An Insight into the Interface through Excited-State Carrier Dynamics for Promising Enhancement of Power Conversion Efficiency in a Mn-Doped CdZnSSe Gradient Alloy.

To explore the significance of impurity doping in power conversion efficiency, quaternary gradient CdZnSSe alloy nanocrystals (NCs) and its Mn-doped analogues were synthesized by high-temperature pyrolysis. The undoped and Mn-doped CdZnSSe alloy NCs have been characterized by employing high-resolution TEM, X-ray diffraction, energy-dispersive X-ray spectroscopy, and electron paramagnetic resonance spectroscopy measurements. A low-temperature injection of chalcogens led to a gradient interface in the alloy, comprised of a CdSe/CdS/ZnSe/ZnS nanostructure. Both steady-state and ultrafast time-resolved absorption studies suggested the formation of a charge-transfer (CT) state due to the inner quasi-type II CdSe/CdS part of the gradient CdZnSSe alloy NCs, in which electrons are delocalized throughout the conduction band (CB) of both CdSe and CdS. The CT-state bleach recovery kinetics gave an additional slow electron cooling component (8 ps) in the undoped alloy NCs, which has been assigned to electron equilibration in the delocalized CB before recombination (or trapping). Interestingly, in the presence of dopant Mn, the slow electron cooling component became even more sluggish at 10 ps due to Mn-mediated electron cooling, in which Mn acts as an electron storage center. An unprecedented increase in the photocurrent conversion efficiency (PCE) of approximately 30 % from (3.3±0.11) to (4.29±0.07) % was observed in the Mn-doped gradient alloy compared with the undoped alloy.