Exciton species, fundamental quantum quasiparticles, are crucial in light-emitting and light-harvesting systems. Interlayer excitons (IXs), formed at heterojunctions (HJs) with type-II band alignment, exhibit unique properties compared to Frenkel excitons (XFs) in organic semiconductors and Wannier excitons in inorganic semiconductors. Charge transfer (CT) excitons, such as IXs, influence the quantum efficiency of optoelectronic devices. In 2D transition metal dichalcogenides (TMDCs), the spatially separated electrons and holes of IXs result in a long lifetime. The spatial configuration of IXs at a p-n (or donor-acceptor) HJ leads to unidirectional alignment of electric dipole moments perpendicular to the interface and electric field dependence of the IX binding energy. CT excitons are observed in various type-II band heterostructures (HSs), including TMDC-based HSs, TMDC/perovskite HSs, perovskite/quantum dot HSs, and PbS-CdS-QD/MAPbCl3 HSs. Notably, CT exciton formation is observed in HSs with rough interfaces, exhibiting long lifetimes and directional electric dipole moments. In π-conjugated organic small/macro molecular systems, the wavefunction overlap of π-electrons and π-π stacking enhance charge delocalization and intermolecular CT rates, leading to advanced optoelectronic devices such as OLEDs, organic field-effect transistors, organic phototransistors, OPVCs, and organic waveguiding cavities. Exciplexes (XPs), CT excitons formed in organic donor (D) and acceptor (A) heteromolecules, are extensively studied for OLEDs, particularly in terms of phosphorescence, thermally activated delayed fluorescence (TADF), and broadband emission. XP binding energy (EB) as a function of wavefunction overlap and electron-hole separation is critical for OLED and OPVC efficiency. The direction of dipole moments in D-A molecules and their correlation with electric fields are also crucial. However, the directional features of XF and XP dipole moments haven't been directly evaluated in terms of luminance. In contrast to 2D-TMDCs, CT excitons like IXs and XPs in organic systems form at the molecular scale and have longer lifetimes. The long-lived and aligned XP is a promising energy host in optoelectronic devices or a platform for strong interactions with photons and phonons in organic photonic devices. This study uses the donor molecule 4,4',4''-Tris[(3-methylphenyl)phenylamino]triphenylamine (m-MTDATA) and acceptor molecule 2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine (T2T), which are widely used in OLEDs because of their low ionization potential (m-MTDATA) and electron-withdrawing properties (T2T). The combination exhibits stable coupling, long lifetime, and high quantum efficiency, making it ideal for investigating XP characteristics. The spatial long-range coupling of XPs in organic D-A HSs has been studied extensively, but the directional features of exciton dipole moments have not been systematically investigated. This study directly demonstrates the distinct orientations of dipole moments in m-MTDATA, T2T, and m-MTDATA/T2T bilayer HSs using back focal plane (BFP) photoluminescence (PL) spectra.

The literature review extensively cites previous research on interlayer excitons in various materials, including 2D transition metal dichalcogenides (TMDCs) like MoSe2/WSe2 and WS2/PbI2, and perovskite/quantum dot heterostructures. These studies highlight the long lifetimes and directional dipole moments of charge-transfer excitons in these systems. The review also covers the characteristics of π-conjugated organic molecules and their use in optoelectronic devices. The properties of exciplexes (XPs) in organic donor-acceptor systems, including their importance in OLEDs, TADF, and broadband emission, are discussed. The influence of XP binding energy on the efficiency of OLEDs and OPVCs is reviewed, along with the significance of dipole moment orientation. The cited works establish a foundation for understanding the unique properties of excitons in both inorganic and organic systems, setting the stage for this study's investigation into the directional features of excitons in organic heterostructures.

High-quality m-MTDATA single layers, T2T single layers, and m-MTDATA/T2T bilayers were fabricated using organic molecular beam deposition (OMBD) at an extremely low deposition rate (0.3 nm/min) under high vacuum (5.0 × 10⁻⁶ Torr). This controlled deposition method aimed to reduce defects and promote the epitaxial growth of high-quality organic layers. Laser confocal microscopy (LCM) PL spectroscopy was used to analyze the photoluminescence of the fabricated samples at room temperature (290 K) and low temperatures (down to 3 K), using a 405-nm diode laser for excitation. The HOMO and LUMO levels, crucial for understanding energy-band alignment, were determined using ultraviolet photoelectron spectroscopy (UPS) and UV-visible absorption spectroscopy. Back focal plane (BFP) PL imaging and spectroscopy were employed to directly visualize and analyze the emission patterns and determine the directionality of the dipole moments associated with different exciton species (XFs and XPs). Time-resolved PL (tr-PL) measurements, using time-correlated single-photon counting (TCSPC), were performed to determine the lifetimes of XFs and XPs at various temperatures. The power dependence of PL was investigated by varying the excitation laser power (Pin) over a wide range, providing insights into exciton recombination processes and dipole-dipole interactions. To study the electroluminescence (EL) characteristics, organic light-emitting diodes (OLEDs) using the m-MTDATA/T2T bilayer and co-deposition layer (CDL) were fabricated. The EL spectra were measured as a function of the applied bias voltage, allowing for the investigation of the influence of the electric field on exciplex dipole alignment. Ellipsometry was employed to estimate the refractive indices and determine molecular orientations. Scanning electron microscopy (SEM) was used to analyze the cross-sectional structure of the organic layers. Data analysis involved deconvolution of PL spectra, fitting tr-PL decay curves, and analyzing the power and temperature dependencies of PL and EL emissions. Parabolic fitting of energy-momentum (E-k) dispersion relations was performed to characterize the angular chromism of exciplexes.

The key findings of this research demonstrate the distinct orientations and characteristics of Frenkel excitons (XFs) and charge-transfer excitons (exciplexes, XPs) in the organic m-MTDATA/T2T heterostructures. Back focal plane (BFP) photoluminescence (PL) imaging revealed that the exciplex dipole moments are oriented perpendicularly to the Frenkel exciton dipole moments, confirming their orthogonality. The observation of angular chromism, evidenced by a blue shift of the exciplex emission energy with increasing emission angle (100 meV range), indicates delocalization of the exciplex wavefunction. This angular chromism is absent in Frenkel excitons, confirming their localized nature. The increase in excitation power caused a blue shift in the exciplex PL peak position and a broadening of the full width at half maximum (FWHM), which were attributed to repulsive dipole-dipole interactions between the aligned exciplexes. The long lifetime of the exciplexes (4.65 µs at 3 K), observed in time-resolved PL measurements, supports their significant spatial separation of electrons and holes and strong repulsive dipole interactions. This long lifetime indicates the occurrence of reverse intersystem crossing (RISC), contributing to efficient delayed fluorescence. Temperature-dependent PL measurements showed that the exciplex PL peak position exhibited a blue shift with decreasing temperature, while the Frenkel exciton peak position remained relatively constant, further confirming the dipole anisotropy. Electroluminescence (EL) measurements of OLEDs using the m-MTDATA/T2T bilayer demonstrated that the exciplex EL peak position blue-shifted with increasing forward bias, providing direct evidence for the alignment of exciplex dipole moments perpendicular to the interface. In contrast, OLEDs fabricated using a co-deposition layer (CDL) of m-MTDATA and T2T exhibited no such blue shift, indicating the absence of unidirectional dipole alignment in the randomly oriented exciplexes of the CDL. The study also quantitatively determined the degree of angular dispersion of exciplex PL energy using parabolic fitting of E-k dispersion relations, highlighting the more delocalized nature of exciplexes in the bilayer compared to the CDL. Analysis of PL intensity dependence on excitation power revealed the values of α (in the equation I_PL = I_0P_in^α), which were lower for XPs compared to XFs, reflecting the loosely bound nature of XPs and the ease of saturation of their excited states. SEM and ellipsometry results suggested that the m-MTDATA/T2T bilayer has an in-plane stacking structure at the interface, while the T2T molecules show some degree of out-of-plane orientation. The observed results of this study offer detailed experimental evidence for the existence and control of aligned dipoles and angular chromism in organic molecular heterostructures, which provide potential for various applications in optoelectronics and photonics.

The findings address the research question by demonstrating the distinct directional characteristics of Frenkel and charge-transfer excitons in organic heterostructures. The orthogonal alignment of exciplex and Frenkel exciton dipole moments, confirmed by BFP-PL imaging, is a significant contribution to the understanding of exciton coupling in these systems. The observed angular chromism suggests that the exciplex wavefunction is delocalized across the interface, which offers potential for designing angle-sensitive photonic devices. The power and temperature dependence of PL and EL emissions directly show the impact of repulsive dipole-dipole interactions on the spectral properties of exciplexes. The long lifetime and efficient delayed fluorescence of exciplexes, caused by RISC, point towards their potential use in advanced optoelectronic devices. The observed differences between the bilayer and CDL OLEDs highlight the crucial role of exciplex dipole alignment in enhancing device efficiency. This understanding of exciton dipole orientation and its impact on energy transfer is fundamental for optimizing the performance of OLEDs and other organic optoelectronic devices. The results have implications for designing high-efficiency OLEDs by controlling the alignment of exciton dipole moments, potentially through material selection and device architecture optimization. Furthermore, the demonstrated angular chromism opens new avenues for developing angle-sensitive photonic devices and exploring energy transfer mechanisms in organic molecular heterostructures.

This study successfully demonstrated the distinctive orientation and properties of Frenkel and charge-transfer excitons in organic m-MTDATA/T2T heterostructures. BFP-PL imaging revealed the orthogonal alignment of exciplex and Frenkel exciton dipoles. Angular chromism in exciplexes suggests delocalization and the potential for angle-sensing applications. Power and temperature-dependent PL and EL measurements confirmed the role of repulsive dipole interactions. The long exciplex lifetime and efficient delayed fluorescence highlight their potential for optoelectronic applications. The contrasting behavior of bilayer and CDL OLEDs emphasizes the importance of exciplex alignment for enhancing device efficiency. Future research could explore the impact of different donor-acceptor pairs and heterostructure architectures on exciton behavior, further optimize device efficiency through improved dipole alignment control, and investigate the potential of exciplex angular chromism for specific applications.

The study primarily focuses on a specific donor-acceptor pair (m-MTDATA/T2T). The generalizability of these findings to other organic systems might need further investigation. While the OMBD technique was used to create high-quality films, there could be some variations in the film morphology and molecular ordering, potentially affecting the observed properties. The analysis of molecular orientations relies on a combination of experimental techniques; further techniques may provide a more complete picture. The interpretation of the long exciplex lifetimes assumes the dominance of RISC; alternative mechanisms could contribute. Finally, the exploration of practical applications is mainly conceptual; detailed device fabrication and performance optimization are needed to validate the proposed angle-sensing and improved OLED efficiency.