Observation of aligned dipoles and angular chromism of exciplexes in organic molecular heterostructures

Exciton species such as Frenkel excitons in organic semiconductors and interlayer excitons in type-II heterojunctions are key quasiparticles for light-emitting and harvesting devices. Charge transfer (CT) at donor–acceptor interfaces forms CT excitons with spatially separated electron–hole pairs that exhibit long lifetimes and directional dipole moments. In two-dimensional TMDC systems, interlayer excitons show long lifetimes and out-of-plane dipole alignment perpendicular to the interface, motivating analogous studies in π-conjugated organic systems. In organics, π–π stacking and charge delocalization enhance intermolecular CT rates, enabling devices like OLEDs, OFETs, and OPVs. Exciplexes (CT excitons in donor–acceptor heteromolecular systems) are central to phosphorescence, TADF, and broadband emission in OLEDs, with their efficiency governed by binding energy and electron–hole separation. However, despite their importance, the directional dipole characteristics of Frenkel excitons (XF) versus exciplexes (XP) and their direct correlation with luminance and device geometry have not been systematically evaluated. The m-MTDATA (donor) and T2T (acceptor) pair forms a stable exciplex with long lifetime and high quantum efficiency, offering an ideal platform. This study directly maps the dipole orientations of XF and XP in m-MTDATA, T2T, and their bilayer via back focal plane (BFP) photoluminescence, revealing orthogonal dipole orientations (XP out-of-plane vs XF in-plane), angular chromism of XP, power- and temperature-dependent spectral shifts linked to dipole–dipole repulsion, and electroluminescence blue shifts under forward bias consistent with aligned XP dipoles.

- CT excitons in type-II heterostructures have been widely observed in TMDC-based systems (e.g., MoS2/WSe2, MoSe2/WSe2), TMDC/perovskite (WSe2/(iso-BA)2PbI4), perovskite/quantum-dot (MAPbI3/CdSe-ZnS QD), and PbS-CdS QD/MAPbCl3, often showing long lifetimes and directional dipole moments even with rough interfaces. - In π-conjugated organics, enhanced π–π interactions promote CT and delocalization, enabling efficient OLEDs, OPVs, and photonic structures. Exciplexes in such systems support phosphorescence and TADF, with device performance dependent on CT binding energy and electron–hole separation. - m-MTDATA (hole injection material) and T2T (electron transport material) have been extensively used; prior work reported long-range coupled exciplexes with TADF in m-MTDATA/T2T combinations and sensitivity of exciplex formation to spacer layers and D–A configurations. - Orientation of excitonic dipoles affects optical outcoupling, energy transfer (FRET/Dexter), angular emission, and dissociation efficiency, but direct luminance-based evaluation of dipole orientation for XF vs XP in organics has been lacking. - Analogous interlayer exciton studies in TMDCs report out-of-plane dipoles and blue shifts with increased exciton density due to dipolar repulsion, providing a comparative framework for interpreting exciplex behavior in organics.

- Materials and films: m-MTDATA (donor) and T2T (acceptor) powders (≥98% purity) were used as received. Single layers and m-MTDATA/T2T bilayers, as well as co-deposited layers (CDLs, 1:1), were grown on Si/SiO2 substrates using organic molecular beam deposition (OMBD) under high vacuum (<5.0×10^-6 Torr). Typical deposition rate was 0.3 nm min^-1 (bilayer OLED depositions: m-MTDATA 0.44 nm min^-1; T2T 0.57 nm min^-1). - Structural/optical constants: Cross-sectional SEM/TEM characterized stacking; ellipsometry yielded birefringent refractive indices (m-MTDATA no≈1.72, ne≈1.70 at 550 nm). UPS and UV–Vis absorption determined HOMO/LUMO levels and type-II band alignment. - Photoluminescence: Laser confocal microscopy (LCM) PL measured in a closed-cycle cryostat (3–290 K) using a 405 nm diode laser (and 409 nm long-pass filter). Power-dependent PL used Pin spanning ~nW to µW. BFP PL imaging/mapping employed a customized Fourier-plane setup to record angle-resolved emission; spectra were analyzed versus in-plane wavevector kx/k0. - Time-resolved PL: TCSPC systems (HPM-100, PML-16) with a 375 nm picosecond diode laser and 409/514 nm long-pass filters measured prompt and delayed fluorescence; decay curves fitted bi-exponentially and intensity-weighted average lifetimes computed. - Energy–momentum analysis: k-dependent PL spectra (E–k maps) were fitted with parabolic dispersion E(k)=ħ^2k^2/2m*+E_XP(θ=0). The curvature K_center=d^2E/dk^2 was used to quantify angular dispersion and infer effective mass m*=ħ^2/K_center. - OLED fabrication and EL: Devices on ITO used PEDOT:PSS as hole transport layer, followed by m-MTDATA (20 nm)/T2T (20 nm) bilayer or 1:1 CDL active layers. Cathode: LiF (2 nm)/Al (150 nm) thermally evaporated. Devices were encapsulated. EL spectra recorded with a CCD spectrometer under forward bias; I–V–L and EQE discussed in SI. - Conditions/examples: Representative PL mappings at 290 K and 3 K; deconvolution used to separate XF, XP, and XM contributions; center vs edge BFP spectra compared for angular dispersion.

- Type-II band alignment: UPS/absorption determined HOMO/LUMO levels (m-MTDATA: HOMO −5.00 eV, LUMO −1.90 eV; T2T: HOMO −6.33 eV, LUMO −2.63 eV), yielding type-II alignment and photoinduced CT across the interface. - Spectral signatures: m-MTDATA monomer XF peak at ~2.90 eV (~427 nm); excimer (XM) ~530 nm; bilayer exciplex (XP) at 2.16 eV (~575 nm) at 290 K. - Dipole orientation via BFP: BFP images showed center-bright (convex) profiles for XF (in-plane dipoles) and edge-bright (concave) profiles for XP (out-of-plane dipoles), evidencing orthogonal dipole orientations in the bilayer. - Angular chromism and dispersion: At 3 K, XP emission blue-shifted from 2.089 eV (BFP center) to 2.194 eV (edge). E–k maps showed clear angular dispersion for XP but not XF. Parabolic fits gave K_center≈0.499 (bilayer) vs 0.259 (CDL), indicating stronger delocalization/dispersion in the bilayer. Disordered drop-cast/reprecipitated blends showed negligible dispersion (K_center an order of magnitude smaller), confirming the role of aligned dipoles. - Power dependence (3–50 K examples): XP peak blue-shifted slightly with increased Pin (e.g., 2.145→2.160 eV at 3 K; 2.143→2.154 eV at 50 K), and XP FWHM broadened (0.42→0.47 eV) as Pin increased from 20 nW to 5 µW; XF peak position (~2.913–2.914 eV) and FWHM (~0.098 eV) remained nearly constant. Interpreted as enhanced repulsive dipole–dipole interactions among aligned XPs with increasing XP density. - PL intensity scaling: I_PL∝Pin^α with α≈0.81 (XF) and 0.72–0.73 (XP at 3 K), increasing toward 0.90 at 290 K for XP, consistent with long-lived CT excitons exhibiting easier saturation. - Lifetimes: XF average lifetime τ_avg≈1.05 ns at 3 K, increasing to ~1.22 ns at 290 K. XP exhibited prompt τ_avg≈3.43–7.33 ns and delayed τ_avg≈2.14–4.65 µs (TADF), both decreasing with temperature due to nonradiative channels and RISC activation. Extremely long delayed component persists from 3 K (4.65 µs) to 290 K (2.14 µs). - Temperature dependence: XP PL blue-shifted from 2.14 eV (290 K) to 2.17 eV (3 K), while XF remained ~2.90 eV across temperatures. XP intensity increased from 3 to ~80 K then saturated >100 K, consistent with small ΔE_ST (~3.2 meV) enabling TADF activation. - Electroluminescence: Bilayer OLEDs showed dominant XP EL at ~2.13 eV (V≈9 V) with clear blue shift of XP peak with increasing forward bias; XF EL (~2.93 eV) appeared only at higher bias and was relatively bias-invariant. CDL OLEDs displayed much lower XP EL intensity and negligible peak shift with bias, consistent with random dipole orientations. - Energy transfer implications: Parallel/aligned dipoles enhance FRET/Dexter coupling; observed XP–XM coupling boosted PL near 575 nm in bilayers compared to CDLs.

The study directly addresses how exciton dipole orientation in organic donor–acceptor heterostructures governs emission properties. BFP PL imaging and E–k dispersion unequivocally reveal that exciplex dipoles in m-MTDATA/T2T align out-of-plane across the interface, orthogonal to in-plane Frenkel excitons. This anisotropy explains angular emission patterns and the observed angular chromism of XP, indicative of delocalization across the interface. Power- and temperature-dependent blue shifts and linewidth broadening of XP arise from repulsive dipole–dipole interactions enhanced by increased XP density and improved alignment at lower temperatures. The long microsecond-scale delayed fluorescence confirms TADF-mediated population dynamics and supports the notion of spatially separated electron–hole pairs. In devices, forward-bias-induced blue shifts of XP EL in bilayers but not CDLs demonstrate that controlled dipole alignment modulates internal fields and emission energy. Collectively, these results establish the relevance of dipole orientation to optical outcoupling, exciton transport/diffusion, and energy transfer pathways, suggesting strategies to optimize OLED efficiency via interface engineering and dipole alignment.

This work demonstrates that in m-MTDATA/T2T bilayers, exciplex (XP) dipoles align out-of-plane and are orthogonal to Frenkel exciton (XF) dipoles, producing distinctive angular emission, clear energy–momentum dispersion, and angular chromism. XP emission exhibits power- and temperature-dependent blue shifts and linewidth broadening due to repulsive dipolar interactions, and shows long-lived delayed fluorescence (2.14–4.65 µs) consistent with TADF and spatial separation. In OLEDs, XP EL blue-shifts with forward bias only in bilayers, evidencing unidirectional dipole alignment not present in co-deposited layers. These findings provide a direct, luminance-based view of exciton dipole anisotropy and its device implications. Future directions include engineering molecular orientation and interface order to enhance dipole alignment, exploiting XP angular dispersion for angle-sensitive photonics, integrating with optical cavities to realize XP-polaritons, and tailoring FRET/Dexter pathways by aligning dipoles among hosts, exciplexes, and dopants for higher-efficiency OLEDs.