Field-induced exciton and CT-state dissociation probed by time-resolved luminescence quenching (Conference Presentation)

The microscopic mechanisms of exciton and charge-transfer-state dissociation in organic semiconductors play a major role for the efficiencies of organic solar cells [1]. One of the most direct experiments for probing the dynamics of these processes is luminescence quenching. Here, we present a comprehensive experimental and simulative study of the field and temperature dependence of the dissociation of singlet excitons in PTB7 and PC71BM, and from charge-transfer states generated across interfaces in PTB7/PC71BM bulk heterojunction solar cells. We deduce the relevant data from time-resolving the near infrared emission of the states from 10K to room temperature and for electric fields ranging from 0 to 2.5 MV/cm. To draw qualitative conclusions from our data, we use an analytical field-assisted hopping model in the presence of disorder [2]. We conclude that singlet excitons can be split by high fields, and that disorder plays a large role in broadening the critical threshold field for which singlet excitons are separated. Charge-transfer (CT) state dissociation can be induced by both field and temperature, and the data imply that a strong reduction of the Coulomb binding potential at the interface facilitates their separation. The observations provided herein of the field dependent separation of CT states as a function of temperature offer a rich dataset against which theoretical models of charge separation can be rigorously tested. References: [1] H. Bassler and A. Kohler, Phys. Chem. Chem. Phys. 17, 28451 (2015) [2] O. Rubel, S. D. Baranovskii, W. Stolz, and F. Gebhard, Phys. Rev. Lett. 100, 196602 (2008).