Nanoscale transport of charge-transfer states in organic donor–acceptor blends

Abstract

Charge-transfer (CT) states, bound combinations of an electron and a hole on separate molecules, play a crucial role in organic optoelectronic devices. We report direct nanoscale imaging of the transport of long-lived CT states in molecular organic donor-acceptor blends, which demonstrates that the bound electron-hole pairs that form the CT states move geminately over distances of 5-10 nm, driven by energetic disorder and diffusion to lower energy sites. Magnetic field dependence reveals a fluctuating exchange splitting, indicative of a variation in electron-hole spacing during diffusion. The results suggest that the electron-hole pair of the CT state undergoes a stretching transport mechanism analogous to an 'inchworm' motion, in contrast to conventional transport of Frenkel excitons. Given the short exciton lifetimes characteristic of bulk heterojunction organic solar cells, this work confirms the potential importance of CT state transport, suggesting that CT states are likely to diffuse farther than Frenkel excitons in many donor-acceptor blends.

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

“…The PL also exhibits pronounced MFEs, indicating that CT state dynamics may involve fluctuations in electron-hole separation. These observations led the authors to hypothesize that CT dynamics proceed through the asynchronous motion of localized electrons and holes [19]. Here we apply our model to this system in order to (i ) confirm that the hypothesized description of CT state dynamics is consistent with the observed MFEs, and…”

“…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].…”

“…Recently, Adachi et al [33] and Baldo et al [18,19] films blends of these molecules this radiative processes is evident in the photoluminescence (PL), which exhibits a long time (∼ 30µs) decay that has been attributed to reverse intersystem crossing from a long lived population of CT triplets. Focusing on this long time PL signature, time-resolved fluorescence microscopy has revealed that the PL profile undergoes both a transient spatial broadening and a transient redshift [19], indicating that CT states are mobile along the donor-acceptor interface and sensitive to the presence of static energetic disorder.…”

“…Focusing on this long time PL signature, time-resolved fluorescence microscopy has revealed that the PL profile undergoes both a transient spatial broadening and a transient redshift [19], indicating that CT states are mobile along the donor-acceptor interface and sensitive to the presence of static energetic disorder. The PL also exhibits pronounced MFEs, indicating that CT state dynamics may involve fluctuations in electron-hole separation.…”

“…19, however trajectories could also be terminated at earlier times via a radiative recombination event.…”