Effects of the incorporation of alkali elements on Cu(In,Ga)Se2 thin film solar cells

https://doi.org/10.1016/j.solmat.2016.07.015Get rights and content

Highlights

  • Effects of alkali elements-Na and K-onCIGS are studied using SIMS, XPS, and PL.

  • Alkali elements passivate non-radiative recombination centers at grain boundaries.

  • The device with Na and K shows the highest efficiencies due to surface passivation.

  • Passivation of both external interfaces and internal grain boundaries is crucial.

Abstract

This study describes in detail the effects of sodium and potassium on Cu(In,Ga)Se2 (CIGS) absorbers and solar cells. We report on the influence of these species on the surface and bulk composition as well as bulk defect structure of CIGS films as revealed by X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), and photoluminescence (PL). From the XPS studies it is found that Na and K promote oxygen absorption onto the CIGS films. Furthermore, potassium accelerates the formation of indium and gallium oxides on the film surface, making the surface Cu-deficient. Low temperature PL studies suggest that (i) Na and K help passivate non-radiative recombination centers, presumably at the grain boundaries, and (ii) Na further impacts the bulk defect structure inside of CIGS grains, which is not observed with K. This change in bulk defect structure is attributed to the greater diffusivity of Na in CIGS relative to K due to the smaller atomic size. This in-depth study (integration of XPS, SIMS, PL, and device characteristics) reveals that the surface chemistry and the grain boundary passivation have stronger influences on the device performance than the bulk defect structure.

Introduction

Cu(In,Ga)Se2 (CIGS) thin-film solar cells have now been reported with efficiency values of over 20% [1], [2]. Achieving high-efficiency CIGS solar cells requires the incorporation of less than 1 atomic percentage of sodium (Na) into the CIGS absorber [3], [4], [5], [6]. Na is commonly introduced during CIGS film growth via diffusion from a Na-containing soda-lime glass (SLG) through the Mo bottom electrode. The effects of Na on the CIGS solar cells have been systematically studied and are shown to enhance p-type carrier concentration and/or grain boundary passivation [3], [4], [5], [6], [7], [8]. The beneficial role of Na naturally led to the investigation of other alkali elements. Recently Tiwari and co-workers employed a “post-deposition potassium fluoride (KF) treatment”, where potassium was incorporated by simultaneous evaporation of KF and Se directly after CIGS deposition; this process led to the successful demonstration of flexible CIGS devices on an alkali-free substrates with the efficiency as high as 20.4% [1]. The post-deposition KF treatment was also used in achieving the record efficiency CIGS solar cell (21.7%) by ZSW [2]. Despite the widespread use of potassium to improve performance of CIGS solar cells, there have been few detailed studies of the mechanisms through which potassium-doping improves CIGS performance.

This study examines in detail the effects of Na and K alkali-doping—separately or jointly—on the CIGS absorber. A control sample free of all alkali elements was prepared on a borosilicate glass (BSG) substrate. Na atoms were supplied to some growing CIGS absorbers from the underlying SLG substrate heated to 550 °C during layer growth. Finally, Potassium was supplied to some samples from the top surface of the grown CIGS absorber via post-deposition KF treatment. A total of four sample types were examined: (1) no alkali incorporation (“Sample X”), grown on BSG, (2) Na-only (“Sample Na”), grown on SLG, (3) K-only (“Sample K”), grown on BSG with post-deposition KF treatment, and (4) both Na and K (“Sample Na+K”), grown on an SLG substrate with post-deposition KF treatment. By integrating data from X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), low temperature photoluminescence (PL), and current density-voltage (J-V) characteristics of the finished devices, we show that passivation of the surfaces (both external interfaces and internal grain boundaries) is critical in realizing high efficiency CIGS solar cells.

Section snippets

Experimental details

CIGS films were deposited on the Mo-coated substrates using a three-stage process by a co-evaporation system [9], [10], [11]. In the first stage, an (In,Ga)2Se3 precursor layer with 1 µm thickness was grown at 350 °C by co-evaporation of In, Ga, and Se. In the second stage, a Cu-rich CIGS film was formed by co-evaporation of Cu and Se on the (In,Ga)2Se3 film at 550 °C. In the third stage, In, Ga and Se were co-evaporated again to convert the overall composition of the growing film to Cu-poor. The

Results and discussion

Fig. 1 shows plane-view SEM images and XPS Na 1s spectra of CIGS film grown on two different substrates (with and without sodium). Although the CIGS films grown on both substrates show dense surface morphology without any pinholes (Fig. 1a), the grain size of CIGS film is dependent on the type of substrate; the sample on SLG exhibited larger grains compared to the sample on BSG, consistent with the literature reports [18], [19], [20]. In Fig. 1b, it is seen from the XPS results that sodium is

Conclusion

To understand effects of Na and K alkali-doping on the CIGS absorber, SLG substrate and post-deposition KF treatment were used as Na and K sources, respectively. According to XPS analysis, the surface potassium attracts oxygen forming indium and gallium oxide on the surface of the CIGS while such observation was not made with the incorporation of sodium. The low temperature PL results show that both alkali elements (either Na or K) are capable of passivating non-radiative centers which are

Acknowledgments

This research was supported by the National Research Foundation of Korea (NRF) (Grant No. 2014R1A1A1004282) and by the Climate Change Research Hub of KAIST (Grant No. N11160017).

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    Both authors equally contributed to this work.

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