Waste- and Cd-Free Inkjet-Printed Zn(O,S) Buffer for Cu(In,Ga)(S,Se)2 Thin-Film Solar Cells.

Van Ben Chu,Mohit Sood,Phillip J Dale,Michele Melchiorre,Jérôme Guillot,Alberto Lomuscio,Daniel Siopa,Brahime El Adib,Damilola Adeleye,Jonathan Rommelfangen,Alice Debot,Nathalie Valle

doi:10.1021/acsami.0c16860

Abstract

Thin film semiconductors grown using chemical bath methods produce large amounts of waste solvent and chemicals that then require costly waste processing. We replace the toxic chemical bath deposited CdS buffer layer from our Cu(In,Ga)(S,Se)2 (CIGS)-based solar cells with a benign inkjet-printed and annealed Zn(O,S) layer using 230 000 times less solvent and 64 000 times less chemicals. The wetting and final thickness of the Zn(O,S) layer on the CIGS is controlled by a UV ozone treatment and the drop spacing, whereas the annealing temperature and atmosphere determine the final chemical composition and band gap. The best solar cell using a Zn(O,S) air-annealed layer had an efficiency of 11%, which is similar to the best conventional CdS buffer layer device fabricated in the same batch. Improving the Zn(O,S) wetting and annealing conditions resulted in the best device efficiency of 13.5%, showing the potential of this method.

Highlights

Thin-film inorganic solar cells require a buffer layer to achieve a high light to electric power conversion efficiency (PCE)
A 50 nm n-type CdS thin film deposited by chemical bath deposition (CBD) technique is used as a buffer layer for both CdTe1,2 and Cu(In,Ga)(S,Se)[2] (CIGS)[3] thin film solar cells
For CIGS devices CBD buffer layers fulfill a number of functions: (i) they provide pinhole free coverage of the rough absorber surfaces with only tens of nanometer thin films,[4−6] (ii) they modify the surface chemistry of the absorber surface,[7,8] (iii) they protect the absorber layer from sputter damage during the deposition of the window layers (e.g., zinc oxide (ZnO), (Mg,Zn)O, ITO),[9,10] (iv) they provide a better lattice matching between absorber and window layer[11] and (v) they result in a high interface band gap that reduces interface recombination.[12,13]

Summary

■ INTRODUCTION

Thin-film inorganic solar cells require a buffer layer to achieve a high light to electric power conversion efficiency (PCE). Care is required to formulate the precursor ink with the correct surface tension and viscosity so that it is jettable in a controllable manner and that it wets the surface of the substrate well.[28] Inkjet printers can be integrated into an in-line or a vacuum-line fabrication process which help to scale up industrial production It is a contactless and accurate deposition technique, allowing the patterning of thin films without masks. The printed precursor films require post annealing to evaporate the solvents and convert the wet precursor layers to solid semiconductor thin films by decomposition and/or combustion reactions at elevated temperature The temperature of this annealing step might affect the Zn(O,S) properties and the underlying CIGS absorber layer when applied to solar cells. The red, green, and blue vertical arrows denote the Zn(O,S) film printed at different drop spacing

■ RESULTS AND DISCUSSION

■ ACKNOWLEDGMENTS

■ REFERENCES