1. Q: What is the optimum concentration level for your solar cells?

A: While there is no clear answer to this question, concentration in the range of 500 suns is a good place to start. At lower concentration ratios, silicon solar cells probably make better economic sense. At significantly higher concentration ratios (>1000x), systems run into issues with cooling and series resistance and multijunction cells may run into issues with stability of the tunnel junctions.

2. Q: Are your terrestrial cells the same as your space solar cells?

A: While there are similarities between a space solar cell and a terrestrial CPV cell, there are intentional differences incorporated into the cell structure to account for the different operating conditions on earth versus in space.

3. Q: Do you have standard cell sizes or standard configurations?

A: Our most common cell size is the CDO-100. This is a concentrator cell with dual busbars (the two metal bars on the edge of the solar cell), and an area of 1 cm². This cell can be purchased in three configurations:
1) Bare cell
2) Cell with welded interconnects
3) Concentrator Cell Assembly (CCA): cell soldered to a ceramic substrate with a bypass diode and conformal coating

4. Q: What is the output of the solar cell?

A: Assuming 50 W/cm² flux and efficiency of 37.5% under 25° C, our CDO-100 cell produces 18.8 W. Since actual operating temperature will be higher, counting on 18 W per cell is a reasonable assumption.

5. Q: Can I have some sample of high concentration solar cells to try?

A: We do not offer sample products.

6. Q: Can I visit your site?

A: Our Sylmar, California facility does not have a showroom. As our facility manufactures products for a number of different programs, access to the facility is restricted. The best way to purchase CPV products from Spectrolab is to contact us at terresemail@spectrolab.com.

7. Q: What is the minimum number of cells I can purchase?

A: Spectrolab has a minimum purchase order value of $5,000. The minimum number of cells you can purchase will vary based on the cell type, size, unit price, and technical support required. For the CDO-100 cells, a $5000 order will allow quantities of:
     120 CDO-100 bare cells
     or
     60 CDO-100-ICs (cells with interconnects)
     or
     15 CDO-100-CCA (concentrator cell assemblies)

8. Q: For budgetary purposes, what is the price of the cells at large volume?

A: Taking the CDO-100 cells as an example, at volumes of 10 MW per year, we estimate the bare cells to be approximately $10 each and cells with welded interconnects to be approximately $14 each. These prices will continue to decrease with sales volume over the next few years.

9. Q: Can you export CPV solar cells outside the United States?

A: Yes. Currently, concentrator photovoltaic (CPV) solar cells for terrestrial purposes (i.e., not for space) are controlled by the U.S. Department of Commerce under a commodity jurisdiction classification of EAR 99. This classification allows for the export of CPV solar cells to most destinations unless the receiving party is on the Denied Parties List or there is a country embargo in place.

10. Q: What types of CPV solar cells can I buy?

A: The C1MJ solar cell is our current product. The C2MJ solar cell, with a projected mode efficiency over 38%, is under development for production in 2008. Work at Spectrolab to provide robust, higher-efficiency solar cells is ongoing. The average (mode) efficiency of the C1MJ cells is 37.5% under a concentration of 500X and 25° C.

11. Q: Do you sell lenses, optics, trackers or related concentrator housing materials?

A: No.

12. Q: Do you offer cells for dense arrays?

A. Yes, however, please note that manufacturing of dense arrays is not our business. Hence, we do not offer technical assistance on how to assemble a dense array.

13. Q: Do you offer information on bypass diodes, solder types, or thermal adhesive to use?

A: Information related to our processes is proprietary and can be made available only after a cooperative agreement is established with a customer.

14. Q: I am a potential consumer and I wish to buy something for my home. What can I buy from you?

A: Spectrolab does not offer products intended for residential applications. Our products are components incorporated into CPV systems. We are not a CPV system provider. To purchase a system you will need to contact a system provider

15. Q: Do you manufacture custom CPV solar cells?

A: We prefer to standardize our solar cells. This insures the lowest prices, the shortest delivery times, and the greatest fidelity in qualification testing. However, custom solar cells may be available for large-volume purchases. The customer may pay for non-recurring engineering involved in development and may be responsible for qualifying the assembly to meet unique requirements. There is a minimum requirement of approximately US $40,000 for non-recurring engineering. Delivery time for a non-standard cell is at least five months after acceptance of an order.

16. Q: What is the lead time for your products?

A: Lead times vary, depending on the configuration and quantity of cells ordered. Standard bare cells require about six weeks. Longer lead times are needed for cells with welded interconnects, cell receivers, and cells requiring an export license.

17. Q: What is the maximum temperature a concentrator solar cell can handle?

A: During assembly, the cells can be exposed to temperatures as high as 300° C for a few minutes. During operation, we suggest maintaining a temperature below 100°C. Higher operating temperatures will reduce the cell efficiency and may jeopardize the lifetime of the cells in the field. Cell performance is a function of temperature. For every 10° C rise in cell temperature, the cell efficiency drops by about 0.5% absolute efficiency. That is, a 38% cell at 25° C will be a 37% cell at 45° C.

18. Q: Do you sell reject or surplus CPV solar cells?

A: No.

19. Q: How can I attach CPV cells on cooling plates or ceramic sub-mounts?

A: Assembly details are specific to the CPV system being developed.

20. Q: Is active cooling of solar cells required?

A: The question of how to cool the cells is up to the CPV system designer. Typically dense-array modules are actively cooled; whereas point focus systems may be actively or passively cooled.

21. Q: What kinds of optics are best for concentrating light on your cells?

A: Our solar cells work with reflective or refractive optics. For refractive optics, we recommend the use of a light homogenizer to smooth the flux on the cells and to avoid issues of chromatic aberration. The multijunction cell consists of three subcells in series, each of which absorbs a different segment of the solar spectrum. If the optics fail to transmit certain wavelengths, one subcell may limit the current output of the entire multijunction cell.

22. Q: Does uniformity of light make a difference in terms of cell performance?

A: Yes, with highly non-uniform flux, the efficiency will drop and temperature variation could cause reliability issues.

23. Q: What is the highest-efficiency cell you have produced?

A: Spectrolab has achieved 40.7% conversion efficiency for a champion cell in December 2006. This result was verified independently by the National Renewable Energy Laboratory. This record efficiency was achieved at 240x concentration and 25° C.

24. Q: Can I buy your 40.7% efficient solar cells now?

A: Production cells will always have a lower average efficiency than the champion cells. Also, typical world-record cells are measured at concentrations less than 300 suns. Higher concentration (400-700 suns) is typically needed for CPV systems to be commercially viable. At these higher concentration levels, series resistance will tend to lower the efficiency of the cells. Our current minimum average production efficiency of the C1MJ cells is 36% at 500x and 25° C. We expect that average production efficiencies will rise as we introduce next-generation concentration cells.

25. Q: How do I solder the solar cells onto ceramic substrates?

A: Achieving a good solder joint between the cells and the next level substrate is critical for the robustness of the cells under continuous illumination. A solder joint that has large voids will result in high rates of cell infant mortality due to thermal runaway.

26. Q: Why do you weld interconnects to the solar cells? Is this the only way to do it? Can I solder interconnects to the busbar or do wire bonding?

A: Welding is one way of attaching interconnects to the cell. Both soldering and wire bonding are also acceptable. When it comes to welding, we offer to do it because of the potential impact of the welding process on the integrity of the cells. When we offer cells with welded interconnects, we will measure the performance of the cells after welding to insure no damage has occurred. The welding process is proprietary.

27. Q: What warranty is offered for CPV cells?

A: Warranty clauses are discussed in our terms and conditions for sale. Because of the dependence of the cell reliability on factors beyond our control, including the integrity of the solder joint between the cells and the substrates/heat sinks, the uniformity of the flux, the cooling design, etc., we do not currently warranty the cell for the entire lifetime of a system. There is an initial acceptance period during which the defective cells are replaced if they do not meet the agreed-upon specifications.

28. Q: Where can I buy a concentrator system?

A: Please contact concentrator PV system manufacturers directly. A web search on "CPV" will yield some answers. As Spectrolab does not represent customers, you are encouraged to contact them directly.

For more details please contact Spectrolab.