This document discusses considerations for organic photovoltaic (OPV) thin-film processing and spin coating. It addresses general OPV requirements, practical fabrication issues like cleaning and solvent compatibility, and spin coating parameters that influence film thickness and morphology such as solution concentration, spin speed, and solvent selection. The ideal is to use solvent blends that allow for good surface wetting and rapid drying while also permitting molecular self-organization in the film. Processing conditions like atmosphere, temperature, and substrate treatment are also crucial factors for technologies like perovskite solar cells.
7. Mobility and trap states / impurities
Electron in
acceptor LUMO
Hole in donor
HOMO
affects fill factors, especially as film
thickness increases
Poole-Frenkel (hopping) based mobility
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8. Morphology
E
v
Ev ļ½ļ®ļ½ ļļ
v: velocity of the carrier,
E=VDS/L: electrical field across the OSC
Ī¼: Carrier mobility; [Ī¼] =cm2/(VĀ·s)
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- Long-Chain Polymeric OSC
10. List of requirements - OPV
Requirement Target Reason Defined By
Donor HOMO -5.6 to -6 eV Air stability Materials
Donor bandgap 1.6 eV Light harvesting
efficiency
Materials
Acceptor energy
levels
āE 0.3 to 0.5 eV Efficient charge
separation
Materials
Phase separation 10 to 20 nm Efficient charge
separation
Processing /
materials
Charge transport Āµ > 10-3 cm^2/VS Effective charge
transport
Processing /
materials
Solubility > 4 mg/ml Film forming
properties
Processing /
materials
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11. Planar versus bulk heterojunction
TCO
Glass or PET
Charge selective interface
Light harvesting layer
Charge selective interface
Back contact
Bulk heterojunction
planar heterojunction
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12. Range of architectures - OPV
Substrates TCO Hole interfaces Electron interfaces Back contacts
Standard Glass ITO PEDOT:PSS Calcium Aluminium
Flexible glass IZO CVD PEDOT Aluminium Silver
PET / PEN AZO MoO3 Cs2CO3 PEDOT:PSS
Metal foil Ag nanowires VO3 Ca(caac) Ag nanowires
PEDOT:PSS MoO3 solgel LiF Graphene
Graphene Cl ā ITO TiOx Laminated ITO
O2 ITO ZnOx
ZrOx
PFN
PEIE
C60
BCP
CuPc
For a review see āInterface materials for organic solar cellsā
Roland Steim, F. Rene Kogler and Christoph J. Brabec, J. Mater. Chem., V20, P2499 (2010)
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14. Perovskite
ETL
TCO
HTL
Perovskites ā fantasy vs. reality
Cathode
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Ideal architecture
Energy Environ. Sci., 2014,7, 399-407
ā reality?
Non-perovskite structure
Organic precursor
Lead salt
āThe technology, as it stands, is suboptimal, primarily resulting
from large-scale inhomogeneity in film uniformity and layer
thicknesses...optimization through better control over all of the
processing parameters should push the efficiency...closer to
20%ā ā Henry Snaith (J. Phys. Chem. Lett. 2013, 4, 3623ā3630)
15. Perovskite crystallisation
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Angewandte Chemie International Edition, 2014, 53, pages 9898-9903.
Device structure and
photovoltaic characterization.
a)ā Schematic illustration of a
typical photovoltaic device.
b)ā Crossāsectional SEM image
of an optimized device.
Schematic illustration of fast
crystallisation and conventional
spinācoating process for fabricating
perovskite films.
16. Processing conditions - perovskites
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Process Atmosphere Annealing Precursors Buffer layers
Spin coating Temperature Temperature Purity Composition
Blade coating Humidity Time Molar ratio Orthogonality
Spray coating Environment Method
- oven/hotplate
- solvent
Solvents
- solubility
- orthogonality
Energy level
alignment
1 or 2 step Drying time Environment Concentration Thickness
Substrate temperature Additives Interface
Wettability
Coverage
MAI Procedures
Author Journal Year HI stabiliser? Nitrogen? Temp Time Washed? Drying Efficiency
Xiao Energ. & Envirvon. 2014 Y Y 0Ā°C 2hr Y Oven 15.4
Liang Adv. Mater. 2014 Y Y 0Ā°C 2hr Y Oven 11.8
Eperon Adv. Func. Mater. 2013 ? ? R.T. - ? Oven 11.4
Docampo Adv. Energ. Mater. 2014 ? ? R.T. 1hr Y ? 14.8
Burschka Nature Letter 2013 ? ? 0Ā°C 2hr ? ? 15
Shi Appl Mater. Interfaces 2014 ? ? Ice bath 2hr Y Vacuum 10.5
Kim Nanoscale 2014 ? Y 0Ā°C 2hr Y Vacuum oven 6.2
18. Physically clean vs. chemically clean
H
O
H
O
H
O
H
O
H
O
H
O
H
O
H
O
H
O
H
O
H
O
H
O
H
O
H
O
H
O
H
O
H
O
H
O
H
O
H
O
H
O
H
O
H
O
H
O
H
O
H
O
Chemically clean surface
with low surface energy
Dust contamination
Local change in surface energyPin-hole formed in later layers
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20. Cleaning routines
Remove dust and gross contamination
(fingerprints etc)
Substrate
Dirt/Dust
Surfactant
cleaning Substrate
Residue
Solvent
cleaning
Substrate
Substrate
H
O
H
O
H
O
H
O
H
O
H
O
H
O
H
O
H
O
H
O
H
O
H
O
H
O
H
O
H
O
H
O
H
O
H
O
NaOH or
UV/Ozone
treatment
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21. UV / Ozone
Handbook of Semiconductor Wafer Cleaning Technology, Science Technology and Applications, Edited by Werner Kern, Noyes publications.
Chapter 6 ā āUltraviolet-Ozone Cleaning of Semiconductor Surfacesā, John R .Vig
Contaminant
Molecules
U.V.
Ions
Free Radicals
Excited States
Neutral Molecules
Volatile Molecules
(CO2, H2O, N2 etc)
U.V.O2
O, O3
+
+
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26. Statistics - practise factor
Substrate #
Efficiency
1 2 3 4 5
Substrate #
Efficiency
1 2 3 4 5
Unpractised Fabricator Practised Fabricator
still some
clumping
still some
poor pixels
ā„
Use multiple substrates per processing condition
Substrate to
substrate variation
Pixel to
pixel variation
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27. Process delays and randomisation
A A A A B B B B C C C C
A B C A B C A B C A B C
Always randomise or alternate the substrate order in a device run:
If you donāt then spurious data can be generated with trends that arenāt seen
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28. Process stability
ITO Substrates On shelf > 2 years
ITO substrates Cleaned and stored in IPA or DI water > 3 days
PEDOT:PSS Ambient conditions < 10 mins
PEDOT:PSS Hotplate in air ~ 3 Hours
PEDOT:PSS Glovebox ~ 3 hours
Active layer Ambient conditions >1 hour (material
dependent)
Active layer Glovebox >3 days (material
dependent)
Finished device Ambient unencapsulated < 1 hour
Finished device Ambient encapsulated < 6 months (dependent on
conditions)
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31. General principal of operation
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ā¢ The rotation of the substrate pulls the
liquid into an even coating
ā¢ The solvent evaporates to leave a film of
the material on the substrate
ā¢ Used to coat small substrates (from a
few mm square) to flat panel TVs
ā¢ Can be used for photoresists, insulators,
organic semiconductors, synthetic
metals, nanomaterials, metal and metal
oxide precursors, transparent
conductive oxides and many, many more
32. General principal of operation
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Advantages
ā¢ Simplicity and relative ease
ā¢ Thin and uniform coating
ā¢ Fast drying times
ā lower performance
Disadvantages
ā¢ Batch process
ā low throughput
ā¢ Fast drying times
ā lower performance
ā¢ Wasted material
ā usage is typically very low at around 10%
33. Drying time
Spin cast 1000 RPM Spin cast 300 RPM Drop cast (covered)
~2mm
Right: Effect of P3HT solvent
(drying time) on absorption
spectra.
Below: Microscope images of the
effect of TIPS-Pentacene casting
conditions (drying time) on
crystal size.
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34. Film thickness
The exact thickness of a film will depend
upon:
ā¢ Solution concentration
ā¢ Solvent evaporation rate:
ā¢ viscosity
ā¢ vapour pressure
ā¢ temperature
Spin thickness curves for new inks are
most commonly determined empirically,
and making a calibration curve:
ā¢ Elipsometry
ā¢ Surface profilometry (Dektak).
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Example spin thickness curve for a solution
36. Common problems ā incomplete coating
Solvent + substrate combination results in difficult wetting and partial coating
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Non-wetting
Negligible wetting
Partial non-wetting
Partial wetting
Complete wetting
Spreading
0
90
180
More Wetting
Less Wetting
http://www.ebatco.com
37. Common problems ā incomplete coating
Solvent + substrate combination results in difficult wetting and partial coating
Solution
ā¢ Larger dispense volume of solution
ā covers the substrate reducing ability to dewet
ā¢ Increase solution temperature
ā reduces the surface tension and increases evaporation rate
ā¢ Leave solution to aggregate slightly
ā aggregates help to pin the meniscus to the surface and stop it from receding
ā¢ Change the solvent
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38. Solvent issues
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Low boiling solvents
(e.g. chloroform):
ā¢ Good surface wetting
ā¢ Quick drying -> disorganised film
High boiling point solvent
(e.g. trichlorobenzene):
ā¢ Slow drying
ā¢ Solution dewet and flung off edge
39. Solvent Blends
Can get best of both worlds by mixing
solvents:
ā¢ Large component of low boiling point
solvent:
ā¢ wets the surface well
ā¢ evaporates quickly
ā¢ Small component of high boiling point
solvent :
ā¢ evaporates slowly allowing time for
molecular self organisation
ā¢ Limit to miscibility if dipole moment too
dissimilar
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A recent meta analysis of the state of organic photovoltaics in the literature showed that the modal efficiency of devices tended towards zero! There are relatively few high efficiency devices but a lot of low efficiency devices. We all know that headline metrics and quality of science shouldnāt be linked but we all know that in practise they are. Science is competitive and the aim of this course is to raise efficiencies and give people scientific competitive advantage.
The purpose of this training course is to transfer practical know-how in OPV fabrication and testing so that people can re-produce the results in their own labs. We intend to cover a range of the āstandardā architectures such that delegates will be familiar and comfortable with their fabrication routines. In general all of the devices that we work on will be R&D focussed where the emphasis is on overall functionality and versatility rather than scale-up or flexibility.
We also aim to cover a number of the common mistakes and problems to help people avoid them.
The course is not intended as a general introduction to organic electronics and/or OPVs and so although we will be reviewing the general concepts to ensure we have a consistent language, we will be covering this at relative speed and refer people to review papers and text-books for a more in-depth study.
Long polymer may look like a spaghetti bowl: messy, with entangled polymer chain. Alkyl side chains are very effective in preventing spaghetti-like OSC.
In general the degree of self organisation may change over the same sample: region of highly (or semi!)-organised polymers are surrounded by amorphous material: this make any theoretical study (or even simple comparison of transistor performance fabricated with the same OSC) quite complicated. In other words: are we studying/measuring the intrinsic OSC performance or the effect of the growth and self-organisation of the OSC layer?
A look at the key requirements for efficient solar cells shows that many of the properties are defined by the materials being used and it is these that will limit the possible performance. For this reason it is materials development that has the largest effect on the progress of the field. However, by fully optimising the processing conditions the maximum performance can be gained from any individual materials set.
Dust is insulating and wonāt kill a OE device in itself as it will just produce a small dead spot which will only reduce the overall performance very slightly as this is an average over the whole area.
However, dust can usually be blown off and the surface properties where dust has been changes. This can lead to pin-holes in later layers and cause device failure even after removal. However, this effect depends upon which layers are being put down next ā i.e if pin-holes will cause critical failure (such as for top-gate dielectrics).
Rubber filters fatal as dissolved by acids (PEDOT:PSS) or chlorinated solvents
In this presentation weāll be referring regularly to two proto-typical materials ā P3HT as an example of a polymeric semiconductor and TIPS-Pentacene as an example small molecule. In the case of P3HT the crystallinity of a film can be seen by eye as a colour change due to a vibronic absorption shoulder appearing at around 620 nm in the absorption spectra. In the case of TIPS-pentacene we can see the crystals easily under a microscope. In both cases higher crystallinity means higher performance but requires longer drying time. Unfortunately, the longer the drying time the larger the opportunity for dewetting.
Weāre relatively familiar with contact angles and wetting from everyday life... Beads of water on a āfurryā leaf such as a lotus leaf lead to droplets forming that simply roll off. Meanwhile the ālegsā (sometimes also called ātearsā) on a glass of wine are a sign of a high alcohol increasing the wettability of the wine on the glass. Good wetting is essential for device fabrication and becomes even more critical for high throughput fabrication techniques. In general if a contact angle is less than 90 degrees we would consider the surface to be āwettable.ā
Solvent blends can also help significantly for solutions that are close to the wetting envelope. Using a low boiling point solvent at high spin speed will enable wetting as the solvent will evaporate quickly leaving insufficient time to dewet. However, on itās own this would give poor performance as there is not time for the molecules to organise themselves. However, by adding a small amount of a high-boiling point solvent the film remains slightly wet but in a gelatinous state that is less likely to dewet but still gives the molecules time to organise themselves.