Passivation of Grain Boundary by Squaraine Zwitterions for Defect

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Passivation of Grain Boundary by Squaraine Zwitterions for Defect Passivation and Efficient Perovskite Solar Cells Zhen Wang, Anusha Pradhan, Muhammad Akmal Kamarudin, Manish Pandey, Shyam Sudhir Pandey, Putao Zhang, Chi Huey Ng, Atul Tripathia, Tingli Ma, and Shuzi Hayase ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22044 • Publication Date (Web): 18 Feb 2019 Downloaded from http://pubs.acs.org on February 18, 2019

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ACS Applied Materials & Interfaces

Passivation of Grain Boundary by Squaraine Zwitterions for Defect Passivation and Efficient Perovskite Solar Cells Zhen Wang,† Anusha Pradhan,† Muhammad Akmal kamarudin,† Manish Pandey,† Shyam S. Pandey,† Putao Zhang,† Chi huey Ng,† Atul S.M. Tripathia,† Tingli Ma,† Shuzi Hayase†*

† Department of Life Science and Systems Engineering, Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu-ku, Kitakyushu, Fukuoka 808-0196, Japan.

KEYWORDS. squaraine zwitterions passivation, symmetrical coordination, DFT calculation, traps distribution, efficient perovskite solar cell.

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ABSTRACT. Unavoidable defects in grain boundaries (GBs) are detrimental and critically influence organometal halide perovskite performance and stability. To address this issue, semiconducting molecules have been employed to passivate traps along perovskite GBs. Here, we designed and synthesized three squaraine molecules (SQ) with zwitterionic structure to interact with under-coordinated Pb2+ and passivate Pb-I antisite defects. Density functional theory (DFT) calculation shows symmetric O atoms could coordinate with perovskite grains simultaneously, resulting in continuous charge distribution at SQperovskite interface. The energetic traps distribution in CH3NH3PbI3 perovskite is influenced significantly by the interaction between SQ and perovskite as analyzed by thermally stimulated current (TSC), in which the deep-level defects are considerably reduced due to efficient SQ passivation. In addition, we explore that how SQ molecules


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with different energy offset affect the charge extraction, which is suggested to facilitate exciton separation at the perovskite-SQ interface. These benefits lead to enhanced perovskite efficiency from 15.77% to 18.83% with the fill factor approaching 80%, which is among the highest efficiency reported for MAPbI3 solar cells fabricated in ambient environment at 60% relative humidity (RH). Considerable retardation of perovskite device degradation was achieved, retaining 90% of initial efficiency when kept 600 h at 60±5% RH.

Introduction


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Last one decade have been witnessing the tremendous improvement in development of organic-inorganic lead halide based perovskite solar cells (PSCs) due to its high dielectric constant, large absorption coefficient and high power conversion efficiency (PCE). Since its inception in 2009, the PCE of PSCs progressed rapidly and reached beyond 23%.1-2 The presence of unavoidable trap sites at the interface and perovskite grain boundary (GB) renders carrier recombination and intrinsic instability during the nucleation and crystal growth, leading to reduced carrier lifetime, ion migration and strong hysteresis behaviors, which are the major challenge for the enhancement and stability of PCE and their scale-up technology in the near future. A recent study has clearly shown that CH3NH3PbI3 (MAPbI3) single crystals without GBs and low surface defects exhibited much-enhanced stability for several years.3-4 Unwanted defects in polycrystalline perovskite films along GBs could create pin-hole and nonradiative recombination center. Defects-induced GBs create a convenient diffusion passage for oxygen and moisture in perovskite films, which accelerate performance degradation and hamper the stability of the photovoltaic devices. The low formation energy of organometal halide perovskite and the tendency of the organic components to decompose during the


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annealing process results in the formation of vacancies defects, which would eventually become the nonradiative recombination centers.5-6 Passivation materials , such as fullerene derivatives,7 polymers8-10 and sulfonate-carbon nanotube,11 have been employed to passivate these defects on GBs in order to control the crystal growth, annihilate the charge traps and eliminate the hysteresis behavior. These materials are expected to have a weak interaction with Pb atoms in perovskite because most of semiconducting materials possess O or S atoms with lone pairs, and directly coordinate with Pb2+ through Van der Waals force. However, most of these materials are commercially unavailable due to the synthetic cost and involve extensive purification process, increasing the fabrication cost of perovskite device and thus limiting the commercialization in near future.12

The presence of functional group such as carboxyl (-COOH),13 amino group14 and carbonyl (C=O) groups can coordinate with Pb, which is an important criterion for selecting a molecule as the passivation materials (known as passivator). These functional groups can interact with Pb to form passivator-PbI2 adduct, bridging the neighboring


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crystal grains through passivator and influencing the film crystallization process. However, these passivators could only coordinate with either the negative-charge Pb-I antisite defects or the positive-charged under-coordinated Pb2+.15-17 To address this issue, Huang and co-workers demonstrated that quaternary ammonium halide could passivate the positive and negative charged defects simultaneously, effectively reducing the trap density and enhancing perovskite solar cell performance compared with phenylC61-butyric acid methyl ester (PCBM) passivation.18 Huang group also applied zwitterion molecules in all-inorganic perovskite to impede the fast crystallization of perovskite for stabilized cubic phase and efficient photovoltaic.19 These efficient defect passivation of the perovskite layer is achieved to date leave much room for further improvement. Therefore, selection of a well-designed and suitable materials to passivate perovskite film is of utmost requirement.

Squaraine (SQ) zwitterions exhibit high absorption coefficient with tunable bandgaps, excellent photochemical and thermal stabilities,20 These functional molecules exhibit substantial bond delocalization and donor-acceptor architecture with notably


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electron-deficient four-membered ring21, which have been extensively employed as the hole transport layers or donor materials in dye-sensitized solar cells (DSSC)22-23 and organic photovoltaic devices.24 In addition, the electron-deficient central four-membered ring functional group in SQ zwitterions could interact with metal ions to form SQ-metal complexes25-28 as revealed by the change in fluorescence spectra. The symmetrical oxygen atoms in four-membered ring functional group could interact with two metal ions, making them suitable for chemosensor application.29-30 Herein, three different p-type SQ zwitterions functionalized with bromide or fluoride (SQ45, SQ63 and SQ81) were designed and synthesized by a simple and cost-effective approach for perovskite films GBs passivation, SQ molecules with resonance stabilized zwitterions are proposed to passivate the under-coordinated Pb2+ and Pb-I antisite defects simultaneously on perovskite boundary as indicated by experimental data and density functional theory (DFT) simulation, leading to reduced trap density and controlled crystal growth. Furthermore, SQ molecules could enhance charge separation at the perovskite grain/SQ interface due to the fact that the highest occupied molecular orbital (HOMO) of SQ molecules aligned well with valence band (VB) of MAPbI3 perovskite. Additionally, we


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employed thermally stimulated current (TSC) measurement to study the effect of the SQ passivation on the traps distribution in MAPbI3 perovskite. Prepared in ambient atmosphere with more than 60% RH condition, the efficiency of SQ63 passivated perovskite solar cells was improved up to 18.83% with approaching 80% fill factor (FF) compared with that of pristine perovskite without SQ passivation (15.77%). This strategy also enhanced the stability of perovskite devices, in which 90% of initial performance was still retained after 600 h storage in 60±5% RH conditions.

Result and Discussion

The procedure for the synthesis of SQ45, SQ63 and SQ81 with different electron-withdrawing group is shown in Figure 1a (SQ synthesis details are given in Supporting Information), SQ molecules information including absorption spectra, fluorescence spectra and carrier mobility estimated from Organic Field-Effect Transistor (OFET) measurement were shown in Figure S1-S2. All three molecules showed p-type behavior as observed from OFET characteristics. SQ molecules were introduced into perovskite films through one-step antisolvent method (Figure 1b). It is worth mentioning


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here that the introduction of the passivator during film preparation process proved to be more effective than mixing in perovskite precursor solution.7 The absorbance spectra of the perovskite exhibited nearly identical features, indicating the same light harvesting capability without the influence of SQ molecules (Figure S3, optimum concentration of 103

mg/ml). Scanning electron microscopy (SEM) images of perovskite films with and

without different SQ treatment is shown in Figure 1c, where the negligible morphological difference was observed with SQ passivation of the perovskite in contrast to the pristine perovskite film, and can be understood due to the extremely low SQ concentration (103mg/ml).

Interestingly, SQ-passivated perovskite films show smooth morphology

compared with the pristine sample as evidenced by atomic force microscopy (AFM). The roughness of films prepared with SQ45, SQ63 and SQ81 passivation were found to be 11.1 nm, 7.8 nm and 8.9 nm respectively (Figure S6) compared to the pristine perovskite (15.4 nm). The roughness of the perovskite surface in the case of SQ passivation decreased due to the fact that SQ molecules with cyclobutene group could coordinate perovskite and further control the crystallization of perovskite.31


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X-ray diffraction (XRD) pattern of perovskite films with and without SQ passivation shows that all XRD pattern of perovskite films exhibited identical diffraction peaks at 14.1°, 28.2°, 31.8° corresponding to (110), (220), (310) facets respectively (Figure S3), which is consistent with the typical main MAPbI3 lattice structure.31 The diffraction peaks of perovskite film when prepared with SQ molecules were found to be intensified with narrow full width at half maximum (FWHM) of 0.103° (SQ45), 0.101° (SQ63) and 0.106° (SQ81) at (110) peak in comparison to the pristine perovskite (0.128°). This clearly indicates that the introduction of SQ molecules in the antisolvent improved the crystallinity of perovskite films, which can be attributed to the interaction between


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Figure 1. a) Synthesis route for squaraine molecules (SQ) with fluorination (SQ45) and bromination (SQ63, SQ81). b) Fabrication of perovskite films with SQ in anti-solvent. c) Top-view SEM images of perovskite films without SQ and with SQ45, SQ63 and SQ81 passivation.


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perovskite and SQ molecules to control perovskite crystal growth during the annealing. We measured the energy dispersive X-ray spectroscopy (EDS) to examine the distribution of SQ molecules, for SQ63 passivation, Br in SQ63 was uniformly distributed on the perovskite film (Br-mapping shown in Figure S4). When we further closely observed the (110) peak at 14.1° (Figure S5), the peak position did not change upon SQ molecules passivation, this implies that SQ molecules incorporation did not influence the perovskite lattice and participate in the formation of MAPbI3 lattice.32 Thus SQ molecules are most likely involved in the passivation on the surface or GBs.

As shown in figure 2a, there are unavoidable I- and MA+ vacancies during the crystal growth in perovskite films, which is critical important for a complete defect passivation to obtain higher performance of PSCs. SQ molecules with zwitterionic structure have separated positive and negative ions centers, which can act as electron acceptors and donors respectively to interact with opposite charges for defects passivation.18 The negative-charged squaraine functional group is expected to interact with under-coordinated Pb2+ originating from halide ions loss during the thermal


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annealing, meanwhile two oxygen atoms of diolate oxygen anion can interact with two Pb atoms in perovskite grains,26, 29 resulting in reduced trap sites. Additionally, the positive charge center in aromatic ring of SQ molecules was proposed to interact and passivate Pb-I antisite defects, compensating the MA+ loss in perovskite host lattice.18 To examine the interaction of perovskite and SQ molecules at the surface of boundary, Fourier transform infrared spectroscopy (FT-IR) was conducted to probe the change in vibration peak of squaraine group when SQ63 molecule was introduced into perovskite film (Figure 2b). Typically squaraine groups do not exhibit C=O stretching vibrations due to the resonance-stabilized zwitterionic form of cyclobutene moiety,33 and as a result, C=C stretching vibrations show the strong absorption peak at 1592 cm-1. The new C=O characteristic peak appeared at 1682cm-1 after dipping 10-5 M PbI2 in SQ63 solution in chlorobenzene (molar ratio PbI2ᵒSQ45=2ᵒ1), which is consistent with previous report proposing that interaction between metal ion and diolate oxygen anion was responsible for the new strong stretching vibration.34 In addition, C=C stretching vibrations shifted to 1587cm-1 due to the existence of bonding of Pb and oxygen anion (inset in Figure 2b), SQ45 and SQ81 also exhibited similar interaction with PbI2 as shown in Figure S7. To


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clarify the properties of the interaction between SQ molecules and perovskite further, the interaction of SQ63-PbI2 was calculated by density functional theory (DFT, Gaussian 09 software, details for the calculation are given in Experimental Section). Considering one SQ63 molecule can coordinate with two PbI2, we first calculated the interaction between SQ63 and one PbI2. As shown in Figure 2c, the optimized geometry demonstrates the interaction of one PbI2 and squaraine group in SQ63, showing that C=O bond length with PbI2 interaction (1.28 Å) is slightly longer than that of another C=O bond length (1.24 Å) without PbI2 interaction. To further visualize the final interaction between SQ63 and two PbI2, the charge distribution of the optimized geometry before and after coordination is shown in Figure S8. The electron density of oxygen in SQ63 and SQ63-PbI2 complex was found to be -0.385 and -0.460 respectively, which can be clearly understood by the increased electronegativity of oxygen after bonding with Pb, resulting in higher electron density in oxygen atoms for the interaction with Pb. It was also found that the electron density in cyclobutene ring decreased, drawing the polarizable π electrons from cyclobutene ring due to the Pb coordination with oxygen atoms. We then mapped the electrostatic potential on the SQ63 molecule surface (Figure 2d), PbI2 coordination draws


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C=O delocalized electrons from cyclobutene ring and neutralized electrons on O atoms due to the Pb-O bond formation, leading to continues charge distribution at SQ63-PbI2 junction. According to the calculation, symmetrical squaraine group could interact with PbI2 strongly, which is expected to passivate defects and reduce trap density on perovskite GBs.


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Figure 2. a) Schematic diagram of SQ molecules with resonance-stabilized zwitterionic structure interacting and passivating the under-coordinated Pb2+ and Pb-I antisite defects to suppress carrier recombination. b) FT-IR spectra of SQ63 and PbI2 complex in


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chlorobenzene, a new C=O stretching peak at 1682cm-1 appears due to the interaction between Pb and O atoms. c) DFT calculation for an optimized geometry of coordination between SQ63 and one PbI2. d) Electrostatic potential mesh maps of SQ63 (left) and SQ63-two PbI2 complex (right). It has been demonstrated that the introduction of semiconducting molecules contributed to efficient charge separation and extraction along perovskite GBs.35 To examine this, steady-state photoluminescence (PL) measurement was performed for the perovskite films with and without SQ passivation as shown in Figure 3a, It can be noted that the PL intensities for SQ45, SQ63 and SQ81-passivated perovskite films were quenched about 50%, 72% and 63% respectively compared to the reference one, suggesting the suppression of radiative recombination as a result of efficient charge separation at the interface between perovskite grains and SQ molecules, allowing for faster charge transport and collection at external electrode.36 Additionally, the PL peaks of SQ45, SQ63 and SQ81-passivated perovskite films were blue-shifted to 766 nm, 768 nm and 768 nm as compared to that of the control sample (770 nm), indicating effective traps passivation after SQ treatment along the perovskite GBs,37-38 which agrees well with the assumption that the coordination between Pb and squaraine oxygen passivates traps.


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Time-resolved PL (TRPL) was further performed to evaluate charge extraction between perovskite and SQ molecules as shown in Figure 3b. TRPL curves were fitted with a biexponential function,39 where the fast (τ1) decay is mainly correlated to charge transfer, and the slow (τ2) decay is caused by radiative emissions of the bulk perovskite film involving trap-assisted recombination.

The PL decay fitting parameters are given in Table S1, the fast decay (τ1) of the SQ45, SQ63 and SQ81-passivated perovskite films decreased from 141.0 ns to 90.0 ns, 60.1 ns and 81.4 ns respectively, strongly indicating that coordination between the two symmetrical O atoms on cyclobutene ring and Pb in perovskite assisted in decreasing defects. This was achieved by efficient passivation along perovskite GBs and simultaneously contribution to the rapid charge transfer at perovskite-SQ interface. The slow decay (τ2) constants, which is related with perovskite carrier lifetime,10 decreased from 562.4 ns in pristine perovskite to 414.2 ns, 245.0 ns and 331.4 ns for SQ45, SQ63 and SQ81 passivation respectively, demonstrating that these three molecules possess excellent charge transfer efficiency. It is worth to note here that SQ63 molecules


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minimized charge losses more efficiently by fast charge separation in comparison with that of SQ45 and SQ81-passivated perovskite films.

Figure 3. a) Steady-state photoluminescence (PL) spectra of perovskite film with and without SQ45, SQ63 and SQ81 passivation respectively on glass substrates and the corresponding b) Time-resolved photoluminescence

The outstanding effects of interaction between SQ and PbI2 are expected to improve perovskite solar cell performance and stability. To demonstrate this potential, we fabricated PSCs in the device configuration of ITO/SnO2/MAPbI3 (with and without SQ)/Spiro-OMeTAD/Au in ambient atmosphere with more than 60% RH condition (Figure 4a). The passivated perovskite films were prepared by spin-coating using dripping ethyl


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acetate (EA) anti-solvent containing SQ45, SQ63 and SQ81 respectively. The photovoltaic performance of perovskite fabricated with various SQ concentrations is shown in Figure S9. The SQ concentration significantly affected the performance of the respective perovskite solar cells and the optimum concentration for all of the three SQ molecules was found to be 10-3 mg/ml. The drop in the efficiency upon more SQ molecules treatment maybe resulted from zwitterion aggregation and led to hampered device performances. The representative current density-voltage (J-V) characteristics of PSCs passivated with different SQ molecules is displayed in Figure 4b. Efficiency of PSCs without SQ passivation was 15.77% with short- circuit current density (Jsc), opencircuit voltage (Voc) and fill factor (FF) of 21.32 mA cm-2, 1.03 V and 71.64% respectively, which is lower than most reported data in the literature since our perovskite fabrication process was carried in ambient environment with about 60% RH. With the corresponding optimum concentration of SQ45, SQ63 and SQ81 molecules in anti-solvent, the perovskite devices performance based on SQ45, SQ63, and SQ81 passivation showed enhanced efficiency of 17.06%, 18.83% and 17.66% respectively (J-V parameters are given in Table 1). All SQ-passivated perovskite devices exhibited higher PCE compared


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with the pristine sample, all of the photovoltaic parameters were found to be increased for final efficiency increment, current density of PSCs increased from 21.32 mA cm-2 for the reference device to 21.55 mA cm-2, 22.22 mA cm-2 and 21.64 mA cm-2 when the perovskite films were passivated by SQ45, SQ63 and SQ81 respectively, which is in agreement with the integrated current densities calculated from the external quantum efficiency (EQE) measurement (Figure 4c). The FF of SQ63-passivated PSCs was boosted up to nearly 80% compared with the pristine sample (71.64%), which may be ascribed to efficient charge transfer and reduced traps.40 The best performing was obtained with SQ63 passivation and to confirm the reproducibility, the histogram comparing the reference and SQ63-passivated solar cells is shown in Figure 4d. It is evident that SQ63 passivation exhibited excellent reproducibility and maximum PCE of 18.83% with Jsc, Voc and FF of 22.22 mA cm-2, 1.07 V and 79.03% respectively, clearly demonstrating the positive contribution of SQ63 to perovskite device performance. The evaluation of the maximum power output of pristine and SQ63-passivated PSCs was carried out under continuous AM 1.5G illumination (Figure 4e). SQ63-treated perovskite devices showed a stable efficiency of 18.5% at 0.90 V under continuous illumination.


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Conversely, the efficiency of about 15.5% for the reference device was observed. From these, we could deduce that SQ45, SQ63 and SQ81 passivation enhanced the PCE of MAPbI3 perovskite in varying degree, and SQ63 passivation enhanced the efficiency of perovskite by a remarkable of 16%.

To further understand the role of SQ molecules in perovskite, we investigated the energy offset between the perovskite and SQ molecules, which is also critically important for charge injection.10 The photoelectron yield spectroscopy (PYS) were performed to estimate the highest occupied molecular orbital (HOMO) of the three SQ molecules as shown in Figure S10. SQ45, SQ63 and SQ81 molecules have the HOMO of -5.52 eV, 5.41 eV and -5.12 eV respectively, as a result, the n-p junction was formed at the perovskite-SQ interface.41 This contributes to the efficient charge separation and extraction near perovskite GBs, which agrees well with the steady-state PL on glass/perovskite+SQ discussed above. Figure 4f shows the energy level diagram of SQ molecules-treated perovskite device, the HOMO of SQ45 possessed energetic disparity of 0.09 eV with the valence band (VB) of MAPbI3 perovskite, which was expected to


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exhibit unimpressive exciton dissociation in comparison with that of SQ63 and SQ81 passivation with 0.02 eV and 0.31eV energetic parity, benefiting efficient charge separation. These results are in agreement with the observed PL quenching discussed above. We speculate that the different impact on the charge extraction along perovskite GBs finally contributed to the photovoltaic performance.


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Figure 4. a) Schematic diagram of the fabricated PSCs having the structure of ITO/SnO2/perovskite (with or without SQ)/Spiro-OMeTAD/Au. b) Current density-voltage characteristics of best performing device based on the pristine and SQ45, SQ63, SQ81 passivation dissolved in ethyl acetate (EA) anti-solvent. c) External quantum efficiency (EQE) spectra and integrated Jsc of pristine and SQ-passivated PSCs. d) Histogram of


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PCE and e) steady-state output power of of PSCs based on the pristine and SQ63passivated PSCs. f) Energy level diagram of the PSCs.

Table 1. Photovoltaic device parameters for the pristine and SQ45, SQ63, SQ81 passivation under optimum SQ concentration.

[a]

Jsc

Jsc[a]

Voc (V)

FF (%)

PCE (%)

(mA cm-2)

(mA cm-2)

Pristine

21.32

21.04

1.03

71.64

15.77

SQ45

21.55

21.13

1.03

76.83

17.07

SQ63

22.22

22.18

1.07

79.03

18.83

SQ81

21.64

21.32

1.05

77.87

17.66

Calculated current density from the IPCE spectrum Thermally stimulated current (TSC) measurement has been used to quantify

the trap density in methylammonium lead iodide perovskite materials.42 Four different perovskite films-pristine perovskite, SQ45, SQ63 and SQ81 passivated perovskite-were cooled to a low temperature below their respective activation energy of electronic trap states, at which point upon light illumination, the trap sites were filled with charge carriers. Subsequently, charge carriers in the traps were released upon heating up to 350K at a constant rate, during which current was gradually generated inside the pristine or SQ-


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passivated perovskite films. The corresponding Arrhenius plot of TSC spectra is shown in Figure S11.

Figure 5a showed the TSC spectra of pristine and three SQ-passivated perovskite films. The electrical current is plotted as a function of the temperature for four different perovskite films, supposing all trap sites were occupied at the starting temperature for simplification, e.g. negligible re-trapping of charge carriers,[42] the number of traps（Nt）are equal to the integration of current over time[43], as described in equation 1.

∫𝐼𝑑𝑡 = 𝑞𝑁

𝑡

(1)

where q is the elementary charge and I denotes the electrical current generated in perovskite film. The trap density can be calculated through the following equation 2.43

𝑁𝑡 = Q/qAL

(2)

where Q refers to the area under the TSC peak, q represents the electronic charge, A is the area of the device, and L is the thickness of the perovskite film. As shown in Figure 5a, introduction of SQ63 inside the perovskite films substantially reduced the overall TSC


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signal, suggesting the reduction of trap density in perovskite films and effective passivation effect of SQ63. We also calculated the trap density from the equation above and the trap density dramatically decreased from 2.75×1017 for the pristine perovskite to 4.69×1016 cm-3 for SQ63 passivated perovskite (all trap density results are given in Figure S12). This results proved that squaraine functional group remediates the defects along perovskite GBs, suppressing carrier recombination and thus contributing to increased FF in SQ-treated perovskite devices.35 In addition, we investigated the impact of SQ molecules treatment on deep traps and shallow traps. Figure 5b shows the trap density distribution in perovskite bandgaps. We calculated the vacuum level of perovskite film according to the following equation 3 supposing the conduction band of MAPbI3 perovskite to be at -3.93 eV.

T4

Evac = ―3.93 ― KTln β

(3)

where β denotes the heating rate employed in TSC measurement, T is the temperature and K is the Boltzmann constant. As shown in Figure 5, the addition of SQ63 has great


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impact in reducing the deeper trap states. It shows that a slight reduction in trap densities at the shallow area near the conduction band was first observed upon the addition of SQ, implying the passivation effect of the SQ molecules. Interestingly, among the SQs, the SQ63 reveals its greatest impact in reducing the deep trap density (red line in Figure 5b), which is nearly 1 order of magnitude lower than that of the pristine perovskite (as observed at the energy level of -4.5 eV to -4.77 eV). This phenomenon has again proven the effectiveness of SQ63 in passivating the defect states, typically the deeper traps, which is in great agreement with the EIS results (discuss below). It is notable that the deep traps are associated with the structural defects originating from interstitial and antisites defects (Pb-I antisites).2 Recent study has demonstrated that the deep traps are mainly originated from the energetically less stable polycrystalline GBs.44 This considerable decreased trap density at the deep area maybe arises from the more efficient passivation using SQ63 functionalized with bromide on the perovskite GBs, leading to decreased interstitial or Pb-I antisite defects within the perovskite film, which has been verified by DFT calculation.45-46 Therefore, we confirmed that SQ63 was a more effective material to passivate GBs and surface of perovskite films compared with that of


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SQ45 and SQ81 passivation. It has been also suggested that the hydrogen bonding interaction between MA+ and aromatic moiety or bromide restricted organic anion from migration.47 We further measured the electrochemical impedance spectroscopy (EIS) of perovskite devices to characterize the charge transfer and carrier recombination. A 0.6 V bias was applied to examine the perovskite solar cells with and without SQ molecules passivation, we estimated the recombination resistance (Rrec) and series resistance (Rs) from the Nyquist plots with diacritical characteristic arcs as shown in Figure S11. We noted the decreased Rs in all three cases of SQ passivation compared with pristine perovskite, suggesting better charge injection from the perovskite into the charge transporting materials.48 Furthermore, the Rrec increased significantly with SQ molecules passivation, which is attributed to the suppressed recombination in perovskite devices.10 These results indeed confirmed that SQ molecules on perovskite GBs contributed to a more efficient charge injection and at the same time mitigated the non-radiative recombination process.


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Figure 5. a) TSC spectra of pristine, S45, SQ63 and SQ81 perovskite, the colored area demonstrates the amount of trap density in corresponding perovskite films. b) Vacuum level of perovskite as a function of trap density and trap density distribution of perovskite with and without SQ molecules passivation. c) Performance stability of the reference cell and SQ-passivated PSCs without encapsulation stored in air with humidity of 60% RH. d) XRD pattern of corresponding perovskite films after 20 days kept in air at 60±5% RH. e) Photographic images of perovskite films with and without SQ molecules passivation exposed to air at 60±5% RH after 20 days


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Recent studies have demonstrated that the defects on perovskite GBs initialized the degradation due to the sensitivity of organometal halide perovskite to moisture and oxygen.49 Therefore, the strong interaction of perovskite films with SQ molecules is expected to enhance the stability of perovskite devices because SQ molecules could inhibit the diffusion of oxygen and moisture through the defects. Ambient stability of perovskite solar cells without sealing was investigated, perovskite solar cells with three SQ molecules passivation showed excellent air stability, retaining 92% of the initial PCE for SQ63-passivated perovskite respectively after 600 hours in air at 60%±5% RH (Figure 5c). In comparison, the pristine PSCs degraded to 50% of its initial performance under the same condition. We have also performed XRD measurement for the perovskite films with and without SQ passivation stored under ambient atmosphere with about 60% RH. The XRD patterns (Figure 5d) confirmed the well-maintained crystal perovskite structure after SQ passivation compared with the pristine sample with observable PbI2 signals in the film, this indicates enhanced moisture stability of perovskite film through passivation by SQ molecules. From Figure 5e, we observed a clear discoloration of the pristine perovskite film after 20 days in ambient atmosphere, whereas the passivated film still


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retained the black perovskite phase. These results clearly demonstrated that SQpassivated perovskite films possess better moisture resistance and much slower degradation of perovskite layer, which confirmed that SQ molecules could effectively reduce trap sites and enhance the moisture stability of photovoltaic devices.

Conclusion

In conclusion, we have synthesized squaraine molecules and proposed a strategy to passivate the defects in perovskite through the coordination between SQ and perovskite grains. SQ with zwitterion structure could simultaneously passivate the undercoordinated Pb2+ vacancies and Pb-I antisite defects between adjacent perovskite grains during the crystal growth, resulting in considerable reduced deep trap density of perovskite as evidenced by TSC measurement. These strong interactions also control the crystal growth to obtain relatively smooth and uniform perovskite film during the annealing process. In addition, SQ63 molecules with better energy alignment yielded more efficient charge transfer and much reduced deep-level trap density, which resulted in considerable enhancement of the PCE from 15.77% for the pristine device to 18.83% with the FF


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approaching 80%. Furthermore, considerable retardation of solar cell degradation was achieved by SQ molecules passivation, retaining 87%~90% of its initial efficiency after 600 h of exposure in air at 60% RH without encapsulation. This work provides a new strategy for SQ molecular design and synthesis with suitable energy level to effectively aid charge transfer and passivate the perovskite GBs, making it possible for realizing highly efficient and stable PSCs.

Experimental Section

Materialsᵒ Methylamine Hydroiodide (MAI, TCI), PbI2 (99.99%, TCI), dimethylformamide (DMF, 99.99%, Sigma-Aldrich), N, N-Dimethyl sulfoxide (DMF, 99.99%, Sigma-Aldrich), Spiro-OMeTAD, 4-tert-butylpyridine (TBP), ITO substrate, SnO2 nanoparticles from Alfa. All chemicals were purchased and used without further purification.

Device fabricationᵒ ITO-coated glass substrates were patterned by etching with zinc powder and HCl solution, then substrates were sequentially sonicated with distilled water, ethanol, acetone and water for 15 minutes in each solvent. Diluted SnO2 colloid precursor (2.67% in water) was spin-coated on glass/ITO after 15 min UV-O3 treatment at 2000 rpm


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for 30s, which was followed by annealing in ambient condition at 150 ᵒC for 30 min. MAPbI3 perovskite precursor solution was prepared by dissolving MAI (1.5M) and PbI2 (1.5M) in mixed solvent DMFᵒDMSO (4ᵒ1). 100 μL of the precursor solution was spincoated on ITO/SnO2 substrate at 4000 rpm for 25s under about in air at 60% RH condition. 0.5 ml of ethyl acetate (EA) with or without SQ45,SQ63 and SQ81 molecules was quickly dropped on the spinning SnO2 substrate, followed by annealing on a hot plate at 100 ᵒC for 10 min. Spiro-OMeTAD in chlorobenzene (72 mg/ml) was spin-coated on ITO/SnO2/ perovskite substrate (4000 rpm for 30s) after cooling down to room temperature. SpiroOMeTAD

was

prepared

with

the

following

additives,

17.5

μL

lithium

bis(trifluoromethane)sulfonimidate (Li-TFSI) in acetonitrile (520 mg/ml) and 28.8 μL 4tertbutylpyridine. Finally, 80 nm gold was deposited by thermal evaporation onto the spiroOMeTAD layer.

Details for DFT calculationᵒ The theoretical molecular orbital calculation was performed using the Time-dependent Density functional Theory program under G09 program package. The squaraine only structure as well the squaraine with the incorporated lead


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iodide (PbI2) were programmed for the theoretical calculation. For the squaraine zwitterions alone, the Linear Spin density Approximation was used (LSDA) which uses the Slater exchange functional and the correlation function for its implication and the basis set 6-311G was employed. It can be clearly visualized by the figure S6 that the computed structure of the SQ alone has the Self Consistent Field (SCF) electron density more concentrated on the oxygen of the central squaric acid moiety. Also, the Mullikan charge distribution was found to be -0.381 on that particular oxygen. On the other hand, the approximation B3LYP and the basis set LANL2DZ was implemented when the PbI2 molecule was introduced to the structure of the SQ zwitterions. B3LYP approximation holds good and reliable in terms of enthalpies of formation. The final SQ-PbI2 complex showed the bond formation between the central core oxygen and metal Pb as given by the increase in the bond length of the carbon and oxygen, which is expected to coordinate with the Pb. The electron density computed by the SCF on the central oxygen was found to be diminished as shown in the figure 2d. Moreover, the Mulliken Charge of on it is found to increase from 0.381 to 0.460, which can be clearly understood by the


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electronegativity of oxygen on bonding to Pb. Therefore, the theory proves the bond formation between the central oxygen and the Pb.

Characterizationᵒ The current density-voltage characteristics of solar cells were measured under AM 1.5G simulated solar illumination. The external quantum efficiencies were measured by illuminating solar cells under monochromatic light from 900 nm to 300 nm (300W Xenon lamp with a monochromator, Newport 74010). Morphology of perovskite film was characterized by SEM (HITACHI S4800). XRD pattern was analyzed by D8 X-ray diffractometer, using Cu Kɑ radiation. Fourier Transform Infrared Spectroscopy (FTIR) measurement was performed by JASCO FTIR 4100 for samples in transmission mode. Reflective FTIR for thin film samples was measured using ATR PRO400-S mode. The steady-state photoluminescence (PL) and time-resolved PL measurement were conducted by the Fluorolog-3-p spectrophotometer by the excitation light. Electrochemical impedance spectra were measured by an electrochemical workstation (Parstat 2273, Princeton) under 0.6V positive bias. Thermally stimulated


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current (TSC) was performed using TS-FET electron trapping measuring system (Rigaku).

ASSOCIATED CONTENT

Contents of corresponding measurements were supplied as Supporting Information

Detailed synthesis and characterization of SQ molecules, absorbance, XRD and atomic force microscopy (AFM) images, energy dispersive X-ray spectroscopy (EDS) mapping, Fourier Transform Infrared Spectroscopy (FT-IR), charge distribution by DFT calculation, J-V curves, Arrhenius plots of thermally stimulated current (TSC), Nyquist plots, values of trap density and carrier lifetime.

AUTHOR INFORMATION

Corresponding Author

E-mail: [email protected] ; [email protected]

Notes There are no conflicts to declare.


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ACKNOWLEDGMENT

This research was supported by Graduate School of Life Science and System Engineering, Kyushu Institute of Technology

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Table of Contents Graphic


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Passivation of Grain Boundary by Squaraine Zwitterions for Defect

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