Charting progress: putting solar cell efficiency to the test
Solar efficiency records are constantly being broken and advancing technology is making them more and more efficient. But how is the process being tracked, and how can it we be sure that companies are telling the truth about their accomplishments?
Since the early 1980s, The National Renewable Energy Laboratory (NREL) has developed a publicly available chart of the highest reported energy conversion efficiencies of research grade solar cells in all major PV technologies in various forms. NREL principal scientist Dr Dean Levi has worked on the project since the outset, along with principle engineer Dr Keith Emery, to make the process of recording record efficiencies of solar cells and modules more accurate. Levi explains the work they do and why the tables are so important for the renewable industry.
An online version of this chart can be found here.
Lindsay Dodgson: What are solar efficiency tables and what are they for?
Dean Levi: The chart provides a historical record of the evolution of the highest achieved efficiency in all of the major recognised photovoltaic (PV) technologies. In order for a cell efficiency to qualify for inclusion in this chart the measurement must be performed or confirmed by one of three internationally recognised reference PV performance measurement laboratories. This chart identifies the institution that produced the solar cell for each of the record efficiencies on the chart. In addition, the scientific journal Progress in Photovoltaics has been publishing an official 'solar cell efficiency table' biannually since its inception in September 1992. This publication is co-authored by principal investigators at each of the three internationally recognised PV performance laboratories in the US, Germany and Japan. The US lab is the NREL, the German lab is the Fraunhofer Institute for Solar Energy or ISE, and the Japanese lab is the National Institute of Advanced Industrial Science and Technology, or AIST.
This publication provides an internationally recognised and agreed upon reference table of the highest reported efficiencies in all major PV technologies. Nearly all of the PV journals have published editorials on reporting efficiencies. They universally suggest or require measurements by a recognised independent test lab. This is to keep the community on the same page when it comes to efficiencies. The NREL chart also helps with this.
LD: When did records begin, and what have been the general trends you have seen?
DL: The first record efficiencies were established for several early technologies in 1976-1977. Trends that we have seen since recording began is that there are periods of steady sustained incremental increases in efficiency, periods where the efficiency will plateau for years at a time, and periods following these plateaus where technological innovation will spur new periods of rapid increases in efficiency.
The most thoroughly established technologies such as wafer-based crystalline silicon have reached a plateau that is with a few percentage points of its maximum theoretical efficiency. Other technologies lacking the deep technological basis of silicon have languished at efficiencies well below their theoretical maximum. There has been renewed rapid progress in industrially relevant PV technologies, such as polycrystalline cadmium telluride, or CdTe, in recent years as the PV industry has grown large enough to fund large corporate research programmes. There have also been new exciting developments of radically new technologies with unprecedented increase in efficiency to over 20%. The most dramatic of these is the case of perovskite solar cells that have recently exceeded 22% efficiency in less than three years since their discovery.
LD: How many companies are currently listed on the tables? Has there been an increase in interest in recent years?
DL: There are currently about 15 recognised world-record efficiencies in different PV technologies. Each of these is held by a different entity. Over the years there have been dozens of different record holders. Overall the number of record holders has remained in the ten-15 range, as defined by the number of recognised PV technologies. The actual number of companies engaged in research and development of PV technologies peaked in the 2005-2010 timeframe. As global PV prices have plummeted by 80% over the past seven years, there has been a significant decrease in the number of companies engaged in PV research and development.
LD: How are the efficiencies of different solar technologies measured?
DL: The independently measured efficiencies that appear in the Progress in PV table or the NREL chart are with respect to the standard reporting conditions at the time of measurements. These conditions have changed slightly over the years. The measurements are performed with respect to internationally recognised standards such as IEC 60904. For cells and modules without concentration the current reference conditions are 1,000Wm-2, 25°C temperature, ASTM G173 or IEC 6090-3 edition three global reference spectrum. For concentration, the irradiance is reported at the maximum efficiency and the reference spectrum is the direct component of the ASTM G173 or IEC standards. The area is defined by an aperture or the total cell area.
LD: Has the technology you use to measure efficiency improved over time and what have been some of the improvements?
DL: The actual laboratory equipment used to measure solar cell efficiency is not highly sophisticated, but the methods for reducing uncertainty and increasing standardisation have continuously evolved over the past three decades. For example, between 1988 and 1992 Independent PV test labs developed spectrally adjustable simulators for all PV technologies, not just amorphous silicon. In 2011, Multijunction Quantum Efficiency (QE) Measurement procedures were established to correct for shunting and luminescent coupling for more than 40% of efficient PV technologies.
LD: How do you ensure that the records are independent and unbiased?
DL: The labs that provide the results all have quality systems that require them to be unbiased and independent. In general if a record from one of the recognised labs is measured by another recognised independent lab the data from the other recognised lab would be used. None of the groups that test cells or modules are involved in fabricating them. As part of the quality system for the independent test labs all data must be reviewed and approved by a person fully knowledgeable in the measurement but did not perform the measurement.
LD: Why is independent adjudication of this so important?
DL: There is frequently a significant impact of measured efficiency values, either financially or in terms of prestige as the 'world record holder' in a particular technology. University research programmes depend on leading the field for access to research funding. The multibillion-dollar PV industry sells its product in units of Watts at standard test conditions. The wattage output of a solar panel is proportional to the measured efficiency.
An uncertainty of 1.0 % in module efficiency can translate into $5m of lost income for a large PV manufacturer producing 1GW per year PV and selling its modules at $0.50 per watt. Cost of financing for multi-million dollar PV power plants can be very sensitive to module efficiency and reliability. Hence the stakes can be very high for the outcome of an efficiency measurement, demanding reliably independent and accurate measurement laboratories.
LD: Has anyone ever tried to cheat to make their technology appear more efficient than it actually is?
DL: Are you at all familiar with the American sport of baseball? Teams are constantly trying to gain an advantage over one another through actions that may skirt the rules, and this is accepted.
PV researchers are nowhere near as devious as baseball managers, but there are many subtle aspects of how a device is set up that can influence the efficiency measurement in negative or positive ways. Savvy researchers will tend to use every tool at their disposal, there is a great deal on the line for them in trying to get that next world record.
Part of the job of the certified labs is to thoroughly understand the measurement process in order to ensure that all devices are measured in a way that provides a level playing field.
I would say that 99% of all researchers are honest and straightforward and seek to get an accurate measurement of their device efficiency. As in any human endeavour, there are those who will try to rig the system.