A Nontoxic and Low‐Cost Solution‐Processed CZTS Absorber Layer for Solar Photovoltaic Applications: Solvent Effects on the Physical Properties

Abstract

Nowadays, the compound Cu 2 ZnSnS 4 (CZTS) semiconductor shows great ability to be considered as an alternative of the CIGS thin film in solar cells. Despite the excellent environment stability and the higher power coefficient efficiency which is around 22.6% represented by CIGS [1] and the new updated recorded efficiency (23.4%) according to the NREL site showed by CIGS solar cell, [2] its potentials are limited by scarcity of indium (In) and gallium (Ga), the relatively high cost of In, and also the toxicity issue represented by this element. To solve these issues, it is necessary to search nontoxic and element-abundant light absorber materials. CZTS quaternary semiconductor is constituted by Earth-abundant (Cu:50, Zn:75, Sn:2.2, S:260 ppm) and environmentally benign elements and also the CZTS properties match well with CIGS ones. In fact, the kesterite-based chalcogenide CZTS has a direct bandgap E g ¼ 1.5 eV, which promotes the bandgap of the absorbent layer for high efficiency, an absorption coefficient greater than 10 4 cm À1 , and is characterized by p-type conductivity, which make them ideal among all the secondgeneration thin-film solar cells. [3,4] Moreover, the simulation calculation proves that the CZTS solar cell has theoretical limit power conversion efficiency over 30%. [5,6] Meanwhile, the experimental efficiency of CZTS kept increasing from 6.7% in 2008 [7] to 11.1% in 2012. [8] Wang et al. achieved the highest photovoltaic conversion efficiency of CZTS(Se), which is 12.6%, by exploiting the hydrazine-based approach. [9] In addition to that, reasonable efficiency and great quality were observed for CZTS thin films deposited by many methods, such as sputtering deposition, [10] pulsed laser deposition, [11] and thermal evaporation. [12] However, these techniques require high temperature, high vacuum, and sophisticated tools that may have toxic chemical elements, which have an impact on the fabrication cost of CZTS thin films of a solar cell. To balance the cost and the solar efficiency, several studies tried to elaborate CZTS thin films with low cost and easily synthetic routes. [13][14][15] Among these methods, sol-gel spin coating represents many advantages such as being nonvacuum, facile, low cost, environmentally friendly, and also suitable for large-scale fabrication. It is well known that the properties of thin films prepared with spin coating depend on various parameters. Several articles reported the deposition of CZTS with spin coating and investigated the effect of layer thickness, [15,16] speed of coating, [17] preheating environment, [18] and annealing temperature, [19] on the structural, optical, and electrical properties of CZTS thin films. One of the most important parts of the elaboration of CZTS thin films with spin coating is solution preparation. However, in this step, the type of solvent used has a great impact on the reaction involved, by influencing its nucleation and growth process, which varied the composition and structure of CZTS thin films and controlled the optical and e...