4.7 Article

Reactively sputtered Zn(O,S) buffer layers for controlling band alignment of Cu(In,Ga)Se2 thin-film solar cell interface

Journal

JOURNAL OF ALLOYS AND COMPOUNDS
Volume 842, Issue -, Pages -

Publisher

ELSEVIER SCIENCE SA
DOI: 10.1016/j.jallcom.2020.155986

Keywords

Zn(O,S); Reactive sputtering; Thin-film solar cell; Band alignment

Funding

  1. Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) - Ministry of Science, ICT & Future Planning [2016M1A2A2936754]
  2. 'New & Renewable Energy' of a Korea Institute of Energy Technology Evaluation and Planning (KETEP) - Ministry of Trade, Industry Energy [20193010014830]
  3. Institute for Information & Communications Technology Promotion (IITP) - Korean government (MSIT) [2016-0-00576]
  4. Korea Evaluation Institute of Industrial Technology (KEIT) [20193010014830] Funding Source: Korea Institute of Science & Technology Information (KISTI), National Science & Technology Information Service (NTIS)

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Buffer layers in Cu(In,Ga)Se-2 (CIGS) solar cells are highly preferable to be vacuum-compatible and use non-toxic materials not only for the film but also for the manufacturing process. Moreover, high controllability for the energy levels of buffer layers is greatly desirable for achieving higher solar cell efficiency because the buffer layer is formed directly onto the absorber layer which generates carriers. In this study, the S/(O + S) composition ratio of the Zn(O,S) thin films was finely controlled by varying O-2/(Ar + O-2) gas flow ratio using a reactive sputtering method including an Ar/O-2 mixture gas and a single ZnS sputter target. The structural, electrical, and photovoltaic performance properties of the Zn(O,S) thin films and their CIGS solar cells were investigated at varied S/(O + S) composition ratios. As the S/(O + S) composition ratio of the Zn(O,S) films increased, bandgap bowing occurred with the increase of the conduction band and valence band energy levels. The photovoltaic performance was greatly influenced by the difference of the conduction band energies between the Zn(O,S) buffer and the CIGS absorber. When the conduction band energy of the Zn(O,S) was too high or too low compared to that of the CIGS, it increased the carrier recombinations or series resistance, respectively, which induced losses of the open-circuit voltage and the fill factor. The controlling and optimizing the energy levels at the Zn(O,S)/CIGS interface were crucial for improved solar cell performances. (C) 2020 Elsevier B.V. All rights reserved.

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