Article pubs.acs.org/JPCC
Molecular Ordering of Conjugated Polymers at Metallic Interfaces Probed by SFG Vibrational Spectroscopy Francisco C. B. Maia† and Paulo B. Miranda* Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, São Carlos − SP 13560-970, Brazil S Supporting Information *
ABSTRACT: Organic−metal interfaces play a fundamental role in charge injection to organic electronic devices, and their performance significantly affects the device efficiency. Therefore, these interfaces have been studied by numerous experimental techniques, but most of them are either not strictly interface-specific or require special samples, with ultrathin metal or molecular coatings, which may not be representative of real devices. Here we use sum-frequency generation vibrational spectroscopy (SFGVS), a surface-specific tool based on nonlinear optics, to probe the molecular arrangement of conjugated polymers (CPs) at metallic interfaces with architectures that are typical of organic devices. The CPs investigated were the widely used poly(3hexylthiophene) (P3HT) and poly(octylfluorene) (PF8), in contact with either gold (Au) or aluminum (Al). On the basis of a simple model for the optical nonlinearity of polymer chains, we were able to quantitatively determine the orientation of CP chains at metallic interfaces. The results show that the polymer orientation is different in the two types of CP−metal interfaces (CP spin-coated on metal or metal thermally evaporated on the CP film), but in all of them the polymer backbone is not planar, on average, having a preferential torsion either toward or away from the interface. For PF8/Au interfaces, SFG spectra are reported for the first time and indicate spontaneous charge transfer. For the interfaces between PF8 and Al, the polymer chains are nearly parallel to the substrate, with the conjugated rings lying almost flat on the surface. Although charge transfer to/from the metal would be favored by this molecular configuration, the SFG spectra suggest the absence of polymer doping, which may be explained by the coincidence of Fermi levels for Al and PF8. In the case of P3HT, the chains are either quite parallel to the interface for polymer-on-metal samples, with thiophene rings nearly perpendicular to the metal substrates (edge-on orientation), or quite tilted from the interface, mainly exposing their ends to the metal, for thermally evaporated metal-on-polymer samples. For both P3HT/Al and P3HT/Au interfaces, SFG spectra indicate that spontaneous charge transfer is hindered by these molecular arrangements. These results may shed light on the mechanisms for charge injection across organic/metal interfaces and could also guide the design of novel organic devices with molecularly tailored interfaces to improve their efficiency.
1. INTRODUCTION In recent years there have been great advances in optoelectronic devices based on conjugated polymers (CPs).1−3 Ubiquitous in the structure of such devices are organic/metal interfaces, whose role in charge transfer from the external circuit significantly influences the device performance.4−6 Considerable improvements of CP-based devices have been achieved from studying and tailoring those interfaces.7−9 In this regard, it has been argued that molecular order at the interface may facilitate or hinder charge transport10 in such devices. Furthermore, the alignment of Fermi levels for each material may lead to charge accumulation at the organic/metal interface, resulting in the formation of interfacial dipoles and doping of the organic material,11 both of which significantly affect charge transport across the interface of a working device. Therefore, the detailed properties of CP/metal interfaces are directly relevant to charge injection inorganic devices. Although understanding CP/metal interfaces is crucial to device performance, studying them has been challenging from the experimental point of view because accessing the interface is © XXXX American Chemical Society
not trivial and requires suitable probes. Techniques commonly used for investigating these interfaces are X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS),11,12 which have pointed out evidence of chemical reactions or doping of the interfacial polymer. However, in these techniques the photoelectrons that originate at the interface have to cross a polymer or metal layer, which may significantly attenuate their signals. As a consequence, to probe buried CP/metal interfaces, XPS and UPS require samples with an ultrathin outermost layer (a few nanometers), whose interfaces may not be the same as those found in real devices. Fourier transform infrared absorption (FTIR),13 Raman scattering,14 and surface-enhanced Raman spectroscopy (SERS)15,16 have also been used to study interfacial properties of semiconducting polymer films. In these cases, however, discriminating the interfacial signal from that of the bulk molecules is frequently difficult, and in the case of SERS, rough Received: February 13, 2015
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DOI: 10.1021/acs.jpcc.5b01527 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C metal films must be used, which are usually not desirable in real devices. In addition, theoretical calculations have reported doping, metal diffusion,17 and chemical reaction18 upon modeling CP/metal interfaces, but they lack experimental confirmation. Recently sum-frequency generation vibrational spectroscopy (SFG-VS) has emerged as a powerful and straightforward probe for buried interfaces in organic devices.19 Because SFG-VS stems from a second-order nonlinear optical phenomenon that is active only when inversion symmetry is broken, it is an intrinsically surface-specific tool for investigating a large number of interfaces, including CP/metal. This technique obtains the vibrational spectrum for an interface (and therefore is closely related to Raman and infrared spectroscopies), with the additional advantage that the signal strength depends on the average molecular order, which allows a quantitative determination of molecular orientation at the interface, if a model for the molecular optical nonlinearity is known. A review of recent nonlinear optical studies of interfaces relevant to organic electronics is available.19 We now briefly outline those that are most relevant to our present study. A molecular order investigation via SFG-VS on the interface between P3HT and different functionalized silica (SiO2) surfaces concluded that surface chemistry directly influences the π-stacking arrangement, which, in turn, correlates with fieldeffect mobilities of polymeric transistors.20 Similar conclusions were reached by an investigation that combined X-ray diffraction to probe polymer crystallinity and SFG-VS to evaluate the quality of surface functionalization.21 On the basis of a quantitative analysis, a recent report10 used SFG-VS to determine the molecular orientation of P3HT rings on SiO2 and AlOx substrates, both for as-cast samples and after thermal annealing. These authors showed that upon annealing P3HT rings lean toward the AlOx surface but tend to adopt a more edge-on orientation on SiO2, therefore suggesting π-stacking of neighboring chains. Finally, another recent letter22 describes the combination of spectroscopic ellipsometry and SFG-VS to investigate the molecular orientation of amorphous films of a small cross-shaped conjugated molecule (anthracene derivative). In this case, a significant anisotropy is found for vacuumevaporated films, which become isotropic after heating to 140 °C. The electrical characteristics of devices indicate that anisotropic films have almost one order of magnitude higher hole mobility. Availing ourselves of SFG-VS capabilities, here we report a study of two types of CP/metal interfaces usually employed in organic devices, which were fabricated by standard procedures: (i) spin-coated (SC) polymer film on a metalized substrate (hereafter referred to as the SC interface) or (ii) thermally evaporated metal on a CP thin film (hereafter, the TE interface). The rather distinct assembly conditions for these interfaces are likely to induce specific molecular order,10 spontaneous charge transfer or favor chemical reactions, which may directly affect the device performance. We aim at obtaining quantitative information about polymer chain orientation or conformation at the two types of CP/metal interfaces. From the vibrational spectra of the polymer, it may also be possible to infer whether chemical reactions or spontaneous charge transfer (doping) occurred at the interface. We investigated polymer/ metal interfaces formed by the metals Al or Au and two CP widely used in organic electronics, namely, regioregular poly(3hexylthiophene-2,5-diyl) (P3HT) and poly(9,9-di-n-octyl-2,7fluorene) (PF8). This choice of materials was motivated by
their technological applicability and the availability of supporting data. In the next section we will describe the sample preparation and give some background on SFG-VS and our experimental setup. Section 3 will present the experimental results for both PF8 and P3HT. For a quantitative analysis of the polarization dependence of the SFG spectra, which yields the average molecular orientation at the interface, we have modified and extended the molecular orientation models previously available in the literature for P3HT10,20 to analyze both types of PF8/metal and P3HT/metal interfaces. Details of the quantitative analysis can be found in the Supporting Information. We then discuss the differences in molecular arrangement caused by the choice of fabrication method (TE or SC interfaces) and their role in hindering or facilitating spontaneous charge transfer at the various interfaces. Section 4 briefly summarizes our main conclusions.
2. MATERIALS AND METHODS The materials used were the metals Al and Au and the commercially available CPs regioregular P3HT and PF8. Both polymers and anhydrous chloroform (for preparing the spincoating solutions) were purchased from Aldrich, USA, and used without any purification. As substrates, we employed either optical glass (B270, 1 mm thick) when the excitation beams came from the air side (spin-coated (SC) type of samples) or 3 mm thick CaF2 windows (Crystran, U.K.) for transmission in the mid-IR range when the beams came from the substrate side (thermal evaporation (TE) type of samples). 2.1. Sample Preparation. Cleaning of the optical glass substrates was performed by the RCA procedure,23 while CaF2 windows were immersed in a solution of KMnO4 (∼0.5 g/L) for 24 h, rinsed with milli-Q water, immersed for ∼30 min in a dilute H2O2 solution (1:20 vol/vol), and washed again with milli-Q water. Upon choosing the sequence of metal and CP deposition on specific substrates, we constructed two different types of CP/metal interfaces for each choice of CP and metal. In one of those, the CP was deposited on a CaF2 substrate by spin coating at 1000 rpm from a chloroform solution in a glovebox under a N2 environment. Metallization of Al or Au (150 to 200 nm thick) was then performed by TE at 10−6 mbar onto the CP film. This produced a CaF2/CP/metal sample, with the TE type of CP/metal interface, whose CaF2 substrate presented a transparent window for investigating the buried CP/metal interface. For the second case, the samples were fabricated under the same conditions, with the metal film evaporated onto glass substrates and a CP film deposited by spin-coating onto the metallic surface. From this glass/metal/ CP sample, the SC type of metal/CP interface was studied. It is worth highlighting that the TE type of CP/metal interface was assembled by metal atoms impinging at high temperatures and condensing at the polymer surface, whereas the arrangement of polymer chains for the SC type of interface was determined by the metal/polymer interaction and the kinetics of solvent evaporation. However, because this latter metallic surface was handled in a room environment prior to spin-coating, superficial oxidation may have formed, in particular for Al, which may have led to an AlOx interfacial layer.10 The CP/glass sample prepared by spin-coating the polymer directly onto the clean glass substrate was investigated as a reference sample, without the polymer/metal interface. To maintain the same average CP thickness (∼30 nm) in all samples, the concentrations of the solutions in anhydrous chloroform were 2 g/L for PF8 and 0.2 g/L for P3HT. As a result, 10 different B
DOI: 10.1021/acs.jpcc.5b01527 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 1. Scheme of the three different types of samples used for SFG-VS: (I) glass/CP, (II) glass/metal/CP (SC type), and (III) CaF2/CP/metal (TE type). For each sample, the layers are listed from bottom to top, and each material is represented by a different color. The visible (green) and the IR (dark red) excitation beams and the generated sum-frequency beam (blue) are shown to illustrate the experimental geometry used for probing each sample.
Ω is the frequency of the light beam), as shown in eq 3. These L⃡ (Ω) factors relate the Cartesian components of the electric fields of the input and output beams to the local field components at the interface for a given polarization combination used in the SFG measurement (êi is the unit vector representing the polarization of beam i).26
samples were investigated: glass/PF8, CaF2/PF8/Al, glass/Al/ PF8, CaF2/PF8/Au, glass/Au/PF8, glass/P3HT, CaF2/P3HT/ Al, glass/Al/P3HT, CaF2/P3HT/Au, and glass/Au/P3HT. Figure 1 illustrates the configuration of the excitation and SFG beams for each kind of sample. 2.2. SFG Vibrational Spectroscopy. SFG vibrational spectroscopy relies on the second-order nonlinear optical processes of SFG. A detailed description is available elsewhere,19,24−28 and here we will describe only the essential points for our data analysis. The SFG process derives from the second-order optical polarization generated at the interface between two media by overlap of visible (ωvis) and tunable infrared beams (ωIR). Such induced polarization irradiates a new beam at a frequency that is the sum (ωsum) of those two input beams (ωsum = ωvis + ωIR). The intensity of the sumfrequency beam is proportional to both visible (Ivis) and infrared (IIR) intensities and to the effective second-order susceptibility of the interface (χ(2) eff ) for a given polarization combination26 of the input and output beams, as shown by eq 1. (2) 2 Isum(ωsum = ωvis + ωIR ) ∝ |χeff | I vis(ωvis)IIR (ωIR )
Ag = [esum ̂ ·L⃡ (ωsum)]·χg⃡ (2) : [e vis ̂ ·L⃡ (ωvis)][eIR ̂ ·L⃡ (ωIR )]
In turn, is related to the molecular hyperpolarizability,24 βg,lmn, through a transformation of coordinates (eq 4) from the molecular axes (l, m, and n) to the laboratory axes (i, j, and k), where the angular brackets represent an average over the molecular orientation distribution and N is the surface density of molecules. Therefore, if the hyperpolarizability tensor βg,lmn for a particular vibration is known, the average orientation of the moiety responsible for that vibration may be deduced from the measurements of Ag values with different polarization combinations. χg(2) =N , ijk
(2) 2 |χeff | =
∑ g
(ωg − ωIR − i Γg )
̂ j ̂ ·m̂ )(k ·̂ n)̂ >β (2) ∑
