Is indium tin oxide a suitable electrode in organic solar cells? Photovoltaic properties of interfaces in organic $p∕n$ junction photodiodes

The charge generation properties at all interfaces of a $p∕n$ junction, bilayer photodiode have been investigated by means of the photoaction spectrum (PAS) as a function of applied bias. The organic photodiode was fabricated with a low-glass transition temperature $(Tg)$ polysiloxane with pendant hydrazone groups as the $p$-type material and a perylene diimide derivative as the $n$-type material. The PAS under short circuit and reverse bias showed an antibatic response at the high-energy region $(3.0–3.5eV)$⁠, and a symbatic response at the low-energy region $(2.0–3.0eV)$⁠. However, under forward bias, the PAS showed the opposite behavior. These results are interpreted in terms of the band structure of tin-doped indium oxide (ITO) that prevents effective photoinjection of electrons at the polymer/ITO interface and the relative energy levels of the constituent materials.