Zhao Xuemei, Xu Chenhui, Wang Haitao, Chen Fei, Zhang Wenfeng, Zhao Zhiqiang, Chen Liwei, Yang Shangfeng
Hefei National Laboratory for Physical Sciences at Microscale, CAS Key Laboratory of Materials for Energy Conversion & Department of Materials Science and Engineering, University of Science and Technology of China (USTC) , Hefei 230026, China.
ACS Appl Mater Interfaces. 2014 Mar 26;6(6):4329-37. doi: 10.1021/am500013s. Epub 2014 Mar 12.
Three amino-containing small-molecule organic materials-biuret, dicyandiamide (DCDA), and urea-were successfully applied as novel cathode buffer layers (CBLs) in P3HT:PCBM bulk heterojunction polymer solar cells (BHJ-PSCs) for the first time, resulting in obvious efficiency enhancement. Under the optimized condition, the power conversion efficiencies (PCEs) of the CBL-incorporated BHJ-PSC devices are 3.84%, 4.25%, and 4.39% for biuret, DCDA, and urea, which are enhanced by ∼15%, ∼27%, and ∼31%, respectively, compared to the reference poly(3-hexylthiophene-2,5-diyl) : [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) BHJ-PSC device without any CBL. The efficiency enhancement is primarily attributed to the increases of both short-circuit current density (Jsc) and fill factor (FF), for which the enhancement ratio is found to be sensitively dependent on the molecular structure of small-molecule organic materials. The surface morphologies and surface potential changes of the CBL-incorporated P3HT:PCBM photoactive layers were studied by atomic force microscopy and scanning Kelvin probe microscopy, respectively, suggesting the formation of an interfacial dipole layer between the photoactive layer and Al cathode, which may decrease the energy level offset between the work function of Al and the lowest unoccipoed molecular orbital level (LUMO) of the PCBM acceptor and consequently facilitate electron extraction by the Al cathode. The difference in the enhancement effect of biuret, DCDA, and urea is due to their difference on the work function matching with P3HT:PCBM. Besides, the coordination interaction between the lone-pair electrons on the N atoms of the amino (-NH2) group and the Al atoms may prohibit interaction between Al and the thiophene rings of P3HT, contributing to the efficiency enhancement of the CBL-incorporated devices as well. In this sense, the different CBL performance of biuret, DCDA, and urea is also proposed to partially originate from the differences on their chemical structure, specifically the number of amino groups.
三种含氨基的小分子有机材料——缩二脲、双氰胺（DCDA）和尿素——首次成功应用于聚（3-己基噻吩-2,5-二亚基）：[6,6]-苯基-C61-丁酸甲酯（P3HT:PCBM）本体异质结聚合物太阳能电池（BHJ-PSCs）中作为新型阴极缓冲层（CBLs），从而使效率显著提高。在优化条件下，含缩二脲、DCDA和尿素的BHJ-PSC器件的功率转换效率（PCEs）分别为3.84%、4.25%和4.39%，与不含任何CBL的参比P3HT:PCBM BHJ-PSC器件相比，分别提高了约15%、约27%和约31%。效率的提高主要归因于短路电流密度（Jsc）和填充因子（FF）的增加，发现其提高比例敏感地依赖于小分子有机材料的分子结构。分别通过原子力显微镜和扫描开尔文探针显微镜研究了含CBL的P3HT:PCBM光活性层的表面形貌和表面电位变化，表明在光活性层和Al阴极之间形成了界面偶极层，这可能会减小Al的功函数与PCBM受体的最低未占据分子轨道能级（LUMO）之间的能级偏移，从而促进Al阴极的电子提取。缩二脲、DCDA和尿素增强效果的差异是由于它们与P3HT:PCBM功函数匹配的差异。此外，氨基（-NH2）基团N原子上的孤对电子与Al原子之间的配位相互作用可能会阻止Al与P3HT的噻吩环之间的相互作用，这也有助于含CBL器件的效率提高。从这个意义上讲，缩二脲、DCDA和尿素不同的CBL性能也被认为部分源于它们化学结构的差异，特别是氨基的数量。
