DOE OSTI2013
The goal of 1366 Direct Wafer™ is to drastically reduce the cost of silicon-based PV by eliminating the cost barrier imposed by sawn wafers. The key characteristics of Direct Wafer are 1) kerf-free, 156-mm standard silicon wafers 2) high throughput for very low CAPEX and rapid scale up. Together, these characteristics will allow Direct Wafer™ to become the new standard for silicon PV wafers and will enable terawatt-scale PV – a prospect that may not be possible with sawn wafers. Our single, high-throughput step will replace the expensive and rate-limiting process steps of ingot casting and sawing, thereby enabling drastically lower wafer cost. This High-Impact PV Supply Chain project addressed the challenges of scaling Direct Wafer technology for cost-effective, high-throughput production of commercially viable 156 mm wafers. The Direct Wafer process is inherently simple and offers the potential for very low production cost, but to realize this, it is necessary to demonstrate production of wafers at high-throughput that meet customer specifications. At the start of the program, 1366 had demonstrated (with ARPA-E funding) increases in solar cell efficiency from 10% to 15.9% on small area (20cm2), scaling wafer size up to the industry standard 156mm, and demonstrated initial cell efficiency on larger wafers of 13.5%. During this program, the throughput of the Direct Wafer furnace was increased by more than 10X, simultaneous with quality improvements to meet early customer specifications. Dedicated equipment for laser trimming of wafers and measurement methods were developed to feedback key quality metrics to improve the process and equipment. Subsequent operations served both to determine key operating metrics affecting cost, as well as generating sample product that was used for developing downstream processing including texture and interaction with standard cell processing. Dramatic price drops for silicon wafers raised the bar significantly, but the developments made under this program have increased 1366 confidence that Direct Wafers can be produced for ~$\$$0.10/W, still nearly 50% lower than current industry best practice. Wafer quality also steadily improved throughout the program, both in electrical performance and geometry. The improvements to electrical performance were achieved through a combination of optimized heat transfer during growth, reduction of metallic impurities to below 10 ppbw total metals, and lowering oxygen content to below 2e17 atoms/cc. Wafer average thickness has been reduced below 200µm with standard deviation less than 20µm. Measurement of spatially varying thickness shortly after wafer growth is being used to continually improve uniformity by adjusting thermal conditions. At the conclusion of the program, 1366 has developed strong relationships with four leading Tier1 cell manufactures and several have demonstrated 17% cell efficiency on Direct Wafer. Sample volumes were limited, with the largest trial consisting of 300 Direct Wafers, and there remains strong pull for larger quantities necessary for qualification before sales contracts can be signed. This will be the focus of our pilot manufacturing scale up in 2014.