Effect of intrinsic polymer properties on the photo sensitive organic field-effect transistors (Photo-OFETs)

    Effect of intrinsic polymer properties on the photo sensitive organic fieldeffect transistors (Photo-OFETs) ¨ urk, Bet¨ul CanZ¨uhal Alpaslan K¨osemen, Arif K¨osemen, Sadullah Ozt¨ imkurbey, SaitEren San, Yusuf Yerli, Ali Veysel Tunc¸ PII: DOI: Reference:

S0167-9317(16)30193-9 doi: 10.1016/j.mee.2016.04.007 MEE 10251

To appear in: Received date: Revised date: Accepted date:

21 December 2015 30 March 2016 7 April 2016

¨ urk, Please cite this article as: Z¨ uhal Alpaslan K¨ osemen, Arif K¨ osemen, Sadullah Ozt¨ Bet¨ ul Canimkurbey, SaitEren San, Yusuf Yerli, Ali Veysel Tun¸c, Effect of intrinsic polymer properties on the photo sensitive organic field-effect transistors (Photo-OFETs), (2016), doi: 10.1016/j.mee.2016.04.007

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ACCEPTED MANUSCRIPT Effect of intrinsic polymer properties on the photo sensitive organic fieldeffect transistors (Photo-OFETs) Zühal Alpaslan KÖSEMEN1,2, Arif KÖSEMEN1,3, Sadullah ÖZTÜRK4, Betül

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CANIMKURBEY1,5, SaitEren SAN1, Yusuf YERLİ1,6,Ali Veysel TUNÇ6*

Department of Physics, Gebze Institute of Technology, Kocaeli, Turkey TUBİTAK UME OpticsLaboratory 41470 Gebze, Kocaeli,Turkey

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Department of Physics, MuşAlparslanUniversity, Muş, Turkey

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Enginnering Department, Fatih Sultan Mehmet Vakif University, 34080, Istanbul, Turkey 6

Deparment of Physics, Amasya University, Amasya, Turkey

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Department of Physics, Yıldız Technical University, Davutpaşa, Turkey

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Department of energysystemsengineering, Istanbul Bilgi University, 34060 Eyup, Istanbul, Turkey

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*[email protected]

Abstract

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In this work, we have demonstrated how the intrinsic properties of a conjugated polymer can influence the electro-optical characteristics of photo sensitive organic field - effect transistors (Photo-OFETs). Photo-OFETs fabricated with three batches of poly[2-methoxy,5-(3′,7′-

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dimethyl-octyloxy)]-p-phenylene vinylene (MDMO-PPV) were investigated in the scope of our work. Photo-OFETs were fabricated with the polymers, than electrically and electrooptically characterized. It was observed that the channel current and the field-effect mobility increase with increasing polymer molecular weight. Interestingly, the electro-optical characteristics and photo switching properties of the transistors were found to depend on the polydispersity (PDI) of the polymer as well. These results are explained in terms of influences of chain packing, ordering and trap density on the FET switching properties and transistor parameters.

Keywords: Photo sensitive organic field-effect transistors (Photo-OFETs), Organic field effect transistor (OFET), MDMO-PPV, Molecular weight

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Introduction

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Organic materials are cheap and easy to product; therefore they have attracted attention for

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potential application in optoelectronic devices last two decades. Organic Field effect Transistors (OFETs) have been widely investigated as a possible candidate for next

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generation electronics and Photosensitive OFETs (Photo-OFETs) are an important electronic device for optical transducers, photo sensing device and image sensors[1,2]. Poly[2methoxy,5-(3′,7′-dimethyl-octyloxy)]-p-phenylene vinylene (MDMO-PPV) is well known

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polymeric semiconductors and have been used to produce OLEDs[3,4], OFETs[5,6] and OPV[7,8] successfully. It has been reported that the electric and electro-optical performance

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in poly(p-phenylenevinylene) (PPV) derivate materials depends strongly on the molecular weight[9,10] and intrinsic material properties. Phthalocyanine[11] ,polythiphene [12] ,poly(pphenylenevinylene) derivates[13] and some of the organic single crystal[14] have been used

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successfully in fabrication of photo-OFETs. Reports of the electro-optical characterization of

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photo-OFETs based on poly(3-hexythiophene) (P3HT) [15] and on pentacene [16,17] show that spectroscopy may find general applications in the analysis of the performance of photo-

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OFETs. However, it is already known that electrical and spectral properties of polymer also depend on intrinsic properties.

Photo-OFETs have same three terminals structure of OFETs, where the terminals are source,

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drain and gate electrodes. In OFETs devices, the current (Ids) in the accumulated channels (between source – drain) can be controlled by the magnitude of gate voltage (Vg) at a given source – drain voltage (Vsd). Likewise in photo-OFETs, light induced charge carriers are added this channel current by using light sensitive active organic semiconductor via absorption of light. Thus, channel conductance is increased in proportional to light intensity [18]. There are two different effects happen under light, firstly photovoltaic effects and then photoconductive effects take place. Under illumination, threshold voltages (Vth) slides toward more positive (negative) values via photo-induced shift for p-type (n-type) semiconductors. For p-type photo-OFETs, when light absorption occur, the photo generated holes move to the drain electrodes and electrons accumulate around the source electrodes, this leads to decrease hole injection barrier between the source electrode and organic semiconductor. The lower injection barrier under light illumination leads to decrease in contact resistance and in a shift

ACCEPTED MANUSCRIPT of Vth and increase in Ids. If the photo-OFET is in the off state, photo generated Ids demonstrates a linear increase with light intensity due to photoconductive effect. [19-22]

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It has been done many studies on photo-OFETs for last decade [23-26] and different kind of

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materials class has been studied from small molecule to polymeric semiconductors. The ideal organic semiconductor for photo-OFET should have both high charge carrier mobility and

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excellent light sensitivity. In a comparative study of the photo-response between small molecule and polymer based OFETs, small molecule OFETs showed superior potential as a photo-OFET in the visible and UV light. However, for the adoption of photo-OFET in low

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cost and large area optoelectronic devices, photo sensitive organic semiconductors have to be soluble in common solvents. Polymer based photo-OFETs can be fabricated by cost effective

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printing method and have an excellent compatibility with flexible substrates. These promising features of the polymeric semiconductors can lead to low cost, easy process and large area optoelectronic integrated circuits. Most of the reports on the photo-OFETs have focused on

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the organic semiconductor materials, the film and dielectric thickness, and contact effects

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[27]. It was demonstrated that the photo responsive performance in photo-OFETs cannot be explained due to device structure alone [28]. It is reasonable to assume that efficient device

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performance is related not only to transistor structure and geometry but also depends on the intrinsic properties of the organic semiconductor, such as molecular weight and polydispersity.

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One of the most investigated polymers in organic electronics is the poly (pphenylenevinylene)s (PPV) and its derivatives. PPV is an insoluble polymer, but the addition of side-chains in the PPV reduces the rigidity of the backbone and increases its solubility [29]. One of the forms of soluble PPV derivatives is poly [2-methoxy,5-(3′,7′-dimethyloctyloxy)]1,4-phenylene vinylene (MDMO-PPV). The polymer derivates of PPV are widely used in organic electronics, and MDMO-PPV can be considered as a representative of a wide class of polymer semiconductors. MDMO-PPV is a well known and promising candidate for p-type organic electronics as a result of air stability, solubility and moderate mobility. In addition to this, soluble PPV derivatives are highly isotropic, which leads significant consistency in device performance. PPV and its derivatives usually act asa p- type material, namely, hole conducting materials. Even though it is not clearly understood intrinsic properties impact on organic device performance, there is some work that showed device performance is improved by increasing molecular weight [30, 31]. This phenomenon was elucidated that higher

ACCEPTED MANUSCRIPT molecular weight polymer have longer conjuge polymer chains that make charge carriers transfer easily. Therefore, it has claimed that increasein mobility is the reason of this process[32]. Some studies done with MDMO:PPV have shown that not only molecular

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weight but also polydispersity affect OFET performance. These results are explained in terms

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of chain packing, which is more efficient in high molecular weight and low PDI polymers [33]. MDMO:PPV with n-type materials widely used in fabrication of organic light sensitive

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devices. This kind of devices have 23mA/W responsivity [34

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16µs device response

time [35]. Our previous report was first paid attention to intrinsic properties on OFET performance [33]. However, to the best of our knowledge, electro optical properties of photo-

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OFET have not detailed investigated in terms of intrinsic properties of conjugated polymers. In this study, we have investigated how the molecular weight and polydispersity of MDMO-

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PPV influence photo-OFET performance. MDMO-PPV has three different molecular weight used as an active layer and PMMA used as a dielectric material. Transfer and out-put characteristics of photo-OFETs were investigated under dark and illuminated conditions.

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Photo-switching and time resolved photo switching properties were investigated as well. We

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have demonstrated that the intrinsic properties of the polymer influence the electrical characteristics of the photo-OFET significantly. The results show that molecular weight and

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PDI influences the transport properties of the polymer, so photo sensitivity properties could have been modulated by it. This work is a continuation of our previous result[33].

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Experimental

We fabricated MDMO-PPV photo-OFETs on glass substrates. Firstly, Cr (5nm) was coated on glass substrate and then Au (100nm) was coated on Cr layer by using thermal evaporation method. The electrodes were etched with standard photo lithography to produce interdigitated source-drain electrode fingers with a channel length of 50 µm and a channel width of 12 cm. Figure 1 depicts the schematic illustration of produced photo-OFET device.

The MDMO: PPV, which has different molecular weights (25.000,450.000 and 85.000) and PDI (3.5, 4.2 and 16.8) were used as an active layer; it was named Polymer A, Polymer B and Polymer C respectively. All MDMO-PPV polymers are commercial available and are used as received. MDMO:PPV was dissolved in 1.2 dichlorobenzene and spin coated on interdigitated Au electrodes. The samples were annealed at 110 oC for 10 minutes. To achieve the same thickness for all samples, MDMO:PPV was dissolved in 1,2 dichlorobenzene at different

ACCEPTED MANUSCRIPT concentrations (5, 12, 6mg/ml respectively). In this study, PMMA was used as a dielectric layer. PMMA was dissolved in ethyl acetate (60mg/ml) and spin coated on MDMO:PPVactive layer. The samples annealed at 110 oC again and Al (40nm) coated on

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PMMA as a gate electrode.

The electrical and photo responsive properties of photo-OFET devices were characterized in

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ambient condition and all measurements were performed at room temperature. The transfer and output characteristic of the OFET were measured with Keithley 4200 semiconductor characterization system and photo-OFET properties was analyzed under dark and under white

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lightat different light power by using silicon photodiode and halogen lamp. The device was illuminated from the bottom side of the device (as presented in figure 1). The photo switching

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and time resolved characteristics of Photo-OFETs were measured with CHI 660D workstation.

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Results and discussions

The photo-OFETs of different MDMO-PPV with a top gate bottom contact configuration

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were fabricated to measure various device properties under dark and under illumination, such as field effect mobility (µ), the current ratio of the on and off states (Ion/Ioff), threshold voltages (Vth), the ratio of photocurrent to dark current (P=IPh/IDark) and photoresponsivity (R) [36]. Figures 2 present output characteristics of all polymers under dark and under light

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irradiation of an intensity of 17 mW/cm2 for different gate voltages.All the polymers photoOFETs present saturation in the dark. A linear gradient of charge density from the carrier injecting source to the extracting drain forms when a small source-drain voltage is applied (Vds<
el is “pi ch off”. Further i cre si g V s will not considerably

increase Ids but leads to an expansion of the depletion region and thus a slight shortening of the channel. Since the potential at the pinch-off point remains Vg - VTh and thus the potential drop between that pinch off point and the source electrode remain nearly the same, thus the current saturates at a level Ids. However, there is no saturation under illumination with these polymers as shown in fig.2a,b and c. Figure 2 d shows gate voltage dependent drain current under dark and light irradiation for all polymer to better visualization. A number of light induced charge carriers are generated when organic semiconductor absorb the light. This

ACCEPTED MANUSCRIPT additional charges lead to an increase in Ids. This indicates that light can play a role as an additional terminal that control the charge carrier concentration inside the channel, along with

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source, drain and gate terminal.

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In order to obtain a good ohmic contact, the work function of the injecting metal must be close to the HOMO or LUMO level of the semiconductor [37] .Au are most commonly used

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for source and drain contacts in p-type OTFTs, form ohmic contacts with organic semiconductors, as expected by comparing gold's work function (~5.1–5.2 eV [38]) to the valence band (or highest occupied molecular orbital, HOMO) (4.90-4.97 eV [39]) energy

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level of MDMO-PPV. Output characteristics for different gate voltages are shown in figure 2 and present ohmic behavior at low drain voltages and drain current saturation above the

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pinch-off point [40]. The characteristics demonstrate excellent linearity at low voltage which indicates the quality of contact.

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OFETs can be assumed a simple metal-insulator semiconductor (MIS) diode (in case of no

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potential difference between source and drain) with a voltage Vg applied to the gate contact. Negative gate voltage Vg will induce and accumulate positive charges (holes) at the

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insulator/semiconductor interface that were injected from the grounded electrodes. The number of accumulated positive charges is proportional to applied Vg and the capacitance Ci of the insulator. Before source-drain voltage is applied, the charge carrier concentration in the transistor channel is uniform. Figure 3 shows the transfer characteristics (Ids versus Vgs at

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constant Vds) of the transistors. The transistors present linear and saturation regime from small to large Vgs value[41].

The values of the molecular weight and polydispersity (Mn, Mw, and PDI), were taken from ref [33] for polymers A, B, and C respectively. In addition to field effect charge carrier mobility [42], threshold voltage, on/off ratio, the photosensitivity P and the responsivity R[43] were determined and these results are summarized in Table.1. Polymer films with higher polydipersity have lower mobility, since longer chains are statistically more likely to have a region with longer conjugation length that can act as a trap site due to its reduced bandgap[44,45]. Mobility could increase with MW because charge carriers can travel farther along longer chains before they have to hop to another chain, and because longer chains give charge carriers more opportunities for hopping to neighboring chains. This net reduction in hopping events could result in an increased mobility[45]. Figure 3- a,b and c show the transfer

ACCEPTED MANUSCRIPT characteristic and leakage current of the photo-OFETs under dark and illumination with 17 mW/cm2 for a drain voltage of -40V. The leakage currents are nearly same order which indicate that all the dielectric thicknesses are nearly same. A ratio of photocurrent to dark

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current as high as 124 was obtained for polymer B, which has the highest molecular weight

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and moderate polydispersity. Polymer A has the poorest performance with the lowest molecular weight, 25.000. Polymer C shows moderate ratio of photocurrent to dark current,

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72, which the molecular weight of polymer C is in between the polymer A and B. The transfer characteristics show clearly that the ratio of photocurrent to dark current strongly depends on the molecular weight and polydispersity of polymer, this result is explicitly presented in

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figure 3d.

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By changing the sweep direction, a hysteresis was observed in the transfer curve. In addition to the hysteresis, a shift of the threshold voltages towards higher voltages was observed, figure 3. The threshold voltage shifts are commonly attributed to a built-in electric field near

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to the dielectric – semiconductor interface caused by the presence of a sheet of charges [46].

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Therefore, the threshold voltage and hysteresis may depend on the charge carrier density, trap density, Fermi levels of contact materials, bias stress, and on the exposure of light. It is also

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supposed that different dielectric/polymer surface and semiconductor morphology may change the threshold voltage, which could influence the trap formation.

It is clearly seen that from table 1, the molecular weight of the polymers that used as active

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layer very influential on the OFET parameters. Polymer B has nearly ideal intrinsic properties (high molecular weight and low PDI) as a polymer semiconductor so it has the best performance: the highest current of channel, Ion/Ioff ratio and the lowest Vth value. When the transistor parameters of the devices fabricated Polymer A, B and C was analyzed with each other, Polymer B has the highest mobility (µsat =1.41×10-5 cm2/Vs), the highest Ion/Ioff ratio (1.1×103) and lowest Vth value (4 V). Polymer A has the lowest molecular weight and the lowest mobility (µsat =1.73×10-7 cm2/Vs), Ion/Ioff ratio (0.6×102) and the highest Vth (27 V). Mobility could increase with MW because charge carriers can travel farther along longer chains before they have to hop to another chain, and because longer chains give charge carriers more opportunities for hopping to neighboring chains. This net reduction in hopping events could result in an increased mobility[45]. The leakage current for all polymers is presented in the same plot. All the leakage current are nearly same order, this behavior indicate that the dielectric thickness and morphology almost same for all devices. In this

ACCEPTED MANUSCRIPT study, we also observed that when there is a decrease inPDI, photo transistor parameters are getting better. Figure 3 presents that the transfer characteristics of polymers under dark and illumination and IPh(under light)/IDark(under dark) ratio at VG=0V for all polymers. Molecular

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weight and PDI highly influenced on the IPh / IDarkratio at VG=0V of MDMO:PPV. Polymer B

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has highest molecular weight with moderate PDI and highest IPh / IDark ratio (1,24×103), polymer A has lowest molecular weight with lowest PDI and has lowest IPh / IDarkratio (4,32).

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The current difference between under dark and illumination were high at initially, then the difference getting smaller. This indicates that Ids governed by light at low gate voltage. After filling all traps by high gate voltage, Ids governed by gate voltage. IPh / IDark ratio of polymer C

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is 7.23×101 and this value is between the Polymer A and Polymer B. Long polymer chains can cause more photon absorbance and high photo current. Likewise, lower PDI can cause

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less end point of polymer chain and better chain conformation which lead to less trap states. Thus polymer B has the highest IPh / IDarkratio and it can be said for polymer Bcan be used

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potential application for transistors in optoelectronic devices.

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Figure 4 shows the photo-switching behavior of all polymers under different powers of illumination intensity without a gate connection. The transistors were controlled by the

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incident light, and their output characteristics were similar to OFET, as shown fig 4 a, b and c. The photo-OFETs output follows a similar trend for all three polymers. At dark condition, all devices demonstrate nanoampere region Ids with increasing Vds. As demonstrated in figures, the transistor Ids were controlled by incident light intensity andtheir behavior of output

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characteristics were very similar except current magnitude. It is clearly seen that the magnitude of the Ids depends on the molecular weight of the polymers. These results suggest that the light could substitute Vg to control the output of the OFET. Figure 4 d shows the power of illumination intensity dependent Ids current for Vds=-80V. Polymer B has highest molecular weight and moderate PDI showed excellent output characteristic with incident light. This result also supported by different gate voltages in figure 5. Figure 5 present that light has similar effectsas if gate voltage drives the devices.

Figure 6 shows the photo switching properties of all three polymers under V G=0V. Without gate voltages, the devices can be controlled by switching the light source on/off. When measured all polymers photo-OFETs in the dark, the devices showed a maximum Ids in the nanoampere region because the device was in a turn-off state for VG=0 and Vds= -10V , as shown in figure 6 a, b, and c. As shown in figures, the increase of Ids is caused by the creation

ACCEPTED MANUSCRIPT of a large number of charge carriers due to the photo generation. The light induced Idswas reached form 1.5 to 15 nA as soon as the light was switched on and Ids was restored to the 3nA after 50s the light was switched off for polymer B. The cycles continue to keep going, Ids

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was at nearly same values. The polymer A, which presents the poorest performance at VG=0.

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The light induced Idswas reached from 0.1 to 0.48nA after 50s the light switch on and Ids was restored to the 0,2nA after 50s the light was switched off for polymer A. The cycles continue

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to keep going, Ids was increase for both on and off state around 0.1nA. Polymer C has similar behavior as Polymer B, but lowers Ids. The current was reached from 2 to 6nA, Ids was nearly

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same values when the cycles continue.

We also measured the time-resolved response of the photo-OFETs, as shown in Fig. 7 a. The be escribe by

si gle respo se time (Kohlr usch’s l w), the curre t i cre se

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respo se c

and decay can be fitted forcefully to stretch exponential behavior [47]. For light on, the current response time constants are7.53, 3.63 and 6.15s for polymers A, B, and C

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respectively. The relaxation after switching the light off, the time constants are a rather long

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namely12, 6.36 and 11.45s, respectively. Since this relaxation time should be related to the recombination of the photo separated charges, it can be improved by intrinsic material

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properties, thus allowing faster detection speeds. As shown in figure 7 b, the time constants depend on molecular weight and PDI of the polymers. Polymer B has best intrinsic properties in terms of molecular weight and PDI, thus the time constants are the lowest among all

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polymers.

Conclusions

In this work, we have investigated three batches of MDMO-PPV, which varied in molecular weight (Mn and Mw) and in PDI. We have demonstrated that the electro-optical characteristics and parameters of MDMO-PPV-based photo-OFETs depend strongly on the molecular weight and PDI of the polymer. High molecular weight results in higher light absorption, channel currents and photoresponsivity. A high PDI, on the other hand, correlates to a decrease in the quality of the photoresponsivity characteristics by large amount of trap site. These results are explained in terms of light absorption, charge transport and charge trapping-release, which is more efficient in high molecular weight and low PDI polymers.The promising opportunity for photo-OFET applications canbe further opened by the tuning of the molecular weight and PDI for sensing.

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ACCEPTED MANUSCRIPT Table Caption: Table 1. Field Effect Mobility µ(cm2/Vs) (Saturation Values), Threshold voltage Vth, Ion/Ioff

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and responsivity values for photo-OFETs made with Polymers A, B, and C.

ACCEPTED MANUSCRIPT Figure captions Figure.1: Schematic illustration of photo-OFET and direction of light incidence.

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Figure 2. Output characteristic of the MDMO:PPV based PhotoOFETs in dark (dark square)

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and under light (open circle, 17 mW/cm2), a) Polymer A b) Polymer B c) Polymer C. d) Gate

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voltage dependent drain current in the dark and under light irradiation for all polymer.

Figure 3. Transfer characteristic and corresponding leakage current of the MDMO:PPV based photo-OFET’s f bric te o Au i ter igit te source-drain electrodes in dark (black line) and

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under light (red line, 17 mW/cm2), PMMA was used as gate insulator, a) Polymer A, b) Polymer B c) Polymer C. The arrows indicate the sweep direction, starting at 10V. d)

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Molecular weight dependent mobility under dark and illumination condition.

Figure 4. The output characteristic of the transistors at VG=0V and under light irradiation

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with different illumination power (0 to 17 mW/cm2).

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Figure 5. Dependency of photocurrent on light intensity a) VG=0V b) VG=-40V Figure 6. Photo-swichting properties of MDMO:PPV photo transistors under 17 mW/cm2 power of illumination intensity with different molecular weight and PDI.

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Figure 7.Time resolved photo-switchi g properties MDMO:PPV photo-OFETs for polymer A(▲), B(■),

ec y time co st ts (τ)of C(●)

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Fig. 1

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Fig. 2

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Fig. 3

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Fig. 4

Fig. 5

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Fig. 6

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Fig. 7

ACCEPTED MANUSCRIPT Table 1 Mw

(g/mol)

PDI

(g/Mol)

Eg, abs

(Mn/Mw) (eV)

µ

Vth

2

(cm /V.s)

25,000

88,000

3,5

2,16

1.73×10

B

450,000

1,900,000

4,2

2,14

C

85,000

1,400,000

16,8

2,15

0.6×10

1.41×10-5

4

9.86×10-7

15

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(mA/W) (IPh/IDark) 2

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(V)

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A

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Mn

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Polymers

1.1×103

1.26×102

0.015

4.32×100

0.12

1.24×102

0.054

7.23×101

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Graphical abstract

ACCEPTED MANUSCRIPT Highlights:

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Photo-OFETs fabricated with three batches of poly[2-methoxy,5-(3′,7′-dimethyloctyloxy)]-p-phenylene vinylene (MDMO-PPV) Photo-OFETs were fabricated with the polymers, than electrically and electrooptically characterized It was observed that the channel current and the field-effect mobility increase with increasing polymer molecular weight The electro-optical characteristics and photo switching properties of the transistors were found to depend on the polydispersity (PDI) of the polymer

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