Nanoscale mapping of thermal and mechanical properties of bare and metal-covered self-assembled block copolymer thin films

We report on the structural, mechanical and thermal analysis of 40 nm thick polystyrene-block-poly (ethylene oxide) (PS-b-PEO) block copolymer (BCP) films coated with evaporated chromium layers of different thicknesses (1, 2 and 5 nm). Solvent annealing processes allow the structural control of the BCP films morphology by re-arranging the position of the PEO cylinders parallel to the substrate plane. High-vacuum scanning thermal microscopy and ultrasonic force microscopy measurements performed in ambient pressure revealed that coated ultrathin metal layers strongly influence the heat dissipation in the BCP films and the local surface stiffness of the individual BCP domains, respectively. The measured tip-sample effective thermal resistance decreases from 6.1×107 to 2.5×107 KW-1 with increasing Cr film thickness. In addition, scanning probe microscopy measurements allow the thermal and mechanical mapping of the two segregated polymer domains (PEO-PS) of sub-50 nm characteristic sizes, with sub-10 nm thermal spatial resolution. The results revealed the effect of the surface morphology of the BCP and the incorporation of the metal film on the nanoscale thermal properties and volume self-assembly on the mechanical properties. The findings from this study provide insight in the formation of high aspect ratio BCP-metal structures with the more established applications in lithography. In addition, knowledge on the thermal and mechanical properties at the nanoscale is required in emergent applications, where BCPs, or polymers in general, are part of the structure or device. The performance of such devices is commonly related to the requirement of increased heat dissipation while maintaining mechanical flexibility

Electrical and Optical Properties of Upgraded Metallurgical Grade Silicon Solar Cells.

Silicon(Si) accounts for more than ~ 90 % of solar cell market due to its advantages of earth abundance, good reliability, performance, and a wealth of Si materials processing knowledge. However, as the photovoltaic industry matures, there have been more demands on lowering the cost of solar cells, which is mainly dominated by the cost of starting materials. Currently two major approaches are pursued to reduce the cost of Si- based solar cells per watt: the adoption of low-cost silicon such as metallurgical-grade (MG) Si or upgraded metallurgical-grade (UMG) Si, and reducing the usage of Si by producing ultrathin solar modules. UMG-Si is generally obtained by special heat treatment of MG- Si and is a much cost–efficient material compared to the solar-grade Si. However, UMG-Si contains high level of various metal impurities and defects which leads to diminished diffusion length and poor performance. Therefore, in order to achieve efficient photo-generated charge collection from a p-n junction made from low quality Si, the thickness of the solar cell should be within the diffusion length, particularly less than ~ 20 µm for the application of UMG-Si. Si thickness in this range does not allow sufficient light absorption and thus, designing of the structure of ultrathin solar cells to have optically thick active layer, so that the light absorbance can be improved, becomes very important. Strategies to enhance optical absorbance in the solar cells include dielectric-anti reflection coating, surface texturing and exploitation of surface plasmon resonance. Among them, the surface plasmon resonance, which is the collective oscillation of conduction electrons stimulated by incident light at the interface between a metallic (Ag, Au, Pt) nanostructure and a dielectric, has been an emerging method for achieving the light trapping in ultrathin Si solar cells. This thesis presents ultrathin Si solar cells generated from UMG-Si wafers incorporating combinations of nanostructures that enable use of surface plasmon resonance, light scattering feature, and anti-reflection layers. Detailed studies of electrical and optical properties of the resulting solar cells provide useful design considerations for future MG-Si based and any classes of solar cell systems.PHDMaterials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/99886/1/jykwon_1.pd