Introduction
The great aspect of solar cells is that they are producing non-polluting renewable energy by absorbing photons i.e. sunlight, and by extension converting the energy of the photon to electrical energy for applications or conservation.
One of the main challenges in the photovoltaic industry is the 33% efficiency limit for any traditional solar cell [1], which is where quantum dot solar cells (QDSCs) come into the spotlight as a solution to break the efficiency limit.
The efficiency limit is due to excess energy lost to heat (thermalization loss), which could have been utilized [2]. The QDSC solves this issue with “quantum confinement” which will be elaborated further in the following.
What are quantum dot solar cells?
Quantum dot solar cells utilize crystalline nanoparticles known as quantum dots (QDs) as the absorbing photovoltaic material, and are investigated to replace common bulk materials such as silicon. QDs are typically in the size of 1-10 nanometers and are made from semiconductor materials or metals, e.g. cadmium selenide (CdSe) and lead sulfide (PbS).
One of the biggest attractive attributes of QDSCs, is that they can cover the whole spectrum of wavelengths from sunlight. This is possible since the band gap i.e. the absorption spectrum is related to the size of QDs due to the quantum confinement effect. Therefore, the band gap can be tuned to match the solar radiation spectrum by adjusting the size of QDs, thereby providing great flexibility of light absorption and better efficiency in solar power generation [3].
Quantum dots made from lead sulfide are of most interest due to its great tunability [4]. In the case of bulk materials, the band gap is fixed in terms of what material is used, which makes thermalization loss inevitable.
Due to the very small size of QDs, it is also possible to print them into thin flexible sheets and make transparent solar cells more of a reality. These could possibly be incorporated into screens or windows as a form of electricity generation. The technology has also improved substantially in terms of efficiency, from 2.7% in 2010 to a reported 16.6% in 2020 [5].
Technical specifications of QDSCs
The QDSC is typically sandwich structured, with a photoanode, a counter electrode and an electrolyte. The photoanode consists of a transparent conducting electrode (TCE, typically indium tin oxide glass) and a metal oxide semiconductor (usually TiO2) coated with a layer of quantum dots [3].
The photoanode is where the sunlight is absorbed and as a result generates both positively charged electron vacancies (holes) and negatively charged electrons in the solar cell. The positive charges are transferred between the photoanode and the counter electrode with the use of an electrolyte.
The purpose of the counter electrode is to transfer electrons from the external circuit (the electrical device) to the electrolyte, which results in catalyzing the redox reactions of the electrolyte [3]. A schematic of the QDSC is shown on Figure 1.
Figure 1: A schematic of the quantum dot solar cell, which shows all the main components of a QDSC. The electrons transfer from the photoanode through the external circuit to the counter electrode.
Outlook on Market
QDSCs are currently yet to be commercially viable in mass scale due to substantial challenges, such as reducing QD material cost and developing high-throughput deposition methods for mass production [6]. Although, several small companies have begun to produce QDSCs with the most notable ones being QD Solar, and Solterra Renewable Technologies, a subsidiary of the QD mass-manufacturer Quantum Materials Corp.
Solterra gets a headstart in the immature market currently and has accordingly the proprietary technology to mass-produce QDSCs at hundreds of meters per minute using roll-to-roll printing technology. [7]
End note
QDSCs are currently seen as the next generation of photovoltaic devices, which has the most potential to break the thermodynamic efficiency limit of 33% with its excellent optoelectronic properties and the use of quantum confinement. QDSCs also have a maximum potential conversion efficiency of 66%, which is double the possible efficiency of traditional single junction solar cells [8], demonstrating that quantum dots could be the future of solar power.
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Sources
[1] Rühle, Sven “Tabulated values of the Shockley–Queisser limit for single junction solar cells”, published in ScienceDirect (2016).
[2] Semalti, Pooja et. al. “Advancements in Quantum Dot Solar Cells: Synthesis and Applications”, published in Sigma-Aldrich (2020).
[3] Technical University of Denmark (DTU): Department of Chemistry. “Chemistry at the Nanoscale: Chapter 4”. (2020)
[4] Lutfullin, Marat et. al. (King Abdullah University of Science and Technology) . “Quantum Dots for Electronics and Energy Applications”, published in Sigma-Aldrich (2020)
[5] Hutchins, Mark. PV Magazine. “A quantum dot solar cell with 16.6% efficiency”. (2020)
[6] Jean, Joel. “Getting high with quantum dot solar cells”. Published in Nature (2020).
[7] Solterra. “Solterra: Business Objective”. (2020)
[8] NREL. “Quantum Dots Promise to Significantly Boost Solar Cell Efficiencies” (2013).