Self-Seeding Growth for Perovskite Solar Cells with Enhanced Stability

Self-Seeding Growth for Perovskite Solar Cells with Enhanced Stability

Article Self-Seeding Growth for Perovskite Solar Cells with Enhanced Stability Fei Zhang, Chuanxiao Xiao, Xihan Chen, Bryon W. Larson, Steven P. Harv...

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Article

Self-Seeding Growth for Perovskite Solar Cells with Enhanced Stability Fei Zhang, Chuanxiao Xiao, Xihan Chen, Bryon W. Larson, Steven P. Harvey, Joseph J. Berry, Kai Zhu [email protected]

HIGHLIGHTS Self-seeding growth (SSG) to realize high-quality perovskite films are developed SSG devices show improved efficiency from 17.76% (control) to 20.30% (SSG) Unsealed devices show better stability under high humidity, heat, and illumination SSG approach can be applied to different substrates and perovskite compositions

A general approach—self-seeding growth (SSG)—that utilizes a typical one-step perovskite precursor ink formulation to create perovskite active layer is reported and can be applied to different substrates and perovskite compositions for preparing high-quality perovskite thin films. Compared to the standard one-step solution-deposited devices, SSG perovskite thin films exhibit reduced defect density, fewer apparent grain boundaries, and improved charge-carrier transport and lifetime. The SSG devices present much improved performance along with better stability under high humidity, heat, and sun illumination.

Zhang et al., Joule 3, 1452–1463 June 19, 2019 ª 2019 Elsevier Inc. https://doi.org/10.1016/j.joule.2019.03.023

Article

Self-Seeding Growth for Perovskite Solar Cells with Enhanced Stability Fei Zhang,1 Chuanxiao Xiao,2 Xihan Chen,1 Bryon W. Larson,1 Steven P. Harvey,2 Joseph J. Berry,2 and Kai Zhu1,3,*

SUMMARY

Context & Scale

Hybrid organic-inorganic lead halide perovskite solar cells have shown a remarkable rise in power conversion efficiency over a short period of time; however, long-term stability remains a key challenge hindering the practical application of these cells. Here, we report an approach to sequentially apply a typical one-step solution formulation—self-seeding growth (SSG)—to realize highquality perovskite thin films with reduced defect density, fewer apparent grain boundaries, improved charge-carrier transport and lifetime, and enhanced hydrophobicity for enhanced stability. Using FA-MA-Cs-based perovskite, SSG devices showed improved efficiency from 17.76% (control) to 20.30% (SSG), with an unencapsulated device retaining >80% of its initial efficiency over 4,680-h storage in an ambient environment with high relative humidities. The SSG devices also exhibited much improved thermal and operational stabilities. In addition, SSG can be applied to different substrates and perovskite compositions, which makes it a viable method for preparing high-quality perovskite thin films for device applications.

The issue of poor long-term stability against moisture is still a key challenge hindering perovskite solar cells for practical applications. Here, we report an approach to sequentially apply a typical one-step solution formulation—self-seeding growth (SSG)—to realize high-quality perovskite thin films with reduced defect density, fewer apparent grain boundaries, improved charge-carrier transport and lifetime, and enhanced hydrophobicity for enhanced stability. Using FA/MA/Cs-based perovskite, SSG devices showed improved efficiency from 17.76% (control) to 20.30% (SSG), with an unencapsulated device retaining >80% of its initial power conversion efficiency over 4,680-h storage in an ambient environment with high relative humidity. In addition, SSG can be applied to different substrates (e.g., SnO2 versus TiO2; planar versus mesoporous) and perovskite compositions, making it a viable method for preparing high-quality perovskite thin films for device applications.

INTRODUCTION Hybrid organic-inorganic lead halide perovskite solar cells (PSCs) have shown a remarkable rise in power conversion efficiency (PCE) from the first-reported 3.8% in 2009 to the certified 23.7% in 2018.1–4 The outstanding photovoltaic performance of PSCs originates from the excellent photophysical and chemical properties of perovskite absorbers, which exhibit tunable band gaps, high absorption over the visible spectrum, long charge-carrier diffusion lengths, low exciton-binding energies, and low non-radiative recombination losses.5–9 The issue of poor long-term stability against moisture is still a key challenge hindering PSCs for practical applications.10 Much effort devoted to improving the stability of PSCs has yielded impressive progress during the past several years, such as compositional engineering,11,12 interfacial modification,13 additives processing,14 tolerance factor adjustment,15 perovskite nanostructure,16 contact layer development,17,18 encapsulation,19 defects passivation,20,21 and self-second growth by the Ostwald ripening phenomenon.22 With all the advances in these systems, the defect tolerance of the metal halide active layer is viewed as enabling solar cell performance23; however, improvement in the details of the active layer clearly impact the optoelectronic properties.24 In this paper, we report a general approach—self-seeding growth (SSG)—that utilizes a typical one-step perovskite precursor ink formulation to create perovskite active layer and tune the formation of these thin films as illustrated in Figure 1. The use

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of processing and templating, seed layers, or other approaches to aid in the control of the structure to impact optoelectronic properties is a common theme in many semiconductor systems.25–27 In the last few years, perovskite seed crystals have also been examined in the initial PbI2 film to facilitate perovskite crystallization during two-step sequential deposition of perovskite thin films28 or in printing approaches to selectively create millimeter-sized perovskite single crystal.29 However, in the case of preparing perovskite thin films, the seeding technique only works using colloidal seeds added to the PbI2 solution, and the crystalline perovskite seeds fail to form in the PbI2 film if powder-form precursors are mixed within the PbI2 solution.28 We show that SSG can be applied to different substrates (e.g., SnO2 and TiO2; planar and mesoporous) and perovskite compositions (e.g., triple-cation formamidiniummethylammonium-cesium (FA-MA-Cs), double-cation FA-MA, and single-cation MA-based perovskites), making it a general approach for preparing high-quality perovskite thin films. Compared to standard one-step solution-deposited devices, SSG perovskite thin films exhibit reduced defect density, fewer apparent grain boundaries (GBs), improved charge-carrier transport and lifetime, and enhanced ability to repel moisture. Using FA-MA-Cs-based perovskite, the PCE increases from 17.76% for the control to 20.30% for the SSG device. In addition, SSG-based PSCs exhibit much improved robustness against high humidity without any encapsulation. When stored in ambient environment, the PCE showed no degradation after 720 h in 10%–20% relative humidity (RH), then remained at 96% of the initial PCE after 1,440 h in 30%–50% RH, and finally dropped to 84% of the initial PCE after another 2,520 h in 50%–75% RH (total 4,680 h of test). The unencapsulated SSG devices also showed much improved thermal and operational stabilities in 10%–30% RH by maintaining 85% of initial PCE after 1,200 h at 80 C under dark and maintaining 81% of initial PCE after 580 h with maximum power point tracking under one-sun illumination.

RESULTS AND DISCUSSION To create the seeds to facilitate perovskite film growth, we repeated the standard coating process with a typical one-step precursor solution based on FA/MA/Cs perovskite composition. We refer to this repeated coating-growth process as SSG; in this study, we use SSG1–SSG4 to indicate that SSG is repeated 1–4 times. Figure 1 schematically illustrates the SSG process for growing perovskite thin films. The triple-cation perovskite precursor consists of CsI, FAI, MABr, PbI2, and PbBr2 in a mixed solvent of dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO) with 1.4 M Pb2+. Then, 60 mL of as-prepared precursor solution is spin coated on mesoporous TiO2-coated fluorine-doped tin oxide (FTO) glass substrates. During the last 15 s of the spin-coating step, 140 mL of chlorobenzene (CB) is dropped onto the above film to promote the formation of an intermediate phase. Without transferring the film to the hot plate for annealing, another 60 mL of as-prepared precursor solution is spin coated on the film, followed by the same antisolvent treatment again. This process can be repeated for multiple times before conducting the annealing at an elevated temperature. During the SSG process, we hypothesize that the perovskite film will regrow based on a self-seeding process, leading to improved structural and optoelectronic properties. To observe the impact of the self-seeding process, we performed a series of analyses. First, we examined the surface potential and morphology by scanning Kelvin probe force microscopy (KPFM), with results shown in Figure 2. We prepared pristine TiO2, and on the same film, we deposited the SSG2 (SSG process repeated twice) perovskite without heating; we then washed it by the same amount of

1Chemistry

and Nanoscience Center, National Renewable Energy Laboratory, Golden, CO 80401, USA

2Materials

Science Center, National Renewable Energy Laboratory, Golden, CO 80401, USA

3Lead

Contact

*Correspondence: [email protected] https://doi.org/10.1016/j.joule.2019.03.023

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Figure 1. Schematic Illustration of Perovskite Crystallization Growth Process SSG, self-seeding growth; CB, chlorobenzene.

DMSO/DMF as used in perovskite deposition. Figures 2A and 2B show that both samples have the same morphology at the same location. However, the corresponding KPFM images (Figures 2C–2E) show that there is a substantial difference in the surface potential between these two samples. We observed a significant decrease of the KPFM potential value after the perovskite precursor coating and washing process. We compared line scans in the atomic force microscopy (AFM) images in Figures 2A and 2B, taking the morphology along with coincident profiles of the potential images in Figures 2D and 2E to quantitatively evaluate the potential change. Figure 2C shows that the mean of potential profile changes from 800 to 400 mV between these two samples, with some degrees of fluctuation. To map the inhomogeneity of the potential change, we used the potential distribution in Figure 2D to subtract that in Figure 2E to generate the change of the potential distribution as shown in Figure 2F. Note that because there was a shift before and after washing of the SSG2 film, the images in Figures 2D and 2E are not exactly for the same location. To compensate for the shift, we selected a matching sub-section from the images and subtracted the potential within these images, pixel by pixel; the selected region is marked by the red dashed-line rectangles in Figures 2A, 2B, 2D, and 2E. In Figure 2F, the pseudocolor image shows the non-uniformity of potential change, where the green background represents a potential value of 300 mV and the other color represents higher potential value. The KPFM tip used is Pt-Ircoated and the tip work function is 5.1 eV.30 KPFM measures the contact potential difference between the tip and sample, and the sample surface work function is

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Figure 2. Atomic Force Microscopy and Kelvin Probe Force Microscopy Measurement (A) AFM image of pristine TiO2 . Scale bar, 200 nm. (B) AFM image of SSG2 perovskite thin films without heating and followed by washing with DMSO/DMF. Scale bar, 200 nm. (C) Line profile of KPFM. (D) KPFM surface potential image of pristine TiO2 . Scale bar, 200 nm. (E) KPFM surface potential image of SSG2 perovskite thin film without heating and followed by washing with DMSO/DMF. Scale bar, 200 nm. (F) The subtracted figure of (D)–(E). Scale bar, 200 nm.

obtained from the tip work function subtracting the measured potential value. So, the work function of pristine TiO2 is 4.3–4.4 eV, which is consistent with a previous report.31 In contrast, the work function of the washed SSG2 sample is 4.6–4.7 eV. The increase of work function indicates that some perovskite is left over on the TiO2 surface (however, the AFM topography does not change), which we hypothesize provides seeds for subsequent perovskite crystallization—and hence we call this process self-seeding. Note that when only the solvent is used, the amount of residual perovskite materials is expected to be lower than when the perovskite precursor solution is used during the SSG process. However, this does not affect the conclusion regarding the SSG process. The potential impact of the precursor concentration on the SSG process and device characteristics will be discussed in the device section. We also performed time-of-flight secondary ion mass spectrometry (TOF-SIMS)32 to identify the distribution of elements in the washed SSG2 sample on FTO/ compact-TiO2/mesoporous-TiO2 substrates. Figure S1 provides clear evidence of residual perovskite elements (e.g., Pb, Br, and Cs) left on the substrate, which is consistent with the KPFM result. To gain insight into optoelectronic properties resulting from the self-seeding effect in the perovskite films, we performed transient absorption (TA)33,34 and

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Figure 3. Comparison of the Control and SSG2 Perovskite Thin Films (A) Normalized transient absorption kinetics at the center of the ground-state bleach. (B) Time-resolved microwave conductivity measurement. (C) Space-charge-limited current measurement based on the ‘‘hole-only’’ geometry (indium-doped tin oxide [ITO] glass/MoO x /perovskite/MoOx /Al). (D) X-ray diffraction patterns.

time-resolved microwave conductivity (TRMC) measurements on these samples. Figures 3A and S2 display the normalized TA kinetics probed at center of the ground-state bleach (760 nm) after 520 nm excitation and the pseudocolor image of the corresponding perovskite films. To extract the lifetime of charge carriers, the transient kinetics was modeled with a bi-exponential decay function; the result is shown in Table S1. Two decay components can be obtained for the pristine control film: the fast component (25 ns) and the slow component (160 ns). Irrespective of the origins of the decay kinetics, after SSG treatment, the samples exhibited longer lifetime for both components. In particular, the SSG2 sample showed a 33 increase in lifetime for both the fast component (83 ns) and slow component (543 ns) as shown in Figure S2. The reduced lifetime of SSG3 and SSG4 was presumably caused by the over-application of solvent, which has a negative impact on crystallization35,36 and will be discussed more in the device section. The improved lifetime is consistent with a reduction in defects resulting from an enhancement in crystallinity with a commensurate reduction in the number of apparent GBs, which, in turn, reduces the number of trapped charge carriers. This should slow the recombination of free carriers, representing a longer carrier lifetime, which could account for the increased open-circuit voltage (Voc) and fill factor (FF).37 TRMC decay data are presented in Figures 3B and S3 and fitted to bi-exponential fittings; the average lifetime and mobility values are summarized in Table S2. Similar to the trend observed in TA measurement, TRMC results showed that SSG1 improved both carrier mobility and lifetime (Figure S3). The improvement was maximized for the SSG2 sample. With SSG3 and SSG4, the carrier mobility and lifetime were reduced in comparison to the SSG2 sample, but they were still better than the control sample. The improved carrier mobility and lifetime further confirmed

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the SSG effect on reducing trap density in perovskite films, which again is consistent with the higher Voc, short-circuit current density (JSC), and FF of the corresponding PSCs. To assess the SSG impact on trap density, we conducted the space-charge-limited current (SCLC) measurement of the corresponding perovskite films, with details shown in the Supplemental Information. The current-voltage (I-V) characteristics of the different devices are shown in Figures 3C and S4. SCLC is commonly used to determine the hole and electron mobilities of perovskites in different device architectures. Both electrical conductivity and defect densities can be obtained by simply scanning I-V curves in the dark. It is worth noting that the SCLC method is generally applicable to perovskites with intrinsic charge densities lower than 1014 cm 3, and therefore it is more suitable for the measurement of perovskite single crystals.38 However, it provides a relative measure of the trap densities, and we can estimate the relative change of trap densities for the corresponding perovskite films. We calculated the trap-state density (Nt) using the trap-filled-limit voltage equation Nt = 233 0 VTFL =qd 2 according to a previous report.39–41 The hole trap density and electron trap density of SSG2 perovskite film (1.4 3 1016 and 4.7 3 1015 cm 3, respectively) are substantially lower than the control (2.1 3 1016 and 6.4 3 1015 cm 3, respectively), which is consistent with the TA and TRMC results as well as the higher Voc and FF in solar cells. The decrease of defect density with the SSG process is further confirmed by a reduced dark carrier density as measured by dark microwave conductivity (DMC) for SSG2 perovskite sample (nd = 6.0 3 1015 cm 3) in comparison to pristine control perovskite film (nd = 9.7 3 1015 cm 3), as shown in Figure S5. Thus, the combination of these measurements then provides clear indication of a reduced trap density after the SSG process. The X-ray diffraction (XRD) measurements were performed to study the crystalline structure of perovskite films on FTO/compact-TiO2/mesoporous-TiO2 substrates (Figures 3D and S6). The XRD patterns exhibited similar, strong, and sharp perovskite characteristic peaks. The intensity of the SSG perovskite films at the (110) peak (14.02 ) became stronger, and the full width at half maximum (FWHM) was decreased when compared with the pristine perovskite film, which also suggests that the SSG process can improve the crystallization of perovskite and/or provide a more preferred (110) orientation as shown in the 2D XRD measurement (Figure S7). The SSG2 perovskite film shows the strongest intensity and smallest FWHM. We attribute the improved crystallization process to the self-seeding effect of SSG on the crystal growth. The reduced intensity and increased FWHM of SSG3 and SSG4 compared to SSG2 is consistent with the result of TA and TRMC and is presumably caused by the over-application of solvent with negative impact to the TiO2 substrate for perovskite growth, which will be discussed in detail in the device section. We ascribe the high-quality crystallization as one of the main factors for improved device performance.42 Scanning electron microscopy (SEM) images of the corresponding perovskite films deposited on FTO substrate are presented in Figures 4 and S8. As illustrated from the top-view SEM, the change in morphology is consistent with an increase in the homogeneity of the active layer in cross-section view and an increase of the apparent grain size from the top view for the SSG perovskite film. There are few inhomogeneities in the SSG cross-section, which we label GBs here for simplicity43; most of these GBs exhibit a perpendicular orientation to the substrate. We further compared the SEM and XRD of the coated perovskite films without heating. Figure S9 shows that the SSG2 sample also presents larger grain size and greater crystallinity than

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Figure 4. Perovskite Film Morphology Comparison (A) Top-view scanning electron microscopy (SEM) image of control perovskite thin films. Scale bar, 1 mm. (B) Cross-sectional view SEM image of control perovskite devices. Scale bar, 500 nm. (C) Top-view SEM image of SSG2 perovskite thin films. Scale bar, 1 mm. (D) Cross-sectional view SEM image of SSG2 perovskite devices. Scale bar, 500 nm.

the control sample without heating, which is consistent with the result for perovskite films prepared with heating. To check whether the SSG process depends on the type of substrate, we compared the SEM and XRD of samples deposited on FTO/ compact-TiO2 and FTO/SnO2 nanoparticle substrates. Figures S10 and S11 show that the SSG2 samples also have larger apparent grain size, fewer GBs, and greater crystallinity than the control samples for both substrates, while the devices also show some obvious improvement. These results indicate that the SSG process is a general approach for various types of substrates. We also checked the SEM for perovskite thin films deposited on FTO/compact-TiO2/mesoporous-TiO2 substrates with and without washing by DMF/DMSO solvent, and the results are shown in Figure S12. No obvious changes were observed in the apparent grain sizes and morphologies, suggesting that the improved crystallization process was caused by the self-seeding effect not by the residual solvent. The cell architecture in this study adopted the traditional mesoporous structure with a full device stack Au/spiro-OMeTAD/perovskite/mesoporous-TiO2/compact-TiO2/ FTO/glass. The typical photocurrent density-voltage (J-V) curves of the control and SSG2 PSCs under AM 1.5 G illumination with the light intensity of 100 mW/cm2 are shown in Figures 5A and 5B with a full comparison of SSG1–SSG4 PSCs shown in Figure S13; all the corresponding photovoltaic parameters are summarized in Table S3. The control device delivers a PCE of 17.76%, with Voc of 1.12 V, Jsc of 21.98 mA/cm2, and FF of 0.72, which is consistent with our previous report.17 In contrast, SSG1, SSG2, SSG3, and SSG4 perovskite-based devices displayed better performance with PCEs of 19.06%, 20.30%, 19.74%, and 18.83%, respectively. We ascribed the improvement in Jsc, Voc, and FF to primarily be due to higher mobility, longer lifetime, lower trap density, and fewer boundaries produced by the SSG process.44,45 It is worth noting that the SSG process also significantly reduces the hysteresis of PSCs. The hysteresis in PSCs is generally reported to be caused by the charge

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Figure 5. Solar Cell Performance Comparison (A and B) Photocurrent density-voltage curves of perovskite solar cells based on the control (A) and SSG2 (B) perovskite thin films. (C) External quantum efficiency (EQE) spectra with integrated current curves of the corresponding devices. (D) Stabilized power output (SPO) of the corresponding devices.

trap states in the bulk or interfaces, mobile charged ions, and ferroelectricity, which are related to grain size, GBs, and defect density.46,47 The SSG perovskite films have the larger grain size, lower defect density, higher carrier mobility, and longer lifetime, which are expected to result in more negligible hysteresis. Figure 5C shows external quantum efficiency (EQE) spectra of the corresponding PSCs. The integrated current densities estimated from the EQE spectra are 21.4 and 22.8 mA/cm2 for control pristine and SSG2 perovskite-based solar cells, respectively. These are in good agreement with the Jsc values obtained from the J-V curves. The stabilized power outputs (SPOs) from the control and SSG2 PSCs are 17.01% and 20.20%, respectively (Figure 5D), which are consistent with the J-V measurements. The reproducibility of the device performance was evaluated by characterizing about 20 cells. Histograms of the PCE parameters of these devices (Figure S14) indicate good reproducibility. To obtain more insight about the SSG process, we further examined the impact of varying the loading time and the concentration of precursor solution on the device characteristics. The standard loading time of SSG process is about 0 s, i.e., spin coating immediately after loading the precursor solution on the substrate. As the loading time was increased from about 0 to 60 s, the device performance was slightly but systematically decreased (Figure S15). For the longer loading time, the seeds will likely be dissolved and decreased, leading to poorer device performance. Consistent with the loading time dependence study, the precursor solution concentration also affects the SSG process and the corresponding device characteristics. As shown in Figures S16 and S17, the device performance reaches the maximum with SSG3 for the 1.2 M precursor and SSG1 for the 1.6 M precursor. In contrast, it is optimum with

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SSG2 for the 1.4 M precursor as discussed in connection with Figures 5 and S14. Taken together, these results suggest that a balance between dissolution and crystallization is needed for the optimum device performance. During the SSG step, some residual perovskite materials will be left after depositing the perovskite precursor and will work as the heterogeneous nucleation sites to improve the subsequent perovskite crystallization. The energy barrier for the crystal growth with seeds is thermodynamically lower than that without seeds.28 Subsequent SSG steps can affect the residual perovskite materials or seeds. The grain morphology is expected to be affected by the nucleation density or seed concentration during the crystallizing process.48 Regardless of the precursor solution concentration, the device performance dropped after the optimum number of repeated coatings, which is attributed to the over-application of the solvent. To confirm this, we examined the impact of over-application of the neat solvent on the device characteristics (Figure S18). To mimic the SSG process, the solvent was repeatedly applied to the TiO2/FTO/glass substrate before coating the perovskite precursor. After applying the solvent once, the PCE of the corresponding devices almost does not change, which further confirmed that the improved PCE was not resulting from the residual solvent. After applying the solvent three times before coating of the perovskite precursor, the PCE of the resulting device was significantly decreased. These results suggest that the over-application of the solvent could have a negative impact to the TiO2 substrate for perovskite growth. In addition to the triple-cation perovskite composition, we further studied two additional common perovskite compositions (single cation: MAPbI3 and double cation: [FAI]0.81[PbI2]0.85[MABr]0.15[PbBr2]0.15) based on the SSG approach. Figure S19 shows that these devices both displayed improved PCE and reduced hysteresis compared to the control, indicating the general applicability of this approach for a broader range of perovskite compositions. Humidity is known to be a challenging issue for the long-term stability of PSCs.49 Figure 6 shows the stability tests of unencapsulated PSCs under the ISOS-D-1 shelf conditions but with the ambient environment held at different relative humidity levels. The unencapsulated SSG2-based PSC maintained its initial PCE after 720 h in 10%–20% RH, then 96% of the initial PCE after an additional 1,440 h in 30%–50% RH, and finally dropped to 84% of the initial PCE after a subsequent 2,520 h test in 50%–75% RH. In contrast, the unencapsulated control PSC showed PCE degradation to 98%, 90%, and 51% of the initial value after sequential tests in 10%–20% RH for 720 h, then in 30%–50% RH for 1,440 h, and finally in 50%–75% RH for 2,520 h, respectively. In addition, we further conducted stability tests under ISOS-L-1 (ambient environment of 10%–30% RH at 45 C–50 C) of the control and SSG2 PSCs under an open-circuit load condition, without encapsulation. Figure S20 shows that the SSG2 PSC maintained 80% of its initial PCE after 576 h, whereas the control PSC maintained only 32% of its initial PCE under the same test conditions. The stability under continuous maximum power point (MPP) tracking also showed the same trend (Figure S21): The SSG2 device maintained 81% of its initial PCE after 580 h under one-sun illumination with MPP tracking, whereas the control device displayed more than 70% degradation under the same conditions. For the thermal stability comparison, an unencapsulated SSG2 PSC maintained 85% of its initial PCE after 1,200 h of 10%–30% RH at 80 C under dark, whereas the control PSC only maintained 53% of its initial PCE under the same test conditions (Figure S22). This significant improvement in the SSG2 PSC is consistent with improved perovskite properties such as enhanced hydrophobicity (Figure S23), larger grain size, fewer GBs, and reduced trap density.14,21,50–53

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Figure 6. Long-Term Moisture Stability Stability comparison of perovskite solar cells based on the control and SSG2 perovskite thin films with normalized PCE (A), J sc (B), FF (C), and V oc (D). These devices were not encapsulated and were held in dark at room temperature in ambient environment of different relative humidity (RH, as indicated).

CONCLUSIONS In summary, we have demonstrated PSCs with simultaneously improved device performance and stability by using a solution-deposition approach based on SSG of high-quality perovskite thin films. The optimization of this SSG process depends on both the concentration of the starting precursor solution and the loading time of subsequent depositions. This process can be applied to a broad range of different substrates and perovskite compositions, and the resulting films exhibit substantially improved optoelectronic properties with a decreased defect density, fewer apparent GBs, enhanced charge-carrier transport and lifetime, and decreased vulnerability to moisture. These beneficial factors lead to much improved device efficiency as well as enhanced stability under high humidity, heat, and light illumination. This promising approach provides a simple route for fabricating highly efficient and stable PSCs.

EXPERIMENTAL PROCEDURES Full details of experimental procedures can be found in the Supplemental Information.

SUPPLEMENTAL INFORMATION Supplemental Information can be found online at https://doi.org/10.1016/j.joule. 2019.03.023.

ACKNOWLEDGMENTS The work was supported by the U.S. Department of Energy (DOE) under contract no. DE-AC36-08GO28308 with Alliance for Sustainable Energy, Limited Liability

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Company (LLC), the Manager and Operator of the National Renewable Energy Laboratory. We acknowledge the support on perovskite synthesis and device fabrication and characterization from the De-risking Halide Perovskite Solar Cells program of the National Center for Photovoltaics, funded by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Solar Energy Technologies Office, and support on the transient absorption study from the Center for Hybrid Organic Inorganic Semiconductors for Energy (CHOISE), an Energy Frontier Research Center funded by the Office of Basic Energy Sciences, Office of Science within the U.S. Department of Energy. We thank Mr. He Zhao from Tianjin University for the SCLC test and contact angles test. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. government.

AUTHOR CONTRIBUTIONS F.Z. designed the experiments, carried out the experimental study on device fabrication, and performed basic characterization. C.X. performed the AFM and KPFM tests. X.C. performed the TA and analyzed the data. B.W.L. performed the TRMC and DMC and analyzed the data. F.Z. performed SEM, XRD measurements, and stability tests. S.P.H. performed the TOF-SIMS. J.J.B. performed supplemental XRD measurements. F.Z. and K.Z. wrote the first draft of the paper. All authors made a substantial contribution to the discussion of the content and reviewed and edited the manuscript before submission. K.Z. supervised the project.

DECLARATION OF INTERESTS The authors declare no competing interests. Received: January 8, 2019 Revised: March 4, 2019 Accepted: March 24, 2019 Published: April 23, 2019

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