In the BHJ active layer, high charg

In the BHJ active layer, high charge mobilities guarantee efficientcharge transfer and collection. Many existing polymer donorsdemonstrated a hole mobility of 10
4–10
3 cm2 V
1 s
1 [169],whereas, the electron mobilities of most NFAs are lower thanthe hole mobility of their polymer donor counterparts. The lowerelectron mobilities and unbalanced hole/electron mobility ratioswill lead to lower Jsc and FF. The FF depends on the hole/electronmobilities of donor–acceptor blends and also the thickness of theblend layers. The electron mobility issue of polymer acceptorshas been discussed meticulously by Kim et al. [62]. Herein, wefocus our attention on the electron mobility of SM NFAs.Fig. 15 shows the electron mobilities of some high-performanceSM NFAs in the blend films. The electron mobility of NFAs cannormally be tuned to match with the hole mobility of the donor.The balanced charge carrier mobility in NFA solar cells favors ahigh FF.