In-depth analysis of chloride treatments for thin-film CdTe solar cells

Major, Jonathan D.; Turkestani, Mohammed Al; Bowen, Leon; Brossard, M.; Li, Chen; Lagoudakis, Pavlos G.; Pennycook, Stephen J.; Phillips, Laurie J.; Treharne, Robert E.; Durose, K.

doi:10.1038/ncomms13231

Cited by 80 publications

(35 citation statements)

References 35 publications

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“…The presence of grain-to-grain contrast in all films we studied suggest that there may be an inherent grain-to-grain work function variation. Though grain-to-grain non-uniformity is not an entirely new finding [3], [6], it has been less explored as a factor impacting local junction formation between CdTe and neighboring buffer layers and electrodes. It should be expected that carrier drift and diffusion could vary significantly depending on the orientation of CdTe crystallites, as suggested by our spatially varying work function measurements.…”

Section: Resultsmentioning

confidence: 99%

“…The prevailing description is that Cl from the CdCl 2 treatment segregates to grain boundaries in p-type CdTe, depleting or inverting them with respect to grain interiors. Since the formation of junctions between grain interiors and grain boundaries are expected to conduct charges to the electrodes more efficiently, individual grain boundary properties have been a subject of intense study, which have been measured using an array of techniques including scanning probe microscopy [2]- [3], electron beam-induced current [4], timeresolved photoluminescence [5], and time-of-flight secondary ion mass spectroscopy (ToF-SIMS) [6].…”

Section: Introductionmentioning

confidence: 99%

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Effects of CdCl2 treatment on the local electronic properties of polycrystalline CdTe measured with photoemission electron microscopy

Berg

Kephart

Sampath

et al. 2017

2017 IEEE 44th Photovoltaic Specialist Conference (PVSC)

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To investigate the effects of CdCl 2 treatment on the local electronic properties of polycrystalline CdTe films, we conducted a photoemission electron microscopy (PEEM) study of polished surfaces of CdTe films in superstrate configuration, with and without CdCl 2 treatment. From photoemission intensity images, we observed the tendency for individual exposed grain interiors to vary in photoemission intensity, regardless of whether or not films received CdCl 2 treatment. Additionally, grain boundaries develop contrast in photoemission intensity images different from grain interiors after an air exposure step, similar to observations of activated grain boundaries using scanning Kelvin probe force microscopy studies. These results suggest that work function varies locally, from one grain interior to another, as well as between grain boundaries and grain interiors.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Effects of CdCl2 treatment on the local electronic properties of polycrystalline CdTe measured with photoemission electron microscopy

Berg

Kephart

Sampath

et al. 2017

2017 IEEE 44th Photovoltaic Specialist Conference (PVSC)

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“…It was found that the presence of O 2 is helpful in achieving high efficiencies after this heat treatment [47]. Research community has tried many other chlorides to replace CdCl 2 but the closest result was found only with MgCl 2 [48]. Researchers have also spent lot of time in optimising both the heat treatment temperature and treatment duration.…”

Section: Summary Of Reported Results On Cdcl 2 Treatmentmentioning

confidence: 99%

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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“…It has been reported that the chloride treatment is an essential manufacturing process in the CdTe thin lm solar cells to achieve high device efficiencies by passivating the grain boundaries, triggering the recrystallization and p-type doping passivation of the CdTe lm etc. [37][38][39][40] Therefore, the chloride treatment was introduced to investigate the inuence of chloride treatment on the device performance of PFBTBr : CdTe based HSCs (Fig. 5b and Table S6 †).…”

Section: Device Optimizationmentioning

confidence: 99%