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Abstract
Doping Cd element into perovskite materials is an effective strategy to improve the photoelectric property. However, the further discussion for carrier dynamic behavior in perovskites affected by Cd element remains not sufficient. In this research letter, based on steady and transient spectroscopy, it is found that adding Cd element into CsPbBr3 nanocrystals can enhance the activity of photo-generated carriers and accompany with the optimization of crystal structure. The former improves the carrier heating effect, which makes carrier keep high temperature for a long time and accelerate the bimolecular and the Auger recombination simultaneously. The latter can restrict the monomolecular recombination through passivating the defect states. Finally, they together improve the photoluminescence characteristics of the Cd doped CsPbBr3 nanocrystals and make them exhibit a huge potential in the fields of optoelectronics or photo-catalysis.
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1. Introduction
Recently, lead halide perovskites (LHPs), acting as a type of promising semiconductor material, have shown huge potential in the fields of solar cell, [1,2] light emitting diodes (LED), [3] photodetectors, [4] lasers [5] and light communication, [6] owing to their numerous merits, such as the broadband absorption, [7] easily tunable optical band gap, [8] high luminescence yield, [9] efficient charge generation and transportation characteristics [10]. Therefore, the performance of optoelectronic device based on perovskite materials exhibits continuous growth. For example, the power conversion efficiency of perovskite solar cells has increased from 4% in 2009 [11] to 25.5% in 2020, [12] approaching that of crystalline silicon counterparts. The external quantum efficiency (EQE) of blue, red and green LED based on perovskite materials has enhance to 13.8%, [13] 28.1% [14] and 22.2% respectively [15]. As visible light communication sources, the white LED based on CsPbBr3 exhibits ‒3 dB bandwidth of 2.75 MHz, and achieves a high data rate of 33.5 Mbps [16].
In order to early achieve commercialization of perovskite-based optoelectronic devices, numerous studies have attempted to discover new perovskite materials or develop new strategies to further improve the photo-physical characteristics of perovskite. Lin et al. prepared CsPbBr3 glass ceramic film sintered on sapphire plate, guaranteed good luminescent behavior while achieving high stability [17]. Shen et al. indicated that the ETA interlayer can promote the formation of pure cubic CsPbBr3 phase, improve device efficiency of CsPbBr3-based LEDs [18]. The current researches point out that adding metal ions, involving Sn2+, Sr2+, Sb3+, Ni2+, Mn2+, Bi3+, into perovskite materials is an effective strategy to improve the performance compared to that of the un-doped counterparts [19–21]. Among them, Cd2+ doping is an important example in metal ion doping engineering. Navendu et al. shown that the PLQY of Cd-doped CsPbCl3 greatly enhanced from 3% to 98%, making it by far the most attractive blue-emitting perovskite [22]. Meanwhile, the enhanced performance of CsPbxCd1-xBr3 nanocrystals in two-photon excited amplified spontaneous emission (ASE) was observed by Li et al, where the threshold decreased from 1.58 to 1.23 mJ cm−2 [23]. Apparently, the enhancement of photo-physical properties in Cd doping perovskite materials should be much sensitive to the modulation of carrier characteristics. Even though the carrier dynamic behavior affected by Cd2+ has been reported, [22,23] the further discussion still remains no sufficient, which not only affect the understanding of the strategy of metal ions doping into perovskite NCs, but also discourage further expansion of such materials in optoelectronic applications.
In this paper, the activity of carriers in Cd doping CsPbBr3 NCs is effectively enhanced, which is confirmed by employing the steady and transient spectroscopy. In Cd doping CsPbBr3 NCs, the free movement of photo-generated carriers can be accelerated, which increases the rate constant of bimolecular and Auger recombination. And, the monomolecular recombination of Cd doping CsPbBr3 NCs is restricted since the addition of Cd element passivates the defect. Meanwhile, the heating effect of photo-generated carriers is enhanced compared to that of pure CsPbBr3 NCs, which is also much sensitive to the environment temperature. These results provide help to understand the physical origin of improved optoelectronic characteristics in Cd element doped CsPbBr3 NCs and dig its potential application in future.
2. Experiment
2.1 Synthesis of CsPb1‒xCdxBr3 Perovskite NCs
To prepare Cs-OA solution, Cs2CO3 (0.814 g), 1-octadecene (ODE, 30.0 mL) oleic acid (OA, 2.5 mL) were combined in a three-neck round bottomed flask, stirred, and heated at 150 °C under N2 flow until completely dissolved after being dried for 30 min at 120 °C under vacuum.
CsPb1‒xCdxBr3 NCs were synthesized by a hot injection method [23]. A mixture of PbBr2 (0.138 g), CdBr2 (0 g for x = 1, 0.051g for x = 0.93), oleylamine (OAm, 1.0 mL), OA (1.0 mL), and ODE (10.0 mL) was added into a three-necked flask (50 mL) at room temperature. The mixture was heated to 120 °C under vacuum with vigorous magnetic stirring for 60 min. After the mixture was completely dissolved, the temperature was increased to 170 °C. Cs-OA solution (1.0 mL) was quickly injected into the mixture, and the mixture was cooled down to room temperature using a water bath after 5 s. For purification, 20.0 mL of ethyl acetate was added to the crude solution followed by centrifugation for 10 min at 7,000 rpm. The resulting precipitate was dispersed in 10.0 mL hexane. The NC solution was centrifuged again for 5 min at 4,500 rpm and the clear supernatant containing NCs was collected for further characterization.
2.2 Characterization
X-Ray Diffraction (XRD) patterns were obtained using an R-AXIS RAPIDII single crystal X-ray diffractometer operated at 40 KV and 30 mA with Cu Kα radiation. Steady-state absorption measurements were carried out with UV-Vis absorption spectrometer (Purkinje, TU-1810PC). Typical transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) images analysis are taken on a FEI Tecnai G2 F20.
2.3 Transient absorption spectroscopy
The laser pulse was generated from laser system (Coherent) and the output laser was split into two parts. One of them was used to generate a white light continuum and the other was used to pump optical parameters amplifier (Coherent, TOPAS) to generate pump pulse to excite the sample. The time-dependent TA spectra was collected using a highly sensitive spectrometer (Avantes AvaSpec-2048 × 16), whose spectral response range was 200−1100 nm.
3. Results and discussion
Considering the CsPb0.93Cd0.07Br3 NCs owns much better PL characteristics than the pure CsPbBr3 NCs, [23] CsPb0.93Cd0.07Br3 NCs is chosen as the research target (named #1) and CsPbBr3 NCs (named #2) acts as a reference sample in this work. The UV-vis absorption and PL spectra of #1 and #2 measured at room temperature are given in Fig. 1(a), exhibiting that adding a slight amount of Cd2+ causes absorption edge blue shifts, meanwhile the PL peak also blue shifts from 516 to 511 nm. This blue shift phenomenon originates from the broadening of band gap, which is attributed to lattice shrinkage, since a part of the Pb is replaced by Cd with a smaller particle radius [24]. (Schematic representation is shown in Fig. 1(b)). As shown in Fig. S1, the transmission electron microscopy (TEM) images and high resolution TEM (HR-TEM) exhibit that the size of #1 and #2 are both about 13 nm, reflecting that adding a slight amount of Cd element has almost no effect on size of nanocrystals. Their XRD patterns are given in Fig. 1(c), showing that the diffraction peak shifts to the high degree region and the crystallinity is improved after adding a slight amount of Cd element, which is also assigned to the lattice shrinking and the restoring of distorted [PbBr6]4+ octahedra [23]. The PLQYs of #1 and #2 in hexane solution are measured based on an integrating sphere setup, showing that the PLQY increases from 73% to 91% at room temperature, which should be attributed to the improvement of photo-generated carrier radiative recombination rate mediated by adding Cd element. The environment temperature (TE)-dependent PL spectra of #1 is probed as shown in Fig. 1(d), showing that their PL signal gradually enhance and PL peak red shift simultaneously. The PL spectra of #2 at different TE follows the same tendency (as shown in Supplement 1 Fig. S2). The full width at half maxima (FWHM) of #1 and #2 as a function of TE is given in Fig. 1(e), offering that the longitudinal optical phonon energy (ELO) of #1 and #2 is 18.0 and 15.7 meV, according to the Bose-Einstein statistics whose calculation details are shown in the Supplement 1. This implies that adding Cd element can really restrict the lattice vibration of CsPbBr3 NCs and enhance the ELO in comparison with that of un-doped counterpart, which should be originated from the variance of XRD in Fig. 1(c).
Fig. 1. (a) Absorption and PL spectra of #1 and #2; (b) Schematic representation of B-site substitutional doping of lead halide perovskites with Cd2+; (c) XRD patterns of #1 and #2; (d) TE -dependent steady-state PL spectra of #1; (e) FWHM of PL spectra as a function of TE for #1 and #2. Inset: the reference bars in the bottom of (b) corresponds to the cubic phase (PDF #75-412).
Herein, the transient absorption (TA) spectroscopy is employed to probe the photo-generated carriers behaviors occurring in #1 and #2, detailed information of TA measurement is provided in the Experement. The two-dimensional TA plot of #1 in Fig. 2(a) exhibits a red photo-bleaching (PB) signal at about 2.5 eV and a blue photo-induced absorption (PIA) below bandgap (2.36 eV), where the PIA signal disappears rapidly in comparison with PB signal. As shown in Supplement 1 Fig. S4(a), the TA spectra of #2 shows a similar spectral feature. The pump intensity-dependent bleaching signal of #1 is given in Fig. 2(b), which can offer the absorption cross section (σ) of #1, based on the Poisson distribution. And the σ of #1 and #2 at different TE are exhibited in Fig. 2(c). Therefore, the N corresponding to the average photons of per nanocrystals can be calculated. Figure 2(b) offers the PB signal dynamics curves of #1 at different N, which becomes more and more complicated with N, suggesting that the high-order carrier combination kinetics participate in the bleaching relaxation behavior. Herein, the carrier density dependence of carrier recombination kinetics can be deeply analyzed based on Eq. (1): [25]
where n denotes the carrier density, k1 is the monomolecular recombination rate constant related to carriers trapping, k2 is the bimolecular (electron–hole) recombination rate constant, and k3 is the third-order Auger recombination rate constant. After fitting, the recombination rate constants of #1 and #2 as a function of TE can be obtained and given in Figs. 2(e)-(g).Fig. 2. (a) TA spectra of #1 at 295 K; (b) pump intensity-dependent bleaching amplitude for #1 under 295 K; (c) σ of #1 and #2 at different TE. (d) charge carrier dynamics measured with TA spectroscopy of #1 as a function of 〈N〉; (e) Monomolecular recombination rate (k1), (f) bimolecular recombination rate (k2) and (g) Auger recombination rate constant (k3) of #1 and #2 at different TE. Note: The wavelength of pump laser is 400 nm.
Figure 2(e) displays that k1 of #1 is lower than that of #2, even at low temperatures. This effect results from that the doping of Cd element optimizes the lattice structure of CsPbBr3 NCs and passivates the defect states in CsPbBr3 NCs [23]. The related monomolecular recombination process is limited in this situation. The probability of annihilated carriers coming from defect states in #1 should be less than that of #2, thus numerous carriers can left in electronic energy band, accelerating the bimolecular recombination probability. As seen in Fig. 2(f), the k2 of #1 is higher than that of #2, which should be attributed to two reasons. Firstly, the restricting of monomolecular recombination process may cause a large number of photo-generated carriers left in the conduction band and participate the bimolecular recombination process. Secondly, the optimization of crystal structure induced by Cd element promote the free movement of photo-generated carriers through increasing their activity at the same excitation condition. In addition, the variance of k2 can be responsible for the improvement of PLQY, considering k2 is beneficial to the PL originated from the electron-hole recombination. In addition, it is found that the evolution of k3 as a function of temperature is much similar to that of k2, which may be assigned to the same reasons. If these Cd doped nanocrystals, acting as the active layer, are in the operating state of the optoelectronic device, the enhancement of carrier movement can facilitate the transport of carriers and further improve the performance of device.
The $N$-dependent TA spectra of #1 at initial 0.4 ps is given in Fig. 3(a), whose width gradually broadens owing to the enhancement of carrier heating and band filling effect [26,27]. The Maxwell-Boltzmann distribution (as described in the Supplement 1) is used to fit the broadening bleaching peak in the blue region, which offers the carrier temperature (TC) of photo-generated carriers. The $N$-dependent ΔTC (TC‒TE) is summarized in Fig. 3(b), in which the contribution of TE is excluded in TC and the pure carrier heating effect can be directly reflected by ΔTC. The ΔTC of #1 almost linear enhance with N and this tendency almost keeps invariable with the variance of TE. Apparently, increasing N can heat the carriers and change TC at the same time, which is suitable for both of #1 and #2 as seen in Fig. 3(c). Based on linear fitting, the slop in Figs. 3(b) and (c) can be extracted, which reflects the capability of heating effect of our samples. The slop of #1 and #2 at different TE is summarized in Fig. 3(d), exhibiting that the capability of heating carrier in #1 is apparently stronger than that of #2 at the same excitation condition, suggesting that adding Cd element can obviously improve the heating effect of carriers. It is known that carriers form a thermalized distribution with TC higher than that of lattice through carrier-carrier scattering after photoexcitation [28]. The addition of Cd element increases the effective dielectric constant of CsPbBr3 NCs, [29] causing the enhancement of carrier-carrier scattering rate through mediating the Coulomb interaction [30]. In this situation, the carriers in #1 can be easily heated through stronger carrier-carrier scattering rate and reach a higher TC in comparison with that of #2. It also can be seen that the ability of heating carrier in #1 and #2 together enhance with TE decreasing. Based on linear fitting, the slop of #1 (‒1.55) is be up to 2.4 times of magnitude larger than that of #2 (‒0.64), reflecting that the heating effect of #1 is much sensitive to TE in comparison with that of #2. If the photogenerated-carriers with high TC can be extracted, Cd doped CsPbBr3 materials own a huge potential in the fields of highly sensitive photo-detector or photo-catalysis, in comparison with the pure CsPbBr3 materials [31].
Fig. 3. (a) 〈N〉-dependent TA spectra of #1; 〈N〉-dependent ΔTc of #1 (b) and #2 (c); (d) heating effect of #1 and #2 as a function of TE. Extracting Tc by fitting higher-energy tails of TA spectra under 295 K.
The ΔTC relaxation process of #1 and #2 at different N are summarized in Figs. 4(a) and (b), reflecting that the TC cooling process in #1 and #2 together slow down with N. In order to analyze the relaxation mechanism of ΔTC, bi-exponential relaxation model is employed to fit the ΔTC decay curve, in which the rapid component corresponds to the phonon energy dissipation owing to the carrier-phonon scattering [32] and the slow component is attributed to the non-radiative Auger heating behaviors, [31] respectively. Their lifetimes as a function of N is summarized in Figs. 4(c) and (d). Apparently, the lifetime of phonon energy dissipation (τ1) and Auger heating effect (τ2) together prolongs with N. Generally speaking, the optical branch phonons can interact with the photo-generated carriers and accelerate its cooling process through phonon emission. When the specific phonon frequency occupancy rate is large enough at high N, the coupling effect between carrier and phonon is weaken. In this case, the energy transfer from carrier to phonon will be restricted and the TC will remain relatively stable for a long time at high N [33]. This can be responsible for the evolution of τ1 at different N. Meanwhile, the high N can prolong τ2 owing to facilitating the Auger reheating [34]. Therefore, the hot-phonon bottleneck effect and Auger heating effect together contribute to the prolonging of ΔTC relaxation lifetime (τa) with N. Actually, #2 also follows the same tendency as that of #1, as seen in Fig. 4(b). Note that τ1 in #1 is shorter than that of #2, reflecting the energy loss rate of carrier in #1 is much rapid owing to the carrier-phonon scattering, which is attributed to the variance of ELO as seen in Fig. 1 (e) [33]. In addition, it is noted that the aforementioned k3 of #1 is larger than that of #2, which can be responsible for the acceleration of related Auger heating process in the ΔTC cooling process of #1. Although the cooling of ΔTC in #1 is rapid compared to that in #2, the photo-generated carriers in #1 can keep high activity for a longer time than that of #2, owing to the strong heating effect of #1. For example, #1 can keep high temperature (≥ 500 K) for about 1.76 ps, but #2 can keep high temperature (≥ 500 K) for only 0.76 ps. In a word, the activity of photo-generated carriers in CsPbBr3 NCs is enhanced after adding Cd element. Therefore, it is reasonable to consider that the improvement of k2 and k3 is attributed to the enhancement of photo-generated carrier activity.
Fig. 4. $N$-dependent ΔTc of #1(a) and #2 (b). τ1, τ1 and τa of #1 (c) and #2 (d) as a function of N.
4. Conclusions
In conclusion, we analyze the role of Cd element in the improvement of photoluminescence quantum yield of CsPbBr3 NCs and attribute this improvement to the restricting of monomolecular recombination and enhancement of bimolecular recombination. Moreover, the heating effect of photo-generated carrier in Cd doping CsPbBr3 NCs is also improved, which apparently increases the carrier temperature and make the carriers keep high temperature for a long time, even though its cooling process is affected by the carrier-photon scattering and the Auger reheating effect. The increasing of carrier temperature after photoexcitation suggests that the activity of carrier apparently improved after introduction of Cd element, which can accelerate the free movement of carriers, leading to the enhancement of carrier recombination. In a word, the improvement of photo-physical characteristics of perovskite materials after adding Cd element should be attributed to the activity enhancement of photo-generated carriers, which is benefit for the extraction of carriers with high temperature and make the Cd doped perovskite materials exhibit a huge potential in the fields of optoelectronics and photo-catalysis. Our results confirm that the strategy of Cd element doping into perovskite NCs is much effective and suitable for further application in future.
Funding
National Natural Science Foundation of China (21573094); National Science Fund for Young Scholars (11904123); Open Project of State Key Laboratory of Superhard Materials, Jilin University (202005).
Disclosures
The authors declare no competing interests.
Data availability
All data in the paper and the supplementary materials are available.
Supplemental document
See Supplement 1 for supporting content.
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