**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 coefficient 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.


#### **Table 2.**

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

## *Recent Developments on the Properties of Chalcogenide Thin Films DOI: http://dx.doi.org/10.5772/intechopen.102429*

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.

The copper rich p-type CuInS2 films were synthesized by using thermal coevaporation 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%.


#### **Table 3.**

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

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%.


#### **Table 4.**

*Power conversion efficiencies of different types of thin films.*

### *Recent Developments on the Properties of Chalcogenide Thin Films DOI: http://dx.doi.org/10.5772/intechopen.102429*

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/cm<sup>2</sup> ), highest shunt resistance (111.1 Ω/cm<sup>2</sup> ) 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/cm<sup>2</sup> ), 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/cm<sup>2</sup> ).

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.
