Photostability and photodegradation processes in colloidal CsPbI3

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Functional Nanostructured Materials (including low-D carbon)

Photostability and photodegradation processes in colloidal CsPbI3 perovskite quantum dots Rui An, Fengying Zhang, Xianshao Zou, Yingying Tang, Mingli Liang, Ihor Oshchapovskyy, Yuchen Liu, Alireza Honarfar, Yunqian Zhong, Chuanshuai Li, Huifang Geng, Junsheng Chen, Sophie Canton, Tõnu Pullerits, and Kaibo Zheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14480 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 23, 2018

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ACS Applied Materials & Interfaces

Photostability and photodegradation processes in colloidal CsPbI3 perovskite quantum dots Rui An†‡⊥, Fengying Zhang†§⊥, Xianshao Zou†, Yingying Tang∥, Mingli Liang∥, Ihor Oshchapovskyy#^, Yuchen Liu†, Alireza Honarfar†, Yunqian Zhong†, Chuanshuai Li†, Huifang Geng¤, Junsheng Chen†, Sophie E Canton¤&, Tõnu Pullerits*†, Kaibo Zheng*†∥ †

Division of Chemical Physics and NanoLund, Department of Chemistry, Lund University, Box

124, SE-22100 Lund, Sweden ‡

Department of Chemistry “G. Ciamician”, University of Bologna, Via F. Selmi 2, 40126

Bologna, Italy § Key

Laboratory of Luminescence and Real-Time Analytical Chemistry of Ministry of Education,

College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, People’s Republic of China ∥

Department of Chemistry, Technical University of Denmark, DK-2800 Kongens Lyngby,

Denmark #

Center for Analysis and Synthesis (CAS), Department of Chemistry, Lund University,

Box 124, SE-22100 Lund, Sweden ^

Department of Inorganic Chemistry, Ivan Franko National University of Lviv, Kyryla i

Mefodiya Str. 6, 79005 Lviv, Ukraine

ACS Paragon Plus Environment

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¤ ELI-ALPS, ELI-HU Non-Profit Ltd., Dugonicster 13, Szeged 6720, Hungary & Attoscience Group, Deutsche Elektronen Synchrotron (DESY), Notkestrasse 85, D-22607 Hamburg, Germany

AUTHOR INFORMATION Corresponding Author *T.P.: E-mail: [email protected] *K.Z.: E-mail: [email protected] Author Contributions ⊥

These authors contributed equally to this work.

ABSTRACT All-inorganic CsPbI3 perovskite quantum dots (QDs) have attracted intense attention for their successful application in photovoltaics and optoelectronics that are enabled by their superior absorption capability and great photoluminescence (PL) properties. However, their photostability remains a practical bottleneck and further optimization is highly desirable. Here we studied the photostability of as-obtained colloidal CsPbI3 QDs suspended in hexane. We found that light illumination does induce photodegradation of CsPbI3 QDs. The steady-state spectroscopy, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM) and transient-absorption spectroscopy (TA) verified that light illumination leads to detachment of the capping agent, collapse of the CsPbI3 QDs’ surface, and finally aggregation of surface Pb0. Both dangling bonds contained surface and Pb0 serve as trap states


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causing PL quenching with dramatic decrease of PL quantum yield. Our work provides a detailed insight about the correlation between the structural and photophysical consequences of the photodegradation process in CsPbI3 QDs and may lead to the optimization of such QDs towards device applications.

KEYWORDS light illumination, trap states, blue-shift emission, surface collapse, photodegradation mechanism 1. INTRODUCTION Hybrid organic-inorganic lead halide perovskites have dominated the development of emerging photovoltaics devices over one decade due to their magnificent optical properties with the state of the art power conversion efficiency exceeding 22%.1–6 However, the chemical instability of these semiconductors, specially towards heat and moisture, has becomes the major obstacle for their future utilization.7,8 In this scenario, all-inorganic cesium lead halide perovskite (CsPbX3, X=I, Br, Cl) is emerging as promising alternative with enhanced stability, while retaining photovoltaic performance compared to their organic-inorganic hybrid conterparts.9–12 However, the stability of bulk CsPbI3 (cubic phase, α-phase) with the most appropriate optical band gap (1.73 eV) for light harvesting in PVs, suffers from the undesired phase transition to the yellowish orthorhombic phase (δ-phase) at room temperature.13–15 In order to solve this problem, carefully engineering of the precursors is needed during the sample crystallization.16 On the other hand, Swarnkar et al. found that colloidal CsPbI3 quantum dots (QDs) can perfectly stabilize the cubic perovskite phase due to


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the large contribution of surface energy and can therefore serve as stable building blocks for efficient PVs.13 Perovskite materials for PV cells were first introduced by Kojima et al. back in 2009,17 and solid state perovskite solar cells with lead iodide perovskite were reported in 2012.18 Solar cells based on CsPbI3 QDs can reach a record PCE of 13.43% among all QDs solar cells realized so far.19 In addition, the high photoluminescence (PL) quantum yield of CsPbI3 QDs (6090%) makes them a promising material for light emitting devices.20–22 Although the α-phase structure has been proven to be stable for CsPbI3 QDs, the correlation between structural stability and photostability still needs to be further studied, especially since the photo-induced charge carrier dynamics would also be dominated by the local imperfection of the lattice (e.g. defects, dangling bonds, etc.) generally acting as sub-band gap trap states. Our previous work has demonstrated that light illumination could generated additional trap states in CsPbBr3 QDs.23 In this letter, we simultaneously monitored the evolution of the structure as well as the photophysical behavior under long-term light illumination by combining structural characterization (TEM, XPS, and FTIR) and spectroscopic investigation (absorption, PL, TRPL, and TA). We found that although the basic cubic phase of QDs can be retained, the light illumination induces the detachment of the capping agent, the collapse of the CsPbI3 QDs’ surface, and finally the aggregation of surface Pb0. The surface states and Pb0 centers both serve as trap states that quench the PL emission. Our work extends the understanding of the photodegradation mechanism in CsPbI3 QDs beyond the well-known phase transition, which could be the reference to optimize the future design of such materials for future applications. 2. RESULTS AND DISCUSSION


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In order to clarify the photodegradation of the QDs, we stored the as-synthesized samples in different light conditions (light illumination, light illumination with a filter, dark) and different temperature conditions (room temperature, 4 °C) for comparison (see Table S1 in supporting information). We first confirmed that no pronounced degradation of the QDs can be observed when stored in the dark conditions up to several days indicating that the QDs are stable without light irradiation. The evolution of the structures and optical properties of the QDs under continuous white light illumination with excitation intensity of 100 mW/cm2 has then been monitored. Figure 1a illustrates the time evolution of the absorption spectra of the CsPbI3 QDs. The optical absorption edge of the CsPbI3 QDs is located at 650 nm, being consistent with the previous report.20 We find that both the absorbance and the absorption edge stay constant over the period of continuous light illumination, which indicates that no change occurs in the concentration and intrinsic structure of the QDs. The X-ray diffraction (XRD) patterns in Figure 1b also prove that the main lattice structure of the QDs is retained during the illumination. This is the typical cubic phase of CsPbI3 QDs (Figure 1b),20,24,25 where no pronounced shift or modification of the diffraction peaks have been observed. This is consistent with the reported conclusion, the colloidal form of the QDs is beneficial to prevent the phase transition from meta-stable α phase to δ phase due to the stabilization of the long-chain carboxylic and oleyammonium surface ligands.26


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Figure 1 Absorption spectra (a) and XRD patterns (b) of CsPbI3 QDs under different period of illumination

Despite of the stable main structure of CsPbI3 QDs, the degradation in photoluminescence (PL) is obvious. As shown in Figure 2, the PL intensity of the sample decreases dramatically after light illumination (Figure 2a). Such a decrease of the PL emission is not due to the decomposition of the QDs as evidenced by the constant absorbance mentioned above but rather the result of the decreased PL quantum yield (QY) of the QDs, which is displayed in Figure 2b (blue line). The absolute PL QY of the QDs decreased from 60% (directly after synthesis) to < 1% (after 24h of illumination). This means that the non-radiative recombination of the charges gradually dominates the recombination dynamics of the photo-generated charges. In order to verify this hypothesis, the emissive state dynamics have been characterized by PL decays using time-correlated single photon counting (TCSPC), as shown in Figure 2c. An obvious decrease of the PL lifetime following light illumination can be observed. All the PL kinetics can be globally fitted by four exponential decay components with lifetime 0.9 ns, 4.9 ns, 19 ns and 65 ns. Figure 2d illustrates the evolution of the contributions from those components during the degradation process represented by the change in the amplitude of the components. At the early stage of the light illumination (< 2h), only three components with lifetimes of 4.9 ns, 19 ns and 65 ns can be extracted (region I of Figure 2d). The


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faster component with lifetime of 0.9 ns emerges after 2h concurrent with the diminishing of the longest 65 ns component (region II of Figure 2d). First, we should notice, the fast components (0.9 ns and 4.9 ns) and slower components (19 ns and 65 ns) should not represent the dynamics coexistent in the same QD otherwise the huge time difference will make the slow decay processes dominantly surpassed and cannot be resolved in TRPL. In the regard, the overall PL QY (η) of all the QDs could be calculated as the fraction of radiative recombination among all the recombination processes of the charge carrier as:

η = ∑ 𝐴 𝑘𝑖

∑𝑖𝐴𝑖𝑘𝑖𝑟𝑎𝑑

𝑖 𝑖 𝑟𝑎𝑑

+ ∑𝑗𝐴𝑗𝑘𝑗𝑛𝑜𝑛𝑟𝑎𝑑

(1)

where krad and knonrad refer to the rates of the radiative and non-radiative recombination processes, respectively, and Ai,j represents the amplitude of each decay component. If we assume that the 0.9 ns and 4.9 ns components are related to non-radiative recombination processes and the 19 ns and 65 ns components to radiative processes, the calculated PLQY (red curve in Figure 2b) complies well with the experimental data (blue curve in Figure 2b). In this scenario, two different emission processes with lifetime of 19 ns and 65 ns should exist in the QDs. As shown in Figure 2d, the emission of the QDs should merely be mirrored by 19 ns component, while the 65 ns component becomes negligible after 5h light exposure. Concurrently, a clear blue shift of the PL spectra with maximum at 1.85 eV (672 nm) can also be observed after 6 h of light illumination in Figure 2a. We can then deconvolute the PL spectrum of the fresh sample into two main components with peak position at 1.85 eV and 1.82 eV (inset of Figure 2a). As the first PL component at 1.85 eV exhibits a narrower bandwidth, it should be attributed to the intrinsic band edge exciton emission correlating with the 19 ns PL decay component measured in TCSPC. The second PL component at 1.82 eV with boarder bandwidth should be assigned to the band to band emission of dissociated


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free charge carriers in the QDs with lifetime of 65 ns in TCSPC. Such coexistence of WannierMott excitons and free charge carriers in perovskite QDs resembles the ion-electron balance in a hot plasma according to the Saha Langmuir model and has been discussed in our previous work.27 It should also be noticed that there is a minor non-radiative charge recombination process with lifetime of 4.9 ns accounting for the PLQY of the fresh samples not being 100%. Due to the highly ionic structure of the all-inorganic CsPbX3 perovskites (X=Cl, Br, or I), the volume defects from the lattice deficiency should be negligible in CsPbI3 QDs.20 Therefore, the quenching is more likely induced by the surface dangling bonds at the places where the surface atoms are not covered by the capping agent, which is unavoidable in hot injection synthesis.28 Apart from the surface trapping by the dangling bonds, another nonradiative recombination process with faster rate (0.9 ns) emerges after photo-degradation as shown in Figure 2d, meaning that a new quenching channel different from the surface trapping has been introduced during the degradation. For comparison, besides the sample under direct LED irradiation, we changed the irradiation power by using a 720 nm long pass filter in the stability test (see supporting information). The results (Figure S1 and S2) show that, the light excitation is accountable for the main effect of the degradation process, even with low excitation intensity, light can still induce photodegradation to some extent.


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Figure 2 CsPbI3 QDs under illumination: photoluminescence spectra (a), quantum yield (QY) change (b), time-correlated single photon counting (TCSPC) spectra (c) and ratio of amplitude of the 4 components respectively (d)


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Figure 3 Transient absorption spectra of fresh sample (a) and degraded sample (b) and the respective SVD fittings (c) and (d)

We measured the optical transient absorption (TA) of both fresh and degraded (i.e. illuminated after 24h) samples in order to confirm the evolution of their photophysics during the degradation process. The TA spectrogram of both samples exhibit characteristic band edge ground state bleach at around 700 nm (Figure 3a&b), which is due to the state filling near the conduction band maximum (CBM) and valence band minimum (VBM) of the QDs after excitation. Singular value decomposition (SVD) fitting of the TA data yields three decay components for both samples. The fast component with lifetime of 215 ps for the fresh sample and 87 ps for the degraded sample should be assigned to the recombination of multiexcitons which is not present in TCSPC as the laser fluence in TA measurement (1×1013 ph/cm2) is about two orders of magnitude higher. In addition, we can extract slow components with lifetimes of 5 ns and 20 ns for the fresh sample and 1 ns and 5 ns for the degraded sample, respectively. The lifetimes of the TA decay components here are consistent with the TRPL results in Figure 2d confirming the proposed charge carrier


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dynamic model during the degradation process mentioned above and summarized in Figure 4. For as prepared QDs, after photoexcitation, one fraction of the photo-generated excitons directly recombines radiatively with lifetime of 19 ns while another dissociates into free charges, with much slower band to band recombination time of 65 ns (Figure 4a). In the same time, some QDs would have surface dangling bands leading to the PL quenching within 4.9 ns. During the degradation process, in the major pool of the QDs, all the long-lived free charge radiative recombination and the excitonic recombination are quenched by the new 0.9 ns trapping process (Figure 4b).

Figure 4 Illustration of the photophysical pathway of fresh QDs (a) and degraded QDs (b) after photoexcitation above the band gap of the QDs

In order to visualize the structural origins of the changes in photophysics during degradation, we performed a series of characterization on the QDs extracted from different period during the degradation process. The transmission electron microscopy (TEM) images for those samples (Figure 5a-h) shows the gradual change of QD morphology from perfect cubic shape to blurred-


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edged sphere shape during the degradation., The mean size of the QDs (13.7 nm) and main lattice structure (evidenced by the same spacing between crystal planes of 0.62 nm) nevertheless keep constant. These are clear indications that the bulk structure of the QDs retain while the surface is decomposed. Furthermore, from the Fourier-transform infrared (FTIR) spectra (Figure 6a) which is in accordance with previous research29, we could easily find that both N-H stretching band and COO- stretching band were greatly diminished during the photodegradation process, which means that both capping agent oleylamine and oleic acid are largely removed. It should be noted that the photo-degradation process mentioned above is negligible if the photon energy of the illumination light is below the optical band gap of the QDs (for details see SI). In other words, it should be the photo-excited charges rather than heat that induce the photochemical reaction in the QDs. We notice the thermal induced quenching of perovskite QDs has been observed in solid form in LED devices. However, the heat dissipation for the suspended individual QDs in solution form should be more efficient due to the large surface area which could be the reason for the less pronounced heat effect in our sample. Similar light-induced removal of surface capping agent has also been observed in CsPbBr3 QDs in our previous research.23 According to the above discussion, the dissociated free charge carrier should be initially delocalized through the QD volume and diffuse to the QD surface. The surface charges could protonate the amino groups or carboxyl groups in the capping agents which would then weaken their strong affinity to the surface atoms and facilitate the detachment of the capping agents.30 Consequently, dangling bonds will be induced on the surface, serving as the PL quenchers as discussed above.


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Figure 5 TEM images and diameter distribution of fresh sample (a) and (b), degraded sample (c) and (d); (e) -(h) show the magnification of corresponding individual NPs.

Besides the surface defects induced by the detachment of the capping agent, we can also observe a gradually enhanced lattice deficiency exhibited as black spots in the TEM images during the photo-degradation as shown in Figure 5e-h (yellow dashed cycles). Such high contrast particle have been widely observed in the TEM images of lead halide perovskite NPs (typical CsPbBr3 NPs). They are interpreted as an electron-beam induced transformation, which involves electron stimulated desorption of Br and reduction of Pb2+ to Pb0.31 As the TEM measurements were performed at high incident electron energy (200 keV) with weak inelastic interaction and the TEM images are taken on the fresh spots without long term beam exposure (< 1 min), an electron-beam induced transformation can be excluded here. On the other hand, the formation of the Pb clusters can be induced by the photo-illumination, resembling the photochemical reaction under electronbeam but with much slow rate. In addition, we further proved the formation of Pb0 clusters in the QDs from the red shift of Pb 4f core level peaks in X-ray photoelectron spectroscopy (XPS) as


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shown in Figure 6b. These shifts reflect the oxidation states changing from Pb2+ to Pb0 after photoillumination. The density function theory (DFT) calculation for conventional lead halide perovskites (e.g. MAPbI3 or FAPbI3) confirmed that the energy level of interstitial Pb point defects resides within the bandgap of the perovskites as deep trap states,32 which indicates the Pb0 clusters in our QDs could also be effective traps for the charge carriers. The schematic in Figure 6c illustrates the structural evolution of the CsPbI3 QDs upon photodegradation. The surface decomposition together with the formation of Pb0 cluster both contribute to the photo-degradation and the declining PLQY in our CsPbI3 quantum dots.

Figure 6 (a) FTIR spectra, (b) XPS Pb 4f core level spectra of the samples upon photo degradation, and (c) Schematic illustration of the morphology evolution and surface ligand upon photodegradation.


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3. CONCLUSIONS We have investigated the photo-stability of hot injection prepared colloidal CsPbI3 QDs in solution under light irradiation. We found that in spite of the stable size and intrinsic cubic lattice structure, the PL emission is drastically quenched with reduced PLQY during continuous light illumination of the QDs. Time-resolved spectroscopic studies revealed that light illumination not only enhances the surface trapping of the photogenerated charges but also creates new trapping centers in the volume of the QDs. The structural characterization of the samples illustrated that the enhancement of the surface trapping is attributed to the detachment of the capping agents, leaving defective surface while the new trapping centers originate from the formation of the Pb0 under light illumination. It should also be noted that in this paper we focused on the intrinsic photodegradation process in solution form of the QDs. Such degradation of QDs could be more serious when assembled in the solids due to the additional influence from the atmosphere. Our results provide inspiration for possible strategies to improve the photostability of CsPbI3 QDs : (1) To engineer the surface ligand to improve their bonding to the QDs surface free from the detachment under light illumination; (2) To design a core-shell structure for CsPbI3 QDs, which could passivate the surface of QDs leading to better photostability.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Experimental details, characterization methods, the thermal degradation as comparison and the


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XPS spectra (PDF)

AUTHOR INFORMATION Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the Independent Research Fund Denmark-Sapere Aude starting grant (No. 7026-00037A) and Swedish Research Council VR starting grant (No. 2017-05337). R.A. acknowledges financial support from the European Commission's Erasums+ Program, F.Z., L. L, Y. L., C. L., and X.Z. acknowledge financial support from China Scholarship Council, SEC acknowledges funding from the Helmoltz recognition award. The ELI-ALPS project (GINOP2.3.6-15-2015-00001) is supported by the European Union and co-financed by the European Regional Development Fund.

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Photostability and photodegradation processes in colloidal CsPbI3

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