Status review and the future prospects of CZTS based solar cell – A novel approach on the device structure and material modeling for CZTS based photovoltaic device

Renewable and Sustainable Energy Reviews 94 (2018) 317–329

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Status review and the future prospects of CZTS based solar cell – A novel approach on the device structure and material modeling for CZTS based photovoltaic device

T

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M. Ravindirana, , C. Praveenkumarb a b

Electronics & Communication Engineering Department, Vinayaka Mission's Research Foundation, Aarupadai Veedu Institute of Technology, Chennai 603104, India School of Engineering, Saveetha University, Chennai 602105, India

A R T I C LE I N FO

A B S T R A C T

Keywords: CZTS Heterojunction Perovskite CVD and Sputtering

Cu2ZnSnS4 (CZTS) based devices has become increasingly popular due to the better efficiency with different architectures for various types of solar cells. The present work reviews and analyzes the different CZTS based solar cells and its synthesis methods. The possible future prospects in the performance improvement of the CZTS based solar cell is analyzed in the present work with the approach based on the novel device architecture and material property. The novel device architecture using CZTS has electron blocking and hole blocking band offset, which can significantly improve the efficiency of the solar cell. Similarly, the first principle calculation on various composites of the CZTS based compound has revealed some interesting property, which has shown a new route for the CZTS based composite.

1. Introduction 1.1. About CZTS CZTS is a kind of semiconducting compound which looks like a greenish black crystal. It is a new version of thin film based on next generation photovoltaic due to fact that it is good solar absorbing compound with better absorption coefficient. CZTS composite has easily available elements which significantly enables in easy fabrication of the composite. Non-poisonous, environment pleasant, low price and better performance are the merits of the CZTS based thin film photovoltaic when compared to other photovoltaic devices. CZTS has very good optical and electronic property which is more suitable for photovoltaic application. Fig. 1 represents the crystal structure of the CZTS composite [1]. Fig. 1 shows the presence of the elements at various coordinates of the crystal lattice, where the Cu is present in the all corners and center with Zn on the top and bottom of the crystal lattice and Sn is placed at the top, bottom and other two sides. S atoms is placed at the center near the Cu atoms. Presence of the atoms in the lattice crystal resembles the ratio of each element. Elements such as Cu, Zn, Sn, and sulfur on the earth crust is 68 ppm, 79 ppm, 2.2 ppm, and 420 ppm respectively, that's much higher when compared to In, Cd, Te of 0.16 ppm, 0.15 ppm, and 0.001 ppm respectively. Hence CZTS based composite is consider to be more efficient

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Corresponding author. E-mail address: [email protected] (M. Ravindiran).

https://doi.org/10.1016/j.rser.2018.06.008 Received 7 December 2017; Received in revised form 5 March 2018; Accepted 4 June 2018

Available online 14 June 2018 1364-0321/ © 2018 Elsevier Ltd. All rights reserved.

when compared to the other materials [2]. From crystallographic rule, CZTS quaternary semiconducting compound has two different kind of structure, such as kesterite structure and stannite structure. These two shapes are identical but differ from its copper, zinc arrangement from the structure in atomic level [3]. Both CZTS and CIGS are very efficient for making thin film based photovoltaic application. But compared to CZTS, CIGS compounds are having some demerits like this compounds availability will be rare and also high cost. Especially the In and Ga materials costs more expensive and less available. CZTS compound also have very good optical properties with better absorption coefficient. Above discussed factors with the abundance availability makes CZTS better when compared to CIGS [4]. The molar mass range of the CZTS compound is 439.471 g/mol. The density of the CZTS compound is 4.59 g/cm3 [5]. Melting point temperature range of the CZTS compound is 1260 k, which makes the composite more suitable for very high temperature synthesis [6]. Band gap energy range of the CZTS compound is around 1.5 eV which is ideal for thin film based photovoltaic applications. There are many approaches used for deposition of thin film like sulfurization, laser ablation, sputtering and evaporation etc… The CZTS based thin film also act as an absorber layer of photovoltaic device due to its better absorbent property when compared to other thin films [7,8]. In the year 1996, the CZTS based thin film efficiency was calculated as 0.66%. Later the efficiency of the CZTS was improved to 5.74% through multisource evaporation technique in the year 2007.

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structures were utilized in CZTS based photovoltaic devices. CZTS based photovoltaic have some precise characteristics more suitable for different kind of photovoltaic devices. In recent years there has been a lot of research on CZTS based thin film photovoltaic. CZTS composites are more economical and easily available in the market [12]. The CZTS has the band gap of 1.4–1.5 eV. CZTS also has a very high absorption coefficient which will very well suit with multiple layer-based structure when employed as the absorption layer of thin film based photovoltaic. CZTS band gap is very close to the better band gap required by the semiconducting photovoltaic of 1.50 eV. Compared to already present commercialized photovoltaic devices such as CdTe and CIGS, CZTS outsmarts because of its easy availability of the elements in the earth crust. It also holds some other interesting properties such as non-poisonous and eco-friendly. Due to these characteristics, CZTS based thin film photovoltaic are one of the better candidate materials for solar absorbing layer. IT is anticipated to turn out to the ideal absorption layer material of future based thin film photovoltaic. [13–15] The production cost of CZTS based thin film photovoltaic technology is very low when compared with other types of photovoltaic devices such as CdTe and CIGS respectively. Hence CZTS based photovoltaic devices are very crucial to be applied in a larger scale. Its high performance, availability, low price, easy to fabricate and proper stability properties are anticipated to be the next generation thin film based photovoltaic [16]. The CZTS is in the form of kesterite structure with promising absorber layer candidature for solar cells which can be more suitable for low price thin film based photovoltaic. Due to the appropriate direct band gap of around 1.5 eV it also have an larger absorption coefficient [17]. The CZTS based thin film photovoltaic device has two different layers, the primary layer is made of organic compound and secondary layer is made of inorganic compounds. This kind of the structure has maximum theoretical efficiency of around 42%. The CZTS based thin film absorber fabricated through different physical and chemical based

Fig. 1. CZTS Crystal structure.

In the year 2008, CZTS efficiency was further enhanced to 6.7% through sulfurization method [9]. In the year 2010, the CZTS based thin film photovoltaic power conversion efficiency was over 9.6% with the fabrication of the device based on ink-based technique [10]. In the year 2012, a team of researchers from IBM was able to reach the power conversion efficiency around 11% using simple ink-based fabrication method which was considered to be the best conversation efficiency for that time. This conversation rate has justified that CZTS based thin film photovoltaic is better solar absorbent when compared to other compounds based thin film photovoltaic [11]. Most commonly used CZTS based solar cell is shown in Fig. 2. CZTS was deposited over the Mo soda lime glass with Cd window layer having ZnO and AZnO layer on the top improving the overall efficiency of the device. 2. CZTS for photovoltaics CZTS has very good optoelectronic property, which makes it more appropriate for photovoltaic application. Different photovoltaic device

Fig. 2. - Schematic representation and characteristics of CZTS thin film based solar cell device a) device structure b) conductivity c) SEM images and d) absorbance. source - http://www.mibsolar.mater.unimib.it/?page_id= 1434) 318

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nanostructures and nanodevices, enabling the proper operation of the device. Many works have been reported in the analysis of the band structure of solar cells to identify the alignment and defects. Crystalline defects in Cu on the interfaces of buffer and absorber layers plays a critical role in the performance of the kesterite solar cells. Cu defects in the CZTS solar cells is analyzed to for the p type conductivity and Ag doping and its defects shows the n type weak conductivity. From the theoretical investigation, it is understood that the Cu and Ag alloying into CZTS will have a better light absorbance resulting in better efficiency with kesterite solar cells. The band alignment of the device is grouped into two types such type I band alignment and type II band alignment. Type I band alignment could be set when the conduction band minimum of absorber layer is less than the window layer. Similarly, it is going be a type II alignment when the conduction band minimum of the absorber layer more than the window layer [25]. Electrical transport properties of hetero junction based photovoltaic highly depends upon the band offsets of the hetero interface. Charge trapping in between the bands and interstatial sites of the device is a major concern in device fabrication. Similarly, the sub band photons can also be utilized effectively with the absorption of the particular wavelength photons. Study on the escape of carriers in InAs/GaAs quantum dot is carried out in order to analyze the effective use the photon energies. Photocurrent measurements under sub bandgap illumination were carried out in the study. The study was carried out to identify the trapping mechanisms in the intermediate bands and to improve the device performance [26]. The haight et al. [27] has measured the valence band offset for Δ Ev of 0.54 eV on the premise of the usage of ultraviolet photoelectron spectroscopy (UPS). The band offsets for CZTS/Cds heterojunction photovoltaic was measured through first-principles calculation (FPC) approach. And then Chen et al. [28] has measured the valence band offset for Δ Ev of 1.01 eV on the premise of the first principles calculation(FPC) approach. Similar to the above approach, the band alignment calculation of the ΔEv of 1.01 eV is the conduction band minimum (CBM) of absorber layer CZTS which is greater than the window layer (CdS). Consequently, the band alignment was type II with ΔEv of 0.54 eV and the conduction band minimum (CBM) of absorber layer CZTS is lesser than the window layer (CdS) resulting in the band alignment as type I [29]. The first principle calculation methods employ the basic idea in deciding the materials composition and the device structure physics involved theoretically. Most of the experimental works fail due to improper analysis of the proposed experiments and its reliability when the device is fabricated. Hence its always preferred to do the theoretical calculation on the proposed device before fabricating the device. The understanding of the proposed device and the elemental composition experimental analysis, fabrication of the device is done by many methods. The following chapter discusses about the various synthesis methods involved in fabricating CZTS based devices.

approaches, includes RF magnetron sputtering, hybrid sputtering, thermal evaporation, pulsar laser deposition, spray pyrolysis approach, sol-gel spin coating approach. The concern of these approaches is to reduce the cost of the photovoltaic and to enhance the efficiency of the solar cell [18,19]. 3. Literature review CZTS has been the choice of material in fabrication of photovoltaic devices due to its very good electronic and optoelectronic properties. CZTS devices had been synthesized and fabricated using different methods. In order to understand the materials property and the device physics, theoretical investigations have been performed using first principle calculations. 3.1. First principle calculation for CZTS Many works have been reported on CIGS based thin film solar cell. Since the efficiency of the kesterite based devices turned into very low, different approaches have been adopted to enhance the overall performance of the kesterite based photovoltaic by different compositions of the crystal lattice to be used as a absorber layer. Hence CZTS based kesterite structure were looked into an alternative for CIGS devices [20]. CZTS compound is typically a p-type semiconductor with a band gap of 1.4–1.5 eV, which is nearer to be optimum band gap of a photovoltaic device. It additionally has very high absorption coefficient ( > 104 cm−1) in the visible region. Hence CZTS based semiconducting compound were used as an absorber layer in heterojunction based photovoltaic [21]. Cd doping were carried out into CZTS to improve the efficiency of the device. To make the investigation even before the synthesis, first principle calculations were performed with the Cd doping into Cu2ZnSnS4 and Cu2ZnSnSe4. Doping of the Cd into CZTS exhibits a n type conduction with the electronic charge structure showing a neutral charge behavior [22]. CdS is a n-type semiconductor compound with a band gap of 2.4 eV. CdS is used as a window layer in a solar cell with CZTS layer as an absorber layer [23]. Working on the energy band difference between the bands makes a significant progress in the device applications. Similar work is carried out with the Cu2CdxZn1_xSnS4 alloy with varying doping concentration of Cd. The films were studied on various properties such as band gap and hall effect. Band gap of the films varied from 1.55 to 1.09 eV as the doping concentration of the Cd is varied with 0–1%. Hall effect results suggests the decrease in the hole mobility with the increasing Cd concentrations. Band alignment of the Cu2ZnSnS4/Cu2CdSnS4 interface has type I alignment with decreasing band gap over increased Cd doping. Hence the structure can be used for multijunction tandem solar cell with improved efficiency [24]. Band gap engineering is the most significant area of research on the Table 1 CZTS Evolution - synthesis methods and its recorded efficiency. Method

Precursor

Efficiency (%)

Year

Reference

Sputtering Electrochemical deposition NP-based method Screen-printing CBD-ion exchange Electron Beam deposition Sol gel-based method Pulsed laser deposition Spray pyrolysis Ink based fabrication Vacuum Sputtering Rapid Thermal Annealing Sputtering

Cu, SnS, ZnS Cu, Zn, Sn Copper(II) acetylacetonate, zinc acetate, tin(II) chloride dehydrate, elemental sulfur CZTS microparticle tin chloride dehydrate, zinc acetate dehydrate, aqueous Cu2+ Cu, Zn and Sn Copper (II) acetate monohydrate zinc (II) acetate dehydrate tin (II) chloride dehydrate in-house fabricated CZTS pellet not available Not available Cu2, Zn, Sn, S4 Cu2, Zn, Sn, S4 Cu2, Zn, Sn, S4

6.77 3.14 0.23 0.49 0.16 5.43 2.23 3.14 1.15 11 6.8 2.56 3.74

2008 2009 2009 2010 2011 2011 2011 2011 2011 2012 2013 2014 2018

[31] [41] [35] [36] [37] [52] [32] [33] [34] [11] [56] [82] [53]

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cumbersome which will have a low yield. Vacuum thermal evaporation is a method used to deposit single quaternary CZTS semiconducting material with an annealing of 300 degrees for 40 min under N2 atmosphere after deposition [46]. In the year 2003, NaS was delivered into the chamber to improve the vacuum background with the annealing temperature in the stainless-steel chamber. Device fabricated with the above technique had an efficiency of 5.43% [47]. In the year 1998, ZSW Company of Germany measured the CZTS based thin film photovoltaic with efficiency of 2.29% through co-evaporation technique [48]. ZSW works on various types of solar cells such as thin film solar cells, flexible solar cells and printed solar cells. The ZSW agency of Germany holds the record in CIGS based photovoltaic with an efficiency of 20.3% [49]. ZSW has achieved different techniques for various layers deposition in fabricating the CIGS solar cell. In the device structure, the CIGS layer functions as the absorber layer of the solar cell. Weber et al. [50] found that the temperature of substrate might be at a variety of 300–600 °C. Bottleneck within the annealing temperature is that, once the substrate temperature reaches above 400 °C the Sn loss will be higher. Hence it had been a big challenge to maintain the process. In the year 2010, CZTS based thin film photovoltaic with an efficiency of 6.79% through co-evaporation was attained. The fabricated device with the above approach has shown a new direction in fabrication of CZTS based cell with better efficiency [51]. In the year 2011, another new approach was adopted by employing the Cu, Zn, and Sn evaporation source with Knudsen type and Veeco S source box in metallic tantalum at the annealing temperature of 540 °C to −570 °C for five minutes. Film made with 600 nm thickness resulted with an efficiency of 8.4%, which is currently the better CZTS based photovoltaic efficiency without Se [23].

3.2. Synthesis methods for CZTS Various synthesis methods adopted and its efficiency chart is shown in the Table 1 [30]. Efficiency of the fabricated CZTS device varies with the synthesis method adopted. The efficiency of the device depends on factors such as deposition rate, layer thickness and synthesis method adopted. The Table 1 illustrates the various methods and respective efficiency achieved. Synthesis method of the CZTS depends on the substrate used and its limitations in using it on the particular synthesis methods. Each method uses its unique deposition of the elements by the order of one after the another or the deposition of all the elements at the same time. 3.2.1. Electrochemical deposition method First ever electrodeposition method for the fabrication of the CZTS based solar was done by Scragg et al. [38] by dissolving CuCl SnCl and ZnCl separately in NaOH and sorbitol and deposited in an order of Cu, Sn and Zn. Electrochemical deposition is a type of coating method used to remit the cations in the aqueous solution, organic solution in the cathode by supplying potential difference through external circuit power. Almost more than 40 years electrochemical deposition process of semiconducting materials was carried out [39]. Bath University established the method for laminating and vulcanizing electro-deposition of Cu/Sn/Zn to acquire the CZTS based thin film photovoltaic in 2008 with a converting efficiency of 0.81% [40]. Further photovoltaic power conversion efficiency of 3.14% was achieved through one-step co-deposited Cu/Zn/Sn alloy and also annealing of 600 °C for 2 h in the carrier gas having sulfur powder. Above method had hiccup to detect the constant sulfur source in the electrodeposition of CZTS based semiconducting compound [41]. In the following years the efficiency of 3.24% was attained with the annealing for 2 h at 575 °C in an atmosphere of N2 carrier gas having Sulfur powder with 10% H2 [42]. Ennaoui in Germany synthesized the CZTS based thin film photovoltaic the efficiency of 3.39% through onestep co-deposition technique of Cu/Zn/Sn in the solution having 3 mM Cu2+, 3 mM Zn2+ and 30 mM Sn2+. Few complexing agents were present with the annealing of 2 h at 550 °C in an atmosphere of Ar gasoline having 5% H2S to synthesize CZTS based thin films with Cupoor etching through washing the CuxS in the KCN solution with 3.49% density. There after light treatment was done for 10 min, which has resulted in enhanced photovoltaic performance of 3.59% [43]. Later commercially plating solution was deposited with Cu/Zn/Sn after which laminated with the annealing for half-hour in N2 at 350 °C. Again Cu/Zn/Sn was achieved with the annealing of 12 min at 585 °C in the N2 environment having sulfur powder. Finally, CdS and ZnO were deposited to get the CZTS based thin film photovoltaic device. Fabricated device had an efficiency of 7.29% [44]. Washio et al. [45] adopted a unique technique to make CZTS based thin film photovoltaic by using oxide precursors with an open atmosphere chemical vapor deposition (CVD) technique. CZTS based thin film photovoltaic are fabricated by using soda lime glass (SLG) and Molybdenum(Mo) coated substrates. Then, the sulfurization of oxide precursor became done to make the thin films (Cu–Zn–Sn–O) in N2 +H2S (4.9%) atmosphere at 520–560 °C for three hours. The device fabricated with the above technique has better efficiency of 6.03%.

3.2.3. Electron beam evaporation method Electron beam evaporation is a type of physical vapor deposition method which bombard of the target anode by electron beam in a high vacuum environment and evaporate the material from anode. The material vaporizes and gets deposited in the surface forming a layer of thin coating. The method is employed for device fabrication and thin film coatings in order to use it for various applications. Layer thickness depends on factors such as the particles size, vacuum pressure and the deposition time takes. In the year 1996, A research group from National College of Technology applied the electron beam evaporation and curing method to fabricate the photovoltaic device of ZnO:Al/CdS/ CZTS/Mo/SLG structure with the open circuit voltage of 400 mV, short circuit current of 6.0 mA/cm2, fill factor of 0.277, and power conversion efficiency of 0.67%. In order to improve the conversion efficiency, Hironori and Katagiri both are employed Cu, Sn (or SnS2), and ZnS as vapor deposition material by the way of the usage of electron beam evaporation technique, changing the order of deposition from an evaporation to multiple cycles evaporation, using soda lime glass (SLG) and ZnO: Al instead of ZnO as a window layer. The proposed method had an eventual expanded efficiency of 5.43% [52]. Electron beam evaporation technique overcomes defects of the resistance heating evaporation, in particular it is more suitable for the manufacturing of high-melting point material and high purity thin film material. At present, preparation of the CZTS based thin film with electron beam evaporation technique is mostly within the research labs. Electron beam deposition method has significant advantages when compared to other methods such as better surface morphology, good phase matching, and better optical performance when fabricated as a thin film [17]. Similar to the CZTS fabrication, electron beam evaporation method is used for many other device fabrications. Being the family of physical vapor deposition there has been many other methods such as sputtering deposition. Following section is introduces about the sputtering deposition of CZTS films.

3.2.2. Vacuum deposition method Vacuum deposition method is similar to physical deposition technique in which the raw materials are loaded into the vacuum chamber and heated to high temperature. This high temperature heating makes the atoms or molecules to escape from the surface. The escaped molecules vaporizes and penetrates into the surface of the substrate. The low temperature of substrate enables the substances to condenses resulting into a solid thin film [17]. This method is very simple with better quality of deposition for fabricating the CZTS based thin films. Maintaining the ratio of the elemental chemistry of the deposited materials is

3.2.4. Magnetron sputtering method Magnetron sputtering is another method in physical vapor 320

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beam to bombard the target and CZTS based thin film turned into deposited in a vacuum chamber, observed through annealing in N2 +H2S gas environment. The thin film-based cells on this approach have open circuit voltage of 585 mV, short-circuit current density of 6.74 mA/cm2 and the fill factor of 0.51, conversion performance of 2.02%, and band gap of 1.52 eV. The study found that once the laser pulse frequency become within the range of 2–10 Hz, the grain length will growth of pulse frequency. Due to the high energy density of the laser and the effect of better crystallization. Uniform, single, and dense crystal grains were obtained from the prepared samples. Compared with other techniques, this method is easy and it can deposit the film with ideal stoichiometric ratio via controlling the composition of ceramic target and the oxygen pressure. It is more suitable for depositing the metal oxide thin films and multi component hetero epitaxial films. Bombardment of high power laser beam makes atoms or molecules to sputtered through the target with high energy resulting in deposition of high quality thin films at low temperature [17]. Synthesis of the thin films involves sophisticated machines with vacuum chambers making the overall process more cumbersome. This can be avoided in the powder synthesis by wet chemical methods such as sol gel process.

deposition which utilizes the magnetic field to deposit the films on the substrate. Magnetron sputtering enables the electrons to crash with Ar atom within a specific electric field subject to ionizing the available argon ions to make the electrons deposited to the substrate. In the year 2011, Japan Nagano National College of Technology had studied the technique of sputtering Cu–Sn–Zn metallic precursor and then vulcanizing it to manufacture the CZTS. The manufactured device has measured the CZTS based photovoltaic with an efficiency of 3.69% [53]. In 2007, Nagaoka University of Technology in Japan, after having successfully manufacturing the CZTS based photovoltaic with an efficiency of 5.43% through electron beam evaporation approach. Then they got successfully produced the efficiency of 5.74% by using RF sputtering technique [54]. In the year 2010, Salome and Femandes et al. [55] fabricated CZTS based thin film photovoltaic with a power conversion efficiency of 0.68% by means of utilizing the sputtering Zn/Sn/Cu with the annealing of S powder for 10 min at 525 °C beneath the N2 carrier gas. Katagiri has fabricated the CZTS based thin film photovoltaic with the best photoelectric power conversion efficiency of 6.8% by using vacuum sputtering technique. Muhunthan et al. [56] introduced a new method by performing co-sputtering of the metal targets and sulfurization in ambient H2S. Metal targets were used to assist in controlling the composition of the film. Compared with the traditional vacuum deposition, sputter coating has many merits which incorporates, manipulation of the stoichiometry of elements, fabrication of films with better density, better use of raw materials, unfastened preference of the deposition site, reduction in the contamination of vacuum chamber, excessive uniformity degree of film and suitability for the practice of larger scale CZTS based thin film photovoltaic. This technique is widely used in mass production of coatings. Various simple approaches were used for thin film deposition apart from sputtering mechanisms, similar approach of technique used in a simple and cost-effective way is spray pyrolysis.

3.2.7. Sol-gel method Sol-gel technique is very simple and cost-effective method of synthesizing the powder samples with needed stichometry. This method makes the hydrolysable metal compound to react with water in certain solvents to form as Sol by hydrolysis and polycondensation. Then the Sol forms as liquid film on substrate through dipping or spin-coating technique. After gelatinization, the substrate may be transformed into amorphous form films through heat treatment. Many works have been reported on the sol gel process in synthesizing composites and ceramics for various applications. In 2007, Tanaka et al. [57] made the dimethyl alcohol as solvent and the ethanolamine as stabilizer to make the sol gelatin with cupric acetate, zinc acetate, and tin chloride and coated it at the Molybdenum(Mo) based glass. To get better thickness, spin coating was done for consecutive 5 times and burned at 300 °C for 5 min in the air and annealed it at 500 °C for 1 h in an atmosphere of N2 gas containing 5% H2S. The above work has been reported with a CZTS based thin film with better element and the crystallinity. In 2009, sol gel method was used and then spin-coat and drying were carried out with the 0.35 M sol for 3 times. Then spin-coated and dried the 1.76 M sol for 5 instances. In the stated work, CZTS based film with uniform surface area and the efficiency of 1.01% was fabricated [59]. In 2011, power conversion efficiency of 2.03% was achieved with the aid of optimizing film components [60]. Addition of MgF2 antireflection layer has resulted in high energy conversion efficiency of 10.1% [61]. Compared with other techniques, sol gel approach has many specific advantages such as, simple process equipment without vacuum conditions, Inexpensive system with large vicinity thin films and substrate with different shapes and materials. Synthesis of the CZTS composite has its unique significances with reference to the type of method adopted. Similarly, the substrate in which the multiple layers deposition is also important to decide the functionality of the deposited film. Different types of substrates are being used for CZTS deposition, which is analyzed in detail on the following sections.

3.2.5. Spray pyrolysis method Spray pyrolysis is a simple and easy method which utilizes the film deposition technique at lower cost. This technique works on by heating the surface of the substrate about 600 °C and then spraying one or more metallic salt solutions onto the substrate surface. The higher temperature gradient will lead pyrolysis of the spray coating ensuring a thin film deposition on substrate surface. The satisfactory and overall performance of thin film manufactured through spray pyrolysis proportional to the substrate temperature. If the substrate temperature is the too high, it will be difficult for the film to be adsorbing on the substrate. when the substrate temperature is very low, the crystallization of film may be deteriorated. CZTS based thin film made with the spray pyrolysis have better optical property through controlling the substrate temperature inside the range of 500–650 °C in pyrolysis [17]. Kamoun made a reaction in CuCl2, ZnCl2, SnCl2 and vulcanized them in SC (NH2)2 solution by using spray pyrolysis technique. The substances reacted for 1 h at the substrate temperature of 340 °C and have been annealed for 120 min at 550 °C. Eventually, the CZTS based thin films with a band gap of 1.5 eV were fabricated [57]. Spray pyrolysis is a simple and smooth technique to perform with simple experimental process without vacuum and gas protection devices. Hence, it's a without problems low priced method with less fee concerned to obtained thin-film with better performance.

4. CZTS on different substrates CZTS based photovoltaic devices were fabricated on different substrates depending on the availability and research ideas. Efficiency of the fabricated device will depend on various factors, which includes the substrates used. Adhesion of the deposited layers and its conductive nature is very important for good device performance. Band alignment if the devices have a significance in the performance due to its heterointerfaces. The band alignment of the layers and its substrates are very important in defining the device characteristics. Charge trapping

3.2.6. Pulsed laser deposition method Pulsed laser deposition approach is a physical vacuum deposition process that produce high energy pulsed laser focus at the target surface to obtain high pressure and high temperature plasma. The plasma emission expands in the directional nearby location and deposits the substrate to obtain a thin film. Moholkar et al. [61] measured the Cu2S, ZnS, and SnS2 powder by grinding technique, and the powder is made to CZTS target via the solid-state reaction; they used an excimer laser 321

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barrier at the Mo/CIGS interface, resistive losses have occurred which affected the device efficiency. Tong et al. [81] investigated the vacuum thermal annealing approach based on Molybdenum (Mo) back contacts to enhance the crystalline properties of Molybdenum (Mo) films and CuInS2 (CIS) absorber which leads to enhance the photovoltaic efficiency. CZTS based thin film photovoltaic was fabricated using Molybdenum (Mo) as a back-contact electrode. Rapid thermal annealing technique based on Molybdenum (Mo) back contact electrode has enhanced the crystalline properties of evaporated CZTS absorber with better efficiency [82]. Mostly CZTS based thin film photovoltaic were made using metal foils substrate like stainless steel, Molybdenum (Mo) and aluminum (Al) as a back contacts electrode [83]. Primary choice is Molybdenum (Mo) foil substrate as a back contact because of its well-matched coefficient of linear expansion is 5.2 × 10−6 K−1, which is a better expansion range compare to other metal foil substrate [84]. The purity of Molybdenum (Mo) foil will be high so no need for barrier layer at the photovoltaic device structure and high purity level Molybdenum (Mo) foil to enhance the electrical properties of the absorbers of the photovoltaic [85]. The Molybdenum (Mo) foil is also a flexible metallic substrate with low cost, less-weight, durable and resistant to high temperature fabrication processes. Hence the total cost of fabrication is less to construct a photovoltaic module with flexible metallic substrates. Finally, Molybdenum (Mo) foil is used to enhance the performance of absorber with improved photovoltaic performance [83].

and the interstate bands should be analyzed well before fabricating the device. Large recombination at the interface of the device will not allow the solar cell to perform the expected operations. Band off set study on CdS/Cu2ZnSnS4 was carried out with the synchrotron radiation photoemission spectroscopy. The study on the interface has revealed that the band alignment is type II, which will have large recombination at the interface resulting in non-suitable nature for solar cell application [62]. Better band alignment of the device will have improved performance. Heterojunction made of p-Cu2CdSnS4/n-ZnS has better photosensitivity at the UV wave lengths which makes the device more suitable for photodetector application. X-ray photoelectron spectroscopy measurements have revealed that the device is type I band aligned and more suitable for UV photodetector applications [63]. Investigation on the experimental and theoretical analysis were carried out for Ag2ZnSnSe4 (AZTSe)/CdS heterojunction. Results reveal that the CdS had higher conduction band minimum than the AZTS interface which results in an ideal band alignment for photovoltaic application [64]. Band offset analysis using x-ray photoelectron spectroscopy is carried out for the heterojunction CZTS/CdS/ZnO using first principle calculations method. The bands forms as type II with the band gap of 0.13 eV and 1 eV which makes less barrier to electrons allowing the recombination better [65]. Conduction band offset study on the CZTS and CdS is calculated with the 0.2 eV allowing the CdS to be used as buffer layer in solar cells [66]. Device properties of the solar cell is analyzed with the identification of the defects involved in the layers and the type of bands [67]. To identify the device property with the band alignment of the layers and substrates of the solar cells its performance on various substrates has been studied in the following sections.

4.2. CZTS on Si substrate The first-generation photovoltaic is likewise known as conventional, traditional or wafer-based cells are crafted from crystalline silicon. Commercially to be had and fabricated photovoltaic devices includes substances inclusive of polysilicon and mono crystalline silicon. The maximum commonplace bulk fabric for photovoltaic is crystalline silicon (c-Si), which is likewise known as solar grade silicon. Bulk silicon is separated into the multiple categories in step with crystallinity and crystal length within the ensuring ingot, ribbon or wafer. These cells are functionally primarily based on the concept of p-n junction photovoltaics [86]. Monocrystalline silicon photovoltaics are more efficient and more expensive than most other kind of cells. The corners of the cells look clipped, like an octagon due to the method worried in cutting the wafer fabric from the cylindrical ingots, that are typically grown through the Czochralski process technique. Solar panels using mono-Si cells display a one of a kind sample of small white diamonds [87]. Epitaxial wafers of crystalline silicon may be grown on a monocrystalline silicon "seed" wafer by means of chemical vapor deposition (CVD). Then the wafers may be indifferent as self-supporting wafers of some standard thickness (e.g., 250 µm) and manipulated through hand. Then the wafers are immediately substituted for wafer cells cut from monocrystalline silicon ingots [88]. Photovoltaic made with this "kerfless" have efficiencies drawing close the one of wafer-cut cells. Instead, reduced cost of fabrication was accomplished when the chemical vapor deposition (CVD) can be performed at atmospheric pressure in a high-throughput inline manner. The surface of epitaxial wafers is textured to improved light absorption [89]. Polycrystalline silicon cells manufactured from the forged rectangular ingots and huge blocks of molten silicon are carefully cooled and solidified. They consist of small crystals giving the fabric its standard metal flake effect. Polysilicon cells are the maximum common substances utilized in photovoltaics because of its monetary availability. These cells are better than monocrystalline silicon. Ribbon silicon is a sort of polycrystalline silicon which is formed through the drawing flat thin films from molten silicon which results in a polycrystalline shape. These cells are inexpensive to make than multi-Si because of extraordinary discount in silicon waste, as this method does not require sawing from ingots [90]. Silicon thin-film cells were

4.1. Fabrication of CZTS solar cell on Molybdenum(Mo) foil Molybdenum (Mo) is the tremendous preference for the use of back contact electrode of CIGS based photovoltaic. Because of its relative stability is better at processing temperature after which its low contact resistance to CIGS based photovoltaic. Molybdenum (Mo) is a silver like appearance and it has the 6th highest melting point of all elements. The resistivity value of Molybdenum (Mo) is nearly 5 × 10−5 Ωcm [68–70]. The Molybdenum (Mo) is deposited on soda lime glass (SLG) through vacuum techniques such as electron gun approach and sputtering technique with an inexpensively [70,71]. CuInS2 (CIS) based photovoltaic device also use Molybdenum (Mo) as back contact electrode [72]. CIGS/Mo interface having ohmic contact conduct with MoSe2. Raud and Nicolet work on the Mo/Se, Mo/In, and Mo/Cu diffusion pairs has shown Se to react with Mo to form MoSe2 with lesser quantity at annealing temperature of nearly 600 °C [73]. Assmann et al. [74] has shown the presence of MoSe2 at the Mo/ CIGS interface and its better mechanical strength at the interface which has the significance of higher adhesion. Shimizu et al. [75] has suggested that the Molybdenum (Mo) thickness will be various between the 0.2–0.07 µm from the properties of CIGS based photovoltaic, whereas the optimum thickness of the Molybdenum (Mo) is 0.2 µm. Introducing water vapor during CIGS growth increases the overall photovoltaic properties [74,75]. Kim et al. [76] has attempted and suggested that Mo/Mo bilayer with Na doped combination deposited on Alumina substrate. It will be enhanced the photovoltaic performance. Guillen et al. [77] has suggested the properties of Molybdenum (Mo) based thin films evaporated onto huge area of 30 × 30 cm2 on soda lime glass (SLG) substrate at different depositions. During the formation of films, sodium (Na) ion diffuse from the soda lime glass (SLG) substrate through the Mo based back contact into the absorber layer. The diffusion of sodium (Na) into absorber film depends upon the deposition conditions of the Mo based back contact [78]. Jaegaermann et al. [79] and Wada et al. [80] investigated the schottky barrier in Molybdenum (Mo) films when deposited on the CIGS films. Due to the schottky

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Fig. 3. Basic structure of CZTS based solar cell with characteristics results, a) device structure, b) absorbance, c) conductivity and d) CV characteristics. (source http://theconversation.com/getting-more-energy-from-the-sun-how-to-make-better-solar-cells-54090).

window layer. A unique architecture for liquid junction-based photovoltaics made from ZnO/Al: ZnO/ZnS or ZnS/CZTS core/shell vertically aligned nanorods array have been reported. Over fluorine-doped tin oxide (FTO) coated glass, vertically aligned Al-doped ZnO nanorods had been grown over ZnO seed layer and it turned into followed with the resource of the surface transformation of the ZnO based nanorods to ZnS or ZnSe via solubility constant (Ksp) distinction prompted anion trade in a S2- or Se2-solution to produce ZnO/ZnS and ZnO/ZnSe core–shell(CS)structures. One after the other, CZTS nanoparticles are synthesized from the excessive temperature arrested precipitation and subsequently used for sensitization of ZnO/ZnS CS-VANR nanostructures. Cu2S and polysulfide had been employed as the counter-electrode and electrolyte respectively for the fabrication of liquid junction photovoltaics. FE-SEM, HRTEM, X-ray diffraction (XRD) and Raman spectroscopy strategies have been hired for microstructural, morphological and compositional characterization of the extraordinary factor materials. The J–V measurements of the photovoltaics correspond to a numerous fold's better power conversion performance than similar device with thin film based multilayer planar configuration related to same amount of materials. The aligned core/shell nanorods configuration offers an increase in the interfacial location through numerous folds, shorter direction way for charge transport and efficient photon absorption [96]. Basically, used device configuration of SLG/Mo/CZTS/ CdS/Al: ZnO/Al/Ni for CZTS thin film-based photovoltaics to have look at the solar cell performance. The device consists of Molybdenum(Mo) coated soda lime glass (SLG), an electrical contact, a layer of CZTS as a light absorber layer in contact with n type CdS to form p–n junction, a thin layer of Al: ZnO as a window layer on top of the CdS layer, and finally an Al/Ni layer as an electrical contact as shown in the Fig. 5 [97].

particularly deposited through chemical vapor deposition (CVD) from silane gas and hydrogen gas. Deposition parameters determines the form of silicon inclusive of amorphous, polycrystalline, nanocrystalline and microcrystalline silicon [91]. Amorphous silicon is the most well-advanced thin film technology until now utilized in fabrication of devices. An amorphous based silicon (a-Si) photovoltaics is made upon non-crystalline or microcrystalline silicon. Amorphous based silicon has a greater band gap (1.7 eV) than crystalline silicon (c-Si) (1.1 eV), because of it absorbs the visible part of the solar spectrum more strongly than the high-energy density infrared portion of the spectrum. The manufacturing of amorphous silicon (a-Si) thin film photovoltaics uses glass as a substrate and deposits completely thin layer of silicon through deposition process. Procrystalline silicon with a low quantity fraction of nanocrystalline silicon is most beneficial for high open circuit voltage [92]. CZTS thin film changed into deposited on the n-type silicon substrate through spin-coating to fabricate a Mo/p-CZTS/n-Si/Al heterostructure based photovoltaic. The p-CZTS/ n-Si heterostructure based photovoltaic as shown in the Fig. 5 and suggest a conversion efficiency of 1.13% with Voc = 520 mV, Jsc = 3.28 mA/cm2, and fill-factor (FF) = 66% [93]. 4.3. CZTS on ZnO substrate ZnO changed into one of the metal oxides used in photovoltaics which exhibits interesting properties such as high bulk electron mobility, wide band gap and transparent. ZnO based nanostructures were manufactured with huge range of the synthesis routes. Efficiencies of the ZnO based photovoltaics are better while as compared to TiO2 based solar cells. Advanced performance must be analyzed and mentioned with the perspective of future applications of the ZnO in dye-sensitized photovoltaics [94]. CZTS devices are likely to CIGS based devices. The architecture is usually SLG/Mo/CZTS/CdS/i-ZnO/ZnO: Al, as presented in Fig. 5. Commonly CZTS based ZnO substrate is encompass Molybdenum(Mo) coated soda lime glass (SLG) as the electrical contact, a thin CZTS based light absorber layer which is in contact with an n-type CdS layer to create a p-n junction, and the thin i-ZnO/Al: ZnO layer on top of the CdS layer acting as a window layer and electrical contact [95]. In 1997, Friedlmeier et al. [22] produce thin film photovoltaics using a CZTS layer as the light absorber in contact with an n-CdS/ZnO

4.4. CZTS as absorber layer CZTS is taken into consideration for best absorption layer material in subsequent generation thin film-based photovoltaics because of the considerable aspects elements inside earth crust being non-poisonous, environmentally pleasant, low price and high overall performance photovoltaic [1]. CZTS based on lighting absorber as shown in Fig. 6. The synthesis of copper, zinc, tin sulfide (CZTS) with kesterite shape 323

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Fig. 4. Schematic representation of CZTS/Si based solar cell with characteristics results a) device structure, b) SEM image, c) conductivity and d) resistancetemperature characteristics. (Source - https://www.intechopen.com/books/solar-cells-research-and-application-perspectives/cu2znsns4-thin-film-solar-cells-present-status-and-future-prospects).

into used to manufacture the CZTS based absorber layers on the glass substrates. The films have been hastily thermally annealed at 500 °C in a nitrogen atmosphere for 20 min to enhance their crystallinity. The formation of the kesterite shape changed into confirmed usage of X-ray diffraction measurements (XRD). The stepped forward crystallinity of the CZTS changed into determined with (112) oriented phase. The band gap of deposited and annealed films turned into discovered to be 1.97 eV and 1.55 eV respectively [100]. Ultrasonic Spray Pyrolysis is the deposition approach, wherein the solution is atomizing ultrasonically, thereby giving a fine mist having a narrow length distribution which may be used for the uniform coatings on substrates. An Ultrasonic Spray Pyrolysis system turned into evolved and CZTS based absorber layers had been efficaciously grown with this approach on soda lime glass (SLG) substrates usage of aqueous solutions. Substrate temperatures ranging from 523 K to 723 K had been used to deposit the CZTS based layers and these films have been characterized using scanning electron microscope (SEM), X-ray diffraction measurements (XRD). It changed into located that the film crystallized in the kesterite

and composition CZTS has attracted high quality attention. The huge range of interest is because of its viable use as absorber layer in photovoltaics as a result of numerous benefits. CZTS has a high absorption coefficient, of the order of 104 cm–1, with a band gap between 1.45 eV and 1.6 eV which facilitates in high conversion efficiencies [21,98]. The photovoltaics manufactured with some of these methods have distinctive performance because the composition of CZTS thin film become quite special and besides many secondary phases which includes ZnS have been present. In order to remit those secondary phases, thermal treatment at exclusive temperatures both in inert or reactive atmosphere is performed. Thermal treatment was also carried out to govern the crystallinity of the deposit, because a number of above cited manufacture techniques produce either amorphous or nanocrystalline CZTS based film [99] (Figs. 7 and 8). A new synthetic approach became employed to obtain a CZTS based totally absorbing layer the usage of a quaternary compound target which has comparable properties to that of copper indium gallium selenide (CIGS). A traditional R-F magnetron sputtering device turned

Fig. 5. Schematic representation and SEM image of CZTS/ZnO based solar cell, a) device characteristics and b) SEM image of the device. (source -) 324

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Fig. 6. CZTS solar cell on Molybdenum(Mo) foil, a) device structure, b) SEM image of device and c) absorbance. (source - https://www.hindawi.com/journals/ijp/ 2011/801292/Fig. 3/).

4.5. CZTS on perovskite based solar cell Perovskite based photovoltaic is a kind of photovoltaic that's in the form of perovskite based structured. First invented perovskite compound is oxide perovskite (CaTiO3) in the year of 1839. This oxide perovskite was available in nature. And then perovskite-based materials mostly used as a light absorbing layer of the photovoltaic device. Because it is the grater solar absorbent compound. Compared to fabrication of perovskite based photovoltaic device is easier than traditional silicon based photovoltaic. Since silicon-based device fabrication involves more precision and cost due to the defects during the fabrication process, alternative materials and device structures were analyzed. Perovskite based photovoltaic device fabrication is an alternative approach which may not need a high precision sophisticated equipment's and also very economical to fabricate. [106,107]. Quantum dots were used with the perovskite layers to improve the efficiency of the solar cells. CsPbCl3:Mn quantum dots were synthesized for boosting and improving the stability of the perovskite solar cells. CsPbCl3:Mn layer is used to downshift the energy which converts the wasted energy in the UV region to visible light of approximately 590 nm. Hence the overall efficiency of the cell is improved to 3.34% [108]. In general lead halide perovskite-based compounds are combination of organic and inorganic substances. It has a general formula of ABX3, where A, B denoted as cations and X denoted as halide anions. The lead halide based perovskite compounds mostly act as a light absorbent of the photovoltaic device. The frequently using perovskite compound is methylammonium lead halide have a chemical formula of CH3NH3PbX3. it will be like a ABX3 Structure, Where X is a halide anion (iodine, bromine). The lead halide is fabricated from the easily available sources in nature like carbon (C), nitrogen (N), lead (Pb), halides. These perovskite substances having specific optical properties, electrical conductivity properties. The band gap of lead halide perovskite structure is between the range of 1.55–2.3 eV. The band gap of lead halide perovskite structure depends upon how much amount of halide contains in the compounds. If vary the halide content in the compounds vice versa vary the band gap range into between 1.55 eV and 2.3 eV [109,110].

Fig. 7. – Crystal lattice of the methylammonium lead halide (CH3NH3PbX3) perovskite structure. (source - https://en.wikipedia.org/wiki/Perovskite_solar_cell).

shape and best crystallites have been received at 613 K. It was found that the grain length step by step elevated with temperature having optical band gap of 1.54 eV [101,102] (Fig. 4). The CZTS based absorbing layers have been grown copper-wealthy, requiring a KCN etch step to dispose the excess of copper sulfide. The compositional ratios as determined through the energy-dispersive X-ray spectroscopy (EDX) after the KCN etch are Cu/(ZnþSn): 1.0 and Zn/Sn: 1.0. A photovoltaic with the performance of 4.1% and an open-circuit voltage of 541 mV became acquired [103]. Copper (Cu)-based quaternary kesterite compounds CZTS, CZTSe, and blended chalcogenide Cu2ZnSn (SxSe1-x)4 (CZTSSe) have emerged as the capability opportunity to current CIGS and CdTe absorbers in thin film based photovoltaic. CZTS (Se) is an appealing preference for thin film based photovoltaic absorber fabric owing to tunable direct band gap of 1.0–1.5 eV with a huge optical absorption coefficient (> 104 cm−1) and p-type conductivity [104]. Among numerous deposition strategies, electrodeposition is one of the promising techniques for making CZTS based absorber films because of its low-fee equipment's, huge scale production and morphology [105]. 325

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Fig. 8. perovskite based solar cell device structure and its characteristics, a) device structure, b) SEM image of device, c) band alignment and d) XRD results. (source https://phys.org/news/2014-09-liquid-inks-solar-cells.html).

treatment on the edge sites. Fabricated cells have shown very good efficiency with better electrocatalytic activity and electrochemical stability [117]. Device performance can be further improved by nanomesh by CVD technique. 3D Nanomesh can improve the conductivity of the cell when fabricated over the perovskite layer [118]. Fig. 9 shows the modified CZTS/perovskite device structure with Au and FTO as the top and bottom layers of the cell. Due to the better optical absorption of the CZTS and perovskite layers, the power conversion efficiency of the device will be better.

Polymer perovskite solar cells are becoming very popular with the ability to be used in flexible substrates. Improving the stability of the polymer perovskite solar cells are very important in terms of its commercial use. Solar cell efficiency is improved from 17% to 19% when the perovskite cells are doped with polymers. These doped polymers act as bridge between the grains which helps in improving the charge transport. The use of the polymers is not limited, hence different polymer doping with perovskite can be tried for improved efficiency of the cell [111]. The perovskite-based substances having different properties, including piezoelectric, conducting, superconducting and insulating properties. And also, conventional perovskite compounds manufactured from high temperature involving method like solid state synthesis technique [112,113]. Halide perovskite-based substances is recently discovered as a desired substance for high performance with low price photovoltaic. The perovskite based photovoltaic was highly mature and then its efficiency enhanced rapidly, from 3.8% in the year 2009–22.1% in the year 2016 [109], [114–116]. Nitrogen doping enhances the conductivity of the device which can further improve the overall efficiency. This can be achieved by doping nitrogen into graphene by N2

5. Proposed structure Proposed device structure with the CZTS as top layer is shown in the Fig. 10. The band structure alignment of the device is shown in Fig. 10(b). It is understood from the band structure that the band offset between the top layer and the middle layers has effective electron blocking alignment which will make the top electrode as cathode allowing maximum number of electron to be collected at the top contact. Similarly, the holes will be collected at the bottom contact of the device allowing maximum number of carriers generated by the incident light

Fig. 9. - Perovskite based CZTS absorbing solar cell, a) device structure, b) conductivity and c) absorbance. (source - https://www.sparrho.com/item/kesteritecu2znsns4-as-a-low-cost-inorganic-hole-transporting-material-for-high-efficiency-perovskite-solar-cells/2903/). 326

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Fig. 10. Proposed device and band structure with CZTS as top layer.

Fig. 11. Proposed device and band structure with CZTS as middle layer.

to be converted into electrical energy resulting in very good power conversion efficiency. Fig. 11 shows the inverted solar cell structure with CZTS layer at the middle of the solar cell structure with the P type ZnO on the top. Band alignment of the proposed device is shown in the Fig. 11(b). The band alignment shows an offset between the top layer and the middle layer resulting in an electron blocking setup. The electrons generated from the incident light make the electrons to move towards the bottom electrode and blocking the electrons to travel towards the top layer. Since the carrier collection is inverted, the top layer is anode and the bottom layer are cathode. Due to the electron blocking layer, there will be an improved carrier collection on the top electrode and the bottom electrode. Hence the power conversion efficiency of the proposed solar cell be much higher when compared to the other CZTS based solar cells.

Table 2 Comparison of various composites for energy application.

6. Proposed composites for energy application

be done with the different compositions of the elements in the composite. Hence with the optimized composite property, a better composite material than CZTS can be identified for energy application.

Sl. no

Composite

Electron density

Absorption doefficient

Optical conductivity

1 2 3 4 5 6 7 8 9

CuZnSnS CuZnSnSe CuZnSnSsS (X)ZnSnS (X)ZnSnSe (X)ZnSnSeS (X)Zn(Y)S (X)Zn(Y)Se (X)Zn(Y)SeS

35 J/m3 35 J/m3 28 J/m3 35 J/m3 37 J/m3 28 J/m3 35 J/m3 38 J/m3 30 J/m3

270000Au 220000Au 270000Au 270000Au 250000Au 280000Au 250000Au 270000Au 270000Au

4 mS 4 mS 4 mS 8 mS 6 mS 8 mS 6 mS 6 mS 6 mS

(X) – Metal, (Y)- Ferromagnetic Metal.

Density Functional Theory (DFT) calculations has been performed with various composites in order to identify better absorber layer for heterojunction solar cell fabrication. Considerable preliminary works has been carried out to identify the absorber layer with better electronic and optoelectronic property. The proposed work will make a new advancement in the solar cell technology with better power conversion efficiency. The Table 2 illustrates the various composites and its respective electron density, absorption coefficient and optical conductivity. Composites made with the X and Y represents the metal and ferromagnetic metal with similar composition of the CZTS. It is understood from the above table that the composites made with (X)ZnSnSe and (X) Zn(Y)Se have improved electron density and absorption coefficient. Further investigation of the proposed composite will make a better composite for energy application. Optimization of the composites can

7. Conclusion Power conversion efficiency of solar cells can be improved with the approaches adopted based on the materials used and the band structure alignment. CZTS has been a promising material of choice over the recent years. This manuscript has extensively analyzed the various CZTS based solar cell structures and synthesis methods. To make an improved device performance with the CZTS based device structure an alternative approach of the device structure is proposed in the present work. To further improve the device performance, an extensive analysis of the composite material is done with the DFT calculations and the composites with better electron density (37 J/m3), absorption coefficient (280000Au) and optical conductivity is analyzed (8 mS). 327

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