title: A donor-supply electrode (DSE) for colloidal quantum dot photovoltaics.
abstract: The highest-performing colloidal quantum dot (CQD) photovoltaics (PV) reported to date have relied on high-temperature (>500°C) annealing of electron-accepting TiO2. Room-temperature processing reduces energy payback time and manufacturing cost, enables flexible substrates, and permits tandem solar cells that integrate a small-bandgap back cell atop a low-thermal-budget larger-bandgap front cell. Here we report an electrode strategy that enables a depleted-heterojunction CQD PV device to be fabricated entirely at room temperature. We find that simply replacing the high-temperature-processed TiO2 with a sputtered version of the same material leads to poor performance due to the low mobility of the sputtered oxide. We develop instead a two-layer donor-supply electrode (DSE) in which a highly doped, shallow work function layer supplies a high density of free electrons to an ultrathin TiO2 layer via charge-transfer doping. Using the DSE we build all-room-temperature-processed small-bandgap (1 eV) colloidal quantum dot solar cells having 4% solar power conversion efficiency and high fill factor. These 1 eV bandgap cells are suitable for use as the back junction in tandem solar cells. The DSE concept, combined with control over TiO2 stoichiometry in sputtering, provides a much-needed tunable electrode to pair with quantum-size-effect CQD films.
authors.affiliation: Department of Electrical and Computer Engineering, University of Toronto, 10 King's College Road, Toronto, Ontario M5S 3G4, Canada.
authors.forename: Ghada I
authors.initials: GI
authors.lastname: Koleilat
authors.forename: Xihua
authors.initials: X
authors.lastname: Wang
authors.forename: Andre J
authors.initials: AJ
authors.lastname: Labelle
authors.forename: Alexander H
authors.initials: AH
authors.lastname: Ip
authors.forename: Graham H
authors.initials: GH
authors.lastname: Carey
authors.forename: Armin
authors.initials: A
authors.lastname: Fischer
authors.forename: Larissa
authors.initials: L
authors.lastname: Levina
authors.forename: Lukasz
authors.initials: L
authors.lastname: Brzozowski
authors.forename: Edward H
authors.initials: EH
authors.lastname: Sargent