Bandgap engineered Cu2ZnGexSn1−xS4 solar cells using an adhesive TiN back contact layer

https://doi.org/10.1016/j.jallcom.2021.160478Get rights and content

Highlights

  • The insertion of TiN interlayer improved the adhesion of CZGTS thin films.

  • Cu-poor, Zn-rich, bi-layered CZTS on top CZGS precursors was fabricated by RF magnetron sputtering.

  • A Ge-Sn compositional gradient is evident in GDOES profiles, but STEM/EDS showed the presence of only a slight Ge gradient.

  • Ge induced bandgap widening and ordered CZTS was identified as the cause of charge carriers blocking in one of the samples.

Abstract

Kesterite-based solar cells are mainly restricted by their lower than expected open-circuit voltage (Voc) due to non-radiative recombination. Therefore, an approach to reduce bulk and interface recombination through band gap grading to induce a back surface field is attempted. This contribution presents the challenges in the formation of compositional grading of the wide bandgap material Cu2ZnGexSn1−xS4 (CZGTS) and successful fabrication of solar cells with an additional adhesive TiN interlayer. It is observed that the TiN interlayer improves adhesion between CZGTS and the back contact. The microstructure of the Cu2ZnSnS4 (CZTS) film is significantly affected by the concentration of Ge, and the existence of a Ge concentration gradient is strongly correlated to the formation of smaller Ge-rich and larger Sn-rich grains. The bandgap grading is exploited with a moderate Ge concentration of up to (Ge/(Ge+Sn) = 0.25) in CZTS. As the Ge profile stretched all the way to the front interface, the cliff-like band alignment at the front interface of the absorber could negate the beneficial effect of Ge inclusion in the bulk and back interface of the absorber. Ordering the absorber can introduce an additional downward shift in the valence band. In one of the samples, the increased ordering and high concentration of Ge in CZTS are suggested to enhance the hole barrier at the back interface. It is concluded that the effect of the bandgap grading with Ge can only be realized with optimization of interface band alignment and back contact formation.

Introduction

Multi-element chalcogenides offer tunable electronic, optical, and defect properties to influence photovoltaic performance [1], [2], [3], [4], [5], [6]. Cu2ZnSnS4 (CZTS) is a promising alternative to established CdTe or Cu(In,Ga)Se2 (CIGS) based solar technologies because of its good absorption coefficient, earth-abundant elements, and non-toxicity [7]. Compatibility with industrial fabrication methods is also an advantage of CZTS.

Theoretical and experimental investigations have shown that the replacement of cations in mixed chalcogenides can modify the optical and electronic properties [8], [9], [10]. Cu2ZnGexSn1−xS4 (CZGTS), a kesterite or stannite quinary compound semiconductor, is emerging as a potential candidate for next-generation technology due to the possible band gap grading. The inclusion of Ge is also suggested to minimize the formation of deep defects due to Sn+2 associated with low open-circuit voltage (Voc) [6]. Various methods for synthesizing Cu2Zn(GexSn1−x)(SySe1−y)4 (CZGTSSe) polycrystalline films using a low concentration of germanium have been used to improve the photovoltaic performance [11], [12], [13]. A CZGTSSe device with 40% Ge was reported to achieve power conversion efficiency (PCE) 9% with antireflective coating [12]. In that case, there was an apparent improvement in Voc, but with a Voc deficit equivalent to that of CZTS, a decrease in short-circuit current density (Jsc), and a worse fill factor (FF), leading to a similar performance in solar cell devices. With a higher [Ge]/([Ge]+[Sn]) (GGS) ratio of 0.7, Cu-poor and Zn-rich CZGTSSe solar cell devices have achieved a PCE of 6.8% [5]. Several authors have reported a PCE drop with notable Ge addition [13]. Consequently, to the best of our knowledge, the best Cu2ZnGeS4 (CZGS) solar cell has achieved only PCE 0.7% [14], leaving plenty of room for optimization of wide band gap solar cells for tandem solar cell applications.

Band gap grading can be used to improve the overall performance of single-junction solar cells. For example, the [In]/[Ga] ratio changes the band gap of CIGS, and optimization of the [In]/[Ga] depth profile can be used to improve the PCE [15]. The incorporation of Ge into CZTS thin films can adjust the band alignment between CZTS and buffer to reduce the Voc deficit, minimize the effect of deep defects, and improve the quality of crystallization [16], [17]. However, due to the rapid diffusion of Ge at high temperatures, it is difficult to restrict the redistribution of Ge. The PCE of graded CZGTSe solar cells was as high as 9.2% [18] due to increased charge carrier collection and Voc [6], [18], [19]. A mild Ge-Sn gradient might have enhanced the drift electrical field, resulting in an increased charge carrier collection [18]. Although the compositional gradient was claimed to be the cause of device improvement, a more detailed analysis of elemental distribution is required. Highly depth-resolved compositional profiles can be obtained by secondary-ion mass spectrometry (SIMS) or glow discharge optical emission spectroscopy (GDOES) on a microscale [20]. However, it is necessary to verify the compositional distribution on a nanoscale with other techniques such as scanning transmission electron microscopy (STEM)/energy-dispersive x-ray spectroscopy (EDS) [10]. Therefore, further research is necessary to understand the formation of compositional gradients and their effect on solar cell performance.

Our previous work showed that controlled sulfurization of co-sputtered CZGS/CZTS precursor stacks could form a Ge-Sn gradient [10]. The controlled atomic diffusion is critical during sulfurization for manufacturing compositionally graded compound semiconductors. The diffusion is typically much faster through grain boundaries than in grains. At high temperatures, and for long periods, the diffusion of atoms through the bulk increases substantially so that the diffusion profile becomes uniform. Nevertheless, diffusion occurs faster through grain boundaries, which dominates at a temperature lower than the melting point [21]. During sulfurization, the substrate temperature is well below the melting point of the absorbers so that the diffusion through the grain boundaries is greater than within the grains. Since recrystallization of CZGTS grains occurs during annealing, this strongly affects the compositional distribution in addition to interdiffusion. The fabricated films with a steep gradient in our previous study [10] showed poor adhesion. The interfacial adhesion between absorber and substrate is essential for the fabrication of thin-film solar cells. The adhesion is a macroscopic property of the film that depends on interfacial bonding and the nature of local stress or strain [22]. Different fabrication steps can affect delamination at various processing stages; for example, delamination may be caused by the dissolution of water-soluble phases when the samples are immersed in a chemical bath deposition (CBD) solution or KCN solution.

Bilayered stacks of CZTS on CZGS with different thicknesses were prepared and annealed for a different duration to probe the effect on the formation of a gradient. To improve adhesion, we investigated the use of a TiN layer between Mo and CZGS films, and varied the substrate temperature during sputter deposition of precursors. STEM and GDOES characterization were used to reveal the elemental distribution profiles. Moreover, solar cell devices were fabricated to investigate the impact of Ge-gradient formation on solar cell performance.

Section snippets

CZGS adhesion optimization

Substrates were prepared by depositing Mo (approx. 350 nm thick) films onto 1 mm thick cleaned soda-lime glass (SLG) by direct current (DC) sputtering (Material Research Corporation sputter system) from a Mo target (purity 99.97%) in the presence of 0.8 Pa Ar (purity 99.99%). The sheet resistance of Mo back contact layer was 0.5–0.65 Ω/□. Mo/SLG substrates were divided into 25 mm × 25 mm pieces. Some of the substrates were coated with TiN (20 nm) by reactive DC sputtering (Von Ardenne sputter

Results

The results from experiments to evaluate the TiN adhesion layers are first presented, followed by the annealing experiments of the CZGS/CZTS stacks.

Factors affecting adhesion of CZGTS absorbers

The adhesion of CZGTS layers can deteriorate due to numerous factors. I.) The microstructure of CZGS is quite different from CZTS, as shown by disrupted columnar growth in SEM (see Fig. 2 and Fig. S1). After annealing, voids appeared, as shown in STEM, which might result in worse adhesion. II.) Adhesion can be affected by the incompatible thermal expansion coefficient of different films. The thermal expansion coefficient of TiN and CZTS is found to be very similar [54], [55]. Therefore,

Conclusion

CZGTS solar cells with adhesive TiN interlayer have been successfully fabricated. The delamination of CZGTS and CZGS is found to be unaffected by temperature during sputter deposition and change in sulfurization temperature. However, a TiN interlayer is found to increase adhesion between the CZGTS and Mo back contact. Using CZGS/CZTS precursor stacks and varying annealing time, a process is developed where a slight Ge gradient is retained in the form of smaller Ge-rich grains towards the back

CRediT authorship contribution statement

Nishant Saini: Conceptualization, Investigation, Resources, Data curation, Methodology, Validation, Formal analysis, Writing - original draft, Visualization, all authors discussed the results, Writing - review & editing. Jes K. Larsen: Analysis, Methodology, Resources, Validation, Data curation, Supervision, Writing - original draft, all authors discussed the results, Visualization, contribution to write-up, reviewed, and edited. Kristina Lindgren: Analysis, Methodology, Resources,

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors would like to thank the Swedish Foundation for Strategic Research (SSF) project RMA15-0030 and the Swedish Research Council (VR, 2019-04793) for their financial support.

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