MANUFACTURINGMETROLOGY
Surface roughness A thin film Transmitting Conducting Oxide (TCO) layer is vital to the design of thin film CdTe and CIGS photovoltaics. The layer is optically transparent but electrically conducting with a sheet resistance typically in the range 5 to 20 ohm/sq. In thin film CdTe, the TCO is the first layer to be deposited on the glass substrate. The surface roughness of this layer is important because if the TCO is too rough it can penetrate the next thin CdS layer and shunt the device. Indium tin oxide (ITO) is an important TCO material and although its use in photovoltaics is limited by cost, its performance advantages are such that it is widely used in flat panel displays (LCD and OLED), touch screens and electrochromic devices.. Low optical scatter is required in all these applications.
The CCI images from a 365nm thick layer of ITO deposited using closed field magnetron sputtering is shown in Figure 2. This film of ITO deposited at room temperature was measured to give a sheet resistance of 10 ohm/sq. The peak to valley roughness is 4.4nm and the rms roughness is 0.453nm. This ITO film is super-smooth and its smoothness is comparable with the original glass substrate surface.
Surface Texture
A key process in the manufacture of crystalline or polycrystalline silicon is the introduction of surface texture to provide anti-reflective properties to the surface. Texture is introduced by caustic etching
Figure 3: Surface texture developed on a silicon surface following a caustic etch cleaning process
the surface to form pyramid structures which reflect light back into the surface. The CCI images shown in Figure 3 shows the texture developed after a light etching. These analyses can be made quickly and the tool is suitable for use in quality control in a manufacturing environment. Laser ablation is another important process in solar module production. Laser Buried Groove Contacts (LGBC) are used for high performance crystalline silicon and although it is a relatively expensive process it is necessary for the small high performance silicon cells used in medium (x 50) Concentrating Photovoltaics (CPV). Laser ablation also has potential uses for interconnection in thin film CdTe photovoltaics where it is necessary to adjust laser power to ablate either to the glass substrate or to the TCO layer. Figure 4 shows the shape of a laser trench ablated through a CdTe device precisely into the TCO layer (Flourine doped Tin oxide). This process is used to isolate
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Figure 2: A CCI images from the surface of a
355nm thick layer of Indium Tin Oxide (ITO). The rms roughness is Sq = .0.453nm
www.solar-pv-management.com Issue X 2010
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