This paper has investigated the effects of montmorillonite (MMT) as filler and post treatment in optical properties of bulk heterojunction P3HT:PCBM thin films. Annealing temperatures varied from 130 to 150 degrees C while annealing time was kept constant for 10 min. The absorption data has shown an increment in peak absorption at around 602 nm, suggesting that the addition of MMT improved the degree of crystallinity of P3HT. The addition of MMT exhibited absorption in the infrared region, suggesting that MMT improved the optical properties of the thin films. The post treatment effects have shown interesting results, with 130 degrees C demonstrating the highest degree of PL quenching with acceptable light absorption.