Synthesis, thermal characterization, and gas barrier properties of UV curable organic/inorganic hybrid nanocomposites with metal alloys and their application for encapsulation of organic solar cells

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Synthesis, thermal characterization, and gas barrier properties of UV curable

organic/inorganic hybrid nanocomposites with metal alloys and their application

for encapsulation of organic solar cells

Chen-Ming Chen

a

, Ming-Hua Chung

a

, Tsung-Eong Hsieh

a,*

, Mark O. Liu

b

, Jen-Lien Lin

b

, Wei-Ping Chu

c

,

Rong-Ming Tang

c

, Yu-Sheng Tsai

c

, Fuh-Shyang Juang

c

a

Department of Materials Science and Engineering, National Chiao-Tung University, Hsinchu 300, Taiwan, ROC

b

Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu 300, Taiwan, ROC

c

Institute of Electro-Optical and Materials Science, National Formosa University, Huwei, Yunlin 63208, Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 9 January 2008

Received in revised form 1 May 2008 Accepted 18 June 2008

Available online 8 July 2008 Keywords:

A. Polymer-matrix composites (PMCs) B. Curing

B. Electrical properties

D. Thermogravimetric analysis (TGA) D. Transmission electron microscopy (TEM)

a b s t r a c t

We have successfully prepared ultraviolet (UV) curable organic/inorganic hybrid nanocomposites with good thermal stability, moderate adhesion strength, and excellent barrier capability. In order to improve the gas blocking properties, the metal alloy (Invar) has been blended with lab-made organic/inorganic hybrid nanocomposites, dramatically not only raising the gas resistance but also reducing the shrinkage after UV curing. Moreover, we have also applied lab-made nanocomposite IV for the encapsulation of organic solar cells. The experimental results reveal that introduction of nanocomposite IV can effectively block the penetration of moisture as well as oxygen in the air into the devices and consequently promote the lifetime of organic solar cells.

1. Introduction

Organic/inorganic hybrid nanocomposites, which are composed of polymer matrices, ﬁllers, and initiators, have attracted lots of attention for several decades because they can resist the penetra-tion of oxygen as well as moisture in the atmosphere and are used in the preservation of food as well as beverages[1]. Pure polymers, such as acrylics and epoxy resins, have been extensively applied for the polymer matrices of organic/inorganic hybrid nanocomposites due to their low cost, high processing facility, and moderate toughness.

Acrylics resins [2] are well-known polymeric materials since they exhibit quick curing speed, high structural strength, and excellent processing convenience. Their fast curing performances are suitable for some electronic applications, such as the bonding of magnets into speaker assemblies, and others with high assembly line speed. Furthermore, they can be cured with heat or ultraviolet (UV) irradiation[3,4]by the addition of thermal initiators or photo-initiators, respectively. UV curable acrylics resins have shorter cur-ing time, weaker structure strength, and higher contraction than thermal curable ones. Although acrylics resins are good candidates

for polymer matrices of organic/inorganic hybrid nanocomposites, they exhibit low viscosity. This extremely restricts their practicability.

Epoxy resins[5]are also famous polymeric materials because of their chemical modiﬁability, high transparency, good thermal stability, and moderate viscosity. In recent years, they have been used in electronic, mechanic, and biological industrials since the oxirane groups of epoxy resins are active to various functional groups such as alcohol, thiol, anhydride, amine, etc., facilitating the curing reaction. They can be cured with heat or UV irradia-tion [6] by the addition of curing agents or photoinitiators, respectively. Compared with UV curable epoxy resins, thermal curable ones have longer curing duration and the color stain takes place while they are cured at high temperature. Despite the epoxy resins have many merits as the polymer matrices of organic/inorganic hybrid nanocomposites; they exhibit low cur-ing speed and high brittleness after curcur-ing.

In order to increase the gas barrier performances of polymer matrices, some researchers have recently added fillers (e.g. silica, metal oxide, clay, etc.) into them to enhance their gas barrier capa-bility[7–11]. Nonetheless, the resistance of gas for polymer matri-ces/fillers was not satisfied. Therefore, we have tried to blend metal alloys with polymer matrices/nano-fillers to further reduce the gas penetration.

* Corresponding author. Tel.: +886 3 5712121x55306; fax: +886 3 5724727. E-mail address:[email protected](T.-E. Hsieh).

Contents lists available atScienceDirect

Composites Science and Technology

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As described above, acrylics resins possess low viscosity and high contraction after curing. However, epoxy resins exhibit mod-erate viscosity and low contraction after curing. In addition, epoxy resins have long curing time and high brittleness after curing. Oppositely, acrylics resins possess high curing speed and low brit-tleness after curing. Thus, we have chosen these two complemen-tary materials as the polymer matrices to modulate the physical, chemical, and mechanical properties.

In this paper, we have ﬁrstly synthesized thermal curing acryl-ics resins with appropriate viscosity by addition of thermal initia-tors. Then epoxy resins and photoinitiators have been added into the acrylics resins and eventually the polymer matrices can be pre-pared. Since UV curable acrylics resins have weaker structure strength and thermal curable epoxy resins are prone to coloration, we have manipulated acrylics and epoxy resin to be thermal and UV curable, respectively. Finally, we have blended the nano-ﬁller (silica) as well as the metal alloy (Invar) with the polymer matrices to accomplish the manufacture of lab-made organic/inorganic hy-brid nanocomposites. After UV irradiation, lab-made organic/inor-ganic hybrid nanocomposites can be cured and bonded with the

glass. The experimental results manifest that addition of Invar into polymer matrices/silica effectively increases the gas barrier

capa-CH

2

CHC

O

(CH

2

)

6

_O

_C

O

CH

2

O

C

OCH

2

_O

O

C

O

OOC

O

S S S+ + _SF 6 SF6- -O O S n

S

(CH

2

)

5

CH

3

n

C60 O OCH3 1,6- Hexanedol diacrylate 3,4-Epoxycyclohexane carboxylode

Resin monomers

Benzoyl peroxide

Triaryl sulfonium hexafluoroantimonate

Initiators

Polyethylene dioxythiophene Poly(3-hexylthiophene-2,5-diyl) [6,6]-phenyl C61-butyric acid methyl ester

(PEDOT) (P3HT) (PCBM)

Materials for organic solar cells

Fig. 1. Chemical structures of the resin monomers, initiators, and materials for organic solar cells.

Fig. 2. Structures of lab-made organic solar cells. (a) ITO glass/PEDOT (30 nm)/ P3HT:PCDM (50 nm)/LiF (10 nm)/Al (80 nm); (b) ITO glass/PEDOT (30 nm)/ P3HT:PCDM (50 nm)/LiF (10 nm)/Al (80 nm)/encapsulating adhesive (100lm).

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bility of lab-made organic/inorganic hybrid nanocomposites but decreases their shrinkage and adhesion strength. Furthermore,

we have also utilized lab-made organic/inorganic hybrid nanocom-posites for the encapsulation of organic solar cells, demonstrating they can effectively resist the invasion of moisture and oxygen in the air into the devices. Consequently, the lifetime of organic solar cells with their package is much longer than that without package. 2. Experimental

2.1. Materials

All of resin monomers, initiators, materials for organic solar cells, solvents, and fillers (silica; 30–100 nm) used in the experiment (Fig. 1) were purchased from Aldrich Co. and used without further purifi-cation. 1,6-Hexanedol diacrylate and 3,4-epoxycyclohexane carb-oxylode were the monomers for acrylics and epoxy resins, respectively. Benzoyl peroxide and triaryl sulfonium hexafluoroan-timonate were utilized as thermal initiators and photoinitiators, respectively. Propylene glycol monomethyl ether acetate (PGMEA) was the solvent. The metal alloy (Invar) purchased from Goodfellow Co. was iron/nickel composite (64/36 = wt%/wt%, size = 100 nm).

Fig. 3. TGA of lab-made organic/inorganic hybrid nanocomposites in air.

Table 1

Physical properties of lab-made organic/inorganic hybrid nanocomposites

Material Td(°C) Adhesion strength (Kgf/cm2) Gas penetration (%) Electrical conductivity (S/cm)

Nanocomposite I 345 117.5 10.71 1.5 1012

Nanocomposite II 328 107.4 8.17 2.3 1010

Nanocomposite III 340 90.7 7.34 4.7 1010

Nanocomposite IV 335 87.3 5.12 6.8 1010

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2.2. Preparation of lab-made organic/inorganic hybrid nanocomposites 1,6-Hexanedol diacrylate (10 g), benzoyl peroxide (0.1 g), and PGMEA (100 mL) were heated at 100 °C for 3 h. After blended with 3,4-epoxycyclohexane carboxylode (10 g) and triaryl sulfonium hexaﬂuoroantimonate (0.1 g), the acrylics/epoxy resins (polymer matrices) were obtained. Then we added silica (50 wt%) and Invar (0–15 wt%) into the polymer matrices and lab-made organic/inor-ganic hybrid nanocomposites were ﬁnally prepared (nanocompos-ite I: with 0 wt% Invar; nanocompos(nanocompos-ite II: with 5 wt% Invar; nanocomposite III: with 10 wt% Invar; nanocomposite IV: with 15 wt% Invar). After coated on the glass with spin-coating tech-nique and irradiated by UV for 1 min, lab-made organic/inorganic hybrid nanocomposites could be cured and bonded with the glass. 2.3. Fabrication of organic solar cells

We ultrasonically washed the ITO glass (5X/e) with the ace-tone, methanol, and de-ionized water for 5 min, dried it with a stream of nitrogen as well as the oven, and treated it with O2

plas-ma for 90 s. Polyethylene dioxythiophene (PEDOT) was then dis-solved in water (3 wt%), ﬁltered with a 0.2

l

m ﬁlter, and deposited onto the ITO glass by spin-coating (stage I: 1000 rpm for 20 s; stage II: 3000 rpm for 30 s). Afterwards, the P3HT powder (molecular weight = 87,000) was ground together with PCBM (pur-ity = 98%) to form P3HT/PCBM mixture (weight ratio = 1/1). Then the P3HT/ PCBM mixture was dissolved in dichloromethane (2 wt%), ﬁltered with a 0.2

l

m ﬁlter, and deposited onto the PEDOT layer by spin-coating (stage I: 1000 rpm for 20 s; stage II: 2000 rpm for 30 s). Finally, we deposited LiF and Al electrode onto the P3HT/ PCBM layer by vacuum evaporation as shown inFig. 2(a).

The encapsulation of lab-made organic solar cells was executed with spin-coating of nanocomposite IV on the Al electrode (stage I: 500 rpm for 10 s; stage II: 1500 rpm for 20 s) and under UV illumi-nation for 10 s (Fig. 2(b)). The thickness of encapsulating adhesive was 100

l

m.

2.4. Instruments

Thermogravimetric analysis (TGA), gas penetration, transmis-sion electron micrograph (TEM), and electrical conductivity were measured by a JHS-100, an Illinois-8501, a Philips/Tecnai F20, and an Everbeing SR-4, respectively. Thermomechanic analysis (TMA), adhesion strength, and ﬁlm thickness were recorded on a Seiko SSC 5000, a micro-computer universal testing machine (Hung Ta Co.) with the standard test method (ASTM D1002) and a surface proﬁler (TENCOR P-10), respectively. The current–voltage (I–V) curves for organic solar cells were measured in the air by an

instrument (Keithley 238), whose accuracy can reach picoampere, under illumination of white light from a 300 W halogen lamp (Sat-urn Co.) whose intensity was recorded on a radiometer (IL-1700). The UV lamp utilized for UV curing was an Entela UVP 100 W. 3. Results and discussion

3.1. Polymer matrices

Although 1,6-hexanedol diacrylate exhibits good mechanic prop-erties, low brittleness after curing, and short curing time, it possesses weak thermal stability, low viscosity, and high contraction after UV

Fig. 5. TMA of lab-made organic/inorganic hybrid nanocomposites.

Fig. 6. I–V curve of lab-made organic solar cells ((a) without encapsulation, (b) with nanocomposite IV, and (c) with EPO-TEK 301).

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curing. In contrast, 3,4-epoxycyclohexane carboxylode is a superior cycloaliphatic epoxy resin with high thermal resistance, good trans-parency, low dimension change, and moderate viscosity. In addition, the cycloaliphatic epoxy resin has lower chloride content than other epoxy resins because it is synthesized by peroxidation rather than by the reaction of hydroxyl group and epichlorohydrin[12]. The chlo-ride ion is capable of eroding the electronic device in high humidity. Therefore, we have blended 3,4-epoxycyclohexane carboxylode with the acrylics resin to heighten the thermal resistance, diminish the shrinkage, and increase the viscosity.

3.2. Thermal characterization, gas barrier properties, and electrical conductivities of lab-made organic/inorganic hybrid nanocomposites

Thermal characterization of lab-made organic/inorganic hybrid nanocomposites was executed under air by TGA instrument (Fig. 3). Their decomposition temperatures (Td; at 5% weight loss) are

tabulated inTable 1, revealing they have excellent thermal stabil-ity since all of their Tdare larger than 300 °C. This is very crucial on

the encapsulation of the electronic device because they can endure high temperature and resist the penetration of gas despite the elec-tronic device liberates much heat when actuated.

Lab-made organic/inorganic hybrid nanocomposites also exhi-bit good gas resistance and introduction of Invar drastically pro-motes gas barrier effect as shown inTable 1. Moreover, the gas penetration decreases with the increase of Invar because the poly-mer matrices/nano-silica were well dispersed (Fig. 4(a)) and the metal alloys lying on the polymer matrices/nano-silica may further ﬁll in the vacancy of the polymer matrices/nano-silica and thus the gas penetration can be improved (Fig. 4(b)–(d)).

We also have investigated the electrical conductivities of lab-made organic/inorganic hybrid nanocomposites with a 4-point probe. Although the metal alloy (Invar) has been blended with the polymer matrices/nano-silica, no signiﬁcant electrical conduc-tivities can be observed in case of organic/inorganic hybrid nano-composites I, II, III, and IV as shown inTable 1. This represents that the polymer matrices/nano-silica possess high electrical resis-tance so that even addition of 15 wt% Invar into them cannot be electrical conductive. The electrical insulation may avoid electrical interference while the electronic device is actuated.

3.3. Thermal mechanic properties and adhesion strength of organic/ inorganic hybrid nanocomposites

As shown inTable 1andFig. 5, addition of Invar not only en-hances the gas barrier capability but also reduces the shrinkage. Furthermore, the dimension change in planar direction decreases

with the increase of Invar as shown in Fig. 5. Nevertheless, the adhesion strength drops with the raising of Invar owing to the poor adhesion energy of metal with the glass.

3.4. Encapsulation of organic solar cells

Although silicon-based solar cells have been found to exhibit good efﬁciency for the conversion of sunlight into electrical energy, their cost is very high. In recent years, organic solar cells have be-come alternative candidates for photoelectric conversion because of low cost and high processability. Nevertheless, the lifetime of or-ganic solar cells has been an essential hindrance for the commer-cialization since oxygen and moisture in the air erode the organic materials and metal electrodes of devices, dramatically dropping the lifetime. In addition, most researches focus on the improve-ment of the efﬁciency for solar cells but the studies about the pro-motion of lifetime are less reported.

As shown inFig. 6(a) andTable 2, lab-made organic solar cell without encapsulation exhibits the photoelectric conversion prop-erties initially and its efﬁciency reaches to be 0.42%. While the actuating time proceeds, nonetheless, the photoelectric conversion capability gradually decays because the moisture and oxygen in the atmosphere invade into the device, causing the corrosion of or-ganic materials and metal electrode. After actuated for 48 h, the efﬁciency merely remains to be 0.016% (decay ratio = 96.2%).

In order to improve the lifetime of organic solar cells, we have tried to applied nanocomposite IV for the package of the devices. With the encapsulation of nanocomposite IV, as manifested in the Fig. 6(b) andTable 2, the decay ratio sharply decreases and can be only 13.0% and 37.0% when the actuating time are 15 and 48 h, respectively. The result proves that nanocomposite IV is capa-ble of resisting the penetration of oxygen as well as moisture in the air into the device, effectively prolonging the lifetime of organic so-lar cells. Furthermore, the simiso-lar result can also be observed in the case of commercial encapsulating adhesive (EPO-TEK 301, Epoxy Technology Co.) as shown inFig. 6(c) andTable 2. Compared with the photoelectric conversion properties of organic solar cells with EPO-TEK 301, the decay ratio of the device with the package of nanocomposite IV is lower, revealing that the nanocomposite IV exhibits better gas barrier capability than EPO-TEK 301.

4. Conclusions

In conclusion, novel UV curable organic/inorganic hybrid nano-composites (acrylics resins/epoxy resins/silica/Invar) with excel-lent gas barrier performance, moderate adhesion strength, low

Table 2

Photoelectric conversion properties of lab-made organic solar cells

Encapsulating material Actuating time (h) VOCa(V) ISCb(mA/cm2) Fill factorc(%) Efﬁciencyd(%) Decay ratio (DR)e(%)

No encapsulation 0 0.44 3.13 31.6 0.42 – 15 0.40 1.28 25.5 0.13 69.0 48 0.30 0.22 23.1 0.016 96.2 Nanocomposite IV 0 0.44 3.33 48.1 0.54 – 15 0.42 3.30 44.9 0.47 13.0 48 0.40 3.05 37.2 0.34 37.0 EPO-TEK 301 0 0.44 3.18 34.6 0.52 – 15 0.40 1.88 33.4 0.28 46.2 48 0.38 0.92 28.5 0.11 78.8 a

Open circuit voltage is deﬁned as the voltage when the current density is zero.

b _{Short circuit current is deﬁned as the current density when the voltage is zero.} c _{Fill factor (FF) is deﬁned as FF ¼}ðVIÞmax

VOCISC.

d _{Efﬁciency (Eff) is deﬁned as Eff ¼}VOCISCFF

Pin 100% where Pinis the power intensity of white light source.

e

Decay ratio is deﬁned as DR ¼ðEffoEffeÞ

Effo 100% where Effoand Efferepresent the efﬁciency of organic solar cell for original state (actuating time = 0 h) and experimental

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shrinkage, good thermal resistance, and fast curing speed have been successfully synthesized and applied for the encapsulation of organic solar cells. We have found that lab-made nanocompos-ites effectively impede the penetration of moisture and oxygen in the atmosphere into the device and consequently prolong the life-time. In the near future, we will further explore their mechanical, chemical, and physical properties and utilize them for the package of other electronic devices (e.g. LEDs, semiconductor chips, LCDs, and so on).

Acknowledgement

Financial support by ITRI 7354DC4300 is highly appreciated. References

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