Journal
ADVANCED FUNCTIONAL MATERIALS
Volume 23, Issue 15, Pages 1935-1946Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/adfm.201201874
Keywords
vertical segregation; solvent additives; surface energy; morphology; organic photovoltaic devices
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Funding
- U.S. Department of Energy EERE Solar America Initiative [DE-FG3608GO18018]
- Nation Science Foundation [0933435]
- ConocoPhilips
- DOE Office of Basic Energy Sciences
- Los Alamos National Laboratory under DOE [DE-AC52-06NA25396]
- U. S. Department of Energy by Lawrence Livermore National Laboratory [DE-AC52-07NA27344]
- Directorate For Engineering
- Div Of Chem, Bioeng, Env, & Transp Sys [0933435] Funding Source: National Science Foundation
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The composition of polymer-fullerene blends is a critical parameter for achieving high efficiencies in bulk-heterojunction (BHJ) organic photovoltaics. Achieving the right materials distribution is crucial for device optimization as it greatly influences charge-carrier mobility. The effect of the vertical concentration profile of materials in spin-coated BHJs on device properties has stirred particularly vigorous debate. Despite available literature on this subject, the results are often contradictory and inconsistent, likely due to differences in sample preparation and experimental considerations. To reconcile published results, the influence of heating, surface energy, and solvent additives on vertical segregation and doping in polymer-fullerene BHJ organic photovoltaics are studied using neutron reflectometry and near edge X-ray absorption fine structure spectroscopy. It is shown that surface energies and solvent additives greatly impact heat-induced vertical segregation. Interface charging due to Fermi level mismatch increases (6,6)-phenyl-C61-butyric acid methyl ester (PCBM)-enrichment at the BHJ/cathode interface. Currentvoltage measurements show that self-assembly of interfaces affects the open-circuit voltage, resulting in clear changes to the power conversion efficiency.
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