Charge Transfer Dynamics

Charge Transfer State Dynamics and Transport

ONE Lab: Wendi Chang, Parag Deotare, Vladimir Bulović
Collaborators: Dan Congreve, Eric Hontz, Li Shi, Chee-Kong Lee, Brian Modtland, Adam Willard, Troy Van Voorhis, Marc Baldo

Technological applications of organic semiconductors, such as organic light emitting diodes (OLEDs) and organic photovoltaics (OPVs), drive the need to further understand the fundamental exciton dynamics in the material systems. As a crucial transitional excited state in nanostructured optoelectronic devices, charge transfer (CT) states mediate both light emission in OLEDs and charge generation in OPVs. Currently, OPVs demonstrate ultrafast formation of CT states from excitons, and efficient dissociation of CT states into charge under short circuit conditions. Thus, understanding the CT state dynamics under various conditions would ultimately lead to designing better devices.

In this project, we examine the CT state spin dynamics by monitoring the CT exciplex emission under various conditions. By applying external pressure and magnetic field on a thin film of organic donor-acceptor blend, we gain the experimental insight to build a model of the spin dynamics of CT states to validate our working theories. Additional work on observing the spatial diffusion of the exciplex emission gave insight into possible transport mechanism of CT states, which is accurately modelled using kinetic Monte Carlo method. By focusing on experimental observations of CT dynamics and transport, we develop simple design rules for material selection for OPV and further our understanding of exciton dynamics as applied to organic optoelectronic devices.

Schematic of charge transfer state dynamics and energy diagram and chemical structures of the donor-acceptor material system.

 

Schematic of expected changes in singlet and triplet CT states as a function of their size. The possible triplet dynamics are dependent on the presence of a triplet quenching state or ‘drain’ on the donor or the acceptor. (inset) The measurement apparatus used to apply pressure to the films.

 

Related publications and links

  • W. Chang*, P. Deotare*, E. Hontz*, D. Congreve*, L. Shi*, P. Reusswig, B. Modtland, C. Lee, A. Willard, M. Balke, V. Bulovic, T. Van Voorhis, M. A. Baldo, “Nanoscale Transport of Charge Transfer State in Organic Donor-Acceptor Blends,” Nat. Mat., vol. 14, 1130 (2015).
  • E. Hontz, W. Chang, D. Congreve, V. Bulovic, M. A. Baldo, T. Van Voorhis, “The Role of Electron-Hole Separation in Thermally Activated Delayed Fluorescence in Donor-Acceptor Blends,” J. Phys. Chem. C, vol. 119, 25591-25597 (2015).
  • W. Chang*, D. Congreve*, E. Hontz, M. Balke, D. McMahon, S. Reineke, T. C. Wu, V. Bulovic, T. Van Voorhis, M. A. Baldo, “Rational Charge Transfer State Design in Organic Photovoltaic,” Nat. Comm., vol. 6, 6415 (2015).