Effect of Nanocrystal Size and Carbon on Grain Growth during Annealing of Copper Zinc Tin Sulfide Nanocrystal Coatings

Abstract

Polycrystalline films were prepared by annealing coatings cast from colloidal dispersions of Cu 2 ZnSnS 4 (CZTS) nanocrystals in sulfur vapor. This nanocrystal dispersion-based route is a promising potential low-cost approach for production of low-cost thin-film solar cells. We studied the effects of nanocrystal size, sulfur pressure, and carbon concentration on the microstructure development and grain growth during annealing. Coatings prepared from dispersions of CZTS nanocrystals with an average diameter of either 5 or 35 nm were annealed for 10−60 min at 600 °C in 50 or 500 Torr of sulfur. The CZTS nanocrystal size influenced both the rate and mechanism of grain growth. When coatings composed of 5 nm nanocrystals are annealed, abnormal grain growth forms micrometer-scale CZTS grains on the surface of the coating. In contrast, when CZTS coatings composed of 35 nm nanocrystals are annealed, grains grow uniformly via normal grain growth. Grain growth rates increased with sulfur pressure regardless of the nanocrystal size. The presence of carbon, originating from ligands used to stabilize nanocrystal dispersions, enhances abnormal grain growth, but too much carbon eventually inhibits all grain growth. On the basis of these observations, we propose a mechanism for microstructure development during annealing of CZTS nanocrystal coatings in sulfur. While much research effort has been expended on the reduction of carbon from nanocrystal coatings prior to sulfidation or selenization by means of ligand exchange or preannealing treatments in the belief that reduced carbon concentration aids CZTS microstructure development and solar cell efficiencies, this work indicates that carbon plays a more complex and significant role in CZTS grain growth than previously assumed: carbon may be beneficial or even required for rapid grain growth during sulfidation.

Role of carbon quantum dot for enhanced performance of photo-absorption in Cu2CoSnS4 superstrate solar cell device

“…Therefore, the CZTS reveals abnormal grain growth at high carbon concentrations (>35 at. %) (Some grains grow faster than others) [53]. Based on this, the presence CQD could limit the grain growth of CCTS and leads to a rod-shape.…”

“…Therefore, CZTS reveals abnormal grain growth at high carbon concentrations (435 at%) (some grains grow faster than others). 53 Based on this, the presence of CQD could limit the grain growth of CCTS and lead to a rod-like shape. The root mean square roughness value of CCTS and CCTS:CQD thin films was measured as 4.8 nm and 2.1 nm.…”

Anisotropic Grain Growth in (111) Nanotwinned Cu Films by DC Electrodeposition

“…In the last few decades, there has been a dramatic proliferation of research related to the mechanism of crystal growth, since it affects the properties of crystal structure in the thermal annealing process [7][8][9][10][11][12][13][14][15][16][17]. A lot of research has been done in this field to observe large grain growth [18][19][20][21][22][23][24][25][26][27][28]. The extra-large grain growth in polycrystalline Cu is called abnormal grain growth.…”

Coordination engineering of Cu-Zn-Sn-S aqueous precursor for efficient kesterite solar cells

“…42 This may arise from residual anions such as chloride ions and long-chain stabilizers, which have a significant impact on CZTSSe crystallization. [43][44][45][46] In addition, these aqueous precursor solutions usually suffer from low stability since the formed metal-thiourea complexes and the dispersed colloids easily precipitate due to the lack of strong solute-solvent interactions. 47 Therefore, more judicious engineering of the CZTSSe aqueous precursors is desired to increase the current device performance.…”