Solid state PbS Quantum dots /TiO 2 Nanoparticles heterojunction solar cell
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Solid state PbS Quantum dots /TiO2 Nanoparticles heterojunction solar cell Lioz Etgar* and Michael Grätzel Laboratoire de Photonique et Interfaces, Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne (EPFL), Station 6, CH-1015, Lausanne, Switzerland. *[email protected] ABSTRACT Solid state PbS Quantum Dots (QDs)/TiO2 Nanoparticles heterojunction solar cells were produced by depositing PbS QDs on a 500nm thick Mesoscopic TiO2 films using layer-by-layer deposition. The heterojunction solar cells show photovoltaic response from the visible to the near infra-red region. Importantly, the PbS QDs act here as photosensitizers and at the same time as hole conductors. The PbS QDs/TiO2 device produces a remarkable short circuit photocurrent (Jsc) of 16.3 mA/cm2, an open circuit photovoltage (Voc) of 0.54 V and a fill factor (FF) of 0.41, corresponding to a light to electric power conversion efficiency (η) of 4.04% under 0.9 sun intensity. INTRODUCTION Dye sensitized solar cells (DSSC), which yield 11% power conversion efficiency, are low cost alternative to traditional silicon solar cells1. Upon illumination the dye absorbs photons, and goes to excited state generating electron and hole pairs. The electrons are injected into the TiO2 conduction band and diffuse to the front contact, simultaneously the holes are injected into redox couple. Replacing the dye molecule by inorganic Quantum Dots (QDs), offers several advantages such as: excellent optical properties, which can be tuned by controlling the semiconductor dimensions2, a high extinction coefficient,3 and a large intrinsic dipole moment. 4 Because of their excellent light-harvesting properties, QDs are ideal sensitizers for solar cells. Furthermore, there is evidence of multiple exciton phenomena in quantum-confined inorganic semiconductors (QDs). 5 Several of research projects attempt to integrate QDs into solar cells devices, including nanocrystal (NC)-polymer hybrid solar cells, NC-Schottky solar cells, NC-sensitized titanium dioxide (TiO2) solar cells, NC hybrid bilayer solar cells, and heterojunction device architecture of placing oxide NCs as a thin spacer layer between the QDs and the FTO.6-19 QD Schottky solar cells show power conversion efficiencies (PCEs) of 1.8-2.1% under AM1.5G illumination.20 However the open circuit voltage (Voc) in a QDs Schottky solar cells is low. For example a Voc of ~0.05 V was obtained in a PbSe QDs Schottky solar cell with an Au contact, due to the high work function of the Au.7 In order to increase the open circuit voltage of the Schottky junction, air sensitive contacts of Ca or Mg metal coated with Al were required (Reaching Voc of 0.2-0.3V).7 Further increase in QDs Schottky solar cell efficiency was reported lately reaching a Voc of 0.51V introducing Al/LiF contact.21 Recently Schottky solar cells incorporating PbSxSe1-X ternary QDs were presented, in order to improve the Voc and Jsc. These ternary Schottky solar cells yield PCEs of 3.3% under AM 1.5 illumination.8 QDs that belong to group IV
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