Open-Circuit Voltage in Organic Solar Cells: The Impacts of Donor Semicrystallinity and Coexistence of Multiple Interfacial Charge-Transfer Bands
AuthorsNgongang Ndjawa, Guy Olivier
Wu, Di M.
Rose, Bradley Daniel
McGehee, Michael D.
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AbstractIn organic solar cells (OSCs), the energy of the charge-transfer (CT) complexes at the donor-acceptor interface, E , determines the maximum open-circuit voltage (V ). The coexistence of phases with different degrees of order in the donor or the acceptor, as in blends of semi-crystalline donors and fullerenes in bulk heterojunction layers, influences the distribution of CT states and the V enormously. Yet, the question of how structural heterogeneities alter CT states and the V is seldom addressed systematically. In this work, we combine experimental measurements of vacuum-deposited rubrene/C bilayer OSCs, with varying microstructure and texture, with density functional theory calculations to determine how relative molecular orientations and extents of structural order influence E and V . We find that varying the microstructure of rubrene gives rise to CT bands with varying energies. The CT band that originates from crystalline rubrene lies up to ≈0.4 eV lower in energy compared to the one that arises from amorphous rubrene. These low-lying CT states contribute strongly to V losses and result mainly from hole delocalization in aggregated rubrene. This work points to the importance of realizing interfacial structural control that prevents the formation of low E configurations and maximizes V .
CitationNdjawa GON, Graham KR, Mollinger S, Wu DM, Hanifi D, et al. (2017) Open-Circuit Voltage in Organic Solar Cells: The Impacts of Donor Semicrystallinity and Coexistence of Multiple Interfacial Charge-Transfer Bands. Advanced Energy Materials: 1601995. Available: http://dx.doi.org/10.1002/aenm.201601995.
SponsorsThe Office of Competitive Research Funds at the King Abdullah University of Science and Technology supported this work in part under the CRG-3 program (A.A. and J.-L.B.). J.-L.B. acknowledges support in part from the Office of Naval Research–Global under Award No. N62909-15-1-2003. This work was also supported in part by the ONR Award Nos. N00014-14-1-0580 and N00014-16-1-2520. Portions of this work were done at the Cornell High Energy Synchrotron Source (CHESS). G.O.N.N., K.R.G., M.D.M., and A.A. acknowledge the Office of Competitive Research Funds for a GRP-CF award. K.R.G. and A.A. acknowledge SABIC for a postdoctoral fellowship. A.A. acknowledges SABIC for the Career Development SABIC Chair. The authors thank Dr. Detlef-M. Smilgies for help with acquisition of GIWAXS data at CHESS. CHESS was supported by the NSF & NIH/NIGMS via NSF Award No. DMR-1332208. The authors also acknowledge Dr. Sean Ryno for helpful discussions. Figure 3 was updated on January 17, 2017 to remove a formatting error. The scientific content was not changed.
JournalAdvanced Energy Materials