Correlation of photoluminescence measurements with the composition and electronic properties of chemically etched CdTe surfaces

Sobiesierski, Zbigniew; Dharmadasa, I. M.; Williams, R. H.

doi:10.1063/1.100178

Cited by 63 publications

(29 citation statements)

References 12 publications

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“…It is also worth re-visiting the PL measurements carried out on bulk CdTe wafers purchased from Eagle Pitcher, in the past by the main author [10]. Figure 10 shows the PL spectra obtained at 4 K, together with corresponding X-ray photo-electron spectra (XPS) of the surfaces investigated.…”

Section: Bulk Cdte From Eagle Pitchermentioning

confidence: 99%

Effects of CdCl2 treatment on deep levels in CdTe and their implications on thin film solar cells: a comprehensive photoluminescence study

Dharmadasa

Echendu

Fauzi

et al. 2015

J Mater Sci: Mater Electron

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This work is aimed at studying defect level distributions in the bandgap of CdTe thin films, used for solar cell development. In particular, the effects of CdCl 2 treatment on the defect levels are the main objectives of this research. Four different CdTe thin films were electroplated using three different Cd-precursors (CdSO 4 , Cd(NO 3 ) 2 and CdCl 2 ), and bulk CdTe wafers purchased from industry (Eagle Pitcher and University Wafers in US) were studied using low temperature photoluminescence. The finger prints of defects, 0.55 eV below the conduction band down to the valence band edge were investigated. In all of the CdTe layers, four electron trap levels were observed with varying intensities but at very similar energy positions, indicating that the origin of these defects are mainly from native defects. CdCl 2 treatment and annealing eliminates two defect levels and the mid-gap recombination centres are reduced drastically by this processing step. The optical bandgap of all four as-deposited CdTe layers is ~1.50 eV, and reduces to ~1.47 eV after CdCl 2 treatment. The material grown using the CdCl 2 precursor seems to produce CdTe material with the cleanest bandgap, most probably due to the built-in CdCl 2 treatment while growing the material.

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Section: Bulk Cdte From Eagle Pitchermentioning

confidence: 99%

Effects of CdCl2 treatment on deep levels in CdTe and their implications on thin film solar cells: a comprehensive photoluminescence study

Dharmadasa

Echendu

Fauzi

et al. 2015

J Mater Sci: Mater Electron

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“…7 Schematic diagrams summarising CdTe grain growth patterns, corresponding XRD spectra and approximate solar cell efficiencies against growth or heat treatment temperature. Note the three regions identified with a sudden phase transition at ST1 = 385 °C, and a slow phase transition taking place after ST2 >430 °C [35] in a comprehensive research programme [3,56,58,59] and a peculiar effect based on fermi level (FL) pinning at n-CdTe/metal interfaces was observed. This work confirmed Schottky barrier formation at the above interface is heavily dependent on Fermi level pinning at one of the five experimentally observed defect levels E1, E2,… E5 (see Fig.…”

Section: Cds/cdte Device Issuesmentioning

confidence: 99%

“…8b). Furthermore, it was found that E1, E2 and E3 dominate when the CdTe surface is rich in Te, and E4 and E5 dominate when the CdTe surface is rich in Cd [56,58]. Therefore, in order to form large Schottky barriers, Fermi level should be pinned at E5, closer to the valence band maximum.…”

Section: Cds/cdte Device Issuesmentioning

confidence: 99%

“…Detailed studies on chemical etching have been carried out using CdTe wafers [3,56,58,59]. Review articles show that acidic etchants preferentially removes metallic element, Cd and leave Te-rich CdTe surface.…”

Section: Chemical Etching Prior To Metallisationmentioning

confidence: 99%

“…NaOH, KOH, hydrazene hydrate etc are good examples of these etchants. A few seconds etch in oxidizing etch (dilute K 2 Cr 2 O 7 in H 2 SO 4 ) followed by ~2 min treatment in reducing etch (aqueous solution of NaOH + Na 2 S 2 O 3 ) proved to produce Cd-rich surfaces, leading to high Schottky barrier formation at n-CdTe/metal interfaces [3,[58][59][60][61].…”

Section: Chemical Etching Prior To Metallisationmentioning

confidence: 99%

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Unravelling complex nature of CdS/CdTe based thin film solar cells

Dharmadasa

Ojo

2017

J Mater Sci: Mater Electron

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and high-temperature growth techniques. The wide bandgap (E g = 2.42 eV) window material, CdS was mainly grown by chemical bath deposition (CBD) and narrow bandgap (E g = 1.45 eV) absorber material, CdTe was grown by over 14 different methods [1]. High efficiencies were achieved by the combination of CdS with CdTe layers grown by high temperature growth methods such as closed space sublimation (CSS) or close spaced vapour transport (CSVT) for a long period. Depending on the growth technique, material growth mechanisms, micro-structure and impurity inclusions are going to vary. Although there are a large number of publications reporting material characterisation, deep understanding is required on material issues such as defects and doping concentrations.The next area needing deeper understanding is the postgrowth chemical and heat treatment. The devices fabricated using as-made CdTe materials show poor device performance in the range ~(0-6)% depending on the growth technique used. However, from 1976 [2], the researchers were aware of the drastic improvement of the device performance after heat treatment around 450 °C in air, in the presence of CdCl 2 . This was known as "CdCl 2 treatment" and device efficiencies improved to mid-teens achieving good solar cell performance. Although there were tremendous efforts to explore the complexity of changes during this treatment, full understanding has not yet been achieved.The third main area of complexity comes from the CdTe/electrical back contact formation. It is essential to clean the CdTe surface after CdCl 2 treatment, and researchers were using different chemicals to clean the surface prior to metallisation. Various electrical contacts have been used to complete the devices and varying results have been reported in the literature [3]. Initial high efficiency, reproducibility and yield, stability and lifetime are main features of fully fabricated devices to establish. Some of these Abstract Thin film solar cells based on CdS/CdTe hetero-structure has shown a drastic improvement changing from 16.5 to 22.1% efficiency during a short period of time from ~2013 to ~2016. This has happened in the industrial environment and the open research in this field has stagnated over a period of two decades prior to ~2013. Most of the issues of this hetero-structure were not clear to the photovoltaic (PV) community and research efforts should be directed to unravel its complex nature. Issues related to materials, post-growth treatment, chemical etching prior to metallisation and associated device physics are the main areas needing deeper understanding in order to further develop this device. After a comprehensive research programme in both academia and in industry on these materials, surfaces and interfaces and fully fabricated devices over a period of over three decades by the main author, the current knowledge as understood today, on all above mentioned complex issues are presented in this paper. Full understanding of this structure will enable PV developers to further improve the ...

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Diffusion-Controlled Electrodeposition and Characteristics of CdTe Films

Rakhshani

Ramazaniyan

1999

phys. stat. sol. (a)

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Cadmium telluride was deposited on various conducting substrates from an acidic bath of CdSO4 (1 M) containing up to 200 ppm of TeO2. The deposition process which is a six‐electron‐transfer reaction, is described satisfactorily by a model developed based on the diffusion of HTeO2+ cations. The film structure, composition and the effect of annealing were studied. The optical dispersion of films which agrees well with that for the single crystal can be described by the classical bound‐electron dispersion model. The number of bound electrons per CdTe molecule which participate in the dispersion process was determined to be 3.0. Annealing in air at 350 °C has no influence on dispersion.

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Correlation of photoluminescence measurements with the composition and electronic properties of chemically etched CdTe surfaces

Cited by 63 publications

References 12 publications

Effects of CdCl2 treatment on deep levels in CdTe and their implications on thin film solar cells: a comprehensive photoluminescence study

Effects of CdCl2 treatment on deep levels in CdTe and their implications on thin film solar cells: a comprehensive photoluminescence study

Unravelling complex nature of CdS/CdTe based thin film solar cells

Diffusion-Controlled Electrodeposition and Characteristics of CdTe Films

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