A Model of Charge-Transfer Excitons: Diffusion, Spin Dynamics, and Magnetic Field Effects

Abstract

In this letter we explore how the microscopic dynamics of charge transfer (CT) excitons are influenced by the presence of an external magnetic field in disordered molecular semiconductors. This influence is driven by the dynamic interplay between the spin and spatial degrees of freedom of the electron-hole pair. To account for this interplay we have developed a numerical framework that combines a traditional model of quantum spin dynamics with a stochastic coarse-grained model of charge transport. This combination provides a general and efficient methodology for simulating the effects of magnetic field on CT state dynamics, therefore providing a basis for revealing the microscopic origin of experimentally observed magnetic field effects. We demonstrate that simulations carried out on our model are capable of reproducing experimental results as well as generating theoretical predictions related to the efficiency of organic electronic materials.1 arXiv:1603.08160v2 [physics.chem-ph] Apr 2016Charge transfer (CT) states play a fundamental role in mediating interconversion between bound electronic excitations and free charge carriers in organic electronic materials.For processes that require this interconversion, such as electroluminescence in organic light emitting diodes (OLEDs) and photocurrent generation in organic photovoltaics (OPVs), low-energy (thermalized) CT states are often implicated as a precursor to efficiency loss pathways [1][2][3][4][5][6][7][8][9][10][11][12]. Despite this, much remains to be understood about the properties of CT states and how they contribute to various energy loss mechanisms. Due to their short lifetime and low optical activity, attempts to interrogate CT states directly have brought limited success. Notably, however, recent experiments that probe CT states indirectly via their response to an applied magnetic field have demonstrated the potential to reveal new information about this elusive class of excited states [13][14][15][16][17][18][19][20][21]. Unfortunately, extracting this information is challenging because it is encoded by a complex interplay of electronic and nuclear spin dynamics [15,22,23]. This interplay is further complicated when the dynamics of the electron-hole spin state (or the specific experimental observable) is coupled to a source of fluctuating microscopic disorder such as charge transport or molecular conformational dynamics [21]. In this letter we focus on disentangling this interplay.The dependence of an experimental observable on an applied magnetic field is generically referred to as the magnetic field effect (MFE). For CT-mediated processes, MFEs require that the observed physical property depends either directly or indirectly on the spin state of the electron-hole pair. For instance, spin selection rules for radiative electron-hole recombination can give rise to a magnetic field-dependent electroluminescence yield [20,[24][25][26]. To understand specifically how CT state properties are influenced by the presence of a magnetic field it is na...

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