Effect of different alkali (Li, Na, K, Rb, Cs) metals on Cu 2 ZnSnSe 4 solar cells

Abstract

It is well established that the addition of sodium (Na) to chalcopyrite or kesterite based solar cells markedly increases the solar cell performance. In this work, we explore the effect of Na and other alkali metals like potassium (K), rubidium, caesium and lithium (Li)on pure selenide Cu2ZnSnSe4 (CZTSe) solar cells. We demonstrate the deposition of alkali metals using spin coating on e-beam evaporated metal precursors. The stack of metal precursors with alkali layer was then selenised at high temperatures to obtain a good quality CZTSe absorber. The diffusion of alkali metals into the absorber layer was confirmed using glow discharge optical emission spectroscopy. Samples doped with Na or K have shown improvement in the open circuit voltage. A maximum power conversion efficiency of 8.3 % (without anti-reflection coating) and a top open circuit voltage 430 mV was achieved for combination of K and Na. Amongst the rest of alkali metals, Li looks the most promising dopant as far as optoelectronic properties are concerned.

Complex Interplay between Absorber Composition and Alkali Doping in High‐Efficiency Kesterite Solar Cells

“…Strikingly, the dependency of the Sn content on the highest device performance of different alkali elements [10] Na > Cs > K > Rb > Li "Efficiency enhancement of Cu 2 ZnSn(S,Se) 4 solar cells via alkali metals doping" [11] K > Rb > Na > Li > Cs "Influence of alkali metals (Na, Li, Rb) on the performance of electrostatic spray-assisted vapor deposited Cu 2 ZnSn(S,Se) 4 solar cells" [12] Li > Na > Rb "Alkali doping strategies for flexible and lightweight Cu 2 ZnSnSe 4 solar cells" [13] K > Na has neither been reported before nor taken into account in earlier publications that compared the effects of alkali elements on kesterite solar cells. [10][11][12][13] The influence of Sn content on V OC , J SC , and FF is shown in Figure 3a-c for the "high" alkali content samples. As a result of the qualitatively distinct behavior of Li, Na, and K (light alkali elements) on the one hand and Rb and Cs (heavy alkali elements) on the other, it is helpful to separate them into two groups.…”

“…[8,9] Several studies comparing different alkali elements and their effect on solar cell properties and device performance have recently been published. [10][11][12][13] Table 1 compares the effects on device performance by extracting a ranking of the various alkali elements in each publication in the order of their capability to improve device performance. It is apparent from these rankings that no consistent experimental results have been obtained, which triggers two questions: (i) Why do the published results differ so much?…”

Doping and alloying of kesterites

“…Although the results and the explanation on the effect of Li doping/alloying differ and even contrast to each other, partially due to variation in material preparation conditions and matrix composition, some conclusions can be drawn based on the available reports [23][24][25][26][27][28][29][30][31][32]: (1) Li alloys with CZTS and widens the band gap of (Li x Cu 1−x ) 2 ZnSn(S,Se) 4 [24,25,27]; (2) incorporation of Li to kesterite is sensitive to Na so that Li alloying is more easily achieved without Na, in the ceramic route [25] or on quartz substrate, or using a blocking layer to prevent Na diffusion from SLG [24,27]. The presence of Na diffused from SLG greatly reduces the Li doping concentration [23,26]; (3) Li doping/alloying improves photovoltaic performance regardless the doping concentration, however, the mechanism on how Li doping/alloying improves device performance remains unclear.…”

“…Rubidium and caesium are alkali elements with ionic radii of 1.66 Å and 1.81 Å, respectively, which is significantly larger than Cu with 0.74 Å. XRD measurements on Rb and Cs treated CZTSSe absorber layers indicated that Rb and even more so Cs are not replacing CZTSSe atoms in the kesterite lattice [32]. The impact of Rb and Cs on kesterite absorbers and respective device performance was studied several times [28,29,32]. Equivalent to the lighter alkali metals, Rb and Cs can form alkali poly-selenide liquid phases at comparably low temperatures.…”

“…Several studies comparing different alkali elements and their effects on kesterite properties and device performance have been published [29,31,32,49]. However, no consistent experimental results have been obtained concerning the impact of alkali treatments on the device properties and performance.…”