University of Utah material science and engineering associate professor Mike Scarpulla

In a world where renewables are the future of energy generation, researchers around the world are constantly chasing the best, most efficient technologies for each kind of clean energy source.

For solar photovoltaic energy generation, solar panels use semiconductor materials to convert light into electrical power. Each material technology has a different world record efficiency (which is always higher than what is achievable in full-sized commercial modules). Is the best material to use for solar panels silicon? Cadmium telluride? Or is it halide perovskite-based cells, which have caught the attention of many researchers of late? Academic and corporate scientists and engineers are spending billions of dollars researching and commercializing what they hope to be the best technologies.

Working with physics professor Phillip Dale at the University of Luxembourg, University of Utah associate professor Mike Scarpulla in the departments of Materials Science & Engineering and Electrical & Computer Engineering has learned that our conventional method of tracking solar cell record efficiencies versus year belies the amount of research effort behind efficiency gains for each particular type of photovoltaic technology. Dale and Scarpulla’s work was recently published in the latest issue of Solar Energy Materials and Solar Cells. The paper, “Efficiency versus effort: A better way to compare best photovoltaic research cell efficiencies?” can be read here.

Most photovoltaic-technology tracking within the community of researchers has used the efficiency-vs-year Best Research-Cell Efficiency Chart from the U.S. Department of Energy’s National Renewable Energy Laboratory. In just 10 years, the remarkable halide perovskite materials have been used in solar cells that went from literally nothing to efficiencies matching or exceeding those form materials with decades of R&D history. The NREL efficiency chart might suggest overall that the halide perovskite technology is more promising because of the incredible rate of efficiency increase versus time compared to the trends for other materials that are more incremental before plateauing.

But Scarpulla and Dale have found evidence that there is a nearly universal correlation between record solar cell efficiencies and the amount of R&D effort devoted to them for single-junction photovoltaic material technologies that have achieved record efficiencies above 20%. Each of these technologies have needed essentially the same amount of research-and-development effort for each increment of efficiency increase.

“One way of saying it is that there is no free lunch — every increment of solar cell device performance has to be hard-won with more effort than the last increment,” Scarpulla said. “This is a whole new dimension for how we should be looking at the development of PV technologies.”

Scarpulla and Dale used the number of research publications as a proxy for the sum of research dollars and person-hours put into each material technology over its lifetime. They found that the two leading PV technologies that have been commercialized, silicon and cadmium telluride, as well as other leading research and early commercial-stage technologies such as CuInGaSe2 (CIGSe) and halide perovskites have essentially taken the same amount of R&D effort to achieve each solar-cell-device-efficiency milestone; it took ten papers to reach 5%, one hundred papers to reach 10%, and so on.

Analyzing these R&D-stage learning curves, “may offer an effective method for assessing the potential of emerging PV materials in the R&D stage where cumulative production learning curves do not exist,” according to the paper. “An improvement to the metric could include elements related to patents, citations or total publication word count.”