Journal
JOURNAL OF MATERIALS CHEMISTRY
Volume 22, Issue 45, Pages 24067-24074Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/c2jm35199d
Keywords
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Funding
- National Natural Science Foundation of China [90921013, 21132007]
- 100 Talents Programme of CAS from Chinese Academy of Sciences
- USTC-NSRL Association [KY2060140003]
- Fundamental Research Funds for the Central Universities [WK2060140005]
- Key Laboratory of Novel Thin Film Solar Cells [KF201103]
- Chinese Academy of Sciences
- National Basic Research Program of China [2010CB923300, 2011CB921400]
- US NSF [ECCS-0950731]
- South Dakota Board of Regent CRGP
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An amphiphilic surfactant oleamide was incorporated into P3HT:PCBM bulk heterojunction polymer solar cells (BHJ-PSCs) as a novel cathode buffer layer (CBL) for the first time by doping in the P3HT:PCBM photoactive layer followed by self-assembly. The power conversion efficiency (PCE) of the annealed P3HT:PCBM/oleamide BHJ-PSC device is enhanced by similar to 28% at the optimum oleamide doping ratio of 2.5%, which is primarily due to the increase of fill factor (FF) by similar to 22%. The surface morphologies of the oleamide-incorporated P3HT:PCBM photoactive films were studied by transmission electron microscopy (TEM), atomic force microscopy (AFM), and scanning Kelvin probe microscopy (SKPM), revealing that oleamide molecules initially doped in the P3HT:PCBM layer may undergo self-assembly and migrate to the top surface of the P3HT:PCBM layer, leading to the formation of a cathode buffer layer (CBL) as an interfacial dipole layer between the photoactive layer and Al cathode electrode. Such an oleamide interfacial dipole layer lowers the work function of Al, thus the energy level offset between the work function of Al and the LUMO level of the PCBM acceptor is decreased, facilitating the electron extraction by the Al cathode. Furthermore, we found that the crystallinity of P3HT upon the incorporation of oleamide was almost unchanged according to X-ray diffraction (XRD) characterization. It is noteworthy that, this phenomenon is completely different from the case of the previously reported analogous surfactant oleic acid, which was doped in the P3HT:PCBM photoactive layer and led to the efficiency enhancement as well due to the increased crystallinity of P3HT; suggesting the strong influence of the terminal group of the surfactant on its function in P3HT:PCBM BHJ-PSC devices.
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