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* **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, like the CdS, polycrystalline, but is p-type doped. Its energy gap (1.5 eV) is ideally suited to the solar spectrum, and it has a high absorption coefficient for energies above this value. It acts as an efficient absorber and is used as the p side of the junction. 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. The thickness of this layer is typically around 10 µm.   * **Back contact** Usually of gold or aluminium, a metal due to high conductivity,  the back contact proves a low resistance electrical connection to the CdTe. P-type CdTe is anotoriously  difficult material on which to produce an ohmic contact, and so the junction will inevitably display some Schottky diode (rectifying) characteristics. Due to its high conductivity, the metal This  layer needs only be a few tens of nanometres in thickness.