Improving the photovoltaic performance of co‐evaporated Cu₂ZnSnS₄ thin‐film solar cells by incorporation of sodium from NaF layers

Abstract

We have investigated the influence of sodium (Na) on the properties of co-evaporated Cu 2 ZnSnS 4 (CZTS) layer microstructures and solar cells. The photovoltaic performance and diode properties were improved by incorporating Na from NaF layers into the CZTS layers, while Na had a negligible effect on the microstructural properties of the layer. The best cell fabricated by using an optimal CZTS layer (Cu/(Zn + Sn) = 0.70, Zn/Sn = 1.8) yielded an active area efficiency of 5.23%. The analysis of device properties suggests that charge-carrier recombination at CZTS/CdS interface is suppressed by intentional Na incorporation from NaF layers.

Highly efficient solution-processed CZTSSe solar cells based on a convenient sodium-incorporated post-treatment method

“…In general, alkali metals (especially sodium) incorporation into CZTSSe absorbers can be realized by pre-deposition [23,25,28] , post-deposition [27,29,30] or co-existence in the precursor solution [24,26] , in contrast to fabricating CZTSSe precursor layers. Unlike CIGS, although some experimental and theoretical effort s have been done to confirm positive roles of the sodium incorporation in the CZTSSe solar cells, fundamental understanding toward the nature of the interaction between Na and other elements or layers in the device and the underlying mechanism have not been fully clear [11] .…”

Ultra-thin Cu2ZnSnS4 solar cell by pulsed laser deposition

“…Still, the current record efficiency of 12.6% for CZTSSe [3] and of 9.1% for pure-sulfide CZTS [4] is far below the 21.7% efficiency demonstrated by the very similar CIGS technology [5], from which they borrow most of the device architecture. Regarding the pure sulfide CZTS, different vacuum deposition techniques have been successfully employed, such as cosputtering [4,[6][7][8] and co-evaporation [9][10][11][12]. The most successful strategies to date consist of a two stage process, where precursors are prepared at a substrate temperature below 300 • C, followed by a high temperature annealing (>500 • C) done separately at much higher pressures.…”

Effect of Sb and Na Incorporation in Cu2ZnSnS4 Solar Cells

“…One of the reason that can explain lower performances of sulfur devices lies in the smaller grains observed in these absorbers which induces a higher concentration of detrimental grain boundaries . Addition of Na in CZTS absorber has already shown beneficial effects in term of reduction of non‐radiative recombination, enhancing the photovoltaic properties and as well as in term of increasing the grain size . However, in almost all the studies dealing with Na‐doping in kesterite devices, Na is already present during the absorber synthesis, namely either introduced from the substrate or in the precursor stack .…”

“…Addition of Na in CZTS absorber has already shown beneficial effects in term of reduction of non‐radiative recombination, enhancing the photovoltaic properties and as well as in term of increasing the grain size . However, in almost all the studies dealing with Na‐doping in kesterite devices, Na is already present during the absorber synthesis, namely either introduced from the substrate or in the precursor stack . Thus it has an impact both on the material structural properties and on defect passivation.…”