Open-circuit voltage dependency on hole-extraction layers in planar heterojunction organic solar cells

The open-circuit voltage (V oc ) dependency on hole-extraction layers (HELs) with different energy levels and mobility was investigated in a single stack heterojunction subphthalocyanine chloride (SubPc)/C 60 organic solar cells. The HELs having about 0.2-0.3 eV higher highest occupied molecular orbital (HOMO) level than that of a donor material can significantly enhance the V oc in SubPc/C 60 device due to a corresponding built-in potential increase. The high mobility of HELs can also increase V oc with increasing J sc according to the simple diode equation. Among all HELs we utilized, N,N,N′,N′ -tetra(biphenyl-4-yl)biphenyl-4,4 ′ -diamine (TBBD) illustrates a largest increase in V oc (from 0.90 to 1.15 V) with an improvement in efficiency compared to a reference SubPc/C 60 device without HEL. This increase is mainly attributed to easy and rapid extraction of holes by TBBD due to its proper HOMO level and high mobility.