ISSN 10637834, Physics of the Solid State, 2012, Vol. 54, No. 12, pp. 2508–2513. © Pleiades Publishing, Ltd., 2012. Original Russian Text © A.N. Aleshin, I.P. Shcherbakov, F.S. Fedichkin, P.E. Gusakov, 2012, published in Fizika Tverdogo Tela, 2012, Vol. 54, No. 12, pp. 2388–2393.
POLYMERS
Electrical and Optical Properties of LightEmitting FieldEffect Transistors Based on MEHPPV Polymer Composite Films with ZnO Nanoparticles A. N. Aleshin*, I. P. Shcherbakov, F. S. Fedichkin, and P. E. Gusakov Ioffe PhysicalTechnical Institute, Russian Academy of Sciences, Politekhnicheskaya ul. 26, St. Petersburg, 194021 Russia * email:
[email protected] Received May 14, 2012
Abstract—The optical and electrical properties of lightemitting fieldeffect transistor structures with an active layer based on nanocomposite films containing zinc oxide (ZnO) nanoparticles dispersed in the matrix of the soluble conjugated polymer MEHPPV have been investigated. It has been found that the current–voltage char acteristics of the fieldeffect transistor based on MEHPPV : ZnO films with a composite component ratio of 2 : 1 have an ambipolar character, and the mobilities of electrons and holes in these structures at a temperature of 300 K reach high values up to ~1.2 and ~1.4 cm2/V s, respectively, which are close to the mobilities in field effect transistors based on ZnO films. It has been shown that the ambipolar fieldeffect transistor based on MEHPPV : ZnO films emits light at both positive and negative gate bias voltages. The mechanisms of injection, charge carrier transport, and radiative recombination in the studied structures have been discussed. DOI: 10.1134/S1063783412120025
1. INTRODUCTION Nanocomposite materials based on conducting polymers and inorganic nanoparticles are of great interest due to their practical application in various devices of organic electronics, such as organic light emitting diodes (OLEDs), solar cells, memory cells, etc. [1–4]. The use of nanocomposite materials for the design and fabrication of organic fieldeffect transis tors (OFETs) is a promising but still underresearched area in organic electronics. A new trend in this field is associated with the lightemitting organic fieldeffect transistors (LEOFETs) first fabricated in 2003, which combine the emission properties of OLEDs with the switching properties of OFETs [5]. LEOFETs can operate in both the unipolar [5, 6] and ambipolar [7– 9] regimes. In recent years, there have been fabricated LEOFETs with an active layer consisting of monopo lymers [5–9], polymer blends [10], two polymer layers [11], or three polymer layers [12]. The emission effi ciency of threelayer LEOFETs is comparable to or even higher than the emission efficiency of equivalent OLEDs [12]. It should be noted that, prior to the beginning of our investigations, in the literature there had been no information about LEOFETs based on nanocomposite materials, including semiconducting polymers and inorganic nanoparticles. We have recently demonstrated such LEOFETs with an active layer based on a soluble polymer, namely, polyfluorene (PFO), and nanoparticles of zinc oxide (ZnO), which can operate in both the unipolar and ambipolar regimes at high and moderate concentrations of ZnO nanoparticles, respectively. It has been found that LE
OFETs based on PFO : ZnO films are characterized by abnormally high values of the charge carrier mobility [13, 14]. OFETs based on the matrix of another poly mer widely used in polymer optoelectronics, namely, poly[2methoxy5(2ethylhexyloxy)1,4phenylene vinylene] (MEHPPV), with ZnO nanoparticles embedded in it have also been demonstrated recently [15]. On the one hand, rather high values of the hole mobility, up to ~0.15 cm2/V s, were achieved in the OFETs based on the MEHPPV : ZnO films [16]. On the other side, LEOFETs were fabricated using only films of pure MEHPPV [17]. The properties of the LEOFETs based on MEHPPV : ZnO composite films have so far not been investigated in detail. The importance of these investigations is associated with the fact that the fabrication of LEOFET structures is a significant step in the development of new light emitting devices, such as polymer and composite injection lasers. The purpose of this work was to investigate the opti cal and electrical properties of LEOFETs structures with an active layer based on nanocomposite films con taining ZnO nanoparticles dispersed in the matrix of the soluble conjugated polymer MEHPPV. It has been established that the current–voltage characteristics of the OFET structures based on the MEHPPV : ZnO films with a composite component ratio of 2 : 1 have an ambipolar character, and the mobilities of electrons and holes in these structures at a temperature of 300 K reach high values up to ~1.2 and ~1.4 cm2/V s, respectively. These values are close to the mobilities in fieldeffect transistors based on ZnO films. It has been shown that
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2. OBJECTS AND METHODS OF INVESTIGATION In this work, we investigated the optical and electri cal properties of composite films based on a soluble conjugated polymer (namely, poly[2methoxy5(2 ethylhexyloxy)1,4phenylenevinylene] (MEH PPV): (C18H28O2)n, the average molecular weight is Mw ~ 4–7 × 104, and the band gap is Eg ~ 2.24 eV) with ZnO nanoparticles (the diameter is ~50–70 nm, the band gap is Eg ~ 3.3 eV, and the density is 5.6 g/cm3). The structure of the molecule of the MEHPPV poly mer is shown in the inset to Fig. 1. The MEHPPV polymer and ZnO nanoparticles studied in our exper iments were purchased from Sigma–Aldrich and used without additional treatment. During the fabrication of the OFET structures, MEHPPV was dissolved in chloroform, which was also used in the preparation of a colloidal solution of ZnO nanoparticles. The result ant solutions were mixed and, after stirring with ultra sound for 8–10 min (at a frequency f ~ 22 kHz), were deposited on silicon (nSi with a SiO2 layer ~200 nm thick) substrates with thermally evaporated Au and Al electrodes. In the nSi/SiO2/Au/MEHPPV : ZnO/Al OFET structures thus prepared, nSi was the gate, the Au electrode was the source, and the Al electrode was the drain. The distance between the Au and Al elec trodes was ~7 μm, and the electrode width was ~1 mm. The layers were precipitated from a 20 wt % solution of the MEHPPV polymer and ZnO nanoparticles in chloroform and then were dried at a temperature of 80°C for 20–30 min in a nitrogen atmosphere. The layer thickness was estimated from the results of the investigations of the films with an atomic force micro scope and amounted to ~ 0.6 μm. The concentration of ZnO nanoparticles in the composites was ~10–33 wt %. The absorption spectra of the composite films were investigated using a Cary50 (Varian) spectrometer. For these investigations, the films were deposited on quartz substrates. The film thickness was ~1 μm. The photoluminescence spectra of the MEH PPV : ZnO composite films were investigated using a setup based on an SPM2 mirror monochromator operating in the wavelength range λ ~ 300–830 nm with a spectral resolution from 0.5 to 5.0 nm. Photolu minescence was excited with an LGI21 pulsed ultra violet nitrogen laser operating at a radiation wave length of 337.1 nm with a pulse energy density of more than 10–4 J/cm2 and a pulse duration of ~10–8 s. The photoluminescence spectra at the output slip of the SPM2 monochromator were recorded in the spectral range λ ~ 300–830 nm with a FEU136 photomulti plier, which was also used to detect signals of the inte
OCH3
1.0 Absorption, arb. units
the ambipolar OFETs based on the MEHPPV : ZnO films emit light at both positive and negative gate bias voltages. The mechanisms of charge carrier injection and light emission in the LEOFETs structures based on the MEHPPV : ZnO films have been discussed.
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n O
0.6 0.4 0.2 0 200
400
600 λ, nm
800
1000
Fig. 1. Spectra of absorption and photoluminescence of the MEHPPV film (thin solid lines) and the MEHPPV : ZnO (2 : 1) film (thick solid lines). The film thickness is ~0.6 mm. The inset shows the chemical structure of MEHPPV.
grated electroluminescence intensity at 300 K. In these measurements, a special mirror was used to increase the intensity of photoluminescence and elec troluminescence of the samples. The resolution of the entire system was ~2 nm. The kinetics of the photolu minescence of the MEHPPV : ZnO composite films was examined using dual channel bandwidth storage oscilloscopes TEKTRONIX TDS 2002B and ASK 3106. The current–voltage characteristics of the OFET structures based on the MEHPPV : ZnO composites placed on the holder of the nitrogen cryostat were mea sured in the dark at a temperature of 300 K in a vacuum (3 × 10–3 Torr) at a direct current in the voltage range from –30 to +30 V with the use of an automated setup for measuring current–voltage characteristics on the basis of a Keithley 6487 picoammeter and an AKIP 1124 programmable power supply unit. Electrical con tacts to the samples were provided using a silver wire and an SPI conductive carbon paint. The charge carrier mobility μFET of the composite active layer was esti mated from the current–voltage characteristics of the OFET under saturation conditions and in weak electric fields, respectively, according to the relationships [18] 2
I DS = ( W/2L )μ FET C 1 ( V G – V th ) ,
(1)
I DS = ( W/L )μ FET C 1 ( V G – V th )V DS ,
(2)
where IDS and VDS are the current and the voltage between the drain and the source, respectively; W is the width of the channel; L is the length of the chan nel; CI is the capacitance per square of the area of SiO2 (for a thickness of ~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 accumulation regime. 2012
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IDS, 10−4 A
5
IDS, 10−4 A
7
3 MEHPPV : ZnO (2 : 1) 2 1 0
4
(a) MEHPPV : ZnO (2 : 1) VG = −25 V
VG = −10 V 10 20 30 VDS, V
VG = −20 V
3
VG = −15 V
2 1 0
10
20 VDS, V
VG = −10 V VG = −5 V VG = 0 V 30
4
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(b) VG = 15 V
IDS, 10−4 A
3 Au
2
Al SiO2 ndroped Si
VG = 10 V 1 VG = 5 V 0
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30 VDS, V
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50
Fig. 2. (a) Current–voltage characteristics of the OFET based on MEHPPV : ZnO (2 : 1) at a temperature of 300 K for different negative gate voltages VG. The inset shows the increase in the drain–source current IDS at VDS > 25 V and VG = –10 V. (b) Current–voltage charac teristics of the same sample at 300 K for different positive gate voltages VG. The inset shows the structure of the LE OFETs based on the MEHPPV : ZnO film.
3. RESULTS AND DISCUSSION The absorption and photoluminescence spectra of the MEHPPV and MEHPPV : ZnO (the concentra tion of ZnO nanoparticles is ~33 wt %, and the weight ratio of the polymer and nanoparticles is 2 : 1) films measured at a temperature of 300 K are shown in Fig. 1. As can be seen from Fig. 1, the absorption spec tra of both the MEHPPV film and the MEH PPV : ZnO composite film exhibit maxima at a wave length λ ~ 500 nm. In this case, when the ZnO nano particles are embedded in the matrix of the MEH PPV polymer, the intensity of absorption in the com posite film decreases compared to that of the MEH PPV film. This confirms the occurrence of the interac tion between the molecules of the polymer and ZnO nanoparticles. The photoluminescence spectra of the MEHPPV and MEHPPV : ZnO films (Fig. 1) indi
cate that the introduction of ZnO nanoparticles into the polymer matrix leads to a shift in the maximum of the photoluminescence band from λ ~ 600 nm (MEH PPV) to λ ~ 640 nm (MEHPPV : ZnO). An increase in the concentration of ZnO nanoparticles gives rise to a photoluminescence peak at λ ~ 380 nm, which is associated with the photoluminescence of ZnO nano particles [19]. Our investigations of the kinetics of the photoluminescence of the MEHPPV and MEH PPV : ZnO (ZnO ~ 33 wt %) films allowed us to esti mate the characteristic relaxation times of excited – τ/t states according to the relationship IPL ~ e , where τ is the lifetime of the excited states. The values of τ obtained from our investigations have demonstrated that, for excited states in MEHPPV at a wavelength corresponding to the photoluminescence maximum (λ ~ 650 nm), the lifetime is τ ~ 2.5 ms, which is 1.4 times longer than the lifetime of charge carriers in the MEHPPV : ZnO composite film at a wavelength of the maximum in the photoluminescence spectrum (τ ~ 1.8 ms at λ ~ 650 nm). Therefore, the lifetime of excited states in the film of pure MEHPPV decreases when ZnO nanoparticles are embedded in the polymer matrix, which suggests an increase in the concentration of defects in the MEHPPV polymer chain [20]. Figures 2a and 2b show typical current–voltage characteristics of the OFET structure based on the MEHPPV : ZnO (2 : 1) composite film, which were obtained in a vacuum at different gate voltages. The OFET structure with an active composite layer based on the MEHPPV : ZnO film is shown in the inset to Fig. 2b. As can be seen from Fig. 2a, the shape of the current–voltage characteristics of the OFET structure based on MEHPPV : ZnO at negative gate voltages VG is typical of the hole transport in a closetosatura tion regime. In this case, the dependences of the drain–source current IDS on the drain–source voltage VDS are not completely flattened, which is probably due to the influence of leakage currents between elec trodes, i.e., between the source and the gate. For suffi ciently high voltages VDS > 25 V, a sharp increase in the current IDS is observed for the OFETs based on the MEHPPV : ZnO films (inset to Fig. 2a), which cor relates with the shape of the current–voltage charac teristics previously observed in OFETs based on other organic materials [6, 14]. This behavior of the cur rent–voltage characteristics indicates that there arises a channel associated with the electron transport at negative values of VG. For positive values of VG, the current–voltage characteristics exhibit a similar behavior. However, in this case, an increase in the cur rent IDS (due to the onset of hole transport) is more pronounced, as is shown in Fig. 2b. The observed behavior of the current–voltage characteristics at neg ative and positive gate voltages VG correlates with the results of the numerical simulation carried out in [21], which predicts a superlinear increase in the saturation current of OFETs in the ambipolar regime.
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drain–source current I DS on the gate voltage VG at the drain–source voltage VDS = –10 V were found to be ~+0.5 and ~+2.0 V for negative and positive values of VG, respectively (inset to Fig. 3). The electron and hole mobilities at 300 K for the OFET based on the MEHPPV : ZnO (2 : 1) film according to the calcu lation from formula (1) proved to be ~1.2 and ~1.4 cm2/V s, respectively. The values of μFET esti mated for the same sample at 300 K from formula (2) proved to be ~ 0.32 and ~ 0.63 cm2/V s for electrons and holes, respectively. These values are lower than the mobilities obtained in the saturation current regime. This can be associated with the influence of leakage currents on the current–voltage characteristics of OFETs in the saturation current regime. The ON/OFF ratio, which characterizes the ratio of the currents through the OFETs without a bias at the gate and with a bias applied to the gate, was determined from the transfer characteristics and proved to be ~104 for VG ~ –30 V, which is higher than the ON/OFF ratios obtained earlier for the OFETs based on PFO : ZnO [13, 14]. A similar behavior of the current– voltage and transfer characteristics was also observed for the other OFETs based on the MEHPPV : ZnO composite films containing ZnO nanoparticles with concentrations ~10 and ~25 wt %. The observed rela tively low values of the threshold voltage Vth and the ON/OFF ratio indicate a low concentration of trap states and a small value of injection barriers in the con tacts. The values of μFET (300 K), which were obtained in our experiments for the OFETs based on MEH PPV : ZnO with an intermediate concentration of PHYSICS OF THE SOLID STATE
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The transfer characteristics for OFETs in the satu ration current regime at positive and negative gate voltages VG are shown in Fig. 3. The values of VG were varied from +30 to –30 V in steps of 0.5 V at a constant voltage VDS = –10 V. It can be seen from Fig. 3 that, at a relatively low concentration of ZnO nanoparticles, the OFET structure based on the MEHPPV : ZnO (2 : 1) film operates in the accumulation regime for both electrons and holes. In this case, the OFET struc ture based on the MEHPPV : ZnO film exhibits transfer characteristics with a small reversible hystere sis, whose amplitude is considerably smaller than the hysteresis observed earlier in the OFET structure based on pure MEHPPV [16]. This shape of current– voltage characteristics means that the density of traps in the OFETs based on MEHPPV : ZnO is less than that in the OFET structure based on pure MEHPPV, which should lead to an improvement of the electrical characteristics of the composite OFET structure. The electron and hole mobilities μFET for the OFETs based on MEHPPV : ZnO were calculated for the satura tion current regimes and weak fields according to rela tionships (1) and (2). For the OFET based on the MEHPPV : ZnO (2 : 1) film (Fig. 3), the threshold voltages Vth estimated from the dependences of the
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−7
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0 VG, V
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Fig. 3. Transfer characteristics of the OFET based on MEH PPV : ZnO (2 : 1) at a temperature of 300 K for VDS = –10 V. The inset shows the dependence of the drain–source cur 0.5
rent I DS on the gate voltage VG for the same sample.
ZnO nanoparticles, are considerably higher than those usually observed for the OFETs based on films of pure MEHPPV (~4 × 10–4 cm2/V s [22]). This can be asso ciated with the contribution from both the pcompo nent and the ncomponent of the MEHPPV : ZnO composite to the charge transfer. The obtained high values of the mobility μFET (300 K) in the OFETs based on the MEHPPV : ZnO composite films, in our opin ion, can be associated with the contribution to the overall mobility of charge carriers from their mobility in ZnO nanoparticles, which being embedded in the polymer matrix can lead to a decrease in the density of traps in the polymer, as well as to the formation of ZnO–MEHPPV complexes and agglomerates between the Al–Au electrodes. This explanation corre lates with the high values of μFET (300 K) obtained for polycrystalline ZnO (~20 cm2/V s) [23] and for the OFET based on ZnO (~7.2 cm2/V s) [24]. The mecha nism of transport due to the chargetransfer complexes formed at the boundary between the polymer and inor ganic nanoparticles requires further investigation. The optical output characteristics (dependence of the integrated electroluminescence intensity on the voltage VDS) at negative and positive gate voltages VG for the ambipolar LEOFET based on MEHPPV : ZnO (2 : 1) are presented in Fig. 4. As can be seen from this figure, the integrated electroluminescence intensity at a temperature of 300 K increases with an increase in the voltage VDS for both negative and positive gate volt ages VG. The LEOFET structures based on the MEHPPV : ZnO composite films have an asymmet ric dependence of the integrated electroluminescence intensity on the voltage VDS with a higher integrated electroluminescence intensity for the positive values of VDS as compared to the negative values, which is prob 2012
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Energy level diagram, eV
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EL intensity, arb. units
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2
1
−2 −3 −4
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−4.2 eV −5 −4.3 eV −6 −7 −8
VG = +30 V
LUMO −2.8 eV Au HOMO −5.1 eV −5.0 eV
VG = +20 V
−7.2 eV VG = +10 V
VG = −30 V
VG = +0 V
VG = −20 V VG = −10 V VG = 0 V
0 −60
−40
−20
0 VDS, V
20
40
60
Fig. 4. Optical output characteristics of the LEOFET based on MEHPPV : ZnO (2 : 1): the integrated elec troluminescence intensity as a function of the drain– source voltage VDS for different gate voltages VG at 300 K. The inset shows the band diagram of the Au–MEH PPV : ZnO–Al structure.
ably due to the formation of polymer–nanoparticle complexes. There is a fixed voltage at the onset of the electroluminescence for VDS ~ 10 V, which does not depend on the concentration of ZnO nanoparticles and on the polarity of the gate voltage VG. This cutoff voltage can be observed for values of VG down to –30 V (Fig. 4). This dependence of the integrated electrolu minescence intensity on the voltage VDS correlates with the previously observed behavior of the integrated electroluminescence intensity in LEOFETs based on tetracene, polyfluorene, and PFO : ZnO [5, 13, 14]. However, in the case of LEOFETs based on the MEHPPV : ZnO films, the cutoff voltage VDS is two times higher than that for LEOFETs based on the PFO : ZnO films. We also measured the dependence of the integrated electroluminescence intensity on the electric field strength (up to fields ~2.5 × 103 V/cm) for the LEOFETs based on MEHPPV : ZnO in different spectral ranges: (1) the entire range—integral, (2) λ ~ 600–830 nm, (3) λ ~ 450–620 nm, and (4) λ ~ 300– 400 nm [19]. As follows from the obtained results, the spectral distribution of the electroluminescence in the LEOFET structure based on MEHPPV : ZnO pre dominantly lies in the red spectral region, which cor responds to the emission from the MEHPPV poly mer matrix. In particular, for the LEOFET structure based on MEHPPV : ZnO (2 : 1), the fractions of radiation in the red and blue spectral regions amount to ~60 and ~10% of the integrated electrolumines cence intensity, respectively, which correlates with the photoluminescence spectrum of these composite films (Fig. 1). The mechanism of generation of excited states in the MEHPPV : ZnO composite films includes the excitation of charge carriers from the highest occupied molecular orbital (HOMO) to the
lowest unoccupied molecular orbital (LUMO) in the MEHPPV polymer, the contribution to the radiative recombination from the emission of ZnO nanoparti cles (at λ ~ 380 nm), and the contribution from the emission of polymer–nanoparticle complexes. As was shown in [19], some of the recombination channels in these systems can be suppressed by changing the con centration of ZnO nanoparticles. In order to elucidate details of the mechanism of charge carrier injection, we consider the band diagram of the Au–MEHPPV : ZnO–Al structure shown in the inset to Fig. 4. As follows from this band diagram, the work functions for the Au and Al electrodes are equal to ~5.1 and ~4.3 eV, respectively, whereas the energies of the HOMO and LUMO levels in MEH PPV are ~5.0 and ~2.8 eV, respectively. Therefore, the barrier for holes in the Au–MEHPPV contact is ~0.1 eV, while the barrier for electrons in the Al– MEHPPV contact is ~1.5 eV. This makes the Au con tact preferable for injection of holes into the MEH PPV polymer. On the other hand, the energies corre sponding to the edges of the valence band (~7.2 eV) and the conduction band (~4.2 eV) in ZnO clearly indicate the presence of a significant energy barrier for holes, which, however, can be overcome in the trans port of charge carriers from ZnO to MEHPPV; in this case, the energy of the injection barrier for electrons in the Al–ZnO contact is ~0.1 eV. These data demon strate that the Au–MEHPPV : ZnO–Al structure should serve as an ambipolar LEOFET. This is in good agreement with our experimental results for the LE OFETs based on the MEHPPV : ZnO films with a rel atively low concentration of ZnO nanoparticles. It can be assumed that the introduction of ZnO nanoparticles decreases the density of traps in the MEHPPV poly mer, which is probably responsible for the increase in the charge carrier mobility of such OFET structures. According to the mechanism of electrolumines cence of LEOFET structures, which was proposed in [25, 26], the emission in the unipolar regime is associ ated with the presence of a stable reservoir of electrons in the polymer in the immediate vicinity of the Al con tact. The transport of charge carriers in the Al–MEH PPV structure occurs after the formation of a contact and a constant electrochemical potential. The diffu sion of electrons from the metal into MEHPPV ceases when the electric field is selfinduced by the accumulated charge; in this case, the mass of excess electrons is located near the Al contact. In the accu mulation regime, holes move from the source (Au electrode) to the drain (Al electrode), where they either can be absorbed by the metal or can recombine with electrons of the reservoir (space charge). As was shown recently in [9, 25, 26], the transition from the unipolar to ambipolar transport regime leads to a change in the position of the recombination zone of charge carriers in their motion from one contact to the other one. In the ambipolar regime, the recombina tion of charge carriers with a higher probability occurs
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at the polymer/dielectric and polymer/ZnO nanopar ticle interfaces rather than in the contacts. The recom bination zone can be displaced from the metal/poly mer interface to the polymer/dielectric and poly mer/ZnO nanoparticle interfaces, which leads to a change in the transport regime from unipolar to ambi polar. Moreover, the spatial heterogeneities of the channel in LEOFETs based on MEHPPV : ZnO can affect the electroluminescence and the injection of electrons and holes from the Al and Au electrodes. The obtained results have demonstrated that LEOFETs based on soluble conjugated polymers, such as MEH PPV, and ZnO semiconductor nanoparticles can operate as multifunction devices whose fabrication technology is compatible with the modern technology of printed organic electronics. 4. CONCLUSIONS Thus, we obtained and investigated the optical and electrical characteristics of LEOFETs with an active layer based on the MEHPPV : ZnO composite films with an intermediate content of ZnO nanoparticles (10–33 wt %). It was established that the LEOFET structures based on the MEHPPV : ZnO films exhibit an ambipolar behavior of the current–voltage charac teristics and operate in the saturation current regime with the accumulation of holes and electrons. It was shown that the inclusion of ZnO nanoparticles in the polymer matrix significantly increases the hole and electron mobilities of charge carriers in these struc tures, so that, for the ambipolar LEOFET based on MEHPPV : ZnO (ZnO ~ 33 wt %), the mobilities of electrons and holes at a temperature of 300 K reach high values up to ~1.2 and ~1.4 cm2/V s, respectively, which are close to the mobilities of fieldeffect transis tors based on the ZnO films. The LEOFETs based on the MEHPPV: ZnO composite films demonstrate cur rent–voltage characteristics with a small hysteresis. The ambipolar LEOFET based on the MEHPPV : ZnO films emits light at both positive and negative gate bias voltages. The implementation of the ambipolar regime in LEOFETs based on MEHPPV : ZnO makes these structures promising for the use in logic circuits and memory elements, as well as for the fabrication of polymer injection lasers. ACKNOWLEDGMENTS This study was supported by the Presidium of the Russian Academy of Sciences within the framework of the Basic Research Program no. P8 (direction “Multi functional Materials for Molecular Electronics”), the Russian Foundation for Basic Research (project no. 11 0200451a), and the State Program for Support of Leading Scientific Schools (NSh3008.2012.2). REFERENCES 1. C. Sanchez, B. Julian, P. Belleville, and M. Popall, J. Mater. Chem. 15, 3559 (2005). PHYSICS OF THE SOLID STATE
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Translated by O. BorovikRomanova 2012
