Solar Panel Manufacturing

The sun provides 1 kW / m² of free, non-polluting power for several hours every day. Thermal and photovoltaic systems take advantage of this as does the biomass. Coal, oil, plant ethanol, and wood are all forms of stored solar energy. While each energy conversion process has a unique spectral responsivity curve, most laboratory development work has concentrated on photovoltaic (PV) systems. Detailed knowledge of solar irradiance through the VIS and near IR is needed for cell optimization and economic viability assessment.

Crystalline-Silicon (c-Si) solar cells are the predominant solar cell technology used in fabricating solar panels because of the availability of silicon, well-developed manufacturing processes, and the high conversion efficiencies that can be achieved. Passivated Emitter Rear Contact (PERC) technology has been shown to generate absolute cell efficiency gains of a percentage point or more compared to conventional cells. In a conventional solar cell, there is an aluminum metallization layer that makes contact across the full area of the back of the cell. PERC technology involves creating a dielectric passivation layer on the rear side of the cell with openings to allow electrical contact to the metallization layer. PERC cells achieve higher efficiencies for a few reasons. First, the passivation layer significantly reduces electron recombination near the back of the cell, where the electrons would otherwise experience a strong attraction to the aluminum metallization layer. This allows more electrons to reach the front-surface emitter, thereby increasing current density in the cell structure. Secondly, the passivation layer enhances the cell's ability to capture light, particularly at longer wavelengths, because it causes multiple reflections, thereby enhancing the probability of absorption. In this way, the absorbing length of the cell is effectively doubled and current density is further improved. Finally, the passivation layer reduces heating of the backside metallization layer by reflecting IR light out of the cell. IR light that is not absorbed by the silicon (. > 1180 nm) would otherwise be absorbed by the aluminum and increase the heat in the cell. Cells are more efficient when operating at a lower temperature.

Lasers are used for creating openings in the dielectric passivation layer for ohmic contact. Aluminum paste is screen printed to this surface and a subsequent thermal annealing process alloys the aluminum with the laser-exposed silicon to form a good ohmic contact. A typical 6 inch PERC cell will have about 155 laser-scribed lines which are longer than 156 mm, 40-50 µm wide, and are separated by 1 mm. The aggregate length of the PERC scribes on a single wafer is approximately 25 m. The target processing time can be as fast as one second per wafer, which equates to a 25 m/s required scribing speed. Fast two-axis galvo scanners as well as spinning polygon scanners can achieve such speeds. MKS Spectra-Physics has demonstrated high-quality solar cell scribing for PERC processing as shown in Figure 1.

Industrial PERC processing also requires a laser that can keep up with the high scan speeds demanded. For this, MKS has tested the Spectra-Physics Quasar 532-75 high-power hybrid-fiber laser with TimeShiftª programmable pulse technology. The laser offers both high power and high PRF along with short pulse widths below 5 ns. For thin film removal, shorter pulse durations have been shown to be more energy efficient compared to longer pulse durations. With the Quasar laser, the shortest pulse durations are generated at the highest operating frequencies, which is ideal for PERC processing. Less heating and reduced risk of thermal damage to the underlying c-Si lattice are additional benefits of short-pulse processing. Further throughput improvement is also possible via beam-splitting. When a Quasar laser is operated at 850 kHz PRF, corresponding to one wafer per second (WPS) throughput, the available pulse energy is approximately three times more than necessary. Therefore, a three-beam split can effectively triple the throughput of a single laser thus allowing for throughputs of three WPS. Irrespective of the final system configuration, the Quasar laser platform and its unique TimeShiftª pulse-tailoring technology has the power, speed, and flexibility to meet the demands of industrial PERC solar cell manufacturing.

One technology which shows promise of meeting the growing demands of solar panel quantity, performance and cost-effectiveness is TOPCon, or tunnel oxide passivated contact, technology. Incorporating some laser operations to the TOPCon process can further improve photovoltaic performance.

First, the LDSE process, which stands for "laser-doped selective emitters," creates a shallow, highly doped n++ emitter region by adding an n-type dopant to the wafer and then using a laser to both ablate the anti-reflective coating layer and drive the dopant into the underlying silicon. This would be performed before growing the tunneling silicon oxide layer. The benefits of LDSE include improved overall cell efficiency compared to standard methods such as furnace-based doping.

Characterization of photovoltaics involves measurement of current voltage relationships under standard illumination and temperature conditions. Surface reflectance, deep level traps, carrier diffusion, crystalline structure and boundaries, junction type depth and temperature, optical absorption and scattering, series and shunt resistance and photon degradation all influence efficiency. The spectral responsivity curve takes many of these fundamental effects into account, but should record the temperature and intensity level and other measurement conditions for completeness. For example, voltage sweep rates and direction and contact resistivity also affect I-V measurements. Simulator pulse duration is important for some heterojunction and electrochemical cells.

Figure 2: A photoresponse scan of an experimental CIGS PV cell. A drop off of the signal is seen in the upper right-hand corner even though upon visual inspection the cell appeared to be uniform. The colors represent photocurrent.

Solar Simulators

Solar Cell Testing and Characterization

Photovoltaic Manufacturing Lasers