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

Abstract

We examined the mechanisms underlying the free carrier generation in a very topical PM6/Y6 organic solar cell. We observed slow yet efficient spatial charge dissociation driven by downhill energy relaxation through the interfacial energy cascade.

Red-Light-Mediated Photocatalytic Hydrogen Evolution by Hole Transfer from Non-Fullerene Acceptor Y6

“…Figure shows the steady-state UV–visible absorption spectra of neat Y6, neat PM6, and 1:1 PM6:Y6 NPs prepared with TEBS or SDS, and thin films of the same materials. The spectrum of the neat Y6 thin film (Figure a) is similar to films previously reported in the literature. ,,− In chloroform solutions of Y6, the lowest energy peak (∼815 nm) and its shoulder (∼730 nm) have been assigned to the vibronic progression of the S 0 → S 1 transition and the 670 nm peak has been assigned to the S 0 → S 2 transition. , Although a redshift and broadening of the absorption peaks is present for Y6 films compared to solutions in chloroform, the relative positions and intensities of the three main peaks are similar and hence their origins are likely to be the similar. The spectrum of neat Y6 NPs also shows the three main peaks, but the relative amplitudes are significantly changed, resulting in similar absorbance values from 650 to 800 nm.…”

“…The PM6 bleach continues to increase until ∼100 ps and is accompanied by the emergence of an ESA in the 650 to 780 nm region. This ESA signal is stronger than the >650 nm ESA formed in neat PM6 and has been previously attributed to the ESA of free polarons in these PM6:Y6 films. − , Additionally, the Y6 ESA at <500 nm appears depleted, but this depletion is likely due to the growth of the PM6 bleach in this region. Tracking the rise of the PM6 GSB at ∼580 nm gives a time constant for hole transfer of ∼20 ps.…”

“…A donor:NFA system that has demonstrated high performance, reaching PCEs greater than 17% in OPV cells − and having high photocatalytic H 2 production rates, consists of donor–acceptor copolymer PM6, and NFA Y6 (Figure a). In PM6:Y6 blend films, results from ultrafast spectroscopy have shown that free charges can be generated following excitation of either PM6 or Y6. − …”

Energetically Trapped Triplet Excitons and their Generation Pathways in Organic Solar Cell Blends based on (Non-)Halogenated PBDB-T and Y Series

“…This has previously been assigned to the electro-absorption of the donor polymer PM6, indicating the separation of bound interfacial CT states into free charges. [3,52,57] All blends with NFA Y6 show a considerable HT yield, whilst it decreases upon halogenation of the donor in these blends (Figure S1) and with NFA Y7 (Figure S3), further discussed below and in Sections 2 and 3 of SI.…”

“…The presence of the polymer GSB already at 0.2 ps in all blends suggests that some ultrafast HT occurs. [52,56] However, as the Y6 GSB falls, likely due to some spectral overlap with the negative sign PIAs of Y6 electron and PM6 hole polarons forming at 780 nm and 920 nm [57] , respectively, the polymer GSB continues to increase, peaking around 100 ps. This indicates that a significant population of the NFA excitons require additional time to dissociate.…”