Metal-Insulator-Metal type devices based on poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) have been examined to understand the effect of degradation on the charge injection and transport phenomena. The Fowler Nordheim (FN) plots with time have been investigated to analyze the behavior of charge tunneling through the potential barrier at the interfaces. A clear transition from thermionic emission to tunneling is observed, and the transition voltage at which this occurs reduces with time. Furthermore, there is appearance of multiple inflexion points with time in the FN plots, and this is explained on the basis of recombination and majority charge carriers within the device. To validate the observations, comparative studies have been carried out on multi-walled carbon nanotube doped samples, which indicate similar behavior for fresh and degraded samples. It is, thus, ascertained that defect states, recombination of charge carriers and the mobility plays a significant role in governing the device behavior.