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

UC-Argonne SCI Workshop - June

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

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

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

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

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 standard BHJidealized BHJ

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

Uniting state-of-the-art materials and characterization UC-Argonne SCI Workshop - June  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: 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

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

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

New materials for OPV UC-Argonne SCI Workshop - June 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

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

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

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 Funding  UChicago-Argonne SCI  NSF  AFOSR  DOE  Solarmer Energy Inc.