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

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Original Article

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

and stamping

Sheng Bi

a,b

, Zhongliang Ouyang

c

, Qinglei Guo

d

, Chengming Jiang

a,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 ( />

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:(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

/>

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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 (2.5

m

l, 5

m

l, 10

m

l, 20

m

l,
100

m

l) 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 6_torr.

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 (0.0

m

l, 2.5

m

l, 5.0

m

l, 10.0

m

l,
20.0

m

l, 100.0

m

l) 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 2.5

m

l to 5

m

l,
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 100

m

l of GLYMO was added, the OSCs short circuit
current decreases to a negligible amount, as illustrated inFig. 4(b).
It was found that 5.0

m

l 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

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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.
Current_{evoltage measurement was performed on a pattern}
transferred device with 460

m

m 1000

m

m dimensions, prepared
with a 5.0

m

l:1 ml GLYMO to PEDOT:PSS ratio, and results are shown
inFig. 5. We achieved a Vocof 0.57 V, a Jscof 1.7

m

A/cm2, FF of 21.44%

and an efficiency of 2  10 4_{%. The V}

ocseems 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 offilms 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

Voc(V), Jsc(mA/cm2), FF(%), and efficiency values of spin-coated solar cells with

various amounts of GLYMO added.

Voc(V) Jsc(mA/cm2) FF (%) Efficiency (%)

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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 5.0

m

l 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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