Economic growth in developing countries and the rising needs of developed countries have resulted in increased demand for energy across the world. According to International Energy Agency (IEA) data (figure 1(a)), a steady increase is predicted in the global energy demand, and historical data collated over the past three decades (figure 1(b)) shows that electricity generation in the world has increased gradually and about 60 to 70% of electricity is produced using fossil fuels.
However, with increased use of fossil fuels, high levels of CO2 gas are released into the atmosphere which can lead to global warming (figure 2). Over the past few years, the prices of these natural energy resources have also increased significantly.
Figure 1. (a) World energy demand in future and (b) past 30 years of world electricity generation data.
Figure 2. Worldwide energy-related CO2 emissions by fuel type.
To this end, researchers across the world are exploring other means to develop alternative energy resources, such as wind, solar power, and bio-fuels. Among these solutions, solar power technology, also called photovoltaic (PV) technology, offers a suitable solution for producing electrical power.
Over the past five years, significant research has been carried out to increase both the efficiency and manufacturing cost of solar cells. It is assumed that present and future success achieved in these areas will drive its implementation on a wider scale (figure 3). This article shows how lasers are helping solar cell technology to meet the economic and technical targets needed to replace the costly oil and coal burning energy alternatives used today.
Figure 3. Forecast for surge in photovoltaic cell implementation market.
Types of PV Solar Cell
Solar cell technology comes in three basic types, namely, crystalline silicon, multi-junction gallium arsenide (GaAs), and other types of thin films on metallic or glass substrates. Among these, crystalline silicon cells are extensively produced and used on a commercial scale. Made from mono-crystalline or multi-crystalline silicon wafers, these cells have efficiencies in the range of 13-22%.
GaAs cells have the highest electrical conversion efficiencies, usually in the range of 28-29%, but can be as high as 40% in some cases. They are manufactured using MOCVD methods, but quite expensive when compared to other technologies. As a result, they are normally used in applications where cost is not a major issue, such as solar concentrators and satellites. Thin film cells are the latest products that can be manufactured cost-effectively and have slightly lower efficiencies, ranging from 8-18%. Figure 4 shows the estimated relationship between cell efficiency and cost of today’s competing technologies.
Figure 4. Relative cost and efficiency of various solar cell technologies.
Thin film-based and crystalline silicon-based structures are the two major structures that are currently used for solar cell manufacture (figure 5).
Figure 5. Schematic of various types of solar cells
Crystalline Silicon Cells
At present, crystalline silicon cells account for over 93% of the PV market. The increased demand for silicon wafers has further increased the cost of silicon. As a result, the industry has started to embrace alternative thin film technologies as they have little or no silicon.
Thin Film Solar Cells
Although thin film solar cells have lower efficiencies when compared to other technologies, they offer huge potential in terms of reduced manufacturing costs. Also, research in thin film solar technology using innovative structures and materials promises to provide higher efficiency in future.
Laser Structuring of Thin Film Solar Cell
In the manufacture of thin film solar cells, lasers play a major role by scribing the preferred patterns in each layer of the cell. Higher power laser sources have also been developed to obtain the desired scribing processes at speeds necessary to exploit existing FPD infrastructure. Therefore, the combination of laser technology and present FPD manufacturing infrastructure offers a practical solution for manufacturing low-cost thin film solar cells.
Common absorber materials used are cadmium telluride, amorphous silicon, and copper indium gallium diselenide (CIGS). Figure 6 shows a standard process sequence for developing a Si-based thin film solar cell. Diode-pumped solid state (DPSS) lasers are usually employed for scribing each layer of the cell. These layers are also used during the edge scribing process.
Figure 6. Typical process steps in manufacturing of a Si-based thin film solar cell panel.
P1 Level Laser Scribe
In the first step of thin film solar cell production, the front electrode is coated onto a glass substrate 2-3mm in thickness. This usually includes a 1-1.5µm thick layer of tin oxide, indium tin oxide (ITO) or zinc oxide, commonly referred to as a transparent conductive oxide (TCO). In the second step, a 25-50µm wide P1 electrode pattern is scribed, which is performed using a 1064nm, 15-20W power q-switched DPSS Nd-vanadate laser irradiated either from the oxide side or from the glass side.
For higher throughput, the beam must be scanned quickly and the laser must be operated at high repetition rates; typically 80-100khz but sometimes even higher. The laser should have a narrow pulse width (15-50ns) to ensure that the peak power is above the material ablation threshold even at high repetition rates.
Also, to realize a clean scribe and a consistent, repeatable process, pulse-to-pulse stability and beam quality are very important. Based on these laser parameters, space scribing speed of 1-2m/s can be effectively realized. The application of flat top beam shaping optics also aids in achieving higher scribing speed by decreasing the beam overlap needed for scribing.
Figure 7. P1 layer ITO scribed using 1064nm, HippoTM H10-106Q laser at 1.25m/s.
Figure 7 illustrates both the top view and 3D profile of 40µm wide P1 level scribe sliced at 1.25m/s using 20W average power, 1064nm q-switched laser Hippo™ H10-106QM at 80kHz.
P2 and P3 Level Laser Scribes
After the front electrode layer on the glass is designed, the panel retreats back into the CVD machine and is coated by means of a semiconductor, such as amorphous silicon. With the help of a green 532nm Nd-vanadate laser, this P2 layer is patterned suitably. A short pulse width of 15-30ns is perfect for this purpose, high repetition rates are again required to achieve higher throughput.
Since a silicon material has a lower material ablation threshold, the power requirement for the P2 scribe is considerably lower, typically less than 1W. Here, either a green laser of low average power can be utilized or the beam from a 4-6W laser can be divided and multiple scribes can be performed simultaneously.
After the P2 scribes are over, the panel is coated with less than 1µm thick aluminum and zinc oxide or ITO and finally laser-scribed with 25-50µm wide P3 pattern. Again, pulse-to-pulse stability and beam quality are very important to obtain a good scribe and to prevent any damage to the underlying layers. Similar to P1 layer scribing, higher scribing speed can be obtained using flat top beam shaping optics and higher repetition rate laser.
Figure 8. P2 and P3 level scribed using 532nm, HippoTM H10-532Q laser at 1.5m/s.
Figure 8 illustrates a top view of 40µm wide P2 and P3 level scribes obtained at scribing speed of 1.5m/s utilizing the 532nm Hippo™ H10-532Q laser, operating at 80kHz.
In certain conditions, once the patterning of P3 level is over, one more cleaning operation is performed to remove the material from the perimeter of the glass substrate. This process ensures electrical isolation and provides space to house subsequent panel packaging. A high average power q-switched 1064nm Nd-YAG laser is ideal for this purpose.
Solar cells are a clean and alternative source to fossil fuels for producing electricity. Thin film solar cells integrated onto a glass panel provide a viable option for developing solar cells cost-effectively. Also, the application of FPD manufacturing infrastructure can be exploited to produce inexpensive thin film solar cells.
Lasers can be utilized to scribe all three layers of a si-based thin film solar cell structure. DPSS lasers at 532nm frequency and 1064nm wavelengths provide a perfect means for scribing layers at high scribing speeds. Higher throughput can be realized by implementing higher repetition rate and flat top beam shaping optics.
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