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Additive Enhanced Crystallization of Solution-Processed Perovskite for Highly Efficient Planar-Heterojunction Solar Cells Po-Wei Liang, Chien-Yi Liao, Chu-Chen Chueh, Fan Zuo, Spencer T. Williams, Xu-Kai Xin, Jiangjen Lin, and Alex K.-Y. Jen* Despite occurring only less than a year ago, the breakthrough of over 15% power conversion efficiency (PCE) in organometal halide perovskite solar cells has attracted significant attention and this hybrid system has been considered a viable member of next generation photovoltaics that can address the scalability changes with a low-cost solution process.[1–13] Organometal halide perovskite absorbers possess several appealing features such as intense light absorption, decent ambipolar charge mobility, and small exciton binding energy. The band-gap of organometal perovskites can be easily tailored through the choice of metal cation,[14] inorganic anion,[1] and organic ligand.[15,16] Both p- and n-type conductivity of this class of perovskites are measured to be on the order 10−3 to 10−2 S/cm.[3,16,17] The small exciton binding energy (∼20 meV) of these perovskites enable long exciton diffusion lengths (100-1000 nm) and lifetimes (∼100 ns) as compared with the poor exciton diffusion lengths (∼10 nm) and lifetimes (∼10 ns) of organic semiconductors caused by tightly bounded electron-hole pairs (>100 meV).[18–20] Complementary to this and central to their commercial viability is the low-temperature (∼100 °C) solution processability of organometal halide perovskites. All these advantages reveal their great potential to rival silicon-based solar cells for solar energy. To meet the commercial requirement of high throughput manufacturing processes, researchers are interested in developing thin-film perovskite solar cells through simple, scalable, and low-temperature processing techniques.[18,20–24] Snaith et al. first discovered the feasibility of the planar thin-film architecture of solution processed perovskite solar cells as an evolution from dye-sensitized photovoltaic systems.[10] This demonstrates
P.-W. Liang, C.-Y. Liao, Dr. C.-C. Chueh, Dr. F. Zuo, S. T. Williams, Dr. X.-K. Xin, Prof. A. K.-Y. Jen Department of Materials Science and Engineering University of Washington Seattle, WA 98195, USA E-mail: [email protected] Prof. A. K.-Y. Jen Department of Chemistry University of Washington Seattle, WA 98195, USA C.-Y. Liao, Prof. J. J. Lin Institute of Polymer Science and Engineering National Taiwan University Taipei 106, Taiwan
DOI: 10.1002/adma.201400231 3748
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the potentiality of the planar p-i-n heterojunction comprised of thin-film intrinsic perovskite between solution processable p- and n-type charge transporting interlayers like PEDOT:PSS or Spiro-MeOTAD, and TiO2 or PCBM, respectively. Several planar-heterojunction configurations with no mesoporous TiO2 layer have been reported with increasing frequency.[11–13,19–24] Device architectures can be divided according to p-i-n heterojunction sequence as conventional (PEDOT:PSS (p)/Perovskite (i)/PCBM (n))[13,20,21,23] and inverted (compact TiO2 (n)/ Perovskite (i)/organic semiconductors (p))[12,19,22,24] structure. Recently, a high efficiency perovskite planar-heterojunction inverted solar cell with power conversion efficiency (PCE) over 12% was demonstrated.[11,19] At present, one of the main challenges encountered in perovskite thin film fabrication is the control of the crystallization process and its impact on film quality. Poor perovskite morphology has been cited as very detrimental to device performance because it not only causes electrical shorting but also deleteriously impacts charge dissociation/transport/recombination.[21–24] Because of the sensitive dependence of growth kinetics on interfacial energy, solution concentration, precursor composition, solvent choice, and deposition temperature, improving perovskite morphology and coverage through controlling crystallization during film deposition and annealing is an attractive route to device optimization. It is possible to achieve optimal perovskite film morphology by finding effective ways to manipulate its nucleation and growth.[21–24] Burschka et al. have recently showed that enhanced perovskite crystallinity can be achieved in DSSCs by pre-deposition of PbI2 from solution onto meso-porous TiO2.[7] The crystallization in the two-step process improved as a result of enhanced perovskite nucleation at the meso-porous TiO2 surface compared to that through the direct one-step deposition of the composite precursor. This example demonstrates that the surface properties of the substrate have a strong influence on the nucleation and growth of a deposited film. As another example of the impact of phase transformation control, Snaith et al. have recently demonstrated that perovskite crystallization rate can be controlled by changing precursor composition.[2,9,10,19,22,23] By partially substituting I− with Cl− in CH3NH3PbI3 to form CH3NH3PbI3-xClx, crystallization is prolonged as a result of the lattice distortion caused by Cl− doping. This is evident by the increased time necessary to fully anneal deposited films, specifically less than 1 h for pure iodine perovskite and between 2 to 3 h for the mixed halide perovskite. More significantly, Cl− also increases the conductivity and charge diffusion length
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COMMUNICATION Figure 1. Device configuration of the planar-heterojunction solar cell, the cross-section SEM of the planar-heterojunction, and the surface SEM images of the CH3NH3PbI3−xClx layer.
(∼1000 nm) of the perovskite domain without affecting its optical properties.[19,25] In this study, we develop a simple and very effective method to enhance the crystallization of solution-processed perovskite. We demonstrate that crystallization rate of perovskite can be controlled by incorporating additives into its precursor solution to modulate thin film formation. The capacity of bidentate halogenated additives to temporarily chelate with Pb+2 during crystal growth is evidenced by the improved solubility of PbCl2 in the presence of 1,8-diiodooctane (DIO). The influence of this chelation encourages homogenous nucleation and likely modifies interfacial energy favorably, ultimately altering the kinetics of crystal growth. As a result, the morphology of the perovskite thin films processed from the solution with 1 wt% DIO show a much smoother and more continuous surface than that obtained from the pristine solution, as is illustrated in Figure 1. The crystal size and homogeneity formed under the influence of this halogenated additive lead to very high PCE (∼12%) in planar-heterojunction perovskite solar cells. Motivated by its low-temperature solution processability, we adopted a conventional p-i-n heterojunction architecture of ITO or FTO/PEDOT:PSS (35–40 nm)/CH3NH3PbI3-xClx (∼400 nm)/PC61BM (∼55 nm)/Bis-C60 (10 nm)/Ag (150 nm) to study the influence of perovskite thin-film morphology on device performance (Figure 1). Currently, the processing of compact TiO2 in the inverted architecture involves high-temperature annealing (∼500 °C) which negates some of the benefits offered by solution-based manufacturing process.[12,19,22] The conventional planar-heterojunction also offers the benefit of efficient charge transfer between the interfaces of PEDOT:PSS/PC61BM interlayers and perovskite thin films, ensuring sufficient charge dissociation and extraction for high performance provided high quality perovskite films can be grown.[19–21,23] Notably, the Bis-C60 surfactant is employed as an efficient electron-selective interfacial layer that aligns the energy levels at the organic/cathode interface and enables the utilization of stable metals such as Ag as the top electrodes, providing respectable environmental stability.[26,27] The mixed halide CH3NH3PbI3-xClx perovskite was chosen for study by virtue of the fore mentioned merits demonstrated by Snaith et al.[2,9,10,19,22,23] Detailed information regarding the preparation of the perovskite precursor solution, deposition of the perovskite thin-film, and the fabrication of devices is described in the experimental section.
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Solvent additives have enabled significant efficiency enhancements in bulk-heterojunction (BHJ) organic solar cells (OPVs) by modulating BHJ morphology.[28–30] Bazan et al. first discovered that the morphology of BHJ layers could be effectively optimized by simply incorporating additives like alkane dithiols or 1,8-di(R)octanes into the processing solution.[28,29] Two important features of the processing additives were identified for the further optimization of BHJ morphology: the selective solubility of fullerenes and the higher boiling point with respect to the host solvent.[28,29] Inspired by this finding, we are interested in exploring the additive's influence on both the crystallization of perovskite thin films and device performance. The additive studied here is DIO as it bears iodide in common with CH3NH3PbI3-xClx and as a soft Lewis base may interact with Pb2+, a soft Lewis acid. We hypothesize that this additive can temporarily coordinate with Pb2+ during crystal growth and modulate crystallization kinetics as the transient capture of additive in the growing crystal lattice will increase both the internal energy and entropy of the crystals.[12,19,25] The solvent additive was incorporated into the CH3NH3PbI3-xClx precursor solution prior to the thin film deposition. The optimized blending ratio of DIO in the precursor solution was found to be around 1 wt% with respect to the weight of perovskite (denoted as 1% DIO in the following discussion). At first, we constructed the planar-heterojunction on an ITO substrate. Encouragingly, the device processed from the precursor solution containing 1% DIO showed a ∼30% PCE enhancement compared to the control as shown in Figure 2a. Open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and PCE of all devices are summarized in Table 1. The control device fabricated from the pristine composite solution showed a PCE of 7.8% with a Voc of 0.90 V, a Jsc of 15.0 mA/cm2, and a FF of 0.58 while that derived from the solution containing 1% DIO exhibited a significantly enhanced PCE of 10.3% with a Voc of 0.92 V, a Jsc of 15.6 mA/cm2, and a FF of 0.71. All parameters improved as a result of the incorporation of DIO in the precursor solution, which suggests the generation of improved perovskite morphology due to the influence of DIO. It is well known that the crystallinity of perovskite absorber domains determines ultimate performance of the fabricated devices since defects in the crystals will create severe shorting and trapping sites for charge recombination. Crystallinity will also greatly affect the efficiency of charge dissociation, transport, and diffusion length.[19,22] As shown in Figure 1, the film
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Figure 2. (a) Current−voltage characteristics and (b) external quantum efficiency (EQE) spectra of the studied solar cells. (c) UV-vis absorption spectra and (d) XRD spectra of the solution-processed perovskite with and without additive.
coverage), benefiting from the rough surface of FTO (100 nm) relative to ITO (∼5 nm, herein).[23] This resulted in the superior performance of the device constructed on FTO which motivated us to explore our additive’s influence on this favored system. Very impressively, an increased PCE with similar enhancement factor (∼31%) to that of the ITO case was achieved on FTO, suggesting that DIO’s beneficial influence on crystal growth kinetics is not limited by interfacial structure. The device derived from the 1% DIO solution on FTO possessed a promising PCE of 11.8% with a Voc of 0.92 V, a Jsc of 17.5 mA/cm2, and a FF of 0.73, compared to the 9.0% PCE of the control device with a Voc of 0.90 V, a Jsc of 16.0 mA/cm2, and a FF of 0.62. Similar to the case on ITO substrates, the improved quality and surface coverage of the crystalline perovskite thin films caused by the presence of DIO significantly contributes to the enhancement of Jsc (16.0 to 17.5 mA/cm2) and FF (0.62 to 0.73) (Surface images were shown in Figure S1). The results of our top-performing devices are among the best reported for state-of-the-art low−2 Table 1. Performance of the studied solar cells under AM 1.5G Illumination (100 mW cm ). temperature solution-processed photovoltaics. The external quantum efficiency (EQE) FF PCE Voc Jsc plotted in Figure 2b confirms the increased 2 (%) (V) (mA/cm ) Jsc of these devices, in which the integrated On ITO Substrate (Roughness: ∼5 nm; 15 ohm/sq) Jsc for the devices derived from 1% DIO conPristine Perovskite 0.90 0.58 15.0 7.9 taining solution on FTO and ITO is 17.3 and Perovskite-1% DIO 0.92 0.71 15.6 10.3 15.4 mA/cm2, respectively (the integrated Jsc of control device on ITO is 14.5 mA/cm2). As On FTO Substrate (Roughness: ∼100 nm; 8 ohm/sq) can be seen, the maximum EQE peaks of the Pristine Perovskite 0.90 0.62 16.0 9.0 top-performing devices can reach over 70% Perovskite-1% DIO 0.92 0.73 17.5 11.8 (ITO-substrate) and 80% (FTO-substrate)
prepared from the 1% DIO solution showed better coverage, less surface roughness, and more regular crystallites with more ordered growth directions than the pristine thin film. It is worthwhile to point out that the high Voc over 0.90 V for both devices implies the small potential loss for photoexciton dissociation (band-gap of CH3NH3PbI3-xClx is ∼1.5 eV, estimated from Figure 2c). This highlights prominent benefits of perovskites: small exciton binding energy (∼20 meV) and respectable ambipolar charge conduction.[11–13,16–20] The high FF of over 0.70 further demonstrates the efficient charge transfer and extraction of such a planar p-i-n heterojunction.[13,20] Recently, Snaith et al. revealed that the perovskite performance is strongly dependent on the surface roughness of the substrate due to the sensitivity of crystallization on interfacial structure.[23] They found that on FTO/PEDOT:PSS the perovskite film was more homogeneous (with ∼90% surface coverage) than the one on ITO/PEDOT:PSS (with only ∼80% surface
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COMMUNICATION Figure 3. (a) Time resolved SEM images of the surface of the evolving CH3NH3PbI3− xClx films. The scale bars are all 5 µm. (b) Time resolved XRD spectra of the evolving CH3NH3PbI3− xClx films.
EQE. This high photon-to-electron conversion together with the panchromatic absorption over the visible range (Figure 2c) highlights the intense bandgap light harvesting and excellent ambipolar transport properties of the perovskites. The additive enhanced crystallization of CH3NH3PbI3-xClx is evident in the absorption and X-ray diffraction (XRD) spectra in Figure 2c–d. As can be seen, the perovskite processed with 1% DIO solution exhibits clearly increased light absorption across the visible range into the near-infrared wavelengths, consistent with the increase in the EQE curves. The increased absorption should be the result of improved surface coverage and more uniform crystal formation in perovskite thin films. The rising band-edge absorption around 780–790 nm proves the increased crystallinity of perovskites processed with additive. This can also be observed in the enhanced intensity of the reflections at 14.2° and 28.5° in the XRD spectrum of the DIO processed perovskite in Figure 2d. Although intensity increase in XRD is possibly due to several factors, in this case, identical instrumental parameters, sample quantities, and compositions were used for analysis. After being annealed for 3 h, we are confident that most DIO has evaporated; which, taken with the above statement, points to increased crystallinity in the DIO sample as the cause for the markedly increased observed peak intensity as compared to the pristine sample.
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These strong reflections at 14.2° and 28.5° are assigned to the (110) and (220) crystal planes of the orthorhombic lattice of mixed halide perovskites.[8] In the XRD analysis of all samples we fabricate on PEDOT:PSS, we see a significant degree of texture with the (110) plane preferentially oriented parallel to the film surface. A close analysis of the data in Figure 2d reveals that in addition to the overall intensity increase in the DIO sample, there is a noticeable increase in the ratio between the intensity of both the (110) and (220) peaks and that of the (310), (224), and (314) peaks located at 31.9°, 40.7°, and 43.3° respectively, as inserted in Figure 2d.[21,31] This indicates that in addition to improving the degree of crystallinity, DIO enhances the generation of texture already encouraged by the PEDOT:PSS/ perovskite interface. This can be explained either through a modification of interfacial energy caused by DIO or as a consequence of the role DIO chelation of Pb2+ plays in phase transformation, but currently the texture inherent to these systems is so strong that most reflections do not extend far above the noise threshold. A pole figure analysis is necessary to quantitatively characterize this phenomenon. To get more in-depth understanding of the function of DIO during perovskite crystallization, time resolved morphological characterization was made and recorded with scanning electron microscopy (SEM) and XRD, as presented in Figure 3. The
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Figure 4. (a) Schematic diagram for the transient chelation of Pb2+ with DIO. (b) Solubility of the PbCl2 and PbI2 in DMF with or without additives. From left to right, PbCl2 in DMF, PbCl2 in DMF/DIO, PbI2 in DMF, and PbI2 in DMF/DIO. The blending ratio of DIO to DMF for PbCl2 and PbI2 is 1:1.6 and 1:2.8 by mol, respectively.
preparation of these perovskite thin films was identical to the conditions used for the device fabrication. As cast, both films contain many pin-holes and voids, but the additive assisted film already demonstrates markedly improved surface coverage. Additionally, the unique contrast features in the SEM image of the as cast DIO film along with its dense XRD pattern suggest a generation of order before annealing that is unique to the DIO sample. The increased coverage and smoothness of the DIO assisted film suggest that DIO encourages homogenous nucleation and reduces the kinetics of the transformation enough to allow the thermodynamic influence of the interface between PEDOT:PSS and CH3NH3PbI3-xClx to play a more dominant role. The presence of DIO may also modify this interfacial energy, making it more favorable for the crystal to grow in contact with the surface. These improvements may also be attributed to the increased solubility of PbCl2 in the mixed solvent DMF/DIO, as shown in Figure 4. We speculate that this improvement results from the temporary coplanar chelation of Pb2+ with DIO as Cl− ligands reside in axial octahedral positions on Pb2+.[8,25] As soft Lewis bases, iodocarbons can coordinate with soft metal ions based on the hard-soft acid-base principle.[32] Thus it can be envisioned that the transient metal-solvent coordination can improve the solubility of PbCl2, as proposed in Figure 4a (and Scheme S1).[32,33] On the other hand, bidentate chelation is also more favorable than monodentate chelation from a thermodynamic perspective as observed in some metal complex systems.[34,35] The formation of the chelated ring structures of the former allow less configurational entropy loss during coordination, resulting in a much smaller Gibbs free energy. Therefore, the temporary chelation of Pb2+ with DIO will participate in the evolving dynamic equilibrium of the drying and annealing film, co-existing with the coordination of Pb2+ with methyl ammonium iodide (MAI) during crystal growth until DIO fully evaporates. As a result, transformation kinetics are retarded enough to allow and encourage more defect-free crystal growth. Moreover, it can be seen in the SEM images that the crystallites of the additive-assisted film display more regular faceting and improved interconnectivity during crystal growth, which can also be attributed to the prolonged growth caused by temporary chelation of Pb2+ by DIO. The distorted crystal lattice induced by captured DIO will increase the internal energy and configurational entropy of growing crystals thus modulating their growth rate and shape. The impact of these modifications on growth kinetics is apparent in the device data discussed above, correlating well with the increase of all relevant parameters.
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Time resolved photoluminescence (PL) behavior was characterized to probe the influence of the enhanced crystallization of perovskite thin films on charge dissociation. Detailed information regarding the preparation, measurement, and fitting methodology can be found in the experimental section. The PL lifetime of the samples was fitted with a bi-exponential decay function containing a fast decay and a slow decay process. We consider the fast decay process to be the result of the quenching of free carriers in the perovskite domain through transport to PEDOT:PSS or PC61BM, and the slow decay process to be the result of radiative decay. Figure 5 displays the PL decay and the related parameters are summarized in Table 2. For the pristine thin-film perovskite, the fast decay lifetime is 12.9 ± 0.8 ns and the slow decay lifetime is 104 ± 3 ns while their weight fractions are 81% and 19% respectively, indicating that charge transfer is the dominating decay mechanism. To mimic the real charge behavior in our p-i-n planar heterojunction device, the bilayer CH3NH3PbI3-xClx/PC61BM systems were examined. The existence of the electron quenching PC61BM layer atop the perovskite significantly decreases the fast decay lifetime from 12.9 ± 0.8 ns to 3.4 ± 0.4 ns and increases the weight fraction of fast decay from 81% to 94%. This suggests that most of the free carriers generated by illumination are efficiently transferred to PEDOT:PSS and PC61BM, which confirms the potential of the conventional p-i-n heterojunction.
Figure 5. Time resolved photoluminescence characterization of the solution-processed perovskite with and without additive.
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Subs. ITO/PEDOT:PSS
ETM
Additive
τ1 (ns)
Fraction 1
τ2 (ns)
Fraction 2
Average (ns)
-
-
12.9 ± 0.8
81%
104 ± 3
19%
30
PC61BM
DIO
8.5 ± 0.5
88%
88 ± 3
12%
18
-
3.4 ± 0.4
94%
139 ± 10
6%
12
DIO
1.2 ± 0.1
99%
18 ± 2
1%
1.4
As for the thin-film perovskite prepared from the 1% DIO solution, the fast decay lifetime decreased from 12.9 ± 0.8 ns to 8.5 ± 0.5 ns and the weight fraction increased from 81% to 88%. The same trend was observed in the bilayer CH3NH3PbI3-xClx/PC61BM film with lifetime decreasing from 3.4 ± 0.4 ns to 1.2 ± 0.1 ns and weight fraction increasing from 94% to 99%. As evidenced in Figure 3, the addition of DIO improved the coverage and crystallinity of the perovskite thin films on ITO/PEDOT:PSS, thus facilitating the diffusion of free carriers and increasing the efficiency of charge transfer. Consequently, the weight fraction of fast decay, which relates to the charge transfer process, increased and the lifetime of fast decay decreased. For perovskite processed from the 1% DIO precursor with PC61BM, the weight fraction related to quenching was almost 100%, which indicated a very efficient charge dissociation in the perovskite domain with PEDOT:PSS and PC61BM. The observed high Jsc (15.6 – 17.5 mA/cm2) and FF (0.71 – 0.73) of the devices derived from the 1% DIO solution can also be attributed to this enhanced charge separation and collection. Based on these findings, the enhanced crystallization of the solution processed perovskite films should be mainly due to the additive chelating effect and is less sensitive to the interfacial structure and the congruent improvement on both ITO and FTO substrates. The chelation efficiency may be further modulated by changing the additive's properties: alkane chain length and the nature of the end groups. Given the efficient coordination between Pb2+ and halides (Cl−, Br−, and I−) in perovskite structures,[1] the effect of alkane chain length will affect bidentate chelation as it will influence the thermodynamics and kinetics of the formation of the proposed chelated ring structures. More in-depth works regarding the understanding of the optimization of processing additives are ongoing. In summary, we have described a significantly enhanced PCE (∼31%) of the planar-heterojunction perovskite solar cells from 9.0% up to 11.8% by incorporating small amount of rationally chosen additives into the perovskite precursor solutions to improve the crystallization of perovskite thin films. We showed that incorporated additives facilitate homogenous nucleation and modulate the kinetics of growth during crystallization, as evidenced from the surface SEM images and XRD spectra. The enhanced crystallization facilitates the charge transfer efficiency between the charge transporting interlayers and the perovskite absorber. As a result, very promising PCEs (∼12%) were achieved in planar-heterojunction solar cells, fabricated through the low-temperature solution processes (under 150 °C). These results are among the best reported for the stateof-the-art solution-processed photovoltaics. We expect that even greater performance enhancement can be achieved through
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further rational design of processing additives as revealed by our work.
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Table 2. Time resolved photoluminescence characterization of the solution processed perovskite.
Experimental Section Materials and Sample Preparation: Methylammonium iodide (MAI) was synthesized by reacting 24 mL of 0.20 mol methylamine (33 wt% in absolute ethanol, Aldrich), 10 mL of 0.04 mol hydroiodic acid (57 wt% in water with 1.5% hypophosphorous acid, Alfa Aesar), and 100 mL ethanol in a 250 mL round bottom flask under nitrogen at 0 °C for 2 h with stirring. After reaction, the white precipitate of MAI was recovered by rotary evaporation at 40 °C and then dissolved in ethanol followed by sedimentation in diethyl ether by stirring the solution for 30 min. This step was repeated three times and the MAI powder was finally collected and dried at 50 °C in a vacuum oven for 24 h. To prepare the perovskite precursor solution, MAI and lead chloride (PbCl2, Aldrich) powder were mixed in anhydrous dimethylformamide (DMF, Aldrich) with a molar ratio of 3:1. The perovskite/1,8-diiodooctane (DIO, Aldrich) solution was prepared via adding 1 wt% of DIO with respect to perovskite weight into the perovskite precursor solution. The solutions (40 wt%) were stirred overnight at 80 °C and filtered with 0.45 µm PVDF filters before device fabrication. Fabrication of thin-film perovskite solar cells: The devices were fabricated in the configuration of indium tin oxide (ITO) or fluorine-doped tin oxide (FTO)/PEDOT:PSS/ CH3NH3PbI3-xClx/[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM)/fullerene surfactant (C60-bis)/Ag. ITO (15 ohm/ sq) and FTO (8 ohm/sq) glass substrates were cleaned sequentially with detergent and deionized water, acetone, and isopropanol under sonication for 10 min. After drying under a N2 stream, substrates were further cleaned by a plasma treatment for 30 s. PEDOT:PSS (Baytron P VP Al 4083, filtered through a 0.45 µm nylon filter) was first spin-coated onto the substrates at 5k rpm for 30 s and annealed at 150 °C for 10 min in air. To avoid oxygen and moisture, the substrates were transferred into a N2-filled glovebox, where the thin-film perovskite layers were spincoated from a homogeneous 40 wt% CH3NH3PbI3-XClX and CH3NH3PbI3XClX/DIO precursor solution at 6k rpm for 45 s (300-500 nm thickness) and then annealed at 90 °C for 2-3 h. Afterward, the PC61BM (15 mg/ mL in chloroform) and C60-bis surfactant (2 mg/mL in isopropyl alcohol) were then sequentially deposited by spin coating at 1k rpm for 60 s and 3k rpm for 60 s, respectively. Silver electrodes with a thickness of 150 nm were finally evaporated under high vacuum (
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Additive Enhanced Crystallization of Solution-Processed Perovskite for Highly Efficient Planar-Heterojunction Solar Cells Po-Wei Liang, Chien-Yi Liao, Chu-Chen Chueh, Fan Zuo, Spencer T. Williams, Xu-Kai Xin, Jiangjen Lin, and Alex K.-Y. Jen* Despite occurring only less than a year ago, the breakthrough of over 15% power conversion efficiency (PCE) in organometal halide perovskite solar cells has attracted significant attention and this hybrid system has been considered a viable member of next generation photovoltaics that can address the scalability changes with a low-cost solution process.[1–13] Organometal halide perovskite absorbers possess several appealing features such as intense light absorption, decent ambipolar charge mobility, and small exciton binding energy. The band-gap of organometal perovskites can be easily tailored through the choice of metal cation,[14] inorganic anion,[1] and organic ligand.[15,16] Both p- and n-type conductivity of this class of perovskites are measured to be on the order 10−3 to 10−2 S/cm.[3,16,17] The small exciton binding energy (∼20 meV) of these perovskites enable long exciton diffusion lengths (100-1000 nm) and lifetimes (∼100 ns) as compared with the poor exciton diffusion lengths (∼10 nm) and lifetimes (∼10 ns) of organic semiconductors caused by tightly bounded electron-hole pairs (>100 meV).[18–20] Complementary to this and central to their commercial viability is the low-temperature (∼100 °C) solution processability of organometal halide perovskites. All these advantages reveal their great potential to rival silicon-based solar cells for solar energy. To meet the commercial requirement of high throughput manufacturing processes, researchers are interested in developing thin-film perovskite solar cells through simple, scalable, and low-temperature processing techniques.[18,20–24] Snaith et al. first discovered the feasibility of the planar thin-film architecture of solution processed perovskite solar cells as an evolution from dye-sensitized photovoltaic systems.[10] This demonstrates
P.-W. Liang, C.-Y. Liao, Dr. C.-C. Chueh, Dr. F. Zuo, S. T. Williams, Dr. X.-K. Xin, Prof. A. K.-Y. Jen Department of Materials Science and Engineering University of Washington Seattle, WA 98195, USA E-mail: [email protected] Prof. A. K.-Y. Jen Department of Chemistry University of Washington Seattle, WA 98195, USA C.-Y. Liao, Prof. J. J. Lin Institute of Polymer Science and Engineering National Taiwan University Taipei 106, Taiwan
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the potentiality of the planar p-i-n heterojunction comprised of thin-film intrinsic perovskite between solution processable p- and n-type charge transporting interlayers like PEDOT:PSS or Spiro-MeOTAD, and TiO2 or PCBM, respectively. Several planar-heterojunction configurations with no mesoporous TiO2 layer have been reported with increasing frequency.[11–13,19–24] Device architectures can be divided according to p-i-n heterojunction sequence as conventional (PEDOT:PSS (p)/Perovskite (i)/PCBM (n))[13,20,21,23] and inverted (compact TiO2 (n)/ Perovskite (i)/organic semiconductors (p))[12,19,22,24] structure. Recently, a high efficiency perovskite planar-heterojunction inverted solar cell with power conversion efficiency (PCE) over 12% was demonstrated.[11,19] At present, one of the main challenges encountered in perovskite thin film fabrication is the control of the crystallization process and its impact on film quality. Poor perovskite morphology has been cited as very detrimental to device performance because it not only causes electrical shorting but also deleteriously impacts charge dissociation/transport/recombination.[21–24] Because of the sensitive dependence of growth kinetics on interfacial energy, solution concentration, precursor composition, solvent choice, and deposition temperature, improving perovskite morphology and coverage through controlling crystallization during film deposition and annealing is an attractive route to device optimization. It is possible to achieve optimal perovskite film morphology by finding effective ways to manipulate its nucleation and growth.[21–24] Burschka et al. have recently showed that enhanced perovskite crystallinity can be achieved in DSSCs by pre-deposition of PbI2 from solution onto meso-porous TiO2.[7] The crystallization in the two-step process improved as a result of enhanced perovskite nucleation at the meso-porous TiO2 surface compared to that through the direct one-step deposition of the composite precursor. This example demonstrates that the surface properties of the substrate have a strong influence on the nucleation and growth of a deposited film. As another example of the impact of phase transformation control, Snaith et al. have recently demonstrated that perovskite crystallization rate can be controlled by changing precursor composition.[2,9,10,19,22,23] By partially substituting I− with Cl− in CH3NH3PbI3 to form CH3NH3PbI3-xClx, crystallization is prolonged as a result of the lattice distortion caused by Cl− doping. This is evident by the increased time necessary to fully anneal deposited films, specifically less than 1 h for pure iodine perovskite and between 2 to 3 h for the mixed halide perovskite. More significantly, Cl− also increases the conductivity and charge diffusion length
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COMMUNICATION Figure 1. Device configuration of the planar-heterojunction solar cell, the cross-section SEM of the planar-heterojunction, and the surface SEM images of the CH3NH3PbI3−xClx layer.
(∼1000 nm) of the perovskite domain without affecting its optical properties.[19,25] In this study, we develop a simple and very effective method to enhance the crystallization of solution-processed perovskite. We demonstrate that crystallization rate of perovskite can be controlled by incorporating additives into its precursor solution to modulate thin film formation. The capacity of bidentate halogenated additives to temporarily chelate with Pb+2 during crystal growth is evidenced by the improved solubility of PbCl2 in the presence of 1,8-diiodooctane (DIO). The influence of this chelation encourages homogenous nucleation and likely modifies interfacial energy favorably, ultimately altering the kinetics of crystal growth. As a result, the morphology of the perovskite thin films processed from the solution with 1 wt% DIO show a much smoother and more continuous surface than that obtained from the pristine solution, as is illustrated in Figure 1. The crystal size and homogeneity formed under the influence of this halogenated additive lead to very high PCE (∼12%) in planar-heterojunction perovskite solar cells. Motivated by its low-temperature solution processability, we adopted a conventional p-i-n heterojunction architecture of ITO or FTO/PEDOT:PSS (35–40 nm)/CH3NH3PbI3-xClx (∼400 nm)/PC61BM (∼55 nm)/Bis-C60 (10 nm)/Ag (150 nm) to study the influence of perovskite thin-film morphology on device performance (Figure 1). Currently, the processing of compact TiO2 in the inverted architecture involves high-temperature annealing (∼500 °C) which negates some of the benefits offered by solution-based manufacturing process.[12,19,22] The conventional planar-heterojunction also offers the benefit of efficient charge transfer between the interfaces of PEDOT:PSS/PC61BM interlayers and perovskite thin films, ensuring sufficient charge dissociation and extraction for high performance provided high quality perovskite films can be grown.[19–21,23] Notably, the Bis-C60 surfactant is employed as an efficient electron-selective interfacial layer that aligns the energy levels at the organic/cathode interface and enables the utilization of stable metals such as Ag as the top electrodes, providing respectable environmental stability.[26,27] The mixed halide CH3NH3PbI3-xClx perovskite was chosen for study by virtue of the fore mentioned merits demonstrated by Snaith et al.[2,9,10,19,22,23] Detailed information regarding the preparation of the perovskite precursor solution, deposition of the perovskite thin-film, and the fabrication of devices is described in the experimental section.
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Solvent additives have enabled significant efficiency enhancements in bulk-heterojunction (BHJ) organic solar cells (OPVs) by modulating BHJ morphology.[28–30] Bazan et al. first discovered that the morphology of BHJ layers could be effectively optimized by simply incorporating additives like alkane dithiols or 1,8-di(R)octanes into the processing solution.[28,29] Two important features of the processing additives were identified for the further optimization of BHJ morphology: the selective solubility of fullerenes and the higher boiling point with respect to the host solvent.[28,29] Inspired by this finding, we are interested in exploring the additive's influence on both the crystallization of perovskite thin films and device performance. The additive studied here is DIO as it bears iodide in common with CH3NH3PbI3-xClx and as a soft Lewis base may interact with Pb2+, a soft Lewis acid. We hypothesize that this additive can temporarily coordinate with Pb2+ during crystal growth and modulate crystallization kinetics as the transient capture of additive in the growing crystal lattice will increase both the internal energy and entropy of the crystals.[12,19,25] The solvent additive was incorporated into the CH3NH3PbI3-xClx precursor solution prior to the thin film deposition. The optimized blending ratio of DIO in the precursor solution was found to be around 1 wt% with respect to the weight of perovskite (denoted as 1% DIO in the following discussion). At first, we constructed the planar-heterojunction on an ITO substrate. Encouragingly, the device processed from the precursor solution containing 1% DIO showed a ∼30% PCE enhancement compared to the control as shown in Figure 2a. Open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and PCE of all devices are summarized in Table 1. The control device fabricated from the pristine composite solution showed a PCE of 7.8% with a Voc of 0.90 V, a Jsc of 15.0 mA/cm2, and a FF of 0.58 while that derived from the solution containing 1% DIO exhibited a significantly enhanced PCE of 10.3% with a Voc of 0.92 V, a Jsc of 15.6 mA/cm2, and a FF of 0.71. All parameters improved as a result of the incorporation of DIO in the precursor solution, which suggests the generation of improved perovskite morphology due to the influence of DIO. It is well known that the crystallinity of perovskite absorber domains determines ultimate performance of the fabricated devices since defects in the crystals will create severe shorting and trapping sites for charge recombination. Crystallinity will also greatly affect the efficiency of charge dissociation, transport, and diffusion length.[19,22] As shown in Figure 1, the film
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Figure 2. (a) Current−voltage characteristics and (b) external quantum efficiency (EQE) spectra of the studied solar cells. (c) UV-vis absorption spectra and (d) XRD spectra of the solution-processed perovskite with and without additive.
coverage), benefiting from the rough surface of FTO (100 nm) relative to ITO (∼5 nm, herein).[23] This resulted in the superior performance of the device constructed on FTO which motivated us to explore our additive’s influence on this favored system. Very impressively, an increased PCE with similar enhancement factor (∼31%) to that of the ITO case was achieved on FTO, suggesting that DIO’s beneficial influence on crystal growth kinetics is not limited by interfacial structure. The device derived from the 1% DIO solution on FTO possessed a promising PCE of 11.8% with a Voc of 0.92 V, a Jsc of 17.5 mA/cm2, and a FF of 0.73, compared to the 9.0% PCE of the control device with a Voc of 0.90 V, a Jsc of 16.0 mA/cm2, and a FF of 0.62. Similar to the case on ITO substrates, the improved quality and surface coverage of the crystalline perovskite thin films caused by the presence of DIO significantly contributes to the enhancement of Jsc (16.0 to 17.5 mA/cm2) and FF (0.62 to 0.73) (Surface images were shown in Figure S1). The results of our top-performing devices are among the best reported for state-of-the-art low−2 Table 1. Performance of the studied solar cells under AM 1.5G Illumination (100 mW cm ). temperature solution-processed photovoltaics. The external quantum efficiency (EQE) FF PCE Voc Jsc plotted in Figure 2b confirms the increased 2 (%) (V) (mA/cm ) Jsc of these devices, in which the integrated On ITO Substrate (Roughness: ∼5 nm; 15 ohm/sq) Jsc for the devices derived from 1% DIO conPristine Perovskite 0.90 0.58 15.0 7.9 taining solution on FTO and ITO is 17.3 and Perovskite-1% DIO 0.92 0.71 15.6 10.3 15.4 mA/cm2, respectively (the integrated Jsc of control device on ITO is 14.5 mA/cm2). As On FTO Substrate (Roughness: ∼100 nm; 8 ohm/sq) can be seen, the maximum EQE peaks of the Pristine Perovskite 0.90 0.62 16.0 9.0 top-performing devices can reach over 70% Perovskite-1% DIO 0.92 0.73 17.5 11.8 (ITO-substrate) and 80% (FTO-substrate)
prepared from the 1% DIO solution showed better coverage, less surface roughness, and more regular crystallites with more ordered growth directions than the pristine thin film. It is worthwhile to point out that the high Voc over 0.90 V for both devices implies the small potential loss for photoexciton dissociation (band-gap of CH3NH3PbI3-xClx is ∼1.5 eV, estimated from Figure 2c). This highlights prominent benefits of perovskites: small exciton binding energy (∼20 meV) and respectable ambipolar charge conduction.[11–13,16–20] The high FF of over 0.70 further demonstrates the efficient charge transfer and extraction of such a planar p-i-n heterojunction.[13,20] Recently, Snaith et al. revealed that the perovskite performance is strongly dependent on the surface roughness of the substrate due to the sensitivity of crystallization on interfacial structure.[23] They found that on FTO/PEDOT:PSS the perovskite film was more homogeneous (with ∼90% surface coverage) than the one on ITO/PEDOT:PSS (with only ∼80% surface
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COMMUNICATION Figure 3. (a) Time resolved SEM images of the surface of the evolving CH3NH3PbI3− xClx films. The scale bars are all 5 µm. (b) Time resolved XRD spectra of the evolving CH3NH3PbI3− xClx films.
EQE. This high photon-to-electron conversion together with the panchromatic absorption over the visible range (Figure 2c) highlights the intense bandgap light harvesting and excellent ambipolar transport properties of the perovskites. The additive enhanced crystallization of CH3NH3PbI3-xClx is evident in the absorption and X-ray diffraction (XRD) spectra in Figure 2c–d. As can be seen, the perovskite processed with 1% DIO solution exhibits clearly increased light absorption across the visible range into the near-infrared wavelengths, consistent with the increase in the EQE curves. The increased absorption should be the result of improved surface coverage and more uniform crystal formation in perovskite thin films. The rising band-edge absorption around 780–790 nm proves the increased crystallinity of perovskites processed with additive. This can also be observed in the enhanced intensity of the reflections at 14.2° and 28.5° in the XRD spectrum of the DIO processed perovskite in Figure 2d. Although intensity increase in XRD is possibly due to several factors, in this case, identical instrumental parameters, sample quantities, and compositions were used for analysis. After being annealed for 3 h, we are confident that most DIO has evaporated; which, taken with the above statement, points to increased crystallinity in the DIO sample as the cause for the markedly increased observed peak intensity as compared to the pristine sample.
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These strong reflections at 14.2° and 28.5° are assigned to the (110) and (220) crystal planes of the orthorhombic lattice of mixed halide perovskites.[8] In the XRD analysis of all samples we fabricate on PEDOT:PSS, we see a significant degree of texture with the (110) plane preferentially oriented parallel to the film surface. A close analysis of the data in Figure 2d reveals that in addition to the overall intensity increase in the DIO sample, there is a noticeable increase in the ratio between the intensity of both the (110) and (220) peaks and that of the (310), (224), and (314) peaks located at 31.9°, 40.7°, and 43.3° respectively, as inserted in Figure 2d.[21,31] This indicates that in addition to improving the degree of crystallinity, DIO enhances the generation of texture already encouraged by the PEDOT:PSS/ perovskite interface. This can be explained either through a modification of interfacial energy caused by DIO or as a consequence of the role DIO chelation of Pb2+ plays in phase transformation, but currently the texture inherent to these systems is so strong that most reflections do not extend far above the noise threshold. A pole figure analysis is necessary to quantitatively characterize this phenomenon. To get more in-depth understanding of the function of DIO during perovskite crystallization, time resolved morphological characterization was made and recorded with scanning electron microscopy (SEM) and XRD, as presented in Figure 3. The
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Figure 4. (a) Schematic diagram for the transient chelation of Pb2+ with DIO. (b) Solubility of the PbCl2 and PbI2 in DMF with or without additives. From left to right, PbCl2 in DMF, PbCl2 in DMF/DIO, PbI2 in DMF, and PbI2 in DMF/DIO. The blending ratio of DIO to DMF for PbCl2 and PbI2 is 1:1.6 and 1:2.8 by mol, respectively.
preparation of these perovskite thin films was identical to the conditions used for the device fabrication. As cast, both films contain many pin-holes and voids, but the additive assisted film already demonstrates markedly improved surface coverage. Additionally, the unique contrast features in the SEM image of the as cast DIO film along with its dense XRD pattern suggest a generation of order before annealing that is unique to the DIO sample. The increased coverage and smoothness of the DIO assisted film suggest that DIO encourages homogenous nucleation and reduces the kinetics of the transformation enough to allow the thermodynamic influence of the interface between PEDOT:PSS and CH3NH3PbI3-xClx to play a more dominant role. The presence of DIO may also modify this interfacial energy, making it more favorable for the crystal to grow in contact with the surface. These improvements may also be attributed to the increased solubility of PbCl2 in the mixed solvent DMF/DIO, as shown in Figure 4. We speculate that this improvement results from the temporary coplanar chelation of Pb2+ with DIO as Cl− ligands reside in axial octahedral positions on Pb2+.[8,25] As soft Lewis bases, iodocarbons can coordinate with soft metal ions based on the hard-soft acid-base principle.[32] Thus it can be envisioned that the transient metal-solvent coordination can improve the solubility of PbCl2, as proposed in Figure 4a (and Scheme S1).[32,33] On the other hand, bidentate chelation is also more favorable than monodentate chelation from a thermodynamic perspective as observed in some metal complex systems.[34,35] The formation of the chelated ring structures of the former allow less configurational entropy loss during coordination, resulting in a much smaller Gibbs free energy. Therefore, the temporary chelation of Pb2+ with DIO will participate in the evolving dynamic equilibrium of the drying and annealing film, co-existing with the coordination of Pb2+ with methyl ammonium iodide (MAI) during crystal growth until DIO fully evaporates. As a result, transformation kinetics are retarded enough to allow and encourage more defect-free crystal growth. Moreover, it can be seen in the SEM images that the crystallites of the additive-assisted film display more regular faceting and improved interconnectivity during crystal growth, which can also be attributed to the prolonged growth caused by temporary chelation of Pb2+ by DIO. The distorted crystal lattice induced by captured DIO will increase the internal energy and configurational entropy of growing crystals thus modulating their growth rate and shape. The impact of these modifications on growth kinetics is apparent in the device data discussed above, correlating well with the increase of all relevant parameters.
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Time resolved photoluminescence (PL) behavior was characterized to probe the influence of the enhanced crystallization of perovskite thin films on charge dissociation. Detailed information regarding the preparation, measurement, and fitting methodology can be found in the experimental section. The PL lifetime of the samples was fitted with a bi-exponential decay function containing a fast decay and a slow decay process. We consider the fast decay process to be the result of the quenching of free carriers in the perovskite domain through transport to PEDOT:PSS or PC61BM, and the slow decay process to be the result of radiative decay. Figure 5 displays the PL decay and the related parameters are summarized in Table 2. For the pristine thin-film perovskite, the fast decay lifetime is 12.9 ± 0.8 ns and the slow decay lifetime is 104 ± 3 ns while their weight fractions are 81% and 19% respectively, indicating that charge transfer is the dominating decay mechanism. To mimic the real charge behavior in our p-i-n planar heterojunction device, the bilayer CH3NH3PbI3-xClx/PC61BM systems were examined. The existence of the electron quenching PC61BM layer atop the perovskite significantly decreases the fast decay lifetime from 12.9 ± 0.8 ns to 3.4 ± 0.4 ns and increases the weight fraction of fast decay from 81% to 94%. This suggests that most of the free carriers generated by illumination are efficiently transferred to PEDOT:PSS and PC61BM, which confirms the potential of the conventional p-i-n heterojunction.
Figure 5. Time resolved photoluminescence characterization of the solution-processed perovskite with and without additive.
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Subs. ITO/PEDOT:PSS
ETM
Additive
τ1 (ns)
Fraction 1
τ2 (ns)
Fraction 2
Average (ns)
-
-
12.9 ± 0.8
81%
104 ± 3
19%
30
PC61BM
DIO
8.5 ± 0.5
88%
88 ± 3
12%
18
-
3.4 ± 0.4
94%
139 ± 10
6%
12
DIO
1.2 ± 0.1
99%
18 ± 2
1%
1.4
As for the thin-film perovskite prepared from the 1% DIO solution, the fast decay lifetime decreased from 12.9 ± 0.8 ns to 8.5 ± 0.5 ns and the weight fraction increased from 81% to 88%. The same trend was observed in the bilayer CH3NH3PbI3-xClx/PC61BM film with lifetime decreasing from 3.4 ± 0.4 ns to 1.2 ± 0.1 ns and weight fraction increasing from 94% to 99%. As evidenced in Figure 3, the addition of DIO improved the coverage and crystallinity of the perovskite thin films on ITO/PEDOT:PSS, thus facilitating the diffusion of free carriers and increasing the efficiency of charge transfer. Consequently, the weight fraction of fast decay, which relates to the charge transfer process, increased and the lifetime of fast decay decreased. For perovskite processed from the 1% DIO precursor with PC61BM, the weight fraction related to quenching was almost 100%, which indicated a very efficient charge dissociation in the perovskite domain with PEDOT:PSS and PC61BM. The observed high Jsc (15.6 – 17.5 mA/cm2) and FF (0.71 – 0.73) of the devices derived from the 1% DIO solution can also be attributed to this enhanced charge separation and collection. Based on these findings, the enhanced crystallization of the solution processed perovskite films should be mainly due to the additive chelating effect and is less sensitive to the interfacial structure and the congruent improvement on both ITO and FTO substrates. The chelation efficiency may be further modulated by changing the additive's properties: alkane chain length and the nature of the end groups. Given the efficient coordination between Pb2+ and halides (Cl−, Br−, and I−) in perovskite structures,[1] the effect of alkane chain length will affect bidentate chelation as it will influence the thermodynamics and kinetics of the formation of the proposed chelated ring structures. More in-depth works regarding the understanding of the optimization of processing additives are ongoing. In summary, we have described a significantly enhanced PCE (∼31%) of the planar-heterojunction perovskite solar cells from 9.0% up to 11.8% by incorporating small amount of rationally chosen additives into the perovskite precursor solutions to improve the crystallization of perovskite thin films. We showed that incorporated additives facilitate homogenous nucleation and modulate the kinetics of growth during crystallization, as evidenced from the surface SEM images and XRD spectra. The enhanced crystallization facilitates the charge transfer efficiency between the charge transporting interlayers and the perovskite absorber. As a result, very promising PCEs (∼12%) were achieved in planar-heterojunction solar cells, fabricated through the low-temperature solution processes (under 150 °C). These results are among the best reported for the stateof-the-art solution-processed photovoltaics. We expect that even greater performance enhancement can be achieved through
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further rational design of processing additives as revealed by our work.
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Table 2. Time resolved photoluminescence characterization of the solution processed perovskite.
Experimental Section Materials and Sample Preparation: Methylammonium iodide (MAI) was synthesized by reacting 24 mL of 0.20 mol methylamine (33 wt% in absolute ethanol, Aldrich), 10 mL of 0.04 mol hydroiodic acid (57 wt% in water with 1.5% hypophosphorous acid, Alfa Aesar), and 100 mL ethanol in a 250 mL round bottom flask under nitrogen at 0 °C for 2 h with stirring. After reaction, the white precipitate of MAI was recovered by rotary evaporation at 40 °C and then dissolved in ethanol followed by sedimentation in diethyl ether by stirring the solution for 30 min. This step was repeated three times and the MAI powder was finally collected and dried at 50 °C in a vacuum oven for 24 h. To prepare the perovskite precursor solution, MAI and lead chloride (PbCl2, Aldrich) powder were mixed in anhydrous dimethylformamide (DMF, Aldrich) with a molar ratio of 3:1. The perovskite/1,8-diiodooctane (DIO, Aldrich) solution was prepared via adding 1 wt% of DIO with respect to perovskite weight into the perovskite precursor solution. The solutions (40 wt%) were stirred overnight at 80 °C and filtered with 0.45 µm PVDF filters before device fabrication. Fabrication of thin-film perovskite solar cells: The devices were fabricated in the configuration of indium tin oxide (ITO) or fluorine-doped tin oxide (FTO)/PEDOT:PSS/ CH3NH3PbI3-xClx/[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM)/fullerene surfactant (C60-bis)/Ag. ITO (15 ohm/ sq) and FTO (8 ohm/sq) glass substrates were cleaned sequentially with detergent and deionized water, acetone, and isopropanol under sonication for 10 min. After drying under a N2 stream, substrates were further cleaned by a plasma treatment for 30 s. PEDOT:PSS (Baytron P VP Al 4083, filtered through a 0.45 µm nylon filter) was first spin-coated onto the substrates at 5k rpm for 30 s and annealed at 150 °C for 10 min in air. To avoid oxygen and moisture, the substrates were transferred into a N2-filled glovebox, where the thin-film perovskite layers were spincoated from a homogeneous 40 wt% CH3NH3PbI3-XClX and CH3NH3PbI3XClX/DIO precursor solution at 6k rpm for 45 s (300-500 nm thickness) and then annealed at 90 °C for 2-3 h. Afterward, the PC61BM (15 mg/ mL in chloroform) and C60-bis surfactant (2 mg/mL in isopropyl alcohol) were then sequentially deposited by spin coating at 1k rpm for 60 s and 3k rpm for 60 s, respectively. Silver electrodes with a thickness of 150 nm were finally evaporated under high vacuum (
