Effect of electric field on the electrical properties of a self-assembled perylene bisimide

A functionalised perylene bisimide forms two different self-assembled structures in water depending on the solution pH. Structure 1 (formed at pH 6.2) consists of a fibrous structure, whilst structure 2 (formed at pH 9.4) consists of disordered aggregates. Despite being formed from the same molecule, structure 1 shows higher stability under illumination and electric field than structure 2, demonstrating that the nature of the self-assembled aggregate is critical in devices. Interestingly, both structures show p-type behaviour.

Conjugated small molecules have shown promising results in optoelectronic devices such as photovoltaics (PVs), 1 eld effect transistors (FETs), 2,3 light emitting diodes (LEDs), 4 and photodetectors. 5Perylene bisimides (PBIs, also called perylene diimides) are well-known electron transporting/accepting n-type organic semiconductors for optoelectronic devices. 6,7PBIs could show high electron conductivity and are the best nonfullerene n-type materials for organic photovoltaic applications. 8These materials also have high extinction coefficients, high thermal and chemical stability, and chemical tunability. 9,10BIs can be used to form useful FETs with high electrical conductivity.The change of electrical properties of PBI is due to the different stacking of the molecules which is inuenced by the structure, solvent and concentration used to self-assemble the PBIs.For example, N,N 0 -1H,1H-peruorobutyl dicyanoperylenecarboxydiimide (PDIF-CN 2 ) forms large grains upon postthermal annealing of a spin-coated lm at 110 C under vacuum, resulting in efficient n-type channel eld effect transistor. 11Jones et al. prepared efficient air-stable n-type FETs based on a core-cyanated PBI derivative. 12They showed a signicantly higher conductivity for the thermally evaporated PBI derivative in a top contact conguration, while substantial drop of conductivity observed for the solution-processed PBIbased FET for a bottom gate structure.
The formation of controlled crystalline structures of PBIs to achieve high charge carrier mobility is difficult.Where the selfassembly can be controlled, this can lead to enhancement of their electrical conduction.Oh et al. observed solution based thin lm formation of a PBI derivative in an OFET structure. 13e OFET device showed eld effect n-type property with good electrical conductivity as a result of the slip-stacked face-to-face molecular packing of the PBI molecules and their dense parallel arrangement.Another study reported a liquid crystal (LC) PBI with space-charge limited current shows higher conductivity under ambient conditions. 14These LC PBIs form onedimensional columnar stacks with intermolecular p-p orbital overlap to enhance mobility.Theoretical work reported by Delgado et al. showed the change of electron and hole conductivity upon addition of different end-substituted and core-substituted groups to a PBI. 15The change is due to different structural forms of the PBI.
PBIs structure are intrinsically insoluble and mostly used as uorescent dyes with high uorescent quantum yield.These materials are however well-known for their excellent n-type behaviour for different optoelectronic devices such as solar cells and eld effect transistors (FETs).Water-based PBIs are promising for biofriendly optoelectronic device application with the possibility of PBI thin lm formation in PVs and FETs.However, there is limited information in the literature regarding the lateral eld effect conductivity of water-based perylene structure in a FET conguration at dark and under illumination.
PBIs in general can form a range of supramolecular structures, which depend on different types of intermolecular forces such as hydrogen bonding, p-p stacking and metal-ligand interactions. 10Among these non-covalent interactions, p-p stacking plays an important role in self-assembly of PBI derivative in both solution and lms. 16,17The dynamics of the supramolecular structure can be controlled via different conditions such as the pH, temperature and concentration.PBIs can form either H-or J-type aggregates, [18][19][20] although we and others have recently highlighted that this assignment needs to be completed with great care. 21,22e have been working with a series of amino acid functionalised PBIs.These have the advantage of being watersoluble, and we have shown that it is possible to control the aggregation type and electronic behaviour by varying the amino acid substituents. 23The chemical structure of the water-soluble alanine-appended PBI (PBI-A) studied in this work is shown in Fig. 1.The aggregation of PBI-A in water is driven by the hydrophobicity of the PBI core in the aqueous environment.The structures formed depend on the pH of the solution.We have previously shown for this molecule that worm-like micelles are formed at a pH of less than 7, with gels being formed by a transition to bres below a pK a of 5.4. 24The control of selfassembled structure formation of PBI-A lm along with its promising photoconductivity 25,26 makes it as an interesting candidate for the next generation semiconductor devices.Previously, we have focussed on preparing lms from PBI-A in the mono-deprotonated state.Here, we specically compare lms prepared from PBI-A at two different solution pH.The degree of deprotonation is different at these two pH values, which affects the aggregation and self-assembly.We show that this directly affects the lm quality.We also show that this ntype semiconductor can show p-type behaviour.
PBI-A was synthesised as described previously. 27This molecule has two apparent pK a . 24The PBI can be dispersed in water by raising the pH above the lowest pK a of the molecule.This can be achieved by using a single equivalent of a base (formally to deprotonate a single carboxylic acid), or with two equivalents of base to form the doubly deprotonated species.Solutions of PBI-A were prepared at a concentration of 5 mg mL À1 .On adding a single equivalent of base, the pH of this solution was 6.2.Slightly viscous solutions with a shear-thinning behaviour were formed (Fig. 1b) as can be seen the viscosity measurements (Fig. S1, ESI †).Shear thinning can be assigned to the presence of worm-like micelles as they align at high shear rates. 24Films can be formed from these solutions by simply drying on a surface.Long, anisotropic structures are present aer drying, as shown by SEM (Fig. 2a).We refer to these as structure 1 throughout this report.
Adding two equivalents of base results in a solution at pH 9.4.The doubly deprotonated PBI-A does not self-assemble into dened structures, resulting in a lower viscosity (Fig. S1, ESI †).We have shown previously by small angle scattering that there is limited self-assembly under these conditions. 28On drying, ill-dened aggregates are formed, which we refer to as structure 2 (Fig. 2b).The differences between structures 1 and 2 arise from the charge on the PBI-A, with the 2 being more negatively charged and so more soluble in water, and 1 being less charged and therefore more hydrophobic.There are slight differences in the lm morphologies for both structures compared to our previous reports; this is due to us using hydrophilic surfaces here whilst our previous data used hydrophobic surfaces (comparative data are shown in Fig. S2, ESI †).
UV-Vis absorption and photoluminescence (PL) spectra of lms formed from both structures are shown in Fig. 3.The absorption spectra of both lms are similar, with a slightly stronger 0-0/0-1 vibronic band ratio for structure 2 as compared to 1.The different ratio of the peaks indicates different molecular packing in the structures. 9,21he PL spectra for both lms (excited at 500 nm) appear similar with a more resolved shoulder at longer wavelength for structure 2, which is due to the stronger 0-0 vibronic band   absorption.The non-normalised PL spectra with maximum absorption for two structures are compared in Fig. S3 (ESI †).The maximum PL peak is quenched signicantly for structure 1 in comparison with structure 2. This substantial decrease in PL intensity peak can be explained as a result of bre formation in structure 1 and a better charge separation upon photoexcitation.For structure 2, the stronger PL is an indication of amorphous structure and consequently more exciton quenching.
We prepared devices with either structure 1 or structure 2 as the active layer.The fabrication procedure for our devices is shown as a schematic diagram in Fig. 4. Briey, p-doped Si coated by 300 nm SiO 2 was used as a substrate and a FET device with bottom gate top contact architecture is fabricated.A PBI-A lm with either structure 1 or structure 2 acted as the active layer.Gold contacts with 25 nm thicknesses were evaporated as the top source and drain contacts via thermal evaporation system.Using these devices, we examined the effect of electric eld on the photoconductivity and structure of lms comprised of either structure 1 or 2. The electrical properties of these two structures are compared in the dark and UV light illumination under different applied electric elds.
The electrical conductivity of both structures was measured in the dark, and under irradiation with 365 nm light at different gate applied positive voltages of between À40 to 50 V (Fig. 5).Illumination with this wavelength was chosen on the basis of our previous reports. 9We observe an increase in conductivity under UV illumination, which is in agreement with our previous work 9,21,29 and for related PBIs by other groups. 25This is due to the formation of radical anions and dianions, which are long-lived charged species under UV illumination and enhance the conductivity of PBI-A lm. 9,30,31The device formed using structure 1 shows a substantial source-drain current from 160 nA in dark to 20.4 mA under UV illumination.These currents are under À40 V gate bias voltage.A weak p-type behaviour is observed for both dark and light currents.The source-drain current in dark decreases from 160 nA at À40 V to 37.5 nA at 50 V.Under 365 nm illumination, the currents for structure 1 changes from 20.4 mA at À40 V to 19 mA at 50 V.As a result, the p-type eld effect transistor for structure 1 is stronger in the dark (Fig. 5a).
For devices formed using structure 2, the conductivity in both the dark and under illumination also showed weak p-type behaviour.The dark current drops from 135 nA at À40 V to 80 nA at 50 V gate voltage.The conductivity under 365 nm illumination changes from 500 nA at À40 V to 200 nA at 50 V.Structure 2 shows the same p-type behaviour in both dark and under illumination as shown in Fig. 5b.This is similar to the effect observed by Besar et al. in OFET devices based on quaterthiophene core and the assembled peptide forming 1D nanostructures.The high off current between sourcedrain is due to the signicant ionic current in the material due to the amino acid groups. 30o explain the better conductivity of structure 1 compared to structure 2, we investigated the lms in the dark under an applied electric eld and aer the simultaneous irradiation and an applied electric eld.To irradiate the lms, we used a 365 nm LED as we have previously shown that there is a signicant enhancement of the conductivity of a lm of structure 1 under this wavelength. 9,21,29Under an applied electric eld (gate) through the lm in the dark, structure 1 does not show any signicant morphological change (compare Fig. 6a with the structure shown in Fig. 2a).On application of an electric eld and the LED, the lms change morphology, but continuous domains can still be seen; the lm shows the presence of signicantly smaller bres compared to before the application of the eld and LED.These are however still connected to each other.Whilst we are unaware of other examples of changes in PBI lms on application of an electric eld, it is well known that electromechanical forces can cause changes in other systems. 32,33n comparison, application of the electric eld to structure 2 results in the domains becoming smaller (compare Fig. 6c with   2b).The lm of structure 2 under the applied electric eld in the dark (Fig. 6c) shows structures with more dispersed white bright objects.As observed in the SEM image in Fig. 6d, these white features become less dispersed over the lm aer illumination and an applied electric eld.The presence of these features could be due to the presence of sodium salts formed in structure 2. However, powder X-ray diffraction (pXRD) measurements (Fig. S4, ESI †) of lms of both structure 1 and structure 2 are similar and show no peaks which could be ascribed to sodium salts.Hence, the lower lm conductivity and current stability can be explained by the formation of more disordered, small aggregated domains, which importantly are not making a continuous pathway between two electrodes.
The eld effect transistors based on both structure 1 and 2 show p-type behaviour in a bottom gate top contact conguration.This behaviour is not expected, as PBI is known to be an ntype material due to high electron affinity of the perylene core of PBI. 11,12In a recent study presented by Draper et al., PBI-A showed an ionisation potential of À5.72 eV and an electron affinity of À3.91 eV. 23Additionally, we have signicant evidence for n-type behaviour of this molecule. 23,24As such, the reason for the observed p-type behaviour here is unclear.A weak p-type behaviour of a peptide-functionalised, self-assembled PBI was previously observed by Eakins et al. 34 Silberbush et al. 35 found a substantial increase of hole transport of a peptide bril network under various relative humidity conditions.Hence, this p-type behaviour might be due to the role of the amino acid (or peptide in the case of Eakins et al. 27 ), or the existence of ions in the lm, which modulate charge injection, and transport in PBI.Alternatively, as discussed by Delgado et al., 15 the presence of functional group in a PBI can result in a lowering of the reorganisational energy of holes and consequently improved hole conductivity.It may be that the molecular packing on drying on the surfaces here leads to suitable morphological changes that favour p-type behaviour.Finally, we note that recent work by Zhang et al. have suggested that PBI lms can show either p-type or n-type behaviour depending on the ratio of dianion to radical anion in the lm. 36This behaviour is the subject of further investigation.
In conclusion, a water-dispersible perylene bisimide can form different structures depending upon the absolute solution pH.In a bottom gate top contact FET conguration, this material shows p-type behaviour with a substantial increase of current under 365 nm UV illumination.The ambipolar behaviour of water-based perylene bisimide derivative under different processing conditions may provide a route toward developing ambipolar FET devices based on the single material.

Fig. 2
Fig. 2 SEM images of PBI-A for (a) structure 1 and (b) structure 2 as formed by drying on a silicon substrate.The scale bar represents 500 nm in both cases.

Fig. 3
Fig.3Normalised UV-Vis absorption (solid lines) to absorption peak at 505 nm and normalised PL intensity to the emission peak at 674 nm (dashed lines) for PBI-A film excited at 500 nm for structure 1 (black data) and (b) structure 2 (red data).

Fig. 4
Fig. 4 Schematic of device fabrication procedure: (a) p-doped silicon/ SiO 2 , (b) ultrasonic bath in acetone and isopropanol each for 10 minutes, (c) oxygen plasma treatment at 50% power for 10 seconds, (d) drop-casting PBI-A solution on top of SiO 2 and (e) evaporation of 25 nm gold as a source and drain contact.

Fig. 5
Fig. 5 Source-drain current versus gate voltage for PBI-A film with (a) structure 1 and (b) structure 2 in the dark (black line) and illuminated with 365 nm light (red line).

Fig. 6
Fig. 6 SEM image of films formed from (a) structure 1 at dark and after applied electric field; (b) structure 1 after UV and applied electric field; (c) structure 2 at dark and after applied electric field; (d) structure 2 after UV and applied electric field.