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Solvent-infiltration imprint lithography: a novel method to prepare large areapoly(3-hexylthiophene) micro/nano-patterns

Jinhe Wang,*a Guoquan Min,b Zhitang Song,c Xiuyuan Ni,d Weimin Zhou,b Jing Zhan,b Yanping Zhang,b

Jianping Zhanga and Liyi Shi*a

Received 31st July 2012, Accepted 21st August 2012

DOI: 10.1039/c2jm35086f

A new method is developed in this work to fabricate large area poly(3-hexylthiophene) (P3HT) micro/

nano-patterns. The method is based on solvent-infiltration imprint lithography (SIIL). P3HT micro/

nano-patterns, including micro-prism arrays, nano-grating and nano-hole arrays, have been

successfully prepared with features ranging from 400 nm to 3 mm. The regularity of the patterns was

studied by SEM and the morphology of the pattern surfaces was investigated by atomic force

microscope (AFM). The solidification mechanism of this method is based on solvent infiltration instead

of external UV or heat to form the polymer pattern. This mechanism makes SIIL more universal, of

lower cost than hard-mold imprint lithography, and more efficient than capillary force based

lithography since high glass transition temperature (Tg) polymers or even polymer/nano-particle blends

can be patterned quickly. Polymer solar cells with nano-hole arrays and nano-grating are prepared

using SIIL, respectively, and the power conversion efficiency is improved compared with traditional

film-structure polymer solar cells.

1 Introduction

Conjugated polymers are considered as organic semiconductor

materials that could lead to the next generation of electronic and

optical devices. Conjugated polymers have many advantages

over inorganic semiconducting materials, such as low cost and

flexibility of use. Poly(3-hexylthiophene) (P3HT) is an important

conjugated polymer that has been widely used in solar cells,1–4

organic transistors,5–7 and polymer light-emitting diodes

(LEDs).8,9 In order to improve performance, micro or nano

patterns are usually required in these devices.10 For example,

micro-textures are employed in solar cells to improve sun light

absorption,11–13 where micro-arrays are used in organic thin film

transistors to increase carrier mobility,14–16 and photo crystals are

fabricated on the surface of LEDs to improve the light

output.17–19

Up to now, photolithography with subsequent dry and wet

etching processes, is the most common micro-pattern fabrication

method in the state-of-the-art semiconducting industry.

aResearch Center of Nano-science and Nano-technology, ShanghaiUniversity, Shangda Road 99, Shanghai, 200444, P. R. China. E-mail:wangjinhe@gmail.combNanotechnology Promotion Center, Jiachuan Road 245, Shanghai 200237,P. R. ChinacLaboratory of Nanotechnology, Shanghai Institute of Microsystem andInformation Technology, Chinese Academy of Sciences, Changning Road865, Shanghai 200050, P. R. ChinadPolymer Department of Fudan University, Handan Road 220, Shanghai200433, P.R. China

21154 | J. Mater. Chem., 2012, 22, 21154–21158

However, this method is hardly applicable to conjugated poly-

mers because the polymer will be significantly damaged or have

its intrinsic properties altered under the harsh patterning

conditions. Some other non-destructive patterning methods,

including the anodic aluminum oxide (AAO) template

method,20,21 dip-pen nanolithography22,23 and nanoimprint

lithography24–27 have been used to fabricate micro/nano-patterns

of polymers.28–30 However, these methods usually rely on

expensive state-of-the-art equipment or cannot fabricate patterns

over large areas.31 Take nanoimprint lithography for example,

the conjugated polymer is heated above its glass transition

temperature (Tg) and is then imprinted by a hard master which

has been patterned previously. Once the temperature drops back

below the Tg, the conjugated polymer is solidified again and the

pattern is transferred from the hard master to the conjugated

polymer. This nanoimprint process is usually carried out on

expensive equipment which can provide high temperatures (50–

100 �C above the Tg of the polymer), high pressures (ca. 40 bar, 1

bar ¼ 100 000 Pa) and full area contact at the same time.32,33

Even so, some high Tg polymers are hard to imprint using this

process. Many papers have aimed at solving this problem. One

method is by using solvent vapor to lower the Tg of the polymer

and reduce the imprint temperature and pressure,32,34 but this

method usually requires long times (for example 3–6 hours) to

swell the polymer film, which will reduce the process efficiency

inevitably.

In this paper we report a novel imprint lithography method,

solvent infiltration imprint lithography (SIIL), by combining the

This journal is ª The Royal Society of Chemistry 2012

Fig. 1 Schematic of SIIL. (a) Spin-coating substrate with P3HT solu-

tion. (b) Imprint P3HT solution film with PDMS mold. (c) Solvent

infiltrates through the PDMS mold. (d) P3HT pattern formed on

substrate.

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essential feature of nanoimprint lithography—imprinting a

polymer film—with the key element of soft lithography—

molding a prepolymer or polymer solution with an soft stamp.

This idea is similar to capillary force lithography (CFL), which

was developed by simply combining nanoimprint and soft

lithographies.35 The differences between SIIL and CFL are (1)

the resist layer, which for the soft mold in SIIL is a polymer

solution or polymer paste rather than solid polymer film. So

heating is not needed to ‘‘melt’’ the polymer and the filling time is

much reduced. (2) The solidification of the polymer in SIIL is

achieved by solvent infiltration into the soft mold instead of

waiting for the temperature to drop below the polymer Tg or

chemical cross-linking reactions. So the heating or UV is not

necessary in SIIL. It combines the wide adaptability of nano-

imprint lithography and the low cost of soft lithography and can

pattern polymers or polymer/particle blends without heating or

UV radiation. Using this method, micro/nano-patterns of P3HT

and P3HT/PCBM polymer solar cells with nano-grating and

nano-hole arrays were fabricated.

2 Experimental

2.1 Materials

Poly(3-hexylthiophene) (P3HT, green power,Mw: approximately

55–60 kg mol�1, regioregular >95%, metal content <0.02%) and

[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) (black

powder) was purchased from Luminescence Technology Corp.

1,2-Dichlorobenzene was obtained from Shanghai Guoyao

Corp. Poly(ethylenedioxythiophene) doped with poly-

(styrenesulfonate) (PEDOT:PSS) was purchased from Sigma-

Aldrich. Polydimethylsiloxane (PDMS) Sylgard 184 and its

curing agent were purchased from Dow Corning Corp. Two

nano-patterned polycarbonate masters were provided by

Shanghai Institute of Optics and Fine Mechanics. The micro-

structure master is fabricated by photolithography following dry

and wet etching processes.

2.2 Replication of the PDMS soft mold

Replication of the PDMS mold was processed as follows: PDMS

and its curing agent 30 g (10 : 1 by weight) was mixed and

degassed. The mixture (1 g) was diluted with toluene (60 wt%) to

decrease the viscosity. The diluted mixture was spin coated at

3000 rpm for 30 s on the master. Then, the rest of the mixture was

carefully poured onto the surface, followed by curing at 80 �C for

120 min. After that, the soft PDMSmold was peeled off carefully

from the master. The detailed experimental process can be found

in the work of Zhou et al.36

Fig. 2 SEM images of patterned P3HT film fabricated by solvent-infil-

tration nanoimprint lithography. (a) Six-sided cylinder array with sides of

3 mm and a period of 15 mm. (b) Four-sided cylinder array with sides of

3 mm and a period of 10 mm. (c) Tri-prism array with sides of 3 mm and a

period of 15 mm. (d) Quadrilateral sink with sides of 10 mm and a depth of

about 700 nm. (e) Nano-hole array with diameters of 400 nm and a period

of 800 nm. (f) Nano-grating with a line width of 400 nm and a period of

800 nm.

2.3 Solvent-infiltration imprint micro/nano patterns of P3HT

P3HT solution in 1,2-dichloriobenzene (50 mg ml�1) was spin-

coated (500 rpm) on ITO glass. The imprinting process was

carried out immediately. Imprint parameters were a temperature

of 25 �C, imprinting pressure of 0.3 bar and imprinting time of

3 min. A schematic of the entire solvent-infiltration imprinting

process is shown in Fig. 1.

This journal is ª The Royal Society of Chemistry 2012

2.4 Polymer solar cells fabrication

An approximately 40 nm thick layer of PEDOT:PSS was spin-

coated on ITO and annealed at 120 �C for 15 min to remove

water. P3HT and PCBM (weight ratio of 1 : 1) was dissolved in

chlorobenzene at a concentration of 20 mg ml�1. The P3HT/

PCBM solution was filtered using 0.45 mm syringe filter before

spin-coating on the PEDOT:PSS layer, then, the SIIL was

carried out to fabricate the nano-grating and nano-hole patterns,

followed by with annealing at 120 �C for 15 min. To complete the

device fabrication, 1 nm of LiF, 30 nm of Al and 70 nm of Ag

were evaporated in order onto the patterns under high vacuum

through a shadow mask. The active device area was measured to

be 0.1 cm2. A blank solar cell was fabricated under the same

conditions, except the SIIL step.

J. Mater. Chem., 2012, 22, 21154–21158 | 21155

Fig. 3 AFM images of PDMSmolds and corresponding P3HT patterns.

(A) Six-sided cylinder mold of PDMS; (a) six-sided cylinder of P3HT. (B)

Four-sided cylinder mold of PDMS; (b) four-sided cylinder of P3HT. (C)

Tri-prism mold of PDMS; (c) tri-prism of P3HT.

Fig. 4 Cross-section curves of different shape molds and corresponding

P3HT structures.

Fig. 5 AFM images of PDMS mold and P3HT with concentric circles.

Fig. 6 Cross-section curves of PDMSmold and P3HT concentric circles

fabricated with different concentrations of P3HT solution.

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2.5 Measurement and characterization

The imprinted patterns were measured by FE SEM (Hitachi S-

4800). The morphologies of the imprinted patterns were inves-

tigated by atomic force microscope (Veeco, Dimension 3100) in

tapping mode. The J–V curve was tested by East China Normal

University with a PVIV-412V solar cell test system.

3 Results and discussion

SIIL successfully produced various P3HT micro/nano-structure

patterns on ITO glass without hot embossing or reactive ion

etching, as shown in Fig. 2. A six-sided cylinder (Fig. 2a), four-

sided cylinder (Fig. 2b), tri-prism (Fig. 2c), quadrilateral sink

21156 | J. Mater. Chem., 2012, 22, 21154–21158

(Fig. 2d), nano-hole (Fig. 2e), and nano-grating (Fig. 2f) with

figure sizes ranging from 400 nm to 3 mm are visible over the large

area. The details of these micro/nano-structures are given in the

figure captions. These figures indicate that highly uniformed

micro/nano-structures can be fabricated on large areas through

SIIL.

This journal is ª The Royal Society of Chemistry 2012

Fig. 7 Cross-section of the nano-grating polymer solar cell.

Fig. 8 J–V curves of polymer solar cells.

Table 1 Characteristics of the three polymer solar cells

Solar cells Voc (V) Jsc (mA cm�2) FF PCE (%)

Film structure 0.57 8.07 46.31 2.13Nano-grating 0.58 9.05 44.37 2.31Nano-hole array 0.61 8.87 53.86 2.93

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In order to study the pattern transfer fidelity of SIIL, AFM

was used to test the surfaces of the PDMS mold and fabricated

P3HT micro/nano patterns. Fig. 3(A)–(C) shows the AFM

images of the three PDMS molds which have different hole-

shapes and hole-size, but the same hole-depth. The AFM images

of the three corresponding P3HT patterns are also shown in

Fig. 3(a)–(c). A cross-section was made in each graph and the

position of the cross-section was shown in each 2D image. The

cross-section curve of each structure was also given below each

graph. These 2D images of the mold and the corresponding

P3HT patterns show that the shapes of the three micro-structures

are transferred very well. The cross-section curves of each

structure were intercepted (between the two vertical broken lines

of each cross-section curve) to compare the mold and the cor-

responding P3HT structure further (Fig. 4, the cross-section

curves of molds are inverted for convenient comparison). This

figure shows that the mold and the corresponding P3HT

structure match each other very well. The height of each

This journal is ª The Royal Society of Chemistry 2012

micro-structure is lower than the depth of each PDMS mold,

which results from the infiltration of the solvent.

The influence of the polymer solution concentration on the

fidelity of SIIL was also studied. The AFM images of the PDMS

mold with concentric circles and one of the P3HT concentric

circles are shown in Fig. 5. The cross-section curves of the PDMS

mold and P3HT concentric circles fabricated with different

concentrations of P3HT solution are shown in Fig. 6. This figure

shows that as the concentration of the polymer solution

decreases the P3HT structure becomes smaller and the meniscus

becomes more obvious. This phenomenon can be used to fabri-

cate small size polymer structures with relatively large-size

molds, which we will study next.

3.1 The properties of polymer solar sells

The aim of a micro/nano-structure fabrication method is to

fabricate micro/nano-structures which can be used in optical,

photovoltaic or biochip devices. In this work, we used SIIL in

polymer solar cells. Fig. 7 is the SEM image of the cross-section

of the nano-grating P3HT/PCBM polymer solar cell after the Al

cathode layer had evaporated. The thickness of the active layer is

in the range of 70–110 nm and the Al layer is about 30 nm. The

nano-grating structure is still visible.

To investigate the effect of the nano-structures on the

performance of the polymer solar cells, photocurrent density–

voltage (J–V) curves were measured (Fig. 8) and the values of

short circuit current densities (Jsc), fill factors (FF), open circuit

voltage (Voc) and power conversion efficiency (PCE) are shown

in Table 1. The increase of Jsc can be attributed to light

diffraction in the nano-structures of the active layer and light

reflection on the nano-texturing Al rear electrode, which

improves light trapping within the active layer.

4 Conclusions

In summary, we have demonstrated an extremely simple, highly

efficienct, universal method for fabricating high Tg polymer

micro/nano patterns, including micro-prism arrays, nano-grating

and nano-hole arrays over large areas. In the proposed method,

the P3HT structure is formed by imprinting the P3HT solution

and solvent infiltration, rather than by heating the polymer

beyond its glass transition temperature or by chemical cross-

linking reactions. The transfer fidelity of SIIL depends on the

concentration of the polymer solution. When the concentration

of the P3HT solution is 50 mg ml�1, the lateral fidelity of SIIL is

good and the height of the P3HT structures are smaller than the

PDMS mold resulting from the solvent infiltration. Lower

concentrations of P3HT solution can be used to fabricate smaller

P3HT structures using the same PDMS mold. SIIL can be used

in P3HT/PCBM polymer solar cells, and the nano-grating and

nano-hole arrays improve the PCE of the polymer solar cell.

Acknowledgements

This work was China Postdoctoral Science Foundation funded

project and supported by Shanghai Postdoctoral Science Foun-

dation (11R21420900) and Natural Science Foundation of

Shanghai (no. 11ZR1432100).

J. Mater. Chem., 2012, 22, 21154–21158 | 21157

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