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Toward clean production of plastic perovskite solar cell: Composition-tailored perovskite absorber made from aqueous lead nitrate precursor
Nano Energy 65 (2019) 104036
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Toward clean production of plastic perovskite solar cell: Compositiontailored perovskite absorber made from aqueous lead nitrate precursor
T
Peng Zhaia,∗∗, Tzu-Sen Sub, Tsung-Yu Hsiehb, Wei-Yen Wangb, Lixia Rena, Jiayi Guob, Tzu-Chien Weib,∗ a b
School of Natural and Applied Sciences, Northwestern Polytechnical University, Xi'an, 710072, PR China Department of Chemical Engineering, National Tsing-Hua University, Hsinchu, 300, Taiwan
ARTICLE INFO
ABSTRACT
Keywords: Aqueous lead nitrate precursor Intermediate ion exchange Compositional engineering Conversion kinetics Scaffold-type plastic perovskite solar cell
Even though the power conversion efficiency (PCE) of plastic perovskite solar cells (P-PSCs) is increased to 18.40%, the majority of solvent systems implemented for deposition of perovskites are hazardous to handle, which will greatly hinder the future development of plastic photovoltaic devices. In this study, compositiontailored hybrid perovskite from a low-toxicity aqueous lead nitrate precursor was fabricated by regulating the conversion kinetics. We systematically investigated the interplay among NO3− and mixed cation/anion in the intermediate ion exchange and renovated the interpretation of hybrid-composition perovskite conversion. The fully ambient-processed hybrid-composition perovskite with high crystallinity and less defects was applied in a brookite TiO2 scaffold-based P-PSCs, which achieved a record-high PCE of scaffold-type P-PSC of 16.50%. The interaction of environmentally-friendly aqueous lead nitrate precursor with hybrid ions advanced the understanding of perovskite conversion mechanism and had a great potential to realize the low-toxic fabrication process by using water as a processing solvent in the ambient atmosphere.
1. Introduction Plastic and lightweight thin film solar cells have been anticipated to have great market prospects for portable electronics such as remote power chargers, bendable displays, and wearable electronic devices, in view of their capability to generate electricity on curved surface using a low temperature solution process [1–3]. Recently, plastic perovskite solar cells (P-PSCs) have made great progress in pursuing high power conversion efficiency (PCE) up to 18.40%, [4] revealing its powerful competitiveness with other flexible photovoltaic technology including copper indium gallium selenide, [5] cadmium telluride, [6] organic photovoltaics, [7] and dye-sensitized solar cells [8,9]. However, numerous concerns of PSCs regarding to scalability, stability, and toxicity remain unsolved or challenging till now. Besides the toxicity of leadcontaining organic material itself, the chemicals involved during the fabrication process is particularly important in P-PSC because plastic photovoltaics are expected to have more chances to contact human skin directly. The possible chemicals left from low temperature process may add another healthy concern during product lifecycle. For instance, the most widely used solvents for the lead-containing precursor in the literature reporting high PCEs are aprotic organic polar solvents with high ∗
boiling points such as N,N-dimethylformamide (DMF), [10] dimethyl sulfoxide (DMSO) [11] and N-methyl-2-pyrrolidone (NMP), [12,13] most of them are skin penetrating and carcinogenic toxicants. P-PSCs are expected to consume large amounts of organic solvent in production scale, which is harmful not only to the factory operators but also to the end-product users. Many endeavors have been carried out to avoid or diminish the use of these hazardous solvents in the fabrication process. Electrospinning [14] and electrodepositing [15] lead-containing compounds onto the TiO2 scaffold without the use of harmful solvents were reported. However, the poor morphological control of the perovskite capping layer and the additional iodination treatment limit their photovoltaic performance and add another complexity of fabrication process, respectively. Gardner et al. developed a set of nonhazardous organic solvent systems for single-step coating of methylammonium lead iodide (MAPbI3) layers based on blending γ-butyrolactone, cyclic carbonates, alcohols, acids, and other protonated carbon chains [16]. Unfortunately, the efficiency of these devices was far inferior to their counterpart using toxic DMF. We have developed and optimized an ecofriendly process to prepare MAPbI3 on FTO glass using aqueous lead nitrate precursor and have achieved a PCE of 16.02% [17,18]. T. Park et al. followed this method and reported that the Pb(NO3)2 residues in
Corresponding author. Corresponding author. E-mail addresses: [email protected] (P. Zhai), [email protected] (T.-C. Wei).
∗∗
https://doi.org/10.1016/j.nanoen.2019.104036 Received 27 March 2019; Received in revised form 29 July 2019; Accepted 16 August 2019 Available online 19 August 2019 2211-2855/ © 2019 Published by Elsevier Ltd.
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were 25*10−3 M bis(trifluoromethane)-sulfonimide lithium salt (LiTFSI, 99.95%, Sigma-Aldrich) and 120*10−3 M 4-tert-butylpyridine (96%, Sigma-Aldrich). The device was subsequently stored in a dry atmosphere (temperature: 25 °C, relative humidity: 10%) overnight, after which the gold terminals were thermally evaporated through a stainless steel shadow mask.
the MAPbI3 could function to retard perovskite decomposition toward moisture as a water scavenger [19]. Notwithstanding of these achievements, there is no report to fabricate P-PSC from aqueous lead precursor until now. Moreover, compositional engineering on perovskite ingredient such as mixing cations or halides is acknowledged as an applicable method to enhance both PCE and stability. Johansson et al. attempted to synthesize CsyFA1-yPb(I1-xBrx)3 perovskite materials based on aqueous Pb(NO3)2 precursor on rigid substrate but the PCE was only 13%, [20] indicating there still has plenty improvement to be engineered. Consequently, in this report, we pioneered to fabricate PPSC based on composition-tailored hybrid perovskite absorber from aqueous lead nitrate precursor in the ambient atmosphere. By understanding and regulating the conversion kinetics of the FAxMA1-xPbI3yBry, a P-PSC made from green process with record-high PCE of 16.50% was achieved.
2.5. Material characterization and photovoltaic performance measurement UV–vis absorption was recorded with a Hitachi U-4100 Spectrophotometer. X-ray diffraction (XRD) data were collected through a Brucker powder diffractometer (D8 Discover) using Cu Kα1 radiation. The topography of different perovskites was investigated by field-emission scanning electron microscope (FESEM, S-4800, Hitachi, Japan). The blocking effect was investigated by CV using a computercontrolled potentiostat (Solartron SI 1286, UK) in a three-electrode system with Ag/AgCl reference electrode and 2 cm2 platinum counter electrode at 50 mV s−1 scan rate. In CV scan, an aqueous electrolyte containing 0.5 mM potassium hexacyano-ferrate(II) trihydrate and 0.5 mM potassium hexacyano-ferrate(III) was utilized with 0.5 M KCl as supporting electrolyte. A photo-mask (0.06 cm2 and 1 cm2) was used to control the area of light exposure in the subsequent current-voltage (IV) scan, which was recorded with a computer-controlled digital source meter (Keithley 2400) under exposure of a solar simulator (AM 1.5G, 100 mWcm−2, PEC-L15, PECCELL Technologies, Japan). The voltage scan rate was 50 mV s−1 in forward and reverse scans. The currentvoltage response of P-PSCs was examined using a PECL15 solar simulator (Peccell, Japan) equipped with a Keithley 2400 digital source meter (USA) under standard 100 mW/cm2, AM 1.5 G illumination. The light intensity was calibrated through the 91150V model (Newport, UK). Before measurement, a non-reflective metal mask with aperture area (0.06 cm2 and 1 cm2) was covered on the top of the device to precisely define the active area and ensure that no illumination or voltage biasing condition had been introduced. In the light intensitydependent VOC examination, the light intensity was controlled using a ND filter (Shibuya Optical Co., LTD, Japan) that covered the top of the device. All photovoltaic parameters were extracted from the average IV curve of forward (from short-circuit to open-circuit) and reverse (from open-circuit to short-circuit) scans.
2. Experimental section 2.1. Materials and reagents The SnO2 colloid precursor was obtained from Alfa Aesar (tin (IV) oxide). Pb(NO3)2 (> 99%) was purchased from J. T. Baker. Formamidinium iodide (FAI), methylammonium bromide (MABr), and methylammonium iodide (MAI) were purchased from Xi'an Polymer Light Technology Corp. 2.2. Substrate preparation ITO-PEN sheets (14–15 Ω•sq−1 Peccell Technologies, Inc.) were etched by using a laser etcher (LMF-020F, Taiwan) to obtain the required electrode pattern. The sheets were then sequentially cleaned with acetone and isopropanol (IPA) under sonication for 15 min. 2.3. Electron-collecting bilayer fabrication Before deposition, each ITO-PEN sheet was treated with UV-ozone (customized, Kingo Electrical Enterprise Co., LTD.) for 15 min to remove the organic residues. Then, the substrate was spin coated with a thin layer of SnO2 nanoparticle film (2.67%) at 3000 rpm for 30 s, and annealed in ambient air at 150 °C for 30 min. A mesoporous scaffold using a 6.5 wt% binder-free brookite suspension (PECC-B01, Peccell Technologies, Inc.) was spin coating on the prepared SnO2/ITO-PEN at 6000 rpm for 30 s, followed by baking at 150 °C for 30 min.
3. Results and discussion Because of conductive plastic substrate (ITO-PEN) is used, the whole fabrication is conducted under the thermal budget, ie. 150 °C of the PEN film. Fig. 1 outlines a schematic of the device architecture and the process flow detailing the fabrication of P-PSC based from Pb(NO3)2/ H2O precursor. Removal of NO3− ions is carried out by two routes: on the one hand, NO3− ions diffuse outwardly from the inner Pb(NO3)2 crystals to bulk solution; on the other, NO3− ions remaining on the surface can be removed by pure isopropanol (IPA) rinsing. The topography of every step is recorded by atomic force microscopy (AFM). The electron transport bilayer on ITO-PEN is prepared by sequentially spincoating an aqueous SnO2 colloidal precursor and a commercial brookite TiO2 (BK-TiO2) solution to form the compact layer (CL) and the mesoporous scaffold (MS), respectively. The root mean square surface roughness (Ra) of the SnO2-coated ITO-PEN is 0.91 nm, indicating that the SnO2 CL is extremely smooth. Ra of BK-TiO2-coated substrate is increased to 3.02 nm with a few white spots revealing on the AFM topography, which probably results from the aggregation of the BK-TiO2 dispersion. After a UV-ozone treatment, an aqueous Pb(NO3)2 precursor is spin-coated on BK-TiO2-coated substrate. The topography of Pb (NO3)2 is island-like rather a continuous film, demonstrating that an intact perovskite capping layer made from aqueous Pb(NO3)2 protocol is inherently challenging. After drying, the film is immersed into an IPA solution containing MAI or FAI/MAI/MABr. The total incubation time is divided into 4 sectors in light of our previous work [13b]. After MAI or
2.4. P-PSC fabrication The as-prepared BK-TiO2/SnO2/ITO-PEN film was treated under exposure to a UV-ozone for 15 min. Subsequently, an aqueous solution containing 1.5 M Pb(NO3)2 was spin coated onto the substrate at 6000 rpm for 20 s and dried at 70 °C on a hot plate for 30 min. After cooling to room temperature, the Pb(NO3)2-infiltrated mesoporous BKTiO2 film was immersed in an IPA solution containing MAI and mixed composition system (FAxMA1-xI0.9Br0.1-IPA; x = 0.1, 0.2, 0.3) with the concentration around 15 mg/mL. The total incubation time (tC) was split into 4 sectors. For instance, the total dipping time for FA0.1 was 400 s, in which the first cycle lasted 200 s and the other three cycle needed 65 s for each. For MAI, FA0.2 and FA0.3, the total dipping time was 500 s, in which the first cycle was 200 s and the other three cycle was 100 s for each. After each dipping cycle, the film was rinsed with IPA and dried in air. Finally, the as-prepared pure MAPbI3 and hybridcomposition perovskites were annealed at 80 °C and 120 °C for 20 min on a hot plate, respectively. The HTL was spin-coated onto a previously prepared photoelectrode in a solution containing 75 × 10−3 M 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene (spiro-OMeTAD, > 99%, Lumtec, Taiwan) in chlorobenzene (99.8%, Sigma-Aldrich) at 3000 rpm for 20 s. The dopants in the HTL solution 2
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Fig. 1. (a) P-PSC structure in this study, the photograph exhibits Pb(NO3)2 and resultant perovskite films on plastic BK-TiO2/SnO2/ITO-PEN substrate; (b) Process flowchart of fabricating P-PSC in this study and the AFM topographic image of each step in the process.
FAI/MAI/MABr incubation, a continuous perovskite capping layer can be obtained as shown in the final step of Fig. 1. In Fig. S1, cyclic voltammetry (CV) is scanned on a three-electrode electrochemical cell to probe the hole blocking capability of SnO2 CL and BK-TiO2/SnO2 bilayer, in which these two test samples serve as the working electrodes. The CV wave representing the redox reaction of the redox couple (Fe (CN)62+/Fe(CN)63+) is mimicked as the charge recombination behavior of hole transport layer (HTL) [21–23]. The CV wave obtained from bare ITO-PEN exhibits a typical Nernstian shape, implying the bare ITOPEN sheet has nearly no blocking capability for the target redox reaction. For the case of SnO2-coated ITO-PEN, the CV wave shrinks and deforms considerably, suggesting that the redox reaction has been effectively retarded. For BK-TiO2/SnO2 bilayer, the CV curve displays the same shape but becomes narrower at the redox potential region compared to that of SnO2 CL, which is an indicative of additional hole blocking capability provided from BK-TiO2. It is generally accepted that to achieve a high PCE for a PSC, compositional engineering on perovskite absorber is essential. Since pristine MAPbI3 has a unstable tetragonal structure and a wide bandgap of 1.57 eV, [24] incorporating dual or even triple cations in lead mixed halide octahedral like FA-MA (FAxMA1-xPbI3-yBry) [25] is a common strategy to enhance the structural stability and light harvesting for realizing high PCE. However, the benefit of compositional engineering has not been verified in Pb(NO3)2/H2O system yet. Two reasons account for this; one is the coverage of MAPbI3 capping layer made from Pb(NO3)2/H2O system is flawed as mentioned above and the other is the slow conversion kinetics of Pb(NO3)2 to MAPbI3 in MAI incubation when compared with conventional PbI2/DMF protocol [26]. This slow formation kinetics forces long MAI incubation time in order to achieve high conversion yield of perovskite. However, long MAI incubation tends to induce Ostwald ripening that devastates the coverage of capping layer again. We have paid attention to solve the MAPbI3 coverage issue by splitting the MAI incubation into several short cycles but maintaining the total time unchanged. This arrangement is designed to interfere the chemical equilibrium of reacting species, which is a criterion to trigger Ostwald ripening, whereas the conversion of MAPbI3 can be high because the total immersion time is identical [18]. Herein, the crystallization mechanism of mixed cations and halides perovskite in Pb(NO3)2/H2O protocol is examined in detail for the first time. As the conversion of Pb(NO3)2 to hybrid perovskite is diffusion rate-dependent, it is imperative to investigate the kinetics of intermediate ion exchange among NO3− and mixed cation/anion. First, Br− is added
into the MAI solution. Fig. 2a presents the in situ UV–Vis absorption spectra, which record the consecutive conversion of Pb(NO3)2 to perovskite in IPA solution containing MAI and MAI0.9Br0.1 (hereafter MAI/ IPA and MAI0.9Br0.1/IPA, respectively) for different periods. The characteristic 780 nm onset wavelength for perovskite and the 500 nm absorbance for PbI2 are observed in both samples initially, indicating the existence of perovskite formation even in 30 s incubation and PbI2 and perovskite coexist for a period of time. PbI2 characteristic peak vanishes after 300 s for the sample immersed in MAI0.9Br0.1/IPA while persists in MAI/IPA. Fig. 2b depicts the variations in the absorbance at 720 nm of the sample as a function of incubation time. There are two features in Fig. 2b: first it shows that the absorbance increases rapidly until the kink point (tC), while above tC the rate of increase is alleviated. We speculate that the sharp slope region corresponds to the perovskite conversion and the mild slope region is related to the light-scattering effect from large perovskite crystals induced by Ostwald ripening effect. In order to confirm our speculation, the time-dependent morphological evolution of Pb(NO3)2 in MAI0.9Br0.1/IPA is investigated and shown in Fig. S2. At initial stages (100 s and 200 s), the formation of perovskite gradually heals the pin-hole defects stemming from the discontinuous Pb(NO3)2 film because of volume expansion during perovskite formation. When the reaction approaches endpoint of first part (300 s), Ostwald ripening effect commences. A few cuboidal perovskite crystals start to appear and coarsen the film surface. With prolonged incubation (400 s and 600 s), the large, anisotropic cuboidal perovskite crystals dominate the appearance of the surface cap. This continuous crystallization is in good agreement with its absorption variation. Consequently, by monitoring the absorbance at 720 nm with incubation time, the determination of optimal incubation time, which is referred to the time before the commencement of Ostwald ripening effect, is convenient and reliable. Fig. 2c shows a semi-logarithmic plot of the absorbance at 720 nm versus (t-tC), confirming the tC of Pb(NO3)2 immersed in MAI0.9Br0.1/IPA and MAI/IPA is 300 s and 500 s, respectively. The complete conversion of Pb(NO3)2 to MAPbI3-yBry takes approximately 300 s, which is significantly shorter than that of Pb (NO3)2 to MAPbI3 (tC ~ 500 s). Due to the concentration gradient of the ions involved in the incubation process, MA+ and Br−/I− diffuse from the bulk solution to inner crystals while NO3− moves outwardly. In contrast to the large I− (2.20 Å), [27] the smaller Br− (1.96 Å) has been reported to possess higher association constant for the complexation with Pb2+ and is expected to diffuse more swiftly from the skin toward the core, leading to accelerate the formation of perovskite [28]. 3
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Fig. 2. (a) In situ UV–Vis spectra during the incubation of the Pb(NO3)2-coated BK-TiO2/SnO2/ITO-PEN substrate in MAI/IPA and MAI0.9Br0.1/IPA solution in different periods; the left configuration is the sample setup for the in situ examination. Variations in the absorbance at 720 nm of (b) MAI/MAI0.9Br0.1-treated and (d) FAxMA1-xI0.9Br0.1-IPA (x = 0.1, 0.2, 0.3)-treated Pb(NO3)2 films on BK-TiO2/SnO2/ITO-PEN substrate as a function of incubation time. (c) and (e) Semi-logarithmic plot of absorbance at 720 nm versus (t−tC), which indicates a kink point. The red line is the linear fit to the data with a correlation factor of over 0.998; (f) Mechanism of perovskite crystal development. Schematic in situ crystal development for perovskite formation in the Pb(NO3)2 protocol via sequential deposition. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
As noted, Hoke et al. reported that phase segregation can occur in materials containing both Br and I under light exposure [29]. The phenomenon typically only manifests itself for compositions with Br contents greater than 20%, so perovskite that use small amounts of Br to facilitate crystallization or perform minimal amounts of band gap tuning tend to not suffer from the Hoke Effect [30]. To determine the optimized Br substitution, 5% and 10% Br contents are tried for comparison. I-V performance demonstrates that P-PSC with 10% Br substitution is better than that with 5% Br substitution due to the enhanced VOC and FF (shown in Fig. S3). Therefore, based on the fixed MAI0.9Br0.1 composition, different amount of FA+ is added to intercalate with the MA+ within the PbI6 octahedra. Fig. S4 shows the in situ UV–Vis absorption spectra of Pb(NO3)2-coated substrates immersed in the FAxMA1-xI0.9Br0.1-IPA (x = 0.1, 0.2, 0.3, hereafter named as FA0.1, FA0.2 and FA0.3). As observed in Fig. 2d, e, the conversion is retarded as the tC increases from 300 s in the absence of FA+ (MAI0.9Br0.1) to 400 s and 500 s for FA0.1 and FA0.2, respectively. This is because that FA+ has a larger ionic size (2.53 Å) than MA+ (2.17 Å), which makes it
more difficult to penetrate into the PbI6 framework at the early stage of crystallization. Upon thermal annealing, the uncoordinated residual FA+ ions gain additional activation energy to replace MA+ in the αphase perovskite framework [24]. In slow intermediate ion exchange process like Pb(NO3)2 protocol, FA+ ions not only have to compete with MA+ to coordinate into PbI6 but also hinder the outward diffusion pathways of hydrated NO3− ions (3.35 Å). Both effects are intensified in high FA+ addition (similar to the case of FA0.2), rendering undesired film quality because of long incubation. FA0.3 has similar result to FA0.2, implying the amount of FA+ is saturated. Perovskite crystal development from Pb(NO3)2 via sequential deposition in FAxMA1xI0.9Br0.1 is cartooned in Fig. 2f. Even though the conversion mechanism of FAxMA1-xI0.9Br0.1 is unveiled, the entire incubation requires from 300 to 500 s depending on the presence of Br− or FA+, which inevitably suffers from Ostwald ripening effect and results in a flawed perovskite morphology as shown in Fig. S2. By using multiple-cycle immersion to refine the morphology of perovskite films, a fully covered, cuboidal crystal-free perovskite 4
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Fig. 3. SEM images of perovskite capping layer on BK-TiO2/SnO2/ITO-PEN obtained by multiple-cycle immersion in (a) MAI-IPA, (b)–(d) FAxMA1-xI0.9Br0.1-IPA (x = 0.1, 0.2, 0.3) solution.
Fig. 4. XRD patterns of pure MAPbI3 and hybrid-composition perovskites on BK-TiO2/SnO2/ITO-PEN.
capping film can be obtained [18]. Fig. 3 shows SEM images of the perovskite capping layer formed in FAxMA1-xI0.9Br0.1/IPA after applying multiple-cycle immersion technique. It can be seen uniform perovskite layers exhibiting void-free are obtained. Notably, as FA+ increases, the average size of the grains increases, in consistent with previous study [31]. Cross-sectional SEM image of FA0.1 P-PSC in Fig. S5 reveals that the total thickness of the device including PEN plastic substrate (125 μm), [32] SnO2 compact layer (0.04 μm), perovskite absorber (0.5 μm), HTL (0.24 μm), and Au layer (0.1 μm) is 125.88 μm. Fig. 4a shows the X-ray diffraction (XRD) patterns of pure MAPbI3 and hybrid-composition perovskites. In addition to the background diffraction peaks of ITO-PEN at 2θ = 27.16°, two peaks at 2θ = 14.42° and 32.24° indicate diffraction at (110) and (310) planes of the MAPbI3 [33]. It is worth noting that a tiny peak of Pb(NO3)2 is shown at around 2θ = 19.5° in all perovskite samples and the Pb(NO3)2 peak slightly increases when FA+ incorporates, which plays a crucial role in the enhanced environmental stability in the perovskite crystallites [19]. Zoom-in XRD pattern at 2θ = 14.42° is shown in Fig. 4b, it is found the (110) diffraction peak continuously shifts from 14.42° to 14.34° when the addition of FA+ increases, which confirms the successful incorporation of FA cation into the perovskite crystal lattice since the FA
cation has a relatively larger ionic radius than MA. The full width half maximum (FWHM) of the (110) diffraction peak increases monotonically with increasing FA content (0.1 ≤ x ≤ 0.3). Typically, decreased grain size and/or increased density of defects (lower crystallinity) are responsible for the increase of FWHM [34]. Because the average grain size increase as the FA content increases shown in Fig. 3, the increased FWHM indicates decreased film crystallinity due to the sluggish conversion kinetics induced by excessive FA incorporation. The UV–Vis absorption spectra of pure MAPbI3 and hybrid-composition perovskites are displayed in Fig. 5a. When Br− is added in MAPbI3, the absorption onset experiences a blue-shift from 780 nm to 774 nm, which is attributed to structural distortion induced by stress and biaxial strain of Pb-I bonds as Br is substituted. The gradual red-shift is present with increased FA+ in FAxMA1-xI0.9Br0.1 from 778 nm to 784 nm, suggesting that the incorporation of FA+ into MAPbI3-yBry helps relax the anisotropic strained lattice and stabilizes the perovskite phase. The slight increment in the onset absorption wavelength for FA+ incorporation again corroborates the fact that Br− dominates the conversion kinetics and FA+ intercalates into PbI6 in an energetically unfavorable way. The corresponding band gap is presented in Fig. 5b. The normalized steady-state photoluminescence (PL) spectra are shown in 5
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Fig. 5. (a) UV–vis absorption and (b) the corresponding band gap variation; (c) the steady-state PL spectra of pure MAPbI3 and hybrid-composition perovskites and (d) the shift of characteristic peak.
Fig. 5c and d. In agreement, a significant blue-shift from 779.5 nm to 773.0 nm for FA0.1 perovskite is observed with respect to the pure MAPbI3, further disclosing that the trap density and energetic disorder are reduced by the compositional engineering [4]. After incorporating of Br− and FA+ into pristine MAPbI3 perovskite absorber, the photovoltaic performance of the FAxMA1-xI0.9Br0.1 based P-PSC made from aqueous Pb(NO3)2 precursor are examined and the result is presented in Fig. 6. All data are statistical results of 10 devices made from different batches. The average values with standard deviations are listed in Table 1. Apparently P-PSC based on FA0.1 shows highest JSC, VOC, FF and consequent PCE. The average PCE of FA0.1based P-PSC improves from 12.72% to 15.45%, including a champion device having 16.50% obtained from backward scan. As shown in Fig. S6, the integrated JSC obtained from IPCE spectrum is 19.25 mA/cm2 for FA0.1-based P-PSC, which is slightly lower than the JSC of 20.70 mA/cm2 obtained from J-V measurement, representing an acceptable mismatch of less than 10%. Even though there is a large optical loss at wavelengths below 450 nm due to intrinsic absorption by the PEN film, the EQE of FA0.1-based P-PSC among 450–750 nm maintains at high plateau of 75–85% and warrants high JSC. The hysP max, b 1; where Pmax,b is the teresis index, HI, is defined as HI = P max, f maximum power obtained from backward scan and Pmax,f is the maximum power obtained from forward scan. Fig. 6e summarizes the HI calculating from the IV data. The FA0.1-based devices show the lowest HI, suggesting that adding 10% FA+ can considerably stabilize the structure, limiting possible light- or field-induced ion movement and impeding the halide segregation. Considering that all the devices share the same ETLs and HTLs, the increase in PCE for FA0.1-based PPSC can be solely ascribed to the improvement of perovskite crystal quality. As mentioned above, the formation of a stable “quasi-cubic” phase at room temperature accompanied by improved crystallinity and reduced energetic disorder is the reason behind the high JSC, VOC and FF. On the other hand, excess FA+ addition (FA0.2 and FA0.3 in this
study) can disturb intermediate ion exchange in the solid-state diffusion, which intensifies the hindrance of crystallization and results in poor crystallinity. Therefore, FA0.2 and FA0.3-based P-PSCs fail to further improve their photovoltaic performance. The finding of low allowance of FA implantation may treat as a special feature of Pb(NO3)2 protocol. Fig. S7 shows a semi-logarithmic plot of VOC and the light intensity of the pristine MAPbI3 and polished FA0.1-based P-PSCs, where the ideality factor (n) in the diode equation can be obtained from nk T the slope of the fitting line (~ qB ; where kB is the Boltzmann constant, T is the absolute temperature, and q is the elementary charge). According to a previous study, [35] the light-intensity dependence of the VOC can provide insights into the role of trap-assisted recombination versus bimolecular recombination at open-circuit. As observed, both devices exhibit high ideality factors of over 1.5, which implies that trapassisted Shockley–Read–Hall (SRH) recombination is present at moderate level in both devices. Compared with larger n value (~1.99) for MAI-based device, n = 1.51 is obtained from FA0.1 P-PSC, once again proving that the compositional optimization effectively reduces the trap density and suppresses SRH recombination. The mechanical stability of FA0.1 P-PSC grown on low-temperature BK-TiO2/SnO2/ITO-PEN substrate has been investigated by repeatedly bending the complete device. First, FA0.1 P-PSC with an initial PCE of 15.75% is bent with a curvature radius of 20 mm for 3 s as one cycle. The I–V characteristics are measured after several bending cycles and this is repeated for 1000 cycles. The results are depicted in Fig. S8a. It is evident from the bending test that the devices retains over 80% of PCE after 1000 bending cycles. In particular, the VOC maintains an impressively constant value of 1.02 V after 1000 cycles, suggesting that the device remains structurally stable and all of the functional layers and their interfaces are mechanically robust throughout the entire duration of the bending test. From an initial value of 20.75 mA cm−2, JSC decreases to 18.22 mA cm−2 after a 1000-cycle bending test; similarly, FF drops from 0.73 to 0.68, resulting in a decay of PCE from 6
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Fig. 6. (a)–(e) Box plots of I–V characteristics of the BK-TiO2 scaffold-based P-PSCs made with different perovskite; (f) I–V curves of the optimal P-PSC employed with FA0.1 perovskite; I–V parameters are depicted in the table.
15.75% to 12.64%. According to our previous work, [23] damage to the fragile ITO coating is to blame for the JSC and FF loss. An SEM examination shown in Figs. S8b and c is conducted on a piece of purposely prepared BK-TiO2/SnO2/ITO-PEN sheet, which has been subjected to
1000 bending cycles. The unambiguous microcracks in the fragile ITO occurs, which obstructs current collection and increased internal resistance; thus, JSC and FF decrease. The stability trail of FA0.1- and MAbased P-PSCs using Pb(NO3)2/H2O protocol is tested. The FA0.1 device
Table 1 Summary of the IV data obtained using different perovskite. Type of perovskite
JSC (mA/cm2)
VOC (V)
MAI FA0.1 FA0.2 FA0.3
20.11 20.70 19.27 19.41
1.006 1.028 1.011 1.008
± ± ± ±
0.51 0.55 0.43 0.49
± ± ± ±
FF 0.020 0.013 0.026 0.011
0.629 0.726 0.677 0.676
7
PCE (%) ± ± ± ±
0.048 0.020 0.036 0.050
12.72 15.45 13.18 13.21
± ± ± ±
HI 0.96 0.38 0.74 0.89
0.13 0.04 0.07 0.10
± ± ± ±
0.07 0.01 0.04 0.05
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Fig. 7. (a) Stability trails of the photovoltaic parameters of the Pb(NO3)2-based device made using the FA0.1 and MAI recipes; (b) XRD patterns of FA0.1 and MAI perovskites after 50 days aging test; (c) UV–vis absorbance of FA0.1 perovskite film before and after 50 days aging.
with initial JSC, VOC, FF, and PCE values of 21.61 mA/cm2, 1.03 V, 0.70, and 15.64%, is stored in ambient darkness without additional sealing under temperature and humidity control at 23–25 °C and 10%–25%, respectively. The results are summarized in Fig. 7a. After 50 days, PCE drops to 11.86%, corresponding to a 75.8% retention. In contrast, MAbased device presents the relatively poor stability trail with only 58.4% PCE retention after 50 days aging. From the XRD pattern (shown in Fig. 7b) of the perovskites after aging test, the FA0.1 film maintains the same perovskite phase as that shown in Fig. 4 while the PbI2 peak ~ 12.7° occurs in aged MAI film due to the decomposition of MAPbI3 under moisture exposure. In addition, a tiny peak of Pb(NO3)2 residue can be observed in FA0.1 film before and after aging test, which corroborates the fact that FA+ ions diffuse inwardly will hinder the outward diffusion pathways of hydrated NO3− ions during the intermediate ion exchange process. Consequently, the high-quality perovskite benefiting from the compositional engineering and “water scavenger” Pb(NO3)2 residue simultaneously contribute to the robust environmental stability. We list major published works relating to the PSC stability in Table S1. From the UV–Vis absorption spectra (shown in Fig. 7c) of the FA0.1 perovskite sample before and after aging test, the optical property of the perovskite film maintains robust optical properties, which implies that the perovskite absorber and its PV activity are inherently stable during the 50 days. Hence, the mild PCE decay in device may come from the poor airproof ability of ITO-PEN, or from hygroscopic spiro-OMeTAD with the lithium dopant, or from gold diffusion into the HTM because of unsealed device. Finally, an attempt to upscale the device size was preliminarily studied. Up to now, high PCE P-PSCs reported in literature are mostly based on an active area < 0.1 cm2, [36,37] it is still challenging to manufacture the P-PSCs with large size. Because the processing solvent in coating lead source is water now, it is a simple decision for us to fabricate FA0.1-based P-PSCs with larger area without the concern of
vast toxic solvent usage. Shown in Fig. 8, the PCE drops to 10.61% when the device area is enlarged to 1 cm2. From the digital photo of spin-coated Pb(NO3)2 film and SEM images of the resultant FA0.1 perovskite film, poor uniformity of spin-coated Pb(NO3)2 film, particularly in the edges of the substrate, accounts for the loss. The thin Pb (NO3)2 coating at the edge intensifies the Ostwald ripening effect for the resultant perovskite as it is supposed to complete the conversion in advance compared with that in the center region. In the edge zone, small FA0.1 perovskite grains in mesoporous BK-TiO2 tend to dissolve first and then recrystallize as large grains. As this phenomenon proceeds, perovskite crystals in the mesoporous BK-TiO2 gradually deplete, accompanied by enlargement of large cuboidal crystals on or above the surface cap at the edge. During this stage, significant loss on JSC and FF can be observed on the IV characteristic because of poor contact between the FA0.1 perovskite absorber and BK-TiO2 scaffold [38]. This result clearly points out that uniform Pb(NO3)2 coating is essential to further develop large area P-PSC using this green, environmentallyfriendly fabrication process. 4. Conclusions In summary, we reported an eco-friendly method from an aqueous lead nitrate precursor to fabricate hybrid-composition perovskite absorber on plastic substrate. By investigating the kinetics of intermediate ion exchange among NO3− and mixed cation/anion, the perovskite conversion was largely enhanced and the resultant perovskite with the optimal composition was verified. Based on the BK-TiO2 scaffold, the fully ambient-processed P-PSCs achieved an optimized PCE of 16.50%, which was the highest in P-PSC made from water-based precursor. Clean production of large-scale P-PSCs using a minimal quantity of harmful chemicals is expected to be realized using Pb(NO3)2/H2O protocol.
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Fig. 8. I–V curves of the large area P-PSC employed with FA0.1 perovskite; the film demonstrates non-uniform morphology between the center and the edge, which degrades the JSC and FF.
Acknowledgments
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This research was supported by the Ministry of Science and Technology, Taiwan (MOST105-2628-E-007-021-MY2 and 107-2119M-007-001). Financial support from National Natural Science Foundation of China (51702263) and the Fundamental Research Funds for the Central Universities, China (3102018zy048) is also appreciated. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.nanoen.2019.104036. References [1] F.D. Giacomo, A. Fakharuddin, R. Jose, T.M. Brown, Energy Environ. Sci. 9 (2016) 3007–3035. [2] M. Park, H.J. Kim, I. Jeong, J. Lee, H. Lee, H.J. Son, D.-E. Kim, M.J. Ko, Adv. Energy Mater. 5 (2015) 1501406. [3] B.J. Kim, D.H. Kim, Y.-Y. Lee, H.-W. Shin, G.S. Han, J.S. Hong, K. Mahmood, T.K. Ahn, Y.-C. Joo, K.S. Hong, N.-G. Park, S. Lee, H.S. Jung, Energy Environ. Sci. 8 (2015) 916–921. [4] J. Feng, X. Zhu, Z. Yang, X. Zhang, J. Niu, Z. Wang, S. Zuo, S. Priya, S.F. Liu, D. Yang, Adv. Mater. 30 (2018) 1801418. [5] A. Salavei, D. Menossi, F. Piccinelli, A. Kumar, G. Mariotto, M. Barbato, M. Meneghini, G. Meneghesso, S.D. Mare, E. Artegiani, A. Romeo, Solar Energy 139 (2016) 13–18. [6] J. Ramanujam, U.P. Singh, Energy Environ. Sci. 10 (2017) 1306–1319. [7] M. Kaltenbrunner, M.S. White, E.D. Głowacki, T. Sekitani, T. Someya, N.S. Sariciftci, S. Bauer, Nat. Commun. 3 (2012) 770. [8] H.C. Weerasinghe, F. Huang, Y.-B. Cheng, Nano Energy 2 (2013) 174–189. [9] A. Khan, Y.-T. Huang, T. Miyasaka, M. Ikegami, S.-P. Feng, W.D. Li, ACS Appl. Mater. Inter. 9 (2017) 8083–8091. [10] C.-G. Wu, C.-H. Chiang, Z.-L. Tseng, Md K. Nazeeruddin, A. Hagfeldt, M. Gratzel,
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Tsung-Yu Hsieh is a Ph.D. candidate in Department of Chemical Engineering, National Tsing Hua University, and he was in Institute of Chemical Research of Catalonia for 1 year exchanging. His research interests are in dye-sensitized solar cells and perovskite solar cells, including process engineering, electrochemical analysis and photophysical analysis.
[31] Y. Zhang, G. Grancini, Y. Feng, A.M. Asiri, M.K. Nazeeruddin, ACS Energy Lett. 2 (2017) 802–806. [32] T. Singh, M. Ikegami, T. Miyasaka, ACS Appl. Energy Mater. 1 (2018) 6741–6747. [33] A. Kogo, M. Ikegami, T. Miyasaka, Chem. Commun. 52 (2016) 8119–8122. [34] C. Wang, D. Zhao, Y. Yu, N. Shrestha, C.R. Grice, W. Liao, A.J. Cimaroli, J. Chen, R.J. Ellingson, X. Zhao, Y. Yan, Nano Energy 35 (2017) 223–232. [35] F. Xie, C.-C. Chen, Y. Wu, X. Li, M. Cai, X. Liu, X. Yang, L. Han, Energy Environ. Sci. 10 (2017) 1942–1949. [36] C. Wang, L. Guan, D. Zhao, Y. Yu, C.R. Grice, Z. Song, R.A. Awni, J. Chen, J. Wang, X. Zhao, Y. Yan, ACS Energy Lett. 2 (2017) 2118–2124. [37] Effciency chart, https://www.nrel.gov/pv/assets/images/effciencychart.png (accessed: October 2017). [38] T.-Y. Hsieh, C.-K. Huang, T.-S. Su, C.-Y. Hong, T.-C. Wei, ACS Appl. Mater. Inter. 9 (2017) 8623–8633.
Wei-Yen Wang came from Miaoli County and now he is a Ph.D. candidate in the Department of Chemical Engineering in National Tsing Hua University. His research interests include surface modification by silane derivative, electrochemical metalization on inorganic/organic material and dye-sensitized solar cells.
Lixia Ren was born in Shanxi province and received her M.S. degree in 2016 from Northwestern Polytechnical University. And now she is a Ph.D. candidate at the same University. Her research interest focus on developing novel functional material as electrode for the third generation thin film solar cells, e.g. dye-sensitized solar cells and perovskite solar cells.
Jiayi Guo is a master student in the Department of Chemical Engineering in National Tsing Hua University. Her expertise includes surface treatment and fracture analysis on interfaces. Her current research topic is passivation of perovskite films using silane coupling agents.
Peng Zhai is an associate professor of the School of Natural and Applied Sciences in Northwestern Polytechnical University. He received his Ph.D. degree in The University of Hong Kong in 2015. Dr Zhai has once worked as a visiting scholar in the Department of Chemical Engineering in National Tsing-Hua University. His current research interests include perovskite solar cells, dye-sensitized solar cells (DSCs) and functional graphene materials.
Tzu Chien Wei is an associate professor in Department of Chemical Engineering at National Tsing-Hua University, Hsinchu, Taiwan. After serving in industry from 2007 to 2011 and a postdoctoral training in 2012, he joined NTHU since 2012. He published over 50 peer-reviewed papers regarding dye-sensitized solar cells, perovskite solar cell and surface treatment & metallization.
Tzu-Sen Su received his M.S. degree in 2015 from Department of Chemical Engineering in National Tsing-Hua University. Now, he is a Ph.D. candidate at the same University. His research interest focuses on electrodepositing different metal oxide as electrode for perovskite solar cells.
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Full paper
Toward clean production of plastic perovskite solar cell: Compositiontailored perovskite absorber made from aqueous lead nitrate precursor
T
Peng Zhaia,∗∗, Tzu-Sen Sub, Tsung-Yu Hsiehb, Wei-Yen Wangb, Lixia Rena, Jiayi Guob, Tzu-Chien Weib,∗ a b
School of Natural and Applied Sciences, Northwestern Polytechnical University, Xi'an, 710072, PR China Department of Chemical Engineering, National Tsing-Hua University, Hsinchu, 300, Taiwan
ARTICLE INFO
ABSTRACT
Keywords: Aqueous lead nitrate precursor Intermediate ion exchange Compositional engineering Conversion kinetics Scaffold-type plastic perovskite solar cell
Even though the power conversion efficiency (PCE) of plastic perovskite solar cells (P-PSCs) is increased to 18.40%, the majority of solvent systems implemented for deposition of perovskites are hazardous to handle, which will greatly hinder the future development of plastic photovoltaic devices. In this study, compositiontailored hybrid perovskite from a low-toxicity aqueous lead nitrate precursor was fabricated by regulating the conversion kinetics. We systematically investigated the interplay among NO3− and mixed cation/anion in the intermediate ion exchange and renovated the interpretation of hybrid-composition perovskite conversion. The fully ambient-processed hybrid-composition perovskite with high crystallinity and less defects was applied in a brookite TiO2 scaffold-based P-PSCs, which achieved a record-high PCE of scaffold-type P-PSC of 16.50%. The interaction of environmentally-friendly aqueous lead nitrate precursor with hybrid ions advanced the understanding of perovskite conversion mechanism and had a great potential to realize the low-toxic fabrication process by using water as a processing solvent in the ambient atmosphere.
1. Introduction Plastic and lightweight thin film solar cells have been anticipated to have great market prospects for portable electronics such as remote power chargers, bendable displays, and wearable electronic devices, in view of their capability to generate electricity on curved surface using a low temperature solution process [1–3]. Recently, plastic perovskite solar cells (P-PSCs) have made great progress in pursuing high power conversion efficiency (PCE) up to 18.40%, [4] revealing its powerful competitiveness with other flexible photovoltaic technology including copper indium gallium selenide, [5] cadmium telluride, [6] organic photovoltaics, [7] and dye-sensitized solar cells [8,9]. However, numerous concerns of PSCs regarding to scalability, stability, and toxicity remain unsolved or challenging till now. Besides the toxicity of leadcontaining organic material itself, the chemicals involved during the fabrication process is particularly important in P-PSC because plastic photovoltaics are expected to have more chances to contact human skin directly. The possible chemicals left from low temperature process may add another healthy concern during product lifecycle. For instance, the most widely used solvents for the lead-containing precursor in the literature reporting high PCEs are aprotic organic polar solvents with high ∗
boiling points such as N,N-dimethylformamide (DMF), [10] dimethyl sulfoxide (DMSO) [11] and N-methyl-2-pyrrolidone (NMP), [12,13] most of them are skin penetrating and carcinogenic toxicants. P-PSCs are expected to consume large amounts of organic solvent in production scale, which is harmful not only to the factory operators but also to the end-product users. Many endeavors have been carried out to avoid or diminish the use of these hazardous solvents in the fabrication process. Electrospinning [14] and electrodepositing [15] lead-containing compounds onto the TiO2 scaffold without the use of harmful solvents were reported. However, the poor morphological control of the perovskite capping layer and the additional iodination treatment limit their photovoltaic performance and add another complexity of fabrication process, respectively. Gardner et al. developed a set of nonhazardous organic solvent systems for single-step coating of methylammonium lead iodide (MAPbI3) layers based on blending γ-butyrolactone, cyclic carbonates, alcohols, acids, and other protonated carbon chains [16]. Unfortunately, the efficiency of these devices was far inferior to their counterpart using toxic DMF. We have developed and optimized an ecofriendly process to prepare MAPbI3 on FTO glass using aqueous lead nitrate precursor and have achieved a PCE of 16.02% [17,18]. T. Park et al. followed this method and reported that the Pb(NO3)2 residues in
Corresponding author. Corresponding author. E-mail addresses: [email protected] (P. Zhai), [email protected] (T.-C. Wei).
∗∗
https://doi.org/10.1016/j.nanoen.2019.104036 Received 27 March 2019; Received in revised form 29 July 2019; Accepted 16 August 2019 Available online 19 August 2019 2211-2855/ © 2019 Published by Elsevier Ltd.
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were 25*10−3 M bis(trifluoromethane)-sulfonimide lithium salt (LiTFSI, 99.95%, Sigma-Aldrich) and 120*10−3 M 4-tert-butylpyridine (96%, Sigma-Aldrich). The device was subsequently stored in a dry atmosphere (temperature: 25 °C, relative humidity: 10%) overnight, after which the gold terminals were thermally evaporated through a stainless steel shadow mask.
the MAPbI3 could function to retard perovskite decomposition toward moisture as a water scavenger [19]. Notwithstanding of these achievements, there is no report to fabricate P-PSC from aqueous lead precursor until now. Moreover, compositional engineering on perovskite ingredient such as mixing cations or halides is acknowledged as an applicable method to enhance both PCE and stability. Johansson et al. attempted to synthesize CsyFA1-yPb(I1-xBrx)3 perovskite materials based on aqueous Pb(NO3)2 precursor on rigid substrate but the PCE was only 13%, [20] indicating there still has plenty improvement to be engineered. Consequently, in this report, we pioneered to fabricate PPSC based on composition-tailored hybrid perovskite absorber from aqueous lead nitrate precursor in the ambient atmosphere. By understanding and regulating the conversion kinetics of the FAxMA1-xPbI3yBry, a P-PSC made from green process with record-high PCE of 16.50% was achieved.
2.5. Material characterization and photovoltaic performance measurement UV–vis absorption was recorded with a Hitachi U-4100 Spectrophotometer. X-ray diffraction (XRD) data were collected through a Brucker powder diffractometer (D8 Discover) using Cu Kα1 radiation. The topography of different perovskites was investigated by field-emission scanning electron microscope (FESEM, S-4800, Hitachi, Japan). The blocking effect was investigated by CV using a computercontrolled potentiostat (Solartron SI 1286, UK) in a three-electrode system with Ag/AgCl reference electrode and 2 cm2 platinum counter electrode at 50 mV s−1 scan rate. In CV scan, an aqueous electrolyte containing 0.5 mM potassium hexacyano-ferrate(II) trihydrate and 0.5 mM potassium hexacyano-ferrate(III) was utilized with 0.5 M KCl as supporting electrolyte. A photo-mask (0.06 cm2 and 1 cm2) was used to control the area of light exposure in the subsequent current-voltage (IV) scan, which was recorded with a computer-controlled digital source meter (Keithley 2400) under exposure of a solar simulator (AM 1.5G, 100 mWcm−2, PEC-L15, PECCELL Technologies, Japan). The voltage scan rate was 50 mV s−1 in forward and reverse scans. The currentvoltage response of P-PSCs was examined using a PECL15 solar simulator (Peccell, Japan) equipped with a Keithley 2400 digital source meter (USA) under standard 100 mW/cm2, AM 1.5 G illumination. The light intensity was calibrated through the 91150V model (Newport, UK). Before measurement, a non-reflective metal mask with aperture area (0.06 cm2 and 1 cm2) was covered on the top of the device to precisely define the active area and ensure that no illumination or voltage biasing condition had been introduced. In the light intensitydependent VOC examination, the light intensity was controlled using a ND filter (Shibuya Optical Co., LTD, Japan) that covered the top of the device. All photovoltaic parameters were extracted from the average IV curve of forward (from short-circuit to open-circuit) and reverse (from open-circuit to short-circuit) scans.
2. Experimental section 2.1. Materials and reagents The SnO2 colloid precursor was obtained from Alfa Aesar (tin (IV) oxide). Pb(NO3)2 (> 99%) was purchased from J. T. Baker. Formamidinium iodide (FAI), methylammonium bromide (MABr), and methylammonium iodide (MAI) were purchased from Xi'an Polymer Light Technology Corp. 2.2. Substrate preparation ITO-PEN sheets (14–15 Ω•sq−1 Peccell Technologies, Inc.) were etched by using a laser etcher (LMF-020F, Taiwan) to obtain the required electrode pattern. The sheets were then sequentially cleaned with acetone and isopropanol (IPA) under sonication for 15 min. 2.3. Electron-collecting bilayer fabrication Before deposition, each ITO-PEN sheet was treated with UV-ozone (customized, Kingo Electrical Enterprise Co., LTD.) for 15 min to remove the organic residues. Then, the substrate was spin coated with a thin layer of SnO2 nanoparticle film (2.67%) at 3000 rpm for 30 s, and annealed in ambient air at 150 °C for 30 min. A mesoporous scaffold using a 6.5 wt% binder-free brookite suspension (PECC-B01, Peccell Technologies, Inc.) was spin coating on the prepared SnO2/ITO-PEN at 6000 rpm for 30 s, followed by baking at 150 °C for 30 min.
3. Results and discussion Because of conductive plastic substrate (ITO-PEN) is used, the whole fabrication is conducted under the thermal budget, ie. 150 °C of the PEN film. Fig. 1 outlines a schematic of the device architecture and the process flow detailing the fabrication of P-PSC based from Pb(NO3)2/ H2O precursor. Removal of NO3− ions is carried out by two routes: on the one hand, NO3− ions diffuse outwardly from the inner Pb(NO3)2 crystals to bulk solution; on the other, NO3− ions remaining on the surface can be removed by pure isopropanol (IPA) rinsing. The topography of every step is recorded by atomic force microscopy (AFM). The electron transport bilayer on ITO-PEN is prepared by sequentially spincoating an aqueous SnO2 colloidal precursor and a commercial brookite TiO2 (BK-TiO2) solution to form the compact layer (CL) and the mesoporous scaffold (MS), respectively. The root mean square surface roughness (Ra) of the SnO2-coated ITO-PEN is 0.91 nm, indicating that the SnO2 CL is extremely smooth. Ra of BK-TiO2-coated substrate is increased to 3.02 nm with a few white spots revealing on the AFM topography, which probably results from the aggregation of the BK-TiO2 dispersion. After a UV-ozone treatment, an aqueous Pb(NO3)2 precursor is spin-coated on BK-TiO2-coated substrate. The topography of Pb (NO3)2 is island-like rather a continuous film, demonstrating that an intact perovskite capping layer made from aqueous Pb(NO3)2 protocol is inherently challenging. After drying, the film is immersed into an IPA solution containing MAI or FAI/MAI/MABr. The total incubation time is divided into 4 sectors in light of our previous work [13b]. After MAI or
2.4. P-PSC fabrication The as-prepared BK-TiO2/SnO2/ITO-PEN film was treated under exposure to a UV-ozone for 15 min. Subsequently, an aqueous solution containing 1.5 M Pb(NO3)2 was spin coated onto the substrate at 6000 rpm for 20 s and dried at 70 °C on a hot plate for 30 min. After cooling to room temperature, the Pb(NO3)2-infiltrated mesoporous BKTiO2 film was immersed in an IPA solution containing MAI and mixed composition system (FAxMA1-xI0.9Br0.1-IPA; x = 0.1, 0.2, 0.3) with the concentration around 15 mg/mL. The total incubation time (tC) was split into 4 sectors. For instance, the total dipping time for FA0.1 was 400 s, in which the first cycle lasted 200 s and the other three cycle needed 65 s for each. For MAI, FA0.2 and FA0.3, the total dipping time was 500 s, in which the first cycle was 200 s and the other three cycle was 100 s for each. After each dipping cycle, the film was rinsed with IPA and dried in air. Finally, the as-prepared pure MAPbI3 and hybridcomposition perovskites were annealed at 80 °C and 120 °C for 20 min on a hot plate, respectively. The HTL was spin-coated onto a previously prepared photoelectrode in a solution containing 75 × 10−3 M 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene (spiro-OMeTAD, > 99%, Lumtec, Taiwan) in chlorobenzene (99.8%, Sigma-Aldrich) at 3000 rpm for 20 s. The dopants in the HTL solution 2
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Fig. 1. (a) P-PSC structure in this study, the photograph exhibits Pb(NO3)2 and resultant perovskite films on plastic BK-TiO2/SnO2/ITO-PEN substrate; (b) Process flowchart of fabricating P-PSC in this study and the AFM topographic image of each step in the process.
FAI/MAI/MABr incubation, a continuous perovskite capping layer can be obtained as shown in the final step of Fig. 1. In Fig. S1, cyclic voltammetry (CV) is scanned on a three-electrode electrochemical cell to probe the hole blocking capability of SnO2 CL and BK-TiO2/SnO2 bilayer, in which these two test samples serve as the working electrodes. The CV wave representing the redox reaction of the redox couple (Fe (CN)62+/Fe(CN)63+) is mimicked as the charge recombination behavior of hole transport layer (HTL) [21–23]. The CV wave obtained from bare ITO-PEN exhibits a typical Nernstian shape, implying the bare ITOPEN sheet has nearly no blocking capability for the target redox reaction. For the case of SnO2-coated ITO-PEN, the CV wave shrinks and deforms considerably, suggesting that the redox reaction has been effectively retarded. For BK-TiO2/SnO2 bilayer, the CV curve displays the same shape but becomes narrower at the redox potential region compared to that of SnO2 CL, which is an indicative of additional hole blocking capability provided from BK-TiO2. It is generally accepted that to achieve a high PCE for a PSC, compositional engineering on perovskite absorber is essential. Since pristine MAPbI3 has a unstable tetragonal structure and a wide bandgap of 1.57 eV, [24] incorporating dual or even triple cations in lead mixed halide octahedral like FA-MA (FAxMA1-xPbI3-yBry) [25] is a common strategy to enhance the structural stability and light harvesting for realizing high PCE. However, the benefit of compositional engineering has not been verified in Pb(NO3)2/H2O system yet. Two reasons account for this; one is the coverage of MAPbI3 capping layer made from Pb(NO3)2/H2O system is flawed as mentioned above and the other is the slow conversion kinetics of Pb(NO3)2 to MAPbI3 in MAI incubation when compared with conventional PbI2/DMF protocol [26]. This slow formation kinetics forces long MAI incubation time in order to achieve high conversion yield of perovskite. However, long MAI incubation tends to induce Ostwald ripening that devastates the coverage of capping layer again. We have paid attention to solve the MAPbI3 coverage issue by splitting the MAI incubation into several short cycles but maintaining the total time unchanged. This arrangement is designed to interfere the chemical equilibrium of reacting species, which is a criterion to trigger Ostwald ripening, whereas the conversion of MAPbI3 can be high because the total immersion time is identical [18]. Herein, the crystallization mechanism of mixed cations and halides perovskite in Pb(NO3)2/H2O protocol is examined in detail for the first time. As the conversion of Pb(NO3)2 to hybrid perovskite is diffusion rate-dependent, it is imperative to investigate the kinetics of intermediate ion exchange among NO3− and mixed cation/anion. First, Br− is added
into the MAI solution. Fig. 2a presents the in situ UV–Vis absorption spectra, which record the consecutive conversion of Pb(NO3)2 to perovskite in IPA solution containing MAI and MAI0.9Br0.1 (hereafter MAI/ IPA and MAI0.9Br0.1/IPA, respectively) for different periods. The characteristic 780 nm onset wavelength for perovskite and the 500 nm absorbance for PbI2 are observed in both samples initially, indicating the existence of perovskite formation even in 30 s incubation and PbI2 and perovskite coexist for a period of time. PbI2 characteristic peak vanishes after 300 s for the sample immersed in MAI0.9Br0.1/IPA while persists in MAI/IPA. Fig. 2b depicts the variations in the absorbance at 720 nm of the sample as a function of incubation time. There are two features in Fig. 2b: first it shows that the absorbance increases rapidly until the kink point (tC), while above tC the rate of increase is alleviated. We speculate that the sharp slope region corresponds to the perovskite conversion and the mild slope region is related to the light-scattering effect from large perovskite crystals induced by Ostwald ripening effect. In order to confirm our speculation, the time-dependent morphological evolution of Pb(NO3)2 in MAI0.9Br0.1/IPA is investigated and shown in Fig. S2. At initial stages (100 s and 200 s), the formation of perovskite gradually heals the pin-hole defects stemming from the discontinuous Pb(NO3)2 film because of volume expansion during perovskite formation. When the reaction approaches endpoint of first part (300 s), Ostwald ripening effect commences. A few cuboidal perovskite crystals start to appear and coarsen the film surface. With prolonged incubation (400 s and 600 s), the large, anisotropic cuboidal perovskite crystals dominate the appearance of the surface cap. This continuous crystallization is in good agreement with its absorption variation. Consequently, by monitoring the absorbance at 720 nm with incubation time, the determination of optimal incubation time, which is referred to the time before the commencement of Ostwald ripening effect, is convenient and reliable. Fig. 2c shows a semi-logarithmic plot of the absorbance at 720 nm versus (t-tC), confirming the tC of Pb(NO3)2 immersed in MAI0.9Br0.1/IPA and MAI/IPA is 300 s and 500 s, respectively. The complete conversion of Pb(NO3)2 to MAPbI3-yBry takes approximately 300 s, which is significantly shorter than that of Pb (NO3)2 to MAPbI3 (tC ~ 500 s). Due to the concentration gradient of the ions involved in the incubation process, MA+ and Br−/I− diffuse from the bulk solution to inner crystals while NO3− moves outwardly. In contrast to the large I− (2.20 Å), [27] the smaller Br− (1.96 Å) has been reported to possess higher association constant for the complexation with Pb2+ and is expected to diffuse more swiftly from the skin toward the core, leading to accelerate the formation of perovskite [28]. 3
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Fig. 2. (a) In situ UV–Vis spectra during the incubation of the Pb(NO3)2-coated BK-TiO2/SnO2/ITO-PEN substrate in MAI/IPA and MAI0.9Br0.1/IPA solution in different periods; the left configuration is the sample setup for the in situ examination. Variations in the absorbance at 720 nm of (b) MAI/MAI0.9Br0.1-treated and (d) FAxMA1-xI0.9Br0.1-IPA (x = 0.1, 0.2, 0.3)-treated Pb(NO3)2 films on BK-TiO2/SnO2/ITO-PEN substrate as a function of incubation time. (c) and (e) Semi-logarithmic plot of absorbance at 720 nm versus (t−tC), which indicates a kink point. The red line is the linear fit to the data with a correlation factor of over 0.998; (f) Mechanism of perovskite crystal development. Schematic in situ crystal development for perovskite formation in the Pb(NO3)2 protocol via sequential deposition. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
As noted, Hoke et al. reported that phase segregation can occur in materials containing both Br and I under light exposure [29]. The phenomenon typically only manifests itself for compositions with Br contents greater than 20%, so perovskite that use small amounts of Br to facilitate crystallization or perform minimal amounts of band gap tuning tend to not suffer from the Hoke Effect [30]. To determine the optimized Br substitution, 5% and 10% Br contents are tried for comparison. I-V performance demonstrates that P-PSC with 10% Br substitution is better than that with 5% Br substitution due to the enhanced VOC and FF (shown in Fig. S3). Therefore, based on the fixed MAI0.9Br0.1 composition, different amount of FA+ is added to intercalate with the MA+ within the PbI6 octahedra. Fig. S4 shows the in situ UV–Vis absorption spectra of Pb(NO3)2-coated substrates immersed in the FAxMA1-xI0.9Br0.1-IPA (x = 0.1, 0.2, 0.3, hereafter named as FA0.1, FA0.2 and FA0.3). As observed in Fig. 2d, e, the conversion is retarded as the tC increases from 300 s in the absence of FA+ (MAI0.9Br0.1) to 400 s and 500 s for FA0.1 and FA0.2, respectively. This is because that FA+ has a larger ionic size (2.53 Å) than MA+ (2.17 Å), which makes it
more difficult to penetrate into the PbI6 framework at the early stage of crystallization. Upon thermal annealing, the uncoordinated residual FA+ ions gain additional activation energy to replace MA+ in the αphase perovskite framework [24]. In slow intermediate ion exchange process like Pb(NO3)2 protocol, FA+ ions not only have to compete with MA+ to coordinate into PbI6 but also hinder the outward diffusion pathways of hydrated NO3− ions (3.35 Å). Both effects are intensified in high FA+ addition (similar to the case of FA0.2), rendering undesired film quality because of long incubation. FA0.3 has similar result to FA0.2, implying the amount of FA+ is saturated. Perovskite crystal development from Pb(NO3)2 via sequential deposition in FAxMA1xI0.9Br0.1 is cartooned in Fig. 2f. Even though the conversion mechanism of FAxMA1-xI0.9Br0.1 is unveiled, the entire incubation requires from 300 to 500 s depending on the presence of Br− or FA+, which inevitably suffers from Ostwald ripening effect and results in a flawed perovskite morphology as shown in Fig. S2. By using multiple-cycle immersion to refine the morphology of perovskite films, a fully covered, cuboidal crystal-free perovskite 4
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Fig. 3. SEM images of perovskite capping layer on BK-TiO2/SnO2/ITO-PEN obtained by multiple-cycle immersion in (a) MAI-IPA, (b)–(d) FAxMA1-xI0.9Br0.1-IPA (x = 0.1, 0.2, 0.3) solution.
Fig. 4. XRD patterns of pure MAPbI3 and hybrid-composition perovskites on BK-TiO2/SnO2/ITO-PEN.
capping film can be obtained [18]. Fig. 3 shows SEM images of the perovskite capping layer formed in FAxMA1-xI0.9Br0.1/IPA after applying multiple-cycle immersion technique. It can be seen uniform perovskite layers exhibiting void-free are obtained. Notably, as FA+ increases, the average size of the grains increases, in consistent with previous study [31]. Cross-sectional SEM image of FA0.1 P-PSC in Fig. S5 reveals that the total thickness of the device including PEN plastic substrate (125 μm), [32] SnO2 compact layer (0.04 μm), perovskite absorber (0.5 μm), HTL (0.24 μm), and Au layer (0.1 μm) is 125.88 μm. Fig. 4a shows the X-ray diffraction (XRD) patterns of pure MAPbI3 and hybrid-composition perovskites. In addition to the background diffraction peaks of ITO-PEN at 2θ = 27.16°, two peaks at 2θ = 14.42° and 32.24° indicate diffraction at (110) and (310) planes of the MAPbI3 [33]. It is worth noting that a tiny peak of Pb(NO3)2 is shown at around 2θ = 19.5° in all perovskite samples and the Pb(NO3)2 peak slightly increases when FA+ incorporates, which plays a crucial role in the enhanced environmental stability in the perovskite crystallites [19]. Zoom-in XRD pattern at 2θ = 14.42° is shown in Fig. 4b, it is found the (110) diffraction peak continuously shifts from 14.42° to 14.34° when the addition of FA+ increases, which confirms the successful incorporation of FA cation into the perovskite crystal lattice since the FA
cation has a relatively larger ionic radius than MA. The full width half maximum (FWHM) of the (110) diffraction peak increases monotonically with increasing FA content (0.1 ≤ x ≤ 0.3). Typically, decreased grain size and/or increased density of defects (lower crystallinity) are responsible for the increase of FWHM [34]. Because the average grain size increase as the FA content increases shown in Fig. 3, the increased FWHM indicates decreased film crystallinity due to the sluggish conversion kinetics induced by excessive FA incorporation. The UV–Vis absorption spectra of pure MAPbI3 and hybrid-composition perovskites are displayed in Fig. 5a. When Br− is added in MAPbI3, the absorption onset experiences a blue-shift from 780 nm to 774 nm, which is attributed to structural distortion induced by stress and biaxial strain of Pb-I bonds as Br is substituted. The gradual red-shift is present with increased FA+ in FAxMA1-xI0.9Br0.1 from 778 nm to 784 nm, suggesting that the incorporation of FA+ into MAPbI3-yBry helps relax the anisotropic strained lattice and stabilizes the perovskite phase. The slight increment in the onset absorption wavelength for FA+ incorporation again corroborates the fact that Br− dominates the conversion kinetics and FA+ intercalates into PbI6 in an energetically unfavorable way. The corresponding band gap is presented in Fig. 5b. The normalized steady-state photoluminescence (PL) spectra are shown in 5
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Fig. 5. (a) UV–vis absorption and (b) the corresponding band gap variation; (c) the steady-state PL spectra of pure MAPbI3 and hybrid-composition perovskites and (d) the shift of characteristic peak.
Fig. 5c and d. In agreement, a significant blue-shift from 779.5 nm to 773.0 nm for FA0.1 perovskite is observed with respect to the pure MAPbI3, further disclosing that the trap density and energetic disorder are reduced by the compositional engineering [4]. After incorporating of Br− and FA+ into pristine MAPbI3 perovskite absorber, the photovoltaic performance of the FAxMA1-xI0.9Br0.1 based P-PSC made from aqueous Pb(NO3)2 precursor are examined and the result is presented in Fig. 6. All data are statistical results of 10 devices made from different batches. The average values with standard deviations are listed in Table 1. Apparently P-PSC based on FA0.1 shows highest JSC, VOC, FF and consequent PCE. The average PCE of FA0.1based P-PSC improves from 12.72% to 15.45%, including a champion device having 16.50% obtained from backward scan. As shown in Fig. S6, the integrated JSC obtained from IPCE spectrum is 19.25 mA/cm2 for FA0.1-based P-PSC, which is slightly lower than the JSC of 20.70 mA/cm2 obtained from J-V measurement, representing an acceptable mismatch of less than 10%. Even though there is a large optical loss at wavelengths below 450 nm due to intrinsic absorption by the PEN film, the EQE of FA0.1-based P-PSC among 450–750 nm maintains at high plateau of 75–85% and warrants high JSC. The hysP max, b 1; where Pmax,b is the teresis index, HI, is defined as HI = P max, f maximum power obtained from backward scan and Pmax,f is the maximum power obtained from forward scan. Fig. 6e summarizes the HI calculating from the IV data. The FA0.1-based devices show the lowest HI, suggesting that adding 10% FA+ can considerably stabilize the structure, limiting possible light- or field-induced ion movement and impeding the halide segregation. Considering that all the devices share the same ETLs and HTLs, the increase in PCE for FA0.1-based PPSC can be solely ascribed to the improvement of perovskite crystal quality. As mentioned above, the formation of a stable “quasi-cubic” phase at room temperature accompanied by improved crystallinity and reduced energetic disorder is the reason behind the high JSC, VOC and FF. On the other hand, excess FA+ addition (FA0.2 and FA0.3 in this
study) can disturb intermediate ion exchange in the solid-state diffusion, which intensifies the hindrance of crystallization and results in poor crystallinity. Therefore, FA0.2 and FA0.3-based P-PSCs fail to further improve their photovoltaic performance. The finding of low allowance of FA implantation may treat as a special feature of Pb(NO3)2 protocol. Fig. S7 shows a semi-logarithmic plot of VOC and the light intensity of the pristine MAPbI3 and polished FA0.1-based P-PSCs, where the ideality factor (n) in the diode equation can be obtained from nk T the slope of the fitting line (~ qB ; where kB is the Boltzmann constant, T is the absolute temperature, and q is the elementary charge). According to a previous study, [35] the light-intensity dependence of the VOC can provide insights into the role of trap-assisted recombination versus bimolecular recombination at open-circuit. As observed, both devices exhibit high ideality factors of over 1.5, which implies that trapassisted Shockley–Read–Hall (SRH) recombination is present at moderate level in both devices. Compared with larger n value (~1.99) for MAI-based device, n = 1.51 is obtained from FA0.1 P-PSC, once again proving that the compositional optimization effectively reduces the trap density and suppresses SRH recombination. The mechanical stability of FA0.1 P-PSC grown on low-temperature BK-TiO2/SnO2/ITO-PEN substrate has been investigated by repeatedly bending the complete device. First, FA0.1 P-PSC with an initial PCE of 15.75% is bent with a curvature radius of 20 mm for 3 s as one cycle. The I–V characteristics are measured after several bending cycles and this is repeated for 1000 cycles. The results are depicted in Fig. S8a. It is evident from the bending test that the devices retains over 80% of PCE after 1000 bending cycles. In particular, the VOC maintains an impressively constant value of 1.02 V after 1000 cycles, suggesting that the device remains structurally stable and all of the functional layers and their interfaces are mechanically robust throughout the entire duration of the bending test. From an initial value of 20.75 mA cm−2, JSC decreases to 18.22 mA cm−2 after a 1000-cycle bending test; similarly, FF drops from 0.73 to 0.68, resulting in a decay of PCE from 6
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Fig. 6. (a)–(e) Box plots of I–V characteristics of the BK-TiO2 scaffold-based P-PSCs made with different perovskite; (f) I–V curves of the optimal P-PSC employed with FA0.1 perovskite; I–V parameters are depicted in the table.
15.75% to 12.64%. According to our previous work, [23] damage to the fragile ITO coating is to blame for the JSC and FF loss. An SEM examination shown in Figs. S8b and c is conducted on a piece of purposely prepared BK-TiO2/SnO2/ITO-PEN sheet, which has been subjected to
1000 bending cycles. The unambiguous microcracks in the fragile ITO occurs, which obstructs current collection and increased internal resistance; thus, JSC and FF decrease. The stability trail of FA0.1- and MAbased P-PSCs using Pb(NO3)2/H2O protocol is tested. The FA0.1 device
Table 1 Summary of the IV data obtained using different perovskite. Type of perovskite
JSC (mA/cm2)
VOC (V)
MAI FA0.1 FA0.2 FA0.3
20.11 20.70 19.27 19.41
1.006 1.028 1.011 1.008
± ± ± ±
0.51 0.55 0.43 0.49
± ± ± ±
FF 0.020 0.013 0.026 0.011
0.629 0.726 0.677 0.676
7
PCE (%) ± ± ± ±
0.048 0.020 0.036 0.050
12.72 15.45 13.18 13.21
± ± ± ±
HI 0.96 0.38 0.74 0.89
0.13 0.04 0.07 0.10
± ± ± ±
0.07 0.01 0.04 0.05
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Fig. 7. (a) Stability trails of the photovoltaic parameters of the Pb(NO3)2-based device made using the FA0.1 and MAI recipes; (b) XRD patterns of FA0.1 and MAI perovskites after 50 days aging test; (c) UV–vis absorbance of FA0.1 perovskite film before and after 50 days aging.
with initial JSC, VOC, FF, and PCE values of 21.61 mA/cm2, 1.03 V, 0.70, and 15.64%, is stored in ambient darkness without additional sealing under temperature and humidity control at 23–25 °C and 10%–25%, respectively. The results are summarized in Fig. 7a. After 50 days, PCE drops to 11.86%, corresponding to a 75.8% retention. In contrast, MAbased device presents the relatively poor stability trail with only 58.4% PCE retention after 50 days aging. From the XRD pattern (shown in Fig. 7b) of the perovskites after aging test, the FA0.1 film maintains the same perovskite phase as that shown in Fig. 4 while the PbI2 peak ~ 12.7° occurs in aged MAI film due to the decomposition of MAPbI3 under moisture exposure. In addition, a tiny peak of Pb(NO3)2 residue can be observed in FA0.1 film before and after aging test, which corroborates the fact that FA+ ions diffuse inwardly will hinder the outward diffusion pathways of hydrated NO3− ions during the intermediate ion exchange process. Consequently, the high-quality perovskite benefiting from the compositional engineering and “water scavenger” Pb(NO3)2 residue simultaneously contribute to the robust environmental stability. We list major published works relating to the PSC stability in Table S1. From the UV–Vis absorption spectra (shown in Fig. 7c) of the FA0.1 perovskite sample before and after aging test, the optical property of the perovskite film maintains robust optical properties, which implies that the perovskite absorber and its PV activity are inherently stable during the 50 days. Hence, the mild PCE decay in device may come from the poor airproof ability of ITO-PEN, or from hygroscopic spiro-OMeTAD with the lithium dopant, or from gold diffusion into the HTM because of unsealed device. Finally, an attempt to upscale the device size was preliminarily studied. Up to now, high PCE P-PSCs reported in literature are mostly based on an active area < 0.1 cm2, [36,37] it is still challenging to manufacture the P-PSCs with large size. Because the processing solvent in coating lead source is water now, it is a simple decision for us to fabricate FA0.1-based P-PSCs with larger area without the concern of
vast toxic solvent usage. Shown in Fig. 8, the PCE drops to 10.61% when the device area is enlarged to 1 cm2. From the digital photo of spin-coated Pb(NO3)2 film and SEM images of the resultant FA0.1 perovskite film, poor uniformity of spin-coated Pb(NO3)2 film, particularly in the edges of the substrate, accounts for the loss. The thin Pb (NO3)2 coating at the edge intensifies the Ostwald ripening effect for the resultant perovskite as it is supposed to complete the conversion in advance compared with that in the center region. In the edge zone, small FA0.1 perovskite grains in mesoporous BK-TiO2 tend to dissolve first and then recrystallize as large grains. As this phenomenon proceeds, perovskite crystals in the mesoporous BK-TiO2 gradually deplete, accompanied by enlargement of large cuboidal crystals on or above the surface cap at the edge. During this stage, significant loss on JSC and FF can be observed on the IV characteristic because of poor contact between the FA0.1 perovskite absorber and BK-TiO2 scaffold [38]. This result clearly points out that uniform Pb(NO3)2 coating is essential to further develop large area P-PSC using this green, environmentallyfriendly fabrication process. 4. Conclusions In summary, we reported an eco-friendly method from an aqueous lead nitrate precursor to fabricate hybrid-composition perovskite absorber on plastic substrate. By investigating the kinetics of intermediate ion exchange among NO3− and mixed cation/anion, the perovskite conversion was largely enhanced and the resultant perovskite with the optimal composition was verified. Based on the BK-TiO2 scaffold, the fully ambient-processed P-PSCs achieved an optimized PCE of 16.50%, which was the highest in P-PSC made from water-based precursor. Clean production of large-scale P-PSCs using a minimal quantity of harmful chemicals is expected to be realized using Pb(NO3)2/H2O protocol.
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Fig. 8. I–V curves of the large area P-PSC employed with FA0.1 perovskite; the film demonstrates non-uniform morphology between the center and the edge, which degrades the JSC and FF.
Acknowledgments
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This research was supported by the Ministry of Science and Technology, Taiwan (MOST105-2628-E-007-021-MY2 and 107-2119M-007-001). Financial support from National Natural Science Foundation of China (51702263) and the Fundamental Research Funds for the Central Universities, China (3102018zy048) is also appreciated. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.nanoen.2019.104036. References [1] F.D. Giacomo, A. Fakharuddin, R. Jose, T.M. Brown, Energy Environ. Sci. 9 (2016) 3007–3035. [2] M. Park, H.J. Kim, I. Jeong, J. Lee, H. Lee, H.J. Son, D.-E. Kim, M.J. Ko, Adv. Energy Mater. 5 (2015) 1501406. [3] B.J. Kim, D.H. Kim, Y.-Y. Lee, H.-W. Shin, G.S. Han, J.S. Hong, K. Mahmood, T.K. Ahn, Y.-C. Joo, K.S. Hong, N.-G. Park, S. Lee, H.S. Jung, Energy Environ. Sci. 8 (2015) 916–921. [4] J. Feng, X. Zhu, Z. Yang, X. Zhang, J. Niu, Z. Wang, S. Zuo, S. Priya, S.F. Liu, D. Yang, Adv. Mater. 30 (2018) 1801418. [5] A. Salavei, D. Menossi, F. Piccinelli, A. Kumar, G. Mariotto, M. Barbato, M. Meneghini, G. Meneghesso, S.D. Mare, E. Artegiani, A. Romeo, Solar Energy 139 (2016) 13–18. [6] J. Ramanujam, U.P. Singh, Energy Environ. Sci. 10 (2017) 1306–1319. [7] M. Kaltenbrunner, M.S. White, E.D. Głowacki, T. Sekitani, T. Someya, N.S. Sariciftci, S. Bauer, Nat. Commun. 3 (2012) 770. [8] H.C. Weerasinghe, F. Huang, Y.-B. Cheng, Nano Energy 2 (2013) 174–189. [9] A. Khan, Y.-T. Huang, T. Miyasaka, M. Ikegami, S.-P. Feng, W.D. Li, ACS Appl. Mater. Inter. 9 (2017) 8083–8091. [10] C.-G. Wu, C.-H. Chiang, Z.-L. Tseng, Md K. Nazeeruddin, A. Hagfeldt, M. Gratzel,
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Tsung-Yu Hsieh is a Ph.D. candidate in Department of Chemical Engineering, National Tsing Hua University, and he was in Institute of Chemical Research of Catalonia for 1 year exchanging. His research interests are in dye-sensitized solar cells and perovskite solar cells, including process engineering, electrochemical analysis and photophysical analysis.
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Wei-Yen Wang came from Miaoli County and now he is a Ph.D. candidate in the Department of Chemical Engineering in National Tsing Hua University. His research interests include surface modification by silane derivative, electrochemical metalization on inorganic/organic material and dye-sensitized solar cells.
Lixia Ren was born in Shanxi province and received her M.S. degree in 2016 from Northwestern Polytechnical University. And now she is a Ph.D. candidate at the same University. Her research interest focus on developing novel functional material as electrode for the third generation thin film solar cells, e.g. dye-sensitized solar cells and perovskite solar cells.
Jiayi Guo is a master student in the Department of Chemical Engineering in National Tsing Hua University. Her expertise includes surface treatment and fracture analysis on interfaces. Her current research topic is passivation of perovskite films using silane coupling agents.
Peng Zhai is an associate professor of the School of Natural and Applied Sciences in Northwestern Polytechnical University. He received his Ph.D. degree in The University of Hong Kong in 2015. Dr Zhai has once worked as a visiting scholar in the Department of Chemical Engineering in National Tsing-Hua University. His current research interests include perovskite solar cells, dye-sensitized solar cells (DSCs) and functional graphene materials.
Tzu Chien Wei is an associate professor in Department of Chemical Engineering at National Tsing-Hua University, Hsinchu, Taiwan. After serving in industry from 2007 to 2011 and a postdoctoral training in 2012, he joined NTHU since 2012. He published over 50 peer-reviewed papers regarding dye-sensitized solar cells, perovskite solar cell and surface treatment & metallization.
Tzu-Sen Su received his M.S. degree in 2015 from Department of Chemical Engineering in National Tsing-Hua University. Now, he is a Ph.D. candidate at the same University. His research interest focuses on electrodepositing different metal oxide as electrode for perovskite solar cells.
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