Identifying Charge-Transfer States in Polymer:Fullerene Heterojunctions by Their Emission Polarization Anisotropy

We investigate the time-resolved photoluminescence (PL) polarization anisotropy of the organic photovoltaic model systems P3HT:PC61BM and PTB7:PC71BM, and their corresponding neat polymer films. Both blends show a strong emission from interfacial charge-transfer states (CTSs); the CTS emission can be uniquely identified by its negative polarization anisotropy, which is indicative of a significant rotation of the transition dipole moment upon charge transfer at the donor acceptor interface. We also observe a spectral region showing negative anisotropy in the PL of the neat PTB7 film; this spectral region is at slightly higher energy than the blend CTS emission. This negative anisotropy is not observed in the emission spectrum of the polymer in solution. This suggests that some emissive states in the pristine PTB7 film have transition dipole moments rotated relative to the absorbing states; we interpret these to be states with a significant interchain charge-transfer character.