Analysis and optimisation of the glass/TCO/MZO stack for thin film CdTe solar cells

Bittau, Francesco; Potamialis, Christos; Togay, Mustafa; Abbas, Ali; Isherwood, Patrick J. M.; Bowers, Jake W.; Walls, John M.

doi:10.1016/j.solmat.2018.07.019

Cited by 38 publications

(23 citation statements)

References 21 publications

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“…1 c. It is not observed to penetrate the FTO 5 . The CdS buffer layer in recently improved devices has been replaced with wide-band-gap metal-oxides such as magnesium-doped zinc oxide (MZO) to reduce optical absorption 6 , 7 . The chlorine does not decorate the MZO grain boundaries to the same extent and the chlorine accumulates mainly at the interface with the CdSeTe absorber.…”

Section: Introductionmentioning

confidence: 99%

Chlorine activated stacking fault removal mechanism in thin film CdTe solar cells: the missing piece

et al. 2021

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The conversion efficiency of as-deposited, CdTe solar cells is poor and typically less than 5%. A CdCl2 activation treatment increases this to up to 22%. Studies have shown that stacking faults (SFs) are removed and the grain boundaries (GBs) are decorated with chlorine. Thus, SF removal and device efficiency are strongly correlated but whether this is direct or indirect has not been established. Here we explain the passivation responsible for the increase in efficiency but also crucially elucidate the associated SF removal mechanism. The effect of chlorine on a model system containing a SF and two GBs is investigated using density functional theory. The proposed SF removal mechanisms are feasible at the 400 ∘C treatment temperature. It is concluded that the efficiency increase is due to electronic effects in the GBs while SF removal is a by-product of the saturation of the GB with chlorine but is a key signal that sufficient chlorine is present for passivation to occur.

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Section: Introductionmentioning

confidence: 99%

Chlorine activated stacking fault removal mechanism in thin film CdTe solar cells: the missing piece

et al. 2021

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“…The transparency of this layer determines the number of photons reaching the absorber and consequently, the number of electron-hole pairs generated, thereby increasing the short circuit current (Jsc). An optimised buffer/absorber interface is essential to maximise the open circuit Voltage(Voc) and Fill Factor (FF) [6]- [8], and the quality of this interface depends on the interface chemistry, the lattice mismatch and the band alignment between the semiconductors. The device is completed with a back contact, deposited on top of the absorbing material, normally consisting of a buffer layer and a metal contact.…”

Section: Introductionmentioning

confidence: 99%

“…Mg-doped ZnO (MZO) has been shown to be an effective buffer layer for both CdTe and CdSeTe alloyed cells, due to its high transparency. It is also possible to tune its energy band structure to fit the absorber material [6], [8]- [13]. By adding Mg to the ZnO lattice, the substitution of Zn ions with Mg to form MgO causes an upward shift of the conduction band level of the semiconductor.…”

Section: Introductionmentioning

confidence: 99%

Degradation of Mg-doped zinc oxide buffer layers in thin film CdTe solar cells

et al. 2019

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Cadmium Sulphide is the conventional n-type buffer layer used in thin film Cadmium Telluride solar cells. It is well known that Cadmium Sulphide causes optical losses and sulphur diffuses into the absorber during high temperature activation. Sputterdeposited Mg-doped ZnO (MZO) has been shown to be an attractive buffer layer for Cadmium Telluride solar cells due to its transparency and tuneable band gap. It is also stable to high temperature processing and avoids diffusion of elements into the cadmium telluride absorber during the cadmium chloride activation treatment. However, degradation is observed in solar cells incorporating MZO buffer layers. Analysis of the MZO film surface potential has revealed significant fluctuations in the thin film work function once the layer is exposed to the atmosphere following deposition. These fluctuations are attributed to the high reactivity to water vapour of the MgO contained in the MZO films. This has been analysed using X-ray Photoelectron Spectroscopy to determine corresponding changes in the surface chemistry. The Zinc Oxide component is relatively stable, but the analysis shows that MgO forms a Mg(OH)2 layer on the MZO surface which forms a secondary barrier at the MZO/CdTe interface and/or at the interface between MZO and the Fluorine-doped SnO2. This affects the Fill Factor and as a consequence it degrades the conversion efficiency.

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“…It is not observed to penetrate the FTO [5]. The CdS buffer layer in recently improved devices has been replaced with wideband gap metal-oxides such as magnesium-doped zinc oxide (MZO) to reduce optical absorption [6,7]. The chlorine does not decorate the MZO grain boundaries to the same extent and the chlorine accumulates mainly at the interface with the CdSeTe absorber.…”

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confidence: 99%

Chlorine activated stacking fault removal mechanism in thin film CdTe solar cells: the whole story

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et al. 2021

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The efficiency of as-deposited CdTe solar cells is typically <5 %. An activation process involving the post-treatment of the CdTe surface with cadmium chloride at approximately 400 deg C improves the absorber microstructure by removing stacking faults, fills the grain boundaries with chlorine and leads to efficiencies of up to ~22 %. Stacking fault removal and improvement in device efficiency are thus correlated but the question of whether this correlation is direct or indirect has to date not been established. Although some types of stacking fault could be hole traps, those most commonly observed are tetrahedral in nature which recent work has shown to be electrically benign. For the first time, in this paper, we not only explain the passivation responsible for this efficiency increase but crucially elucidate the associated stacking fault removal mechanism. Experimental work shows that as chlorine is added to the system, the stacking faults gradually disappear from the absorber layer upwards to the surface with some grains being fault-free in the intermediate state before full saturation. At saturation chlorine decorates all the grain boundaries and so the effect of chlorine on a model system of a tetrahedral stacking fault bounded by two grain boundaries is investigated using density functional theory (DFT). We find that if the stacking faults between the grain boundaries were to be removed without Cl, this would result in an energy increase for the system. Increasing the chlorine concentration in the grain boundaries decreases the energy difference between the faulted and unfaulted system until a cross-over occurs close to the point at which chlorine saturates the grain boundaries. The atomic mechanisms and energy profile for the stacking fault removal is presented. The removal process is via a cascade effect whereby the system energy gradually increases as the fault is sequentially removed until the layers snap into place. The energy barrier for this to occur is easily overcome with the 400 deg C temperature treatment. Density of States (DOS) plots of the chlorine saturated structures show that defect passivation occurs in the highly reconstructed grain boundaries due to both interstitial and substitutional chorine. Chlorine saturation is shown to disconnect the two sides of the grain boundary avoiding electronic defects which arise from the interaction of dangling bonds across this region. It is thus concluded that the cell efficiency increase observed is due to the electronic effects of chlorine in the grain boundaries and that the observed stacking fault removal is a bi-product of the chlorine grain boundary saturation.

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Analysis and optimisation of the glass/TCO/MZO stack for thin film CdTe solar cells

Cited by 38 publications

References 21 publications

Chlorine activated stacking fault removal mechanism in thin film CdTe solar cells: the missing piece

Chlorine activated stacking fault removal mechanism in thin film CdTe solar cells: the missing piece

Degradation of Mg-doped zinc oxide buffer layers in thin film CdTe solar cells

Chlorine activated stacking fault removal mechanism in thin film CdTe solar cells: the whole story

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