Low-Temperature Annealed Perovskite Films: A Trade-Off between

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Functional Inorganic Materials and Devices

Low Temperature Annealed Perovskite Films: A Trade-Off Between Fast and Retarded Crystallization Via Solvent Engineering Zulqarnain Arain, Cheng Liu, Yingke Ren, Yi Yang, Muhammad Mateen, Xuepeng Liu, Yong Ding, Zulfiqar Ali, Xiaolong Liu, Songyuan Dai, Tasawar Hayat, and Ahmed Alsaedi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019 Downloaded from http://pubs.acs.org on March 26, 2019

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ACS Applied Materials & Interfaces

Low Temperature Annealed Perovskite Films: A Trade-Off Between Fast and Retarded Crystallization Via Solvent Engineering Zulqarnain Arain,a,b Cheng Liu,a Yingke Ren,a Yi Yang,a Muhammad Mateen,a Xuepeng Liu,a,* Yong Ding,a,* Zulfiqar Ali,c Xiaolong Liu,a Songyuan Dai,a,d,* Tasawar Hayat,d and Ahmed Alsaedid [a]Key

Laboratory of Novel Thin-Film Solar Cells, North China Electric Power University, Beijing, P. R.

China. [b]Energy

System Engineering Department, Sukkur IBA University, Sukkur, Pakistan.

[c]Renewable [d]NAAM

Energy School, North China Electric Power University, Beijing, P. R. China.

Research Group, Department of Mathematics, Faculty of Science, King Abdulaziz University,

Jeddah 21589, Saudi Arabia. E-mail: [email protected], [email protected] and [email protected] Abstract In the current field of photovoltaic, researchers are working hard to produce efficient, stable and commercially feasible devices. The prime objective behind the innovation of any photovoltaic device is to yield more energy with the easy manufacture and less process cost. Perovskite solar cells (PSCs) are prominent in the field of photovoltaic, owing to its low material cost, simple fabrication process, and ideal optoelectronic properties. Despite rapid augmentation in progress of PSCs, it is still a bottleneck to produce high-quality perovskite layer at low temperature in a short time. Herein, a facile solvent engineering technique is used to produce high-quality perovskite layer at 50 °C in just 30 mins. We employed solvents coordination strength to form intermediate state as well as their sensitive behavior against anti-solvent to establish a trade-off between fast and retarded crystallization. Dimethylsulphoxide (DMSO), a traditional co-solvent is used as an additive instead of co-solvent, in contrast, the mixed 1methyl-2-pyrrolidinone (NMP) and dimethylacetamide (DMAC) are employed as principal solvents for perovskite precursors. Different volume ratios of DMSO as a fraction of NMP are added to examine the evolution of the perovskite layer at low temperature. It is noted that the mixed solvent with 30% DMSO shows a pin-hole free, uniform and compact layer with strong absorption spectrum. Promisingly, the corresponding device with 30% DMSO shows a high efficiency reached to 18.19%, which is even comparable to traditionally high temperature annealed PSCs. These findings may provide a way to produce low-temperature annealed, high-quality perovskite films and subsequently facilitate the production of cost-effective and efficient devices. Keywords: Low-temperature annealing, solvent engineering, perovskite solar cell, fast and retarded crystallization Introduction In the last few years, perovskite solar cells (PSCs) have gained enormous attention due to their low material cost, simple fabrication techniques, and high power conversion efficiency (PCE).1 Since the PSCs with an efficiency above 9% were reported, this solar cell has gained much attention from the scientific community due to its exceptional photovoltaic performance.2,3 Perovskites with the structure

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ABX3 (A = CH3NH3+, HC(NH2)2+, B = Pb, Sn; X = I, Br, Cl) symbolize an innovative and novel photoactive material greatly exploited in the new class of photovoltaic. Up to date, solution-based processing methods remain the most common practice to produce quality perovskite films as it shows many advantages, such as simple and robust deposition process.4 Various solution-based deposition techniques such as one-step deposition, two-step deposition, vapor assisted deposition, etc. have been studied to produce high-quality perovskite films.5,6,7,8 Amongst these techniques, the formation of an intermediate phase (IP) to retard the rapid reaction between precursors in solvents has been proved to be a key factor in producing compact and smooth perovskite films in onestep deposition method.1,9 The reported techniques require annealing at elevated temperature for longer duration to relieve formed stable IP. Nevertheless, these techniques can certainly induce thermal convection because the solvent must be wiped out by annealing. Moreover, the thermal convection sometimes produces low-quality films with microscopic trap states (protruded boundaries or crevices) as well as adds energy cost and time to the fabrication process.10 To encourage the commercialization of PSCs, it is required to find such a scalable technique which facilitates low-temperature processing in optimal time for crystal evolution. One of the key factors to control the morphology of perovskite film is the annealing process. An extended approach is adding additives into the solvent system, which leads to either retarded or rapid crystallization, depending on the additives.11,12 Recently, numerous groups have studied the impact of mixed solvents on the perovskite film morphology.13,14,15 The reported results show that the solvent type has a substantial influence on both the crystallinity and morphology of the perovskite. However, a thermal annealing step is always needed to attain the complete crystallization of the perovskite films, which can induce inhomogeneous nucleation and consume time and cost, making these methods not suitable for industrial manufacturing. Therefore, the development of a facile way for producing highquality perovskite films without or at low annealing temperature can be helpful for the commercialization of PSCs. In this respect, many groups have tried to develop new techniques to produce quality perovskite films at low temperature under varying conditions. Bin et al.16 reported a novel room-temperature, airexposure technique that eliminates the need of annealing step and initiate the inter-diffusion of PbI2 and MAI layers to produce pinhole-free and highly crystalline perovskite films. This room-temperature air exposure technique achieved a PCE of 15.6%, which is relatively comparable to that of many thermally annealed PSCs. Wang et al.17 revealed a unique water-vapor annealing (WVA) technique to fabricate efficient planar heterojunction PSCs based on pure MAPbI3. The WVA technique produced a pin-hole free highly crystalline perovskite film at room temperature with PCE of 16.4% and an open-circuit voltage (Voc) of 1.0 V. It was advocated that the water molecules in vapor catalyzed the dissolution and re-crystallization of the perovskite at void area and grain edges, resulting in large crystals and void-free perovskite and the enhanced photovoltaic performance. This WVA technique offers an efficient, simple, and energy conservation fabrication method for the commercialization of planar PSCs. Xiang et al.18 developed a solvent engineering assisted method for uniform perovskite films without high-temperature annealing. 1-methyl-2-pyrrolidinone (NMP) was used as a complexant and a high-boiling-point solvent, along with the dimethylacetamide (DMAC), to enhance the quality of the films and resulting device PCE.


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Notably, an efficiency of 17.09% was reached even without any annealing process. The witnessed phenomenon is primarily because the NMP-included solvents can activate an immediate crystallization once the non-polar anti-solvent drips on the deposited film at room temperature. In the above-discussed studies, it is either fabrication environment or solvent additives that trigger crystallization without any post-annealing stage. Most of these techniques promote perovskite crystalisation by lengthy ambient drying process or under preciously maintained processing conditions which are difficult to replicate at industrial scale. Means, either annealing temperature can be reduced at the cost of time or annealing time at the cost of temperature, but simultaneously not both in normal processing conditions. Normally, solvents resilient (strong) against anti-solvent tends to retard crystallization by forming IP and then to produce smooth morphology. While, solvents with affinity (weak) towards anti-solvent promotes fast crystallization and produces grains substantial in size at the cost of film homogeneity. A solvent can initiate the instant crystallization, or it can retard, depending on its coordination strength and removal process.19 Perovskite device based on a scalable and reproducible technique is necessary for the commercialization of this novel photovoltaic technology. A trade-off between fast and retarded crystallization of perovskite film is therefore needed. Herein, we extended an annealing free technique by adding traditional co-solvent DMSO as an addidtive into DMAC/NMP to produce a ternary based solvent system. By switching the role of DMSO in the solvent system, we enable exploiting solvent abilities to form IP as well as their affinity towards non-polar anti-solvent to develop a low-temperature annealing technique for uniform and efficient perovskite film. This feasible technique involves the use of DMAC, a weak primary solvent and homologs of DMF, with high boiling point NMP as a co-solvent. DMSO is used as an additive unlike in traditional way where it plays an important role as a co-solvent. Though both NMP and DMSO are high boiling solvents, their affinity towards strong non-polar anti-solvent is different. NMP’s affinity towards anti-solvent promotes fast crystallization, while, DMSO coordination strength retards crystallization.18 Different volumes of DMSO added into the mixed DMAC/NMP solvents, so perovskite precursors could form a stable IP and produce a uniform layer with homogeneous coverage. An appropriate volume percent of DMSO dopants in the mixed solvents can relieve the formed IP even at a low temperature in optimal time. The developed technique is relatively different from the both traditional as well as Xiang18 method which either requires annealing at an elevated temperature or drying at room temperature for a longer period to release the remaining solvents and promote crystallization. The prepared film shows improved crystallinity, reduced spontaneous non-radiative recombination and solid absorption coefficient consistent with the stronger spectral response. Experimental Section In a typical synthesis: 1.2 mol of PbI2 (553 mg) and 1.17 mol of MAI (186 mg) were dissolved in a solvent system (1 mL) containing DMAC and NMP in 4:1 volume ratios respectively, then the prepared solution was continuously stirred and heated at 65 °C for 12 hours. Change volume proportions of DMSO (%) as a fraction of NMP in the mixed DMAC/NMP solvent system were added as 10%, 20%, 30% up to 50%. The prepared solution was filtered and spin-coated onto the mp-TiO2 scaffold in two stages as 1100 rpm for 10 s and 4200 rpm for 30 s. 100 μL chlorobenzene (CB) as an anti-solvent was dripped onto the as-deposited film at the last 12th second of the 2nd stage (4200 rpm for 30 s). Then, the prepared



films were heated at 50 °C for 30 mins. The detailed device fabrication and characterizations are available in supporting information. Results and Discussion. It is very necessary to employ a facile technique to produce PSCs at low temperature and in a short time for the commercialization. Solvents coordination strength, as well as their behavior against nonpolar anti-solvent defining their release process, can be a key factor for such a scalable method. In this study, we added DMSO as an additive into the mixed DMAC/NMP solution in a controlled amount to form a stable IP and to retard crystallization. Then, high-quality perovskite film is produced at a low temperature in less time in such a way, which was not possible to achieve from only DMAC/NMP triggered fast crystallization. This low-temperature technique produces high-quality film with enhanced performance characteristics comparable to that of the high temperature annealed films.11,20,21 The employed fabrication process is shown in Figure 1. The solution containing only DMAC/NMP starts fast crystallization due to quenching of both DMAC and NMP solvent after dripping of the non-polar antisolvent. Fast crystallization can lead to a poor-quality film with trap states for charge carriers in the shape of pinholes and irregular grain orientation. Addition of optimized DMSO volume slows down the crystal evolution and results in a more uniform and high-quality film. The film containing DMAC/NMP solvents turns dark grey instantaneously after anti-solvent washing, indicating the formation of the perovskite layer. This immature film further needs either to anneal or to dry under a controlled environment for a longer time to produce a mature layer, as reported earlier.18 DMSO-containing layers undergo slow crystallization due to its sturdiness against anti-solvent washing (IP) and evolve into a high-quality mature layer after annealing at 50 °C for 30 mins.

Figure 1. Schematic elaboration of the fabrication process for low-temperature annealed perovskite film. Figure 2 shows X-ray diffraction (XRD) patterns of the perovskite films without and with annealing. As expected, film without DMSO exhibits a strong peak at 14.2° (Figure 2a), indicating immediate formation of MAPbI3 perovskite phase due to anti-solvent washing, which removes DMAC and NMP instantly out of the deposited film.18,22,23 While, films with DMSO exhibit co-existence of MAPbI3 perovskite peaks and lower angle peaks at 6.7°, 7.4° and 9.3° simultaneously, which rises along with increasing proportion of DMSO in the mixed solution. These lower angle peaks indicate the formation of MAI-PbI2-DMSO intermediate phase (IP), which would retard the nucleation rate of the films as


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reported in earlier studies.1,24,25 The annealed films show raised intensities of the peaks at 14.2° for both cases (Figure 2b). While there were no any visible lower angle IP peaks except for the films containing 40% and 50% DMSO, demonstrating that IP still existing and suggests an incomplete nucleation process due to the mass of PbI2-DMSO IP. These observations reveal that wise control of the solvent proportions can get an optimum combination which can produce a trade-off between fast and retarded crystallization, resulting in a low-temperature annealed highly crystalline perovskite layers.

Figure 2. XRD patterns of the films with different proportions of DMSO (%) as a fraction of NMP. (a) without annealing and (b) after annealing at 50 °C for 30 min.

Figure 3. Top SEM images of the perovskite films prepared by different proportions of DMSO (%) as a fraction of NMP after annealing at 50 °C for 30 mins. Figure 3 shows scanning electron microscopy (SEM) images of the perovskite films fabricated from different proportions of DMSO (%) as a fraction of NMP in the mixed solvents and after annealing at 50 °C for 30 mins. The low magnification SEM image (1 μm) of the perovskite film prepared without DMSO exhibits a non-uniformity regarding multiple pinholes and shows a non-aligned growth of grains in any single particular dimension. Grain size is quite large, perhaps due to the rapid crystallization. Films prepared by 10% and 20% DMSO showed slight better morphology than the former one, however, which still lacks uniformity as an unusual non-aligned growth of grains in any single particular dimension. This irregularity could be attributed to the fast crystallization triggered by the anti-solvent washing of DMAC and NMP solvents. Perovksite film with 30% DMSO exhibits much improved morphological



characteristics with uniform grains size and pinhole free closely packed grain distribution. It shows no sign of non-aligned growth of grains in any single particular dimension. This enhancement is indicative of controlled and homogeneous nucleation process imposed by DMSO action. Films obtained with 40% and 50% DMSO reflect a similar pinhole free morphology like the one with 30% DMSO, however, which reveals a loosely packed grain distribution. The grains are not completely grown and still in the process of nucleation, which needs further high-temperature annealing. Rapid removal of solvent can drastically decrease the precursor solubility, leading to non-homogeneous nucleation, as shown in the cases of the films without and with 10%, 20% DMSO. Perovskite grain expands either by uncontrolled crystallization triggered by instant quenching of weak molecule or by controlled crystallization due to any strong retarding molecule additive. Uncontrolled crystallization produces larger grain at the cost of film homogeneity, while, controlled crystallization can produce large grain with quality homogeneous film.19,24 Change in grain size observed in DMSO based films (10%, 20% and 30%) compared to without DMSO based film is indicative of controlled crystallization caused by an optimum combination of each solvent in the system in terms of volume, which not only optimizes grain size but relatively also produces homogeneous coverage of the films.

Figure 4. Cross-sectional SEM images of the perovskite films prepared by different proportions of DMSO (%) as a fraction of NMP. Ra is the corresponding root-mean-square roughness of the perovskite films. Furthermore, cross-sectional SEM images of perovskite films deposited on m-TiO2 scaffold are shown in Figure 4. The results show dissimilar thicknesses of perovskite films prepared from varied DMSO to NMP volume ratio, 30% DMSO based film in particular, probably due to the different solvent removal rate.26,27 Perovskite layers without and with 10% DMSO exhibit a bumpy surface with irregular crystal growth. Sample with 20% DMSO shows a bit improved surface characteristics than the former samples, but rod-shaped grains are still visible. While the sample with 30% DMSO displays a fairly better surface uniformity, evidenced by closely packed grains with no visible rod-shaped grains. Perovskite layers made of 40% and 50% DMSO exhibit loosely bound grains, reflecting an immature perovskite layer. The AFM images of the films shown in Figure S1 also support the cross-sectional results. The corresponding root-mean-square roughness (Ra) values of perovskite layers are 34.6, 23.8, 15.7, 9.08, 19.6 and 23.3 nm, respectively. The AFM results are quite consistent with the respective surfaces and cross-sectional morphologies.


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Figure 5. (a) Fourier transform infrared spectroscopy (FTIR) of 30% DMSO solvent and its solution. (b) The fingerprint regions for S═O stretching vibration. (c) The schematic illustration of the evolution mechanism for low-tremperature annealed films. (d) Schematic device structure of the final prepared devices. Fourier transform infrared spectroscopy (FTIR) of 30% DMSO solvent and its solution are presented in Figure 5a to study how DMSO incorporates with DMAC/NMP in the solvent system to obtain qualified crystallization. The peaks at 1679 cm-1 and 1644 cm-1 are the C═O stretching vibrations for NMP and DMAC, respectively, while the peak at 1058 cm-1 is the S═O stretching vibration of DMSO in the 30% DMSO solvent system.21,20,24 The C═O stretching vibration peaks for NMP and DMAC in 30% DMSO solution shifted to 1671 cm-1 and 1637 cm-1, respectively, and the S═O stretching vibration peak for DMSO shifted to lower wavenumber of 1024 cm-1 in 30% DMSO solution. Thus, such significant shift in the S═O stretching vibration of DMSO as compared to the C═O stretching vibrations of NMP and DMAC is indicative of a stable precursor-DMSO adduct in the solution (Figure 5b & Table 1).28,19 This adduct formation also advocates the strength of controlled amount of DMSO to interact with precursor in the presence of other solvents in the system, which eventually leads to the evolution of a quality perovskite layer at low temperature. Table 1. Summarized detail of the change in stretching vibration of each solvent in 30% DMSO solvent system and its solution. Stretching Vibration (ν)

Solvent System

Solution (IP)

cm-1

DMAc

C═O

1679

1671 cm-1

NMP

C═O

1644 cm-1

1637 cm-1

DMSO

S═O

1058 cm-1

1024 cm-1



As schemed in Figure 5c, we have proposed the evolution mechanism of the low-tremperature annealed perovskite film. The ternary solvent based as-deposited solution boost fast crystallisation once DMAC and NMP washes out during the process of anti-solvent washing. However, DMSO remains in the depostied films as a strong retarding molecule. Thanks to its ability to produce precursor-DMSO adduct, which results in a controlled fast crystallisation at low temperature. Figure 5d shows schematic diagram of the final device architecture. Where active layer is sandwitched between hole transfer layer (HTL) and electron transfer layer (ETL) in a mesoscopic device configuration. To further dig into the mechanism, top SEM images of the non-annealed films are also studied. All the non-annealed films (Figure S2) show proximity to their respective final annealed films. Films without DMSO exhibits pin holes with seed crystals of rod-shape grains, most likely due to the absence of the retarding agent like DMSO in the solution triggers the formation of such irregular film template. While films with 10% and 20% DMSO show a bit improved templates for film formation but still lack uniformity. Films with 30%, 40% and 50% DMSO reveal gradual improvement in uniformity with alligned seed crystals distribution in the film templates with the increased DMSO volume. These non-annealed film morphologies confirm that even after the removal of DMAC/NMP the presence of controlled DMSO amount in the film keeps seed crystals aligned without any major irregularity, which could produces high-quality perovskite layer at desired low-temperature annealing conditions. UV-vis spectroscopy measurement was conducted to evaluate light absorption ability of the prepared films, as shown in Figure 6a. Compared with the pristine film (without DMSO), the films with DMSO show stronger absorption throughout the entire light wavelength, except for the film with 50% DMSO. Among all the films, the one with 30 % DMSO has the strongest absorption, revealing a better light management ability of the perovskite layer. While the films without and with 50% DMSO stand weaker, indicative of a poor light management ability. Magnified UV-vis spectra of the films without DMSO and with 30% DMSO are shown in Figure 6b. The film with 30% DMSO shows a positive shift in absorption edge of the perovskite energy level compared to pristine one, thus which can capture photons with low energy due to its narrower band gap. It has been reported previously that even without compositional engineering (mixed cation), absorption edge can be tuned by improving the crystallinity and film homogeneity.21,29,30 However, such red shifts in absorption edge are usually of little magnitude (

Low-Temperature Annealed Perovskite Films: A Trade-Off between

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