Single elementary target-sputtered Cu2ZnSnSe4 thin film solar cells

https://doi.org/10.1016/j.solmat.2014.08.043Get rights and content

Highlights

  • First attempt of single elementary target-sputtering of Cu2ZnSnSe4 thin films.

  • Excess Cu and Zn in sputter targets are essential for quality cells.

  • A conversion efficiency of ~4.16% was obtained for excess Cu/Zn target.

  • TEM/AES line profiles reveal elemental distributions over the absorber.

  • Surface potential by AFM confirms the benefit of the excess Cu/Zn target.

Abstract

A simple single elementary target sputtering method is proposed as an alternative to the multi-target sputter approach for preparing Cu2ZnSnSe4 absorber films. Several single targets utilizing excess Cu and Zn, including Cu2ZnSnSe4, Cu2Zn1.5SnSe4, Cu2.5Zn1.5SnSe4 and Cu3Zn1.5SnSe4, were investigated to determine the absorber film with the most promising structural and photovoltaic performance. A conversion efficiency of ~4.16% was obtained from the Cu2.5Zn1.5SnSe4 single target, which displayed well-defined grain structures with desirable Zn/Sn and Cu/(Zn+Sn) ratios. Critical roles of excess Cu and Zn during sputtering are discussed in conjunction with microstructural evolution, elemental distribution, photovoltaic characteristics and grain boundary contributions, which are specified for the sputtering method.

Introduction

Quaternary p-type semiconducting Cu2ZnSnSe4 (CZTSe) compounds have been studied recently as alternative absorber materials for high efficiency thin film solar cells due to the abundance of non-toxic constituents and high absorption coefficient (>104 cm−1) [1], [2]. A variety of processing techniques, such as thermal evaporation [2], solution-based process [3], [4], pulsed-laser deposition [5], electrodeposition [6] and sputtering [7], have been investigated to prepare high quality CZTSe thin films. Sputtering is one of the most viable deposition techniques for producing a large-scale panel of thin film solar cells with demonstrable productivity and easy adjustment. So far, all reported sputtering methods for CZTSe absorber are primarily based on the utilization of multi-targets of the elements of Cu, Zn and Sn and the subsequent selenization of the deposited precursors [7], [8], [9]. Control of the thickness of each deposited layer by sequential deposition of Cu, Zn and Sn on Mo-coated substrates has been critical in producing a conversion efficiency of 3.2–4.8% [7], [8]. However, there has not been any report on a simple single-target approach that can avoid the complex sequential deposition process. A simple sputtering method is preferred for a large-scale production and cost-effectiveness.

Here we present an effective sputtering method for producing high quality absorber films by adjusting the relative content of Cu, Zn, Sn and Se in a single-target without the extra control of deposition sequence. A cell efficiency of ~4.16% obtained for a Cu/Zn-rich target demonstrates the great potential of the elementary single target approach. The effects of substantial variations in target compositions are primarily discussed in conjunction with desirable grain structures and phase distributions for better photovoltaic characteristics.

Section snippets

Experimental

Single targets of four selected compositions, Cu2ZnSnSe4 (designated as 2Cu 1Zn), Cu2Zn1.5SnSe4 (2Cu 1.5Zn), Cu2.5Zn1.5SnSe4 (2.5Cu 1.5Zn) and Cu3Zn1.5SnSe4 (3Cu 1.5Zn), were prepared from elementary metal sources. Cu (99.85% Kanto Chem. Co., Inc.), Zn (>99%, Aldrich), Sn (>99%, Aldrich) and Se (>99%, Dae Jung) were mixed by simple ball milling in ethanol for 6 h. The mixture was dried and uniaxially pressed into a 2-in. disk pellet. The pellet was fired at 300 °C for 30 min in N2 atmosphere. Thin

Results and discussion

Fig. 1 shows XRD patterns of the absorber thin films prepared on Mo-coated glass substrates using different sputtering targets of 2Cu 1Zn, 2Cu 1.5Zn, 2.5Cu 1.5Zn and 3Cu 1.5Zn. The XRD patterns exhibit several distinct CZTSe peaks [JCPDS file: 52-0868] including (1 1 2), (2 2 0)/(2 0 4) and (3 1 2)/(1 1 6) planes at approximately 27.1°, 45° and 53.5°, respectively. As shown in insets of the XRD pattern, the clear peak splitting of the (2 2 0)/(2 0 4) and (3 1 2)/(1 1 6) planes indicates the stannite structure (space

Conclusions

We have demonstrated the practical application of one-stage sputtering of Cu2ZnSnSe4 using a single elementary target with a conversion efficiency of 4.16%. A critical step for the successful demonstration is based on the adoption of excessive Zn and Cu contents, which are assumed to decrease significantly during the active deposition. The best target composition Cu2.5Zn1.5SnSe4 resulted in enlarged grains with promising ratios of Zn/Sn ~1.06 and Cu/(Zn+Sn) ~0.81. Excess Zn seems to bring

Acknowledgements

This work was financially supported by a grant (no. 2011-0020285) from the National Research Foundation of Korea funded by the Korean government.

References (33)

  • K. Moriya et al.

    Fabrication of Cu2ZnSnS4 thin-film solar cell prepared by pulsed laser deposition

    Jpn. J. Appl. Phys.

    (2007)
  • L. Guo et al.

    Electrodeposited Cu2ZnSnSe4 thin film solar cell with 7% power conversion efficiency

    Prog. Photovoltaics

    (2014)
  • G. Zoppi et al.

    Cu2ZnSnSe4 thin film solar cells produced by selenisation of magnetron sputtered precursors

    Prog. Photovoltaics

    (2009)
  • A. Fairbrother, E. Saucedo, X. Fontané, V. Izquierdo-Roca, D. Sylla, M. Espindola-Rodriguez, F.A. Pulgarin-Agudelo, O....
  • F. Luckert et al.

    Optical properties of high quality Cu2ZnSnSe4 thin films

    Appl. Phys. Lett.

    (2011)
  • S. Jeong et al.

    An 8.2% efficient solution-processed CuInSe2 solar cell based on multiphase CuInSe2 nanoparticles

    Energy Environ. Sci

    (2012)
  • Cited by (38)

    • Ni addition effects on physical properties of spin-coated Sb<inf>2</inf>S<inf>3</inf> semiconducting compound thin films

      2023, Applied Surface Science
      Citation Excerpt :

      Although new compound semiconductor thin film solar cells such as Cu-Zn-sn-S have been studied [2–4], problems such as difficult process conditions and secondary phase formation are being raised. Therefore, simple binary semiconductors, especially chalcogenide materials such as Sb2S3, Sb2Se3, SnSe, SnS, Ni2S, Cu2S, Cu2Se and ZnS, have received renewed interest for chalcogenide materials, which are composed of elements abundant on Earth and easy to fabricate [5–8]. Among them, antimony sulfide (Sb2S3, belongs to group V-VI elements with an orthorhombic crystal structure and an optimal indirect optical band gap) has mainly attracted attention in the field of solid-state sensitized solar cells, and has recently been applied to thin-film solar cells.

    • A review on binary metal sulfide heterojunction solar cells

      2019, Solar Energy Materials and Solar Cells
      Citation Excerpt :

      As an alternative to CdTe and CIGS, earth-abundant Cu2ZnSnS4 (CZTS) compound has been receiving great attention. However, a relatively large number of the constituent elements (four for pure sulfide and selenides and five for mixed sulfo-selenide) with a high possibility for the formation of unwanted secondary phases such as Cu2SnS3, ZnS, and SnxSy has resulted in significant deterioration of the device efficiency [9–12]. In this regard, simple binary semiconductor compounds, consisting of cheap elements, have gained renewed attention as promising light-absorbing materials [2,13–15].

    View all citing articles on Scopus
    1

    Present address: School of Physics & Materials Science, Thapar University, Patiala 147004, India.

    View full text