Overcoming Shockley–Queisser Limit: Why and How?

Every 24 hours, enough solar energy reaches the Earth's surface to meet the world's annual electricity needs... five times! [1] Solar energy is abundant, inexhaustible and clean. However, converting it directly into electricity is not trivial. The semiconductor technology currently in use is approaching the fundamental Shockley–Queisser (SQ) limit, which sets the maximum energy conversion efficiency for a single-junction solar cell at ~33.7%. [2]

In a recent article published in Science [3], the author briefly presents two new strategies to overcome the SQ limit. Namely, carrier multiplication and hot-carrier extraction. Both are fascinating concepts, each worthy of their own discussion.

In this post, however, I would like to bring everyone who is not familiar with photovoltaics up to date and briefly explain what the SQ limit is, how we have dealt with it so far and what is the current hotest-topic in photovoltaics. With this post, I hope to bring more attention to the rapidly developing PV research sector, which to my surprise is still highly underrepresented in the RH community.

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The SQ limit was derived in 1961 by William Shockley and Hans-Joachim Queisser. The key concept of the SQ limit is that solar cells can only convert photons with energies equal to or greater than the band gap energy of the semiconductor into electricity. Photons with energies below the band gap energy cannot be absorbed, while photons with energies significantly above the band gap energy generate excess energy in the form of heat and not electricity. This limit takes into account not only bandgap energy but also factors such the temperature of the cell and the spectrum of sunlight incident on the cell. Importantly, it applies to ideal solar cells, meaning that practical solar cells have efficiencies even lower than the SQ limit due to material imperfections and unavoidable extraction losses related to the balance between collecting carriers at a high electrical potential and collecting those carriers before they recombine (Figure 1). 
 

Figure 1. Taken from [4].

Solar spectrum on Earth's surface ranges from deep-UV to IR with it's peak in visible range. No single semiconductor could utilize the whole spectra efficiently, therefore, most common way to overcome SQ limit is using multi-junction (or tandem) architecture, where several multiple layers of semiconductor materials are stacked on top of each other, each with a different bandgap energy, allowing the solar cell to capture a broader spectrum of sunlight and convert it into electricity more efficiently. According to similar considerations as for SQ limit for a single junction cell, a two-layer cell can reach 42% efficiency, three-layer cells 49%, and a theoretical infinity-layer cell 86% in non-concentrated sunlight. [5] 

Most tandem cells that have been produced so far use three layers that are tuned to blue (top), yellow (middle) and red (bottom) and are made from gallium arsenide (GaAs) compounds. They are expensive and require similar techniques to the construction of microprocessors on the order of several centimeters in size. These cells are often used in applications where performance is of paramount importance. For example, in satellite applications.
 

Figure 2. Taken from [6].

The most promising recent technological breakthrough for large-scale photovoltaics is perovskite/silicon tandem solar cells, which have now achieved certified efficiencies of more than 33% for lab-scale devices, surpassing SQ limit for a single–junction cell technology. [7] In a perovskite/silicon tandem solar cell, the perovskite solar cell is usually placed on top of the silicon solar cell. This arrangement allows the perovskite layer to efficiently absorb high-energy photons (blue and green light), while the silicon layer absorbs lower-energy photons (red and infrared light). However, long-term stability, scalability and manufacturing processes remain the biggest challenges for this technology.

Following in the footsteps of Pierfrancesco Butti's post – if this post arouses sufficient interest, I would be happy to follow up with more sophisticated photovoltaic concepts, state-of-the-art technologies and the main challenges in their further development.
 

References:
[1] IEA (2023), World Energy Outlook 2023, IEA, Paris https://www.iea.org/reports/world-energy-outlook-2023
[2] https://doi.org/10.1016/j.solener.2016.02.015
[3] https://doi.org/10.1126/science.ado4308
[4] doi:10.1117/2.1201203.004146 
[5] https://doi.org/10.1088/0022-3727/13/5/018
[6] https://www.bbvaopenmind.com/en/technology/innovation/photovoltaic-paradigm-silicon-perovskite-tandem/
[7] https://doi.org/10.1126/science.adh3849