Alleviating excessive aggregation of a non-fullerene acceptor by delaying and shortening the crystallization time to reduce the energy loss of ternary organic solar cells

Abstract

The key factor restricting the power conversion efficiency (PCE) of organic solar cells (OSCs) is the energy loss (E_loss), which is the difference between the optical bandgap (E_g) of the active layer and open-circuit voltage (V_OC) of the device. To achieve lower E_loss, it is necessary to obtain an appropriate donor–acceptor phase separation size to accelerate exciton dissociation and inhibit the recombination process. However, in most high-efficiency non-fullerene systems, acceptors often exhibit excessive aggregation phenomena. The decrease in the interface area leads to a decrease in exciton dissociation efficiency, which increases the energy loss. Herein, we report a ternary strategy to decrease the crystallization time of the acceptor and inhibit the excessive aggregation condition of a non-fullerene acceptor. We chose a donor poly{[4,8-bis[5-(2-ethylhexyl)-4-fluoro-2-thienyl]benzo[1,2-b:4,5-b′]-dithiophene-2,6-diyl]-alt-[2,5-thiophenediyl[5,7-bis(2-ethylhexyl)-4,8-dioxo-4H,8H-benzo[1,2-c:4,5-c′]dithiophene-1,3-diyl]]} (PM6) and a non-fullerene acceptor (2,2′-((2Z,2′Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2′′,3′′:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile) (Y6) as the model system. Y6 is prone to forming a tightly packed structure due to its planar curved skeleton. To suppress the excessive aggregation, we chose poly[2,2′-((2Z,2′Z)-((12,13-bis(2-octyldodecyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2′′,3′′:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile-co-2,5-thiophene] (PY-IT) as a second acceptor, which has good compatibility with Y6. By using in situ UV-visible absorption spectroscopy to monitor the film formation kinetics of Y6, it was found that after adding 15 wt% PYIT, the total crystallization time of Y6 decreased and the excessive aggregation of Y6 was inhibited. In the PM6:Y6 system, Y6 only had one crystallization and film-forming process. While in the PM6:Y6+15 wt% PYIT system, the process of film formation became more complex, with two stages of aggregation. PYIT crystallized before Y6, when Y6 began to crystallize, PYIT has occupied a portion of the crystallization growth space. What is more, PYIT delayed the crystallization process of Y6, and the change in the acceptor peak position showed a stable region. After that, Y6 began to aggregate and the crystallization time of Y6 was shorter than that of the binary system. As a result, PYIT alleviated the excessive aggregation of Y6, resulting in better mixing between the non-fullerene acceptor and the donor, increasing the interface area and enabling faster dissociation of excitons. In addition, the vertical phase separation of the active layer has also been optimized, allowing more donors enriched near the anode, enhancing the efficiency of charge extraction. The improved morphology of the active layer results in a better interface area, which can not only ensure exciton dissociation and charge generation, but also reduce the transfer time, which is conducive to reducing energy loss. As a result, E_loss reduced from 0.559 eV to 0.539 eV, and the optimized ternary OSC exhibited a PCE of 17.05%.