Impact of one step alloying on the carrier relaxation and charge separation dynamics of CdxZn1-xSe graded nanocrystals

Abstract

Slow carrier relaxation and extent of charge separation in semiconductor nanocrystals (NCs) are the desired parameters, which control the power conversion efficiency in photovoltaic. Although several synthesis approaches have explored, the spectroscopic analysis of the fundamental understanding of charge carrier dynamics in the quantum confined states of cadmium zinc selenide (CdxZn1-xSe) graded alloy NCs remain relatively unexplored. Here, we study the charge carrier dynamics of a series of CdxZn1-xSe alloy NCs by changing the metal (Cd and Zn; M) to selenide ratio adopting one-step synthesis. Our findings underline that both excitonic absorption and band edge photoluminescence (PL) of all the alloys are red shifted concerning the binary analogs (CdSe and ZnSe), which implies the formation of gradient alloy structure where the core is CdSe rich that leads to increasing light absorption cross-section in the solar spectrum. Femtosecond transient absorption (fs-TA) measurements of the alloy NCs reveal that the CdxZn1-xSe alloy show three distinct bleaches due to the different electronic transition like 1S (1Se-1S3/2), 2S (1Se-2S3/2), and 1P (1Pe-1P3/2) while only CdSe shows two bleaches due to 1S, and 1P. The distinctive band structure (quasi type-II) of the alloy leads to slow down the intraband carrier cooling time and prolonging the charge separation due to specially decoupled photo-excited electron and hole. This finding offers new insights to understand the photophysical behavior and suggest that CdxZn1-xSe graded NCs can be used as efficient light harvesting materials in quantum dot solar cell.