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 Journal of Micromechanics and Microengineering

J. Micromech. Microeng. 27 (2017) 015019 (9pp) doi:10.1088/0960-1317/27/1/015019

Silicon micro venturi nozzles for

cost-efficient spray coating of thin
organic P3HT/PCBM layers
Michael A Betz1, Patric Büchele2,3, Manfred Brünnler1, Sonja Deml1
and Alfred Lechner1
1
  Competence Center Nanochem, Faculty of General Studies and Microsystems Technology,
OTH Regensburg, Seybothstraße 2, 93053 Regensburg, Germany
2
  Siemens Healthcare GmbH, Technology Center, Günther-Scharowsky-Straße 1, 91058 Erlangen,
Germany
3
  Light Technology Institute and Institute of Microstructure Technology, Karlsruhe Institute
of Technology, Kaiserstraße 12, 76131 Karlsruhe, Germany

E-mail: michael.andreas.betz@gmail.com

Received 4 August 2016

Accepted for publication 14 October 2016
Published 16 November 2016

Abstract
Improvements on spray coating are of particular interest to different fields of technology as
it is a scalable deposition method and processing from solutions offer various application
possibilities outside of typical facilities. When it comes to the deposition of expensive and
film-forming media such as organic semiconductors, consumption and nozzle cleaning issues
are of particular importance. We demonstrate the simple steps to design and fabricate micro
venturi nozzles for economical spray coating with a consumption as low as 30–50 µl · min−1.
For spray coating an active area of 25 cm2 a 2.45–4.01 fold coating efficiency is observed
compared to a conventional airbrush nozzle set. The electrical characterization of first diodes
sprayed with an active layer thickness of ~750 nm using a single micronozzle at a coating speed
of 1.7 cm2 · min−1 reveals a good external quantum efficiency of 72.9% at 532 nm and a dark
current of ~7.4 · 10−5 mA · cm−2, both measured at  −2 V. Furthermore, the high resistance of the
micronozzles against solvents and most acids is provided through realization in a silicon wafer
with silicon dioxide encapsulation, therefore allowing easy and effective cleaning.

Keywords: deep reactive ion etching, microfluidics, organic semiconductors,

organic photodiodes, spray coating, venturi nozzles

S Online supplementary data available from stacks.iop.org/JMM/27/015019/mmedia

(Some figures may appear in colour only in the online journal)

1. Introduction the world continue to work on scalable deposition methods

such as spray coating, as the benefits from solution pro-
The spread of organic photodiodes (OPDs) and organic pho- cessing and application possibilities potentially outweigh the
tovoltaics (OPVs) is limited because silicon devices still offer disadvantages by far. The benefits, for example, are lattice-
higher efficiencies and are available at a lower price, partially independent large area coating, continuous flow processing
due to mass production effects. Many research groups around and therefore a reduction in cost per area [1]. Furthermore,
spray coating enables the use of fluids with different rheolo-
Original content from this work may be used under the terms
of the Creative Commons Attribution 3.0 licence. Any further
gies, and leaves the user independent from substrate material
distribution of this work must maintain attribution to the author(s) and the title and form [2]. Krebs describes spray coating as one of the
of the work, journal citation and DOI. coating methods compatible with roll-to-roll processing in

0960-1317/17/015019+9$33.00 1 © 2016 IOP Publishing Ltd  Printed in the UK

J. Micromech. Microeng. 27 (2017) 015019 M A Betz et al

his review of printing and coating techniques for OPV, which Table 1.  Dimension parameters of tested nozzles. Etch depth
we gladly recommend as overview of applicable coating T of all nozzles was 100 µm. Droplet size is geometric mean of
probability density function of dried droplets sprayed from diluted
methods [3]. Still, he highlights the difficulty of obtaining
photoresist.
smooth surfaces, which will also be addressed in this paper.
Another intensively discussed alternative application method M N D W L Droplet size
is ultrasonic spray coating, which is similar to spraying with Nozzle (µm) (µm) (µm) (°) (µm) (µm)
micronozzles in terms of generated droplet size and drying 2k (2.7) 50 300 100 60 0 17.81d
behavior [4], and therefore shares similar characteristics. 4 50 300 100 170 50 23.63
A promising approach to better market OPDs and OPVs 4.05 50 300 50 170 50 28.68
is to reduce the amount of expensive organic semiconductors 10 50 300 100 170 50 32.16
(OSCs) required to form the active layer. For conventional 4b 50 500 100 170 50 32.45
spin coating (resulting in best layer quality), more than 90% of 2.br (2.2) 150 300 100 120 150 41.22
the organic semiconductors are spun off the substrate and lost 2 50 300 100 120 50 64.19
[5]. As long as OSCs such as the electron-conductor PCBM 2k.br.05 150 300 50 60 150 69.45e,f
2–50 (2.1) 50 300 50 120 50 n.a.a
(phenyl-C61-butyric-acid-methyl-ester) cost up to 1850 €/g
5b 50 300 50 0 50 n.a.a
depending on purity [6], making them the main matter of
2–25 50 300 25 120 50 n.a.a,b
expense, such loss rates are highly uneconomic and impede
3 50 300 100 0 300 n.a.a,b
the further spread of OPDs and OPVs. By using our micronoz- 9 50 500 100 120 300 n.a.a,b
zles the overspray shall be reduced and the coating efficiency 7 50 500 100 120 300 n.a.b,c
for the spray coating OSC layers increased accordingly. 2k.05 50 300 50 60 0 n.a.c
Another important issue when spray coating film-forming 11 50 500 100 120 50 n.a.c
organic semiconductors is the guarantee of thorough nozzle 1 50 300 100 120 300 n.a.c
cleaning as small residues may have a serious impact on the 8 50 500 100 0 300 n.a.c
spray result, e.g. doping or reduced layer thickness. For those 2k.br 150 300 100 60 0 n.a.e
reasons, we realize micro venturi nozzles in a silicon wafer 4.br 150 300 100 170 150 n.a.e
with silicon dioxide encapsulation allowing easy cleaning and 4.br.05 150 300 50 170 150 n.a.e
economical spray coating without performance losses. a
Lack of suction,
In this paper, we will show the design of micro venturi b
Unsteady spraying,
c
nozzles based on FEM (finite element method) simulations Low maximal pressure,
d
and several nozzle evaluation steps (section 2.1). We dem- Insufficient suction,
e
Too high mass flow,
onstrate the simple fabrication of the nozzles using a single f
Values at 8 cm distance.
dry etch process to structure a silicon wafer, anodic bonding
with a borosilicate glass wafer to perform capping and a wafer
saw for nozzle separation (section 2.2). We will picture the suction, FEM simulations on first nozzle layouts were com-
spray cone, drop size distribution and layer properties using pleted and nozzles were evaluated and adapted according
profilometer scans and optical microscopy (section 3.1). We to their suctioning behavior and generated droplet distri-
also visualize droplet dispersion and compare it to conven- bution with diluted photoresist (AZ9260 1:3 in PGMEA).
tional airbrush technology applying a CCD-based high-speed Considerations and calculations on the simulation were
camera (section 3.1). Furthermore, we demonstrate how to accomplished taking into account results by Dagaonkar et al
economically spray coat with a single nozzle and a 2.45–4.01 of simulations on a venturi liquid/liquid dosage system [9].
times enhanced coating efficiency (compared to the conven- A dynamic viscosity of 0.42 mPa · s at room temperature
tional airbrush nozzle set) at a speed of 1.7 cm2 · min−1 only was assumed, as the most appropriate solvent for the OSCs
consuming 30–50 µl · min−1 of OSC chlorobenzene solution was yet to be determined.
(section 3.2). Finally, we show the electrical characterization Table 1 shows some of the tested nozzles with their geo-
of first sprayed diodes and discuss how to further increase metric parameters, as illustrated in figure 1.
coating speed and improve atomization and layer homoge- SEM pictures of the selected micronozzle design are shown
neity (sections 3.2 and 4). in figure 2, featuring a medium inlet (a2) in an atomizing gas
restriction (a1). Higher gas velocity (red arrow) caused by
the smaller restriction width results in a pressure drop and
a medium flow towards restriction and nozzle opening (blue
2.  Fabrication and experimental methods
arrow with broken line).
2.1. Design The pressure drop is dependent on the ratio of atomizing
gas tube width to restriction width as well as on the width
Our goal was to transform a basic venturi nozzle design, as of the medium inlet. By increasing the medium inlet width,
e.g. described in [7], to the microscale, suitable for struc- the media flow may be increased. The used design features
turing by photolithography and dry etching (similar in setup a nozzle opening and medium inlet width of 50 µm and an
to that presented in [8]). In order to achieve sufficient venturi atomizing gas tube width of 300 µm.

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J. Micromech. Microeng. 27 (2017) 015019 M A Betz et al

spray coater, developed and constructed at the competence

center Nanochem of OTH Regensburg, is depicted in figure 4,
equipped with a spray head and micronozzle. Upscaling in
terms of output and coating velocity is easily possible by real-
izing several nozzles aligned next to each other on the wafer
level, with separate inlets for atomizing gas and medium, or
even by melding channels to single inlets for atomizing gas
and medium. The aforementioned substrate cleaning pro-
cess is also applied for nozzle cleaning after spraying, as it
oxidizes and removes all organic residues contained in the
organic semiconductor solution. Flushing with chlorobenzene
prior to this is convenient as a pre-cleaning process.
Reproducible etch rates of 1.1 µm per step and slightly
negative sidewall angles of 87–90° were achieved using a
SF6/CHF3 Bosch process, two-precedent chamber cleaning
and one conditioning process. The chamber walls were
cleaned with 30/30 sccm SF6/O2 [10] and 50 sccm of O2 both
for 20 min at 20 mTorr, 300 W ICP power and 200 W RF for-
ward power. Conditioning was accomplished with 150 steps
of the following Bosch process on a photoresist (AZ9260)
Figure 1.  Illustrated nozzle dimension parameters. coated non-structured dummy wafer [11]. The gas flow of all
process gases during the Bosch process was 100 sccm with
2.2. Fabrication 300 W ICP power and a turbopump valve opening of 40°.
The nozzle design was realized using photolithography After a first 5 s strike (20 sccm SF6/50 sccm CHF3, 10 mTorr,
(10 µm AZ9260, 208 mJ i-line) and deep reactive ion etching 35 W forward power) pulsed Bosch etching loops consist of
(DRIE) on silicon wafer substrates (phosphor doped, single 5 s CHF3-passivation (10 W forward, 90 W bias power) and
side-polished (SSP)) Czochralski grown silicon wafer with a 8 s SF6-dry-etching (30 W forward, 140 W bias) separated by
diameter of 100  ±  0.3 mm (4″), a resistivity of 5–10 Ω · cm and two 3 s purge steps with the following process gas. 100 µm
a thickness of 525  ±  20 µm) utilizing an Oxford Plasmalab etch depths were achieved by 91 loops accordingly. All pro-
80plus. The process was optimized by variation of exposure cesses were kept at a temperature of 20 °C. Previous results
time, film thickness, gas chopping intervals and inductive cou- showed a resist removal of about 3.6–3.7 µm during dry
pled plasma (ICP) power to achieve a reproducible sidewall etching, which corresponds to a selectivity (silicon to resist
angle and channel depth. Furthermore, a conditioning process etch rate) of about 27.
was found to obtain reproducible etch rates using a versatile
utilized R&D RIE device. By connecting the nozzles with 2.3.  Nozzle tests
attached fluidports (the nozzle and fluidport body are shown
in figure  2(b)) instead of glued fused silica capillary tubes, 2.3.1.  Spray cone.  To obtain an area distribution of the spray
it was possible to realize the nozzles in only one etch depth, cone, P3HT/PCBM solution in chlorobenzene (1:0.75:98.25
wihout requiring a connection plane with tube OD depth. wt%), referred to as OSC solution in the following, was
After substrate cleaning (peroxomono-sulfuric acid (140 °C, sprayed on a glass substrate for defined periods of time at
10 min), ultrasonic 2-propanol bath (10  min)), the struc- a constant position using the micronozzle and conventional
tured silicon wafer is capped using a glass wafer (ultrasonic airbrush nozzle set (Harder & Steenbeck, 0.15 mm diameter)
cleaned (2-propanol, 10 min)), which is pre-drilled (1.2 mm for the purpose of comparison. Chlorobenzene with relatively
in diameter) for media connection, and anodic bonding at low vapor pressure and high boiling point was used as the
400 °C and 1.1 kV. The cleaning with piranha etch becomes sole solvent for P3HT and PCBM in all experiments. Solvents
possible here because the nozzles only consist of the crystal- such as chlorobenzene or dichlorobenzene (DCB) have to
line silicon substrate and a glass wafer. The acid is a widely be deployed because the large surface of small drops greatly
applied cleaning solution for both materials. Nozzles are then enhances evaporation during deposition. The spray spot was
separated by wafer dicing alongside designated scribe lines measured using a P-16 KLA Tencor profilometer.
(figure 3) and as mentioned connected using fluidports and a
nozzle-clamping mount (figure 4(c)) to reversible clamp the 2.3.2. Drop size.  For further nozzle characterization, the
fluidports tight onto the nozzles. The nozzle clamping mount, OSC solution was sprayed at 7 bar on a glass substrate with
media reservoirs and perfluoroalkoxy alkane (PFA) tubings a high stage velocity of 100 mm · s−1 to receive distinct drop-
are installed on the spray head featuring a four-way L-valve for lets. The diameter of at least 1000 dried droplets (after 5 min
immediate after-spray flushing (figure 4(b)). The spray head is of 140 °C annealing) was measured using optical microscopy
easily screwed onto the x-axis stage of the spray coater and the to achieve a proper distribution. In order to receive compa-
atomizing gas tube is connected with the nitrogen supply. The rable values, droplet diameter was calculated back to the drop

3
J. Micromech. Microeng. 27 (2017) 015019 M A Betz et al

Figure 2.  (a) SEM image of the nozzle layout with atomizing gas tube restriction (a1) and media inlet (a2). Fluid flow is indicated with red
(atomizing gas) and blue (broken line) arrows (medium). (b) Separated nozzle with fluidport body and coin. (c) SEM image of the nozzle
opening compiled from three pictures at different planes of focus.

Figure 4.  (a) Spray coater with acrylic spray chamber doors and
control units on the right side. (b) Spray head with L valve (1),
syringe barrel reservoirs for OSC solution (2), chlorobenzene (3),
and clamping mount (4). All mounted on the x axis stage inside
the spray chamber. (c) Top view of clamping mount with clamped
Figure 3.  Photolithographic design of the overall nozzle chip with
micronozzle and connected OSC and atomizing gas tubes.
different nozzle layouts facing each other. Nozzles will be separated to
airbrush nozzle using a Photron FASTCAM SA5 high speed
20 mm  ×  25 mm chips alongside plotted scribe lines. The blue shape
depicts a 100 mm wafer alignable at mask openings on each side. camera, magnification lenses and 50 W LED backlight. Both
nozzles were installed and connected to the atomizing gas
size with the known original chlorobenzene concentration and supply in a setup outside the spray coater to allow the place-
the droplet volume. It seems the relationship is roughly a qua- ment of camera equipment.
dratic correlation for smaller drops, while larger drops form
crater-like droplets instead of flat circular ones, and show a 2.3.4.  Coating efficiency.  Consumption of the OSC solution
higer degree of correlation. A dry droplet with a diameter of was determined by measuring the time for consuming a given
9.89 µm (flat) sprayed at a distance of 3.5 cm for example volume represented by a defined length of the PFA supply tub-
shows a volume of 195.5 µm³ corresponding to a wet spheri- ing. Coating time for homogeneously coated 50 mm  ×  50 mm
cal drop diameter of 7.2 µm. substrates was used to calculate solution consumption. Rever-
sal points of the coating meander pattern with 1 mm spray line
2.3.3. Atomization.  In order to visualize and compare atom- distance (raster) were chosen so as to just not affect sprayed
ization, spraying was recorded for the micronozzle and the layers. The total covered area was 66 mm  ×  66 mm and the

4
J. Micromech. Microeng. 27 (2017) 015019 M A Betz et al

Figure 5.  Diode design with cross section along cut line A.

average layer thickness was measured using the profilometer.

Active volume was calculated from layer thickness and utiliz-
able area (substrate area) and divided by the volume fraction
of organic semiconductors dissolved in the coating solution to
receive the coating efficiency. Calculation assumes the com-
plete evaporation of the solvent during spray coating and the
subsequent annealing. Density values of PCBM (1.33 g · ml−1)
and P3HT (1.15 g · ml−1) are obtained from literature [12].
Spraying was performed using the best parameters for
the conventional airbrush nozzle, set with higher consump-
tion and the micronozzle to receive dry layers. To obtain
reasonably thin layers, the airbrush nozzle was only opened
by 125 µm (pullback length of the needle) and a high nozzle
movement of 100 mm · s−1 was applied. Dry layers could be
obtained at a nozzle distance of 8 cm with an atomizing gas
pressure of 1 bar. The applied operating atomizing gas pres­
sures differ strongly in value as the micronozzle requires up
to 7 bar whereas the airbrush nozzle can only spray coat at a
maximum pressure of ~2–3 bar. Parameters for spray coating
with the micronozzle were 7 bar at smaller distances and suit-
able velocities (2.5 cm at 45 mm · s−1, 4.5 cm at 25 mm · s−1
and 5.5 cm at 10 mm · s−1) as this nozzle shows enhanced Figure 6.  Spray cone diameter. Inset shows spray spot at 3.5 cm
atomization and therefore dry layers are obtained at a much distance after 10 s of spraying. Average numerical eccentricity was
0.213.
shorter nozzle distance.

2.4.  Spray coated layers

Figure 5 schematically shows the design and layer struc-
ture with corresponding thicknesses of a diode chip with four
To acquire spray parameters and evaluate spray results of active areas of 10 mm2 and two of 1 mm2. The OSC layer
the micronozzle, P3HT interlayers (to improve the hole-­ forms a bulk heterojunction (BHJ) nanostructure with large
selectivity of the indium tin oxide (ITO) electrode [13]) and interfaces between donor and acceptor [14].
P3HT/PCBM layers were sprayed at room temperature and in
section 2.3.4 tested distances and velocities to deposit layers
3.  Results and discussion
of about 200 nm thickness per spray step. The P3HT inter-
layer was sprayed twice from a P3HT:chlorobenzene solution
3.1.  Nozzle tests
(1:99 wt%) to guarantee complete coverage of the ITO contact
while accepting additional light absorption without exciton 3.1.1.  Spray cone.  The resulting shape was a ­quasi-circular
generation. P3HT with a molecular weight of 57 kg per mole, ellipse with an average numerical eccentricity of 0.213,
a regio-regularity of 91% and a polydispersity index of  ⩽2.4 though dry particles settle outside this area (figure 6). Shape
was applied (Rieke Metals LLC). The aforementioned P-16 broadening is observed over time as excessive medium is
KLA Tencor profilometer was utilized to measure cross laterally blown away. An opening angle from 8.9 to 13.6°
­section of the OSC layers. is calculated and most likely is about 10°, taking into
Used glass substrates featured 120 nm of structured ITO as account shape broadening at longer spray times and poor
bottom electrode passivated with 1.2 µm of SU-8 photoresist. ­ detectability at shorter spray times. Airbrush spraying
Finally, 200 nm aluminum was vapor coated as top contact and did result in larger spray spots ranging from 14.75 mm to
the diode was encapsulated with epoxy glue inhibiting water 20.84 mm after 5 s at distances from 5 to 11 cm, but a ­similar
and oxygen diffusion. Current density was then measured opening angle (10.59°, without a value at 5 cm due to exces-
over voltage (−2 V to 1 V) with and without green 532 nm sive shape broadening), and average numerical eccentricity
irradiation (780 µW · cm−2). (0.216).

5
J. Micromech. Microeng. 27 (2017) 015019 M A Betz et al

Figure 7.  Probability density function (PDF) and cumulative density function (CDF) of wet drop diameters sprayed at 7 bar and 2.5, 3.5,
4.5 and 5.5 cm distance.

Figure 8.  High-speed camera frames with Δt  =  20 µs for the micro venturi and the airbrush nozzle. Tearing is visible for both nozzles.
While drops next to the micro venturi nozzle opening (red circles) are drawn back into the atomizing gas stream, atomization will
eventually create greater droplets resulting in observed dry particle deposition outside the dedicated spray cone. See appendix A for
animated frames.

Table 2.  Measured and calculated consumption values.

Tubing ID Marked tubing length Calculated volume Consumption time Consumption

(mm) (cm) (µl) (min:s) (µl · min−1)
16.65 134.99 2:37 51.59
1.016 (0.04″) 16.65 134.99 2:48 48.21
16.95 137.42 4:29 30.65
4 (airbr.) 15 1884.96 1:57 1966.64

3.1.2.  Drop size.  The wet drop diameter follows a lognormal as well, which, in our view, might be contributed to by the
distribution as can be seen in figure 7. The probability density agglomeration of drops.
function of the drop diameter distribution shows a geomet-
ric mean of 7.4/7.9/10.4/11.6 µm (2.5/3.5/4.5/5.5 cm) and a 3.1.3. Atomization.  In order to investigate atomization, the
geometric standard deviation of 1.40/1.34/1.46/1.34 µm. With spray performance was recorded with a high-speed camera. As
increasing nozzle distance the wet drop diameter increases velocities are very high and the depth of field is shallow, drops

6
J. Micromech. Microeng. 27 (2017) 015019 M A Betz et al

Table 3.  Calculations for coating efficiencies of micronozzle and airbrush.

Spray distance Nozzle velocity Pattern Consumption Coating Thickness Coating

(cm) (mm · s−1) (mm2) Spray steps (µl min−1) time (min) (nm) efficiency (%)
3 45 66  ×  66 5 51.59 12 821 20.66
3 45 56  ×  56 5 48.21 9:20 1085 35.1
2.5 45 66  ×  66 5 30.65 12:01 453 19.16
4.5 66  ×  66 1 3:31 166 24.02
25 30.65
66  ×  66 5 17:35 673 19.46
5.5 66  ×  66 1 7:37 341 22.73
10 30.65
66  ×  66 6 45:42 2007 22.32
8 (airbrush) 100 66  ×  66 1 1966.64 1:22 741 8.74

Figure 9.  Photo diode device with four diodes of 10 mm2 and two of 1 mm2 active areas according to the design depicted in figure 5. The
profile shows the organic layer with a root mean square (rms) of 206.3 nm. Subjacent layers have been subtracted but may have caused an
increase in roughness.

themselves could not be visualized during flight, but constant

or high frequency tearing from drops right below and above
the micronozzle opening could be observed. These drops form
either due to the condensing of the atomized medium or sim-
ply due to not yet atomized medium. Tearing becomes more
and more frequent with higher atomizing gas pressure, result-
ing in quasi constant spraying at 7 bar and higher. The airbrush
nozzle set also shows this pressure-dependent tearing though,
at lower pressure values up to 2 bar and with higher outputs. In
all cases, the medium is atomized outside of the nozzle when
it enters or re-enters the atomizing gas flow. Medium drops
accelerated perpendicular to the propellant flow are drawn
back to the stream. Atomization inside the nozzle could not be
observed by the means applied. As the 3D cone forms, spray
drops move out of focus quickly and only a small part of the
pictured spray cone can be visualized clearly at the same time
(figure 8).
Figure 10.  Current density over voltage for first sprayed diodes
with a dark current (black curve) of ~7.41 · 1025 mA · cm−2 and an
3.1.4. Coating efficiency.  The determined consumption val- external quantum efficiency (EQE) of 72.9% (both measured at
ues are given in table  2. The airbrush nozzle uses a larger   −2 V), calculated from the illuminated green curve. Standard
medium tubing and shows much higher consumption. The deviation of the error (SE) results from averaging four diodes.

7
J. Micromech. Microeng. 27 (2017) 015019 M A Betz et al

consumption of the micronozzle was approximately 50 µl · min−1 Performance is comparable to reported values of ~70% for
for the first experiments at 3 cm distance. When supplement- OPDs with ~450 nm P3HT/PCBM layer at  −5 V bias [17].
ing results at additional distances, a reduced consumption of The quantum yield could be further increased by reducing the
just above 30 µl · min−1 was detected but similar efficiencies thickness of the P3HT interlayer, which absorbs light without
were achieved, as given in table 3. In our view this might be as generating charge carriers.
result of reduced solvent content due to evaporation.
By measuring the time and thickness of the respective 4. Conclusion
coating, the coating efficiency can be calculated as described
in section 2.3.4. For all micronozzle coatings, an average effi- Micro venturi nozzles have been realized in a silicon wafer
ciency of 21.39%  ±  1.94% is calculated, which corresponds using a single dry etch process and anodic bonding. Produced
to a 2.45 fold increase compared to airbrush efficiency. The nozzles featuring a rectangular nozzle opening of 50 µm
required amount of OSCs to achieve the same layer thickness width and 100 µm depth generate a quasi-circular spray cone,
is hence reduced by 59%, allowing economical spray coating. which is important for the overlap of spray lines in orienta-
As the spray cone diameter using the micronozzle is smaller tion-independent coating (from the spray line center gradually
essentially due to the smaller nozzle distance, reversal points decreasing in thickness in all directions). The cleaning of
may be set closer to the substrate without affecting the active nozzles with aggressive acids (peroxomono-sulfuric acid) and
area. Using a 56 mm  ×  56 mm spray pattern, coating efficiency strong solvents (e.g. chlorobenzene) was easy and fast. The
further increases to 35.1% and consumed volume is reduced best spray results were achieved at a short nozzle distance as
by 75% in total (a 4.01 fold coating efficiency). The coating solvent evaporation is enhanced by the small drop diameter
efficiency for large substrates should approach a theoretical with a distance dependent geometric mean of 7.4 µm–11.6 µm.
maximum of ~15.16% for the airbrush nozzle set and ~44% The average coating efficiency of the micronozzles was
using the micronozzle (a 2.9 fold coating efficiency) with the 21.39%  ±  1.94% (35.1% for smaller pattern) for 25 cm2 active
overlapping area of the spray pattern eventually becoming area and therefore 2.45 times (4.01) higher than spraying with
negligible. These values almost reach and considerably exceed a conventional airbrush nozzle set. Presumably, the micro noz-
the coating or ‘transfer’ efficiency of ~20% empirically deter- zles feature a more directional spray jet and a more efficient
mined for an ultrasonic spray nozzle [15], even though this use of the atomized solution as no deposition of dry particles/
value may vary noticeably with different nozzles. dust is observed in the spray coater in contrast to spraying with
the airbrush nozzle. This results in a 59% (75%) lower con-
3.2.  Sprayed layers sumption and hence lower material costs when manufacturing
organic photovoltaic or organic photodiodes. To achieve a suf-
OSC layer homogeneity decreases with greater micronozzle ficient coating speed, multiple nozzles or nozzle arrays may
distance, which indicates small drops with low solvent fraction be deployed. Diodes produced from sprayed P3HT/PCBM
after greater distances. Droplets were distinguishable partially organic semiconductor layers show state-of-the-art perfor-
through transmitted optical microscopy, as further drops do mance with a comparably low dark current of ~7.41 · 10−5
not dissolve already dried subjacent droplets. Their size ranges mA · cm−2 and an external quantum efficiency of 72.9% at
from about 5 to 50 µm and matches the droplet size measured 532 nm, both measured at  −2 V. Performance may even be
in section 3.1. Results are shown for the smallest nozzle dis- improved and layer thickness minimized by optim­ization of
tance of 2.5 cm as for greater distances with fewer spray steps, solution and solvent (e.g. DCB) and higher atomizing gas
layer inhomogeneity partially resulted in nonclosed layers, pressure. It is reported that layer roughness may be consider-
which causes short circuits during electrical characterization. ably decreased by subsequent solvent coating steps [18, 19]
Even at a short distance a high roughness root mean square and that this is likely compatible to continuous, e.g. roll-
(rms) of 206.3 nm of the P3HT interlayer and OSC stack was to-roll, processing [20]. To improve the variance of the drop
measured (figure 9). size distribution and reduce drop formation as seen in figure 8,
No short-circuits were detected for the small nozzle dis- a design with a central medium channel and surrounding
tance of 2.5 cm with a comparably high stage velocity of atomizing gas channels is being tested at present.
45 mm · s−1 and five spray steps. J/V characteristics of 10 mm2
active area diodes are shown in figure 10.
Current density over voltage of produced diodes shows an Acknowledgment
average dark current of 7.41 · 10−5 mA · cm−2 (−2 V), which
is within a common magnitude for organic photodiodes, e.g. We would like to thank the German Federal Ministry for Edu-
[16]. Under green illumination (532 nm, 780 µW · cm−2), cation and Research (BMBF) for funding this work (FHPro-
an average current density of 0.244 mA · cm−2 (−2 V) was fUnt grant 03FH040PX2). Furthermore, we would like to
measured. An EQE of 72.9% is calculated from the ratio of thank the Georg-August University Göttingen for providing
generated charge carriers to photons at this wavelength. the high-speed camera and related optical equipment.

8
J. Micromech. Microeng. 27 (2017) 015019 M A Betz et al

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