Performance improvement of DBP-based solar cells by introducing a luminescent sensitizer bis[(4,6-difluorophenyl)-pyridinato-N,C 2′ ]c(picolinate)iridium(III) (FIrpic)

In this work, a sky-blue luminescent down-shifting (LDS) layer bis[(4,6-difluorophenyl)-pyridinato-N,C2′]c(picolinate)iridium(III) (FIrpic) was inserted between tetraphenyldibenzoperiflanthene (DBP) and MoO3 as UV-screen and sensitizer for small molecule DBP/C60 based planar heterojunction (PHJ) solar cells. With 8-nm FIrpic the short circuit current (Jsc) and power conversion efficiency (PCE) of the device are enhanced by 28% and 15%, respectively, probably originating from the re-absorption of the photons emitted from FIrpic. The Voc linearly increases over 1-nm FIrpic, ascribed to the deeper HOMO level of FIrpic than DBP, while the fill factor continuously declines from 3- to 10-nm FIrpic. The EQE spectra prove that the Jsc is mainly contributed by the photocurrent generated in DBP and C60 layers. When the FIrpic thickness is 8 nm, the film surface is very uniform with the smallest water contact angle. The impedance spectroscopy demonstrates that the device resistance gradually increases from 4.1×104 Ω (without FIrpic) to 4.6×104 Ω (with 10-nm FIrpic) with the FIrpic thickness rise, simultaneously the device transits from the insulating state into the conductive state faster for the thin FIrpic layer than the thick layer.