Employing microspectroscopy to track charge trapping in operating pentacene OFETs
Graphical abstract
Introduction
Ultrathin pentacene films, bearing high potential as active semiconducting elements in electronic devices such as organic field effect transistors (OFETs) [1], [2], are promising and benchmark materials for OFETs due to their high charge carrier mobilities and paramount semiconducting behaviour [3], [4], [5], [6]. To this date, pentacene is clearly the most intensively studied and still best-working organic substance in terms of device performance [7]. Charge transport in thin organic films is a crucial issue for pushing the performance to state-of-the-art levels [8]. However, its exact mechanism still remains one of the key challenges for future investigations [9], [10], [11]. The overall device performance of OFETs generally scales with the product of the charge carrier density and the charge mobility. The latter is often strongly influenced by the microstructure of the active film. Its microstructural order has hence to be carefully considered for the engineering of high-performance devices, especially targeting ultra-thin layers [4]. With the interdependency between microstructure, molecular and/or structural reorientation and charge transport being controversially discussed in the literature [2], [4], [12], [13], [14], this topic still provides a huge playground for further investigations.
Although there are several studies on the characterization of OFETs under operation, most of them address the device performance either in the light of the morphology or focus on spectroscopic changes, i.e., the vast majority of reports deals with these questions separately. For instance, scanning transmission X-ray microscopy (STXM), atomic force microscopy (AFM), grazing incidence X-ray diffraction or Raman spectroscopy enable to study the granular microstructure of pentacene films [2], [3], [15], [16], whereas Raman spectroscopy, near-edge X-ray absorption fine structure (NEXAFS) or sum frequency generation have disclosed average spectroscopic changes induced by transistor operation [13], [14], [17]. Raman spectroscopy furthermore provides hints that changes in the structural order occur within the active channel and may affect charge-transport in OFETs [12].
However, most of these spectroscopic studies are methodically limited to average across the electrically active semiconductor film, which means that they are not capable of probing the changes locally. Apart from scanning Kelvin probe microscopy (SKPM) [4], the local impact of charges on spectroscopic features remains widely unexplored. This lack gives rise to the demand for microspectroscopic probes which provide spectroscopic specificity combined with high spatial resolution. We consider this to be a precondition to obtain true insights into the interdependency of charge transport and morphology. STXM and confocal Raman microspectroscopy ideally meet these requirements by combining highly resolving imaging techniques with the acquisition of chemical information [18], [19], [20].
Within this study, we gather comprehensive insights into local changes of the electronic structure within the polycrystalline pentacene film by combining these two complementary methods. STXM delivers better insight into morphology, grains and domain orientation due to improved lateral resolution whereas Raman spectroscopy has been shown to be sensitive to charges densities [21]. Using these techniques, we deal with two different, yet related processes, namely resonant absorption in STXM and (inelastic) light scattering in Raman. For STXM, varying absorption of linearly polarized synchrotron radiation with the azimuthal orientation of individual crystalline pentacene domains is known as linear dichroism [15], [18]. For Raman scattering, we have to consider a photon-in/photon-out process with intensities that depend on the polarization vectors of both incident and outgoing waves and the polarisability tensor of the molecules [22]. In our Raman experiments, we can neglect the scattered light as we do not analyse its polarization, leaving the product of the polarization vector of the incident light and the – non-equal – matrix elements of the polarisability tensors [22] perpendicular to the long axis of the pentacene molecule (the tensor along the main axis only plays a minor role as the molecules are almost upright standing [3]). In other words, we can treat the Raman scattering process in this special case similar to absorption of linearly polarized synchrotron radiation in STXM, with the intensity directly dependent on azimuthal crystalline orientation with respect to the laser polarization. Thus, the information obtained from Raman microspectroscopy is in direct correlation with the STXM results. The experiments aim to observe spectral differences induced by charge and orientation with local information and thus to shed light on the correlation of microstructure and local electronic structure in an OFET under different operation modes.
Section snippets
Materials and methods
Pentacene OFETs were fabricated in typical top electrode geometry by subsequent evaporation of a pentacene layer (thickness: 17–30 nm) and gold electrodes (thickness: 30 nm, channel length: 30 μm) onto silicon wafers or silicon nitride membranes under high vacuum [18]. The transport characteristics of the prepared devices were determined with a Keithley SCS-4200 parameter analyser. For Raman microspectroscopy, we used a WITec Alpha R 500 microscope (excitation wavelength of 532 nm). Gate and drain
Results and discussion
We employed STXM as microspectroscopic probe with high lateral resolution for evaluation of the well-described polycrystalline structure of pentacene films grown by vapour deposition. Note that this thin film structure on inert substrates consists of crystalline domains of almost upright-standing pentacene molecules in the well-known herringbone arrangement [4], [16], [23], [24]. The individual domains exhibit arbitrary azimuthal rotation, featuring absorption dependent on the crystallographic
Conclusions
The findings of our experiments show in essence that X-ray and Raman scanning probes using linearly polarized light can be employed not only for the structural investigation of ultrathin organic films but also to locate charges trapped in the active layer of an OFET under operation. This becomes possible as both molecular orientation in microcrystalline films and charge-induced effects impinge on the vibrational fingerprint. However, an estimation for a homogenous film taking into account a
Acknowledgements
We acknowledge experimental support at the PolLux beamline by B. Watts and J. Raabe, and C. Hub and K. Ludwig for experimental assistance. The STXM study was funded by the BMBF project 05K10WEA. NZ obtained financial support through the Bavarian initiative “Solar technologies go hybrid”. BR was supported by the Graduate School of Molecular Science (GSMS). The study was performed within the framework of the GRK 1896.
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