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.

1. Organic photovoltaics
thin-flm processing considerations
Dr Max Reinhardt
Ossila Ltd.
11/03/2015 1

2. Purpose
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3. Content
• General OPV considerations and requirements
• Practical fabrication issues
– Cleaning and processing conditions
• Spin coating considerations
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4. General OPV considerations

5. OPV (Organic Photovoltaic)
Buffer
Acceptor phase
Donor phase
Light
Buffer layer
Anode Cathode
Buffer layer
Blended donor and
acceptor phases
Power supply
OLED Stack
OPV stack
OPV: conversion of the photon energy into
electrical energy (power) exploiting the properties
of the conjugate molecules
Increasing efficiency increase device complexity
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6. Ideal Morphology
Donor
Acceptor
Anode
Cathode
interface material
Perovskite
Cathode
Interface material
Anode
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Organic bulk heterojunction solar cell
Pure perovskite phase solar cell

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  
v: velocity of the carrier,
E=VDS/L: electrical field across the OSC
μ: Carrier mobility; [μ] =cm2/(V·s)
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- Long-Chain Polymeric OSC

9. Some degree of organisation....
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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
∆E 0.3 to 0.5 eV Efficient charge
separation
Materials
Phase separation 10 to 20 nm Efficient charge
separation
Processing /
materials
Charge transport µ > 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 – ITO TiOx Laminated ITO
O2 ITO ZnOx
ZrOx
PFN
PEIE
C60
BCP
CuPc
For a review see “Interface materials for organic solar cells”
Roland Steim, F. Rene Kogler and Christoph J. Brabec, J. Mater. Chem., V20, P2499 (2010)
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13. Solvent compatibility
Solvent
MP
(°C)
BP
(°C)
Density
(g/cm3)
Refracti
ve index
Er
Dipole
moment
Surface
Tension
(dyn/cm)
Viscosity
(mPa.S)
Water 0 100 0.997 1.333 80.2 1.85 72 1
Dimethyl Sulfoxide 19 189 1.100 1.48 48 3.96 43 2.14
Glycerol 17.8 290 1.261 1.473 42.5 63.4 1069
Methanol -98 65 0.792 1.328 32.7 1.7 22.6 0.593
Ethanol -114 78 0.789 1.36 24.6 1.69 22.3 1.144
Acetone -95 56 0.791 1.359 20.7 2.88 23.7 0.308
IPA -89 82.5 0.785 1.378 18 1.66 21.7 1.96
1,2 Dichlorobenzene -17 180.5 1.3 1.551 9.8 2.14 35.7 1.32
Dichloromethane -96.7 39.6 1.33 1.425 9.1 1.6 26.5 0.41
Tetrahydrofuran -108.4 66 0.889 1.404 7.5 1.75 26.4 0.456
Chlorobenzene -45 131 1.11 1.524 5.7 1.54 33 0.753
Chloroform -63.5 61.2 1.48 1.49 4.8 1.04 26.7 0.563
Toluene -93 110.3 0.865 1.497 2.4 0.36 28.5 0.550
Benzene 5.5 80.1 0.874 1.501 2.3 0 28.9 0.652
p-Xylene 13 138 0.861 1.496 2.2 0.07 28.3 0.648
1,2,4 trichlorobenzene 16.9 214.4 1.46 1.572 2.2 0 39.1 1.611
Cyclohexane 6.9 80.74 0.779 1.426 2.0 0 25.3 0.93
Hexane -95 69 0.655 0.375 1.9 0 18.4 0.326
P3HT
PEDOT:PSS
PFN
PCBM
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14. Perovskite
ETL
TCO
HTL
Perovskites – fantasy vs. reality
Cathode
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Ideal architecture
Energy Environ. Sci., 2014,7, 399-407
– reality?
Non-perovskite structure
Organic precursor
Lead salt
“The 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%” – Henry Snaith (J. Phys. Chem. Lett. 2013, 4, 3623−3630)

15. Perovskite crystallisation
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Angewandte Chemie International Edition, 2014, 53, pages 9898-9903.
Device structure and
photovoltaic characterization.
a) Schematic illustration of a
typical photovoltaic device.
b) Cross‐sectional SEM image
of an optimized device.
Schematic illustration of fast
crystallisation and conventional
spin‐coating 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 0°C 2hr Y Oven 15.4
Liang Adv. Mater. 2014 Y Y 0°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 ? ? 0°C 2hr ? ? 15
Shi Appl Mater. Interfaces 2014 ? ? Ice bath 2hr Y Vacuum 10.5
Kim Nanoscale 2014 ? Y 0°C 2hr Y Vacuum oven 6.2

17. Practical fabrication issues

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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19. Effect of dust/dirt
PEDOT:PSS
OSC
ITO
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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 – “Ultraviolet-Ozone Cleaning of Semiconductor Surfaces”, 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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22. Filtration
Polymer Aggregates
CB DCB TCB
PCBM Crystallites
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23. Syringe filters
Rubber filters fatal !
Use all polypropylene
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24. Vials / Septa’s
PTFE
Solvent
Vial on hotplate
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25. Effect of residues / impurities
Erratic JV curves
Sources:
•Dirty substrates / grease
•Cleaning agents
•Solvent contaminants
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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
≥
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 don’t then spurious data can be generated with trends that aren’t 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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29. Spin coating considerations

30. Digital Signal
Solution deposition techniques
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31. General principal of operation
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• The rotation of the substrate pulls the
liquid into an even coating
• The solvent evaporates to leave a film of
the material on the substrate
• Used to coat small substrates (from a
few mm square) to flat panel TVs
• 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
• Simplicity and relative ease
• Thin and uniform coating
• Fast drying times
– lower performance
Disadvantages
• Batch process
– low throughput
• Fast drying times
– lower performance
• Wasted material
– 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:
• Solution concentration
• Solvent evaporation rate:
• viscosity
• vapour pressure
• temperature
Spin thickness curves for new inks are
most commonly determined empirically,
and making a calibration curve:
• Elipsometry
• Surface profilometry (Dektak).
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Example spin thickness curve for a solution

35. Wetting
θ > 90
θ tangent θ tangent
θ = 90 θ < 90
θ
tangent
Hydrophobic Hydrophillic
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36. Common problems – 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 – incomplete coating
Solvent + substrate combination results in difficult wetting and partial coating
Solution
• Larger dispense volume of solution
– covers the substrate reducing ability to dewet
• Increase solution temperature
– reduces the surface tension and increases evaporation rate
• Leave solution to aggregate slightly
– aggregates help to pin the meniscus to the surface and stop it from receding
• Change the solvent
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38. Solvent issues
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Low boiling solvents
(e.g. chloroform):
• Good surface wetting
• Quick drying -> disorganised film
High boiling point solvent
(e.g. trichlorobenzene):
• Slow drying
• Solution dewet and flung off edge

39. Solvent Blends
Can get best of both worlds by mixing
solvents:
• Large component of low boiling point
solvent:
• wets the surface well
• evaporates quickly
• Small component of high boiling point
solvent :
• evaporates slowly allowing time for
molecular self organisation
• Limit to miscibility if dipole moment too
dissimilar
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40. www.Ossila.com
11/03/2015 40
Spin coating guide Perovskite materials
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