Enhanced Organic Photovoltaic Cell Enhanced Organic Photovoltaic Cell Performance using Transparent Performance using Transparent

Microlens ArraysMicrolens Arrays

Jason D. Myers, Sang-Hyun Eom, Vincent Cassidy, and Jiangeng Xue

Department of Materials Science and EngineeringUniversity of FloridaGainesville, FL, USA

jdmyers@ufl.edujxue@mse.ufl.edu

Outline

• Introduction– Photovoltaic technology– Organic photovoltaics– Performance limitations

• Enhancement concept• Results– Experimental– Simulation

• ConclusionsImages 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%

Inorganics• Expensive processing• High installation costs• Efficiency: >20% (c-Si), 10-

20% (thin film)

Image courtesy of Konarka, Inc.

Organic Photovoltaic (OPV) Basics

• 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

Substrate

Transparent ElectrodeTransparent ElectrodeActive LayersActive Layers

Metal Electrode

Illumination

Absorption ≈ 1- e-αd α = absorption coefficientd = light path length

Glass or plastic

OPV Operation1. Light Absorption - ηA 2. Exciton Diffusion - ηED

3. Exciton Dissociation - ηCT 4. Charge Collection - ηCC

Donor

Acceptor

hv Exciton

Fundamental Tradeoffs

• There is a fundamental tradeoff between light absorption and exciton diffusion/charge collection.

Substrate

Transparent ElectrodeTransparent ElectrodeActive LayersActive Layers

Metal Electrode

Increase layer thickness:

Light absorption ↑Charge collection ↓

Substrate

Transparent ElectrodeTransparent Electrode

Active LayersActive Layers

Metal Electrode

Substrate

Transparent ElectrodeTransparent ElectrodeActive LayersActive Layers

Metal Electrode

Decrease layer thickness:

Light absorption ↓Charge collection ↑

Improvement Routes1. Develop new active materials2. Improve device architectures3. Manipulate light propagation and absorption

Substrate

Transparent ElectrodeTransparent Electrode

Active LayersActive Layers

Metal Electrode

Active LayersActive LayersActive LayersActive Layers

Microlens Arrays for OPVs

Effectively increase light absorption without altering active layer

path length = layer thickness

(1)Refraction due to incident angle and index of refraction(2)Surface reflection into neighboring features

SubstrateTransparent ElectrodeTransparent Electrode

Metal ElectrodeActive LayersActive Layers

Microlens array

(2)(1)

path length > layer thickness

Array Fabrication

(a)

(c)

PSPS

PDMS(b)

PS = 100μm

(d)

UV-glass or SiO2

PDMS

UV-glass or SiO2

(a)

(b) Cured PDMS

a) Convective self-assembly of PS microspheresb) Cure PDMS, make moldc) Scotch tape to remove spheresd) Mold optical adhesive and cure, form array

SubstrateMicrolens Array

(d)

Substrate Optical Adhesive

PDMS mold

(c)Concave PDMS mold

Experimental Results

Enhancement is more significant in regions of poor spectral response

Absorption ≈ 1- e-αd If α is small, path length increase is more significant

GlassITOITO

Aluminum

CuPcC60C60

BCPBCP

30nm

60nm

8nm

80nm

100nmCuPc C60

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%

θ

GlassITOITO

Aluminum

P3HT:PCBMP3HT:PCBM

80nm

100nm

100nm

Device Area Dependence

0.0 0.5 1.0 1.5 2.0 2.50

5

10

15

20

25

30

35

Enhance

ment (

%)

Device Area (cm2)

Thick Device Thin Device

Enhancement increases with device area

GlassITOITO

Aluminum

CuPcC60C60

BCPBCP

40nm

70nm

8nm

80nm

100nm

Laboratory-scale devices: mm x mmProduction-scale devices: cm x cm

Ray Tracing Simulations

More rays are being absorbed after multiple passes through the device area

Air

Air

DeviceITOITO

GlassBuffer

Illumination

n = 1Lens layer, n = 1.5n = 1.5, 0.5mm thickn = 1.5, 0.5mm thickn = 2.0, 100nm thick

n = 1.7, 100nm thickn = 1

Excellent qualitative agreement with experiment

• In-house code• Rays fired at the stack• Propagation behavior is tracked

Large Area Enhancement

Larger devices allow for:1. increased light trapping 2.multiple absorption opportunities

Small area device:Large area device:

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

Very promising for future developmentImage courtesy of Frederik Krebs

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