Article pubs.acs.org/cm
Trap-clearing Spectroscopy in Perylene Diimide Derivatives Louisa M. Smieska,† Zhong Li,‡ David Ley,§ Adam B. Braunschweig,§ and John A. Marohn*,† †
Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, United States Department of Chemistry and The Molecular Design Institute, New York University, New York, New York 10003, United States § Department of Chemistry, University of Miami, Coral Gables, Florida 33124, United States ‡
S Supporting Information *
ABSTRACT: Perylene diimide (PDI) derivatives are important components of organic circuits and photovoltaic systems, but the electron trapping mechanism(s) in higher-LUMO PDIs (−3 to −4 eV, where radical anions are expected to react with atmospheric water and oxygen) are not well characterized. Here, we examine the spatial distribution of traps in transistors made from PDIs with these higher LUMO levels and measure trap-clearing spectra in n-type organic materials using timeand wavelength-resolved frequency-modulated Kelvin probe force microscopy (FM-KPFM). We find that the rate of trapclearing under visible light does not follow a LUMO-level trend, and we observe spectral evidence for different trapping mechanisms on bare versus HMDS-treated SiO2.
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INTRODUCTION Although a few high-performing organic hole-transport materials are known, better and more robust organic electron-transport materials are needed to realize complementary circuits and enable devices with high operating speeds and low power consumption. The family of perylene-3,4:9,10tetracarboxylic diimides (PDIs) has relatively high n-channel mobilities compared with other small molecule semiconductors and are well-known in transistor applications.1−4 PDIs can also serve as the electron acceptor in donor−acceptor photovoltaic blends5,6 and self-assembled systems.7−11 However, PDIs can suffer performance instabilities in the form of threshold voltage shifts and lowered mobilities due to charge trapping.12,13 By employing frontier molecular orbital design rules10,14,15i.e., making materials with LUMO levels below about −4 eV relative to vacuumnew PDIs have been synthesized and devices have been made with some success.4 These design rules result from treating charge trapping in ntype organic semiconductors as an electrochemical reaction in which the organic anion radical serves as a reducing agent (see the Supporting Information for representative reactions). In the prototypical reaction, PDI•− reacts with water to yield the neutral PDI, hydroxide, and hydrogen gas.16,17 There is spectroscopic evidence18 for a related reaction in which PDI•− reacts with hydroxyl groups on the underlying silicon dioxide dielectric to yield silanoxide (SiO−) and H2. These electrochemical trapping mechanisms predict that introducing electron-withdrawing substituents to lower the PDI LUMO2,13,19,20 should result in decreased trapping and improved device performance; many devices made from lower-LUMO PDIs do indeed show improved performance. © XXXX American Chemical Society
Yet the loss of hydrogen gas in the above reactions would lead to a seemingly irreversible trapping of charge that is at odds with the finding that trapping in most devices is reversible.12 Reversible charge-trapping reactions involving PDI•− are not hard to imagine (see the Supporting Information for representative reactions). One such reaction is the protonation of PDI•− by water to produce hydroxide. As in the electrochemical mechanism, the trapped charge resides off the PDI; in contrast, a new chemical bond is made to the PDI. While protonating the PDI in this example reaction comes at the cost of breaking resonance, no free hydrogen is generated, and so the reaction should be reversible. In the prototypical ptype organic semiconductor pentacene, spectroscopic evidence supports the hypothesis that trapped charge arises from an atom-transfer reaction involving the pentacene cation and a chemical impurity.21,22 We think it is plausible that an analogous reactive trapping mechanism may be at play in ntype materials. In principle, the feasibility of a proposed electrochemical or reactive trapping mechanism can be assessed by computing the driving force for the reaction. However, the findings of refs 21 and 22 indicate that in practice the most thermodynamically stable product may not form in the solid state due to an unfavorable activation barrier. In ref 21, Luria and co-workers used scanning Kelvin probe microscopy to nonperturbatively observe optically induced detrapping of charge in an organic thin-film transistor. Plotting the trap-clearing rate versus Received: October 16, 2015 Revised: January 20, 2016
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DOI: 10.1021/acs.chemmater.5b04025 Chem. Mater. XXXX, XXX, XXX−XXX
Chemistry of Materials
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wavelength revealed a spectrum with peaks arising from photoexcitation of a charge trapping species. The observed spectrum could be compared with ab initio predictions to rule in (or rule out) proposed trapping species. In ref 22, Smieska et al. added trap precursors to a pentacene transistor, operated the transistor, and used trap-clearing spectroscopy to interrogate the reaction products. Surprisingly, the precursor predicted to produce the lowest-energy product was observed to be essentially unreactive, presumably because the associated chemical reaction has a comparatively high activation barrier. In n-type materials, both electrochemical and reactive trapping reactions could have large overpotentials or activation barriers. Since activation barriers for chemical reactions of large πconjugated molecules in the solid state are at the present time difficult or impossible to compute, new experimental data are needed to better understand charge-trapping mechanisms in organic semiconductor films. In this work, we examine charge trapping in transistors fabricated with “high-LUMO” PDI derivatives whose anion radicals are expected to react with water and oxygen (Figure 2 and Table 1). We examine four different PDI derivatives; all
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METHODS
Materials and Sample Preparation. The molecular structures of the compounds studied in this paper are shown in Figure 2. The series
Table 1. HOMO and LUMO Levels for PDI Derivatives Molecule
LUMO [eV]
HOMO [eV]
dPyr PDI 1 dEO PDI 2 dCH dEO PDI 3 dCH dBr PDI 4
−3.11 −3.61 −3.62 −3.86
−4.76 −5.39 −5.10 −6.11
show a high degree of charge trapping. Using frequencymodulated Kelvin-probe force microscopy (FM-KPFM), we demonstrate through maps of trapped charge that trapping reactions take place throughout the channel upon a positive gate bias. We moreover present the first trap-clearing spectra measured in n-channel organic semiconductors. The measurement is sketched in Figure 1: we measure a light-induced conversion of trapped charge to free charge as a function of wavelength, monitored via the surface potential. In one PDI derivative, we observe possible spectroscopic evidence for different trapping mechanisms on bare versus HMDSpassivated SiO2. Only some PDI films exhibit traps whose clearing is accelerated by exposure to light. The observed trap clearing behavior shows no LUMO-level trend.
Figure 2. PDI derivatives discussed in this study. is numbered according to decreasing LUMO level as shown in Table 1, with PDI 1 expected to be the least stable and PDI 4 the most stable. The synthesis and characterization of PDI 1 is reported in the Supporting Information. PDIs 2−4 were synthesized and purified as previously reported.8 LUMO levels were measured using cyclic voltammetry in tetrahydrofuran (THF) with 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) as the electrolyte, in the presence of ferrocene as an internal reference.10 The PDIs were deposited on bottom-contact transistor substrates via drop-casting or thermal evaporation. Transistor substrates consisted of an n+-doped silicon gate with 315 nm thermal oxide
Figure 1. Sketch of trap-clearing spectroscopy experiment. Trapped charge is induced in a PDI film by applying a positive gate bias to the bottomcontact transistor substrate. Trapped charge in the semiconductor film (circled e−) is converted to free charge (free e−) under visible light. The progress of this trap-clearing is monitored by tracking the surface potential versus time with FM-KPFM. B
DOI: 10.1021/acs.chemmater.5b04025 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
Figure 3. Long-lived electron traps in PDI thin-film transistors. Top row: FM-KPFM images of trapping in the transistor channel after a 2 minute positive gate bias of (a,b,d) +5 V or (c) +15 V. The traps clear over time as each scan progresses from left to right. Dashed lines mark the edges of the transistor channel measured with atomic force microscopy. Bottom row: Several rows of data in the center of each channel were averaged to produce surface potential transients of thermal trap-clearing. In a−d, y is the fast scanning direction, while the x is slow scanning direction that corresponds to the time axis in e−h. (a,e) PDI 4 thermally deposited on bare SiO2. (b,f) PDI 2 thermally deposited on HMDS-treated SiO2. (c,g) PDI 2 drop-cast on bare SiO2. (d,h) PDI 2 thermally deposited on bare SiO2. Scale bar in a−d is 2 μm. and 30-nm-tall gold electrodes with a 5 nm chromium adhesion layer. The electrodes were 15-μm-wide, and the channel length was 5 μm. Laid out in an interdigitated array with an active area of 3 × 6 mm, the total channel width was 19.8 cm. Drop-cast PDI films were prepared from solutions in pyridine. The PDI concentration in solution was 10−5 M or approximately 6 mg/mL. A drop of PDI solution was placed on the transistor active area, and the solvent was allowed to evaporate under ambient atmospheric conditions. Thermally deposited thin films of PDI were prepared in a custom-built glovebox evaporator. PDI powders were heated in ceramic crucibles, and films were deposited onto a shutter until a deposition rate of 0.1 Å/s was achieved. A copper substrate heater was used to hold the substrates at 125 °C before, during, and for a few minutes after deposition. High substrate temperatures are important for achieving the highest possible mobilities when thermally depositing PDIs.12,23−26 These high temperatures should promote molecular reorganization into crystalline domains during film deposition.27 Attempts to examine film mobility using transistor current−voltage measurements largely failed; in all but one device, the observed current was too small to be meaningful. Observing small, injection-limited currents is not surprising, given the large injection barrier expected for electrons moving from gold to the PDIs. For PDI 4, the current was nevertheless large enough that a mobility could be estimated by jointly analyzing the measured current and the voltage map obtained using FM-KPFM.28 In this way, we estimated a mobility of 1 × 10−4 cm2/(V s) for PDI 4.29 We emphasize that it is possible to examine the dynamics of trap-clearing processes using scanning probe methods even in devices such as ours where the low mobilities make typical bias-stress measurements impossible. Surface Potential Measurements. All scanning probe measurements were performed in a high vacuum (10−6 mbar) in our custombuilt microscope.30,31 The cantilever displacement was monitored with a 1490 nm laser interferometer. This signal was demodulated, and the cantilever was driven via base excitation in positive feedback (phase locked loop mode) using an RHK PLLPro 1.0 controller with a bandwidth of 400 Hz. We employed platinum-coated probes from MikroMasch (DPE 18/NO AL) with a typical tip radius
