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Negative differential resistance in doped poly(3-methylthiophene) devices
View the table of contents for this issue, or go to the journal homepage for more
Received 7 May 2010, in final form 17 September 2010Published 7 October 2010Online at stacks.iop.org/JPhysD/43/425103
AbstractThe current density–voltage (J–V ) characteristics of poly(3-methylthiophene) devices show anegative differential resistance (NDR) at room temperature with a large peak to valley currentratio (∼507). This NDR can be tuned by two orders of magnitude by controlling the carrierdensity due to the variation of the space-charge region in the device. The temperature and scanrate dependent J–V measurements infer that the NDR is mainly driven by the trapping andde-trapping of carriers. The photo-generation of carriers is observed to reduce the NDR effect.
(Some figures in this article are in colour only in the electronic version)
1. Introduction
Negative differential resistance (NDR) has been observedin many devices based on inorganic, organic, polymeric,molecular, nano-composites and bio-molecular systems[1–6]. The reason for its origin is diverse and isunder debate especially in polymeric and organic materials[7–10]. Polymeric devices which show NDR have numerousadvantages over the other systems due to the ease ofpreparation, low cost, light weight and flexibility. NDRfinds applications in low-power devices, memory elements,oscillators, etc [3, 11]. Several mechanisms such as filamentformation [10], trapping and de-trapping of carriers [7],tunnelling through defect states [3], space-charge formationand band bending [9] were used to explain the observed NDRmechanism in polymer devices. Usually it is believed thatin single-layer metal–polymer–metal devices the trap statespresent at the interface and in the bulk of the sample areresponsible for the observed NDR [7, 9].
Although several groups have observed NDR in variouspolymer systems [7–10], the tuning and controlling of NDRremains a challenge [7]. If the interfacial and bulk trapsare responsible for the NDR, one can control its peakto valley current ratio (PVCR) by varying the rate oftrapping/de-trapping of carriers due to the limited lifetime oftrapped states [7]. Practical applications of NDR in low-powerdevices, memory elements, oscillators, etc need a robust PVCR[11]. Here we report NDR, with a large PVCR value, in poly(3-methylthiophene) [P3MeT] devices at room temperature. By
varying the carrier density or scan rate, the PVCR can be variedby two orders of magnitude; also, the NDR can be modifiedwith light. The photo-tuning of NDR peak current is quiteuseful in optoelectronic applications. An investigation of theNDR as a function of carrier density, temperature and scanrate can help in identifying the carrier trapping and de-trappingmechanism in these polymeric devices.
2. Experimental
P3MeT thin films were electrochemically deposited onstainless steel (SS) foil under inert atmosphere by passinga current of 2.5 mA cm−2 for 3 min. P3MeT films dopedwith PF−
6 ions were de-doped for 600, 180, 150 and 120 s bypassing a current of 0.25 mA cm−2 in the reverse direction toobtain samples with resistivity values of ∼4 × 108 � m (D1),7.6 × 107 � m (D2), 1.9 × 106 � m (D3) and 2.4 × 105 � m(D4), respectively. This systematic decrease in resistivity, fromD1 to D4, is mainly due to the increase in carrier density asthe doping level increases [12]. The top contact with a contactarea of ∼0.8 mm2 is made by semi-dry silver paste on the driedP3MeT films. The SS foil (50 µm) is polished and cleanedusing triple distilled water, acetone and isopropyl alcohol inultrasonic bath. The thickness (∼1 µm) of the P3MeT thinfilms is measured using a Dektak surface profilometer. Thecurrent–voltage (I–V ) measurements were carried out with asource meter (model Keithley-2400) and electrometer (modelKeithley-6514). Low temperature I–V measurements downto 120 K were carried out in a custom made cryostat. A
J. Phys. D: Appl. Phys. 43 (2010) 425103 P Anjaneyulu et al
Calibrated Pt-100 sensor is used to measure the temperatureand a Lakeshore temperature controller (Model DRC 91C) isused for controlling the temperature to an accuracy of 0.03 K.Further the I–V measurements were performed under 532 nmillumination using a diode laser. The schematic of the P3MeTdevice is shown in the inset of figure 1(a).
3. Results and discussions
The current density–voltage (J–V ) characteristics of D1 at twodifferent temperatures are shown in figure 1(a). The voltageis swept from −10 to 10 V with SS as anode. In the positivebias region (0 to 10 V) current value increases as the voltageis increased to 1.6 V (at 315 K) and 2.8 V (300 K); whereasat higher voltages the current decreases (i.e. the NDR region)up to a voltage of 5 V (at 315 K) and 7.5 V (300 K). This ismainly due to the filling of trap states; once the trap states arefilled, the current increases with voltage [9, 13]. The slopes ofthe log (J ) versus log (V ) curves in the positive bias regionare 2.2 (315 K) and 3.2 (300 K), whereas in the negative biasregion (−10 to 0 V) the slopes are 2.8 (315 K) and 3.1 (300 K).The slope values >2 indicate that the conduction mechanism isdominated by trap-controlled space-charge limited conduction(SCLC) at higher voltages. This NDR and SCLC are mainlydue to the presence of carrier traps at the electrode–polymerinterfaces [9]. Thermal activation of the filling of trap statesis verified from the shift in the peak position of the J–V
profiles at 300 and 315 K. The peak-to-valley ratio (∼3.6)hardly varies with temperature. However, the NDR in reversebias is observed to be significantly larger, hence investigatedin more detail as given below. To study the effect of carrierdensity and scan rates, we have carried out the measurementsin the negative sweep direction, since the PVCR is much largerin the reverse bias.
The possibility for any contact related contribution toNDR is being verified by carrying out a controlled experimentin polypyrrole (PPy) devices fabricated by similar technique,and no NDR has been observed in these devices [14].Both P3MeT and PPy samples are prepared very identically,with similar thickness around a micrometre. Since bothare prepared in very similar conditions, so there shouldnot be any significant variation in the morphology of thesesamples. Further, we have deposited aluminium and gold, bythermal evaporation, as a top electrode and studied the I–V
characteristics of P3MeT devices, which also showed NDRbehaviour may be due to the intrinsic trapping or diffusionof evaporated metal atoms [15]. However, these results arenot that consistent and need more investigation in future. Ingeneral the diffusion of metal atoms into the polymer is arandom process and if NDR is due to the presence of diffusedmetal atoms, then its reproducibility is also expected to berandom. However, a consistent and reproducible result isobtained in dry silver contact devices while this reproducibilityis rather poor in our metal evaporated devices which couldbe due to the possibility of diffusion. The Ag contacts areprepared by dry silver paste to avoid any solvent-assisteddiffusion, and measurements were carried out immediatelyto avoid any diffusion over a period of time. Also we have
Figure 1. (a) J–V characteristics of poly(3-methylthiophene)device D1 at 300 and 315 K with SS as anode. The inset shows theschematic diagram of the HOMO–LUMO level and band bending(under equilibrium and applied bias, not to scale). (b) I–Vcharacteristics (−3 → 0 V scan) of P3MeT devices D2, D3 and D4at 300 K. The inset shows the log–log plot of J–V (0 → −3 V scan)for D2 and D3. (c) J–V characteristics (−3 → 0 V scan) of D2 atdifferent scan rates. The inset shows the PVCR as a function of scanrate.
2
J. Phys. D: Appl. Phys. 43 (2010) 425103 P Anjaneyulu et al
repeated the measurements over a period of time and theNDR results are being reproduced. This is because, in drysilver paste the grain sizes are around a micrometre, andthey are clustered together, hence its diffusion is significantlyretarded; unlike in the case of evaporated contacts. Hencewe propose that the trap-induced band bending is the mostplausible explanation for the NDR in these devices.
The NDR of P3MeT devices (D2, D3 and D4) is presentedin figure 1(b). The current values are normalized with respectto the current at 0.4 V to display the prominent NDR region.The PVCR, at a scan rate of 20 mV s−1, is ∼170 in D2 device;and in the other two it is considerably reduced (∼2.3 in D3and ∼1.2 in D4). The NDR region is suppressed as thecarrier density increases. When the voltage is swept (SS asanode) from −3 to 0 V, the NDR region is observed until∼−2.5 V in all the three devices. This observation of NDRis quite reproducible, although the peak current and peakposition can slightly vary from device to device. Whereas,in the 0 to −3 V sweep hardly any NDR could be seen (theinset of figure 1(b)), which is due to the charge de-trappingeffects [9]. This NDR is explained in terms of band bending,charge trapping, de-trapping and transit time effects of thetrap states. The interface states act as traps in addition tothe usual traps present in the bulk of the polymer. Whena large negative bias voltage is applied, the interface trapstates get filled-up by the injected carriers, and these trappedcarriers will increase the built-in potential, which induces aband bending to occur at the electrode–polymer interface, asshown in the inset of figure 1(a) [9, 16]. These processes canresult in the accumulation of a large number of carriers at theinterface. Since mobility in polymers is rather low (10−3–10−5 cm2 V−1 s−1), [17, 18] the carriers will form a space-charge region and this opposes the current flow. When thebias is applied at −3 V, the quasi-Fermi level shifts from theHOMO (see the inset of figure 1(a) (not to the scale)), hence allthe trap states are filled-up to this level. As the bias voltage isdecreased from −3 V, the de-trapped carriers contribute to thetotal current since the quasi-Fermi level moves to the valenceband edge (i.e. up to ∼−2.5 V) [16, 19, 20]. As a result thenumber of carriers in the space-charge region is lowered, andits effect in the injection and transport is also observed to beless, as the bias voltage is reduced from −3 to 0 V. However, atvoltages less than −2.5 V, the current decreases as in the caseof normal injection. When the voltage is increased from 0 to−3 V, all the trapped carriers get de-trapped and this explainsthe absence of NDR [9]. This mechanism also explains howthe carrier density influences the NDR in these devices. It isknown that carrier density determines the location of Fermilevel in polymer devices, and accordingly the width of thespace-charge region varies which affects the band bending.And also as the carrier density increases, the trapped states willget filled and this leads to the increase in space-charge regionin the device, and hence the NDR effect gets suppressed. Allthese contribute to the reduction in the PVCR as the dopingincreases.
The effect of traps in these devices can be probed from thetemperature dependent J–V characteristics. The temperaturedependence of NDR of D4 is presented in figure 2. NDR
Figure 2. J–V characteristics of D4 at various temperatures. Theinset shows NDR peak voltage versus T . The solid line is a guide tothe eye.
behaviour is observed in the temperature range 300–120 K, butits peak position shifts to higher voltages as the temperatureis decreased, as shown in the inset of figure 2. The slopesof the J–V curves are observed to increase from ∼2.2 (at300 K) to 4.5 (at 160 K) with a decrease in temperature, andthis behaviour is attributed to the presence of exponentialdistribution of trap states in these devices [19, 20]. Thetrap density and mobility values are estimated from theSCLC theory as 1016 cm−3, 10−3 cm2 V−1 s−1, respectively, asreported elsewhere [14, 16, 20]. As the temperature decreasesthe phonon-assisted de-trapping of carriers becomes less,hence higher electric-fields are required to lower the potentialbarriers to de-trap the carriers, as observed in the data (see theinset of figure 2). This highlights the role of trap states, sincehigher voltages are required to de-trap the carriers at lowertemperatures [21].
The scan rate dependence of NDR for D2 is presented infigure 1(c). The NDR decreases by more than two orders ofmagnitude as the scan rate is increased. The PVCR is observedto be ∼507 for a scan rate of 4 mV s−1 and it diminishes to∼2 as the scan rate is increased to 200 mV s−1, which is dueto the limited lifetime of the trapped carriers; and at higherscan rates (>200 mV s−1) the NDR almost disappears. It isalso observed that the PVCR decreases at scan rates below4 mV s−1 (see the inset of figure 1(c)). This scan rate dependentNDR behaviour can be explained in terms of the lifetime ofthe trapped or de-trapped carriers, and their transit time [7].The transit time [22] and lifetime of the carriers [23, 24] areestimated to be ∼30 ms and ∼40 ms, respectively. If the transittime is nearly equal to the lifetime of the trapped carriers,the carriers will have enough time to respond to the appliedvoltage, causing a larger PVCR [7]. As the scan rate increases(200 mV s−1) only a few trapped carriers can respond quickly,due to the limited lifetime of trapped states; hence the currentand PVCR are less in the NDR region. If the scan rate isvery low (below 4 mV s−1), the de-trapped carriers may gettrapped again before reaching the collecting electrodes, andthereby causing a decrease in PVCR [7]. This variation of
3
J. Phys. D: Appl. Phys. 43 (2010) 425103 P Anjaneyulu et al
Figure 3. J–V characteristics of D3 under dark and illumination.
NDR as a function of carrier density, temperature and scan ratecan be used to investigate the carrier trapping and de-trappingmechanisms in semiconducting polymers.
To further understand this carrier density dependence ofNDR we have investigated its photo-response, since it is knownthat photo-generation of carriers occurs in semiconductingpolymers [25]. It is observed that the PVCR diminisheswith optical illumination, as in figure 3. The decrease inphotocurrent is consistent with the tuning of carrier densityby doping, as in both cases the space-charge region widens asthe number of carriers increases, hence the NDR peak currentdecreases. Although the scan rate dependent studies haveshown that the NDR is suppressed at a higher scan rate, in thepresence of light it occurs at a much lower value of the scan rate.This photo-response of NDR can be used in optoelectronicswitching applications.
4. Conclusions
In summary the carrier dependent J–V characteristics onSS/P3MeT/Ag devices show that trap states at the electrode–polymer interface play a major role in NDR, as inferred fromthe temperature and scan rate dependent J–V measurements.The peak position of NDR shifts to higher voltages at lowertemperatures. The decrease in NDR, as the carrier densityincreases, is mainly due to the widening of the space-chargeregion. The photo-induced J–V measurements also show therole of trapped states in tuning the NDR.
Acknowledgments
PA and CSSS are grateful to the CSIR, New Delhi, for financialassistance. PA is grateful to J Singh for sample preparation.
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