Magnetic Field Effects of Charge Transfer Excitons in Organic Semiconductor Devices

Abstract

The topic of this review is the effect of magnetic field on photo‐current (PC) in organic semiconductor devices. Magnetic field modifies the fraction of the singlet/triplet population in the charge transfer excitons (CTE) whose lifetime is spin configuration dependent yielding magneto‐PC (MPC). We review MPC in the framework of the polaron pair model and emphasize the effect of CTE lifetime on MPC. In addition to a detailed calculation of the MPC response we find a range of physical parameters in which the response follows a simple ′fit function′. For example, a sum of wide and narrow Lorentzian functions, MPC(B)=cwB2/(B2+bw2)+cnB2/(B2+bn2), in the case of hyperfine interaction; or a Voigt profile V=L*G, where the sign * stands for the convolution operation between a Lorentzian (L) and a Gaussian (G) functions, in the case of thermal spin polarization. We provide expressions for the fit parameters in terms of physical parameters.

Insights into Magneto-Photocurrent and Coherent Spin Mixing for Binary and Ternary Nonfullerene Bulk Heterojunction Organic Solar Cells

“…13 On the contrary, (CTS) T is likely to undergo PBT and subsequently form TE since it has a long lifetime and its energy level is usually lower than that of (CTS) T . 13,14,32 In OSCs, the existence of TE mainly contributes to the energy loss and degradation because of its long lifetime and oxygen reaction. 52−55 Nevertheless, it may help to generate the photocurrent through the triplet-polaron reaction (TPR, sometimes also referred to as triplet-charge reaction).…”

“…At the moment, polaron spins are preferably in the triplet configuration because of the paramagnetism (Figure b case II). The lineshape of the LFMPC can be described by normalL normalF normalM normalP normalC ( % ) = ( c w B 2 B 2 + b w 2 ︸ f a s t d e c a y + c n B 2 ( B 2 + b n 2 ) ︸ s l o w d e c a y ) × 100 % …”

“…The same is applied for c n . The expressions for the quantities b w and b n are b normalw = true[ true( a normalH normalF μ normalB true) 2 + true( normalℏ γ μ normalB true) 2 true] 0.5 / p 1 and b normaln = [ ( p 2 ) 2 + true( p 3 J normale normalx μ normalB true) 2 + ( ℏ γ μ B ) 2 ] 0.5 in which μ B is the Bohr magneton. J ex is the exchange energy.…”

“…Most experiments that involve the MPC measurements in organic BHJ solar cells are in fact due to the HFMPC. Its lineshape can be written by normalH normalF normalM normalP normalC ( % ) = c B 2 false( B 2 + b 2 false) × 100 % in which c = false( k normalT 0 γ normals − k normals γ normalT 0 false) / false[ k normals ( γ s + γ T 0 ) false] and b 2 = [ 4 J e x 2 + ℏ 2 ( γ s + γ T 0 ) 2 ] γ ...…”

“…If the SE is not quenched by the germinate recombination at the initial stage, it migrates to an intermolecular donor–acceptor interface. By comparing with the SE, the largely separated electron–hole pair, which resides at the interfacial CTS, is coined as a PP. , The PP that bounds electrostatically may dissociate into free polarons for electrical conduction or may fuse into an exciton for radiative and nonradiative recombination. Analogous with the excitonic spin states, the PP has the singlet and triplet characters calling them PP singlet ((PP) S ) and PP triplet ((PP) T ).…”