Organic Electronics 14 (2013) 1663–1672

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Organic Electronics

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Morphology optimization for achieving air stable and highperformance organic field effect transistors

1566-1199/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.orgel.2013.03.027

⇑ Corresponding author. Tel.: +91 1126738771.E-mail addresses: subhasis@mail.jnu.ac.in, ghosh.subhasis@gmail.com

(S. Ghosh).

Pramod Kumar, Akanksha Sharma, Sarita Yadav, Subhasis Ghosh ⇑School of Physical Sciences, Jawaharlal Nehru University, New Delhi 110 067, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 23 August 2012Received in revised form 19 March 2013Accepted 19 March 2013Available online 6 April 2013

Keywords:Small moleculeMorphologyOTFT

To develop an all organic active matrix light emitting display required for large area thindisplay, electronic paper and electronic paints, Si-based thin film transistor has to bereplaced with organic thin film transistor (OTFT). The most important issues in OTFT arethe low charge carrier mobility and poor stability under ambient conditions, which criti-cally depend on how organic thin films are grown on different substrates. Here we showthat both these issues are correlated and can be overcome by certain surface morphologywhich can only be achieved through anisotropic growth. Careful control of different growthparameters can lead to unprecedented control on thin film morphology which has beenshown to be engineered reversibly and reproducibly. High temperature and low evapora-tion rate increase the diffusive mobility of molecules, which are responsible for the stack-ing of molecules to higher length scales. By carefully choosing a temperature andevaporation rate, elongated rod-like grains were grown for achieving high performanceand stable thin film transistors.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

The p-conjugated small organic molecules have beensuccessfully used as active layers in organic light emittingdiodes, organic thin film transistors (OTFTs) and organicsolar cells. As organic materials in all these devices are sup-ported by substrate, so the preferred orientation of themolecules giving rise to particular morphology [1–3] hasstrong influence on the device performance. Though, it isbeing realized that the optimization of growth processfor getting specific thin film morphology is extremelyimportant to achieve high performance devices in termsof charge carrier mobility and stability, but compared tothe large number of studies directed towards how to in-crease carrier mobility by engineering organic/gate dielec-tric interface [4–6], few investigations [7,8] have beendirected towards how to improve the carrier mobility

and stability of devices intrinsically. This is highly desir-able since exact role of extrinsic parameters on the perfor-mance of organic molecule based devices will bemisunderstood and misguided, unless the role of intrinsicparameters are understood and optimized. M}ullen et al.[8] have shown that polymers which is generally a lowmobility materials, can be designed in such a way to havecertain thin film morphology with very high carrier mobil-ity. Poor charge carrier mobility in organic materials is themajor impediment for the development of high perfor-mance OTFT. Though organic single crystal due to theirefficient p–p stacking, as in rubrene single crystal, canshow a maximum charge carrier mobility of 40 cm2/V s[9], but several problems, such as, poor device integration,cross-talk between devices, and low mechanical flexibilitymake organic single crystal unsuitable for large area dis-play. There are two main reasons for low charge carriermobility in OTFT based on polycrystalline organic thinfilms. Firstly, the p-conduction network is not extendedbetween source and drain in OTFT. Secondly, the presenceof multiple transport barriers for charge carriers due to

Gate

Drain

SiO2

n++Si

A

Source

Gate

DrainA

Source

n++SiSiO2

a bFig. 1. Schematic representation of charge carrier transport between source and drain in OTFT based on CuPc (a) polycrystalline thin film grown at roomtemperature, a large no of barriers at the interface between isotropic spherical grains exists in the current conduction path between source and drain, asshown by multiple curly arrows; (b) polycrystalline anisotropic thin film achieved at higher growth temperature (�100 �C), in this case rod like elongatedgrains provide very few soft grain boundaries in the current conduction path between source to drain, as shown by curly arrow.

a b c dFig. 2. AFM topographic images (1 lm � 1 lm) of CuPc thin films deposited on SiO2 (column a and b) and Si (column c and d) substrates with variablethickness at different substrate temperatures and at constant deposition rate 0.1 Å/s. The elongation in grains is more in case of films grown on H-passivated Si-substrate.

1664 P. Kumar et al. / Organic Electronics 14 (2013) 1663–1672

30oC 60oC

90oC105oC

120oC 135oC

Fig. 3. AFM topographic images of 1 lm � 1 lm of CuPc thin films grown at different growth temperatures with a fixed evaporation rate of 0.1 Å/s.

P. Kumar et al. / Organic Electronics 14 (2013) 1663–1672 1665

large numbers of grain boundaries between source anddrain in OTFT, as schematically shown in Fig. 1. The grainboundaries are also responsible for the temporal instabilityof the devices under ambient conditions. It has been shown[10–17] that grain boundaries limit the performance of de-vices based on small molecules like, pentacene and oligoth-iophenes. In particular, Salleo et al. [14,15] have shown thatdifferent orientations of grains in thin films modulate thecharge carrier mobility by two orders of magnitude in n-typesmall molecule organic semiconductors. Moreover, theOTFTs fabricated on polycrystalline thin film show poorreproducibility, as the performance varies from device to de-vice due to random behavior of grain boundaries. Frisbieet al. [16,17] have shown that voltage drop across grainboundaries is substantial due to spatially sharp interface be-tween grains, which result large variation of charge carriermobility and other transport properties. Mainly there aretwo types of grain boundaries [10–17] viz. soft and sharp,as shown schematically in Fig. 1. The soft grain boundariesmarginally modify the p–p stacking and hence do not affectthe charge carrier transport substantially, whereas sharpgrain boundaries are characterized with abrupt collapse ofthe p-conduction network.

The morphology of the organic thin film plays impor-tant role on the performance of the devices. Compared toinorganic counterpart, it is difficult to control the morphol-ogy of organic thin films due to the intrinsic anisotropy in

molecular structure, crystal packing, and interfacial inter-actions. The highly anisotropic growth of organic thin filmis often governed by kinetic processes, rather than by ther-modynamics [18]. It has been shown that the growth ofpentacene thin film is governed by interfacial interaction[19] and molecular anisotropy [20] and overall growth isgoverned by kinetic processes. Furthermore, the chargecarrier mobility in organic materials is highly anisotropic,the difference between in and out of plane mobilities canbe as high as �105 in organic molecular semiconductorand �102 in organic polymer [21]. These problems can becircumvented by, (i) reducing the number of grain bound-aries between source and drain in OTFT and (ii) making thealignment and orientation of grains to maximize the p–poverlapping along the direction of current flow. Here, weshow that this can be achieved in certain morphology oforganic thin films using intrinsic anisotropy in organic sys-tems. The anisotropic growth, which leads to certain thinfilm morphology with (i) rod-like elongated grains, (ii) bet-ter p–p stacking, (iii) connected with soft grain boundaries,and (iv) higher crystallinity, results better charge carriermobility and stability in OTFT, as schematically shown inFig. 1. Further, it has been shown how anisotropic growthcan lead to high performance OTFTs without modifyingthe interface and any other extrinsic parameters.

Whenever a molecule comes close to the substrate itgets physisorbed or chemisorbed depending upon the

0.1Å/sec 0.5Å/sec

1Å/sec 5Å/sec

10Å/sec20Å/sec

Fig. 4. AFM topographic images of 1 lm � 1 lm of CuPc thin films grown at different deposition rates at a fixed growth temperature of 105 �C.

1666 P. Kumar et al. / Organic Electronics 14 (2013) 1663–1672

interaction between substrate and deposit. For nucleation,the surface energy of the deposit should be high comparedto that of the substrate while vice versa is true for mono-layer [22]. If the deposit is highly strained, it leads to defec-tive monolayer which serves as nucleation site resulting inlayer plus island growth. This is the most suitable growthmode for growing different anisotropic topological struc-tures including organic based nanostructures. The dynam-ics of layer plus island growth is determined by thediffusion coefficient, D = Doexp(�Ea/kBT) of the moleculeson the substrate, and flux, F of the evaporated molecules[23]. The prefactor, D0 and diffusion energy barrier, Ea de-pend on the molecule–substrate interaction. The growthtemperature, TG determines the kinetic energy of theadsorbates to overcome the diffusion barrier and the depo-sition rate F determines the density of molecules diffusingon the substrate. Hence, the engineering of thin filmgrowth requires appropriate balancing between diffusionand deposition flux to optimize the growth process to ob-tain desired film morphology. At relatively low TG whenthe distance between adsorbed molecules is less than intermolecular spacing, the diffusion of molecules reduces andthe island mobility is too small to allow the island migra-

tion and/or rotation, and this result into randomly orientedgrainy structures separated by sharp grain boundaries. Inthis way nucleation growth gives rise to isotropic grainsof organic molecules resulting polycrystalline thin film.The main effect of increasing TG is to increase the diffusionlength of molecules prior to their burial under the arrivingflux of molecules enhancing the migration and rotation ofgrains or clusters. This leads to coalescence of grains orclusters that have the lowest surface energy parallel tothe substrate surface, resulting anisotropic growth charac-terized with improved crystallinity and less grain bound-aries. Hence, by varying three parameters, substrate-molecule interaction, TG and F appropriately, thin filmswith different morphologies can be achieved.

2. Results and discussions

In case of thin films grown under nucleation favoredconditions (at low TG), carriers encounter large number ofbarriers while going from source to drain, as shown inFig. 1a. In case of anisotropic growth, carriers encounterfewer grain boundaries while going from source to drain

0

1x104

2x104

3x104

6.0 6.5 7.0 7.5 8.0

0.1Å/sec0.5Å/sec1.0Å/sec10 Å/sec

2θ (o)

Inte

nsity

(a.u

)

Inte

nsity

(a. u

.)

5 6 7 8 9 10 3060

90 120

105oCa

b

Fig. 5. (a) X-ray diffraction results for CuPc thin films grown at differenttemperatures with a fixed evaporation rate of 0.1 Å/s using Cu Ka line.The intensity of the XRD peak shows nonmonotonic dependence peakingat TG = 105 �C. (b) XRD of CuPc thin films grown with different evapora-tion rate at a fixed growth temperature 105 �C. The intensity of the XRDpeak is the maximum at the lowest evaporation rate (0.1 Å/s).

P. Kumar et al. / Organic Electronics 14 (2013) 1663–1672 1667

in OTFT. As shown schematically in Fig. 1b, the OTFT fabri-cated on anisotropic thin film would be most desirable toachieve high performance OTFT. We have chosen copperphthalocyanine (CuPc) for its exceptional thermal andchemical stability as well as intrinsic anisotropy in itsmolecular structure. CuPc is a macrocyclic metal complexconsisting of a fourfold p-conjugated ring in which fouriso-indoline groups are bounded by aza-nitrozen. Like allother small organic molecules, CuPc molecules are bondedby weak van-der Waals forces in thin films. Here, littlemore elaboration is required regarding the particularchoice of organic molecule for this investigation. In viewof the main goal of this investigation which is to engineera certain morphology of thin film to be used as active layerfor OTFT, the planar organic molecules that have twodimensional extended aromatic system are required. CuPcis an ideal choice for this. For example, it is almost impos-sible to achieve the desired morphology with anisotropicstructures using linear molecule, like pentacene or anyother acences.

In order to understand the effect of surface interaction,which affects the diffusion constant and orientation of the

molecules, we have compared the surface morphologies ofCuPc thin films grown on hydrogen passivated Si which is ahydrophobic surface and on SiO2 which is a hydrophilicsurface. Fig. 2 shows the AFM topographic images whichreveal the evolution of morphology of CuPc thin filmsdeposited on SiO2 and Si substrates at different substratetemperatures and at constant deposition rate of 0.1 Å/s. Itis observed that anisotropic growth could be initiated evenat a very low thickness and serve as a template for the sub-sequent growth. The origin behind more elongation ofgrains at lower temperature and at lower thickness forthe Si surface compared to those on SiO2 surface lies withthe difference of molecule–substrate interaction due tohydrophobicity of passivated Si surface. In case of Si, mol-ecule–substrate interaction is weak compared to that incase of SiO2 surface, so the physisorbed molecules on Silead to elongated structures even at room temperaturedue to higher surface mobility. However, in the case ofhydrophilic SiO2 surface, few more monolayers are re-quired to suppress the substrate interaction and henceelongation is observed at higher substrate temperatureand higher film thickness.

Fig. 3 shows the AFM images of CuPc thin films grownat different TG (30–135 �C) with a fixed evaporation rateof 0.1 Å/s on SiO2 surfaces. These images show how themorphology evolves with TG. The grain size increases withgradual transition from isotropic to aniostropic morphol-ogy at higher TG. The CuPc films deposited at room temper-ature shows nucleation growth having nearly uniformdistribution of grains. The grains in films deposited at105 �C show elongation with larger dimensions, so as theTG increases, grains transform to elongated rod-like lamel-lae. This is due to the fact that, at higher temperature,nucleation rate decreases and rate of lateral growth in-creases due to higher mobility of the adsorbed moleculesand preferred unidirectional attachment of the molecules.This preferential growth is along the p–p interactions inthe facets of adjacent molecules and gives rise to largersticking coefficient along this direction. These conditions(higher TG and low F) lead to anisotropic rod-like grainswith large lateral dimension. Further increase in TG leadsto change in van der Waals interaction between molecules,causing phase changes [24]. TG = 90 �C shows more grainalignment in the form of network over whole surface. AtTG = 120 �C, the interconnectivity between grains is sub-stantially reduced resulting elongation with sharp grainboundaries, which make charge carrier migration difficultfrom one grain to another. Fig. 4 shows the AFM imagesof CuPc thin films grown at different evaporation rates(0.1–20 Å/s) at a fixed growth temperature of 105 �C. It isclear that evolution of anisotropic growth to isotropicgrowth is completely reversible. At high TG, as the evapora-tion rate increases, in spite of higher kinetic energy of mol-ecules, nucleation rate increases due to higher flux ofincoming molecules giving rise to polycrystalline isotropicgrainy morphology at highest evaporation rate of 20 Å/s,similar to what is observed (Fig. 3) in thin film morphologyachieved with low evaporation rate (0.1 Å/s) at low TG. Inview of the comprehensive data presented in Figs. 3 and4, it could be concluded that the morphology in CuPc thinfilms can be engineered reversibly and reproducibly by

-15

-10

-5

0

-60 -40 -20 0First Day

105oC

VDS (V)

I DS

(μA

)

-15

-10

-5

0

-60 -40 -20 0After One Year

105oC

VDS (V)

I DS

(μA

)

-1.0

-0.6

-0.2

-60 -40 -20 0

30oC

VDS (V)

I DS

(μA

)

-1.0

-0.6

-0.2

-60 -40 -20 0

First Day30oC

VDS (V)

I DS

(μA

)

a b

c d

e f

After One Year

Fig. 6. The dependence of source to drain current (IDS) on applied bias between source to drain (VDS) at different gate voltage (VG) of OTFTs fabricated on thinfilms grown (a and b) at TG = 30 �C and (c and d) at TG = 105 �C. (a and c) show the output characteristics of CuPc OTFTs grown at TG = 30 �C and TG = 105 �C,respectively with gate voltages varying from 0 V to �50 V at a step of �10 V on the first day. (b and d) show the output characteristics of above mentioneddevices after one year. The current and mobility decrease by an order of magnitude, subthreshold swing doubles in the devices with TG = 30 �C after 1 year,however device grown at TG = 105 �C is marginally affected over time and show only a marginal increase in subthreshold swing. (e and f) schematicallyshows the presence and absence of voids is sharp and soft grain boundaries, respectively. Atmospheric gases and unwanted analytes diffuse into relativelylarge number of voids between sharp gain boundaries.

1668 P. Kumar et al. / Organic Electronics 14 (2013) 1663–1672

controlling growth parameters. The crystalline nature ofCuPc thin films grown at different substrate temperaturesis determined by XRD, as shown in Fig. 5. The XRD datashow strong peak orientation of [200] plane with a diffrac-tion peak at 2h � 6.8�. This is due to a-phase monoclinicstructure, which is known as herringbone structure alongthe b-axis [25,26]. It is interesting to note that the 2h peakintensity shows nonmonotonic dependence on TG, peakingat 105 �C. Fig. 5 also shows the decrease in XRD intensityand increase in width of X-ray peak with evaporation rate.The main effect of lower deposition rate is to increase thediffusion length of molecules prior to their burial underthe arriving flux of molecules, so that incoming moleculesget enough time to find out preferred orientation resultinglimited number of structural disorder and better crystalin-ity. The optimum TG, at which anisotropic growth withhigher crystallinity is obtained for particular molecule, de-

pends on the molecular cohesive energy (Ecoh) of the organ-ic molecular crystal. Ecoh is the energy required for briningisolated molecules together to form a crystal and is typi-cally around 1 eV per molecule, whereas the same for inor-ganic crystal is around 5 eV per atom. This is the reasonwhy enhanced crystallinity in organic thin film has beenobserved at 100 �C, compared to typically 500 �C in caseof inorganic thin films.

Generally, the performance of OTFT is determined byfive important parameters, (i) mobility l, which is ‘‘figureof merit’’ of thin film and thin film based transport devices,(ii) threshold voltage Vth, which is required to inducecharge for complete filling of traps in thin film and/or or-ganic/dielectric interface, (iii) subthreshold swing S, whichis the measure of how easy or difficult to switch a transi-tion from off-state to on-state, and (iv) on–off ratio, whichis the measure of switching efficiency. In addition to these

101

102

103

104

105

0 50 100TG(oC)

ON

-OFF

ratio

10-3

10-2

10-1

0 50 100

TG(oC)

μ(c

m2 /V

s)0

25

50

0 50 100TG (oC)

-VT

(V)

2

5

8

0 50 100

TG (oC)S

(V/d

ec)

Fig. 7. Characterization of CuPc thin film OTFTs. Dependence of (a) threshold voltage (Vth), (b) on–off ratio, (c) charge carrier mobility (l) in the linear regionof output characteristics (IDS � VDS), and (d) subthreshold swing (S) on growth temperature TG of the CuPc thin films. Connecting lines are guides for eyes.Solid symbols show values of the different parameters of the best device out of those devices fabricated on thin film grown at TG = 105oC.

P. Kumar et al. / Organic Electronics 14 (2013) 1663–1672 1669

four parameters, it is essential that the stability of OTFTs,in terms of temporal dependence of the performance,should be high enough to work under realistic environ-mental and electrical operational conditions [27]. The envi-ronment stability is major concern for pentacene [28] andpoly (3-hexythiophene)-2,5diyl [29] based OTFTs, whichsuffer degradation and instabilities when exposed to atmo-sphere. One of the main reasons for this instability is pres-ence of high density of voids at the grain boundaries, whichfacilitate the diffusion of atmospheric gases inside organicmaterials. Fig. 6 summarizes the effect of environment andoperational stress on OTFT performance. Fig. 6a and c showthe output characteristics of OTFTs fabricated on CuPc thinfilms grown at 30 �C and 105 �C, respectively, obtained onfirst day and Fig. 6b and d show output characteristics ob-tained after 1 year for the same devices. The prolongedexposure to the environment over a long period has mar-ginal effect on devices fabricated on thin films grown at105 �C. There is a marginal increase in saturation current.The devices fabricated on thin films grown at 30 �C showa notable degradation over time. Over a year, the mobility,l � 5 � 10�4 cm2/V s and the maximum saturation currentat VG = �50 V, IDS � 1.0 lA were lowered by an order ofmagnitude to 2 � 10�5 cm2/V s and 0.4 lA, respectively.As discussed before, the reason behind this degradationmay be due to large number of voids at the sharp grainboundaries in thin films grown at 30 �C, whereas numberof voids in thin film grown at 105 �C substantially reducedresulting diffusion of unwanted analytes difficult into theconduction pathways, as shown schematically in Fig. 6.Hence, TG plays an extremely important role on device sta-bility. As TG increases, IDS increases and reaches a maxi-

mum at 105 �C and then decreases. We have also variedevaporation rate from 0.1 Å/s to 10 Å/s and observed thatbest devices can be achieved with lowest evaporation rateof 0.1 Å/s. Fig. 7 summarizes the effect of TG on OTFT per-formance. The l, Vth, S and on–off ratio obtained at differ-ent TG are shown in Fig. 7. The values different parametersof the best OTFT fabricated on CuPc thin film grown at105 �C with a deposition rate of 0.1 Å/s are representedby solid symbols. We have obtained the room temperaturel of 0.12 cm2/V s, Vth of 0.9 V, on–off ratio of 5 � 104 and Sof 1.1 V/decade in the best performing device. After 1 year,except l which became half (�0.06 cm2/V s) of what wasobserved on 1st day, others parameters remained moreor less unchanged. In OTFT, the charge carrier conductiontakes place within few monolayers adjacent to the dielec-tric, but AFM only provides the surface morphology of thinfilms. To ensure the similar anisotropic unidirectionalgrowth starts from few monolayers from the substrate sur-face, the AFM morphology of very low thickness CuPc filmshave been studied. We have observed that at highergrowth temperature even very low thickness of 1 nm filmmorphology shows elongated rod-like structures, whichserve as templates for subsequent layer by layer growth.The performance of the OTFTs depend nonmonotically onTG (as summarized in Fig. 7). By comparing the crystallinenature of CuPc thin films with different surface morphol-ogy, grown at different TG and the output characteristicsof OTFTs, it can be concluded that highest performanceOTFT could be obtained on CuPc thin films grown ataround 100 �C. At 120 �C, the correlations among the de-crease of XRD peak intensity, drain current and carriermobility could be attributed to the transformation to other

a

b

-17

-14

-11

-8

-5

3 5 7 9

VGS = -20 VVGS = -50 V

TG=30oC

1000/T

-15

-12

-9

-6

-3

3 5 7 9

VGS = -50 VVGS = -20 V

TG=105oC

1000/T

-25

-20

-15

-10

-5

3 5 7 9

VG=-50VVG=-20V

TG=120oC

1000/T

c

( μ)

ln ( μ

)ln

( μ)

ln

Fig. 8. Arrhenius plot of the dependence of mobility l on temperature obtained from IDS � VDS characteristics in the linear region. (a) There are two slopes inOTFT fabricated CuPc thin film at 30 �C. The activation energies are 0.22 eV and 0.16 eV in high temperature region (300–200 K) and are 0.09 eV and 0.08 eVin low temperature region (200–120 K), when VGS are �20 V and �50 V, respectively. (b) A single slope has been observed for the films grown at 105 �C. Theactivation energies are 0.1 eV and 0.15 eV, when VGs are �20 V and �50 V, respectively, over the whole temperature range of 300–120 K. (c) There are twoslopes in OTFT fabricated on CuPc thin film at 120 �C. The activation energies are 0.34 eV and 0.22 eV in high temperature region (300–200 K), and 0.14 eVand 0.12 eV in the low temperature region (200–120 K), when VGS are �20 V and �50 V, respectively. The right side figures show the schematicrepresentation of the alignment of grain boundaries for three different devices in which the organic layers are grown at different substrate temperatures.

1670 P. Kumar et al. / Organic Electronics 14 (2013) 1663–1672

phase [32] resulting a mixed phase and misorientation ofthe anisotropic structures at higher TG(>105 �C). A com-plete transformation from a-phase to b-phase happens at320 �C [30].

The highest mobility observed in thin films grown athigher temperature is observed due to two reasons, firstly:anisotropic growth provides better crystallinity as evidentfrom X-ray data and secondly: continuous conductionpathways along rod-like elongated grains between source

and drain in OTFT, giving rise to better p–p overlap withreduced effect of grain boundaries [13,14]. The effect ofgrain boundaries on carrier transport is clearly evidentfrom the comparison of temperature dependent l (Fig. 8)in three OTFTs, fabricated on thin films grown at (i)30 �C, (ii) 105 �C and (iii) 120 �C. It is clear from Fig. 8 thatthe Arrhenius plots of temperature dependence of l (lnlvs. 1/T) in OTFTs (i) and (iii) have two slops indicatingtwo different transport processes whereas Arrhenius plot

Table 1Activation energies calculated from temperature dependence of mobility inthree OTFTs, fabricated on thin films grown at (i) 30 �C, (ii) 105 �C and (iii)120 �C.

VG = �20 V VG = �50 V

300–120 K 300–120 K

300–200 K

200 K-120 K

300–200 K

200–120 K

Tsub = 30 �C 0.22 eV 0.09 eV 0.16 eV 0.07 eVTsub = 105 �C 0.15 eV 0.1 eVTsub = 120 �C 0.34 eV 0.14 eV 0.22 eV 0.11 eV

P. Kumar et al. / Organic Electronics 14 (2013) 1663–1672 1671

for OTFT (ii) shows single slope indicating single transportprocess over entire range of temperature. It has beenshown [31] the activation energy at higher temperatureis due to hopping transport and the activation energy atlower temperature is due to grain boundary betweeneither two grains, or lamella. Similar grain boundary effectwas previously reported in polycrystalline silicon films[32]. The activation energies calculated from the Arrheniusplots given in Fig. 8 have been summarized in Table 1. Thesingle activation energy over the entire range of tempera-ture (300–120 K) obtained in case of OTFT in which organicthin films were grown at TG = 105 �C from Arrhenius plot ofmobility vs. temperature, shown in Fig. 8b, provides an evi-dence for absence of barrier due to sharp grain boundariesand in this case, conduction mechanism is predominantlygoverned by hopping. The high activation energies ob-tained from the temperature dependence of field effectmobility in OTFT in which thin films were grown 30 �Cand 120 �C can be attributed to the barriers due to sharpgrain boundaries along the conduction path from sourceto drain in OTFT. The decrease in activation energy withgate bias in case of device with TG = 105 �C over wholetemperature range and the devices with TG = 30 �C andTG = 120 �C in the high temperature region is due to the risein Fermi level with increasing carrier density. The rise inFermi level leads to lowering of hopping barrier to trans-port level thereby decreasing the activation energy [33].The activation energies in the low temperature region forthe devices with TG = 30 �C and 120 �C are lower than thosein devices with TG = 105 �C. It has been shown that this isdue to the presence of low angle grain boundaries[13,14,31] in thin film grown at 105 �C and high anglegrain boundaries in thin films grown at 30 �C and 120 �C.The decrease of activation energies due to barriers at grainboundary with increase in gate bias is caused by thescreening of grain boundary interfaces with carriers avail-able at higher gate bias.

3. Conclusions

In this paper, we have shown that the growth of smallmolecule organic thin film by vacuum evaporation can leadto unprecedented control over the morphology of the thinfilms by varying different growth conditions. We haveshown that morphology of CuPc thin films on SiO2 surfacescan be engineered reversibly and reproducibly by tuning

certain growth parameters. If the appropriate growth con-ditions are chosen, unique morphology whose characteris-tics depend on the detailed balance between phase growth,growth kinetics and substrate film interactions, can beachieved. The OTFTs fabricated on these organic thin filmswith rod like elongated grains connected with soft grainboundaries result enhanced field effect mobility, high on–off ratio, low threshold voltage and subthreshold swingand higher stability. The engineering of thin film morphol-ogy achieved by anisotropic growth help in overcomingthree intrinsic problems, (i) limited p-conduction network,(ii) multiple transport barriers for charge conduction be-tween source and drain in OTFT and (iii) stability of organicthin films based devices.

4. Experimental section

4.1. Sample preparation and characterization

Triple sublimed high purity (>99.999%) CuPc was ob-tained from Sigma Aldrich Company, USA. Thin films weregrown by oil free vacuum deposition system under a basepressure of 2 � 10�6 Torr with evaporation rate varyingfrom 0.1 Å/s to 20 Å/s. CuPc thin films were grown on dif-ferent substrates, such as quartz, thermally grown SiO2 onhighly doped n-type silicon and gold coated glass. The sub-strate temperature was varied from 30 �C to 135 �C. Thethin film morphology was analyzed using Park SystemsXE70 atomic force microscope (AFM). The thickness ofthe organic layers was measured in situ by quartz crystalthickness monitor and ex situ by AFM. Crystalline natureof the thin films has been characterized by XRD in thereflecting mode at grazing angle using Bruker X-ray dif-fractometer. Simple OTFT test structures with CuPc as ac-tive layer have been fabricated with top contactconfiguration. CuPc thin film of 100 nm was evaporatedon n++ Si/SiO2 substrate. n++ Si side was coated with alu-minum and used as gate. Gold source and drain contactswere deposited on the CuPc thin films. The transistor char-acteristics were measured at room temperature usingKeithley picoammeter and Keithley/Agilent voltagesources.

4.2. Device characterization

CuPc is a p-type organic molecular semiconductor, sowhenever the negative gate voltage (VG) is sufficient toaccumulate the charge carriers close to dielectric surface,a p-type channel is formed at dielectric/organic interface,and this value of VG is defined as threshold voltage (Vth).The channel length (L) and width (W) were fixed at20 lm and 3 mm, respectively. The output characteristics(IDS � VDS) follow two regions viz. linear and saturation.The currents in linear and saturation regions are given byEqs. (1) and (2), respectively

IDS ¼WL

� �Cil VGS � VT �

VDS

2

� �VDS ð1Þ

IDS ¼W2L

� �CilðVGS � VTÞ2 ð2Þ

1672 P. Kumar et al. / Organic Electronics 14 (2013) 1663–1672

where l is the field effect mobility, Ci is the dielectriccapacitance per unit area. The charge carrier mobility iscalculated in the linear region by fitting the linear part ofthe output characteristics and Vth has been determinedby transconductance change method developed by Wonget al. [34], which is independent of factors like contactresistance and field dependent mobility, given by:

VT ¼@2IDS

@2V2GS

�����max

ð3Þ

Acknowledgements

This work was supported by the DST India. Pramod Ku-mar, Akanksha Sharma and Sarita Yadav acknowledge sup-port from CSIR through fellowship.

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