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

Abstract

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

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

“…Thus, much effort has been dedicated to studying the low-offset hole transfer process from the NFA to the D, which appears to be significantly slower than the charge transfer in FA OSCs; in FA OSCs, charge transfer and separation into FCs is typically completed on sub-ps timescales, [87,93,94,103,104] while hole transfer alone can take tens of ps in low-offset NFA systems. [29,109,111,114,[116][117][118][119] In order to explain how hole transfer and CT state dissociation can occur in NFA systems with apparently minimal driving energy, the involvement of intermolecular NFA excited states with a reduced binding energy, [118][119][120] as well as interfacial electrostatics that can drive the charge transfer, [121][122][123][124][125] have both been invoked in addition to entropic factors. [89,90,[95][96][97] In our recent study, we investigated the effect of the driving energy on the hole transfer process for a series of NFAs blended with the polymer PTB7-Th.…”

“…[29] However, Karuthedath et al have claimed that utilizing the frontier molecular orbital energies determined using cyclic voltammetry (CV) provides an incorrect picture of the true energetics of "low-offset" OSCs. [125] By employing ultraviolet photoelectron spectroscopy (UPS), they discovered that the ionization energy (IE) offsets are often larger than previously thought. For example, the HOMO offset in the PM6:Y6 blend measured by CV is 90 meV; [37] in contrast, the PM6:Y6 IE offset measured by UPS is 700 meV.…”

“…Reproduced with permission. [125] Copyright 2020, Springer Nature. the PTB7-Th:IOTIC systems, [29,61] the charge generation from excitons created on the D was also characterized in-depth.…”

“…As the bandgap of the D is almost always wider than the NFA, this opens up the possibility of Förster resonance energy transfer (FRET) from D to A. [ 11,29,125,129 ] Indeed, this is assisted by the frequent overlap of the D emission and A absorption, as well as the strong absorption coefficient of the NFA. In our studies of the 1% and 52% LMWF PM6:Y6 blends and the PTB7‐Th:IOTIC systems, [ 29,61 ] the charge generation from excitons created on the D was also characterized in‐depth.…”

“…b)A schematic to illustrate the energy level bending at the D:A interface, resulting from the electrostatic interaction between the charges and the quadrupolar moments of nearby NFA molecules. Reproduced with permission [125]. Copyright 2020, Springer Nature.…”