1. Chain conformation, aggregation, and miscibility in polymer/fullerene blends for photovoltaics UChicago-Argonne National Laboratory Strategic Collaborative Initiative Luping Yu, University of Chicago Seth B. Darling, Argonne National Laboratory

2. UC-Argonne SCI Workshop - June 2011 2

3. Current technologies UC-Argonne SCI Workshop - June 2011 Levelized Cost of Energy (LCOE) 3

4. Organic photovoltaics challenges  Absorption of the full solar spectrum  Separation of bound charges (next slide)  Voltage loss in charge transfer  Poor charge transport through the active layer  Charge collection at the electrodes UC-Argonne SCI Workshop - June 2011 4

5. Organic photovoltaics challenge: Excitons + – + + + – – – small work function electrode large work function electrode donor acceptor E h h exciton 5

6. UC-Argonne SCI Workshop - June 2011 Bulk heterojunction (BHJ)  Donor and acceptor materials are mixed –Random phase separation occurs on the nanoscale 6

7. Idealized bulk heterojunction (current model)  Phase-pure domains (polymer and fullerene)  Domain spacing comparable to the exciton diffusion distance  Continuous pathways for charge carrier to traverse the film thickness UC-Argonne SCI Workshop - June 2011 7 standard BHJidealized BHJ

8. PTB polymer series  Set world record in OPV device efficiency (7–8%) UC-Argonne SCI Workshop - June 2011 8 J. Am. Chem. Soc. 131 (2009) 7792 Adv. Mater. 22 (2010) E135 What is the solvent effect and why is it so strong?

9. Uniting state-of-the-art materials and characterization UC-Argonne SCI Workshop - June 2011 9  Variety of x-ray and neutron scattering measurements to probe the internal structure at multiple length scales and correlate with device performance  Polymer-only studies don’t capture polymer/fullerene blend structure  Domains far larger than expected: 100-200 nm (these devices shouldn’t work!)  We can rationalize why certain solvent mixtures produce better devices—turning an art into a science Chen et al., submitted

10. Expected insights: Solvent effects UC-Argonne SCI Workshop - June 2011 10 CBCB+DIODCBDCB+DIO Crystallinity ( θ c (°)) 0.1800.1820.1860.188 Phase Separation (R g (nm)) 176.8 ± 0.8227.7 ± 0.3216.8 ± 0.3236.7 ± 0.3 Heterogeneity (L (nm)) 7565–45 Domain Purity (P max (r)) 0.4490.1190.1250.121 Short-Circuit Current Density (J SC (mA/cm 2 )) 10.4 ± 0.511.1 ± 0.59.5 ± 0.710.9 ± 0.4 Series Resistance (R S ( Ω ∙cm 2 )) 5.73.36.11.7 PCE (%)3.0 ± 0.24.0 ± 0.23.6 ± 0.34.4 ± 0.2 device I-V (# of crystallites, not size) GIWAXS & XRR RSoXS

11. Rethinking the idealized morphology  Hierarchical morphology  Substantial intermixing of donor and acceptor materials –Expands exciton separation volume UC-Argonne SCI Workshop - June 2011 11 Chen et al., submitted

12. New materials for OPV UC-Argonne SCI Workshop - June 2011 12 J. Am. Chem. Soc. 133 (2011) 3284 (Several others in preparation) HF series XT series enhancing π-stacking (flexible) d-PTB series enhanced contrast for neutron scattering PTAT series enhancing π-stacking (rigid) PTB series extension building in solvent interactions

13. Future/Ongoing work  Deciphering the structure-property relationships in a rational, modular way –Manipulating molecular structure and probing its effect on hierarchical morphologies and device performance  Correlating structure in solution to structure in films –Determining when structural features form will guide optimization efforts  Ascertaining internal structural details such as chain interdigitation and specific fullerene/polymer interactions UC-Argonne SCI Workshop - June 2011 13

14. Summary  Organic photovoltaics are among the most promising candidates for next-generation energy supply, but a deeper understanding of structure–property relationships is needed  The collaboration between UofC and Argonne brings together unique, world-class synthesis and characterization capabilities  In nine months, we have investigated numerous novel materials and developed a new paradigm for the idealized internal structure in organic photovoltaic devices UC-Argonne SCI Workshop - June 2011 14

15. Acknowledgments  Synthesis and device work (UofC) –Feng He, Tao Xu, Wei Wang, Hae Jung Son  X-ray and neutron characterization (Argonne) –Wei Chen  X-ray and neutron scattering facility support –Joe Strzalka (Argonne) –Yun Liu (NIST) –Cheng Wang (LBNL) –Joerg Neuefeind and Yuri Melnichenko (ORNL) –Katharine Page (LANL)  d-monomer synthesis –Kunlun Hong and Peter Bonnesen (ORNL) UC-Argonne SCI Workshop - June 2011 15 Funding  UChicago-Argonne SCI  NSF  AFOSR  DOE  Solarmer Energy Inc.