Direct Observation of the Charge Transfer States from a Non-Fullerene Organic Solar Cell with a Small Driving Force

Abstract

For organic solar cells (OSCs), the charge generation mechanism and the recombination loss are heavily linked with charge transfer states (CTS). Measuring the energy of CTS (E CT) by the most widely used technique, however, has become challenging for the non-fullerene-based OSCs with a small driving force, resulting in difficulty in the understanding of OSC physics. Herein, we present a study of the PM6:Y6 bulk heterojunction. It is demonstrated that electro-absorption can not only reveal the dipolar nature of Y6 but also resolve the morphology-dependent absorption signal of CTS in the sub-bandgap region. The device with the optimum blending weight ratio shows an E CT of 1.27 eV, which is confirmed by independent measurements. Because of the charge transfer characteristics of Y6, the charge generation at PM6:Y6 interfaces occurs efficiently under a small but non-negligible driving force of 0.14 eV, and the total recombination loss is as low as 0.43 eV.

Cascaded energy landscape as a key driver for slow yet efficient charge separation with small energy offset in organic solar cells

“…A recent study showed that PM6 exhibits a steady-state EA spectrum in this wavelength region. 51 Therefore, in line with previous studies, 18,42,51,52 this band is attributable to the EA of PM6. When an exciton dissociates to form an electron–hole pair at the D/A interface, the electron–hole pair generates a dipole-like local electric field in the surroundings.…”

“…Note that the positive PIA in the B800 nm region, which we attribute to Y6 anions in crystalline domains, cannot be assigned to an EA signal of Y6 because no (steady-state) EA peak was reported at B800 nm for pristine Y6 or PM6/Y6 blend. 52,53 As the EA amplitude depends on the strength of the local electric field, which is a function of the separation distance between the electron and hole, we can directly probe the dissociation kinetics of the electron-hole pair. The fact that the EA signals reached their maximum value slightly after the occurrence of hole transfer, as shown in Fig.…”

The Intrinsic Role of the Fusion Mode and Electron‐Deficient Core in Fused‐Ring Electron Acceptors for Organic Photovoltaics

“…0.12 eV reported for a typical polymer donor [45] . In addition, considering a low driving force for exciton dissociation via hole transfer (Δ G ED ) in these nonfullerene OSCs, [46–48] the Δ G ED is assumed to be −0.1 eV. As seen in Figure 5c, the calculated k ED for Y6 is on the order of 10 12 s −1 , which is in good agreement with the previous results for nonfullerene‐based blends [48, 49] .…”

The Intrinsic Role of the Fusion Mode and Electron‐Deficient Core in Fused‐Ring Electron Acceptors for Organic Photovoltaics

“…0.12 eV reported for a typical polymer donor. [45] In addition, considering a low driving force for exciton dissociation via hole transfer (ΔG ED ) in these nonfullerene OSCs, [46][47][48] the ΔG is assumed to be À 0.1 eV. As seen in Figure 5c, the calculated k ED for Y6 is on the order of 10 12 s À 1 , which is in good agreement with the previous results for nonfullerene-based blends.…”