Abstract
In this work, we employed a convenient one-step synthesis method for synthesizing Cu2ZnSnSe4 (CZTSe) nanocrystals (NCs) in an excess selenium environment. This excess selenium situation enhanced the reaction of metal acetylacetonates with selenium, resulting in the burst nucleation of NCs at relatively low temperatures. The phase morphology and surface and optoelectronic properties of NCs before and after ligand exchange were discussed in depth. It was found that pure tetragonal-phase structure CZTSe NCs with approximately 1.7-eV bandgap could be synthesized. The removal of large organic molecules on CZTSe NCs after ligand exchange by S2− decreased the resistivity. The bandgap of the films after ligand exchange by 550°C selenization was also decreased due to better crystallinity. For potential application in CZTSe solar cells, we constructed an energy level diagram to explain the mutual effect between the absorption layer and CdS layer. Using cyclic voltammetry (CV) measurement, we found that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of CZTSe films shifted down after ligand exchange. After energy level alignment at the CdS/CZTSe interface, a type I band alignment structure was more conveniently formed after ligand exchange. This structure acted as the barrier against injection electrons from ZnO to the CZTSe layer, and recombination would subsequently be depressed.
Highlights
Nontoxic and earth-abundant I2-II-IV-VI4 quaternary compounds such as Cu2ZnSnS4 and Cu2ZnSnSe4 (CZTSe) have been considered as the most promising ‘next-generation’ photovoltaic materials to substitute for CIGSe absorber materials, due to their excellent properties such as high absorption coefficients (1 × 105 cm−1) [1,2,3], suitable absorption bandgap for the solar spectrum, high radiation stability, and considerable cell efficiency [4,5,6]
Using cyclic voltammetry (CV) measurements, we found that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of CZTSe films shifted down after ligand exchange
For potential application in CZTSe solar cells, the physical mechanism of utilizing energy level alignment for reducing recombination was discussed in depth after ligand exchange
Summary
Nontoxic and earth-abundant I2-II-IV-VI4 quaternary compounds such as Cu2ZnSnS4 and Cu2ZnSnSe4 (CZTSe) have been considered as the most promising ‘next-generation’ photovoltaic materials to substitute for CIGSe absorber materials, due to their excellent properties such as high absorption coefficients (1 × 105 cm−1) [1,2,3], suitable absorption bandgap for the solar spectrum, high radiation stability, and considerable cell efficiency [4,5,6]. We showed the energy level movement of CZTSe films before and after ligand exchange. Using cyclic voltammetry (CV) measurements, we found that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of CZTSe films shifted down after ligand exchange. Utilizing energy level alignment at the CdS/CZTSe interface, we constructed an energy level diagram to explain the physical mechanism of reducing recombination in CZTSe solar cells. This provides a different approach to the design of the absorption layer, which is generally not afforded by previous reports applying interface passivation and the control of trap states, focuses on the problem of recombination, and holds for a more convenient way to optimize interface properties
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