A non-conjugated polymer poly(vinylpyrrolidone) (PVP) was applied as a new cathode buffer layer in P3HT:PCBM bulk heterojunction polymer solar cells (BHJ-PSCs), by means of either spin coating or self-assembly, resulting in significant efficiency enhancement. For the case of incorporation of PVP by spin coating, power conversion efficiency (PCE) of the ITO/PEDOT:PSS/P3HT:PCBM/PVP/Al BHJ-PSC device (3.90%) is enhanced by 29% under the optimum PVP spin-coating speed of 3000 rpm, which leads to the optimum thickness of PVP layer of ~3 nm. Such an efficiency enhancement is found to be primarily due to the increase of the short-circuit current (J(sc)) (31% enhancement), suggesting that the charge collection increases upon the incorporation of a PVP cathode buffer layer, which originates from the conjunct effects of the formation of a dipole layer between P3HT:PCBM active layer and Al electrodes, the chemical reactions of PVP molecules with Al atoms, and the increase of the roughness of the top Al film. Incorporation of PVP layer by doping PVP directly into the P3HT:PCBM active layer leads to an enhancement of PCE by 13% under the optimum PVP doping ratio of 3%, and this is interpreted by the migration of PVP molecules to the surface of the active layer via self-assembly, resulting in the formation of the PVP cathode buffer layer. While the formation of the PVP cathode buffer layer is fulfilled by both fabrication methods (spin coating and self-assembly), the dependence of the enhancement of the device performance on the thickness of the PVP cathode buffer layer formed by self-assembly or spin coating is different, because of the different aggregation microstructures of the PVP interlayer.

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 $28% at the optimum oleamide doping ratio of 2.5%, which is primarily due to the increase of fill factor (FF) by $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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