Contrastive manipulations on vertical stratifications by a fluorescent guest component in ternary nonfullerene and fullerene organic solar cells

Herein, a simple fluorescent material TPA2O (5,5′-((4′-(diphenylamino)-[1,1′-biphenyl]-2,6-diyl)bis(methaneylylidene))bis(1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione)) is designed and successively introduced into both polymer:nonfullerene and polymer:fullerene blends to construct ternary organic solar cells with an inverted structure of ITO/ZnO/active layer/MoO₃/Ag. Remarkably improved power conversion efficiency by up to 25% (from 9.21% to 11.47%) and sufficiently enhanced photocurrent (from 21.01 to 25.25 mA cm⁻²) are realized in nonfullerene ternary devices by incorporating a small amount of TPA2O, while reduced PCE is found in ternary fullerene systems. Characterizations have proven that efficient Förster resonance energy transfer (FRET) exists between TPA2O and the host PTB7-Th donor. And the distinguishable device performance in different ternary systems is determined by the contrastive influences on lateral and longitudinal direction morphologies. Specifically, by incorporating TPA2O, the cross-section morphology is degenerated in ternary fullerene blends but stable in ternary nonfullerene blends. More interestingly, for vertical-section morphology, PTB7-Th is more enriched in the upper layers in ternary nonfullerene blends, which is close to hole-transport MoO₃ and inducing more efficient hole transport. Whereas PTB7-Th is less concentrated within the top layers in ternary PC₇₁BM-based blends. Moreover, remarkably enhanced thermal stability is observed in TPA2O-based nonfullerene devices, with 94% of the initial PCE after baking at 80 °C for 144 h, compared to 75% and 62% of IEICO-4F and PC₇₁BM binary devices, respectively.