2Dâ3D Mixed OrganicâInorganic Perovskite Layers for Solar Cells

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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2D−3D Mixed Organic−Inorganic Perovskite Layers for Solar Cells with Enhanced Efficiency and Stability Induced by N‑Propylammonium Iodide Additives Disheng Yao,† Chunmei Zhang,† Shengli Zhang,† Yang Yang,† Aijun Du,† Eric Waclawik,† Xiaochen Yu,‡ Gregory J. Wilson,§ and Hongxia Wang*,†

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†

School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology, Brisbane 4001, Australia ‡ School of Materials Science and Engineering, Chang’an University, Xian 710061, China § Solar Technologies, CSIRO Energy, Mayfield West, NSW 2304, Australia S Supporting Information *

ABSTRACT: Device instability has become an obstacle for the industrial application of organic−inorganic metal halide perovskite solar cells that has already demonstrated over 23% laboratory power conversion efficiency (PCE). It has been discovered that the sliding of A-site cations in the perovskite compound through and out of the three-dimensional [PbI6]4− crystal frame is one of the main reasons that are responsible for decomposition of the perovskite compound. Herein, we report an effective method to enhance the stability of the FA0.79MA0.16Cs0.05PbI2.5Br0.5 perovskite film through the incorporation of n-propylammonium iodide (PAI). Both density functional theory calculation and the X-ray diffraction patterns have confirmed the formation of two-dimensional (PA)2PbI4 with the Ruddlesden−Popper perovskite as a result of the reaction between PAI and PbI2 in the perovskite film. X-ray photoelectron spectroscopy reveals less −COOH (carboxyl) groups on the surface of the perovskite film containing (PA)2PbI4, which indicates the suppressed penetration of oxygen and moisture into the perovskite material. This is further confirmed by the surface water wettability test of the (PA)2PbI4 film that exhibits excellent hydrophobic property with over 110° contact angle. Ultraviolet photoelectron spectroscopy demonstrates the introduction of PAI additives that resulted in the upshift of the conduction band minimum of the perovskite by 160 meV, leading to a more favorable energy alignment with an adjacent electron transporting material. As a consequence, enhanced 17.23% PCE with suppressed hysteresis was obtained with the 5% PAI additive (molar ratio) in perovskite solar cells that retained nearly 50% of the initial efficiency after 2000 h in air without encapsulation under 45% average relative humidity. KEYWORDS: perovskite solar cells, device stability, 2D Ruddlesden−Popper perovskites, n-propylammonium iodide, aging against moisture

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INTRODUCTION

The perovskite compound adopts a chemical formula of ABX3, where A = monovalent cation such as methylammonium (MA+), formamidinum (FA+), and Cs+;13−15 B = metal cation such as Pb2+ and Sn2+;16,17 X = halide anion such as Cl−, Br−, and I−.18−20 In this structure, the A-site cations that have a small ionic radius is believed to be one of the main reasons that cause the material decomposition, thus device instability. A-site cations are located in the interstitial space of the [PbI6]4− octahedral frame and have relatively smaller decomposition activation energy.21−23 Therefore, changes of the external environment such as temperature and humidity can easily trigger the vibration of the A-site cations that increases lattice defects and even leads to molecular dissociation.24 So far,

The past decade has witnessed the skyrocketing progress of perovskite solar cells (PSCs). Although PSCs using methylammonium iodide (MAPbI3) as the light absorber started with humble energy conversion efficiency of less than 4% in 2009,1 the advancement in material engineering and device engineering has dramatically improved their power conversion efficiency (PCE) with the most recently reported world record of 23.7%,2−7 which is already comparable to commercial crystalline silicon cells and CIGS, CdTe thin film solar cells. PSCs are considered as one of the most cost-effective and promising photovoltaic technologies in the future, owing to their outstanding photoelectric properties and simple solutionprocessed fabrication procedure.8 However, the unsatisfactory stability of perovskite materials in the presence of moisture, ultraviolet light, and thermal stress has become an obstacle that impedes the practical application of PSCs.9−12 © XXXX American Chemical Society

Received: April 16, 2019 Accepted: May 28, 2019 Published: May 28, 2019 A

DOI: 10.1021/acsami.9b06305 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. XRD patterns of (a) (PA)2xFA0.79MA0.16Cs0.05Pb1+xI2.5+4xBr0.5 (2x = 0, 0.025, 0.05, 0.075, 0.1) mixed perovskite films containing different amounts of PA+ on FTO substrates, (b) (101) lattice plane of FAPbI3, (c) (202) lattice plane of FAPbI3, and (d) (211) lattice plane of FAPbI3.

bI3)0.88(CsPbBr3)0.12 perovskite and the hole transporting layer by Park et al.34 The initial efficiency of 16.75% of the solar cells declined by only 5% under extremely high relative humidity of 85 ± 10% after 24 h. Despite the efforts mentioned above, the efficiency of PSCs with the 2D perovskite material is generally lower compared to PSCs using 3D materials only. According to the Goldschmidt’s equation, only cations or ligands that have a suitable ionic radius to maintain the perovskite crystal phase with a tolerance factor in the range of 0.78−1.03 can form a stable perovskite phase.35 Thus, when doping perovskite with long-chain ligands, such as BA and PEA, secondary phases, such as (BA)2PbI4 and (PEA)2PbI4, which are photoinactive and weaken the light harvesting of perovskite films will form in the perovskite.36,37 Furthermore, the inhibition of out-plane charge transport of 2D perovskite induced by the large ionic radius of BA or PEA acts like an insulating layer that reduces device efficiency.38 To solve these problems, controlling the crystal growth orientation of BA+-based RPPs using a hot-cast method was reported to improve device performance.39 Nevertheless, unexpected defects or cracks on the film surface are often observed during the hot-cast process, which is probably due to different crystallization rates between BA + and MA + components under high temperature (∼150 °C). The complexity of controlling the crystallization process of 2D− 3D perovskite materials makes it difficult to repeat the result. Furthermore, regarding the interfacial engineering of PSCs via 2D RPP intercalation, it is still an open question regarding the location of the 2D RPPs. The lack of clear evidence demonstrates that the 2D RPPs form an intermediate layer with a satisfactory morphology giving rise to the speculation that PEA-based 2D RPP compounds may grow at grain boundaries of bulk 3D perovskites.40 This is because the in situ formation of the 2D RPPs relies on the reaction of PEAI with excess PbI2, which often exists at grain boundaries in the perovskite film. Recently, a novel 2D RPP was synthesized using the shorter-chain n-propylamine (PA) to form

attempts have been made to address this issue. One of the methods is to substitute the A-site cations with relatively stable ions or ligands.25−27 Recently, two-dimensional (2D) Ruddlesden−Popper perovskites (RPPs) with a generic structural formula BA2MAn−1PbnI3n+1 (BA is n-butylammonium) have drawn the attention of researchers because of their better material stability. Cao et al. first demonstrated the good stability of the 2D RPPs in PSCs although the initial PCE of 4.02% was poor with BA2MA2Pb3I10-based cells.28 Tsai et al. optimized the material synthesis method for 2D BA2MA3Pb4I13 and obtained a near-single-crystalline material quality.29 12.5% PCE was achieved owing to a strongly preferential out-plane alignment to fulfill efficient charge transport. Most importantly, the devices showed impressive stability under light, humidity, and heat stress tests. An efficiency retention of 90% was reported with the encapsulated BA+-based solar cells under 65% relative humidity after 2250 h. Furthermore, Zhang et al. incorporated Cs+ to BA2MA3Pb4I13 and achieved 13.7% PCE with outstanding humidity resistance.30 The optimized devices could retain 89% of the initial PCE without encapsulation under 30% humidity after 1400 h. Meanwhile, Wang et al. found that BA+-doped Cs−FA-based PSCs retained 80% of initial average efficiency (18.1%) after 1000 h in air and 4000 h when the device was encapsulated.31 Besides BA+, other aliphatic or aromatic ammonium groups have also been introduced to form a hydrophobic layer on top of perovskites to hinder the penetration of water into the perovskite film. Smith et al. fabricated 2D−three-dimensional (3D) (PEA)2MA2Pb3I10 (PEA+ = C6H5C2H4NH3+) mixed perovskites films, which maintained the stable crystal phase for 46 days in an ambient environment.32 Based on this work, Chen et al. employed (PEA)2PbI4 as a capping layer for the Cs0.05(FA0.83MA0.17)Pb(I0.83Br0.17)3 perovskite that achieved 18.5% PCE and retained nearly 90% efficiency after 1000 h in ambient conditions.33 More recently, (5-AVA)2PbI4 (5-AVA = 5-ammoniumvaleric acid) was reported to form in situ 2D compound at the interfaces between the (FAPB


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(2D) planes, stacking along the archetypal three-dimensional (3D) perovskite structure. This characteristic peak can be probably buried under the (101) plane diffraction of α-FAPbI3 at 14.1° because of the extremely similar peak position. Therefore, the diffraction of these compounds most likely results in the overlapping and shift of the perovskite XRD patterns observed above. Compared with the XRD patterns of (PA)2PbI4 and PAPbI3, the diffraction peak belonging to the (001) plane of PbI2 at 12.6° is observed with the XRD pattern of the PAPbI3 sample. In contrast, no peak of PbI2 is found in the XRD pattern of (PA)2PbI4. This indicates that PbI2 can be fully transformed into the (PA)2PbI4 Ruddlesden−Popper phase in the 1:2 molar ratio of PbI2 and PAI. To determine the favorable phase that PA exists, we first calculate the Goldschmidt tolerance factor (t) of the PAPbI3 perovskite compound according to the equation below.44 RA + RX t= 2 (RB + RX)

PA2MA4Pb5I16. Although solvent engineering was used to optimize the material properties, the solar cells based on this material showed an efficiency of merely 10.4% due to the low fill factor.41 Furthermore, the large band gap (1.85 eV) of this material limits the conversion efficiency of single phase PA2MA4Pb5I16-based solar cells. Ahmad et al. reported the Dion−Jacobson 2D organometal halide perovskite phase by the incorporation of 1,3-propanediamine (PDA) in MAPbI3 to form (PDA)(MA)3Pb4I13. The related device shows a better efficiency of 13.3% and excellent stability owing to the elimination of the van der Waals gap in the 2D perovskite framework.42 Nevertheless, compared with the conventional 3D perovskite-based device, the relatively low efficiency indicates that more effort is needed to improve the performance of solar cells. Herein, we report a 2D−3D mixed PSC using npropylammonium iodide (PAI) as the additive in FA0.79MA0.16Cs0.05PbI2.5Br0.5 perovskite films. The perovskite film was fabricated via a simple one-step method without hotcast treatment (synthesis details in the Supporting Information). We have found that the shorter alkyl chain of PAI gradually forms a capping layer through the reaction with PbI2 with the Ruddlesden−Popper crystal structure of (PA)2PbI4 on top of the perovskite film with the increase of the doping concentration. The hydrophobic (PA)2PbI4 showed a large water contact angle of 115°, which dramatically enhanced the stability of the perovskite against moisture. As a result, optimum 17.23% PCE with suppressed hysteresis was obtained in our unencapsulated solar cells that retained nearly 50% of initial efficiency under 45% average relative humidity after 2000 h.

Where RA, RB, and RX are the radii of PA, Pb, and I ions with values of RPA = 3.70 Å, RPb = 1.02 Å, and RI = 2.20 Å. The calculated tolerance factor of PAPbI3 is 1.30, which is far beyond the ideal scope of 0.78−1.05 to form the stable cubic perovskite. Therefore, PAPbI3 is not a favorable phase formed by PAI and PbI2. We further employed density functional theory to determine the structural configuration of possible PA-based compounds. In this case, to focus on the coordination between the A-site ion and the [BXm]n ligand, two independent crystal forms of PAPbI3 (ABX3 perovskite, Figure S2a) and (PA)2PbI4 (AX· nABX3 Ruddlesden−Popper, Figure 2) and two composite

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RESULTS AND DISCUSSION Figure 1a shows the X-ray diffraction (XRD) pattern of the synthesized (PA)2xFA0.79MA0.16Cs0.05Pb1+xI2.5+4xBr0.5 (2x = 0, 0.025, 0.05, 0.075, 0.1) mixed perovskite films on fluorinedoped tin oxide (FTO) substrates. The characteristic peaks at around 14.1, 28.5, and 31.8° are consistent with typical (101), (202), and (211) lattice planes of α-FAPbI3 reported before.43 Meanwhile, no observable δ-FAPbI3 (yellow phase) peak at around 11.6° indicates high phase purity of the perovskite film, which is attributed to the incorporation of Cs+ that can effectively suppress the presence of the photoinactive δ-FAPbI3 phase. Although no new peak is observed with the perovskite with PAI additive, a clear shift of all of FAPbI3 peaks representing (101), (202), and (211) lattice planes toward lower angles (Figure 1b−d) by over 0.08° (much more than the resolution of 0.02° applied step size) is noted with an increase of the amount of PA+ from 0 to 10%. This demonstrates the enlargement of the d-spacing and lattice distortion of perovskite crystals. However, whether this peak shift is due to PA+ incorporation into the perovskite lattice or overlapping of PA-based material characteristic patterns is still unresolved. To gain further insight into this aspect, we measured XRD patterns of the two possible compounds formed by the interaction of PAI and PbI2. One is (PA)2PbI4 and the other is the PAPbI3 compound, which were made by mixing the stoichiometric ratio of PbI2 and PAI (molar ratio of PbI2/PAI = 1:2 for (PA)2PbI4 and 1:1 for PAPbI3) (Figure S1). Compared to the reported (111) lattice plane of BA2MAn−1PbnI3n+1,29 a strong peak at around 14.0° is clearly observed in both types of films that belong to two-dimensional

Figure 2. Top view of the optimized structure of PA2PbI4. White: H, blue: N, light gray: C, dark gray: Pb, red: I. The lattice parameters for the unit cell of PA2PbI4 are a = 11.616 Å; b = 6.150 Å; c = 7.514 Å; α = 105.557°; β = 110.664°; and γ = 97.809°.

structures including PAFA7Pb4I24 (PA replacing FA, Figure S2b) and PAFA8Pb4I24 (interstitial PA, Figure S2c) are considered in the calculation. Comparison of the formation energy of these four models (for methodology, see the Supporting Information) shows that only PAPbI3 (perovskite, formation energy −5.40 eV) and (PA)2PbI4 (Ruddlesden− Popper, −7.17 eV) may exist because they both have negative energy of formation. The large positive energy of formation values for the PAFA 7Pb4I24 (replacing, 8.67 eV) and PAFA8Pb4I24 (interstitial, 4.95 eV) indicate that they are thermodynamically less probable. This result suggests that PA is preferential to form an independent compound instead of C


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Figure 3. High-resolution XPS for the C 1s peak in (PA)2xFA0.79MA0.16Cs0.05Pb1+xI2.5+4xBr0.5-mixed perovskite films with (a) 0%, (b) 5%, (c) and 10% PA+ additive. (d) (PA)2PbI4 film without FA+ or MA+.

incorporation into the FAPbI3 lattice. The thermal stability of PAPbI3 and (PA)2PbI4 were then evaluated via ab initio molecular dynamics simulations with the canonical ensemble. Both (PA)2PbI4 and PAPbI3 were optimized from their crystal structures with the lowest energy of formation.45 It is found that the model of PAPbI3 collapsed within 1.5 ps at 500 K temperature (Figure S2d), whereas (PA)2PbI4 maintained the initial crystal structure for 10 ps (Figure S2e). Clearly, the lower energy of formation and apparent thermal stability of (PA)2PbI4 indicate that it is the most favorable product when PbI2 reacts with PAI. This is consistent with the observed XRD results and the tolerance factor calculation above. To further confirm the existence of (PA)2PbI4 in the perovskite films and investigate the effect of PAI additive on the perovskite surface, X-ray photoelectron spectroscopy (XPS) was employed as a less-destructive method to detect the surface composition of the perovskite film (detecting depth ∼20 nm). Figure S3a is the XPS survey spectra of 0, 5, and 10% PAI-contained films that were calibrated using the standard O 1s position (C 1s is variable in our case). Compared to the reference sample without PAI, there is no shift in the binding energy of Pb 4f, Pb 4d, I 4d, Br 3d, and I 3d (Figure S3a,b) in the PAI-contained films. This implies that PAI additive does not cause a significant change in the chemical valence state of each element.46 However, peak differentiating analysis of high-resolution spectra of C 1s (Figure 3a−c) shows that there are three different types of carbon-based chemical bonds in the perovskite films, whereas only two peaks are detected in pure (PA)2PbI4 (Figure 3d). These chemical bonds can be well identified as C−C (or C−H at the similar binding energy), C−N, and −COOH (or O− CO at the similar binding energy) at 285.5, 287.1, and 288.9 eV, respectively. Despite the uncertainty of C−C (C−H) from carbon contamination and/or perovskite self-contamination, it is clear that the C−N results only from FA+, MA+, and PA+ ligands. The percentage increase of the area (Table 1) of C−N with the PA+ additive is observed. Although the amounts of

Table 1. Percentage of Chemical Bonds Related to Carbon in the Perovskites Analyzed with Different Amounts of PA+ Additives sample

C−C(C−H) (%)

C−N (%)

−COOH (O−CO) (%)

0% PA+ 5% PA+ 10% PA+ (PA)2PbI4

54.91 55.27 47.65 70.01

12.62 16.43 27.39 29.99

32.47 28.30 24.96 0

C−N remain constant in both MA+ and FA+ in all films analyzed, it increases because of 2-fold C−N in the chemical format of (PA)2PbI4, which is attributed to the increasing amount of PA+ on the surface. This result provides strong evidence of the accumulation of the PAI-based material phase on the surface of the perovskite films. Meanwhile, the peak reflecting −COOH (O−CO) at 288.9 eV is indicative of the effect of moisture (H2O) and/or oxygen (O2) on perovskite films due to the exposure of the samples in air before it was transferred to the vacuum chamber for XPS measurement.33 The content of −COOH (O−CO) decreases with the increased PA+ amount. In particular, there is no −COOH (O− CO) in the (PA)2PbI4 indicating that PA+ additives can improve the stability of the perovskite against moisture. To further understand the effects of PA+ on the optical properties of the perovskite films, UV−vis absorption spectra of the perovskite films containing different amounts of PA+ were measured (Figures 4a,b and S4a,b). As depicted in Figure 4a, a sharp absorption change of the samples at around 750 nm is consistent with the light absorption edge of FAPbI3.47 With the increased concentration of PA+, the data indicates that the optical band gap shows no significant change (±0.01 eV) around 1.60 eV (Figure 4b) due to a very small amount of PA+ additive in our case. Ultraviolet photoelectron spectroscopy (UPS) of the samples was then performed to investigate the energy band position and band alignment between TiO2 and perovskite layers. As shown in Figure 4c, the cut-off edge of the D


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Figure 4. (a) Ultraviolet−visible absorption spectra of perovskite films with different PA+ contents. (b) Evolution of the optical band gap of the films according to the absorption spectra. (c) Ultraviolet photoelectron spectra (UPS) positioning the cut-off edge of perovskite films with different PA+ contents. (d) UPS spectra showing Fermi edge of the films. (e) Band energy diagram of TiO2 and the perovskite without and with 5% PA additives based on the UPS and UV−vis spectra results.

perovskite film slightly shifts toward the lower binding energy level by 0.03 eV for the film containing PA+. Meanwhile, a significant change of the Fermi edge (0.3 eV) is observed with the PA-based sample (Figure 4d). Combining the value of cutoff energy and Fermi edge in UPS with the optical band gap derived from the UV−vis spectrum, the energy level of both the valence band maximum (EVBM) and conduction band minimum (ECBM) of the perovskite materials can be determined (Figure 4e) according to the following equations.

mismatch is effectively reduced through the introduction of PAI into the perovskite that should benefit the charge injection from the perovskite to the adjacent TiO2 layer. To investigate the influence of PA+ incorporation on the morphology of the perovskite film that profoundly affects the J−V performance of solar cells, top-view scanning electron microscopy (SEM) images were obtained for the perovskite films containing 0−10% PAI (Figure 5a−e). For comparison, the morphology of pure (PA)2PbI4 is also shown in Figure S5a. Clearly, a highly compact film surface without pinhole is observed with the 0% PA+ reference sample (Figure 5a). The grain sizes of the 3D perovskite crystals are in the range from 100 to 300 nm. With the incorporation of PAI, a schistose-like crystalline structure with a smaller size (20 mA/cm2). However, after aging for over 2000 h, the device with 5% PA+ could still retain nearly 50% of PCE, whereas the efficiency of the ref-0% PA+ cell dropped

increase of the PAI additive from 0 to 10%, the value of θ gradually increases from 59.8 to 74.2°, which indicates enhanced ability against moisture on the film surface. Nevertheless, the contact angles of all of the samples slightly reduced after a few seconds, partially due to liquid surface tension (Figure S9). Furthermore, the surface wettability of the pure (PA)2PbI4 film (Figure 9f) shows an extraordinary hydrophobic surface with a contact angle of 115.2°. These results further confirm that the capping layer incorporating (PA)2PbI4 is most likely responsible for the improved

Figure 8. (a−d) Device stability of the reference and the 5% PA+-doped solar cells in a 2060 h-long aging process. H


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Figure 10. (a−e) Nyquist plots of the solar cells with 0 and 5% PA+ under various light illumination intensities. (f) Surface recombination resistance (R2) under open-circuit voltage and different light illumination intensities. (g) Extracted interfacial charge accumulation capacitance (Cs) and fitted ideality factors. (h) Open-circuit voltage as a function of light illumination intensity in the PSCs with different amounts of PA+.

durability due to surface repellence of the perovskite film against moisture. To further reveal the effect of the PAI additive on interfacial charge carrier transport and film surface recombination, impedance spectroscopy (IS) of the devices under different light illumination intensities was measured (Figure 10a−e). It is found that there are two semicircles in all of the Nyquist plots. R2 of the first semicircle at high frequency represents charge transport resistance in bulk perovskites, whereas Cg is the dielectric capacitance of the perovskite layer. R1 of the second semicircle at low frequency serves as the resistance at perovskite/electrode interfaces, whereas Cs corresponds to the interfacial charge accumulation capacitance. In this equivalent circuit, surface recombination in the PSCs is mainly determined by R2.52,53 As shown in Figure 10f, the related

R2 shows a decreasing trend with the enhancement of light illumination. Though the trends of R2 under open-circuit voltage are similar in both the cells, compared with the ref-0% PA+ cell, a larger value with 5% PA+ is obtained at the same illumination intensities. This result suggests that the charge extraction process is more efficient in the device in the PAIcontained perovskite films that are in agreement with the improved efficiency in J−V plots. On the other hand, it is worth noting that Cs increases linearly with the enhancement of illumination intensities and open-circuit voltage (Figure 10g). Zarazua et al. reported that photogenerated holes build up the measured photovoltage upon light illumination, which can accumulate at interfaces.53 Hence, increased Cs in the 5% PA+-doped device can be associated with the enhanced Voc and FF. I



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The ideality factor of the solar cells, which is indicatively dominated by trap-related recombination, is evaluated from the plots of the interfacial charge accumulation capacitance (Cs) according to the below equation.54

*E-mail: [email protected]. ORCID

Aijun Du: 0000-0002-3369-3283 Hongxia Wang: 0000-0003-0146-5259

The fitted curves show that the ideality factor (mc) of the 5% PA+-doped cell is 1.58 that is smaller than that of the ref-0% PAI sample (1.79). The reduced value indicates a higher photovoltage at the interface and less trap-associated charge recombination in the device with the incorporation of PAI. The open-circuit voltage (Voc) as a function of light illumination intensity (Figure 10h) was also measured to estimate the ideality factor. As mentioned above, trap-assisted recombination exists in PSCs besides direct and radiative recombination. Then, the ideality factor can be calculated from the plots following the equation.54

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Australian Research Council (FT120100674) and the Queensland government DSITI QCAS project. D.Y. thanks joint Ph.D. scholarship supported by the Queensland University of Technology (QUT) and CSIRO through the Low Emissions Technology Program. The data of XRD, SEM, EDS, XPS, and UPS reported in this paper were obtained at the Central Analytical Research Facility (CARF), QUT. Access to CARF was supported by the generous funding from the Science and Engineering Faculty, QUT. We also acknowledge the measurement of TAS by Prof. Qing Shen from the Faculty of Informatics and Engineering, The University of Electro-Communications in Japan.

md kBT ln Φph q

The value of the ideality factor (md) is 1.75 in the 5% PAIbased fresh sample, while md = 1.89 in the ref-0% PA+ device. Compared with the ideality factor calculated from the IS measurement, a similar trend is observed indicating that the trap-assisted recombination is suppressed owing to the formation of (PA)2PbI4. Although the same cells are applied in both the measurements, we note that the estimated ideality factors from Voc under light illumination intensities are a bit larger than the result of the IS, probably due to the lightinduced polarization of the devices after series of measurements under light illumination.55

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REFERENCES

(1) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050−6051. (2) Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Humphry-Baker, R.; Yum, J.-H.; Moser, J. E.; Gratzel, M.; Park, N. G. Lead Iodide Perovskite Sensitized AllSolid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, No. 591. (3) Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Gratzel, M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, 316−319. (4) Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T.-B.; Duan, H.-S.; Hong, Z.; You, J.; Liu, Y.; Yang, Y. Interface Engineering of Highly Efficient Perovskite Solar Cells. Science 2014, 345, 542−546. (5) Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. High-Performance Photovoltaic Perovskite Layers Fabricated through Intramolecular Exchange. Science 2015, 348, 1234−1237. (6) Yang, W. S.; Park, B.-W.; Jung, E. H.; Jeon, N. J.; Kim, Y. C.; Lee, D. U.; Shin, S. S.; Seo, J.; Kim, E. K.; Noh, J. H.; Seok, S. I. Iodide Management in Formamidinium-Lead-Halide-Based Perovskite Layers for Efficient Solar Cells. Science 2017, 356, 1376−1379. (7) National Renewable Energy Laboratory USA. Best Research-Cell Efficiencies, 2018, https://www.nrel.gov/pv/assets/pdfs/pvefficiencies-chart.201812171.pdf. (8) Grätzel, M. The Light and Shade of Perovskite Solar Cells. Nat. Mater. 2014, 13, 838−842. (9) Shahbazi, M.; Wang, H. Wang. Progress in Research on the Stability of Organometal Perovskite Solar Cells. Sol. Energy 2016, 123, 74−87. (10) Kaltenbrunner, M.; Adam, G.; Głowacki, E. D.; Drack, M.; Schwödiauer, R.; Leonat, L.; Apaydin, D. H.; Groiss, H.; Scharber, M. C.; White, M. S.; Sariciftci, N. S.; Bauer, S. Flexible High Power-perWeight Perovskite Solar Cells with Chromium Oxide-Metal Contacts for Improved Stability in Air. Nat. Mater. 2015, 14, 1032−1039. (11) Tiong, V. T.; Pham, N. D.; Wang, T.; Zhu, T.; Zhao, X.; Zhang, Y.; Shen, Q.; Bell, J.; Hu, L.; Dai, S.; Wang, H. OctadecylamineFunctionalized Single-Walled Carbon Nanotubes for Facilitating the Formation of a Monolithic Perovskite Layer and Stable Solar Cells. Adv. Funct. Mater. 2018, 28, No. 1705545.

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CONCLUSIONS We have demonstrated that the introduction of suitable amounts of PAI additives into the Cs0.05MA0.16FA0.79PbI2.5Br0.5 perovskite film effectively forms a 2D (PA)2PbI4 capping layer with the Ruddlesden−Popper crystal structure. Owing to the relatively long and immobile alkyl chain, the 2D material can significantly change the wettability of the film surface to be hydrophobic against the ambient moisture. Further investigation indicates that the (PA)2PbI4 capping layer also led to an improved energy alignment between TiO2 and perovskite layers as a result of upshifting the conduction band minimum of the perovskite materials, which benefits the interfacial charge transfer between the perovskite film and the charge extraction layer. In addition, the PAI additive also reduces recombination and trap states in the perovskite film. As a result, the performance of the PSCs improves from 16.86% for 0% PA to 17.23% for 5% PAI with negligible hysteresis. After 2000 h aging in an ambient environment, the 5% PA contained device demonstrated much better stability with nearly 50% of PCE retention. This work opens a new approach to fabricating PSCs with enhanced stability for industrial application in the future.

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AUTHOR INFORMATION

Corresponding Author

C = C0 eqV / mckBT

Voc =

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b06305. Fabrication, characterization, and supporting results of the PSCs referred to the paper (PDF) J


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