|Fig. 1: Module manufacturing cost per peak watt versus module efficiency.|
According to the US Energy Information Administration, global demand for energy is expected to increase by 50% by 2050. PV module production must increase from 10-15 gigawatts (GW) per year in 2010 to 100-200 GW per year to satisfy a meaningful portion of this expected new demand. Failure to achieve these production levels will likely lead to an increased dependence on fossil fuels and further risk global climate change. Furthermore, Americans have become increasingly concerned about the loss of domestic manufacturing jobs and reliance on the import of fossil fuels from countries with unstable leadership. Large-scale adoption of PV would help mitigate environmental damage, reduce reliance on foreign oil imports and promote domestic job creation. There are clear benefits to developing high risk but disruptive technologies, which cannot be easily replicated abroad, in the United States with the potential for domestic manufacturing.
|Fig. 2: Design of a 2-Terminal Tandem Solar Cell.|
The Department of Energy (DOE), at its most recent "$1/W" workshop held in Washington DC in August of 2010, estimated that at a total installed cost of $2/Wp PV will be competitive with natural gas and at $1/Wp PV will compete directly with coal. Though I have access to the literature from this workshop it is not yet available for public consumption. However, one can arrive at this conclusion based on a simple cost model that assumes 25 year lifetimes, financing costs of 8%/year and an average of 5.5 hours per day of one-sun light intensity. Fig. 1 shows the module production costs for leading PV module manufacturers. The cost per watt number for First Solar is based on the latest publicly available quarterly investor report while the numbers for Sunpower and Trinasolar are based on publicly available 2010 shareholder presentations available on each of their websites. Suntech Power's numbers are more challenging to obtain given the range of products offered (including polycrystalline and monocrystalline silicon solar cells) but the estimates are based on the silicon cost per watt, as reported in their latest investor quarterly report, and related back to Trinasolar's costs. The 2015 projected cost/efficiency line is based on First Solar's 2014 Technology Roadmap and Sunpower's 2014 Cost Reduction Roadmap. With balance of system (BOS) costs projected to be $1~2/Wp by 2015, which, again based on guidance from the largest producers in the industry, total installed costs will be closer to $2~3/Wp. This is encouraging but insufficient to be disruptive and enable PV to be adopted on a large scale without substantial government subsidies. Fig. 1 highlights the region where a PV technology becomes disruptive and enables direct competition with natural gas and coal without the use of subsidies.
Tandem solar cells have the potential to be a disruptive technology that can allow PV to become competitive with fossil fuel. In a standard tandem, as shown in Fig. 2, two individual solar cells, one utilizing a higher bandgap semiconductor (top cell) and the other with a lower bandgap semiconductor (bottom cell), are vertically stacked. The top cell absorbs higher energy photons while allowing lower energy (sub-bandgap) photons to pass through the device to the bottom cell where they can be absorbed. Two-terminal tandems that are connected in series require current matching while four-terminal tandems, which can be mechanically stacked, remove the constraint of current matching.
|Fig. 3: Power conversion efficiency of two-junction, four-terminal thin film tandem solar cell as a function of the band gap of the top cell. The bottom cell has a fixed bandgap of 1.14 eV and assumes an open-circuit voltage of Eg - 0.3 V, and 100% internal quantum efficiency FF of 0.80.|
Two-junction tandem solar cells have a theoretical efficiency approaching 40% under one-sun intensity and can, in principle, be made compatible with low cost manufacturing techniques.  Fig. 3 shows a plot of the efficiency of a 4-terminal TSC as a function of the bandgap of the bottom cell based on a simple model that I created. The underlying calculation uses the solar spectrum under AM 1.5 conditions (not concentrated light) and assumes that a photon with an energy above the bandgap of the material will be absorbed and converted to an electron out of the devices (i.e., an external quantum efficiency of 100% is assumed). The model also uses a bottom cell with a bandgap of 1.14eV (typical of a CIGS solar cell) and a Voc and FF that are also typical of high efficiency CIGS solar cells. Though a simplistic model, it matches the theoretical predictions quite well an provides the essential insight needed for understanding why tandems offer significant potential if they can be made cheaply. From the figure we see that a wide range of bandgaps can be used for the top cell. This is mainly due to the 4-terminal configuration, which removes the constraint of current matching between the cells. The model shows that the ideal bandgap for the top cell is ~1.8eV, which again matches quite well with more rigorous models produced in the literature (e.g., ). However, this model assumes a monocrystalline material to which perfect ohmic contact can be made on both electrodes. The reality of thin film devices is that various losses occur that lower both the voltage and FF. A more realistic model for polycrystalline thin film tandem solar cells that account for these losses leads to an efficiency closer to 25%.  Though lower than the theoretical efficiency, this is still significantly higher than single junction cells that are being produced by industry.
As a final note, higher efficiency tandem devices are possible when either more junctions are used (e.g., three junction tandem solar cell) or when light concentration is employed. The record tandem efficiencies is 41.6%, which was achieved with a three junction GaInP/GaInAs/Ge two-terminal solar cell under 364-sun concentration.  However, these devices required epitaxial growth of single crystal material (e.g., GaAs) using expensive single crystalline substrates (e.g., single crystalline Ge), which has relegated this technology to concentrated solar applications.
© Craig Peters. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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