Potential of CuS cap to prevent decomposition of Cu₂ZnSnS₄ during annealing

Abstract

One of the challenges associated with processing of Cu2ZnSnS4 (CZTS) is the thermal decomposition reaction that causes loss of S and SnS from the absorber surface. To reduce the decomposition a sufficiently high SnS and S partial pressure must be supplied during annealing. The absorber surface can alternatively be protected with a thin cap. Aiming to obtain a more flexible process, CZTS precursors were capped with a thin CuS layer before annealing. The cap was subsequently removed with a KCN etch before device finishing. It was found that the cap coverage decreased during annealing, exposing a part of the absorber surface. At the same time, the initially Cu poor absorber took up Cu from the cap, ending up with a stoichiometric Cu content. Devices made from capped precursors or precursors annealed without sulfur had poor device characteristics. An increased doping density of almost one order of magnitude could be the reason for the very poor performance. CuS is therefore not a suitable cap material for CZTS. Other cap materials could be investigated to protect the CZTS absorber surface during annealing. A thin CuS cap is deposited on CZTS to prevent surface decomposition during annealing.

Surface modification through air annealing Cu2ZnSn(S,Se)4 absorbers

“…With an efficiency of 2-3 % this sample performs worse than typical devices produced with the baseline process that are typically above 6 % (see e.g. reference [10,11]). For the samples treated with the air annealing of 100 °C and 200 °C a significant FF and V OC increase is observed as well as a small increase in short circuit current (J SC ).…”

Passivation of CdS/Cu2ZnSnS4 Interface from Surface Treatments of Kesterite‐Based Thin‐Film Solar Cells

“…The bulk composition of the precursors measured by X‐ray fluorescence showed composition ratios of Cu/Sn = 1.89 and Zn/(Cu + Sn) = 0.36, typical for this type of solar cells. After deposition, the precursors were sulfurized for 13 min at 585 °C in a pyrolytic carbon‐coated graphite box containing 250 mg of elemental sulfur, as described further in the study by Larsen et al [ 12 ] After sulfurization, the samples were exposed to different surface treatments, as shown in Table 1 , and described in the following paragraphs. For the first set of samples (F), the direct deposition of the buffer layer took place after a short air exposure (<10 min).…”

Calculation of point defect concentration in Cu2ZnSnS4: Insights into the high-temperature equilibrium and quenching

“…18 Experimental structural analysis of the CZTS shows that the substitution of Cu atoms by Zn atoms significantly prevails in both Zn-rich/Cu-poor (A-type) and Cu-poor/Sn-poor (Btype) materials. 12,13 [44][45][46] is rather related to formation of the secondary phases than due to changes in concentration of the native point defects.…”