Defect behind early degradation of solar cells revealed
Oregon Department of Transportation
About half of the solar panels collecting energy today are made of boron-doped silicon, and they have a flaw. After their first few hours in the Sun, the panels’ sub-20% energy efficiency falls by an additional 2 percentage points due to light-induced degradation (LID). The efficiency drop amounts to losses of tens of gigawatts and millions of dollars annually, and despite 40 years of work, researchers don’t agree on how it happens. A team of experimentalists and theorists led by Michelle Vaqueiro-Contreras and Vladimir Markevich of the University of Manchester, UK, recently adapted existing instruments and methods and found the culprit.
To determine the efficiency of a solar cell, researchers typically measure how long electrons persist before recombining with positively charged electron holes on the lattice. Vaqueiro-Contreras (now at the University of New South Wales in Australia), Markevich, and their colleagues combined several electron-lifetime measurement methods with deep-level transient spectroscopy (DLTS). They measured the thermal emission spectra resulting from an electrical pulse that dislodged electrons trapped in recombination centers. By applying long pulses and taking measurements at up to 430 K, the researchers were able to close in on an impurity that leads to LID.
When a boron-doped silicon panel works as designed, electrons move between holes in the boron atoms’ highest valence levels. The Manchester team’s observations and ab initio calculations from a collaborative group at the University of Aveiro in Portugal revealed that even before a panel is exposed to sunlight, some boron atoms form bonds with oxygen dimers in the cell, which creates defects. Then during operation, the influx of energy turns the boron–oxygen defects into potent traps that force electrons to recombine with holes before they can reach the panel’s electrodes; thus the traps decrease the cells’ efficiency.
The Manchester team is applying its understanding of LID to another efficiency-sapping phenomenon with an unknown mechanism: light and elevated temperature induced degradation, known as LeTID, which occurs over years and affects a wider range of solar cells than does LID. (M. Vaqueiro-Contreras et al., J. Appl. Phys. 125, 185704, 2019 .)