Cu2O/TiO2 Nanorod Heterojunctions: Synthesis, Characterization and Applications as Solar Cells on the Nanoscale
Sammanfattning: Novel solar cells are being synthesized from sustainable, non-toxic, and economic materials. All metal oxide semiconductors are one such class of these materials. Synthesis of different combinations of p and n type MO semiconductors heterojunctions as well as high throughput characterization is crucial to improve their applications in fields such as solar cells. A Cu2O/TiO2 Nanorods heterojunction is synthesized on a fluorine doped tin oxide substrate. The TiO2 Nanorods are synthesized via a two-step, solvothermal method. The Cu2O is deposited conformally on the TiO2 NRs via a physical vapor deposition method known as RF magnetron sputtering, with thicknesses of 100, 50 and 25 nm. Characterization methods are used to first determine that the correct materials were synthesized and deposited. Scanning electron microscopy demonstrated that nanorods were made of length, 750 nm, and width, 45 nm. Optical measurements were taken, including: absorbance, transmittance, and reflectance; trends followed the optical data as the thickness of the p-type material increased. From the absorbance data, the bandgap of the materials could be calculated from the generated Tauc plot. The bandgap of TiO2 was calculated to be 3.0 eV which agreed with literature values. The bandgap of Cu2O was calculated to be 2.76 eV which is not in agreement with literature values. X-ray diffraction demonstrated that TiO2 rutile phase was grown, with diffraction angles at: 36.45, 62.747 and 69.766 with their lattice planes being (011), (002) and (112) respectively. Raman spectroscopy demonstrated TiO2 in the rutile phase with Raman shifts at both 447 cm-1 (Eg) and 609 cm-1(A1g). There is a minor peak at 522 cm-1 (T2g) which correlates to Cu2O. Macro-electrical measurements were taken to plot a current vs voltage curve (IV curve), under dark and light conditions. From the macro-electrical measurements Isc, Voc and η (photoconversion efficiency) were calculated: 2.38 E-09 A, -0.18 V, 7.25E-07 respectively (under light, 1 sun equivalent). Atomic force microscopy (AFM) was used to attain topographical images, force/deflection curves, IV curves/maps, and surface potential maps. Conductive-AFM (c-AFM) and Kelvin Probe Force Microscopy (KPFM) were the specific AFM techniques used. From the KPFM measurements it was possible to measure the work functions of TiO2 NRs and Cu2O/TiO2 NRs by using highly oriented pyrolyzed graphite as a reference. The work function for TiO2 NRs was: 4.24 eV and 4.14 eV under dark and light conditions respectively. The work function of the 100 nm Cu2O/TiO2 NRs heterojunction was 4.44 eV and 4.35 eV under dark and light conditions respectively. The apparent work functions that were calculated via this KPFM method were not in agreement with literature values of the respective materials. This thesis has proved that Cu2O/TiO2 Nanorod Heterojunctions can be synthesized using previously known solvothermal synthetic methods. Furthermore these Cu2O/TiO2 Nanorod Heterojunctions have an increase in current under illumination. This current response under illumination has been studied on the nanoscale, using KPFM and C-AFM, as well as on the macroscale. Further investigations on the nanoscale are to be done, which can shed light on how and why these all metal oxide nanorod heterojunctions are functioning as solar cells.
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