This paper describes the details of surface reactions producing >100 nm thick conjugated polymer films. When 2,5-diiodothiophene films deposited on copper are irradiated with UV at room temperature in Ar environments, oligothiophene films are synthesized. The average conjugation length of the produced film varies from ~7 to 3~4 as the film thickness increases from ~100 nm to ~500 nm. The x-ray photoelectron spectroscopy analysis of the produced films reveals the evidences for formation of organo-copper intermediate species at the copper–monomer film interface and their diffusion from the copper surface into the monomer film during the photochemical process. A one-dimensional diffusion-reaction model is presented to explain the formation, diffusion and reaction of organo-copper intermediates in the multilayer film during the photochemical reaction. The model simulation results qualitatively explain the decrease of the Ullmann coupling contribution in the photochemical reaction with the film thickness.