Crystalline-Silicon/Organic Heterojunctions for Solar Photovoltaics
Speaker: Sushobhan Avasthi
Series: Final Public Orals
Location: Engineering Quadrangle J401
Date/Time: Monday, October 10, 2011, 2:30 p.m. - 4:30 p.m.
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 tho 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 molc cules 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 increas ingly important. At a bare silicon surface, some of the silicon valencies remain unsat isfied. These "dangling-bonds" cause midgap states at the silicon surface where pho togenerated carriers can recombine, resulting in lower performance. Typically, a layer of silicon oxide/nitride is deposited on the silicon, at high-temperatures (>350 'C), to passivate the dangling-bonds and reduce surface recombination. Organic semiconduc tors 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 cm2 /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 typ ically 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-hcxylthiophene) heterojunction that can be manufactured at low temperatures ( <150 oc) with a sim ple spin-coating process. The key design rules to achieve a high quantum-efficiency and high open-circuit voltage arc discussed and experimentally demonstrated. Finally we highlight the importance of reducing minority-carrier currents in these het erojunction 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.