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A British-Australian research group has built an intermediate band solar cell with a quantum ratchet semiconductor nanostructure that appears to increase the lifetime of the devices ratchet band state. Its new design is based on a cell structure known as the Vaquero-Stainer Device (VSD).
An international research team has developed a novel intermediate-band solar cell (IBSC) design that includes a quantum ratchet (QR) semiconductor nanostructure. Apparently, this new element makes it possible to store photoelectrons in a long-lived state, which enables efficient optical reabsorption.
Intermediate band solar cells (IBSCs) are believed to have the potential to exceed the Shockley-Queisser limit, that is, the maximum theoretical efficiency that a single pn junction solar cell can achieve. It is calculated by examining the amount of electrical energy that is extracted per incident photon.
The devices are usually designed to provide a large photogenerated current and maintain a high output voltage. They incorporate an energy band that is partially filled with electrons within the bandgap of a semiconductor. In this cell configuration, photons with insufficient energy to push electrons from the valence band to the conduction band use this intermediate band to generate an electron-hole pair.
The scientists note that previous research demonstrated a QR-IBSC device using a quantum well superlattice (QWSL) at low temperature.
“This approach involved adding a set of ratchet band (RB) states, in which electrons are scattered irreversibly, at the cost of a small energy penalty,” they explained.
Its new design is based on this cellular structure, also known as a Vaquero-Stainer Device (VSD). The new design of the high barrier device (CHB) supposedly increases the lifetime in the RB state and makes possible the operation of the cell at room temperature.
In the proposed cell setup, an additional 2 nm-thick layer of aluminum arsenide (AlAs) films is inserted between the final quantum well of the QWSL and a broad layer made up of multiple aluminium-gallium-arsenide-based quantum wells ( Al0.3Ga0.7As), which act as conduction band (CB).
“This AlAs barrier increases the confinement of the electrons in the RB, reducing thermal runaway,” state the academics, noting that they used a high-speed double demodulation two-photon spectroscopy setup to measure the two-photon photocurrent ( TPP).
They fabricated the cell with an undoped GaAs substrate by molecular beam epitaxy and a buffer layer of the same material. They also used 200-nanometer-thick gold rings on 20-nanometer-thick titanium all around for electrical contact on the front side and metallized the front of the device with layers of gold and zinc.
They also used partial etching for the electrical contact on the back, which was plated with indium (In) and germanium (Ge) and connected to a gold strip.
“The back of the device was polished with a 45-degree bevel to refract the intraband beam impinging on the back of the device,” the researchers explain.
The scientists found that the lifetime of the RB state in the HBD design is more than 100 s at 12 Kelvin (K), which they said is an improvement of seven orders of magnitude compared to that of the Vaquero-Stainer cell. and other VSD designs.
“This led to the successful operation of the device at 300 K, which represents a significant advance in the field of IBSCs,” they stated.
The group presented the new cell design in “ Room Temperature Operation of a Quantum Ratchet Intermediate Band Solar Cell,” recently published in RRL Solar. Scientists from Imperial College London (United Kingdom) and the University of New South Wales (UNSW) (Australia) participate in it.
"Now future generations of the device must be developed, whose bandgaps are better adjusted to the solar spectrum, and with designs that improve the efficiency of photon capture and extraction from RB to CB," they conclude. |
