All-Inorganic CsPbBr3 Perovskite Solar Cells with 10.45% Efficiency

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All-Inorganic CsPbBr3 Perovskite solar cells with 10.45% Efficiency by Evaporation-Assisted Deposition and Setting Intermediate Energy Levels Xin Li, Yao Tan, Hui Lai, Shuiping Li, Ying Chen, Suwei Li, Peng Xu, and Junyou Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b06356 • Publication Date (Web): 30 Jul 2019 Downloaded from pubs.acs.org on July 31, 2019

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ACS Applied Materials & Interfaces

All-Inorganic CsPbBr3 Perovskite Solar Cells with 10.45% Efficiency by Evaporation-Assisted Deposition and Setting Intermediate Energy Levels Xin Li,1 Yao Tan,1 Hui Lai,1,2 Shuiping Li,1 Ying Chen,1 Suwei Li,1 Peng Xu1,2 and Junyou Yang*,1 1. State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, P.R. China. E-mail: [email protected] 2. China-Eu Institute for Clean and Renewable Energy, Huazhong University of Science & Technology, Wuhan 430074, P.R. China

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ABSTRACT: Nowadays, inorganic CsPbBr3 perovskite is emerging to be a promising candidate as light-absorbing layer in photovoltaic devices due to its excellent photoelectric property and superior stability under humidity and thermal attacks in comparison with organic cation based hybrid perovskites. However, the impure perovskite phase and severe interfacial charge recombination have limited the further improvement of device performance. In this work, a vapor-assisted solution technique was introduced to prepare high-purity CsPbBr3 film in perovskite solar cell (PSC). To further reduce the electron-hole recombination and enhance charge 2


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extraction, we introduced the novel intermediate energy level of manganese sulfide (MnS) as hole transport layer (HTL) in CsPbBr3 PSC. The as-optimized CsPbBr3 PSC based all inorganic transport layers delivers a PCE of 10.45% in comparison with 8.16% for the device free of intermediate layer, which is one of the highest PCEs achieved among the CsPbBr3 based PSC to date. Moreover, the optimized device retained 80% PCE of its initial efficiency over 90 days under 80% relative humidity (RH) at 85 C, indicating an excellent environmental tolerance to boost the commercial application of low cost, efficient and stable all-inorganic PSCs. KEYWORDS: Vapor assisted solution method, CsPbBr3 solar cell, MnS hole transport layer, interfacial engineering, long-term stability INTRODUCTION Recently, one of the most potential inorganic perovskite materials (CsPbBr3) has been employed as abosrber active layers in solar cells.1 Among organic-inorganic hybrid or other inorganic perovskites,2-12 it possesses stable crystalline structure at room temperature and high carrier mobility,13-17 which are significant to the efficiency of solar cells.18-20 However, the PCE improvement of inorganic CsPbBr3 PSCs is markedly limited by two aspects as follows.21-24 On the one hand, it was hard to obtain high CsPbBr3 3



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film quality and purity by typical spin-coating solution process due to huge solubility differences between CsBr and PbBr2 solutions.19, 25

That is to say, PbBr2 is easy to dissolved in DMF, but the

solubility of CsBr in methanol or ethanol is rather small. Tang et al. demonstrated that a multistep solution-processed strategy was introduced to obtain pure CsPbBr3 film via controlling deposition cycles of multilayer CsBr with monolayer PbBr2.19 However, this method is difficult to precisely control the amount of spin-coated CsBr precursor at each step, resulting in difficulty in duplication. Moreover, the crystallization of CsPbBr3 thin film usually needs high-temperature annealing process around 250

C,

leading to

increased fabrication cost and it will limit the selection materials for each layer in rigid or flexible devices.26-30 Yan et al. reported that a pyridine-vapor treated CsPbBr3 thin films can reduce the annealing temperature to 160 C.31 Please note that, the pyridine was identified by the World Health Organization as a carcinogen in the year of 2017. In a word, these methods are not appropriate for practical application. On

the

other

hand,

although

the

device

based

on

FTO/c-TiO2/m-TiO2/CsPbBr3/carbon shows superior stability under various conditions, its low PCE also limits its commercialization. 4


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The reason for low efficiency is that the large energy barrier at the CsPbBr3 (EVB= -5.6 eV)/Carbon (WF= -5.0 eV) interface significantly limits hole extraction, thus poor efficiency. Generally, organic Spiro-OMeTAD and P3HT can be used as HTLs in PSC to solve this problem. However, the high cost and chemical instability of organic conductive materials are detrimental to long-term operational perovskite devices. Recently, inorganic hole transport materials attract more and more attention in organic-inorganic hybrid PSC. Unfortunately, few inorganic materials have been reported as HTL in inorganic PSC. Hence, it is crucial to develop simple and cost effective technique to get high-purity inorganic perovskite and inorganic HTL for efficient and stable PSCs. In this scenario, a vapor-assisted solution technique to fabricate consistently uniform and pure CsPbBr3 films was demonstrated. By tuning the deposition thickness of CsBr, the phase conversion from CsPb2Br5 to CsPbBr3 and then to Cs4PbBr6 was precisely controlled to obtain high-quality CsPbBr3 film. To further reduce the electron-hole recombination and enhance charge extraction, we introduced the intermediate energy levels of MnS as HTL at the interface of CsPbBr3/carbon. The as-optimized CsPbBr3 PSC delivers a PCE of 10.45% in comparison with 8.16% for the device 5



without HTL. Moreover, the optimized device retained 80% PCE of its initial PCE over 90 days under harsh condition (80% RH, 85 C). Overall, this work not only presents a new avenue to fabricate high-purity CsPbBr3 films, but also provides an effective interface design strategy to enhance the device performance in all-inorganic PSCs that is likely to be applicable to improve other optoelectronic devices that use metal halide perovskites.

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RESULTS AND DISCUSSION

Figure 1. Schematic illustration of the evaporation-assisted solution method (EAS) to fabricate CsPbBr3 thin film. Figure 1 indicates the illustration of EAS method. The PbBr2/DMF solution was firstly spin-coated on the FTO/compact TiO2 (c-TiO2)/mesoporous TiO2 (m-TiO2) substrate. After annealing process, the white PbBr2 film was then placed into the vacuum chamber for the deposition of CsBr. In this evaporation process, the deposition parameters of CsBr film (e.g., deposition rate and thickness) can be precisely tuned. Subsequently, the as-deposited CsPbBr3 film turned to yellow and demonstrate large-scale 7



uniformity, shown in Figure S1, indicating that the PbBr2 is easy to react with the vapor of CsBr without further annealing temperature. Please note that, the thickness of as-prepared CsBr is of great importance to obtain efficient CsPbBr3 based solar cell because deficient or excessive evaporated CsBr will lead to the residue of PbBr2 or CsBr, which will be detrimental to the carrier transportation and efficiency of PSC. Therefore, the thickness parameter of CsBr should be precisely modulated by a thermal evaporator equipment to explore the change of morphology and composition as well as device performance.

Figure 2. (a) SEM image of PbBr2 film on mesoporous TiO2 substrate. SEM images of CsPbBr3 films processed by EAS method 8


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with different CsBr deposition thicknesses of (b) 100 nm, (c) 150 nm, (d) 200 nm, (e) 250 nm, and (f) 300 nm. The morphology of as-deposited PbBr2 film on the m-TiO2 layer is shown in Figure 2a, which is quite different from the SEM image of CsPbBr3 perovskite film with different CsBr thicknesses, depicted in Figure 2b-f. When the deposition thickness of CsBr film is too thin, as shown in Figure 2b and 2c, respectively, some pinholes are obviously found. That is to say, there is not enough CsBr to fully react with PbBr2 layer. By increasing the deposition thickness from 150 nm to 250 nm, the more uniform and larger average crystal grains are formed (Figure 2d and 2e). Nevertheless, when the thickness further increased to 300 nm (Figure 2f), the perovskite morphology and grain size are not change significantly, which may be attributed to the excess of CsBr. Figure S2 further shows the roughness of the various CsPbBr3 films measured via AFM, indicating that perovskite film based 250 nm CsBr exhibits more compact perovskite film. In addition, XRD patterns were used to further confirm this effect and deliberate the formation mechanism of this inorganic CsPbBr3 perovskite, shown in Figure 3a, suggesting a phase conversion of CsPb2Br5→CsPbBr3→Cs4PbBr6 under different CsBr deposition 9



thicknesses from 100 nm to 300 nm. The JCPDS cards of CsPb2Br5, CsPbBr3, and Cs4PbBr6 are exhibited in Figure 3b. Apparently, the main peaks of all samples are consistent well with cubic structure of CsPbBr3 (JCPDS card no. 00-018-0364).

Figure 3. (a) XRD patterns of CsPbBr3 perovskite films prepared by EAS method with different CsBr deposition thickness. (b) The JCPDS cards of CsPb2Br5, CsPbBr3, and Cs4PbBr6, respectively. (c) 3D crystal models of CsPb2Br5, CsPbBr3, and Cs4PbBr6 structures. However, for instance, with 100 nm CsBr, there exists a peak near 29.4° corresponding to CsPb2Br5 (JCPDS card no. 00-025-0211), 10


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which is ascribed to the PbBr2 residue (2PbBr2+CsBr→CsPb2Br5). As the thickness of CsBr increases from 100 nm to 200 nm, the peak intensity of CsPb2Br5 has reduced. When it reaches to 250 nm, the the CsPb2Br5 can fully react with CsBr to form high-quality CsPbBr3 perovskite (CsPb2Br5+CsBr→2CsPbBr3). Further increasing the thickness of CsBr (300 nm), the CsPbBr3 will be transformed into a CsBr-rich

Cs4PbBr6 (JCPDS

card

no.

01-073-2478)

phase

(CsPbBr3+3CsBr→2Cs4PbBr6), indicating the excess of CsBr. The EDS mappings of Ti, O, Cs, Pb, and Br elements (Figure S3), confirm that the perovskite film has a homogeneous composition distribution. A obvious phase conversion can also be observed in the crystallographic structures shown in Figure 3c. This deduction can also be proved by the atomic Cs/Pb ratio obtained by XPS spectra (Figure S4) summarized in Figure S5. As discussed above, it can be clearly indicated that 250 nm CsBr will be the optimized thickness with both ideal film coverage and grain size via EAS method. Moreover, the absorbance spectra of various CsPbBr3 samples revealed in Figure S6, are also consistent with the results of SEM and XRD. The cross-sectional SEM image of typical CsPbBr3 film deposited on the m-TiO2 substrate is shown in Figure S7a. It is worth noting that the thickness of carbon electrode pasted onto the 11



perovskite layer is above ten micrometers,which is hard to display all in the cross-sectional SEM. Thus, we just show the top-view image of carbon in Figure S7b. Schematically, the typical structure of the inorganic PSC is demonstrated in Figure S8a, and the energy level alignment of the devices is exhibited in Figure S8b.

Figure 4. (a) J-V curves for the photovoltaic devices based on different CsBr layers. (b) IPCE spectra of the various solar cells. (c) Steady-state PCE measured as a function of time for the fabricated PSCs. (d) Histograms of PCEs for PSCs. The data were statistically analyzed from 20 cells per sample type (CsBr deposition thickness). 12


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As presented in Figure 4a, the J-V curves based on the various deposition thicknesses of CsBr film was first observed to optimize photovoltaic parameters of PSC, summarized in Table 1. Obviously, the device based on the the CsBr deposition thickness up to 250 nm shows the champion PCE of 8.16% with maximized Jsc of 7.32 mA cm-2, Voc of 1.43 V, and FF of 0.74 due to compact and high-purity CsPbBr3 perovskite. The relatively low PCEs in the PSCs with CsBr thickness of 100 , 150, and 200 nm can be ascribed to the insufficient coverage of CsPbBr3 perovskite, the existence of heterozygous CsPb2Br5 phases, and many grain boundaries, leading to the serious interface recombination and suppressing charge transportation. Although the inorganic perovskite based on CsBr 300 nm has similar SEM morphology with that of perovskite with the CsBr thickness of 250 nm, it exists some second phase Cs4PbBr6, which is not conducive to device performance. Furthermore, the measured IPCE spectra of the various solar cells are shown in Figure 4b. The integrated current densities from these curves are 5.07, 5.92, 6.38, 7.06, and 6.82 mA cm-2 for the devices based on CsBr thicknesses of 100, 150, 200, 250, and 300 nm, respectively, which are consistent well with the Jsc obtained from the J-V curves. To evaluate device reliability, as shown in Figure 4c, it can be seen that 13



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a PCE of 8.09% are obtained at a constant bias of 1.25 V for the optimized PSC, which is higher than that of other comparative device. Furthermore, devices also exhibits good reproducibility with relatively low SD presented in Figure 4d and Figure S9. The average photovoltaic parameters are listed in Table S1.

Table 1. Photovoltaic parameters for various PSC devices made with different CsBr deposition thicknesses. CsBr Thickness (nm)

Voc (V)

Jsc (mA cm-2)

FF

η (%)

100

1.29

5.21

0.67

4.50

150

1.32

6.12

0.74

5.97

200

1.36

6.52

0.75

6.65

250

1.43

7.32

0.78

8.16

300

1.39

7.12

0.76

7.52

However, the large energy barrier at the the interface of CsPbBr3 (EVB= -5.6 eV)/Carbon (WF= -5.0 eV) evidently restricts hole 14


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transfer and thus PCE improvement.32, 33 To address this issue, a thin intermediate MnS layer was employed in PSC device as HTL, illustrated in Figure 5a. According to our previous research,34 MnS has been employed as an cost-effective inorganic HTL of organic-inorganic hybrid PSC. By UPS spectra, it also has better band alignment with inorganic pervoskite layer as displayed in Figure S10 and Figure 5b.35-39

15



Figure 5. (a) Schematic view of the inorganic cell structure. (b) Energy level diagram of the PSC. (c) J-V curves of inorganic PSCs with and without MnS intermediate layer. (d) Stabilized power output of the PSC devices. (e) PL and (f) time-resolved PL spectra of the perovskite films with and without MnS HTL. Obviously, a dependence of device performance on the thickness of MnS layer is observed shown in Figure S11 and Table S2. The 16


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solar cell based on the optimal thickness of 70 nm MnS yields a champion PCE of 10.45% with Jsc of 8.28 mA cm-2, Voc of 1.52 V, and FF of 0.83, which is much higher than that of referenced PSC without the MnS HTL in Figure 5c. The FF improvement is attributed to a reduced interfacial energy barrier and increased carrier transportation at the interface between perovskite and carbon when introducing MnS. More importantly, it shows good reproducibility (Figure S12). The

corresponding

champion

photovoltaic

parameters

are

summarized in Table S3. As is shown in Figure S13, The calculated current densities from IPCE spectra are 7.73 and 7.07 mA cm-2 for the devices with and without MnS HTL, respectively, which are consistent with the corresponding Jsc. Furthermore, the CsPbBr3 PSC with MnS as the interfacial layer shows less hysteresis shown in Figure S14, indicating the efficient charge collection and transportation. In Figure 5d, the steady-state PCEs are 10.24% and 8.09% for the PSC with and without MnS HTL at its the maximum power point, respectively. To the best of our knowledge, the all-inorganic PSCs in this work suggest a reproducible device performance with a champion PCE of 10.45%, which is one of the highest efficiencies achieved among the CsPbBr3 based PSC to date, 17



clearly seen in Table S4. The steady-state photoluminescence (PL) spectra are performed to further explore the carrier transport rate at the HTL/perovskite interface.11 As illustrated in Figure 5e, the inorganic CsPbBr3 film covered with MnS shows lower PL intensity than the CsPbBr3 without any intermediate layer, indicating a stronger hole extraction capability at the interface between perovskite and MnS HTL. Moreover, the time-resolved PL spectra of CsPbBr3 with and without MnS were measured to determine the PL decay lifetimes shown in Figure 5f. As listed in Table S5, a shorter carrier decay lifetime (7.58 ns) for CsPbBr3 film covered with MnS compared with 17.16 ns for

CsPbBr3 free with MnS also confirms enhanced

hole transfer and suppressed interface recombination between perovskite layer and carbon.11, 19

18


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Figure 6. (a) Jsc and (b) Voc as a function of illuminated light intensity for the PSCs. Long-term stability of the devices with and without MnS HTL under 25 C and 80% RH (c) or under 85 C and 80% RH (d). Furthermore, the charge recombination mechanism of PSCs with and without MnS as intermediate layer is investigated by plotting Jsc or Voc as a function of incident light intensity shown in Figure 6a and Figure 6b, respectively. According to Jsc∝ Iα (α≤ 1), the charge recombination at the interface PSC should be minimum while the α is close to 1.40 It is worth noting that the α yields from the device 19



modified by MnS HTL is around 0.99 compared with α =0.96 for HTL-free device. It is suggested that suitable matching of intermediate energy level between CsPbBr3 and carbon enables the carrier to be transferred from perovskite layer faster without charge accumulation inside the PSC device. This result is consistent well with measured Voc in Figure 6b, where the slop is reduced from 1.62 KT/q to 1.25 KT/q after incorporating MnS thin layer, suggesting that the used of MnS HTL significantly suppressed the trap assisted recombination process.41 Besides the high PCE of PSC, the stability under high humidity or thermal attacks is also important for the commercial applications of all-inorganic PSCs. As is shown in Figure 6c, compared with MAPbI3 device, both CsPbBr3 devices have superior moisture tolerance due to the hydrophobicity of carbon electrode and excellent stability of CsPbBr3 itself. Moreover, Figure 6d displays the thermal stability of PSCs under 85 C and 80% humidity. It is obviously indicated that the inorganic PSC with MnS HTL as the intermediate layer shows a superior stability under harsh condition (80% RH, 85°C) retaining 80% of the its initial PCE after 100 days without any encapsulation. For the referenced device without MnS HTL, its poor stability is attributed to perovskite layer eroded by 20


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water vapor due to the high temperature and humidity. By introduction of MnS, the better stability has been achieved. As is well-known, the surface wettability of the top HTL is an important factor that significantly affects the stability of PSCs. According to our previous work34, it was indicated that MnS film showed the good waterproofing capabilities, because of a higher water contact angle and good chemical-stability of inorganic hole transport material itself. Please note that, there is still value in vacuum processed perovskite active layers and, from that perspective, it may be more streamlined to use vacuum-compatible HTLs. In particular, many state-of-art textured silicon/perovskite tandem solar cells use vacuum-processed perovskites (for conformal coating) and Oxford PV has stated that they find the overall cost of ownership for the cells does not change whether you use vacuum or solution processed active layers. Therefore, this result demonstrates that low-cost, high efficiency and stable all-inorganic PSCs can be made by evaporation-assisted deposition and setting intermediate energy levels.

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CONCLUSION In summary, a vapor-assisted solution method has been first introduced to prepare high-purity CsPbBr3 film applied in PSC as light-absorbing layer and then the photovoltaic performance has further been improved by setting an intermediate energy level of MnS as HTL to promote charge extraction in PSC. Due to the enhanced hole extraction, the champion inorganic CsPbBr3 PSC based on all inorganic transport layers delivers a PCE of 10.45% compared with 8.16% for the device free of intermediate layer. Moreover, the optimized device retained 80% PCE of its initial efficiency over 90 days under 80% relative humidity (RH) at 85 C, indicating an excellent environmental tolerance to boost the commercial application of low cost, efficient and stable all-inorganic PSCs.

EXPERIMENTAL DETAILS Materials. Unless stated otherwise, all materials were purchased from Alfa Aesar and used without further purification.Lead iodide (PbI2), lead bromine (PbBr2), and cesiun bromide (CsBr) were purchased from Xi’an Polymer Light Technology Corp. DMF was 22


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purchased from Ying Kou You Xuan Trade Co., Ltd. Device Fabrication. Prior to fabrication of solar cells, the etched FTO substrate was ultrasonically rinsed using detergent and deionized water, ethanol, and acetone step by step before drying in a nitrogen flow and followed by oxygen plasma treatment for 15 min. Next, a compact TiO2 layer (c-TiO2) was deposited on the FTO by spin-coating 0.2 M solution of titanium isopropoxide in ethanol, and annealed at 450

C

for 30 min. Afterward, the Na-treated

mesoporous TiO2 film (m-TiO2) and the CH3NH3PbI3 (MAPbI3) film were prepared according to our preliminary research.35 The CsPbBr3 film was fabricated by a vapor-assisted solution method. 1M PbBr2 (or PbI2) in DMF was spin-coated onto the m-TiO2 layer at 3500 rpm for 30 s under 90 C. Then, the PbBr2 (or PbI2) film was placed into the vacuum chamber for the CsBr deposition. Proper amount of CsBr was evaporated thermally onto the PbBr2 (or PbI2) layer without further annealing process. For inorganic MnS HTL, it was prepared according to our previous work.34 Finally, the carbon was coated by doctor-blade method to form the whole device. Characterizations. The XRD patterns were utilized to exam the the crystal structure of the perovskite films (XRD-7000s, Shimadzu). SEM was used to measure the surface morphology of the perovskite 23



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films (GeminiSEM300, Carl Zeiss). A UV-vis spectrophotometer were obtained to test absorption spectra of the films (Lambda 950, PerkinElmer).

XPS

and

UPS

spectra

were

performed

(AXIS-ULTRA DLD-600W, Shimadzu). AFM was applied to characterize RMS roughness of perovskite films (SPM9700, Shimadzu). PL and TRPL spectra were obtained using a laser spectometer (FLS 980, Edinburgh Instruments Ltd). The IPCE spectrum was measured using Newport-74125 system (Newport Instrument). The J-V curves were measured using a Keithley 2400 source with a solar light simulator (Model 71675-71580, Oriel Company).

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Photographs, AFM images, SEM images, XPS spectra, EDX spectra, Absorption spectra, UPS spectra, Box charts of photovoltaic parameters, J-V curves, IPCE spectra.

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AUTHOR INFORMATION Corresponding Author *Email:

[email protected].

ORCID Xin Li: 0000-0003-4720-6180 Junyou Yang: 0000-0003-0849-1492 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is co-financed by National Natural Science Foundation of China (Grant No. 51572098, 51632006, 51772109 and 51872102), the Fundamental Research Funds for the Central Universities (No. 2018KFYXKJC002). The technical assistance from the Analytical and Testing Center of HUST is likewise gratefully acknowledged.

REFERENCES 25



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GRAPHICAL ABSTRACT

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All-Inorganic CsPbBr3 Perovskite Solar Cells with 10.45% Efficiency

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