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Solar Cells Thin Film Technologies Part 13

Tham khảo tài liệu 'solar cells thin film technologies part 13', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Large Area a-Si pc-Si Thin Film Solar Cells 349 at the glass-side of the film Fig. 9a . Typical laser energy is 1x106 W cm2 and calculation shows more than 80 of that energy is absorbed and converted to heat building up in the film. 2. Decomposition of H from a-Si H. The absorbed heat induces the decomposition of a-Si H and releases hydrogen at a temperature of 600 C Fig. 9b . In fact the local temperature in the film can be heated up to 700 C by the laser. 3. Destruction of the PV layers and back contact. The gaseous H2 quickly expends its volume and pressure under the high temperature. The pressure of the H2 gas can amount to 1x107 Pa inducing enormous shear stress on the layers above the heated zone. In one estimation applying a 532 nm 12 kHz and 9.5 x 106 W cm2 laser beam on a-Si single junction module created shear stress of 3.9x108 Pa enough to break the layers on top of the heating zone among which the most ductile Ag layer has a shear strength of 107 - 108 Pa Fig. 9c . 4. Formation of heat affected zone HAZ . Along with the H2 volume expansion the film cracks quickly followed by blasting off effectively removing the a-Si gc-Si layers and the back contact layers above the local heated zone. The laser heating also damages the film around the removed region creating a HAZ with high density of defects and poor electrical properties Fig. 9d . By using high-frequency pulsed laser the HAZ is limited to less than a few tens of nm wide. It is important to note that the laser scribing removal is not a true thermal process but the mechanical blasting off of the film. By applying different wavelengths of lasers the laser energy is absorbed by different layers thus selectively removes those layers without affecting other underlying layers. a Back reflector AZO a-Si TCO Glass Laser beam c Fig. 9. Laser-scribing mechanism. a absorption of laser beam incidents through the glass b decomposition of H from a-Si H c destruction of the photovoltaic layer and back electrode d .