ISSN 10637834, Physics of the Solid State, 2012, Vol. 54, No. 8, pp. 1693–1698. © Pleiades Publishing, Ltd., 2012. Original Russian Text © A.N. Aleshin, I.P. Shcherbakov, F.S. Fedichkin, 2012, published in Fizika Tverdogo Tela, 2012, Vol. 54, No. 8, pp. 1586–1590.
POLYMERS
Photosensitive FieldEffect Transistor Based on a Composite Film of Polyvinylcarbazole with Nickel Nanoparticles A. N. Aleshin*, I. P. Shcherbakov, and F. S. Fedichkin Ioffe PhysicalTechnical Institute, Russian Academy of Sciences, Politekhnicheskaya ul. 26, St. Petersburg, 194021 Russia * email:
[email protected] Received January 18, 2012
Abstract—The electronic and optoelectronic properties of fieldeffect transistor structures with an active layer based on composite films of a semiconducting polymer, namely, polyvinylcarbazole (PVC), with nickel nanoparticles have been investigated. It has been shown that these structures at low nickel concentrations (5– 10 wt %) possess current–voltage characteristics that indicate an ambipolar transport. For the fieldeffect transistor structures based on PVC : Ni (Ni ~ 5 wt %) films, the mobilities of electrons and holes are found to be ~1.3 and ~1.9 cm2/V s, respectively. It has been established that the photosensitivity observed in these structures is associated with the specific features of transport in the film of the polymer with nickel nanopar ticles. The mechanism of this transport is determined by the modulation of electrical conductivity of the working channel of the fieldeffect transistor by applying a combination of incident light and gate voltages. DOI: 10.1134/S1063783412080033
1. INTRODUCTION Fieldeffect transistors based on organic materials (organic fieldeffect transistors (OFETs)) have attracted increasing attention from the viewpoint of their application in various devices of organic elec tronics [1]. The field of application of OFETs includes actual fieldeffect transistors [2, 3], lightemitting OFETs [4], memory elements [5, 6], etc. An impor tant advantage of OFETbased devices is the possibil ity of their direct integration into standard logic elec tronic circuits. The possibility of controlling the trans port of charge carriers, including the charge carrier concentration in the OFET channel, by applying and varying the voltage at the gate, as well as the concen tration of photoinduced charge carriers by the light incident on the OFET structure, has led to the advent of photosensitive OFETs and optically controlled memory cells [7, 8]. In recent years, phototransistors based on conjugated polymers have been actively investigated [9, 10]. At the same time, composites based on organic polymers with embedded inorganic nanoparticles have attracted the particular attention of researchers as promising materials for active layers of OFETs. An important advantage of composite films is their high functionality and electrical stability in com parison with pure polymer analogs. Recently, we have demonstrated the possibility of designing and fabricat ing a lightemitting OFET with a composite active layer based on a soluble conjugated polymer, namely, polyfluorene, with semiconductor nanoparticles of zinc oxide (ZnO) [11, 12]. A characteristic feature of these composite OFET structures is a significant
increase in the mobility of charge carriers, which has been observed with an increase in the concentration of ZnO nanoparticles in the composite active layer. It was also shown that the introduction of metal (gold) nano particles with strong acceptor properties into an active or dielectric layer of the OFET structure results in the emergence of memory effects, which manifest them selves in a hysteresis of the current–voltage character istics and transfer characteristics of OFETs [13, 14]. The introduction of nanoparticles of metals, such as gold [14] or nickel [15], into the polymer matrix leads to a sharp increase in the mobility of charge carriers in these composite systems. The use of composite films based on polymers with metal nanoparticles as active layers of OFETs, in our opinion, should also lead to the manifestation of photosensitivity effects in these structures. However, the photosensitivity effects in composite OFETs are still not clearly understood. The purpose of this work was to investigate the optical, electrical, and photoelectric properties of the OFET structures with a composite active layer based on a widebandgap semiconducting polymer, namely, polyvinylcarbazole, with nickel nanoparticles. It has been shown that the photosensitivity observed in these films is associated with the specific features of trans port in the polymer–nickel nanoparticle structure. The mechanism of photosensitivity in these struc tures, which is based on the modulation of electrical conductivity of the working channel of the fieldeffect transistor by applying a combination of incident light and gate voltages, has been discussed.
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បω
PVC N
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n 0.6 PVC : Ni
0.4 0.2 0 200
Polymer + nanoparticles Au
PVC 300
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500 λ, nm
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Al SiO2
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ndoped Si
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Fig. 1. Absorption spectrum of polyvinylcarbazole (PVC) and photoluminescence spectra of the PVC and PVC : Ni (Ni ~ 10 wt %) films. The inset shows the structure of the PVC molecule.
Fig. 2. Structure of the fieldeffect transistor with a com posite active layer based on PVC : Ni.
2. SAMPLE PREPARATION AND EXPERIMENTAL TECHNIQUE In this work, we investigated the optical, electrical, and photoelectric properties of composite films of the polymer polyvinylcarbazole (poly(9vinylcarbazole) (PVC): the molecular weight is MW ~ 1.1 × 106, the density at 25°C is 1.2 g/mL, and the band gap is Eg ~ 3.6 eV) with nickel nanoparticles (the diameter is less than 100 nm, the electrical resistivity at 20°C is equal to 6.97 μΩ cm, and the density at 25°C is 8.9 g/mL). The structure of the molecule of the PVC polymer is shown in the inset to Fig. 1. The PVC polymer and nickel nanoparticles studied in our experiments were purchased from Sigma–Aldrich and used without additional treatment. During the fabrication of the fieldeffect transistor structures, PVC was dissolved in chloroform, which was also used in the preparation of a colloidal solution of nickel nanoparticles. The resultant solutions were mixed and, after stirring with ultrasound for 8–10 min (at a frequency f ~ 22 kHz), were deposited on silicon (Si with a SiO2 layer ~200 nm thick) substrates with thermally evaporated Au and Al electrodes. The distance between the elec trodes was ~7 μm, and the electrode width was ~1 mm. The layers were precipitated from a 20 wt % solution of the polymer and nickel nanoparticles in chloroform and then were dried at a temperature of 80°C for 10 min. The layer thickness was estimated from the results of the investigations of the films with an atomic force microscope and amounted to ~0.6 μm. The con centration of nickel nanoparticles in the composites was ~5–10 wt %. The structure of the organic field effect transistor with a composite active layer based on PVC : Ni is shown in Fig. 2. The photoluminescence spectra of the PVC : Ni 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 res olution from 0.5 to 5.0 nm. Photoluminescence was excited with an LGI21 pulsed ultraviolet nitrogen laser operating at a wavelength 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 monochroma tor were recorded in the spectral range λ ~ 300– 830 nm with a FEU136 photomultiplier. The resolu tion of the entire system was ~2 nm. The kinetics of the photoluminescence of the PVC : Ni composite films was examined using dual channel bandwidth storage oscilloscopes TEKTRONIX TDS 2002B, and ASK3106. The current–voltage characteristics of the field effect transistor structures based on the PVC : Ni com posites placed on the holder of the nitrogen cryostat were measured in the dark at a temperature of 300 K in a vacuum (3 × 10–3 Torr) at a direct current in the volt age range from –30 to +30 V with the use of an auto mated setup for measuring current–voltage character istics on the basis of a Keithley 6487 picoammeter and an AKIP1124 programmable power supply unit. Electrical contacts to the samples were provided using a silver wire and SPI conductive carbon paint. The charge carrier mobility μFET of the composite active layer was estimated from the current–voltage charac teristics of the fieldeffect transistor under saturation conditions and in weak electric fields, respectively, according to the relationships [2, 3] 2
I DS = ( W/2L )μ FET G 1 ( V G – V th ) ,
(1)
I DS = ( W/L )μ FET G 1 ( V G – V th )V DS ,
(2)
where W is the width of the channel; L is the length of the channel; 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
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threshold voltage, which corresponds to the onset of the accumulation regime. The photosensitivity of fieldeffect transistor structures with a composite active layer based on PVC : Ni was measured under irradiation with light from a halogen lamp with a power of ~10 W so that a part of the spectrum of the lamp in the range λ ~ 350–700 nm with the maximum at a wavelength λ ~ 540 nm, which is close to the spec trum of the solar radiation, was cut using an SZS21 filter. The total irradiation area was ~10 cm2.
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3. RESULTS AND DISCUSSION
The PVC : Ni (Ni ~ 5–10 wt %) composite films were used as the active layer of the fieldeffect transis tor whose structure is presented Fig. 2. Figures 3a and 3b show the current–voltage characteristics of the OFET with a composite active layer based on PVC : Ni (Ni ~ 5 wt %), which operates at different gate voltages VG, and the transfer characteristics (dependences of PHYSICS OF THE SOLID STATE
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The absorption and photoluminescence spectra of the PVC and PVC : Ni (Ni ~ 10 wt %) films measured at a temperature of 300 K are shown in Fig. 1. As can be seen from Fig. 1, the absorption edge of the initial PVC polymer lies in the spectral region of ~350 nm. The photoluminescence spectra of the PVC and PVC : Ni films consist of bands with maxima at wavelengths of ~390 and ~ 450 nm due to the radiative processes in carbazole chromophore and bands with maxima at ~550 nm (PVC) and ~560–570 nm (PVC : Ni) due to the formation of PVC agglomerates and also, in the latter case, due to the formation of complexes with embedded nickel nanoparticles. The observed photo luminescence spectra indicate charge transfer between the carbazole fragment of the polymer and nickel nanoparticles and the formation of chargetransfer complex, which manifests itself in the photolumines cence spectrum of the PVC : Ni composite film. Our investigations of the kinetics of the photolumines cence of the PVC and PVC : Ni (Ni ~ 10 wt %) films allowed us to estimate the characteristic relaxation times of excited states according to the relationship IPL ~ e–t/τ, where τ is the lifetime of the charge carriers. The values of τ obtained from our investigations have demonstrated that, for excited states in pure PVC at a wavelength corresponding to the photoluminescence maximum λ ~ 430 nm, the lifetime is τ ~ 7.9 μs, which is 1.2 times longer than that for the PVC : Ni (Ni ~ 10 wt %) composite film at λ ~ 430 nm (τ ~ 6.6 μs). The same behavior of the lifetime τ is observed for the photoluminescence band with a maximum at λ = 550 nm, where τ ~ 10 and ~ 9.2 μs for the PVC and PVC : Ni (Ni ~ 10 wt %) films, respectively. Therefore, the lifetime of the excited states in the PVC film decreases when nickel nanoparticles are embedded in the polymer matrix, which indicates an increase in the concentration of defects in the PVC polymer chain [16].
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0 VG, V
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PVC : Ni (Ni ~ 5 wt %) VDS = –5 V
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Fig. 3. (a) Current–voltage characteristics of the field effect transistor with a composite active layer based on PVC : Ni (Ni ~ 5 wt %) for different gate voltages VG in the dark and the dependence of the drain–source current IDS on the drain–source voltage VDS at VG = –10 V for the same sample under illumination. (b) Transfer characteris tics of the same OFET without illumination at VDS = ⎯5 V. 0.5
The inset shows dependence of I DS on VG for the same OFET at VDS = –5 V.
IDS on VG) of the same device, which were measured at the voltage between the drain and source VDS = –5 V. The obtained current–voltage characteristics are typi cal of ambipolar fieldeffect transistors operating under current saturation conditions at voltages slightly exceeding a certain threshold value Vth. The mobility of charge carriers of the composite active layer μFET was estimated from relationships (1) and (2). For the OFET based on the PVC : Ni (Ni ~ 5 wt %) composite film, the threshold voltages Vth estimated from the 0.5
dependence of I DS on VG at the drain–source voltage VDS = –5 V (inset to Fig. 3b) proved to be ~–2.5 and ~–0.7 V for negative and positive values of VG, respec tively. The ratio ON/OFF, which characterizes the
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Fig. 5. Kinetics of relaxation of the drain–source current IDS of the fieldeffect transistor based on PVC : Ni (Ni ~ 10 wt %) after turning on and off the light with a power of ~ 60 µW/cm2) at VDS = –10 V and VG = –8 V.
Fig. 4. Dependences of IDS on VG for the fieldeffect tran sistor based on PVC : Ni (Ni ~ 5 wt %) at VDS = –5 V in the dark and under illumination by a sunlight simulator. The inset shows the dependences of IDS on VG for the OFET based on PVC : Ni (Ni ~ 10 wt %) at VDS = –10 V in the dark and under illumination.
ratio of the currents through the transistor without a bias at the gate and with a negative voltage at the gate, was found to be ~102 for VG = –5 V and increased with an increase in VG. The observed relatively low thresh old voltages and ON/OFF ratios indicate a low con centration of traps and a small value of the contact barrier during the charge carrier injection. The mobil ity of charge carriers at 300 K μFET(300 K) was esti mated using formula (1) for VG = –10 V and VG = +10 V and for VDS = –5 V. The obtained values of μFET (300 K) for the OFETs based on the PVC : Ni compos ite films with a nickel concentration of ~5 wt % were found to be ~1.3 and ~1.9 cm2/V s for electrons and holes, respectively. The mobilities estimated for the same samples in weak electric fields according to for mula (2) have close values. The values of μFET (300 K) obtained in our experiments for the OFETs based on the PVC : Ni films with a nickel concentration of ~5– 10 wt % proved to be considerably higher than the values of μFET (300 K) for pure PVC (~10–5–10–6 cm2/V s), which, in our opinion, as in the case of polymer– nickel complexes [15], is associated with a significant contribution to the total mobility of one of the compo nents of the composite, namely, nickel nanoparticles. Figure 3a shows the current–voltage characteris tics (in the coordinates IDS–VDS) of the fieldeffect transistor with a composite active layer based on PVC : Ni (Ni ~ 5 wt %), which were obtained at differ ent gate voltages VG (from 0 to –10 V) without illumi nation, and the dependence of IDS on VDS measured at the gate voltage VG = –10 V for the same sample under irradiation with light from a halogen lamp so that a
part of the spectrum of the lamp, which is close to the spectrum of the solar radiation, was cut out. Figure 4 shows the dependences of IDS on VG for the same OFET based on PVC : Ni (Ni ~ 5 wt %), which were measured at VDS = –5 V in the dark and under illumi nation. The inset to Fig. 4 shows similar dependences for the OFET based on PVC : Ni (Ni ~ 10 wt %), which were measured at VDS = –10 V in the dark and under illumination. For the OFET based on PVC : Ni, there is a small hysteresis of the transfer characteristics both in the dark and under illumination of the sam ples; in this case, the amplitude of the hysteresis is sig nificantly less than that for the OFETs based on PEPC : Au films [14]. As can be seen from Fig. 4, the illumination of the OFET based on PVC : Ni (Ni ~ 5 wt %) by the sunlight simulator leads to a noticeable increase in IDS (by a factor of 2–3), which indicates an increase in the electrical conductivity of the active layer of the OFET under irradiation with light. A typ ical character of the relaxation kinetics of the current IDS for the OFET based on PVC : Ni (Ni ~ 10 wt %) after turning on and off the light (with a power ~60– 70 μW/cm2) at VDS = –10 V and VG = –8 V is shown in Fig. 5. The effects observed under illumination of the fieldeffect transistors based on PVC : Ni (Ni ~ 5– 10 wt %) indicate an effective light absorption and photoinduced generation of charge carriers (holes and electrons) in the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), respectively, as is shown in the energy band diagram of the studied structure pre sented in Fig. 6. The effect is more pronounced after applying a negative gate voltage VG. This suggests that photoinduced holes dominate over electrons due to the more effective capture of photoinduced electrons
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Thus, the fieldeffect transistors with an active layer based on PVC : Ni, which have been studied in this work, demonstrate a significant photosensitivity in addition to the ambipolar transport and high charge carrier mobility in the working channel. The results of the performed investigations are of interest for the development of OFETbased reusable memory cells with optical recording and electric erasing (by apply ing the bias to the OFET gate) of information by anal ogy with polymer fieldeffect transistors [8], as well as for the design and fabrication of photosensitive ele ments for optocouplers. It is necessary to perform fur ther investigations of the processes of transport and relaxation of photoinduced charge carriers in the OFET composite structures with the aim of optimiz ing their performance characteristics.
PVC LUMO –2.2 eV
−
−2
Ni –5.04 eV
−5 −6 −7
Au –5.1 eV + HOMO –5.8 eV
Fig. 6. Energy band diagram of the fieldeffect transistor with the Au–PVC : Ni–Al structure.
at the interface between the active and dielectric layers of the fieldeffect transistor, as well as at the polymer– nanoparticle interface. A significant increase in the current IDS (see Fig. 4) is associated both with the presence of charge carriers accumulated in the region of the polymer (PVC)–dielectric (SiO2) interface after applying the gate bias VG and with the presence of lightinduced charge carriers in the active layer of the fieldeffect transistor. Our investigations have revealed that, for the OFET based on PVC : Ni, the transfer characteristics exhibit a small hysteresis and do not reach saturation in the range of gate voltages VG from +15 to –15 V, which suggests an insignificant capture of charge carriers by traps at the interface. As can be seen from Fig. 4, the illumination of the OFET based on PVC : Ni (Ni ~ 5 wt %) by the sunlight simulator leads to a shift of the threshold voltage Vth toward lower values: from Vth ~ 2.5 V (without illumination) to Vth ~ –1.5 V (under illumination). In this case, the ratio of the photoinduced current to the dark current (Iph/Idark) at VG = –10 V is ~ 2–3. From the analysis of the obtained transfer characteristics, we can estimate the photosensitivity of the OFETs based on PVC : Ni as the ratio ΔIDS/Pin, where ΔIDS = IDS, light – IDS, dark and Pin is the power of the radiation incident on the fieldeffect transistor. As was noted above, the power of the halogen lamp used in the experiment is ~ 10 W, and the total irradiation area is ~10 cm2. The working area of the active region of the OFET based on PVC : Ni is ~7 × 10–4 cm × 10–1 cm ~ 7 × 10–5 cm2. There fore, when the OFETs based on PVC : Ni are irradiated by the sunlight simulator with a power of ~60– 70 μW/cm2 at VG = –10 V and VDS = –10 V, their effi ciency reaches ~ 1.2 and ~ 5.3 A/W for the samples with nickel concentrations of ~5 and ~10 wt %, respectively; in this case, the efficiency of the photo sensitive OFETs based on PVC : Ni increases with an increase in the gate bias VG. PHYSICS OF THE SOLID STATE
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4. CONCLUSIONS In this work, we have investigated the electronic and optoelectronic properties of the OFET structures with an active layer based on composite films of a semiconducting polymer, namely, polyvinylcarbazole, with nickel nanoparticles. It has been shown that the OFET structures at low nickel concentrations (5– 10 wt %) possess current–voltage characteristics that are typical of ambipolar transport. For the OFET structures based on PVC : Ni (Ni ~ 5 wt %) films, the mobilities of electrons and holes are found to be ~1.3 and ~1.9 cm2/V s, respectively. It has been established that the photosensitivity observed in these OFET structures is associated with the specific features of transport in the film of the polymer with nickel nano particles. The mechanism of this transport is deter mined by the modulation of electrical conductivity of the working channel of the fieldeffect transistor by applying a combination of incident light and gate volt ages. The obtained results have demonstrated that the fieldeffect transistors with a composite active layer based on the soluble PVC polymer and Ni nanoparti cles can operate as photosensitive OFETs whose fabri cation technology is compatible with the modern technology of printed organic electronics. 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 “Mul tifunctional Materials for Molecular Electronics”) and the Russian Foundation for Basic Research (project no. 110200451a).
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Translated by O. BorovikRomanova
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