Plastic Solar Cells: current status and future prospects

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Alvin Kwiram Symposium Seattle, June 24, 2003
Plastic Solar Cells current status and future
prospects
Bernard Kippelen, Neal R. Armstrong, and Seth
Marder Optical Sciences Center, and Department
of Chemistry, University of Arizona, Tucson, AZ
85721, USA
NREL, ONR, NSF
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Collaborators
Kippelen Group Armstrong Group Marder Group
Benoit Domercq Britt A. Minch Steve Barlow
Seunghyup Yoo Wei Xia Yadong Zhang
Carrie Donley
Chet Carter
Prof. David OBrien, deceased
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Organic Electronics
Low temperature processing of organic
semiconductors, metals and dielectrics on
flexible substrates low cost (0.01)
Metal deposition on plastics from solution,
micro-size features using soft lithography and
transfer
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A Complementary Material Platform
Light-weight, high versatility, low cost, large
area
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Technology Opportunities

• Low cost scanners
• Optical isolators
• Devices that take advantage of the integration
  of photodetectors on light-weight flexible
  substrates

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Outline

• Introduction to photovoltaic technologies
• Organic excitonic solar cells
• Requirements for conversion with high efficiency
• An approach based on self-assembly

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LUMO
Electrode
HOMO
Electrode
Semiconductor
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Solar cell parameters

• short-circuit current ISC
• open circuit voltage VOC
• fill factor FF

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Evolution of PV Technologies
A.M. air mass G global, direct scattered
angle of 48.2, zenith angle (sec(48.2)
1/cos(48.2) 1.5)
AM 1.5 G, 25 ?C, 1 sun 100mW/cm2
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State-of-the-art in organic photovoltaics

• Grätzel cell (liquid electrolyte, solid)
• Small molecules (bi-layers)
• Polymer blends (interpenetrated networks)
• Hybrid approaches (Inorganic sc doped in organic
  matrix)

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The Challenge
Harvesting the solar spectrum and
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maintain simultaneously high open circuit
voltage and high fill factor

• optimize absorption, charge generation, charge
  collection photocurrent
• optimize relative energy levels built-in
  voltage
• optimize electrical characteristic fill factor

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Step 1 Achieve efficient dissociation of
excitons in organic materials
vacuum
vacuum
LUMO
electrode
HOMO
electrode
Double layer
Single layer
Overcome exciton binding energy
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Ansatz the maximum value for Voc is the smallest
band gap minus the exciton binding energy (0.5
eV)
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Equivalent Circuit Model
Understanding Key Factors for Efficient Organic
Photovoltaic Cells

• Finite conductance of materials and contact
  resistance nonzero Rs
• Leakage path finite Rp

(J0)
(V0)
OPEN-CIRCUIT VOLTAGE
SHORT-CIRCUIT CURRENT DENSITY
What determines the fill factor ?
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Effects of Rp
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Effects of Rs
A need for high mobility materials
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Self-Assembly a Path for Controlled Morphology
in Wet Processed Materials
Adapted from D. Haarer
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Existing other approaches
Choice of hexabenzocoronene (HBC) driven by large
core that can lead to large mobility
John Warman, Adv. Mater. 13, 130 (01)
Voc 0.69 V FF 0.4 Jsc 30
?A/cm2 Saturation for illumination gt 1 mW/cm2
Mixtures of HBC and perylene
K. Müllen, R. Friend et al. Science, 293, 1119,
(01)
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Our materials choice phthalocyanines

• Good Thermal stability
• Strong Molar absorptivity
• Good Light stability

Alkoxy substituted Pc known to form discotic
hexagonally ordered mesophases. Problems K?I gt
350C, difficult to align, no photocurrent when
combined with PTCDI
Skoulios et al. J. Am. Chem.Soc. 1982, 104,
5245-5247
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Molecular design
Metal
Tuning of spectroscopic and electronic properties
Core
Provides large core for strong ?-orbitals
coupling and cohesive forces through Van der
Waals interactions
Arms
Influence the solid-to-mesophase (K?Dh) and
mesophase-to-isotropic liquid (Dh?I) transition
temperatures
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Molecular optimization
O-Et-O-Bz CuPc
S-Et-O-Bz CuPc
K?Dh 134C Dh?I 320C easy to process into
thin films by spin-coating (chloroform)
K?Dh 111C Dh?I gt 400C difficult to process
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Optical properties
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Material Characterization
Small-angle X-ray scattering
Data show that Pc form three different types of
crystalline phases dependent on surface treatment
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Before annealing
AFM studies
Possibility to form nanostructured surfaces by
thermal annealing to create high area networks
for improved exciton dissociation
After annealing
Spin-coated at 4000 rpm on PEDOTPSS/ITO 180C
for two hrs.
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Device Configuration
C60
DLC-CuPc
Energy scale in eV w.r.t vacuum
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Experimental Results

• Annealing of DLC-CuPc film resulted in 3.7-fold
  increase in Jsc.
• Estimation of RsA values by inverse slopes of
  J-V curves at V Voc suggests increase of
  mobility in annealed device.
• Reduction of Voc is considered due to creation
  of pinholes in DLC-CuPc film caused by dewetting
  while being annealed.

Result for device with ITO/PEDOTPSS (30nm)
/DL-CuPc (20nm)/C60 (40nm)/BCP (10nm)/Al, under
50mW/cm2 (AM1.5Direct illumination)
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Self-assembled electron transport materials
Star-like discotic LC oxadiazole materials with
good electron mobility
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TOF experiments
N2 laser, 337 nm, 6 ns R 102 104 ?, C 10 pF,
RC ltlt ?
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TOF mobility results at room temperature
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Cyclicvoltammetry of Discotic LCs
0.6 volt shift in reduction potentional
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OE Testing Facilities
Fully automated high vacuum deposition system
with four organic sources and two high power
sources for metals and oxides (co-deposition
capabilities). Integrated with double glove box
(one dry and one wet with integrated
spin-coater).
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Conclusions and future work

• Transport properties of organic semiconductors
  often limit power conversion efficiency in
  organic solar cells. High mobility required in
  materials that can be processed from solution.
• DLC-CuPc is solution-processible, and we
  demonstrated that its transport property can be
  improved in the discotic liquid crystalline
  phase.
• Photocurrents reaching mA/cm2, significant
  improvement over HBC-based devices
• Development of discotic electron-transport
  oxadiazole-based materials.
• Optimization of parameters will require control
  of interfaces, relative orbital energies, control
  of morphology through use of self-assembly.

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Additional supporting information
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Stabilization of geometry and patterning
Photo-crosslinking between adjacent side chains
through cyclobutane links gt 50 conversion of
styryl groups
254 nm
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Effects of Rs and Rp on Fill Factor in High
Photocurrent Regime
Minimize Rs and Maximize Rp
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Excitonic Solar Cells Energy Level Engineering
B)
A)
C)
D)
Band offset lt exciton binding energy
Band offset gt exciton binding energy
Working hypothesis the maximum value for Voc is
the smallest band gap minus the exciton binding
energy (0.5 eV)
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