基于PTB7,Bis-PC_(70)BM,PC_(70)BM的高效率有机三元太阳能电池

我们将Bis-PC_(70)BM作为第二种电子受体混入基于PTB7:PC_(70)BM的聚合物太阳能电池中,制备了三元混合聚合物太阳能电池.相比于PC_(70)BM,Bis-PC_(70)BM的最低未占分子轨道(lowest unoccupied molecular orbital,LUMO)能级更高,所以掺入Bis-PC_(70)BM后器件的开路电压(V_(oc))得到了提升.Bis-PC_(70)BM在PTB7和PC_(70)BM之间起到桥梁的作用,因此在给体/受体界面创造了更多的电荷传递通道.而且从原子力显微镜中得到的结果来看,当混入质量比为3%的Bis-PC_(70)BM后薄膜的表面形貌更为平整,平均粗糙度从原来的1.87 nm降到了1.80 nm.能量转换效率(power conversion efficiency,PCE)达到7.00%,其中器件的V_(OC)为0.77 V,短路电流(J_(SC))为13.92 mA·cm~(-2),比PTB7:PC_(70)BM的器件效率6.07%提高了15%.

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]benzo1, 2-b:4, 5-b']dithiophene-2, 6-diyl-alt-3-fluoro-2-(2-ethylhexyl) carbonyl]thieno3, 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 (V_OC) is 0.77 V, the short-circuit current (J_SC) 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 V_OC 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 J_ph-V_eff 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.