الفهرس 
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Abstract
Summary and Conclusions
The thesis deals with the study of the effect of quantum dots ZnO, ZnS, Mn:ZnS, G, GO, and their doping in PVA on increasing the efficiency of luminescent solar cells. It consists of three chapters:
The first chapter contains a general introduction on the subject of the thesis.
The second chapter consists of two parts:
In the first part ZnO has been synthesized by means of hydrothermal method and investigated by using XRD, SEM, and FTIR analysis. The used preparation method formed ZnO with hexagonal Wurtzite structure in nanoparticle range with size of 40 nm. It means that it did not form in quantum dot size. On the other hand, pure ZnS and 3 wt% Mn:ZnS were successfully synthesized via hydrothermal method in the range of QDs. Therefore, they have been investigated for QD LSCs. The XRD patterns indicated that the two quantum dots have a cubic crystal structure. The crystallite size for ZnS and Mn:ZnS was 4.9 and 3 nm, respectively, as calculated by the Scherrer equation. UV-Vis absorption spectra showed a blue shift for both ZnS and Mn:ZnS QDs compared with bulk ZnS due to the quantum confinement effect. The bandgap values (4.85 and 4.3 eV) of ZnS and Mn:ZnS QDs were found to be greater than the bandgap value (3.68 eV) of bulk ZnS. The intensity of fluorescence peaking around 570 nm for Mn:ZnS QDs magnifies threefold compared to the Mn2+-free ZnS QDs. At 470 nm, a broad FL peak was noticed for both QDs. The FL results are an indication of an interaction between the Mn2+ ions’ d-electron states and the ZnS lattice’s s-p states, providing a better path for the electrons-holes transfer. PLQY of ZnS and Mn:ZnS QDs were calculated to be about 0.86 and 0.85, respectively. The high PLQY may be due to surface passivation. The properties that the particles emit intensive and broad yellow luminescence indicates that the ZnS and Mn:ZnS QDs would be suitable materials for luminescent solar collector applications. At 370 nm, the IPCE (%) values are 1.2 % and 2.1 % for ZnS and Mn:ZnS, respectively, showing that the Mn2+ dopant boosts the photoconversion efficiency of ZnS QDs. Mn2+ ions increase the number of excited electrons and suppress the dark produced current, leading to a higher IPCE (%) value for Mn:ZnS QDs compared with ZnS QDs.
ZnS and Mn:ZnS QDs have been doped into PVA with 3 different concentrations, each, to be recruited for LSC applications; the LSC sheets have been studied and investigated. UV–Visible absorption spectra showed that the intensities of absorption bands increase with increasing the concentration of our QDs in the PVA matrix due to an increased number of the absorbing species. For ZnS QDs, the bandgap is 4.6 eV for 0.1 wt% ZnS/PVA, 4.5 eV for 0.5 wt% ZnS/PVA, and 3.8 eV for 1 wt% ZnS/PVA. The band gaps of 0.1 wt% Mn:ZnS/PVA, 0.5 wt% Mn:ZnS/PVA, and 1 wt% Mn:ZnS/PVA are 4.55, 4.5, and 4.4, respectively. The particle size increases by increasing the content of QDs in PVA. The PL emission of ZnS/PVA LSCs shows a broad emission band at 460 and 545 nm. The possible emission centers are related to either surface/lattice defects or native impurities. The high intense peak observed at 460 nm could be attributed to transitions from sulfur vacancy to different surface states, while the low intense peak observed at 545 nm may be associated with the deeper electron and hole trap states. Furthermore, the intensity increases for all composite samples compared to PVA. Such a raise could be because of the fact that the n-type ZnS can trap electrons and permit more holes to recombine through the interface of PVA and ZnS. Mn:ZnS QDs exhibit blue FL emission at 470 nm and orange-red emission at 570 nm. The f values increased with the increasing concentration of the QDs, with better results for Mn-doped QDs, due to the increase of luminophores in the PVA film. The photovoltaic properties (I-V curve) of a solar cell with and without our QDs was studied. The results showed that the solar cell parameters, Jsc, Voc, FF, Vmax., Imax, and improved after coupling with QDs/PVA LSCs. The percent of efficiency increase is improved from 8.4 % for 0.1 wt% ZnS/PVA to 8.5 % for 0.5 wt% ZnS/PVA to 14.28 % for 1 wt% ZnS/PVA. For Mn:ZnS/PVA LSCs, the efficiency increase percent for 0.1 wt% Mn:ZnS/PVA, 0.5 wt% Mn:ZnS/PVA, and 1 wt% Mn:ZnS/PVA are 11.07 %, 13.95 %, and 16.63 %, respectively. The Mn:ZnS/PVA LSCs have higher efficiency compared with ZnS/PVA LSCs with the same concentration due to its high fluorescence properties.
The second part of chapter two deals with the preparation, characterization, and study of the optical properties of GQDs, GOQDs, polyvinyl alcohol (PVA), GQDs/PVA, and GOQDS/PVA. It contains also the photovoltaic properties of a solar cell with and without the C-dots.
An easy and low-cost bottom-up method has been developed to prepare fluorescent GQDs and GOQDs by tuning the carbonization degree of CA. The prepared C-dots were characterized by FTIR, Raman, and XRD analysis. The UV-Vis spectrum of GQDs and GOQDs showed that both GQDs and GOQDs have a weak absorption peak at 280 nm, attributed to the 𝜋-𝜋∗ transition of the C–C conjugated aromatic domains and a strong absorption peak at 350 nm due to 𝑛-𝜋∗ transition of C=O bond. The bandgap energy of GQDs and GOQDs is 2.3 and 3.28 eV, respectively. The calculated diameter of GQDs and GOQDs is 1.8 and 1.3 nm, respectively, confirming that G and GO have been introduced onto QDs form. The PL spectra showed that the GQDs show bright blue emission at 470 nm, while GQODs exhibit stronger green emission at 570 nm. The PL of GOQDs is much stronger than that of GQDs owing to the opening of the former’s energy bandgap. The PLQYs of GQDs and GOQDs were calculated to be about 0.1 and 0.2, respectively. The GOQD exhibited the highest quantum yield and narrowest full width at half maximum. The GOQDs has the highest IPCE compared with GQDs due to its small size.
PVA doped with various amounts of GQDs and GOQDs for LSC applications has been also studied. The UV–Visible absorption spectra showed that intensities of absorption bands increase with increasing the concentration of CQDs in the PVA matrix due to increasing the number of absorbing molecules. The FWHM of all CQDs in the PVA matrix is higher than that of the solution and increase by increasing CQDs content. For GQDs, the bandgap is 2.1 eV for 2 wt% GQDs/PVA and 1.66 eV for 4 wt% of GQDs/PVA. The band gaps of 2 wt% GOQDs/PVA and 4 wt% GOQDs/PVA are 2.2 and 2.15 eV, respectively. The particle size increases by increasing the content of C-dots in PVA. Also, the particle size of GQDs/PVA composites is higher than GOQDs/PVA composites with the same content. The PL intensity of the GOQDs/PVA is higher than GQDs/PVA, due to the hydrogen bonds produced from various functional groups on the graphene layers with the hydroxyl-rich PVA chains. The fill factor of the GQDs and GOQDs in PVA was found to improve compared with the C-dots aqueous solution, which is due to the enhancement of the passivation of non-radiative defects on the C-dots surface. Moreover, the fill factor is increased with increasing C-dots concentration due to the increase of photoluminescent species in the PVA film. The photovoltaic properties of a solar cell with and without our C-dots were studied. The results showed that the solar cell parameters, Jsc, Voc, FF, Vmax, Imax, and improved after coupling with C-dots/PVA LSCs. The percent of increasing efficiency is improved from 5.1% for 2%wt GQDs/PVA to 7.9% for 4%wt GOQDs/PVA LSC and from 7.1% for 2%wt GOQDs/PVA to 9.5% for 4%wt GOQDs/PVA LSC. The GOQDs/PVA LSCs have the highest efficiency compared with GQDs/PVA LSCs with the same concentration due to its high PL properties and broad IPCE.
The third chapter contains the summary and conclusions.