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Title: Crystalline-Silicon/Organic Heterojunctions for Solar Photovoltaics
Authors: Avasthi, Sushobhan
Advisors: Sturm, James C
Contributors: Electrical Engineering Department
Keywords: Heterojunction
Solar Cellls
Subjects: Engineering
Issue Date: 2011
Publisher: Princeton, NJ : Princeton University
Abstract: Solar cells based on crystalline silicon offer high efficiency but they are expensive due to the high temperatures required in their fabrication. The alternative approach using low-temperature processable organic-semiconductors is potentially cheaper, but the organic solar cells are not very efficient. In this thesis we explore if organic semiconductors can be integrated with silicon to form hybrid organic/silicon solar cells that are both efficient and low-cost. Specifically, we demonstrate that a) organic molecules can be used to reduce carrier recombination at the silicon (100) surface and b) a solution-processed organic/silicon heterojunction can replace the conventional silicon p-n junction to yield solar cells with high power conversion efficiencies (> 10 %). With decreasing wafer thicknesses and improving bulk lifetimes of silicon solar cells, losses due to carrier recombination at the silicon surface are becoming increasingly important. At a bare silicon surface, some of the silicon valencies remain unsatisfied. These ``dangling-bonds'' cause midgap states at the silicon surface where photogenerated carriers can recombine, resulting in lower performance. Typically, a layer of silicon oxide/nitride is deposited on the silicon, at high-temperatures (> 350 &deg; C), to passivate the dangling-bonds and reduce surface recombination. Organic semiconductors can be deposited at much lower temperatures, but in general organic materials do not react with the silicon dangling-bonds and the surface remains unpassivated. In this work, we demonstrate that the organic molecule 9,10 phenanthrenequinone (PQ) reacts with and satisfies the silicon dangling bonds, leading to a relatively defect-free silicon surface with a very low surface recombination velocity (~150 cm/s). Electrical measurements of the metal/insulator/silicon devices show that the Fermi-level at the PQ-passivated silicon surface can be modulated and an inversion layer can be induced in silicon. High electron mobility of 600 cm<super>2</super>/Vs is measured at the Si/PQ interface further proving the electronic quality of the PQ-passivated surfaces. To generate a photovoltage in a solar cell, the photogenerated carriers need to be spatially separated at two electrodes of opposite polarity. In solar cells this is typically accomplished using a p-n junction. While the p-n junction technology is well understood, the fabrication of p-n junctions on silicon is an expensive process because it requires ultra-clean furnaces, pure precursors and high temperatures. In this thesis we successfully replace the silicon p-n junction with an silicon/poly(3-hexylthiophene) heterojunction that can be manufactured at low temperatures (< 150 &degC) with a simple spin-coating process. The key design rules to achieve a high quantum-efficiency and high open-circuit voltage are discussed and experimentally demonstrated. Finally we highlight the importance of reducing minority-carrier currents in these heterojunction devices, which gives a pathway for further improving the efficiency of heterojunction solar cells. Using the prescribed design rules and optimizing device structure, a silicon/organic heterojunction solar cell with an open-circuit voltage of 0.59 V and power conversion efficiency of 10.1 % is demonstrated.
Alternate format: The Mudd Manuscript Library retains one bound copy of each dissertation. Search for these copies in the library's main catalog
Type of Material: Academic dissertations (Ph.D.)
Language: en
Appears in Collections:Electrical Engineering

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