Zn ( O , S ) buffer layers by atomic layer deposition in Cu(In,Ga)Se2 based thin film solar cells: Band alignment and sulfur gradient

Abstract

Thin film solar cells with the structure sodalimeglass∕Mo∕Cu(In,Ga)Se2∕Zn(O,S)∕ZnO∕ZnO:Al are studied for varying thickness and sulfur content of the Zn(O,S) buffer layer. These Zn(O,S) layers were deposited by atomic layer deposition (ALD) at 120°C. Devices with no or small concentrations of sulfur in the buffer layer show low open-circuit voltages. This is explained by the cliff, or negative conduction-band offset (CBO), of −0.2eV measured by photoelectron spectroscopy (PES) and optical methods for the Cu(In,Ga)Se2 (CIGS)∕ZnO interface. Devices with ZnS buffer layers exhibit very low photocurrent. This is expected from the large positive CBO (spike) of 1.2eV measured for the CIGS∕ZnS interface. For devices with Zn(O,S) buffer layers, two different deposition recipes were found to yield devices with efficiencies equal to or above reference devices in which standard CdS buffer layers were used; ultrathin Zn(O,S) layers with S∕Zn ratios of 0.8–0.9, and Zn(O,S) layers of around 30nm with average S∕Zn ratios of 0.3. The sulfur concentration increases towards the CIGS interface as revealed by transmission electron microscopy and in vacuo PES measurements. The occurrence of this sulfur gradient in ALD-Zn(O,S) is explained by longer incubation time for ZnO growth compared to ZnS growth. For the Zn(O,S) film with high sulfur content, the CBO is large which causes blocking of the photocurrent unless the film is ultrathin. For the Zn(O,S) film with lower sulfur content, a CBO of 0.2eV is obtained which is close to ideal, according to simulations. Efficiencies of up to 16.4% are obtained for devices with this buffer layer.

The Zn(S,O,OH)/ZnMgO buffer in thin‐film Cu(In,Ga)(Se,S)2‐based solar cells part II: Magnetron sputtering of the ZnMgO buffer layer for in‐line co‐evaporated Cu(In,Ga)Se2 solar cells

“…The oxygen concentration in the buffer layer is very important for an optimal band alignment. 9 The composition of the CBD ZnS film can be influenced by the concentration of the precursors as well as by temperature ramps. The excess Zn(OH) 2 overlayer which is mainly formed during the rinsing procedure with water is found to cause transient effects and must be removed by ammonium hydroxide.…”

“…Several groups have demonstrated the potential of this material in the laboratory using different deposition techniques such as chemical bath deposition (CBD) [4][5][6][7][8] or atomic layer deposition (ALD). 9 Furthermore, CBD ZnS is already used as a standard buffer layer in industrially produced CIGS modules. 10 Zn 1-x Mg x O has also been demonstrated to be a suitable buffer material by magnetron sputtering on sequentially deposited Cu(In,Ga)(Se,S) 2 absorbers 11 or by the soft ALD technique with low deposition rates on co-evaporated CIGS absorbers.…”

Enhancing the efficiency of SnS solar cells via band-offset engineering with a zinc oxysulfide buffer layer

“…21,22 One of the approaches to adjust the CBO is to vary the constituent elements in the semiconductor-alloy buffer layer. For example, (Zn,Cd)S, 23 (Zn,Mg)O, 24 (Zn,Sn)O x , 25 and Zn(O,S) 26 were used in an attempt to replace CdS in CIGS solar cells. In this letter, we present a SnS device with a record power conversion efficiency of 2.04% (total area) using Zn(O,S) as an n-type buffer layer, and evaluate the effect of CBO on device performance.…”

Valence band structure and optical properties of ZnO1−хSх ternary alloys

“…Sulfur is intriguing isoelectronic impurity for anion substitution in ZnO [3]. Recently, much attention has been paid to the synthesis of ZnO1-хSх alloys [4][5][6][7][8].…”