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 [1], high absorption coefficients [2], high carrier mobilities [3], and long diffusion lengths [4]. 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 et al. demonstrated the reduced non-radiative recombination of perovskite film by surface defect passivation with an organic halide salt [7]. Moreover, we previously reported the improved luminescence behavior and stability of surface-passivated perovskite film with CdSe/ZnS quantum dots (QDs) [8].

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 N,N-dimethylformamide (0.7 mL; DMF; Sigma-Aldrich). The CH₃NH₃PbI₃ solution was spin-coated onto the ITO substrate at 3500 rpm (5 s acceleration time) for 20 s, and then annealed at 100 °C for 1 min. Diethyl ether (DE; Sigma-Aldrich) was applied dropwise to the surface during spinning. For the synthesis of CH₃NH₃PbBr₃, methylammonium bromide (112 mg; Great Cell Solar Ltd.), lead bromide (367 mg; Sigma-Aldrich), and DMSO (0.3 mL) were dissolved in DMF (0.7 mL). The CH₃NH₃PbBr₃ solution was spin-coated at 3500 rpm for 20 s onto the ITO substrate, and the resulting spin-coated CH₃NH₃PbBr₃ film was immersed in DE for 2 min. For fabrication of the QD/PS hybrid structures, CdSe/ZnS core–shell QD powder (5 mg; PlasmaChem GmbH) was dissolved in toluene (1 mL), and then spin-coated onto the PS films at 3500 rpm for 20 s. Figure 1 shows a schematic of the procedure for synthesizing the QD/PS hybrid structures.

Figure 1. Schematic procedure of synthesizing the QD/PS hybrid structures.

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 [11]. 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.

Figure 2. Room temperature PL spectra of the bare PS and the QD/PS hybrid structures for (a) CH₃NH₃PbI₃ and (b) CH₃NH₃PbBr₃. Absorption spectra of (c) bare CH₃NH₃PbI₃ film (red dashed line) and (d) bare CH₃NH₃PbBr₃ film (blue dashed line). The PL spectrum of the CdSe/ZnS QDs is also shown (black solid line). The inset shows the absorption spectrum of the CdSe/ZnS QDs (black dashed line) and the PL spectrum of CH₃NH₃PbBr₃ (blue solid line).

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.

Figure 3. Energy-level diagrams of (a) QD/CH₃NH₃PbI₃ and (b) QD/CH₃NH₃PbBr₃ hybrid structures.

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., I(t) = A ₁ exp(−t/τ ₁) + A ₂ exp(−t/τ ₂), where τ ₁ and τ ₂ are the fast and slow decay times, respectively, and A ₁ and A ₂ are the pre-exponential constants of T ₁ and T ₂, respectively. The average decay time (τ _ave) was calculated using τ _ave = Σ A _i τ i²/Σ A _i τ i. The detailed decay parameters measured at RT are listed in Table I. It was determined that τ _ave = 13.92 ns for the bare CH₃NH₃PbI₃, while τ _ave = 19.24 ns for QD/CH₃NH₃PbI₃. The longer decay time for the QD/PS hybrid structure than for the bare PS was attributed to ET from the QDs to the PS. In contrast to the QD/CH₃NH₃PbI₃ hybrid structure, the decay time of QD/CH₃NH₃PbBr₃ (2.23 ns) was shorter than that for the bare CH₃NH₃PbBr₃ (4.46 ns). This was attributed to ET from the PS to the QDs. It is important to note that the ET processes of the QD/CH₃NH₃PbI₃ and QD/CH₃NH₃PbBr₃ hybrid structures occurred in opposite directions. This finding suggests that the optical properties of the PS or the adjacent active layer can be improved using the ET phenomenon, since the bandgap energy of the PS films can be controlled by changing the composition ratio of the halide elements.

Table 1 . Estimated PL decay parameters of the bare PS and QD/PS hybrid structure samples.

Sample	τ 1 (ns)	τ 2 (ns)	A 1 (%)	A 2 (%)	τ ave (ns)
Bare CH3NH3PbI3	4.00	14.68	22	78	13.92
QD/CH3NH3PbI3	2.58	20.27	33	67	19.24
Bare CH3NH3PbBr3	3.46	7.09	84	15	4.46
QD/CH3NH3PbBr3	0.71	3.02	68	31	2.23

Figure 4. PL decay curves of the bare PS and QD/PS hybrid structures for (a) CH₃NH₃PbI₃ and (b) CH₃NH₃PbBr₃, taken at the corresponding PL peaks of 763 nm and 536 nm, respectively.

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.