Next-Generation Solar Power Technology (Part 2 of 2)

Tanuj Tiwari is research assistant at University of Texas, USA -- Sanjay Tiwari is professor at Photonics Research Lab, Pt Ravishankar Shukla University, Raipur -- Tanya Tiwari is working at R&D Division of Samsung India

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Read Part 1

Quantum-dot solar cells based on perovskites materials will convert not just rooftops but complete buildings, including windows, into solar energy generators. Up to 26% efficiency has been achieved by mechanically combining perovskite with silicon solar cells.

Quantum dots are already being used in quantum-dot televisions. These have revolutionised the display industry with ultra-high-definition colours and greatly increased effective viewing angles, eliminating the need for rare earth elements—minerals where China has a virtual monopoly in the market.

Quantum dots, however, can also be used to absorb light to boost the output of photovoltaics, photocatalysts, light sensors and other optoelectronic devices. In fact, quantum-dot solar cells can absorb energy 24×7. While conventional silicon modules can absorb visible light during peak times of the day, quantum-dot solar cells can absorb ultraviolet through visible to infrared lighting spectrum to produce power day and night.

Through a process called multiple exciton generation, quantum dots capture excess photon energy that is normally lost to heat generation. The incident light radiations enter through the transparent electrode of a quantum-dot solar cell onto a light-absorbing layer of dots and create electron-hole pairs (e–/h+). The charged particles then separate and eventually travel to their respective electrodes, thereby producing electric current.

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Quantum dots are made from electro-optically active materials, whose size as well as composition controls the photon energy that they interact with, allowing their absorption (and emission) wavelength to be tuned by particle size. These can be tuned to deliver multiple electron-hole pairs from one photon—a process called multiple exciton generation that has allowed quantum-dot solar cells to operate at 10 per cent efficiency.

In quantum-dot solar cells, the function of light-absorbing material is carried out by nanocrystals of semiconducting metal chalcogenides such as CdS, CdSe, PbS and PbSe. Like other emerging photovoltaic technologies, quantum-dot cells can be prepared via low-cost solution-phase chemistry methods and are amenable to high-speed printing techniques.

The structure of quantum dots is peculiar with generally cadmium selenide as the inner core and a Cd1−xZnxS outer shell, coated in silica to avoid oxidation. The outer shell acts like an absorber. When a photon interacts with a quantum dot, the electron transfers from its valence band into the conduction band, leaving a hole in the valence band. The cell is designed such that the shell only absorbs high-energy photons, and the new photon from the core can propagate via internal reflections throughout the glass and quantum-dot layers. Finally, the propagating photons would reach the glass edges, where one or more solar cells could pick them up.

Skyscraper buildings with large windows could harness light for electricity. Researchers at the Los Alamos National Laboratory in New Mexico have reported that a thin film of quantum dots on glass could be the solution to achieving satisfactory efficiency in window photovoltaic systems at a low cost.

A thin layer of quantum dots on normal glass could have a lifetime of up to 14 years but low overall energy conversion efficiency of 1.9 per cent, which should be raised to 6 per cent for practical use. It’s amazingly easy to add quantum dots to window glass as a machine pours slurry of quantum dots and polyvinylpyrrolidone polymer onto the glass and a blade spreads it out to form a thin sheet.

Quantum-dot solar cells may bring breakthrough innovation in the design of solar arrays with their favourable power-to-weight ratio and high efficiency. Mass and area savings as well as flexibility will result in miniaturisation, lower power consumption, increased efficiency, versatility and improved functionality of future satellites. The research also paves the way for new opportunities in the terrestrial renewable energy sector.

Quantum-dot solar window technology converts not just rooftops but complete buildings, including windows, into solar energy generators. It is envisaged that highly efficient and large-area colourless luminescent solar concentrators using heavy-metal-free colloidal quantum dots will soon be available in the market. Luminescent solar concentrators use a new sunlight harvesting technology that has the potential to turn any window into a daytime power source. Many academic research groups have shown that this technology holds promise. But very few companies such as Solterra Renewable Technologies in Texas claim to be competent in this area as it requires unique material chemistry and manufacturing know-how.

Solterra’s cells are designed to exploit Quantum’s low-cost synthesis methods for making four-armed quantum dots. The unique shape of these crystals reduces the probability of electron-hole recombination, which leads to greater charge transport and photovoltaic efficiencies than with spherical quantum dots of the same material.

Quantum-dot solar cells can be made more efficient but further progress in this area has been impeded by the challenge of understanding the mechanisms of electrical conductance in quantum-dot solids and the processes that limit the charge transport distance and low efficiency. The Los Alamos Lab of USA has recently solved this problem. This will lead to a dramatic boost in the photo-voltage and overall device efficiency of quantum-dot solar cells. As economies of scale are achieved, quantum-dot solar cells will reach the market at competitive prices.

Structure and working mechanism of a quantum-dot solar cell
Fig. 7: Structure and working mechanism of a quantum-dot solar cell

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