Effect of manganese dopants on defects, nano-strain, and photovoltaic performance of Mn–CdS/CdSe nanocomposite-sensitized ZnO nanowire solar cells

This study reports the mechanism of power conversion efficiency (PCE) enhancement in CdS/CdSe quantum dot (QD) composite-sensitized ZnO nanowire (NW) solar cells when Mn2+ ions are doped in CdS QDs. Various ZnO NWs sensitized with different types of QD-composites including CdS, CdS/CdSe and Mn–CdS/CdSe QDs were successfully synthesized using an in-situ sequential assembly process involving successive ionic layer adsorption and reaction and chemical bath deposition at room temperature. QD solar cells with a ZnS/CdSe/Mn(0.02M)-CdS/ZnO NW structure exhibited a PCE of 3.47% (Voc = 0.74 V, Jsc = 14.56 mA/cm2, and FF = 36.92%), which is 33% higher than that (2.61%) of their counterparts without Mn2+ dopants. Electrochemical impedance spectroscopy studies of the devices reveal that the main role of the Mn2+ dopant was to reduce the recombination of photo-excited carriers. Similar improvement in PCE is traditionally indicated as being a result of the generation of the mid-band (4T1−6A1) region owing to the atomic d orbitals of Mn (4T1 and 6A1); however, no evidence is available. Rather, we focused on quantifying the nanoscale strain in the QD composites with and without Mn2+; this is because the formation of nanoscale strain in QDs implies the presence of considerable imperfections functioning as charge traps and recombination centers. Analyses of the interface, interior, and crystallinity of the QDs by using high-resolution electron microscopy combined with geometric phase analysis (GPA) revealed that Mn2+ doping significantly reduced the lattice nano-strain in the QD nano-composites. This result is probably owing to Mn2+ cation diffusion (or exchange) to the defective CdS/CdSe layer assisted by the differences among the solubility products (Ksp) in the MnS, CdS, and CdSe phases; this diffusion reduces the nanoscale strain resulting from the presence of defects and misfit dislocations in QD composites and thereby decreases carrier recombination.