Theoretical and experimental investigations of lead chalcogenides quantum confined structures for solar cell application

Solar cell has gained much attention due to its simple design and similarity to dye sensitized solar cells (DSSC), in which the QCS replaces the dye molecules. The QCSs are able to yield more than one exciton upon absorption of a single photon with sufficient energy, a multiple exciton generation (MEG). Theoretically, MEG could increase the efficiency of a PV device ≥ 60%, however QCSCs could deliver an insignificant PV conversion efficiency (PCE) of only ca. 16.6%. This doctoral research therefore aims to: (i) investigate the effect of energy level alignment of the lowest unoccupied molecular orbital of the fluorophore (LUMOfluorophore), with the conduction band minimum of MOS (CBMMOS) on the electron injection efficiency from the fluorophore to the MOS, (ii) determine the geometry of lead chalcogenides QCS (< their exciton Bohr radius) that would exhibit MEG using ab-initio density functional theory (DFT) calculations, (iii) identify the simulated geometries of lead chalcogenides that could be synthesized using a vacuum thermal evaporator (TE) and (iv) investigate the effect of the addition of activated carbon (AC) on the morphology and optoelectronic properties of the lead sulphide (PbS) fabricated using a vacuum TE, and the electron injection efficiency from the fluorophore to the MOS. The results of the study show that the ideal energy level alignment between LUMOfluorophore (-4.0 eV) and CBMMOS (-4.1 eV) supported an efficient electron injection from the fluorophore to the MOS, with an injection efficiency as high as ca. 97%. The structural geometry of PbS, PbSe and PbTe that exhibit MEG were identified viz., (PbS)40, (PbS)74, (PbS)80, (PbSe)16, (PbSe)30, (PbSe)32, (PbSe)50, (PbSe)74, (PbTe)12, (PbTe)16, (PbTe)44, (PbTe)50 and (PbTe)74 with the size of 3.49 nm, 4.86 nm, 4.58 nm, 2.63 nm, 3.20 nm, 3.29 nm, 4.03 nm, 5.02 nm, 2.52 nm, 2.69 nm, 3.90 nm, 4.16 nm and 4.84 nm respectively. The optoelectronic properties of (PbS)80, (PbSe)30 and (PbTe)50 QCS that were obtained from ab-initio DFT calculations were in good agreement the PbS, PbSe and PbTe thin films; which compared based on the first excitonic peaks of the fabricated thin films (nano-sphere morphology) to the PbS, PbSe and PbTe realistic cluster models, which resulted in similarities of 92.93%, 99.38% and 95.49% respectively. The PbS nano-tubules with a size range of 41-76 nm were yielded after the addition of AC with a specific surface area of 80 m 2/g (AC80). PbS nano-sheets with a size range of 36-95 nm were yielded after the addition of AC with a specific surface area of 650 m2/g (AC650). PbS nano-sheets (size range: 33-63 nm) were yielded after the addition of AC with a specific surface area of 1560 m2/g (AC1560). Optoelectronic properties of the fabricated PbS thin films with the addition of AC80, AC650 and AC1560 were similar to that of the (PbS)80 realistic model; determined based on the positions of the first excitonic peaks, which recorded 90.1%, 96.1% and 92.8% of similarity, respectively. The electron injection efficiencies from PbS-AC80, PbS-AC650 and PbS-AC1560 conjugates to the MOS were determined to be 18.48%, 62.71% and 87.18%, respectively. In conclusion, a PbS thin film possessing a nano-sphere morphology and exhibiting MEG could be fabricated using TE without the addition of AC

Student’s acceptance of e-learning during the COVID-19 pandemic

Electronic learning (e-learning) has become the main method of teaching and learning for many educational institutions during the COVID-19 pandemic. The purpose of this study is to investigate the students’ acceptance of e-learning during the COVID-19 pandemic. Quantitative research methods were utilized in order to obtain the necessary data. A total of 100 respondents from three science courses, which were Biology (ED247), Chemistry (ED260), and Physics (ED248) in the Faculty of Education, University Teknologi MARA, Puncak Alam answered a questionnaire given via Google Forms. The IBM SPSS version 25.0 software was used for data analysis. The results show that the students’ acceptance towards e-learning is low based on research question 1 (mean=3.05, SD=.796), research question 2 (mean=1.18, SD=.386), research question 3 (mean=1.18, SD=.435) and research question 4 (mean=1.44, SD=.608). In conclusion, the student’s acceptance of e-learning during the COVID-19 pandemic is significant to improve an educational process by creating and sustaining meaningful learning

Student’s acceptance of e-learning during the COVID-19 pandemic

Electronic learning (e-learning) has become the main method of teaching and learning for many educational institutions during the COVID-19 pandemic. The purpose of this study is to investigate the students’ acceptance of e-learning during the COVID-19 pandemic. Quantitative research methods were utilized in order to obtain the necessary data. A total of 100 respondents from three science courses, which were Biology (ED247), Chemistry (ED260), and Physics (ED248) in the Faculty of Education, University Teknologi MARA, Puncak Alam answered a questionnaire given via Google Forms. The IBM SPSS version 25.0 software was used for data analysis. The results show that the students’ acceptance towards e-learning is low based on research question 1 (mean=3.05, SD=.796), research question 2 (mean=1.18, SD=.386), research question 3 (mean=1.18, SD=.435) and research question 4 (mean=1.44, SD=.608). In conclusion, the student’s acceptance of e-learning during the COVID-19 pandemic is significant to improve an educational process by creating and sustaining meaningful learning

Characterizations of MoS2 nanosphere fabricated using vacuum thermal evaporation at steady and rapid heating

Two-dimensional MoS2 has been speculated to be the best material to replace graphene due to its peculiar structural-electronic properties. The MoS2 with size smaller than its exciton Bohr radius (ca. 1.61 nm) would favor multi exciton generation upon absorption of photon with sufficient energy, Ephoton ≫ Egap (1.89 eV); which would increase the efficiency of an excitonic solar cell greater than 60%. Despite promising properties of the MoS2, however an excitonic solar cell with high efficiency is yet to be exhibited. In this work, the MoS2 thin films were fabricated using vacuum thermal evaporation technique and characterized. Four objectives have been outlined i.e., to study the effect of heating rate (steady, and rapid) on the (i) morphology, (ii) size, (iii) optoelectronic and (iv) crystal properties of the fabricated thin films. The MoS2 precursor was heated at the rate 2.027 A/s (steady), and 18.75 A/s (rapid), 1.5 × 10−3 Torr, 1.48 A, and 4.58 V. The deposited films later were characterized using Field Emission Scanning Electron Microscope with Energy Dispersive X-ray attachment, photoluminescence spectrometer, UV–vis-NIR spectrometer, and X-ray Diffractometer. The fabricated thin films exhibited nanosphere morphology with different size distributions i.e., wide (steady heating), and narrow (rapid heating). Two hypotheses were made based on the optoelectronic properties i.e., the basic building block of the MoS2 thin film fabricated under steady heating is (i) experiencing stronger quantum confinement effect, and (ii) dominated by nanocrystals which are smaller than that of the rapid heating. Similar energy loss could be expected in both MoS2 thin films i.e., ca. 0.15 to 0.17 eV, indicating the existence of shallow trap states. The MoS2 thin films were dominated by (0 0 2), (0 0 4), and (1 0 6) crystal planes. Therefore, the vacuum thermal evaporation technique would offer materials with unique size, crystal arrangement, and optoelectronic properties upon change of heating rate

Multiple exciton generation in MoS2 nanostructures: A density functional theory study

Excitonic solar cell which fabricated using quantum confined semiconducting material that exhibits multiple exciton generation (MEG) is speculated could achieve theoretical photovoltaic conversion efficiency more than 60%. However, the expected efficiency has yet to be reached to date. Specific size and morphology of a quantum confined semiconducting material needs to be studied to determine the presence of MEG. The objective of this study is to verify the occurrence of MEG in few realistic cluster models of MoS2 using density functional theory (DFT) calculations. Small MoS2 nanocrystals were modelled using GaussView 5.0 software, which later validated as realistic using harmonic frequency calculations analysis executed by Gaussian 09W software. The presence of MEG in realistic models of MoS2 nanocrystals was studied using time-dependent density functional theory (TD-DFT) calculations. The output of the work is summarized as the followings, (i) (MoS2)n with n = 2, 4, 6, 8 and 12 models were established as realistic, (ii) the size of the nanocrystal models are smaller than its exciton Bohr radius (ca. 1.61 nm) i.e., 0.54, 0.62, 0.95, 1.09 and 1.57 nm respectively, and (iii) all calculated MoS2 nanoparticle models exhibit MEG. Therefore, a practical technique that could synthesize MoS2 nanocrystals with similar structure or geometry with that of the evaluated models would materialize a device with practical photovoltaic conversion efficiency more than 60%

Industrial applications of quantum dots

Practical strategies of utilizing typical semiconducting materials with size smaller than the exciton Bohr radius—known as quantum confined structures, including the quantum dots (QDs) are needed. Numerous techniques have been explored to fabricate devices that contain specific size of the QDs, theoretically would exhibit favorable effects to the performance of the devices. The QDs have been proven could be synthesized using various techniques. However, there was always a debate whether the unique optoelectronic properties of a quantum-confined structure (e.g., multiple exciton generation and discrete energy levels) could be maintained in different batches of QDs with different size distributions. It is peculiar that numerous reports have shown the ability to synthesized QDs, however a technique that could isolate a specific size of QDs efficiently and/or a method that could maintain the size of the freshly synthesized QDs without altering their unique optoelectronic properties are yet to be demonstrated

A study of the electron regeneration efficiency of solar cells fabricated using CMC/PVA-, Alginate-, and Xanthan-based electrolytes

A photovoltaic (PV) mechanism consists of three important steps, i.e., (i) electron excitation upon absorption of photon with energy higher than the bandgap of fluorophore, (ii) excited-state electron injection from the fluorophore to the photoelectrode, and (iii) electron regeneration from the electrolyte to the fluorophore. An efficient electron regeneration could be achieved upon fulfillment of the requirements of energy alignment, i.e., lowest unoccupied molecular orbital of fluorophore (LUMOfluorophore) > redox potential of electrolyte > highest occupied molecular orbital of fluorophore (HOMOfluorophore). This study investigated the electron regeneration efficiency of excitonic solar cells fabricated using three polymer-based electrolytes, i.e., (i) 60% carboxymethyl cellulose (CMC) blended with 40% polyvinyl alcohol (PVA), (ii) alginate, and (iii) xanthan. The redox potentials of the electrolytes (Eo ) were calculated using quantum chemical calculations under the framework of density functional theory. The compatibility of fluorophore and electrolyte was analyzed in terms of the energy level alignment. The cells fabricated using the three polymer-based electrolytes were analyzed, with the CMC/PVA-based cell yielding the highest efficiency, η, of 1.39% under the illumination of the sun. The low η of the cells can be attributed to the incompatible Eo of the electrolytes, which exhibited a higher energy level than the LUMOfluorophore . The alginate- and xanthan-based cells exhibited inferior PV properties (i.e., open circuit voltage, short circuit current, fill factor, and η) to that of the CMC/PVA-based cell. This finding can be attributed to the increment of energy offset between Eo and HOM

A Study of Interest in Astronomy Among University Students in Malaysia

The misconception related to astronomy is fretfully rising in society. This study aims to investigate, (i) level of misconception towards astronomy among university students, (ii) significant difference in students’ interest towards astronomy between male and female, (iii) significant difference between science and non-science students’ level of knowledge in astronomy, (iv) significant relationship between students’ faculties and misconception in astronomy, and (v) significant relationship between the educational background of the respondents' parents and their misconceptions towards astronomy. A qualitative approach was implemented using a set of questionnaires for data collection. The respondents were undergraduate students of with different courses and backgrounds from four different faculties: Faculty of Education, Faculty of Pharmacy, Faculty of Art and Design, and Faculty of Business Management. The data were analyzed using Statistical Package for the Social Science (SPSS). Findings showed that there is a significant difference in students’ interest towards astronomy between male and female students, there is a significant different between level of knowledge in astronomy and students’ major course, there is significant difference between students’ faculties and misconception in astronomy, and there is no significant relationship between the educational background of the respondents’ parents and their misconception in astronomy. This study benefits society by clarifying phenomena, distinguishing myth from reality. It aids Science teachers in addressing astronomy misconceptions and helps educators curb their spread

A atudy on the optoelectronic properties of lead chalcogenides nanospheres using a combination of experimental and theoretical approach

Various morphologies and cluster geometries of lead chalcogenides (PbX, X = S, Se, Te) have been studied in the size range of 2 - 200 nm. The nanosized PbX clusters that are smaller than their exciton Bohr radius would experience deviation of optoelectronic properties (bandgap and energy levels) in comparison to that of the bulk. The multi-exciton generation (MEG) could be resulted upon expansion of energy levels of the quantum confined PbX; would expedite their application in photovoltaic field. The MEG would favor the increment of photo-generated current and therefore an increment of efficiency (η) of a photovoltaic device could be expected. The characterization of the electronic and emitting states of the quantum confined PbX is however received less attention. This paper aims to validate realistic models of PbX and establish a correlation between the validated models with the experimentally fabricated PbXs based on their optical properties. The narrow bandgap PbX models i.e., (PbS)n, (PbSe)n and (PbTe)n; which n = 4 - 80 were evaluated as realistic models using geometry optimizations and harmonic frequency calculations at the level of B3LYP functional and lanl2dz basis set. The PbX thin films were fabricated using thermal evaporator at vacuum pressure of 1.0 × 10-5 Torr. A nanosphere morphology of the yielded PbXs was observed using Field Emission Scanning Electron Microscopy (FESEM). The realistic models of (PbS)80, (PbSe)30 and (PbTe)50 were successfully established and validated as a basic building block of the fabricated thin film based on their crystal structure of the synthesized PbXs; supports the morphological observations made using FESE

Investigating TikTok as A Learning Tool for Learning Chemistry: A Study Among Secondary School Students in Malaysia

TikTok is a social media, video-based phone application, which enables creative and engaging videos that are rapidly growing in popularity. TikTok has been used to create fun, exciting, and engaging 15-60 second duration educational chemistry and science outreach videos. This study aims i.e., (i) to investigate the effectiveness of TikTok as a learning tool for learning chemistry among secondary school students in Malaysia, and (ii) to compare the effectiveness of TikTok and traditional mind-map technique in facilitating secondary school students’ learning chemistry and determine which approach is preferred by students. Non-random purposive sampling was used to select 40 participants from two different secondary schools, namely Sekolah Menengah Integrasi Sains Tahfiz, Cheras, Kuala Lumpur, and Sekolah Menengah Tinggi Sultan Salahuddin Abdul Aziz Shah, Sabak Bernam, Selangor. The instruments used were document and interview sessions. The results showed that the overall response to the survey was promising as most of the respondents gave positive feedback on the use of the TikTok application in their chemistry learning process. In this respect, the potential of TikTok to be used as a learning tool should be further explored since it enables the designing and delivering of creative and high-quality e-learning content