Scientific Method / Science & Exploration

Solar cells must emit light to attain perfection

Researchers find balancing absorption and emission works best.

Berkeley researches claim to have hit upon a counter-intuitive means of boosting the efficiency of solar cells: making them emit light

Researchers at the University of California-Berkeley claim to have hit upon a counterintuitive means of boosting the efficiency of flatplate solar cells by making them emit light. "What we demonstrated is that the better a solar cell is at emitting photons, the higher its voltage and the greater the efficiency it can produce," said principal researcher, UC Berkeley Professor of Electrical Engineering Eli Yablonovitch.

To briefly recap the mechanism behind the photovoltaic effect itself, photons from some external light source (the sun, preferably) entering a solar cell excite the electrons in the semiconductor into higher energy states. This frees them from confinement so that they can convey current. (The charge itself is created by using two materials. Free electrons find it easier to move in one direction between the materials, creating a negative charge in one and a positive charge in the other.)

Berkeley researchers Eli Yablonovitch and Owen Miller
Berkeley researchers Eli Yablonovitch and Owen Miller
Eli Yablonovitch

But in some semiconductors, when the excited electrons return to lower energy states they have a knack for emitting photons, which is a desirable property for a semiconductor in, say, an LED. But Yablonovitch argues this is also crucial to solar cells—an argument his team is the first to make, a recent press release claims. Unlike an LED, the electrons in a solar cell are absorbing photons from an exterior source as well as emitting their own.

But the emitted photons find it hard to escape the semiconductor's surface due to their narrow escape cone. These trapped photons are likely to be re-absorbed, causing yet another subsequent photon emission. Yablonovitch asserts that the more photons that can be made to escape, the greater the voltage achievable in the cell. How? "Fundamentally, it's because there's a thermodynamic link between absorption and emission," explained team member Owen Miller.

Yablonovitch goes into more detail about this thermodynamic link in a 2011 paper (which, it should be pointed out, has yet to be peer reviewed). Due to "basic thermodynamics," a material that absorbs light must also emit light, to an extent determined by the material's absorptivity. "External photon emission is part of a necessary and unavoidable equilibration process," it explains, "which does not represent loss at all."

Yablonovitch has likened this to maximizing power from a water wheel driven by water from a faucet. In this analogy, the water tank has an open top and is continually filled by rainfall. If the faucet is fully open, the tank will drain too quickly, losing pressure and failing to turn the wheel. With a closed (or almost closed faucet), the pressure in the tank may be high, but the flow of water is insufficient to turn the wheel. The optimal setup is to have the faucet part open so that the water in and out of the tank is balanced and the pressure is equalized. You may be losing water, but the energy output of the water wheel over time is maximized.

The external luminescence from the solar may also be thought of as a gauge of the voltage in the cell. "My preferred way of explaining this is to say that the external luminescence is like a contactless volt-meter in the cell," he told Ars. "To say that we want more external luminescence is like saying we want more voltage."

An Alta Devices solar cell
An Alta Devices solar cell
Joe Foster, Alta Devices

How does one go about expelling these unwanted photons? A highly reflective rear mirror can help to expel new photons, as can an "optically textured" front surface that facilitates photon escape, the paper claims. And Yablonovitch has done exactly that, with the help of some friends.

In June, 2011, Alta Devices, a company cofounded by Yablonovitch, announced it had achieved an efficiency of 28.2 percent in its gallium arsenide-based solar panels (the previous record of 26.4 percent having been achieved in 2010).

The boost of almost two percent may sound modest, but when closing in on the Shockley-Queisser limit, every tenth of a percent counts. The Shockley-Queisser limit is the theoretical maximum efficiency—33.7 percent—at which single p-n junction flatplate cells can operate. Receiving 1000 W/m2 of solar radiation at noon on a clear day, these ideal cells would be capable of producing 337 W/m2 of electrical power. Multilayer cells are capable of greater efficiencies.

The voltage increase offered by the emission of photons may be sufficient to explain the shortfall between real-world achievements and the Shockley-Queisser limit—a limit which the team's research indicates should be perfectly possible in gallium arsenide, which has internal fluorescence approaching 100 percent. For the Shockley-Queisser limit to be achieved, solar cells require a 100-percent external fluorescence to "balance" the light coming in. In other words, a perfect system would emit one photon for every photon absorbed. Needless to say, a voltage increase equates to a power increase, since power (in watts) is the product of the current (in amps) and potential difference (in volts).

To achieve cell efficiencies greater than 30 percent, the optical performance of solar panels "will need to be very carefully designed," the 2011 paper, Intense Internal and External Fluorescence as Solar Cells Approach the Shockley Queisser Efficiency Limit, concludes. Yablonovitch hopes to see 30-percent efficiency cracked in the next few years. The team is set to present its latest findings in May in its presentation, The Opto-Electronics which Broke the Efficiency Record in Solar Cells, at the Conference on Lasers and Electro Optics in San Jose, California.

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