The p-i-n a-Si:H photodiode is promising device as transducer in biosensors. The native and light induced localized state density and energy distribution in the energy gap of a-Si:H have a large effect on the photoconductivity of thin films photodiode. Depends on their nature they play a crucial role in trapping and recombination processes and consequently influence the photodiode capacitance. The optical bias dependence of modulated photocurrent method - OBMPC using the blue LED light is applied to clarify the energy gap density of state nature and energy distribution from photodiodes transient response and their influence on the photodiode capacitance. It is observed, that the deep defect states of i- layer contribute to the capacitance at various bias voltages. Also, the capacitance achieves the upper limit around the built-in potential. Based on this method and obtained results, the a- Si:H p-i-n photodiode is used as biosensor transducer in detection of mammalian cells chemiluminescence.