Ultrafast spectroscopic investigation of the effect of solvent additives on charge photogeneration and recombination dynamics in non-fullerene organic photovoltaic blends

Abstract

The effects of DIO and CN additives on the neat and PBDB-TF:IT-4F blend film morphologies and the related ultrafast photophysical processes are studied.

The Path to 20% Power Conversion Efficiencies in Nonfullerene Acceptor Organic Solar Cells

“…[ 138,143,173 ] Indeed, owing to the negligible geminate recombination losses in high performance NFA OSCs, we expect the geminate T 1 formation pathway to be insignificant. [ 10,121,174 ] However, there have been a number of reports where nongeminate T 1 formation in NFA OSCs has been observed, [ 165,175,176 ] confirming that this potential loss pathway is still active. As the NFA OSCs in these studies exhibited good performance (PCEs ≈11–12%), it is apparent that T 1 formation does not preclude efficient photocurrent generation, consistent with the observations of Karuthedath et al.…”

Ultrafast Fluctuations in PM6 Domains of Binary and Ternary Organic Photovoltaic Thin Films Probed with Two-Dimensional White-Light Spectroscopy

“…Thus, we refer to the process that depletes the intensity of the diagonal peak as “charge transfer”, keeping in mind that it could also be energy transfer. Charge transfer is known to be highly efficient ,, in these photovoltaic films and has been previously measured to be ultrafast. ,,, Thus, one likely reason for faster dynamics is electron transfer from PM6 excitons to the acceptor molecules.…”

“…25 Yang et al found that use of solvent additives in preparation of the bulk heterojunctions can suppress recombination losses. 35 A more advanced form of TA spectroscopy, ultrafast twodimensional electronic spectroscopy (2DES), probes both structures, and dynamics. By measuring 2D lineshapes as a function of delay time, the magnitude and time scales of the fluctuations of the environment around the photogenerated excitons are measured, which are created by molecular differences in structure and their motions.…”

“…Charge transfer is known to be highly efficient 13,18,22 in these photovoltaic films and has been previously measured to be ultrafast. 24,25,34,35 Thus, one likely reason for faster dynamics is electron transfer from PM6 excitons to the acceptor molecules.…”

“…Ultrafast charge transfer and other femtosecond energy transfer processes are the first processes to occur upon absorption of a photon and often play important roles in device efficiency. The time scale of these processes often correlate with materials or processing methods and differences in efficiency, open circuit voltage, current density, and other devices metrics. − Several transient absorption (TA) studies of the PM6:Y6 and PM6:IT4F systems have identified hole transfer from excitons in the acceptor material to be a process that contributes to the overall photocurrent as well as electron transfer from the donor material. ,,,− Chen et al varied the morphology of their bulk heterojunction films and found that the grain size controls the hole transfer time scale . Yang et al found that use of solvent additives in preparation of the bulk heterojunctions can suppress recombination losses …”