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Abstract Exciton‐polaritons in organic microcavities are applied in devices including lasers, light‐emitting devices, and photodetectors, as well as in structures capable of tuning exciton kinetics and energy transfer. To enable a broader tailoring of polariton properties, it is important to develop means to better control molecular orientation and tune the intensity of the exciton–photon interaction. Vapor‐processed, glassy organic thin films are previously shown to have tunable molecular orientation as evidenced by phenomena including birefringence and transition dipole moment (TDM) alignment. Here, this tunability in TDM orientation with thin film processing conditions is exploited to continuously vary the interaction between the exciton and confined cavity photon mode. By embedding a thin film of 4,4′‐bis[(N‐carbazole)styryl]biphenyl (BSB‐Cz) in a metal‐reflector microcavity, ultrastrong coupling and hybridization of multiple electronic transitions of BSB‐Cz are demonstrated with a common cavity mode. Increasing the temperature during BSB‐Cz deposition tunes the TDM orientation from predominantly in‐plane to random to slightly vertical. This leads to a corresponding ≈30% variation in the associated Rabi splitting, consistent with theoretical predictions. This work demonstrates a means to continuously tune coupling strength from a materials perspective while also providing a handle to tune orientation disorder in thin film. 

Abstract Significant work has been directed at measuring the exciton diffusion length (L_D) in organic semiconductors due to its significance in determining the performance of photovoltaic cells. Several techniques have been developed to measureL_D, often probing photoluminescence or charge carrier generation. Interestingly, in this study it is shown that when different techniques are compared, both the diffusive behavior of the exciton and active carrier recombination loss pathways can be decoupled. Here, a planar heterojunction device based on the donor–acceptor pairing of boron subphthalocyanine chloride‐C₆₀is examined using photoluminescence quenching, photovoltage‐, and photocurrent‐basedL_Dmeasurement techniques. Photovoltage yields the device relevantL_Dof both active materials as a function of forward bias subject to geminate recombination losses. These values are used to accurately predict the photocurrent as a function of voltage, suggesting geminate recombination is the dominant mechanism responsible for photocurrent loss. By combining these measurements with photocurrent and photoluminescence quenching, the intrinsicL_D, as well as the voltage‐dependent charge transfer state dissociation and charge collection efficiencies are quantitatively determined. The results of this work provide a method to decouple all relevant loss pathways during photoconversion, and establish the factors that can limit the performance of excitonic photovoltaic cells.