Elucidating the factors that determine the open circuit voltage in discrete heterojunction organic photovoltaic cells

The operation of discrete heterojunction organic photovoltaic (OPV) cells employing chloroaluminium phthalocyanine (ClAlPc) as the electron donor and C-60 as the electron acceptor is reported and the characteristics are correlated with the energy level structure of the devices determined using X-ray photoelectron spectroscopy. The results give new insight into the origin of the open circuit voltage (V-oc) in discrete heterojunction OPVs. The measured V-oc in this system is found to be determined by: (i) the frontier orbital energy offsets between the donor and acceptor materials, accounting for the likely formation of an abrupt vacuum level shift at the heterojunction interface and (ii) the degree of alignment between the hole-extracting electrode Fermi level and the highest occupied molecular orbital energy of the electron donor material. The generality of the findings is demonstrated by rationalising the V-oc in OPVs employing the archetypal electron donor, copper phthalocyanine.