Chalcopyrite solar cells —state-of-the-art and options for improvement

Abstract

Chalcopyrite solar cells will have to play an important role to mitigate the climate crisis, because of their particularly low carbon emissions. Doping in these semiconductors is due to native defects and intentional alkali impurities. The recent progress in efficiency has been made possible by post-deposition treatments with heavy alkalis. Tail states and band gap distribution are the main limitations for the open circuit voltage in state-of-the-art chalcopyrite solar cells. Further efficiency limitations are due to the increased diode factor because of metastable defect transitions. Alloying with Ag opens new possibilities of band-edge engineering, as well as seems to improve the diode factor. In state-of-the-art cells the back contact is passivated by a Ga gradient; considerable research has been done to passivate the back contact by structured or continuous dielectric layers. A leap forward in efficiency can be expected from tandem cells. Chalcopyrite solar cells show promising potential as bottom cells as well as top cells.

Composition dependence of electronic defects in CuGaS2

“…The k exponent results in $ 3 2 at low excitation intensities and in $ 2 2 at high excitation intensities. As previously mentioned, the exponents take on multiples of 1 2 , and a change in the exponent occurs when competing transitions or a defect involved in the transition…”

“…F I G U R E 6 (A) Excitation intensity with respect to integrated PL flux for transition lines 2.474 (E), 2.481 (D), 2.488 (C), and 2.496 eV (B) fitted with the double power law. (B) Fit of emission line at 2.496 eV (B) with two power-law exponents k ¼ 3 2 and k ¼ 2 2 at high and low excitations, respectively. ϕ 0 denotes the turning point between the two excitation regimes, that is, the flux at which one of the defects is saturated.…”

“…Cu(In,Ga)S 2 is not only a promising material for single‐junction solar cells but also a strong candidate for top cells in tandem applications to absorb high‐energy photons in the solar spectrum 1–3 . Barreau et al reported an efficiency of 16%, 4 which is considerably lower than the efficiencies of the selenide chalcopyrites Cu(In,Ga)Se 2 which have reached 23.6% (M. Edoff et al, in preparation) 5 .…”

Shifting the Paradigm: A Functional Hole‐Selective Transport Layer for Chalcopyrite Solar Cells

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Chalcopyrite-based thin-film solar cells are industrially produced [1][2][3] and have achieved stable efficiencies exceeding 23%. [4,5] Remarkably, these high efficiencies employ a full-area metal back contact [4][5][6] with high recombination velocity ranging from 10 5 to 10 7 cm s À1 .

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