Recent Developments on the Properties of Chalcogenide Thin Films

Abstract

Chalcogenide thin films have attracted a great deal of attention for decades because of their unique properties. The recent developments on thin film-based supercapacitor applications were reported. As a result of sustained efforts, the experimental findings revealed remarkable properties with enhanced fabrication methods. The properties of perovskite solar cells were discussed in terms of crystal structure and phase transition, electronic structure, optical properties, and electrical properties. Perovskite solar cell has gained attention due to its high absorption coefficient with a sharp absorption edge, high photoluminescence quantum yield, long charge carrier diffusion lengths, large mobility, high defect tolerance, and low surface recombination velocity. The thin film-based gas sensors are used for equally the identification and quantification of gases, and hence should be both selective and sensitive to a required target gas in a mixture of gases. Metal chalcogenide materials are considered excellent absorber materials in photovoltaic cell applications. These materials exhibited excellent absorption coefficient and suitable band gap value to absorb the maximum number of photons from sun radiation. The photovoltaic parameters were strongly dependent on various experimental conditions.

1. Introduction

Chalcogenide thin films have attracted a great deal of attention for decades because of several reasons such as earth abundancy, environmental friendly [1, 2, 3], excellent structural [4, 5, 6], electrical [7, 8, 9], and optical properties [10, 11, 12]. These materials could be employed in various applications such as solar cells [13, 14, 15], ultraviolet light emitters, laser devices [16], spin functional devices, gas sensors, transparent electronics, corrosion resistant coating [17], microelectronics [18], optics, magnetic and acoustic wave devices. Several deposition methods including chemical vapor deposition [19], physical vapor deposition [20], sputtering [21], SILAR, spray pyrolysis [22], chemical bath deposition [23, 24, 25, 26], vacuum evaporation method [27], and electro deposition method [28, 29] have been used to produce thin films. Generally, these deposition methods could be divided into physical method and chemical techniques. Researchers highlighted that the chemical deposition method has many advantages such as inexpensive, and convenient for large area deposition [30, 31, 32].

Supercapacitor has been considered as one of the potential energy storage systems. The redox electrochemical capacitors and the electrochemical double layer capacitor have been extensively investigated by many researchers. Researcher reported that transition metal oxide, conducting polymers, and metal oxide thin films have been tested in supercapacitors. The perovskite structure consisted of the crystal structure of calcium titanium oxide. This material showed high absorption coefficient with a sharp absorption edge. The organic-inorganic hybrid perovskites based solar cell was made from sandwiching a perovskite absorber layer between the electron transport layer and hole transport layer. The obtained solar cells showed some unique advantages such as low-temperature processes for all sub cells, compatibility with flexible and lightweight applications. The thin film-based sensor could be used to convert physical or chemical quantity into equivalent electrical for measurement. Sensor is critical in improving the reliability and efficiency of manufacturing operations by providing faster and more accurate feedback regarding product quality.

In this book chapter, thin film based solar cell, thin film based supercapacitor and thin film based sensor will be discussed. The properties of the obtained films were reported. Lastly, the advantages and limitations of these materials will be highlighted.

2. Thin film based supercapacitors

Recently electronic devices such as computers, roll-up displays smartwatches, mobile phones and other portable devices abound in the twenty-first century. For greater performance, improved energy storage devices are required to reduce the energy consumption of these smart electronic devices [33]. As a result, devices with long-lasting battery, high power outputs, and quick recharge times are required. As a consequence, it is critical to create innovative energy storage materials and devices. The realities of scarcity of fossil fuels, and environmental damage should all be considered in this endeavor [34]. By modify the surface properties of the electrodes with a long life cycle, the supercapacitor (SC) is such an effective energy-saving technology that is environmentally friendly with quick charging, and high energy density are just a few of the benefits [35]. However, this redeemer (supercapacitor) has issues. Nevertheless, in comparison to lithium batteries, such savior (supercapacitor) has challenges such as poorer energy density, unavailability, and the high cost of ruthenium (IV) oxide (RuO₂) and platinum electrode materials, all of whom have stymied the supercapacitor development. Supercapacitors, which are versatile, compact, ecologically benign, and yet still economical energy storage devices, are in growing market. The flexible supercapacitors, which bridges the gap between batteries and traditional capacitors, is a bright spot in the realm of energy-saving engineering. Flexible-all-solid-state thin film supercapacitor, an innovative novel thing, has gotten a lot of interest as unique energy storage devices because of its friendly construction, compact size, easy handling, and excellent power density with a quick charging-discharging rate. The supercapacitor is called as electrochemical capacitors it has a fast charging and discharging properties, excellent power density and high specific capacitance with compact construction, and inexpensive cost of maintenance. The three primary mechanisms of supercapacitor can be classified (Figure 1), which is depending of the reversible redox reactions and the accumulation of charge. There is electric double layer capacitors (EDLC), pseudo capacitor, and the combination of EDLC and pseudo capacitor called the hybrid supercapacitor [36].

Figure 1.

Classification of supercapacitors.

Thin films are very intriguing in modern research for a variety of applications in ethanol sensor, photocatalytic, thermoelectric and supercapacitor [37, 38, 39, 40]. The supercapacitors can store the electrical energy for all the electronic devices to stabilize the power supply. Generally, to prepare a pseudo capacitive electrode transition metal oxide (TMO) is the most popular approach, however relatively higher electrical resistivity restricts whose use several fields. As a consequence, the focus of researchers is turning to metal chalcogenides, which have a lower electrical resistivity than oxygen due to sulfur’s low electronegativity. The preponderance of these metal chalcogenides, mostly sulfides, are made from inexpensive and abundant transition metals. For example, Dai and co-workers [41] have prepared hierarchically structured Ni₃S₂ and multi-walled carbon nanotube (MWCNT) composites using the hydrothermal methods and the prepared device can have obtained the maximum C_s of 55.8 F g⁻¹, it provides a highest energy density of 19.8 Wh/kg at power density of 789 W/kg. Xiao and co-workers [42] prepared a nickel cobalt sulfide nanoparticle graphene-based sheet (NiCo₂S₄@GR) there is no surfactant through simple one-step solvo thermal method, which results revealed the maximum C_s of 1708 F g⁻¹ at a current density of 1.0 A g⁻¹, while comparing without graphene. Mukkabla and co-workers [43] reported a Poly(3,4-ethylenedioxypyrrole) (PEDOP) Enwrapped bismuth sulfide (Bi₂S₃) nano flowers hybrid flexible SC_S, and composite offered a maximum C_s of 329 F g⁻¹ at 0.4 A g⁻¹. Furthermore, these are usually undergoing redox reactions between the metallic ions valence states. Besides, TMO and transition metal chalcogenides, various metal nitrides have previously been observed has outstanding results as electrodes in supercapacitors and lithium ion batteries with impressive results. Recently, metal nitrates also have superior abilities in electrochemical properties with excellent chemical stability. Metal nitrides have gotten a lot of interest as supercapacitors electrodes since they have a lot of benefits. Metal nitrates have three major advantages. (1) It has a high σ (electrical conductivity) of 55,500 S/cm⁻¹ while compared to the metal oxides as a result shows the excellent power density, (2) compared to the metal oxides and carbon based materials metal nitrates have a higher specific capacitance, which results shows the higher energy density, and (3) high mechanical stability. These characteristics make them extremely promising as high-performance supercapacitor electrodes. Balogun and co-workers [44] have summarized the performance of different metal nitrides like molybdenum nitrides (MoN), nickel nitride, titanium nitride. Among these metal nitrides, molybdenum nitride was considered as the first metal nitride which could be used as supercapacitor electrode materials. However, for supercapacitor applications, researchers mostly considering their materials cost and electrochemical performance. There are many transition metals and metal oxides are considerable for supercapacitor applications such as CuO, NiO, Mn₃O₄, Co₃O₄, Ni or CuCo₂O₄ and Ni or CuFe₂O₄ [45, 46, 47, 48, 49, 50]. Compared to the other metal oxides, the metal ferrite based materials much attracted to the researchers. For example, Fe, Ni or Cu based Fe₂O₄ materials have an excellent performance in the energy storage applications. There are two major methods could be used to prepare the thin films supercapacitors, namely physical technique (physical vapor deposition and sputtering) and chemical method. The successive ionic layer adsorption and reaction (SILAR), spin coater, and chemical bath deposition (CBD) are some examples for chemical deposition method (Figure 2).

Figure 2.

Thin films deposition techniques.

Bandgar and co-workers [51] studied the nature of starting materials on the properties of NiFe₂O₄ thin films for flexible supercapacitors. There are several morphologies could be observed (nanosheet, flower, and feather) through different salts such as nickel(II) chloride hexahydrate (NiCl₂·6H₂O), nickel nitrate [Ni(NO₃)₂·6H₂O], and nickel sulfate hexahydrate (NiSO₄·6H₂O), respectively. The nanosheet based electrode material received the maximum C_s of 1139 Fg⁻¹, nanoflower and feather achieved the good C_s of 677 and 435 F g⁻¹, respectively. Immanuel and co-workers [52] have optimized the Cr doped Mn₃O₄ thin films for high performance supercapacitors using the SILAR method. The experimental results showed that 3 wt % of Cr doped Mn₃O₄ thin films exhibited the maximum C_s of 181 Fg⁻¹ at the current density of 1 Ag⁻¹.

Jesuraj and co-workers [53] studied the pristine and Li doped NiO thin films using the spin coating method. Kin and co-workers [54] prepared the carbon based flexible supercapacitors using the chemical vapor deposition. Yu and co-workers [55] have prepared the cobalt nickel oxide and sulfide heterostructure thin films through electrodeposition method for supercapacitor applications. The obtained findings revealed the maximum energy density of 78.2 Wh·kg⁻¹ at 542.8 W·kg⁻¹ and the high power density of 5440.2 W·kg⁻¹. Recently, Immanuel and co-workers [56] synthesized Mn₃O₄ nanorod thin films via SILAR method. The prepared Mn₃O₄ thin films showed the maximum C_s value of 295 Fg⁻¹ at the scan rate of 2 mVs⁻¹. Vivek and co-workers [57] prepared a reliable electrode material, and results obtained a maximum C_s of 426.40 Fg⁻¹ at a current density of 1 Ag⁻¹. Arulraj and co-workers [58] prepared the cubic shaped Ag₂S using the CBD method on Ni mesh. The prepared Ag₂S used a working electrode, which electrochemical performance showed the highest C_s of 179 C/g at constant charge and discharge current density of 1 A/g.

3. Thin film based perovskite solar cells

Any materials which have the crystal structure of calcium titanium oxide (CaTiO₃), were known as the perovskite structure and the materials have stoichiometry of ABX₃; where “A” is the larger cation, “B” is the smaller cation and “X” is the anion. Each unit cell of ABX₃ crystal comprises of corner sharing BX₆ octahedra, with the “A” moiety cubo-octahedral cavity. In case of organic-inorganic hybrid perovskites (OIHP), halide anions (I⁻, Br⁻, Cl⁻) are found at the “X”-site anion instead of oxygen, while monovalent (CH₃NH₃⁺, CH(NH₂)₂⁺) and bivalent (Pb²⁺, Sn²⁺) cations occupy the “A” and “B” sites, respectively. Halide perovskites were first reported by Moller in 1958 for cesium lead halides [59]. Further, it was also observed that small organic molecules with effective radii less than 260 pm [methylammonium (MA), formamidinium (FA), hydrazinium, hydroxylammonium) can also accommodate inside the PbX₆ octahedrons. The word “hybrid” indicates that the crystal is made specifically by the combination of “organic” and “inorganic” components. The architecture of OIHP-based solar cell is quite simple and prepared by sandwiching a perovskite absorber layer between the electron transport layer (ETL) and hole transport layer (HTL). A standard OIHP based solar cell device has a structure composed of glass/ transparent conductive oxide (TCO)/TiO₂ (ETL)/ mesoporous TiO₂ (mp-TiO₂)/ perovskite (~500 nm)/ HTL/ metal and a quite high efficiency exceeding 20% can be realized without including complicated processing steps. The operation of the perovskite device is sstraight forward; namely, the photo-electrons and holes created by light absorption are collected in the ETL and HTL, respectively, and the electrons flow through the outer circuit and recombine with holes at the HTL/metal interface. The efficiencies of OIHP-based solar cells have increased all the way from 3.8% in 2009 to 25.5% for single-junction solar cells, and 29.15% for the highest publicly disclosed perovskite/silicon (Si) tandem [60].

The properties of perovskite solar cells were discussed in terms of crystal structure and phase transition, electronic structure, optical properties and electrical properties. One of the interesting aspect of the crystal structure of halide perovskite is the structural flexibility of organic cation. Taking MAPbI₃ as an example, the disorder-order transition of MA⁺ cation is believed to trigger the phase transition with the decrease of temperature. At high temperature MAPbI₃ takes a cubic structure (space group: Pm-3 m; Z = 1). Since MA⁺ has a lower symmetry of C_3v, the orientation of MA⁺ ion should be disordered to satisfy the O_h symmetry. As the temperature is lowered, tetragonal and orthorhombic phases are realized by an accompanying ordering of methylammonium ion. Structural transition from cubic to tetragonal phase occurs due to the reorientation of MA⁺ ion, as observed by nuclear magnetic resonance (NMR) studies where lowering the number of disorder states of MA⁺ was observed from 24 in the cubic phase to 8 in tetragonal phase [61]. Below a critical temperature (tetragonal-orthorhombic phase transition), the MA⁺ molecule is frozen (only 1 degree of freedom) and the symmetry of MAPbI₃ become orthorhombic. Similar crystallographic phase transition can be realized with replacing I by Br and Cl [62].

The band structure of MAPbI₃ exhibits a direct bandgap of 1.6 eV at the R point. Calculated band structure suggests conduction band minima (CBM) is dominated by the Pb-p orbital, whereas the valence band maxima (VBM) is constituted by I-p states mixed with a small amount of Pb-s states, which is consistent with the photoemission results [63]. The optical transition of MAPbI₃ relies on a direct bandgap p-p transition, leading to a strong optical absorption coefficient. Strong s-p antibonding enhances dispersion of the upper valence bands [64], which resulted in small effective masses of electrons (m_e*) and holes (m_h*). Further, it is believed that the role of MA⁺ cation is to maintain the overall charge symmetry and as dictated by the crystal structure of the system [65]. However, it was reported that MA⁺ cation has an indirect impact on the shape and orbital composition of the band edges. The molecular orientation of MA⁺ cation can distort the PbI₆ octahedral and affected the cell size and bonding of Pb-I, which modulated the density of states near the band edges [66]. Other halide perovskites also possess similar ways of electronic band structure.

Organic-inorganic hybrid perovskites are direct band gap semiconductor and the direct transition produces large absorption coefficients of the order of 10⁴–10⁵ cm⁻¹. In the case of perovskite thin films, the optical properties of perovskites are dramatically affected by the quality, composition and morphology of the film [67]. Sizes of the halide anions (X = I, Br, Cl) affected the electronic band structure of the system. Large anion (iodine based materials) showed a smaller bandgap and corresponded the absorption edge at 780 nm; whereas substituting iodine with smaller bromine (chlorine) anion shifts the absorption edge to 535 nm (408 nm) for MA⁺ based perovskite system [68]. A systematic blue shift of the PL emission peak is observed with the increase of Br concentration in mixed halide perovskite of the type MAPb(I_1−xBr_x)₃. Further, replacing MA with CH(NH₂)₂ red shifts the absorption spectra by 40 nm, which makes CH(NH₂)₂PbI₃ more suitable for high-performance solar cell applications [69]. Intermediated solid solutions of MASn_1−xPb_xI₃ with x = 0.25 and 0.5 exhibited the smallest band gap of 1.17 eV [70]. Irrespective of bandgap tuning, fundamental understanding of absorption and PL spectra are essential to study the basic photo physical properties of hybrid perovskite. In spite of several optical investigations performed at different temperatures, there have been a lot of ambiguities in the data as well as its interpretations, especially observation of multiple peaks in the photoluminescence (PL) spectrum of organic-inorganic hybrid perovskites. Literature reports excitonic emission, tetragonal inclusion in orthorhombic phase, order-disorder transition, surface-bulk effects are responsible for these multiple PL emissions [71].

Space charge limited current (SCLC) is one of the effective approaches to measure mobility, diffusion length and trap density of hybrid perovskites. Due to the advancement in fabrication techniques, the diffusion length of hybrid perovskite has increased from 1 to ~10 μm in about 3 years [72]. This improvement reflects the progress that has been recently made in producing samples with better structural order and morphology. Further, it is also observed that the diffusion length has a strong dependence on the grain size of the film. The results showed that more than 1 μm diffusion length has been achieved by realizing films with an average grain size of 2 μm. The perovskite single crystal was found the highest measured diffusion length (10 μm) [73]. Carrier mobility of hybrid perovskite has also been improved over the years and exhibited morphology dependence. Mobility values exceeding 10 cm²V⁻¹ s⁻¹ have been measured in perovskite film [74] and above 100 cm²V⁻¹ s⁻¹ in perovskite single crystals. Further, it is also observed that the mobility (and also diffusion length) did not exhibit a strong dependence on the material composition. Further, the dielectric constant (relative permittivity) is a complex number given by, ε = ε^/−ε^//, where the real part ε^/ is the charge storage ability and the imaginary part ε^// is the energyloss. For MAPbI₃, a small ε^/ is obtained (ε^/= 6.5 in experiment, while 5.6 to 6.5 in calculation) at optical frequency and only electronic polarization takes part in dielectric process [75]. With the decrease of frequency, ionic polarization and dipolar polarization (contribution from MA⁺ dipoles) leading to enhanced ε^/ (ε^/_low ~ 60 at 100 KHz).This large dielectric constant facilitates the screening effect of Coulombic attraction between photoexcited electron-hole pairs (excitons), so that they can be separated easily. Also, noncentro symmetric crystal structure in tetragonal and orthorhombic phases proposed OIHP are ferroelectric in nature. It is also believed that ferroelectricity may give rise to hysteresis observed in current-voltage (I-V) curves. However, observation of ferroelectricity in hybrid perovskite is not well justified from polarization-electric field (P-E) hysteresis loop and second harmonic generation experiments. Despite the above controversies, it is of great interest to study the order-disorder transition of hybrid perovskites due to MA⁺ orientation inside the PbX₆ octahedral [76].

Perovskite solar cell has gained attention due to favorable material properties of OIHP, which include a high absorption coefficient with a sharp absorption edge, high photoluminescence quantum yield, long charge carrier diffusion lengths, large mobility, high defect tolerance, and low surface recombination velocity. At the same time, easy solution processability and completely tunable optical bandgap from blue to red regions of wavelength just by mixing the B-site cation (Pb-Sn) and the X-site anion (I-Br-Cl), while maintaining the sharp absorption edge makes the OIHP family a potential candidate for application in multijunction/tandem solar cells. Another strong advantage of hybrid perovskite solar cells is quite high V_oc, which can be explained by the suppression of the defect formation in the bulk layer as well as at the interfaces. It is observed that OIHP solar cells with E_g < 1.65 eV, the open circuit current (V_oc) is remarkably high and V_oc increases with band gap (E_g) without significant V_oc loss. In particular, a quite high V_oc of 1.26 V has been reported for a pure MAPbI₃ cell, which is very close to the theoretical limit of 1.32 V, with V_oc loss of only 60 mV. High absorption coefficient and low nonradiative recombination rate of OIHP solar cell resulted very small short circuit current (J_sc) loss. We know that photovoltaic devices rarely operate at room temperature, and high power output is necessary even under high-temperature operation conditions. It is observed that OIHP solar cell shows the lowest temperature coefficient (TC) of −0.17%, which is far better than other photovoltaic devices. Also nearly 90% efficiency is maintained even at a high operating temperature of 85°C. Further, OIHP-based solar cell exhibited some unique advantages such as low-temperature processes for all sub cells, compatibility with flexible and lightweight applications, low life-cycle environmental impacts and embodied energy, and potentially low fabrication costs.

Although OIHP solar cells produced quite impressive efficiency, they have several limitations too and to overcome these limitations are the major challenge for the commercialization of these devices. One significant drawback of OIHP is degradation of these perovskite materials under a range of environmental factors such as humidity, illumination, oxygen, and thermal stress. OIHP solar cells are ionic crystals, and the presence of H₂O leads to the decomposition of the perovskite structures to hexagonal-shaped PbI₂/hydrate crystals; which can be suppressed by introducing protective (passivation) layers. In case of mixed halide perovskites strong photo-induced phase segregation occurred under illumination and judicial choice of A-site cation can minimize this instability. Further, it is observed that a higher level of performance in OIHP solar cell is hindered by anomalous hysteretic behavior and large discrepancy between the forward and reverse scans put a question on the reproducibility of power conversion efficiency (PCE) of the device. In searching for the possible origins of hysteresis, several explanations such as ion migration, charge trapping/detrapping, photoinduced capacitive effect, and ferroelectricity have been imposed. Among them, ion migration and ferroelectricity are believed as feasible origins of the hysteresis in transport measurements. Extensive research efforts continue to find the long-term stability of OIHP solar cells.

Another major challenge is the realization of large-area module due to its fabrication limitations. Till now high efficiency of 17.9% has been realized for the large-area module with a size of 30 × 30 cm² (aperture area: 80² cm²), which was formed by an inkjet printing technology. Thus development of proper fabrication technique is essential to make pinhole free large-area OIHP devices. Also in the large area tandem cells, current matching conditions for the top and bottom cells as well as each sub cell need to be established; which can be improved through technological advances.

High toxicity of heavy metal (lead) is a serious problem which cannot be neglected in OIHP-based solar cells. Although the content of lead (Pb) in OIHP solar panel (~1m²) is only a few hundred milligrams, could be severe problems in environmental impact. As an alternative people are trying to replace Pb²⁺ with Sn²⁺; but the efficiency of Sn-based photovoltaic devices are extremely poor. Thus, roof-top application of OIHP modules is difficult and large-area operations as solar farms are more appropriate. Also, encapsulation of photovoltaic module and environmentally friendly 100% recycling programs are essential for OIHP-based solar modules.

The future of perovskite solar cells was highlighted. As discussed earlier, the significantly reduced efficiency upon solar module area scaling-up is still the main challenge to face for the commercialization of OIHP-based solar cell. It is observed that efficiency decreases to 19.6% when the aperture area increases from 0.1 cm² to about 10 cm², and further drops to 17.9% with the area approaching 1000 cm², which still lags far behind that of the crystalline silicon cells (26.7% at 79 cm² and 24.4% at 13,177 cm²). Thus, intensive works should be conducted to precisely control the uniformity of the crystallization process in large-area perovskite films. Also, the fundamental photophysical mechanisms relative to the efficiency loss in OIHP modules should be further studied to understand role of surface and interface. Development of green solvent systems or the solvent-free deposition technology for fabricating large-area perovskite film will be an important research topic in the future. Besides the efficiency, more and more attention need to invest in the long-term stability of OIHP solar modules. Recently, Okinawa Institute of Science and Technology Graduate University in Japan reported over 1100-h operational lifetime for a 10 × 10 cm² solar module. Although many research groups and companies claimed that their devices have passed International Electro Technical Commission (IEC) standard test, there are still some stability issues needed to be addressed at the next stage. Thus proper development of encapsulation technology is essential and we believe that a growing number of studies will move to exploit such multifunctional encapsulation materials in the near future. The single-junction OIHP cells with efficiency above 24% and long-term stability can be more cost-effective than tandem cells which may work at a PCE of 27–28%. Thus, more efforts should be made in fabrication and scaling up of single-junction OIHP-based solar cells with high efficiency, high yield, and long-term stability. Development of low-cost large-scale fabrication methods with highly reproducible results is required for commercialization of OIHP-based photovoltaic cells.

4. Thin film based sensor

A thin film-based sensor is a type of transducer which converts a physical or chemical quantity into equivalent electrical for measurement. It is used to detect the presence of stimulus to very low concentrations of toxic or harmful target environment (gases) of importance, such as ammonia [77], carbon monoxide [78], carbon dioxide [79], nitrogen dioxide [80], sulfur dioxide [81], propane [82], liquefied petroleum gas [83], hydrogen sulfide [84], and volatile organic compounds. Worldwide thin film gas sensing technology is playing a major role in protecting the environment and improving homeland security. Sensors are also critical in improving the reliability and efficiency of manufacturing operations by providing faster and more accurate feedback regarding product quality. In the area of environmental health and safety, lowering the limits of detection can improve the quality of life through precise information regarding the pollutants in air, water and soil. High-performance thin-film sensors and systems are essential to monitoring various kinds and quantities of analysts.

The typically thin film-based sensors are described using the main characteristics such as sensing response, stability, repeatability, reproducibility, linearity, response time, and recovery time. An efficient thin-film sensor;

• Must have a high sensing response towards a very low concentration of target gas.

• Would give the same sensing characteristics after repeated usage (stability) and for different sensors of the same kind.

• Should be capable of responding fast towards a target gas.

• Must regain initial characteristics as soon as the target gas is flushed.

• Thin-film sensor response should increase linearly with increasing the concentration of target gas.

The thin film-based gas sensors are used for equally the identification and quantification of gases, and hence should be both selective and sensitive to a required target gas in a mixture of gases. Sensitivity defines the smallest concentration of gas/vapour that can be fruitfully and repeatedly sensed by a thin film sensor.

Thin film-based semiconductor is commonly used materials as sensor application as indicated in Table 1. This is because of its versatile advantages like high sensitivity and low manufacturing is metal oxide which contain the elements having one oxidation state because it requires more energy to form more than one oxidation states. Semiconductor metal oxide films have been exploited for the sensing of various toxic and harmful gases in the form of ceramics, thick films, thin films or nanostructures. Sensors based on ceramics have shown advantages in terms of their mechanical strength, large resistance to chemical attack and good thermal and physical stability and most of the available commercial sensors are based on ceramics only. One of the additional attractive features associated with low temperature operated semiconductor thin film sensor is that it can lead to a complete integration with well-established Si based micro-electronics technology.

S. No.	Material/modifier	Temperature (°C)	Gas concentration (ppm)	Response	Response/recovery time	Reference
1.	SnO2/Pt	200	1000	89	20/27 sec	[85]
2.	ZnO/PANI	36	1000	13	3.3/9.8 min	[86]
3.	TiO2/PANI	273	0.1 vol%	0.63	3.3/3.0 min	[87]
4.	TiO2/Ni	250	1000	37	−/−	[88]
5.	ZnO/PEDOT: PSS	27	1000	0.58	3.7/3.1 min	[89]
6.	ZnO/MWCNT	30	1500	61	5.8/3 min	[90]
7.	h-BN/−	RT	3.0 vol%	6.17	55/40 sec	[91]
8.	PANI/−	RT	100	12.10	11/07 sec	[92]
9.	ZnO-TiO2/PANI	30	20	412	35/54 sec	[93]
10.	CdS/−	70	20	173	5.52/3.46 min	[94]
11.	Ag-BaTiO3/CuO	250	5000	0.28	15/10 min	[95]
12.	CuO-CuxFe3−xO4	250	3000	0.50	9.5/− min	[96]
13.	CdO	250	5000	0.01	3.33/5 min	[97]
14.	PEDOT-BPEI	RT	1000	0.03	−/60 min	[98]
15.	La1−xSrxFeO3	380	2000	0.25	11/15 min	[99]
16.	ZnO	200	3000	0.03	8 /40 sec	[100]
17.	ZnO-La (50%)	400	5000	0.65	90/38 sec	[101]
18.	SnO2/PANI/Ag	30	1000	67	1000/900 sec	[102]
19.	TiO2/Zn	RT	1.5 vol%	2.92	120/− Sec	[103]
20.	Fe2O3/ PANI	RT	20	229	2.35/3.8 min	[104]

Table 1.

Literature survey of various gas sensing characteristics of different metal oxidebased nanomaterials with different modifiers.

The limitation of thin film based sensor was described. A number of thin film sensors might be recognized from sensor arrays which yield slightly different responses to various target gases. The availability of thin film gas sensor potentially creates a complicated selection problem, and is more important in view of cost and technology limitations. Many researchers have self-sufficiently confirmed practical limitations to thin film gas detection at low temperature and have attributed it to the requirement of high activation energy which can be attained only at elevated temperatures. A reduction in the number of sensors to be involved in E-Nose is advantageous due to several reasons as discussed. Sensors which exhibit an insignificant response to target gases, increase variance (noise) in E-Nose and do not assist pattern recognition process. Furthermore, sensors exhibiting similar responses to the target gases provide no additional information and are redundant.

In future, low temperature operation of the thin film sensors is an attractive proposition for the industry since it not only holds a promise to cut down the costs but also overcome technological limitations of miniature heaters of high wattage. In order to identify the target gases other classification technique such as artificial neural network approach is required where the selected features/variables obtained from principal component analysis (PCA) could be used as input features, and will be carried out in future. Therefore, a new methodology or novel design approach is essentially required in order to fulfil the essential requirements of future sensor in the market.

5. Metal sulphide, metal telluride and metal selenide thin film based solar cells

Metal chalcogenide materials are considered as excellent absorber materials in photovoltaic cell applications. These materials exhibited excellent absorption co-efficient and suitable band gap value to adsorb the maximum number of photons from sun radiation. Photovoltaic cell can be used to convert sunlight into electricity. These materials have a several advantages such as flexible, lower in weight, have less drag and very thin layer (from nanometer to micrometers). Preparation of the films has been reported by many researchers via different deposition methods. The properties of obtained films were studied by using various tools. The obtained experimental findings revealed that these materials could be classified into two groups, namely p-type and n-type materials. Experimental results confirmed that electron (n-type material) can absorb the energy from photons, following that, jump to the p-type materials, to produce electric potential.

Metal chalcogenide materials are considered as excellent absorber materials in photovoltaic cell applications [105, 106]. These materials exhibited excellent absorption co-efficient and suitable band gap value to adsorb the maximum number of photons from sun radiation [107, 108]. Photovoltaic cell can be used to convert sunlight into electricity. These materials have a several advantages such as flexible, lower in weight, have less drag and very thin layer (from nanometer to micrometers). Preparation of the films has been reported by many researchers via different deposition methods [109]. The properties of obtained films were studied by using various tools [110]. The obtained experimental findings revealed that these materials could be classified into two groups [111, 112], namely p-type and n-type materials. Experimental results confirmed that electron (n-type material) can absorb the energy from photons, following that, jump to the p-type materials, to produce electric potential.

Based on the global photovoltaic market [113], the market shares of silicon based solar cell decreased from 92% (in 2014) to 73.3% in 2020. Silicon based solar cell accountable for the highest percentage of market share due to the abundant raw material availability and high efficiency value. The thin film based solar cells increased from 2014 (7%) to 2020 (10.4%). Solar cell market is expected to growth rapidly due to the rising demand for commercial, residential and utility applications. According to the market share of thin film technologies [114], there are three common thin film materials such as amorphous silicon, cadmium telluride and copper indium gallium selenide. Amorphous silicon based solar cell was the oldest thin film technologies, and it dominates overall market from 2000 to 2003. This type of solar cell can absorb a wide range of the light spectrum, did excellent in low light, but loses efficiency rapidly. The CdTe films have been deposited successfully onto glass. Quaternary thin films such as copper indium gallium selenide were prepared via co-evaporation method. The global demand for CdTe films and CIGS films was expected to drive the market start from 2004 and onwards [114].

The cadmium telluride thin films could be used as solar absorber due to suitable band gap value and high absorption coefficient in the visible light region [115]. The materials have high absorption coefficient was able a low absorber thickness (about 1 μm) to absorb sufficient sunlight. Generally, several researchers reported the synthesis of CdTe films by using various deposition methods such as chemical bath deposition [116], spray pyrolysis [117], thermal evaporation [118], molecular beam epitaxy [119], close space sublimation [120], pulsed laser deposition method [121], hydrothermal method, electrochemical deposition technique. Researchers pointed out that the CdTe films deposited onto glass substrates showed some problems such as heavy and fragile. Currently, more and more research activities are focusing on the synthesis of CdTe films onto metal foils in order to lower the investment in equipment and infrastructure. The thin film deposited onto flexible substrates could be folded in any shape, and the researcher concluded that the supporting structure requirements are minimum if compared to heavy glass substrates. Table 2 showed the advantages, limitations, power conversion efficiency of CdTe films. Also, the solar power plant was described in the table. So far, the First Solar Company is the main producer of CdTe film.

Advantages	Limitations	Solar power plant	Power conversion efficiency (%)
CdTe has band gap about 1.5 eV, it can absorb sunlight at close to ideal wavelength, it captures energy at shorter wavelength.	Toxic effect of cadmium	The Topaz Solar Farm was located in California, United States. The photovoltaic power station includes 9 million CdTe thin film modules [122].	19% as reported by Gloeckler and co-workers [123]
The cadmium is abundant	Very limited availability of tellurium	In the Desert Sunlight Solar Farm (California, United States), it employed 8.8 million CdTe film modules [124].	13.3% as highlighted by Kamala and coworkers [125]
CdTe film based solar cell showed the shortest energy payback time and the smallest carbon footprint.	It does not remain stable under severe stress	The Waldpolenz Solar Park was located in Germany, has used CdTe film modules, was 52megawatt photovoltaic power station [126].	15% as pointed out by Devendra and co-workers [127]
	It is very important to enhance the efficiency of solar cells	Templin solar power plant was located in Germany, has installed more than 1.5 million CdTe film modules [128].	17.8% as concluded by Deb and coworkers [129]
			9.59% as described by Xixing and coworkers [130]

Table 2.

Advantages, limitations, power conversion efficiencies and CdTe film based solar power plant.

The copper indium gallium selenide (CIGS) thin films have been prepared by using different deposition methods such as thermal evaporation method [131], spray pyrolysis [132], solvothermal method [133], physical vapor deposition [134], and electro deposition method [135]. Table 3 showed the advantages, limitations and power conversion efficiency of CIGS thin films. These films showed p-type absorbing layer materials and the tunable band gap (1.07–1.7 eV) value [141]. Researcher highlighted that there are 99% of the light will be successfully absorbed in the first micrometer of the materials [142]. The solar cell is classified as heterojunction structures [143]. Generally, the junction is produced between thin films having various band gap values. Experimental results showed that the addition of small amount of gallium can improve the voltage, boost band gap value and enhance the power conversion efficiency of solar cell [144]. There are several companies produced CIGS solar cell such as Solar Frontier, Solyndra, SoloPower, Global Solar, SulfurCell, MiaSole and Nanosolar. The solar cell showed open circuit voltage, short circuit current and the maximum power values of 5 V Dc, 95 mA and 0.25 watts, respectively.

Advantages	Limitations	Power conversion efficiency (%)
CIGS thin films have been deposited onto substrates (flexible)	Less efficient if compared to silicon based solar panels	Conventional solar cell: 22.67%. Adding the BSF (PbS) layer in solar cell: 24.22% as reported by Barman and Kalita [136]
The active layer could be deposited in polycrystalline form.	Higher production costs if compared to other thin film technologies.	The highest efficiency is 25.5% as highlighted by Sobayel and coworkers [137]
Much lower level of cadmium will be used during the synthesis of thin films	Complex structures	The highest power conversion efficiency was 26.4% as concluded by Sobayel and co-workers [138]
CIGS thin films based solar panel indicated better resistance to heat.		Boubakeur et al. have achieved power conversion efficiency of 21.08%. [139]
Much less expensive if compared to silicon based solar cells.		Nour and Patane reported the highest power conversion efficacy about 24.5%. [140]

Table 3.

The advantages, limitations and power conversion efficiency of CIGS thin films.

The copper rich p-type CuInS₂ films were synthesized by using thermal co-evaporation method. The obtained results showed that small (less than 10%) solar to electrical conversion losses when the copper to indium ration between 1 and 1.8. The highest power conversion efficiency was 10.2% as reported by Scheer and co-workers [145].

The chemical bath deposition was used to produce Ni₃Pb₂S₂ thin films [146]. The photovoltaic parameters such as open circuit voltage (0.61 V), short circuit current density (9.9 mA/cm²), fill factor (0.47) and power conversion efficiency (2.7%) were studied. The band gap was calculated based on the absorption spectra and was about 1.4 eV.

The atomic layer deposition was employed to produce SnS films [147] as highlighted by Rafael and co-workers. These materials are non-toxic solar cell, and the power conversion efficiency was 4.36%. Vera and co-workers [148] reported that SnS heterojunction solar cell was made, and reached power conversion efficiency about 3.88%.

The performance of p-type InSe films for solar cell was reported. The open circuit voltage (0.55 V), short circuit current density (7.09 mA/cm2), fill factor (53.85%, and power conversion efficiency (0.52%) were highlighted. Researchers explained that higher series resistance and reduced shunt resistance lead to lower value of efficiency. The band gap values are in the range of 1.75–1.95 eV in as-deposited films, annealed films at 250 and 300°C as concluded by Teena and co-workers [149].

The electrochemical technique was used to produce CdSe film MnCdSe films as described by Shinde and co-workers [150]. XRD analysis showed the obtained films are polycrystalline with hexagonal crystal phase. The SEM images revealed that nanosphere morphology and nanonest structure for CdSe and MnCdSe films respectively. The band gap value was measured, and the reduced from 1.81 eV (CdSe) to 1.6 eV (MnCdSe). The fill factor and power conversion efficiency of CdSe films 0.71 and 0.67%, respectively. The MnCdSe films showed power conversion efficiency about 0.37%.

The ternary compound such as Cu₂SnS₃ (CTS) films showed high absorption coefficient (10⁴ cm⁻¹) and wider range of band gap energy (0.9–1.7 eV). Researchers reported that easy to control the secondary phase during the synthesis of CTS films. The formation of cubic, monoclinic, tetragonal and orthorhombic structure strongly depended on deposition method and annealing process. The magnetron sputtering method was used to produce CTS films. The films reached the highest power conversion efficiency about 2.2%, due to the formation of pure phase of CTS, lowest sheet resistance (8.2 Ω/cm²), highest shunt resistance (111.1 Ω/cm²) and uniform morphology [151]. The p-type CTS films have been produced via co-evaporation method [152]. The photovoltaic parameters such as open circuit voltage (248 mV), short circuit current density (33.5 mA/cm²), fill factor (0.439) and power conversion efficiency (3.66%) were highlighted. Mingrui and co-workers [153] described the preparation of CTS films by using sputtering method. The films prepared at 2812 seconds indicated the highest efficiency value (2.39%), with fill factor (39.7%), open circuit current voltage (208 mV) and short circuit current density (28.92 mA/cm²).

The Cu₄SnS₄ films showed p-type electrical conductivity and the band gap values (0.93–1.84 eV). Chen et al., have reported the synthesis of thin films by a combination of mechanochemical and doctor blade techniques [154]. The highest power conversion efficiency reached 2.34%. The influence of the film thickness on the properties of samples was study. Based on the absorption spectra, the absorption edge moved towards longer wavelength with increasing the film thickness (0.25–1 μm). Also, band gap reduced (1.47–1.21 eV) due to reduction of structural disorder and the increase in the crystalline size.

The Table 4 showed the power conversion efficiency of the various thin films. The obtained experimental results confirmed that metal sulfide, metal selenide and metal telluride thin films could be used in solar cell applications. The photovoltaic parameters were strongly depended on various experimental conditions. Researchers also highlighted a lot of research activities have been carried put in order to enhance the power conversion efficiency of thin film based solar cell.

thin films	Power conversion efficiency (%)	References
Cu2ZnSnS4	5.74	Kazuo and co-workers [155]
Cu2ZnSnS4	2.62	Hironori and co-workers [156]
Cu2ZnSnS4	6.8	Wang and co-workers [157]
Cu2ZnSnS4	4.1	Schubert and co-workers [158]
Cu2ZnSnS4	0.23	Chet and co-workers [159]
Cu2ZnSnS4	3.2	Jonathan and co-workers [160]
Cu2ZnSnS4	3.4	Ennaoui and co-workers [161]
Cu2ZnSnS4	0.396	Sawanta and co-workers [162]
Cu2ZnSnS4	6.03	Tsukasa and co-workers [163]
Cu2ZnSnS4	0.12	Shinde and co-workers [164]
CuS	0.39	Donghyeok and co-workers [165]
CdS	8	Karl [166]
ZnS	8.83	Qiu and co-workers [167]
PbS	2.02	Omer and co-workers [168]
PbS:Mo	2.16	Omer and co-workers [168]
Sb2Se3	7.6	Wen and co-workers [169]
Sb2Se3	5.93	Liang and co-workers [170]
Sb2Se3	5.6	Chao and co-workers [171]
CuInTe2	3.8	Manorama and co-workers [172]
CuInTe2	4.13	Lakhe and co-workers [173]
CuInTe2	1.22	Jia and co-workers [174]
CuInSe2	1.75	Hyun and co-workers [175]
CuInSe2	2	Se and co-workers [176]
CuInSe2	4.57	Prabukanthan and co-workers [177]
MnCuInSe2	6.38	Prabukanthan and co-workers [177]

Table 4.

Power conversion efficiencies of different types of thin films.

6. Conclusions

Chalcogenide thin films have received a great deal of attention for decades due to some unique properties. The thin film based supercapacitor can have store the electrical energy for all the electronic devices to stabilize the power supply. Metal nitrates have gotten a lot of interest as supercapacitors electrodes due to showed higher electrical conductivity, higher specific capacitance, good energy density, and excellent mechanical stability. Perovskite solar cell indicated higher power conversion efficiency value. The organic inorganic hybrid perovskite solar cells are very simple, and prepared by sandwiching a perovskite absorber layer between the electron transport layer and hole transport layer, reached power conversion efficiency exceeding 20%. The thin film-based sensors showed high sensitivity and low manufacturing cost. In future, low temperature operation of the thin film sensors is an attractive proposition for the industry. The market shares of silicon based solar cell decreased, while thin film based solar cells increased in the global photovoltaic market due to the low material consumption, low manufacturing cost, shorter energy pack back period. Solar cell market is expected to growth rapidly due to the rising demand for commercial, residential and utility applications.

Acknowledgments

The author (HO SM) gratefully acknowledge the financial support provided by the INTI International University.

Conflict of interest

The authors declare no conflict of interest.