J. Phys. Chem. C 2008, 112, 12993–12999
12993
Investigation of the Optoelectronic Properties of Organic Light-Emitting Transistors Based on an Intrinsically Ambipolar Material Raffaella Capelli,*,† Franco Dinelli,‡ Stefano Toffanin,† Francesco Todescato,† Mauro Murgia,† Michele Muccini,† Antonio Facchetti,§ and Tobin J. Marks§ Consiglio Nazionale delle Ricerche (CNR), Istituto per lo Studio dei Materiali Nanostrutturati (ISMN), Via P. Gobetti 101, I-40129 Bologna, Italy, Consiglio Nazionale delle Ricerche (CNR), Istituto per i Processi Chimico Fisici, Via Moruzzi, 1 I-56124 Pisa, Italy, and Department of Chemistry and the Materials Research Center, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208 ReceiVed: December 17, 2007; ReVised Manuscript ReceiVed: March 31, 2008
The realization of organic light-emitting transistors (OLETs) based on R,ω-dihexylcarbonylquaterthiophene (DHCO4T), an intrinsically ambipolar and luminescent semiconductor, is reported. In this device structure, optimization of the hole/electron density ratio in the channel region has been identified as the major issue to optimize light emission.. Therefore, the focus of this study is to understand how DHCO4T optoelectronic response vary with semiconductor film growth conditions as well as the selection of the gate dielectric and metal contact materials. Our results demonstrate that DHCO4T hole and electron mobilities and the IDS-VDS hysteresis mainly depend on the gate dielectric material composition. Atomic force microscopy analysis of the semiconductor film reveals a layer-by-layer growth mechanism, giving rise to the formation of a continuous and homogeneous charge transport layers. With Au as the source and drain contact material, the best carrier mobilities have been measured for the poly(methyl methacrylate)-coated SiO2 gate dielectric devices. Metals with Fermi energy ranging from -5.1 to -2.87 eV have also been investigated. Metal deposition on the semiconductor film does not significantly affect film morphology as evidenced by the topography of the electrode top surface. However, for a given dielectric material, the OLET performance strongly depends on the metal/dielectric combination employed and marginally correlates with the contact Fermi energy. Electroluminescence has been observed in DHCO4T-based OLETs but principally in concert with unipolar transport. The hole and electron large gate threshold voltage values have been identified as the principal limitation to high electroluminescence performances. Introduction Organic light-emitting transistors (OLETs) represent a new class of optoelectronic devices of increasing interest for their potential in large-area display technologies.1–3 In addition, they are also some of the most promising candidates for the realization of electrically pumped organic laser sources.4 In these devices, light emission can be tuned by applying suitable bias voltages to the drain, source, and gate electrodes.5,6 In this way, all the operational requirements of the basic electronic and optoelectronic elements in active matrix displays can be satisfied in a single device structure. In conventional electronics, such a high degree of integration cannot be achieved and, for each pixel, an electrical switch (a field effect transistor, FET) and a separate light source (a light-emitting diode, LED) must be combined. Furthermore, OLETs have a planar architecture (see Figure 1), substantially different from the vertical OLED structure. A planar geometry is ideal for the realization of a resonant microcavity where the active region is separated by distances of a few microns from the injecting metal electrodes. This device configuration prevents exciton quenching, thus making OLETs well suited for the development of electrically pumped organic lasers.4 * Corresponding author. E-mail:
[email protected]. † Istituto per lo Studio dei Materiali Nanostrutturati. ‡ Istituto per i Processi Chimico Fisici. § Northwestern University.
Figure 1. A schematic of the top contact device structure employed here to realize organic light-emitting transistors based on DHCO4T.
The realization of OLETs has been successfully achieved using either conjugated oligomers (small molecules having a well-defines structure) or polymers.2 In most of the cases, these devices exhibit unipolar transport, in which only one type of charge carriers contributes to the current. However, in unipolar OLETs, the exciton formation is always located at the drain contact and is adversely affected by strong quenching due to the proximate metal. In order to obtain efficient OLETs, ambipolar transport is therefore required with both electrons and holes flowing through the device active region. This latter condition enables exciton formation via direct electron-hole recombination in the areas where the two types of charge carriers coexist. The ratio of the two carrier densities and the location of the recombination region can be controlled via the gate bias.
10.1021/jp7118235 CCC: $40.75 2008 American Chemical Society Published on Web 07/31/2008
12994 J. Phys. Chem. C, Vol. 112, No. 33, 2008 This process has been recently demonstrated in polymer-based OLETs,7–9 along with spatial control of the light formation zone inside the channel. Nevertheless, the realization of ambipolar transport in oligomer-based OLETs is of great importance for the future development of OLET technology. Note that organic thin film transistors (OTFTs) based on vapor-deposited conjugated oligomers can achieve electrical performance approaching that of amorphous silicon, generally greater than that of polymerbased ones.9–12 The realization of ambipolar transport using semiconducting oligomers can be achieved either by combining two unipolar transport materials or by employing a single organic material capable of transporting electrons as well as holes. OLETs based on a bilayer structure, obtained by superposing two layers of unipolar materials, have been successfully demonstrated.13 Similarly, OLETs based on a bulk heterojunction, fabricated by simultaneous coevaporation of two unipolar materials, have been also realized.14,15 However, in both cases, the flow of holes in the electron transporting material and electrons in the hole transporting material is strongly disadvantaged and reduces the probability of exciton formation. In the case of bilayer structures, there is a physical separation between holes and electrons that are confined in each of the two unipolar layers. In the case of the bulk heterojunction, the disordered mixing of electron- and hole-transporting materials in the active region does not guarantee the formation of the percolation pathways, thus reducing charge carrier transport efficiency.16 In principle, all of the aforementioned challenges could be addressed by fabricating OLETs based on intrinsically ambipolar oligomers: no interface would separate the electrons from the holes, and the film microstructural order could be enhanced by controlling the growth process of a single material. In polymerbased OTFTs, for a small number of materials, control of the electron-hole recombination region inside the device channel has been already demonstrated with high electroluminescence efficiency in correspondence to ambipolar transport.17 Molecular materials showing ambipolar behavior in OTFT structures have been reported 18–21 but light emission has been demonstrated only in devices with charge mobility of the order of 10-3 cm2 V/s for the electrons and 10-5 cm2 V/s for the holes.22 Recently, it has been shown that R,ω-dihexylcarbonylquaterthiophene (DHCO4T, for structure see Figure 2) has intrinsic ambipolar character.23 DHCO4T-based OTFTs exhibit field-effect mobility values on the order of 10-3 cm2 V/s for holes and of 10-1 cm2 V/s for electrons. At present, DHCO4T is one of the few materials exhibiting high-mobility ambipolarity in OTFTs. Therefore it represents a good candidate for molecular ambipolar OLETs, considering that it is also luminescent. In this work, we explore the possibility to achieve electroluminesce in OTFT structures based on DHCO4T molecules. The focus is on the understanding and optimizing of the optoelectronic response in terms of key issues such as the organic-gate dielectric and organic-metal interfaces, and the organic film growth conditions. Experimental Section Organic light-emitting transistors (OLETs) were fabricated in the top electrode configuration (see Figure 1). The substrates consisted of heavily doped n+2 silicon with a layer of thermal oxide (SiO2) grown on top as dielectric layer. The oxide thickness varied from 100 to 300 nm (electrical capacitance per unit area, Cox ) 9-36 × 10-4 F/cm). The substrate cleaning procedure consisted of sonication in chloroform, acetone, and UHP water in order to remove possible organic contamination. The SiO2 surfaces were then chemically treated with hexam-
Capelli et al.
Figure 2. Top: chemical structure of the R,ω-dihexylcarbonylquaterthiophene (DHCO4T) molecule. Bottom: plots of the absorption and photoluminescence (PL) spectra of DHCO4T molecules as a dichloromethane solution (a) and as a 100 nm film, at room temperature (b) and 20 K (c).
ethyldisilazane (HMDS) or octadecyltrichlorosilane (OTS). These treatments were performed by keeping the substrates under controlled argon atmosphere saturated with HMDS and OTS molecules for one day. The layers of poly(vinyl alcohol) (PVA) and poly(methyl methacrylate) (PMMA) were obtained by spin-casting the polymers on top of the SiO2 surfaces. For PVA, we spun 200 µL of a 10 g/L water solution at 1500 rpm; for PMMA, we spun 150 µL of a 3 wt % ethyl lactate solution at 6000 rpm. The films were then annealed for 12 h at 120 °C for PMMA and at 80 °C for PVA. The thickness and Cox values for 50 and 130 nm thicknesses were 1.1 × 10-7 and 1.9 × 10-8 F cm-1, respectively. The R,ω-dihexylcarbonylquaterthiophene films were grown by sublimation in high vacuum at a base pressure of 5 × 10-7 mbar in a homemade chamber directly connected to a nitrogen glovebox to prevent sample exposure to air.24 Each layer had a nominal thickness of 20 nm as measured with a quartz microbalance. The substrate temperature during growth was 90 °C. The growth rate was fixed at 0.1-0.2 Å/s (0.6-1.2 nm/ min). Differences in the sticking coefficients at these substrate temperatures can be considered to be negligible. The Au, Ag, Cu, and Ca electrodes were deposited in high vacuum at a base pressure of 2 × 10-4 mbar at a growth rate of 0.5 Å/s with the sample held at room temperature. The electrode thickness was 50 nm nominally. The device geometry factors were the following: width (W) ) 10 mm and length (L) ) 150 to 600 µm. Opto-electronic measurements were carried out in a glovebox under nitrogen atmosphere with 50 50 10 20
s >50 40 15 18
In DHCO4T-based OTFTs, the organic-dielectric interface profoundly affects the transport properties of both charge carriers in terms of OFET carrier mobility (µ) and gate threshold voltage (Vth).31 We have therefore treated the SiO2 gate dielectric surfaces either chemically by exposure to a gaseous reagent or by coating it with a nonconducting polymer. In all of these devices, 50 nm thick Au source-drain contacts are used. Figure 3 shows the saturation transfer characteristics of three OLETs along with simultaneously measured EL data. The drain-source current (IDS) is measured for the gate voltage (VGS) equal to the drain voltage (VDS).32 The only difference between the three plots of Figure 3 is in the SiO2 surface treatment: (a) vapor phase treatment with hexamethyldisilazane (HMDS); (b) spincoating a ∼50 nm thick poly(vinyl alcohol) (PVA) layer; (c) spin-coating a ∼130 nm thick poly(methyl methacrylate) (PMMA) layer. Table 1 collects µ and Vth values for DHCO4T devices fabricated with these dielectrics. In addition, the results obtained for devices fabricated on bare SiO2 and SiO2 treated with an octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) are included for comparison. Note that a small and unstable IDS is measured when operating the devices fabricated on bare SiO2, while the performance is also very poor for the OTS case. A morphological analysis carried out by atomic force microscopy (AFM) does not evidence significant variation in the rms roughness for the OTS-treated surfaces, and the measured contact angle is always >100°. We also fabricated OTFTs on OTS surfaces using other p-type (R,ωdihexylquaterthiophene) and n-type materials (N,N′-ditridecylperylene-3,4,9,10-tetracarboxylic diimide, PTCDI-C13H27). In both cases, no significant difference in the electrical characteristics was observed between SiO2/OTS and the other dielectrics. This suggest that the OTS treatment was carried out correctly, and that there is some interaction between OTS and DHCO4T that introduces charge carrier traps and consequently affects OTFT performance. The electrical behavior of the DHCO4T devices based on HMDS and PVA is in agreement with previously reported results.31 For the present study, it is clear, comparing HMDSand PVA-based devices, that the electron µ is enhanced by ∼5× when using PVA and the electron Vth is reduced. However, the hole µ is reduced by ∼ 10 × for PVA, whereas the hole Vth is reduced by ∼40 V for the same insulator. In conclusion, for PVA the hole/electron ratio is very unbalanced with electrons predominating. With respect to the previous work reported on DHCO4T based FETs,31 we have obtained an improvement in the electrical characteristics by using PMMA to treat the SiO2 surface. As Figure 3b shows, the current hysteresis is clearly reduced with respect to the HMDS case. The hole and electron mobility values are substantially unvaried and consequently the holes/electrons ratio. However, the variations in the threshold voltage values guarantee a better balance of the hole/electron ratio in the ambipolar region (VDS ) VGS/2). The phenomenon of ‘stress’ in the OTFT active region has been observed for most organic semiconductor-based TFTs examined to date.32,33 This degradation process takes place when
the OTFT is repeatedly operated. The effect on device performances manifests itself in a progressive Vth shift correlated to the operation time. The initial performance characteristics can be partially recovered after leaving the transistor unbiased for certain periods of time. Part of the deterioration is, however, irreversible depending on the materials system under examination. A clear and detailed explanation in terms of charge trapping and nanoscale modifications in molecular organization is still elusive at present despite important observation.34,35 Even if the device stress is a continuous memory-dependent process, in order to quantify its impact on device performance, the transfer curves of Figure 3 can be considered to be composed of two independent branches: (1) a ‘forward’ component, corresponding to increasing VGS (in absolute value); (2) a ‘reverse’ component, corresponding to decreasing VGS. This allows estimation of the two Vth values for electrons and holes for each of the two branches. The difference between the forward and reverse Vth values thus obtained represents a reasonable estimation of the IDS-VDS hysteresis. From these plots, it is clear that there is a great difference in the IDS-VDS hysteresis exhibited by the three dielectrics. With PMMA, the plot hysteresis is strongly suppressed. This indicates that the trap density is lower on PMMA compared to HMDS and PVA. Electroluminescence Characteristics versus Device Structure. Table 1 also collects the variations in Vth for all five dielectrics examined. Together with the IDS, in Figure 3 it is shown the corresponding EL signal measured during OLET operation. In particular, within the sensitivity of the photodiode used to collect the emitted photons, the EL signal is detectable for IDS g 1 µA. The IDS transfer curves of Figure 3 are characterized by a central minimum (for VDS ) VGS/2) with monotonic current increase in the left and right bias regions around it, a clear signature of ambipolarity. Roughly, we can assign the left part of the transfer curves as relating to hole transport, the right part to electron transport, and the central region to simultaneous coexistence of holes and electrons in the active zone. This latter situation is the most important for efficient OLET operation. In fact, when EL is associated with the flow of a single charge carrier (the left and right parts of the transfer curves), exciton formation and light emission result from the recombination processes occurring at the organic-metal contact interface as previously reported for unipolar OLETs.36 In contrast, when EL is associated with ambipolar transport (the central part of the transfer curves), exciton formation can be ascribed to electron-hole recombination processes in the active channel. With reference to Figure 3, we performed an evaluation of the optoelectronic efficiency calculated as the ratio between the EL and IDS data. The calculated plots show peaks in correspondence of the minima of the transfer curves for all the graphs reported in Figure 3. This fact might suggest the presence of a direct electron-hole recombination process in the ambipolar region of the DHCO4T based OLET. However the EL signal is of the same order of the photodiode background noise and the ratio between the EL signal, corrected by the background noise of the photodiode, and the IDS current has no clear physical
Organic Light-Emitting Transistors
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Figure 4. AFM images of the early growth stages of DHCO4T films sublimed on (a) bare SiO2, and SiO2 treated with (b) HMDS and (c) PMMA.
sense. The absence of a clearly detectable EL signal around the minimum of the transfer plots can be ascribed to the low values of IDS in this bias region. The hole/electron carrier density ratio present in the ambipolar current is related not only to the µ values but also to the Vth values of each of the charge carriers. Table 1 shows that the principal limitations to efficient ambipolar transport in DHCO4Tbased devices are the large Vth values. In optimum cases, the hole µ is 3 orders of magnitude lower than the electron µ (Table 1). This implies that exciton formation is dominated by holes so that the light emission is hole-limited. In spite of this, a µ of ∼10-3 cm2 V/s should afford a clearly detectable EL signal.13 On the other hand, high Vth values drastically compress the bias regime where the active channel is open for both electrons and holes. This restricts photon formation to relatively low VGS values and to the saturation condition (VGS ) VDS). The intermediate zone therefore remains nearly inactive. This corresponds to the central bias region where IDS is near the instrument detection limit (Figure 3). The gate threshold voltage depends on several factors.37 Some of these can be ascribed to the morphology and/or materials composition of the dielectricsemiconductor and metal contact-semiconductor interfaces. In OTFTs, charge injection occurs when the highest occupied molecular orbital (HOMO) for holes and the lowest unoccupied molecular orbital (LUMO) for electrons, respectively, are aligned with the Fermi energy level (EF) of the electrode metal. In principle this should correspond to the initial VGS needed to form the transport channel in the OTFT. However, the existence of energy levels acting as charge traps at the semiconductordielectric interface (in absence of a current flow) increases Vth until their complete filling. The origin of these trap states are likely related to the semiconductor film morphology as well as to chemical interactions with the dielectric surface. In particular, it has been demonstrated that grain boundaries and changes in molecular organization at the microscopic scale can create a distribution of spurious electronic states.25,38,39 Regarding the role of the chemical interaction between the semiconductor and the dielectric layers in determining the Vth needed to open the transport channel, note in Table 1 that changing the dielectric
surface composition principally affects µ, which varies over 3 orders of magnitude. The differences in Vth as a function of the substrate treatment can be rather large, specifically for the case of PVA. We can therefore conclude that the chemical species present at the dielectric-semiconductor interface are not the principal cause of Vth variations. Concerning the morphology of the DHCO4T-dielectric layer interface where charge transport occurs, we have analyzed the early stages of DHCO4T growth by AFM. The images in Figure 4 reveal an initial layer-by-layer growth mechanism with a high density of nucleation centers. This is substantially independent of the various SiO2 treatments employed. The height distribution of the DHCO4T islands is peaked around a characteristic value of 3 nm. This bidimensional growth affords good film connectivity, essential for a good charge transport. We therefore reasonably conclude that lowering Vth requires optimizing the metal-organic interface. Effect of the Metal Contact on Charge Injection and Gate Threshold Voltage. We have also fabricated devices using different metals as the top contact electrodes. In previous studies on DHCO4T based OTFTs the only metal used to realize the device electrodes was gold (Au). On the other hand, the understanding of the mechanism of the metal-organic interface formation and the charge injection process is one of the key points in organic electronics technology.40 In particular, the energetic matching between the metal Fermi energy and the organic transport levels, the HOMO for the hole and the LUMO for the electrons, is considered to be crucial to control the device gate threshold voltage. The Fermi energy (EF) of the metals employed in this study ranged between -2.87 (Ca) and -5.1 eV (Au). The HOMO and LUMO values of DHCO4T have been estimated to be -5.38 and -3.78 eV, respectively.20 Therefore the energy alignment of the HOMO level should be more favorable for high EF metals such as Au, whereas, the alignment of the LUMO level should be more favorable for low EF metals such as Ca. With regards to Vth, one expects a progressive shift of the hole Vth and the electron Vth toward higher and lower values, respectively, on moving from Au to Ca. Consistently, the hole
TABLE 2: Electron and Hole Field-Effect Mobility (µ) Data Together with the Corresponding Gate Threshold Voltage (Vth) as a Function of the Fermi Energy (EF) of the Metal Used To Make the Top Contact OLET Electrodes HMDS EF (eV) µ p-type (cm2/V s) µ n-type (cm2/V s) Vth p-type (V) Vth n-type (V)
PMMA
Αu
Cu
Ag
Ca
Αu
Cu
Ag
Ca
-5.1 8 × 10-3 0.1 -60 53
-4.65 2 × 10-4 6 × 10-4 -36 40
-4.26 8 × 10-5 0.2 -50 30
-2.87 -
-5.1 3 × 10-3 0.15 -46 63
-4.65 4 × 10-3 56
-4.26 2 × 10-3 0.1 -50 55
-2.87 7 × 10-5 11
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Figure 5. AFM images of the electrodes (including film thickness) grown with Ag in a and a′; Cu in b and b′; Au in c and c′. The dielectric functionalization layers are HMDS, a, b, c, and PMMA, a′, b′, c′.
µ would be expected to decline and the electron µ to increase on progressing from Au to Ca. Table 2 illustrates that the electron Vth trend is as expected: electron injection efficiency is increased on lowering the EF. The hole Vth is largely unaffected implying that hole injection is not degraded by lowering the EF. An exception is represented by Cu on HMDS where the hole Vth is unexpectedly reduced. Devices made with Ca do are inactive although all the deposition process is made in high vacuum and in a glovebox with atmosphere controlled
