Research ArticleMATERIALS SCIENCE

Confining metal-halide perovskites in nanoporous thin films

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Science Advances  04 Aug 2017:
Vol. 3, no. 8, e1700738
DOI: 10.1126/sciadv.1700738
  • Fig. 1 Metal-halide perovskites confined in nanoporous films.

    (A) Schematic of nanoscale solid film templates (npAAO on the left and npSi on the right) infused with perovskite nanocrystals. The chemical structure of methylammonium lead trihalide (ncMAPbX3) and cesium lead trihalide perovskites (ncCsPbX3), with chloride, bromide, and iodide (X = Cl, Br, I), is shown in the middle. (B) BF-STEM image of a 170-nm-high, ~100-nm-thick npAAO filament, indicating alumina nanopores of 6 to 8 nm in diameter, partially filled with conjoined perovskite nanocrystals. (C) Exfoliated flake of npSi filled with MAPbI3, imaged by BF-STEM. Perovskite nanocrystals appear dark because of their diffraction and mass contrast. Nanocrystals on the right appear aggregated; this is an imaging artefact caused by an increase in flake thickness. (D) PL of ncMAPbI3 grown in npAAO (solid red line), blue-shifted by 62 nm. ncMAPbBr3 (solid green line) and ncCsPbBr3 (solid cyan line) are shifted by 14 and 16 nm, respectively. Dashed lines show bulk film PL. (E) PL of perovskite-infiltrated npSi. These smaller pores result in a 150-nm shift for ncMAPbI3 (solid red line), 52 nm for ncMAPbBr3 (solid cyan line), 51 nm for ncCsPbBr3 (solid blue line), and 14 nm for ncMAPbCl3 (solid purple line). Dashed lines indicate bulk film PL. (F and G) Photographs of square centimeter–scaled thin films of nanocrystalline perovskites under UV illumination: npAAO on glass slides (F) and npSi on Si wafers (G). The circular areas in (G) are nanoporous.

  • Fig. 2 PL tuning and stability of perovskites confined in nanoporous films.

    The dashed lines represent the bulk PL, whereas the dotted lines are the PL of perovskites in npAAO. Solid lines correspond to the PL of perovskites grown in npSi, with successive blue-shifted peaks originating from samples with progressively smaller pore sizes. (A) MAPbI3 bulk versus crystals confined in npAAO and in eight differently sized npSi scaffolds. A clear transition from NIR emission to visible red is observed. (B) MAPbBr3 bulk versus crystals confined in npAAO and six differently sized npSi scaffolds, showing emission shifting from green to blue. (C) CsPbBr3 bulk versus confined nanocrystals in the same set of npAAO and npSi. (D) The UV-emitting MAPbCl3 bulk compared with confined crystals in two of the smallest npSi matrices. Even this already wide-bandgap material can be blue-shifted via spatial confinement to emit below 400 nm. (E) Time evolution of the PL intensity during illumination at 405 nm under ambient conditions for ncMAPbBr3 in npAAO (cyan empty circles) compared to bulk planar films of MAPbBr3 on glass (green empty squares), evidencing encapsulation-like effects of the npAAO thin film. Sealing ncMAPbBr3 samples with epoxy and glass slides (dark blue empty triangles) results in an essentially constant PL signal, demonstrating stability under continuous illumination without light-induced degradation. (F) PL signal for ncMAPbBr3 in npAAO after 1-min (dark green), 30-min (green), and 1-day (light green) illumination at 405 nm. (G) Time evolution of the PL intensity during illumination at 405 nm under ambient conditions for ncCsPbBr3 in npAAO (blue empty circles) and bulk CsPbBr3 on glass (cyan empty squares). Epoxy- and glass-encapsulated ncCsPbBr3 samples (dark blue empty triangles) again show stable PL and no photodegradation. (H) PL signal for ncCsPbBr3 in npAAO after 1-min (light blue), 30-min (blue), and 1-day (dark blue) illumination at 405 nm.

  • Fig. 3 Depth-resolved structural characterization of perovskite nanocrystals in npSi films.

    (A) HAADF-STEM image of an exfoliated npSi/ncMAPbI3 flake. The bright regions correspond to perovskite (due to z-contrast), as confirmed by EDX analysis. The Pb signal is plotted as orange overlay. (B) EDX line scan (Pb L line) of a single crystallite from the thinnest part of the flake [indicated as red line in (A)]. We find a crystallite size of 4 nm for 30–mA cm−2 npSi. (C) Background corrected, azimuthally averaged x-ray diffraction profiles of MAPbI3 in 30–mA cm−2 npSi (violet line), in 25–mA cm−2 npSi (blue line), and in 15–mA cm−2 npSi (green line) as a function of the scattering vector q. The curves are normalized and vertically shifted for clarity. The log-scale highlights the increasing broadening of all MAPbI3 peaks with decreasing current density due to crystallite size reduction, suggesting increasingly strong confinement. The peaks are indexed to a tetragonal structure with the lattice constants a = b = 8.90(3) Å and c = 12.71(5) Å. (D to F) Crystallite size and diffracted power of the MAPbI3 signal as function of depth for (D) 30–mA cm−2 npSi/ncMAPbI3, (E) 25–mA cm−2 npSi/ncMAPbI3, and (F) 15–mA cm−2 npSi/ncMAPbI3. Red circles: Depth-dependent crystallite size. Black line: Integrated intensity of the perovskite diffraction signal as a measure for the amount of perovskite at a specific depth. Dashed red line: Weighted average of the crystallite size. Error bars correspond to the 1-σ values of the uncertainty resulting from data analysis and the initial resolution of the experiment. (G) PL peak emission energy against the average size of crystallites formed in three npSi layers prepared with indicated anodization current density. Error bars correspond to the SD of the crystallite size.

  • Fig. 4 Perovskite-infused nanoporous thin films for LEDs.

    (A) Schematic of npPeLEDs. Devices are fabricated on glass/FTO by anodizing a titanium/aluminum bilayer. This electrochemical procedure produces npAAO with compact TiO2 below. Precursor solution infused to the top of the pores yields conducting and electroluminescent nanocrystalline perovskite domains. A polymeric HTL and hole-injecting contact of MoOx/Ag complete the device stack. (B) SEM of anodized Ti/Al, showing the npAAO layer with pores of ~6-nm diameter on top of the scale-like Ti/TiO2 domains below. (C) PL micrograph image taken of a Ti/npAAO layer infused with CsPbBr3, evidencing uniform light emission across large areas of the sample. Excitation filter, 465 to 495 nm; barrier filter, 515 to 555 nm. (D) EL of an ncMAPbI3 diode, (filled curve) centered around 731 nm, with bulk PL emission (dashed line) and PL of the ncMAPbI3 diode (solid curve) plotted for comparison. (E) EL (filled curve) of an ncCsPbBr3 diode with PL emission of the bulk (dashed line) and the ncCsPbBr3 diode is shown (solid line). Cyan-green narrowband EL peaking at 518 nm is observed. (F) J-V characteristic of an ncCsPbBr3 diode (black trace) plotted together with luminance (cyan). Devices turn on at ~2.5 V with a luminance of ~330 cd m−2 at 5 V. (G) Photograph of a CsPbBr3 diode while operating at 4 V, displaying cyan-green EL over the entire area of the pixel (15 mm2).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/8/e1700738/DC1

    fig. S1. Bandgap tuning via size effects and halide compositional stoichiometry in npSi and npAAO.

    fig. S2. PL of native npSi and MAPbI3 nanocrystals in npSi.

    fig. S3. Fine-tuning of the bandgap in npSi via variation of pore size.

    fig. S4. Time-resolved PL lifetime.

    fig. S5. Structural characterization of MAPbBr3 nanocrystals in npAAO.

    fig. S6. Wide-angle x-ray scattering (WAXS) study of MAPbI3 nanocrystals in 30–mA cm−2 npSi: Experiment and data analysis.

    fig. S7. WAXS study of MAPbI3 nanocrystals in 15–mA cm−2 npSi and 25–mA cm−2 npSi.

    fig. S8. Pore size estimation by Porod analysis of SAXS data of 15–mA cm−2 npSi.

    fig. S9. Surface SEM of npAAO on compact TiO2.

    References (6367)

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Bandgap tuning via size effects and halide compositional stoichiometry in npSi and npAAO.
    • fig. S2. PL of native npSi and MAPbI3 nanocrystals in npSi.
    • fig. S3. Fine-tuning of the bandgap in npSi via variation of pore size.
    • fig. S4. Time-resolved PL lifetime.
    • fig. S5. Structural characterization of MAPbBr3 nanocrystals in npAAO.
    • fig. S6. Wide-angle x-ray scattering (WAXS) study of MAPbI3 nanocrystals in 30–mA cm−2 npSi: Experiment and data analysis.
    • fig. S7. WAXS study of MAPbI3 nanocrystals in 15–mA cm−2 npSi and 25–mA cm−2 npSi.
    • fig. S8. Pore size estimation by Porod analysis of SAXS data of 15–mA cm−2 npSi.
    • fig. S9. Surface SEM of npAAO on compact TiO2.
    • References (63–67)

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