Charge Transfer Exciton Lifetime Determined by low-field Magneto-Photocurrent

  Daniel Nikiforov [1,2]  ,  Himanshu Shekhar [3]  ,  Nir Tessler [3]  ,  Eitan Ehrenfreund [1,2]  
[1] Physics Department, Technion-Israel Institute of Technology, Haifa 32000, ISRAEL
[2] Solid State Institute, Technion-Israel Institute of Technology, Haifa 32000, ISRAEL
[3] Department of Electrical Engineering, Technion-Israel Institute of Technology, Haifa 32000, ISRAEL

Decay rate of the charge transfer exciton (CTE) in donor-acceptor organic semiconductor photodiode devices has been measured by means of magnetic field effect. Following photoexcitation transient CTE states are formed at the donor-acceptor interface where an electron (e) and hole (h) are residing on neighboring acceptor and donor molecules respectively. Although this photogenerated e-h pair is spatially separated it is still electrostatically bound. Dissociation efficiency of the CTEs is an important component in overall efficiency of photodiodes and photovoltaic cells. However, the overall device photocurrent (PC) is determined not only by the CTE dissociation but also by the free charge carrier extraction efficiency; hence direct investigation of CTE state dynamics is problematic.

In the presented work, we demonstrate that the CTE decay rate may be determined from magneto-PC (MPC) measurements in photodiode with an active layer made of 1,1-bis [(di-4-tolylamino) phenyl] cyclohexane (TAPC) and C70 fullerene donor-acceptor interface. The MPC response is caused by spin dependent decay of the CTE states; whereas the e-h pair (polaron pair) spin states are modified by hyperfine and Zeeman interactions. This is a well-known model (“polaron pair model”) explaining magnetic field effects in organic semiconductors. We show that the linewidth of MPC response at low-fields (< 200 mT) may be modified by the CTE decay rate. We conduct MPC experiments at various temperatures and find CTE lifetimes of about 0.5 ns at room temperature and 5 ns at 100 K; in the intermediate temperatures the CTE decay rate is thermally activated with activation energy of about 32 meV. In summary, we have again shown that magnetic field effect is a useful tool in investigation of e-h pair dynamics in organic semiconductors; in particular, we demonstrated that the pair lifetime can be determined from low-field magnetic field response.