High-efficiency polymer solar cells with Sm/Ca bilayer cathode buffers

The Sm/Ca bilayer buffers together with Al as the cathode, instead of Al or Ca/Al cathodes are applied into bulk heterojunction polymer solar cells. Accompanied with the optimization of Sm/Ca buffer thicknesses, a high power conversion efficiency of 3.98%, an enhanced short-circuit current density (J_SC) of 10.87 mA/cm², and a fill factor (FF) of 0.61 are achieved in the poly(3-hexylthiophene): [6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM)-based cell. Analysis on the film's scanning electron microscopy images, light absorption characteristics, the electromagnetic fields distribution, and scattering illustrates that absorption enhancement induced by Sm clusters via the localized surface plasmon resonance and scattering plays a major role in the enhancement of J_SC, while a highly efficient electron extraction and a further resulting large FF is dominated by the Ca buffer.