Four-junction Tandem Thin Film Cells Have Huge Potential for Rooftop Solar: MIT Study

Due to its high energy yield, this PV technology is a great option for residential rooftop solar systems


Scientists working in the field of solar energy are constantly trying to find a balance between efficiency and cost. Recently, a team of researchers at the Massachusetts Institute of Technology (MIT), First Solar, and Siva Power conducted a study that suggests a four-junction tandem cell has the best potential use for residential rooftop systems, regardless of different climate conditions and its high cost.

The research group compared two types of thin-film solar cells. First, a standard design that uses single-junction thin-film solar cells and second, a design that has two different materials and uses either two-junction or four-junction thin-film solar cells, also known as tandem cells.

“Standard single-junction cells have a maximum efficiency limit of about 30 percent, whereas tandem cells, using two materials, can have much higher efficiency, above 40 percent,” said Sarah Sofia, an MIT graduate student and research coordinator on the study.

The study was conducted at three different geographical locations in the United States, each having distinct environments – Arizona (arid environment), South Dakota (temperate environment), and Florida (humid environment). As the amount of water vapor in the air can determine how much sunlight reaches the solar cell, a total of four different technologies were analyzed in each environment.

The researchers found that while the four-terminal tandem system was the best option for household-scale rooftop system given its high energy yield, the cells with the lowest production costs are best suited for utility-scale installations as the cost of installation and the control systems can be spread over many more panels.

“This paper breaks new ground because it precisely quantifies the cost of solar energy for different solar-panel technologies, in different climate zones, and for different application scales,” said Raffi Garabedian, Chief Technology Officer at First Solar.

Tandem cells, though more efficient, are also costly to manufacture. The study also factored the costs of installation and associated equipment. Another area that researchers looked at is how a project’s levelized cost of electricity (LCOE) could be affected if overall energy prices remain constant or decline over time.

The findings of the researchers may be applicable to a variety of thin-film solar technologies, the study concluded.

The materials used in the case study are already in commercial use. It is up to manufacturers how quickly they bring in this practical, economical four-junction tandem system for application in the rooftop sector.

A bottom-up analysis performed by Mercom in its 2017 Q4 Solar Market Update found that India’s rooftop installations grew by 56 percent year-over-year to reach a cumulative total of nearly 1.6 GW on December 31, 2017.