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Color tunable halide perovskite CH3NH3PbBr3−xClx emission via annealing
Organic Electronics 26 (2015) 260–264
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Color tunable halide perovskite CH3NH3PbBr3xClx emission via annealing Mingyang Wei a,1, Yao-Hsien Chung a,1, Yi Xiao b, Zhijian Chen a,c,⇑ a
State Key Laboratory for Mesoscopic Physics and Department of Physics, Peking University, Beijing 100871, PR China The Affiliated High School of Peking University, Beijing 100871, PR China c New Display Device and System Integration Collaborative Innovation Center of the West Coast of the Taiwan Strait, Fuzhou 350002, PR China b
a r t i c l e
i n f o
Article history: Received 5 July 2015 Received in revised form 21 July 2015 Accepted 27 July 2015
Keywords: Perovskite PeLED Annealing Blue Color tuning
a b s t r a c t A fundamental step to design perovskite light emitting device (PeLED) is to properly control its emission color of lead halide perovskite layer. In this paper, we find the color of perovskite layer can be tunable with the annealing temperature. By decreasing the annealing temperature, a blue shift of emission peak for perovskite CH3NH3PbBr3xClx layer can be observed. Possible reasons for such phenomena were also investigated. We excluded the affinity difference of CH3NH3Cl and CH3NH3Br to get into perovskite. By synthesizing perovskite powder using precursors heated under vacuum or ambient condition, emission peak can also be tunable, suggesting decomposition of chloride may make contribution to the color tuning. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Very recently, inorganic–organic hybrid perovskite was investigated enthusiastically for solar cells due to its excellent optical and electronic properties [1,2]. The exciton binding energy of these perovskites is very small compared to that of organic semiconductors, as well as the long carriers diffusion length of 100–1000 nm and lifetime of 100 ns [3], which are beneficial for light harvesting. Besides the use for photovoltaic device, lead halide perovskite can also be applied as emitter [4] and quantum dots (QDs) phosphors [5,6] in light-emitting devices (LEDs) due to its high photoluminescence efficiency [7,8]. Interests for such application, especially perovskite light-emitting devices (PeLEDs) have grown intensively since last year (2014) [9–12]. The reported PeLEDs mainly focused on perovskite CH3NH3PbBr3, which is a green emitter [4,9–12]. However, emission color of PeLEDs also has the potential to be tuned to blue through chlorine doping in perovskite CH3NH3PbBr3xClx to result in wider energy gap [5,10]. Usual method to achieve such color tuning is by changing the molar ratio of PbCl2 and CH3NH3Br in precursors [10,13]. However, even using a mixture of PbCl2 and CH3NH3Br with the same molar ratio in precursors, literatures ⇑ Corresponding author at: State Key Laboratory for Mesoscopic Physics and Department of Physics, Peking University, Beijing 100871, PR China. E-mail address: [email protected] (Z. Chen). 1 State Key Laboratory for Artificial Microstructures and Mesoscopic Physics, Department of Physics, Peking University, Beijing 100871, PR China. http://dx.doi.org/10.1016/j.orgel.2015.07.053 1566-1199/Ó 2015 Elsevier B.V. All rights reserved.
reported different emission color of the final form of perovskite [13,14]. This phenomenon, which is unfavorable for PeLEDs, hasn’t been discussed explicitly as far as we know. In this paper, we found different annealing condition is the main reason for such emission color difference. Generally, lower annealing temperature will result in wider band width of perovskite, causing the blue shift of emission color. Our result suggests annealing temperature should be carefully considered for PeLEDs and varying annealing temperature can also be a useful way to tune emission color of PeLEDs. Through further investigation, we suggested the decomposition of chloride during annealing may cause emission color shift. 2. Experiment 2.1. Syntheses of CH3NH3Cl and CH3NH3Br CH3NH3Cl was synthesized by reacting methylamine (33 wt% in ethanol) and 33 wt% hydrochloride acid with the molar ratio of 1:1 in an ice bath for 2 h with stirring followed by vacuum drying and washing with diethyl. CH3NH3Br was synthesized in the same way by using hydrobromide acid (48 wt% in water) to replace hydrochloride acid. 2.2. Film fabrication The film of CH3NH3PbBr3xClx on quartz was formed by dropping precursor solution of CH3NH3Br (0.75 M) and PbCl2 (0.25 M)
M. Wei et al. / Organic Electronics 26 (2015) 260–264
in dimethylformamide (DMF) onto quartz, which was cleaned sequentially with detergent, pure water, and acetone for 20 min, respectively. The film formation was realized by spin-coating at 2000 rpm for 30 s under ambient condition. Then the film was annealed in air on a hot plate for 5 min under different temperature. Film of CH3NH3PbBr3xClx using precursor solution of CH3NH3Br (1.50 M) and PbCl2 (0.25 M) or CH3NH3Br (1.00 M), CH3NH3Cl (0.50 M) and PbBr2 (0.25 M) in DMF was formed on quartz in the same way under annealing temperature of 60 °C. 2.3. Preparation of perovskite powder Precursor solution of CH3NH3Br (0.75 M) and PbCl2 (0.25 M) in DMF was heated at 90 °C for 10 min under ambient condition or under low pressure (10 kPa), respectively. Then 1 mL solution under different pre-treatment was directly dropped into 30 mL acetone. After carefully removing the solvents, the remained powder was dried in air and collected for further characterization. Perovskite powder using precursor solution of CH3NH3Br (0.75 M) and PbBr2 (0.25 M) in DMF was formed for comparison using the same method without any pre-treatment. 2.4. Characterization PL spectra were detected by F-2500 fluorescence spectrophotometer. Structure analysis of powder and films were carried out by X-ray diffractometer (XRD, Philips Xpert) using Ni filtered Cu Ka (k = 0.154 nm, radiation at 45 kV and 40 mA) over the 2h range of 10–90°. 3. Results and discussion A mixture of PbCl2 (0.25 M) and CH3NH3Br (0.75 M) in DMF was spin-coated at 2000 rpm for 30 s on quartz. To study the influence of annealing temperature, the films were then heated on a hot plate under different temperature for 5 min. By varying the annealing temperature from 90 °C to 150 °C, PL peak wavelength of the final obtained perovskite thin films shifted from 470 nm to 550 nm, as shown in Fig. 1. It suggests that lower annealing temperature results in shorter wavelength (blue shift) emission. X-ray diffraction (XRD) pattern indicate that all the films of CH3NH3PbBr3xClx were perovskite structure as shown in Fig. 2(a). However it can be seen from Fig. 2(b) that the center of
(1 1 0) and (2 2 0) peaks shifted under different annealing temperature, suggesting smaller lattice constant for perovskite under lower annealing temperature. Comparing to the previously reported results [13,15], blue shift of emission and smaller lattice constant were resulted from higher chlorine content in perovskite. As a result, decreasing the annealing temperature can directly increase chorine content in perovskite CH3NH3PbBr3xClx, thus the PL peak wavelength of perovskite CH3NH3PbBr3xClx can be tuned. On this stage, we mainly suggested and examined two proposals for the loss of chlorine under high annealing temperature: (i) formation of CH3NH3Cl under high annealing temperature and low affinity of to get into perovskite comparing to CH3NH3Br; (ii) CH3NH3Cl decomposition and sublimation during the growth of perovskite film. The first proposal is based on the following reasons. During the growth of perovskite, chemical reaction for the case using PbCl2 as lead source in precursors may happen generally in the form:
2CH3 NH3 Br þ PbCl2 ¼ PbBr2 þ 2CH3 NH3 Cl
ð1Þ
The similar reaction between CH3NH3I and PbCl2 had been reported in literature of [16–18]. It was shown before that CH3NH3Cl may have lower affinity to get into perovskite comparing to CH3NH3Br during the growth of single crystal [15]. If such difference also exists for the case of perovskite thin films, the formation of CH3NH3Cl will direct lead to loss of chlorine content comparing to without CH3NH3Cl containing. To testify the first proposal, perovskite films from two types of precursors where CH3NH3Cl and PbCl2 were used as the chlorine resource, respectively, with the same Cl/Br molar ratio, were prepared (detail preparation methods were shown in Section 2.2). Annealing temperature was controlled to be 60 °C in order to eliminate the influence of possible CH3NH3Cl decomposition above 100 °C [19]. No great difference of emission peak for two films can be observed as shown in Fig. 3, considering large red shift of spectra under high annealing temperature in Fig. 1. Comparing to perovskite using PbCl2 as chlorine source, even blue shift of emission peak is observed for perovskite using CH3NH3Cl as chlorine source, suggesting larger chlorine content in perovskite. As a result, even though we didn’t exclude the existence of CH3NH3Cl under high annealing temperature, it is unlikely that affinity difference of CH3NH3Cl and CH3NH3Br to get into perovskite is the reason for large chlorine loss under high annealing temperature. The second proposal that decomposition of CH3NH3Cl under high annealing temperature results in chlorine loss is mainly based on the following reason. Thermal analysis suggested that during the growth of perovskite film using PbCl2 as lead source, CH3NH3Cl decomposition may happen in the form [19]:
CH3 NH3 Cl ¼ CH3 NH2 þ HCl
Fig. 1. PL spectra of perovskite films under different annealing temperature.
261
ð2Þ
And sublimation of CH3NH3Cl may also happen under high annealing temperature [17,20]. As escape of HCl can be significant under high temperature, reaction (2) will be enhanced greatly by increasing annealing temperature. High temperature can also increase sublimation rate of CH3NH3Cl. As both processes result in direct chlorine loss in perovskite, improving annealing temperature will cause lower chlorine content to give narrower band width of perovskite. To examine the second proposal, we heated a mixture of solution of PbCl2 (0.25 M) and CH3NH3Br (0.75 M) in DMF under 90 °C for 10 min under ambient pressure or under low pressure (10 kPa), respectively. The main reason for heating precursors under different conditions is to control the decomposition process happened in precursors. For chloride may exist in precursors either PbCl2 or CH3NH3Cl, their bulk counterparts have relatively high decomposition temperature [18,20,21]. As a result, even heating solutions under 90 °C, escape of chloride from decomposed
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Fig. 2. (a) XRD profiles, and (b) XRD profiles near (1 1 0) and (2 2 0) of perovskite films under different annealing temperature.
Fig. 3. PL spectra of perovskite using CH3NH3Cl and PbCl2 respectively as chlorine resource.
Fig. 4. PL spectra of perovskite powder.
CH3NH3Cl isn’t obvious. However, as low pressure heating can significantly enhance the decomposition process due to the enhanced escape of chloride such as HCl, thus through different treatment of precursors, we can observe whether decomposition and sublimation of CH3NH3Cl in precursors can result in chlorine loss in perovskite. To further clarify, solvent was also evaporated but no perovskite crystals were formed in precursors under such treatment. After different treatment, perovskite powder was prepared under ambient temperature from treated precursors. The main reason for the synthesis of perovskite powder under ambient temperature rather than spin-coating perovskite film is to make sure that decomposition can only happen during thermal treatment for precursors while spin-coating thin films need annealing process. In comparison, CH3NH3PbBr3 perovskite powder was also synthesized. PL spectra in Fig. 4 suggest decrease in band width for low pressure heated precursors comparing to precursors heated under ambient pressure. Though XRD pattern shown in Fig. 5(a) indicates the formation of perovskite for all the powder, center of
(1 1 0) and (2 2 0) of the XRD profiles in Fig. 5(b) shifted using different precursors. Perovskite powder from low pressure heated precursor had larger lattice constant than obtained from precursors heated under ambient pressure. These results are similar to the spin-coated perovskite under different annealing temperature, suggesting chlorine loss happened in perovskite from precursors under low pressure treatment with physical properties closer to CH3NH3PbBr3. As the main difference for the two precursors was that decomposition and sublimation of CH3NH3Cl was greater for low pressure treatment, and considering such process in spin-coated films can be especially enhanced by increasing annealing temperature [21], we suggest lower Cl/Br ratio in perovskite under higher annealing temperature may be a result of CH3NH3Cl decomposition and sublimation, though we cannot exclude other effect under higher annealing temperature on this stage. Thus, a more detailed chemical analysis is needed further to find out other possible process relating to chlorine loss. Based on the results above, we have shown the importance of CH3NH3Cl decomposition to the final obtained perovskite.
M. Wei et al. / Organic Electronics 26 (2015) 260–264
263
Fig. 5. (a) XRD profiles, and (b) XRD profiles near (1 1 0) and (2 2 0) of perovskite powder.
Though our proposal needs examined further, the results that annealing temperature can have an important impact on the Cl/Br molar ratio for spin-coated perovskite is clear. As a result, annealing temperature should be controlled carefully for the color stability of PeLEDs. And it is also an alternative way to tune the color of PeLEDs comparing to the traditional solutes variation methods. As we shown above, by using precursors with Cl/Br molar ratio 2:3, PL peak of perovskite layer from green (540 nm) to blue (470 nm) can be obtained under different annealing temperature. 4. Conclusion In conclusion, we found that during the growth of solution-processed perovskite CH3NH3PbBr3xClx using PbCl2 as lead source, lower annealing temperature can cause higher chlorine content of final formed perovskite layer which results in the blue shift of emission peak. We mainly proposed two mechanisms to explain such phenomena: (i) low affinity of CH3NH3Cl to get into perovskite; (ii) decomposition and sublimation of CH3NH3Cl during perovskite growth. Preparing perovskite thin film under low annealing temperature from precursor containing CH3NH3Cl as chlorine source, no chlorine loss can be observed comparing to precursor using PbCl2 as chlorine source. Thus, affinity difference of CH3NH3Cl and CH3NH3Br to get into perovskite may not be the main cause for chlorine loss under high annealing temperature. Synthesizing perovskite powder from precursors heated under low pressure using PbCl2 as lead source, a significant chlorine loss can be observed comparing to the precursors heated under ambient pressure. Since lower pressure can enhance decomposition and sublimation of CH3NH3Cl, we believe the direct decomposition and sublimation of CH3NH3Cl was the main reason for Cl/Br molar ratio decrease by increasing annealing temperature. As a result, annealing temperature should be carefully considered to get a proper Cl/Br molar ratio in CH3NH3PbBr3xClx perovskite which is especially important for PeLEDs. And controlling the annealing temperature is also an alternative way to tune the emission color of PeLEDs. Acknowledgements We acknowledge the financial support from the National Natural Science Foundation of China (61177020 and 11121091) and the National Basic Research Program of China
(2013CB328704). M. Wei is also supported by the President’s Fund for Undergraduate Research of Peking University.
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Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Color tunable halide perovskite CH3NH3PbBr3xClx emission via annealing Mingyang Wei a,1, Yao-Hsien Chung a,1, Yi Xiao b, Zhijian Chen a,c,⇑ a
State Key Laboratory for Mesoscopic Physics and Department of Physics, Peking University, Beijing 100871, PR China The Affiliated High School of Peking University, Beijing 100871, PR China c New Display Device and System Integration Collaborative Innovation Center of the West Coast of the Taiwan Strait, Fuzhou 350002, PR China b
a r t i c l e
i n f o
Article history: Received 5 July 2015 Received in revised form 21 July 2015 Accepted 27 July 2015
Keywords: Perovskite PeLED Annealing Blue Color tuning
a b s t r a c t A fundamental step to design perovskite light emitting device (PeLED) is to properly control its emission color of lead halide perovskite layer. In this paper, we find the color of perovskite layer can be tunable with the annealing temperature. By decreasing the annealing temperature, a blue shift of emission peak for perovskite CH3NH3PbBr3xClx layer can be observed. Possible reasons for such phenomena were also investigated. We excluded the affinity difference of CH3NH3Cl and CH3NH3Br to get into perovskite. By synthesizing perovskite powder using precursors heated under vacuum or ambient condition, emission peak can also be tunable, suggesting decomposition of chloride may make contribution to the color tuning. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Very recently, inorganic–organic hybrid perovskite was investigated enthusiastically for solar cells due to its excellent optical and electronic properties [1,2]. The exciton binding energy of these perovskites is very small compared to that of organic semiconductors, as well as the long carriers diffusion length of 100–1000 nm and lifetime of 100 ns [3], which are beneficial for light harvesting. Besides the use for photovoltaic device, lead halide perovskite can also be applied as emitter [4] and quantum dots (QDs) phosphors [5,6] in light-emitting devices (LEDs) due to its high photoluminescence efficiency [7,8]. Interests for such application, especially perovskite light-emitting devices (PeLEDs) have grown intensively since last year (2014) [9–12]. The reported PeLEDs mainly focused on perovskite CH3NH3PbBr3, which is a green emitter [4,9–12]. However, emission color of PeLEDs also has the potential to be tuned to blue through chlorine doping in perovskite CH3NH3PbBr3xClx to result in wider energy gap [5,10]. Usual method to achieve such color tuning is by changing the molar ratio of PbCl2 and CH3NH3Br in precursors [10,13]. However, even using a mixture of PbCl2 and CH3NH3Br with the same molar ratio in precursors, literatures ⇑ Corresponding author at: State Key Laboratory for Mesoscopic Physics and Department of Physics, Peking University, Beijing 100871, PR China. E-mail address: [email protected] (Z. Chen). 1 State Key Laboratory for Artificial Microstructures and Mesoscopic Physics, Department of Physics, Peking University, Beijing 100871, PR China. http://dx.doi.org/10.1016/j.orgel.2015.07.053 1566-1199/Ó 2015 Elsevier B.V. All rights reserved.
reported different emission color of the final form of perovskite [13,14]. This phenomenon, which is unfavorable for PeLEDs, hasn’t been discussed explicitly as far as we know. In this paper, we found different annealing condition is the main reason for such emission color difference. Generally, lower annealing temperature will result in wider band width of perovskite, causing the blue shift of emission color. Our result suggests annealing temperature should be carefully considered for PeLEDs and varying annealing temperature can also be a useful way to tune emission color of PeLEDs. Through further investigation, we suggested the decomposition of chloride during annealing may cause emission color shift. 2. Experiment 2.1. Syntheses of CH3NH3Cl and CH3NH3Br CH3NH3Cl was synthesized by reacting methylamine (33 wt% in ethanol) and 33 wt% hydrochloride acid with the molar ratio of 1:1 in an ice bath for 2 h with stirring followed by vacuum drying and washing with diethyl. CH3NH3Br was synthesized in the same way by using hydrobromide acid (48 wt% in water) to replace hydrochloride acid. 2.2. Film fabrication The film of CH3NH3PbBr3xClx on quartz was formed by dropping precursor solution of CH3NH3Br (0.75 M) and PbCl2 (0.25 M)
M. Wei et al. / Organic Electronics 26 (2015) 260–264
in dimethylformamide (DMF) onto quartz, which was cleaned sequentially with detergent, pure water, and acetone for 20 min, respectively. The film formation was realized by spin-coating at 2000 rpm for 30 s under ambient condition. Then the film was annealed in air on a hot plate for 5 min under different temperature. Film of CH3NH3PbBr3xClx using precursor solution of CH3NH3Br (1.50 M) and PbCl2 (0.25 M) or CH3NH3Br (1.00 M), CH3NH3Cl (0.50 M) and PbBr2 (0.25 M) in DMF was formed on quartz in the same way under annealing temperature of 60 °C. 2.3. Preparation of perovskite powder Precursor solution of CH3NH3Br (0.75 M) and PbCl2 (0.25 M) in DMF was heated at 90 °C for 10 min under ambient condition or under low pressure (10 kPa), respectively. Then 1 mL solution under different pre-treatment was directly dropped into 30 mL acetone. After carefully removing the solvents, the remained powder was dried in air and collected for further characterization. Perovskite powder using precursor solution of CH3NH3Br (0.75 M) and PbBr2 (0.25 M) in DMF was formed for comparison using the same method without any pre-treatment. 2.4. Characterization PL spectra were detected by F-2500 fluorescence spectrophotometer. Structure analysis of powder and films were carried out by X-ray diffractometer (XRD, Philips Xpert) using Ni filtered Cu Ka (k = 0.154 nm, radiation at 45 kV and 40 mA) over the 2h range of 10–90°. 3. Results and discussion A mixture of PbCl2 (0.25 M) and CH3NH3Br (0.75 M) in DMF was spin-coated at 2000 rpm for 30 s on quartz. To study the influence of annealing temperature, the films were then heated on a hot plate under different temperature for 5 min. By varying the annealing temperature from 90 °C to 150 °C, PL peak wavelength of the final obtained perovskite thin films shifted from 470 nm to 550 nm, as shown in Fig. 1. It suggests that lower annealing temperature results in shorter wavelength (blue shift) emission. X-ray diffraction (XRD) pattern indicate that all the films of CH3NH3PbBr3xClx were perovskite structure as shown in Fig. 2(a). However it can be seen from Fig. 2(b) that the center of
(1 1 0) and (2 2 0) peaks shifted under different annealing temperature, suggesting smaller lattice constant for perovskite under lower annealing temperature. Comparing to the previously reported results [13,15], blue shift of emission and smaller lattice constant were resulted from higher chlorine content in perovskite. As a result, decreasing the annealing temperature can directly increase chorine content in perovskite CH3NH3PbBr3xClx, thus the PL peak wavelength of perovskite CH3NH3PbBr3xClx can be tuned. On this stage, we mainly suggested and examined two proposals for the loss of chlorine under high annealing temperature: (i) formation of CH3NH3Cl under high annealing temperature and low affinity of to get into perovskite comparing to CH3NH3Br; (ii) CH3NH3Cl decomposition and sublimation during the growth of perovskite film. The first proposal is based on the following reasons. During the growth of perovskite, chemical reaction for the case using PbCl2 as lead source in precursors may happen generally in the form:
2CH3 NH3 Br þ PbCl2 ¼ PbBr2 þ 2CH3 NH3 Cl
ð1Þ
The similar reaction between CH3NH3I and PbCl2 had been reported in literature of [16–18]. It was shown before that CH3NH3Cl may have lower affinity to get into perovskite comparing to CH3NH3Br during the growth of single crystal [15]. If such difference also exists for the case of perovskite thin films, the formation of CH3NH3Cl will direct lead to loss of chlorine content comparing to without CH3NH3Cl containing. To testify the first proposal, perovskite films from two types of precursors where CH3NH3Cl and PbCl2 were used as the chlorine resource, respectively, with the same Cl/Br molar ratio, were prepared (detail preparation methods were shown in Section 2.2). Annealing temperature was controlled to be 60 °C in order to eliminate the influence of possible CH3NH3Cl decomposition above 100 °C [19]. No great difference of emission peak for two films can be observed as shown in Fig. 3, considering large red shift of spectra under high annealing temperature in Fig. 1. Comparing to perovskite using PbCl2 as chlorine source, even blue shift of emission peak is observed for perovskite using CH3NH3Cl as chlorine source, suggesting larger chlorine content in perovskite. As a result, even though we didn’t exclude the existence of CH3NH3Cl under high annealing temperature, it is unlikely that affinity difference of CH3NH3Cl and CH3NH3Br to get into perovskite is the reason for large chlorine loss under high annealing temperature. The second proposal that decomposition of CH3NH3Cl under high annealing temperature results in chlorine loss is mainly based on the following reason. Thermal analysis suggested that during the growth of perovskite film using PbCl2 as lead source, CH3NH3Cl decomposition may happen in the form [19]:
CH3 NH3 Cl ¼ CH3 NH2 þ HCl
Fig. 1. PL spectra of perovskite films under different annealing temperature.
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ð2Þ
And sublimation of CH3NH3Cl may also happen under high annealing temperature [17,20]. As escape of HCl can be significant under high temperature, reaction (2) will be enhanced greatly by increasing annealing temperature. High temperature can also increase sublimation rate of CH3NH3Cl. As both processes result in direct chlorine loss in perovskite, improving annealing temperature will cause lower chlorine content to give narrower band width of perovskite. To examine the second proposal, we heated a mixture of solution of PbCl2 (0.25 M) and CH3NH3Br (0.75 M) in DMF under 90 °C for 10 min under ambient pressure or under low pressure (10 kPa), respectively. The main reason for heating precursors under different conditions is to control the decomposition process happened in precursors. For chloride may exist in precursors either PbCl2 or CH3NH3Cl, their bulk counterparts have relatively high decomposition temperature [18,20,21]. As a result, even heating solutions under 90 °C, escape of chloride from decomposed
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Fig. 2. (a) XRD profiles, and (b) XRD profiles near (1 1 0) and (2 2 0) of perovskite films under different annealing temperature.
Fig. 3. PL spectra of perovskite using CH3NH3Cl and PbCl2 respectively as chlorine resource.
Fig. 4. PL spectra of perovskite powder.
CH3NH3Cl isn’t obvious. However, as low pressure heating can significantly enhance the decomposition process due to the enhanced escape of chloride such as HCl, thus through different treatment of precursors, we can observe whether decomposition and sublimation of CH3NH3Cl in precursors can result in chlorine loss in perovskite. To further clarify, solvent was also evaporated but no perovskite crystals were formed in precursors under such treatment. After different treatment, perovskite powder was prepared under ambient temperature from treated precursors. The main reason for the synthesis of perovskite powder under ambient temperature rather than spin-coating perovskite film is to make sure that decomposition can only happen during thermal treatment for precursors while spin-coating thin films need annealing process. In comparison, CH3NH3PbBr3 perovskite powder was also synthesized. PL spectra in Fig. 4 suggest decrease in band width for low pressure heated precursors comparing to precursors heated under ambient pressure. Though XRD pattern shown in Fig. 5(a) indicates the formation of perovskite for all the powder, center of
(1 1 0) and (2 2 0) of the XRD profiles in Fig. 5(b) shifted using different precursors. Perovskite powder from low pressure heated precursor had larger lattice constant than obtained from precursors heated under ambient pressure. These results are similar to the spin-coated perovskite under different annealing temperature, suggesting chlorine loss happened in perovskite from precursors under low pressure treatment with physical properties closer to CH3NH3PbBr3. As the main difference for the two precursors was that decomposition and sublimation of CH3NH3Cl was greater for low pressure treatment, and considering such process in spin-coated films can be especially enhanced by increasing annealing temperature [21], we suggest lower Cl/Br ratio in perovskite under higher annealing temperature may be a result of CH3NH3Cl decomposition and sublimation, though we cannot exclude other effect under higher annealing temperature on this stage. Thus, a more detailed chemical analysis is needed further to find out other possible process relating to chlorine loss. Based on the results above, we have shown the importance of CH3NH3Cl decomposition to the final obtained perovskite.
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Fig. 5. (a) XRD profiles, and (b) XRD profiles near (1 1 0) and (2 2 0) of perovskite powder.
Though our proposal needs examined further, the results that annealing temperature can have an important impact on the Cl/Br molar ratio for spin-coated perovskite is clear. As a result, annealing temperature should be controlled carefully for the color stability of PeLEDs. And it is also an alternative way to tune the color of PeLEDs comparing to the traditional solutes variation methods. As we shown above, by using precursors with Cl/Br molar ratio 2:3, PL peak of perovskite layer from green (540 nm) to blue (470 nm) can be obtained under different annealing temperature. 4. Conclusion In conclusion, we found that during the growth of solution-processed perovskite CH3NH3PbBr3xClx using PbCl2 as lead source, lower annealing temperature can cause higher chlorine content of final formed perovskite layer which results in the blue shift of emission peak. We mainly proposed two mechanisms to explain such phenomena: (i) low affinity of CH3NH3Cl to get into perovskite; (ii) decomposition and sublimation of CH3NH3Cl during perovskite growth. Preparing perovskite thin film under low annealing temperature from precursor containing CH3NH3Cl as chlorine source, no chlorine loss can be observed comparing to precursor using PbCl2 as chlorine source. Thus, affinity difference of CH3NH3Cl and CH3NH3Br to get into perovskite may not be the main cause for chlorine loss under high annealing temperature. Synthesizing perovskite powder from precursors heated under low pressure using PbCl2 as lead source, a significant chlorine loss can be observed comparing to the precursors heated under ambient pressure. Since lower pressure can enhance decomposition and sublimation of CH3NH3Cl, we believe the direct decomposition and sublimation of CH3NH3Cl was the main reason for Cl/Br molar ratio decrease by increasing annealing temperature. As a result, annealing temperature should be carefully considered to get a proper Cl/Br molar ratio in CH3NH3PbBr3xClx perovskite which is especially important for PeLEDs. And controlling the annealing temperature is also an alternative way to tune the emission color of PeLEDs. Acknowledgements We acknowledge the financial support from the National Natural Science Foundation of China (61177020 and 11121091) and the National Basic Research Program of China
(2013CB328704). M. Wei is also supported by the President’s Fund for Undergraduate Research of Peking University.
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