distinct transitions which are not distinguishable either due to the limited energy resolution of the technique or due to broadening of peaks caused by (for example) different bonding environments and different bond lengths (particularly for * features). Such peak fitting does enable the intensity of resonant transitions to be more reliably determined through the area of the fitted peaks, and is particularly helpful for discerning the different contributions from overlapping peaks such as in the * manifold, or the intensity of transitions above the step-edge can be disentangled from that of the stepedge. Generally, a fit with the fewest number of Gaussians is preferred, with the width of the Gaussians restricted to reflect physically realistic transitions (for example avoiding Gaussians that cover much of the spectrum). Note that in figure 5 the highest energy Gaussian is an exponentially-modified Gaussian, modelling the asymmetric broadening of

* resonances which results from vibrational motion of atoms in the molecule. For analysis of NEXAFS spectra, beamlines such as the Soft X-ray beamline at the Australian Synchrotron have produced freely available analysis tools to enable curve fitting and other analysis of NEXAFS spectra.32

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Figure 5. Experimental P3HT NEXAFS spectrum (black, squares) fitted to a series of Gaussian peaks and a step-edge. Note that an exponential-modified Gaussian is used for the highest energy peak.

Figure 6. NEXAFS spectra of the polyfluorene copolymers PFB, F8BT and F8TBT. A feature of NEXAFS spectroscopy is the chemical fingerprint that is provided, with different conjugated polymers exhibiting different NEXAFS spectra. For example, figure 6 presents the NEXAFS spectra of the polyfluorene copolymers PFB, F8BT and

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F8TBT. While being based on similar conjugated building blocks, the NEXAFS spectra of these polymers exhibit clear differences, particularly at energies corresponding to C1s to

* transitions (highlighted in the inset). In particular, PFB has a rather sharp * resonance due to the fact that most -bonded carbon atoms in PFB find themselves in a similar chemical environment (within a phenyl ring). In contrast to PFB, for F8BT there are -bonded carbon atoms in the benzothiadiazole moiety which results in a splitting of the sharp * resonance and the formation of a low energy shoulder. The spectra of F8TBT can be seen as a hybrid of F8BT with P3HT, with the low energy shoulder associated with the benzothiadiazole unit diluted, and more prominent peak at 285 eV associated with the additional thiophene rings. For further details on differences in the NEXAFS spectra of different conjugated polymers and indeed for calibrated reference spectra of many common conjugated polymers the reader is directed to the article of Watts et al.33 The ability to distinguish between different polymers with only a subtle difference in chemical bonding is important as it allows for chemical contrast to be achieved for materials that otherwise have similar chemical composition and density. 34 While not the focus of this article, differences in the NEXAFS spectra of different conjugated polymers can be used to provide contrast in scanning transmission X-ray microscopy (STXM) experiments.35, 36 Differences in the X-ray optical constants in general near an absorption edge can furthermore be used to generate material-specific scattering contrast in resonant soft x-ray scattering (R-SoXS) experiments.37 Furthermore, surface-sensitive NEXAFS spectroscopy can be used to determine the surface composition of blends. 38-40 Another key feature of NEXAFS spectroscopy, and of particular relevance for studying polymers used in OFETs, is that the resonance intensity of a particular transition can be sensitive to the relative orientation of the molecule to the electric field vector of the synchrotron beam. Specifically for a particular transition, the transition probability (and hence resonance intensity) is dependent on the overlap between the electric field vector, E, associated with the linear polarisation of the beam and the transition dipole moment, O of the transition:

(1) where  is the angle between E and O. The transition dipole moment encodes (along with other information) the directionality or polarisation of a transition from the initial state to the final state. The directionality of O is determined by the symmetry of the initial and final orbitals. For K-edge transitions the initial state is a 1s orbital which has spherical symmetry and hence the directionality of O is determined by the symmetry of

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the final state. For C 1s  * transitions the * molecular orbitals are oriented perpendicular to the C-C bond axis. Thus for aromatic structures such as thiophene rings and phenyl rings which tend to make up the backbone of conjugated polymers, the C 1s

 * transition dipole moment is oriented perpendicular to the plane of the aromatic ring (and hence perpendicular to the conjugated backbone), see figure 7. In contrast, for C 1s  * transitions the * molecular orbitals are oriented along the C-C bond axis and hence the C 1s  * transition dipole moment is oriented along the C-C bond axis. Therefore by measuring the NEXAFS spectrum of a conjugated polymer for different incident polarisations, changes in the resonance intensity of a transition can be used to determine the orientation of the transition dipole moment of that transition. The variation of resonance intensity as a function of polarisation is referred to as the dichroism of a transition. When the orientation of the transition dipole moment with respect to the structure of the molecule is known, then the observed dichroism of a particular transition can be used to determine molecular orientation (see chapter 9 in Stöhr26).

Figure 7. Geometry of an angle-resolved NEXAFS spectroscopy experiment, to determine the molecular orientation of a thiophene ring with respect to the plan of the substrate.

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Figure 8. (a) Angle-resolved NEXAFS spectra of a spin-coated P3HT film. (b) Dichroism of the C 1s  * transition at 285 eV with a fit to equation 2.

Figure 8(a) presents angle-resolved NEXAFS spectra of a spin-coated P3HT film, acquired by measuring the NEXAFS spectrum for different X-ray angles of incidence. Note that these spectra represent the normalised, energy-dependent partial X-ray absorption cross-section, with the value of the pre-edge spectrum (280 eV to 282 eV) set to 0 and the value at 320 eV set to 1. The pre-edge region is generally identified as being sufficiently before the onset of the first resonant transitions in the spectrum, while the post-edge is identified as being sufficiently above the ionisation threshold such that there are no longer any resonant transitions. Since there is no dichroism associated with ionisation of electrons to the continuum, spectra acquired at different angles (which have different beam footprints) can be normalised by measuring the NEXAFS spectrum out to high enough energy such that there are no orbital-specific transitions, with the measured absorption strength at 320 eV being an indication of how much carbon is being sampled. By normalising at this energy, we are then normalising the spectra to the number of carbon atoms contributing to the collected intensity. Note that in this approach the pre-

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edge background is assumed to be essentially flat though in reality will have a decaying form associated with absorptions from lower energy transitions. It is possible to fit a decaying background to better account for the background signal, and further details can be round in the article of Watts et al.41 Note also that detector efficiency will also affect the shape of the spectrum and is often assumed that detector efficiency does not significantly vary over the spectral range of interest though this should also be considered. The geometry of the angle-resolved experiment is schematically represented in figure 7, although for simplicity only a single thiophene ring is represented rather than a P3HT chain with side-chains. In this geometry the X-ray beam is incident in the xz plane with an angle of incidence of

. For simplicity only the case of perfectly linearly

polarised

X-rays are considered (as is typically the case for undulator beamlines) with the polarisation vector of the beam, E, also in the xz plane. The xy plane defines the substrate plane, with the thiophene ring oriented such that the tilt angle of the transition dipole moment, O, from the surface normal (the surface normal is along the z axis) is given by

. The tilt angle  ranges from  = 90 for the thiophene ring being completely

‘edge-on’ relative to the substrate, to

 = 0 for the thiophene ring being completely

‘face-on’ to the substrate. Note that for many conjugated polymer thin films the film thickness (of order 20 – 100 nm) is typically smaller than the length of an extended polymer chain with polymer chains packing such that the polymer backbone lies in the plane of the film.17 With conjugated polymers being characterised by having rigid, planar backbones,

 then characterises the tilting of the planar conjugated backbone

with

respect to the substrate. For spin-coated polymer films, on average there is no preferential alignment of polymer chains along any direction in the plane of the substrate. Furthermore, the size of the X-ray beam in NEXAFS spectroscopy is typically larger than a single crystalline or liquid crystalline domain, meaning that the beam interrogates an ensemble of polymer backbones conformations, sampling backbones with a large distribution of backbone conformations. That is, the beam samples thiophene rings with essentially every possible value of

 which has the effect of azimuthally averaging the -dependence of EO. For

such samples with rotational symmetry about the surface normal (typical of most spincoated polymer samples) the resonance intensity, I, of a C 1s  * transition depends only on the tilt angle of the conjugated ring structures (conjugated backbone) from the surface normal. Indeed, it can be shown that (equation 9.16a in Stöhr):26

(2)

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where TRI is the total resonance intensity (the intensity measured for EO = 1). Therefore the extent of the measured dichroism (the solely by

 dependence of I) is determined

, and to characterise the tilt angle of the system it is sufficient

to measure I()

for an arbitrary azimuthal angle . Figure 8(b) presents a plot of the resonance intensity of the C 1s  * transition at ~ 285 eV (quantified by fitting Gaussian peaks and using the fitted peak area) as a function of . By fitting of the data to equation 2, a tilt angle of

 = 68.6 is determined. Note that since there is an appreciable amount of disorder in even semicrystalline polymer thin films, the determined value for

 actually represents

an ensemble-averaged tilt angle, designated as <>. A weakness with NEXAFS spectroscopy is that it provides information about the average tilt angle but not the distribution of tilt angles. For highly crystalline samples where there is uniform organisation of molecules, one can be confident that the measured tilt angle reflects a specific orientation of the molecule with respect to the substrate. For polymer films with a range of confirmations and potentially a large distribution of tilt angles, if the average tilt angle returned is intermediate, there can be ambiguity as to whether the polymer backbone has a genuine preference to orient with an intermediate tilt angle or whether there is a large distribution of tilt angles with no strong preference for an edge-on or face-on configuration. For the case of rotational substrate symmetry (typical for spincoated polymer films) the tilt angle that coincides with zero dichroism (sometimes referred to as the “magic angle”) is tilt angle of

 ≈ 54.7. Thus if an experiment returns an average

 = 54.7 one cannot be sure whether a genuine preference for a slightly

edge-on orientation is being exhibited (54.7  is greater than 45) or whether there is a disordered polymer configuration. However, the closer the measured average tilt angle is to the extremes of

 = 0 and  = 90 the higher the certainty that the measured tilt

angle indeed corresponds to a preferred molecular orientation. Interestingly, in certain cases it is possible to determine the distribution of tilt angles using scanning transmission X-ray (spectro)microscopy (STXM) at the carbon edge.42 For samples with mesoscale domain structure, STXM can resolve individual liquid crystalline domains. STXM experiments are usually performed at an angle of incidence of

 = 90. By

acquiring images at different linear polarisations, the azimuthal orientation of the transition dipole moment for a given domain,

, as well as the in-plane component of O,

can be deduced. With knowledge of the TRI of the transition, the tilt angle

 can thus be

deduced. (The TRI can be found from an angle-resolved NEXAFS experiment with

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at

 = 54.7 according to equation 2.) A distribution of tilt angles can then be

built up from calculation of

 for the many different pixels that make up the STXM image.

It should be noted that the average tilt angle measured by NEXAFS spectroscopy cannot be identified with the average orientation of the relevant crystallographic axis as probed by grazing incidence wide-angle X-ray scattering (GIWAXS). For P3HT, for example, the backbone is tilted within the unit cell with calculations predicting a tilting of the transition dipole moment of

 = 67 with respect to the lamellar stacking direction.43

Thus for perfectly edge-on oriented P3HT crystallites a tilt angle of the transition dipole moment of

 = 67 is to be expected.

Figure 9. Angle-resolved NEXAFS spectra of a highly-aligned P(NDI2OD-T2) thin film: (a) Azimuthal dependence taken at a polar angle of

 = 90; (b) Plot of the azimuthal

dependence of the C 1s  * resonance intensity; (c) Polar dependence taken at an azimuthal angle of  = 0; (d) chemical structure of P(NDI2OD-T2). Reprinted with permission from Ref.

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. Copyright 2013 American Chemical Society.

An interesting aspect of polymers is their ability to be aligned with the majority of polymer backbones directed along the same axis. Experimentally such alignment can be achieved by mechanical rubbing,45, 46 directional solidification47 (for example with blade coating or zone casting), mechanical imprinting5 or through the use of nanogrooved substrates.5 This alignment of polymer chains produces interesting anisotropic optical and electronic properties. In such cases where all (or the vast majority) of polymer

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chains are aligned along a particular direction, the associated NEXAFS dichroism must be characterised as a function of both azimuthal and polar angles. Figure 9 presents the NEXAFS spectra of highly aligned films of the donor-acceptor copolymer P(NDI2OD-T2) produced via zone-casting.44 Figure 9(a) shows the variation in the spectrum as the sample is azimuthally rotated with respect to the X-ray beam (and hence with respect to the electric field vector). Pronounced dichroism is observed in both * and * resonances. Figure 9(b) plots the azimuthal dependence of the * resonance with a near-extinction of the * resonance intensity at an azimuthal angle of

 = 90.

Dichroism is also observed for the polar scan (Figure 9(c)) with a higher * resonance intensity for normal incidence compared to grazing incidence. For such highly aligned films, the azimuthal and polar dependence of the resonance intensity is given by (equation 9.14a in Stöhr):26





 (3)

For the case of

 = 90 (normal incidence) equation 

3 simplifies to:

 (4)

giving the cosine2 dependence seen in figure 9(b). For the case of  = 0 (the azimuthal angle which gives the greatest resonance intensity) equation 3 simplifies to:



 (5)

and hence from the different angle of

 dependence of I the tilt angle can be deduced. Note that a

 = 45 produces the special case of zero dichroism meaning that

aligned samples and samples with azimuthal symmetry have different “magic angles.” Indeed, for the observed dichroism in figure 9(c) an average tilt angle of <> = 57.5 is determined. Measuring the azimuthal dependence of the C 1s  * transition is also a way to quantify the degree of polymer backbone alignment. By noting the maximum and minimum intensities corresponding to the electric field vector being perpendicular and parallel to the backbone axis, the dichroic ratio, D, can be calculated:

(6) with D ranging from 0 for no alignment to 1 for perfect alignment. For the data shown in figure 9, a dichroic ratio of D = 0.86 is calculated. Alternatively, the degree of anisotropy can be calculated as

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(7) With a value of R = 16 calculated for the data of figure 9. Note that the above treatment of aligned polymer films assumes that the polymer chains are lying flat in the plane of the substrate. For cases where the polymer chains have a characteristic tilt (in addition to the edge-on / face-on tilt) a fuller characterisation has to be undertaken with more parameters defined, with an example given by Stöhr and Sammat for rubbed polyimide films.48

Figure 10. Schematic of the different NEXAFS measurement modalities. 2.2. Measurement modalities 2.2.1 Transmission The X-ray absorption spectrum of a material can be measured in a number of ways, see figure 10. Similar to optical spectroscopy, the NEXAFS spectrum of a material can be measured in transmission mode, where the intensity of the X-ray beam before (Io) and after (IT) the film is measured. The X-ray optical density is then calculated as:

(8) which is proportional to the extinction co-efficient. Note that in the soft X-ray community it is conventional to take the natural logarithm, rather than the base-10 logarithm used in the optical community. The optical density can be expressed as:

(9) where

(E) is the energy-dependent mass absorption co-efficient (with units of cm2/g),

is the mass density (in units of g/cm3) and t is the film thickness (in cm). Sufficiently

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far above the absorption edge where the NEXAFS spectrum is dominated by the stepedge,

(E) is determined by the absorption cross-sections of the bare atoms that make

up the polymer. The bare atomic cross-sections (in units of cm2/atom) have been tabulated,49 and thus it is possible to calculate

(E) for a given material based on the

elemental composition of the material.32 If the optical density is measured for a film of unknown thickness and density, comparison with the calculated mass absorption coefficient based on the bare atom scattering cross sections (see figure 11) enables the measured OD spectrum to be converted to a unit of cm 2/g. Alternatively, if the film thickness or density is already accurately known, measurement of the OD along with calculation of the mass absorption co-efficient enables either the material density or thickness to be determined.50

Figure 11. NEXAFS spectrum of P(NDI2OD-T2) (red graph) scaled to fit the bare atom mass absorption coefficient (black curve). Transmission measurements are obviously a bulk sensitive technique with the NEXAFS spectrum acquired by measuring the proportion of the X-ray beam absorbed along the path of the X-rays through the sample. In order to measure the transmission NEXAFS spectrum of a polymer film, the film either needs to be free-standing such as supported by a TEM grid, or on an X-ray transparent substrate such as a silicon nitride membrane. Free standing films are generally preferred as silicon nitride membranes contribute a background to the spectrum of the polymer, particularly across the nitrogen edge where the background is resonant. Conjugated polymers have similar extinction coefficients at the carbon edge to optical energies, such that films of ~ 100 nm are ideal for transmission measurement. Films that are much thinner will not absorb enough light, while films that are too thick will absorb too much light making accurate measurement of

IT difficult. Transmission measurements can be measured under low vacuum or under a partial pressure of helium such that there is minimal scattering of X-rays. Transmission

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measurements are most commonly performed in a transmission X-ray microscopy environment, where a normal angle of incidence is usually used.51 2.2.2 Fluorescence Yield Fluorescence Yield (FY) is another bulk-sensitive technique which measures X-ray absorption through detection of fluorescent X-rays emitted subsequent to photoabsorption. Following absorption of an X-ray, the excited electron may be excited to a bound state or to a continuum state. Regardless of whether a bound or continuum state is excited, the core hole that is generated is filled by an electron from a higher shell. The energy lost by the electron as it fills the core hole is either carried away by a fluorescent photon or by an Auger electron, as shown in figure 12. For a given core hole, a constant fraction of recombination events will lead to emission of an Auger electron as opposed to a fluorescent photon, meaning that in both cases the number of Auger electrons or fluorescent photons emitted will be directly proportional to the number of Xrays absorbed by that core electron. For low-Z elements such as carbon, the emission of an Auger electron is more than two orders of magnitude more likely than the emission of a fluorescent photon, making fluorescent yield measurements for carbon-based polymers more challenging compared to other materials. The characteristic energy for a transition from a carbon 2p orbital to a 1s orbital (the carbon K line) is 277 eV meaning that these fluorescent photons are X-rays.

Figure 12. After a core electron is promoted to an anti-bonding state (or excited to a continuum state above EV) a core hole is created. The core hole is subsequently filled by an electron from a higher lying bonding orbital with the energy lost given to either an Auger electron or a fluorescent photon. The detection of fluorescent photons can be achieved using a variety of detectors, such as photodiodes that have high quantum yields at soft X-ray energies (over 100 electrons per photon) or channel plate detectors that detect cascades of photoelectrons generated by X-rays that impinge upon them. FY measurements can also be performed under atmospheric conditions making this technique more versatile than electron yield measurement which requires high vacuum. Reported FY spectra typically plot the measured fluorescent photon flux detected as a function of excitation energy normalised to the incident photon flux (I0). Plotting in this way, the reported FY signal is not

18

proportional to the extinction co-efficient of the material. For polymer films of increasing thickness, the FY signal increases with film thickness but not in a linear fashion. Rather, the number of absorbed photons per unit time IA increases with increasing film thickness as:

(10) hence to covert a measured FY signal to something that is proportional to an extinction coefficient (such as the product

(E) ) requires accurate knowledge of the thickness of

the illuminated region. Moreover, FY spectra may be distorted by self-absorption, whereby a fluorescent X-ray generated in the bulk of the sample is absorbed before it leaves sample. The probability of self-absorption varies with escape depth, such that the degree of self-absorption will also vary for the different angles of an angle-resolved FY NEXAFS experiment. For these reasons, FY NEXAFS measurements of polymer films do not easily produce spectra where the reported signal is proportional to the extinction coefficient and hence proportional to resonance intensity. This limits the use of FY for quantitative determination of average tilt angles, though such spectra still may provide qualitative information regarding whether a molecular plane is orienting on average more face-on or more edge-on to the substrate. 2.2.3 Electron yield As shown in figure 12, the relaxation process that fills the core hole can also produce an Auger electron, which for low-Z elements such as carbon is much more probable than the production of a fluorescent photon. Detection of Auger electrons emitted from the sample thereby provides another way to infer the X-ray absorption spectrum of a material. Auger electrons have characteristic kinetic energy, with Auger Electron Yield (AEY) mode using an energy analyser (such as used in an X-ray photoelectron spectroscopy system) to directly detect Auger electrons discriminating against other emitted electrons. Since only Auger electrons produced at the very surface of the film are able to escape without experiencing an inelastic scattering event that would reduce their kinetic energy, AEY detection selectively detects photoelectrons emitted from the very top surface of the film with a surface sensitivity of order 1 nm. Auger electrons produced deeper within the film are inelastically scattered, transferring some of their energy to other electrons in the sample. If enough energy is transferred from the initial Auger electron to a scattered electron, this secondary electron may acquire enough energy to leave the sample as well. Increasing the energy window over which photoelectrons are detected thereby enables elastic and inelastically scattered Auger electrons to be detected, and ultimately secondary electrons as well. In a partial electron yield (PEY) experiment, an electron detector such as a channeltron is used, with a retarding voltage applied to a grid in front of the channeltron

19

such that electrons with kinetic energy above a certain threshold, Ep, are detected. PEY detects electrons from slightly deeper within the film, but is still highly surface sensitive with the electron escape depth in solids being typically < 5 nm. Finally, total electron yield (TEY) detection measures the flux of all electrons that are emitted from the sample by measuring the drain current flowing to the sample to maintain the electrical neutrality of the sample. The TEY signal is dominated by low energy secondary electrons with energy below 20 eV (see section 5.1 in Stöhr).26 TEY measurement is also very surface sensitive with Chua et al. determining a sample depth of ~ 3 nm for P3HT.27 For all electron yield modalities the measured signal is proportional to X-ray absorption, with TEY and to a lesser extent PEY having a greater background signal than AEY due to their including electrons with a broader kinetic energy range. Since the sampling depth of all electron yield measurements is very small compared to the X-ray penetration depth, the electron yield signal normalised to the incident photon flux is essentially proportional to the extinction co-efficient. Since electrons only escape from the surface, the overall electron yield signal varies with illumination area and hence the X-ray foot print on the sample. When performing angleresolved NEXAFS measurements a larger signal is observed at glancing incidence compared to normal incidence, however relative resonance intensities can be compared by setting the signal just below pre-edge to 0 and the intensity well above the edge ) to 1, as shown in figure 8. Electron yield measurements typically require a high vacuum, if not an ultra high vacuum (UHV) environment in order to enable detection of emitted photoelectrons (a low vacuum or ambient conditions prevent photoelectrons from reaching the detectors). Due to the high surface sensitivity of electron yield methods a UHV environment is also important to prevent surface contamination during measurement (for example, the intense synchrotron beam can lead to beam-induced deposition of material from the atmosphere on the sample surface). Samples on conducting substrates are generally necessary for electron yield measurements, especially for TEY measurement, however sample charging can be mitigated through use of a flood gun enabling the PEY measurement of insulating samples. 2.3 Beamline and endstation requirements Only general considerations are given here and further technical details can be found in other publications.41, 52 In general, the beamline must be capable of delivering monochromatic X-rays of variable photon energy with sufficient resolving power to distinguish different resonant transitions. Plane grating monochromators are typically used to produce a monochromatic beam, with mirrors and slits used to shape and focus

20

the beam. An undulator or bending magnet can be used to produce the X-ray beam, with undulators being able to achieve higher X-ray flux as well as higher degrees of linear polarisation. A gold mesh is often used to measure the intensity of the incident X-ray beam, sampling a small portion of the beam before it enters the analysis chamber. Measuring the drain current to the mesh thereby provides a measure of the photoelectrons generated per unit time and hence the intensity of the X-ray beam. In order to monitor the energy calibration of the beamline a reference sample can also be placed upstream from the sample chamber, cutting a portion of the beam. This enables a NEXAFS reference spectrum to be acquired simultaneous with sample measurements, enabling proper energy calibration. For carbon, highly oriented pyrolytic graphite (HOPG) makes a suitable reference sample with the energy of the graphite exciton in HOPG determined to be 291.65 ± 0.025 eV.53 Within the sample chamber the sample is mounted such that it may be rotated with respect to the X-ray beam achieving different X-ray angles of incidence, , as depicted in figure 13. Different detectors may be mounted in the analysis chamber (see also figure 13) enabling simultaneous measured of AEY, PEY and FY spectra. A photodiode placed behind the sample holder can also enables transmission measurements; measurement of the drain current flowing into the sample upon X-ray illumination enables measurement of TEY.

Figure 13. Schematic diagram of a possible geometry of a NEXAFS analysis chamber. 2.4 Proper normalisation of carbon K-edge spectra A key challenge facing the reliable measurement of carbon K-edge spectra is contamination on beamlines of the optical components with adventitious carbon. Even for relatively clean beamlines this carbon contamination results in a severe variation of incident X-ray intensity across the carbon K-edge, with pronounced spectral dips corresponding to resonant absorption by the adventitious carbon coating the optical elements. While in principle the gold mesh that samples the beam upstream to the analysis chamber can account for variations in I0 with energy, this mesh will also be

21

contaminated with adventitious carbon. A successful carbon K-edge experiment therefore requires a clean measurement of the X-ray intensity at the sample position prior to being able to measure reliable spectra. Measurement of the true incident X-ray spectrum can be achieved by using a photodiode at the sample position (or just behind the sample position) with mild carbon contamination on the surface of the photodiode only absorbing a negligible amount of the transmitted beam, and surface photoelectrons not contributing to the photodiode signal. (Severe carbon contamination of the photodiode may lead to strong variation in detector efficiency.) Alternatively the photoemission of a sputtered gold sample at the sample position can be measured as a function of energy as a way of measuring the incident flux free from carbon contamination. It is crucial that the gold sample be sputter cleaned immediately prior to measurement as molecules will slowly adsorb onto the surface even in a UHV environment. Comparison of the true I0 signal measured at the sample position with either a photodiode or sputtered gold sample with the I0 signal simultaneously recorded by the gold mesh enables calibration of the gold mesh. That is, such a measurement quantifies the degree of carbon contamination on the gold mesh enabling a correction to be applied and subsequent determination of the true I0 signal. The process of acquiring a reliable NEXAFS spectrum is demonstrated with the following example, based on data taken at the soft X-ray beamline at the Australian Synchrotron.

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Figure 14. (a) Signals measured simultaneously from direct illumination of a photodiode at the sample position and the gold mesh used to monitor I0. (b) Mesh signal divided by photodiode signal revealing the carbon contamination of the mesh. Figure 14(a) shows signals measured from a photodiode directly illuminated by the X-ray beam and from the mesh upstream of the analysis chamber (see figure 13 for system geometry). The signal from the mesh is nominally measuring the incident intensity of the X-ray beam, however a large discrepancy is seen between the profile measured by the photodiode with that measured by the mesh. As noted above, the photodiode is measuring the true I0 signal with the signal from the mesh contaminated by adventitious carbon (the absorption of X-rays by carbon on the mesh leads to an apparent I0 that is higher than that recorded by a clean gold mesh). Dividing the mesh signal by the photodiode signal provides the NEXAFS spectrum of the carbon that is coating the mesh (see figure 14(b)) and provides the correction factor required to correct the signal measured by the mesh to the true incident X-ray intensity. Note that even the true incident X-ray profile as measured by the photodiode dramatically varies across the carbon edge due to X-rays being absorbed by carbon contamination of optical elements. Figure 15(a) shows the raw TEY NEXAFS spectrum measured from a P3HT film, plotting the raw drain current measured as a function of photon energy. As should be evident, this raw spectrum does not resemble the true NEXAFS spectrum of this polymer, as shown in figure 8. To correct, the spectrum is first normalised by the incident X-ray intensity as measured by the gold mesh, to give the spectrum in figure 15(b). However as noted above, the I0 signal measured by the gold mesh is itself distorted by the adventitious carbon on the mesh. Thus the raw spectrum additionally needs to be corrected for the carbon-sensitisation of the mesh, by multiplying the spectrum shown in figure 15(b) by the appropriate correction factor (as shown in figure 14b), to give the spectrum in figure 15(c). To enable comparison with spectra taken at other angles, the pre-edge of the spectrum is set to 0 and the post-edge set to 1, giving the final spectrum shown in figure 15(d).

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Figure 15. (a) Raw drain current (TEY) spectrum of a P3HT film. (b) P3HT TEY spectrum corrected by mesh current. (c) P3HT TEY spectrum corrected for carbon contamination of the mesh. (d) P3HT TEY spectrum with pre-edge (280 eV) set to 0 and post-edge (320 eV) set to 1. This method of normalising the measured signal to the incident flux is known as the ‘stable monitor method,’ as it assumes that the contamination of the mesh used for monitoring variations in X-ray flux is stable over the duration of the experiment, and hence only a single measurement of the carbon contamination of the mesh is required. 41 It should be noted that it is also often necessary to measure and subtract zero offsets from all channels, that is, a measurement of the I0 signal, drain current, photodiode signal etc should be taken with no X-ray beam present and these values subtracted from the measured values. There may also be drifts in the energy calibration of the beamline over the duration of an experiment which can influence normalisation. Fortunately analysis tools exist (as the upstream HOPG discussed above) to help with the monitoring of energy calibration and normalising of data during acquisition, allowing normalised data to be quickly processed and viewed during an experimental beamtime.32 Further details regarding the potential pitfalls of carbon K-edge NEXAFS spectroscopy can be found in the paper by Watts et al.41 Another important experimental consideration is minimising beam damage. Repeated measurement of the same position of the sample can lead to changes in the observed NEXAFS spectrum, as shown in figure 16 for a P(NDI2OD-T2) sample. X-ray absorption and the subsequent relaxation processes leads to the generation of high

24

energy secondary electrons within the sample which have more than enough energy to alter chemical bonds. In order to minimise the effects of beam damage, a fresh spot on the sample should be measured each time, since changes in apparent resonance intensity could otherwise be mistaken for anisotropy in the sample. Additionally, as fast a scan rate as possible should be adopted to minimise the X-ray dose. As seen in figure 16, the * region of the spectrum is most susceptible to beam damage. Different conjugated polymers have different resilience to beam damage, depending upon their chemical composition. Fluorine containing polymers in particular tend to be especially sensitive to beam damage,35 due to the photo-dissociation of fluorine atoms.54

Figure 16. Changes in the shape of the NEXAFS spectrum of a P(NDI2OD-T2) film caused by repeated measurement of the same spot on the sample. 3. Case Studies In this section a number of case studies are presented that highlight the unique capabilities and information provided by NEXAFS spectroscopy for studying conjugated polymer-based OFETs. 3.1. Interfacial molecular orientation in poly(3-alkylthiophenes) An important topic in OFET research is how molecular orientation affects charge transport mobility. An edge-on orientation of polymer chains with respect to the substrate is thought to be beneficial for charge transport, since the crystallographic axes with the highest charge transport mobilities (along the backbone and - stacking directions) are aligned with the plane of the accumulation layer.13 A strong link between interfacial molecular orientation and charge transport mobility is provided by the graph in figure 17. In this graph, the FET charge carrier mobility of different polythiophene films is compared with the average tilt angle of the C 1s  * transition dipole moment measured using angle-resolved NEXAFS spectroscopy.43 As introduced above, a larger

25

average tilt angle corresponds to the polymer chains being more ‘edge-on.’ It should also be noted that mobility is plotted on a log-scale with mobility varying by several orders of magnitude. A bottom-gate bottom-contact transistor geometry was used, which then measures the mobility corresponding to the bottom interface of the film with the SiO2 dielectric layer. The open symbols in figure 17 correspond to the tilt angles measured of the bottom of films prepared directly on the SiO2 dielectric. The bottom interface was exposed by dissolving of the SiO2 in HF, with the polymer films that floated off picked up bottom-side up with another silicon wafer. The different numbers correspond to poly(3-alkylthiophenes) (P3AT) with different alkyl side chain length, with n = 4 being poly(3-butylthiophene), n = 5 being poly(3-pentylthiophene), n = 6 being poly(3-hexylthiophene), etc. Also compared are films spin-coated from solutions of the polymer dissolved in chlorobenzene (‘CB’) and films prepared from a nanofiber dispersion where P3AT chains are pre-aggregated in solution prior to film deposition (‘NF’). The different alkyl side chain length and different preparation conditions provide different films with different interface morphology related to how long the polymer chains have been available to re-orient and order during deposition. Strikingly for the open symbols there is a strong correlation between the measured FET mobility and the average tilt angle measured, with higher tilt angles corresponding to higher mobilities. The NF films exhibit a systematically higher tilt angle and mobility associated with ordered aggregates being produced before film deposition. What is most striking about the plot is the average tilt angle and mobility of the closed symbols. The closed symbols correspond to bottom-gate bottom-contact devices where films were first spin-coated from CB onto SiO2, floated off and then deposited top-side down onto an OFET substrate. Thus the polymer/air surface is utilised as the charge transporting interface in a bottomcontact bottom-gate transistor. For all polymers, high tilt angles ( 64) are measured with mobilities of 0.1 cm2/Vs achieved (except for P3BT). Thus the same film can exhibit vastly different charge carrier mobilities depending upon which interface (top or bottom) is utilised for charge transport. The polymer/air interface has a very low surface energy enabling the polymer chain to adopt a preferential edge-on orientation similar to that achieved using superhydrophobic gate dielectrics.55 The measured tilt angles of the top surfaces therefore correspond closely to the tilting of the polymer backbone within the crystalline unit cell.43 In contrast, the bottom interface of films prepared directly on SiO 2 exhibit average tilt angles significantly below the preferential edge-on orientation (and approaching the magic angle) reflecting a mixture of face-on and edge-on configurations, or a distribution of tilt angles. The low mobilities of films prepared directly on the OFET substrates is attributed to the disordered molecular orientation with even a small population of face-on oriented chains having a dramatic impact on hole mobility. It is noted that an edge-on orientation of polymer chains may not be strictly necessary, with

26

strain-aligned P3HT films showing high mobilities with a preferential face-on orientation.56 In this case, the high degree of chain alignment of the strain-aligned films means that the charges can travel predominantly along the polymer backbone without recourse to lateral jumps across the side chains. Note that figure 17 also is a good example to highlight the limitations of NEXAFS spectroscopy for studying OFETs. While a strong correlation between average tilt angle and mobility is certainly evidenced, the average tilt angle as determined by NEXAFS spectroscopy is limited in being able fully explain charge transport properties. While NEXAFS spectroscopy is able to quantify the average tilt angle, many other parameters are important such as the degree of backbone planarity,57 and the length scale over which the alignment of polymer backbones are correlated.58

Figure 17. Plot of field-effect mobility (log scale) as a function of average tilt angle, <> determined from NEXAFS spectroscopy for a range of poly(3-alkylthiophene) films. Open symbols refer to films prepared on bare SiO2 bottom-gate bottom-contact transistors, while closed symbols refer to transistors with the same geometry, but where the poly(3alkylthiophene) has been prepared on a separate substrate and laminated top-side-down onto the SiO2 dielectric. The numbers refer to the length of the alkyl side chain (e.g. ‘6’ = P3HT). Circles refer to conventionally spin-coated films while squares refer to ‘nanofiber’ films deposited from nanofiber dispersions. Reproduced with permission from Ref.

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. Copyright 2014 John Wiley and Sons.

3.2. Measurement of surface and bulk molecular orientation The development of the naphthalene-diimide bithiophene co-polymer P(NDI2ODT2) sparked a lot of interest after its first report in 2009, with unprecedented electron mobilities of 0.85 cm2/Vs reported at the time.59 GIWAXS results indicated that the polymer chains were packing face-on in the bulk representing an unconventional microstructure for a high performance polymer.16 Initial NEXAFS results60 measured tilt

27

angles close to the magic angle suggesting a lack of preferential orientation at the surface, despite TEM results indicating a remarkably high degree of order in the bulk.19

Figure 18. (a) Schematic of the experimental setup enabling simultaneous probing of surface and bulk molecular orientation. (b) Comparison of the NEXAFS spectra of P(NDI2OD-T2) measured using transmission, AEY, TEY and FY modes acquired at an angle of incidence of 55. Reprinted with permission from Ref.44. Copyright 2013 American Chemical Society. A difficulty with comparing NEXAFS and GIWAXS results is the different sampling depths of these techniques. Furthermore NEXAFS directly probes the orientation of the polymer backbone while GIWAXS probes the orientation of the crystallographic axes with the polymer backbone generally titled within the unit cell. To provide a more direct comparison between these two techniques, simultaneous measurement of surface and bulk molecular orientation with NEXAFS was devised, see figure 18(a).44 In this experiment, P(NDI2OD-T2) films are floated onto TEM grids with the transmitted photon flux measured along with the AEY signal, TEY signal and FY signal. Spectra are then acquired for different orientations of the film with respect to the X-ray beam. Figure 19 presents the angle-resolved spectra taken using transmission (optical density), AEY, TEY and FY modalities. Figure 18(b) compares the shape of the spectra taken at an angle of incidence of

 = 55 for the different measurement modes. The AEY, TEY and

transmission spectra have very similar shape, in agreement with what was discussed in section 2 with the corrected AEY, TEY and transmission signals proportional to resonance intensity. In contrast the FY signal appears distorted due to the difficulty in being able to

28

correct for film thickness (which influences the number of photons absorbed) and selfabsorption effects (which influences the number of photons that leave the film and are detected). Similar to previous results, there is little dichroism seen in the AEY and TEY spectra, with average tilt angles of <> = 52.7 and <> = 54.6 for AEY and TEY modes. In contrast, significant dichroism is observed in the transmission and FY spectra, with a stronger C 1s  * resonance intensity observed for grazing incidence ( = 20) compared to normal incidence ( = 90). While quantitative determination of an average tilt angle from the FY data is problematic due the FY spectra not being properly corrected for absorbed and emitted photon number, the transmission spectra enable a quantitative determination of the average tilt angle in the bulk of <> = 40.9. GIWAXS measurements on an identically prepared portion of film confirmed a face-on orientation of crystallites consistent with the more face-on orientation in the bulk. Thus the previous GIWAXS results were not in agreement with the NEXAFS results since they were sampling different portions of the film. Measurement of uniaxially aligned films (see figure 9) confirm that the average tilt angles of ~ 55  correspond to a genuine preferential edge-on orientation of polymer chains at the surface. Therefore distinct surface and bulk molecular orientations can spontaneously arise in solution-processed conjugated polymer thin films, suggesting separate nucleation of bulk, face-on crystallites and surface, edge-on chains. Different surface and bulk molecular orientations have been observed in other polymer systems, notably in a high performance polymer/fullerene photovoltaic blend where edge-on oriented polymer chains were found at the surface of a film with face-on oriented crystallites.61

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Figure 19. Angle-resolved NEXAFS spectra of an as-cast P(NDI2OD-T2) film acquired using (a) AEY, (b) TEY, (c) transmission and (d) FY modalities. Reprinted (modified) with permission from Ref.

44

. Copyright 2013 American Chemical Society.

3.3. Side-chain surface enrichment For P(NDI2OD-T2) comparison of the AEY, TEY and transmission spectra taken at

 = 55, a similar shape is seen indicating that each technique is sampling a similar proportion of carbon atoms in aromatic and saturated environments. In contrast, figure 20 presents the PEY, TEY and FY spectra of the polymer DPP-BTz.62 Comparing the PEY and TEY spectra, the PEY spectrum has a much stronger relative intensity for the C 1s  C-H * at 287.5 eV with a very flat spectrum past 293 eV where transitions are associated with C 1s  C=C * bonds. The C 1s  C=C * transition is also weaker in the PEY spectrum compared to the TEY spectrum. These observations taken together indicate that the PEY and TEY measurements, with their different surface sensitivities, are sampling different proportions of aromatic to saturated carbon atoms. In particular the more surface sensitive PEY measurement is sampling a higher proportion of carbon atoms in saturated environment. DPP-BTz has rather long side chains with the PEY measurement detecting an enrichment of side chains at the surface relative to bulk composition. The presence of side-chain enriched interfaces is likely to have implications for device operation, helping to screen the buried conjugated core from interfacial effects and dipolar disorder in the dielectric. This surface sensitivity and bonding sensitivity enable NEXAFS spectroscopy to discern such an enrichment, which may enable new

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insight into the way conjugated polymers pack at surfaces, particularly when combined with atomistic simulations.

Figure 20. Comparison of the PEY, TEY and FY spectra of the polymer DPP-BTz acquired at an angle of incidence of

 = 55. Reproduced (modified) with permission from Ref. 62.

Copyright 2014 John Wiley and Sons. 3.4. Potential for discerning dihedral angles Combining the chemical sensitivity of NEXAFS spectroscopy with the sensitivity of NEXAFS spectroscopy to bond orientation affords the possibility of measuring dihedral angles in conjugated polymers. Donor-acceptor co-polymers have a repeat unit with two or more planar aromatic moieties. The torsional or dihedral angle between different planar sub-units will strongly influence the optoelectronic properties of donor-acceptor polymers. For example, the dihedral angle influences electronic coupling along the backbone as well as the -stacking of polymer chains.17, 63, 64 Dihedral angles are typically assessed using quantum chemical calculations however these values do not necessarily reflect those that actually occur in the solid state. Raman spectroscopy is able to provide qualitative information regarding torsional angles but measurements typically need to be interpreted with the help of simulations, and cannot provide a direct experimental measurement of dihedral angle.17, 57 In an X-ray scattering experiment there are insufficient diffraction orders to solve the structure within the unit cell. The IRbased method of infrared transition moment orientational analysis does provide a direct measurement of dihedral angle by analysing structure-specific vibrational bands. Similar to NEXAFS, examining the polarisation and inclination dependence of different vibronic transitions the average tilting of different aromatic planes can be assessed.25 While this

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IR-based technique has superior chemical sensitivity to NEXAFS spectroscopy, films much thicker (~ 1 m) than used in OFETs (< 100 nm) have to be used, with IR measurements being bulk sensitive. The potential for NEXAFS to distinguish dihedral angles in thin films, and furthermore at the surface of thin films is thus of interest.

Figure 21. (a) Chemical structures of a series of NDI-based co-polymers where the number of thiophene units is increased from 1 (T2) to 4 (T4). (b) NEXAFS spectra of polymers T1 – T4. (c) *-region highlighting differences in the NEXAFS spectra of T1 – T4. The NEXAFS spectrum of the NDI monomer is shown for comparison. Reprinted with permission from Ref.

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. Copyright 2014, AIP Publishing LLC.

In order for NEXAFS spectroscopy to be able to discern dihedral angles, sensitivity to the different sub-units in the donor-acceptor polymer must first be demonstrated. As an example, figure 21 shows the NEXAFS spectra of different naphthalene-diimide (NDI)-based co-polymers where the number of thiophene units has been increased from 1 (polymer T1) to 4 (polymer T4).66 Examining the *-region of the

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spectra, a systematic increase in resonance intensity is observed at ~ 285 eV which can be associated with the thiophene units.65 Peak fitting was employed to separate the resonance intensity of the thiophene unit from the NDI unit, and the average tilt angles of the different transitions assessed. Despite being able to separate out the contribution due to thiophene and NDI moieties, similar average tilt angles were measured for the different sub-peaks in all cases, with <>  56 for T1, <>  61 for T2, <>  64 for T3 and <>  62 for T4. Although similar anisotropy and hence average tilt angles are measured, the results are not necessarily inconsistent with a significant dihedral angle, since what is measured is the average tilt angle of the transition dipole moment from the surface normal. Thus the NDI and thiophene units may have a significant dihedral angle (an angle of ~ 40 - 60 between the aromatic planes)25, 67 with each unit tilted by a similar angle away from the surface normal (such as a titling of the aromatic plane of ~ 20 - 30 from the surface normal), but tilted away from each other. 68 A significant distribution of tilt angles also hinders discernment of dihedral angles, though the NEXAFS results are in general in broad agreement with recent IR measurements of edge-on oriented polymer chains.25 In contrast to the results on P(NDI2OD-T2), NEXAFS measurements on the polymer BFS4 do provide direct evidence for a significant dihedral angle existing between different chemical subunits. Figure 22(a) presents the chemical structure of the polymer BFS4 which was developed for use in organic solar cells, achieving good efficiencies as a donor in combination with both fullerene 69 and polymer acceptors.40 BFS4 is an interesting polymer based on dithienyl-benzodithiophene (DT-BDT) and fluoro-benzothiadiazole (FBT) sub-units. For the sample examined here, the film was prepared by spin-coating from a dichlorobenzene solution directly onto a polydimethylsiloxane (PDMS) substrate and then ‘stamping’ the film from the PDMS onto a silicon substrate to expose the bottom BFS4 interface. Examining the angle-resolved PEY carbon K-edge NEXAFS spectra, figure 22(b), there is dichroism at * energies with grazing incidence in general giving a higher * resonance intensity compared to normal incidence. Decomposing the * manifold into 4 peaks located at 284.1 eV, 284.9 eV, 285.4 eV and 285.7 eV (which optimise the empirical peak fitting, see inset in figure 22(b)) the peak located at 285.4 eV shows different dichroism compared to the other peaks, as confirmed by the plot of peak intensity vs angle as shown in figure 22(c). In particular the peak located at 285.4 eV is associated with a distinct ‘edge-on’ orientation corresponding to an average tilt angle of <> = 64.2 compared to average tilt angles of <>  43 - 47 for the other peaks with an average tilt angle averaging over the entire

* manifold of <> = 47.4. The low energy peaks in the * manifold are expected to be associated with the FBT unit indicating that a more face-on orientation is at least

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associated with the DT-BDT unit. Previous computational calculations predict a very small dihedral angle between the FBT and DT-BDT units (5 – 8) suggesting the polymer should adopt a rather planar backbone.69 Interestingly however, these calculations found that a large dihedral angle of ~ 55 should exist between the pendant thiophene rings on the DT-BDT unit and the dithienyl-benzo core. The observed NEXAFS trends are then consistent with a slightly face-on orientation of the planar polymer backbone with edgeon oriented pendant thiophene rings tilted away from the planar backbone.

Figure 22. (a) Chemical structure of BFS4. (b) Angle-resolved carbon K-edge PEY spectra of BFS4. (c) Plot of peak area vs X-ray angle of incidence for the different sub-peaks indentified in the inset on part (b) with fits to determine average tilt angles. For donor-acceptor co-polymer systems with different heteroatoms on different chemical subunits, performing angle-resolved NEXAFS spectroscopy at different edges may provide greater clarity discerning dihedral angles. For example, with BFS4 the nitrogen and fluorine units on the FBT sub-unit permit separate probing of the orientation of the FBT sub-unit by acquiring fluorine-edge and nitrogen-edge spectra.

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Figure 23(a) presents the angle-resolved nitrogen K-edge spectra of BFS4. Strong dichroism is clearly observed at the lowest energy peak at 398.4 eV associated with the N 1s - * transition. From the dichroism of this peak, an average tilt angle of the associated transition dipole moment of <> = 34.4 is determined, confirming a more ‘face-on’ orientation of the FBT unit with respect to the substrate. The angle-resolved fluorine edge spectra are shown in figure 23(b). The fluorine edge spectra are dominated by * peaks, with a much higher local density of states associated with px-py orbitals compared to pz orbitals.70 The position of the lowest energy * peaks also tend to overlap with * peaks making it more reliable to use the dominant * peaks for determining molecular orientation at the fluorine edge.70 Based on the dichroism of the peak at 691.4 attributed to F 1s  C-F * transitions, an average tilt angle of <> ~ 62 for the associated transition dipole moment is calculated. As this transition dipole moment is expected to be oriented in the plane of the FBT moiety (along the axis of the C-F bond) it is broadly in agreement with the orientation of the FBT unit as probed by at the nitrogen edge. Unfortunately for the case of BFS4 there are not distinct heteroatoms on the core and pendant units of the DT-BDT unit which both contain carbon and sulphur. Sulphur Kedge data were also collected, see figure 23(c), however due to the low intensity of photons at the sulphur K-edge (at the limit of the range of the soft X-ray beamline at the Australian Synchrotron) less reliable data are obtained. These data however do suggest a more edge-on orientation when selectively exciting sulphur atoms compared to carbon atoms, consistent with the conclusions drawn from the other absorption edges.

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Figure 23. Angle-resolved nitrogen K-edge, (a), fluorine K-edge, (b) and sulphur K-edge PEY spectra of BFS4. 4. Future Perspectives Although NEXAFS spectroscopy is a relatively mature technique, there is still room for future innovations that will provide further insight into the material properties of conjugated polymer thin films. As discussed in section 3.4 routine multi-edge NEXAFS investigations will provide fresh insight into surface conformations of conjugated polymers, particularly when combined with theoretical calculations. Less has been made of the electronic information provided by NEXAFS spectroscopy and more theoretical calculations will certainly help to this end. In-depth fluorescence yield studies will also provide new insight into X-ray photophysical processes, particularly by combining NEXAFS spectroscopy with emission spectroscopy. For example it would be interesting to compare the X-ray fluorescence spectrum of a conjugated polymer when exciting into a

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* peak compared to a * peak or well above the ionisation threshold. For well-ordered films with a high degree of backbone alignment angularly resolving photoemission and X-ray emission could also provide new insight into electronic structure. The combination of NEXAFS spectroscopy with imaging is a particularly interesting area, and there is not space in this review to separately introduce X-ray spectromicroscopy (the reader is directed to a recent review on this topic51). While X-ray spectromicroscopy is also a relatively mature technique, the use of an X-ray microscope as a local probe of molecular orientation is relatively new to the polymer electronics community. 42 As briefly alluded to in section 2, probing a polycrystalline film with a spot-size smaller than the characteristic domain size enables the following to be mapped: domain orientation, the degree of local liquid crystalline order, and the local tilt angle.42 This combination of microscopy and spectroscopy also enables for the distribution of tilt angles of a sample to be computed, overcoming a limitation of NEXAFS spectroscopy whereby the average tilt angle is known but not the distribution of tilt angles. To date only transmission measurements have been effective, but such imaging with surface sensitivity would be clearly highly interesting to OFET research, and improvements in detector efficiency and experiment design may enable this in the future.

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Conclusions NEXAFS spectroscopy is a versatile tool for probing molecular orientation in conjugated polymer thin films and for relating molecular orientation to OFET performance. Using different detection modalities, surface-sensitive and bulk sensitive detection are possible, providing a way to distinguish between different surface and bulk molecular orientations. The surface sensitivity and chemical sensitivity of NEXAFS spectroscopy make it a unique probe of surface composition and molecular orientation enabling it to exclusively probe the microstructure of the accumulation layer in an OFET. As well as providing a general introduction to NEXAFS spectroscopy and the experimental requirements for a successful NEXAFS experiment, this feature article has also presented several case studies highlighting the contribution of NEXAFS spectroscopy to OFET research. Acknowledgements This work was supported by the Australian Research Council (DP130102616). This research was undertaken on the Soft X-ray Beamline at the Australian Synchrotron, Victoria, Australia. The contributions of the beamline scientists (Dr. Bruce Cowie, Dr. Lars Thomsen, and Dr. Anton Tadich) in providing a first-rate beamline for carbon Kedge NEXAFS spectroscopy are gratefully acknowledged. The authors thank Dr. Tianshi Qin for the supply of BFS4. References

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Figure Captions Figure 1. (a) Chemical structure of poly(3-hexylthiophene) (P3HT), a prototypical conjugated polymer. (b) Schematic diagram showing the way conjugated polymers pack to form crystalline lamellae with the lamellar stacking and - stacking directions shown. Figure 2. Schematic diagram of the three most common OFET geometries. Figure 3. Schematic energy diagram showing possible electronic transitions from a core state to anti-bonding and continuum states following absorption of an X-ray by a conjugated polymer, and the associated NEXAFS spectrum. For simplicity Rydberg states are not shown. Figure 4. Comparison of the NEXAFS spectra of P3HT, thiophene in the gas phase and OTS. The NEXAFS data of thiophene is taken from Ref. from Ref.

30

while the OTS data is taken

31

.

Figure 5. Experimental P3HT NEXAFS spectrum (black, squares) fitted to a series of Gaussian peaks and a step-edge. Note that an exponential-modified Gaussian is used for the highest energy peak. Figure 6. NEXAFS spectra of the polyfluorene copolymers PFB, F8BT and F8TBT. Figure 7. Geometry of an angle-resolved NEXAFS spectroscopy experiment, to determine the molecular orientation of a thiophene ring with respect to the plan of the substrate. Figure 8. (a) Angle-resolved NEXAFS spectra of a spin-coated P3HT film. (b) Dichroism of the C 1s  * transition at 285 eV with a fit to equation 2. Figure 9. Angle-resolved NEXAFS spectra of a highly-aligned P(NDI2OD-T2) thin film: (a) Azimuthal dependence taken at a polar angle of

 = 90; (b) Plot of the azimuthal

dependence of the C 1s  * resonance intensity; (c) Polar dependence taken at an azimuthal angle of  = 0; (d) chemical structure of P(NDI2OD-T2). Reprinted with permission from Ref.

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. Copyright 2013 American Chemical Society.

Figure 10. Schematic of the different NEXAFS measurement modalities. Figure 11. NEXAFS spectrum of P(NDI2OD-T2) (red graph) scaled to fit the bare atom mass absorption coefficient (black curve). Figure 12. After a core electron is promoted to an anti-bonding state (or excited to a continuum state above EV) a core hole is created. The core hole is subsequently filled by an electron from a higher lying bonding orbital with the energy lost given to either an Auger electron or a fluorescent photon. Figure 13. Schematic diagram of a possible geometry of a NEXAFS analysis chamber. Figure 14. (a) Signals measured simultaneously from direct illumination of a photodiode at the sample position and the gold mesh used to monitor I0. (b) Mesh signal divided by photodiode signal revealing the carbon contamination of the mesh.

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Figure 15. (a) Raw drain current (TEY) spectrum of a P3HT film. (b) P3HT TEY spectrum corrected by mesh current. (c) P3HT TEY spectrum corrected for carbon contamination of the mesh. (d) P3HT TEY spectrum with pre-edge (280 eV) set to 0 and post-edge (320 eV) set to 1. Figure 16. Changes in the shape of the NEXAFS spectrum of a P(NDI2OD-T2) film caused by repeated measurement of the same spot on the sample. Figure 17. Plot of field-effect mobility as a function of average tilt angle, <> determined from NEXAFS spectroscopy for a range of poly(3-alkylthiophene) films. Open symbols refer to films prepared on bare SiO2 bottom-gate bottom-contact transistors, while closed symbols refer to transistors with the same geometry, but where the poly(3alkylthiophene) has been prepared on a separate substrate and laminated top-side-down onto the SiO2 dielectric. The numbers refer to the length of the alkyl side chain (e.g. ‘6’ = P3HT). Circles refer to conventionally spin-coated films while squares refer to ‘nanofiber’ films deposited from nanofiber dispersions. Reproduced with permission from Ref.

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. Copyright 2014 John Wiley and Sons.

Figure 18. (a) Schematic of the experimental setup enabling simultaneous probing of surface and bulk molecular orientation. (b) Comparison of the NEXAFS spectra of P(NDI2OD-T2) measured using transmission, AEY, TEY and FY modes acquired at an angle of incidence of 55. Reprinted with permission from Ref.

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. Copyright 2013

American Chemical Society. Figure 19. Angle-resolved NEXAFS spectra of an as-cast P(NDI2OD-T2) film acquired using (a) AEY, (b) TEY, (c) transmission and (d) FY modalities. Reprinted (modified) with permission from Ref.

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. Copyright 2013 American Chemical Society.

Figure 20. Comparison of the PEY, TEY and FY spectra of the polymer DPP-BTz acquired at an angle of incidence of

 = 55. Reproduced (modified) with permission from Ref. 62.

Copyright 2014 John Wiley and Sons. Figure 21. (a) Chemical structures of a series of NDI-based co-polymers where the number of thiophene units is increased from 1 (T2) to 4 (T4). (b) NEXAFS spectra of polymers T1 – T4. (c) *-region highlighting differences in the NEXAFS spectra of T1 – T4. The NEXAFS spectrum of the NDI monomer is shown for comparison. Reprinted with permission from Ref.

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. Copyright 2014, AIP Publishing LLC.

Figure 22. (a) Chemical structure of BFS4. (b) Angle-resolved carbon K-edge PEY spectra of BFS4. (c) Plot of peak area vs X-ray angle of incidence for the different sub-peaks indentified in the inset on part (b) with fits to determine average tilt angles. Figure 23. Angle-resolved nitrogen K-edge, (a), fluorine K-edge, (b) and sulphur K-edge PEY spectra of BFS4.

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Graphical abstract

Highlights     

Comprehensive introduction to the technique of Near-Edge X-ray Absorption Fine-Structure (NEXAFS) spectroscopy in the context of the study of conjugated polymers. Experimental requirements for a successful NEXAFS experiment reviewed. Several case studies presented highlighting the unique information provided by NEXAFS spectroscopy. Relevance of the microstructural information provided by NEXAFS spectroscopy to the operation of organic-field effect transistors reviewed. Potential for NEXAFS spectroscopy to discern dihedral angles in conjugated polymers highlighted.

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