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Title:	Growth optimization of tungsten-di-sulfide (WS2) thin films by physical vapor deposition technique for thin film solar cells
Authors:	Md Khan Sobayel Rafiq (P86077)
Supervisor:	Afida Ayob, Dr.
Keywords:	Universiti Kebangsaan Malaysia -- Dissertations Dissertations, Academic -- Malaysia Thin films Solar cells Tungsten-di-sulfide
Issue Date:	10-Feb-2019
Description:	Tungsten di-sulfide (WS2) has attracted growing research interests as photovoltaic material to the researcher's community due to its non-toxicity, well-matched bandgap (1.5 to 2 eV), conductivity (n-type) and high absorption coefficient (> 105 cm-1). Over the past decades, fewer researchers have shown interest in WS2 thin film growth for photovoltaic applications. Unfortunately, up until now, these efforts did not lead to any solid-state based solar cells with any reasonable efficiency. Merely one group reported solar-cell device parameters of planar cells with an efficiency well below 1% (i.e. <0.31%). Hence, the aim of this study is to identify the suitable incorporation of WS2 in photovoltaic devices. This study encompasses two parts, which are numerical modelling and practical implementation upon experimental studies. Numerical studies for both as absorber layer and window layer have been conducted by solar cell simulation software named SCAPS-1D. WS2 thin film as absorber layer has exhibited conversion efficiency as high as 25.71% with short-circuitcurrent (Jsc) of 32.70 mA/cm2, open-circuit-voltage (Voc) of 0.90 V and fill-factor (FF) of 86.5%. On the other hand, WS2 thin film as a window layer in CdTe solar cell has exhibited conversion efficiency of 16.02%. All these results hold strong theoretical rationale for WS2 to be a potential photovoltaic material. In practical fabrication, WS2 has been deposited and optimized by Physical Vapor Deposition (PVD) method namely radio-frequency magnetron sputtering. Growth optimization has been executed by optimizing all the important deposition parameters such as power, temperature and operating gas-flow rate in the radio frequency magnetron sputtering technique. Moreover, sulfurization process, an additional step in achieving stoichiometric metal chalcogenides, has also been adopted to improve its structural and opto-electronic properties. A number of characterization methods such as Xray diffraction (XRD), Atomic Force Microscopy (AFM), Hall Effect Measurement, Field Emission Scanning Electron Microscope (FESEM), Raman Spectroscopy, Energydispersive X-ray spectroscopy (EDX) and UV-VlS were used to characterize WS2 films. All these results, however, suggest WS2 to be incorporated as window layer instead of absorber layer which has been later successfully introduced in lieu of usual CdS window layer in CdTe thin film solar cells. Eventually, it facilitates non-toxic use of materials by avoiding toxic Cd in CdTe solar cell as well. As for initial investigation, newly incorporated WS2 window layer in CdTe solar cells successfully demonstrated photovoltaic conversion efficiency of 1.2% for an active area of 0.205 cm2 with Voc of 379 mV, Jsc of 11.5 mA/ cm2 and FF of 27.1%. This study therefore paves the way for WS2 thin film as potential buffer layer to be used in thin film solar cell.,Ph.D.
Pages:	133
Publisher:	UKM, Bangi
Appears in Collections:	Faculty of Engineering and Built Environment / Fakulti Kejuruteraan dan Alam Bina

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