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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 60, NO. 3, MARCH 2013
Improved Performance of Photosensitive Field-Effect Transistors Based on Palladium Phthalocyanine by Utilizing Al as Source and Drain Electrodes Yingquan Peng, Wenli Lv, Bo Yao, Jipeng Xie, Ting Yang, Guoying Fan, Deqiang Chen, Pengjie Gao, Maoqing Zhou, and Ying Wang
Abstract—In conventional photosensitive organic field-effect transistors (FETs) (OFETs) (photo-OFETs) based on p-type organic semiconductors, high work function metals such as Au are generally used as source/drain electrodes, whose photosensitivity is generally low. We report on the performance improvements of photo-OFETs based on palladium phthalocyanine (PdPc) by using Al as source and drain electrodes. It is concluded that the dark currents of the photo-OFET based on PdPc with Al as source and drain electrodes (denoted as Al-PdPc device) are about only one-thousandth of that of the photo-OFET based on PdPc with Au as source and drain electrodes (denoted as Au-PdPc device). This tremendous decrease of dark current results in about a three-order-of-magnitude increase for photosensitivity at the gate and drain voltages of −50 V. The enormous decrease of dark current is ascribed to the Schottky contacts between the Al source/drain electrodes and PdPc. In addition, the Al-PdPc devices show also larger photoresponsivity compared with the Au-PdPc devices. Index Terms—Field-effect transistor (FET), palladium phthalocyanine (PdPc), photosensitivity.
copper hexadecafluorophthalocyanine (F16CuPc) [7], and 2,5bis-biphenyl-4-yl-thieno[3,2-b]thiophene (BPTT) [8], and by spin coating of some polymers, such as poly(3-hexylthiophene) (P3HT) [9]–[11], and donor–acceptor blend, such as P3HT: [6,6]-phenyl C61-butylyic acid methyl ester (PCBM) [12], have been reported. An important parameter of the phototransistor is photosensitivity P , which is defined as the ratio of the photocurrent (Iph ) to the dark current (Idark ) [13] P =
Iill − Idark Iph = Idark Idark
where Iill is the drain current under illumination. Another important parameter is photoresponsivity R, defined as the ratio of the photocurrent to the incident optical power on the channel of the device Popt R=
I. I NTRODUCTION
P
HOTOSENSITIVE organic field-effect transistors (FETs) (OFETs) (photo-OFETs) are an important electronic device for optical transducers, photosensing devices, and highly sensitive image sensors [1]. In a photo-OFET, light is used as an additional control quantity to create photogenerated charge carriers in addition to the carriers induced by the gate voltage. Compared to organic diodes, photo-OFETs exhibit high sensitivity and low noise [2]. Photo-OFETs fabricated by the thermal evaporation of some small molecules, such as pentacene [3], [4], rubrene [5], copper phthalocyanine (CuPc) [6],
Manuscript received July 17, 2012; revised October 8, 2012; accepted December 21, 2012. Date of publication January 14, 2013; date of current version February 20, 2013. This work was supported in part by the National Natural Science Foundation of China under Grant 10974074 and in part by the Research Fund for the Doctoral Program of Higher Education of China under Grant 20110211110005. The review of this paper was arranged by Editor D. J. Gundlach. Y. Peng, W. Lv, B. Yao, T. Yang, G. Fan, D. Chen, P. Gao, M. Zhou, and Y. Wang are with the Institute of Microelectronics, School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China (e-mail:
[email protected];
[email protected];
[email protected]; yangtin09@ lzu.edu.cn;
[email protected];
[email protected];
[email protected];
[email protected];
[email protected]). J. Xie is with the Institute of Microelectronics, School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China, and also with the Department of Foundation, Dalian Air force Communication NCO Academy, Dalian 116600, China (e-mail:
[email protected]). Digital Object Identifier 10.1109/TED.2012.2237405
(1)
Iph . Popt
(2)
Conventional photo-OFETs adopt the standard structures of OFETs, in which ohmic source and drain contacts to the channel material are strived. Based on this consideration, Au is usually used as source and drain electrodes for OFETs with ptype organic semiconductors as the active channel material [6], [14], [15] because the work function of Au (∼5.1 eV [16]) is close to the highest occupied molecule orbital (HOMO) of most p-type channel materials. However, in the case of the photoOFET, the ohmic or near ohmic contacts of source and drain electrodes to the channel material result in reasonable high dark current due to the reduced resistance of the conduction channel between the source and drain electrodes, which leads to low photosensitivity. Thus, minor photosensitivity usually appears in the condition of high gate voltage in most literatures about photo-OFETs [2], [3], [6], [7], [9], [13]. However, photoinduced currents increase with the increase of the gate voltage, which could result in high photoresponsivity of the photo-OFETs at a high gate voltage [9], [17]. Therefore, it is difficult to obtain the high photosensitivity and photoresponsivity synchronously for the conventional photo-OFET. Metal phthalocyanines (MPcs) are widely used as active materials for organic solar cells [18]–[21] and OFETs [22]– [25]. In organic photovoltaic devices, charge carriers are generated through the dissociation of photogenerated excitons in the region where the electric field is sufficiently strong. A longer exciton diffusion length is beneficial to producing large
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PENG et al.: IMPROVED PERFORMANCE OF PHOTOSENSITIVE FETs BASED ON PdPc
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III. R ESULTS AND D ISCUSSIONS
Fig. 1. UV-Vis absorption spectrum of a 20-nm-thick PdPc film on quartz substrate. The insert is the molecular structure of PdPc.
photocurrent. It was reported that the replacement of central metal ions with palladium in widely used planar MPcs such as CuPc and zinc phthalocyanine (ZnPc) resulted in a longer exciton diffusion length [26]. In this paper, we report on the performance improvement of the photo-OFET based on palladium phthalocyanine (PdPc) by utilizing Al as source and drain electrodes (denoted as AlPdPc device hereafter). It is concluded that, by utilizing Al as the source/drain electrode, the dark current is tremendously reduced by at least three orders of magnitude and the photocurrent is also increased compared with the PdPc photo-OFET with Au as source and drain electrodes (denoted as Au-PdPc device hereafter), which resulted in a great enhancement for photosensitivity. II. E XPERIMENT PdPc was synthesized following procedures detailed in the literature [27]. Bottom-gate top-contact geometry was used to fabricate the photo-OFET. A heavily n-doped Si substrate with a resistivity of 0.03 Ω · cm acts as the gate electrode with a 1000-nm thermally grown SiO2 layer as the gate dielectric. The substrate was ultrasonically cleaned by acetone, ethanol, and deionized water and was dried with N2 gas blowing and baking in an oven with a temperature of 60◦ C for 20 min. PdPc of thickness 53 nm was vacuum deposited on top of SiO2 . The vacuum during the deposition was 4 × 10−3 Pa. Au or Al source/drain electrodes were deposited through a shadow mask by thermal evaporation defining a channel length/width of 25 μm/5 mm. Two samples with the same channel length, channel width, and organic thickness are fabricated in total, in which one is with Au source and drain electrodes (Au-PdPc device) and the other is with Al source and drain electrodes (Al-PdPc device). For the UV-Vis absorption measurement, a 20-nm-thick PdPc film was vacuum deposited on a cleaned quartz substrate. A TU-1901 UV-Vis spectrometer was used for absorption spectra measurements (see Fig. 1). The spectrum has an absorption maximum at a wavelength of 616 nm in the visible region. For the measurement of photoeffects, a laser diode with a wavelength of 655 nm and a power density of 100 mW/cm2 was used.
The output and transfer characteristics of the Au-PdPc device in the dark and under illumination are plotted in Fig. 2(a) and (b). Typical p-type unipolar FET characteristics were observed. Without illumination, the drain current Id at a drain voltage of Vd = −50 V and a gate voltage of Vg = −50 V was 29 nA and was increased by a factor of 2.8 upon illumination to 81 nA [see Fig. 2(a) and (c)]. Under light illumination, two different effects, i.e., photoconductivity and photovoltaic effects, are assumed to occur depending on the gate voltage Vg . The threshold voltage shift ΔVth , defined as the difference between the threshold voltage under illumination Vth,ill and that in the dark Vth,d , is caused by the photovoltaic effect which results mainly from the photocurrent and charge carrier trapping. For p-type photo-OFETs, the photovoltaic effect results from the transport of photogenerated holes and trapping of photogenerated electrons near the source electrode [28]. In the accumulation mode, if the contribution of charge trapping is negligibly small, then ΔVth can be expressed as [29] ΔVth = Vth,ill − Vth,d =
AkB T ln q
ηqλPopt +1 Idark hc
(3)
where η is the quantum efficiency, Idark is the dark current, hc/λ is the photon energy, kB T /q is the thermal voltage, and A is the fit parameter. Equation (3) indicates that, for p-type photo-OFETs operating in enhancement mode, the illumination of the device always results in a decrease in the threshold voltage and the threshold voltage shift increases linearly with the logarithm of optical power and decreases linearly with the logarithm of the dark current. It is seen from the insert of Fig. 2(b) that the threshold voltage shift ΔVth due to illumination is 20 V (from 6 to 26 V). Fig. 2(c) and (d) shows the gate and drain voltage dependence of the photosensitivity and photoresponsivity of the Au-PdPc device, respectively, from which two drawbacks can be seen. First, the photosensitivity is small due to the large dark current caused by the small energy barrier for hole transport at the Au/PdPc interfaces, and second, the photosensitivity and photoresponsivity vary toward opposite directions with the increase of the gate and drain voltages. That is, with the increase of the gate and drain voltages, the photoresponsivity increases, while the photosensitivity decreases. At zero gate voltage and Vd = −50 V, the photosensitivity is as large as 59, while the photoresponsivity takes only a small value of 69 μA/W; at Vg = −50 V and Vd = 50 V, the photoresponsivity reaches a high value of 578 μA/W, while the photosensitivity was only 1.8 [see Fig. 2(c)]. It is to be noted that, at Vg = 0 V and Vd = −50 V, a photocurrent of 6 nA was measured, which means that a part of photoexcited excitons are dissociated into free electrons and holes and collected by the respective electrodes. The dissociation of excitons occurs most probably near the Au electrodes because an electric field which is high enough for the exciton to dissociate exists in the PdPc film near the Au electrodes that is generated by the energy difference between the PdPc HOMO, −5.4 eV [30], and the Fermi level of the Au electrode, −5.1 eV [16].
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Fig. 2. Characteristics of the Au-PdPc device. (a) Output characteristics (filled symbols) in the dark and (open symbols) under illumination. (b) Transfer characteristics (filled symbols) in the dark and (open symbols) under illumination. The insert displays the (−Id )1/2 versus Vg curves for determination of threshold voltage in the saturation regime. (c) Drain- and gate-voltage-dependent photoresponsivity. (d) Drain- and gate-voltage-dependent photosensitivity.
IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 60, NO. 3, MARCH 2013
The output characteristics of the Al-PdPc device under illumination and in the dark are shown in Fig. 3(a) and in the insert. Under the dark condition, the drain current is too small to observe any FET characteristics [the insert of Fig. 3(a)]. The drain current Id at Vg = −50 V and Vd = −50 V was 0.03 nA, which is much smaller than that of the Au-PdPc device at the same gate and drain voltages [∼29 nA in Fig. 2(a)]. The transfer characteristics of the Al-PdPc device are shown in Fig. 3(b), and the (−Id )1/2 versus Vg relation for the determination of the threshold voltage in the saturation regime is plotted in the insert. Under light illumination, the device exhibited a typical p-type FET characteristic with a threshold voltage of 44 V. While the photoresponsivity of the Al-PdPc device was only about several tens percent larger than that of the Au-PdPc device, the photosensitivity was much higher than that of the Au-PdPc device due to the hole Schottky contact between Al source/drain electrodes and PdPc. For example, the photocurrent at Vg = −50 V and Vd = −50 V was 68 nA [see Fig. 3(a)], which was 30% larger than that of the Au-PdPc device at the same gate and drain biases [52 nA; see Fig. 2(a)], but the dark current was about three orders of magnitude lower than that of the AuPdPc device, which resulted in higher photosensitivity. It should be noted that, at Vg = 0 V and Vd = −50 V, a photocurrent of 6 nA was measured, which equals to that of the Au-PdPc device. The physical origin why the equal photocurrent was measured at zero gate voltage for the Au-PdPc and Al-PdPc device is not clear. Fig. 3(c) and (d) shows the calculated gate and drain voltage dependence of the photoresponsivity and photosensitivity of the Al-PdPc device. In the calculation of photosensitivity for a given gate voltage, the root mean square of the dark currents at different drain voltages was used. For given gate and drain voltages, the photoresponsivity is generally several tens percent larger than that of the Au-CuPc device [see Fig. 2(d)]. The photosensitivity is much higher than that of the Au-PdPc device [see Figs. 3(d) and 2(d)] due to low dark current caused mainly by the Schottky contacts between the Al source and drain electrodes and PdPc. A photosensitivity larger than 1.44 × 103 was obtained at Vg = −50 V and Vd = −50 V, which is about three orders of magnitude larger than that of the Au-CuPc device at the same gate and drain voltages (at Vg = −50 V and Vd = −50 V, P = 1.8). Aside from being able to work with high photosensitivity, an important advantage of Al-PdPc devices in comparison with Au-PdPc devices is that the photosensitivity and photoresponsivity increase synchronously with the gate and drain voltages, which indicates that the Al-PdPc device can work at gate and drain voltages with both high photosensitivity and high photoresponsivity. For example, at Vg = −50 V and Vd = −50 V, the photosensitivity and photoresponsivity reach a large value of 1.44 × 103 and 761 μA/W, respectively. In most works on photo-OFETs, a relatively low photosensitivity value occurs in the condition of the maximum photoresponsivity. For example, Hamilton et al. reported a photo-OFET based on F8T2 [poly(9,9-dioctylfluorene-co-bithiophene)], and its photoresponsivity can reach 700 μA/W in the same level with the Al-PdPc device in this paper, but the photosensitivity of that device is far lower than that of the Al-PdPc device at Vg = Vd = −40 V, only ∼1.7 [13].
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Due to the fact that the photosensitivity and photoresponsivity of the Au-PdPc device vary in opposite directions with the increase of the gate and drain voltages, the photosensitivity of the Au-PdPc device will be poor when it works at gate and drain voltages with high photoresponsivity. That is, Au-PdPc devices can work with only large photoresponsivity but poor photosensitivity, while Al-PdPc devices can work with both high photoresponsivity and high photosensitivity. IV. C ONCLUSION In conclusion, photoeffects of OFETs based on PdPc with Al and Au as source/drain electrodes were investigated. It is concluded that, by utilizing Al as source/drain electrodes, the dark current was reduced tremendously by a factor of 10−3 and the photocurrent was increased by 30% compared with the Au-PdPc device, which resulted in about a three-order-ofmagnitude increase for photosensitivity. The decrease of dark current in the Al-PdPc device is ascribed to the Schottky contact between the Al source/drain electrode and PdPc. Compared with Au-PdPc devices, Al-PdPc devices have the advantage of being able to work at very high photosensitivity and high photoresponsivity. R EFERENCES
Fig. 3. Characteristics of Al-PdPc device. (a) Output characteristics (filled symbols) in the dark and (open symbols) under illumination. (b) Transfer characteristics under illumination. The insert displays the (−Id )1/2 versus Vg curve for determination of threshold voltage in the saturation regime. (c) Drainand gate-voltage-dependent photoresponsivity. (d) Drain- and gate-voltagedependent photosensitivity.
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Yingquan Peng received the Ph.D. and Dipl.Ing. degrees from Humboldt University of Berlin, Berlin, Germany, in 1992. He is currently a Professor and the Vice-Director of the Teaching Center for Experimental Physics with Lanzhou University, Lanzhou, China.
Wenli Lv, photograph and biography not available at the time of publication.
Bo Yao, photograph and biography not available at the time of publication.
Jipeng Xie, photograph and biography not available at the time of publication.
Ting Yang, photograph and biography not available at the time of publication.
Guoying Fan, photograph and biography not available at the time of publication.
Deqiang Chen, photograph and biography not available at the time of publication.
Pengjie Gao, photograph and biography not available at the time of publication.
Maoqing Zhou, photograph and biography not available at the time of publication.
Ying Wang, photograph and biography not available at the time of publication.
