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Investigation the Effect of Formamidinium Lead Halide on the Stability and Performance of Graphene Phototransistors Based on Lead Sulfide

Aynehband, Samaneh | 2021

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 54477 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Simchi, Abdolreza
  7. Abstract:
  8. Enhancing the detection range, sensitivity and stability of phototdetectors due to their vast applications are in great importance. In recent years, Lead sulfide quantum dots, in order to their absorption over a wide range of wavelengths including Near-infrared (NIR), multiple exciton generation effect and solution processability are promising materials.However, long organic surface ligands which are necessary in synthesis process can suppress the carrie transport and the common method to overcome this weakness, is ligand exchange process. In this way, using short organic ligands because of their low electron mobility and methylammonium lead iodide ligands in order to their instability, could not play the effective role.In this project, the carrier transport of 4.5 nm PbS QDs synthesized by hot injection method was improved by replacing the long organic surface ligands with FAPbI3 ligands. After ligand exchange, the oxygen atomic percent extracted from XPS reached from 5.93 to 0.6% and the superlattice spacing was changed from 5.6 to 4.8 nm. On the other hand, the photodetectors fabricated with PbS QDs with MAPbI3 and FAPbI3 ligands implied the 10% higher thermal stability of PbS-FAPbI3. TEM results proved that achieving a high-quality 100nm-thick film of QDs is achievable via solid-state ligand exchange. Using this method, the specific detectivity was improved up to 34% and reached 6.3×107 Jones and the ON/OFF ratio was enhanced from 26.5 for Liquid ligand echanged-sample to 34 for solid-ligand exchanged sample.Furthermore, the existance of SOx2- compounds on the QDs surface after ligand exchange, which enhanced the hole transport by trapping the electrons, was confirmed by XPS. Halide of the perovskite ligand is another influential item which can affect the self-assembly, ordering and stability of QDs and also the performance of optoelectronic devices. In present project the I- ions of the ligand were replaced with Br- ions to improve the stability. Based on the TEM results PbS-FAPbBr3 QDs create the cube assembly on graphene substrate with less defects and better quality than PbS-FAPbI3 QDs. Considering these properties, phototransistors fabricated with QDs and FAPbBr3 ligands are stable about 60 days in ambient condition and their specific detectivity was 1010 Jones which is about 60% higher than the phototransistors with QDs-FAPbI3. Finally, EIS results claimed the 0.219 Ω interfacial resistance for PbS-FAPbI3 which is 21% higher than PbS-FAPbBr3
  9. Keywords:
  10. Lead Sulfide ; Stability ; Self Assembly ; Photodetector ; Carrier Transport ; Perovskite Halide ; Formamidinium Lead Iodide

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