Author

Pranab Sarker

ORCID Identifier(s)

0000-0001-9283-6398

Graduation Semester and Year

2015

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Physics and Applied Physics

Department

Physics

First Advisor

Muhammad N Huda

Abstract

Our present work represents a systematic theoretical and computational research to find an affordable material for solar energy application using density functional theory (DFT) and post-DFT such as DFT+U and DFT-HSE06. Here, we predict a new photovoltaic material (CuSnW2O8) and a new photocatalyst (ZnSnW2O8). In addition, a new method for predicting the higher power conversion efficiency (PCE) optimized growth conditions will be presented. It is well known that all physical properties are calculated once crystal structure is known. However, knowing crystal structure is extremely challenging and to date, there is no success has been claimed in the case of a material which is yet to exist. Under this circumstance, an existing method was adopted and extended to a great extent in predicting the crystal structures of new materials. This method invokes global optimization of possible candidates (motif structures), which was accomplished by means of DFT; the motif structures were obtained through an in-house algorithm that takes an existing structure in mineral database as an input and generates a possible candidate of the material of interest as an output. This method was initially tested to few existing structures and the success of which was extended to the new materials. In addition to crystal prediction, determining suitable growth conditions before the material is synthesized is another challenge. To tackle this difficulty, a method has been developed, which includes chemical potential (Gibbs free energy) landscape analysis (CPL) as well as defects calculation. Like other scientific processes, the method was used to reproduce the available data for the existing systems and due to success on those it was employed to the predicted materials. The DFT-derived opto-electroinc properties and stability analysis of CuSnW2O8 show that it can be a perfect alternative of currently commercialized solar cell such as silicon (Si) and CuInxGa1-xSe2 (CIGS). It possesses band gaps of 1.25 eV (indirect) and 1.37 eV (direct), which were evaluated using the hybrid functional (HSE06) as a post-DFT method. The hole mobility of CuSnW2O8 was higher than that of Si. Further, optical absorption calculations demonstrate that CuSnW2O8 is a better absorber of sunlight than other promising solar cells, namely Cu2ZnSnS4 (CZTS) and CIGS. In addition, it exhibits rigorous thermodynamic stability comparable to WO3. CPL analysis further revealed that CuSnW2O8 can be synthesized at flexible experimental growth conditions although the co-existence of at least one secondary phase is likely. The formation of Cu vacancies was found to be highly probable, even at Cu-rich growth condition, which could introduce p-type activity in CuSnW2O8. Like CuSnW2O8, ZnSnW2O8 also exhibited primary features of the PEC process such as moderate band gap (~2.34 eV in DFT-HSE06), proper band edges positions, and higher stability. In addition, the higher optical absorption ability and dispersive band features embodied it a very attractive candidate for PEC process. According to CPL analysis, ZnSnW2O8 could also be synthesized at flexible experimental growth; however, at least two secondary phases were likely. Defects were found to be less probable at cationic-rich growth conditions in which the probable defects could be the Zn at Sn site and O-vacancy which give rise to n-type activity in ZnSnW2O8. Finally, the results presented here reveal that considering our predicted materials for the synthesis of the next general solar cells could be an effective choice.

Keywords

Materials design, Crystal structure prediction, Phase stability, Chemical potential analysis, Optimized growth conditions prediction, PV/PEC materials

Disciplines

Physical Sciences and Mathematics | Physics

Comments

Degree granted by The University of Texas at Arlington

Included in

Physics Commons

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