Highly efficient and affordable solar elements, long considered the holy grail of the renewable energy industry, can appear at any moment, according to Mike McGehee, Stanford’s professor of material science and engineering and senior fellow, by courtesy, at the Precourt Institute for Energy. In his public lecture at the Stanford’s Energy Seminar on March 3rd, 2014, McGehee covered emerging solar cell technologies and rebutted public opinion that solar energy might not live long. In fact, scientists are full of hope and zeal to find the best combination of efficiency and cost for photovoltaic (PV) modules in the near future. This hope is also shared by the Department of Energy, which set a Sunshot goal to reach $1 per Watt in Solar energy by 2017, McGehee said. $1 per Watt can make solar energy fully cost-competitive with traditional energy sources such as coal and natural gas.
As of now, we are less than halfway there. In 2010, one solar Watt cost about $3.40 including $1.48 for balance of system (BOS) and installation, $0.22 for an AC/DC inverter, and $1.70 for the PV module itself. The Sunshot goal breakdown is as follows: $0.40 for BOS & installation, $0.10 for an inverter, and $0.50 for the PV module. McGehee’s research group is focused on improving the module, and their chief idea is to find the best combination of layers for the PV panel in order to marry costs and efficiency.
As McGehee said, the most effective existing solar panel is made of Epitaxial Crystalline, which has about 45% performance and costs $40,000 per sq.m. This price point makes it unlikely to reach the market. The cheapest option is organic film for $30 per sq.m, but its efficiency is only 12%. So, a new hybrid technology with 30% efficiency and a $100/sq.m price tag is what McGehee group is aiming at.
Instead of the 4 to 20 layers that occur in most efficient multi-junction cells, the hybrid design will have only two. The lower layer will be based on an established technology, either silicon or copper indium gallium selenide (CIGS) films. For the upper layer a low-cost defect-tolerant technology is needed, and here McGehee has a choice of tried and true organic cells, nanowires, II-VI compounds (those formed from the elements of group II and VI of the periodic table) or Perovskite, which he prefers over all others.
Perovskite, although a newbie in solar energy, is showing great efficiency improvement. Usually, it takes over a decade of hard work for 100 scientists to get a material over 15% efficiency, McGehee said, but with Perovskite a team of 5 or 6 were able to break 15% in less than 5 years. Preliminary cost estimate of silicon-Perovskite Tandem Cell that the professor provided in March was $0.67 per Watt, or $167 per sq.m with 25% efficiency. Expected improvements in silicon technology should take the cost below the desired 50 cents per Watt.
Not only Perovskite is promising. Indium Phosphide employed in the tandem CIGS technology can be very efficient too. McGehee believes that Stion, a Khosla Ventures backed company, uses this material in its Simply Better solution. In February, Stion has demonstrated a 23.2% efficiency thin film cell based on its proprietary junction technology. However the company is closed lipped about the projected price per Watt.
My take-away lesson from the Energy seminar is that solar panels have an unexplored ocean of resources to reduce their cost per Watt, and a new disruptive technology may come out any moment.