Enhanced Organic Photovoltaic Cell
Performance using Transparent
Microlens Arrays
Jason D. Myers, Sang-Hyun Eom,
Vincent Cassidy, and Jiangeng Xue
Department of Materials Science and Engineering
University of Florida
Gainesville, FL, USA
jdmyers@ufl.edu
jxue@mse.ufl.edu
Outline
• Introduction
– Photovoltaic technology
– Organic photovoltaics
– Performance limitations
• Enhancement concept
• Results
– Experimental
– Simulation
• Conclusions
Images courtesy of Global Photonic Energy Corp.
Solar Energy
• Sunlight is an ubiquitous, clean and abundant
energy source.
• Readily available energy source for:
– Remote locations
– Developing nations
– Outer space
Photovoltaic Technology
Organics
•
•
•
•
Inexpensive substrates
High-throughput processing
Flexible
Efficiency : 8%
Image courtesy of Konarka, Inc.
Inorganics
• Expensive processing
• High installation costs
• Efficiency: >20% (c-Si), 1020% (thin film)
Organic Photovoltaic (OPV) Basics
Illumination
Glass or plastic
Substrate
Transparent Electrode
Active Layers
Metal Electrode
Absorption ≈ 1- e-αd
α = absorption coefficient
d = light path length
• Active layer materials can be small molecules,
polymers, inorganic nanoparticles, or blends
• Two different materials are required: electron donor
and electron acceptor
• Materials are generally neat layers or intermixed
OPV Operation
1. Light Absorption - ηA
Exciton
hv
2. Exciton Diffusion - ηED
Donor
Acceptor
3. Exciton Dissociation - ηCT 4. Charge Collection - ηCC
Fundamental Tradeoffs
• There is a fundamental tradeoff between light
absorption and exciton diffusion/charge
collection.
Substrate
Transparent Electrode
Active Layers
Metal
ActiveElectrode
Layers
Metal
ActiveElectrode
Layers
Metal Electrode
Increase
Decreaselayer
layerthickness:
thickness:
Light absorption ↑
↓
Charge collection ↓
↑
Improvement Routes
1. Develop new active materials
2. Improve device architectures
3. Manipulate light propagation and absorption
Substrate
Transparent Electrode
Active Layers
Metal Electrode
Microlens Arrays for OPVs
Microlens array
(1)
(2)
Substrate
Transparent Electrode
Active Layers
Metal Electrode
path length >= layer thickness
(1)Refraction due to incident angle and index of refraction
(2)Surface reflection into neighboring features
Effectively increase light absorption without
altering active layer
Array Fabrication
(a)
UV-glass or SiO2
PDMS
a)
b)
c)
d)
Convective self-assembly of PS microspheres
Cure PDMS, make mold
Scotch tape to remove spheres
Mold optical adhesive and cure, form array
UV-glass or SiO2
(b)
PS
Cured PDMS
(c)
(a)
Concave PDMS mold
(b)
PS = 100μm
PDMS mold
Optical Adhesive
Substrate
(c)
(d)
Microlens Array
Substrate
PDMS
(d)
PS
Experimental Results
Glass
Enhancement
is more
80nm
ITO
30nm
significant CuPc
in regions of
Absorption ≈ 1- e-αd
If α is small, path length
increase is more significant
C60 response
poor spectral
60nm
8nm
100nm
BCP
Aluminum
CuPc
C60
Results, cont.
• Enhancement is seen with a variety of active layer materials.
Enhancement in current
Small Molecule
Polymer
Hybrid
(CuPc/C60)
(P3HT:PCBM)
(P3HT:CdSe)
30%
29%
7%
• Enhancement is also present at all angles of incidence.
80nm
100nm
100nm
Glass
ITO
P3HT:PCBM
Aluminum
θ
Device Area Dependence
Laboratory-scale devices: mm x mm
Production-scale devices: cm x cm
35
40nm
70nm
8nm
100nm
C60
BCP
Aluminum
Enhancement (%)
80nm
Glass
ITO
CuPc
30
25
20
15
Thick Device
Thin Device
10
5
0
0.0
0.5
1.0
1.5
2.0
2
Device Area (cm )
Enhancement increases with device area
2.5
Ray Tracing Simulations
• In-house code
Excellent
qualitative
• Rays fired
at the stack
agreement
with
experiment
• Propagation
behavior
is tracked
Illumination
More rays are being absorbed
Air
n = 1 through
after multiple
passes
Bufferarea Lens layer, n = 1.5
the device
n = 1.5, 0.5mm thick
Glass
ITO
Device
Air
n = 1.5, 0.5mm thick
n = 2.0, 100nm thick
n = 1.7, 100nm thick
n=1
Large Area Enhancement
Small
Large area device:
Larger devices allow for:
1.increased light trapping
2.multiple absorption opportunities
Practical Applications
• Lens arrays provide large-area enhancement
• Optical enhancement effect is not specific to
one material system
• Soft lithography is compatible with roll-to-roll
production
Image courtesy of Frederik Krebs
Very promising for future development
Conclusions
• Controlling light propagation is a viable
route for enhancing organic photovoltaic
device performance.
• Enhancement is due to increased path
length in active layer
• Mechanisms are compatible with different
active materials, and production-scale
processing and device sizes.
Acknowledgements
• Funding:
– NSF CAREER Grant
– DOE SETP
• UF OTL
• Xue Group