On the Physical Origins of Charge Separation at Donor–Acceptor Interfaces in Organic Solar Cells: Energy Bending versus Energy Disorder

Abstract

Charge separation (CS) is a central process in the working of organic solar cells (OSC). Despite the strong electron–hole (e–h) Coulombic attraction at the donor–acceptor (D–A) interface, the bound e–h pairs do separate into free charges following an ultrafast process. To explain these results, several models have been proposed. By means of kinetic Monte Carlo simulations, the energy bending (EB) at the D–A interface is considered as the driving force for CS against the impact of energy disorder. The results suggest that, while entropy and energy disorder alone allow for the bound e–h pairs to escape charger recombination, the efficiency of CS increases by several times and its timescale decreases significantly in the presence of EB, approaching experimental findings. The impact of external electric fields on CS efficiency is found to stem from insufficient amount of EB. Importantly, the results indicate that, in the absence of EB, simply improving charge carrier mobility in bulk has no effect on the performances of OSC, thus orienting the new materials design strategy toward preferably targeting interfacial properties. Guidelines regarding how to introduce EB at the D–A interface are also discussed.

Intrinsic efficiency limits in low-bandgap non-fullerene acceptor organic solar cells

“…This energy level bending is due to the gradual change in the electrostatic potential created by the quadrupole moments of the NFA and the donor polymer. [45][46][47][48][49][50][51] The energy level is bent in the direction that corresponds to the (negative) sign of the in-plane quadrupole moment of the acceptor-donor-acceptor (A-D-A) NFA molecule. Because of energy level bending, the CT state energy increases, and the IE difference narrows by approximately the same amount at the interface.…”

Static and Dynamic Disorder of Charge Transfer States Probed by Optical Spectroscopy

“…[3,4] This does not rule out that a more expanded electron-hole pair may initially be generated but it indicates that the vast majority of such "hot" electron-hole-pairs relax to cold states within the coulomb well before they execute a random walk to escape from it. [1,2,[5][6][7][8][9][10][11][12] The crucial entity in the dissociation process is therefore the CT state of the donor-acceptor couple and to unravel its properties is an important issue. [13,14] The usual formalism to describe the dynamics of CT states rests upon the classic Marcus theory of electron transfer [15] and its extension by Levich, Jortner, and Marcus.…”

Binding Energy of Triplet Excitons in Nonfullerene Acceptors: The Effects of Fluorination and Chlorination

“…However, photoexcitation of organic semiconductors generates excitons that are coulombically bound electron–hole pairs. , The excitons formed in D or A can diffuse up to the D/A interface, where they are more easily dissociated, thus generating free charges. − The free charges are then transported to their respective electrodes, producing the device’s photocurrent response …”