<p>The diffusion length of photo-generated excitons in organic semiconductors is a fundamental parameter that governs photoelectronic processes in devices. However, the conventional electro-optical method for determining the exciton diffusion length typically requires multiple device fabrications and is subject to significant uncertainty, as several critical parameters are often unknown or assumed. Here, we present a single-device approach to simultaneously measure both the diffusion length and the charge-generation yield in organic semiconductors using a planar heterojunction structure. This methodology relies on fitting the external quantum efficiency spectra, whose shapes are strongly influenced by both the light-absorption profiles and the probability that excitons reach the charge-separation interfaces. We demonstrate that non-fullerene acceptors exhibit relatively long diffusion length, which correlates with the high power conversion efficiencies observed in organic photovoltaics. This approach enables unprecedented accuracy in probing the relationship between material structure and exciton diffusion length, providing a powerful tool for the rational design of high-performance organic semiconductor devices.</p>