organic photovoltaics thin-film processing considerations
TRANSCRIPT
Organic photovoltaicsthin-flm processing considerations
Dr Max ReinhardtOssila Ltd.
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Purpose
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Content
• General OPV considerations and requirements
• Practical fabrication issues
– Cleaning and processing conditions
• Spin coating considerations
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General OPV considerations
OPV (Organic Photovoltaic)
Buffer
Acceptor phase
Donor phase
Light
Buffer layer
Anode Cathode
Buffer layerBlended 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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Ideal Morphology
Donor
Acceptor
Anode
Cathodeinterface material
Perovskite
CathodeInterface material
Anode
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Organic bulk heterojunction solar cell
Pure perovskite phase solar cell
Mobility and trap states / impuritiesElectron in
acceptor LUMO
Hole in donor HOMO
affects fill factors, especially as film thickness increases
Poole-Frenkel (hopping) based mobility
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Morphology
E
vEv
v: velocity of the carrier,E=VDS/L: electrical field across the OSC
μ: Carrier mobility; [μ] =cm2/(V·s)11/03/2015 8
- Long-Chain Polymeric OSC
Some degree of organisation....
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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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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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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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Solvent compatibility
SolventMP (°C)
BP (°C)
Density (g/cm3)
Refractive index
ErDipole 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
P3
HT
PED
OT:
PSS
PFN
PC
BM
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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)
Perovskite crystallisation
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Angewandte Chemie International Edition, 2014, 53, pages 9898-9903.
Device structure andphotovoltaic characterization.a) Schematic illustration of atypical photovoltaic device.b) Cross‐sectional SEM imageof an optimized device.
Schematic illustration of fastcrystallisation and conventionalspin‐coating process for fabricatingperovskite films.
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
Practical fabrication issues
Physically clean vs. chemically clean
HO
HO
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HO
HO
HO
HO
HO
HO
HO
HO
HO
HO
HO
HO
HO
HO
HO
HO
HO
HO
HO
HO
HO
HO
HO
Chemically clean surface with low surface energy
Dust contamination
Local change in surface energyPin-hole formed in later layers11/03/2015 18
Effect of dust/dirt
PEDOT:PSS
OSC
ITO
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Cleaning routinesRemove dust and gross contamination (fingerprints etc)
Sub
stra
te
Dir
t/D
ust
Surfactant cleaning
Sub
stra
teR
esid
ue
Solvent cleaning
Sub
stra
te
Sub
stra
te
H OH O
H OH O
H OH O
H OH O
H OH O
H OH O
H OH O
H OH O
H OH O
NaOH or UV/Ozone treatment
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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.IonsFree RadicalsExcited StatesNeutral Molecules
Volatile Molecules(CO2, H2O, N2 etc)
U.V.O2O, O3
+
+
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FiltrationPolymer Aggregates
CB DCB TCB
PCBM Crystallites
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Syringe filters
Rubber filters fatal !Use all polypropylene
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Vials / Septa’s
PTFE
Solvent
Vial on hotplate
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Effect of residues / impurities
Erratic JV curves
Sources:•Dirty substrates / grease•Cleaning agents•Solvent contaminants
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Statistics - practise factor
Substrate #
Effi
cien
cy
1 2 3 4 5Substrate #
Effi
cien
cy
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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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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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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Spin coating considerations
Digital Signal
Solution deposition techniques
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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
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%
Drying time
Spin cast 1000 RPM Spin cast 300 RPM Drop cast (covered)
~ 2
mm
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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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
Wetting
θ > 90
θ tangent θ tangent
θ = 90 θ < 90
θ
tangent
Hydrophobic Hydrophillic
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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
Spreading0
90
180
More Wetting
Less Wetting
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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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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
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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