Growth of MAPbBr3 perovskite crystals and its interfacial properties with Al and Ag contacts Mansoor Ani Najeeba, Zubair Ahmada*, R.A. Shakoora, Abdulla Alashraf a, Jolly Bhadraa, Noora AL Thania and Shaheen A. Al Muhtasebb and A. M. A. Mohamedc a
Center for Advanced Materials (CAM), Qatar University, Doha 2713, Qatar. Department of Chemical Engineering, College of Engineering, Qatar University, 2713, Doha, Qatar c Department of Metallurgical and Materials Engineering, Faculty of Petroleum and Mining Engineering, Suez University, 43721 Suez, Egypt b
*Corresponding author:
[email protected]
Abstract In this work, the MAPbBr3 perovskite crystals were grown and the interfacial properties of the poly-crystalline MAPbBr3 with Aluminium (Al) and Silver (Ag) contacts has been investigated. MAPbBr3 crystals are turned into the poly-crystalline pellets (PCP) using compaction technique and the Al/PCP, Al/interface layer/PCP, Ag/PCP, and Ag/interface layer/PCP contacts were investigated. Scanning Electron Microscopic (SEM), Energy-dispersive X-ray spectroscopy (EDX) and current-voltage (I-V) characteristic technique were used to have an insight of the degradation mechanism happening at the Metal/perovskite interface. The Ag/PCP contact appears to be stable and could be potential alternative to the gold (Au) whereas Al is found to be highly reactive with the MAPbBr3 perovskite crystals due to the infiltration setback of Al in to the perovskite crystals. The interface layer showed a slight effect on the penetration of Al in to the perovskite crystals however it does not seem to an appropriate solution. It is noteworthy that the stability of the underlying metal/perovskite contact is very crucial towards the perovskite solar cells with extended device lifetime.
Key words: Perovskite crystals, perovskite crystals/metal contact, interfacial properties
Introduction
Since the invention of the use of perovskite materials for solar cells by Miyasaka and co-workers in 2009 [1], a significant advancement has been observed in this field of solar cells which lead to the improvement of efficiency from 3.8% to a qualified value of 22.1% on laboratory-scale cells in 2016. Perovskite solar cells consist of organic-inorganic hybrid system with general formula ABX3, where A is an organic/inorganic cation (methylammonium (CH3NH3)), B is the metal cation (Pb), and X is a halide anion (I, Br, Cl) [2-4]. However due to number of factors, the perovskite solar cells are facing the issue of low stability. The high affinity of CH3NH3PbX3 for moisture leads to degradation of the material to PbX2 and the biggest hindrance on the way towards commercialization of perovskite solar cells is lack of environmental stability [5, 6]. The reaction of the metal contact with the perovskite films is another important reason behind the instability of the perovskite solar cells. To overcome these discrepancies and to obtain stable perovskite solar cells, diffusion barrier layers are inserted between metal electrodes and the perovskite layer, to keep away from each other [7-10]. However, the shielding lifetime of buffer layers is very short and cannot be used to resolve the problem in the long run. Since the repetitive illumination or thermal cycles, the electrode materials tend to diffuse through the buffer layers after several months or years of operation [11, 12]. To limit the diffusion effect, various approaches including device hardening by introducing alternative perovskite active layer chemistries [13-15], integrating buffer layers [11, 12, 16] or by initiating more robust or hydrophobic hole-transporting layers (HTLs) and top electrodes [17-19] have been studied.
A careful and proper selection of a top electrode and a corresponding hole-transport layers on top of the perovskite layer, might help to safeguard the active layer [20-22]. Preceding studies have revealed stability issues with Ag [23, 24] or Al [25] electrodes in perovskite devices. Kato et al.[26], have reported that MAPbI3 iodizes Ag electrodes. Back et al.[27], employed a chemical inhibition layer to counteract corrosion of Ag and Al electrodes. 2,2′,7,7′-tetrakis(N,N-di-pmethoxyphenylamine)- 9,9′-spirobifluorene (spiro-OMeTAD) is the most extensively used HTL in high performance perovskite solar cells [28-30]. When utilized in perovskite solar cells as a HTL, the spiro-center aggravates crystallization while safeguarding appropriate penetration of the material into the mesoporous layer [31]. Nevertheless, the glass-forming affinity of spiroOMeTAD layer remains as a major holdup in the progression of perovskite solar cell efficiencies. Recently, a molecularly engineered novel dopant-free hole transporting materials
(HTMs) for perovskite solar cells (PSCs) have been reported [32-34]. These materials are more stable and efficient as well. For instance, the PSC based FA-CN showed exceptional stability up to 500 h. These reports opens a new avenue for efficient and stable PSCs exploring new materials in an alternative to Spiro-OMeTAD. In this work, the interfacial properties of MAPbBr3 perovskite crystals were investigated with Al and Ag metal contacts. MAPbBr3 crystals are turned into the poly-crystalline pellets (PCP) using compaction technique. The contacts were studied using Scanning Electron Microscopic (SEM), Energy-dispersive X-ray spectroscopy (EDX) and current-voltage (I-V) characteristic technique. The aim of this work is to have an insight of the degradation mechanism happening at the Metal/perovskite interface.
EXPERIMENTAL Materials: Lead (II) bromide (PbBr2)(for perovskite precursor) is purchased from TCI chemicals, Japan,
Dimethylformamide
DMF
Bis(trifluoromethane)sulfonimide
(anhydrous,
lithium
salt
99.8%),
(LiTFSI)
Chlorobenzene, and
Acetonitrile,
2,2′,7,7′-tetrakis(N,N-di-p-
methoxyphenylamine)- 9,9′-spirobifluorene (spiro-OMeTAD) were purchased from Sigma Aldrich. MABr were purchased from Dyesol Limited (Australia). We used the chemicals as received without further purification. Preparation of CH3NH3PbBr3 poly-crystals: The perovskite crystals were grown by the inverse temperature crystallization method. In brief, 1 M solution of the Lead (II) bromide (PbBr2) and Methylammonium bromide (MABr) in 5 ml DMF solution is mixed at room temperature. Then the solutions were filtered using 0.2-mm pore size PTFE filters. The filtered solution is transferred to glass vials and kept in oil bath at 80 C for 30 minutes. Then 60µl of Formic acid (FA) was added to the solution and the crystallization was started within 5 min as shown in Figure 1. Preparation of CH3NH3PbBr3 pellets: The small crystals (size 0.5 mm) grown using inverse temperature crystallization method were then filtered without altering the temperature using filter paper. The dried crystals were then pressed under a pressure of 20 MPa using a hydraulic press which resulted in the formation of perovskite crystals pellet of diameter 1 cm and thickness 4mm.
Deposition of spiro-MeOTAD HTL layer: Spiro-MeOTAD solution was prepared following previously reported method by Nam-Gyu Park et al [35]. Briefly 72.3 mg of spiro-MeOTAD is dissolved in 1 ml of chlorobenzene, to which 28.8 µl of 4-tert-butyl pyridine and 17.5 µl of lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) solution (520 mg LI-TSFI in 1 ml acetonitrile) were added. After overnight stirring, the solution is spin coated over the crystal pellets using a one-step spin coating process at 2000 rpm for 30 seconds. Characterization: X-ray diffractometer (PAN analytical EMPYREAN, Netherland) with Cu Kα radiation was used to obtain the crystal structure of CH3NH3PbBr3. Scanning Electron Microscope (SEM) instrument (model: FEI NOVA NANOSEM 450) equipped with EDX were used to obtain the surface morphology, cross-sectional and elemental composition of materials. JCM-6000PLUS NeoScope Benchtop SEM was also used to record the interface morphology in some cases. I-V characteristic were measured using Keithley 2400 SMU.
Figure 1: Schematic diagram showing the process of perovskite crystal growth and the pellet formation. The grown perovskite poly-crystals and SEM images of the same are given as inset.
RESULTS AND DISCUSSION
Figure 2(a) shows the X-ray diffraction (XRD) patterns of a CH3NH3PbBr3 crystals (black) and CH3NH3PbBr3 compressed pellets (blue and red). The XRD patterns of MAPbBr3 crystals reveals that they have 𝑃𝑚3̅𝑚 cubic perovskite phases at room temperature, that are in perfect understanding with the previously reported structures [13, 36, 37]. All the peaks of poly crystals, specifically peaks of (100) at 14.87°, (110) at 21.33°, (200) at 30.29°, (220) at 42.87°, (300) at 48.21° are mimicked in the XRD of pellets which confirms that under the 20 MPa compression the crystalline structure unaffected and the properties remain same. CH3NH3Br and PbBr2 residuals are not detected in the crystallized CH3NH3PbBr3, signifying after all the PbBr2 is transformed into CH3NH3PbBr3. The intensity variations may be associated to the outcome of solid fraction and compact thickness on the crystal structure [38]. Figure 2(b) shows the SEM image of the of the pellet (taken at the cross-section of the pellet) made by compaction technique.
(a)
(b)
Figure 2: (a) XRD of grown poly crystals and pellets made by compaction technique, (b) SEM image (cross-sectional) of the of the pellet made by compaction technique. Aluminium/perovskite interface: Aluminum, because of its low cost and good electrical conductivity, is the most common material used to fabricate the contacts of solar cells. Aluminum electrodes are widely utilized in polymer-based solar cells due to their corresponding work function [39-45]. Nonetheless, due to its reactivity toward the formation of aluminum
oxide, the device stabilities are not adequate for steady performance. To study the effect of Aluminum/perovskite interface, a thin layer of Al was deposited on both sides of the perovskite pellet and observed the interface with during the course of time. Figure 3 (a) shows the crosssectional SEM view of fresh Al contact on perovskite pellet. It is noteworthy that initially the Al layer on the perovskite pellet doesn’t make any observable changes at the interface. However, after 48 hours (Figure 3 (b)) there has been significant changes in the interfacial layer of Alperovskite junction. Figure 3(b) shows that the Al has been penetrated in the perovskite crystal pellet up to 15 µm. This may be due to the “Diffusion effect of Aluminium”, which has been reported in various theoretical and experimental studies previously [46-48]. A closer look uncovers the intrinsic reaction which took place at the Al/perovskite junction resulting in the cracking and perversion of the perovskite crystal lattice Figure 3(c & d). It is considered as a fundamental phenomenon in most metallurgical processes which may result in recrystallization, homogenization, phase changes, age hardening etc. Here, after 48 hours, it has been observed that, there is a redistribution of crystal structure and a 'random-walk' phenomenon of Al electrodes deep in to the crystals up to 15 µm, which may be due to the tendency of Al atoms to move preferentially toward the regions of lower concentration. The effect of humidity and presence of oxygen also favors this process. This effect of metal penetration not only affect the stability and conductivity, it also changes the device parameters like active-layer thickness (15 µm in our case). Figures 3(c) and (d) indicates that, after 15 µm, there has been cracks in the crystals. This may likely result in increased exciton quenching [49] which result in the variations of the active-region energy band structure [50]and eventually affects device stability [51]. The penetration of Al metal in to the crystal structure has also been confirmed by EDX as shown in Figure 4. The EDX data spectrum for the region of the perovskite crystal affected by the Al penetration is given in Table 1. The acidic nature of the perovskite crystal due to the addition of Formic acid may be one cause of fast degradation of Al/perovskite crystal contact.
(a)
(b)
(c)
(d)
Figure 3: (a) Fresh Aluminum layer on perovskite crystal, (b) Aluminum layer on perovskite crystal after 48 hours, (c) and (d) Magnified view of Aluminum layer on perovskite crystal after 48 hours, which shows penetration of Aluminum layer into the perovskite crystal and resultant cracking of crystals
Table 1: EDX data spectrum for perovskite crystal with Al metal electrode.
El
AN
Series
unn. C [wt.%]
norm. C [wt.%]
Atom. C [at.%]
(1 Sigma) [wt.%]
C O Al Br Pb
6 8 13 35 82
K-Series K-Series K-Series K-Series L-Series
7.18 17.19 3.31 73.32 42.61
5.00 11.97 2.31 51.06 29.67
20.48 36.82 4.21 31.45 7.05
2.05 3.19 0.20 2.77 1.67
Figure 4: EDX analysis of the Al penetration at the Al/perovskite interface. Silver/perovskite interface: In turn to study the penetration effect of Ag electrode in to the perovskite layer, a thin layer of silver paste is applied over the perovskite crystal pellet under the same environmental conditions used for preparing Al electrodes. Figure 5(a) shows the crosssectional image of silver/perovskite interface, which disclosed the harmonized accumulation of silver pattern over the crystal pellet. The SEM analysis established that the silver particles are strappingly interlocked with each other with void free surface morphology, which may be due to micro-structured silver pattern [52]. The SEM images taken after the same samples after 48 hours (Figure 5(b)) tells that the underlying perovskite crystals are less provoked by the Ag electrode. Even after a closer evaluation, the interfacial layer remains intact, but the granule/droplet formation of the side of the perovskite crystals may be because of the moisture on the exposed crystals of the pellet.
(a)
(b)
Figure 5: (a) Silver paste on perovskite crystal (Fresh), (b) Silver paste on perovskite crystal (After 48 Hours). Spiro-OMeTAD interfacial layer: To investigate the effect of spiro-OMeTAD, the most commonly HTL, interface layer on the penetration of Al and Ag electrodes into perovskite crystals, a layer of spiro-MeOTAD is applied over the perovskite crystal pellets using spin coating. Spiro layer acts as a protective layer over the perovskite material from the air and inhibits the diffusion of exterior moieties or elements inside the absorber. It also properly fills the porosity of the absorber layer, which in turn keeps the interfacial junction intact. Differences between the penetration of Al and Ag electrodes on spiro-MeOTAD coated perovskite crystal pellets were imaged by scanning electron microscopy (Figure 6 (a & b) and Figure 6 (c & d) respectively). The Figure 6 (a) clearly reveals the profound penetration of Al into the perovskite crystal structure resulting in the distorting of the crystal structure and causing damaging to the interfacial layer whereas in the case of Ag-perovskite configuration, both the layers remained intact and flawless. Even though the spiro-MeOTAD sandwiched between the Al-perovskite junction was not strong enough to counteracts the penetration of Al, however, the damage is limited to few m in depth (after 48 hours) as shown in Figure 6(b). This may be due to the presence of of “pin-holes” in the HTL, which leads to the permeation of oxygen and moisture in to the active layer [53]. The corresponding images for Ag with spiro layer (Figure 6 (c&d)) shows no significant differences even after 48 hours.
(b)
(a)
Al/HTM/Perovskite junction
Al/Perovskite junction
(c)
(d)
Figure 6: (a) Aluminum layer on perovskite crystal after 48 hours, (b) Aluminum layer on perovskite crystal after 48 hours spiro-MeOTAD layer spin coated on perovskite crystal. (c) Silver paste on spiro-MeOTAD layer spin coated on perovskite crystal (Fresh), (d) Silver paste on spiro-MeOTAD layer spin coated on perovskite crystal (After 48 Hours)
Current-voltage (I-V) characteristics: To back up the findings obtained from SEM results, a parallel study on IV characteristics of perovskite crystal pellet (PCP) coated with Al and Ag metal electrodes along with the effect of spiro-MeOTAD was also conducted. Crystal pellets of Al/PCP/Al and Ag/PCP/Ag configurations were used for the dark I-V study. When connected
with the freshly applied Al electrodes on both sides, the dark IV curve showed a linear behavior as expected (Figure 7(a)). After passage of 48 hours, there was drastic change in the IV curve. The disorientation of the linearity of the curve can be associated to the previously obtained SEM images, which shows the penetration of the Al electrode into the perovskite crystal and deforming the crystal arrangement. The resultant cracks may have created traps between the interfacial layer and may result in the disrupted charge transfer properties. Nonetheless, when comes to the Ag electrode, Figure 7(b), the curve shows very stable behavior, even after 48 hours. This indicates that the Ag electrode maintain good contact with the perovskite crystal layer.
(a)
(b)
Figure 7: Dark IV characteristics of (a) Al/PCP/Al and (b) Ag/PCP/Ag structure.
Conclusion
To conclude, in this study we demonstrate that Ag electrode can serve as a better alternative for the Al electrode which has the shortcoming of penetration effect into the perovskite layer. We also verify that Spiro-MeOTAD HTL has limited the Al penetration but cannot completely. A
deep insight to the interfacial morphology and the changes happening there may provide a key to understanding the long-term performance of perovskite solar cells. While this work is focused on the effects of the Al and Ag metal electrode interfacial properties and stability for the MAPbBr3 perovskite crystals, a detailed study on the other interfacial layer and their degradation mechanism will be a required for other perovskite materials.
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