| AVS 60th International Symposium and Exhibition | |
| Energy Frontiers Focus Topic | Wednesday Sessions |
| Session EN+SE+SS+TF-WeM |
| Session: | Thin Films for Energy |
| Presenter: | A. Dolocan, The University of Texas at Austin |
| Correspondent: | Click to Email |
Charge separation at small molecule organic donor/acceptor (D/A) interfaces is controlled, among other factors, by morphology and molecular packing. These buried interfaces are both crystalline and amorphous and exhibit a considerable degree of molecular intermixing. This combination prevents any single technique from solving the detailed picture of D/A interfaces. Here, a general methodology that combines Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) and Atomic Force Microscopy (AFM) is shown. Since AFM is little sensitive to buried interfaces, it is employed here to investigate the surface after sequential profiling using TOF-SIMS. The unique combination of high (few nanometers) resolution depth profiling and ultra-high (parts-per-billion) chemical selectivity of TOF-SIMS and nanometer spatial resolution of AFM can assess the degree of molecular intermixing, local crystallinity and molecular packing at such interfaces. We will present results for two model interfaces: copper phthalocyanine/fullerene (CuPc/C60) [1] and squaraine/fullerene (SQ/C60) [2]. We find that both interfaces deviate considerably from the cartoon like picture with sharp interfaces. The CuPc/C60 interfaces are characterized by an interdiffusion of the C60 molecules as far as 6.5 nm into the CuPc amorphous regions. A similar interdiffusion scenario is found for SQ/C60 interfaces that can be correlated with the resulting 60% increase in the external quantum efficiency (EQE).
References:
[1] Na Sai, Raluca Gearba, Andrei Dolocan, John R. Tritsch, Wai-Lun Chan, James R. Chelikowsky, Kevin Leung, and Xiaoyang Zhu, “Understanding the Interface Dipole of Copper Phthalocyanine (CuPc)/C60: Theory and Experiment”, The Journal of Physical Chemistry Letters 3, 2173 (2013).
[2] Jeramy D. Zimmerman, Brian E. Lassiter, Kai Sun, Andrei Dolocan, Raluca Gearba, Keith J. Stevenson, David Vanden Bout, and Stephen R. Forrest, “Control of interface order by inverse quasi-epitaxial growth of squaraine/fullerene thin film photovoltaics”, submitted to Proceedings of the National Academy of Sciences of the USA (2013).