PbS/CdS Core/Shell Nanocrystals For Solution-Processed Colloidal Quantum Dot Solar Cells
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PbS/CdS Core/Shell Nanocrystals For Solution-Processed Colloidal Quantum Dot Solar Cells Darren C. J. Neo1, Cheng Cheng1, Hazel E. Assender1, Andrew A. R. Watt1 1 Department of Materials, University of Oxford, 16 Parks Road, Oxford OX1 3PH, United Kingdom. ABSTRACT An epitaxial shell of cadmium sulphide is grown on lead sulphide quantum dots in order to reduce the concentration of surface defects. Thin solid films of these core/shell materials are found to have low carrier concentrations due to effective surface passivation which reduces the number of dangling bonds. In this paper PbS/CdS is used as a quasi-intrinsic layer in p-i-n photovoltaic devices where PbS acts as the p-layer and ZnO the n-layer. By studying different permutations of these layers and the degree of PbS p-type doping by annealing we optimise fill factor and open-circuit voltage. INTRODUCTION At present, lead sulphide (PbS) is the material choice for high efficiency colloidal quantum dot (CQD) solar cells due to its band gap tunability, ease of production and low cost fabrication techniques1,2. Through better understanding of device physics3, improved synthetic chemistry and surface engineering of quantum dots4,5, device efficiencies have seen a steady rise. As of December 2013, the record reported efficiency is 8.6%6. For further improvement, the problem of a high defect concentration7, which increases the probability for recombination losses, has to be addressed. Unpassivated surface states or dangling bonds serve as traps sites or, worse, recombination centres for photogenerated carriers. These shorten carrier lifetimes, limit charge transport efficacy and modify the degree of Fermi-level splitting2. To push power conversion efficiencies even higher, there is a need to passivate the surface while preserving quantum confinement and solution processability. The main source of post-synthesis defects arises from the solid state ligand exchange process, where a substitution reaction swaps the bulky organic ligand used in the synthesis for a shorter one 4. This technique makes the highest efficiency solar cells by far6. However, solid state ligand exchange is not perfect: not every surface is passivated and the incoming ligand may be highly labile, making the surface chemically unstable8,9. These problems need a strategy which removes surface defects but does not deteriorate the transport properties of the quantum dots. To this end, we adopted the approach of introducing a cadmium sulphide (CdS) shell around PbS CQDs and showed that the core/shell CQDs outperform PbS-only CQD based devices if the shell thickness is optimized and further halide surface functionalization is applied10. Device performance in our previous studies was limited by the thickness of the absorber layer as the core/shell CQDs have poor carrier properties. In this paper PbS/CdS is used as a quasiintrinsic layer in p-i-n photovoltaic devices where PbS acts as the p-layer and ZnO the n-layer. By studying different permutations of these layers and the degree of PbS p-type doping by
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