Contact effects in organic solar cells: Towards low-cost photovoltaics Erik Garnett Amps Current, Department of Materials Science Stanford University Cui, McGehee, Brongersma groups Solar Cell Basics 1.4 eV The Sun 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 silicon 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 '"'$ '$ !"&$ !"%$ !"#$#%&''()$*'+,#* !"#$ (!!$ )!!$ *!!$ #!!$ %!!$ -+.()/()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 band gap, 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 .