19.6% efficiency of layer-by-layer organic photovoltaics with decreased energy loss via incorporating TADF materials with intrinsic reverse intersystem crossing

In this work, a thermally activated delayed fluorescence (TADF) material BN-STO is incorporated into the PM1 layer for preparing layer-by-layer organic photovoltaics (LOPVs) due to the intrinsic reverse intersystem crossing and long emission lifetime of BN-STO. The power conversion efficiency (PCE) of LOPVs can be enhanced from 18.54% to 19.65% by introducing 0.5 wt% BN-STO in the PM1 layer, originating from the increased exciton diffusion distance and reduced energy loss. The exciton diffusion distance in the PM1 layer can be increased from 33.06 nm to 59.93 nm by introducing 0.5 wt% BN-STO, which can be deduced from the photoluminescence dynamic decay process of PM1:BN-STO films and special layered PM1:BN-STO/C₆₀ films. The energy loss of optimal LOPVs is reduced from 0.5539 eV to 0.5379 eV due to the reverse intersystem crossing in L8-BO induced by BN-STO incorporation, which can be confirmed from the variation of singlet and triplet exciton excited state absorption peaks and intensity according to transient absorption spectra of L8-BO, L8-BO:PtOEP and L8-BO:PtOEP:BN-STO films. This work indicates that the performance improvement of LOPVs can be enhanced through improving the exciton diffusion distance assisted by energy transfer and decreasing energy loss via incorporating TADF materials with intrinsic intramolecular reverse intersystem crossing.