title: Narrow Band Gap Lead Sulfide Hole Transport Layers for Quantum Dot Photovoltaics.
abstract: The band structure of colloidal quantum dot (CQD) bilayer heterojunction solar cells is optimized using a combination of ligand modification and QD band gap control. Solar cells with power conversion efficiencies of up to 9.33 ± 0.50% are demonstrated by aligning the absorber and hole transport layers (HTL). Key to achieving high efficiencies is optimizing the relative position of both the valence band and Fermi energy at the CQD bilayer interface. By comparing different band gap CQDs with different ligands, we find that a smaller band gap CQD HTL in combination with a more p-type-inducing CQD ligand is found to enhance hole extraction and hence device performance. We postulate that the efficiency improvements observed are largely due to the synergistic effects of narrower band gap QDs, causing an upshift of valence band position due to 1,2-ethanedithiol (EDT) ligands and a lowering of the Fermi level due to oxidation.
authors.affiliation: Department of Materials, University of Oxford , 16 Parks Road, Oxford OX1 3PH, United Kingdom.
authors.forename: Nanlin
authors.initials: N
authors.lastname: Zhang
authors.affiliation: Department of Materials, University of Oxford , 16 Parks Road, Oxford OX1 3PH, United Kingdom.
authors.forename: Darren C J
authors.initials: DC
authors.lastname: Neo
authors.affiliation: Department of Materials, University of Oxford , 16 Parks Road, Oxford OX1 3PH, United Kingdom.
authors.forename: Yujiro
authors.initials: Y
authors.lastname: Tazawa
authors.affiliation: Department of Chemistry, Physical & Theoretical Chemistry Laboratory, University of Oxford , Oxford OX1 3QZ, United Kingdom.
authors.forename: Xiuting
authors.initials: X
authors.lastname: Li
authors.affiliation: Department of Materials, University of Oxford , 16 Parks Road, Oxford OX1 3PH, United Kingdom.
authors.forename: Hazel E
authors.initials: HE
authors.lastname: Assender
authors.affiliation: Department of Chemistry, Physical & Theoretical Chemistry Laboratory, University of Oxford , Oxford OX1 3QZ, United Kingdom.
authors.forename: Richard G
authors.initials: RG
authors.lastname: Compton
authors.affiliation: Department of Materials, University of Oxford , 16 Parks Road, Oxford OX1 3PH, United Kingdom.
authors.forename: Andrew A R
authors.initials: AA
authors.lastname: Watt
keywords: band alignment optimization; band gap control; quantum dot; solar cell; surface treatment