1. 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

2. Outline Introduction Enhancement concept Results Conclusions
Photovoltaic technology
Organic photovoltaics
Performance limitations
Enhancement concept
Results
Experimental
Simulation
Conclusions
Images courtesy of Global Photonic Energy Corp.

3. Solar Energy
Sunlight is an ubiquitous, clean and abundant energy source.
Readily available energy source for:
Remote locations
Developing nations
Outer space

4. Photovoltaic Technology
Organics
Inorganics
Inexpensive substrates
High-throughput processing
Flexible
Efficiency : 8%
Expensive processing
High installation costs
Efficiency: >20% (c-Si), 10-20% (thin film)
Image courtesy of Konarka, Inc.

5. Organic Photovoltaic (OPV) Basics
Substrate
Transparent Electrode
Active Layers
Metal Electrode
Illumination
Absorption ≈ 1- e-αd
α = absorption coefficient
d = light path length
Glass or plastic
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

6. OPV Operation 1. Light Absorption - ηA 2. Exciton Diffusion - ηED hv
Donor
Acceptor hv
Exciton
3. Exciton Dissociation - ηCT
4. Charge Collection - ηCC

7. Fundamental Tradeoffs
There is a fundamental tradeoff between light absorption and exciton diffusion/charge collection.
Substrate
Transparent Electrode
Active Layers
Metal Electrode
Substrate
Transparent Electrode
Active Layers
Metal Electrode
Substrate
Transparent Electrode
Active Layers
Metal Electrode
Increase layer thickness:
Light absorption ↑
Charge collection ↓
Decrease layer thickness:
Light absorption ↓
Charge collection ↑

8. Transparent Electrode
Improvement Routes
Develop new active materials
Improve device architectures
Manipulate light propagation and absorption
Substrate
Transparent Electrode
Active Layers
Active Layers
Active Layers
Metal Electrode

9. Microlens Arrays for OPVs
(2)
(1)
Substrate
Transparent Electrode
Active Layers
path length > layer thickness
path length = layer thickness
Metal Electrode
Refraction due to incident angle and index of refraction
Surface reflection into neighboring features
Effectively increase light absorption without altering active layer

10. Array Fabrication (a) Convective self-assembly of PS microspheres
UV-glass or SiO2
PDMS
(a)
Convective self-assembly of PS microspheres
Cure PDMS, make mold
Scotch tape to remove spheres
Mold optical adhesive and cure, form array
(a) PS
PDMS
(b)
(b)
Cured PDMS
(c)
Concave PDMS mold
(c)
PS = 100μm
(d)
Substrate
Microlens Array
(d)
Optical Adhesive
PDMS mold

11. Enhancement is more significant in regions of poor spectral response
Experimental Results
Glass
ITO
Aluminum
CuPc
C60
BCP
Absorption ≈ 1- e-αd
If α is small, path length increase is more significant
Enhancement is more significant in regions of poor spectral response
CuPc
C60
80nm
30nm
60nm
8nm
100nm

12. Results, cont.
Enhancement is seen with a variety of active layer materials.
Enhancement is also present at all angles of incidence.
Small Molecule
Polymer
Hybrid
(CuPc/C60)
(P3HT:PCBM)
(P3HT:CdSe)
Enhancement in current
30%
29% 7%
Glass
ITO
Aluminum
P3HT:PCBM θ
80nm
100nm
100nm

13. Device Area Dependence
Laboratory-scale devices: mm x mm
Production-scale devices: cm x cm
Glass
80nm
ITO
CuPc
40nm
70nm
C60
8nm
BCP
100nm
Aluminum
Enhancement increases with device area

14. Ray Tracing Simulations
Air
Device
ITO
Glass
Buffer
Illumination
n = 1
Lens layer, n = 1.5
n = 1.5, 0.5mm thick
n = 2.0, 100nm thick
n = 1.7, 100nm thick
In-house code
Rays fired at the stack
Propagation behavior is tracked
More rays are being absorbed after multiple passes through the device area
Excellent qualitative agreement with experiment

15. Large Area Enhancement
Small area device:
Large area device:
Larger devices allow for:
increased light trapping
multiple absorption opportunities

16. 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

17. 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.

18. Acknowledgements
Funding:
NSF CAREER Grant
DOE SETP
UF OTL
Xue Group