Why are kesterite solar cells not 20% efficient?

Abstract

Although kesterite solar cells show the same range of band gaps as the related chalcopyrites, their efficiencies have so far reached only 10%, compared to 20% for the chalcopyrites. A review of the present literature indicates that several non-ideal recombination channels pose the main problem: (i) recombination at the interface between the kesterite and the CdS buffer. This is very likely due to an unfavourable clifflike band alignment between the absorber and the buffer. However, for pure selenide absorbers, this recombination path is not dominating, which could be due to a spike-like band alignment at the absorber-buffer interface. (ii) A second major recombination becomes obvious in a photoluminescence maximum well below the band gap, even in record efficiency absorbers. This is either due to a very high density of defects, comparable to the density of states in the band, or to stannite inclusions. In view of the phase diagram, secondary phases are not likely the source of the low energy emission.Only in sulphide kesterite a non-stoichiometric SnS phase could also cause this low energy radiative recombination.

Absorption Coefficient of a Semiconductor Thin Film from Photoluminescence

“…The thickness plays an important role in this consideration, but band tailing is also an important parameter. It is well known that the stronger the band tailing, the stronger the redshift of the PL compared to the band-gap energy [41,42]. Therefore, it is more challenging to measure a reliable PL signal in the high absorption range, where Θ w or Θ 1−R may be used to derive Δμ for a semiconductor with strong band tailing.…”

Formation of nanometer-sized Cu-Sn-Se particles in Cu2ZnSnSe4 thin-films and their effect on solar cell efficiency

“…In general, they are considered to be detrimental to the cell's performance. Cu 2 SnS(e) 3 , Cu 2 S(e) and SnS(e) have smaller band gaps than CZTS(e) and should be avoided by any means as their presence will decrease the open circuit voltage [3]. In agreement with this presumption, CZTS(e) based solar cells with the highest efficiencies are found to be Cu-poor ( [3,17].…”

“…Secondary phases that were observed are predominantly ZnS(e) [9e12], Cu x S(e) [13,14], Cu 2 SnS(e) 3 [15,16], and SnS(e) x [9,14]. In general, they are considered to be detrimental to the cell's performance.…”

“…Under such growth conditions ZnS(e) can be easily formed. However, ZnS(e) has a larger band gap than CZTS(e) and its detrimental effect on cell efficiency is moderate [3].…”

Extended antisite defects in tetrahedrally bonded semiconductors

“…A technologically important example is CZTS which has attracted high interest as a solar absorber material 32,33 . The formation of Cu Sn anti-site point defect in this material leads to the creation of four unfavorable S-Cu 3 Zn motifs, which violate the octet rule by 0.75 electrons.…”