Progress towards high efficiency thin film CdTe solar cells

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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“…Some of the most popular methods for growing CdTe layer are CSS [16], CSVT [17], sputtering [18] and electrodeposition [19,20].…”

Section: Materials Issuesmentioning

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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“…The theoretical maximum photocurrent available from a CdTe solar cell under the standard global spectrum, 100 mW/cm2 (air mass lS), is 30.5 mA/cm2 [21]. It is found that a short-circuit current density (JJ of 26-28 mA/cm2 [22] can be achieved by minimizing the reflection and absorption losses.…”

Section: Solar Cellsmentioning

confidence: 92%

“…Sites [21] showed that the major photocurrent loss in the present fiont wall CdTe/CdS solar cells is associated with the reflection and the absorption in the CdS window layer. Since the CdS film acts as a dead layer, resulting fiom recombination of most of the photo-generated carriers in the CdS, its thickness has significant impact on the cell efficiency.…”

Section: Efficiency Limiting Defects and Mechanisms In Cdtdcds Solar mentioning

confidence: 99%

“…Since there are limited choices for p-type widegap window materials, heterojunctions with n-type CdTe are rarely investigated. However, a number of n-type wide-bandgap materials like CdS, IT0 [20], SnO, [21], ZnO [22], and ZnSe [23] can be used with p-type CdTe to form heterojunctions.…”

Section: Historical Survey Of Cdte Solar Cellsmentioning

confidence: 99%

“…An integrating sphere was used for accurate reflectance measurement in order to reduce the errors due to flux losses [21] and the scattered from the polycrystalline cell structure.…”

Section: Reflectance Measurementsmentioning

confidence: 99%

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