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

HO

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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Spin coating guide Perovskite materials

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