Charge transfer (CT) states play evidently an important role at the interface of organic heterostructures but their identification and characterization is often experimentally less obvious and challenging.
We studied two exemplary material systems which both represented a benchmark within the research of organic photovoltaics at their time: the homopolymer P3HT blended with PC61BM and the copolymer PTB7 blended with PC71BM. In both heterostructures, we could identify a distinct CT state emission by the use of NIR time-resolved photoluminescence (PL) , . The selectivity of this technique enables us to clearly probe the energetics and dynamics of weak emitting interfacial states and therefore to prove differences in the CT state characteristics between the two systems.
We went beyond this previous work and investigated the time and temperature dependent emission anisotropy as well as the electric field dependence of the time-resolved PL for both blends and the pristine polymers, respectively. In both cases the CT state emission clearly deviates from the one of the primarily excited singlet excitons: the emission anisotropy reveals an additional relaxation pathway for the exciton which is connected with a change of the transition dipole moment of the emission, and under applied bias different quenching thresholds can give access to varying binding energies of the emissive excitons involved.
Finally, we think that our findings demonstrate how interfacial CT state emission can be clearly identified as such and how it can be unambiguously distinguished from singlet exciton emission.
Andreas P Arndt, Marina Gerhard, Ian A. Howard, Martin Koch, and Ulrich Lemmer, "Identifying interfacial charge transfer states in organic heterostructures
(Conference Presentation)," Proc. SPIE 9923, Physical Chemistry of Interfaces and Nanomaterials XV, 99231G (Presented at SPIE Nanoscience + Engineering: August 31, 2016; Published: 10 November 2016); https://doi.org/10.1117/12.2238126.5161498061001.
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