Effects of the incorporation of alkali elements on Cu(In,Ga)Se2 thin film solar cells
Graphical abstract
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).
References (28)
- et al.
Sodium incorporation strategies for CIGS growth at different temperatures
Thin Solid Films
(2005) The effect of Na in polycrystalline and epitaxial single-crystal CuIn1−xGaxSe2
Thin Solid Films
(2005)- et al.
Surface modification of CIGS film by annealing and its effect on the band structure and photovoltaic properties of CIGS solar cells
Curr. Appl. Phys.
(2015) - et al.
Fabrication of Cu(In,Ga)Se2 solar cell with ZnS/CdS double layer as an alternative buffer
Curr. Appl. Phys.
(2010) - et al.
Growth of a high-quality Zn(S,O,OH) thin film via chemical bath deposition for Cd-free Cu(In,Ga)Se2 solar cells
Sol. Energy Mater. Sol. Cells
(2013) - et al.
Why do we make Cu(In,Ga)Se2 solar cells non-stoichiometric?
Sol. Energy Mater. Sol. Cells
(2013) - et al.
Potassium-induced surface modification of Cu(In,Ga)Se2 thin films for high-efficiency solar cells
Nat. Mater.
(2013) - et al.
Properties of Cu(In,Ga)Se2 solar cells with new record efficiencies up to 21.7%
Phys. Status Solidi RRL
(2015) - et al.
Effects of sodium on polycrystalline Cu(In,Ga)Se2 and its solar cell performance
Adv. Mater.
(1998) - et al.
Efficiency enhancement of Cu(In,Ga)Se2 solar cells due to post-deposition Na incorporation
Appl. Phys. Lett.
(2004)
Na impurity chemistry in photovoltaic CIGS thin films: investigation with x-ray photoelectron spectroscopy
J. Vac. Sci. Technol. A
Na-induced variations in the structural, optical, and electrical properties of Cu(In,Ga)Se2 thin films
J. Appl. Phys.
Effects of Se flux on the microstructure of Cu(In,Ga)Se2 thin film deposited by a three-stage co-evaporation process
Electrochem. Solid State Lett.
Control of the preferred orientation of Cu(In,Ga)Se2 thin film by the surface modification of Mo film
J. Electrochem. Soc.
Cited by (0)
- 1
Both authors equally contributed to this work.