Guest Post: Twelve Good Reasons Why High Efficiency Matters in Photovoltaics

Historically, the high cost of solar cells has limited the widespread use of PV. However, the cost of PV modules has declined by a factor of approximately 50 since 1975 and is currently within striking distance of grid parity in some parts of the world. In addition, PV has been growing at an average annual rate of approximately 40 percent since 1998, in tandem with an experience curve that displays a 20 percent reduction in module ASP for every doubling in cumulative installed PV in the world.

At this rate, PV will attain grid parity in many parts of the world in the next one to four years. Production of higher efficiency cells, at very little or no additional processing cost, is the key to achieving that target.

This can be accomplished by clever cell design and manufacturing technology innovation, both of which can increase cell efficiency without appreciably increasing cost. Cells and modules are the DNA of the PV value chain because higher efficiency reduces the use of semiconductor material and the number of wafers, cells and modules required to build a given size system. There are 12 good reasons why higher efficiency cells, processed at little or no additional cost, can reduce levelized cost of electricity (LCOE) for the customer and increase the profit margin, which can be shared by cell/module manufacturers and system installers. These reasons are listed below in chronological order from reduced material cost to LCOE.

1. An increase in cell efficiency increases the power output of a cell. This reduces grams-per-watt of silicon and dollar-per-watt of a silicon wafer because wafer price and size are independent of cell efficiency.
2. Higher cell efficiency may reduce the cell conversion cost ($/W) if the cell technology or processing cost in dollars-per-square meter ($/m2) remains unchanged.
3. Higher cell efficiency reduces module assembly cost ($/W). This is because module assembly cost in $/m2 remains unchanged but the module power output (W/m2) increases.
4. The combination of the above-listed three items reduces the module manufacturing cost.
5. Increased module power reduces the system area and number of modules required for a given size system. This is most important for rooftop applications where space is limited or is at a premium.
6. Higher efficiency increases the power density (kW/m2) of a ground-mounted system. This is highly desirable even if the land cost and area are not as important.
7. Higher efficiency lowers the total BOS cost because two of the four BOS components, mounting hardware and labor, are efficiency- or area-dependent, while inverter and indirect costs are not.
8. Higher efficiency reduces the O&M cost because fewer panels need to be maintained.
9. The combination of all the above-listed factors reduces the installed system cost ($/W), which is a strong function of Si wafer cost ($/W), cell processing cost ($/W), module assembly cost ($/W), BOS cost ($/W) and O&M cost ($/kW-yr).
10. Higher efficiency generally corresponds to higher Voc, which reduces the absolute value of temperature coefficient for efficiency degradation (AC=-%/°C).
11. Higher efficiency increases the capacity factor (kWh/kW), because lower AC modules produce more energy (kWh).
12. Finally, higher efficiency modules reduce LCOE because it is proportional to installed system cost and inversely related to energy production or capacity factor for a given size system.

All of these effects are quantitatively depicted in the table below for a PV system in Atlanta with total BOS cost of $2.80 per watt and an area-dependent BOS cost of $0.97 per watt for a 16-percent-efficient reference module. In addition, a wafer cost of $2.24, cell manufacturing cost of $56.4/m², and module assembly cost of $56/m² is assumed for all three cases. While the accuracy of these cost assumptions will surely change (especially with respect to the wafer cost), it is important to note that they are constant for all module percentages compared here. As such, a relative comparison is meaningful.

Table: A Quantitative Analysis of the Impact of High Efficiency on Manufacturing Cost, Energy Production and LCOE of a Fixed-Size PV System

Suniva’s mission is to manufacture high-efficiency monocrystalline Si cells that are at the right balance of cost and efficiency to attain grid parity. In 2008, we started production with 17-percent-efficient cells and raised efficiency to 18 percent in 2009 without any additional cost. This was achieved through the process and design optimization of each layer of solar cell without altering the number of processing steps. In the period 2010 to 2011, we raised the cell efficiency to ~19 percent in production by pioneering the use of ion implantation for emitter formation. This technology innovation reduced the number of cell processing steps from nine to eight, while also producing a one-percent enhancement in cell efficiency. Suniva is currently working on manufacturable 20-percent-efficient cells with little or no additional cost.

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Dr. Rohatgi is the Founder and CTO of Suniva, Inc. and has positioned the company as a U.S. leader in the research, development and manufacturing of high-efficiency, low-cost monocrystalline PV cells, using unique processes and techniques that evolved from his work at the University Center of Excellence for Photovoltaic Research and Education (UCEP) at Georgia Tech. Dr. Rohatgi continues to pursue his research interests in the development of cost and efficiency roadmaps for attaining grid parity with silicon PV, as well as innovations in cell design and technology.