The microscopic mechanisms of exciton and charge-transfer-state dissociation in organic semiconductors play a major role for the efficiencies of organic solar cells . 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 . 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.
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Uli Lemmer, Marina Gerhard, Andreas P. Arndt, Martin Koch, and Ian A. Howard, "Field-induced exciton and CT-state dissociation probed by time-resolved luminescence quenching (Conference Presentation)," Proc. SPIE 10348, Physical Chemistry of Semiconductor Materials and Interfaces XVI, 103480L (Presented at SPIE Nanoscience + Engineering: August 08, 2017; Published: 21 September 2017); https://doi.org/10.1117/12.2274345.5583366392001.
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