Vol.:(0123456789) 1 3 Applied Physics A ( 2020) 126:398 https://doi.org/10.1007/s00339-020-03591-6 Crystallite size and intrinsic strain contribution in band gap energy redshift of ultrasonic‑sprayed kesterite CZTS nanostructured thin flms M. Khammar 1  · F. Ynineb 2  · S. Guitouni 3  · Y. Bouznit 4  · N. Attaf 3 Received: 25 February 2020 / Accepted: 28 April 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract The efect of crystallite size and intrinsic strain on the band gap shift of ultrasonically sprayed Cu 2 ZnSnS 4 flms has been discussed. The flms have been deposited over glass substrates under various temperatures, namely 280, 320, 360, and 400 °C. X-ray difraction revealed the presence of kesterite single phase with preferential orientation along (112) plan. The band gap energy decreases from 1.84 to 1.51 eV with increasing substrate temperature, where an increase in crystallite size from 5.92 to 21 nm was observed as well as relaxation of intrinsic strain from 6.82 × 10 −3 lines −2  m −4 to 2.11 × 10 −3 lines −2  m −4 . A concurrency behavior between crystallite size and intrinsic strain efect on the band gap shift has been noticed. The intrinsic strain efect is dominant in higher substrate temperature and that of crystallite size in low temperatures owing to the pres- ence of a strong quantum confnement. Keywords CZTS · Band gap · Redshift · Intrinsic strain · Crystallite size 1 Introduction Cu 2 ZnSnS 4 (CZTS) semiconductor has emerged as a poten- tial candidate for the next-generation solar cells. With a stable kesterite structure with a space group of I-4 [1], this material exhibits p-type conductivity with excellent optical absorption of about 10 4  cm −1 and direct optical band gap energy of 1.4 eV, which make it a suitable candidate for developing low-cost and environmentally friendly nontoxic thin flm solar cells (TFSCs) [2–4]. Since CZTS has a nar- row compositional stability window of stoichiometric sin- gle phase, most deposition methods require post treatment procedure [5–7]. The deposition of one step stoichiometric single CZTS phase is really difcult. Up to now, CZTS nanostructures have been grown by numerous techniques such as sputtering method [8, 9], co- evaporation [10], pulsed laser deposition [11], photo-chemi- cal deposition [12], solgel [13], spray pyrolysis [14]. Among these methods, ultrasonic spray pyrolysis has the advantages of high productivity, short processing cycle time and deposi- tion of nanostructures. Moreover, ultrasonic spray pyrolysis process is easy control of composition and substrate temper- ature, cost-efective, and environmentally friendly approach [15, 16]. The optical band gap of nanostructured semiconductor materials depends on the deposition parameters and struc- tural quality. It has been found to depend upon the size of nanocrystals and defects in nanostructured flm [17]. The frst is attributed to the quantum confnement efect [18, 19] and the second to the presence of intrinsic strain defects [20]. In CZTS nanostructures single phase, non-stoichio- metric structures can cause strain, which can modify the electrical and optical properties. A detailed study on the correlation between the crystallite size, intrinsic strain and optical band gaps of nanostructured CZTS flms would be interesting and can provide useful informations related to optical band gap evolution, which is very important to better understanding and enhancing the CZTS flm quality. * Y. Bouznit bouznit80@gmail.com 1 Development Center of Advanced Technologies (CDTA), Research Unit in Optics and Photonics, Sétif 1 University, 19000 El Bez, Sétif, Algeria 2 Research Center on Semiconductors Technology for Energetic, CRTSE - 02, Bd. Dr. Frantz FANON, 7 Merveilles, Box 140, 16038 Algiers, Algeria 3 Material Sciences and Applications Research Unit, Physics Department, Constantine 1 University, Constantine, Algeria 4 Faculty of Science, Department of Chemistry, M’sila University, M’sila, Algeria