2.1. Experimental Part I
Based on the following two criteria that the minor component in the solvent mixture has: (1) a higher boiling point and (2) a lower surface tension than the major solvent in the solvent mixture, along with two additional constraints which were: (1) the solubility of TIPS-pentacene (as shown in
Figure 1 and
Figure 2a large area processing compatibility in an Roll-to
-Roll (R2R) environment, we chose 26 solvent combinations with varying major to minor solvent ratios to initiate characterization of the µ-dispensing process and film morphology. The list of the solvent combinations is shown in
Table 1. In this table, all the solvents that are used in the present work, sorted by major and minor solvents, are listed. The basis for the solvent ratios is taken from the research paper of Jung Ah Lim
et al. [
11], where it was inferred that if the boiling point of the two solvents are far apart (temperature difference >20 °C), the minor solvent should have a share in the mixture of 25% or less. On the other hand, if the boiling points are close to one another (temperature difference <20 °C), the minor solvent should have a share between 25% and 50%.
Figure 1.
Teas Chart showing the solubility of TIPS-pentacene in common solvent groups.
Figure 1.
Teas Chart showing the solubility of TIPS-pentacene in common solvent groups.
Using each of these solutions, a 1% (weight) solution of TIPS-pentacene was formulated. Subsequently, the solution was filtered through a 0.45 µm syringe filter and dispensed (volume of 0.3 µL) onto a planarized polyethylene naphthalate (PEN) foil substrate. The PEN substrate was mounted onto a carrier wafer to maintain a flat surface. Afterwards, the samples were allowed to dry in the ambient laboratory air (23 °C and 25% RH) to allow a slow and undisturbed drying process, so that the self-assembly of the small molecules could take place successfully.
After the drops have dried completely, images of the crystallization were taken using a Zeiss Axioplan optical microscope (Carl Zeiss AG, Oberkochen, Germany) in a dark field setting. The dark field setting was preferred as it allows the source light to illuminate the sample and only the scattered light enters the objective and produces the image, whereas the directly transmitted light is omitted. This allowed for a better visualization of the drop crystallization. The results of the complete set are shown in
Figure 2.
Figure 2.
Dark field microscope images of the drop crystallization in each of the 26 solvent mixtures as specified in
Table 1.
Figure 2.
Dark field microscope images of the drop crystallization in each of the 26 solvent mixtures as specified in
Table 1.
Table 1.
List of the 26 different solvent combinations.
Table 1.
List of the 26 different solvent combinations.
# | Major Solvent | Boiling Point (°C) | Minor Solvent | Boiling Point (°C) | Ratio | Short Name |
---|
A1 | Toluene | 111 °C | n-Butyl acetate | 126 °C | 50%–50% | Tol-But-50-50 |
A2 | Toluene | n-Butyl acetate | 75%–25% | Tol-But-75-25 |
A3 | Toluene | 4-Methyl-2-pentanone | 117 °C | 50%–50% | Tol-Met-50-50 |
A4 | Toluene | 4-Methyl-2-pentanone | 75%–25% | Tol-Met-75-25 |
A5 | Toluene | m-Xylene | 139 °C | 75%–25% | Tol-m-Xyl-75-25 |
A6 | Toluene | m-Xylene | 90%–10% | Tol-m-Xyl-90-10 |
A7 | Toluene | p-Xylene | 138 °C | 75%–25% | Tol-p-Xyl-75-25 |
A8 | Toluene | p-Xylene | 90%–10% | Tol-p-Xyl-90-10 |
B1 | Chlorobenzene | 132 °C | o-Xylene | 144 °C | 50%–50% | CB-o-Xyl-50-50 |
B2 | Chlorobenzene | o-Xylene | 75%–25% | CB-o-Xyl-75-25 |
B3 | Chlorobenzene | m-Xylene | 139 °C | 50%–50% | CB-m-Xyl-50-50 |
B4 | Chlorobenzene | m-Xylene | 75%–25% | CB-m-Xyl-75-25 |
B5 | Chlorobenzene | p-Xylene | 138 °C | 50%–50% | CB-p-Xyl-50-50 |
B6 | Chlorobenzene | p-Xylene | 75%–25% | CB-p-Xyl-75-25 |
C1 | Chloroform | 61 °C | n-Butyl acetate | 126 °C | 75%–25% | CF-But-75-25 |
C2 | Chloroform | n-Butyl acetate | 90%–10% | CF-But-90-10 |
C3 | Chloroform | Heptane | 98 °C | 75%–25% | CF-Hep-75-25 |
C4 | Chloroform | Heptane | 90%–10% | CF-Hep-90-10 |
C5 | Chloroform | 2,2,4-Trimethylpentane | 99 °C | 75%–25% | CF-Tri-75-25 |
C6 | Chloroform | 2,2,4-Trimethylpentane | 90%–10% | CF-Tri-90-10 |
C7 | Chloroform | 4-Methyl-2-pentanone | 117 °C | 75%–25% | CF-Met-75-25 |
C8 | Chloroform | 4-Methyl-2-pentanone | 90%–10% | CF-Met-90-10 |
D1 | Chloroform | 61 °C | 1-Propanol | 97 °C | 75%–25% | CF-1-Pro-75-25 |
D2 | Chloroform | 1-Propanol | 90%–10% | CF-1-Pro-90-10 |
D3 | Chloroform | 2-Propanol | 82 °C | 75%–25% | CF-2-Pro-75-25 |
D4 | Chloroform | 2-Propanol | 90%–10% | CF-2-Pro-90-10 |
An optical evaluation of the crystallization was performed to make a pre-selection of expedient compositions out of the high variety of solvent mixtures. Solvent combinations are referred to as expedients when the drying result exhibit homogeneous and distinct crystal formations. As part of the optical evaluation three compositions meet the requirements: “Tol-But-75-25”, “CB-o-Xyl-75-25” and “CF-But-90-10”. The selected solvent mixtures were used for the next experimental step.
2.2. Experimental Part II
In this experimental step, the expedient compositions of experimental step 1 were used and were attempted to be improved. Furthermore, a new solvent pair (
o-dichlorobenzene with tetralin) was added. Although the temperature difference between toluene and butyl acetate and between chlorobenzene and
o-xylene is less than 20 °C, the proportion of the minor solvent was further reduced, since the composition of 75% to 25% has shown more promising results for both solutions than the composition of 50% to 50%. The solution of chloroform and butyl acetate (90% to 10%) showed a good semiconductor layer as the drying result and was therefore repeated in this experimental step.
Table 2 shows an overview of the combinations and solvent ratios.
The drops were then processed identically to the process mentioned in the experimental part I. After the drops had dried completely, microscope images of the crystallization were taken and is shown in
Figure 3, but unlike experimental step I, a bright field setting was chosen as it showed better contrasting images than the dark field setting for the drop crystallization. The bright field setting here refers to the fact that the samples were illuminated from below, the better contrast in the samples is caused by absorbance as the samples are observed from above.
Table 2.
List of the shortlisted solvent combinations for experimental part II.
Table 2.
List of the shortlisted solvent combinations for experimental part II.
# | Major Solvent | Boiling Point (°C) | Minor Solvent | Boiling Point (°C) | Ratio | Short Name |
---|
A1 | Toluene | 111 °C | n-Butyl acetate | 126 °C | 90%–10% | Tol-But-90-10 |
A2 | Toluene | n-Butyl acetate | 80%–20% | Tol-But-80-20 |
B1 | Chlorobenzene | 132 °C | o-Xylene | 144 °C | 90%–10% | CB-o-Xyl-90-10 |
B2 | Chlorobenzene | o-Xylene | 80%–20% | CB-o-Xyl-80-20 |
C1 | Chloroform | 61 °C | n-Butyl acetate | 126 °C | 90%–10% | CF-But-90-10 |
D1 | o-Dichlorobenzene | 180 °C | Tetralin | 207 °C | 95%–5% | o-DCB-Tet-95-5 |
D2 | o-Dichlorobenzene | Tetralin | 90%–10% | o-DCB-Tet-90-10 |
Analysis of the images of “Tol-But-90-10” and “Tol-But-80-20” show that the crystal growth was directed towards the center. The crystallization is quite closed. A coffee-stain is present. Particularly, in the crystal structure of “Tol-But-80-20”, small gaps were observed along with areas that have no visible deposits, suggesting that there is no semiconductor material.
The semiconductor layer of “CB-o-Xyl-90-10” and “CB-o-Xyl-80-20” shows a coffee-stain. The coverage crystal growth is not well directed and separated, and curved ring-like areas with crystal deposits can be seen.
The surface layer of “CF-But-90-10” shows strong crystal growth directed from the outside to the inside. They meet and merge in the center of the circle area. However, there are gaps between the crystal structures at the outer edge area.
Figure 3.
Bright field microscope images of the drop crystallization of the shortlisted solvent mixtures as specified in
Table 2.
Figure 3.
Bright field microscope images of the drop crystallization of the shortlisted solvent mixtures as specified in
Table 2.
The drop surfaces of “o-DCB-Tet-90-10” shows a thick coffee-stain and distinct crystals directed towards the center, but gaps are visible between the individual crystals. The layer of “o-DCB-Tet-95-5” shows an even thicker coffee-stain from which distinct crystals grow into the circle area. However, in the circle area, there are also many smaller crystallization formation centers that form separate areas.
Figure 4 shows the profilometer µscan
® from NanoFocus AG (NanoFocus AG, Oberhausen, Germany). This non
-contact profilometry method enables surface analysis in the micrometer and nanometer ranges [
18]. Although some problems did emerge during the measurements due to the PEN substrate properties of transparency and absorbance of part of the light spectrum, it was accurate enough to indicate the discontinuities of the crystal layers. Although only the drop for CF-But-90-10 is shown in
Figure 4 to highlight the surface discontinuities, it is to be noted that all the solvent combination except CB-
o-Xyl suffered from the same problem.
Figure 4.
Profilometry results of the CF-But-90-10 drop using a NanoFocus µscan®. It can be seen from the surface profile that the surface is quite rough due to the crystal boundaries and that there are areas where no material is present.
Figure 4.
Profilometry results of the CF-But-90-10 drop using a NanoFocus µscan®. It can be seen from the surface profile that the surface is quite rough due to the crystal boundaries and that there are areas where no material is present.
Although the results of
Figure 3 show there is a directed growth of the crystal structures, they were far from being either homogenous or reproducible. Additionally, the use of just TIPS-pentacene in the solvent results in discontinuities in the crystal layer, as shown in
Figure 4. The profilometry results further indicate the following: (1) the surface is rather rough
i.e., a high degree of variance in the surface morphology due to the crystal structures; (2) the layer is discontinuous
i.e., areas are present with no OSC material on the surface; (3) the surface thickness is too high to be viably used in an organic thin film transistor (OTFT) configuration.
2.3. Experimental Part III
To consequently tackle the problem of surface discontinuities, it was decided to use two insulating binders directly in the solvent mixture to improve surface homogeneity. The two chosen binders were: (1) polystyrene (PS) and (2) PAMS. One percent (by weight) of the two binders were added to three of the shortlisted solvent mixtures from experimental part II respectively. The shortlisted solvent mixtures from experimental part II were: (1) Tol-But-90-10, (2) CF-But-90-10 and (3)
o-DCB-Tet-90-10. The solvent mixtures and binder details are shown in
Table 3.
Table 3.
List of the three sets of short listed solvent combinations for experimental part III.
Table 3.
List of the three sets of short listed solvent combinations for experimental part III.
# | Major Solvent | Boiling Point (°C) | Minor Solvent | Boiling Point (°C) | Ratio | TIPS-pentacene | Binder |
---|
A1 | Toluene | 111 °C | n-Butyl acetate | 126 °C | 90%–10% | 1% | None |
A1_PS | Toluene | n-Butyl acetate | 90%–10% | 1% | PS (1% wt) |
A1_PAMS | Toluene | n-Butyl acetate | 90%–10% | 1% | PAMS (1% wt) |
C1 | Chloroform | 61 °C | n-Butyl acetate | 126 °C | 90%–10% | 1% | None |
C1_PS | Chloroform | n-Butyl acetate | 90%–10% | 1% | PS (1% wt) |
C1_PAMS | Chloroform | n-Butyl acetate | 90%–10% | 1% | PAMS (1% wt) |
D1 | o-Dichloro benzene | 180 °C | Tetralin | 207 °C | 90%–10% | 1% | None |
D1_PS | o-Dichloro benzene | Tetralin | 90%–10% | 1% | PS (1% wt) |
D1_PAMS | o-Dichloro benzene | Tetralin | 90%–10% | 1% | PAMS (1% wt) |
The results of experimental part III, as shown in
Figure 5, shows an improvement of the surface continuity for each combination with a binder when compared to one without the use of a binder. In addition, the profilometry results as shown in
Figure 6, further validate that the adding of the binder solves the surface discontinuity problem. The drops of A1_PS, C1_PS and D1_PAMS (refer to
Table 3) are the ones with the most continuous drop surfaces of the whole set. These results show that the addition of the binder solves one of the problems
i.e., the surface discontinuity problem as faced in the experimental part II.
Figure 5.
Microscope images of drops of experimental part III with and without binders.
Figure 5.
Microscope images of drops of experimental part III with and without binders.
Figure 6.
Profilometry results of the D1_PAMS drop. It can be seen that the surface still exhibits discontinuities at the crystal boundaries.
Figure 6.
Profilometry results of the D1_PAMS drop. It can be seen that the surface still exhibits discontinuities at the crystal boundaries.
The problem that could not be addressed effectively with these solvent mixtures is the repeatability in the drop crystallization. When a set of these drops was dispensed onto the substrate, the crystallization growth appeared to be different in each dried drop. The crystal growth process was not fully repeatable. This problem is exemplified in
Figure 7, where nine drops of the D1_PAMS solution were µ-dispensed onto the PEN substrate. The crystallization albeit improved and better packed in terms of fewer surface discontinuities suffers from a large inter-drop variation, albeit our best efforts in the variation in crystallization could not be improved.
Figure 7.
Microscope images of nine D1_PAMS drops indicating the variance in crystallization repeatability.
Figure 7.
Microscope images of nine D1_PAMS drops indicating the variance in crystallization repeatability.
2.4. Experimental Part IV
With a view to improve the repeatability of the dispensing process, we made a modification to the solvent pair after evaluation of all of the results of the three experimental parts above. The selection criteria of the major and minor solvent was modified such that the difference between the boiling points of the minor solvents to major components is made as higher. In selecting
o-Xylene as the major solvent and
o-Dichlorobenzene as the minor solvent in the solvent mixture, the resulting mixture has the property that the minor component in the solvent mixture has a higher boiling point and a higher surface tension than the major solvent.
Table 4 details the solvent mixture. This solution mixture was then optimized by the same process as described in the three experimental parts above and the µ-dispensed drop results are shown in
Figure 8. From
Figure 8, it is evident that the combination of
o-Xylene (75% by volume) with
o-Dichlorobenzene (25% by volume) in which 1.5% (by weight) TIPS-pentacene and 1.5% (by weight) PAMS is dissolved, produces the desired crystallization results. Here, it can be seen that the crystals grow in a homogenous manner from the edge of the drops converging towards the center. The crystals also appear to be tightly packed leaving no discontinuities on the film surface.
Table 4.
Selected solvent mixture pair for experimental part IV and their properties.
Table 4.
Selected solvent mixture pair for experimental part IV and their properties.
Type | Name | Boiling Point (°C) | Surface Tension @ 20 °C (dyn/cm) |
---|
Major Solvent (75%) | o-Xylene | 144.4 | 30.53 |
Minor Solvent (25%) | o-Dichlorobenzene | 180.5 | 37 |
Figure 8.
Microscope images of the drops of experimental part IV (a) without binder, (b) with 1.5% (wt) PS and (c) with 1.5% (wt) of PAMS.
Figure 8.
Microscope images of the drops of experimental part IV (a) without binder, (b) with 1.5% (wt) PS and (c) with 1.5% (wt) of PAMS.
Figure 9 shows the reproducibility of the process using this improved solvent mixture. Here, it can be seen that this solvent mixture is superior to all the previous solutions tried out in this paper. It effectively solves the problem of homogenous crystal growth and is reliably reproducible.
Figure 9.
Microscope images of the drops from experimental part IV indicating a high degree of reproducibility in the different dispensed drops.
Figure 9.
Microscope images of the drops from experimental part IV indicating a high degree of reproducibility in the different dispensed drops.
However, for these drops to be viably used in OTFTs, their film thickness needs to be reduced. To enable the reduction in film thickness and to allow for a uniform surface thickness, the concentration of the TIPS-pentacene was reduced in the solution, keeping the other components the same.
Figure 10 shows the results of the profilometry results and microscopic images of the drops at concentration levels varying from 1.5% to 0.125% of TIPS-pentacene. Correspondingly, the layer thickness varied from about 2 µm to 300 nm. Using the 0.125% concentration solution, we were able to create ~300 nm thick layers without any discontinuities. It is to be noted that one side effect of the reduction in concentration is the increase in diameter of the dispensed drops although the same volume was dispensed of each solution.
Figure 10.
Microscope images of the drops and their corresponding profilometry results with the conc. of TIPS-pentacene varying from (a) 1.5%, (b) 0.5%, (c) 0.25% and (d) 0.125%.
Figure 10.
Microscope images of the drops and their corresponding profilometry results with the conc. of TIPS-pentacene varying from (a) 1.5%, (b) 0.5%, (c) 0.25% and (d) 0.125%.