Applied Science and Convergence Technology 2020; 29(2): 28-30
Published online March 30, 2020
Copyright © The Korean Vacuum Society.
Il-Wook Cho and Mee-Yi Ryu*
Department of Physics, Kangwon National University, Chuncheon 24341, Republic of Korea
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-CommercialLicense (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution,and reproduction in any medium, provided the original work is properly cited.
The luminescence properties of surface-passivated methylammonium lead halide perovskites (PS) combined with CdSe/ZnS core–shell quantum dots (QDs) were investigated using photoluminescence (PL) and time-resolved PL (TRPL) spectroscopies. We synthesized two different PS films covered with QDs, namely QD/CH3NH3PbI3 and QD/CH3NH3PbBr3. Both the PL intensity and decay time were higher for the QD/CH3NH3PbI3 hybrid structure than those of the bare CH3NH3PbI3 film, which was attributed to an energy transfer (ET) process from the CdSe/ZnS QDs to CH3NH3PbI3. In contrast, for the QD/CH3NH3PbBr3 hybrid structure, the reduced PL intensity and decay time compared to those of the bare CH3NH3PbBr3 film were attributed to ET from CH3NH3PbBr3 to the QDs. These PL and TRPL results demonstrated that the optical properties of PS films can be improved by the addition of QD coatings, which could enable the application of such materials as support layers for adjacent active layers in optoelectronic devices.
Keywords: Perovskite, CdSe/ZnS core–,shell quantum dots, Energy transfer, Photoluminescence, Time-resolved photoluminescence
Methylammonium lead halide perovskites (PS) have been demonstrated as prospective materials for light-emitting diodes and solar cells because of their inherent advantages, such as a tunable bandgap , high absorption coefficients , high carrier mobilities , and long diffusion lengths . PS films can be easily fabricated using low-cost solution-based methods, such as spin-coating; however, solution-processed PS films contain charge traps at the grain boundaries, which results in increased non-radiative recombination [5,6]. In particular, the instability of PS films under humid conditions must be overcome to enable their widespread application. To address this, a surface passivation method was previously reported [7,8]. Jiang
Here, we report an investigation into the effect of surface passivation and the luminescence properties of PS films covered with CdSe/ZnS core–shell QDs using photoluminescence (PL) and time-resolved PL (TRPL) spectroscopy methods. For this purpose, the energy transfer (ET) phenomenon is introduced into the QD/PS hybrid structure to verify the luminescence properties. In this context, fluorescence resonance energy transfer (FRET) is a process involving transfer of the absorbed photon energy from a donor to an acceptor, where charge transfer (CT) involves transfer of the photo-generated electrons and/or holes from a donor to an acceptor. These processes are known to be affected by the spectral overlap between the emission of the donor and the absorption of the acceptor, and the difference in energy band offsets between the donor and the acceptor, respectively. To date, ET processes have been demonstrated in hybrid structures, such as graphene/semiconductor and graphene/QD, MoS2/QD, and QD/PS systems [9–11]. Thus, we herein report the fabrication of two different QD/PS hybrid structures, namely QD/CH3NH3PbI3 and QD/CH3NH3PbBr3, and the subsequent investigation of the ET processes between the PS and the CdSe/ZnS QDs.
Indium tin oxide (ITO)-coated glass substrates were cleaned by ultra-sonication (UCP-02, Lab companion) for 10 min in acetone, isopropyl alcohol, and deionized water, sequentially, and then exposed to UV-ozone cleaning (PSDP-UV4T, Novascan, USA) for 15 min at 100 °C. For the synthesis of CH3NH3PbI3, methylammonium iodide (159 mg; Great Cell Solar Ltd.), lead iodide (461 mg; Alfa Aesar), and dimethyl sulfoxide (71 μL, DMSO, Sigma-Aldrich) were dissolved in
PL measurements were conducted using a charge-coupled device detector (Andor DV420-BU2) and a 325-nm He-Cd laser (Kimmon Koha Co., Ltd.) as the excitation source. TRPL measurements were performed using a FLS 920 spectrometer (Edinburgh Instruments Ltd., UK), a micro-channel-plate photomultiplier tube detector, and a 375-nm picosecond-pulsed diode laser (pulse width = 48 ps, EPL-375, Edinburgh Instruments Ltd.) as the excitation source. UV-visible absorption spectra were obtained using a Libra S80 spectrometer (Biochrom Ltd., UK).
Figure 2(a) shows the PL spectra of the bare CH3NH3PbI3 and QD/CH3NH3PbI3 films measured at room temperature (RT) on a logarithmic scale. The PL peak of the CH3NH3PbI3 film was located at 763 nm (1.62 eV), and the PL intensity of the QD/CH3NH3PbI3 film was stronger than that of the bare CH3NH3PbI3 film. This enhanced PL intensity was attributed to the combined effect of the surface passivation of the PS, and ET from the CdSe/ZnS QDs to CH3NH3PbI3 . The RT PL spectra of the bare CH3NH3PbBr3 and QD/CH3NH3PbBr3 films are shown in Fig. 2(b) on a logarithmic scale, where the PL peak of the CH3NH3PbBr3 film was observed at 536 nm (2.31 eV). After coating the CH3NH3PbBr3 films with the QDs, the PL intensity decreased significantly, and a new PL peak was observed at ~600 nm (2.06 eV) originating from the CdSe/ZnS QD emission. Figures 2(c) and 2(d) show UV-Vis spectra of the bare CH3NH3PbI3 and the CH3NH3PbBr3 films, respectively, along with the PL spectrum of the QDs. The overlap between the PL spectrum of the QDs and the absorption spectrum of the bare CH3NH3PbI3 can be seen in Fig. 2(c), which leads to a FRET from the QDs to the PS. In contrast, the emission of the QDs lies outside the absorption edge of CH3NH3PbBr3, as seen in Fig. 2(d). The inset of Fig. 2(d) shows the PL spectrum of the bare PS and the absorption spectrum of the QDs. The PL emission of CH3NH3PbBr3 overlapped with the absorption spectrum of the QDs, allowing FRET from the PS to the QDs to take place. The decreased PL intensity of QD/CH3NH3PbBr3 was therefore attributed to the FRET from CH3NH3PbBr3 to the CdSe/ZnS QDs.
Figures 3(a) and 3(b) show the energy level diagrams of the QD/CH3NH3PbI3 and QD/CH3NH3PbBr3 hybrid structures, respectively. The valence band maximum (VBM) values for CH3NH3PbI3 (-5.43 eV), CH3NH3PbBr3 (-5.60 eV), and the CdSe/ZnS QDs (-5.42 eV) were taken from the literature [12–14]. The conduction band minimum (CBM) values were then determined using the PL peak energies for the CH3NH3PbI3 (-3.81 eV), CH3NH3PbBr3 (-3.29 eV), and CdSe/ZnS QD (-3.36 eV) species. The obtained energy-level diagrams suggested that CT could occur due to the difference in the energetic offset between the QDs and the PS. For the QD/CH3NH3PbI3 hybrid structure, the photo-excited electrons from QDs were transferred to CH3NH3PbI3 since the CBM of CH3NH3PbI3 is located at a lower energy than that of the CdSe/ZnS QDs. Therefore, the increased PL intensity of the QD/CH3NH3PbI3 hybrid structure was also attributed to CT from the QDs to the PS, in addition to FRET, as shown in Fig. 2(a). In contrast, in the case of the QD/CH3NH3PbBr3 hybrid structure, the photo-excited electrons and holes from the CH3NHPbBr3 are transferred into the QDs due to the CBM of CH3NH3PbBr3 being located at a higher energy than that of the QDs, and the VBM of CH3NH3PbBr3 being located at a lower energy than that of the QDs. Therefore, the PL intensity in the QD/CH3NH3PbBr3 hybrid structure was reduced because of CT occurring prior to radiative recombination.
To investigate the ET process between the QDs and the PS in the QD/PS hybrid structures, TRPL measurements were conducted. Figures 4(a) and 4(b) show the PL decay curves of the bare PS and the QD/PS hybrid structures for CH3NH3PbI3 and CH3NH3PbBr3, respectively, which were taken at the PL peaks of CH3NH3PbI3 (763 nm) and CH3NH3PbBr3 (536 nm), respectively. The PL decay curves were fit well using a double-exponential decay function, i.e.,
We herein reported our investigation into the luminescence properties of methylammonium lead halide PS films (CH3NH3PbI3 and CH3NH3PbBr3) covered with CdSe/ZnS core–shell QDs using PL and TRPL spectroscopy techniques. Following coating of the PS films with the QDs, the PL intensity and decay time of the QD/CH3NH3PbI3 hybrid structure increased compared to those of the bare PS, while the corresponding values for the QD/CH3NH3PbBr3 hybrid structure decreased compared to those of the bare PS film. These observations were attributed to ET processes between the QDs and the PS. Hence, such hybrid structures can be tailored to take advantage of ET processes in the required direction, to optimize the optical properties of the PS or adjacent active layers. We propose that these QD/PS hybrid structures showing ET processes are suitable for use as active layers in high-performance optoelectronic devices.
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (NRF-2019R1A2C1086813). Time-resolved photoluminescence and UV-Vis absorption measurements were performed at the Central Lab of Kangwon National University.