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1. Static disorder-induced renormalization of polariton group velocity

   https://doi.org/10.1063/5.0288551  

   Aroeira, Gustavo_J R; Ribeiro, Raphael F (September 2025, The Journal of Chemical Physics)

   Molecular exciton-polaritons exhibit long-range, ultrafast propagation, yet recent experiments have reported far slower propagation than expected. In this work, we implement a nonperturbative approach to quantify how static energetic disorder renormalizes polariton group velocity in strongly coupled microcavities. The method requires no exact diagonalization or master equation propagation and depends only on measurable parameters: the mean exciton energy and its probability distribution, the microcavity dispersion, and the Rabi splitting. Using parameters corresponding to recently probed organic microcavities, we show that exciton inhomogeneous broadening slows both lower and upper polaritons, particularly when the mean exciton energy fluctuation approaches the collective light–matter coupling strength. A detailed discussion and interpretation of these results is provided using perturbation theory in the limit of weak resonance scattering. The magnitude of the effects examined in this work supports the conclusion that most of the reported polaritonic slowdown arises from dynamical (phonon-assisted) disorder, with static energetic disorder contributing only secondarily. 

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2. Structural properties, polymorphism, and multiscale disorder unravel energy transport limitations in perylene diimide semiconductors

   https://doi.org/10.1126/sciadv.aed0037  

   Smalley, Christopher_J H; Hughes, Colan E; Willhammar, Tom; Pandya, Raj; Saikin, Semion K; Johnstone, Duncan N; Gorman, Jeffrey; Sung, Jooyoung; Jacucci, Gianni; Midgley, Paul A; et al (May 2026, Science Advances)

   Organic semiconductors continue to make substantial performance gains from photovoltaics to electronics. However, understanding how differences in solid-state structure give rise to large differences in energy transport properties remains unresolved. We report that microcrystals of two perylene diimide (PDI) derivatives differing only in their terminal groups [cyclohexyl (CH) and 4-heptyl (ST)] have exciton diffusion coefficients differing by more than two orders of magnitude. Applying state-of-the-art techniques for microcrystal structure determination, we report the crystal structures of CH-PDI and two polymorphs of ST-PDI. Scanning electron diffraction reveals a range of crystallographic defects in ST-PDI microcrystals, attributed to polymorph intergrowths, while electron energy loss spectroscopy links these defects to nanoscale electronic structure changes. Computational modeling demonstrates that rotational disorder explains the difference in exciton diffusion coefficients. Our observations establish the importance of defect-induced orientational disorder as a source of extrinsic energetic disorder, highlighting the need for defect management in organic semiconductor technologies. 

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3. Carrier Screening Controls Transport in Conjugated Polymers at High Doping Concentrations

   https://doi.org/10.1103/PhysRevLett.131.248101  

   Duhandžić, Muhamed; Lu-Dìaz, Michael; Samanta, Subhayan; Venkataraman, Dhandapani; Akšamija, Zlatan (December 2023, Physical Review Letters)

   Transport properties of doped conjugated polymers (CPs) have been widely analyzed with the Gaussian Disorder Model (GDM) in conjunction with hopping transport between localized states. These models reveal that even in highly doped CPs, a majority of carriers are still localized because dielectric permittivity of CPs is well below that of inorganic materials, making Coulomb interactions between carriers and dopant counter-ions much more pronounced. However, previous studies within the GDM did not consider the role of screening the dielectric interactions by carriers. Here we implement carrier screening in the Debye-H¨uckel formalism in our calculations of dopant-induced energetic disorder, which modifies the Gaussian density of states (DOS). Then we solve the Pauli Master Equation using Miller-Abrahams hopping rates with states from the resulting screened DOS to obtain conductivity and Seebeck coefficient across a broad range of carrier concentrations and compare them to measurements. Our results show that screening has significant impact on the shape of the DOS and consequently on carrier transport, particularly at high doping. We prove that the slope of Seebeck coefficient vs electric conductivity, which was previously thought to be universal, is impacted by screening and decreases for systems with small dopant-carrier separation, explaining our measurements. We also show that thermoelectric power factor is underestimated by a factor of ∼ 10 at higher doping concentrations if screening is neglected. We conclude that carrier screening plays a crucial role in curtailing dopant-induced energetic disorder, particularly at high carrier concentrations. 

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4. Covalently Tethered Assemblies Improve Energetic Homogeneity and Exciton Transport in Organic Materials

   https://doi.org/10.1021/acsmaterialslett.4c00279  

   King, Alexander J.; Paulino, Victor A.; Hollinbeck, Skyler R.; Tsironi, Ifigeneia; Maleszka, Jarek A.; Olivier, Jean-Hubert; Grumstrup, Erik M. (March 2024, ACS Materials Letters)

   Structural and functional heterogeneity is a consequence of the weak noncovalent interactions that direct the formation of organic materials from solution precursors. While covalent tethering of solution-phase assemblies provides a compelling strategy to enhance intermolecular order, the effects of this tethering strategy on the formed solid-state materials remain unestablished. This work uses pump–probe microscopy to compare excited-state dynamics in thin films fabricated from tethered perylene bisimide assemblies to those fabricated from noncovalent assemblies. On average, tethered films exhibit faster and more homogeneous excited-state lifetimes, consistent with stronger and more uniform intermolecular coupling. Optical measurements of excited-state diffusion show that the tethered film has ∼75% faster transport than the control film. Kinetic Monte Carlo modeling suggests that the reduction of site energetic disorder is sufficient to quantitatively explain the difference in diffusion coefficients. These results provide strong support that covalent tethering is a promising strategy to enhance the structural and energetic ordering in molecular materials. 

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5. Polymorphism in Non‐Fullerene Acceptors Based on Indacenodithienothiophene

   https://doi.org/10.1002/adfm.202103784  

   Marina, Sara; Scaccabarozzi, Alberto D.; Gutierrez‐Fernandez, Edgar; Solano, Eduardo; Khirbat, Aditi; Ciammaruchi, Laura; Iturrospe, Amaia; Balzer, Alex; Yu, Liyang; Gabirondo, Elena; et al (July 2021, Advanced Functional Materials)

   Abstract Organic solar cells incorporating non‐fullerene acceptors (NFAs) have reached remarkable power conversion efficiencies of over 18%. Unlike fullerene derivatives, NFAs tend to crystallize from solutions, resulting in bulk heterojunctions that include a crystalline acceptor phase. This must be considered in any morphology‐function models. Here, it is confirmed that high‐performing solution‐processed indacenodithienothiophene‐based NFAs, i.e., ITIC and its derivatives ITIC‐M, ITIC‐2F, and ITIC‐Th, exhibit at least two crystalline forms. In addition to highly ordered polymorphs that form at high temperatures, NFAs arrange into a low‐temperature metastable phase that is readily promoted via solution processing and leads to the highest device efficiencies. Intriguingly, the low‐temperature forms seem to feature a continuous network that favors charge transport despite of a poorly order along the π–π stacking direction. As the optical absorption of the structurally more disordered low‐temperature phase can surpass that of the more ordered polymorphs while displaying comparable—or even higher—charge transport properties, it is argued that such a packing structure is an important feature for reaching highest device efficiencies, thus, providing guidelines for future materials design and crystal engineering activities. 

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