Research Article www.acsami.org
Solvent-Mediated Intragranular-Coarsening of CH3NH3PbI3 Thin Films toward High-Performance Perovskite Photovoltaics Wu-Qiang Wu,† Dehong Chen,*,†,∥ William A. McMaster,† Yi-Bing Cheng,*,‡ and Rachel A. Caruso*,†,§,⊥ †
Particulate Fluids Processing Centre, School of Chemistry, The University of Melbourne, Melbourne, Victoria 3010, Australia Department of Materials Science and Engineering, Monash University, Clayton, Victoria 3800, Australia § CSIRO Manufacturing, Private Bag 10, Clayton South, Victoria 3169, Australia ‡
S Supporting Information *
ABSTRACT: The deposition of dense and uniform perovskite films with large grains is crucial for fabricating highperformance perovskite solar cells (PSCs). High-quality CH3NH3PbI3 films were produced by a self-induced intragranular-coarsening approach. The perovskite precursor solution contained a Lewis base, N,N-dimethyl sulfoxide (DMSO), and was deposited using a gas-assisted, one-step, spin-coating method that was followed by a solvent vapor-assisted annealing treatment using a mix of DMSO and chlorobenzene (CBZ). Combining solvent-engineering with gas-assisted deposition helps to form intermediate crystalline entities upon evaporation of the parent solvent but retards the otherwise fast reaction between the precursor ingredients. Subsequent cosolvent annealing induces further grain-coarsening via a facilitated dissolution−precipitation process. This technique produced flat CH3NH3PbI3 films featuring large grain microstructures, with well-coarsened subgrains and a reduction of intragranular defects that minimized carrier recombination. The optimized CH3NH3PbI3 films exhibited enhanced crystallinity, excellent carrier transport and injection, as well as suppressed charge recombination. Benefiting from these advantages, PSCs based on the optimized perovskite films delivered a power conversion efficiency of 17.99% and a stabilized power output above 17.30%. This study presents an effective strategy for the fabrication of high-quality, hybrid perovskite films with potential applications in optoelectronic devices. KEYWORDS: solvent engineering, grains, Ostwald ripening, intermediate phase, solar cells
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INTRODUCTION Following the introduction in 2009 of solution-processed, organic−inorganic hybrid perovskite materials as light absorbers, the photovoltaic field has undergone a significant revolution.1−4 A surge of progress in device architecture,2,3,5 perovskite film fabrication processing,4,6,7 and composition8,9 has led to rapid improvements in the power conversion efficiency (PCE) of perovskite solar cells (PSCs) with certified values of up to 22.1%.10 Methylammonium lead iodide (CH3NH3PbI3, MAPbI3) has been the most frequently used light harvester in perovskite photovoltaic devices.11,12 The growth of a uniform perovskite film with well-defined grain structure, full surface coverage, flat surface, and high crystallinity is regarded as essential for the realization of an efficient PSC.13,14 Various methodologies such as one-step spin-coating,15 twostep sequential deposition,4,16 and vapor deposition6 have been developed to produce high-quality MAPbI3 films. In particular, the solution-processed, one-step spin-coating approach tends to be more promising than the other methods due to its simplicity and potentially low fabrication cost. This technique was successful in the preparation of perovskite films, especially for those with n-i-p planar device architectures.14,15 However, conventional one-step deposition, that is, spin-coating of the © XXXX American Chemical Society
perovskite precursor containing lead iodide (PbI2) and methlyammonium iodide (CH3NH3I, MAI) in N,N-dimethylformamide (DMF), often results in films with uncontrolled crystal sizes and surface morphology, and also incomplete surface coverage. These deficiencies are caused by simultaneous evaporation of excess solvent and crystal nucleation and growth during the film formation. Hence, it is a challenge to form a flat and uniform perovskite film with full surface coverage.17 A great deal of effort has been invested in controlling the rapid crystallization behavior of perovskite materials to obtain uniform and pinhole-free perovskite films, including designing the precursor composition (e.g., mixed cations and/or anions,18,19 additive-assisted,20,21 and adduct-approach22,23), solvent engineering,7,14 different deposition methods (e.g., gas-assisted15 and solvent−solvent extraction24), and optimizing the annealing conditions (e.g., solvent-annealing13,25 and air-exposure26−28). Yet, due to the poor thermal stability of the perovskite materials, films obtained by the optimized one-step, solution-processed spin-coating approach are usually composed of packed small grains (approximately 100−500 nm in Received: July 6, 2017 Accepted: August 29, 2017
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DOI: 10.1021/acsami.7b09822 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
(CBZ), a well-coarsened, mirror-like perovskite film with large microscale grains and well-interconnected subgrain morphology was obtained. As a result, an efficient PSC with excellent reproducibility was achieved, yielding a top PCE of 17.99% and a stabilized power output above 17.30%. This work gathers together the solvent engineering of the perovskite precursor during the gas-assisted perovskite deposition and subsequent cosolvent vapor annealing for further grain-coarsening via Ostwald ripening. This protocol produces a high-quality, smooth, pinhole-free perovskite film with high crystallinity and an interconnected granular network.
diameter)15 containing a number of crystal defects and grain boundaries. Moreover, the defects and grain boundaries in polycrystalline perovskite films hinder the diffusion of photogenerated carriers, thus leading to undesirable recombination and energy loss. This reduces the charge carrier lifetime and is detrimental to the photovoltaic performance of PSCs.29,30 Therefore, there is still plenty of scope for the optimization of one-step, solution-processed perovskite films composed of large grains but with fewer intragranular imperfections, which would benefit PSC performance. In this context, self-induced intragranular-coarsening of pristine films during the perovskite deposition process is a rather attractive concept for the formation of high-quality perovskite films with few intragranular defects and imperfections, yet there is a dearth of knowledge in this area. Regarding the formation of one-step, solution-processed perovskite films, the selection of polar solvents to dissolve both inorganic lead halides (PbX2, where X = I, Br, or Cl) and organic iodide (MAI) plays a critical role in determining the perovskite crystal growth. A strongly coordinative solvent like the Lewis base N,N-dimethyl sulfoxide (DMSO), which has a relatively high boiling point (189 °C), low volatility, and low saturated vapor pressure (0.76 kPa at 60 °C), has recently been employed to control the crystal growth and surface morphology of the perovskite films.31−33 Taking advantage of the strong coordination and interactivity of DMSO with PbI2 and/or I−, the crystallization of perovskite materials could be effectively retarded.31 Central to the control of crystallization is the formation of an intermediate precursor complex that inhibits the rapid reaction between PbX2 and I− (from the MAI) in the solution, which is beneficial in fabricating a high-quality dense and flat perovskite film.22 Very recently, a Lewis acid−base adduct approach was developed toward depositing high-quality, pinhole-free MAPbI3 films for improved PSC performance with high reproducibility using the adduct CH3NH3I·PbI2·DMSO, an intermediate phase formed during the one-step spin-coating process.22,34 Moreover, Yang et al. demonstrated that a direct intramolecular exchange of DMSO intercalated in PbI2 with formamidinium iodide (FAI) could result in extremely uniform FAPbI3 films, producing PSCs with a certified PCE over 20%.32 Li et al. proposed a molecular self-assembly strategy toward fabricating perovskite films with controllable grain morphology by adjusting the DMF:DMSO ratio.35 All of these studies illustrate the favorable effect DMSO has on controlling the morphology and crystallization of perovskite thin films. In the present study, a gas-assisted, one-step, spin-coating method using a mixed DMF/DMSO solvent for the perovskite precursor solution was employed to fabricate high-quality MAPbI3 perovskite films. By systematically adjusting the DMF:DMSO ratio and annealing temperatures, self-induced intragranular coarsening of the perovskite occurred, which led to the formation of highly crystalline and uniform MAPbI3 films composed of enlarged grain sizes (approximately 1−4 μm) with reduced grain boundaries and less intragranular defects. A DMF:DMSO volume ratio of 4:1 effectively controlled the grain morphology and crystallization of the perovskite films. The PCE of the corresponding planar PSCs was increased from 13.13% (pure DMF) to 16.08% (DMF:DMSO = 4:1). A plausible mechanism for the perovskite crystal growth and film formation by self-induced intragranular coarsening in the absence or presence of DMSO was proposed in view of the microscopic dynamics. In combination with the mixed solventassisted annealing technique using 1:1 DMSO:chlorobenzene
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EXPERIMENTAL SECTION
Device Fabrication. Layers of TiO2 nanoparticles (TNP) were deposited on cleaned and laser-patterned fluorine-doped tin oxide (FTO) glass (TEC8, Dyesol) via a previously reported hydrothermal process.25 Typically, an FTO glass electrode was immersed in 14 mM potassium titanium oxide oxalate dihydrate (Sigma-Aldrich) aqueous solution inside a Teflon-lined steel autoclave. The autoclave was sealed and heated at 140 °C for 3 h. After being cooled, the as-prepared TNP films were rinsed with Milli-Q water and ethanol several times, followed by annealing in air at 500 °C for 1 h. The TNP films were treated with an aqueous 40 mM TiCl4 (Sigma-Aldrich) solution at 70 °C for 30 min, cleaned with Milli-Q water, and then sintered at 500 °C for 30 min. The 50 wt % CH3NH3PbI3 perovskite precursor solution was prepared by mixing 350.2 mg of PbI2 (Sigma-Aldrich) and 121.0 mg of CH3NH3I (MAI, in-house) in a 500 μL mixture of anhydrous N,Ndimethylformamide (DMF, Sigma-Aldrich) and N,N-dimethyl sulfoxide (DMSO, Sigma-Aldrich) with different volume ratios of 1:0, 4:1, 3:2, 1:1, and 0:1. The mixture (25 μL) was then spin-coated on the FTO/TNP films via an argon gas-assisted method15 at 6500 rpm for 30 s. The perovskite films were denoted as DMSO-x, where x is the DMSO μL volume in the solvent mixture. The transparent CH3NH3PbI3·DMSO·DMF films were then heated in the presence (cosolvent annealing, CSA) or absence (thermal annealing, TA) of 10 μL of DMSO/CBZ 1:1 (volume ratio) at 70−150 °C for 10 min to obtain well-coarsened CH3NH3PbI3 perovskite films. The coarsened films are denoted DMSO-x-CSA or DMSO-x-TA. The hole-transporting layer (HTL) solution was then deposited via spin-coating of a spiro-MeOTAD solution with 41.6 mg of spiroMeOTAD (Lumtec) dissolved in 500 μL of CBZ, 7.5 μL of lithium bis(trifluoromethylsulfonyl)imide in acetonitrile (500 mg mL−1), and 14.4 μL of 4-tert-butylpyridine (Sigma-Aldrich). To complete the device fabrication, 80 nm gold electrodes were thermally evaporated onto the films under a vacuum of approximately 10−5 Torr. Characterization. Scanning electron microscopy (SEM) was performed using an FEI Quanta 200F field emission scanning electron microscope to study surface morphologies and obtain cross-sectional images. X-ray diffraction (XRD) patterns were acquired on a Bruker D8 Advance diffractometer using Cu Kα radiation to determine sample phase composition and crystallinity. UV−visible absorption spectra of the DMSO-x-TA or -CSA films were recorded with a PerkinElmer Lambda 1050 UV/vis/NIR spectrophotometer equipped with a 150 mm integrating sphere. Photocurrent−photovoltage (J−V) characteristics of the PSCs were measured with an Oriel solar simulator equipped with a Keithley 2400 source meter. Before each measurement, the light intensity was calibrated by a silicon reference diode (Peccell Technologies) equipped with an infrared cutoff KG3 filter and adjusted to 1 sun (AM 1.5 G, 100 mW cm−2). While measuring, the cell was covered with a metal mask with an aperture of 0.16 cm2. An incident photon-to-electron conversion efficiency (IPCE) spectrum was recorded from 330 to 800 nm on a Keithley 2400 source meter under the irradiation of a 300 W xenon lamp fitted with an Oriel CornerstoneTM 260 1/4 m monochromator. The steady-state photoluminescence (PL) emission spectra were recorded on a FluoroMax-4 spectrofluorometer from 650 to 850 nm with an excitation wavelength of 532 nm. Time-resolved PL measurements B
DOI: 10.1021/acsami.7b09822 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. Schematic view of the crystal growth process of perovskite films in (a) DMF and (b) a DMF/DMSO mixture. The MAI·PbI2·DMF (orange-colored film) or MAI·PbI2·DMSO·DMF (light yellow and transparent film) intermediate complex was formed after the gas-assisted spincoating step in the absence or presence of DMSO, which plays a crucial role in film formation and crystal growth. (c) Schematic illustration of the self-induced intragranular-coarsening of the perovskite film in the presence of DMSO, in which the small grains are gradually consumed and merge with the remaining large grains, accompanied by the release of DMSO molecules, leading to a smooth and homogeneous MAPbI3 film with coarsened large grains.
perovskite films were thermally annealed at 100 °C for 10 min. The film samples are referred to as DMSO-0-TA, DMSO-100TA, DMSO-200-TA, DMSO-250-TA, and DMSO-500-TA, where the appended number represents the actual volume of DMSO present in the 500 μL solvent mixture and TA indicates standard thermal annealing conditions. When the DMF/ DMSO perovskite precursor solution was combined with the gas-assisted technique during spin-coating, highly densified perovskite films composed of fine grains (Figure S3) and improved crystallinity, determined from XRD (Figure S4), were obtained. The perovskite films produced using an increasing DMSO volume showed coarsening of grain morphology similar to that of DMSO-100-TA, but also slightly increased surface roughness (Figure S5). Additionally, the film prepared with pure DMSO, that is, DMSO-500-TA, had voids and incomplete surface coverage (Figure S5c), which was most likely due to solvent dewetting. Therefore, a 4:1 volume ratio of DMF:DMSO was considered to be ideal to both control the grain morphology and retard crystallization of the perovskite film during the spin-coating deposition. The formation of a flat and uniform perovskite film with wellcoarsened grains can be attributed to the controlled dynamics of solvent evaporation, crystal growth, and crystallization during the spin-coating and annealing processes. The presence of DMSO appeared to be responsible for grain-coarsening within the perovskite films. Furthermore, it is known that DMSO coordinates with PbI2 and MAI to form an intermediate complex of MAI·PbI2·DMSO after spin-coating, but the DMSO ligand is sufficiently labile to be released upon heating.22 When fabricating a PSC, perovskite deposition is followed by annealing. Significant perovskite grain-coarsening behavior may occur during the annealing step while the DMSO is gradually evaporated. Specifically, the annealing conditions (i.e., temperature and time) may affect the crystal growth of the
were performed using an in-house setup. Samples were photoexcited by 532 nm laser pulses (30 ps fwhm, 1 μJ cm−2 per pulse) at 50 Hz, and the signal was recorded at 780 nm with a 10 nm bandwidth. Intensity-modulated photovoltage spectroscopy (IMVS) measurements were recorded on a Zahner Zennium electrochemical workstation with a frequency response analyzer under modulated green light emitting diodes (474 nm) driven by a Zahner PP211 source supply, yielding light with frequency ranging from 10 Hz to 10 kHz and illumination intensity from 8 to 273 mW cm−2.
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RESULTS AND DISCUSSION Perovskite films were deposited by spin-coating a 25 μL precursor solution containing equimolar amounts of PbI2 and MAI in a variable ratio solvent mixture of DMF/DMSO onto a film of uniform TiO2 nanoparticles (TNP, Figure S1), which were observed by SEM. The TNP films were previously grown on fluorine doped tin oxide (FTO) glass substrates by a hydrothermal process. Conventional spin-coating of the perovskite precursor solution using only DMF led to dendrite-like MAPbI3 perovskite crystals that did not fully cover the substrate, with voids in the film up to several micrometers across (Figure S2a). The poor surface coverage was likely induced by differences in crystal growth rate, originating from the incompatible solubility of PbI2 and MAI in DMF,15,22 because DMF rapidly evaporated during spinning. However, partial substitution of DMF by DMSO in the solvent mixture (i.e., DMF:DMSO = 4:1) resulted in improved surface coverage of the MAPbI3 crystal on the substrate (Figure S2b), indicating that the presence of DMSO plays a crucial role in regulating the crystal growth, and thus the morphology, of MAPbI3 films. In the present study, the effects of the DMF:DMSO volume ratio on the perovskite film morphology, crystallization quality, and photovoltaic performance were investigated. DMF:DMSO volume ratios of 1:0, 4:1, 3:2, 1:1, and 0:1 were used for the perovskite precursor solution, and the C
DOI: 10.1021/acsami.7b09822 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces perovskite films. Hence, an as-prepared DMSO-100 film that was annealed directly at 100 °C for 10 min (DMSO-100-TA, Figure S3c) was compared to one initially annealed at 65 °C for 1 min and then at 100 °C for 10 min (Figure S6). Graincoarsening was not obvious for the film that was initially annealed at 65 °C. At low-temperature, the removal of DMSO took the primary role, which meant that DMSO evaporated faster than the crystal growth of MAPbI3. Hence, the resulting film featuring small packed grains had a morphology similar to a film produced in pure DMF, that is, DMSO-0-TA (compare Figure S6 and Figure S3a). Contrastingly, when the DMSO-100 film was directly annealed at 100 °C, crystal growth was faster than solvent evaporation, and the coarsening of grains (Figure S3c) was beneficially assisted by a large volume expansion of the precursor solution at the higher temperature. To further explore the relationship between annealing temperature and grain-coarsening, DMSO-100 films were annealed directly at 70 or 150 °C for 10 min. Figure S7 shows the surface morphology, and Figure S8 shows the XRD patterns of these perovskite films. Again, the results confirmed that low temperature annealing (i.e.,
