New Breakthrough Could Revolutionize Solar Panel Efficiencies

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Researchers recently unveiled a new breed of materials that could catapult PV solar panels into entirely new thresholds of conversion efficiency—far past the limitations of the Shockley-Queisser limit.

The secret lies in the use of a barium titanate crystal to convert sunlight into electric power. Laying the groundwork for this breakthrough was Russian physicist Vladimir M. Fridkin, who nearly 50 years ago discovered a mechanism for converting light into electrical power. He suggested that such materials could collect the “hot” electrons that carry extra energy in a PV material when excited by sunlight, and they would do so before they lose their energy.

Current Conversion Efficiencies Hit 25 percent Wall

 It’s no secret that solar PV manufacturers have been leapfrogging barriers to efficiency for years now. First it was 10 percent, then 17 percent and later 22 percent. SunPower currently holds the record for the world’s most efficient rooftop solar module, reaching an NREL-confirmed efficiency of 24.1 percent.  The record-breaking SunPower module was constructed “using laboratory solar cells” of 25 percent mean efficiency. The design is based on commercially available module architecture. It seems that current PV materials had run into a wall, and that conversion efficiencies would never cross a certain threshold.  At least with materials currently in use, PV solar was stuck in a rut.  But that has changed, thanks to an old physicist and a new breed of materials.

 Using UV Light to Generate Electricity

Ordinary semiconductor solar cells absorb sunlight between two regions, one with a surplus of negative-charge carriers (electrons), and another with a surplus of positive-charge carriers (holes). To generate electricity, photons from sunlight need to energize the electrons to a certain threshold called the band gap. In most PV materials, some of the solar spectrum energy merely generates heat and not electricity. Overcoming this limitation, researchers discovered that a barium titanate crystal could convert sunlight into electric power with far greater efficiencies. Such materials could exceed the Shockley-Queisser limit, for instead of absorbing light in the visible spectrum, they would absorb ultraviolet light to dislodge electrons and create electric current.

Harnessing “Hot Electrons”

Fridkin and his colleagues discovered that crystal symmetry was a key element in improving the photovoltaic properties of a material. He noted that a “bulk photovoltaic effect” could transport photo-generated hot electrons in a certain direction without necessarily colliding and “cooling off.” This then could push the limits of solar power conversion beyond the Shockley-Queisser limit, since the excess energy of hot electrons is needn’t be lost but extracted as power before it’s wasted as heat.

An Insulator That Boosts PV Energy Conversion

When one thinks of PV generated electricity, the last thing that comes to mind is using an insulator to raise conversion efficiencies. But that’s exactly what researchers found. Barium titanate absorbs less than 10 percent of the sun’s solar energy. Yet this material—essentially an insulator—converts the sun’s energy 50 percent more efficiently than current theoretical limits of materials used in conventional solar cells. What has become known as the bulk photovoltaic effect allows free electrons generated by sunlight to harness the excess heat energy they possess. These electrons can be tapped for energy before they cool off or ‘thermalize’ and lose their excess energy.

All of this validated Fridkin’s decades-old assumption that eventually led to the current breakthrough. As a leader in ferroelectricity and piezoelectricity, he paved the way for a deeper understanding of how light interacts with ferroelectrics. It’s breakthroughs like these and those who piggyback on them that will revolutionize PV materials science and ultimately lead to astonishing conversion efficiencies in tomorrow’s solar panels.

Posted on Wednesday, October 19th, 2016