Organic-based light harvesting

electronic devices

Maddalena Binda

Organic Electronics: principles, devices and applications

Milano, November 26-29th, 2013

SENSORISTICS

OPTICAL COMMUNICATIONS

Photodetector Light source

Optical Fiber

IMAGING

REMOTE CONTROL

From signal detection to power generation...

Organic-based light harvesting devices

High speed

Spectral selectivity

High signal-to-noise ratio

Broad responsivity

From signal detection to power generation...

Hardiness in critical conditions

Bottom transparent electrode Glass

Top Electrode

Active material

CW operation

Bias voltage allowed Bias voltage not allowed

Organic-based light harvesting devices

Photogeneration efficiency: EQE

Light sensors:

Organic PhotoDetectors (OPD)

Light sensors: sensitivity

signal

We want to go from this... ...to this:

High signal-to-noise ratio (SNR)

noise

Increase signal Increase EQE

Reduce parasitic effects like e.g. leakage current

ITO+PEDOT:PSS Glass

Vbias

Al

Active material

-4.7eV

-4.0eV

+

-4.3eV Al

ITO -

-5eV M. Binda et al. Appl. Phys. Lett. 98 (7), 073303, 2011.

Light sensors: sensitivity

signal

We want to go from this... ...to this:

High signal-to-noise ratio (SNR)

noise

Increase signal Increase EQE

Reduce parasitic effects like e.g. leakage current

M. Binda et al. Appl. Phys. Lett. 98 (7), 073303, 2011.

-

ITO Glass

Vbias

MEH-PPV

Al

Active material

-4.7eV +

-4.3eV Al

ITO

-5eV

Cross-linked

MEH

-PP

V

0,1 1

100p

1n

10n

100n

I da

rk [A

/cm

2]

V [V]

without PPV

with PPV

400 500 600 700 800 900 1000

1

10

with PPV

without PPV

EQ

E [%

]

Wavelength [nm]

M. Binda et al., Appl. Phys. Lett. 98 (7), 073303, 2011.

SEE ALSO: P. E. Keivanidis et al., Appl. Phys. Lett. 94, 173303, 2009. X. Gong et al., Science 325, 1665, 2009.

Reduced leakage current while EQE is conserved

SNR

Light sensors: sensitivity

1E-9

1E-7

1E-5

1E-3

electrons

holes

[cm

2/V

s]

AlkSQ:PCBM

1:1

GlySQ:PCBM

1:3GlySQ:PCBM

1:1

Play with: •device geometry –> Reduced interelectrode spacing •active material composition

Light sensors: response speed E.g. APPLICATION IN OPTICAL DATA TRANSMISSION

-500 0 500 1000 1500 2000

0,0

0,2

0,4

0,6

0,8

1,0

GlySQ:PCBM 1:1

No

rma

lize

d p

ho

tocu

rre

nt

Time [ s]

AlkSQ:PCBM 1:1

GlySQ/PCBM 1:3

Fast reponse is required!

M. Binda et al., Organic Electronics 10 (2009) 1314–1319

Adv. Mater. 2013, 25, 4335–4339

BW~10kHz

Organic (and hybrid) Solar Cells

scoc

mppmpp

IV

IVFF

Power Conversion Efficiency (PCE)

Power Conversion Efficiency of a solar cell

LOAD

Series and Shunt resistance of a solar cell

Photocurrent flow

Rs

Rs

Rsh

We want: Rs low Rsh high

poorly conductive paths in the semiconductor, contact resistance, resistivity of the electrode,...

Leakage paths: metal infiltration, pinholes,...

scoc

mppmpp

IV

IVFF

Power Conversion Efficiency (PCE)

Power Conversion Efficiency of a solar cell

Important notes on cell characterization:

2. A standard path must be adopted

Standard AM1.5 - 1 sun

air-mass: optical path through the Earth athmosphere

scattering absorption zL

LX

cos

1

0

L0=zenit path length at see level L=effective path length Es. z=0 AM1

z

AM1.5 z=48.2º (middle latitudes)

~1000W/m2

Power Conversion Efficiency of a solar cell

AM X L L0

1. The solar emission must be reproduced

Different parameters must be simoultaneously optimized:

IN

SCOC

P

IVFFPCE

Power Conversion Efficiency of a solar cell

But they are indeed strongly correlated!!!

Debated...

Open Circuit Voltage (VOC)

See: Adv. Funct. Mater. 11, 5, 2001. Adv. Funct. Mater. 2011, 21, 2744–2753 Adv. Mater. 2010, 22, 4987–4992 Nat. Materials, 8, 904-909, 2009

D A

LUMO, A

HOMO, D

Adv. Mater. 18, 789 (2006)

empirical

at solar intensity

Voc=(1/q)(HOMOD-LUMOA – 0.3)

Voc=(1/q)(HOMOD-LUMOA – 0.3) empirical

Jnet(x)=0

Voc,max=HOMOD-LUMOA

G=R(x)

G=Rgeminate G=Rlangevin G=RSRH

....

?

Es. R=Rlangevin=γnh(x)ne(x)

eh

BgNN

GTkEqVoc ln

PHYSICAL REVIEW B 84, 075210 (2011)

Recombination -0.2 -0.5

Logaritmic dependence on G, often reported in the literature

Open Circuit Voltage (VOC): recombination

*Boltzmann approx., no disorder

*

qVoc=EFh-EFe

EFh

EFn

not the complete story...egergetic disorder plays a role!!!

Recombination via localized states

Open Circuit Voltage (VOC): energetic disorder

Carriers relaxed into the tail of the DOS reduce the quasi-Fermi level splitting

......ln.

GTmkconstqVoc B

HOMOD -LUMOA-ΔE

m>1

Example. PHYSICAL REVIEW B 84, 075210 (2011)

Hp. Langevin modified for disorder, exponential tail distribution

LUMO

EFe

EFh

HOMO

Origin of the VOC: role of the electrodes

No net current flow. Voc given by the difference between the metal workfunctions

MIM: Metal Intrinsic Metal model

Short-circuit condition Open-circuit condition or “flat band” condition

Built-in field due to metals workfunction mismatch

JPHOTO: photogenerated carriers need a driving force

•Internal Electric FieldDrift current

•Charge concentration gradientDiffusion current ideal BHJ

Origin of the VOC: role of the electrodes

No net current flow. Voc given by the difference between the metal workfunctions

MIM: Metal Intrinsic Metal model

Short-circuit condition Open-circuit condition or “flat band” condition

Built-in field due to metals workfunction mismatch

JPHOTO: photogenerated carriers needs a driving force

•Internal Electric FieldDrift current

•Charge concentration gradientDiffusion current ideal BHJ

Voc,max=(1/q)(HOMOD-LUMOA– ΔE)

Voc,max=(1/q)(ΦM1-ΦM2)

built-in voltage

Vs

Origin of the VOC: role of the electrodes Contacts have little effect… if I choose them right!

J. Appl. Phys., Vol. 94, No. 10, 15 November 2003

Ag

Au nominal

Au «effective» due to interf. dipole

1.4eV

0.76eV

Fermi-level pinning

PHYSICAL REVIEW B 84, 075210 (2011)

Origin of the VOC: role of the electrodes

Voc can exceed the built-in of the contacts if the bulk-heterojunction is not so “ideal”

Current flow can be driven by diffusion! D. Cheyns, Phys. Rev. B 77, 165332 (2008).

Waldauf et al., J. Appl. Phys. 99, 104503 (2006)

Compex, more realistic device architectures provide asymmetry to the system (i.e. blocking layers)

Jphoto

Voc

DONOR

ACCEPTOR

1. D/A energy level alignment 2. Reducing recombinations

Improving the VOC: active material

TRADE OFF with charge dissociation!!!

? material level ? Increase phase separation: slow down the rate at which charges meet each other... But TRADE OFF with charge generation (reduced D/A interface)!!!

BLOCKING LAYERS FOR SELECTIVE COLLECTION AT THE ELECTRODES

Improving the VOC: device

Dr. Carvelli

Renee Janssen-University of Eindhoven, Plastic Electronics 2011.

Voc,1

Voc,2

Voc,3

Voc,4

Voc,5

Voc,6

Improving the VOC: tandem cells

S. Sista et al., Adv. Mater. 22, 380, 2009.

Tandem solar cells for increased Voc

Equivalent Voc= sum of single cells Voc

Equivalent Jsc= Jsc of the worst cell in the stack

REVIEW ART.: Energy Environ. Sci., 2013,6, 2390-2413

Short-circuit current (Jsc)

EQE Light absorption

Charge photogeneration efficiency

Charge transport and collection

Number of collected charges/s

Number of incoming photons/s EQE=

nc/s

nv/s = = abs· ed· cc

Active material Device (architecture) level

Extending the responsivity to low energy photons

As much as half of the energy of the solar spectrum is carried by long wavelengths photons!

ΔLUMO

Voc

DONOR

ACCEPTOR

Short-circuit current (Jsc): light absorption – active material

optimum bandgap Progress in Polymer Science 38 (2013) 1929–1940

Red-light responsivity

X. Gong et al., Science, 325, 25, 2009.

Other works: Muhlbacher et al., 2006; Kooistra et al., 2006; Yao et al., 2006; Soci et al., 2007; Wang et al., 2008; Hou et al., 2008)

J. Peet et al., Nature Materials 6, 497 - 500 (2007)

PDDTT-PCBM

PCPDTBT-PCBM

PDDTT

EXAMPLES 1: POLYMERS

PCE>5%

EXAMPLES 2: SMALL MOLECULES

G. Wei et al., ACSNano 4, 4, 2010.

REVIEW: L. Beverina et al., Eur. J. Org. Chem. 2010, 1207–122

SQUARAINES

Red-light responsivity

η=6% (certified by NREL)

Requirements:

• Transparent

• Electron transporter

• Conduction band lower than LUMO of acceptor and higher than cathode Fermi level

Short-circuit current (Jsc): light absorption - device

Maximum of the stationary optical field inside the active region

Park et al., Nat. Phot. 3 (2009) 297

1. Additives (solvents mixtures):

selective solubility

A. de Sio et al., 95, 3536–3542, 2011.

Short-circuit current (Jsc): photogeneration and charge transport

BULK-HETEROJUNCTION OPTIMAL FOR EXCITON DISSOCIATION... ...BUT REQUIRES CAREFUL CONTROL OF MORPHOLOGY!

Charges percolation must be allowed!

How?

One phase still in solution while the other is already in solid state

Higher degree of phase separation, higher cristallinity

XRD, increased P3HT crystallinity

2. Post deposition treatments: thermal annealing

X. Fan et al., Appl. Phys. A, 2011.

Short-circuit current (Jsc): photogeneration and charge transport

BULK-HETEROJUNCTION OPTIMAL FOR EXCITON DISSOCIATION... ...BUT REQUIRES CAREFUL CONTROL OF MORPHOLOGY!

Charges percolation must be allowed!

How?

No continuous paths between the electrodes for leakage carriers...

▫ Higher Shunt resistance

...but continuous and directional paths for photogenerated carriers:

▫ Higher mobility

▫ Lower recombination

▫ Low series resistance

Fill Factor (FF)

Going towards the ideal “comb-like” structure

1 2

Hybrid devices: organic/inorganic Inorganic nanostructured semiconductors exploited in combination with organics for their

Optical properties Morphological and electrical properties

CdSe, CdS, PbSe, PbS,… nanocrystals Strong and tunable absorption Tunable shape

Morphologically stable Higher mobilities

•A.C. Arango et al., Nano Letters 9, 860-863 (2009) •B. Sun et al., J. Appl. Phys., 2005, 97, 14941. •D.M.N.M. Dissanayake, Nanotechnology 20 (2009) 245202

Transparent High dielectric constant

Metal oxides: TiO2, ZnO

•K. Coakley, Adv. Funct. Mater., 2003, 13, 301. •S.S. Li, J. Am. Chem. Soc. 2011, 133, 11614–11620. •S.D. Oosterhout, Nat. Mater. 2009, 8, 818 – 824.

Recent review paper: Energy Environ. Sci., 2013,6, 2020–2040

1 2

Hybrid devices: organic/inorganic Inorganic nanostructured semiconductors exploited in combination with organics for their

Optical properties Morphological and electrical properties

CdSe, CdS, PbSe, PbS,… nanocrystals Tunable absorption High dielectric constant Tunable shape

Morphologically stable Higher mobilities

•A.C. Arango et al., Nano Letters 9, 860-863 (2009) •B. Sun et al., J. Appl. Phys., 2005, 97, 14941. •D.M.N.M. Dissanayake, Nanotechnology 20 (2009) 245202

Transparent High dielectric constant

Metal oxides: TiO2, ZnO

•K. Coakley, Adv. Funct. Mater., 2003, 13, 301. •S.S. Li, J. Am. Chem. Soc. 2011, 133, 11614–11620. •S.D. Oosterhout, Nat. Mater. 2009, 8, 818 – 824.

Recent review paper: Energy Environ. Sci., 2013,6, 2020–2040

solution processed! Colloidal NCs

Solubility!

Hybrid devices: heterojunction structure

BHJ Mimicking comb-like

enabled by high morphological stability after thermal post-processing: organic ligands removal, nanoparticles sintering

Planar HJ

ZnO, TiO2, Al2O3

Quantum confinement effect:

MEG? Multiple Exciton Generation

Extreme spectral tunability by tuning the dots dimension

Debated...

Nanocrystals based hybrid solar cells

Science, Vol. 334 no. 6062 pp. 1530-1533, 2011.

EQE>100% reported!

Optical properties

Charge transport

interparticle hops

direct transport intra-crystal

Nanocrystals based hybrid solar cells

Charge trapping at surface states Distance between nanocrystals (hopping sites)

careful tuning of capping ligands! (shorter, conductive,...)

Nano Lett., 2011, 11, 3998 – 4002

Nanocrystals based hybrid solar cells blended with polymer for extended absorption

Nanocrystals based hybrid solar cells blended with polymer for extended absorption

Effectiveness of D/A interface? Energy transfer between polymer and NCs? Poor charge transport?

Metal oxides based hybrid solar cells

TiO2

Mesoporous TiO2

Polymer-MetalOxide

TiO2

Dye-sensitized

Dye-doped with perovskites

TiO2/Al203

Solution processed!

Metal oxides based hybrid solar cells

TiO2

polymer

Mesoporous TiO2

Polymer-MetalOxide

TiO2

HTM

Dye monolayer

Dye-sensitized

Dye-doped with perovskites

TiO2/Al203

HTM Perovksite layer

Solution processed!

Metal oxides based hybrid solar cells

polymer - metal oxide

TiO2

polymer Poor performance: PCE ~ 0.5%

Improvement with interlayers chemisorbed onto TiO2 surface to: - Induce order in the polymer phase at the interface (Energy Environ. Sci., 2012, 5, 9068–9076)

- Mediate charge transfer process (monolayer of a standard organic acceptor molecule) (Adv. Funct. Mater. 2012, 22, 2160–2166)

A

D

< 1 μm Possible explanation: - Polymer infiltration inside the mesoporous scaffold - Poor efficiency of charge generation at D/A interface

PCE ~ 1%

- No exciton diffusion lossesExtended interface for charge transfer,

readily available - Dye’s charge transport properties are not involved in the process - High surface mesoporous TiO2 ensures good light absorption from the dye monolayer

Light absorption and charge transport are separated

Metal oxides based hybrid solar cells

TiO2

HTM

Dye monolayer

Dye-sensitized (solid state)

+

- -

+

Dye ETM HTM Electrode 1 Electrode 2 μm

REVIEW ARTICLE: H.J. Snaith, Adv. Mater. 2007, 19, 3187–3200

Room for improvement: New dye with higher absorption Charge transport in TiO2 and HTM

PCE ~ 7%

HTM: spiroOMeTAD molecule

Metal oxides based hybrid solar cells

Dye-doped based on perovskites

TiO2/Al203

HTM Perovksite layer

PCE ~ 15 % Record efficiency (not certified)

Solution processed from precursor Polycrystalline morphology

A=CH3NH3

B=Pb X=Cl, I

Organometal halide perovskite

Science, 338, 643-646, 2012

Nature, 501 (7467) , pp. 395-398

Metal oxides based hybrid solar cells

Dye-doped based on perovskites

TiO2/Al203

HTM Very new... many things to be understood: - charge transport: features and mobility values? - primary photoexcitation: excitonic or not? - role of morphology - stability ...

...but

strong panchromatic absorption long range crystallinity (>100 nm)

Efficient migration of the species (excitons, charges)

+