Electrodeposition of ternary compounds for novel PV application and optimisation of electrodeposited CdMnTe thin-films

The study targets development of a p-type, wide-bandgap window material for graded bandgap CdTe-based thin-film photovoltaics fabricated by low-cost electrodeposition. CdTe thin-film PV leads the market, with commercial success via CSS and electrodeposition; electrodeposition offers simplicity, scalability, and cost advantages, especially for developing regions. The Sheffield Hallam University group has demonstrated high-efficiency graded bandgap devices and proposes p-type window architectures (p+-p-i-n-n*) that can yield higher open-circuit voltages. A suitable p-type window must be electrodepositable, possess a bandgap exceeding CdTe’s (~1.45 eV), exhibit good crystallinity, and have minimal mid-gap defects. Prior work alloying Mg into CdTe (CdTe:Mg) produced wide bandgaps (~2.8 eV) and p-type conductivity but severely degraded crystallinity and introduced deep levels (~−1.45 eV), making it unsuitable. Motivated by the good lattice match of MnTe with CdTe and the relevance of Cd1−xMnxTe in electronics, this work explores, for the first time, electrodeposited CdMnTe (CMT) thin films as a prospective p-type window for CdTe PV.

- Commercial CdTe PV has been realized via CSS (First Solar) and electrodeposition (BP Solar), with electrodeposition noted for low-cost manufacturing, scalability, and compatibility with developing-world infrastructure. - Graded bandgap, multi-layer device architectures on p-type windows can deliver higher Voc; prior demonstrations in GaAs/AlGaAs and perovskites achieved Voc up to 1.17 V and 1.00 V, respectively. - Previous electrodeposited ternary attempt: CdTe:Mg achieved bandgap widening but resulted in amorphous layers and deep defect levels, compromising device utility. - MnTe has high lattice compatibility with CdTe; Cd1−xMnxTe is established for electronic and radiation detector applications, suggesting potential as a wide-bandgap window if electrodeposited with sufficient quality.

- Substrates: Glass/FTO (TEC 7, 7 Ω/sq). Cleaning by soap solution, DI rinse, N2 dry. - Electrolyte preparation: 400 mL DI water containing 1.00 M CdSO₄ (99.999%), 0.12 M MnSO₄ (99.996%), and 5 mL of TeO₂ solution. TeO₂ solution prepared by adding 2 g TeO₂ (99.995%) to 200 mL DI water with 30 mL concentrated H₂SO₄ to aid dissolution; stirred and heated ~45 min to clarity. - Deposition conditions: Two-electrode potentiostatic cathodic electrodeposition (GillAC ACM potentiostat), FTO as cathode, carbon as counter, bath pH 2.00 ± 0.02 (adjusted with NH₄OH/H₂SO₄), bath temperature ~85 °C, moderate magnetic stirring. Growth potentials selected from cyclic voltammetry. - Cyclic voltammetry: Sweep 0 to 2000 mV at 180 mV/min in the deposition electrolyte on FTO to identify onset and co-deposition regions (forward and reverse cycles analyzed). Expected sequential deposition based on redox potentials: Te (~+0.59 V), Cd (−0.40 V), Mn (+1.19 V vs SHE), with reactions: HTeO₃⁺ + 4e⁻ + 3H⁺ → Te + 2H₂O; Cd²⁺ + 2e⁻ → Cd; Mn²⁺ + 2e⁻ → Mn; co-deposition: H₂TeO₃²⁺ + 3H⁺ + Cd²⁺ + Mn²⁺ + 8e⁻ → CdMnTe + 2H₂O. - Post-growth treatments: As-deposited (AD) layers subjected to heat treatment in air at 400 °C for 20 min in presence of CdCl₂ (CCT) or GaCl₃ (GCT); additional CCT temperature series at 320, 350, 380, 400 °C for 20 min for resistivity studies. - Structural characterization: XRD (Philips PW X’Pert Pro, Cu Kα, λ = 1.54 Å, 40 kV, 40 mA). Peaks indexed with ICDD 98-017-4576 (cubic phase). - Composition: Initial XPS lacked Mn sensitivity; SNMS performed (HIDEN SIMS Workstation, quadrupole MS) with Ar ion beam (5 keV, 100 µA), 45° incidence, raster 800×800 µm², gate 100×100 µm². Isotopes monitored: ⁵⁵Mn, ¹¹⁴Cd, ¹³⁰Te, ¹²⁰Sn (substrate marker). Data presented with 5-point FFT smoothing. - Optical: UV-Vis absorption (Cary 50 Scan, 300–1000 nm). Bandgaps via Tauc-like plots (α² vs hν) using tangent extrapolation. - Electrical type: PEC cell measurements to determine p/n conduction type for AD, CCT, GCT across growth potentials. - DC I–V and conductivity: Glass/FTO/p-CdMnTe/Au structures (100 nm Au, 0.20 cm diameter) measured with Keithley 619; ohmic assessment and resistivity/conductivity extraction. - Schottky diodes: Glass/FTO/p-CdMnTe/Al (0.20 cm diameter Al by evaporation) characterized under dark I–V; series/shunt resistances and ideality factor estimated. - Morphology: SEM (Quanta 3D FEG, 20.0 kV, 30,000×) for AD and CCT samples grown at optimal potential.

- Cyclic voltammetry identified deposition sequence and co-deposition window: Te begins deposition at ~300 mV; Cd deposition onset at ~1100 mV (point A) with a peak near ~1300 mV (point B) where Mn deposition starts; stable forward current region ~1320–1450 mV indicates co-deposition of Cd, Mn, Te to form CdMnTe. Selected growth window ~1300–1560 mV; Cd-rich films near the high end of this range. Reverse scan shows dissolution features for Cd/Mn (point C) and Te (D, E). - Structural (XRD): AD films are polycrystalline cubic with clear (111), (022), (113) peaks; CCT introduces additional (004), (133) and increases intensities, indicating improved crystallinity. Best crystallinity at growth potential 1430 mV, selected as optimized Vg. Possible coexistence of CdTe and CdMnTe phases due to bath chemistry and similarity of diffraction patterns. - Composition (SNMS): Mn is included but concentrated toward upper regions of the film; Te is relatively uniform; Cd peaks near the substrate; Sn rises upon reaching substrate. Due to thin films, depth not quantified. Overall qualitative composition indicates Cd-rich CdTe with Mn inclusion, consistent with CV. - Optical bandgap: Bandgaps range ~1.72–2.22 eV depending on growth voltage and treatment (Table 1); for Vg = 1430 mV, AD/CCT/GCT all ~1.95–1.96 eV. A sub-bandgap absorption near ~930 nm (≈1.33 eV) consistent with CdTe signature (~1.45 eV) indicates mixed phases; post-treatments sharpen absorption edges. Table highlights: e.g., at 1450 mV, AD Eg ~2.22 eV; CCT Eg ~1.94 eV. - Conductivity type (PEC): AD layers switch from p- to n-type at ~1330 mV, then revert to p-type at ~1370 mV as Mn incorporation increases; post-treatments (CCT, GCT) yield p-type across examined voltages (GCT damaged lower-voltage films). Optimal window layer identified as CCT-CdMnTe grown at 1430 mV with stable Eg ~1.95 eV and strong (111) texture. - DC conductivity versus CCT temperature (for Vg = 1430 mV): Resistivity decreases with higher CCT temperatures: AD ~2.771×10³ Ω·cm; CCT320 ~2.302×10³ Ω·cm; CCT350 ~1.677×10³ Ω·cm; CCT380 ~6.25×10² Ω·cm; CCT400 ~3.94×10² Ω·cm. Corresponding conductivities increase, highest for CCT400 (~2.536×10⁻⁴ Ω⁻¹·cm⁻¹). - Schottky diodes (p-CdMnTe/Al): Rectifying behavior with Rs ≈ 1.46×10⁶ Ω, Rsh ≈ 2.87×10⁶ Ω, rectification factor ≈ 10^2.5, ideality factor n ≈ 1.19, indicating thermionic emission with some recombination-generation contribution; confirms p-type conduction and device-quality interfaces. - Morphology (SEM): AD films show small, cauliflower-like grains; CCT promotes significant grain growth up to ~0.80 µm through crystallite agglomeration. - Comparative insight: Compared with Mg incorporation, Mn widens the bandgap to ~2.2 eV while retaining polycrystallinity, avoiding amorphization seen with Mg; however, mixed-phase behavior and sub-bandgap absorption persist.

The results demonstrate that Mn can be incorporated via electrodeposition into CdTe to form CdMnTe films exhibiting p-type conductivity and widened direct bandgaps suitable for window layers in graded-bandgap CdTe devices. Cyclic voltammetry guided selection of a co-deposition potential window, leading to optimized growth at 1430 mV where XRD shows enhanced crystallinity and optical bandgap remains stable (~1.95 eV) regardless of post-treatment. Electrical characterization confirms p-type behavior after chloride treatments and functional device interfaces: Au forms ohmic contacts, Al forms Schottky barriers with favorable rectification and near-ideal transport. Compared to the prior Mg-alloyed system, Mn alloying preserves crystallinity while providing the necessary p-type conversion and bandgap widening. Nevertheless, optical signatures and SNMS indicate mixed-phase films (CdTe plus CdMnTe) and non-uniform Mn distribution concentrated toward the film surface, implying that further compositional control is needed. These findings directly address the goal of identifying an electrodepositable p-type wide-bandgap window for CdTe PV and substantiate CdMnTe as a promising candidate within a manufacturable, low-cost process framework.

This work provides the first demonstration of electrochemical incorporation of Mn into polycrystalline CdTe to produce p-type CdMnTe thin films with a widened bandgap around ~1.95 eV, suitable as window layers for graded bandgap CdTe solar cells. Optimized deposition at 1430 mV and subsequent CdCl₂ treatment yield improved crystallinity, increased conductivity, and device-quality contacts, with Schottky diodes exhibiting strong rectification and an ideality factor ~1.19. While the films show mixed-phase characteristics and non-uniform Mn distribution, the overall results validate CdMnTe as a promising, low-cost, electrodepositable window material for future CdTe-based PV architectures. Future work should focus on controlling composition and Mn incorporation depth profiles, reducing mixed phases to suppress sub-bandgap absorption, and integrating these layers into full device stacks to evaluate PV performance.

- XPS lacked sensitivity to detect low Mn content; SNMS provided only qualitative confirmation of Mn presence and distribution. - SNMS depth profiling could not reliably quantify film thickness due to thin layers; Mn found non-uniform, concentrated near the surface. - Evidence of mixed phases (CdTe and CdMnTe) from optical absorption (sub-bandgap feature near CdTe) and potential peak overlap in XRD complicates unambiguous phase identification. - PEC is surface sensitive; p-type signals after CCT may reflect surface/grain-boundary doping, not necessarily bulk grain interiors; thus conductivity type inference requires corroborating electrical contacts (performed here for select samples only). - GaCl₃ treatment damaged films grown at lower potentials (<1370 mV), preventing their characterization, limiting comparative post-treatment analysis. - Compositional control (Cd-rich tendencies) and Mn uniformity remain challenges for optimizing electronic properties and eliminating defect-related absorption.