Controllable growth and photovoltaic applications of group II-VI semiconductor nanomaterials

Abstract

Group II-VI semiconductors with ordered nanostructure arrays or higher level nanoarchitectures are expected to not only have enhanced applications in electromagnetic, optical, and green energy fields; but also to enable synergetic multifunctionalities as part of an integrated system or device. For example, tunable band gap alloyed ternary nanostructures, covering the 340-710 nm range, are promising candidates for nano-optoelectronic devices. Likewise, semiconductor nanocrystal quantum dots (QDs) are considered promising photosensitizers for TiO2 and ZnO-based quantum dot sensitized solar cells (QDSCs) due to their intrinsic attractive properties including: (i) tunable bandgap both by the choice of material and by size, thus offering the possibility to match the solar spectrum; (ii) potential to align energy levels both in respect to the conduction level of the electron-conducting nanostructure and to the redox potential of the electrolyte; and (iii) high extinction coefficients. The first section reviewed our work toward the development of efficient roomtemperature optically pumped nanolasers covering the NIR-UV spectral region. Nanostructures based on ternary II-VI composition alloys, ZnYCd1YS and CdS1XSeX, can provide efficient room-temperature lasing at any predetermined wavelength between 710 and 340 nm with tuning resolution, which could be realized via composition modulation in X and Y, demonstrating the possibility of continuous tuning in the lasing wavelength throughout the complete 340-710 nm spectral range. In the second part, we have investigated the influence from preparation temperature, size and thickness of catalyst film, on the morphology and density of CdS1−XSeX nanostructures. Five different metals were selected as potential catalyst to facilitate nanoribbons growth and spatial composition gradient. Then, we select Au and Au/Ag catalyst as potential candidates to achieve CdS1−XSeX nanoribbons composition gradient over a several millimeter range on a single substrate. The PL results show that peak positions from the band gap emission of CdS1−XSeX have about 15 nm shifts between Au and Au/Ag region. We demonstrate that it is possible to control the spatial composition gradient with sub-mm resolution thus opening the possibility for the fabrication of devices with full spectral optimization. In the third part, large-scale vertically aligned ZnO nanorod arrays (NRAs) have been prepared on FTO glass through a hydrothermal growth approach. The length, diameter and density of ZnO nanorods can be controlled by adjusting the growth time, the number of seeding layers, and addition of the organic solution. CdS nanoparticles/ZnO NRAs heterostructures have been fabricated by the relatively simple vapor-solid (VS) method, using Au catalyst-induced growth of CdS nanowire (NW) branches on ZnO NRAs surfaces through the vapor-liquid-solid (VLS) approach. PL and CL properties of two different CdS/ZnO NRAs heterostructure demonstrated the luminescence quenching and blue shifts due to type II band alignment between CdS and ZnO. Photovoltaic devices based on different kinds of CdS/ZnO NRAs heterostructures were fabricated using iodide electrolyte and Pt counter electrode. Open circuit voltage (VOC) ~0.77 V was obtained for heterostructures based on CdS nanoparticles (NPs) and short ZnO nanorods with 4 μm length. Power conversion efficiencies of 1.15% were achieved using ZnO NRAs/Al2O3/CdS NWs electrode with a somehow lower VOC value ~0.65 V. In the fourth part, vertically aligned single crystalline ZnO nanorod arrays, with ~3 μm in length and 50-450 nm in diameter are grown by a simple solution approach on a Zn foil substrate. CdS and CdSe colloidal quantum dots are assembled onto ZnO nanorod arrays using water-soluble nanocrystals capped as-synthesized with a shortchain bifuncional linker thioglycolic acid. The solar cells co-sensitized with both CdS and CdSe quantum dots demonstrate superior efficiency compared with the cells using only one type of quantum dots. A thin Al2O3 layer deposited prior to quantum dot anchoring successfully acts as a barrier inhibiting electron recombination at the Zn/ZnO/electrolyte interface, resulting in power conversion efficiency of approximately 1% with an improved fill factor of 0.55. Furthermore, in situ growth of ZnO nanorod arrays in a solution containing CdSe quantum dots provides better contact between two materials resulting in enhanced open circuit voltage. The fifth section focus on the effects of ZnO NRAs density on the photovoltaic conversion efficiency (PCE) of QDSCs sensitized with CdS and CdSe QDs. By tuning the nucleation density of ZnO NRAs, from ~60 NRs /μm2 to ~8 NRs /μm2, with the use of a Ti film nucleation barrier, it is demonstrated that the performance in these QDSCs is an extremely sensitive function of the NR density and can be of great interest in nano array-based solar cells. Optimized values yield CdSe QDSCs with open circuit voltage (VOC) as large as ~0.81 V and high fill factor (FF) ~0.68. These findings where then used to guide the fabrication of CdS, CdSe co-sensitized QDSCs with high power conversion efficiency (PCE) ~4.22% even for relatively short NRAs with 5 μm length, thus highlighting the relevance of NRAs density as an important parameter in NRAbased QDSCs. Finally, the future work was described based on the present experiments and findings. It is advanced that solar cell conversion efficiency in could be improved by increasing the length of ZnO nanorods thus resulting in larger effective QD absortion and enhanced photocurrents, or tuning the light absorption in QD to match the solar spectrum.

Date of Award	3 Oct 2012
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Juan Antonio ZAPIEN (Supervisor)