Photoresponses and memory effects in organic thin film transistors incorporating
poly(3-hexylthiophene)/CdSe quantum dots
Chen-Chia Chen, Mao-Yuan Chiu, Jeng-Tzong Sheu, and Kung-Hwa Wei
Citation: Applied Physics Letters 92, 143105 (2008); doi: 10.1063/1.2899997
View online: http://dx.doi.org/10.1063/1.2899997
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/92/14?ver=pdfcov Published by the AIP Publishing
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Photoresponses and memory effects in organic thin film transistors
incorporating poly
„3-hexylthiophene…/CdSe quantum dots
Chen-Chia Chen,1,2Mao-Yuan Chiu,2Jeng-Tzong Sheu,1,a兲and Kung-Hwa Wei2,b兲
1
Institute of Nanotechnology, National Chiao Tung University, Hsinchu 30050, Taiwan
2Department of Materials Science and Engineering, National Chiao Tung University, 1001 Ta-Hsueh Road, Hsinchu 30050, Taiwan
共Received 28 October 2007; accepted 25 February 2008; published online 9 April 2008兲
This paper describes the optical responses and memory effects of poly共3-hexylthiophene兲 共P3HT兲/ CdSe quantum dot共QD兲 thin-film transistors 共TFTs兲. TFTs incorporating P3HT/CdSe QD blends as the active layer exhibited higher photocurrents than did the corresponding P3HT-only devices because the heterojunction between P3HT and the CdSe QDs enhanced the separation of excitons. Moreover, the CdSe QDs served as trap centers so that the memory effect was maintained for several hours, even when the device was operated without a gating voltage. Here, we demonstrate the potential applicability of such P3HT/CdSe QD TFTs through repeated optical programming and electrical erasing. © 2008 American Institute of Physics. 关DOI:10.1063/1.2899997兴
During the past few years, a tremendous amount of ef-fort has been devoted to studies of polymer-based optoelec-tronic devices. The use of polymer memory has several ad-vantages, including ease of fabrication and low cost. Many research teams are actively pursuing polymer phototransis-tors, but few are focusing on memory effects in the polymer devices, especially for polymer devices operated using opti-cal programming and electriopti-cal erasing. A simple optiopti-cally writeable memory device incorporating poly共alkylthiophene兲 as the active layer has been proposed, with the optically induced charge being trapped at the polymer-dielectric interface.1Carbon nanotube networks have also been coated with polymers to form optoelectronic memory devices that are written optically and read and erased electrically, but these blended polymer/carbon nanotube devices lost their memory capability because the carbon nanotubes separated from the substrate and because the nanotube bundles con-tained metallic nanotubes.2 In polymer-based memory de-vices, the dynamic switch phenomenon depends strongly on the gate effect.3The “on” state can be returned to the “off” state by removing the gate voltage.4 Unfortunately, the on states of these devices were not maintained for very long in the absence of an applied gate voltage, which limits their potential use in commercial applications. In this paper, we describe poly共3-hexylthiophene兲 共P3HT兲/CdSe quantum dot 共QD兲 thin-film transistors 共TFTs兲 that exhibit long retention times for their on states even in the absence of a gate voltage. This behavior differs significantly from that reported previ-ously.
Composite films of CdSe QDs and P3HT have been widely used in photovoltaic devices5in which exciton disso-ciation and charge separation occur at the interface between the CdSe QDs and P3HT. Based on this principle, the elec-trons become trapped in the CdSe QDs when the light is turned off. This behavior enhances the retention time of polymer-based phototransistors. In this paper, we demon-strate that hole-only transport occurs in P3HT/CdSe QD TFTs. The drain-source currents共IDS兲 of both the P3HT and P3HT/CdSe TFTs increased by up to several orders of
mag-nitude upon irradiation with light when operated in the depletion mode. After turning off the light, the current de-cayed to a metastable state, where it remained for several hours, when the devices were subjected to a positive or zero gate voltage.
Trioctylphosphine oxide 共TOPO兲-capped CdSe QDs were synthesized using a modification of a procedure re-ported previously.6 A solution of P3HT 共Rieke Metals, used as received兲 in chloroform 共5 mg/mL兲 was blended with a solution of CdSe QDs 共diameter of 3.5⫾0.5 nm兲. The P3HT and P3HT/CdSe TFT devices were fabricated in a bottom-gate configuration共Fig.1兲. An n+silicon wafer 共⬍0.005 ⍀ cm兲 was used as the substrate and gate. 900 Å thermal SiO2 共capacitance of 38.4 nF/cm2兲 was the gate in-sulator. It was hydrophobically modified using hexamethyl-disilazane vapor. The source and drain fingerlike electrodes 共W=3000m and L = 10m兲 were defined using standard photolithography. A bilayer of Au/Ti 共thickness of 1000/100 Å兲 was thermally evaporated and then lifted off. The P3HT and P3HT/CdSe films共thickness of 100 nm兲 were deposited through spin coating. The density of CdSe QDs in the thin P3HT film is about 9.93⫻1017cm−3. The films were subsequently annealed at 150 ° C under N2 for 5 min. The performance of each device was measured under vacuum 共⬍1⫻10−5torr兲 in the dark using a Hewlett-Packard 4156C semiconductor parameter analyzer and a cryogenic probe sta-tion共VFTTP4, Lakeshore兲. The devices were illuminated un-der vacuum using a tungsten halogen lamp.
Figure 2 displays the transfer curves 共drain-to-source voltage VDS= −20 V兲 of the P3HT-only and P3HT/CdSe QD
a兲Electronic mail: [email protected]. b兲Electronic mail: [email protected].
FIG. 1. 共Color online兲 Schemes of devices structure and organic active layers.共a兲 Schematic representation of the bottom-gate organic TFT con-figuration with an active polymer layer and interdigitated source and drain 共S for source, D for drain; and G for gate兲. 共b兲 Schematic representations of the P3HT-only and P3HT/CdSe blend films.
APPLIED PHYSICS LETTERS 92, 143105共2008兲
0003-6951/2008/92共14兲/143105/3/$23.00 92, 143105-1 © 2008 American Institute of Physics
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blend TFTs in the dark and under a white light of 0.26 mW/cm2. Both the P3HT and P3HT/CdSe TFTs exhibit the characteristic behavior of p-channel field-effect transis-tors. Ambipolar-type TFTs based on blended donor- and acceptor-type materials have been reported previously.7 The work function of Au共5.1 eV兲 matched the highest occupied molecular orbital of P3HT共4.9 eV兲,8forming an Ohmic con-tact for hole injection. The drain currents measured by sweeping gate voltage at VDS= 20 V. We observed a weak electron current in the P3HT/CdSe and pure P3HT TFTs. The electron current remains weak even if higher positive gate voltages. Whereas Au strongly suppressed electron injection into CdSe 关lowest unoccupied molecular orbital 共LUMO兲: 4.3 eV兴9
and P3HT 共LUMO: 3.0 eV兲8 because of a large mismatch between its work function and the LUMO band of CdSe and P3HT. In addition, the current in the PH3T/CdSe TFTs is about one order of magnitude larger than for the pure PH3T TFTs in the whole voltage range. Incorporation of CdSe QDs into the P3HT lightly enhances the hole mobility of the devices, it is possible that CdSe reduced the density of traps in the polymer.10
The inset of Fig.2 displays phase images—obtained by AFM 共Multimode, DI兲 in the tapping mode—of both the P3HT and P3HT/CdSe thin films. The P3HT/CdSe compos-ite thin film had a rougher surface morphology共rms rough-ness of 3.6 nm兲 in comparison with that of the P3HT-only film共2.0 nm兲. The surface morphology of the film incorpo-rating the CdSe QDs was rough because of CdSe aggrega-tion. The contrast in the phase image of the film blend indicates phase separation. The transmission electron micros-copy image of the blend film共not shown here兲 revealed that the CdSe QDs had unexpectedly aggregated into clusters. To avoid aggregation of the CdSe QDs, a suitable ligand must be used to passivate the QDs to enhance the performance of the memory devices.
We observed photoconductivity and the photovoltaic ef-fect in the active layer of the transistors upon illumination.11 We attribute the significant increase in the drain current in the off state, when the device was being illuminated, to the enhancement of the drain current caused by the excitons in the polymer and nanocrystal. When illuminated, the excitons were generated in the CdSe QDs and P3HT. The electrons and holes eventually separated as a result of the electrical field. The drain currents of the P3HT/CdSe devices were larger than those of the P3HT-only devices because of the built-in field present at the P3HT-CdSe interface. The photo-excitation hole density within the thin film also contributed to the drain current and increased the threshold voltage to a large positive value.
Threshold voltages共Vth兲 were determined from the inter-cepts of the
冑
IDS-VGSplot. In general, the values of Vthof the blend P3HT/CdSe devices shifted to more-positive values,indicating the existence of a permanent electric field at the interface. The IDS-VGS curve shifted toward a positive volt-age under illumination. As shown in Fig.2, the value of Vth of the P3HT/CdSe device shifted to 10.0 V 共illuminated兲 from 4.2 V 共darkness兲, i.e., ⌬Vth= 5.8 V. In contrast, the value of ⌬Vth of the P3HT-only device was ⬃1.9 V. The ⌬Vthextracted from the backward sweep curve is more pro-nounced than that extracted from the forward sweep. For the forward sweep, when an initial negative gate voltage is ap-plied, the trapped electrons in the P3HT/SiO2 interface and CdSe QDs would easily detrap and recombine with the hole in P3HT, leading to a reduction in the carrier density. Whereas, the recombination process of electrons and holes in backward sweep took place in the region that is not con-cerned with the memory functionality.
The increase in the value of the carrier density N*in the active layer could be estimated using the equation N* = Ci⌬Vth/e, where Ci is the capacitance per unit area of the dielectric layer,⌬Vthis the shift of the threshold voltage, and e is the elementary charge. The values of N*of the P3HT/ CdSe device and the P3HT-only device were ⬃1.39 ⫻1012/cm2 and ⬃4.53⫻1011/cm2, respectively, indicating that the P3HT/CdSe devices were more efficient at separat-ing excitons, presumably because of the heterojunction at the P3HT-CdSe interface in the active layer.
Upon sweeping the voltage from positive to negative and then back to positive, we observed obvious hysteresis under illumination, but indistinct hysteresis in the dark 共Fig. 2兲. This behavior was a consequence of the trapped charges present at the polymer-dielectric interface or in the dielectric material. The hysteresis for a P3HT device was typically less than 1.1 V. For the P3HT/CdSe devices, it was ⬃0.8 V. These similar values indicate a very small difference in their interface trap densities. The manifest hysteresis under illumi-nation has also been observed in P3HT/PCBM phototransis-tors and organic capaciphototransis-tors.10,12Apparent hysteresis occurred upon illumination as a result of an increased number of car-riers in the active layer becoming easily trapped, either at the P3HT-SiO2 interface or in SiO2 itself. In the P3HT/CdSe devices, some of the carriers were trapped in the CdSe QDs, resulting in the reduced degree of hysteresis.
Hysteresis in the illuminated devices resulted from trapped electrons. When applied a positive gate voltage of 7.5 V 共where the hysteresis is most pronounced in the Fig. 2兲, Fig.3共a兲indicates that the current in the P3HT-only de-vices reached a metastable state after turning off the pulse light source共30 s兲. This behavior is consistent with previous observations.1–3 Trapping of electrons in SiO2 or at the P3HT-SiO2interface screened the back-gate potential, result-ing in the metastable state. We attribute the slow current decay to bulk recombination, which is an indication of the slow nonexponential relaxation process inherent to
polymer-FIG. 2.共Color online兲 Transfer characteristics obtained for the共a兲 P3HT-only and 共b兲 P3HT/CdSe blend films 共i兲 initially 共in the dark兲 and 共ii兲 under illumination. Under illumination, all of the devices displayed hyster-esis. Transfer characteristics of the devices in the dark and under illumination共0.27 mW/cm2兲 were measured
at VDS= −20 V. Inset: AFM phase images of P3HT
films in the共a兲 absence and 共b兲 presence of CdSe QDs. Image size was 3⫻3m.
143105-2 Chen et al. Appl. Phys. Lett. 92, 143105共2008兲
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based devices. After turning off the light source, the drain current decayed rapidly when the gate voltage was equal to zero. The current moved back to the initial state in the P3HT devices after turning off the light共100 s兲. In the absence of a gate voltage, there was no external electrical field to induce the electrons; thus, the trapped electrons escaped from the trap centers共i.e., from the SiO2or the P3HT-SiO2interface兲 into the active layer, resulting in decay of the drain current in the P3HT devices.
Relative to the P3HT-only devices, the P3HT/CdSe de-vices displayed entirely different behavior, as shown in Fig. 3共b兲. Under illumination, the electrons were trapped not only in SiO2and at the P3HT-SiO2interface but also in the CdSe QDs. After photoexcitation, the trapped electrons escaped from the trapping centers共SiO2 or P3HT-SiO2兲 after a few minutes. In contrast, the highly localized electrons inside the CdSe QDs had difficulty jumping back into the polymer. Also, the presence of TOPO on the surface of the CdSe QDs enhanced the trapping of electrons. This metastable state was maintained for several hours, even in the absence of a gate bias. The P3HT/CdSe devices exhibited higher ION/IOFF ra-tios 共⬎100兲 than did the P3HT-only devices 共10兲. A large on/off ratio can also be achieved for the P3HT/CdSe and P3HT-only devices providing an appropriate gate voltage is applied. Our observations show that the on/off ratio of P3HT/CdSe TFTs is at least one order of magnitude larger than that the pure P3HT TFTs near the threshold voltage. Moreover, there is a much long retention time for the P3HT/ CdSe devices than the P3HT-only devices in absence of a gate voltage. Although the P3HT/CdSe devices exhibited a memory effect in the absence of a gate voltage, their ION/IOFF ratios were too low to meet the required memory window.
Upon illumination with white light共2.75 mW/cm2兲, the drain current of the P3HT/CdSe device rose from 1.5 to 415 nA共Fig.4兲. After turning off the white light, the drain current dropped slowly and eventually settled at a metastable state of 260 nA. Moreover, this metastable state could be erased efficiently using a single pulse of a gate voltage for a short duration 共−15 V, 100 ms兲. When this negative pulse gate bias was applied, trapped electrons quickly recombined with the majority carriers from the trap centers. After applying the negative electrical field, the Fermi level of CdSe also modulated up toward the conduction band, reducing the built-in field and, hence, enhancing re-combination. Based on this mode of operation, we could repeatedly program the P3HT/CdSe device optically and electrically erase it.
In summary, we have investigated the electrical and op-tical properties of polymer memory TFTs incorporating P3HT and P3HT/CdSe as active layers. Upon illumination,
the P3HT/CdSe TFTs exhibited stronger carrier induction in the channel and greater electron trapping ability than did the P3HT-only devices. This phenomenon resulted in a relatively high ION/IOFFratio. After introducing the CdSe QDs as elec-tron trap centers, the retention time of the metastable memory state of the P3HT/CdSe TFT improved in the ab-sence of a gate voltage. We are currently optimizing the working point of Vthat values of VGSnear 0 V to maximize the photoresponse so that the memory window can be wid-ened further. We are also investigating replacing the TOPO molecules with long alkyl chains to enhance the retention time.
We are grateful to the National Science Council 共NSC-96-2218-E-009-011兲 and the MOE-ATU Program in Taiwan for financial support.
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FIG. 3. 共Color online兲 Time responses of the drain current at VGS= 7.5 V and VGS= 0 V of the共a兲
P3HT-only and 共b兲 P3HT/CdSe devices to a light pulse 共2.75 mW/cm2, 30 s兲. Inset: Illustrations of the
elec-tron trap mechanisms for the共a兲 P3HT-only and 共b兲 P3HT/CdSe devices.
FIG. 4. 共Color online兲 Dynamic responses of the optical programming and electrical erasing of a typical P3HT/CdSe device. Light was turned on at
t = 80 s and turned off at t = 90 s. A short共100 ms兲 negative gate voltage
pulse was applied at t = 260 s to erase the memory.
143105-3 Chen et al. Appl. Phys. Lett. 92, 143105共2008兲
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