Solution-processed organic solar cells (OSCs) have attracted considerable attention because of their advantageous features of lightweight and flexible substrates, colorful and easy manufactory on a large scale. Particularly, non-fullerene acceptor materials have been applied to photovoltaics with promising results and have shown power conversion efficiencies (PCEs) of over 17%.
The tremendous achievements that have been made toward pushing efficiency; however, the commercialization of solution-processed OSCs has been limited by their intrinsic low environmental stability. Of note is that heat has been identified as a vital stress factor for the degradation of OSCs. Because these devices operating under standard conditions are markedly heated by long-term illumination of sunlight, and the actual operating temperature for solar panels can be as high as 50 to 70 oC, even reaching 100 oC in some areas. The thermal issue should be properly addressed to meet the requirements of the practical outdoor application.
In this work, we designed and synthesized a new non-fullerene acceptor, BTTT-2Cl. Using polymer PM6 as the donor, we systematically evaluated the photovoltaic properties of PM6:BTTT-2Cl solar cells with a power conversion efficiency of 13.80%. Importantly, we further introduced a polymer acceptor PZ1 employed as the dual functional additive in the PM6:BTTT-2Cl active layer. The addition of 1 wt% PZ1 not only enhanced π–π stacking and aggregation of BTTT-2Cl acceptors promoting device efficiency up to 15.10%, but also dramatically enhanced its thermal stability (Fig. 1). In contrast with the host blends, whose efficiency reduced from 13.30% to 6.38% after heating at 150 °C for 24 hours, the PM6:BTTT-2Cl blends with 1 wt% PZ1 showed much slower morphology degradation, which represents 12% efficiency loss after heating at 150 °C for 800 hours.
The morphological and physical characterizations demonstrated that PZ1 can significantly suppress the phase separation, fix the blend microstructure and slow down the trap generation of the PZ1-doped blend at high temperatures. Furthermore, the great thermal stability of the PZ1-doped PM6:BTTT-2Cl blends subjected to thermal cycling stress conditions illustrated that this doping photovoltaic system can be used in outer space applications. More importantly, the investigated four other systems demonstrate that PZ1 is a versatile key to enhance photovoltaic efficiency and thermal stability of active layers simultaneously.
Overall, we have developed a PZ1-doped PM6:BTTT-2Cl photovoltaic system with good device efficiency and excellent thermal stability, indicating its potential outdoor and space applications. Moreover, this research also demonstrates that the use of PZ1 as a dual function additive is a simple and general doping strategy for improving the photovoltaic performance of OSCs and boosting their commercial applications.
For more information, please refer to our recent publication in Nature Communications, “Simultaneous Enhanced Efficiency and Thermal Stability in Organic Solar Cells From A Polymer Acceptor Additive” (https://www.nature.com/articles/s41467-020-14926-5).