Additive effect for organic solar cell fabrication by multi layer inking and stamping

Original ArticleAdditive effect for organic solar cell fabrication by multi-layer inking and stamping Sheng Bia,b, Zhongliang Ouyangc, Qinglei Guod, Chengming Jianga,b,* a Key Laboratory

Original Article

Additive effect for organic solar cell fabrication by multi-layer inking

and stamping

Sheng Bia,b, Zhongliang Ouyangc, Qinglei Guod, Chengming Jianga,b,*

a Key Laboratory for Precision and Non-traditional Machining Technology of the Ministry of Education, Dalian University of Technology, Dalian 116024, PR

China

b Institute of Photoelectric Nanoscience and Nanotechnology, Dalian University of Technology, Dalian 116024, PR China

c Department of Electrical and Computer Engineering, Center for Materials for Information Technology, The University of Alabama, Box# 870209,

Tuscaloosa, AL 35487, USA

d Department of Material Science and Engineering, Frederick Seitz Material Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL

61801, USA

a r t i c l e i n f o

Article history:

Received 29 January 2018

Received in revised form

14 March 2018

Accepted 4 April 2018

Available online 11 April 2018

Keywords:

Organic solar cells

Pattern transfer

Additives

Flexible substrate

Power conversion efficiency

a b s t r a c t Large-scale printing fabrication of organic solar cells (OSCs) has attracted much attention in recent de-cades due to its efficient industrial application Additive in the organic layer is one of the crucial factors that promote both quality of transferred pattern and the power conversion efficiency of the solar cell Here, an organic material, 3-Glycidyloxypropyl trimethoxysilane (GLYMO), as an additive was used in cost-efficient multi-layer inking and stamping processes to fabricate OSCs Polydimethylsiloxane (PDMS) was used as a transfer carrier that carries patterns from silicon mold to indium tin oxide (ITO) glass or polyethylene terephthalate (PET) to fabricate rigid orflexible organic solar cell devices By investigating the effects of chemical additives on OSCs performance in a regular procedure, the amount of additive was found which provides the best power conversion efficiency of 1.71% Further refining the multi-layer inking and stamping process by using the amount of additive found in previous experiments, high-resolution transferred patterns with maximum efficiency were produced The overall OSCs efficiency and high yield pattern transfers indicate high potential for future printing processing and will thus reduce OSCs production costs

© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

1 Introduction

Renewable and low-cost energy sources have gained increased

attention as the global supply of fossil fuels decreases and the

modern energy crisis intensifies Since the annual solar radiation

from the sun produces significantly more energy than that

consumed by the entire world's population in a year, much research

has been invested into photovoltaic cells to harvest the energy of

the Sun[1e6] Organic solar cells (OSCs) serve as a more viable

possibility in the future that is both cost and energy efficient to

replace conventional energy sources [7e9] However, the

spin-coating method widely used in laboratory is difficult as well as

relatively expensive for the fabrication of large area devices Furthermore, spin coating technique is unable to fabricate thin films on flexible substrates with the same uniformity as on rigid ITO glass substrates

Recently developed inexpensive high yield pattern transfer techniques have been used to overcome the incompatibility of certain organic electronics on both rigid and flexible substrates [10e18] The inking and stamping pattern transfer method, which uses cost-efficient PDMS elastomer stamps, has been applied to successfully transfer conducting polymer PEDOT:PSS to make organic thin film transistors (OTFT) [19] Multi-layer inking and stamping of metals and polymers in a single step has also been developed to fabricate polymer light-emitting diodes (PLED) on both ITO andflexible substrates[20e23] Direct multilayer pattern transfer is noted to preserve the functionality of the patterned polymer layers in organic devices and still maintain high-resolution transferred patterns[19,24e26] A high yield multi-layer pattern transfer depends on the relative adhesion strengths among the

* Corresponding author Key Laboratory for Precision and Non-traditional

Machining Technology of the Ministry of Education, Dalian University of

Technol-ogy, Dalian 116024, PR China.

E-mail address: jiangcm@dlut.edu.cn (C Jiang).

Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

https://doi.org/10.1016/j.jsamd.2018.04.004

2468-2179/© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license

Journal of Science: Advanced Materials and Devices 3 (2018) 221e225

layers of thin-film, the PDMS stamp and the substrate For an entire

stack of thin-films to be transferred, the adhesion between the

organic layer and the substrate must be the strongest of all

inter-layer attractions and the adhesion between the PDMS and the

stamp must be the weakest Therefore, additive is essential to the

multi-layer pattern transfer The process of the multi-layer inking

and stamping still needs to be optimized

In this study, we utilize a chemical additive in the multi-layer

inking and stamping technique to successfully fabricate OSCs on

both ITO glass and PETflexible substrates We established a reliable

procedure to investigate the effects of the additives on the pattern

transfer and the overall performance of solar cells, and eventually

to fabricate high-resolution multi-layer inked and stamped OSCs

Scanning Electron Microscope (SEM) was used to demonstrate the

quality of the transferred patterns as well as the separated

cross-section layers of the transferred patterns Atomic Force

Micro-scopy (AFM) images document the recessions found between the

transferred patterns A currentevoltage (IeV) curve was measured

and the energy-conversion efficiency was calculated An optical

image of successful OSCs fabrication on PET substrate was also

taken In the experiment, the spin-coating method was used as an

example to deposit organic films onto the PDMS mold Other

methods such as dip-coating, for instance, might also work to

complete the PDMS mold fabrication The pattern transfer

tech-nique is an efficient way of making sub-micro patterns instead of

using photolithography, metal deposition, developing, lift-off, etc It

was found a lot more useful that the soft PDMS mold is

appropri-ately applicable on flexible substrates We anticipate that our

method can improve the development of the devices and promote

industrial production of OSCs

2 Experimental

Poly(3-hexylthiophene-2,5-diyl) (P3HT) and[6,6]

-phenyl-C61-butyric acid methyl ester (PCBM) were purchased from Solarmer

and Nano-C respectively and just used without any further

treat-ment The ITO glass was cleaned in detergent, de-ionized water,

acetone and isopropyl alcohol in sequence, and treated with oxygen

plasma at 30 W for 5 min to increase its surface energy[27,28] A

silicon wafer PDMS master mold, initially etched by

photolithog-raphy and reactive ion etching, was put pattern-side up into a petri

dish Sylgard 184 silicone elastomer base mixed with a curing agent

at a weight ratio of 8:1 was poured into the petri dish and put in a

vacuum oven overnight at room temperature to remove the excess

bubbles and was then heated to 100 C for 1.5 h to completely

solidify the PDMS solution A 30 nm thick layer of gold was

sput-tered onto the PDMS stamp, followed by a 50 nm layer of aluminum

deposited by thermal evaporation at a rate of 2Å/s P3HT and PCBM

(1:1 wt, concentration of 25 mg/mL in chlorobenzene) was

spin-coated onto the PDMS with a spin-speed of 900 rpm for 45 s The

PDMS was then treated with oxygen plasma at 30 W for 10 s

fol-lowed by spin-coating PEDOT:PSS (purchased from HC Stark) onto

the PDMS at 5000 rpm Various amounts (2.5ml, 5ml, 10ml, 20ml,

100ml) of 3-Glycidyloxypropyl trimethoxysilane (GLYMO)

(chemi-cal structure shown inFig 1(b)) were added to 1 ml of PEDOT:PSS

solution and left at room temperature overnight before use to

increase the adhesive properties of the solution The“inked” PDMS

is then immediately stamped onto the pre-cleaned ITO glass on a hot plate at 80C for 2 min and then slowly peeled off The entire process is illustrated inFig 2 All fabrication procedures were un-dertaken in nitrogenfilled glove box

To accurately test the effect of the GLYMO additive to the per-formance of the solar cell, spin-coated solar cells on rigid ITO substrate were fabricated The regular structure of the P3HT/PCBM system was used The ITO glass substrate was first cleaned following the procedure mentioned above A 40-nm-thick PEDOT:PSS anode buffer layer with various amounts of GLYMO was spin-coated on top of the precleaned ITO substrate The P3HT-PCBM solution was then deposited by spin coating at a speed of

900 rpm for 40 s on the top of the PEDOT:PSS layer Then, the entire device was put into the vacuum oven and annealed at 140C for

20 min An 80 nm thick Al layer was subsequently thermally evaporated on it at the vacuum pressure of 3 10 6torr The current-voltage (IeV) characterization of the polymer photovoltaic cells was conducted using a computer-controlled measurement unit (B1500A semiconductor parameter analyzer) from Agilent Technologies under ambient condition with illumi-nation of the AM1.5G, at 100 mW/cm2 The open circuit voltage (Voc) and the short circuit current (Isc) were measured Thefill factor (FF, that is the available power at the maximum power point divided by the open circuit voltage and the short circuit current) and the power conversion efficiency (PCE) were determined The GLYMO acts as a plasticizer, which increases the chain mobility of the polymers, resulting in a lower processing temper-ature and pressure[29] GLYMO is able to prevent the spin-coated PEDOT:PSS thinfilm from completely dry out immediately Also, it helps with sticking the layers on the mold to the substrate More-over, adding glycerol can also enhance the conductivity of PEDOT:PSS[30]

3 Results and discussion

In order to test the effect of GLYMO on the OSCs efficiency, a set

of control experiments were performed on spin-coated solar cells with different amounts of GLYMO (0.0ml, 2.5 ml, 5.0 ml, 10.0ml, 20.0 ml, 100.0 ml) added to 1 ml of PEDOT:PSS Current-voltage characterizations are displayed inFig 3(a) and derived parameters

inTable 1 When the amount of GLYMO increases from 2.5ml to 5ml,

Vocremains relatively constant, while the Jscgreatly increases from 6.84 mA/cm2to 7.03 mA/cm2 The FF raises from 38.58% to 40.24%,

as shown inTable 1, indicating that the highest occupied molecular orbit and lowest unoccupied molecular orbit remain the same, while the resistance inside the devices decreases However, when the GLYMO concentration further increases, a significant change occurrs in the devices, dramatically decreasing their short circuit current When 100ml of GLYMO was added, the OSCs short circuit current decreases to a negligible amount, as illustrated inFig 4(b)

It was found that 5.0ml of GLYMO was the ideal amount required per 1 ml of PEDOT:PSS to achieve both the highest OSCs efficiency and a high yield pattern transfer Higher or lower concentrations of GLYMO would both significantly reduce the OSCs efficiency A small amount of GLYMO additive in the PEDOT:PSS solution is able to enhance the conductivity of PEDOT:PSS However, when further increase the amount of GLYMO, the mismatch of the energy levels between the GLYMO and the P3HT/PCBM will result in a charge transport block, leading to an increase of the recombination ef fi-ciency and a decrease of charge generation efficiency, which causes

a poor performance of the solar cell

Fig 4(a) illustrate that the high yield multi-layer pattern transfer was successfully performed on the ITO glass substrate Each rect-angular pattern represents a separate OSCs device The relatively

Fig 1 The chemical structures of (a) P3HT (b) 3-Glycidyloxypropyl trimethoxysilane

S Bi et al / Journal of Science: Advanced Materials and Devices 3 (2018) 221e225 222

smooth surface and the high yield of the transferred patterns with

the minimal deformities, such as cracks or buckles, signify an

optimal metal deposition, an appropriate additive use (the suf

fi-cient amount of GLYMO added to the PEDOT:PSS solution), and the

careful handling of the PDMS stamp during the spin-coating

pro-cess Pattern transfer onflexible PET substrate is another step that

was achieved As shown in inset of Fig 4(a), successful pattern

transfer with clear separated patterns was observed This

achievement demonstrates a great promise for using the

multi-layer inking and stamping technique to fabricate large amounts of

OSCs through printing method

To prove that the stripes between each two rectangle patterns

were also transferred, AFM was carried out and results are shown in

Fig 4(b) A clear recession as observed indicates good separation

between the patterns due to an optimal pressure applied during the

stamping process and the maximum stress at the corners of the

patterns The dark orange middle section with well-defined top and bottom edges represents a clear recession between the two trans-ferred patterns This pattern separation indicates that only desired layers on the patterned parts of the PDMS stamp were transferred while the remaining layers are still attached to the original stamp and, thus, allowing a more accurate area for each OSCs device to be measured

A clearly separation of each layer is the key to ensure that charge transports can be generated and the electron and hole pairs can be successfully separated and transported to the cathode and anode, respectively In order to reveal the separation of metal, P3HT/PCBM and PEDOT:PSS thin-films on the ITO glass after the transfer pro-cess, an SEM image of the cross-section of this structure was taken

as it is shown inFig 4(c) From the image, distinctive separated edges with sharp contrast are observed It has a an apparent effect

on the charge carrier generation and transportation in OSC devices Currentevoltage measurement was performed on a pattern transferred device with 460 mm 1000 mm dimensions, prepared with a 5.0ml:1 ml GLYMO to PEDOT:PSS ratio, and results are shown

inFig 5 We achieved a Vocof 0.57 V, a Jscof 1.7mA/cm2, FF of 21.44% and an efficiency of 2  10 4% The Vocseems comparable to that of

a spin-coated solar cell, but the Jscis rather low The comparable Voc indicates a good pattern transfer and a functional light absorption layer The low current is likely caused by the oxidation of Al at the interface between the Al and P3HT/PCBM layers, which may have led to significant degradation of the OSCs device Another effect might come from the oxygen plasma treatment on P3HT/PCBM

Fig 2 Schematic of the transfer procedure Deposition of films onto PDMS stamp; Oxygen plasma treatment on ITO glass; Press PDMS onto plasma treated ITO glass; Slowly peel off the PDMS from the ITO glass.

Fig 3 (a) IeV characterizations of spin-coated P3HT/PCBM OSCs devices with 0.0ml, 2.5ml, 5.0ml, 10.0ml, 20.0ml and 100ml of GLYMO added to 1 ml of PEDOT:PSS solution (b) Comparison of OSCs efficiency vs the amount of GLYMO added to 1 ml of PEDOT:PSS solution.

Table 1

V oc (V), J sc (mA/cm 2 ), FF(%), and efficiency values of spin-coated solar cells with

various amounts of GLYMO added.

V oc (V) J sc (mA/cm 2 ) FF (%) Efficiency (%)

0.0ml 0.66 5.93 43.67 1.66 ± 0.081

2.5ml 0.60 6.84 38.58 1.52 ± 0.065

5.0ml 0.61 7.03 40.24 1.67 ± 0.089

10.0ml 0.58 5.69 30.95 1.13 ± 0.173

20.0ml 0.58 3.92 20.73 0.30 ± 0.097

100ml 0.56 2.77E-3 14.18 2.2E-4 ± 1E-4

S Bi et al / Journal of Science: Advanced Materials and Devices 3 (2018) 221e225 223

layer since oxygen could react with the P3HT molecules which will

change its conjugated property, resulting in some disadvantages to

the charge carrier generation and transport

4 Conclusion

In summary, GLYMO was used in the multi-layer pattern

transfer process to print OSCs With an amount of 5.0ml of GLYMO

in 1 ml of PEDOT:PSS, we managed to perform both high-yield

transferred patterns and to reach the maximum power conversion

efficiency Multi-layer patterns were successfully transferred from

PDMS stamp to both ITO glass and PETflexible substrates with the

optimum GLYMO additive Each layer was clearly separated after

the transfer, and recessions between the transferred patterns were

distinct We achieved an overall OSCs efficiency of 2.1  10 4% We anticipate this work may ultimately support the development of the multi-layer inking and stamping pattern transfer technique to a more viable and beneficial option for large-scale OSCs fabrication

Acknowledgments This project wasfinancially supported by National Natural Sci-ence Foundation of China (NSFC, 51702035 and 51602056), and Dalian University of Technology, China, DUT16RC(3)051

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