Understanding the Role of Charge Mobility and

Recombination in Organic Photovoltaics

David Lam1

Advised by Professor Michael D. McGehee2

1. Department of Physics, Stanford University2. Department of Materials Science and Engineering, Stanford University

May 18, 2015

2

Ener

gy C

onsu

mpti

on [1

015 B

TU]

Year

Why Solar?

Energy consumption increase by 56% in 30 years1

Ex: In 2003, 13.1 x 1015 BTU used in U.S.2

With 15% efficiency, only need 100 km x 100 km

1. International Energy Outlook, U.S. EIA, 20132. Shaheen et al., MRS Bulletin, 2005

3

Why Organic Photovoltaics?

Image from popupcity.net, Solar Cells for Cheap, Cheap Soon

Solution processed Low temperatures Inexpensive substrates

Large-scale, wet-processing production Blade-coating Roll-to-roll printing

4

Organic Photovoltaics BasicsEn

ergy

AnodePolymer

Fullerene

Cathode

1) Photon Absorption2) Exciton Diffusion3) Charge Transfer4) Charge Drift

Exci

ton

+

-

+

-

5

Solar Cell Measurements

𝑉 𝑜𝑐

𝑃𝑚𝑎𝑥

𝐽 𝑠𝑐

OPV Goal #1: 90% EQEOPV Goal #2: Thin (70-100 nm) -> Thick (300 nm)

OPV Goal #3: 0.8 FF

: Current at short circuit : External quantum efficiency

: Voltage at open circuit

: Max power : Fill factor

6

Research Questions

1) What causes poor fill factor in thick OPVs?

2) Can we use simulation to model device physics?

7

P3HT:PCBM

PCBMP3HT

Mihailetchi et al., Adv. Funct. Mat., 2006

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Fill Factor and Efficiency vs. Thickness

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Recombination in P3HT:PCBM𝑅𝑏𝑖𝑚𝑜𝑙𝑒𝑐=𝑘𝑛𝑝

: Recombination rate constant

: electron density : hole density

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Factors Affecting Recombination

𝜇𝑒 ,𝜇h 𝑛 ,𝑝 ,𝑅𝐸=Δ𝑉𝑙

𝑛 ,𝑝 ,𝑅

: Recombination rate constant

: electron density : hole density : electron mobility : hole mobility

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Why Simulation?

Glass

ITOPEDOT:PSS

Ca/Al

Polymer:Fullerene Blend (Active Layer)

Measure: Electron, hole mobility Energy levels of

semiconductors Complex index of refraction

Obtain: Recombination rate

constant Electrical and optical

profiles

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Simulated vs Experimental Efficiency

Simulated Experimental

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Simulated vs Experimental Fill Factor

Simulated Experimental

Simulated: Literature: to

14

Simulating H1

15

H1 Experimental vs Simulated Data

Fill Factor Efficiency

Average: Range: to 1

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Conclusion

Fill factor of P3HT:PCBM devices sensitive to thickness and charge mobility

Bimolecular recombination can be decreased by:Decreasing k Increasing charge mobility

Simulation captures device physics

17

Acknowledgements

18

Acknowledgements

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End

20

Langevin vs. Constant Recombination

21

Seftos

]