Contact effects in organic solar cells: Towards low-cost

Erik Garnett Amps Current, Department of Materials Science Stanford University Cui, McGehee, Brongersma groups Basics 1.4 eV The

Transmitted photons Transparent front contact (hole selective)

Wasted energy (heat) Conduction band (LUMO) Solar absorber Load Valence band (HOMO)

Metal back contact (electron selective) Two cells are better than one

1.8 1.1 eV

Organic, 1.8 eV Load Inorganic thin-film, 1.1 eV

Al foil or plastic

Multijunction (tandem) solar cell concept

• 40% maximum theoretical efficiency (vs. 29% for only) • Upper cell requires high output voltage and polarity control Standard vs. Inverted Organic Photovoltaics

PCDTBT

PC70BM

Park et al. Nature Photonics 3, 297 (2009)

Carrier selective contact layers (2-20 nm thick) are critical Varying the Contact Energy Level

ΔE ~ built-in electric field

Large difference in contact energy level leads to high Voc with metal contacts – how about with semiconductors? Semiconductor Electron Contacts

ZnO CdS CdO

In2S3

ZnS

Voc peaks when semiconductor conduction band matches PC70BM LUMO level Temperature dependence of Voc

Carrier depletion in semiconductor at low T hurts Voc Inverted vs. Standard Cell Performance

5.5% power conversion effficiency

Inverted cell with semiconductor electron contact matches performance of standard cell but is stable in air Future work enabled by inverted cells

Photocurrent map

Before After Current, Amps Current,

50 nm 50 nm

Metal nanowires act as transparent electrode, may focus light into the active layer and allow for metal foil substrates Acknowledgements

Mark Brongersma Yi Cui Mike McGehee

Steve Wenshan Judy Jonathan Hui Wu Connor Cai Cha Bakke

YOU! Questions? For further details see my poster, #22 Current, Amps Current,

5.5% Inverted devices on Ag films Plexcore BHJ ZnO Ag glass Au nfs 0 -1 -2 -3 -4 -5 -6 -7 -8 Inverted on Ag film with Au nfs

-9 Inverted control -10 Current Density, mA/cm2 0 0.2 0.4 0.6 0.8 1 Voltage, V Used low density Au nanofibers as transparent top contact; high series resistance (~100 Ohms) from low nf density Photocurrent maps

λ = 500 nm λ = 600 nm λ = 700 nm Wavelength Scans

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!"%$ !"#$#%&''()$*'+,#* !"#$ (!!$ )!!$ *!!$ #!!$ %!!$ -+.()/()0$"1*)2* Higher current away from the nanofiber (~ horizontal polarization) Pseudo-tandem solar cell

p-n Si BHJ FTO glass

Clamp Clamp

Voc for tandem within 20 mV of sum of individual cells Self-limited optical nanowelding simulations

Heat generation focused in bottom wire at junction; moves along seam and drops off during melting process

After melting – 2 nm overlap

FEM simulations from Wenshan Cai, Brongersma group Nanowelding gap and polarization effects

Sudden drop in heating upon welding and strong After meltingpolarization – 2 nm overlap effect suggests plasmonic mechanism

FEM simulations from Wenshan Cai, Brongersma group Plasmonic nanowelding experiments

Before illumination SEM Before illumination TEM

Observed similar change with high power white light and laser source

500 nm 50 nm

After illumination SEM After illumination cross-sectional SEM

500 nm 500 nm TEM after optical nanowelding

Twinning defects continue through junction only for top nanowire

5 nm

50 nm

Bottom nanowire always recrystalizes

30 nm onto top nanowire at the junction, TEM images by Judy Cha (Cui group) consistent with heat generation profile Solar cell efficiency measurements

Open circuit voltage (Voc) – maximum electron energy extracted per photon; depends on , contact selectivity and recombination rates

Fill Factor (FF) – Load depends on cell absorber resistances

Maximum power Efficiency (η) = maximum power out/power in

= (VocxJscxFF)/power in Short circuit current density (Jsc) – maximum photon to electron extraction rate; depends on absorption and charge extraction efficiency