Bandgap engineered Cu2ZnGexSn1−xS4 solar cells using an adhesive TiN back contact layer
Graphical Abstract
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.
References (71)
- et al.
Wide band-gap tuning Cu2ZnSn1−xGexS4 single crystals: optical and vibrational properties
Sol. Energy Mater. Sol. Cells
(2016) - et al.
Cu2ZnSn(S,Se)4 from annealing of compound co-sputtered precursors – recent results and open questions
Sol. Energy
(2018) - et al.
Ge doped Cu2ZnSnS4: an investigation on absorber recrystallization and opto-electronic properties of solar cell
Sol. Energy Mater. Sol. Cells
(2019) - et al.
Bandgap engineering of Cu2ZnSn1−xGexS(e)4 by adjusting Sn-Ge ratios for almost full solar spectrum absorption
J. Alloy. Compd.
(2017) - et al.
Ga-grading and solar cell capacitance simulation of an industrial Cu(In,Ga)Se2 solar cell produced by an in-line vacuum, all-sputtering process
Thin Solid Films
(2017) - et al.
From sputtered metal precursors towards Cu2Zn(Sn1−x,Gex)Se4 thin film solar cells with shallow back grading
Thin Solid Films
(2018) - et al.
Study of polycrystalline Cu2ZnSnS4 films by Raman scattering
J. Alloy. Compd.
(2011) - et al.
Optical properties of reactively sputtered Cu2ZnSnS4 solar absorbers determined by spectroscopic ellipsometry and spectrophotometry
Sol. Energy Mater. Sol. Cells
(2016) - et al.
Extreme radiation hard thin film CZTSSe solar cell
Sol. Energy Mater. Sol. Cells
(2018) - et al.
Light-enhanced reverse breakdown in Cu(In,Ga)Se2 solar cells
Thin Solid Films
(2013)
The mechanical properties of thin films: a review
Thin Solid Films
Annealing behavior of reactively sputtered precursor films for Cu2ZnSnS4 solar cells
Thin Solid Films
Influence of precursor sulfur content on film formation and compositional changes in Cu2ZnSnS4 films and solar cells
Sol. Energy Mater. Sol. Cells
Studies of compositional dependent Cu2Zn(GexSn1−x)S4 thin films prepared by sulfurizing sputtered metallic precursors
J. Alloy. Compd.
Optimization of post-deposition annealing in Cu2ZnSnS4 thin film solar cells and its impact on device performance
Sol. Energy Mater. Sol. Cells
An in-depth investigation on the grain growth and the formation of secondary phases of ultrasonic-sprayed Cu2ZnSnS4 based thin films assisted by Na crystallization catalyst
Sol. Energy
Growth of Cu2ZnSnS4 absorber layer on flexible metallic substrates for thin film solar cell applications
Thin Solid Films
Residual stresses in thin film systems: effects of lattice mismatch, thermal mismatch and interface dislocations
Int. J. Solids Struct.
Grazing incidence X-ray diffraction for the study of polycrystalline layers
Thin Solid Films
Elucidating the role of interfacial MoS2 layer in Cu2ZnSnS4 thin film solar cells by numerical analysis
Sol. Energy
Cu2ZnSn(S,Se)4 from annealing of compound co-sputtered precursors – recent results and open questions
Sol. Energy
Band gap engineering of alloyed Cu2ZnGexSn1–xQ4 (Q = S,Se) films for solar cell
J. Phys. Chem. C
Band gap tuning of Cu2ZnGeSxSe4−x absorbers for thin-film solar cells
Energies
Earth abundant element Cu2Zn(Sn1−xGex)S4 nanocrystals for tunable band gap solar cells: 6.8% efficient device fabrication
Chem. Mater.
Bandgap-graded Cu2Zn(Sn1−xGex)S4 thin-film solar cells derived from metal chalcogenide complex ligand capped nanocrystals
Chem. Mater.
Electrical and optical properties of stannite-type quaternary semiconductor thin films
Jpn. J. Appl. Phys.
Wurtzite-derived polytypes of kesterite and stannite quaternary chalcogenide semiconductors
Phys. Rev. B
Cu2Zn(Sn,Ge)Se4 and Cu2Zn(Sn,Si)Se4 alloys as photovoltaic materials: Structural and electronic properties
Phys. Rev. B
Germanium incorporation in Cu2ZnSnS4 and formation of a Sn–Ge gradient
Phys. Status Solidi (a)
Hydrazine-processed Ge-substituted CZTSe solar cells
Chem. Mater.
7.6% CZGSe solar cells thanks to optimized CdS chemical bath deposition
Phys. Status Solidi (a)
Revealing the beneficial effects of Ge doping on Cu2ZnSnSe4 thin film solar cells
J. Mater. Chem. A
Germanium alloyed kesterite solar cells with low voltage deficits
Chem. Mater.
Rear Band gap grading strategies on Sn–Ge-alloyed kesterite solar cells
ACS Appl. Energy Mater.
Glow discharge optical emission spectrometry: moving towards reliable thin film analysis–a short review
J. Anal. . Spectrom.
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