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: 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
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,...
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)!!!
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
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)
+