(This is a sample cover image for this issue. The actual cover is not yet available at this time.)
This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright
Author's personal copy Organic Electronics 12 (2011) 1285–1292
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Solution-processed polyfluorene–ZnO nanoparticles ambipolar light-emitting field-effect transistor Andrey N. Aleshin ⇑, Igor P. Shcherbakov, Vasily N. Petrov, Alexander N. Titkov Ioffe Physical-Technical Institute of the Russian Academy of Sciences, St. Petersburg 194021, Russia
a r t i c l e
i n f o
Article history: Received 24 March 2011 Received in revised form 25 April 2011 Accepted 26 April 2011 Available online 10 May 2011 Keywords: Organic field-effect transistors Conjugated polymers Zinc oxide Solution processing Charge transport
a b s t r a c t We report on a solution-processed polyfluorene (PFO)–ZnO nanoparticles composite light-emitting organic field-effect transistor (LE-OFET). The behavior of absorption, photoluminescence spectra and electroluminescence intensity of the PFO:ZnO hybrid films with different concentration of ZnO nanoparticles is analyzed. By changing the PFO/ZnO nanoparticles concentration ratio the PFO:ZnO OFET shows either unipolar or ambipolar behavior of current–voltage characteristics and operates in the hole/electron accumulation mode with current saturation behavior. The field effect mobility of charge carriers depends on the ZnO concentration. For the ambipolar PFO:ZnO OFET with modest PFO/ZnO nanoparticles ratio (1:0.2), well balanced electron and hole mobility values at 300 K are 0021 and the 0029 cm2/Vs, respectively, whereas for films with high ZnO concentration (1:1) . mobility (2 cm2/Vs) is close to that of polycrystalline ZnO. The ambipolar PFO:ZnO LE. OFET emits light at both positive and negative gate bias. The working mechanism of the PFO:ZnO LE-OFET is investigated. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Solution-processed organic field effect transistors (OFETs) are of interest because of their application in low-cost fabrication of arrays/circuits via mass manufacturing roll-to-roll processes using a combination of conventional spin coating and printing techniques [1–4]. Such OFETs have been proven appropriate to drive organic-light-emitting diodes (OLEDs) in active integrated OFET–OLED pixels [5,6]. OFETs usually display unipolar charge transport, which indicates either holes or electrons as dominant charge carriers in the channel. It was shown recently that organic heterostructured FETs with an interpenetrating network of two materials fabricated by solution processing, demonstrated ambipolar transport characteristics [7,8]. In such organic heterostructured FETs possible interactions between these two or more components can form an organic heterojunction [9,10], which ⇑ Corresponding author. Tel./fax: +7 8122976245. E-mail address:
[email protected] (A.N. Aleshin). 1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2011.04.017
may lead to carrier redistribution and band bending. Recently demonstrated light-emitting organic field-effect transistors (LE-OFETs) are of particular interest because they combine the emission properties of OLEDs with the switching properties of OFETs [11]. It was shown that LEOFETs can also operate either in the unipolar [11–13] or ambipolar [14–17] regimes. The latter ambipolar LE-OFET regime supports both electron and hole transport that makes these devices useful for technological applications in complementary logic circuits [18]. Unipolar and ambipolar LE-OFETs have been demonstrated with different active layers: mono polymers [11–17], polymer blends [19] and polymer bilayers [20,21]. It was shown that the efficiency of semiconducting polymer trilayers LE-OFETs is much higher than that of the equivalent OLEDs [22]. At the same time, publications on LE-OFETs with hybrid (semiconducting polymer–inorganic semiconductor nanoparticles) active layers are rare to date. In our previous work we have shown that embedding transition metal oxide–ZnO nanoparticles into the polymer (PPV-derivatives) matrix results in the photoluminescence (PL) spectra
Author's personal copy 1286
A.N. Aleshin et al. / Organic Electronics 12 (2011) 1285–1292
broadening and enable the electroluminescence (EL) even in the planar geometry of electrodes [23,24]. Recently we have demonstrated that the LE-OFET based on a polyfluorene (PFO)–ZnO nanoparticles film exhibits very high FET mobility values at high concentration of the ZnO nanoparticles in a unipolar regime [25]. However, the ambipolar regime of the PFO:ZnO LE-OFET was not demonstrated so far. We have focused our attention on the PFO, which is a most promising stable conjugated polymer semiconductor for OLEDs and OFETs active layers and ZnO nanoparticles which is a relatively nontoxic n-type, solution processible inorganic semiconductor. ZnO has extremely high electron mobility (up to 20 cm2/Vs [26]), excellent environmental stability, and high transparency [27]. Most recently, solution-processed ZnO FETs were presented with a high electron mobility up to 5.25 cm2/Vs [28] and 7.2 cm2/Vs [29] and with a high on/off ratio. To reach the ambipolar LE-OFET regime we have decided to use a hybrid structure of composite film consists of n-ZnO and p-PFO semiconductors. The main problem for ambipolar LE-OFETs is the injection of both: electron and holes from the electrodes [7], due to the mismatch in the energy levels of the electrodes and the semiconductors. Injection of holes into the HOMO level of p-type polymer semiconductors can be achieved by using gold electrodes because the HOMO level of many conjugated polymers (4.8–5.3 eV) and the work function of gold (5.1 eV) are very close. However, gold electrodes are not appropriate for electron injection into the LUMO level of conjugated polymers due to very high injection barrier of 2–3 eV [18]. This problem was solved in our present work by using two different metal electrodes (gold and aluminum) with high and low work functions values. In this article we report on a solution-processed PFO– ZnO nanoparticles composite ambipolar LE-OFET. We have analyzed the behavior of absorption, PL spectra and EL intensity of the PFO:ZnO hybrid films with different ZnO concentration. It was found that by changing the PFO/ ZnO nanoparticles concentration ratio the PFO:ZnO LEOFET shows either unipolar or ambipolar behavior of current–voltage characteristics (I–Vs) and operates in the hole/electron accumulation mode with current saturation behavior. The transistor shows transfer characteristics with a small reversible hysteresis. We have found that for the ambipolar PFO:ZnO LE-OFET with the modest PFO/ZnO nanoparticles ratio (1:0.2) well balanced electron and hole mobility values at 300 K are 0021 and 0029 cm2/Vs respectively, whereas for films with high ZnO nanoparticles concentration (1:1) the mobility (2 cm2/Vs) is close to that of polycrystalline ZnO. It was observed that the ambipolar PFO:ZnO LE-OFET emits light at both positive and negative gate bias. The working mechanism of the PFO:ZnO LE-OFET is discussed.
2. Experimental Fig. 1a and b provides chemical structure of the conjugated polymer poly[9,9-bis-(2-ethylhexyl)-9H-fluorene2,7-diyl] (PFO), used in our experiments and schematic illustration of the polymer–inorganic nanoparticles hybrid
FET, which comprise a n-Si/SiO2/Au/PFO:ZnO/Al structure. An n+ silicon substrate was used with 200 nm thermally grown SiO2 as gate dielectric with thermally evaporated gold and aluminum electrodes on top. The distance between the electrodes was 7 lm and the width of the electrode 1 mm. The conjugated polymer, PFO, and ZnO nanoparticles (diameter 50–70 nm) used in our study were purchased from Sigma–Aldrich and used as received. PFO polymer and ZnO nanoparticles were dissolved and dispersed in chloroform respectively, and then they were mixed together and subjected to ultrasonic treatment for 10 min (f 22 kHz). The mixtures with different concentration ratios of components were drop-cast or spin-coated onto a Si substrate. The thickness of the composite films was about 0.6 lm according to Atomic Force Microscopy (AFM) results. The composite films were dried and heated at 80 °C in N2 atmosphere for 15 min. The samples for absorption measurements were drop cast onto quartz substrates (the film thickness 1 lm) and studied by using a Cary-50 (Varian) spectrometer. The detailed microstructure of these films was studied by AFM using P47-Solver NT-MDT. A LGI-21 pulse laser (k = 337, 1 nm, Ei >104 J/ cm2, s 108 s) was used to excite the PL. The laser beam was focused onto the sample using the quartz lens (The diameter of the focused beam was 2–3 mm). The PL spectra and the EL intensity at 300 K were analyzed using a SPM-2 spectrometer (spectral resolution 2 nm) and a photomultiplier with a spectral sensitivity range 300– 850 nm. The PL/EL light collection was about 100% owing to the use of a special mirror. The I–V characteristics of OFETs were measured in vacuum (3 103 torr) in the dark at 300 K using a liquid N2 cryostat holder and a dc electronic computer controlled measuring system with a Keithley 6487 picoammeter/voltage source and AKIP1124 programmable voltage source. The source – drain and gate voltages were varied between – 40 and +40 V in variable steps. Field-effect hole/electron mobility, lFET, was calculated using the following equations for the saturation and low field regimes respectively [2]:
IDS ¼ ðW=2LÞlFET C I ðV G V th Þ2
ð1Þ
IDS ¼ ðW=LÞlFET C I ðV G V th ÞV DS
ð2Þ
Here W is the channel width, L is the channel length, CI is the capacitance per unit area of the SiO2 layer (for SiO2 layer thickness of about 200 nm, CI 7–10 nF/cm2). VG is the gate voltage, and Vth is the threshold voltage, which corresponds to the onset of the strong accumulation regime. 3. Results and discussion Fig. 2 shows the absorption and PL spectra of the PFO pristine and the PFO:ZnO composite (ZnO concentration 17 wt.%, 1:0.2) films. It is evident from Fig. 2, the absorption spectra of both: pristine PFO and PFO:ZnO films have maxima at 380 nm, however in the case of hybrid film this maximum is lower that confirms the presence of weak interaction between the polymer molecules and inorganic nanoparticles. As regards the PL spectra, Fig. 2 shows that
Author's personal copy A.N. Aleshin et al. / Organic Electronics 12 (2011) 1285–1292
1287
Fig. 1. Chemical structure of the PFO polymer (a) and schematic device structure of PFO:ZnO OFET (b).
Fig. 2. Absorption and PL spectra of PFO (thin solid lines) and PFO:ZnO (thick solid lines) films. Concentration of the ZnO nanoparticles for PFO:ZnO 17 wt.% (1:0.2). Film thickness 0.6 lm.
an introduction of the ZnO nanoparticles results in shifting of the PL maximum from 580 nm (pristine PFO) toward the ultraviolet (hybrid PFO:ZnO) spectral region. The increase of the ZnO nanoparticles concentration leads to an increase of intensity of the PL peak at 380 nm related to the emission of the ZnO nanoparticles. There are no distinct signs of complexes formation in visible spectral region that can be explained by high values of the ionization potential ID of PFO (ID >8.2 eV), which results in relatively weak interaction between the PFO molecules and ZnO nanoparticles. Fig. 3a–c shows the AFM micrographs of pure PFO and PFO:ZnO (1:0.2) films between Au and Al electrodes on a n-Si/SiO2 substrate. It is worth noting that the channel
length of 7 lm is much larger with respect to the diameter of the ZnO nanoparticles 50–70 nm. As can be seen from Fig. 3a, detailed AFM surface examination indicates rather smooth pure PFO film structure. Introduction of the ZnO nanoparticles into the PFO matrix results in very rough composite film structure with few pinholes throughout the film between Au–Al electrodes (Fig. 3b and c). There are a number of the ZnO nanoparticles aggregates with a diameter of about several hundreds nm despite the PFO–ZnO solution was subjected to ultrasonic treatment before deposition. The presence of the ZnO nanoparticles aggregates can affect the I–Vs of OFETs and therefore they can influence the lFET (300 K) values as will be shown below. Typical output I–V characteristics of the LE-OFET based on PFO:ZnO nanoparticles film (1:0.2) with a channel length of 7 lm and a channel width of 1 mm operated at different gate voltages in vacuum are displayed in Fig. 4a and b. It is evident from Fig. 4a, the PFO:ZnO OFET exhibits the output I–V characteristics for the holeenhancement mode of a OFET with nearly saturation behavior. As can be seen from Fig. 4b, the output characteristics for the electron-enhancement mode at positive gate voltages are rather similar to those of the hole-enhancement mode. Fig. 4a and b, demonstrate that the saturated drain currents of I–Vs are not flat completely due to probably leakage currents between source and gate electrodes at high gate-to-source voltage (VG). Moreover, there is a sharp increase of the source-drain current, IDS, in the output characteristics at rather high VDS >25 V at modest VG for the PFO:ZnO LE-OFET structures (see inset to Fig. 4a) similar to that found for other LE-OFETs [13,18]. This indicates the onset of electron (negative VG) or hole (positive
Author's personal copy 1288
A.N. Aleshin et al. / Organic Electronics 12 (2011) 1285–1292
Fig. 3. AFM micrographs of PFO (a) and PFO:ZnO (b and c) films between Au–Al electrodes. Concentration of the ZnO nanoparticles 17 wt.%.
VG) transport. Such a behavior correlates well with numerical modeling results which predict an indication of the ambipolar regime by a superlinear current increase on
top of a significant saturation current [30]. The transfer characteristic for the same device in the saturation regime for positive and negative gate voltage is shown in Fig. 4c.
Author's personal copy A.N. Aleshin et al. / Organic Electronics 12 (2011) 1285–1292
1289
Fig. 4. (a) Output I–V characteristics of the PFO:ZnO OFET (1:0.2) at 300 K for different negative VG; (inset) output I–V characteristics for the same sample at high VDS >20 V and VG = 20 V; (b) output I–V characteristics of the PFO:ZnO OFET (1:0.2) at 300 K for different positive VG; (c) Transfer characteristic of the PFO:ZnO OFET (1:0.2) at 300 K for VDS = 10 V; (inset) I0:5 DS versus VG for the same sample; (d) transfer characteristic of the PFO:ZnO OFET (1:1) at 300 K for VDS = 10 V; (inset) I0:5 DS versus VG for the same sample.
The gate-to-source voltage was swept between +20 V to 20 V at 0.5 V step with a constant drain voltage VDS = 10 V. Fig. 4c clearly shows both electron and hole accumulation regimes for the PFO:ZnO OFET with relatively low ZnO nanoparticles concentration (1:0.2). The PFO:ZnO OFET shows the transfer characteristics with a small reversible hysteresis. Electron and hole field-effect hole mobility, lFET, was calculated for the saturation and low field regimes according to Eqs. (1) and (2). For the PFO:ZnO OFET (1:0.2) shown in Fig. 4a and b, the threshold voltages Vth estimated from the slope of the I0:5 DS versus VG plot at VDS = 10 V were found to be about 0.2 and +0.2 V for negative and positive VG, respectively (Fig. 4c). Therefore electron and hole mobility values at 300 K for this OFET estimated according to Eq. (1) were found to be 0021 and 0029 cm2/Vs, respectively. The lFET (300 K) values estimated for the same sample using Eq. (2) are of the same order of magnitude. On/off ratio determined for this device from the transfer characteristics was found to be in the range of 103 for VG 20 V, but it
is obviously higher for higher VG values. As the concentration of the ZnO nanoparticles increases the transfer plot of the PFO:ZnO OFET becomes asymmetric that indicates the transition to the unipolar regime (Fig. 4d). For such PFO:ZnO OFET (1:1), shown in Fig. 4d, we observed the Vth 1 V and on/off ratio 102 for VG = 20 V. The lFET (300 K) values calculated for this PFO:ZnO OFET (1:1) using Eq. (1) at VG = 20 V and VDS = 10 V were found to be unusually high and reached the value of 2 cm2/vs. These results demonstrate that on/off ratio decreases whereas the lFET (300 K) value increases significantly with increase of concentration of the ZnO nanoparticles. The observed rather low threshold voltages and on/off ratios indicate a low concentration of trap states as well as low injection barriers at the contacts. The obtained mobility values for PFO:ZnO OFET (1:0.2 and 1:1) are much higher than those observed in our experiments and reported in the literature for pure PFO OFETs (4 104 cm2/Vs [12]). We have suggested that the lFET (300 K) values obtained in our experiments for PFO:ZnO OFET with modest ZnO nanoparticles
Author's personal copy 1290
A.N. Aleshin et al. / Organic Electronics 12 (2011) 1285–1292
concentration (17 wt.%) reflects the contribution to charge transport from both p- and n-components of the PFO:ZnO film. The lFET (300 K) at high ZnO nanoparticles concentration is mostly related to the mobility of ZnO nanoparticles, which may form some bridges or thin ZnO layers between Al–Au electrodes, according to our AFM data (Fig. 3b and c). This suggestion correlates well the lFET (300 K) values obtained by other authors for the polycrystalline ZnO (20 cm2/Vs [26]) and for the ZnO FETs (up to 7.2 cm2/Vs [29]). The optical output characteristics (EL intensity versus VDS) at different negative and positive gate voltages of the ambipolar PFO:ZnO LE-OFET (1:0.2) are shown in Fig. 5. We found that the EL intensity at 300 K is increasing with increasing of source – drain, VDS, and both: negative and positive gate voltages, VG, at nearly saturated drain currents. It is worth noting that the EL intensity measured at the same conditions of the PFO:ZnO LE-OFET with high ZnO nanoparticles concentration (1:1) is higher by factor of 3 with respect to that of the PFO:ZnO LE-OFET (1:0.2), but it has been observed at negative values of VDS and VG only. That correlates well with an increase of the ZnO nanoparticles concentration that affects strongly the EL intensity. There is the almost fixed onset of the light EL emission at VDS 5 V independent of the ZnO nanoparticles concentration and the gate voltage polarity. This onset have been observed even for gate voltages up to 60 V, where the only intensity of the emitted light changes after application of VG (Fig. 5). This behavior is similar to that observed earlier for tetracene and pure PFO LE-OFETs [11,12], but in our case of the PFO:ZnO LE-OFET the onset of the VDS values is lower significantly. Inset to Fig. 5 presents the EL intensity vs electric field for the PFO:ZnO LEOFET (1:0.2) at different spectral regions: I0 – integral; I1 – 600–830 nm; I2 – 450–620 nm; I3 – 300–400 nm. It is evident from these data that the EL emission of the PFO:ZnO LE-OFET (1:0.2) takes place mainly in the green spectral region. In particular, for the PFO:ZnO LE-OFET (1:0.2) it
Fig. 5. Optical output characteristics of the PFO:ZnO LE-OFET (1:0.2): EL intensity versus VDS at different VG at 300 K. (Inset): EL intensity vs electric field for the PFO:ZnO LE-OFET (1:0.2) at different spectral regions: I0 – integral; I1 – 600–830 nm; I2 – 450–620 nm; I3 – 300–400 nm.
reaches 40% of the integral EL intensity, whereas the EL emission intensity in the blue region reaches 20% of the integral EL that correlates with the PL spectra of such composite films (Fig. 2). The same result was obtained for the PFO:ZnO LE-OFET (1:1). The mechanism for the formation of the excited states in the PFO:ZnO composite films implies the presence of a LUMO–HOMO recombination channel in the PFO polymer as well as the contribution of UV excitonic emission (at 380 nm) from radiative recombination in the ZnO nanoparticles. The latter channel can be suppressed by changing the concentration of ZnO nanoparticles [24]. The PL line observed at 550 nm (Fig. 2) corresponds to the excimer emission from the original polymer aromatic (fluorene) fragments. Our results demonstrate that the light output of the PFO:ZnO LE-OFET is controlled by the ZnO nanoparticles concentration and the gate voltage. To determine the charge carrier injection mechanism one can consider the energy band diagram of the Au–PFO:ZnO–Al structure shown in Fig. 6. It is evident from this band diagram that the work functions for Au and Al are 5.1 and 4.3 eV, respectively, compared with the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of PFO of 5.8 and 2.1, respectively [31]. Therefore the injection barrier for holes in Au–PFO is 0.7 eV and for electrons in Al–PFO is 2.1 eV. From another side the conduction and valence bands energy levels of ZnO are 7.5 and 4.2 eV, respectively. That makes the injection barrier for electrons in Al–ZnO of about 0.1 eV. It is clear evident from these values that the structure Au–PFO:ZnO–Al should work as ambipolar LE-OFET, that is in good agreement with our experimental results for PFO:ZnO LE-OFET with modest concentration of the ZnO nanoparticles (1:0.2). It is worth noting that some signs of ambipolar transport manifest themselves even for high mobility unipolar PFO:ZnO LEOFETs (1:1) but at relatively high values of positive VG only (see Fig. 4d). The reason why the hole accumulation mode still dominates in the unipolar regime of the PFO:ZnO LE-OFET with high concentration of ZnO nanoparticles is not clear completely. One may suggest that this behavior
Fig. 6. Band diagram of the Au–PFO:ZnO–Al composite structure.
Author's personal copy A.N. Aleshin et al. / Organic Electronics 12 (2011) 1285–1292
is due to absence of the robust electrical contact between ZnO nanoparticles and Al/Au electrodes. As regards the model for the light emission mechanism for the composite active layer LE-OFET, it has been developed recently by Schidleja et al. [21,32]. Follow this model we suggest that for the PFO:ZnO LE-OFET the light emission in the unipolar p-type regime is due to a stable reservoir of electrons established in the PFO in the vicinity of the Al contact. Since the work function of Al and PFO differ, a charge transfer occurs after contact formation and a constant electrochemical potential across the contact is the consequence. The diffusion of electrons from the metal into the organic semiconductor is stopped by the self-induced electric field of the transferred charge. Thus, a reservoir of excess electrons establishes close to the Al contact. In the unipolar p-type regime accumulated holes flow from the Au electrode to the Al electrode. At the Al contact they can either be ejected in the metal or recombine with the electrons of the reservoir. Therefore light emission can be observed even in the unipolar p-type regime of the LE-OFET. Switching from the unipolar to the ambipolar transport regime the position of the charge carrier recombination zone can be moved through the channel from one contact to the other [16,18,21,32]. In the ambipolar regime the charge carrier recombination takes rather place at the polymer/dielectric and polymer/ ZnO nanoparticle interfaces than at the contacts. Consequently, the position of the recombination zone is moved from the metal/polymer interface to the polymer/dielectric and polymer/ZnO nanoparticle interfaces by changing the transport regime from unipolar to ambipolar. One may suggest that the spatial heterogeneity of the PFO:ZnO LEOFET channel affects strongly the EL emission and the electrons and holes injections from different electrodes (Al and Au) is not completely balanced especially at high concentration of the ZnO nanoparticles. The results demonstrate that LE-OFETs based on solution processible polymers such as PFO and semiconducting nanoparticles such as ZnO provide a good example of the multifunctional devices obtained by the deposition technique compatible with upto-date printed organic electronic technology.
4. Conclusions We have investigated the solution-processed PFO–ZnO nanoparticles composite light-emitting organic field-effect transistor. We have analyzed the behavior of absorption, PL spectra, EL intensity and I–Vs of the PFO:ZnO hybrid films with different ZnO nanoparticles concentration. The field effect mobility of charge carriers depends on ZnO concentration. It was found that by changing the PFO/ZnO nanoparticles concentration ratio the PFO:ZnO OFET shows either unipolar or ambipolar behavior of I–Vs (at high and low ZnO nanoparticles concentration, respectively) and operates in the hole/electron accumulation mode with nearly current saturation behavior. The FET mobility of charge carriers in PFO:ZnO films depends on ZnO concentration and at high concentration of the ZnO nanoparticles it is close to the mobility of polycrystalline ZnO. The observed ambipolar regime for the PFO:ZnO LE-OFET with
1291
relatively low ZnO nanoparticles concentration makes these devices useful for applications in complementary logic circuits. Acknowledgements Research funding from the Program No. 7 of Presidium of Russian academy of sciences; subprogram: ‘‘Polyfunctional materials for molecular electronics’’ and RFBR #1102-00451a is gratefully acknowledged. We would like to thank I.N. Trapeznikova, F.N. Fedichkin and P.N. Gusakov for help with absorption and electrical measurements, respectively. References [1] H. Sirringhaus, Device physics of solution-processed organic fieldeffect transistors, Adv. Mater. 17 (2005) 2411–2425. [2] C.D. Dimitrakopoulos, P.R.L. Malenfant, Organic thin film transistors for large area electronics, Adv. Mater. 14 (2002) 99–117. [3] D.B. Mitzi, L.L. Kosbar, C.E. Murray, M. Copel, A. Afzali, High-mobility ultrathin semiconducting films prepared by spin coating, Nature 428 (2004) 299–303. [4] H.E. Katz, A.J. Lovinger, J. Johnson, C. Kloc, T. Siegrist, W. Li, Y.Y. Lin, A. Dodabalapur, A soluble and air-stable organic semiconductor with high electron mobility, Nature 404 (2000) 478–481. [5] H. Sirringhaus, N. Tesler, R.H. Friend, Integrated optoelectronic devices based on conjugated polymers, Science 280 (1998) 1741– 1744. [6] A. Dodabalapur, Z. Bao, A. Makhija, J.G. Laquindanum, V.R. Raju, Y. Feng, H.E. Katz, J. Rogers, Organic smart pixels, Appl. Phys. Lett. 73 (1998) 142 (3pp). [7] J.W. Shi, H.B. Wang, D. Song, H.K. Tian, Y.H. Geng, D.H. Yan, nChannel, ambipolar, and p-channel organic heterojunction transistors fabricated with various film morphologies, Adv. Funct. Mater. 17 (2007) 397–400. [8] E.J. Meijer, D.M. de Leeuw, S. Setayesh, E. van Veenendaal, B.-H. Huisman, P.W.M. Blom, J.C. Hummelen, U. Scherf, T.M. Klapwijk, Solution-processed ambipolar organic field-effect transistors and inverters, Nat. Mater. 2 (2003) 678–682. [9] D. Schlettwein, K. Hesse, N.E. Gruhn, P.A. Lee, K.W. Nebesny, N.R. Armstrong, Electronic energy levels in individual molecules, thin films, and organic heterojunctions of substituted phthalocyanines, J. Phys. Chem. B 105 (2001) 4791–4800. [10] J. Wang, H.B. Wang, X.J. Yan, H.C. Huang, D.H. Yan, Organic heterojunction and its application for double channel field-effect transistors, Appl. Phys. Lett. 87 (2005) 093507 (3pp). [11] A. Hepp, H. Heil, W. Weise, M. Ahles, R. Schmechel, H. von Seggern, Light-emitting field-effect transistor based on a tetracene thin films, Phys. Rev. Lett. 91 (2003) 157406 (4pp). [12] M. Ahles, A. Hepp, R. Schmechel, H. von Seggern, Light emission from a polymer transistor, Appl. Phys. Lett. 84 (2004) 428–430. [13] F. Cicoira, C. Santato, M. Melucci, L. Favaretto, M. Gazzano, M. Muccini, G. Barbarella, Organic light-emitting transistors based on solution-cast and vacuum-sublimed films of a rigid core thiophene oligomer, Adv. Mater. 18 (2006) 169–174. [14] J. Reynaert, D. Cheyns, D. Janssen, R. Muller, V.I. Arkhipov, J. Genoe, G. Borghs, P. Heremans, Ambipolar injection in a submicron-channel light-emitting tetracene transistor with distinct source and drain contacts, J. Appl. Phys. 97 (2005) 114501 (5pp). [15] J.S. Swensen, C. Soci, A.J. Heeger, Light emission from an ambipolar semiconducting polymer field-effect transistor, Appl. Phys. Lett. 87 (2005) 253511 (3pp). [16] J. Zaumseil, C.L. Donley, J.-S. Kim, R.H. Friend, H. Sirringhaus, Efficient top-gate, ambipolar, light-emitting field-effect transistors based on a green-light-emitting polyfluorene, Adv. Mater. 18 (2006) 2708–2712. [17] J. Cornil, J.-L. Bredas, J. Zaumseil, H. Sirringhaus, Ambipolar transport in organic conjugated materials, Adv. Mater. 19 (2007) 1791–1799. [18] J. Zaumseil, H. Sirringhaus, Electron and ambipolar transport in organic field-effect transistors, Chem. Rev. 107 (2007) 1296–1323. [19] M.A. Loi, K. Rost-Bietsch, M. Murgia, S. Karg, W. Riess, M. Muccini, Tuning optoelectronic properties of ambipolar organic lightemitting transistors using a bulk-heterojunction, Approach Adv. Func. Mater. 16 (2006) 41–47.
Author's personal copy 1292
A.N. Aleshin et al. / Organic Electronics 12 (2011) 1285–1292
[20] F. Dinelli, R. Capelli, M.A. Loi, M. Murgia, M. Muccini, A. Facchetti, T.J. Marks, High mobility ambipolar transport in organic light-emitting transistors, Adv. Mater. 18 (2006) 1416–1420. [21] E.J. Felmeier, M. Schidleja, C. Melzer, H. von Seggern, A color-tunable organic light-emitting transistor, Adv. Mater. 22 (2010) 3568–3572. [22] R. Capelli, S. Toffanin, G. Generali, H. Usta, A. Facchetti, M. Muccini, Organic light-emitting transistors with an efficiency that outperforms the equivalent light-emitting diodes, Nat. Mater. 9 (2010) 496–503. [23] A.N. Aleshin, I.P. Shcherbakov, E.L. Alexandrova, E.A. Lebedev, Effect of electric field on the photoluminescence of polymer-inorganic nanoparticles composites, Solid State Commun. 146 (2008) 161– 165. [24] A.N. Aleshin, E.L. Alexandrova, I.P. Shcherbakov, Hybrid active layers from a conjugated polymer and inorganic nanoparticles for organic light emitting devices with emission color tuned by electric field, J. Phys. D: Appl. Phys. 42 (2009) 105108 (7pp). [25] A.N. Aleshin, I.P. Shcherbakov, Light-emitting field-effect transistor based on a polyfluorene–ZnO nanoparticles film, J. Phys. D: Appl. Phys. 43 (2010) 315104 (5pp). [26] R. Wang, L.L.H. King, A.W. Sleight, Highly conducting transparent thin films based on zinc oxide, J. Mater. Res. 11 (1996) 1659–1664.
[27] U. Ozgur, Ya.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dogan, V. Avrutin, S.-J. Cho, H. Morkocd, A comprehensive review of ZnO materials and devices, J. Appl. Phys. 98 (2005) 041301 (103pp). [28] B.S. Ong, C. Li, Y. Li, Y. Wu, R. Loutfy, Stable, solution-processed, highmobility ZnO thin-film transistors, J. Am. Chem. Soc. 129 (2007) 2750–2751. [29] B.N. Pal, P. Trottman, J. Sun, H.E. Katz, Solution-deposited zinc oxide and zinc oxide/pentacene bilayer transistors: high mobility nchannel, ambipolar, and nonvolatile devices, Adv. Funct. Mater. 18 (2008) 1832–1839. [30] S. Verlaak, D. Cheyns, M. Debucquoy, V. Arkhipov, P. Heremans, Numerical simulation of tetracene light-emitting transistors: a detailed balance of exciton process, Appl. Phys. Lett. 85 (2004) 2405–2407. [31] S. Janietz, D.D.C. Bradley, M. Grell, C. Giebeler, M. Inbasekaran, E.P. Woo, Electrochemical determination of the ionization potential and electron affinity of poly-9,9-dioctylfluorene, Appl. Phys. Lett. 73 (1998) 2453–2455. [32] M. Schidleja, C. Melzer, M. Roth, T. Schwalm, C. Gawrisch, M. Rehahn, H. von Seggern, The impact of contact formation on the light emission from ambipolar transistors, Appl. Phys. Lett. 95 (2009) 113303 (3pp).
