A Study Of Ternary Organic Solar Cells Based On PTB7:PC₇₀BM

The novel concept of ternary organic solar cells is a promising candidate to improve the performance of the organic solar cells and has attracted attention of more and more research groups around the world in recent years. In this paper, Bis-PC7oBM and PC60BM are used as electron-cascade acceptor materials in PTB7:PC7oBM blend to fabricate ternary blend polymer solar cells, respectively.First we investigate the ternary organic solar cells based on PTB7: Bis-PC7oBM:PC70BM. In the devices doping with 3% Bis-PC7oBM, the VOC is 0.77V, the JSC is 13.94 mA cm-2, the FF is 0.65 and the PCE is 7.00%, respectively, while in the PTB7:PC70BM binary blend, the VOC is 0.74V, the JSC is 13.92 mA cm-2, the FF is 0.59 and the PCE is 6.07%, respectively. The ternary organic solar cells exhibit higher Voc, Jsc, FF and PCE than that of the PTB7:PC7oBM binary blend, among which the PCE increases by 15%. Due to the higher LUMO energy levels of Bis-PC7oBM relative to PC7oBM, the Voc increases when Bis-PC7oBM is used. AFM images show that when 3% Bis-PC7oBM is used, the film of the ternary blend active layer becomes smoother and the RMS roughness decreases from 1.87nm to 1.80nm. The decreased roughness is likely good for the contact between PEDOT:PSS and the active layer, improving the transport rate. Besides, under the short-circuit current conditions, the Jph/Jsat of the ternary organic solar cells and binary blend is 97% and 96%, respectively, suggesting an increased exciton dissociation efficiency.Then we investigate the ternary organic solar cells based on PTB7: PC6oBM:PC70BM. In the devices doping with 3% PC6oBM, the VOC is 0.75V, the JSC is 15.34 mA cm-2, the FF is 0.64 and the PCE is 7.39%, respectively, while in the PTB7:PC70BM binary blend, the VOC is 0.73V, the Jsc is 14.10 mA cm-2, the FF is 0.60 and the PCE is 6.13%, respectively. The ternary organic solar cells exhibit higher Voc, Jsc, FF and PCE than that of the PTB7:PC70BM binary blend, among which the PCE increases by 21%. The surface of the ternary system is smoother than that of the binary system, the RMS roughness decreases from 1.87nm to 1.72nm, which may be beneficial to the charge trafser and the charge collection, thus improving the Jsc. Besides, the exciton dissociation probability in the ternary system is 94%, higher than that (87%) of the binary system, which is beneficial to improve the Jsc and PCE.Our results indicate that using Bis-PC70BM and PC60BM as electron-cascade acceptor materials in PTB7:PC7oBM blend to fabricate ternary blend polymer solar cells is an effective method to improve the PCE.