MANUFACTURINGLASERS
Figure 6: Non optimized P1 scribe
Figure 5: Diagram showing P1, P2 and P3 scribes in a thin-film solar cell
Figure 7: JPSA’s optimized P1 scribe
laser processing point of view is to optimize via quality (Figure 4) while keeping up with the required throughput for a cost effective solution.
Laser annealing and doping Annealing with UV nanosecond pulse duration lasers for the formation of polycrystalline silicon is a proven technique regarding its quality, reliability and economical impact. With laser annealing, amorphous Si is transformed into polycrystalline form by heating a thin layer to just above the melting temperature. The strong absorption of UV radiation together with the short pulse duration limit heating to a thin layer and allows precise control over the heating cycle necessary to achieve polycrystals of sufficient quality and size distribution. In order to make the process efficient large areas need to be irradiated at high speed thus requiring high pulse energy and high repetition rate with very good pulse to pulse stability.
The duration of Si melting following nanosecond laser irradiation is sufficient to allow diffusion of dopant impurities throughout the molten regions of the film. Consequently, laser crystallization can be augmented by including a laser doping procedure, thereby reducing the process steps in fabricating thin-film poly-Si devices [7]. Most current commercial solar cells use wafer-based crystalline silicon. A number of techniques (spray coating, roll-on, etc.) can be used to apply precursors containing phosphorous (P) or Boron (B) onto monocrystalline, n- and p-type wafers. Afterwards, laser processing can be used to melt the silicon to a controlled depth allowing for liquid-phase diffusion of atoms from the films into the silicon. Dopant concentrations above 1019 cm-3, for depths below 1 micron have been reported for both P and B doped wafers, revealing great potential for laser doping applicable to solar cell processing [7] . At JPSA we have developed
techniques that allow simultaneous exposure of large areas with beam homogeneity better than 3% as required for controlled doping profile.
Lasers for thin-film PhotoVoltaics (Generation II)
The increasing cost of silicon, which is half the cost of a finished module, has resulted in a number of new ventures exploiting thin film approaches for manufacturing photovoltaic (PV) panels. Whereas crystalline silicon solar cells are a few hundred microns thick, thin film thicknesses are on the order of a few microns and greatly reduce the cost of raw materials. Furthermore, these films are scalable for processing panels in excess of 1 meter square and enable low-cost high-volume manufacturing. Laser scribing has proven to be critical for high-volume production of thin-film solar devices [8].
Typically the thin film layers are deposited onto glass sheets consisting of three main layers P1, P2, and P3. For a typical panel, a transparent conducting oxide layer (such as Indium Tin Oxide - ITO) forms the front electrical contact (P1), with the semiconductor layer forming P2 and then a metallic layer (Molybdenum – Mo) forming the rear contact (P3). In order to generate useful voltage from thin film panels, they must be isolated into
25
Figure 8: Cross section scan of P1 scribe trench by laser beam with optimized energy profile
www.solar-pv-management.com Issue II 2011
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48