Effect of solvent induced dielectric property

Polymer 130 (2017) 79e87

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Effect of solvent induced dielectric property modulation of poly(methyl methacrylate) insulator on the electrical and photosensing behaviour of p-channel organic transistors D. Panigrahi, S. Kumar, A. Dhar* Department of Physics, IIT Kharagpur, Kharagpur 721302, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 July 2017 Received in revised form 21 September 2017 Accepted 28 September 2017 Available online 29 September 2017

We have investigated the influence of solvent dipole moment on the dielectric properties of poly(methyl methacrylate) (PMMA) gate dielectric and unravel its effect on the OFET performance, environmental stability, and photosensitivity. The device performance improved substantially upon the use of high dipole moment solvents, however, it was also observed that such solvents can exacerbate the operational stability of the transistors. A detailed investigation of polymer bulk and surface properties revealed that the choice of solvents can strongly influence its micro-molecular structure, dipolar orientation and surface chemical composition which consequently, affect the device performance and stability by controlling the leakage characteristics, capacitance density, trap formation mechanism and charge trapping behaviour at dielectric/semiconductor interface. This report illustrates the requirement of proper solvent selection to achieve high electrical performance as well as long term operational stability. Our study also demonstrates that low dipole moment solvents are more favourable in transistors for photo-detection applications. © 2017 Elsevier Ltd. All rights reserved.

Keywords: PMMA Solvent Polymer dielectrics OFET

1. Introduction Organic field effect transistors (OFETs) have drawn immense interest in last few decades due to their potential applications in transparent and flexible devices, complementary circuits, radio frequency identification tags (RFID) and various chemical and biological sensors [1e4]. Moreover, solution processed devices have achieved interesting technological attributes such as compatibility with simple direct-write printing techniques, roll-to-roll fabrication procedures, low cost manufacturing techniques and flexible substrates [5,6]. However, low charge carrier mobility and poor environmental as well as long term operational stability have been the major obstacles in the commercialization of OFET devices. Significant amount of progress has been made towards improving the charge carrier mobility and stability of the devices so that they can exhibit comparable results with their conventional inorganic counterparts. Electrical performance and stability of the devices depend on many important parameters such as molecular ordering and crystallinity of the active semiconducting layer, capacitance

* Corresponding author. E-mail address: [email protected] (A. Dhar). https://doi.org/10.1016/j.polymer.2017.09.065 0032-3861/© 2017 Elsevier Ltd. All rights reserved.

and leakage behaviour of the dielectrics and quality of the dielectric semiconductor interface. Improving capacitance density of the polymer dielectrics and thereby enhancing the device performance is one of the most well-known and facile route to obtain satisfactory outcome from the devices. The use of high k (dielectric constant) polymer dielectrics, crosslinking of the polymers to prepare ultra-thin insulating layer, use of high dipole moment solvents etc. are some of the very familiar strategies adopted by many research groups to enhance the capacitance density and hence to obtain superior device performances [7e12]. Although solvent dipole moments affecting the dipolar orientation and capacitance density of the polymer dielectrics is well reported in the literature [11,12], its effect on the configurational arrangements, free volume distribution, bulk hygroscopicity, and surface properties has not been widely studied. In this work, we have investigated the effect of solvent dipole moment on each of the above mentioned parameters of the polymer dielectrics and also performed an in-depth study to explore the role of these additional factors on the device performance and its degradation mechanism which were not explored before. Pentacene based organic field effect transistors with dual layer dielectric system of PVA (poly(vinyl alcohol)) and PMMA were

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D. Panigrahi et al. / Polymer 130 (2017) 79e87

fabricated by varying the solvents of PMMA, and their performances were compared. We have chosen the solvents, n-butyl acetate (BTA), dimethyl sulfoxide (DMSO) and propylene carbonate (PC) for the processing of PMMA dielectric to cover a wide range of dipole moments as well as dielectric constants of solvents for detailed and accurate analysis (Dipole Moment (D): BTA (1.84) < DMSO (3.96) < PC (4.94)). Impedance analysis was carried out to gauge the extent of variation in the chain packing density as well as the dipolar orientation of the PMMA polymer dielectrics by monitoring the dielectric properties and these results were correlated with the OFET device performances. Operational stability of the transistors was measured by systematically examining the bias stress instability effect and hysteresis behaviour. We observed significant amount of improvement in the device performance parameters upon the use of high dipole moment solvents, nevertheless in contrast to the previous reports, our results revealed that use of such solvents can play a detrimental role in the long term operational stability of the devices. Further insight into the underlying mechanism of charge trapping behaviour was sought by investigating the solvent dependent moisture uptake behaviour and surface characteristics of the PMMA films through Fourier transform infrared spectroscopy (FTIR), x-ray photoelectron spectroscopy (XPS) and energy dispersive x-ray spectroscopic (EDX) analysis. Another unique and important aspect of this study is the investigation on the influence of solvent dipole moments on the photo sensing behaviour of the transistors. We have investigated the photosensitivity of the devices as a function of solvent dipole moment and unveiled the effect of polymer chain packing arrangement on its photo sensing behaviour. 2. Experimental Pentacene based OFETs with bottom-gate and top contact configuration were fabricated on glass substrates which were cleaned sequentially by sonication in acetone, isopropyl alcohol, deionized water and dried in a nitrogen flow. The device structure is shown in Fig. 1. Aluminium (~80 nm) was thermally evaporated on the substrates in a vacuum pressure of 6 106 mbar through shadow mask as gate electrodes. PVA (MW: 31,000e50,000) was deposited by spin coating at 6000 rpm for 45 s from its aqueous solution (60 mg/ml) and annealed at 150 C for 2 h. Subsequently, PMMA (MW: 15,000) was spin coated on top of the PVA films from different solvents (BTA, DMSO and PC) (60 mg/ml) and baked at 120 C for 2 h. The spin coating process for casting the PMMA films

Fig. 1. Schematic of fabricated OFET devices.

from different dipole moment solvents was optimized by varying the spin speeds to maintain an approximately constant thickness (~590 ± 20 nm, Dektak profilometry measurement) of the PVA/ PMMA bilayer. Pentacene (~50 nm) was then thermally evaporated on PMMA as active channel layer. Consequently, Ag (~80 nm) source-drain electrodes were deposited by thermal evaporation at a base pressure of 5 106 mbar on top of the pentacene film. The channel length and width of the devices were 50 mm and 2 mm respectively. Electrical measurements of the OFETs were performed by Keithley 2450 programmable voltageecurrent source under ambient conditions. Metal-Insulator-Metal (MIM) devices with Al/ PVA-PMMA/Al configuration were fabricated for impedance and leakage characteristics measurement following the same procedures described above. For XPS, EDX and FTIR measurements PMMA films were prepared on glass substrates from different solvents. 3. Results and discussions Prior to measuring the electrical performances of the devices, morphological properties of pentacene and PMMA were investigated by AFM (atomic force microscopy) measurements (AFM 5500, Agilent Technology) as these parameters are of utmost importance in determining the device performance. All the PMMA films displayed smooth surface morphologies with r.m.s roughness values around 0.33 ± 0.02 nm (Fig. S1). Fig. 2 shows pentacene film morphologies grown on BTA, DMSO and PC dissolved PMMA films, respectively. In all the devices pentacene showed similar morphological characteristics with dendritic growth and closely packed structures. We observed that the pentacene grain sizes didn't vary significantly in all the three differently grown films which precluded the possibility of semiconductor morphology playing any role in electrical properties and charge carrier transport in the fabricated OFET devices. The crystallinity of pentacene films were further investigated by XRD technique (Fig. S2). The peak position and FWHM of the peaks didn't show any significant changes suggesting that the structural differences were very minimal. Fig. 3 (a) shows the capacitance density and leakage characteristics of various solvent casted PMMA films deposited on PVA layer. Capacitance density was found to be the maximum in PC devices whereas BTA devices exhibited poorest capacitive behaviour. It is well known that when PMMA is dissolved in a high dipole moment solvent, polymer chains remain in more elongated configuration in the solvent due to enhanced polymer-solvent interaction which consequently, increases the end to end distance of the polymer chains and promotes the orientation of the fuctional groups during spincoating and annealing. This proper orientation and well separation of the functional groups enhance the dipole moment of each of the polymer chains and hence improve the over all capacitance of the film [11e13]. On the other hand, in a low dipole moment solvent interchain interaction dominates over the polymer-solvent interaction, which leads to the occurrence of self aggregation of the polymer chains. As a result of such self association, functional groups can not be properly oriented and results in a poor capacitance density [14,15]. Fig. 3(b) shows the leakage characteristics as a function of applied dc bias for all the PVA/PMMA films. The devices with BTA dissolved PMMA shows maximum amount of electrical resistance and correspondingly minimum amount of leakage through the dielectric film. Such improvement in leakage current can be attributed to the less amount of free volume in PMMA when dissolved in low dipole moment solvent [16,17]. We speculate that the inter chain attraction of the polymer when dissolved in BTA, helps the polymer chains to be packed more tightly resulting in a more compact film formation which consecutively reduces the amount of free volume and minimizes the amount of leakage


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Fig. 2. AFM images of pentacene surface morphology grown on PMMA dissolved in (a) BTA (b) DMSO and (c) PC.

Fig. 3. (a) Variation of capacitance density with the variation of frequency (b) variation of leakage current density as a function of applied voltage and (c) variation of dielectric loss as a function of frequency for all the PVA-PMMA dual layer dielectrics. Schematic of the MIM devices are shown in the inset.

current. This solvent dependent stereostructural characteristics of the PMMA polymers are also replicated in the dielectric loss (tand) spectra (Fig. 3 (c)). When an ac electric field is applied, energy is lost through the generation of heat due to the movement of the fuctional groups and segmental motion of the polymer chains. PC devices exhibited maximum amount of loss as the oriented functional groups in PMMA can interact with the alternating electric field more promptly along with the hydroxyl groups of PVA and the presence of free volume allows more segmental motion of the polymer chains. In BTA and DMSO devices, due to close packing of the polymer chains, the motion of the polar functionalities may be restricted and results in a decrease in loss factor [18,19]. Notably, FTIR spectroscopic studies shown in Fig. S4 rule out any possible role of the residual solvents in observed variation of dielectric properties for the annealing conditions used in the study. Fig. 4 shows the gate voltage dependent transfer and output characteristics of all the transistors under light condition. Improvement in each of the performance parameters can be clearly observed in the high dipole moment solvent devices over the BTA dissolved PMMA devices. Comparison of all the parameters are shown in the bar plot below. On/Off ratio of the devices were found to increase with the increasing dipole moments of the solvents. The value of switching ratio increased by two orders in the PC devices compared to the BTA devices. Despite having high leakage current issues, DMSO and PC dissolved PMMA devices were able to show high on/off ratio due to the higher capacitance density of the dielectric layers which ensured better charge accumulation at the dielectric semiconductor interface and hence better gate voltage modulation properties. The effect of higher capacitance density was

also emulated in the lower threshold voltage (VTh) of the high dipole moment solvent devices. Threshold voltage reduced to 1.9 V and 2.7 V in PC and DMSO devices respectively while it was 5.3 V in BTA devices. Other important electrical parameters such as subthreshold swing (S.S.) and saturation hole mobility (m) were also calculated from the following relations,

S:S: ¼

dlogIDS dVG

1 (1)

!2

m¼

2L W

pffiffiffiffiffi d IDS dVG

Ci

(2)

where L is the channel length, W is the channel width, Ci is the capacitance per unit area of the dielectric layer. The value of subthreshold swing was found to decrease gradually with the increasing dipole moment of the solvents, with the value decreasing from 4.08 V/dec in case of BTA to 1.14 V/dec in the PC devices. Lower value of subthreshold swing indicates faster switching of the devices from “off” state to “on” state and efficient collection of the charge carriers at the drain electrode. Saturation hole mobility of all the devices pffiffiffiffiffiffiffiwere estimated by extracting the slope of the straight line in IDS vs. VG curve in the saturation region, where IDS ¼ (W/2L)mCi(VG e VT)2 [12]. The value of mobility was found to increase drastically from 0.01 cm2/V-s to 0.82 cm2/V-s in the transistors using high dipole moment solvents which

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Fig. 4. (a)e(c) Transfer characteristics of the OFET devices; (d)e(f) Output characteristics; (g)e(i) OFET device parameters for different solvents of PMMA.

consequently, resulted in an enhancement of output saturation current to 33 mA in the PC devices from 20 nA obtained in the BTA devices on the application of same gate and drain voltage. These results illustrate the efficacy of high dipole moment solvents in improving the device performance which is consistent with the earlier reports available in the literature [11,12]. Bias stress experiment was carried out to investigate the effect of solvent dipole moments on the long term operational stability of the devices under ambient condition. A constant bias at drain and gate electrode (VG ¼ VD ¼ 20 V) was applied for different time spans from 5 min to 20 min and transfer characteristics were measured quickly just after relaxation. Fig. 5 (a), (b) and (c) shows the device transfer characteristics for BTA, DMSO and PC devices respectively before and after the application of time dependent bias stress condition. Interestingly, the bias stress instabilities in the devices exhibited drastically different trends in contrast to their electrical performances. The effect of bias stress instability was found to be most pronounced in PC devices whereas BTA devices exhibited most stable electrical performance. We have calculated the values of saturation hole mobility from each of the transfer characteristics for all the fabricated devices and plotted the mobility values (m(t)/m(0)) as a function of stress time (Fig. 5 (d)). Mobility value decreased to 88% from its initial value in PC devices

whereas in BTA and DMSO devices mobility showed a decrement of 36% and 25% respectively from their initial values. This result indicates that albeit high dipole moment solvent PC can significantly improve the OFET device performance, it can also play an adverse role in the operational stability of the devices. We suspect that the solvent influenced alternation of the chain packing density and free volume distribution plays a vital role in determining the charge trapping behaviour at the dielectric semiconductor interface since such reduction in mobility is closely related to the trapping of the charge carries at the dielectric semiconductor interface [20,21]. Additional physical insights into the effect of solvent dipole moments on the bias stress stabilities of the OFET devices were sought by performing the drain current decay measurement as a function of time. Fig. 6 (a) shows the time dependent variation of IDS under constant bias stress (VG ¼ VD ¼ 20 V) for about 10 min under ambient condition. BTA and DMSO devices showed the typical drain current decay profile under bias stress condition whereas PC devices showed an anomalous behaviour with an initial increase of drain current followed by the decay. The drain current decayed nearly to 25% of their initial value ID (0) in BTA and DMSO devices in contrast to the PC devices where the decay was found to be most severe in the same time duration. Drain current in PC devices initially increased until a certain time and then decayed to


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Fig. 5. Transfer characteristics after the application of bias stress at different stress time for (a) BTA (b) DMSO and (c) PC devices; (d) Variation of saturation hole mobility as a function of stress time.

Fig. 6. (a) Variation of drain current under bias stress condition as a function of time and (b) estimated values of decay time of all the OFET devices.

46% of the highest value at the end of the measurement time. The drain current decay under bias stress condition occurs generally due to the trapping of charge carriers at the dielectricsemiconductor interface [22]. On the contrary, the observed enhancement of current with stress time in PC devices can be attributed to the slow polarization of dipoles present in the dielectric under applied bias stress [23,24]. We suspect that the slow polarization dipoles in PC devices originate from the absorbed environmental moisture present in the bulk of the PMMA film. The lower chain packing density and presence of large amount of free volume in PC dissolved PMMA dielectrics facilitate the penetration of absorbed water molecules from dielectric surface to the bulk in contrast to the BTA and DMSO dissolved PMMA films, which due to their higher packing density, disrupt such penetration [20]. Furthermore, the IDS (t) curves have been fitted with the following stretched exponential function to determine the characteristic decay time associated with the charge trapping.

" # b IDS ðtÞ ¼ I0 ð0Þexp t=t This model is extensively used to quantify the bias stress effect in both organic and inorganic OFET devices where I0 (0) is the initial drain current when t ¼ 0, b is the dispersion parameter whose value lies between 0 and 1 (0 < b < 1) and t is the characteristic decay time [22,24]. In PC devices, we didn't fit the initial rising part of the drain current as it is associated only with the charge induction due to polarization and doesn't have any significant influence on the characteristic decay time. Fig. 6 (b) shows the estimated decay time for all the devices. The values of decay times in BTA and DMSO devices were found to be one order higher than that of the PC devices. Such decrease in the decay time also indicates that the charge traps produced by air molecules formed much more readily in the PC devices reducing their operational stability compare to

84


BTA and DMSO dissolved PMMA devices. The corresponding values of dispersion parameters (b) which are related to the width of the exponential energy distribution of the trapping sites above the valence band edge increased systematically with solvent dipole moments from 0.47 in BTA and 0.56 in DMSO devices to 0.65 in PC devices. The quality of the dielectric semiconductor interface of the devices is further investigated by performing C-V hysteresis measurements at AC signal frequency of 100 kHz. Fig. 7 shows the C-V hysteresis characteristics of the fabricated devices. We observed minimal hysteresis behaviour in BTA and DMSO devices in contrast to the PC devices which shows maximum degree of hysteresis with a clockwise loop direction. Anticlockwise hysteresis loop direction in the BTA and DMSO devices is associated with the trapping of charge carriers at the dielectric semiconductor interface which generally originates from the structural defects of the semiconductor, PMMA surface functionalities and absorbed air molecules (water, oxygen etc.) at the interface [25]. On the contrary, clockwise hysteresis loop in PC devices may arise due to the following reasons, (1) slow polarization of dipoles present in the polymer dielectric and (2) charge injection from the gate electrode [24,26,27]. Clockwise hysteresis due to slow polarising dipoles occurs as follows (Fig. 7(d)). When gate voltage is swept from off to on state, these dipoles can readily respond to the applied field and get polarised in the polymer dielectric but while sweeping in the backword direction (from on to off) they remain polarised being unable to return to their initial state and results in an increase in capacitance density. This result is in consistence with the anomalous bias stress behaviour of the PC devices and supports our assumption of solvent dependent alternation of packing density and free volume distribution playing the critical role in controlling the absorption and transport of water molecules into the polymer film. Besides, higher rate of charge penetration into the polymer

dielectric bulk in case of PC devices due to their low packing density may also be responsible for this kind of hysteresis. Such hysteresis behaviour can prevent the use of PC devices in the practical applications such as in display backplanes or in logic circuitry where device characteristics must be stable. In order to understand the role of solvent induced surface chemical compositional characteristics in the trap formation mechanism, we have performed XPS measurements (PHI Versa Probe 5000 with a micro focused AlKa X-ray beam with beam energy 25 W, voltage 15 kV and beam diameter 100 mm) of the PMMA thin films spin coated from different solvents. Fig. 8 shows the O 1s XPS core level spectrum of PMMA. This O 1s peak was deconvoluted into two main peaks, one at lower binding energy (~532 eV) is due to the oxygen atom O1 of carbonyl group (C¼O) and another centred at higher binding energy (~534 eV) is associated with the oxygen atom O2 in the ester group (C-O-C) [28]. We have calculated the ratios of the areas under the deconvoluted curves (O2/O1) to estimate the surface polymer composition of various solvent casted PMMA films and observed an increasing trend of carbonyl groups (relative to ester groups) with the increase in solvent dipole moments. Iwamoto et al. previously reported carbonyl groups to be the only groups through which water can interact with PMMA [29]. So we assume that along with the stereo structural changes, such alternation of surface functionalities is also playing a decisive role in the charge trapping behaviour at the interface as these polar carbonyl groups can promptly interact with the ambient moisture and induce bias stress instabilities in the devices by drawing them to the dielectric semiconductor interface. Solvent induced compositional changes occurring in the PMMA films were mapped by SEM-EDX measurements (Zeiss Supra 40 field emission microscope) and analysed by calculating carbon to oxygen atomic ratios. The C/O atomic ratio is found to decrease with the increase in solvent dipole moments, with the ratio decreasing

Fig. 7. Hysteresis behaviour of (a) BTA (b) DMSO and (c) PC devices; (d) schematic of clockwise hysteresis caused by slow polarising dipoles in PC devices. In the schematic, the black, yellow and green boxes represent the gate electrode, dielectric layer and the active semiconducting layer respectively whereas the dipoles shown in the dielectric layer are the slow polarising dipoles. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)


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Fig. 10. FTIR spectra of all the PMMA films casted from BTA, DMSO and PC.

Fig. 8. High resolution O 1s core-level spectra for all the PMMA films casted from (a) BTA (b) DMSO and (c) PC. The O 1s main peak has been deconvoluted into O1 and O2, representing the oxygen of carbonyl group and ester group, respectively.

from 2.1 in case of BTA to 1.5 in PC dissolved PMMA films (Fig. 9). This result could have originated from the solvent dependent compositional properties of the PMMA films as observed in XPS analysis. On the other hand, the increment of oxygen atomic percentage in high dipole moment solvent devices may also arise from the excess amount of ambient water molecules drawn at the surface. Both of the possibilities discussed above are closely related to the charge trapping behaviour at the dielectric semiconductor interface and thus, can strongly exacerbate bias stress instabilities in case of devices using high dipole moment solvents. Moreover, fourier transform infrared spectroscopic studies (Nicolet 6700 FTIR, Thermo Fisher instrument) were carried out to monitor solvent dependent moisture uptake behaviour of the PMMA films. Fig. 10 shows the FTIR spectra of the PMMA films casted from different solvents (Full range spectra are shown in Fig. S3). Peak centred at 3440 cm1 can be assigned to the over tone of hydrated C¼O group. Peaks at 3550 cm1 and 3625 cm1 can be

attributed to the two particular modes of water eOH stretching, the hydrogen bonded O-H associated with a dimer and a free O-H bond respectively [30]. Many research groups have assigned these bands to symmetric and asymmetric O-H stretching of a single water molecule that is hydrogen bonded to two C¼O groups [29,31]. Both of the above mentioned assignments indicate the presence of water in the polymer either as dimers or monomers. In our case we can clearly observe that all these peaks begin to increase in intensity when high dipole moment solvents are used instead of low dipole moment solvent BTA. This result confirms our hypothesis of enhanced moisture absorption in high dipole moment solvent dissolved PMMA films and hence, can explain the aggravated bias stress instabilities in the high dipole moment solvent devices [32,33]. To investigate the effect of solvent dipole moments on the photo sensitivity of the transistors, transfer characteristics of the devices were measured under dark and light (white LED light) conditions. As depicted in Fig. 11 (a), drain current of the devices were found to increase under light due to the generation of photo induced charge carriers. When photons having energy equal or higher than pentacene band gap energy are absorbed, charge carriers are generated and started flowing towards drain electrode under drain to source bias voltage leading to an increase in the drain current [34]. Along with the increase in drain current, a shift in the transfer curves towards more positive voltage was also observed upon illumination. This photo induced shift of transfer curves can be attributed to the accumulation or trapping of the less mobile charge carriers (electrons in case of pentacene). When absorption occurs in the p type semiconducting layer, photo generated holes can easily move towards the drain electrode whereas the electrons due to their less mobility cannot flow through the semiconductor and accumulate at the channel region. These accumulated electrons can effectively

Fig. 9. Carbon to oxygen atomic percentages obtained from EDX mapping for all the PMMA films casted from (a) BTA (b) DMSO and (c) PC.

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Fig. 11. (a) Transfer characteristics of all the devices under dark and light conditions (b) light to dark ratio as a function of gate voltage for all the devices.

reduce the potential barrier between source and the active layer which consequently, reduces the threshold voltage of the devices and results in a shift of transfer curves towards positive voltage [35]. We have estimated photo sensitivity of the devices by calculating the light to dark ratio of the devices using the following relation,

P¼

Iph I Idark ¼ illumination Idark Idark

Where Iph is the photo current, Iillumination and Idark are is the current under light and dark condition, respectively and plotted this ratio as a function of gate voltage for all the devices. The value of this ratio is found to decrease with the increase of solvent dipole moment, with the value of 375 in case of BTA to 2.5 in case of PC devices. Such remarkable changes in light to dark ratios can be ascribed to the solvent dependent stereostructure and insulation properties of the PMMA films. Poor insulation properties of the high dipole moment solvent devices lead to higher off current which is not desirable for attaining high sensitivity whereas less free volume of BTA devices results in lower current in “off” state which is favourable for obtaining high photosensitivity from the transistors. The maximum on/off ratio appears in the “off” state of the device because the conductivity in “on” state is mainly dominated by the field induced charge carriers in contrast to the “off” state when the device remains charge depleted and only the photo current contributes in the enhancement of drain current [36,37]. We confirmed that the absorbance of pentacene didn't alter when deposited on different solvent casted PMMA films and hence, didn't influence the photo sensitivity of the devices (Fig. S5). 4. Conclusion In conclusion, we have thoroughly investigated the effect of solvent-induced dielectric property changes of PMMA polymer on the electrical properties and operational stability behaviour of the OFET devices fabricated with pentacene semiconducting channel. The electrical performance as well as the mechanism of trap formation and charge trapping behaviour of the transistors showed a strong correlation with the solvent dependent chain arrangements in the dielectric polymer, dipolar orientations and surface chemical compositional characteristics of the PMMA dielectrics. Our investigation finds that the high dipole moment solvents like PC play a crucial role in enhancing the OFET performance parameters by altering the dielectric properties at a molecular level; however the operational stability of the devices is subdued. In contrary, low dipolar solvents like BTA show excellent stability in ambient atmosphere; however, their low performance parameters become a

major bottleneck for commercialization purposes. This study thus, reveals the importance of proper optimization of solvent dipole moment so that optimum device performance can be achieved without compromising with the environmental stability. In our case, DMSO was found to be the most efficient solvent of PMMA for obtaining desirable device performances along with excellent environmental stability in ambient condition. Moreover, our study also demonstrates the efficacy of low dipole moment solvent BTA for obtaining high photosensitivity from the OFET devices. Therefore, in contrast to the existing notion that high dipole moment solvents are always favourable for fabricating high performance OFET devices, our study emphasizes on the necessity of proper solvent selection for the processing of polymer dielectrics depending on the device applications. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.polymer.2017.09.065. Reference [1] P. Lin, F. Yan, Adv. Mater. 24 (2012) 34. [2] R. Capelli, S. Toffanin, G. Generali, H. Usta, A. Facchetti, M. Muccini, Nat. Mater. 9 (2010) 496. [3] C. Tanase, J.C. Hummelen, P.W.M. Blom, Appl. Phys. Lett. 18 (2004) 4205. [4] O. Knopfmacher, M.L. Hammock, A.L. Appleton, G. Schwartz, J. Mei, T. Lei, J. Pei, Z. Bao, Nat. Commun. 5 (2014) 2954. [5] B. Kang, W.H. Lee, K. Cho, ACS Appl. Mater. Interfaces 5 (2013) 2302. [6] Y.J. Kwon, Y.D. Park, W.H. Lee, Materials 9 (2016) 650. [7] K. Müller, I. Paloumpa, K. Henkel, D. Schmeisser, J. Appl. Phys. 98 (2005) 056104. [8] Y.R. Su, W.G. Xie, Y. Li, Y. Shi, N. Zhao, J.B. Xu, J. Phys. D. Appl. Phys. 46 (2013) 095105. [9] X. Cheng, M. Caironi, Y.-Y. Noh, J. Wang, C. Newman, H. Yan, A. Facchetti, H. Sirringhaus, Chem. Mater. 22 (2010) 1559. [10] S.Y. Yang, S.H. Kim, K. Shin, H. Jeon, C.E. Park, Appl. Phys. Lett. 88 (2006) 173507. [11] N.B. Ukah, S.P. Senanayak, D. Adil, G. Knotts, J. Granstrom, K.S. Narayan, S. Guha, J. Polym. Sci. Part B Polym. Phys. 51 (2013) 1533. [12] N.B. Ukah, J. Granstrom, R.R. Sanganna Gari, G.M. King, S. Guha, Appl. Phys. Lett. 99 (2011) 243302. [13] K.-L. Tung, K.-T. Lu, R.-C. Ruaan, J.-Y. Lai, Desalination 192 (2006) 380. [14] S. Bistac, J. Schultz, Prog. Org. Coat. 31 (1997) 347. [15] S. Bistac, J. Schultz, Int. J. Adhes. Adhes. 17 (1997) 197. [16] K.-L. Tung, K.-T. Lu, R.-C. Ruaan, J.-Y. Lai, Desalination 192 (2006) 391. [17] M.P. Chenar, H. Rajabi, M. Pakizeh, M. Sadeghi, A. Bolverdi, J. Polym. Res. 20 (2013) 216. [18] L. Zhu, J. Phys. Chem. Lett. 5 (2014) 3677. [19] Z. Ahmad, Polymer Dielectric Materials, Dielectric Material, Dr. Marius Alexandru Silaghi InTech, 2012. [20] J. Lee, H. Min, N. Park, H. Jeong, S. Han, S.H. Kim, H.S. Lee, ACS Appl. Mater. Interfaces 7 (2015) 25045. [21] S.J. Fakher, A.K. Hassan, M.F. Mabrook, Synth. Met. 191 (2014) 53. [22] S.M. Obaidullaa, P.K. Giri, J. Mater. Chem. C 3 (2015) 7118. [23] S. Vasimalla, N.V.V. Subbarao, P.K. Iyer, J. Mater. Chem. C 4 (2016) 7102.

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