Large grain Cu(In,Ga)Se2 thin film growth using a Se-radical beam source

https://doi.org/10.1016/j.solmat.2008.09.043Get rights and content

Abstract

Cu(In,Ga)Se2 (CIGS) thin films were grown by the three-stage process using a rf-plasma cracked Se-radical beam source. CuGaSe2 (CGS) films grown at a maximum substrate temperature of 550 °C and CuInSe2 (CIS) and CIGS films grown at the lower temperature of 400 °C exhibited highly dense surfaces and large grain size compared with films grown using a conventional Se-evaporative source. This result is attributed to the modification of the growth kinetics due to the presence of active Se-radical species and enhanced surface migration during growth. The effect on CIGS film properties and solar cell performance has been investigated. Enhancements in the cell efficiencies of 400 °C-grown CIS and CIGS solar cells have been demonstrated using a Se-radical source.

Introduction

Cu(In,Ga)Se2 (CIGS) thin films can be prepared by a variety of methods. Among these, multi-source evaporation has proven to date to be the most promising method to obtain high-quality photovoltaic-grade CIGS films [1]. Multi-source evaporation, however, has been seen to date as a laboratory-scale production technique. One of the disadvantages of multi-source evaporation holding back scaling up of this method to industrial application has been the large amount of Se source material consumed during growth. This leads to increased production costs, more frequent maintenance of the growth chamber, and increased levels of industrial waste. To solve this issue, we have developed a CIGS growth technique that utilizes a radio frequency (rf)-plasma cracked Se-radical beam source in the multi-source evaporation method [2], [3]. A significant reduction in the amount of Se source material used by more than a factor of 10 over that used by a conventional Se evaporation has been demonstrated. In addition to this merit, CIGS films grown with a Se-radical source exhibited highly dense, smooth surfaces, and large grain size. This is attributed to the modification of growth kinetics due to the high reactivity of the active Se-radical species produced and the resulting enhanced migration during growth. Se radicals are expected to enhance surface migration, which may increase the tendency toward two-dimensional growth similar to the case of ZnSe growth using a thermally cracked Se source [4].

In this study, we have applied the Se-radical source for film growth of CuGaSe2 (CGS), low-temperature growth of CuInSe2 (CIS), and CIGS films. The In–Ga composition ratio of the CIGS and the growth temperature have been known as critical parameters controlling variations in film properties and solar cell performance. In general, CGS (x=1) films and low-temperature (here we note that ‘x’ is the composition ratio of [Ga]/[In+Ga] in CIGS in at% and ‘low-temperature’ is 400 °C where CIGS growth on polyimide films is possible)-grown CIGS films exhibit small grain size compared with CIGS films grown at x<1 or at high temperature (∼550 °C). In view of the previous results seen for CIGS films grown at 550 °C, Se-radical source grown films can be expected to exhibit large grain size and modified surface morphologies. Here, the film properties of CGS grown at 550 °C, low-temperature-grown CIS, and CIGS films using a Se-radical source have been studied. Photovoltaic performance of solar cells fabricated using Se-radical source grown films has been also examined.

Section snippets

Thin film growth

CIGS thin films were grown on Mo-coated soda-lime glass substrates by the three-stage process using a molecular beam epitaxy apparatus equipped with elemental Cu, In, and Ga Knudsen-cell sources and an rf-cracking unit equipped Se-radical source. The growth chamber was also equipped with an elemental Se Knudsen-cell source for conventional CIGS film growth. Detailed growth conditions of CIGS films using a conventional Se-evaporative source and an Se-radical source have been described elsewhere

CGS thin films

Cross-sectional and surface SEM images of CGS thin films grown using a conventional Se-evaporative source and an Se-radical source are shown in Fig. 1(a) and (b), respectively. These films were grown under nominally identical conditions except for difference in Se source. The Se-radical source grown CGS film exhibited large grain size compared with the Se-evaporative source grown CGS film as shown in Fig. 1(b); similar results have been seen for CIGS films grown at a maximum substrate

Summary

We have grown CGS, CIS, and CIGS thin films using an rf-plasma cracked Se-radical source alternative to the conventional Se-evaporative source. All films grown with an Se-radical source exhibited highly dense surfaces and large grain size with strong (1 1 2) texture in comparison with Se-evaporative source grown films. Although further investigation and development is necessary concerning Se-radical source grown CGS films and corresponding devices, solar cells made from low-temperature-grown CIS

Acknowledgements

This work was supported in part by the Incorporated Administrative Agency, New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade and Industry (METI).

References (10)

  • W.N. Shafarman et al.

    Effect of substrate temperature and deposition profile on evaporated Cu(InGa)Se2 films and devices

    Thin Solid Films

    (2000)
  • M.A. Contreras et al.

    Diode characteristics in state-of-the-art ZnO/CdS/Cu(In1−xGax)Se2 solar cells

    Prog. Photovolt: Res. Appl.

    (2005)
  • S. Ishizuka et al.

    Growth of polycrystalline Cu(In,Ga)Se2 thin films using a radio frequency-cracked Se-radical beam source and application for photovoltaic devices

    Appl. Phys. Lett.

    (2007)
  • S. Ishizuka et al.

    Growth of Cu(In,Ga)Se2 absorbers using a Se-radical beam source

  • D.A. Cammack et al.

    Low-temperature growth of ZnSe by molecular beam epitaxy using cracked selenium

    Appl. Phys. Lett.

    (1990)
There are more references available in the full text version of this article.

Cited by (26)

  • Precise Se-flux control and its effect on Cu(In,Ga)Se<inf>2</inf> absorber layer deposited at low substrate temperature by multi stage co-evaporation

    2017, Thin Solid Films
    Citation Excerpt :

    To achieve high solar cell efficiency, careful control of Se supply during the CIGS process is mandatory for layers grown by thermal co-evaporation of elements in vacuum. There are many reports on the effect - not only of the ratio of Se flux to the other metals - but also of the activity of Se during the deposition on the properties of CIGS solar cells [3–10]. However, the measurement of the flux of Se in a repetitive and quantitative manner during the process has always been an issue.

  • Cu(In,Ga)Se<inf>2</inf> thin films annealed with SnSe<inf>2</inf> for solar cell absorber fabricated by magnetron sputtering

    2017, Solar Energy
    Citation Excerpt :

    Se vapor cracking is a common method to get more Se2 clusters. The typical paths of cracking include radio frequency-cracking (Ishizuka et al., 2007; Ishizuka et al., 2011a; Ishizuka et al., 2011 b; Ishizuka et al., 2009) and thermal cracking (Li et al., 2015a; Li et al., 2015 b; Lin et al., 2016) techniques. However, Se vapor cracking is necessary to establish an additional disposal system, cracking zone.

  • Coupled effect of pre-alloying treatment and plasma-assisted Se vapor selenization process in Cu(In, Ga)Se<inf>2</inf> thin film

    2017, Solar Energy
    Citation Excerpt :

    Fig. 9 shows the cross-sectional SEM images of the CIGS film samples based on the precursor films of 200 °C and 400 °C respectively selenized at various plasmas selenization powers. As seen in Fig. 9(a), the CIGS film based on the precursor film of 200 °C selenized by TASVS process exhibits continuous and dense profile which resembles CuInSe2 structure (Ishizuka et al., 2009). It means that a large amount of Ga aggregate at the bottom of the film.

  • Growth and characterization of CIGS thin films by plasma-assisted and thermal-assisted Se vapor selenization process

    2017, Journal of Alloys and Compounds
    Citation Excerpt :

    Recently, a promising alternative selenization process for post-selenized CIGS films has been proposed by employing plasma-assisted technique to increase the activation energy of Se vapor. The activity of traditional thermal-assisted Se vapor is increased by coupling with a radio frequency (RF)-plasma treatment [13] or an inductive coupling plasma (ICP) treatment [14,15]. The activity of plasma-assisted Se vapor is supposed to be higher than that of even hydrogen-assisted Se vapor.

  • 30×40 cm<sup>2</sup> flexible Cu(In,Ga)Se<inf>2</inf> solar panel by low temperature plasma enhanced selenization process

    2016, Nano Energy
    Citation Excerpt :

    It has been claimed that the ionization of Se vapor with increase of reaction activity achieved by inductive coupling plasma (ICP) is much higher than that of dilute H2Se gas or merely Se vapors [7]. Further studies on formation of larger grain size can be achieved by Se radicals through RF-plasma treatment, resulting in enhancing quality of CIGS film at the selenization temperature of 400 °C [8–11]. However, low-temperature and large area of plasma-enhanced CIGS solar cell with mass-production-potentially selenization is still not well reported yet.

View all citing articles on Scopus
View full text