Structure of vapor-deposited glasses within a few nanometers of organic-organic interfaces is bulk-like

Vapor-deposited (PVD) glasses are used in OLED devices as <50 nm layers, each deposited onto the layer below it. The bulk structure of a >100 nm PVD glass depends on the temperature at which it was deposited; it is unaffected by the substrate. The distance over which the substrate perturbs the structure of a PVD glass is unknown. In this work, we prepare PVD films made of 60 alternating 10 nm layers of organic semiconductors, DSA-Ph and Alq3, creating 60 interfaces. The large number of organic-organic interfaces allows us to access the X-ray scattering of the structure near the organic substrate. GIWAXS of films with 60 interfaces is virtually identical to the sum of the scattering of thick films of the pure components, indicating that the organic substrate perturbs the glass structure for <2 nm. When deposited at or below 0.8 T_g, we find that the DSA-Ph molecules at organic interfaces lie parallel to the substrate. We expect that this is true for all PVD glassy films of rigid, flat molecules like DSA-Ph, ensuring good π-π orbital overlap at the organic-organic interface, allowing for efficient charge transport across the interface and into the semiconductor layer.