A British and Australian research team has built an intermediate-band solar cell with a quantum ratchet semiconductor nanostructure that is said to extend the lifetime of the device’s ratchet band mode. Their new design is based on a cellular structure known as the Vaquero-Stainer Device (VSD).
Intermediate bandgap solar cells (IBSCs) are believed to surpass Shockley-Queisser limit – the maximum theoretical efficiency that a solar cell with a single pn junction can achieve. It is calculated by looking at the amount of electrical energy separated per incoming photon.
The devices are generally designed to produce a large photogenerated current and maintain a high output voltage. They contain an energy band that is partially filled with electrons within the band gap of the semiconductor. In this cell configuration, photons that do not have enough energy to push electrons from the valence band to the conduction band use this intermediate band to form an electron-hole pair.
The scientists said that previous research demonstrated a QR-IBSC device using a quantum well superlattice (QWSL) at low temperature.
“This approach included a set of ‘ratchet band’ (RB) states into which electrons scatter irreversibly at the cost of a small energy penalty,” they said.
Their new design is based on this cellular structure, also known as the Vaquero-Stainer Device (VSD). The new High Barrier Device (CHB) structure is claimed to extend the lifetime of the RB mode and enable cell operation at room temperature.
In the proposed cell configuration, an additional 2 nm thick layer of aluminum arsenide (AlAs) films is placed between the final quantum well of the QWSL and a wide layer made of multiple quantum wells based on aluminum, gallium, and arsenide (Al).0.3Ga0.7As) which act as conduction band (CB).
“This AlAs barrier increases electron confinement in RB and reduces thermal runaway,” the researchers noted, noting that they used fast double-demodulation two-photon spectroscopy to measure the two-photon photocurrent (TPP).
They fabricated the cell on an undoped GaAs substrate using molecular beam etch and a buffer layer made of the same material. They also used gold rings 200 nanometers thick with 20 nanometers thick titanium around us in the front electrical contact and metallized the front of the device with layers of gold and zinc.
They also used partial etching for the backside electrical contact, which was then metallized with indium (In) and germanium (Ge) and connected to a gold strip.
“The back of the device was polished to a 45-degree bevel to refract the in-band beam that hit the back of the device,” the researchers said.
The researchers found that the RB mode lifetime in the HBD design is greater than 100 seconds at 12 Kelvin (K), which they say is a seven-order improvement over 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 said.
The team presented the new cell design in “Room-temperature operation of a quantum-fast intermediate-band solar cell,” which was published recently RRL sun. It includes researchers from Imperial College London in Great Britain and the University of New South Wales (UNSW) in Australia.
“Future generations of the device should now be developed with bandgaps more closely matching the solar spectrum and with structures that improve the efficiency of photon capture and RB-CB removal,” they concluded.