Degradation in bulk heterojunction organic solar cells: changes in electrode interface and reduction in the occupation probability of the interface states

We have investigated the degradation of P3HT:PCBM (poly(3-hexylthiophene):6,6-phenylC61 butyric acid methyl ester) solar cell beyond 150?h of fabrication in continuation to our earlier reported work up to 150?h of fabrication. The current- voltage characteristics of degraded Indium tin oxide/poly(3,4-ethylenedioxythiopene):poly(styrenesulfonate)/poly(3-hexylthiophene):6,6-phenylC61 butyric acid methyl ester/Aluminum (ITO/PEDOT:PSS/P3HT:PCBM/Al) solar cell can be explained by considering the tunneling current through electrode interfaces, increase in both the interface states density and the thickness of interface with time for150-200?h. Beyond 200?h of fabrication, a significant reduction in the occupation probability at the electrode interfaces explains the experimental results up to 300?h fairly well. Calculations based on realistic parameters and activity at both the electrode interfaces (ITO/PEDOT:PSS and P3HT:PCBM/Al) confirm that degradation at P3HT:PCBM/Al interface is more prominent than that at ITO/PEDOT:PSS interface.