Novel Nanostructured Conductive Films And Their Applications:Optical And Electrical Research

There are various unique and fantastic optical and electrical properties with nano-structured conductive films,and they have been applied in many important areaes such as transparent electrodes and nano-structured back electrodes.Transparent electrodes are one of the critical components for many optoelectronic devices,while metal nano-structured back electrodes play an important role in absorption enhancement for thin film solar cells.This thesis focuses on the fabrication of large area and uniform transparent electrodes based on random silver nanowire(Ag NW)networks and the improvement of their performance by natural sunlight irradiation.A facile method is also presented to prepare two dimensional(2D)gold(Au)nanobowl arrays,whch have been applied as back electrodes to enhance the optical absorption of amorphous silicon(a-Si)film.This thesis begins with a brief introduction of various kinds of transparent electrodes and non-metal and metal light trapping nano-structrures,followed by a theoretical analysis of the optical and electrical properties of the random Ag NW networks based on percolation theory.Then,several self-built measurement systems and universal experiment methods are exhibited.For transparent electrodes,we propose a PMMA-assisted spin-coating approach to fabricate large area(4-inch glass substrate)and uniform transparent electrodes with high oprical transmission(-0.91),high electrical conductivity(Rsh<20 Ω/□)and high stability based on random Ag NW networks.The mechanism is presented through systematic investigation of the effects of two key processing parameters,i.e.,the volume ratio of the Ag NWs in ethanol to the PMMA,and the second rotary speed of the spin-coating process.This method has been extended to flexible substrates successfully and good uniformity is also achieved over a large area of 5 cm x 5 cm.Aferwards,to improve the sheet resistance that is greatly limited by large wire-to-wire contact resistances,we propose a simple sunlight illumination approach to remarkably improve electrical conductivity of the Ag NW network without any significant degradation of the light transmittance.Because the power density is extremely low(0.1 W/cm2,1-Sun),only slight welding between Ag NWs has been observed.Despite this,a sheet resistance of<20 Ω/sq and transmittance of～87%at wavelength of 550 nm as well as excellent mechanical flexibility have still been achieved for Ag NW networks after sunlight illumination for 1 hour or longer,which are significant upgrades over those of ITO.Slight plasmonic welding together with the associated self-limiting effect has been investigated by numerical simulations and further verified experimentally through varied solar concentrations.Due to the reduced resistance,high-performance transparent film heaters as well as efficient defrosters have been demonstrated,which are superior to the previously-reported Ag NW based film heaters.In terms of nano-structured back electrodes,based on self-assembled polystyrene(PS)sphere arrays,we have developed a facile PMMA-assisted transfer approach to fabricate uniform 2D hexagonal Au nanobowl arrays,which show inferior light trapping ability to their inverted counterparts(Au nanospherical shell arrays)mainly due to the higher reflection induced lower absorption in the long wavelength range.Surprisingly,after being coated with a 60-nm thick a-Si film,an anomalous light trapping enhancement was observed for the Au nanobowl array with greatly enhanced average absorption(0.82),while for the Au nanospherical shells,the light trapping was significantly reduced with avergage absorption of only 0.66.Systematical theoretical and experimetnal results show that the top a-Si coating play an important role in mediating the excitation of SPPs and the electric field distributions in both nanostructures,leading to such opposite light trapping behaviors.Based on the gold nanobowl array,a distinctive a-Si:H solar cell is proposed.An accurate three-dimensional opto-electrical hybrid model is built and simulations are performed to systematically investigate the light trapping behavior as well as the photocarrier generation,transport and recombination behaviors.The numerical results show that our Au nanobowl based solar cell has superior light trapping property over the solar cell based on the inverted nanostructure and the planar counterpart.Despite the greater recombination rate,a much higher short-circuit current density of 14.62 mA/cm2 is still achieved.Its power conversion efficiency up to 13.83%is still 25.73%higher than that of the planar counterpart and also 6.71%higher than that of the Au nanospherical shell based solar cell.Finally,we conclude the thesis and give perspective about transparent electrodes based on random Ag NW networks and nano-structured back electrodes,especially in the terms of where further study may be rewarding.