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High-performance CH3NH3PbI3 inverted planar perovskite solar cells via ammonium halide additives
Journal of Industrial and Engineering Chemistry 80 (2019) 265–272
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High-performance CH3NH3PbI3 inverted planar perovskite solar cells via ammonium halide additives Muhammad Jahandara,b,1,2 , Nasir Khanb,c,1,2 , Muhammad Jahankhanb,c, Chang Eun Songb,c,* , Hang Ken Leec , Sang Kyu Leea,b , Won Suk Shina,b , Jong-Cheol Leea,b , Sang Hyuk Imd,* , Sang-Jin Moona,b,* a
Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT), 141 Gajeong-ro, Yuseong, Daejeon 34114, Republic of Korea Advanced Materials and Chemical Engineering, University of Science and Technology (UST), 217 Gajeongro, Yuseong, Daejeon 34113, Republic of Korea c Energy Materials Research Center, Korea Research Institute of Chemical Technology (KRICT), 141 Gajeong-ro, Yuseong, Daejeon 34114, Republic of Korea d Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 136-713, Republic of Korea b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 21 February 2019 Received in revised form 29 July 2019 Accepted 2 August 2019 Available online 9 August 2019
Organic-inorganic hybrid perovskites have recently attracted substantial attention as a top candidate for use as light-absorbing materials in high-efficiency, low-cost and solution-processable photovoltaic devices owing to their excellent optoelectronic properties. Here, we fabricated inverted planar perovskite solar cells by incorporating small amounts of ammonium halide NH4X (X = F, Cl, Br, I) additives into a CH3NH3PbI3 (MAPbI3) perovskite solution. A compact and uniform perovskite absorber layer with large perovskite crystalline grains is realized by simply incorporating small amounts of additives and by using an anti-solvent engineering technique to control the nucleation and crystal growth of perovskite. The enlarged perovskite grain size with a reduced density of the grain boundaries and improved crystallinity results in fewer charge carrier recombinations and a reduced defect density, leading to enhanced device efficiency (NH4F: 14.88 0.33%, NH4Cl: 16.63 0.21%, NH4Br: 16.64 0.35%, and NH4I: 17.28 0.15%) compared to that of a reference MAPbI3 device (Ref.: 12.95 0.48%) and greater device stability. This simple technique involving the introduction of small amounts of ammonium halide additives to regulate the nucleation and crystal growth of perovskite films translates into highly reproducible enhanced device performance. © 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: CH3NH3PbI3perovskite Inverted planar structure Ammonium halide additives Anti-Solvent engineering Perovskite grain size
Introduction The area of organic-inorganic hybrid perovskite photovoltaics is among the most propitious and most rapidly growing technologies owing to its low costs, solution processability and excellent intrinsic optoelectronic properties [1–3]. Over the past few years, remarkable developments in perovskite material compositions, device architectures and in the interfacial engineering of perovskite solar cells (PvSCs) with the highest ever power conversion efficiency (PCE) of more than 23% have made these devices captivating candidates for future energy applications [4–13]. The
* Corresponding authors. E-mail addresses: [email protected] (C.E. Song), [email protected] (S.H. Im), [email protected] (S.-J. Moon). 1 First authors. 2 M.Jahandar and N.K. have equally contributed to this work.
family of three-dimensional organic-inorganic halide perovskite materials used for photovoltaics has a simple AMX3 perovskite structure normally composed of an organic/inorganic cation (e.g., A = methylammonium (CH3NH3+; MA), formamidinium (CH (NH2)2+; FA) and cesium (Cs+; Cs)), a divalent metal (M = lead (Pb2+), tin (Sn2+)) and a halide anion (X = Cl, Br and I) [14–18]. The chemical flexibility of the organic-inorganic halide perovskite structure makes this material important for optoelectronic devices such as light-emitting diodes, photodetectors, and various photovoltaic applications [19–22]. Although the conventional n-i-p mesoporous PvSCs device architecture has demonstrated the highest ever PCE, it requires a high processing temperature to form a mesoscopic TiO2 electron conductor and often exhibits significant current-voltage (J-V) hysteresis [23,24]. In contrast, planar PvSCs have the simplest device architecture, allowing fabrication at a low temperature with a high PCE (over 20%) and without significant J-V hysteresis [25,26]. Proper control of the J-V curve hysteresis is very important to
https://doi.org/10.1016/j.jiec.2019.08.004 1226-086X/© 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
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realize actual stable device performance. Previous reports suggests that J-V curve hysteresis in PvSCs at different scan rates with respect to the scan direction can arise due to unbalanced electrons and hole flux or due to interphase defects and charge traps in lowquality perovskite film [27,28]. Mohit et al. reported millimeterscale CH3NH3PbI3 (MAPbI3) perovskite grains fabricated by a solution-process hot-casting technique and showed a hysteresisless PCE of 18% by using an inverted planar perovskite device architecture [29]. Similarly, Heo et al. reported hysteresis-less inverted planar PvSCs with a PCE of 18.1% [30]. In their study, they describe the significant role of balancing the electrons and hole flux in the PvSCs to realize reduced hysteresis characteristics by utilizing [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as the electrons and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as the hole-transporting layer with similar levels of conductivity s (sPEDOT:PSS: 0.016 mS/cm and sPCBM: 0.014 mS/cm). In addition, improved surface coverage and a large grain size of perovskite films can effectively reduce defects which act as recombination centers of photo-induced charge carriers and diminish the J-V hysteresis by suppressing charge carrier trapping during the operation of the device. In this regard, the planar PvSC device architecture provides the best opportunity to fabricate uniform and pin-hole free perovskite absorber layers with larger crystalline grains and energy bands well aligned to electron/hole transport layers with similar conductivity levels. When preparing pin-hole free and high-quality polycrystalline perovskite films with larger crystalline grains, the presence of grain boundaries is inevitable [31,32]. The optoelectronic properties and device performance capabilities of polycrystalline PvSCs are highly dependent on the crystallinity and grain size of the perovskite film, as the diffusion length and charge carrier mobility can be significantly improved in perovskite films with large perovskite grains [33]. The improved electronic properties of perovskite films are normally ascribed to decreased grain boundary densities, which can diminish the presence of traps and reduce the number of charge carrier recombinations. However, fabricating large-scale crystal growth and high-quality films with minimized amounts of pinholes in perovskite films is challenging using solutionprocessed PvSCs due to the low molecular kinetic energy that results from the low-temperature process. In general, the quality of the perovskite absorber layer is controlled by nucleation and perovskite crystal growth [34]. Recently, several groups have addressed the issue of nucleation and perovskite crystal growth to obtain high-quality perovskite film, as both rapid nucleation and retarded perovskite crystal growth are key factors when fabricating highly crystalline large perovskite grains [35,36]. To control the nucleation process, several protocols have been reported. Among them, anti-solvent engineering can successfully facilitate rapid nucleation through the quick and well-controlled removal of solvents to balance the precursor precipitation time [6,37]. However, quick crystallization of perovskite films results poor film morphology, increasing the number of recombination sites. To overcome the rapid crystallization of perovskite films, several strategies has been employed to moderate the perovskite growth rate, such as hot-casting, solvent annealing and additive engineering [29,38–40]. Among these strategies, the use of additives such as polymers, fullerenes, metal halide salts, organic halide salts, inorganic acids and ionic liquids have proven to be vital to not only retard the perovskite crystal growth but also reduce the grain boundary density due to the formation of high-quality large crystalline grains [41–46]. In this work, we introduce a small amount of ammonium halide (NH4X, where X = F, Cl, Br, I) additives into the MAPbI3 perovskite absorber layer to fabricate inverted planar PvSCs. The addition of ammonium halide salts in MAPbI3 perovskite solutions and an anti-solvent engineering technique help to control perovskite
nucleation and crystal growth. A compact and uniform perovskite absorber layer with an enlarged perovskite grain size, reduced density of the grain boundaries, and improved crystallinity combine to reduce charge carrier recombinations and lower the defect density level, leading to enhanced efficiency and better device stability in PvSCs. Experimental Materials and preparation of perovskite precursor solution Lead iodide (99.999% trace metals basis), ammonium fluoride (NH4F) (99.99% trace metals basis), ammonium chloride (NH4Cl) (99.99% trace metals basis), ammonium bromide (NH4Br) (99.999% trace metals basis), ammonium iodide (NH4I) (99.999% trace metals basis), dimethyl sulfoxide (DMSO), ɣ-butyrolactone (GBL) and chlorobenzene (CB) were purchased from Sigma-Aldrich. Methylammonium iodide (MAI) was purchased from DyeSol and all the materials were used as received without any further purification. To prepare the perovskite precursor solution, we mixed MAI (159 mg) powder and PbI2 (461 mg) (1:1 molar ratio) in 1 mL mixed GBL:DMSO (0.7:0.3) solvent for the reference perovskite precursor solution. Whereas, for the NH4X (X = F, Cl, Br, I) incorporated MAPbI3 perovskite solution, 0.10 M of NH4F (3.70 mg), NH4Cl (5.34 mg), NH4Br (9.79 mg) and NH4I (14.49 mg) were added in the reference perovskite precursor solution. All perovskite precursor solutions were kept for stirring at 70 C for overnight before use. The important point here to be noted is that the solubility of NH4F is very low. Although, we added very small amount (3.70 mg) in 1 mL reference perovskite precursor solution but it was not well soluble and need to filter to remove the insoluble NH4F. Whereas, other ammonium halide materials showed good solubility with given quantities. Device fabrication For inverted planar perovskite solar cells device fabrication, firstly, the patterned glass/ITO substrates were cleaned with DI water, acetone and isopropanol and dried in drying oven at 140 C for overnight. PEDOT:PSS (Clevios P VP AI4083) was spin-coated on UV-ozone treated glass/ITO substrates at 4000 rpm for 60 s in air and dried at 150 C for 20 min. Then, the samples were transferred to the N2 filled glove-box for further device fabrication steps. Perovskite precursor solution without and with NH4X (X = F, Cl, Br, I) was spin coated in N2 filled glove-box at 2000 rpm for 60-70 s, followed by a step of 1000 rpm for 20 s. During the 2nd step of 2000 rpm for 60 s, a chlorobenzene (CB) solution (400 mL) was dropped on the substrate during spin coating after 40 s and continued the spin for further 20 s. The important point to be noted here that the CB dripping time during 2nd spin-coating step was further delayed approximately 5 to 10 s for NH4X (X = F, Cl, Br, I) containing perovskite precursor solutions as compare to reference solution. Then, the samples were dried on hot plate at 100 C for 3 min. Then PC61BM (purchased from OSM, Republic of Korea) as ETL was deposited on the glass/ITO/ PEDOT:PSS/perovskite substrate by spin coating PC61BM (20 mg/1 mL in CB) solution at 1200 rpm for 30 s followed by a final spin-coating step of 2000 rpm for 2 s. Finally, the LiF/Al (0.5 nm/100 nm) electrode was deposited by thermal evaporation. Results and discussion MAPbI3 perovskite absorber layers with ammonium halide additives dissolved in a ɣ-butyrolactone (GBL) and dimethyl sulfoxide (DMSO) mixed solvent system were prepared by means of a chlorobenzene (CB) anti-solvent dripping technique. This type
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of mixed solvent system can help suitably to dissolve lead iodide (PbI2), methylammonium iodide (MAI) and NH4X (X = F, Cl, Br, I), whereas the anti-solvent CB dripping factor improves the uniform nucleation process of perovskites, resulting in the formation of very smooth film with large perovskite grains. The preparation of the perovskite precursor solution is described in detail in the experimental section (see Supporting Material). Fig. 1a exhibits a cross-sectional transmission electron microscope (TEM) image of inverted planar PvSCs with the composition of ITO/PEDOT:PSS/ perovskite layer/PC61BM/LiF/Al, and Fig. 1b illustrates the corresponding energy band diagram. The inverted planar PvSCs with the PEDOT:PSS hole-transporting layer (HTL) and the PC61BM electrontransporting layer (ETL) offer better energy band alignment and well-balanced hole/electron flux in this device architecture, leading to a high PCE without significant J-V curve hysteresis under forward and reverse scan directions [47,48]. Fig. 1c illustrates the UV-visible absorption spectra of a control sample and the MAPbI3 perovskite films incorporating ammonium halide as an additive. All of the perovskite films were prepared under identical conditions on quartz glass/ITO substrates with a film thickness of approximately 350 nm. The perovskite films with ammonium halide additives show slightly stronger absorption as compared to the control MAPbI3 perovskite absorber layer, whereas a slight blue shift of 2–3 nm was observed when adding NH4Br, which may be due to the presence of Br. Fig. 1d shows the crystal structure of the MAPbI3 and ammonium halide, and Fig. 1e exhibits a schematic diagram of the fabrication process of MAPbI3 perovskite absorber layers with and without ammonium halide additives. Perovskite films without and with 10 mol% NH4X (X = F, Cl, Br, I) additives in the GBL:DMSO mixed solvent system were deposited onto glass/ITO/PEDOT:PSS substrates by means of the CB antisolvent dripping method. During the formation of the perovskite films, we found that the optimized CB anti-solvent dripping time for NH4X (X = F, Cl, Br, I) incorporating MAPbI3 perovskite films is approximately five to ten seconds delayed as compared to that of the reference MAPbI3 perovskite film. A schematic of the spincoating process of the perovskite solutions and the CB anti-solvent dripping process is illustrated in Fig. S1. As the solubility of PbI2 is low compared to that of MAI, the quick precipitation of PbI2 during
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the spin-coating process can occur, possibly resulting in poor film formation with numerous defects [34,35]. In contrast, the addition of NH4X (X = F, Cl, Br, I) to the MAPbI3 perovskite precursor solution can delay the precipitation of PbI2 by retarding the PbI2 nucleation rate during the spin-coating step, leading to better film formation. With NH4X (X = F, Cl, Br, I) additives, the ammonium (NH4+) ion with a small ionic radius favorably diffuses into the [PbI6]4 octahedral layer. This type of NH4+ cation successfully occupies the cavity of the lead coordination complex, contributing to extra heterogeneous nucleation sites and reducing the defects stemming from the absence of MA+ [49]. On the other hand, the CB anti-solvent dripping technique not only promotes uniform rapid nucleation but also helps to form an intermediate MAI-PbI2-DMSO complex as compared to MAPbI3, which transforms into a uniform dense perovskite film with a large grain size upon thermal annealing [40]. Through this process, compact uniform perovskite film can easily be formed, but the CB dripping timing with regard to the perovskite precursor solution during the spin-coating step is critical. To investigate the effect of NH4X (X = F, Cl, Br, I) additives on the MAPbI3 perovskite crystalline grain growth, perovskite films created with the optimized fabrication condition were analyzed using scanning electron microscopy (SEM), as shown in Fig. 2a–e. The SEM images shows compact and homogeneous perovskite films, and it is clear that the MAPbI3 perovskite grains grew from 200 nm to 400 nm to >1 mm in size with the incorporation of the NH4X (X = F, Cl, Br, I) additives. Such an enlarged grain size compared to the control MAPbI3 perovskite film is attributed to the incorporation of the NH4X (X = F, Cl, Br, I) additives. In contrast, among other ammonium halide additives, NH4I when incorporated into the MAPbI3 perovskite film led to uniformly large crystalline grains. As the ionic radii of the halide ions increase, the solubility of the ammonium halides also increases. Among the NH4X (X = F, Cl, Br, I) additives, the solubility of NH4I was the highest, whereas NH4F exhibits the lowest solubility above room temperature [50]. The growth of the MAPbI3 perovskite grains size with the NH4I additive as compared to the non-uniformity of the grain size of NH4F when incorporated into MAPbI3 perovskite films demonstrates that the solubility of the ammonium halide additives can affect the nucleation/grain growth process of perovskite films. The
Fig. 1. (a) Cross-sectional TEM image and (b) energy band diagram of ITO/PEDOT:PSS/Perovskite/PC61BM/LiF/Al inverted planar PvSCs. (c) UV-vis absorption spectra of reference and NH4X (X = F, Cl, Br, I) incorporating MAPbI3 perovskite film. (d) Schematic of perovskite and ammonium halide structure. (e) Schematic diagram of the fabrication process of MAPbI3 perovskite absorber layers with and without ammonium halide additives.
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Fig. 2. (a–e) FE-SEM images, and (f) XRD spectra of reference and NH4X (X = F, Cl, Br, I) incorporated MAPbI3 perovskite films.
increases in the grain size and crystallinity of MAPbI3 perovskite with the incorporation of NH4X (X = F, Cl, Br, I) additives were probed by X-ray diffraction (XRD), as shown in Fig. 2f. All of the perovskite films with or without NH4X (X = F, Cl, Br, I) additives exhibit diffraction peaks at 14.12 , 28.44 and 31.88 , corresponding to the (110), (220) and (310) crystal planes of the tetragonal perovskite structure. No significant new XRD peak or gradual peak shift was observed, which illustrates that the finial acquired MAPbI3 perovskite samples with the NH4X (X = F, Cl, Br, I) additives all maintained the intrinsic crystal structure. Compared to the control MAPbI3 perovskite film, the increased perovskite peak intensity and reduced full width at half maximum (FWHM) of these films clearly indicate the improved perovskite grain size and crystallinity due to the incorporation of the NH4X (X = F, Cl, Br, I) additives. The improved grain size and crystallinity of perovskites can reduce the density of grain boundaries and trap sites, resulting in enhanced photovoltaic properties [51]. Subsequently, to investigate the crystallinity and orientation of the perovskite nanostructure on the substrate, 2D scattering patterns were acquired with the grazing incidence wide-angle X-ray scattering (GIWAXS), which has become an important technique for characterizing organic semiconductor and perovskite thin films
[52–54]. The recorded 2D-GIWAXS patterns of the reference MAPbI3 perovskite films and those incorporating NH4X (X = F, Cl, Br, I) are displayed in Fig. S2a-e. The higher intensity of the (110) ring around q = 0.98 Å1 in the 2D-GIWAXS data reveals that the MAPbI3 perovskite films incorporating NH4X (X = F, Cl, Br, I) exhibit a preferential crystal orientation, with the c-axis perpendicular to the substrate. Furthermore, the azimuthal integration at around q = 0.98 Å1 represents the crystallites of the MAPbI3 perovskite films incorporating NH4X (X = F, Cl, Br, I) with the (110) plane oriented toward the out-of-plane direction compared to the reference MAPbI3 perovskite film (Fig. S2f). To estimate the appropriate amount of NH4X (X = F, Cl, Br, I) in the MAPbI3 PvSCs to optimize device performance capabilities, we fabricated inverted planar PvSCs with different amounts of ammonium halide and determined the J-V characteristics under one sun (100 mW/cm2 AM 1.5 G) light illumination, as shown in Fig. S3. The related photovoltaic parameters are summarized in Table S1. The J-V characteristics, the external quantum efficiency (EQE) curves and photovoltaic properties of the best MAPbI3 PvSCs with and without ammonium halide additives are illustrated in Fig. 3 and are summarized in Table 1, respectively. The reference device exhibited an open-circuit voltage (VOC) of 0.94 V, a short-circuit current
Fig. 3. (a–e) J–V curves of the reference and NH4X (X = F, Cl, Br, I) incorporating MAPbI3 inverted planar PvSCs under 1 sun light illumination with respect to scan direction and (f) corresponding EQE graph with integrated current density.
M. Jahandar et al. / Journal of Industrial and Engineering Chemistry 80 (2019) 265–272 Table 1 Photovoltaic parameters of reference and NH4X (X = F, Cl, Br, I) incorporating MAPbI3 inverted planar PvSCs. Perovskite film
Scan direction
VOC [V]
JSC [mA/cm2]
FF [%]
PCEa [%]
Avg. PCEb [%]
Ref.
Reverse Forward Reverse Forward Reverse Forward Reverse Forward Reverse Forward
0.94 0.94 0.98 0.98 0.99 0.99 1.02 1.02 1.00 1.00
18.56 18.46 19.10 19.13 21.01 21.03 20.33 20.10 21.02 21.04
78.95 77.90 80.42 79.90 80.80 80.31 81.14 80.51 82.43 82.38
13.74 13.47 15.15 15.02 16.88 16.78 16.85 16.53 17.44 17.34
12.95 0.48
NH4F NH4Cl NH4Br NH4I a b
14.88 0.33 16.63 0.21 16.64 0.35 17.28 0.15
Best device performance. Average device efficiency of independent 20 devices.
density (JSC) of 18.56 mA/cm2, a fill factor (FF) of 78.95%, and a PCE of 13.74% under the reverse scan direction, whereas under the forward scan direction, the values were as follows: VOC = 0.94 V, JSC = 18.46 mA/cm2, FF = 77.90% and PCE = 13.47%, with slight J-V curve hysteresis under both the forward and reverse scan directions. Among the MAPbI3 PvSCs incorporating NH4X (X = F, Cl, Br, I), the highest PCE was achieved with the addition of NH4I (VOC: 1.00 V, JSC: 21.02 mA/cm2, FF: 82.43% and PCE: 17.44% under the reverse scan direction and VOC: 1.00 V, JSC: 21.04 mA/cm2, FF: 82.38% and PCE: 17.34% under the forward scan direction) without significant J-V curve hysteresis. On the other hand, the photovoltaic properties when incorporating NH4F, NH4Cl and NH4Br were VOC: 0.98 V, JSC: 19.10 mA/cm2, FF: 80.42% and PCE: 15.15% under the reverse scan direction and VOC: 0.98 V, JSC: 19.13 mA/cm2, FF: 79.90% and PCE: 15.02% under the forward scan direction; VOC: 0.99 V, JSC: 21.01 mA/cm2, FF: 80.80% and PCE: 16.88% under the reverse scan direction and VOC: 0.99 V, JSC: 21.03 mA/cm2, FF: 80.31% and PCE: 16.78% under the forward scan direction; and VOC: 1.02 V, JSC: 20.33 mA/cm2, FF: 81.14% and PCE: 16.85% under the reverse scan direction and VOC: 1.02 V, JSC: 20.10 mA/cm2, FF: 80.51% and PCE: 16.53% under the forward scan direction, respectively.
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The improved device performance of the MAPbI3 PvSCs with NH4X (X = F, Cl, Br, I) incorporated into them additive can be attributed to the enhanced VOC, JSC and FF values. The larger crystal grains, preferential crystal orientation and high-quality perovskite film formation effectively enhanced the photovoltaic properties. The deviation of the VOC, JSC, FF and PCE values of 20 devices are shown in Fig. 4. The deviation outcomes in the efficiency were 12.95 0.48, 14.88 0.33, 16.63 0.21, 16.64 0.35 and 17.28 0.15% for the control sample and for the MAPbI3 PvSCs incorporating NH4F, NH4Cl, NH4Br and NH4I, respectively. The reproducible and noticeably enhanced values of VOC, JSC and FF of the PvSCs processed with NH4X (X = F, Cl, Br, I) additives have facilitated the achievement of consistently high PCEs compared to those of the control device. Fig. 3f displays the EQE spectrum and integrated JSC of the control sample and the MAPbI3 PvSCs incorporating NH4X (X = F, Cl, Br, I). The tendency of the integrated JSC value is in good agreement with the measured JSC value of the J-V curves under 1 sun illumination. A slight change in the shape of the EQE spectra was observed for the optimized PvSCs. The change in the shape of the EQE spectra may be due to the effects of optical interference and the change in the optimized absorber layer thickness [55]. The EQE is a product of the light harvesting, charge separation, and charge collection efficiency. The EQE spectrum also confirmed that the PvSCs incorporating NH4X (X = F, Cl, Br, I) had increased JSC values compared to that of the control cell. This is likely due to the formation of large crystal grains, the improved perovskite crystallinity, and the high-quality perovskite film [56]. These factors effectively reduced the number of charge carrier recombinations and improved the charge separation while also improving the charge collection efficiency. To probe the photo-generated charge recombination processes in MAPbI3 PvSCs with and without NH4X (X = F, Cl, Br, I) additives, we adopted the most common and simplest technique to identify trap-assisted and bimolecular recombinations during the operation of PvSCs by measuring the VOC and JSC as a function of the light intensity. The related J-V curves of PvSCs with respect to the change in the light intensity are shown in Fig. S4. At room temperature, by
Fig. 4. The average photovoltaic parameters of (a) power conversion efficiency (PCE), (b) open-circuit voltage (VOC), (c) short-circuit current density (JSC) and (d) fill factor (FF) for 20 devices of reference and NH4X (X = F, Cl, Br, I) incorporating MAPbI3 inverted planar PvSCs.
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Fig. 5. (a) VOC and (b) JSC values as a function of light intensity for reference and NH4X (X = F, Cl, Br, I) incorporating MAPbI3 inverted planar PeSCs.
linearly fitting VOC versus the logarithm of the light intensity, as shown in Fig. 5a, we found slopes of 1.29 kBT/q, 1.22 kBT/q, 1.18 kBT/q, 1.19 kBT/q and 1.16 kBT/q (where kB is the Boltzmann constant, T is the absolute temperature and q is the elementary charge) for the reference sample and for the MAPbI3 PvSCs incorporating NH4F, NH4Cl, NH4Br and NH4I, respectively. In an ideal case, the slope should be kBT/q in the absence of trap-assisted recombinations, whereas any deviation from the ideal value specifies the presence of trap-assisted recombinations [57]. The MAPbI3 PvSCs with NH4X (X = F, Cl, Br, I) additives showed fewer trap-assisted recombinations as compared to the reference MAPbI3 PvSCs, which indicates that the addition of NH4X (X = F, Cl, Br, I) to MAPbI3 films can reduce the number of recombination trap sites at the grain boundaries due to the formation of large crystalline grains and the reduced density of the grain boundaries. We also studied the dependence of the light intensity of J SC to understand the charge recombination mechanism. The powerlaw dependence of JSC on the light intensity can be represented by JSC / (Plight)α (α 1). The deviation of the slope from unity implies the occurrence of bimolecular recombinations [58]. Fig. 5b shows the relationship between JSC and the light intensity in a double
logarithmic scale with slopes of 0.95, 0.96, 0.97, 0.97 and 0.97 for the reference, NH4F, NH4Cl, NH4Br and NH4I PvSCs, respectively. These results indicate that bimolecular charge recombinations are suppressed due to the incorporation of NH4X (X = F, Cl, Br, I) into the MAPbI3 PvSCs [59]. Therefore, the synergetic negative effects of trap-assisted and bimolecular charge recombinations in the control MAPbI3 PvSCs lead to severe losses of the photovoltaic properties compared to the MAPbI3 PvSCs incorporating NH4X (X = F, Cl, Br, I). Furthermore, to assess the defect density (Ndefects) levels quantitatively, we prepared devices with the ITO/perovskite/ Au device architecture. The results of the evolution of the spacecharge-limited current (SCLC) for different biases were characterized as shown in Fig. 6a-e. The sharp rise of the dark J-V curve is associated with a trap-filled limit, where all the defects are occupied by charge carriers. The Ndefects value is calculated according to the following equation [60,61]. Ndefects = 2ee0VTFL/qL2
(1)
where Ndefects is the defect density, e is the dielectric constant of the perovskite material, e0 is the permittivity of free space
Fig. 6. (a–e) Current density-voltage characteristics of device with ITO/perovskite/Au configuration for estimating the defect density in perovskite films, and (f) steady-state PL spectra of reference and NH4X (X = F, Cl, Br, I) incorporated perovskite films.
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(8.854 1014 F/cm), q is the elementary charge and L is the thickness of the perovskite film. The values of Ndefects, calculated for the control device and for MAPbI3 perovskite devices incorporating NH4F, NH4Cl, NH4Br and NH4I, were 1.98 1016, 1.32 1016, 9.45 1015, 1.02 1016, and 8.38 1015 cm3, respectively. The density levels of the defects of the MAPbI3 perovskite devices incorporating NH4X (X = F, Cl, Br, I) were reduced significantly compared with that of the control device. The defect density of the control device is comparable to those of previous MAPbI3 PvSCs reported in the literature [58]. Moreover, to explain the mechanisms of the defect density and charge carrier recombination, the steady-state photoluminescence (PL) spectra of the control and the MAPbI3 perovskite films incorporating NH4X (X = F, Cl, Br, I) on quartz glass substrates were obtained, as shown in Fig. 6f. The PL intensity levels of the MAPbI3 perovskite films incorporating NH4X (X = F, Cl, Br, I) are higher than that of the control MAPbI3 perovskite film, which indicates that the non-radiative decay is suppressed in the MAPbI3 perovskite films which incorporate NH4X (X = F, Cl, Br, I) [62,63]. In general, PvSCs show J-V curve hysteresis with respect to the forward and reverse scan directions or at certain voltage scan rates. This is due to the unbalanced electron/hole flux, to interphase defects, and to charge traps in low-quality perovskite films [27,28]. Therefore, the PCE of the PvSCs considering the J-V curve hysteresis may not truly signify the actual device performance. To check the J-V curve hysteresis of high-performance of PvSCs, the J-V characteristics of control and MAPbI3 PvSCs incorporating NH4X (X = F, Cl, Br, I) were examined at different scan rates (100 ms per 0.01 V to 500 ms per 0.01 V) in the reverse scan direction, as shown in Fig. S5. For the MAPbI3 PvSCs incorporating NH4X (X = F, Cl, Br, I), no significant J-V hysteresis was observed at different scan rates, whereas the reference MAPbI3 PvSCs showed clear J–V hysteresis. These results suggest that J–V hysteresis is more likely due to defects in the perovskite film and the interface of PEDOT:PSS/ perovskite/PC61BM, which can be partially eliminated by using a high-quality perovskite film realized simply by adding ammonium halide, as we have revealed experimentally that a high-quality perovskite film with fewer charge carrier recombinations and a reduced trap density can be realized by simply adding NH4X (X = F, Cl, Br, I) to the perovskite precursor solution. The incorporation of NH4X (X = F, Cl, Br, I) additives into the MAPbI3 perovskite absorber layer improved the perovskite film morphology and crystalline grain size as compared to the reference MAPbI3 perovskite film. The significant reduction of the trap density, which in fact represents defect sites at which charge recombinations take place, can enhance the stability of perovskite devices. The stability of the reference and the MAPbI3 inverted planar PvSCs incorporating NH4X (X = F, Cl, Br, I) are compared in Fig. S6. All of the PvSCs were kept in air after glass-to-glass encapsulation to avoid the degradation of the LiF/Al electrode, as the LiF/Al metal contact is highly sensitive to moisture and can degrade quickly without proper encapsulation. After five weeks, the MAPbI3 PvSCs incorporating NH4F, NH4Cl, NH4Br and NH4I showed drops in the efficiency of approximately 18, 13, 11 and 9%, respectively, whereas the efficiency of the reference MAPbI3 PvSC fell by approximately 30%. Conclusion In summary, we developed a simple technique to control the nucleation and crystal growth of MAPbI3 perovskite to obtain a compact and uniform perovskite absorber layer by introducing ammonium halide additives and anti-solvent engineering. The incorporation of small amounts of NH4F, NH4Cl, NH4Br, and NH4I additives into MAPbI3 perovskite eventually led to the formation of high-quality perovskite film with an enlarged grain size and
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improved crystallinity, resulting in reduced density levels of the grain boundaries and fewer charge carrier recombinations. The suppressed charge carrier recombinations and reduced defect density level simultaneously enhanced the photovoltaic properties and improved the device stability. Consequently, the MAPbI3 PvSCs incorporating the NH4X (X = F, Cl, Br, I) additives demonstrated greatly improved device efficiency (best PCE = NH4F: 15.15%, NH4Cl: 16.88%, NH4Br: 16.585%, and NH4I: 17.44%) compared to the PCE of the control MAPbI3 device (best PCE = 13.74%) and showed no significant J-V curve hysteresis. Fundamentally, our promising, simple approach of the incorporation ammonium halide as an additive into the MAPbI3 perovskite solution enables the reproducible formation of large and uniform crystalline perovskite films which would further promote the development and improvement of perovskite device performance capabilities. Acknowledgements M.Jahandar and N.K. contributed equally to this work. This research was supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF-2015M1A2A2056214 & 2015M1A2A2055631) and by the Korea Institute of Energy Technology Evaluation and Planning (No. 20173010012960 & 20183010013820) of the Republic of Korea. GIWAXS measurement was performed on the beam line 3C at the Pohang Accelerator Laboratory. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jiec.2019.08.004. References [1] M.M. Lee, J. Teuscher, T. Miyasaka, T.N. Murakami, H.J. Snaith, Science 338 (2012) 643. [2] C. Wehrenfennig, G.E. Eperon, M.B. Johnston, H.J. Snaith, L.M. Herz, Adv. Mater. 26 (2013) 1584. [3] G.E. Eperon, S.D. Stranks, C. Menelaou, M.B. Johnston, L.M. Herz, H.J. Snaith, Energy Environ. Sci. 7 (2014) 982. [4] NREL, PV efficiency chart, (2019) . (accessed 1 January 2019) https://www.nrel. gov/pv/assets/pdfs/pv-efficiency-chart.20181221.pdf. [5] J.Y. Kim, G. Kwak, Y.C. Choi, D.-H. Kim, Y.S. Han, J. Ind. Eng. Chem. 73 (2019) 175. [6] N.J. Jeon, H. Na, E.H. Jung, T.-Y. Yang, Y.G. Lee, G. Kim, H.-W. Shin, S.I. Seok, J. Lee, J. Seo, Nature Energy 3 (2018) 682. [7] M. Long, T. Zhang, W. Xu, X. Zeng, F. Xie, Q. Li, Z. Chen, F. Zhou, K.S. Wong, K. Yan, J. Xu, Adv. Energy Mater. 7 (2017)1601882. [8] G.W. Baek, Y.-J. Kim, K.-H. Jung, Y.S. Han, J. Ind. Eng. Chem. 73 (2019) 260. [9] Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 131 (2009) 6050. [10] NREL, PV efficiency chart, (2017) . (accessed 26 April 2017) https://www.nrel. gov/pv/assets/images/efficiency-chart.png. [11] E.H. Jung, N.J. Jeon, E.Y. Park, C.S. Moon, T.J. Shin, T.-Y. Yang, J.H. Noh, J. Seo, Nature 567 (2019) 511. [12] W.S. Yang, B.-W. Park, E.H. Jung, N.J. Jeon, Y.C. Kim, D.U. Lee, S.S. Shin, J. Seo, E.K. Kim, J.H. Noh, S.I. Seok, Science 356 (2017) 1376. [13] Y.-J. Noh, J.-H. Jeong, S.-S. Kim, H.-K. Kim, S.-I. Na, J. Ind. Eng. Chem. 65 (2018) 406. [14] T.M. Koh, K. Fu, Y. Fang, S. Chen, T.C. Sum, N. Mathews, S.G. Mhaisalkar, P.P. Boix, T. Baikie, J. Phys. Chem. C 118 (2014) 16458. [15] Y.G. Yoo, J. Park, H.N. Umh, S.Y. Lee, S. Bae, Y.H. Kim, S.E. Jerng, Y. Kim, J. Yi, J. Ind. Eng. Chem. 70 (2019) 453. [16] H.-S. Kim, C.-R. Lee, J.-H. Im, K.-B. Lee, T. Moehl, A. Marchioro, S.-J. Moon, R. Humphry-Baker, J.-H. Yum, J.E. Moser, M. Grätzel, N.-G. Park, Sci. Rep. 2 (2012) 591. [17] J.S. Cho, W. Jang, S. Park, B.K. Kim, D.H. Wang, J. Ind. Eng. Chem. 61 (2018) 71. [18] F. Hao, C.C. Stoumpos, D.H. Cao, R.P.H. Chang, M.G. Kanatzidis, Nat. Photonics 8 (2014) 489. [19] Z.-K. Tan, R.S. Moghaddam, M.L. Lai, P. Docampo, R. Higler, F. Deschler, M. Price, A. Sadhanala, L.M. Pazos, D. Credgington, F. Hanusch, T. Bein, H.J. Snaith, R.H. Friend, Nat. Nanotechnol. 9 (2014) 687. [20] R. Kaur, A.L. Sharma, K.-H. Kim, A. Deep, J. Ind. Eng. Chem. 53 (2017) 77. [21] R. Dong, Y. Fang, J. Chae, J. Dai, Z. Xiao, Q. Dong, Y. Yuan, A. Centrone, X.C. Zeng, J. Huang, Adv. Mater. 27 (2015) 1912. [22] J.Y. Do, Y. Im, B.S. Kwak, M. Kang, J. Ind. Eng. Chem. 34 (2016) 89.
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High-performance CH3NH3PbI3 inverted planar perovskite solar cells via ammonium halide additives Muhammad Jahandara,b,1,2 , Nasir Khanb,c,1,2 , Muhammad Jahankhanb,c, Chang Eun Songb,c,* , Hang Ken Leec , Sang Kyu Leea,b , Won Suk Shina,b , Jong-Cheol Leea,b , Sang Hyuk Imd,* , Sang-Jin Moona,b,* a
Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT), 141 Gajeong-ro, Yuseong, Daejeon 34114, Republic of Korea Advanced Materials and Chemical Engineering, University of Science and Technology (UST), 217 Gajeongro, Yuseong, Daejeon 34113, Republic of Korea c Energy Materials Research Center, Korea Research Institute of Chemical Technology (KRICT), 141 Gajeong-ro, Yuseong, Daejeon 34114, Republic of Korea d Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 136-713, Republic of Korea b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 21 February 2019 Received in revised form 29 July 2019 Accepted 2 August 2019 Available online 9 August 2019
Organic-inorganic hybrid perovskites have recently attracted substantial attention as a top candidate for use as light-absorbing materials in high-efficiency, low-cost and solution-processable photovoltaic devices owing to their excellent optoelectronic properties. Here, we fabricated inverted planar perovskite solar cells by incorporating small amounts of ammonium halide NH4X (X = F, Cl, Br, I) additives into a CH3NH3PbI3 (MAPbI3) perovskite solution. A compact and uniform perovskite absorber layer with large perovskite crystalline grains is realized by simply incorporating small amounts of additives and by using an anti-solvent engineering technique to control the nucleation and crystal growth of perovskite. The enlarged perovskite grain size with a reduced density of the grain boundaries and improved crystallinity results in fewer charge carrier recombinations and a reduced defect density, leading to enhanced device efficiency (NH4F: 14.88 0.33%, NH4Cl: 16.63 0.21%, NH4Br: 16.64 0.35%, and NH4I: 17.28 0.15%) compared to that of a reference MAPbI3 device (Ref.: 12.95 0.48%) and greater device stability. This simple technique involving the introduction of small amounts of ammonium halide additives to regulate the nucleation and crystal growth of perovskite films translates into highly reproducible enhanced device performance. © 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: CH3NH3PbI3perovskite Inverted planar structure Ammonium halide additives Anti-Solvent engineering Perovskite grain size
Introduction The area of organic-inorganic hybrid perovskite photovoltaics is among the most propitious and most rapidly growing technologies owing to its low costs, solution processability and excellent intrinsic optoelectronic properties [1–3]. Over the past few years, remarkable developments in perovskite material compositions, device architectures and in the interfacial engineering of perovskite solar cells (PvSCs) with the highest ever power conversion efficiency (PCE) of more than 23% have made these devices captivating candidates for future energy applications [4–13]. The
* Corresponding authors. E-mail addresses: [email protected] (C.E. Song), [email protected] (S.H. Im), [email protected] (S.-J. Moon). 1 First authors. 2 M.Jahandar and N.K. have equally contributed to this work.
family of three-dimensional organic-inorganic halide perovskite materials used for photovoltaics has a simple AMX3 perovskite structure normally composed of an organic/inorganic cation (e.g., A = methylammonium (CH3NH3+; MA), formamidinium (CH (NH2)2+; FA) and cesium (Cs+; Cs)), a divalent metal (M = lead (Pb2+), tin (Sn2+)) and a halide anion (X = Cl, Br and I) [14–18]. The chemical flexibility of the organic-inorganic halide perovskite structure makes this material important for optoelectronic devices such as light-emitting diodes, photodetectors, and various photovoltaic applications [19–22]. Although the conventional n-i-p mesoporous PvSCs device architecture has demonstrated the highest ever PCE, it requires a high processing temperature to form a mesoscopic TiO2 electron conductor and often exhibits significant current-voltage (J-V) hysteresis [23,24]. In contrast, planar PvSCs have the simplest device architecture, allowing fabrication at a low temperature with a high PCE (over 20%) and without significant J-V hysteresis [25,26]. Proper control of the J-V curve hysteresis is very important to
https://doi.org/10.1016/j.jiec.2019.08.004 1226-086X/© 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
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realize actual stable device performance. Previous reports suggests that J-V curve hysteresis in PvSCs at different scan rates with respect to the scan direction can arise due to unbalanced electrons and hole flux or due to interphase defects and charge traps in lowquality perovskite film [27,28]. Mohit et al. reported millimeterscale CH3NH3PbI3 (MAPbI3) perovskite grains fabricated by a solution-process hot-casting technique and showed a hysteresisless PCE of 18% by using an inverted planar perovskite device architecture [29]. Similarly, Heo et al. reported hysteresis-less inverted planar PvSCs with a PCE of 18.1% [30]. In their study, they describe the significant role of balancing the electrons and hole flux in the PvSCs to realize reduced hysteresis characteristics by utilizing [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as the electrons and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as the hole-transporting layer with similar levels of conductivity s (sPEDOT:PSS: 0.016 mS/cm and sPCBM: 0.014 mS/cm). In addition, improved surface coverage and a large grain size of perovskite films can effectively reduce defects which act as recombination centers of photo-induced charge carriers and diminish the J-V hysteresis by suppressing charge carrier trapping during the operation of the device. In this regard, the planar PvSC device architecture provides the best opportunity to fabricate uniform and pin-hole free perovskite absorber layers with larger crystalline grains and energy bands well aligned to electron/hole transport layers with similar conductivity levels. When preparing pin-hole free and high-quality polycrystalline perovskite films with larger crystalline grains, the presence of grain boundaries is inevitable [31,32]. The optoelectronic properties and device performance capabilities of polycrystalline PvSCs are highly dependent on the crystallinity and grain size of the perovskite film, as the diffusion length and charge carrier mobility can be significantly improved in perovskite films with large perovskite grains [33]. The improved electronic properties of perovskite films are normally ascribed to decreased grain boundary densities, which can diminish the presence of traps and reduce the number of charge carrier recombinations. However, fabricating large-scale crystal growth and high-quality films with minimized amounts of pinholes in perovskite films is challenging using solutionprocessed PvSCs due to the low molecular kinetic energy that results from the low-temperature process. In general, the quality of the perovskite absorber layer is controlled by nucleation and perovskite crystal growth [34]. Recently, several groups have addressed the issue of nucleation and perovskite crystal growth to obtain high-quality perovskite film, as both rapid nucleation and retarded perovskite crystal growth are key factors when fabricating highly crystalline large perovskite grains [35,36]. To control the nucleation process, several protocols have been reported. Among them, anti-solvent engineering can successfully facilitate rapid nucleation through the quick and well-controlled removal of solvents to balance the precursor precipitation time [6,37]. However, quick crystallization of perovskite films results poor film morphology, increasing the number of recombination sites. To overcome the rapid crystallization of perovskite films, several strategies has been employed to moderate the perovskite growth rate, such as hot-casting, solvent annealing and additive engineering [29,38–40]. Among these strategies, the use of additives such as polymers, fullerenes, metal halide salts, organic halide salts, inorganic acids and ionic liquids have proven to be vital to not only retard the perovskite crystal growth but also reduce the grain boundary density due to the formation of high-quality large crystalline grains [41–46]. In this work, we introduce a small amount of ammonium halide (NH4X, where X = F, Cl, Br, I) additives into the MAPbI3 perovskite absorber layer to fabricate inverted planar PvSCs. The addition of ammonium halide salts in MAPbI3 perovskite solutions and an anti-solvent engineering technique help to control perovskite
nucleation and crystal growth. A compact and uniform perovskite absorber layer with an enlarged perovskite grain size, reduced density of the grain boundaries, and improved crystallinity combine to reduce charge carrier recombinations and lower the defect density level, leading to enhanced efficiency and better device stability in PvSCs. Experimental Materials and preparation of perovskite precursor solution Lead iodide (99.999% trace metals basis), ammonium fluoride (NH4F) (99.99% trace metals basis), ammonium chloride (NH4Cl) (99.99% trace metals basis), ammonium bromide (NH4Br) (99.999% trace metals basis), ammonium iodide (NH4I) (99.999% trace metals basis), dimethyl sulfoxide (DMSO), ɣ-butyrolactone (GBL) and chlorobenzene (CB) were purchased from Sigma-Aldrich. Methylammonium iodide (MAI) was purchased from DyeSol and all the materials were used as received without any further purification. To prepare the perovskite precursor solution, we mixed MAI (159 mg) powder and PbI2 (461 mg) (1:1 molar ratio) in 1 mL mixed GBL:DMSO (0.7:0.3) solvent for the reference perovskite precursor solution. Whereas, for the NH4X (X = F, Cl, Br, I) incorporated MAPbI3 perovskite solution, 0.10 M of NH4F (3.70 mg), NH4Cl (5.34 mg), NH4Br (9.79 mg) and NH4I (14.49 mg) were added in the reference perovskite precursor solution. All perovskite precursor solutions were kept for stirring at 70 C for overnight before use. The important point here to be noted is that the solubility of NH4F is very low. Although, we added very small amount (3.70 mg) in 1 mL reference perovskite precursor solution but it was not well soluble and need to filter to remove the insoluble NH4F. Whereas, other ammonium halide materials showed good solubility with given quantities. Device fabrication For inverted planar perovskite solar cells device fabrication, firstly, the patterned glass/ITO substrates were cleaned with DI water, acetone and isopropanol and dried in drying oven at 140 C for overnight. PEDOT:PSS (Clevios P VP AI4083) was spin-coated on UV-ozone treated glass/ITO substrates at 4000 rpm for 60 s in air and dried at 150 C for 20 min. Then, the samples were transferred to the N2 filled glove-box for further device fabrication steps. Perovskite precursor solution without and with NH4X (X = F, Cl, Br, I) was spin coated in N2 filled glove-box at 2000 rpm for 60-70 s, followed by a step of 1000 rpm for 20 s. During the 2nd step of 2000 rpm for 60 s, a chlorobenzene (CB) solution (400 mL) was dropped on the substrate during spin coating after 40 s and continued the spin for further 20 s. The important point to be noted here that the CB dripping time during 2nd spin-coating step was further delayed approximately 5 to 10 s for NH4X (X = F, Cl, Br, I) containing perovskite precursor solutions as compare to reference solution. Then, the samples were dried on hot plate at 100 C for 3 min. Then PC61BM (purchased from OSM, Republic of Korea) as ETL was deposited on the glass/ITO/ PEDOT:PSS/perovskite substrate by spin coating PC61BM (20 mg/1 mL in CB) solution at 1200 rpm for 30 s followed by a final spin-coating step of 2000 rpm for 2 s. Finally, the LiF/Al (0.5 nm/100 nm) electrode was deposited by thermal evaporation. Results and discussion MAPbI3 perovskite absorber layers with ammonium halide additives dissolved in a ɣ-butyrolactone (GBL) and dimethyl sulfoxide (DMSO) mixed solvent system were prepared by means of a chlorobenzene (CB) anti-solvent dripping technique. This type
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of mixed solvent system can help suitably to dissolve lead iodide (PbI2), methylammonium iodide (MAI) and NH4X (X = F, Cl, Br, I), whereas the anti-solvent CB dripping factor improves the uniform nucleation process of perovskites, resulting in the formation of very smooth film with large perovskite grains. The preparation of the perovskite precursor solution is described in detail in the experimental section (see Supporting Material). Fig. 1a exhibits a cross-sectional transmission electron microscope (TEM) image of inverted planar PvSCs with the composition of ITO/PEDOT:PSS/ perovskite layer/PC61BM/LiF/Al, and Fig. 1b illustrates the corresponding energy band diagram. The inverted planar PvSCs with the PEDOT:PSS hole-transporting layer (HTL) and the PC61BM electrontransporting layer (ETL) offer better energy band alignment and well-balanced hole/electron flux in this device architecture, leading to a high PCE without significant J-V curve hysteresis under forward and reverse scan directions [47,48]. Fig. 1c illustrates the UV-visible absorption spectra of a control sample and the MAPbI3 perovskite films incorporating ammonium halide as an additive. All of the perovskite films were prepared under identical conditions on quartz glass/ITO substrates with a film thickness of approximately 350 nm. The perovskite films with ammonium halide additives show slightly stronger absorption as compared to the control MAPbI3 perovskite absorber layer, whereas a slight blue shift of 2–3 nm was observed when adding NH4Br, which may be due to the presence of Br. Fig. 1d shows the crystal structure of the MAPbI3 and ammonium halide, and Fig. 1e exhibits a schematic diagram of the fabrication process of MAPbI3 perovskite absorber layers with and without ammonium halide additives. Perovskite films without and with 10 mol% NH4X (X = F, Cl, Br, I) additives in the GBL:DMSO mixed solvent system were deposited onto glass/ITO/PEDOT:PSS substrates by means of the CB antisolvent dripping method. During the formation of the perovskite films, we found that the optimized CB anti-solvent dripping time for NH4X (X = F, Cl, Br, I) incorporating MAPbI3 perovskite films is approximately five to ten seconds delayed as compared to that of the reference MAPbI3 perovskite film. A schematic of the spincoating process of the perovskite solutions and the CB anti-solvent dripping process is illustrated in Fig. S1. As the solubility of PbI2 is low compared to that of MAI, the quick precipitation of PbI2 during
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the spin-coating process can occur, possibly resulting in poor film formation with numerous defects [34,35]. In contrast, the addition of NH4X (X = F, Cl, Br, I) to the MAPbI3 perovskite precursor solution can delay the precipitation of PbI2 by retarding the PbI2 nucleation rate during the spin-coating step, leading to better film formation. With NH4X (X = F, Cl, Br, I) additives, the ammonium (NH4+) ion with a small ionic radius favorably diffuses into the [PbI6]4 octahedral layer. This type of NH4+ cation successfully occupies the cavity of the lead coordination complex, contributing to extra heterogeneous nucleation sites and reducing the defects stemming from the absence of MA+ [49]. On the other hand, the CB anti-solvent dripping technique not only promotes uniform rapid nucleation but also helps to form an intermediate MAI-PbI2-DMSO complex as compared to MAPbI3, which transforms into a uniform dense perovskite film with a large grain size upon thermal annealing [40]. Through this process, compact uniform perovskite film can easily be formed, but the CB dripping timing with regard to the perovskite precursor solution during the spin-coating step is critical. To investigate the effect of NH4X (X = F, Cl, Br, I) additives on the MAPbI3 perovskite crystalline grain growth, perovskite films created with the optimized fabrication condition were analyzed using scanning electron microscopy (SEM), as shown in Fig. 2a–e. The SEM images shows compact and homogeneous perovskite films, and it is clear that the MAPbI3 perovskite grains grew from 200 nm to 400 nm to >1 mm in size with the incorporation of the NH4X (X = F, Cl, Br, I) additives. Such an enlarged grain size compared to the control MAPbI3 perovskite film is attributed to the incorporation of the NH4X (X = F, Cl, Br, I) additives. In contrast, among other ammonium halide additives, NH4I when incorporated into the MAPbI3 perovskite film led to uniformly large crystalline grains. As the ionic radii of the halide ions increase, the solubility of the ammonium halides also increases. Among the NH4X (X = F, Cl, Br, I) additives, the solubility of NH4I was the highest, whereas NH4F exhibits the lowest solubility above room temperature [50]. The growth of the MAPbI3 perovskite grains size with the NH4I additive as compared to the non-uniformity of the grain size of NH4F when incorporated into MAPbI3 perovskite films demonstrates that the solubility of the ammonium halide additives can affect the nucleation/grain growth process of perovskite films. The
Fig. 1. (a) Cross-sectional TEM image and (b) energy band diagram of ITO/PEDOT:PSS/Perovskite/PC61BM/LiF/Al inverted planar PvSCs. (c) UV-vis absorption spectra of reference and NH4X (X = F, Cl, Br, I) incorporating MAPbI3 perovskite film. (d) Schematic of perovskite and ammonium halide structure. (e) Schematic diagram of the fabrication process of MAPbI3 perovskite absorber layers with and without ammonium halide additives.
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Fig. 2. (a–e) FE-SEM images, and (f) XRD spectra of reference and NH4X (X = F, Cl, Br, I) incorporated MAPbI3 perovskite films.
increases in the grain size and crystallinity of MAPbI3 perovskite with the incorporation of NH4X (X = F, Cl, Br, I) additives were probed by X-ray diffraction (XRD), as shown in Fig. 2f. All of the perovskite films with or without NH4X (X = F, Cl, Br, I) additives exhibit diffraction peaks at 14.12 , 28.44 and 31.88 , corresponding to the (110), (220) and (310) crystal planes of the tetragonal perovskite structure. No significant new XRD peak or gradual peak shift was observed, which illustrates that the finial acquired MAPbI3 perovskite samples with the NH4X (X = F, Cl, Br, I) additives all maintained the intrinsic crystal structure. Compared to the control MAPbI3 perovskite film, the increased perovskite peak intensity and reduced full width at half maximum (FWHM) of these films clearly indicate the improved perovskite grain size and crystallinity due to the incorporation of the NH4X (X = F, Cl, Br, I) additives. The improved grain size and crystallinity of perovskites can reduce the density of grain boundaries and trap sites, resulting in enhanced photovoltaic properties [51]. Subsequently, to investigate the crystallinity and orientation of the perovskite nanostructure on the substrate, 2D scattering patterns were acquired with the grazing incidence wide-angle X-ray scattering (GIWAXS), which has become an important technique for characterizing organic semiconductor and perovskite thin films
[52–54]. The recorded 2D-GIWAXS patterns of the reference MAPbI3 perovskite films and those incorporating NH4X (X = F, Cl, Br, I) are displayed in Fig. S2a-e. The higher intensity of the (110) ring around q = 0.98 Å1 in the 2D-GIWAXS data reveals that the MAPbI3 perovskite films incorporating NH4X (X = F, Cl, Br, I) exhibit a preferential crystal orientation, with the c-axis perpendicular to the substrate. Furthermore, the azimuthal integration at around q = 0.98 Å1 represents the crystallites of the MAPbI3 perovskite films incorporating NH4X (X = F, Cl, Br, I) with the (110) plane oriented toward the out-of-plane direction compared to the reference MAPbI3 perovskite film (Fig. S2f). To estimate the appropriate amount of NH4X (X = F, Cl, Br, I) in the MAPbI3 PvSCs to optimize device performance capabilities, we fabricated inverted planar PvSCs with different amounts of ammonium halide and determined the J-V characteristics under one sun (100 mW/cm2 AM 1.5 G) light illumination, as shown in Fig. S3. The related photovoltaic parameters are summarized in Table S1. The J-V characteristics, the external quantum efficiency (EQE) curves and photovoltaic properties of the best MAPbI3 PvSCs with and without ammonium halide additives are illustrated in Fig. 3 and are summarized in Table 1, respectively. The reference device exhibited an open-circuit voltage (VOC) of 0.94 V, a short-circuit current
Fig. 3. (a–e) J–V curves of the reference and NH4X (X = F, Cl, Br, I) incorporating MAPbI3 inverted planar PvSCs under 1 sun light illumination with respect to scan direction and (f) corresponding EQE graph with integrated current density.
M. Jahandar et al. / Journal of Industrial and Engineering Chemistry 80 (2019) 265–272 Table 1 Photovoltaic parameters of reference and NH4X (X = F, Cl, Br, I) incorporating MAPbI3 inverted planar PvSCs. Perovskite film
Scan direction
VOC [V]
JSC [mA/cm2]
FF [%]
PCEa [%]
Avg. PCEb [%]
Ref.
Reverse Forward Reverse Forward Reverse Forward Reverse Forward Reverse Forward
0.94 0.94 0.98 0.98 0.99 0.99 1.02 1.02 1.00 1.00
18.56 18.46 19.10 19.13 21.01 21.03 20.33 20.10 21.02 21.04
78.95 77.90 80.42 79.90 80.80 80.31 81.14 80.51 82.43 82.38
13.74 13.47 15.15 15.02 16.88 16.78 16.85 16.53 17.44 17.34
12.95 0.48
NH4F NH4Cl NH4Br NH4I a b
14.88 0.33 16.63 0.21 16.64 0.35 17.28 0.15
Best device performance. Average device efficiency of independent 20 devices.
density (JSC) of 18.56 mA/cm2, a fill factor (FF) of 78.95%, and a PCE of 13.74% under the reverse scan direction, whereas under the forward scan direction, the values were as follows: VOC = 0.94 V, JSC = 18.46 mA/cm2, FF = 77.90% and PCE = 13.47%, with slight J-V curve hysteresis under both the forward and reverse scan directions. Among the MAPbI3 PvSCs incorporating NH4X (X = F, Cl, Br, I), the highest PCE was achieved with the addition of NH4I (VOC: 1.00 V, JSC: 21.02 mA/cm2, FF: 82.43% and PCE: 17.44% under the reverse scan direction and VOC: 1.00 V, JSC: 21.04 mA/cm2, FF: 82.38% and PCE: 17.34% under the forward scan direction) without significant J-V curve hysteresis. On the other hand, the photovoltaic properties when incorporating NH4F, NH4Cl and NH4Br were VOC: 0.98 V, JSC: 19.10 mA/cm2, FF: 80.42% and PCE: 15.15% under the reverse scan direction and VOC: 0.98 V, JSC: 19.13 mA/cm2, FF: 79.90% and PCE: 15.02% under the forward scan direction; VOC: 0.99 V, JSC: 21.01 mA/cm2, FF: 80.80% and PCE: 16.88% under the reverse scan direction and VOC: 0.99 V, JSC: 21.03 mA/cm2, FF: 80.31% and PCE: 16.78% under the forward scan direction; and VOC: 1.02 V, JSC: 20.33 mA/cm2, FF: 81.14% and PCE: 16.85% under the reverse scan direction and VOC: 1.02 V, JSC: 20.10 mA/cm2, FF: 80.51% and PCE: 16.53% under the forward scan direction, respectively.
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The improved device performance of the MAPbI3 PvSCs with NH4X (X = F, Cl, Br, I) incorporated into them additive can be attributed to the enhanced VOC, JSC and FF values. The larger crystal grains, preferential crystal orientation and high-quality perovskite film formation effectively enhanced the photovoltaic properties. The deviation of the VOC, JSC, FF and PCE values of 20 devices are shown in Fig. 4. The deviation outcomes in the efficiency were 12.95 0.48, 14.88 0.33, 16.63 0.21, 16.64 0.35 and 17.28 0.15% for the control sample and for the MAPbI3 PvSCs incorporating NH4F, NH4Cl, NH4Br and NH4I, respectively. The reproducible and noticeably enhanced values of VOC, JSC and FF of the PvSCs processed with NH4X (X = F, Cl, Br, I) additives have facilitated the achievement of consistently high PCEs compared to those of the control device. Fig. 3f displays the EQE spectrum and integrated JSC of the control sample and the MAPbI3 PvSCs incorporating NH4X (X = F, Cl, Br, I). The tendency of the integrated JSC value is in good agreement with the measured JSC value of the J-V curves under 1 sun illumination. A slight change in the shape of the EQE spectra was observed for the optimized PvSCs. The change in the shape of the EQE spectra may be due to the effects of optical interference and the change in the optimized absorber layer thickness [55]. The EQE is a product of the light harvesting, charge separation, and charge collection efficiency. The EQE spectrum also confirmed that the PvSCs incorporating NH4X (X = F, Cl, Br, I) had increased JSC values compared to that of the control cell. This is likely due to the formation of large crystal grains, the improved perovskite crystallinity, and the high-quality perovskite film [56]. These factors effectively reduced the number of charge carrier recombinations and improved the charge separation while also improving the charge collection efficiency. To probe the photo-generated charge recombination processes in MAPbI3 PvSCs with and without NH4X (X = F, Cl, Br, I) additives, we adopted the most common and simplest technique to identify trap-assisted and bimolecular recombinations during the operation of PvSCs by measuring the VOC and JSC as a function of the light intensity. The related J-V curves of PvSCs with respect to the change in the light intensity are shown in Fig. S4. At room temperature, by
Fig. 4. The average photovoltaic parameters of (a) power conversion efficiency (PCE), (b) open-circuit voltage (VOC), (c) short-circuit current density (JSC) and (d) fill factor (FF) for 20 devices of reference and NH4X (X = F, Cl, Br, I) incorporating MAPbI3 inverted planar PvSCs.
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Fig. 5. (a) VOC and (b) JSC values as a function of light intensity for reference and NH4X (X = F, Cl, Br, I) incorporating MAPbI3 inverted planar PeSCs.
linearly fitting VOC versus the logarithm of the light intensity, as shown in Fig. 5a, we found slopes of 1.29 kBT/q, 1.22 kBT/q, 1.18 kBT/q, 1.19 kBT/q and 1.16 kBT/q (where kB is the Boltzmann constant, T is the absolute temperature and q is the elementary charge) for the reference sample and for the MAPbI3 PvSCs incorporating NH4F, NH4Cl, NH4Br and NH4I, respectively. In an ideal case, the slope should be kBT/q in the absence of trap-assisted recombinations, whereas any deviation from the ideal value specifies the presence of trap-assisted recombinations [57]. The MAPbI3 PvSCs with NH4X (X = F, Cl, Br, I) additives showed fewer trap-assisted recombinations as compared to the reference MAPbI3 PvSCs, which indicates that the addition of NH4X (X = F, Cl, Br, I) to MAPbI3 films can reduce the number of recombination trap sites at the grain boundaries due to the formation of large crystalline grains and the reduced density of the grain boundaries. We also studied the dependence of the light intensity of J SC to understand the charge recombination mechanism. The powerlaw dependence of JSC on the light intensity can be represented by JSC / (Plight)α (α 1). The deviation of the slope from unity implies the occurrence of bimolecular recombinations [58]. Fig. 5b shows the relationship between JSC and the light intensity in a double
logarithmic scale with slopes of 0.95, 0.96, 0.97, 0.97 and 0.97 for the reference, NH4F, NH4Cl, NH4Br and NH4I PvSCs, respectively. These results indicate that bimolecular charge recombinations are suppressed due to the incorporation of NH4X (X = F, Cl, Br, I) into the MAPbI3 PvSCs [59]. Therefore, the synergetic negative effects of trap-assisted and bimolecular charge recombinations in the control MAPbI3 PvSCs lead to severe losses of the photovoltaic properties compared to the MAPbI3 PvSCs incorporating NH4X (X = F, Cl, Br, I). Furthermore, to assess the defect density (Ndefects) levels quantitatively, we prepared devices with the ITO/perovskite/ Au device architecture. The results of the evolution of the spacecharge-limited current (SCLC) for different biases were characterized as shown in Fig. 6a-e. The sharp rise of the dark J-V curve is associated with a trap-filled limit, where all the defects are occupied by charge carriers. The Ndefects value is calculated according to the following equation [60,61]. Ndefects = 2ee0VTFL/qL2
(1)
where Ndefects is the defect density, e is the dielectric constant of the perovskite material, e0 is the permittivity of free space
Fig. 6. (a–e) Current density-voltage characteristics of device with ITO/perovskite/Au configuration for estimating the defect density in perovskite films, and (f) steady-state PL spectra of reference and NH4X (X = F, Cl, Br, I) incorporated perovskite films.
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(8.854 1014 F/cm), q is the elementary charge and L is the thickness of the perovskite film. The values of Ndefects, calculated for the control device and for MAPbI3 perovskite devices incorporating NH4F, NH4Cl, NH4Br and NH4I, were 1.98 1016, 1.32 1016, 9.45 1015, 1.02 1016, and 8.38 1015 cm3, respectively. The density levels of the defects of the MAPbI3 perovskite devices incorporating NH4X (X = F, Cl, Br, I) were reduced significantly compared with that of the control device. The defect density of the control device is comparable to those of previous MAPbI3 PvSCs reported in the literature [58]. Moreover, to explain the mechanisms of the defect density and charge carrier recombination, the steady-state photoluminescence (PL) spectra of the control and the MAPbI3 perovskite films incorporating NH4X (X = F, Cl, Br, I) on quartz glass substrates were obtained, as shown in Fig. 6f. The PL intensity levels of the MAPbI3 perovskite films incorporating NH4X (X = F, Cl, Br, I) are higher than that of the control MAPbI3 perovskite film, which indicates that the non-radiative decay is suppressed in the MAPbI3 perovskite films which incorporate NH4X (X = F, Cl, Br, I) [62,63]. In general, PvSCs show J-V curve hysteresis with respect to the forward and reverse scan directions or at certain voltage scan rates. This is due to the unbalanced electron/hole flux, to interphase defects, and to charge traps in low-quality perovskite films [27,28]. Therefore, the PCE of the PvSCs considering the J-V curve hysteresis may not truly signify the actual device performance. To check the J-V curve hysteresis of high-performance of PvSCs, the J-V characteristics of control and MAPbI3 PvSCs incorporating NH4X (X = F, Cl, Br, I) were examined at different scan rates (100 ms per 0.01 V to 500 ms per 0.01 V) in the reverse scan direction, as shown in Fig. S5. For the MAPbI3 PvSCs incorporating NH4X (X = F, Cl, Br, I), no significant J-V hysteresis was observed at different scan rates, whereas the reference MAPbI3 PvSCs showed clear J–V hysteresis. These results suggest that J–V hysteresis is more likely due to defects in the perovskite film and the interface of PEDOT:PSS/ perovskite/PC61BM, which can be partially eliminated by using a high-quality perovskite film realized simply by adding ammonium halide, as we have revealed experimentally that a high-quality perovskite film with fewer charge carrier recombinations and a reduced trap density can be realized by simply adding NH4X (X = F, Cl, Br, I) to the perovskite precursor solution. The incorporation of NH4X (X = F, Cl, Br, I) additives into the MAPbI3 perovskite absorber layer improved the perovskite film morphology and crystalline grain size as compared to the reference MAPbI3 perovskite film. The significant reduction of the trap density, which in fact represents defect sites at which charge recombinations take place, can enhance the stability of perovskite devices. The stability of the reference and the MAPbI3 inverted planar PvSCs incorporating NH4X (X = F, Cl, Br, I) are compared in Fig. S6. All of the PvSCs were kept in air after glass-to-glass encapsulation to avoid the degradation of the LiF/Al electrode, as the LiF/Al metal contact is highly sensitive to moisture and can degrade quickly without proper encapsulation. After five weeks, the MAPbI3 PvSCs incorporating NH4F, NH4Cl, NH4Br and NH4I showed drops in the efficiency of approximately 18, 13, 11 and 9%, respectively, whereas the efficiency of the reference MAPbI3 PvSC fell by approximately 30%. Conclusion In summary, we developed a simple technique to control the nucleation and crystal growth of MAPbI3 perovskite to obtain a compact and uniform perovskite absorber layer by introducing ammonium halide additives and anti-solvent engineering. The incorporation of small amounts of NH4F, NH4Cl, NH4Br, and NH4I additives into MAPbI3 perovskite eventually led to the formation of high-quality perovskite film with an enlarged grain size and
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improved crystallinity, resulting in reduced density levels of the grain boundaries and fewer charge carrier recombinations. The suppressed charge carrier recombinations and reduced defect density level simultaneously enhanced the photovoltaic properties and improved the device stability. Consequently, the MAPbI3 PvSCs incorporating the NH4X (X = F, Cl, Br, I) additives demonstrated greatly improved device efficiency (best PCE = NH4F: 15.15%, NH4Cl: 16.88%, NH4Br: 16.585%, and NH4I: 17.44%) compared to the PCE of the control MAPbI3 device (best PCE = 13.74%) and showed no significant J-V curve hysteresis. Fundamentally, our promising, simple approach of the incorporation ammonium halide as an additive into the MAPbI3 perovskite solution enables the reproducible formation of large and uniform crystalline perovskite films which would further promote the development and improvement of perovskite device performance capabilities. Acknowledgements M.Jahandar and N.K. contributed equally to this work. This research was supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF-2015M1A2A2056214 & 2015M1A2A2055631) and by the Korea Institute of Energy Technology Evaluation and Planning (No. 20173010012960 & 20183010013820) of the Republic of Korea. GIWAXS measurement was performed on the beam line 3C at the Pohang Accelerator Laboratory. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jiec.2019.08.004. References [1] M.M. Lee, J. Teuscher, T. Miyasaka, T.N. Murakami, H.J. Snaith, Science 338 (2012) 643. [2] C. Wehrenfennig, G.E. Eperon, M.B. Johnston, H.J. Snaith, L.M. Herz, Adv. Mater. 26 (2013) 1584. [3] G.E. Eperon, S.D. Stranks, C. Menelaou, M.B. Johnston, L.M. Herz, H.J. Snaith, Energy Environ. Sci. 7 (2014) 982. [4] NREL, PV efficiency chart, (2019) . (accessed 1 January 2019) https://www.nrel. gov/pv/assets/pdfs/pv-efficiency-chart.20181221.pdf. [5] J.Y. Kim, G. Kwak, Y.C. Choi, D.-H. Kim, Y.S. Han, J. Ind. Eng. Chem. 73 (2019) 175. [6] N.J. Jeon, H. Na, E.H. Jung, T.-Y. Yang, Y.G. Lee, G. Kim, H.-W. Shin, S.I. Seok, J. Lee, J. Seo, Nature Energy 3 (2018) 682. [7] M. Long, T. Zhang, W. Xu, X. Zeng, F. Xie, Q. Li, Z. Chen, F. Zhou, K.S. Wong, K. Yan, J. Xu, Adv. Energy Mater. 7 (2017)1601882. [8] G.W. Baek, Y.-J. Kim, K.-H. Jung, Y.S. Han, J. Ind. Eng. Chem. 73 (2019) 260. [9] Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 131 (2009) 6050. [10] NREL, PV efficiency chart, (2017) . (accessed 26 April 2017) https://www.nrel. gov/pv/assets/images/efficiency-chart.png. [11] E.H. Jung, N.J. Jeon, E.Y. Park, C.S. Moon, T.J. Shin, T.-Y. Yang, J.H. Noh, J. Seo, Nature 567 (2019) 511. [12] W.S. Yang, B.-W. Park, E.H. Jung, N.J. Jeon, Y.C. Kim, D.U. Lee, S.S. Shin, J. Seo, E.K. Kim, J.H. Noh, S.I. Seok, Science 356 (2017) 1376. [13] Y.-J. Noh, J.-H. Jeong, S.-S. Kim, H.-K. Kim, S.-I. Na, J. Ind. Eng. Chem. 65 (2018) 406. [14] T.M. Koh, K. Fu, Y. Fang, S. Chen, T.C. Sum, N. Mathews, S.G. Mhaisalkar, P.P. Boix, T. Baikie, J. Phys. Chem. C 118 (2014) 16458. [15] Y.G. Yoo, J. Park, H.N. Umh, S.Y. Lee, S. Bae, Y.H. Kim, S.E. Jerng, Y. Kim, J. Yi, J. Ind. Eng. Chem. 70 (2019) 453. [16] H.-S. Kim, C.-R. Lee, J.-H. Im, K.-B. Lee, T. Moehl, A. Marchioro, S.-J. Moon, R. Humphry-Baker, J.-H. Yum, J.E. Moser, M. Grätzel, N.-G. Park, Sci. Rep. 2 (2012) 591. [17] J.S. Cho, W. Jang, S. Park, B.K. Kim, D.H. Wang, J. Ind. Eng. Chem. 61 (2018) 71. [18] F. Hao, C.C. Stoumpos, D.H. Cao, R.P.H. Chang, M.G. Kanatzidis, Nat. Photonics 8 (2014) 489. [19] Z.-K. Tan, R.S. Moghaddam, M.L. Lai, P. Docampo, R. Higler, F. Deschler, M. Price, A. Sadhanala, L.M. Pazos, D. Credgington, F. Hanusch, T. Bein, H.J. Snaith, R.H. Friend, Nat. Nanotechnol. 9 (2014) 687. [20] R. Kaur, A.L. Sharma, K.-H. Kim, A. Deep, J. Ind. Eng. Chem. 53 (2017) 77. [21] R. Dong, Y. Fang, J. Chae, J. Dai, Z. Xiao, Q. Dong, Y. Yuan, A. Centrone, X.C. Zeng, J. Huang, Adv. Mater. 27 (2015) 1912. [22] J.Y. Do, Y. Im, B.S. Kwak, M. Kang, J. Ind. Eng. Chem. 34 (2016) 89.
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