Excitons are quasi-particles, which are responsible for energy transport in organic semiconductors. Excitons are therefore instrumental in understanding the photophysics of organic opto-electronic devices. The present work focused on describing the dynamics of spin-forbidden, long-lived triplet excitons in archetypal organic materials such as CBP. Triplet excitons lifetime and diffusion length are here estimated from modelling the results of triplet-triplet photoinduced absorption spectroscopy, steady-state photoluminescence spectroscopy and time-resolved photoluminescence measurements. The last two measurements are performed using a modified time-of-flight method, whereby the investigated material is adjacent to a phosphorescent sensing layer and optically excited from the opposite side. As the thickness of the material is increased, the variations of phosphorescence intensity coming from the sensing layer is correlated to the exciton diffusion parameters. We show that for fluorescent materials such as CBP, the near-field component of this emission couples to guided modes supported by the structure and directly excites the sensitizer - here Ir(ppy)3 doped into CBP - which lead to an overestimation of the diffusion length. In addition, we investigate a strategy to mitigate the effect of guided modes by using an optical quenching layer of C6. This results in an estimated triplet exciton lifetime in the ms range and a diffusion length in excess of 30 nm, based on modelling the steady-state and time-resolved emission of the sensing layer when varying CBP thickness.