Cadmium Telluride (CdTe) Solar Cell

In 1972, first CdTe/CdS solar cell was reported. It is a type of photovoltaic that uses Cadmium Telluride (CdTe) and Cadmium Sulphide (CdS) thin film for light absorption and charge extraction for generating electricity using the photovoltaic effect. Two major companies that are manufacturing CdTe solar cells are "Antec" in Germany and "First Solar" in US.

                CdTe (Cadmium Telluride) solar cell belongs to the family of chalcogenides (ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe). CdTe is formed by reaction of Cd and Te vapours and then deposition on surface using sputtering, thermal evaporation and other physical vapor deposition techniques. CdTe is a direct band gap semiconductor and has high absorption coefficient. As absorption coefficient is inversely proportional to absorption length, only a few micron thick layer of cadmium telluride is required to absorb visible spectrum of sunlight. Band gap of CdTe is 1.45 eV which is most appropriate for solar cell applications because wavelength corresponding to 1.45 eV comes out to be 855 nm (λ = hc/E = 1240/1.45 = 855). Thus, whole visible region will be absorbed and wavelength above 855 nm will not be absorbed.

   CdTe Solar Cell Parameters

Efficiency (η) : 21 %

Open Circuit voltage (V_OC) : 0.88 V

Short Circuit Current Density (J_sc) : 30 mA/cm²

Fill Factor (FF) : 80 %

Mobility of Electrons (μ_e) : 500-1000 cm² / Vs

Mobility of Holes (μ_h) : 50-80 cm² / Vs

Melting Temperature : 1365 K
Absorption Coefficient (600 nm) : >5 * 10⁵ / cm
Density : 6.2 g / cm³

CdTe Solar Cell Structure

Figure 1. CdTe  Solar Cell Structure

                    Figure 1 represents superstrate structure of CdTe solar cell. Superstrate structure is the one in which light enters through the substrate itself. In CdTe solar cell front contact is made by TCO (Transparent Conducting Oxide) and is also called (Window Layer). TCO layer should be thick enough so that it can prevent the impurities from the substrate. On TCO CdS layer is deposited (100 nm) followed by deposition of CdTe layer (5 micro meter). Once the junction is formed device needs to be activated by chlorine treatment using CdCl2. In this treatment CdCl2 is dripped on to CdTe coated substrate and then heating it at 450 degree Celsius for 15 minutes. CdCl2 treatment lowers down the defect density and give rise to holes, promoting P-type character. The last step involves the formation of back contact. Figure 2 represents sequence wise manufacturing process of CdTe solar cell.

Figure 2. Manufacturing process of CdTe Solar Cell

                   CdTe is used as P-type semiconductor and CdS is used as N-type semiconductor leading to the formation of junction and depletion region as shown in figure. Band gap of CdTe and CdS is 1.45 eV and 2.42 eV. As the major portion of the solar spectrum is absorbed by CdTe, CdS is mainly used as partner layer to form the cell and is referred as "Buffer Layer". To enhance the efficiency of solar cell active layer band gap should align with that of buffer layer band gap. In cadmium telluride solar cell, CdS is highly doped and CdTe is lightly doped so that depletion layer portion is extended deep inside CdTe side and recombination will be less probable due to the presence of an electric field in the depletion region as shown in figure 3.

Figure 3. CdTe Solar Cell Energy Band Diagram

Issues with CdTe Solar Cell

1.) Intermixing of CdS-CdTe layer.
2.) Toxic nature of Cd.
3.) Bandgap can not be tailored.