High Performance Non-fullerene Polymer Solar Cells
Samson A. Jenekhe, Ye-Jin Hwang, Taeshik Earmme, Haiyan Li
Department of Chemical Engineering, University of Washington,
Seattle, Washington 98195-1750, United States
New electron transporting (n-type) organic semiconductors are long sought as alternatives to fullerene-based electron acceptors for polymer solar cells. Non-fullerene electron transport materials offer the potential to overcome drawbacks of fullerene acceptors, which include the small open-circuit voltage (Voc), poor light harvesting, high cost, and poor photochemical stability. However, the performance of non-fullerene polymer solar cells is still limited by lack of suitable acceptors and lack of knowledge of the critical factors that determine compatibility, blend morphology, and performance of non-fullerene bulk heterojunction (BHJ) solar cells. All known non-fullerene electron acceptors have so far shown inferior photovoltaic properties compared to the fullerene benchmark [6,6]-phenyl-C70-butyric acid methyl ester (PC70BM).
We have developed several new classes of n-type organic semiconductors, including small molecules and polymers based on strong electron-withdrawing moieties such as naphthalene diimide (NDI), perylene diimide (PDI) and tetraazabenzodifluoranthene Diimides (BFI). Bulk heterojunction (BHJ) solar cells based on the new acceptors with various electron-donating (p-type) polymers, including poly(3-hexylthiophene) (P3HT), thiazolothiazole-dithienylsilole copolymer (PSEHTT), and thieno[3,4-b]thiophene-benzodithiophene copolymers, have been fabricated and their photovoltaic properties were evaluated. The new series of acceptors have high electron affinities in the range of 3.5 – 4.0 eV and energy band gaps of less than 2 eV, forming good energy offsets with various donor polymers for efficient charge separation. The non-fullerene polymer solar cells have a high power conversion efficiency (PCE) in the range of 5 – 7 %, which is superior performance than the similarly evaluated PC70BM-based BHJ solar cells. They also combine high photocurrent (< 18.5 mA/cm2) with high open circuit voltage (< 0.94 V). Furthermore, we demonstrated the critical role of the crystallinity and the nonplanar 3D conformation of the acceptor materials in achieving high-performance non-fullerene solar cells, and thereby provide important criteria for the molecular design of new high performance non-fullerene electron transport materials.