HUI LIU, JINJUN LIU, Department of Chemistry, HEMANT M. SHAH and BRUCE W. ALPHENAAR, Department of Electrical & Computer Engineering, University of Louisville

Outline Introduction – Organic solar cell: PCBM /ITO – Capacitive photocurrent (CPC) spectroscopy Experimental Results Quantum chemistry calculations and interpretation of experimental results – Geometry optimization – Excitation energies – Electron collision induced singlet-to-triplet transitions Summary Future Work

Organic Solar Cell he LUMO HOMO Anode Cathode ITO electron donor PC 60 BM electron acceptor hνhν Light absorption and formation of an exciton (electron-hole pair); Electron promoted to LUMO of electron donor (e.g., ITO) Indium Tin Oxide Electron transfer from electron donor to accepter (e.g., PC 60 BM) [6,6]-phenyl-C 61 -butyric acid methyl ester Subsequently reach to electrode. How does Organic Solar Cell work? [1] e e

PCBM conformers Cis (0°) Gauche (60°)Trans (180°) PC 60 BMPC 70 BM α isomer [2] 85% [2] Wienk, M.; Kroon, J. et al. Angew. Chem. 115, 3493, (2003) G+ G-

Capacitive Photocurrent Spectroscopy Spectra Physics OPA, 130 fs pulsed, Ti:Sapphire laser, rep rate: 1 KHz V out Quartz PC 60 BM Ag ITO Cu [3] Tanesh Bansal; Bruce W. Alphenaar. et al. Carbon, 50(3), 808, (2012)

Spectra of PCBM/ITO Singlet  Transition at ~ 800 nm; Singlet state of PC 60 BM  Low optical absorbance, high dissociation in CPC  New feature observed at 1250nm and 1800 nm New features PC 70 BM PC 60 BM Higher absorbance in PC 70 BM due to relaxed symmetry compared to PC 60 BM Previously unseen features observed in the CPC spectrum Absorption features New features 947 nm and 1667 nm

Quantum Chemistry Calculation Relative energy (cm -1 ) CisTrans (TS) PC 60 BM HF/3-21G G- PC 70 BM HF/3-21 G Gauche B3LYP/3-21G B3LYP/3-21G G+

Singlet—Triplet states (a)CPC spectra. (b)Calculated transitions to singlet states. (c)Calculated transitions to triplet states. PC 60 BM by ZINDO/S Scaled factor: 0.71 Triplet

Singlet—Triplet states (a)CPC spectra. (b)Calculated transitions to singlet states. (c)Calculated transitions to triplet states. PC 70 BM by ZINDO/S Scaled factor: 0.94 Triplet

Proposed mechanism 1.Free carrier excitation 2. Electron collision 3. Transition to T states

New peaks observed in the CPC spectra of PC 60 BM/ITO & PC 70 BM/ITO junction that are absent in the absorption spectroscopy Comparison of experimental results & quantum chemistry calculations suggest singlet-triplet transitions ZINDO/S calculations, when scaled according to the S 1  S 0 excitation energies, well reproduces the CPC spectra A model based on electron collision induced transitions have been proposed to explain the new features in the CPC spectra Summary

Future Work (a)(b) Ultrafast Transient Absorption Spectroscopy Transient spectroscopy to study triplet state dynamics Extend to other absorption organic solar cell

Acknowledgement Professors: Dr. Jinjun Liu Dr. Bruce Alphenaar Friends: Samantha Strom Dustin Cummins Malika Rizmanova Bokhodir Mamedov Technical support: Cardinal Research Cluster Financial support: NSF U.S. Department of Energy

Indium Tin Oxide (20-25 Ω/□) Below gap peaks due to PCBM/ITO interface? Thin insulator should reduce the photocurrent in CPC Varying thickness of insulator should have high impact Quartz Lithium Fluoride PC 60 BM molecules Indium Tin Oxide Spectroscopy of PC 60 BM with varying thickness of insulator

Increasing thickness lithium fluoride New peaks drop Not much change in photocurrent below 400nm CPC of PC 60 BM with varying thickness of LiF