title: Depleted-heterojunction colloidal quantum dot solar cells.
abstract: Colloidal quantum dot (CQD) photovoltaics combine low-cost solution processability with quantum size-effect tunability to match absorption with the solar spectrum. Rapid recent advances in CQD photovoltaics have led to impressive 3.6% AM1.5 solar power conversion efficiencies. Two distinct device architectures and operating mechanisms have been advanced. The first-the Schottky device-was optimized and explained in terms of a depletion region driving electron-hole pair separation on the semiconductor side of a junction between an opaque low-work-function metal and a p-type CQD film. The second-the excitonic device-employed a CQD layer atop a transparent conductive oxide (TCO) and was explained in terms of diffusive exciton transport via energy transfer followed by exciton separation at the type-II heterointerface between the CQD film and the TCO. Here we fabricate CQD photovoltaic devices on TCOs and show that our devices rely on the establishment of a depletion region for field-driven charge transport and separation, and that they also exploit the large bandgap of the TCO to improve rectification and block undesired hole extraction. The resultant depleted-heterojunction solar cells provide a 5.1% AM1.5 power conversion efficiency. The devices employ infrared-bandgap size-effect-tuned PbS CQDs, enabling broadband harvesting of the solar spectrum. We report the highest open-circuit voltages observed in solid-state CQD solar cells to date, as well as fill factors approaching 60%, through the combination of efficient hole blocking (heterojunction) and very small minority carrier density (depletion) in the large-bandgap moiety.
authors.affiliation: Department of Electrical and Computer Engineering, University of Toronto, 10 King's College Road, Toronto, Ontario M5S 3G4, Canada.
authors.forename: Andras G
authors.initials: AG
authors.lastname: Pattantyus-Abraham
authors.forename: Illan J
authors.initials: IJ
authors.lastname: Kramer
authors.forename: Aaron R
authors.initials: AR
authors.lastname: Barkhouse
authors.forename: Xihua
authors.initials: X
authors.lastname: Wang
authors.forename: Gerasimos
authors.initials: G
authors.lastname: Konstantatos
authors.forename: Ratan
authors.initials: R
authors.lastname: Debnath
authors.forename: Larissa
authors.initials: L
authors.lastname: Levina
authors.forename: Ines
authors.initials: I
authors.lastname: Raabe
authors.forename: Mohammad K
authors.initials: MK
authors.lastname: Nazeeruddin
authors.forename: Michael
authors.initials: M
authors.lastname: Grätzel
authors.forename: Edward H
authors.initials: EH
authors.lastname: Sargent
mesh_terms: D003102:Colloids / Q000737:chemistry; D004553:Electric Conductivity; D011211:Electric Power Supplies; D004566:Electrodes; D004735:Energy Transfer; D004867:Equipment Design; D019544:Equipment Failure Analysis; D036103:Nanotechnology / Q000295:instrumentation*; D045663:Quantum Dots; D012993:Solar Energy
chemical_list: D003102:Colloids