Research ArticleAPPLIED OPTICS

Tunable light emission by exciplex state formation between hybrid halide perovskite and core/shell quantum dots: Implications in advanced LEDs and photovoltaics

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Science Advances  22 Jan 2016:
Vol. 2, no. 1, e1501104
DOI: 10.1126/sciadv.1501104
  • Fig. 1 Schemes, SEM, and EDX mapping of the prepared samples.

    (A) Schemes of the samples analyzed in this study formed by single layers of PS and QDs and bilayers of QD/PS and PS/QDs. Two types of QDs with different particle sizes (2.3 and 3 nm) have been analyzed in this study. (B) SEM picture of the cross section of a PS/QD complete device. The picture has been taken using backscattered electrons to enhance the contrast between different layers. The image contrast is increased by using colors for the different layers. The average thickness of each layer is also indicated. A thin layer of 300 nm of transparent conduction SnO2:F (FTO) on glass is used as a substrate. TiO2 and spiro-OMeTAD act as electron- and hole-injecting layers, respectively, and core/shell PbS/CdS 3 nm and the PS bilayer act as the electroluminescent layer. (C) Elemental mapping obtained by EDX for a 3-nm PS/QD sample. (D) Elemental mapping obtained by EDX for a 3-nm QD/PS sample. Si, Sn, Ti, and Au allow the identification of glass, FTO, TiO2, and Au contact layers, whereas I and Cd allow the identification of PS and QD layers.

  • Fig. 2 PL study of single-layer and bilayer samples of PS and core/shell PbS/CdS QDs.

    (A) PL of a thin layer of QDs using two QD sizes: 2.3 and 3 nm. a.u., arbitrary unit. (B) PL of PS single layer and from bilayers using 3-nm QDs (for 2.3-nm QDs, see fig. S2); PL from PS/QD was multiplied by a factor 15, whereas PL from QD/PS was multiplied by a factor 30, for an easier comparison. (C) Proposed relative energy diagram of PS and both kinds of QDs analyzed in this study. Relative positions of CB and VB are in agreement with literature predictions (fig. S3). Solid arrows 1, 2, and 3 indicate the band gap emissions from PS, 2.3-nm QD, and 3-nm QD, respectively. Dotted arrows 4 and 5 indicate the exciplex emission of samples prepared with PS + QD 2.3 nm and PS + QD 3 nm, respectively. Dashed arrows indicate charge transfer processes of electrons, e, and holes, h+, between PS and QD 3 nm (analogous processes, not represented, should be expected for 2.3-nm QDs), causing the PL quenching of bilayers. (D) PL recorded using an InGaAs detector at the IR region between 900 and 1700 nm. (E) PL excitation spectra, around the PS band gap, of the exciplex PL band (integrated) in the QD/PS bilayer with 3-nm QDs. (F) TRPL spectra recorded at the PL peak of PS, QD, and QD/PS samples for 2.3-nm QDs (open symbols) and exponential fitting (solid lines) using a single exponential decay (QD/PS bilayer) or a double exponential decay (PS and QD single layers). Inset, TRPL of the PS layer. a. log. u., arbitrary logarithmic unit.

  • Fig. 3 EL of bilayer samples.

    (A) EL spectra in the visible-NIR (near infrared) region obtained from a QD/PS (3 nm) bilayer at different applied voltages. Inset, EL PS/QD peak intensity ratio as a function of the applied voltage of the same sample. (B) EL spectra in the visible-NIR region obtained from a PS/QD (2.3 nm) bilayer at different applied voltages. Inset, EL PS/QD peak intensity ratio as a function of the applied voltage of the same sample. (C) EL spectra in the IR region obtained from a PS/QD (2.3 nm) bilayer at a different applied bias. Inset, EL peak intensity as a function of the applied bias (solid symbols); the black solid line corresponds to the linear fit, and the red dashed line indicates the extrapolated threshold turn-on potential, Vth.

  • Fig. 4 Implications for advanced photovoltaics.

    Energetic diagram of an IB photovoltaic device, adapted from Luque and Martí (19) and Luque et al. (20). In an IB system, one or more levels in the band gap, EG, of a semiconductor that are not contacted by the extraction contacts exist. Consequently, the open-circuit potential, Voc = EFnEFp/e, is still determined by the Fermi level splitting of electron and holes, EFn and EFp, respectively, associated with the occupation of the CB and VB, as in a single absorber photovoltaic system. However, in contrast with conventional systems, it is possible to define a Fermi level of the intermediate band gap, EFIB, situated at energetic distances EL, from the bottom of CB, and EH, from the top of VB. This isolated IB produces that in addition to photons with energy equal to or higher than EG (process 1), it is also possible to harvest photons with energy lower than EG due to photon absorption through this IB state (processes 2 and 3).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/1/e1501104/DC1

    Fig. S1. SEM picture of the surfaces of the different layers.

    Fig. S2. Normalized PL for QDs in solution, on film, and on film after the ligand exchange.

    Fig. S3. PL of PS single layer and PS/QD and QD/PS using 2.3 nm.

    Fig. S4. Energy band position of PS and QD conduction and VB extracted from literature.

    Fig. S5. Comparison of the exciplex PL for QD/PS and PS/QD samples with 2.3-nm QDs.

    Fig. S6. Comparison between PS/QD and QD/PS configurations for 3-nm QDs.

    Fig. S7. Analysis of PL excitation measurement and estimation of the hole transfer time from PS into QD.

    Fig. S8. Current-potential curve for prepared QDs.

    Fig. S9. Normalized EL for PS, QD, and QD/PS.

    Fig. S10. EL at different applied biases for PS/QD configuration using both 2.3- and 3-nm QDs.

    Fig. S11. EQE of the different LEDs prepared.

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. SEM picture of the surfaces of the different layers.
    • Fig. S2. Normalized PL for QDs in solution, on film, and on film after the ligand exchange.
    • Fig. S3. PL of PS single layer and PS/QD and QD/PS using 2.3 nm.
    • Fig. S4. Energy band position of PS and QD conduction and VB extracted from literature.
    • Fig. S5. Comparison of the exciplex PL for QD/PS and PS/QD samples with 2.3-nm QDs.
    • Fig. S6. Comparison between PS/QD and QD/PS configurations for 3-nm QDs.
    • Fig. S7. Analysis of PL excitation measurement and estimation of the hole transfer time from PS into QD.
    • Fig. S8. Current-potential curve for prepared QDs.
    • Fig. S9. Normalized EL for PS, QD, and QD/PS.
    • Fig. S10. EL at different applied biases for PS/QD configuration using both 2.3- and 3-nm QDs.
    • Fig. S11. EQE of the different LEDs prepared.

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