Insights into the Formation Pathways of Cu₂ZnSnSe₄ Using Rapid Thermal Processes

Abstract

Recent advances in Cu2ZnSn(S,Se)4 (CZTSe) thin film photovoltaics open the possibility for the future industrialization of this technology. Nevertheless, major progresses in CZTSe have been achieved using conventional thermal processing annealing routes (CTP), which rely on timeconsuming processes with tubular furnaces, incompatible with the requisites of fast methodologies for the Industry. Changing from conventional to rapid thermal processes (RTP) using halogen lamps as heating method is not at all obvious, since the system becomes kinetically controlled, and the CZTSe formation mechanisms as well as crystallization pathways can drastically change. In this work we present the transfer of our kesterite production baseline (Cu2ZnSnSe4:Ge) from a conventional thermal process using a tubular furnace, towards a rapid thermal process using an adapted system, by comparing them and analyzing the differences between both processes in terms of formation mechanisms as well as photovoltaic absorber properties. For this purpose, the rapid annealing process is stopped at different steps, analyzing the compositional, structural and morphological properties of the CZTSe absorber at these different stages. Using a combination of XRF, SEM, Raman spectroscopy and XRD characterization techniques it is demonstrated that in contrast to CTP routes, when RTP is used, kesterite is being formed in large amounts in the very early stages. This suggests a fast formation of CZTSe promoted by the higher Se vapor pressure that can be quickly achieved with this methodology. The formation of kesterite seems to proceed via two competitive reactions (binaries vs ternary compound). Additionally, the fast reaction observed in the system avoids the possible Sn-loss in an efficient way. Through the optimization of this RTP treatment a device with 8.3% efficiency has been obtained (the total time of the thermal process is 12 min), comparable with the efficiencies obtained so far with CTP routes. Finally, the consequences of all these changes for the future interpretation of the formation reaction mechanisms of kesterites are discussed.

Point defects, compositional fluctuations, and secondary phases in non-stoichiometric kesterites

“…The assessment of the defects by means of Raman spectroscopy resulted in experimental evidence of the effect of the Cu-substitutional defects on the optoelectronic properties, especially on open circuit voltage [116]. Along this line, interesting results have also been obtained using Raman non-band gap resonant conditions with 325 nm excitation for the assessment of CZTS thin films [67,106]. Under this specific excitation wavelength the authors showed a clear modification of the relative intensity of different Raman peaks with crystallization time.…”

“…The possibilities of Raman spectroscopy were then used for a more complex understanding of the properties of kesterites and formation mechanisms. For instance, Raman spectroscopy allowed revealing the intermediate formation steps and the optimal formation temperatures of kesterite compounds [83,85,[101][102][103][104][105][106]. Here the authors showed how the kesterites are formed by starting from different precursors [101,102,104], or the influence of Ge on the formation steps [85], or how the deposition temperature conditions or thermal treatment could improve the quality of the kesterite phase [83, 102, 106] (e.g.…”

“…Under this specific excitation wavelength the authors showed a clear modification of the relative intensity of different Raman peaks with crystallization time. This has been attributed to changes in the V Cu and Zn Sn point defect concentrations in the absorber surface (<20 nm), which have a strong impact on the final optoelectronics properties of solar cell devices [106]. On the other hand, the correlation of Raman spectra changes with the optoelectronic properties also yielded an understanding of the effect of small Cu cations substitution by Mg in CZTSe-based solar cells [92].…”

Progress and Perspectives of Thin Film Kesterite Photovoltaic Technology: A Critical Review

“…The first species always formed in this system, irrespectively of the subsequent pathway (Equation or 3), is ZnX due to the high stability of these compounds. After formation of the binary or ternary compounds, ZnX is introduced in the structure forming the kesterite . According to this process, one can consider that from the very beginning of the reaction, the kesterite phase is Zn‐poor which in return conditions the formation of associated point defects, as will be discussed later.…”

Effect of Al2O3 Dot Patterning on CZTSSe Solar Cell Characteristics

“…The origin of the barrier was estimated as follows: 1) Cu + Zn disordered kesterite has the lowest formation energy, causing additional problems such as energy bandgap fluctuation and a large amount of defect formation [ 11 , 12 , 13 ]. 2) The highly stable ZnS(e) phase [ 14 , 15 ], which always participates in the reaction pathway [ 16 ], makes it difficult to form a CZTSSe layer with a uniform composition. SnZn, which is a killing detect, is well-formed at conditions of Zn-poor kesterite [ 17 , 18 , 19 ].…”