Relationship between charge transfer state electroluminescence and the degradation of organic photovoltaics

The degradation of archetype organic photovoltaics comprising both vacuum and solution-deposited bulk heterojunction active regions is investigated and quantified using a theory based on detailed balance, which relates the open-circuit voltage to the efficiency of charge transfer state emission. To describe this relationship, we account for the difference between electroluminescent external quantum efficiency and the charge transfer emission efficiency. An empirical factor, m, is introduced to distinguish between nonradiative defect sites both within, m=1, and outside, m>1, of the photoactive heterojunction. The m-factor is used to determine the primary sources of degradation for archetype solution- and vacuum-processed material systems. We conclude that degradation occurs primarily within the donor-acceptor heterojunction for the vacuum-processed devices (where m=1.020 +/- 0.002) and outside of the photoactive heterojunction for the solution-processed devices studied, both with and without an anode buffer layer (where m=2.93 +/- 0.09 and m=1.90 +/- 0.01, respectively).