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Abstract
Improving the optoelectronic characteristics of organic-inorganic perovskites is crucial for fabrication of functional devices. Herein, we demonstrate that the optoelectronic properties of 2D organic-inorganic perovskites can be greatly improved by UV-light illumination during growth. The photoluminescence emission of the 2D perovskite exhibits a 3.1-folds increase in intensity, with a decreased trap-assisted recombination. The improved optoelectronic characteristics can be attributed to the high-quality crystallization and lattice expansion induced by the UV-light illumination. Moreover, the optimized 2D perovskites enable the fabrication of photoconductive devices with improved optoelectronic responses. This work indicates that light illumination is a novel and convenient approach for engineering the fabrication of 2D organic-inorganic hybrid perovskites, which advocates great promise for achieving high-performance functional devices.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Benefited from the low-defect density [1–3], long carrier diffusion [4–6], high conversion efficiency [7] and flexible structures [8–10], organic-inorganic perovskites have shown great promise for the applications in optoelectronic devices such as solar cells [11,12], light-emitting devices [13] and optical detectors [14,15]. Simultaneously, 2D-layered materials have also attracted great attentions due to the unique optoelectronic properties [16–20]. Combining the advantages of the organic-inorganic perovskites and 2D materials, 2D organic-inorganic perovskites are demonstrated to possess a stronger quantum confinement, a larger exciton binding energy and a greater tunability due to the multiple-quantum-well structure [21–24]. As a consequence, 2D perovskites are emerging as an excellent candidate for functional optoelectronic devices nowadays. For instance, color-pure violet LED devices have been fabricated with the 2D perovskite nanoplates [25], and optoelectronic devices have been achieved based on the energy funnel effect in the quasi-2D perovskite [4]. More recently, 2D perovskites have also attracted great interests in the novel applications of photophysics, such as nanolasing, spin-selective Stark effect and laser cooling [26–30]. However, despite great advantages for 2D perovskites, their severe trap states still remain rigorous challenges for achieving high-performance optoelectronic devices [31–33].
Previous reports have led to the realization that the preparation conditions play important roles in controlling the crystallization rate, morphology and crystalline grains of the organic-inorganic perovskites [34–36]. Particularly, light illumination has become a useful approach to improving the morphology and optical properties of 3D perovskites. Proper light soaking will promote a superior surface coverage and an enlarged unit cell of the 3D perovskite films, thus resulting in an improvement of the fill factor and open circuit voltage of the perovskite devices [37,38]. In addition, the properties of the perovskites could be optimized by the photochemical reaction occurred during light exposure [39]. Although widely studied in 3D perovskites, the influence of light illumination on the growth and optoelectronic properties of 2D layered organic-inorganic perovskites still needs detailed understanding, which will lay great importance for fabricating high-quality 2D perovskites for functional devices.
Herein, we report the enhanced optoelectronic performances of 2D organic-inorganic hybrid perovskite through UV-light illumination. UV-light illumination will lead to an improvement of the crystallization and lattice expansion of the 2D perovskite monocrystalline films. Consequently, the PL emission of the 2D perovskite can be enhanced by 3.1 folds, with the trap-assisted recombination greatly decreased. For functional applications, photoconductive devices is fabricated based on the optimized 2D perovskites and the optoelectronic responses have been enhanced.
2. Results and discussion
(PEA)2PbI4 monocrystalline films (PEA)2PbI4 monocrystalline films were synthesized using an anti-solvent vapor-assisted capping crystallization (AVCC) method [40]. During the growth, the precursor solution was placed under white light (Bright), UV light (UV), and in the dark (Dark) respectively. Figure 1(a) shows the microscope images of the as-prepared 2D perovskite monocrystalline films on glass substrates. Clearly, the crystals tend to form assembly in the dark and under white light, with large crystal sizes and inhomogeneous surfaces. In contrast, the crystals tend to form regularly under UV light, with small crystals and uniform surfaces. Scanning electron microscopy (SEM, Nova NanoSEM 450) was used to characterize the morphology of the monocrystalline films. Figure 1(b) shows that the UV-grown samples exhibit a much smoother surface than the other two samples, suggesting a better crystallization of the (PEA)2PbI4 monocrystalline films under UV-light illumination. The better crystallization of the UV-grown samples can also be reflected in the size distribution of the monocrystalline films [Fig. 1(c)], which was obtained based on an analysis of more than 60 randomly-selected crystal grains for each growth condition. Figure 1(c) shows that the crystal size has a wide distribution for the Dark- and Bright-grown samples, while it is more concentrated for the UV-grown samples, which implies a uniform crystallization of the 2D perovskite under UV-light illumination. Figure 1(d) shows that the thickness of the UV-grown samples is about 3.3µm, which is much smaller than those of the Dark- and Bright-grown samples (∼16.5 µm and ∼13.2 µm respectively). Furthermore, X-ray diffraction (XRD, X’Pert PRO) was used to characterize the crystallinity of the 2D perovskites. Figure 1(e) shows that all of the samples have well-defined diffraction peaks corresponding to the (00l) series of (PEA)2PbI4 [22]. For detailed insight, the XRD pattern corresponding to the (004) peak is enlarged and presented in Fig. 1(f). The spectrum of the Bright-grown samples has been broadened a lot with impurity peak, suggesting a poor crystallinity [41,42].
Fig. 1. Characterization of the (PEA)2PbI4 monocrystalline films prepared in different illumination conditions. (a) Microscope images and (b) SEM images of the (PEA)2PbI4 monocrystalline films grown in the dark (Dark), and under the illumination of white light (Bright) and UV light (UV), respectively. (c) Grain size distributions of the (PEA)2PbI4 monocrystalline films.(d) Height profiles of the (PEA)2PbI4 monocrystalline films. (e) XRD patterns, and (f) the enlarged XRD peaks for the (PEA)2PbI4 monocrystalline films, respectively.
XRD results are further used to characterize the differences of the crystal structures of the (PEA)2PbI4 monocrystalline films. Table 1 lists the XRD peaks and the corresponding lattice parameters extracted from the XRD patterns. All the XRD peaks of the bright- and UV-grown samples exhibit a slight blue shift relative to the corresponding peaks of the dark-grown samples, indicating a variation in the crystal structures of the 2D perovskites induced by light illumination. For clear presentation, Fig. 2(a) shows the change of the lattice parameters for the bright- and UV-grown samples relative to those of the dark-grown samples. Notably, the lattice parameters of the Bright- and UV-grown samples are a little larger than those of the Dark-grown samples at the corresponding (00l) plane, indicating a lattice expansion of the samples grown under light illumination. Light illumination can enlarge the Pb-I-Pb bond angle, thus resulting in the lattice expansion, as schemed in Fig. 2(b). The lattice expansion can be also reflected from the photoluminescence (PL) spectra of the perovskite samples. The peak wavelength of the PL spectra under single-photon excitation for the perovskite monocrystalline films grown in the Dark-, Bright- and UV-conditions are analyzed and presented in Fig. 2(c). The PL peak of the samples grown under light illumination show a redshift compared to that of the dark-grown samples. As reported, the redshift of the emission peak can be attributed to the enlarge of the unit cell in the organic-inorganic perovskites [2–3].
Fig. 2. The lattice expansion of the 2D perovskite induced by light illumination. (a) Plot of the increase of lattice parameters (lattice expansion) for the Bright- and UV-grown samples relative to that of the Dark-grown samples. (b) Schematic diagram for the lattice expansion. (c) Distribution of peak wavelengthes of the PL spectra under single-photon excitation for the perovskite monocrystalline films.
Table 1. XRD peaks and the corresponding lattice parameters extracted from the XRD patterns for the 2D perovskite monocrystalline films prepared under different illumination conditions.
To reveal the influence of light illumination on the performances of the (PEA)2PbI4 samples, photoemission properties were investigated in details by PL spectra. PL measurements were performed based on an inverted optical microscope system (Olympus IX73). The laser beam from a mode-locked Ti/Sapphire oscillator (Vitara, Coherent, 800 nm, ∼8 fs, 80 MHz) was directly used for two-photon excitation, or frequency doubled by a BBO crystal for one-photon excitation. The laser beam was focused by a 20× objective (Olympus, NA = 0.4) to the samples. The reflected signal was collected by the same objective and then imported into a spectrometer (Andor 193i) for spectra measurement. Figure 3(a) presents the PL spectra for the Dark-, Bright- and UV-grown samples measured in the same conditions by single-photon excitation at 400 nm. The integrated PL intensity of the UV-grown sample is ∼3.1 times stronger than that of the Dark-grown sample, although the thickness of the UV-grown monocrystalline films are much smaller. The boosted PL emission indicates that UV-light illumination is advantageous for improving the photoemission performance of the 2D perovskites. Previous reports show that the radiative recombination in the 2D perovskite are attributed to the intrinsic exciton recombination and trap states simultaneously [31]. To have a deep insight, the PL spectra were decomposed into two parts by fitting to a two-peak Lorentz model, as shown in Fig. 3(b). The main part (part 1) corresponds to the exciton recombination, and the shoulder part (part 2) represents the trap-assisted recombination. The ratios between the integrated intensity of the two parts (I1/I2) are 0.95, 0.55 and 0.44 for the UV-, Dark- and Bright-grown samples respectively, which indicates that the proportion of trap-assisted recombination in the UV-grown samples is greatly decreased. Similar results could be obtained in the two-photon-absorption-induced PL (TPL) measurement [Fig. 3(c)], in which the PL emission deep inside the crystals can be detected benefiting from the larger penetration depth under two-photon excitation [43,44].
Fig. 3. Influence of light illumination on the PL properties of the (PEA)2PbI4 monocrystalline films. (a) Normalized PL spectra for the (PEA)2PbI4 monocrystalline films grown in the Dark, Bright, and UV conditions, respectively. (b) Single-photon excitation and (c) two-photon excitation induced PL spectra (solid curve). The gray dashed curves are the corresponding decompositions by fitting to a two-peak Lorentz model. (d) Time-resolved PL decay traces for the (PEA)2PbI4 monocrystalline films grown in the Dark, Bright, and UV conditions, respectively.
The transient PL behavior can provide unique information on the charge recombination dynamics [45–47]. Figure 3(d) shows the time-resolved PL decay traces for the (PEA)2PbI4 monocrystalline films. The decay traces were fitted to a biexponential decay function, I = A1exp(−t/τ1)+A2exp(−t/τ2). The UV-grown samples display a fast decay lifetime of 0.68 ns (16%) and a slow decay lifetime of 3.9 ns (84%). Correspondingly, the Dark-grown samples have a fast decay lifetime of 0.7 ns (37%) and a slow decay lifetime of 3.2 ns (63%), and the Bright-grown samples exhibit a fast component of 0.57 ns (68%) and a slow component of 2.6 ns (32%). Notably, the lifetime and fraction of the slow decay component for the UV-grown samples are both increased, which corresponds to a reduction of the trap-assisted recombination, and agrees well with the PL spectra.
As mentioned above, the improved optoelectronic characteristics of the (PEA)2PbI4 monocrystalline films is directly correlated to the optimized crystallization induced by the UV-light illumination. On the one hand, UV-light illumination will promote an uniform crystallization of the 2D perovskites. The high-quality monocrystalline films with smooth surface can largely decrease the defects in the bulk and at the interfaces, which is helpful for decreasing the trap-assisted recombination in the 2D perovskites. On the other hand, light illumination will lead to a little enlarge of the lattice unit cells. The lattice expansion is beneficial for relaxing the local lattice strain in the 2D perovskites, thus resulting in an improvement of the optical performances [33].
In addition, the UV light intensity was tuned to investigate the effect on the crystal quality and PL performance of the (PEA)2PbI4 monocrystalline films. Figure 4(a) presents the microscope images (inset) and the corresponding XRD patterns of the samples grown under UV-light illumination. All of the monocrystalline films exhibit a uniform crystal structure and a narrow XRD spectrum, indicating a high-quality crystallization of the 2D perovskite. Furthermore, the crystal size becomes smaller as the illumination intensity increases, because the crystallization rate is further decreased by the UV-light illumination. Figure 4(b) shows the normalized PL spectra and the corresponding decomposed components for these UV-grown samples. Overall, all the ratios of the decomposed two parts (I1/I2) under UV-light illumination are larger than those of the Dark- and Bright-grown samples, which further demonstrate the depression of the trap states and the improvement of the photoemission. We also noticed that the value of I1/I2 is gradually decreased with the UV-light intensity, because the organic-inorganic perovskites would be slightly degraded under strong UV light [46].
Fig. 4. Controllable crystal quality and PL performance of the (PEA)2PbI4 monocrystalline films through light illumination. (a) Enlarged XRD patterns and (b) PL spectra of the (PEA)2PbI4 monocrystalline films grown under UV-light illumination, with various intensity. The insets show the corresponding microscope images.
In order to examine the potential for functional applications, we fabricated the photoconductive devices based on the (PEA)2PbI4 monocrystalline films. Figure 5(a) shows the microscope image and the linear I-V curves of the as-fabricated device based on the UV-grown (PEA)2PbI4 monocrystalline films, which indicates the high crystallinity of the as-synthesized monocrystalline films and good contact between Au electrodes and perovskite. Notably, the photocurrent increases gradually as the incident power increases, due to the production of photogenerated charge carriers. Figure 5(b) shows the spectral response of the device under different bias, which exhibits a maximum value of 0.13 A/W around 495 nm under the bias of −9 V. The optical switch characteristics under different incident light power at −9 V bias reveal an excellent stability and reversibility of the device based on UV-grown samples, as shown Fig. 5(c). In particularly, the on-off ratio can reach 103 with the incident power of 6 mW/cm2, which is comparable to or even higher than the reported 3D perovskite photodetectors [48–50]. The response speed is also a significant parameter for the photodetectors, which is usually characterized by rise and fall time (defined as the time taken for the photocurrent rising from 10% to 90% of the peak value, and vice versa) [51]. Figure 5(d) displays that the rise/fall time of the as-fabricated device is approximately 136/127 ms respectively. The slow response speed might be due to the large size of the (PEA)2PbI4 monocrystalline films and the small electric field applied in the measurement (0.04 V/µm).
Fig. 5. Optoelectronic characteristics of the photoconductive device fabricated based on the (PEA)2PbI4 monocrystalline films grown under UV condition [(a)–(f)], and dark condition [(g)–(l)]. (a)(g) I-V curves of the device in dark and under a 518-nm light with different powers. The inset shows the optical image of the as-fabricated device. The scale bar is 100 µm. (b)(h) The spectral response of the device under different biases. (c)(i) The optical switch characteristics of the device under a switched-on/off 518-nm light irradiation with different powers at the bias of –9 V. (d)(j) Transient photocurrent response under a 518-nm light illumination at the bias of –9 V. (e)(k) Normalized frequency response of the device at the bias of –9 V. The 3-dB bandwidth is 8.5 Hz. (f)(l) Detectivity (D*) spectrum of the device at the bias of −9 V. The insets show the noise power density (Sn) spectrum of the device.
Detectivity (D*) represents the detecting ability that a detector can distinguish weak signals from the noise, which is an important figure-of-merit for the photodetectors. In order to evaluate the detectivity of the device, the 3-dB bandwidth was first measured to be 8.5 Hz under the bias of −9 V, as shown in Fig. 5(e). In addition, the noise power density (Sn) was acquired to be 2.38×10−26 A2/Hz by a previous reported method at the 3-dB bandwidth [52]. Therefore, the detectivity can be determined by
where A is the active area of the detector, and Δf is the bandwidth of the photodetector. NEP represents the noise equivalent power, NEP = (Sn)1/2/(RΔf1/2), where R is the responsivity. Figure 5(f) shows that the D* spectrum of the as-fabricated device exhibits a peak value of 7.1×1010 Jones around 495 nm under −9 V bias, which is comparable with that for the 3D perovskite devices. We also measured the photoconductive characteristics of the device fabricated based on the Dark-grown samples, as shown in Figs. 5(g)–5(l). The device exhibits a much smaller photocurrent (∼pA), a narrower 3-dB bandwidth (4.3 Hz), a larger noise power density (Sn = 1.1×10−25 A2/Hz) and a worse detectivity (1.25×1010 Jones) compared with that fabricated with the UV-grown samples, indicating the optoelectronic responses of the 2D perovskites can be also greatly improved by UV-light illumination.3. Conclusion
In summary, we demonstrate the enhanced optoelectronic performances of 2D organic-inorganic perovskite through UV-light illumination. UV-light illumination is beneficial for a high-quality crystallization of the (PEA)2PbI4 monocrystalline films. Correlatively, the PL intensity has been increased by 3.1 folds, and the trap-assisted recombination has been greatly depressed in the perovskites. The optimized 2D perovskites also have great potential for photoconductive devices with improved optoelectronic responses. This work demonstrates that light illumination has become a novel and convenient approach for engineering the fabrication of the 2D organic-inorganic hybrid perovskites. In the future work, further studies will take advantage of this effective approach to focus on the development of high-performance functional devices such as light-emitting devices and solar cells.
Funding
National Natural Science Foundation of China (11204097, 11674117, 11804109); Ministry of Education of the People's Republic of China (20130142110078).
Acknowledgments
We acknowledge the Analytical & Testing Center of Huazhong University of Science and Technology (HUST) for XRD measurement.
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