Ion mediated charge carrier transport in a novel radiation sensitive polyoxometalate–polymer hybrid

Abstract

We elucidate the carrier transport mechanism in a novel polyoxometalate–polymer (POM–MAPDST) hybrid containing molybdenum transition metal. Temperature dependent electrical measurements reveal trap free space charge limited conduction at room temperature, but at low temperatures, the conduction becomes space charge limited with exponential distribution of traps. Moreover, pH dependent electrochemical studies provide evidence for ion assisted carrier conduction in POM–MAPDST hybrid polymers.

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Organic semiconducting polymers^1,2 are an important class of materials for organic electronics and optoelectronics such as organic solar cells (OSCs),^3,4 organic light emitting diodes (OLEDs),^5,6 organic thin film transistors (OTFTs)^7,8 and organic memory devices.^9,10 Although, it enables low-cost, light-weight, flexible and large-area devices, there are still limitations in device performances because of improper carrier injection, poor charge transport and material stability. The early twenty first century witnessed a significant evolution of semiconducting polymers in terms of design, synthesis, physical and chemical properties. However, efficient carrier injection and transport in polymers are important to enhance the device performance by maintaining the carrier balance within the device.

Smart properties that increase the applicability of organic polymers can be improved by incorporating inorganic elements in the base polymer framework.^11,12 Polyoxometalates are commonly incorporated into a polymer backbone via covalent binding to develop stable POM–polymer hybrids of diverse cluster types.^13–15 POM represents an important class of polyanionic nano-clusters of early transition metals having the general formula [XM₁₂O₄₀]ⁿ⁻ (X is a heteroatom such as Mn and Si, M is the metal ion such as W and Mo), which can be envisioned as soluble molecular semiconducting oxide.^16,17 Transition metal containing POMs have shown promise for electronic device applications. The semiconductor like behavior of POM cluster has been widely explored towards variety of applications.¹⁸ Cronin and co-workers¹⁹ have shown that core–shell POM molecules could be a good candidate for the storage nodes of flash memory. Makarona et al.²⁰ have reported the charge transport property in metal–insulator–semiconductor devices containing self-assembled monolayers of tungsten POM with its potential application as electron storage media. Mielczarski and co-workers²¹ have demonstrated how to control the charge carrier transport in molecular devices of tungsten based hybrid POM. Varying the inter-electrode spacing and the POM concentration, Diakoumakos et al.²² have shown the tunneling transport in POM based composite materials. Recently, we have reported the synthesis of a POM–polymer hybrid, which is an Anderson type POM cluster hybrid with a organic monomer (methacryloyloxy)phenyl-dimethylfoniumtriflate (MAPDST).²³ The hybrid exhibited photo conductive behavior due to the redox property of the Mn-Anderson cluster as well as the photo generation of ions by MAPDST polymer under illumination and expected to have potential applications in optoelectronic devices. As the charge carrier transport largely influences device performances, a detailed knowledge about carrier conduction, including the role of the charge traps in POM–polymer hybrid is essential for their device applications.

In this communication, we report, for the first time, the electrical transport properties of the thin films of a novel radiation sensitive hybrid polymer POM–MAPDST. The temperature dependent current (I)–voltage (V) characteristics of hybrid polymer sandwiched devices were recorded in the variable temperature range within 320 K to 213 K. The conduction is trap free space charge limited at the room temperature, whereas at low temperatures, it is space charge limited with the presence of exponentially distributed traps (EDT). Both temperature and pH dependent electrical properties demonstrate that the carrier conduction in the POM–MAPDST is mainly governed by the counter-ions produced by POM clusters.

In order to investigate the transport property of POM–polymer hybrid, we employed ITO/POM–MAPDST/Al sandwich device structure (Fig. 1). The energy of the HOMO (highest occupied molecular orbital) level of POM–MAPDST is about 4.99 eV and the work function of ITO is 4.7 eV, while Al is known to have a work function of 4.3 eV.^24,25 It is evident from the energy level alignment (see Fig. S1, ESI†) of different layers of the device that ITO forms an ohmic contact (barrier height <0.4 eV) with POM–MAPDST for hole and the transport of hole from POM–MAPDST to Al is a downhill process. Therefore, the conduction through ITO/POM–MAPDST/Al device is governed by bulk-limited mechanism, which depends on the transport properties of the material.²⁶ Since the conduction is not injection-limited, the effect of electric contact from ITO glass and Al on charge injection during the temperature dependent electrical measurement is very slim. The I–V measurements at different temperatures have been carried out by applying a positive bias to the ITO electrode ensuring hole injection from the ITO layer into the POM–MAPDST. Symmetric current density versus voltage J–V curves (see Fig. S2, ESI†) infer the absence of Schottky barrier in POM–polymer devices. Furthermore, space charge limited conduction (SCLC) takes place in low mobility semiconductors when injected carrier density exceeds the intrinsic free carrier density of the materials. SCLC could be the dominant transport mechanism, if at least one contact is ohmic. These criteria are fulfilled in POM–MAPDST devices. Hence, the conduction is expected to be space charge limited and the experimental data were analyzed using SCLC formalism. In SCLC mechanism, the conduction can be either trap free or trap filling, depending upon the property of the sample and strength of the applied electric field. It is evident from the I–V characteristic (Fig. 2) that the variation of current at a particular voltage is temperature dependent. Each I–V curve shows the nonlinear behavior with a super linear region at higher voltages.

Fig. 1 Schematics of the POM–MAPDST devices.

Fig. 2 Current–voltage characteristics of POM–MAPDST films at different temperatures.

The charge transport in (low mobility) polymeric materials is often limited by the presence of space charges. In an ideal case or trap free materials, SCLC theory predicts the Mott Gurney equation for the current density, (μ, ε_r, and d are mobility, relative dielectric constant, and the separation between two electrodes, respectively).^27,28 However, in the presence of trap states that are exponentially distributed in energy, the current voltage relationship becomes²⁹

where, N_t is the total trap density, N_v is the effective density of states, and l is an exponent.

Fig. 3a represents the J–V characteristics of POM–MAPDST thin film at different temperatures. We fitted measured data with the relation J ∝ V^m to find the exponent m = (l + 1). It was found that, at low voltages (at all temperatures), the exponent m is not exactly unity (see Fig. S3, ESI†). This can be explained in terms of the effective charge carrier density in the active material. In fact, Mn centers of the Mn-Anderson clusters undergo a valence shift from Mn(III) to Mn(IV) in the presence of an applied potential by releasing some tetrabutylammonium (TBA) counter-ions associated with the clusters.²³ These ions act as charge carriers leading to a current flow through the device. As very few ions are produced at low applied fields, current is limited by the low free carrier density. However, at higher voltages (>2.6 V), the slope is increased to higher values because of large current flow through high effective carrier density. At room temperature (302 K), the exponent m becomes 2 implying trap free (TF)-SCLC regime. Above room temperature (320 K), although the exponent remains same (Fig. 3a), the current at a particular applied voltage is higher than that of room temperature due to thermally generated charge carriers. In the higher voltage region, the exponent increases with decreasing the temperature. The values of the exponent were 2.50, 3.39, 3.92 and 4.42 at 293, 253, 233 and 213 K, respectively. This result suggests that at temperatures other than the room temperature, SCLC is governed by traps that exponentially distributed in energy. It is reasonable to observe EDT-SCLC at low temperatures because the number of free carriers is reduced while lowering the temperature and the trap states start dominating by localizing charge carriers.

Fig. 3 (a) J vs. V plots in log–log scale at different temperatures. Solid lines are fits to J ∝ V^m and (b) extrapolation of J–V characteristics at different temperatures (plotted in log–log scale). The extrapolated J–V curves are intersected at ∼14.9 V.

Carrier traps having an energy distribution are gradually filled with increasing applied electric field (at all temperatures) and at a certain bias called critical voltage (V_c), all trap states will be occupied. The critical voltage can be expressed as³⁰ . V_c and hence the trap density (N_c) can be obtained by extrapolating J–V curves in voltage to a common point. Extrapolation of the log J–log V characteristic curves (Fig. 3b) at higher voltages resulted into a V_c of 14.9 V. The trap density was estimated to be ∼1 × 10¹⁸ cm⁻³ (d ∼ 70 nm), which is close to that reported for semiconducting polymers.^31,32

If the carrier conduction in POM–MAPDST is mediated by ions (as we have already mentioned above), then it is expected to be pH dependent. In order to study the pH effect on the carrier transport in POM–polymer hybrid, linear sweep voltammograms (LSVs) were performed. LSVs in Fig. 4a clearly show reduction in current in acidic pH over entire voltage range and current gets saturated at highly acidic condition. It should be noted that in acidic pH, the work-function of ITO may increase to 5.4 eV (ref. 33) by making the ITO contact barrier less for hole. In that case, the conduction mechanism will remain bulk-limited and the ITO contact is expected to have no effect on pH dependent carrier transport study. Observed decrement in the current value with increasing the medium acidity (see Fig. S4, ESI†) could be due to the reduction in effective charge carrier density in the surface electrode material, POM–MAPDST. To understand the individual role of POM cluster and MAPDST polymer unit during the conduction through POM–MAPDST, we recorded LSVs of only MAPDST under identical experimental conditions. An appreciable current flows through MAPDST only at neutral pH (see Fig. S5, ESI†). At ∼1.5 V and pH 7, MAPDST shows a current of 10⁻⁵ A, which is about two orders of magnitude lower than that of POM–polymer hybrids. In our previous report,²³ we have mentioned that a small current flows through MAPDST polymer due to triflate counter ions. However, the large improvement in the transport property of the POM–polymer hybrid may be attributed to the incorporation of POM into the polymer. Moreover, porous structure of POM–MAPDST than MAPDST polymer (see Fig. S6 and S7, ESI†) suggest that ionic conduction is more favorable in POM–MAPDST.

Fig. 4 (a) Linear sweep voltammograms (scan rate 100 mV s⁻¹) of POM–MAPDST at different pH values and (b) Nyquist plots for ITO/POM–MAPDST electrode in pH range 1.00 to 7.00 (inset: equivalent circuit).

Electrochemical impedance spectroscopy (EIS)³⁴ was exploited to further investigate the transport property of POM–polymer hybrid by monitoring the change in the surface property of the electrodes modified by POM–MAPDST. The impedance spectra of ITO/POM–MAPDST (Fig. 4b) consists of a single arc semicircle, which is strongly pH dependent. The real part of the impedance was found to increase at lower pH values implying the enhancement of the circuit resistance. On the other hand, the imaginary component increases monotonically with the variation of pH from neutral to acidic. This could be due to the change in capacitive behavior arising from the adsorption/desorption of ions at the surface of the electrode. Impedance spectra were fitted with an equivalent circuit (inset of Fig. 4b), which includes solution resistance (R_s), electron transfer resistance (R_ct), Warburg impedance (Z_w), and a constant phase element (CPE). The fitting parameter R_ct increases from Ω to few kΩ in decreasing pH value confirming the enhancement of electron transfer resistance in acidic medium (see Table 1, ESI†).

The effect of pH on LSV and EIS measurements could be explained in terms of the changes in the effective charge carrier densities due to the protonation or deprotonation of POM–MAPDST, depending upon the solution pH value. The polyoxometalate can change the charge states by releasing some TBA counter ions associated with the POM cluster. At neutral pH, the TBA counter ions are responsible for the conduction of large current. In contrast, when the polyoxometalate–polymer electrode is exposed to an acidic solution, protons (H⁺) neutralize TBA counter ions and reduce the effective charge carrier densities. This in turn reduces the current by increasing the resistance. Scheme 1 illustrates the effect of protonation on the conduction through the electrode modified by POM–polymer hybrid during EIS study.

Scheme 1 Illustration of the protonation effect on POM–MAPDST.

Conclusion

In conclusion, carrier transport through a novel POM–MAPDST hybrid polymer is temperature dependent. Although the conduction is nonohmic at low voltages, it becomes SCLC at high potentials. Nonetheless, the conduction is governed by trap free SCLC mechanism at room temperature (and high bias), but at low temperature it is controlled by traps that are exponentially distributed in energy. We have estimated an average trap density of 1 × 10¹⁸ cm⁻³ in the POM–polymer hybrid. pH dependent electrical transport infers that the carrier conduction through POM–MAPDST hybrid polymer is predominantly mediated by TBA counter ions, which are released by the POM cluster. We strongly believe that these results including the quantitative information about charge traps are important in rational design of electronic devices with POM–polymer hybrids.

Acknowledgements

ASS is thankful to IIT Mandi for his fellowship.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, results of electrical measurements and microscopic images. See DOI: 10.1039/c6ra04182e

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