Discerning Bulk and Interfacial Polarons in a Dual Electron Donor/Acceptor Polymer

Abstract

The active layer of organic solar cells typically possesses a complex morphology, with amorphous donor/acceptor mixed domains present in addition to purer, more crystalline domains. These crystalline domains may represent an energy sink for free charges that aids charge separation and suppresses bimolecular recombination. The first step in exploiting this behavior is the identification and characterization of charges located in these different domains. Herein, the generation and recombination of both bulk and interfacial polarons are demonstrated in the dual electron donor/acceptor polymer XIND using transient absorption spectroscopy. The absorption spectra of XIND bulk polarons, present in pristine polymer domains, are clearly distinguishable from those of polarons present at the donor/acceptor interface. Furthermore, it is shown that photogenerated polarons are transferred from the interface to the bulk. These findings support the energy sink hypothesis and offer a way to maximize morphology relationships to enhance charge generation and suppress recombination.

Triplet‐Charge Annihilation in a Small Molecule Donor: Acceptor Blend as a Major Loss Mechanism in Organic Photovoltaics

“…Oxygen quenching experiments are standard procedures in µs-TAS and have been applied successfully to many conjugated polymer systems. [5,74,85] The short oxygen exposure times in such experiments usually preclude oxygen-induced degradation. The key to successful use of our TCA methodology is therefore an assessment of the reversibility of the reaction (Figure S7, Supporting Information): any oxygen-induced degradation will prevent a full reversibility.…”

“…It has been shown that certain materials have the ability to form polarons in both crystalline and amorphous regions, showing two different polaron absorbance features. [74,75] For this particular approach, the energy sink phenomenon may play an important role: polarons, once formed, may be rapidly transferred to more crystalline domains. [92,93]…”

“…The two different DRCN5T species seen in the annealed spectrum under oxygen could be attributed to polarons in domains with different crystallinity, as shown before in literature for different OPV materials, and consistent with the presence of both crystalline and amorphous regions showed by the Raman spectroscopy. [74,75] Furthermore, faster kinetics are often associated with polarons in more amorphous domains, where the positive polaron has more accessibility to the fullerene anion. In contrast, slower kinetics are assigned to polarons localized/trapped in more crystalline, and thus lower energy, domains.…”

Intrinsic photogeneration of long-lived charges in a donor-orthogonal acceptor conjugated polymer

“…This is highly unusual because conjugated polymers in solution often display only triplet formation, 32 and for those that are capable of charge generation in solution or pristine films, these charges generally decay on nanosecond timescales due to rapid geminate recombination. 9 The charge generation process in the pristine polymer solution is likely aided by the strongly intramolecular donor/acceptor nature of PCPDT-sFCN , providing significant charge-transfer character to the lowest energy singlet state. The spatial separation of electron and hole in this CT state reduces the exciton binding energy, and this, coupled to the aggregate configuration with multiple interchain interactions, allows the separated charges to form (depicted in Fig.…”

“…Instead, any charges that do form tend to undergo rapid geminate recombination on ps-ns timescales. 8,9 Furthermore, many pristine conjugated polymers exhibit solely triplet formation. 10,11 Triplet states are a key species that commonly form during the operation of organic electronic devices such as light emitting diodes and solar cells.…”