R. Sharma a, Himanshu a, S.L. Patel a, M.D. Kannan b, M.S. Dhaka
a Department of Physics, Mohanlal Sukhadia University, Udaipur, Rajasthan 313001 India
b Department of Physics, PSG College of Technology, Coimbatore 641004 India
## Abstract
The Silicon based single junction and perovskite solar cells have demonstrated high efficiency but Silicon based cells suffer from the drawback of higher cost and perovskite solar cells are not stable and degradable in environment. The thin film based CIGS and CdTe cells have achieved efficiency of more than 22% but, particularly, the CdTe thin film solar cells require a suitable back contact which can be obtained by doping of Zn element in CdTe. The grain growth in CdTe-based films is attained by chloride treatment and therefore, in order to seek applicability of chloride treatment on CdZnTe (CZT) thin films, the present communication reports an evolution of different annealing conditions (in air, CdCl2 and MgCl2 environments) on physical properties to electron beam evaporated CdZnTe thin films. The XRD diffractograms revealed to the dominance of cubic phased peak corresponding to (111) plane and augmentation in grain size with treatment temperature. The optical spectra indicated variation in absorbance and transmittance with temperature of treatments and Tauc's plots demonstrated band gap of all the films in 1.41–1.55 eV range. Current-voltage characteristics divulged Ohmic character of films where conductivity and resistivity are eventually modified with annealing conditions. The FESEM micrographs of pristine films represented bouncy grains with some vacant spaces whereas chloride treated films showed larger grains. The EDS patterns confirmed the deposition of CdZnTe thin films and treatments concerned. The AFM maps demonstrated deep valley-like and hill-like configurations of processed films. The obtained results signify that the films annealed at 300 °C aver their suitability for absorber layer roles in development of efficient solar cells with single junction and tandem architectures.
## Introduction
The power conversion efficiency to the single junction solar cell devices is limited to maximum of ∼33% by the Shockley-Queisser limit based upon the principle of detailed balance while the tandem architecture exceeds this limit where higher band gap sub cell is used as top block and lower band gap sub cell as bottom block to harvest more incident solar irradiance [1,2]. The optical energy band gap of CdTe could be tailored as required for the top block in the tandem architecture, by alloying CdTe with Zn element or using a combination of CdTe and ZnTe materials and the obtained CdZnTe material can provide higher power conversion efficiency when it is applied as absorber layer to top sub cell in a tandem device since, it has tunable energy band gap and suitable properties for solar cell applications [3]. The post deposition thermal treatment in air and chloride atmospheres typically enhances the grain size to CdTe-based constituent layers and consequently improves the device performance [4], [5], [6]. Thin films made up of semiconductors have laid the foundation of various contemporary technologies due to their enviable applications in the field of basic science and marketable products [7]. Compound semiconductors (binary, ternary, quaternary) are used to modify the properties of the materials in accordance with particular functions for the development of devices concerned [7]. Nowadays, researchers are also using novel compound semiconductors such as environmental friendly antimony triselenide (Sb2Se3) and kesterite-based Cu2ZnSnS4 (CZTS) for the fabrication of thin film solar cell devices while the power conversion efficiency (PCE) is limited to ∼9% for the Sb2Se3 based solar cells and ∼11% for the CZTS based solar cells [8], [9], [10]. Cadmium Telluride (CdTe) is one of the promising II-VI compound semiconductors having an optical energy band gap of 1.45 eV and till date, CdTe based solar cells concerned attained higher PCE of 22.1% [10], [11], [12], [13]. Although the CdTe is potential absorber layer material yet by incorporating ZnTe, suitable interface could be established to overcome the problem of difference between CdTe valence band level and metal work function concerned and hence,
Cadmium Zinc Telluride (CdZnTe) is considered [14,15]. The CdZnTe is competent II-VI compound semiconductor not only used in thin film solar cells [13] but in various optoelectronic devices such as radiation detectors, lasers, photodiodes too [16], [17], [18], [19], [20]. Owing to the high absorption coefficient, high mobility, tunable band gap (1.40–2.26 eV) of CdZnTe and higher atomic number of its constituent elements (viz. Cd, Zn and Te), it can be used as an absorber layer to the top block in two-junction tandem solar cells [14].
The physical properties of thin films are strongly affected by the defects present in the films. Apart from the native defects like Cd vacancies and Te interstitials [11] which are already present in the CdZnTe thin films, other defects such as internal stress, lattice mismatch, crystal defects, dislocation density, grain boundaries etc. could also be existed during the growth of CdZnTe thin films which can reduce the films quality and deprive physical properties concerned [21]. Therefore, in order to invert various kinds of defects and to perk up the device performance, different pre- and post-deposition treatments are used e.g. doping, annealing in air atmosphere and vacuum, chloride treatment etc. [22], [23], [24]. Out of these, chloride activation treatment or traditionally CdCl2 treatment is considered as one of the critical steps in modifying the properties and performance of the device concerned [25], [26]. This treatment has gained tremendous attention and is widely used since, it promotes grain growth, recrystallization, passivation of grain boundaries, carrier lifetime and lowers the stacking faults leading to the enhancement in the device performance [27,28]. During the CdCl2 activation treatment on CdTe films, Cl– ions may be released which could lead to formation of Cl-Te complexes those might be existed in liquid or gas phases. These phases induce enhancement in mobility of Cd and Te which may lead to recrystallization that is endorsed by relaxation of volume strain energy. The higher degree of recrystallization favors larger modification in films properties and better performance of device concerned. The recrystallization is also associated with the dissociation energy of chloride compound concerned where higher recrystallization and performance is achieved with chloride compounds those have lower bond energy [29]. The conventional CdCl2 treatment is usually undertaken by two ways viz. wet and dry chloride procedures. In wet chloride treatment, samples are treated by certain concentration of CdCl2 solution by drop cast method and then annealed at suitable temperatures. In case of dry chloride treatment, a thin layer of CdCl2 is usually deposited on already deposited CdTe-based layer followed by annealing at suitable temperatures [12,30]. Although chloride treatment using CdCl2 compound provides some wonderful benefits yet it is inhibited with a drawback of noxious nature i.e. use of CdCl2 compound is harmful to both the operator's health and environment too. Therefore, in order to avoid the use of toxic Cadmium, several other environmentally acceptable and Cd-free chloride compounds or Chlorine salts like MgCl2, NH4Cl, ZnCl2, and NaCl may also be used to perform chloride treatments [5,30] with comparable solar cell performance. Another replacement of these Cd-based chloride activation compounds (e.g. CdCl2) is the halogen carrier gasses (HCl, Cl2 etc.) having the capability of releasing Chlorine or Chlorine based radicals corresponding to the suitable annealing temperature [5,30].
The CdZnTe thin films can be deposited using a variety of techniques such as resistive heating based thermal evaporation, electron beam vacuum evaporation, sputtering, chemical bath deposition, pulsed laser deposition etc. [4,[31], [32], [33]]. Among these techniques, the electron beam evaporation method is highly beneficial as this technique ensures higher materials utilization, controllable deposition rate, impurity free films growth etc. and therefore in the present work, the CdZnTe thin films of thickness 800 nm are deposited on the glass and ITO substrates using electron beam vacuum evaporation technique. The as-deposited (pristine) films are subjected to different annealing conditions viz. air, CdCl2 and MgCl2 treatments at temperature of 200 °C, 300 °C and 400 °C followed by optimization of physical properties of these films for solar cell applications as an absorber layer in single and tandem device architectures.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/effect-of-different-annealing-conditions-on-cdznte-thin-films-for-absorber-layer-applications.html