Triplet Excitons and Associated Efficiency‐Limiting Pathways in Organic Solar Cell Blends Based on (Non‐) Halogenated PBDB‐T and Y‐Series

Abstract

The great progress in organic photovoltaics (OPV) over the past few years has been largely achieved by the development of non‐fullerene acceptors (NFAs), with power conversion efficiencies now approaching 20%. To further improve device performance, loss mechanisms must be identified and minimized. Triplet states are known to adversely affect device performance, since they can form energetically trapped excitons on low‐lying states that are responsible for non‐radiative losses or even device degradation. Halogenation of OPV materials has long been employed to tailor energy levels and to enhance open circuit voltage. Yet, the influence on recombination to triplet excitons has been largely unexplored. Using the complementary spin‐sensitive methods of photoluminescence detected magnetic resonance and transient electron paramagnetic resonance corroborated by transient absorption and quantum‐chemical calculations, exciton pathways in OPV blends are unravelled employing the polymer donors PBDB‐T, PM6, and PM7 together with NFAs Y6 and Y7. All blends reveal triplet excitons on the NFA populated via non‐geminate hole back transfer and, in blends with halogenated donors, also by spin‐orbit coupling driven intersystem crossing. Identifying these triplet formation pathways in all tested solar cell absorber films highlights the untapped potential for improved charge generation to further increase plateauing OPV efficiencies.

A nonfullerene acceptor as a solid additive realizing a record efficiency of 17.74% in quasi-layered all-polymer solar cells

“…S4,† the ground state bleaching (GSB) peaks of neat PM6 films are centered at ∼620 nm, which correspond well with the steady absorption spectra of the corresponding neat films. 44 The PY-IT presents a relatively broad GSB signal with characteristic peaks at ∼710 nm and ∼800 nm. The TA spectra of the PM6/PY-IT and PM6/PY-IT:L8-BO films at different probe delay times are displayed in Fig.…”

NIR‐II Absorbing Monodispersed Oligomers Based on N−B←N Unit

“…[15] The electron mobility of long oligomers also meets the requirements of OPD devices (Figure S27). We use OTB4 as the electron acceptor and select commercially available Poly [(2,6-(4,8-bis(5-(2-ethylhexyl) thiophen-2-yl)-benzo-[1,2-b:4,5-b']dithiophene))-alt- (5,5-(1',3'-di-2thienyl-5',7'-bis(2-ethylhexyl)benzo[1', 2'-c:4',5'c']dithiophene-4,8-dione))] (PBDB-T, see Figure 5b) [16] as the electron donor to construct the OPDs. The device structure is ITO/ZnO/PBDBÀ T: OTB4/MoO 3 /Ag (Figure 5a).…”

Understanding the Role of Triplet‐Triplet Annihilation in Non‐Fullerene Acceptor Organic Solar Cells

“…It has been widely reported that more crystalline phases have increased triplet exciton diffusion lengths and mobilities, and so we propose that the enhanced long-range OOP ordering in PM6:Y11 increases the rate constant of TTA in this blend relative to that in PM6:Y6. [30][31][32][33]68]…”