Passivation of Grain Boundaries and Defects in CZTSSe Solar Cells by In Situ Na Doping

Abstract

Alkali metal doping plays a great role in improving the efficiency of Cu2SnZn(S, Se)4 (CZTSSe) thin film solar cells. However, it is unclear how to realize the ideal distribution of alkali metal in CZTSSe films. Meanwhile, the mechanisms of alkali metal doping are still controversial. Herein, Na‐doped CZTSSe cells are fabricated by magnetron sputtering with Na‐containing Cu2SnZnS4 target which is in situ doping and annealing in selenization atmosphere. The incorporation of Na enhances the diffusion of K from the substrates to the absorbers. Na doping can increase the contact potential at the grain boundaries, which has beneficial effects on reducing the carrier recombination at the grain boundaries. Furthermore, Na incorporation modifies the conduction band offset at the CdS/CZTSSe interface from −0.07 to 0.03 eV. The defects and recombination in different regions are quantitatively extracted. It is demonstrated that the optimized grain boundary electrical properties and the heterojunction band alignment passivate the defects of the depletion region, the heterojunction interface, and the quasineutral region. Finally, a total‐area efficiency of 11.18% has been achieved in CZTSSe solar cell with an in situ Na doping concentration of 0.6 at%.

The Multiple Roles of Na Ions in Highly Efficient CZTSSe Solar Cells

“…The reduction in Urbach energy suggests that the band tailing might originate from defects in CZTSSe. [ 23 ] The carrier density of the CZTSSe has also been measured by capacity–voltage ( C–V ) and deep‐level capacitance profiling (DLCP), and the plots of N CV and N DL against the depletion region width W D have then been extracted (Figure 5e). Similar trends for the Urbach energy and C–V response were also observed in sister devices shown in Figure S12 (Supporting Information).…”

Energy Band Alignment at p–n Heterojunction Interface in CZTSSe Solar Cells with Novel ZnMgO Buffer Layer

“…The details of fabrication of CZTSSe layer can be found in other studies. [ 35,36 ] 600 nm‐thick ZnO:Al(AZO) film was prepared by sputtering ZnO:Al (Al 2 O 3 : 2 wt%k ZnO:Al(AZO) film was substrate temperature of 200 °C. CdS layer was obtained by chemical bath deposition which utilized cadmium surface as the source of Cd, thiourea as the source of S, and ammonia to adjust the pH value of the solution.…”

Pd(II)/Pd(IV) redox shuttle to suppress vacancy defects at grain boundaries for efficient kesterite solar cells

“…Due to the aforementioned reasons, precise regulation of defects at GBs of CZTSSe absorbers has become an essential prerequisite for further enhancing the efficiency of kesterite solar cells. Researchers have attempted several strategies for overcoming issues such as the detrimental Se-Se dimers and the segregation of secondary phases, by using alkali metal incorporation 26 , 27 , surface etching 22 , 28 , composition control 29 – 31 , and optimization of reaction pathways 6 , 7 . These efforts have yielded promising results sequentially; however, the previously generally believed positive effect of GB on charge transport 32 – 34 and the intricate growth process of CZTSSe polycrystalline films at elevated temperatures has led to a dearth of investigations into the characteristics of defects situated in the GB regions and their impacts.…”