The U.S. Department of Energy (DOE) estimates that a $1 per watt installed photovoltaic (PV) solar energy system - equivalent to 5-6¢/kilowatt hour (kWh) — would make non subsidized solar competitive with the wholesale rate of electricity nearly everywhere in the United States. In order to reach this goal, manufacturers in the highly competitive solar manufacturing industry have placed a greater focus on two important aspects of their processes - throughput and efficiency. While all sectors of the industry must focus on process improvements, traditional crystalline silicon based manufacturers stand to benefit the most through process improvements because almost one half of the installed cost of a silicon cell solar module is driven by the cost of the silicon wafer. Silicon wafers break during manufacturing due to a combination of large stress in handling and thermal processing, and the fragile nature of the silicon substrate.
Current wafer screening techniques use highly unreliable infrared imaging or an energy intensive method based on optically induced thermal stress. To remedy these inefficient processes, scientists at NREL have created a low-power system using concepts of an optical cavity furnace with the capacity to operate at throughput levels required by high speed solar cell manufacturers (1200-2000 wafers/hr).
Wafer screening by optically-induced thermal stress is traditionally accomplished through a process containing several water-cooled lamps with parabolic reflectors. As a wafer is transported underneath reflectors via conveyor belt, light sources optically induce a maximum thermal stress that the wafer would be subject to during manufacturing. If the wafer survives the test, it is likely to survive the cell fabrication process. This process requires a tremendous amount of energy and does not allow for high throughput processing.
The proposed method of screening wafers takes advantage of the concepts of an optical cavity furnace. In much the same manner as the traditional method, the wafer in the improved system is transported through the process via conveyor belt. However, in the improved system, an optical cavity is formed by placing optical sources within the reflecting walls of the furnace. This design ensures that energy coming from the light source is transferred to the wafer only, resulting in substantial energy savings over the traditional method. After the wafer exits the cavity furnace, it is subjected to either a jet of cold air or an ambient of a water-cooled camber to release the thermal energy. This dynamic temperature profile of the wafer produces a predetermined time-dependent stress in the wafer, which corresponds to the highest stress the wafer can experience during solar cell processing. This process has the capability to reach the required throughput levels of approximately 2,000 wafers/hr in a modular system that sits quietly on top of any existing line and uses far less energy than the traditional system.
Applications and Industries
- This method is applicable to all traditional wafer solar cell manufacturers
- Increased effective yield of crystalline silicon wafers
- Reduced overall cost of finished modules
- High Throughput of approximately 2,000 wafers/hour
- Reduced energy consumption over existing wafer screening systems
For more information, contact Bill Hadley at Bill.Hadley@nrel.gov
U.S. Patent # 8,780,343