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

CdTe-based thin-film solar cells are commercially successful, primarily due to the Close Space Sublimation (CSS) technique. However, electrodeposition (ED) offers advantages in terms of cost-effectiveness, scalability, and simplicity. The research group at Sheffield Hallam University holds the record for the highest cell efficiency for CdTe-based electrodeposited PV, using a graded bandgap (GBG) structure. GBG devices offer potential for higher performance, particularly those fabricated on p-type windows due to the possibility of higher open-circuit voltage (Voc). This work aims to identify and optimize a suitable p-type window layer material for CdTe-based ED solar cells, requiring a bandgap >1.45 eV, good crystallinity, and minimal defect levels. Previous attempts using Mg-incorporated CdTe (CdTe:Mg) yielded unsatisfactory results due to poor crystallinity and the presence of intermediate defect levels. This study focuses on Mn incorporation in CdTe to form CdMnTe, leveraging the close lattice matching of MnTe and CdTe, and its potential for various electronic applications. This is the first effort to electrodeposit CdMnTe for CdTe-based solar devices. The research seeks to determine the optimal electrodeposition parameters and post-growth treatments to achieve the desired p-type CdMnTe layer properties for integration into a novel GBG solar cell structure.

The literature review extensively covers the existing CdTe-based thin-film PV technology, highlighting the dominant CSS technique and the historically significant, albeit less prevalent, electrodeposition method. The review emphasizes the economic and manufacturing advantages of electrodeposition, particularly for developing countries. The work cites previous research on graded bandgap (GBG) solar cells and the potential for enhanced performance with p-type window layers. It also reviews prior work by the authors on Mg-incorporated CdTe, highlighting the challenges encountered and the rationale for shifting the focus to Mn incorporation in CdTe. This section establishes the context and motivation for the current research by showcasing the existing advancements and limitations in the field.

CdMnTe thin films were electrodeposited onto glass/fluorine-doped tin oxide (FTO) substrates using a GillAC ACM potentiostat in a potentiostatic configuration. The electrolyte contained CdSO₄, TeO₂, and MnSO₄ in an acidic aqueous medium. Cyclic voltammetry (CV) was employed to determine the optimal deposition potential for CdMnTe. The structural properties were characterized by X-ray diffraction (XRD), Mn inclusion was verified using sputtered neutral mass spectrometry (SNMS). Photo-electrochemical (PEC) cell measurements determined the electrical conductivity type. Scanning electron microscopy (SEM) examined surface morphology, and UV-Vis spectroscopy determined optical properties. Two post-growth treatments, heat treatment in air with CdCl₂ (CCT) and GaCl₃ (GCT), were applied to the as-deposited (AD) layers. Ohmic (Au contacts) and rectifying (Al contacts) behaviors were explored to confirm p-type conductivity. DC conductivity measurements were performed on Au-contacted samples, and Schottky diode characteristics were investigated on Al-contacted samples.

Cyclic voltammetry identified the optimal deposition potential range for CdMnTe (around 1300-1560 mV). XRD analysis revealed a polycrystalline cubic structure with (111) preferred orientation. SNMS confirmed the presence of Mn in the deposited layers, showing non-uniform distribution, concentrated towards the surface. Optical bandgap values varied between ~1.90 eV and ~2.20 eV, depending on the deposition potential and post-growth treatment. All layers after post-treatment showed p-type conductivity, while AD layers exhibited both p- and n-type conductivity at different potentials. The optimal deposition potential was determined to be 1430 mV, producing the best crystallinity and stable bandgap (~1.95 eV). The CCT treatment resulted in improved crystallinity and larger grain sizes compared to the AD and GCT samples. DC conductivity measurements showed the lowest resistivity and highest conductivity for samples treated at 400 °C with CdCl₂. Schottky diode measurements confirmed the p-type conductivity and rectifying behavior of the CdMnTe layers with Al contacts. SEM micrographs revealed an increase in grain size after CCT treatment.

The successful Mn incorporation in CdTe through electrodeposition provides a promising pathway to creating a p-type window layer for GBG solar cells. Compared to previous work with Mg, Mn incorporation maintained polycrystallinity, unlike Mg which resulted in amorphous layers. While both Mg and Mn converted n-CdTe to p-type and widened the bandgap, Mn offered better crystallinity. The mixed-phase characteristics observed in the optical absorption studies are typical of ternary compounds and suggest the presence of both CdTe and CdMnTe phases. The formation of ohmic contacts with Au and rectifying contacts with Al validates the p-type conductivity and the absence of Fermi-level pinning. The observed results suggest that the optimized CdMnTe layer grown at 1430 mV and subjected to CCT treatment is suitable for use as a p-type window layer in GBG solar cells.

This research demonstrated the successful electrochemical incorporation of Mn into CdTe to produce p-type CdMnTe thin films with a suitable bandgap for PV applications. The optimized films exhibited desirable electrical properties, confirmed by conductivity and Schottky diode measurements. The findings highlight the potential of electrodeposited CdMnTe as a p-type window layer for advanced GBG solar cells. Future research should focus on integrating the optimized CdMnTe layer into a complete GBG solar cell device structure to assess its performance and investigate further optimization strategies.

The study's limitations include the non-uniform Mn distribution in the CdMnTe layer. Further investigations are necessary to achieve more homogeneous Mn distribution. The limited depth profiling analysis in SNMS prevents precise quantification of Mn concentration. The use of a two-electrode system in the electrodeposition process could introduce some inaccuracies. Also, this study is limited to investigating only two post-growth treatment methods (CCT and GCT). Further exploration of alternative treatments could potentially lead to even better performance.