Saied Md Pratik

The emergence of non‐fullerene acceptors (NFAs), particularly the Y‐type series, has reshaped organic photovoltaics, nowadays enabling ∼21% efficient solar cells with high charge generation and low voltage loss. Yet, the origin of these properties is still not entirely understood. Here, we describe a unified picture of the lowest electronic excited states in Y6 films and contrast them with those prevalent in ITIC and C ₆₀ films. Y6 supports hybrid local‐exciton (LE)–charge‐transfer (CT) states stabilized via intermolecular electronic couplings and short π–π contacts, which result in excimer‐like states delocalized over aggregates. The large change in dipole moment for the S ₀ →S ₁ transition makes this excitation sensitive to the polarizable environment, with dielectric stabilization red‐shifting S ₁ and bringing LE and CT configurations into near resonance. This polarization‐driven LE–CT hybridization contrasts with the situation in ITIC and C ₆₀ , where S ₁ remains LE (Frenkel)‐like and CT states lie energetically higher. Also, reports of intrinsic free‐charge photogeneration in neat Y6 and C ₆₀ films are discussed; devices are found to deliver efficiencies <1% unless aided by transport layers or donor additives. These insights define design rules for NFAs—favoring dipolar transitions, co‐facial packing, and near‐resonant LE‐CT energetics—to realize single‐component photovoltaics with built‐in charge separation.

Efficient exciton dissociation and charge separation at donor/acceptor interfaces as well as balanced charge transport within each phase are critical for advancing next-generation organic photovoltaics. In this work, we study a trifluoromethyl-substituted nonfullerene acceptor (NFA), Y₂CF₃, whose central-core fluorinated side chain modification induces favorable crystal packing and markedly improves device performance (Cho et al., J. Am. Chem. Soc., 147, 758 (2025)). Combining molecular dynamics simulations and density functional theory calculations, we comprehensively describe the electronic processes and charge transport in PM6:Y₂CF₃ blends and compare them to the benchmark PM6:Y6 blends. Y₂CF₃ shows a red-shifted singlet local exciton (¹LE) energy but nearly unchanged charge transfer (¹CT) energies, narrowing the ¹LE-¹CT gap and enabling rapid conversion as precursor of charge separated states. Enhanced electron transfer rates among Y₂CF₃ molecules arise from strong terminal-terminal interactions as dominantly found in the Y₂CF₃ crystal structure. Thermal fluctuations are found to substantially enhance the hole transport rates in PM6, narrowing the disparity between electron and hole transport. These findings clarify how targeted side chain fluorination at the NFA core can optimize packing, charge separation, transport, and OPV efficiency.

A detailed understanding of the composition and polymerization mechanism of elemental sulfur remains a decades long unresolved question for modern chemistry. However, the dynamic nature of molten sulfur significantly complicates its accurate characterization. To overcome this challenge, we performed the first comprehensive molecular dynamics (MD) simulations using a ReaxFF reactive force field specifically parameterized to capture the complex ring‐opening polymerization dynamics of elemental sulfur. Rigorous development of the force field parameters, trained against extensive quantum mechanical datasets, was key to enabling these large‐scale (>10 000 atoms) reactive MD simulations at polymerization‐relevant temperatures. This study provides the first detailed atomic‐level description of liquid sulfur, elucidating temperature‐dependent molecular composition and offering unprecedented insights into sulfur polymerization mechanisms. These are the first simulations to reveal the formation of remarkably large macrocyclic sulfur rings at the onset of polymerization—a discovery that challenges longstanding mechanistic misconceptions, thus reshaping our understanding of sulfur polymerization. Our findings highlight the power of molecular dynamics in exploring complex polymerization processes, with broad impact in dynamic covalent chemistry and covalent adaptable polymers.

A detailed understanding of the composition and polymerization mechanism of elemental sulfur remains a decades long unresolved question for modern chemistry. However, the dynamic nature of molten sulfur significantly complicates its accurate characterization. To overcome this challenge, we performed the first comprehensive molecular dynamics (MD) simulations using a ReaxFF reactive force field specifically parameterized to capture the complex ring‐opening polymerization dynamics of elemental sulfur. Rigorous development of the force field parameters, trained against extensive quantum mechanical datasets, was key to enabling these large‐scale (>10 000 atoms) reactive MD simulations at polymerization‐relevant temperatures. This study provides the first detailed atomic‐level description of liquid sulfur, elucidating temperature‐dependent molecular composition and offering unprecedented insights into sulfur polymerization mechanisms. These are the first simulations to reveal the formation of remarkably large macrocyclic sulfur rings at the onset of polymerization—a discovery that challenges longstanding mechanistic misconceptions, thus reshaping our understanding of sulfur polymerization. Our findings highlight the power of molecular dynamics in exploring complex polymerization processes, with broad impact in dynamic covalent chemistry and covalent adaptable polymers.

We engineer molecular packing in five derivatives of the nonfullerene acceptor Y6. Using transient absorption spectroscopy, we find evidence of the formation of a delocalized exciton in addition to the local exciton in neat films of the acceptors. Following selective photoexcitation of the acceptors in donor/acceptor blends with D18, we observe anion formation on the same timescale as in neat acceptor films, suggesting that D18 is a bystander to charge generation after photoexcitation of the acceptors. We quantify the recombination kinetics of the delocalized excitons with the monomolecular recombination constant (a) and find that both the hole transfer yield and the internal quantum efficiency in photovoltaic devices increase for acceptor films with lower a. In A1, relatively localized excitons with a limited charge transfer character have fast recombination kinetics (a = 3.2 × 10¹⁰ s^-1), leading to the lowest IQE (83.7%). In T1, more delocalized excitons with stronger charge transfer character have slower recombination kinetics (a = 5.3 × 10⁹ s^-1), leading to a higher IQE (97.2%). Grazing incidence wide-angle X-ray scattering of π-π stacking regions reveals that the tendency to pack face-on is a key driver of exciton delocalization across acceptors with similar molecular packing. We anticipate that this newly identified structural lever will help propel organic photovoltaics toward 20% efficiency.

Singlet fission (SF) is a multielectron process in which one singlet exciton S converts into a pair of separated triplet excitons T. SF is widely studied as it may help overcome the Shockley-Queisser efficiency limit for semiconductor photovoltaic cells. To elucidate and control the SF mechanism, great attention has been given to the identification of intermediate states in SF materials, which often appear elusive due to the complexity and fast time scales of the SF process. Here, we apply 14 fs-1 ms transient absorption techniques to high-purity rubrene single crystals to disentangle the intrinsic fission dynamics from the effects of defects and grain boundaries and to identify reliably the fission intermediates. Our data demonstrates that above-gap excitation directly generates a hybrid vibronically assisted mixture of singlet state and triplet-pair multiexciton [S/TT], which rapidly (<100 fs) and coherently branches into pure singlet or triplet excitations. The relaxation of [S/TT] to S is followed by a relatively slow and temperature-activated (48 meV activation energy) incoherent fission process. The SF competing pathways and intermediates revealed here unify the observations and models presented in previous studies of SF in rubrene and offer alternative strategies for the development of SF-enhanced photovoltaic materials.

Multi‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters that display efficient reverse intersystem crossing (RISC) rates and circularly polarized luminescence (CPL) are of great interest for next‐generation organic light‐emitting diode (OLED) applications, owing to their narrowband emission, high efficiency, and remarkable color purity. Here, the photophysical and chiroptical properties of three series of molecules derived from boron/nitrogen‐embedded MR cores by systematically introducing chalcogen atoms (O, S, Se) and/or incorporating ortho‐fused benzo or naphtho groups are investigated. Highly correlated quantum‐chemical calculations reveal that steric repulsions resulting from the ortho‐fused positions induce molecular distortions and twist the molecular backbone into helical structures, which enhances the chiral properties; the incorporation of heavier chalcogens increases spin–orbit coupling (SOC), leading to enhanced RISC rates. These findings demonstrate that several of the molecules that are considered exhibit high radiative decay rates (≈10⁸ s⁻¹), substantial RISC rates (≈10⁴–10⁸ s⁻¹), and values of the dissymmetry factor g of the order of 10⁻³, which makes them potential candidates for CPL applications. Overall, this study highlights the complex interplay among chalcogen substitution, structural modifications, and electronic structure in governing the photophysical and chiroptical properties of MR‐TADF emitters, and offers valuable insight for the rational design of next‐generation CPL‐enabled OLEDs.

This work gives a comprehensive description of the fundamental electronic processes taking place in the active layer of PM6:Y6 solar cells. It also points to the role of triplet states, excimer-like states, and defect states in device performance.