Blade-Coated Hybrid Perovskite Solar Cells with Efficiency > 17%: An

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Blade-Coated Organolead Triiodide Perovskite Solar Cells with Efficiency >17%: An In Situ Investigation Yufei Zhong, Rahim Munir, Jianbo Li, Ming-Chun Tang, Muhammad R. Niazi, Kui Zhao, and Aram Amassian ACS Energy Lett., Just Accepted Manuscript • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 26, 2018

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ACS Energy Letters

Blade-Coated Organolead Triiodide Perovskite Solar Cells with Efficiency >17%: An In Situ Investigation Yufei Zhong,1 Rahim Munir,1 Jianbo Li,2 Ming-Chun Tang,1 Muhammad R. Niazi,1 Kui Zhao,2* Aram Amassian1* 1

King Abdullah University of Science and Technology (KAUST), KAUST Solar

Center (KSC), and Physical Sciences and Engineering Division, Thuwal 23955-6900, Saudi Arabia. E-mail: [email protected] 2

Key Laboratory of Applied Surface and Colloid Chemistry, National Ministry of

Education; Shaanxi Key Laboratory for Advanced Energy Devices; Shaanxi Engineering Lab for Advanced Energy Technology, School of Materials Science and Engineering,

Shaanxi

Normal

University,

Xi’an

710119,

China.

E-mail:

[email protected] Yufei Zhong, Rahim Munir and Jianbo Li contributed equally to this paper.

Abstract Blade coating has recently emerged as a scalable fabrication method for hybrid perovskite solar cells, but it currently underperforms spin-coating, yielding power conversion efficiency (PCE) of ∼15% for CH3NH3PbI3 (MAPbI3). We investigate the solidification of MAPbI3 films in situ during spin/blade-coating using optical and x-ray scattering methods. We find the coating method and conditions profoundly influence 1

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the crystallization process, which proceeds through intermediate crystalline solvates. The polymorphism and composition of the solvates are mediated by solvent removal rate dictated by process temperature in blade-coating. Low to intermediate temperatures (25-80oC) yield solvates with differing compositions and yield poor PCE (~5-8%) and a large spread (±2.5%). The intermediate solvates are not observed at elevated temperatures (>100oC) pointing to direct crystallization of the perovskite from the solgel ink. These conditions yield large and compact spherulitic domains of perovskite and improve the PCE to ∼13-15% with narrower spread (100oC), significantly improving the average PCE (∼13-14%). Coating at even higher temperatures (150oC) yields average PCE >17% despite resulting in PbI2 formation through in situ decomposition of MAPbI3 during bladecoating. This study adds significant new insight into the film formation process that complements several other in situ studies of coating and annealing processes, underscoring the importance and need for these methods in improving the PCE of perovskite solar cells.32-41

We begin by assessing the impact of the coating process, namely spin-coating vs. bladecoating, on the solidification behavior of the common perovskite ink at room temperature. To do so, we investigate in situ the spin-coating and blade-coating processes of the same ink formulation (MAPbI3 in DMF) using time-resolved grazing incidence wide angle x-ray scattering (GIWAXS) and optical reflectometry. In Figure S1, we show selected 2D-GIWAXS snapshots taken with a 0.1 s time resolution at different stages of spin-coating (t = 1, 10, 25, 30 s) and blade-coating (t = 1, 300, 400, 480 s). Figures 1a and 1b present time-resolved GIWAXS intensity maps summarizing the time-evolution of scattering features of the MAPbI3 ink in terms of q ( 𝑞𝑞 =

2 + 𝑞𝑞 2 ) during spin-coating and blade-coating experiments, respectively. The �𝑞𝑞𝑥𝑥,𝑦𝑦 𝑧𝑧

scattering halo at low q values (~1-7 nm-1) early in the coating process is associated to

the colloidal state of the sol-gel precursor (Figures S1 & 1a,b), as previously observed.22, 42

The scattering halo at high q in Figure 1b is associated to the liquid phase solvent 5



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scattering. The emergence of sharp diffraction peaks with q values between 4-8 nm-1 is attributed to the collapse of the sol-gel phase [noted by the disappearance of the colloidal and liquid phase scattering features at low and high q values (Figure 1b)] into formation of one or more crystalline precursor-DMF solvates.22 Diffraction features of PbI2 (110) and MAPbI3 (110), known to appear at ~9.2 nm-1 and 10.0 nm-1, respectively, are extremely weak or absent, proving the as-cast film crystallizes into intermediate solvated phases at room temperature. The measurement time-frame for in situ experiments was chosen according to the time it took for solidification to occur and reach a steady-state, as indicated by the formation and stabilization of diffraction features. This occurred very rapidly during spin-coating, beginning at ~16.5 s after the experiment started and stabilizing within ~1.5 s. Solidification started much later for blade-coating case (~250-300 s) and the ink continued to dry even after 400 s, as indicated by the coexistence of sharp diffraction features (solvate) with the scattering halo at low q (colloidal precursor). The scattering intensity vs. q (azimuthal integration) of as-cast films prepared by both techniques is plotted in the low-q range in Figure 1c. Comparison with the equilibrium MAPbI3-DMF solvate phase obtained by exposing MAPbI3 to DMF vapors reveals a diffraction peaks triplet (Table S1),43 shifted to lower q values in as-cast films as compared to vapor solvated films. Blade-coating appears to yield two sets of triplets, both shifted to lower q values compared to the equilibrium and spin-cast solvates (Table S1). The coating process influences the polymorphism of the MAPbI3-DMF solvate and produces two co-existing polymorphs in blade-cast films. The shift to lower q 6


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ACS Energy Letters

values hints at a unit cell expansion of the solvate, suggesting more solvent is retained in as-cast solvates and particularly in blade-coated films.

Figure 1. In situ diagnostics performed during spin-coating and blade-coating of hybrid perovskite films at room temperature. In situ GIWAXS performed (a) during spincoated and (b) during blade-coating. (c) Intensity vs. q plots of the low-q region corresponding to the crystalline MAPbI3-DMF solvates integrated from an in situ GIWAXS snapshot taken near the end of the coating process. (d) In situ thickness measurements of the blank DMF solvent and perovskite ink performed (d) during spincoating and (e) during blade-coating. (f) TGA analysis of as-cast perovskite films scraped off the substrate immediately after spin-coating and blade-coating experiments. The structural differences in solvates prepared by the two methods are not easy to explain. However, they appear to be correlated to differences in solvent drying and retention rates indicated in Figures 1a and 1b. Differences in the evaporation rate of the solvent are expected in stationary vs. rotating substrates. In situ optical reflectometry was used to monitor the thinning behavior of blank solvents and inks. Figure 1dreveals rapid thinning of the spin-cast solvent within ~7 s, nearly half the time needed for the 7



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perovskite ink to dry (~15 s). The difference in thinning behavior is likely due to the higher viscosity of the ink with respect to the blank DMF and to the presence of solventsolute interactions, which promote solvent retention. The blade-coated solvent and ink dry more slowly. The blank solvent starts off thinner (~4-5 µm) and takes ~15 s to dry, yielding an overall drying rate three times slower than spin-cast one. The loaded solution starts off substantially thicker (~12 µm), most likely due to higher viscosity of the ink, and requires more time to dry (~30 s), resulting in a ~1.5 µm thickness of the as-cast film. The film is still highly disordered and heavily solvated, as revealed by in situ GIWAXS (Figures 1 and S1), whereas the spin-cast film forms a crystalline solvate immediately. We further confirm the differences in solvent retention by performing thermogravimetric analysis (TGA) on as-cast films scraped from the substrate (Figure 1f). We observe a slower rate of solvent loss and a smaller overall amount of solvent removal from spin-cast films as compared to blade-coated ones. The spin-cast film exhibiting a loss of crystalline solvate less than 5 wt.%, whereas the blade-cast film loses 15 wt.% up to 60oC. Heating to 100oC results in a total mass loss of ~17 wt.% and ~25 wt.% for the spin- and blade-cast films, respectively. These results, taken together, confirm that significant structural differences seen in as-cast films are closely linked to differences insolvent evaporation and retention for the sol-gel precursor, which is ultimately subject to the processing method. The critical role of drying kinetics in determining the solidification of hybrid perovskite films is expected to have important implications on thin film microstructure and morphology of the converted perovskite film. We note that the microstructure and morphology of 8


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ACS Energy Letters

hybrid perovskite active layers appear to be significantly more sensitive to the processing technique than organic bulk heterojunction blends.25-26,

44

This issue is

therefore of importance to the upscaling of perovskite solar cell manufacturing as it requires the effective drying rate of the ink to be increased in meniscus-guided coating processes to at least match that of spin-coating. The easiest way to achieve greater evaporation rate is to increase the coating temperature. We have investigated the solidification behavior of perovskite films bladecoated at different temperatures up to 150oC (Figures 2 and S2). At 50oC (Figure S2a), solidification occurs at ∼47 s with the formation of the solvate and PbI2 phases without perovskite phase formation. Solution thinning is much faster at 80 and 135oC, as highlighted in Figures 2a and 2b, respectively. It occurs only after ∼13.4 s at 80oC (Figure 2c) and ∼2.4 s at 135oC (Figure 2d). The former is slightly faster than spincoating, whereas the latter is much faster. As will be shown below, this approach to matching solidification kinetics at T ≤ 80oC fails to yield meaningful device improvements, unlike the case of organic solar cells. Formation of the solvate polymorph at 80oC is accompanied by significant formation of PbI2, unlike spin-cast films, as indicated by in situ GIWAXS measurements. Not surprisingly, the structure of the solvate changes with coating temperature, as shown in plots of integrated intensity vs. q (Figure 2e). The solvate formed at room temperature is replotted for reference and its comparison with the solvate diffraction peaks at 50 and 80oC are shifted significantly toward higher q values, indicating a shrinking solvate unit cell. This is consistent with increasing solvent evaporation rate which also reduced solvent 9



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retention in the sol-gel precursor leading up to solvate crystallization. TGA measurements performed on as-cast films record a much lower solvent mass loss for samples coated at 50oC (8.6 wt.%) and 80oC (6.8 wt.%) as compared to room temperature coated ones. 2D GIWAXS snap shots taken at the end of in situ experiments (Figure S3a-e) also reveal the solvate precursor in spin-cast and blade-cast films tends to be highly textured suggesting substrate-driven nucleation and growth, whereas in situ converted films by high temperature blade coating exhibit primarily a powder texture, indicative of nucleation from the bulk. The structure of the intermediate solvates formed during solution processing is difficult to solve, but can be informed by comparison with recent reports in which single crystals of different solvated intermediates have been prepared and their structure solved. In figure S3f, we have plotted the diffraction features of the solvate phases seen in this study along with those of the resolved structure for equimolar MAI and PbI2 in DMF, as well as MAI-rich and PbI2-rich crystalline solvates in DMF.45-46 Solvateassociated diffraction peak positions are listed in table S1. Films processed at room temperature by spin coating and blade coating exhibit diffraction features with lower q value than the equimolar solvate formed from the MAPbI3 single crystal and polycrystalline film, and appears to exhibit very similar characteristics as the PbI2-rich, indicating the DMF-solvated phase is deficient of MAI. The balance of the MAI remains in the as-cast film and later incorporates the perovskite film upon thermal conversion. Thin films blade coated at intermediate temperatures of 50 and 80oC exhibit gradually higher diffraction features very much consistent with the formation of an 10


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ACS Energy Letters

equimolar MAPbI3-DMF solvated phase incorporating increasingly more MAI. The relationship between the structure of the solvated phase and the coating temperature is indicative of temperature-mediated coordination of lead and DMF. Rapid loss of the solvent at elevated temperature prevents the excessive coordination of lead and DMF at the exclusion of MAI, thus promoting collapse of the disordered sol-gel state into an equimolar crystalline solvated phase rather than a PbI2-rich solvate.

Figure 2. In situ diagnostics performed during blade-coating of hybrid perovskite films at elevated coating temperatures. In situ measurements of thickness vs. time of the blank DMF solvent and perovskite ink performed during blade-coating at (a) 80oC and (b) 135oC. In situ GIWAXS performed during blade-coating at (c) 80oC and (d) 135oC. (e) Intensity vs. q plots of the low-q region corresponding to the crystalline MAPbI3-DMF 11



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solvates formed at different blade-coating temperatures and comparison with equimolar and PbI2-rich crystalline DMF solvates from refs. 47-48. (f) TGA analysis of as-cast blade-coated perovskite films scraped off the substrate immediately after blade-coating at different temperatures. Solidification at 135oC occurs without formation of any solvates or PbI2, resulting in direct conversion of the colloidal sol-gel precursor to MAPbI3. In situ GIWAXS maps obtained at all temperatures (Figure S2) show that solidification occurs via direct transition to the perovskite phase for temperatures equal and above 100oC, circumventing the crystalline solvate phase. Interestingly, blade-coating performed at 150oC results in extremely rapid solidification detected with the onset of perovskite phase formation at ∼1.0-1.5 s and is accompanied with the formation of a weaker PbI2 signal after ∼1.8 s (Figure S2f and S3), which suggests the PbI2 forms after MAPbI3 growth via in situ decomposition and loss of MAI, most likely at grain boundaries, as will be discussed below. An important point to reiterate here is the temporal resolution of in situ GIWAXS measurements was 0.1 s. We have plotted all 2D-GIWAXS snapshots of the blade-cast sample with 0.1 s in figure S4. We observe no evidence of the intermediate crystalline solvate and we also find that the perovskite phase and the disordered colloidal sol-gel state coexist, proving the transformation is direct and that PbI2 formation follows conversion to MAPbI3. This does not prove that the intermediate solvate does not form. Instead, it is the solvate exhibiting long range crystalline order which is not detected on the time scale of measurements (within 0.1 s). Large grains of the solvate responsible for microscopic ribbons at low and intermediate temperatures therefore do not form. Instead, the film forms via macroscopic and polycrystalline spherulites of the perovskite phase. The nature of interactions between solvent and 12


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ACS Energy Letters

solute is complicated.49-50 It can be explained by a previously reported ion cage-based complex.33 Briefly, solvent evaporation leads to the formation of Pb-centred ion-cages (CH3NH3I)X-PbI2-(DMF)Y. This ion cage is stable at room temperature and when the cage concentration increases to a point where the cages are forced into contact, they start to transform into more stable perovskite crystals with PbI2, MAI and another intermediate (CH3NH3I)m-PbI2-(DMF)n. The ion cage should be centered with a [PbI6]4octahedron covered with MAI and DMF shells. High coating temperature promotes DMF evaporation, mitigating DMF complexation with PbI2, and thus controlling the crystalline solvate phase’s composition at low and intermediate temperatures (Figure 2e), and preventing its growth at high temperature (>100oC). These films exhibit increased carrier lifetime and result in improved solar cell performance, as will also be shown below.47-48 The decomposition process appears to be self-limiting as the PbI2 diffraction intensity remains constant after formation, even as the sample is kept at 150oC for several seconds. TGA measurements up to 100oC (Figure 2f) reveal predictably minimal mass loss from these samples when coated at 100 oC (1.4 wt.%) and 120 oC (0.46 wt.%), as the solvent is removed in situ almost entirely. We summarize in Figure 3 the insight gleaned so far into the solidification of MAPBI3 in low, intermediate and high temperature blade-coating conditions. All three scenarios begin with spreading of the colloidal sol-gel ink over the substrate placed on a hotplate. At low temperature (Figure 3a, below 80 oC), slow drying leads to formation of crystalline solvates rich in PbI2, along with the PbI2 phase, both of which can exhibit 13



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ribbon-like morphologies. The solvate ribbons do not provide full coverage of the substrate, nor does the post-deposition thermally converted perovskite film (see SEM in 3a). This scenario is experimentally verified by polarized optical microscopy (Figure 3d, 50 oC) and by in situ optical microscopy performed during drying at 50oC (Figure S5a). The ribbons are composed of fine perovskite grains. At intermediate temperature (Figure 3b, above 80 but below 100oC), drying is faster and there is competition between ribbon-like equimolar solvate formation, PbI2 formation and direct crystallization of the perovskite phase (primarily at 80oC). The annealed MAPbI3 film preserves a mixture of ribbon-like and compact morphologies. At high temperature (Figure 3c, above 100 oC), the perovskite crystals nucleate and grow directly within the colloidal ink, forming compact films with little or no solvent retention, as confirmed in the micrographs shown in Figure S5b. To our surprise, these large, compact and uniform-looking regions are themselves comprised of smaller domains (5-10 μm), as revealed by SEM and polarized optical micrographs (Fig. 3e and 3f) indicating their polycrystalline spherulite nature instead of single crystalline domains. This in situ study distinguishes itself from other reports utilizing in situ x-ray scattering, for instance, to monitor the influence of light and moisture exposure on the microstructure and performance of perovskite solar cells,51-52 or to link the microstructure of various semiconductors to the PCE of solar cells.53 It specifically links the solidification pathway of the hybrid perovskite ink in upscalable processes to its microstructural and morphological outcome and highlights the solidification conditions which yield highly efficient solar cells. 14


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The insights unearthed should significantly impact solar cell performance as well. We have fabricated solar cells with n-i-p architecture of FTO/c-TiO2/ perovskite/SpiroOMeTAD/Au based on blade-coated perovskite layer. The PCEs of dozens of devices fabricated at each temperature condition are summarized in Figure 4a (see figures of merit in Table S2). We observe low efficiency and large variations in devices prepared at temperatures below 80oC, which approach the kinetics of solvent removal and solidification of spin-coating at room temperature (Figures 1 and 2). The solar cell fabricated by spin-coating the DMF-based solution without drip (Figure S6) yields a PCE of 9.5±0.5%, which is somewhat better than devices fabricated by blade-coating at RT (Figure S7) and intermediate temperatures (80oC).

Figure 3. Schematic representation of the MAPbI3 perovskite film formation mechanism and SEM micrographs representing (a) low (~25-50oC), (b) intermediate (~80oC), and (c) high (>100oC) blade-coating temperatures. Polarized and unpolarized 15



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(inset) optical micrographs of as-cast MAPbI3 films blade-coated at (d) 50℃, (e) 135℃, and (f) 150℃. Increasing the coating temperature to 100-135oC yielded much higher average PCE, typically between 13-14% with smaller deviations than at lower temperatures. These results clearly reflect the benefits of the direct crystallization of MAPbI3 and the compact morphology of the as-cast films. A further significant increase in average PCE >17% is observed at 150oC, with champion efficiency of 17.54%, the highest PCE reported to date for solar cells based on blade-coated MAPbI3 film. The appearance of small amounts of PbI2 at 150oC after MAPbI3 growth (see Figures S2f, S3e and inset of Figure 4b) is absent at all other temperatures studied, including 100, 120 and 135oC. The decomposition appears to be self-limiting at 150oC, which we interpret as being due to loss of MAI from the terminal monolayers of MAPbI3 grains, a feature known to passivate grain boundary trap states.47-48 Indeed, closer inspection of plan-view SEMs of the polycrystalline films processed at elevated temperatures (100, 120 and 150oC; see Figure S8) reveals evidence of charging at grain boundaries of the film blade-coated at 150oC, a tell-tale sign of PbI2 formation,54-56 this feature is absent in other samples prepared at elevated temperature. Figures 4b and 4c show the hysteresis and external quantum efficiency (EQE) of the best devices, respectively. The hysteresis was found to decrease upon raising the coating temperature from 25 to 150oC [around 30% change of PCE at 25oC (see figure S7), versus 14% change at 150oC], which we link to formation of higher quality films and better interface between perovskite and carrier transport layer in general (the dark current of devices, the stability of Jsc and the two-week stability of spin-cast and blade16


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ACS Energy Letters

coated devices are presented in figure S6). The short circuit current density calculated from EQE agrees well with the one obtained in J-V sweeps. Meanwhile the bladecoated device at 150oC is more stable after two weeks in dark ambient storage than the spin-cast device.

Figure 4. (a) Power conversion efficiency of devices prepared at different blade-coating temperatures. (b) J-V curves of the best device blade-coated at 150 ℃ and (c) EQE of the same device, along with integrated Jsc. (d) Transient photoluminescence measurements on perovskite films blade-coated at different temperatures.

The dramatic microstructural and morphological differences seen from electron and optical micrographs in Figure 3 have been shown to influence the photovoltaic performance of solar cell devices. Time-resolved photoluminescence (PL) measurements performed on thermally annealed films prepared by blade-coating at 25, 80 and 150oC (Figure 4d) provide useful hints through increasing lifetime at higher 17



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coating temperature. The decay time constant doubles from ∼12 ns at RT, where the perovskite phase transforms from ribbon-like solvate domains, to ∼25 ns at 80oC, where mixed growth (direct and via solvates) was observed. It tops off at ∼33 ns at 150oC. Clearly, direct phase transformation from ink to perovskite appears to be of great benefit and probably the most promising path forward to achieving high quality perovskite films and devices using scalable meniscus-guided coating processes.

We have revealed through in situ investigations significant differences in the process of perovskite film formation during spin-coating and blade-coating at room temperature and further shown that processing temperature mediates the structure, composition and solvent retention by the intermediate solvate, with high temperatures (>100oC) circumventing the solvate phase entirely. These lessons are valid for all meniscusguided coating techniques where anti-solvent dripping method used successfully with spin-coating is difficult to implement. The current study also sheds light into additional benefits of high temperature blade-coating, including partial decoration of MAPbI3 grain boundaries with PbI2, which we have attributed with significant improvements of PCE > 17%, bringing scalable and large-area manufacturing of hybrid perovskites one step closer to reality.

Experimental details See supplementary information

18


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Acknowledgement This work was supported by the King Abdullah University of Science and Technology (KAUST). GIWAXS measurements were performed at D-line at the Cornell High Energy Synchrotron Source (CHESS) at Cornell University. CHESS is supported by the NSF & NIH/NIGMS via NSF Award DMR-1332208. K.Z. acknowledges the National Key Research and Development Program of China (2017YFA0204800, 2016YFA0202403), and the National Nature Science Foundation of China (No. 61604092).

Supplementary information Available: The supporting information is available free of charge on the ACS publications website at DOI: XXXXX Experimental details Figure S1: GIWAXS of samples spin/blade-coated at RT Figure S2: GIWAXS of samples blade-coated at different temperature Figure S3: GIWAXS and integrated solvate peak of samples under different processing condition Table S1: Peak position of ordered solvate Figure S4: in situ GIWAXS of blade-coated film at 150oC with a time step of 0.1 s Figure S5: SEM and OM of blade-coated perovskite film Table S2: Figure of merit of devices processed at different temperature Figure S6: Stability test and dark current of devices 19



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Figure S7: J-V curves of RT blade-coated devices Figure S8. SEM micrographs of perovskite films blade-coated at 100, 120 and 150oC

References (1) Correa-Baena, J.-P.; Abate, A.; Saliba, M.; Tress, W.; Jesper Jacobsson, T.; Grätzel, M.; Hagfeldt, A. The Rapid Evolution of Highly Efficient Perovskite Solar Cells. Energy Environ. Sci. 2017, 10, 710-727. (2) Mengxia, L.; Oleksandr, V.; Randy, S.; F. Pelayo, G. d. A., Rahim, M.; Ahmed, H. B.; Xinzheng, L.; Fengjia, F.; Grant, W.; Ahmad, R. K. et al. Hybrid Organic– Inorganic Inks Flatten the Energy Landscape in Colloidal Quantum Dot Solids. Nat. Mater. 2017, 16, 258–263. (3) Huawei, H.; Philip, C. Y. C.; Guangye, Z.; Tingxuan, M.; Jing, L.; Guofang, Y.; and He, Y. Design of Donor Polymers with Strong Temperature-Dependent Aggregation Property for Efficient Organic Photovoltaics. Acc. Chem. Res. 2017, 50, 2519–2528. (4) Zhiping, W.; Qianqian, L., Francis, P. C.; Nobuya, S.; Laura, M. H. and Henry, J. S. Efficient Ambient-Air-Stable Solar Cells with 2D–3D Heterostructured Butylammonium-Caesium-Formamidinium Lead Halide Perovskites. Nat. Energy 2017, 2, 17135. (5) Hairen, T.; Ankit, J., Oleksandr, V.; Xinzheng, L.; F. Pelayo, G. d. A.; James, Z. F.; Rafael, Q.-B.; Mingjian, Y.; Bo, Z.; Yicheng, Z. et al. Efficient and Stable Solution-Processed Planar Perovskite Solar Cells via Contact Passivation. Science 2017, 355, 722-726. (6) Dongqin, B.; Wolfgang, T.; M. Ibrahim, D.; Peng, G.; Jingshan, L.; Clémentine, R.; Kurt, S.; Antonio, A.; Fabrizio G.; Juan-Pablo C. B. et al. Efficient Luminescent Solar Cells Based on Tailored Mixed-Cation Perovskites. Sci Adv 2016, 2, e1501170. (7) Yuan, M.; Quan, L. N.; Comin, R.; Walters, G.; Sabatini, R.; Voznyy, O.; Hoogland, S.; Zhao, Y.; Beauregard, E. M.; Kanjanaboos, P. et al. Perovskite Energy Funnels for Efficient Light-Emitting Diodes. Nat. Nanotechnology 2016, 11, 872-877. (8) Bush, K. A.; Palmstrom, A. F.; Yu, Z. J.; Boccard, M.; Cheacharoen, R.; Mailoa, J. P.; McMeekin, D. P.; Hoye, R. L. Z.; Bailie, C. D.; Leijtens, T. et al. 23.6%Efficient Monolithic Perovskite/Silicon Tandem Solar Cells with Improved Stability. Nat. Energy 2017, 2, 17009. (9) Yufei, Z.; Rahim, M.; Ahmed, H. B.; Arif, D. S.; Liyang, Y.; Ming-Chun, T.; Hanlin, H.; Frédéric, L. and Aram, A. Mesostructured Fullerene Electrodes for Highly Efficient n–i–p Perovskite Solar Cells. ACS Energy Lett. 2016, 1, 1049– 1056. 20


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Blade-Coated Hybrid Perovskite Solar Cells with Efficiency > 17%: An

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