Germanium Incorporation in Cu₂ZnSnS₄ and Formation of a Sn–Ge Gradient

Abstract

Alloying of Cu2ZnSnS4 (CZTS) with Ge can potentially promote grain growth and suppress the formation of Sn‐related defects. Herein, a two‐step fabrication route based on compound co‐sputtering and sulfurization at a high temperature is used to prepare Ge‐incorporated CZTS (Cu2ZnGexSn1 − xS4 [CZGTS]). For Cu2ZnGeS4 (CZGS), films deposited using elemental Ge and binary GeS targets are compared. The recrystallization is shown to be promoted for the absorbers deposited using Ge target, possibly due to lower sulfur content in the precursor suppressing the formation of wurtzite‐like phases during sputtering. The grain growth and crystallinity in CZGTS are slightly improved for x = 0.2 but not for higher concentration of the incorporated Ge. Owing to the composition‐dependent electronic properties, compositionally graded CZGTS films may be beneficial for reducing recombination towards the back contact. Hence, herein, the successful formation of a steep concentration gradient with Ge and Sn is demonstrated by the deposition of a CZGS/CZTS precursor stack followed by sulfurization with varying time periods.

Optimizing kesterite solar cells from Cu2ZnSnS4 to Cu2CdGe(S,Se)4

“…First, our results show that Ge-substitution does not affect polymorph preference (panel a) with kesterite being the groundstate polymorph (E Stannite À E Kesterite > 0 eV per formula unit) for all x Ge (see Table S5 in Section S4 of the ESI † for CZTS and CZGS wurtzite data). That Ge increases the relative stability of kesterite vs. stannite for all but dilute Ge-substitution is in general agreement with other theoretical work 59,121,122 and the experimental observation of enhanced grain growth in Ge-doped CZTSSe. [123][124][125] Recent scanning electron microscope images show that grain growth and crystallinity in CZTGS is not improved for x Ge > 0.2 121 but this could be due to suboptimal annealing conditions for each Ge composition during fabrication.…”

Comparison of Sulfur Incorporation into CuInSe2 and CuGaSe2 Thin‐Film Solar Absorbers

“…[ 29 ] Other example of such methodology applied to solar absorber alloys can be found in our earlier works. [ 30,31 ] The Brillouin‐zone integrations were performed using 3 × 2 × 2 Γ ‐centered Monkhorst–Pack grid [ 32 ] and a cutoff energy of 350 eV. The ionic force threshold for atomic relaxations was set to 0.01 eV Å −1 .…”

Record 1.1 V Open‐Circuit Voltage for Cu2ZnGeS4‐Based Thin‐Film Solar Cells Using Atomic Layer Deposition Zn1‐xSnxOy Buffer Layers

“…The Zn x Sn 1-x O y (ZTO) can be used as an alternative non-toxic substitute for the commonly used CdS buffer layer due to the possibility of band edge movement by either compositional change [21] or deposition temperature, [22] leading to reduced cliff-like alignment for CZGS absorbers (Figure 1). Moreover, the bandgap of CZGS [10,23] is comparable to CdS, [24] which results in significant parasitic absorption in the CdS. The wide bandgap ZTO [22] can reduce the parasitic short wavelength absorption loss compared with the CdS [24] buffer layer.…”

“…[7][8][9] We have previously reported the formation of a Ge-Sn gradient toward the rear interface of CZGTS absorbers made by sulfurizing a Cu 2 ZnGeS 4 (CZGS) layer buried underneath a CZTS layer. [10,11] Since the diffusion of Ge and Sn occurs faster through grain boundaries than grains, [10,12] the accumulation of Ge on absorber interfaces easily occurs during sulfurization, which modifies the bandgap energy at the interfaces. The importance of the conduction band offset (CBO) at the absorber and buffer interface is well established.…”