Influence Of Semiconductor Nanomaterials On The Performance Of New Type Thin Film Solar Cells

The third-generation inorganic-organic thin film solar cells have emerged as one of the most promising solar cells due to their simple fabrication process, environmental-friendly materials, and low-cost technique. Presently, energy conversion efficiencies of thin film solar cells have reached up to10-12%through numerous efforts from worldwide scientists. For purpose of industrialization, however, we still need to enhance their photovoltaic performance. In view of this, the research focus will be concentrated on improving the photovoltaic performance of inorganic-organic thin film solar cells. This thesis aims at developing highly efficient metal oxide semiconductor nanomaterials as photoanodes, non-toxic and low cost semiconductors as inorganic sensitizers, and finally novel photoelectric devices are also involved. We prepared a series of highly efficient nanocrystalline metal oxide semiconductor materials, including TiO2, N-doped TiO2, multiwall carbon nanotubes (MWCNTs) nanocomposites, SnO2, and ZnSnO4as photoanodes for dye-sensitized solar cells (DSCs). Via a simple chemical bath deposition (CBD) approach, cost-effective and non-toxic inorganic sensitizer SnS was synthesized and applied into the systems of Quantum-dot sensitized solar cells (QDSCs) and all-solid state inorganic-organic heterojunction solar cells (HSCs). The structure and performance of the photoelectric devices were further designed and optimized.We investigated the morphology control and optimization of T1O2and N-doped TiO2nanomaterials, as well as their performance in the photoanodes of DSCs. We prepared a series of nanocrystalline nanoparticles, hierarchically mesoporous microspheres, and nanotube arrays of TiO2nanomaterials. The highly efficient DSCs were fabricated using these TiO2nanomaterials as photoanodes, finally giving efficiency over10%. We prepared a series of N-doped TiO2using dry and wet methods, which improved the efficiency of DSCs by10%. We investigated the effect of starting materials, nitrogen dopant type and amount on the performance of N-doped TiO2photoanodes. The results showed that the starting materials and nitrogen dopant types affectted the process of nitridation, specific surface area, nitrogen dopant amount, and bandgap of N-doped TiO2. The results from IMVS/IMPS (intensity-modulated photovoltage and photocurrent spectroscopy) and EIS (electrochemical impedance spectroscopy) showed that, in N-doped DSCs, the synergic effect of short electron lifetime, fast electron transport, and high dye uptake contributed to an improved photovoltaic performance. Finally, a sol-gel method was employed to prepare TiO2/MWCNTs and TiO2-xNx/MWCNTs nanocomposite photoanodes, in which MWCNTs were expected to serve as highways to guide electron transfer. We systematically studied the photovoltaic performance of DSCs based on MWCNTs nanocomposite photoanodes. The results suggested that MWCNTs facilitate the charge separation and transport, the electron collection efficiency of DSCs was thus improved by20%.We studied the preparation of SnO2and Zn2SnO4nanomaterials with higher electron mobilities than traditional TiO2. Through solvothermal methods, SnO2hollow sphere and sea urchin-like strucutures, Zn2SnO4hierarchical structures composed of cubes or crossed nanosheets were prepared, respectively. The results showed that the specific surface areas of the two SnO2structures are much higher than those of traditional nanoparticles. Also, the exposed crystal facets in the two SnO2structures were different. For the SnO2hollow spheres and seaurchins,they exhibited excellent light-scattering effects as the photoanodes in DSCs and gave efficiencies of3.93%and3.42%, respectively. On the other hand, when using the cube and nanosheet-based Zn2Sn04hierarchical structures in DSCs, conversion efficiencies of5.72%and4.43%were yielded, respectively. The differences of electron transport kinetics between Zn2SnO4and TiO2-based DSCs were studied using stepped light-induced transient measurement of photocurrent and voltage(SLIM-PCV). The Zn2SnCVbased DSCs showed a larger electron diffusion coefficient and more dependence on light intensity than those of TiO2-based DSCs. The Zn2SnO4photoanodes could be further optimized using TiO2thin layer for surface passivation, thus increasing the electron lifetime even electron diffusion coefficient was decreased. By this strategy, both Voc and Jsc of DSCs were improved.We developed new materials and found out new paths for improving the photovoltaic performance of cost-effective QDSCs. A simple modified chemical bath deposition (CBD) method was employed to synthesis low-cost, environmental-friendly, and element abundant inorganic semiconductor SnS as sensitizer. Besides, we introduced transition metal carbides TiC as a novel catalyst for counter electrodes in QDSCs. The crystal structure and optical properties of SnS were characterized by XRD and UV-Vis analysis. The results sugguested that SnS presented a herzenbergite phase. The visible light absportion of SnS ranged from400to550nm, leading to an improved light absorption of HSCs devices. A series of QDSCs were fabricated using SnS-sensitized photoelectrodes, two kinds of counter electrodes (Pt, TiC), and three redox mediators(S2-SX2-,I-I3-, T2/T-). The catalytic selectivity of TiC and Pt towards T?/T-and I-/I3-was investigated using Tafel-polarization and EIS analysis. The results showed that Pt and TiC showed a similar catalytic selectivity towardI-/I3-. However, TiC possesses a better catalytic activity for T2/T-than I-/I3-which is due to a smaller electron transfer resistance. The role of redox couples in the regeneration of sensitizers was also studied using charge extraction measurement, which revealed that T2/T-is more capable than the other two redox couples for SnS regeneration. The electron density of TT-QDSCs is an order of magnitude larger than TS-QDSCs. These results indicate the great potential of transition metal carbide catalysts and organic redox couples for photovoltaic improvements of QDSCs.All-solid-state inorganic-organic HSCs were designed and fabricated using SnS as inorganic sensitizers to enhance the visible light absorption. The HSCs devices were fabricated under ambient conditions using a solution-processable, simple, and low-cost technique. Mesoporous spherical TiO2films were introduced to facilitate polymer infiltration and interface contact. SnS-HSCs showed a promising photovoltaic performance with an energy conversion efficiency of2.8%and a significantly high Voc of0.85V. We examined the effect of different nanostructured TiO2films on the polymer penetration and SnS deposition amount. Futhermore, we also studied the influence of SnS amount, electron acceptor, and surface modification on the photovoltaic performance of HSCs. The flatband of TiO2/SnS films were measured using Mott-Schottky curves to find out the effect of SnS on the flatband of bare TiO2and HSCs.