Effects of composition and thermal treatment on V_OC‐limiting defects in single‐crystalline Cu₂ZnSnSe₄ solar cells

Abstract

Single-crystalline Cu 2 ZnSnSe 4 (CZTSe) solar cells with open circuit voltages reaching 500 mV are achieved through a combination of composition control and a lowtemperature thermal ordering treatment. A comparison of the device results for Cupoor CZTSe with Cu/Zn + Sn ratios of 0.77 and 0.86 is presented with and without the implementation of a 130 C absorber annealing treatment. An increase in bandgap energy is observed via external quantum efficiency measurements with both the decrease in Cu content and the implementation of the order anneal, the latter of which also leads to a decrease in Urbach energy. Defect characterization performed with admittance spectroscopy on devices is demonstrated as insufficient because of low-temperature current barriers. Photoluminescence (PL) on crystal surfaces however enables a qualitative comparison of the defect landscape between crystal compositions and annealing treatments. Both a highly compensated and a lightly doped defect model are used to fit PL as a function of laser fluence to identify defects contributing to each observed recombination channel. The PL signatures attributed to the Zn Sn defect become unresolvable with a decrease in Cu/Zn + Sn ratio from 0.86 to 0.77. Furthermore, a decrease in Cu-Zn disorder is observed upon the implementation of the annealing treatment through both a comparison of potential fluctuation depths and of both PL models.

Emergence of flexible kesterite solar cells: progress and perspectives

“…Although the more defective grain interior yet larger bulk volume must lead to more non-radiative recombination in the quasineutral region of monograin CZTS solar cells, the V OC deficit against the Shockley-Queisser (SQ) limit of monograin CZTS and thin-film CZTS are very close. For pure-selenide CZTSe, the monograin CZTSe fabricated by University of Delaware shows slightly larger E u compared to the record thin-film CZTSe, but much higher V OC and lower V OC deficit than any reported thin-film CZTSe solar cells 77 . These contradictions could be attributed to the difference in non-radiative recombination at grain boundaries of monograin and thin-film solar cells.…”

“…Therefore, surface chemical treatments are critical for achieving high-efficiency monograin layer CZTSSe solar cells. Monograin powders are reported to be etched using 1% Br-MeOH followed by 10% KCN to remove the defective surface layer 34,77 . An air annealing and surface CdS treatment are also found to further improve the quality of the CZTSSe monograins, particularly, making the monograins more ordered (Fig.…”