Phase evolution during annealing of low-temperature co-evaporated precursors for CZTSe solar cell absorbers

Abstract

Systematic investigations into the phase evolution during reactive annealing of copper–zinc–tin–selenide (CZTSe) precursors for the fabrication of kesterite solar cell absorber layers have been paramount in understanding and suppressing the formation of secondary phases that deteriorate device performance. In this study, the phase evolution during annealing of low-temperature co-evaporated CZTSe precursors is investigated. A detailed analysis of films selenized at different temperatures is used to reveal the possible reaction pathway of CZTSe formation. Utilizing a combination of x-ray diffraction, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and energy-dispersive x-ray spectroscopy, it is shown that CZTSe formation starts by Cu out-diffusion to the surface and Cu–Se phase formation at a temperature of 350 °C. An intimate mixing of binaries and ternaries during low-temperature selenization is observed. On the contrary, only binaries are observed at high-temperature selenization. This suggests that the CZTSe formation pathway involves reaction schemes where (i) a competition between binary and ternary phases dominates at low-temperature and (ii) binary reactions dominate the process at high temperatures. However, the number of binary phases decreases with increasing selenization temperature until they become undetectable by XRD and Raman spectroscopy at a temperature of 540 °C (selenization time 10 min). Utilizing the presented selenization conditions, prototype solar cells with an efficiency of up to 7.5%, an open-circuit voltage of 407 mV, and a fill factor of 59%, could be demonstrated. The temperature-dependent current density–voltage characteristics indicate that the performance of the prototype devices is limited by bulk Schottky–Read–Hall recombination.

Facile Approach for Metallic Precursor Engineering for Efficient Kesterite Thin-Film Solar Cells

“…In addition, the Cu–Sn alloy in the metallic precursor significantly prevents the loss of Sn and the diffusion of Cu, suppressing void formation. More importantly, the Cu–Sn alloys could either react with Se to form liquid Cu–Sn–Se phase or decompose into the liquid Cu–Se or/and Sn–Se phases to promote the elemental interdiffusion to hinder the formation of bilayer microstructure. , …”

“…Nevertheless, this PCE falls short compared to its competitors, specifically Cu­(In,Ga)­Se 2 (24.0%) and CdTe (22.7%) . The main reason for the inferior PCE in thin-film solar cells (TFSCs) based on CZTSSe is attributed to the elevated densities of defects and defect clusters, along with suboptimal microstructures featuring large grains on the top and small grains with voids at the bottom. , These flaws serve as nonradiative recombination centers, imposing a substantial restriction on the minority carrier lifetime. − Unfortunately, the thermodynamic characteristics of CZTSSe, stemming from its quaternary compositional nature and a highly restricted narrow single-phase region, accelerate the complex formation pathway of kesterite. − This phenomenon occurs irreversibly during synthesis at high temperatures under a chalcogenide vapor atmosphere, giving rise to a bilayered microstructure and the formation of abundant defect clusters of [2Cu Zn + Sn Zn ]. , Lately, various effective strategies have emerged to tackle challenges encountered in the synthesis of single-phase CZTSSe with dense microstructures. − A pivotal emphasis is placed on facilitating a direct transformation of the precursor into the kesterite phase, thereby circumventing the formation of secondary phases as well as defects. − As an example, Gong et al successfully illustrated that incorporating a fully oxidized Sn source (i.e., Sn 4+ ) in the solution precursor can directly initiate the transformation into the CZTSSe phase without intermediate phases during the synthesis process. This approach can bypass the formation reaction of secondary phases, consequently diminishing the presence of deep-level defects.…”

Influence of Stacking Order on the Structural and Optical Properties of Cu2ZnSnS4 Absorber Layer Prepared from DC-Sputtered Oxygenated Precursors

“…Operated in Bragg-Brentano geometry, samples were scanned in the range of 10 ≤ 2θ ≤ 80. The crystalline phases were analyzed by comparing the measured XRD patterns with the cards provided by ICDD as described by Mwakyusa et al (2021).…”

“…As a result, the CZTS formation proceeds via competition pathways between binaries and ternaries. In the CZTS and its sister materials based on CZTSe thin films, it has been experimentally proven that at low temperatures ternary phase (Cu2SnS(e)3) reacts with ZnS(e) to form the CZTS(e) phase while at high temperatures via reaction of binary phases (Jung et al 2017, Mwakyusa et al 2021.…”

“…For pure sulfide kesterite (CZTS)-based solar cells, the efficiency (11.4 ± 0.3%) is even lower compared to that of the CZTSSe-based solar cells (Green et al 2023). Several studies have shown that the synthesis of kesterite absorber mixed with secondary phases, which act as recombination centres that lower the open-circuit voltage (VOC), is among the major problems narrowing the efficiency gap of kesterite-based solar cells (Mwakyusa et al 2019, Islam et al 2021, Mwakyusa et al 2021. Therefore, dedicates the need to understand and establish a controlled synthesis process for making a kesterite absorber in a way that can limit the formation of secondary phases.…”