Highly crystalline new benzodithiophene-benzothiadiazole copolymer for efficient ternary polymer solar cells with an energy conversion efficiency of over 10

A new alternating polymer P(BDTSi-DFBT), poly(4,8-bis(triisopropylsilylethynyl)-benzo[1,2- b :4,5- b ′]dithiophene- alt -5,6-difluoro-4,7-bis(4-octylthiophen-2-yl)benzo[ c ][1,2,5]thiadiazole), was prepared via Stille polymerization. The determined absorption maximum and optical band-gap ( E g ) of P(BDTSi-DFBT) were 590 nm and 1.74 eV, respectively. The calculated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of P(BDTSi-DFBT) were −5.45 eV and −3.71 eV, respectively. The X-ray diffraction (XRD) analysis confirmed that P(BDTSi-DFBT) is a crystalline polymer. The binary-polymer solar cells, ITO/PEDOT:PSS/P(BDTSi-DFBT) : PC 70 BM (1 : 1.5 wt%) + 3 vol% DIO/Al, made from P(BDTSi-DFBT) gave a power conversion efficiency (PCE) of 5.02% with an open-circuit voltage ( V oc ) of 0.81 V, a short-circuit current ( J sc ) of 11.68 mA cm −2 , and a fill factor (FF) of 53%. Conversely, the ternary-polymer solar cells, ITO/PEDOT:PSS/PTB7-Th : P(BDTSi-DFBT) : PC 70 BM (0.8 : 0.2 : 1.5 wt%) + 3 vol% DIO/Al, made with a synthesized medium band-gap P(BDTSi-DFBT) and low band-gap PTB7-Th, offered a maximum PCE of 10.05%, with a V oc of 0.79 V, a J sc of 17.92 mA cm −2 , and a FF of 71%. A crystalline medium band-gap polymer is prepared. The binary- and ternary-PSCs made with the polymer offered a PCE of 5% and 10%, respectively.