The relationship between the coherent size, binding energy and dissociation dynamics of charge transfer excitons at organic interfaces

Endothermic Charge Separation Occurs Spontaneously in Non-Fullerene Acceptor/Polymer Bulk Heterojunction

Organic photovoltaics (OPVs) based on non-fullerene acceptors (NFAs) have achieved a power conversion efficiency close to 20%. These NFA OPVs can generate free carriers efficiently despite a very small energy level offset at the donor/acceptor interface. Why these NFAs can enable efficient charge separation (CS) with low energy losses remains an open question. Here, the CS process in the PM6:Y6 bulk heterojunction is probed by time-resolved two-photon photoemission spectroscopy. It is found that the CS, the conversion from bound charge transfer (CT) excitons to free carriers, is an endothermic process with an enthalpy barrier of 0.15 eV. The CS can occur spontaneously despite being an endothermic process, which implies that it is driven by entropy. It is further argued that the morphology of the PM6:Y6 film and the anisotropic electron delocalization restrict the electron and hole wavefunctions within the CT exciton such that they can primarily contact each other through point-like junctions. This configuration can maximize the entropic driving force.

Sub-10-fs observation of bound exciton formation in organic optoelectronic devices

Fundamental mechanisms underlying exciton formation in organic semiconductors are complex and elusive as it occurs on ultrashort sub-100-fs timescales. Some fundamental aspects of this process, such as the evolution of exciton binding energy, have not been resolved in time experimentally. Here, we apply a combination of sub-10-fs Pump-Push-Photocurrent, Pump-Push-Photoluminescence, and Pump-Probe spectroscopies to polyfluorene devices to track the ultrafast formation of excitons. While Pump-Probe is sensitive to the total concentration of excited states, Pump-Push-Photocurrent and Pump-Push-Photoluminescence are sensitive to bound states only, providing access to exciton binding dynamics. We find that excitons created by near-absorption-edge photons are intrinsically bound states, or become such within 10 fs after excitation. Meanwhile, excitons with a modest >0.3 eV excess energy can dissociate spontaneously within 50 fs before acquiring bound character. These conclusions are supported by excited-state molecular dynamics simulations and a global kinetic model which quantitatively reproduce experimental data.

Ultrafast action spectroscopies of organic optoelectronic devices reveal that the formation of bound exciton state occurs as fast as 10 fs. Excitons having excess energy can dissociate spontaneously within 50-fs before acquiring bound character.

Dynamics of Excitons in Conjugated Molecules and Organic Semiconductor Systems

The exciton, an excited electron-hole pair bound by Coulomb attraction, plays a key role in photophysics of organic molecules and drives practically important phenomena such as photoinduced mechanical motions of a molecule, photochemical conversions, energy transfer, generation of free charge carriers, etc. Its behavior in extended π-conjugated molecules and disordered organic films is very different and very rich compared with exciton behavior in inorganic semiconductor crystals. Due to the high degree of variability of organic systems themselves, the exciton not only exerts changes on molecules that carry it but undergoes its own changes during all phases of its lifetime, that is, birth, conversion and transport, and decay. The goal of this review is to give a systematic and comprehensive view on exciton behavior in π-conjugated molecules and molecular assemblies at all phases of exciton evolution with emphasis on rates typical for this dynamic picture and various consequences of the above dynamics. To uncover the rich variety of exciton behavior, details of exciton formation, exciton transport, exciton energy conversion, direct and reverse intersystem crossing, and radiative and nonradiative decay are considered in different systems, where these processes lead to or are influenced by static and dynamic disorder, charge distribution symmetry breaking, photoinduced reactions, electron and proton transfer, structural rearrangements, exciton coupling with vibrations and intermediate particles, and exciton dissociation and annihilation as well.

Extracting Electrons from Delocalized Excitons by Flattening the Energetic Pathway for Charge Separation

At organic donor-acceptor (D-A) interfaces, electron and hole are bound together to form charge transfer (CT) excitons. The electron and hole wave functions in these CT excitons can spatially delocalize. The electron delocalization opens up possibilities of extracting free charges from bound excitons by manipulating the potential energy landscape on the nanoscale. Using a prototype trilayer structure that has a cascade band structure, we show that the yield of charge separation can be doubled as compared to the bilayer counterpart when the thickness of the intermediate layer is around 3 nm. This thickness coincides with the electron delocalization size of CT excitons typically found in these organic films. Tight-binding calculation for the CT states in the trilayer structure further demonstrates that electron delocalization, together with the energy level cascade, can effectively flatten the energetic pathway for charge separation. Hence, it is possible to add nanometer-thick layers between the donor and the acceptor to significantly enhance the charge separation yield.

Free Charge Carriers in Homo-Sorted π-Stacks of Donor-Acceptor Conjugates

Inspired by the high photoconversion efficiency observed in natural light-harvesting systems, the hierarchical organization of molecular building blocks has gained impetus in the past few decades. Particularly, the molecular arrangement and packing in the active layer of organic solar cells (OSCs) have garnered significant attention due to the decisive role of the nature of donor/acceptor (D/A) heterojunctions in charge carrier generation and ultimately the power conversion efficiency. This review focuses on the recent developments in emergent optoelectronic properties exhibited by self-sorted donor-on-donor/acceptor-on-acceptor arrangement of covalently linked D-A systems, highlighting the ultrafast excited state dynamics of charge transfer and transport. Segregated organization of donors and acceptors promotes the delocalization of photoinduced charges among the stacks, engendering an enhanced charge separation lifetime and percolation pathways with ambipolar conductivity and charge carrier yield. Covalently linking donors and acceptors ensure a sufficient D-A interface and interchromophoric electronic coupling as required for faster charge separation while providing better control over their supramolecular assemblies. The design strategies to attain D-A conjugate assemblies with optimal charge carrier generation efficiency, the scope of their application compared to state-of-the-art OSCs, current challenges, and future opportunities are discussed in the review. An integrated overview of rational design approaches derived from the comprehension of underlying photoinduced processes can pave the way toward superior optoelectronic devices and bring in new possibilities to the avenue of functional supramolecular architectures.

Collective Effects of Band Offset and Wave Function Dimensionality on Impeding Electron Transfer from 2D to Organic Crystals

Excited-state electron transfer (ET) across molecules/transition metal dichalcogenide crystal (TMDC) interfaces is a critical process for the functioning of various organic/TMDC hybrid optoelectronic devices. Therefore, it is important to understand the fundamental factors that can facilitate or limit the ET rate. Here it is found that an undesirable combination of the interfacial band offset and the spatial dimensionality of the delocalized electron wave function can significantly slow down the ET process. Specifically, it is found that whereas the ET rate from TMDCs (MoS₂ and WSe₂) to fullerenes is relative insensitive to the band offset, the ET rate from TMDCs to perylene molecules can be reduced by an order of magnitude when the band offset is large. For the perylene crystal, the sensitivity of the ET rate on the band offset is explained by the 1D nature of the electronic wave function, which limits the availability of states with the appropriate energy to accept the electron.

Effect of the Interfacial Energy Landscape on Photoinduced Charge Generation at the ZnPc/MoS2 Interface

Monolayer transition-metal dichalcogenide crystals (TMDC) can be combined with other functional materials, such as organic molecules, to form a wide range of heterostructures with tailorable properties. Although a number of works have shown that ultrafast charge transfer (CT) can occur at organic/TMDC interfaces, conditions that would facilitate the separation of interfacial CT excitons into free carriers remain unclear. Here, time-resolved and steady-state photoemission spectroscopy are used to study the potential energy landscape, charge transfer, and exciton dynamics at the zinc phthalocyanine (ZnPc)/monolayer (ML) MoS₂ and ZnPc/bulk MoS₂ interfaces. Surprisingly, although both interfaces have a type-II band alignment and exhibit sub-100 fs CT, the CT excitons formed at the two interfaces show drastically different evolution dynamics. The ZnPc/ML-MoS₂ behaves like typical donor-acceptor interfaces in which CT excitons dissociate into electron-hole pairs. On the contrary, back electron transfer occur at ZnPc/bulk-MoS₂, which results in the formation of triplet excitons in ZnPc. The difference can be explained by the different amount of band bending found in the ZnPc film deposited on ML-MoS₂ and bulk-MoS₂. Our work illustrates that the potential energy landscape near the interface plays an important role in the charge separation behavior. Therefore, considering the energy level alignment at the interface alone is not enough for predicting whether free charges can be generated effectively from an interface.