The conversion of solar energy into electricity does not solely rely on PV cells made from crystalline silicon. There are many alternative technologies including thin-film cells, perovskite cells, and organic cells. Recently, scientists in the US have also made breakthroughs in the development of PV cells based on quantum dots. Previously, the application of quantum dots in PV and other applications was constrained by the fact that this technology uses heavy metals. However, a study released by Los Alamos National Laboratory (LANL) this May offers a design of a quantum-dot solar cell (QDSC) that achieves a decent conversion efficiency rate without the use of heavy metals.
QDSC is a PV cell that has an active layer of nanometer-sized semiconductor particles for generating electricity. These particles, or quantum dots, are usually 2-10nm in diameter and made of compounds that often include heavy metals such as lead and cadmium. Their unique optical and electronic properties are governed by quantum mechanics. For instance, after absorbing the sunlight, quantum dots can emit light of different colors depending on their material composition, shape, size, and structure. Scientists have also found that they can create quantum dots tailored to have certain photonic, electrical, and magnetic qualities. Particles with larger diameters have a narrower energy band gap and produce redder light, whereas those with smaller diameters have a wider energy band gap and produce bluer light.
(Source: Los Alamos National Laboratory)
Since quantum dots are efficient and tunable light emitters, they can support a wide range of applications. Currently, the technology is present in many fields including catalytic processing, electronics, photonics, sensors, data storage, medical devices, and imaging solutions. Display systems are an area where quantum dots have been grabbing a lot of attention lately, as a major brand in consumer electronics is developing TVs and monitors based on this technology. However, there are still some obstacles that prevent quantum dots for being widely adopted in various applications. The environmental hazard posed by the toxic materials that make up the nanoparticles is especially a major concern.
Scientists at LANL have decided to substitute the heavy metals when developing their QDSC. According to their study that was published this May in Nature Energy, they made their quantum dots by first combining copper, indium, and selenium. Afterward, they coated the particles with zinc and inserted them into the voids of a porous titania film that acts as an electrode.
When the quantum dots on the titania film absorb the photons of the sunlight, they become excited and release electrons. These electrons are then transferred to the titania and turned into photocurrent. Victor Klimov, who specializes in semiconductor nanocrystals and is one of the authors of the study, said that the quantum dots made from the four relatively non-toxic elements have a lot of structural defects. These defects are supposed to adversely affect conversion efficiency, but further tests conducted by his team found that the opposite might be true. Under the lab condition, the photo-to-electron conversion efficiency rate of the device was 85% on average (or every 100 photons absorbed resulted in 85 electrons released). This implies that the defects might be assisting rather than impeding electricity generation.
On the other hand, the efficiency of the QDSC developed by the LANL team is much lower when it comes to converting the actual sunlight into electricity. The study points out that the actual solar energy conversion rate of the device is just 9-10%. In contrast, another QDSC developed by the researchers at Australia’s University of Queensland earlier this February has a conversion efficiency rate of 16.6%, which is a new record for this kind of PV cell. Nevertheless, the LANL team stressed that its device is completely non-toxic and can maintain a conversion efficiency rate within the mean range. Furthermore, it has the advantages of having a relatively low production cost and a high tolerance for defects. Thus, the LANL team believes its solution has a lot more potential in the solar energy market compared with other QDSCs.
(News source: TechNews. Top photo credit: Shutterstock.)