Fill factor enhancement of organic solar cells based on a wide bandgap phosphorescent material and C60

Planar heterojunction organic solar cells using wide bandgap phosphorescent material bis[2-(4-tertbutylphenyl)benzothiazolato-N,C2'] iridium (acetylacetonate) [(t-bt)2Ir(acac)] as electron donor and fullerene (C60) as electron acceptor were fabricated. A large open circuit voltage of 0.94 V was achieved due to low highest occupied molecular orbital level of (t-bt)2Ir(acac). The effect of different hole transport layers and substrate heating were investigated to improve fill factor. It is shown that the open circuit voltage is strongly influenced by the interface energy barrier, whereas the fill factor is mainly limited by the charge carrier transport properties in active materials. The fill factor was significantly improved by either using hole transport layer with high carrier mobility or increasing the hole mobility of (t-bt)2Ir(acac). A power conversion efficiency of 2.10% under AM 1.5 solar illumination at an intensity of 100 mW/cm2 was achieved by heating the substrate during the deposition of active materials.

► Organic solar cells based on wide bandgap phosphorescent material were fabricated.
► A relatively high open circuit voltage about 0.94 V was achieved.
► The fill factor (FF) is mainly limited by the charge transport properties.
► The FF can be improved by using hole transport layers with high mobility.
► Increasing the mobility of active materials can also enhance the FF.