Application of Biuret, Dicyandiamide or Urea as Cathode Buffer Layer toward Efficiency Enhancement of Polymer Solar Cells.

ACS applied materials & interfaces

PubMedID: 24575873

Zhao X, Xu C, Wang H, Chen F, Zhang W, Zhao Z, Chen L, Yang S. Application of Biuret, Dicyandiamide or Urea as Cathode Buffer Layer toward Efficiency Enhancement of Polymer Solar Cells. ACS Appl Mater Interfaces. 2014;.
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 (PCE) 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 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 the 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 (AFM) and scanning Kelvin probe microscopy (SKPM), 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 LUMO level of the PCBM acceptor and consequently facilitate the electron extraction by Al cathode. The difference on 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 nitrogen atoms of the amino (-NH2) group with Al atoms may prohibit the 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 their difference on the chemical structure specifically the number of the amino groups.