Surface preparation for 10% efficient CZTSe solar cells

Abstract

Kesterite‐based solar cells suffer from a large open‐circuit voltage deficit, which largely arises from carrier recombination at the buffer interface. In this study, we compare two strategies to passivate the absorber surface in order to fabricate devices with power conversion efficiency higher than 10% and an open‐circuit voltage deficit as low as 306 mV. These two strategies consist of annealing in air or performing a chemical etching of the absorbers before buffer deposition. They lead similarly to a significant reduction of the interface recombination but as well to a shortening of the minority carrier diffusion length from 1 μm to less than 500 nm. This latter effect limits the short‐circuit current and fill factor of the devices but is largely compensated by the open‐circuit voltage gain of more than 100 mV. For the absorber air annealing, which is the simplest solution to implement, absolute photoluminescence measurements reveal that the voltage gain is directly linked to a drop in the nonradiative losses in the absorber and to a small reduction of the band tailing. It is demonstrated that the removal of detrimental secondary phases at the surface of the absorber due to oxidation at elevated temperature and etching in the basic CdS solution is responsible for these improved opto‐electronic properties. On the contrary, the apparent Cu‐depletion observed after air annealing is totally recovered after the chemical bath and cannot be responsible for the improved performances.

Short- and Long-Range Cation Disorder in (AgxCu1–x)2ZnSnSe4 Kesterites

“…Moreover, the XPS profile in Figure e shows the presence of Ag in the ACZTSe samples, with Ag 3d 3/2 and Ag 3d 5/2 peaks at ∼368.1 and 372.1 eV (δ E ≈ 4.0 eV), respectively. This reveals that silver was present in Ag + form, in agreement with other reports. − …”

Toward High Efficient Cu2ZnSn(Sx,Se1−x)4 Solar Cells: Break the Limitations of VOC and FF

“…According to Grenet et al., the peaks of Zn 2p3/2 and Sn 3d5/2 moving toward lower binding energy after AS treatment is considered to be due to the oxidation/deoxidation of the ZnSe and SnSe x phases on the surface, respectively. [ 16 ] Therefore, under the same AS treatment time, more Zn and Sn are etched, resulting in an increase of the Cu/(Zn+Sn) ratio for sample Kes‐Al 2 O 3 ‐AS. Overall, the surface composition of the absorber is within a reasonable range, and the Cu/(Zn+Sn) value for sample Kes‐Al 2 O 3 ‐AS is more ideal as a Cu‐poor surface, which is suitable for the preparation of efficient solar cells.…”

“…It is reported that ammonium sulfide ((NH 4 ) 2 S, referred to as AS) solution treatment on the absorber layer can improve the film's surface properties. [16][17][18][19][20][21][22][23] For example, Buffière et al selectively removed the secondary phase on the surface and modified the surface chemical state by dipping CIGS absorber into AS solution, and the minority carrier lifetime increased about 6-11 ns with different treatment times. [17] The same treatment has been employed for CZTSSe, too.…”

A critical review on rational composition engineering in kesterite photovoltaic devices: self-regulation and mutual synergy

“…Post-heat treatment in air induces oxygen from the ambient atmosphere to form a native oxide layer on the surface 257 and diffuse into CZTSSe absorbers along grain boundaries (GBs), 258 which has been reported to contribute to improving device performance. 91,95,168,259 This approach can cause local composition uctuation of the absorbers, such as Cu and Se decient and Sn-enriched composition at the GBs (Fig. 10a), 168 and Zn enrichment on the surface.…”