Abstarct: Owing to their special electronic properties, energy-disordered organic semiconductors have been widely exploited in the fabrication of various optoelectronic devices. Transport of localized excitons, generated upon photon absorption by these materials, has been subject to many studies in recent years. Although both singlet and triplet excitons are generated in these systems, in the present work we have focused on the phosphorescent systems in which triplet excitons are the dominant emitter population of the excitons in the system. Due to the transport and subsequent encounter of the excitons, the so-called triplet-triplet annihilation can occur in the phosphorescent systems. This process, as is known, leads to a decrease in the efficiency of corresponding optoelectronic devices. Therefore, to improve the performance of these devices, it is important to overcome the parameters that increase the annihilation rate. In this work, to simulate the transport and annihilation of the excitons, we have used the kinetic Monte-Carlo technique to investigate the parameters affecting the efficiency of the phosphorescent devices. We used, in accordance with the experimental reports, the values of 3, 4, and 5 nm for the Forster radius, which is known to be an important factor determining the annihilation rate. Also, in agreement with the working conditions of a diode at high brightness, the simulations were done at two initial exciton densities of 0.008 nm^(-3) and 0.011 nm^(-3). The possibility of the existence of an energetic disorder for the emitter molecules is also considered in the simulations. As a new step to the previous research works in the field, we developed and implemented three methods for the spatial distribution of the emitter dye molecules in the systems: Ags model (in which the dye molecules are well-separated from each other), Rnd model (random distribution of the dye molecules in the system), and Agg model (in which molecules distribute in clusters of dye aggregation). By introducing this simulation scheme that can account for the realistic distribution of the guest molecules, we could provide a quantitative descxription of how and to what extent the prevalent molecular aggregation in the typical phosphorescent devices can affect the efficiency of the phosphorescence process. As a main result, it is found that up to a 16% increase in the internal efficiency is attainable in the typical phosphorescent light emitting diodes by controlling the aggregate formation.