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Letter 2
Strontium Doped Low Temperature Processed CsPbIBr Perovskite Solar Cells Cho Fai Jonathan Lau, Meng Zhang, Xiaofan Deng, Jianghui Zheng, Jueming Bing, Qingshan Ma, Jincheol Kim, Long Hu, Martin A. Green, Shujuan Huang, and Anita W. Y. Ho-Baillie ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00751 • Publication Date (Web): 11 Sep 2017 Downloaded from http://pubs.acs.org on September 12, 2017
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ACS Energy Letters
Strontium Doped Low Temperature Processed CsPbI2Br Perovskite Solar Cells Cho Fai Jonathan Lau, Meng Zhang,* Xiaofan Deng, Jianghui Zheng, Jueming Bing, Qingshan Ma, Jincheol Kim, Long Hu, Martin A. Green, Shujuan Huang, and Anita Ho-Baillie* Australian Centre for Advanced Photovoltaics, School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney 2052, Australia
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Abstract: Cesium (Cs) metal halide perovskites for photovoltaics have gained research interest due to its better thermal stability compared to their organic-inorganic counterparts. However, demonstration of highly efficient Cs-based perovskite solar cells requires high annealing temperature which limits their use in multi-junction devices. In this work, low temperature processed cesium lead (Pb) halide perovskite solar cells are demonstrated. We have also successfully incorporated the less toxic strontium (Sr) at a low concentration that partially substitutes Pb in CsPb1-xSrxI2Br. The crystallinity, morphology, absorption, photoluminescence and the elemental composition of this low-temperature processed CsPb1-xSrxI2Br are studied. It is found that the surface of the perovskite film is enriched with Sr providing a passivating effect. At the optimal concentration (x = 0.02), a mesoscopic perovskite solar cell using CsPb0.98Sr0.02I2Br achieves a stabilised efficiency at 10.8 %. This work shows the potential of inorganic perovskite stimulating further development of this material.
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TOC GRAPHICS
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Lead halide perovskite solar cells have been considered as a competitive photovoltaic technology due to the rapid rise in power conversion efficiency (PCE) from 3.8 % to 22 % and ease of fabrication including low process temperatures. A barrier to wide spread use is the instability of these devices. In particular, organic lead halide perovskites have shown lower thermal stability. It is reported that there is significant decomposition after annealing MAPbI3 at 85 ºC1 resulting in dissociation of MAPbI3 into PbI2 and MAI. One alternative to improve the thermal stability is to replace the organic cation with an inorganic component such as Cs2 which has higher melting point3 (460ºC). Notable results for CsPbBr3 solar cells include the 6.0% CsPbBr3 device by Kulbak et al. 4,5. However, the bandgap of CsPbBr3 is 2.3 eV which is too high to be used even in multi-junction tandem solar cells6. On the other hand, the bandgap of CsPbI3 (1.73 eV) is more suitable for photovoltaic application7,8 but is unstable in the black perovskite phase at ambient temperature. Incorporating Br into CsPbI3 stabilises the perovskite structure and CsPbI2Br for example has a bandgap of ~1.9 eV 9, which is suitable for a top cell in a triple-junction device. Recently, CsPbI2Br solar cells has reached 10% efficiency using solution processing10–12, and 11.8% by co-evaporation13. However, all Cs perovskite devices demonstrating PCE higher than 10% require high annealing temperature (>250ºC), which could limit use in monolithic tandem solar cells since causing degradation of underlying layer or in a flexible device using substrates that tolerate lower processing temperature. It has been shown that partial substitution of organic cation with inorganic Cs or Rb stabilises the perovskite structure as well as producing better performance14,15. Apart from the substitution of the cation in the A site and the substitution of the halide in the X site in an ABX3 perovskite, it is possible to substitute the metal in the B site because of the high defect tolerance of 3dimensional perovskite. There are a few reports that use metals, such as Cd2+, Ca2+, Sn2+, Sr2+ and
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Bi3+ 16–20, to partially substitute Pb in Pb-based perovskites. Incorporating a small amount of different homo or heterovalent cations could substantially change the electronic properties and the optical properties of the perovskite. Motivated by these works, we incorporate strontium (Sr) into CsPbI2Br to fabricate perovskite thin films for the first time. Instead of using high temperature annealing for crystallization, we anneal the CsPbI2Br film at 100 ºC, producing comparable properties compared to high temperature counterparts which normally require annealing temperature more than 250ºC. In addition, we vary the stoichiometric ratio of Sr in CsPb1-xSrxI2Br films for characterization and solar cell device demonstration. Under the optimal condition where the molar concentration of Sr is x=0.02, the Sr doped CsPbI2Br mesoporous perovskite solar cells give the best performance. The champion reverse scan efficiency is 11.3% and the stabilised PCE is 10.8%.
Figure 1. a) XRD patterns of CsPbI2Br films and (b) light J–V characteristics of CsPbI2Br devices using different annealing temperature.
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CsPbI2Br thin films are obtained by dissolving CsBr, PbI2 stoichiometrically in a mixed solvent of N,N dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). Films were spin-coated and then annealed at 100°C or 310ºC on a hot plate to form a 250 nm thick perovskite film. X-ray diffraction (XRD) patterns of low temperature and high temperature annealed CsPbI2Br thin films are shown in Figure 1a. The Bragg peaks at 2θ = 14.6° and 29.5°, show that both films are well-oriented in the cubic (100) direction. CsPbI2Br annealed at low temperature has a broader FWHM than the film annealed at high temperature. This indicates that the crystal size of the low temperature processed CsPbI2Br is smaller than that from the high temperature processed sample. The top view scanning electron microscopy (SEM) in Figure S2 in Supporting Information shows that the crystal size of the low temperature processed CsPbI2Br film is around 200-500 nm while the crystal size of the high temperature processed CsPbI2Br film is around 1-2 µm. Solar cells annealed at these low and high temperatures are then fabricated with the structure of FTO/compact-TiO2 (c-TiO2)/mesoporous-TiO2 (mp-TiO2)/ CsPbI2Br / poly(3-hexylthiophene2,5-diyl) (P3HT)/Au. P3HT is chosen because of better stability compared to the commonly used Spiro-OMeTAD 21. The light current density versus voltage (J-V) characteristics under 1 sun illumination of these devices are shown in Figure 1b. The best device that uses a low temperature processed CsPbI2Br has a higher PCE (>7 %) than the best device that uses a high temperature processed CsPbI2Br (the photovoltaic parameters of the devices are shown in Figure S1 in the supporting information). To investigate the effect of partially substituting Pb in the low temperature processed CsPbI2Br, Sr is incorporated at different Sr2+ concentrations and the crystallinity and morphology of films are studied. XRD patterns of the CsPb1-xSrxI2Br films are shown in Figure
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2a. XRD peak intensity of the 14.6° and 29.5° peaks is reduced with increasing Sr2+ due to degradation of the film in the ambient environment during measurement. As the content of Sr2+ increases, no additional phase can be observed but the XRD peak at 20.4° is stronger, indicating that the preferred orientation of the perovskite is altered. However, it is hard to discern change in the perovskite peak location due to the reduced peak intensity and the similar ionic radius of Sr (118 pm) and Pb (119 pm), The XRD patterns of high temperature processed CsPb1-xSrxI2Br films are also measured (see Figure S3 in the Supporting Information). Identical peaks can be observed. In terms of crystallinity the low temperature CsPb1-xSrxI2Br is comparable to that of the high temperature processed counterpart.
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Figure 2. a) XRD patterns of low temperature processed CsPb1-xSrxI2Br films. SEM images of (b) CsPbI2Br, (c) CsPb0.99Sr0.01I2Br, (d) CsPb0.98Sr0.02I2Br, (e) CsPb0.97Sr0.03I2Br and (f) CsPb0.95Sr0.05I2Br. The inset SEM images are taken in the darker region. Figures 2b-f show the top view SEM of CsPbI2Br film and films with increasing Sr content. The reference CsPbI2Br film shows a rough surface and is composed of densely packed crystalline grains, with an average grain size of ~200-500 nm. The addition of Sr2+ drastically changes the morphology of the perovskite and results in the appearance of “snowflakes” which appear brighter under the SEM, see Figure 2. The amount of “snowflakes” increases with Sr2+ as shown in Figure 2b-f. Energy dispersive X-ray spectroscopy (EDS) mapping was carried out on the CsPb0.98Sr0.02I2Br film, with results shown in Figure S4 in the Supporting Information. As shown by EDS mapping, all the elements including Sr were distributed homogenously throughout out the film. This is also verified by the back-scattered electron (BSE) images of the CsPb0.99Sr0.01I2Br and CsPb0.97Sr0.03I2Br films (see Figure S5 in Supporting Information), showing negligible difference (same average atomic number) in film composition in the “snowflake” regions throughout the bulk of the film.
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Figure 3. XPS spectra for (a) Pb 4f and Sr 3d, and (b) I3d for CsPb1-xSrxI2Br film. Table 1. Sr/Pb atomic ratios of CsPb1-xSrxI2Br films.
Sr/Pb Atomic Ratio (%)
CsPbI2Br
CsPb0.99Sr0.01I2Br
CsPb0.98Sr0.02I2Br
CsPb0.97Sr0.03I2Br
CsPb0.95Sr0.05I2Br
Surface
N.A.
11.4
20.1
28.4
38.2
3 nm etched
N.A.
4.9
6.8
9.8
12.6
In order to study the effect of Sr2+ incorporation on the elemental composition at the surface of CsPb1-xSrxI2Br films, X-ray photoelectron spectroscopy (XPS) was performed, see Figure 3. As shown in Figure 5a, the Pb 4f spectrum for CsPbI2Br film shows 4f5/2 and 4f7/2 peaks at 137.9 and 142.8 eV respectively, corresponding to the Pb2+ cations. The I 3d3/2 and I 3d5/2 peaks are also evident in the I 3d spectrum for the CsPbI2Br film, see Figure 5b. After the incorporation of Sr, the Pb 4f5/2, Pb 4f7/2 , I 3d3/2 and I 3d5/2 peaks shift to higher binding energy, indicating that the chemical structures of the surface have been modified. In
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addition, Sr2+ cations at the surface of the film are evident with the presence of Sr 3d3/2 and 3d5/2 peaks at 134.4 and 136.3 eV respectively. The Sr/Pb atomic ratios at the surface of the CsPb1xSrxI2Br
films are summarized in Table 1. The Sr/Pb atomic ratio at the surface for the
CsPb0.98Sr0.02I2Br and CsPb0.95Sr0.05I2Br films are 0.20 and 0.38. At the depth of 3 nm, the Sr/Pb atomic ratio drops dramatically to 0.07 and 0.13 for CsPb0.98Sr0.02I2Br and CsPb0.95Sr0.05I2Br film. However, these ratios are higher than the molar ratios used in the synthesis of the films which are 0.02 and 0.05 respectively for the CsPb0.98Sr0.02I2Br and CsPb0.95Sr0.05I2Br films. This shows that the film surface is strongly enriched with Sr2+ compared to the bulk of the film. In addition XPS measurements of the film at the surface and at 10nm from the surface shown in Figure S6 show that the binding energy of Sr 3p peak shifted from 268.5 eV (at the surface) to 269.5eV (depth=10 nm) suggesting the possibility Sr oxide formation on the surface because for SrO, the Sr 3p peak has been reported to be between 268-269 eV
22,23
. Further investigation will
be required to ascertain if there is any correlation between the Sr2+ enrichment on the surface and the appearance of snowflakes on the surface under SEM.
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Figure 4. a) Absorbance spectra and (b) time-resolved PL (tr-PL) decay profiles of CsPb1xSrxI2Br films. Table 2. Typical lifetimes extracted from TCSPC for CsPb1-xSrxI2Br. Lifetime (ns)
CsPbI2Br
CsPb0.99Sr0.01I2Br
CsPb0.98Sr0.02I2Br
CsPb0.97Sr0.03I2Br
CsPb0.95Sr0.05I2Br
τ1
2.2
2
2.1
1.6
2.3
τ2
11.1
13.3
17.1
16.7
9.3
Optical and photo-luminescence measurements are also carried out on the CsPb1-xSrxI2Br films, see Figure 4. Figure 4a shows the absorption onset of CsPb1-xSrxI2Br is around 656-664 nm (1.87-1.89 eV) and there is no shift in the absorption onset with Sr content. The absorption of the film improves when Sr is incorporated as long as it is limited to less than 5%. The steady state PL spectra of these films also show peaks at around 660-665 nm (see Figure S7 in the Supporting Information) which are consistent with those in the absorbance spectra. Timeresolved PL (tr-PL) decays for CsPb1-xSrxI2Br/mp-Al2O3 glass are also measured and shown in Figure 4b. Insulating mesoporous Al2O3 layer was coated as the scaffold layer of the perovskite film to eliminate the effect electron extraction on the results 24. Using bi-exponential decay function, the PL decay traces were fitted to determine the decay times of the fast and slow components which are summarized in Table 2. The presence of the fast component (τ1) in the PL decay is commonly assumed to indicate the presence of defect trapping and the slow component (τ2) to correspond to the effective recombination lifetime. Defect trapping lifetime, τ1, of all the films is relatively the same with a value of 2ns. However, as Sr content increases from 1 % to 2 %, τ2, increases from 11.1 ns to 17.1 ns, suggesting better effective recombination lifetime. This suggests better surface
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ACS Energy Letters
passivation provided by the Sr enriched surface as evident in the XPS results. Previous work on organic metal halides has also observed the beneficial passivation effects when Sr is incorporated19. However as the Sr content increases further, e.g., at a concentration of 5%, the excess Sr2+ doping in the perovskite film enhances electron-hole recombination which will have a detrimental effect on photovoltaic performance of the CsPb0.95Sr0.05I2Br device.
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Figure 5. a) Light J–V characteristics under reverse scan and (b) EQE spectra and (c) photovoltaic parameters (with standard deviations) of FTO/c- TiO2/mp-TiO2/ CsPb1-xSrxI2Br /P3HT/Au devices as a function of Sr2+ concentration in the perovskite. Table 3. Photovoltaic parameters of FTO/c- TiO2/mp-TiO2/ CsPb1-xSrxI2Br/P3HT/Au champion devices under reverse scan.
PCE(%)
Jsc(mA/cm2)
Voc(mV)
FF(%)
CsPbI2Br
7.7
13.4
962
59.8
CsPb0.99Sr0.01I2Br
8.3
14.3
938
62.2
CsPb0.98Sr0.02I2Br
11.2
15.3
1043
69.9
CsPb0.97Sr0.03I2Br
9.8
14.2
999
69.2
CsPb0.95Sr0.05I2Br
6.4
11.2
927
61.3
To confirm the effectiveness of the Sr incorporation on device performance, FTO/c- TiO2/mpTiO2/ CsPb1-xSrxI2Br/P3HT/Au solar cells were fabricated. Results are shown in Figure 5 and Table 3. As Sr content in the perovskite increases, the short circuit current density Jsc increases as long as the amount of Sr2+ is small, i.e.,
