Application of biuret, dicyandiamide, or urea as a cathode buffer layer toward the efficiency enhancement of polymer solar cells

Xuemei Zhao; Chenhui Xu; Haitao Wang; Fei Chen; Wenfeng Zhang; Zhiqiang Zhao; Liwei Chen; Shangfeng Yang

doi:10.1021/am500013s

Application of biuret, dicyandiamide, or urea as a cathode buffer layer toward the efficiency enhancement of polymer solar cells

ACS Appl Mater Interfaces. 2014 Mar 26;6(6):4329-37. doi: 10.1021/am500013s. Epub 2014 Mar 12.

Authors

Xuemei Zhao¹, Chenhui Xu, Haitao Wang, Fei Chen, Wenfeng Zhang, Zhiqiang Zhao, Liwei Chen, Shangfeng Yang

Affiliation

¹ 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.

PMID: 24575873
DOI: 10.1021/am500013s

Abstract

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.

Publication types

Research Support, Non-U.S. Gov't