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* **Glass subtrate** Typically around 2-4 mm thick, this protects the active layers from the environment, and provides all the device's mechanical strength. The outer face of the pane often has an anti-reflective coating (ARC) to enhance its optical properties.  * **Transparent conducting oxide (TCO)** Usually of tin oxide or indium tin oxide (ITO), this acts as the front contact to the device. It is needed to reduce the series resistance of the device, which would otherwise arise from the thinness of the CdS layer.   * **Window Layer (CdS)** Polycrystallin cadmium sulphide is n-type doped. The wide band gap of CdS (~ 2.4 eV) means it is transparent down to wavelengths of around 515 nm, and so is referred to as the window layer. Below that wavelength, some of the light will still pass through to the CdTe, due the thinness of the CdS layer (~ 100 nm).   * **Absorber (CdTe)** The CdTe layer is a  polycrystalline, p-type doped and acts as an efficient absorber. material.  CdTe has is  a nearly perfect absorber material since its bandgap is  direct band gap leading to a and closely matches the peak of the solar spectrum and it has  high absorption coefficient ( α > 5 x 10⁵ cm^-1). A layer of 2µm 1-2µm  is enough to absorb around 99% of incident light which means that the amount of active material used is relatively small.CdTe has a bandgap (1.49 eV) which is very close to the theoretically-calculated optimum value for solar cells under unconcentrated AM1.5 sunlight (1.34eV).  Because it is less highly doped than the CdS, the depletion region is mostly within the CdTe layer. This is therefore the active region of the solar cell, where most of both the carrier generation and collection occur. * **Back contact** Usually a metal due to high conductivity, the back contact proves a low resistance electrical connection to the CdTe. P-type CdTe is a difficult material on which to produce an ohmic contact, and so the junction will inevitably display some Schottky diode (rectifying) characteristics. This layer needs only be a few tens of nanometres in thickness.