Depth profiling with SNMS and SIMS of Zn(O,S) buffer layers for Cu(In,Ga)Se₂ thin‐film solar cells

Abstract

Zn(O,S) is a promising candidate to replace the commonly used CdS buffer layer for Cu(In,Ga)Se2 (CIGS) thin‐film solar cells due to its non‐toxicity and its potential to enhance the conversion efficiency of the CIGS solar cell. The composition of chemical bath deposited (CBD) and sputtered Zn(O,S) layers with thicknesses well below 100 nm was determined by sputtered neutral and secondary ion mass spectrometry (SNMS and SIMS). Despite numerous mass interferences of double‐charged atoms and dimers with single Zn, O and S isotopes, we developed an evaluation algorithm for quantification of SNMS depth profiles of Zn(O,S) layers. In particular, the superposition of double‐charged S and Zn atoms with O and S isotopes is accounted for numerically in the quantification procedure. For sputtered Zn(O,S) layers, the S/(S + O) atomic ratio and the vertical composition profile can be controlled by the O2 content in the gas flow and the substrate temperature during sputtering whereas for CBD Zn(O,S) the S/(S + O) ratio is constant around 0.7–0.8. A Cu‐depleted layer of about 5 nm on the CIGS surface after buffer deposition was observed for both preparation methods. With negative SIMS, we found more hydroxides and carbon residues in CBD Zn(O,S) as compared to sputtered layers. Best cell performance with sputtered Zn(O,S) layers was achieved for S/(S + O) ratios of 0.25–0.40, yielding efficiencies up to 13%. Our solar cells with CBD Zn(O,S) buffers exhibit higher efficiencies due to an improved open‐circuit voltage. Copyright © 2013 John Wiley & Sons, Ltd.

Electronic structure of the Zn(O,S)/Cu(In,Ga)Se2thin-film solar cell interface

“…Comparing the binding energies with values from literature, the two components for both samples can be assigned to ZnO and Zn(OH) 2 . Because the Zn(O,S) is deposited onto the CIGSe via a wet‐chemical deposition route, Zn‐OH bonds are not unexpected , suggesting that the dehydrogenation of the Zn(O,S) layer is incomplete for both the thin and thick Zn(O,S)/CIGSe samples . The possibility of a sulfate species was also taken into account because the O 1 s peak location of a sulfate species and a hydroxide species are similar.…”

Valence band offsets at Cu(In,Ga)Se2/Zn(O,S) interfaces

“…In contrast to literature reports , we have not observed the formation of a pure ZnS layer on the substrate surface in the initial stages of deposition. Such a layer is not evident from SIMS and SNMS depth profiling . In addition, the Auger parameter obtained from XPS does not change during sputter depth profiling throughout the Zn(O,S) buffer layer.…”

Deuterium Markers in CdS and Zn(O,S) Buffer Layers Deposited by Solution Growth for Cu(In,Ga)Se2 Thin‐Film Solar Cells

“…Due to different solubility products of the corresponding hydroxides and sulfides, the CBD Zn(O,S) layers contain a higher amount of Zn(OH) 2 compared with Cd(OH) 2 of solution‐grown CdS . Hydroxides play an important role in CIGS buffer layers in terms of efficiency, metastable behavior, and post‐annealing, especially for cells with Zn(O,S) buffers grown by CBD . However, it is difficult to determine whether the hydroxide species in the buffer layers were incorporated during the CBD process, during exposure to air before the deposition of the subsequent ZnO‐based layers, or during the long‐time exposure to air of the final devices and OH incorporation through (micro‐) cracks in the transparent conductive oxide (TCO) on top of the cell stack.…”

“…b) Scheme of reactions between thiourea and cadmium tetraammine complex or thiourea and zinc tetraammine complex to form an oxide, hydroxide, deuteroxide containing cadmium sulfide or zinc sulfide layer with the formal notations Cd(S,O,OH,OD) or Zn(S,O,OH,OD). The typical [S]/([S]+[O]) ratios for both materials are denoted for comparison . The stated [S]/([S]+[O]) ratios include both oxide and hydroxide amounts.…”