Efficient organic ternary solar cells based on PTB7 and PC70BM with Bis-PC70BM

In recent years, solar cells, especially the bulk heterojunction (BHJ) polymer solar cells (PSCs), have attracted considerable attention. BHJ PSCs have several advantages such as easy fabrication, light weight, low cost and flexibility. The research on ternary BHJ PSCs will become a hot topic since incorporating near infrared region (NIR) low bandgap polymer materials into the donor/acceptor system can easily extend the absorption spectral range and improve the photon harvesting. In this paper, we investigate the ternary PSCs based on poly{4, 8-bis[(2-ethylhexyl)-oxy]benzo[1, 2-b:4, 5-b']dithiophene-2, 6-diyl-alt-3-fluoro-2-[(2-ethylhexyl) carbonyl]thieno[3, 4-b]thiophene-4, 6-diyl} (PTB7); Bis adduct of phenyl-C71-butyric acid methyl ester (Bis-PC70BM); [6, 6]-phenyl-C71-butyric-acid-methyl-ester (PC70BM). The performance of PSCs based on PTB7 and PC70BM may be improved by doping with Bis-PC70BM which is used as an electron-cascade acceptor material. Ternary blend PSCs with 3% Bis-PC70BM exhibit a power conversion efficiency (PCE) of 7.00%, higher than that (6.07%) of the PTB7 :PC70BM binary blend. The open-circuit voltage (VOC) is 0.77 V, the short-circuit current (JSC) is 13.92 mA cm-2 and the fill factor (FF) is 65%. However, in our research, the absorption spectra for the films with different amount of Bis-PC70BM are hardly changed, implying that doping with Bis-PC70BM would not improve the photon harvesting. The LUMO (HOMO) energy levels of PTB7, Bis-PC70BM and PC70BM are -3.49 eV (-5.31 eV), -3.80 eV (-6.10 eV) and -3.91 eV (-6.20 eV), respectively. Due to the higher LUMO energy levels of Bis-PC70BM relative to PC70BM, the VOC increases when Bis-PC70BM is used. The cascade-like energy levels of ternary blend PSCs can facilitate the charge transfer at the donor/acceptor interface owing to the bridging effect. There are three routes for charge transfer (PTB7-Bis-PC70BM, Bis-PC70BM-PC70BM and PTB7-PC70BM) in ternary PSCs, more than that one in the binary PTB7:PC70BM counterpart. Moreover, PC70BM can provide a driving force to transfer the electrons on the LUMO of Bis-PC70BM to a lower energy orbital (the LUMO of PC70BM), which can facilitate charge transfer from PTB7 to Bis-PC70BM. Atomic force microscopy (AFM) images show that when 3% Bis-PC70BM is used, the film of the ternary blend active layer becomes smoother and the root-mean-square (RMS) roughness decreases from 1.87 nm to 1.80 nm. The decreased roughness is likely good for the contact between the PEDOT:PSS and the active layer, improving the transport rate. We have fabricated hole-only devices using a high-work-function material (Au) as the cathode to block the back injection of electrons in order to investigate charge carrier transport and collection in the PSCs. Result shows that doping with Bis-PC70BM may not change the hole mobility in the device. Besides, the Jph-Veff characteristics shows that doping with 3% Bis-PC70BM can facilitate exciton dissociation and charge collection at a low voltage. Our results indicate that using Bis-PC70BM as an electron-cascade acceptor material in PTB7 :PC70BM blend to fabricate ternary blend PSCs is a promising way to improve the PCE.