To inhibit coffee ring effect in inkjet printing of light-emitting polymer fi lms by decreasing capillary force

Xinhong Yu,Rubo Xing,Zhongxiang Peng,Yangming Lin,Zhonghui Du,Junqiao Ding,Lixiang Wang,Yanchun Han*

State Key Laboratory of Polymer Physics and Chemistry,Changchun Institute of Applied Chemistry,Chinese Academy of Sciences,Changchun 130022,China

Key words:Inkjet printing Coffee ring effect Polymer fi lm Uniformity Organic light emitting diodes

ABSTRACT Polymer thin fi lm with uniform thickness and fl at surface pro fi le is the key point for polymer light emitting diodes(PLEDs)by inkjet printing.However,the coffee ring effect is usually observed due to the mismatch between the evaporation of the solvent and the decrease of solution volume,which promotes the formation of radial fl ow from the interior of the drop to the edge.In this paper,coffee ring effects of inkjet printed poly(spirobi fl uorene)fi lms were proposed to be restrained by decreasing capillary force by adding co-solvent with high boiling point and high viscosity to the main solvent.The lowevaporation rate of the co-solvent can reduce the driving force of the radial fl ow;Meanwhile the high viscosity of the co-solvent can increase the resistance of the radial fl ow.Thus,polymer fi lms with improve uniformity can be obtained due to the suppression of the radial fl ow.The device performance was greatly improved under the condition of proper fi lm thickness and fi lm uniformity and the maximum luminous ef fi ciency of devices with inkjet printed poly(spirobi fl uorene)can reach 80%of the spin-coated devices.

Organic light emitting diodes(OLEDs)have been attracting much attention in fl at panel displays(FPDs)due to their high brightness,w ide view ing angle,low power consumption and good contrast[1–4].Inkjet printing is considered as one of the most promising technology in the fi eld of controlled deposition of polymers and functional materials in well-de fined patterns,especially in relation to the fabrication of multicolor OLEDdisplays[5–12].For OLED displays,the accuracy and uniformity of organic layer thickness is an important manufacturing issue.

In the thin fi lm forming process,the solutes prefer depositing along the periphery due to the coffee ring effect[13–15].This phenomenon w ould affect thickness uniformity of inkjet printed fi lms and further influence the performance of devices.Deeganet al.[16,17]explained that,w hen the contact line between a droplet and substrate is pinned,there exist a mismatch between the evaporation of the solvent and the decrease of solution volume.The evaporation rate of the solvent is faster at the pinned threephase contact line,m eanwhile the volume decreases occurred mainly in the central region of droplets.Such a mismatch promotes the formation of radial fl ow from the interior of the drop to the edge.This inside-out radial fl ow carries the solutes to the contact line and formed ring-like morphology.

To eliminate coffee ring effect and improve fi lm uniformity of inkjet printed fi lms,several methods have been proposed[18–24].The coffee ring effect can be avoided through weakening the outw ard fl ow[20–22],increasing inw ard Marangoni fl ow[23–27]or prevent the droplet pinning[28,29].Soltmanet al.studied the effect of the substrate temperature on the inkjet printed thin fi lm morphology[20].Morphology adjustment was realized through using cooled substrate to weakened radial capillary fl ow.Schubertet al.also reported that through utilizing gelation of polymer solution to restrain the coffee ring effect[21].Lines with improved morphological control and resolution were achieved due to the reduced outward fl ow.Yodhet al.found that the anisotropic particles can inhibit the coffee ring effect effectively due to the strong long-ranged attractions between ellipsoids[22].Marangoni fl ow can be another effective w ay to suppress the complementary fl ow and regulate thickness uniformity of inkjet printed fi lms.Huet al.obtained reversed coffee ring deposition through Marangoni convection induced by temperature gradient[23].Marangoni fl ow also can be produced with the concentration gradient through adding compositional solvent.Moonet al.fabricated homogeneous conductive patterns by adding co-solvent with high boiling point and low surface tension[25].Pinned contact line is a primary requirement for the coffee ring effect to take place,thus coffee rings can be avoided with the depinned three phase contact line[30].For inkjet printed OLEDs,through modulation of the solvent properties to eliminate the coffee rings maybe most promising.

In this paper,fi lm forming property of poly (spirobi fl uorene)solutions inkjet printed on substrate with low surface energy separators was investigated.Through adding co-solvent with high boiling point and high viscosity to reduce the capillary force of radial fl ow,poly(spirobi fl uorene)fi lms with improved thickness uniformity were obtained due to the inhibition of coffee ring effect.Through modulation of the volume fraction of co-solvent,the thickness of inkjet printed fi lms can be tuned effectively.Such inkjet printed poly(spirobi fl uorene)fi lms are ideal for PLED fabrication and could gain improved device performance.Passive matrix display has been fabricated and the performance of the matrix was characterized.

Isopropylbenzene(IPB)and chlorobenzene(CB)were selected as main solvent due to their good solubility topoly(spirobi fl uorene)and stable inkjet property.Poly(spirobi fl uorene)was dissolved in pure CB or IPB and inkjet printed on the prefabricated PEDOT:PSS fi lms inkjet printed previously under the confinement of the low surface energy separators.Figs.1a and c show the fluorescence images of poly (spirobi fl uorene)fi lm deposited from pure IPB and CB solutions,the deposited fi lms show heavy edge deposition phenomena.Main poly (spirobi fl uorene)were deposited at the edge region of the grooves and the fluorescences were asymmetric.The thickness uniformity of the inkjet printed poly (spirobi fl uorene)fi lms were poor due to the coffee ring effect.

In order to improve the uniformity of the inkjet printed fi lm,cosolvent with high boiling point and high viscosity was mixed with main solvent and their effects on fi lm uniformity were investigated.Here we choose cyclohexylbenzene(CHB)as the co-solvent.The main physical parameters of IPB,CB and CHB are listed in Table 1.Figs.1b and d show the fluorescence images of poly (spirobi fl uorene)fi lmsinkjet printed from CB/CHB(60/40)and IPB/CHB(60/40)solutions,respectively.Uniformity of inkjet printed poly (spirobi fl uorene)fi lms improved dramatically with the addition of co-solvent CHB.The addition of CHB didnot influence the morphology of printed fi lm obviously(AFM images show n in Fig.S2 in Supporting information).Figs.S3 and S4(Supporting information)show the optical images and corresponding fluorescence images of poly (spirobi fl uorene)fi lms inkjet printed from IPB/CHB and CB/CHB solutions with different volume fraction of CHB.

From the cross-section pro files of poly (spirobi fl uorene)fi lms deposited from IPB/CHBand CB/CHBsolvent mixtures,we can see the improvement of fi lm thickness uniformity more obviously.For pure IPB solution,the poly (spirobi fl uorene)fi lm was about 50 nm and more than 120 nm at the center and edge regions respectively.As for IPB/CHB solvent mixtures with volume fraction of 90/10,edge deposition phenomenon is suppressed mostly.The difference of fi lm thicknessbetween the edge and the center region of the fi lm reduced to about 15 nm.With further increasing the volume fraction of CHB,the thickness uniformity further increased to lessthan 10 nm.These results indicate that the coffee ring effect has been restrained.When the volume fraction of CHB increased to 50%,there is convex–lens like cross-section at the center region forpoly(spirobi fl uorene)fi lm maybe due to the excess of CHB.For CB/CHBsolvent mixtures,the thickness fl uctuation reduced from over 100 nm to about 10 nm(Fig.2d).The addition of CHB suppressed the outer-fl ow and edge deposition and increased the average thickness uniformity.

Table 1 Main physical parameters,including boiling point(b.[30_TD DIFF]p.),viscosity(h),surface energy(g),saturated vapor pressure(PV).

In the solvent evaporation process,fast solvent evaporation can induces fast diffusion of the polymer solute and results in maximizes material transfer tow ard the edges.It is the driving force of complementary fl ow via capillarity.Meanwhile,the viscosity of the solution can hamper such transfer;it is the resistance of the radial fl ow.Based on the data in Table 1,the Pv of CHBis 10,which is much lower than IPB and CB.Meanwhile,the[23_TD DIFF]h of CHBis higher than IPBand CB.With the increase of CHBcontent,Pv and[23_TD DIFF]h of solvent mixtures show ing a tendency to decrease and increase,respectively.For pure IPB or CB,the fast solvent evaporation speed during inkjet printing facilitates the transfer of poly[24_TD DIFF](spirobi fl uorene)in fi lms.Meanwhile,the lower viscosity of the solution can not reduce such radial fl oweffectively.Poly(spirobi fl uorene)fi lms with heavy coffee ring effect can be obtained.When adding co-solvent CHB with high boiling point and high viscosity,the driving force of radial fl ow is reduced due to the lowevaporation rate of CHB.Meanwhile,the high viscosity of CHB increases the resistance of the radial fl ow.By adjusting the composition of co-solvent,to tune the volatility/viscosity match,we can realize the inhibition of edge deposition in the inkjet printing process and gain fi lms with uniform thickness.

The poly (spirobi fl uorene)fi lms patterned by inkjet printing were used to fabricate PLEDs.The device structure of the PLEDs is ITO/PEDOT:PSS/[25_TD DIFF]poly(spirobi fl uorene)/Ca/Al(Fig.3a).ITO stripes have a w idth of 200 m m with a space of 50 m m patterned by photolithography and wet-etching were used as anode.Lowenergy separators have a w idth of 50 m m with a space of 200 m m vertical with the ITO stripes were used as con fined pattern.The size of each pixel was 200?200 m m.The fi lms of PEDOT:PSSwere inkjet printed on the substrate with the low surface energy separators fi rstly.Then the poly(spirobi fl uorene)fi lms were deposited on the PEDOT:PSS fi lm by inkjet printing from IPB/CHB(60/40)or IPBsolution and served as emitting layer.Athin layer of calcium(20 nm)/aluminum(100 nm)were deposited through physics vapor deposition.

Fig.1.The fluorescence images ofpoly(spirobi fl uorene)fi lms deposited from(a)CB,(b)CB/CHB(60/40),(c)IPBand(d)IPB/CHB(60/40).The w idth of groove and separator is 200 m m and 50 m m,respectively.

Fig.2.The cross-section pro files ofpoly(spirobi fl uorene)fi lms deposited from(a)IPB/CHBand(b)the corresponding thickness fl uctuation,(c)CB/CHBsolvent mixtureswith different volume fraction of CHB and(d)the corresponding thickness fl uctuation.

Fig.3.(a)[13_TD DIFF]evice con fi guration,(b)optical image of matrix of inkjet printed PLEDs,(c)current density-voltage-luminance curves and(d)their current density dependence of luminous ef fi ciency of devices inkjet printed with pure IPB and IPB/CHB(60/40)solution.

The optical images of the inkjet printed pixel array from IPB/CHB(60/40)solution are show n in Fig.3b.All the pixels are similar to each other in the w hole and the light-emitting intensity in every pixel is uniform.Figs.3c and d show the current density-voltageluminescence characteristics and current density dependence of luminous ef fi ciency of devices inkjet printed with pure IPBand IPB/CHB(60/40)solution.By improving the uniformity of the fi lm thickness,device with improved performance can be obtained.For devices inkjet printed from IPB/CHB(60/40)solution,the turn-on voltage is 3.7 V,the maximum luminance is 4400 cd/m2and the maximum luminous ef fi ciency is 2.33 cd/A.While the performance of the devices inkjet printed from pure IPBis much lower.The turnon voltage is 4.6 V,the maximum luminance is 1160 cd/m2and the maximum luminous ef fi ciency is 0.37 cd/A.

We further compared the performance of the devices inkjet printed from IPB/CHB(60/40)solution and CB/CHB(60/40)solution with the device fabricated by spin coating[31].The corresponding device data are summarized in Table S1 in Supporting information.The CIE of devices inkjet printed from IPB/CHB(60/40)solution and CB/CHB(60/40)solution is comparable with device fabricated by spin coating.The maximum luminous ef fi ciency of devices inkjet printed with poly(spirobifl uorene)from IPB/CHB(60/40)can reach 80%of the spin-coated devices[31].The maximum luminous ef fi ciency of devices inkjet printed from CB/CHB(60/40)solution is relative lower maybe due to the presence of halogen ions in chlorobenzene solvents.

In conclusion,we have improved the thickness uniformity of inkjet printed poly (spirobi fl uorene)fi lms through adding cosolvent with lower volatility and higher viscosity.Coffee ring effect can be overw helmed through modulated the volatility/viscosity match to regulate the driving force and the resistance for evaporation-driven radial fl ow during the evaporation process.By improving the uniformity of the fi lm thickness,device with improved performance can be obtained.The maximum luminous ef fi ciency of devices with inkjet printed poly(spirobi fl uorene)can reach 80%of the spin-coated devices.

Acknow ledgments

This work wasfi nancially supported by the National Natural Science Foundation of China(Nos.21574130,51473161,51873212),the Ministry of Science and Technology of China (No.2015CB655001)and National Key R&D Program of“Strategic Advanced Electronic Materials” (Nos. 2016YFB0401301,2016YFB040110011F]).

Appendix A.Supp lementary data

Supplementary materialrelated to thisarticlecan befound,in the online version,at doi:https://doi.org/10.1016/j.cclet.2018.09.007.