The Curious Case of tBu-Cz-TPA-bis-DPPO: Lowest Structural Order Among Three Deep-Blue Hosts, Yet a Smooth Hole-Only Device Response

Deep-blue phosphine-oxide hosts are compared by energy levels and photophysics, almost never by microstructure—yet order and film morphology are routinely read as proxies for how well a host will carry charge. Here three related hosts are compared from primary particles to film relief with documented, protocol-tested descriptors. The tert-butyl-substituted S3 host emerges as the least ordered on every scale: the lowest X-ray ordered fraction at every baseline setting, the shortest film correlation length at matched scan size, micrometre-scale relief and the lowest decomposition temperature, while only the S1 host shows lattice fringes. This structural disorder coexists with a smooth hole-only device response. The S3 hole-only device shows a single, nearly ideal relaxation with a capacitance close to the geometric value and an apparent hole mobility of about 5 × 10⁻⁷ cm² V⁻¹ s⁻¹, comparable to that of the far better-ordered S1 host, although a bias-independent relaxation time is also consistent with the RC response of the layer stack. Long-range disorder in these hosts is therefore compatible with a clean hole-only response, and microstructure alone is not a reliable screen for transport.