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Directional solvent vapor annealing for crystal alignment in solution processed organic semiconductors Deepak Bharti, Vivek Raghuwanshi, Ishan Varun, Ajay Kumar Mahato, and Shree Prakash Tiwari ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b03432 • Publication Date (Web): 14 Jul 2017 Downloaded from http://pubs.acs.org on July 15, 2017

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ACS Applied Materials & Interfaces

Directional solvent vapor annealing for crystal alignment in solution processed organic semiconductors Deepak Bharti, Vivek Raghuwanshi, Ishan Varun, Ajay Kumar Mahato, and Shree Prakash Tiwari* Department of Electrical Engineering, Indian Institute of Technology Jodhpur, Jodhpur, Rajasthan, India, 342011 KEYWORDS: Solvent vapor annealing (SVA), crystal alignment, organic semiconductors, organic field effect transistors (OFETs), TIPS-pentacene crystal

ABSTRACT

A unified approach of directional solvent vapor annealing (DSVA) for crystal alignment in solution processed organic semiconductors is proposed. Highly crystalline molecular selfassembly of the drop cast technique is further enhanced by post processing scheme of solvent vapor annealing with additional benefit of alignment of crystalline domains. In this technique, a mixture of carrier gas and solvent vapors are made to flow in a certain direction and in the close proximity of the surface of the substrates carrying the solution. Flow of the carrier gas 1

ACS Paragon Plus Environment


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imparts directionality to the semiconducting crystalline ribbons, whereas the influx of solvent vapors improves the crystalline order in the semiconducting film. Flow rate of the carrier gas and position of the substrate in the interaction chamber are the primary regulating factors, which have ability to provide a semiconducting layer with well-aligned and inter-connected assembly of long ribbons. These favorable film properties further materialize in the form of electrical performance of corresponding field effect transistors. Versatility of this technique makes it a viable alternative for the solution processing of organic semiconductors.

1. INTRODUCTION Solution processing methods of organic semiconductors have received special attention in the past few years in the organic electronics community owing to their economic and facile nature and potential to achieve outstanding device performance comparable to complex and expensive vacuum processing techniques. Performance of solution processed devices is governed by numerous microscopic factors such as the degree of crystallinity1, the molecular packing style of the active layer2-3, morphology4-5, and the orientation of crystalline domains67

. These factors are regulated by the molecular self-organization of the organic semiconductor,

which in turn depend on the parameters of semiconductor processing method8-9. Optimization of molecular self-organization in the active semiconductor layer is of vital importance to achieve a high quality semiconducting layer leading to a high performance device. An optimal self-organization with well aligned crystalline domains may cause near-uniaxial charge transport, imperative for high performance. However, controlling the self-organization in the solution processed semiconductors is quite challenging due to three dimensional crystal growth in these materials10. The anisotropy in the crystalline growth and orientation leads to inconsistent charge transport and device behavior11-12. In the commonly used solution deposition schemes of drop cast13-14 and spin coating15-16, self-organization and performance 2


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consistency conflict each other. In drop cast, where large time is available for molecular settlement leading to high degree of crystallinity, uniformity and consistency of the device performance is a challenge17. In contrast, spin coating results into uniform films, however, does not display substantial crystallinity due to fast solvent evaporation18. Various alternative processing and post-processing schemes have also been applied to control the molecular selfassembly which include zone casting19, dip coating20-21, solution shearing22, blade coating23-24, ink-jet printing25, droplet pinning26, electrostatic spraying27, slot die coating28, template assisted

crystallization29,

volatility

controlled

evaporation31, solvent vapor annealing (SVA)30,

deposition30,

32-33

non-isotropic

solvent

, etc. Incorporation of post-processing

methods like SVA in deposition schemes has been found to be advantageous for ameliorating the self-organization by enhancing the crystalline order in the semiconductor films32, 34. To achieve a high performance in device, it is indispensable to incorporate an appropriate deposition and post-processing scheme to produce highly aligned films with high crystalline order. Thus, a unified approach is required where an appropriate deposition and postprocessing schemes are jointly harnessed to produce high performance devices having highly aligned films together with high crystalline order. In this paper, we propose an integrated approach termed as directional solvent vapor annealing (DSVA) for solution processing of organic semiconductors (patent pending)35, where deposition and post-processing are jointly performed in order to obtain high performance devices with highly aligned and crystalline films of the active semiconductor material. In DSVA, a mixture of carrier gas and solvent vapors are made to flow in a certain direction and in the close proximity of the surface of the substrates carrying the solution. Crystalline organic semiconductor TIPS-pentacene has been selected for this study due to its intrinsic ability to form large crystals on solvent evaporation1, 8-9. Flow of the carrier gas helps to achieve a noticeable macroscopic alignment of the crystalline domains of the 3



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semiconductor, whereas the inflow of solvent vapors improves the microscopic molecular self-assembly in the crystalline domains. Flow rate of the carrier gas, proximity of the substrate to the gas flow, and position of the substrate in the interaction tube are some of the explored parameters which affect the overall crystallinity and alignment and hence the performance in the devices very critically. At an optimal flow rate of the carrier gas, increasing the proximity of interaction of the substrate and gas-vapor mixture was found to enhance the device performance. Application of DSVA for organic field effect transistors (OFETs) fabrication resulted in a maximum mobility as high as 0.86 cm2 V-1 s-1 and a high current on-off ratio of 106. Thus, the DSVA can be a potential semiconductor-processing scheme to jointly exploit the advantages of the high crystallinity and alignment in the solution processed organic semiconductors, and ultimately leading to high performance in devices. 2. EXPERIMENTATION a) Design of Experimental Setup for DSVA: Experimental setup for DSVA was carefully designed and customized to perform deposition and post-processing together. Schematic diagram of the setup is shown in Figure 1. Setup consists of three zones; fusion, directive and interaction zone. Lower portion of the fusion zone is essentially a glass vessel filled with the solvent. Bottom of this glass vessel rests on a hot plate which constantly heats the solvent. The glass vessel has two fluid inlets, one for solvent filling and another for carrier gas input. Top portion of the fusion zone is a chamber, which has another gas inlet to accelerate the gas-vapor mixture into the directive zone. Generation of solvent vapor, their mixing with the carrier gas and, the forceful thrust into directive zone takes place in the fusion zone.

4


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Gas Inlet

Carrier Gas Solvent Vapors

Solvent Inlet Vent Gas Inlet

Samples Carrying Solution Support

Detachable Air-tight Door

Heat at 100 C Fusion Zone

Figure 1. Design of a customized experimental setup to perform DSVA. The setup has three main zones, namely; Fusion, Directive and Interaction zone.

Directive zone is a hollow rectangular chamber with very small ceiling height, which provides high directivity to the gas-vapor mixture. Continuous inflow of gas-vapor mixture causes the directional gas-vapor mixture in the directive zone to move ahead to interaction zone. Interaction zone consists of a long and narrow interaction tube, which houses a stage of adjustable height to place the samples at different positions in the tube. The combined interaction of mixture of solvent vapors and the carrier gas from the substrate carrying solution takes place in the interaction zone. Interaction tube ends with a detachable air-tight door to put in or take out the samples. A small vent is provided at the top end surface of the interaction tube for the gaseous discharge. Figure 2 shows the cross sectional view of the hollow interaction tube, with depiction of various parameters. hc,i, denotes the height of tube ceiling with respect to floor of the stage, ds,i represents the distance of mid-point of the sample from the beginning of the interaction tube. The parameter ft,i denotes the flow rate of the carrier gas.

5



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Carrier gas

ft,i Solvent vapors

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TIPS-PEN Solution

hc,i ds,i

Si/SiO2 wafer

Figure 2. Depiction of various parameters for DSVA. hc,i, ds,i and ft,i denote the height of tube ceiling with respect to floor of the stage, the distance of mid-point of the sample from the beginning of the interaction tube and the flow rate of the carrier gas respectively.

b) Processing: Highly doped n-Si wafers with a 300 nm thick SiO2 layer were used as substrates to perform DSVA. 1"×1" substrates were thoroughly cleaned in heated solutions of 2-propanol, trichloro-ethylene and methanol. TIPS-pentacene and polystyrene (PS, Mw~2,80,000) were purchased from Sigma-Aldrich, and were used without further purification. A 0.5 wt.% solution in toluene, of each of TIPS-pentacene and PS were prepared separately by stirring at 70 °C for 2 hours. Blend solutions were prepared by mixing the TIPS-pentacene and PS solutions in 1:1 ratio by volume, followed by stirring for 30 minutes. ~200 µL of blend solution was drop casted on the samples on the stage in the interaction zone and the tube was closed air-tight for further processing. Temperature of the solvent in the fusion zone was maintained at 100 °C throughout the process. Nitrogen was used as the carrier gas. Vapor pressure of the solvent was not considered in total flow rate for simplicity. After 10 min of DSVA, samples were taken out of the interaction tube. 200 nm thick Au S/D contacts were deposited on DSVA films through shadow masks by thermal evaporation under high vacuum of 10-6 torr in order to fabricate OFETs in bottom-gate top-contact configuration. All the 6


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solution and sample processing steps were performed in dark and ambient conditions. X-ray diffraction studies were carried out using D8 Advanced from Bruker. Percentage crystallinity was calculated as the ratio of area under crystalline peaks to that under whole of the diffractogram. Electrical measurements were performed using Keithley 4200 SCS. Fieldeffect mobility (µsat) and threshold voltage (VTH) were extracted from the transfer curves in the saturation regime, using the highest slope of |IDS|1/2 vs. VGS plot using the following equation, 1

𝐼DS = 2 𝜇𝐶i

𝑊 𝐿

(𝑉GS − 𝑉TH )2

(1),

Where W, L and Ci are the width, length and capacitance density respectively. Ci was found to be 10.54 nF/cm2 at 1 KHz.

3. RESULTS AND DISCUSSION Figure 3 shows the optical micrographs of TIPS-pentacene films obtained from DSVA at different positions in the interaction tube at a fixed flow rate, ft,i of 5 L/min. Alignment of the semiconducting crystals along the direction of carrier gas flow can easily be noticed. Similar semiconductor film morphologies were obtained in different batches of the experiment (Figure S1, supporting info). With decreasing hc,i, degree of interaction of gas-vapor mixture with the solution carrying substrate increases. To ascertain the pressure-velocity variation of the carrier gas in the interaction tube, computational fluid dynamics (CFD) simulations were performed using ANSYS Fluent at hc,i = 2 mm (Figure S2, supporting info), which reveals that for hc,i = 2 mm, fluid pressure is maximum at the inlet of the interaction tube, which decreases gradually along the length of the tube, whereas the magnitude of fluid velocity is higher over the samples and near the small vent opening. Though the flow of gas-vapor mixture in the interaction zone is turbulent, as confirmed by large magnitude of Reynolds number 7



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(supplementary note 1), and small turbulent recirculation flow zones are created in the close vicinity of samples36, the ultimate flow of overall gas-vapor mixture still remains unidirectional i.e. from region of high to low pressure.

ds,i (inches)

1.5

4

6.5

hc,i (mm) Direction

2

of flow

100 µm

6

10

Figure 3. Optical micrographs of DSVA TIPS-pentacene films at different positions in the interaction tube at a fixed flow rate, ft,i of 5 L/min. Alignment of the semiconducting ribbons along the direction of carrier gas flow can easily be noticed.

The high pressure exerted by the gas-vapor mixture on the substrate carrying the solution causes formation of a uniform film like morphology along the flow direction, while the long ribbon like shape of the crystals is retained. Shape and morphology of these ribbons appears to remain almost unaffected with the distance ds,i on the stage. However, shape and morphology 8


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of these TIPS-pentacene micro-crystals are significantly influenced by variation in hc,i. As the height from the ceiling, hc,i increases, pressure exerted by gas-vapor mixture on the substrate carrying the solution decreases. Due to reduced pressure, TIPS-pentacene ribbons become irregular and discontinuity between them increases. However, the alignment of ribbons along the flow direction is still preserved. These observations suggest that the ceiling height in the interaction tube, hc,i at the time of solution-gas-vapor interaction is the factor which dictates the length and connectivity in ribbons.

ft,i (L/min) ds,i (inches) 1.5

0.5

5

10

Direction of flow 100 µm

4

6.5

Figure 4. Optical micrographs of DSVA TIPS-pentacene films at different distance ds,i and different flow rate ft,i at a fixed hc,i of 2 mm. At lower flow rates, aligned semiconducting crystals were obtained, whereas at higher flow rate a uniform semiconducting film was obtained due to faster solvent evaporation.

9



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Figure 4 shows the optical micrographs of the crystals at different distance ds,i and different flow rate ft,i in the interaction tube at a fixed hc,i. As the flow rate increases, the connectivity among the ribbons increases. At lower flow rate of 0.5 L/min, drop-cast like crystals are obtained, very long but unconnected ribbons, whereas at a higher flow rate 10 L/min, ribbon like crystals disappears and a very uniform film similar to that in spin coating is obtained. At lower flow rates, rate of solvent evaporation is slower and hence drop-cast-like ribbons are achieved, whereas at higher flow rates, a spin coating like film is obtained due to faster rate of solvent evaporation. In this case also, the shape and morphology of crystals remain largely unaffected by the position of the sample in the interaction tube. In addition, crystals remain aligned even at lower flow rate of 0.5 L/min. However, at an intermediate flow rate of 5 L/min, aligned and well-connected ribbons are obtained. In a simple drop cast method, solvent evaporation is regulated by a thermal gradient induced convection process. To compensate the higher rate of solvent evaporation at the peripheral edges, solvent flows towards these edges under capillary effect, which also governs transfer of solute from the central region to the edges to promote the growth of self-assembled crystals3738

. In the case of DSVA however, it is the flow rate of the carrier gas and the position of the

sample in the closed interaction tube, which causes the drying process to differ significantly from the normal case. Rate of solvent evaporation is higher on the sample side which encounters inflow of gas-vapor mixture first, which causes migration of active material from other regions towards the solution periphery in the faster evaporating side, ultimately resulting in growth of crystalline domains along the flow direction. Incoming solvent vapors from the directive zone are unable to stay in the initial portion of the interaction zone because of the continuous influx of gas-vapor mixture, and hence their degree of interaction with the first sample in the interaction tube is lower. As the gas-vapor mixture moves forward in the interaction zone, additional solvent concentration obtained from solvent evaporation of the 10


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preceding samples is also added in the flow. Near the vent valve, the drop in the pressure of gas-vapor mixture is associated with increases in the flow velocity in these regions (Figure S2, supporting info), as per Bernoulli’s principle. A high velocity carrier gas emerges out of the interaction tube quickly, however, due to lower relative pressure and much higher density with respect to carrier gas, solvent vapors are unable to escape completely out of the small vent opening, and their large concentration remains accumulated in the rear portion of the interaction tube. Their larger concentration in these regions enables a higher interaction with the samples in these regions, leading to a larger SVA treatment. A larger SVA causes them to attain a higher crystallinity, as observed from Figure 5(a) and Figure 5(d), which shows the Xray diffractogram of crystals processed at various positions in the interaction tube and corresponding variation in crystallinity metrics.

ds,i=6.5"

ds,i=4"

ft,i=2 L/min ft,i=1 L/min ft,i=0.5 L/min

5

20

2 (degrees)

(d)

70 ft,i=5 L/min hc,i=2 mm 65 60

0.14 0.13 0.12

55

0.11

50

10

15

20

ds,i (inches)

hc,i=6 mm

5

2 (degrees)

(e)

80 hc,i=2 mm ds,i=6.5 in

0.18 60

0.16 0.14

40

0.12

0.10

10

15

20

2 (degrees)

(f)

0.20

0.20

ft,i=5 L/min

75

ds,i=6.5 in

0.18

60

0.16

45

0.14 0.12

30

20 1 2 3 4 5 6 7

(002) hc,i=2 mm

hc,i=10 mm

FWHM (degree) % Crystallinity

15

FWHM (degree) % Crystallinity

10

ft,i=5 L/min ds,i=6.5"

No Flow

ds,i=1.5"

5

(001)

FWHM (degree)

hc,i=2 mm (002)

(002) (003) f =10 L/min hc,i=2 mm t,i ds,i=6.5" ft,i=5 L/min

Intensity (a.u.)

ft,i=5 L/min

(c)

(001)

Intensity (a.u.)

(b)

(001)

Intensity (a.u.)

(a)

% Crystallinity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

3

6

9

0.10 12

ft,i (L/min)

2

4

6

8

10

0.10

hc,i (mm)

Figure 5. X-ray diffractograms and variation in crystallinity of DSVA TIPS-pentacene films processed at various positions different positions in the interaction tube at a fixed hc,i of 2 mm 11



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and a fixed flow rate, ft,i of 5 L/min (a) & (d), at different flow rate ft,i, fixed hc,i of 2 mm and distance ds,i of 6.5" (b) & (e), and at different ceiling height hc,i, fixed flow rate ft,i of 5 L/min and fixed distance ds,i of 6.5" (c) & (f).

As described earlier, TIPS-pentacene ribbons at ds,i = 6.5" were found to be more crystalline than their counterparts at ds,i = 4" and 1.5", with percentage crystallinity of 68, 66 and 49 and full width at half of the maximum (FWHM) of 0.109, 0.123 and 0.120 for (001) peak for the samples processed at ds,i = 6.5", 4" and 1.5" respectively. Flow rate of the carrier gas is a critical parameter not only for the initial solvent evaporation but also for the overall postprocessing by SVA. Flow of carrier gas not only causes faster solvent evaporation, however also helps in achieving high crystallinity by carrying the solvent vapors to the interaction tube. With flow rates, variation in the crystallinity of the film is shown in Figure 5(b) and Figure 5(e) at a fixed hc,i of 2 mm and a fixed ds,i = 6.5" in the interaction tube. Crystals obtained with no flow and low flow rate of 0.5 L/min were found to be identically crystalline with the percentage crystallinity of 34 and 35 respectively, which indicates that at low flow rates of carrier gas, sufficient solvent vapors are unable to reach from fusion zone to the interaction zone to show any improvement in the crystallinity. As the flow rate increases, the percentage crystallinity of the DSVA film also rises. At a higher flow rate of 5 L/min, when ample amount of solvent vapors reach in the interaction zone, a typical SVA of the samples takes place along with the solvent evaporation and as a result, percentage crystallinity rises to 68 with reduced (001) peak FWHM value of 0.109. However, at further high flow rate of 10 L/min, a faster solvent evaporation becomes prevalent, inhibiting any improvement due to SVA, which leads to a severe decrease in percentage crystallinity to 19 with increased (001) peak FWHM value of 0.169. Qualitatively, it can be stated that at higher flow rates, the inherent faster solvent evaporation supersedes the SVA, whereas at moderate and optimal flow 12


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rates, both solvent evaporation and SVA occur simultaneously. Thus the flow of carrier gas in the interaction tube with an optimum rate becomes crucial for maintaining adequate rate of solvent evaporation and satisfactory solvent vapor annealing. Figure 5(c) and Figure 5(f) show the crystallinity variation with varying hc,i. Percentage crystallinity of 37 and 30 and (001) peak FWHM of 0.174 and 0.187 were obtained for TIPS-pentacene films processed at hc,i of 6 and 10 mm respectively, whereas for the sample at hc,i = 2 mm, crystalline nature of the film increased with percentage crystallinity of 68 and an FWHM of 0.109, due to enhanced interaction of solvent vapors with the solution on the substrate.

(b) 1E-5

VGS = 0 to -30 V Step = -6 V

-IDS (A)

1.2

W/L = 1000/200

1E-6 VDS = -30 V 1E-7 1E-8 1E-9 1E-10 1E-11 1E-12 ds,i = 6.5" h = 2 mm 1E-13 f c,i= 5 L/min t,i 1E-14 -20 0

0.8 0.4 0.0 -30

-20

-10

VDS (V)

0

1.5

(-IDS)0.5 (A)0.5

(a) 1.6 -IDS (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0 0.5 0.0

VGS (V)

Figure 6. Transfer (a), and output characteristics (b) of a representative OFET.

To analyze the effect of DSVA on the device performance, OFETs were fabricated with DSVA film of TIPS-pentacene as the active layer. Figure 6 shows the output and transfer characteristics of a representative OFET with semiconductor film processed by DSVA. An average mobility of 0.29±0.22 cm2 V-1 s-1 with maximum of 0.86 cm2 V-1 s-1 was obtained from a set of 9 devices processed at the position of hc,i = 2 mm, ds,i = 6.5", and with ft,i = 5L/min. Near zero threshold voltage of -0.4±0.8 V was obtained. However, the device 13



performances obtained from DSVA are somewhat lower than that of some of the other methods like solution shearing22, which can be possibly attributed to two reasons. First reason is the higher thickness of the active film (0.5 to 1 µm) and second is the higher morphological roughness of the film (Figure S3, supporting info). Both of these factors suppress the performance of DSVA OFETs. However despite lower mobility values, these devices provide high current on-off ratios (~107) and smaller sub-threshold swing (0.52 ± 0.16 V/dec.) at an operating voltage of -30 V, in comparison to those of solution shearing22 method with inferior or similar values at more than three times higher operating voltage of -100 V. Such devices with high current on-off ratio and small sub-threshold swing enable faster transition between ON and OFF states in comparison to devices obtained with other processing techniques19, 21-22, and can be suitable for solution processed organic digital logic circuitry. (b)

ft,i=5 L/min

hc,i=2 mm

0

1

-4 -8 avg sat

0

max sat

VTH (V) sat (cm2V-1s-1)

4

-12

avg sat

2

3

4

5

6

7

ds,i (inches)

(c) 0

1

-4 -8 0

hc,i=2 mm ds,i=6.5 in

0

3

6

4 ft,i=5 L/min

4

VTH

VTH

1

max sat

-12

9

ft,i (L/min)

ds,i=6.5 in

0 1

avg sat

-4

max sat VTH

-8

VTH (V)

(a)

VTH (V) sat (cm2V-1s-1)

26-28, 31

sat (cm2V-1s-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 22

-12

0 2

4

6

8

10

hc,i (mm)

Figure 7. Performance variation in OFETs with active layer of DSVA TIPS-pentacene films processed at various positions different positions in the interaction tube at a fixed hc,i of 2 mm and a fixed flow rate, ft,i of 5 L/min (a), at different flow rate ft,i, fixed hc,i of 2 mm and distance ds,i of 6.5" (b), and at different ceiling height hc,i, fixed flow rate ft,i of 5 L/min and fixed distance ds,i of 6.5" (c).

Figure 7 shows mobility and threshold voltage variation at different positions in the interaction tube. Variation in the device performance with position in the interaction tube at a 14


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fixed height is shown in Figure 7(a). The average and maximum mobility values are higher at ds,i = 6.5" than those at ds,i = 1.5" and 4". This trend can be directly correlated with the degree of crystallinity shown in Figure 5(d). The DSVA film at ds,i = 6.5", has the highest degree of crystallinity, which results into high average and maximum mobilities in corresponding devices than their counterparts at ds,i = 1.5" and 4". Mobility values of devices for all ds,i at hc,i = 2 mm are higher than that for any other hc,i, due to the highest degree of interaction between the gas-vapor mixture and the drying film at this hc,i = 2 mm which causes semiconductor molecules to arrange in a long and well-connected self-assembly of ribbons (Figure 3 and 4). An efficient charge transport takes place in these ribbons due to high degree of molecular arrangement, as indicated by relative crystallinity in Figure 5(d). Figure 7(b) shows the trend of mobility and threshold voltage with the flow rate of the carrier gas. Following the trend of percentage crystallinity of Figure 5(e), devices with films processed at no or low flow rates show inferior performance. However, at flow rate of 5 L/min, solvent vapors reach to the interaction tube and crystallinity and hence the device performance is improved due to SVA. At higher flow rate of 10 L/min, due to very fast evaporation of solvent and resultant poor crystallinity, performance of the devices decreases. Figure 7(c) shows the mobility and threshold voltage variation with hc,i at fixed ds,i = 6.5", and fixed flow rate of 5 L/min, which is in agreement with crystallinity variation shown in Figure 5(f). As discussed earlier, mobility values are higher at a smaller hc,i, which are due to a high degree of crystallinity and a wellconnected self-assembly of ribbons, formed by a closer interaction of gas-vapor mixture. At larger hc,i, reduced interaction with gas-vapor mixture leads to formation of discontinuous ribbons with decreased percentage crystallinity, resulting into devices with inferior performances. In principle, from Figure 5 and Figure 7 it was observed that higher percentage crystallinity which was associated with a smaller FWHM, resulted in higher field-effect

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mobility values. On the other side, average threshold voltages were found to be relatively independent of all process parameters, with a value of sub-|2| V for most of the cases.

Table 1. Comparison of various solution processing techniques. Attribute

Drop Casting

Material Low wastage Time of film Slow preparation Apparatus Not and its required complexity Separate Required postprocessing Film Poor uniformity Control on Poor film thickness Crystalline Good quality Crystal Poor alignment Control on Poor crystal dimensions Control on Poor morphology

Spin Coating

Dip Coating

Spray Coating

Zone Casting

Solution Shearing

DSVA

High

Very high

Low

Low

Low

Low

Very quick

Moderate

Moderate Moderate

Quick

Moderate

Required, Required, average average

Required, Required, average complex

Required, complex

Required, simple

Required

Required

Required

Required

Required

Not required

Good

Moderate

Moderate

Good

Good

Moderate

Good

Moderate

Moderate Moderate

Good

Moderate

Poor

Moderate

Moderate Moderate

Moderate

Good

Poor

Good

Good

Good

Poor

Moderate

Moderate Moderate

Moderate

Good

Poor

Moderate

Moderate Moderate

Moderate

Good

Poor

Moderate

Thin films, Thin films, Thin films, Discrete Discrete Continuous Achievable Discrete Thin films Discrete Thin films and crystallites crystallites morphologies crystallites crystallites continuous crystallites Poor Moderate Moderate Moderate Good Good Good Repeatability

Table 1 compares DSVA with existing solution processing techniques on several grounds related with processing features and quality of outcomes. Dip coating and spin coating involves high degree of material wastage, whereas other techniques consume lesser amount of active material. Techniques like spin coating and solution shearing which are quick in 16


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fabricating films, often require complex experimental setups and separate post processing steps. However, DSVA with low material consumption requires a simpler apparatus, moderate duration of film preparation and no separate post-processing steps. Spin coating and solution shearing provide good control on film uniformity and film thickness, whereas with moderate control on film uniformity and film thickness, DSVA offers well repeatable morphologies, a better crystalline quality and a fine degree of control on macroscopic alignment, dimensions and continuity of the crystals in comparison to other process techniques such as spin coating, dip coating and solution shearing. With moderate device performance, DSVA is such an adjustable technique which is alone capable of producing different morphologies ranging from continuous assembly of long ribbons (as obtained in solution shearing) to discontinuous array of wider crystals (similar to that obtained in dip coating and drop casting) to uniform film (similar to spin coating), just by fine tuning the deposition parameters. Because of its versatility, it can be used as a generic deposition approach beyond OFET applications, which can provide films with morphologies suitable to specific application and material. 4. CONCLUSION An integrated technique termed as directional solvent vapor annealing (DSVA) for solution processing of organic semiconductors is proposed. DSVA resulted in an improved molecular self-assembly in the semiconductor films by means of solvent vapor annealing, additionally providing alignment to the crystalline domains. In DSVA, flow of the gas-vapor mixture induces a visible alignment in semiconducting ribbons together with the improvement in molecular self-organization in the semiconductor crystalline domains by solvent vapor annealing. Flow rate of the carrier gas and the position of the substrate in the interaction tube are the main parameters which determine the degree of crystallinity and alignment. At an optimum flow rate and at a position of closest interaction with the gas-vapor mixture, DSVA 17



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resulted in a semiconducting layer in form of a well-aligned and inter-connected assembly of long ribbons. Such semiconducting layers in OFETs resulted in high device performance suggesting the viability of the method as a versatile technique for solution processing of organic semiconductors. Supporting Information. Semiconductor film morphologies obtained in different batches of experiment, results of computational fluid dynamics simulations, depicting pressure and velocity profiles in the interaction tube, line profile of a DSVA film, and calculation of Reynolds number. AUTHOR INFORMATION Corresponding Author *Shree Prakash Tiwari. Email: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS Authors acknowledge Mr. Gurveer Singh, Department of Mechanical Engineering, Indian Institute of Technology Jodhpur, for providing support for computaional fluid dynamics simulations. REFERENCES 1. Bharti, D.; Tiwari, S. P. Crystallinity and Performance Improvement in Solution Processed Organic Field-Effect Transistors Due to Structural Dissimilarity of the Additive Solvent. Synth. Met. 2016, 215, 1-6. 2. Reig, M.; Puigdollers, J.; Velasco, D. Molecular Order of Air-Stable p-type Organic Thin-Film Transistors by Tuning the Extension of the [small pi]-Conjugated Core: The Cases 18


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Table of Contents (TOC) Graphic

Gas Inlet

Carrier Gas Solvent Vapors

Solvent Inlet Vent Gas Inlet

Samples Carrying Solution Support

Detachable Air-tight Door

Heat at 100 C Fusion Zone

22


Directional Solvent Vapor Annealing for Crystal ... - ACS Publications

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