MAPbI3 Solar Cells with Absorber Deposited by Resonant Infrared

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Letter

MAPbI3 Solar Cells with Absorber Deposited by Resonant Infrared Matrix-Assisted Pulsed Laser Evaporation Wiley A. Dunlap-Shohl, E. Tomas Barraza, Andrew Barrette, Kenan Gundogdu, Adrienne Stiff-Roberts, and David B. Mitzi ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b01144 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 24, 2017

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MAPbI3 Solar Cells with Absorber Deposited by Resonant Infrared Matrix-Assisted Pulsed Laser Evaporation Wiley A. Dunlap-Shohl,†⁋ E. Tomas Barraza,‡⁋ Andrew Barrette,ǁ Kenan Gundogdu,ǁ Adrienne D. Stiff-Roberts,‡* David B. Mitzi†§* † Department of Mechanical Engineering and Materials Science, Duke University, Durham, North Carolina 27708, United States ‡ Department of Electrical and Computer Engineering, Duke University, Durham, North Carolina 27708, United States ǁ Department of Physics, North Carolina State University, Raleigh, North Carolina 27695, United States § Department of Chemistry, Duke University, Durham, North Carolina 27708, United States ⁋ These authors contributed equally to this work. *Corresponding Author Contact Information: Email: [email protected]; [email protected]

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ABSTRACT. Resonant infrared, matrix-assisted pulsed laser evaporation (RIR-MAPLE) is a gentle thin-film deposition technique that combines the facile chemical control of solution processing with the growth control of vapor-phase deposition, yet one that has not been widely applied to crystalline organic-inorganic hybrid materials. In this work, we investigate the optoelectronic quality of RIR-MAPLE-deposited CH3NH3PbI3 (MAPbI3) perovskite films, and report on the fabrication of perovskite solar cells in which the absorber is deposited by RIRMAPLE. We find the composition, morphology and optical properties of these perovskite films to be comparable to those produced by more conventional methods, such as spin coating. The champion device reaches a stabilized power conversion efficiency of over 12%, a high value for perovskite solar cells deposited by a laser ablation process, highlighting the ability of this new technique to produce device-quality films.

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Hybrid organic-inorganic perovskite (HOIP) materials offer outstanding optoelectronic characteristics and recently have been integrated as the active layer in high-performance devices such as transistors,1,2 photodetectors,3 lasers,4,5 light-emitting diodes,6,7,8 and state-of-the-art thinfilm solar cells.9,10,11 The HOIP family is especially intriguing, not only because of tunable optical and electrical properties, but also due to the great structural and chemical diversity within this family, particularly as afforded by the organic hybrid component.12 While many exciting optoelectronic device architectures based on HOIP materials may be envisioned, the current menu of deposition techniques may not be suitable for all such structures. Solution-processing may be considered undesirable for some device structures because drenching the substrate with a polar solvent may attack the layers beneath (furthermore, many popular perovskite film fabrication recipes also involve washing the substrate with13,14 or immersing the substrate in15 a non-polar solvent, which might also attack other device structure components). Solution processing may also complicate the fabrication of more advanced device architectures, involving for example graded compositions/band gaps or multiple junctions. Vapor-based deposition techniques such as thermal co-evaporation16,17 or pulsed-laser deposition (PLD)18,19 offer resolutions to these challenges through the absence of solvent and the ability to alter deposition parameters to achieve varying compositions as a function of depth in the film. However, these techniques are likely to present difficulties in controlling the deposition rate of the organic precursors and the overall stoichiometry of the hybrid films,20 as well as the potential for thermal damage to the organic components. These limitations motivate the development of novel deposition techniques to address the unique challenges posed by the fabrication of HOIP thin films.

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The resonant infrared, matrix-assisted pulsed laser evaporation (RIR-MAPLE) technique provides a potential solution for the challenges confronting the vapor-phase deposition of hybrid films. RIR-MAPLE, like PLD, uses pulsed laser energy to deposit films, but instead of ablating the deposition material directly, the energy is absorbed by a cryogenically frozen solvent matrix.21 The laser energy is resonant with the vibrational mode of a specific chemical bond in the solvent, which preferentially absorbs the energy and evaporates, carrying the HOIP precursor materials to the substrate. RIR-MAPLE offers several compelling advantages over conventional solution- and vapor-based thin-film deposition techniques, by combining desirable attributes and mitigating flaws of each type of processing. Excess ablated solvent is removed by dynamic vacuum, reducing the possibility of damage to underlying layers by the solvent and enabling the construction of complex features such as composition gradients. Furthermore, the targets may be composed of solutions that are orders of magnitude less concentrated than those required for conventional solution deposition techniques (~0.02 M vs upwards of 1 M, respectively), mitigating challenges associated with solubility. Compared to vapor deposition techniques, RIRMAPLE eliminates risks of damaging sensitive organic materials through excessive heating (as in thermal evaporation) or by direct absorption of laser energy (as in PLD). In addition, RIRMAPLE provides a more controlled ablation of the organic component because reasonably volatile precursors that are susceptible to removal by the dynamic vacuum, like methylammonium iodide (MAI), are transferred to the substrate in proportion to their concentration in solution. As a result, stoichiometric films can be achieved without an excess of the organic component, which can be associated with unwanted secondary phases.22 Thus, RIRMAPLE is well-suited for the deposition of thin films of complex organic-inorganic hybrid materials, including but not limited to HOIPs.23 In this work, we aim to extend this paradigm of

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RIR-MAPLE for hybrids through the deposition of HOIP materials for optoelectronic devices. To do so, we examine the properties of MAPbI3 films deposited on FTO/NiOx substrates and the performance of solar cells using these thin films as the absorber. The best device so prepared has a stabilized power conversion efficiency of over 12%, which is to the best of our knowledge the highest value for any perovskite solar cell in which the absorber is prepared by a laser ablationbased deposition process (a previous study employed PLD and demonstrated 10.9% power conversion efficiency).19 RIR-MAPLE was used to deposit thin films of MAPbI3 onto unheated FTO/NiOx substrates using an optimized method, which we have described in a separate work.22 The target comprises a 0.022 M (10 mg/mL PbI2, equivalent molarity of MAI) solution of PbI2 and MAI in a 1:1 by volume mixture of dimethyl sulfoxide (DMSO) and ethylene glycol (MEG). The components are deposited onto the substrates at approximately 10 °C (without any form of temperature control) for over 4 hours (more details are available in the Experimental Methods section). X-ray diffraction patterns of the as-deposited films (Figure 1a) reveal that a slight amount of solvent remains in the film, as indicated by the weak peaks below 2θ = 10o, which can be assigned to a MAPbI3-DMSO complex;13 however, the film otherwise contains no crystalline phases besides MAPbI3, consistent with our previous results on other substrates.22 The residual solvent may be driven off by annealing the film at 110oC for 10 min on a hot plate in a nitrogen-filled glovebox, and the corresponding XRD pattern confirms the purity of the resulting film, with no peaks belonging to phases other than MAPbI3 or the FTO substrate. The phase-pure XRD patterns indicate that the stoichiometry of the precursor target (i.e., a 1:1 molar ratio of PbI2:MAI) transfers essentially perfectly to the final film,22 setting it apart from other vapor-deposition techniques that require a significant excess of MAI.16,18

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Figure 1: (a) XRD (Cu Kα radiation) of as-deposited and annealed (110oC, 10 min after deposition) RIR-MAPLE-deposited MAPbI3 films on FTO/NiOx substrates, with asterisks in the XRD pattern denoting a MAPbI3-DMSO impurity, § denoting peaks belonging to FTO, and prominent MAPbI3 peaks indexed with the corresponding Miller indices; top-view SEM image comparisons of the same as-deposited (b) and annealed (c) films; UV-Vis absorbance and PL spectra (d) of the annealed film, showing good alignment of the PL peak and absorption onset. Top-view SEM images (Figure 1b) of the as-deposited film indicate a dense and close-packed microstructure, though the grains are quite small, the majority being 100 nm across or less. The grain structure of the annealed film modestly coarsens, with an average grain size of roughly 124 6 ACS Paragon Plus Environment

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nm (estimated using the line intercept method for five transects of the image in Figure S1), while maintaining a compact pinhole-free morphology. This film structure is comparable to that observed for perovskite films prepared by a conventional spin-coating recipe.14 We also note that the grain structure of the RIR-MAPLE films deposited on FTO/NiOx appears similar to that obtained for MAPbI3 films fabricated on glass and FTO/TiO2 substrates,22 demonstrating that the deposition process is relatively insensitive to the substrate chemistry. Motivated by concerns of possible interaction between the target solution and the stainless steel cup used in the RIR-MAPLE process (particularly with regards to the introduction of Fe, which is known to be harmful to performance),24,25 the composition of the MAPLE-grown perovskite was probed by time-of-flight secondary ion mass spectrometry (TOF-SIMS) and compared against that produced by a standard spin-coating procedure14 on FTO/NiOx substrates. TOF-SIMS depth profiles reveal that the predominant metallic components of stainless steel, Fe and Cr, are present in neither the spin-cast nor MAPLE-deposited samples. However, MAPLE samples display elevated levels of Na and, to a lesser extent, Ca in the positive ion profile (Figure S2), and Cl in the negative ion profile relative to the spin-cast films (Figure S3). The Na+, Ca2+, and Cl- species are likely to be benign or even beneficial, as each has been demonstrated to improve film quality when present in small amounts,26,27,28 though it is conceivable that too large amounts could lead to the formation of insulating species such as NaCl, CaCl2 or MAPbCl3, which might impede carrier extraction if present at grain boundaries or interfaces.

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Figure 2: Schematic (a) and cross-sectional SEM image (b) of the best-performing RIR-MAPLE perovskite solar cell; TRPL decay of a MAPLE film on glass, fit to a model curve incorporating effects of mono- and bimolecular recombination (c). We have also examined the optical properties of films prepared as above and annealed at 110oC. The photoluminescence (PL) and UV-Vis absorption spectra are plotted in Figure 1d, from which it is evident that the characteristic band-to-band PL emission coincides closely with the absorption onset. The Stokes shift (as calculated from the inflection point of the absorption onset, in Figure S4) is no more than 10 meV, indicating that the deposition process does not significantly induce band-edge fluctuations or defects, in approximate agreement with previous reports.29,30 We have performed time-resolved photoluminescence (TRPL) measurements on a MAPLE-grown film annealed at 110oC, and estimate a monomolecular recombination lifetime of 68 ns, which we assign to trap-assisted Shockley-Read-Hall (SRH) recombination. This value is comparable to that of other MAPbI3 films prepared by spin-coating, which often display carrier

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lifetimes in the 10-100 ns range.28,31,32 More details of the TRPL data fitting can be found in the Supporting Information. Solar cells were fabricated in the commonly used inverted device architecture FTO/NiOx/MAPbI3/C60/bathocuproine (BCP)/Ag (Fig. 2), using MAPbI3 films deposited by RIR-MAPLE, with annealing after the deposition at 110oC for 10 min in a nitrogen-filled glovebox. Current density-voltage (J-V) curves for six such solar cells prepared on two separate occasions were measured under simulated AM1.5G light, with devices displaying average forward (reverse) scan open-circuit voltage of 0.957 ± 0.021 V (0.939 ± 0.020 V), short-circuit current of 16.3 ± 1.4 mA/cm2 (16.5 ± 1.3 mA/cm2), fill factor of 64.8 ± 1.9% (60.8 ± 3.4%), and power conversion efficiency of 10.1 ± 1.1% (9.4 ± 1.1%). In these statistical results, uncertainty values reflect the sample standard deviation. Device parameters for the best-performing solar cell are, for forward (reverse) scans: open-circuit voltage of 0.966 V (0.934 V), short-circuit current density of 18.2 mA/cm2 (18.3 mA/cm2), fill factor of 67.3% (58.8%), and power conversion efficiency of 11.8% (10.0%); the J-V curves for this cell are plotted in Figure 3a. External quantum efficiency measurement of the device (Figure 3b) indicates excellent agreement with the J-V measurements, yielding a calculated short-circuit current under AM1.5G illumination of 18.2 mA/cm2, and demonstrating a relatively flat curve with good broadband photon-toelectron/hole pair conversion, though some slight attenuation occurs at longer wavelengths. Time-dependent photocurrent measurement of this device yields a stabilized efficiency of 12.2% at 0.8 V (Figure 3c); we believe this efficiency to be the highest so far reported amongst solar cells using a perovskite absorber deposited by a laser ablation process, concretely showcasing the advantages of the RIR-MAPLE process.

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Figure 3: J-V curves (forward and reverse scanning directions are shown) (a), external quantum efficiency (b), and stabilized photocurrent and efficiency (c), of the best-performing RIRMAPLE-deposited device. A comparison of RIR-MAPLE with its cousin PLD reveals several advantages that make up for the challenge of finding a suitable solvent. In principle, RIR-MAPLE should be an inherently gentler process, not only because targeted absorption of the laser in the solvent prevents thermal damage to the perovskite precursors, but also because the individual photons in the Er:YAG laser used in RIR-MAPLE (~0.4 eV) are an order of magnitude less energetic than those produced by the KrF excimer laser (5 eV) used in previous PLD studies,18-19 lessening the probability of unwanted photochemical interactions. In practice, RIR-MAPLE makes considerably more efficient use of the organic component than PLD, for which targets must be prepared with organic halide to lead halide ratios of at least 4:1 (off-axis deposition)18 and up to 12:1 (onaxis)19 in order to obtain phase-pure perovskite films. Furthermore, the morphology of films deposited by RIR-MAPLE are competitive with those deposited by PLD, which can possess prominent voids in the off-axis case18; for on-axis PLD, film continuity is better,19 but this process is less materially efficient, as noted above. The superior performance of the devices produced by RIR-MAPLE may thus be rationalized in view of the improved morphology and reduced potential for damage compared to PLD. However, it is important to note that differences 10 ACS Paragon Plus Environment

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in performance between these techniques may be a consequence of factors unrelated to the deposition method, but rather more broadly to the level of optimization of the devices as a whole, particularly in view of the present scarcity of reports of perovskite devices fabricated by laser ablation. While these initial results are promising, prospective areas to further improve performance are suggested by cross-sectional SEM images of the best-performing device. The relatively small grain size of the perovskite film provides pathways for trapping or scattering of carriers at the grain boundaries. Therefore, the overall device performance may at least in part be limited by the film microstructure, and further increase in the device performance parameters may be contingent on increasing the film grain size. Furthermore, comparison of several SEM crosssections (Figure S5) indicates a rather wide variation of the thickness, with the perovskite thickness ranging from ~230 to ~340 nm. Some photocurrent may be lost by incomplete capture of light by the perovskite, particularly in the thinner regions, as reflected by the reduction of EQE in the infrared part of the spectrum. However, increasing the thickness of the active layer will require increasing the film grain size in order for the thicker films to be useful. If the grain size remains the same, the increased number of grain boundaries that photogenerated carriers originating in the center of the perovskite film must cross to be collected at the contacts negates potential benefit from increased absorption. An additional issue arises from the roughness of the RIR-MAPLE-deposited MAPbI3 films. Atomic force microscopy (AFM) images of these films (Figure 4) demonstrate that the RIR-MAPLE-grown films are rougher than spin-cast perovskite analogs, forming a network of ridges and craters on a 50 µm lateral scale; however, as the AFM scan scale is reduced from 50 to 5 µm, the RMS roughness values for the MAPLE and spin-cast films converge, reflecting their structural similarity at the nanoscale. The maximum variation

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(Rmax) over the 50 µm scan area is 519 nm, a value on the order of the cross-sectional thickness, indicating that there are most likely a number of thin areas which may reduce the shunt resistance. While the J-V curves appear to be relatively flat at short circuit, increasing the thickness of the films (while also improving grain size) may help to increase shunt resistance, as well as boost short-circuit current.

Figure 4: AFM images of a RIR-MAPLE-deposited perovskite film (a, c, e) and a perovskite film prepared by a conventional solvent-engineering spin-coating process14 (b, d, f). In conclusion, we have demonstrated for the first time solar cells using a new deposition process, RIR-MAPLE, to fabricate the perovskite absorber. These perovskite films are phasepure (after a brief post-anneal) and have a compact, though fine-grained, microstructure suitable

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for optoelectronic applications. The highest-performing cell, with stabilized efficiency over 12%, is to the best of our knowledge the highest efficiency perovskite solar cell prepared by a laserablation deposition method. Future improvements are likely if a suitable strategy for increasing the grain size and reducing the roughness of the films can be developed. These results pave the way for future exploration of the RIR-MAPLE process as a means towards deposition of thin films of other high quality, crystalline organic-inorganic hybrid materials, and exploitation of its ability to explore more complex structures and chemistries than are currently possible with existing techniques.

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ASSOCIATED CONTENT Supporting Information. Experimental methods; grain size estimation of RIR-MAPLEdeposited perovskite film annealed at 110oC; TOF-SIMS depth profiles of spin-cast and RIRMAPLE-deposited perovskite films; calculation of Stokes shift for RIR-MAPLE-deposited perovskite film annealed at 110oC; TRPL decay fitting method explanation; cross-section SEM images of solar cells made using RIR-MAPLE-deposited absorber. AUTHOR INFORMATION Corresponding Authors E-mail: [email protected]; [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Science Foundation, Research Triangle MRSEC (DMR-1121107). W.A.D.-S. acknowledges support from the Fitzpatrick Institute for Photonics John T. Chambers Scholarship. This work was performed in part at the Duke University Shared Materials Instrumentation Facility (SMIF) and at the Chapel Hill Analytical and Nanofabrication Laboratory (CHANL). Both are members of the North Carolina Research Triangle Nanotechnology Network (RTNN), which is supported by the National Science Foundation (Grant ECCS-1542015) as part of the National Nanotechnology Coordinated Infrastructure (NNCI).

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(11) Yang, W. S.; Park, B.-W.; Jung, E. H.; Jeon, N. J.; Kim, Y. C.; Lee, D. U.; Shin, S. S.; Seo, J.; Kim, E. K.; Noh, J. H.; et al. Iodide Management in Formamidinium-Lead-Halide-Based Perovskite Layers for Efficient Solar Cells. Science 2017, 356, 1376-1379. (12) Saparov, B.; Mitzi, D. B. Organic-Inorganic Perovskites: Structural Versatility for Functional Materials Design. Chem. Rev. 2016, 116, 4558-4596. (13) Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I. Solvent Engineering for High-Performance Inorganic-Organic Hybrid Perovskite Solar Cells. Nat. Mater. 2014, 13, 897-903. (14) Ahn, N.; Son, D.-Y.; Jang, I.-H.; Kang, S. M.; Choi, M.; Park, N.-G. Highly Reproducible Perovskite Solar Cells with Average Efficiency of 18.3% and Best Efficiency of 19.7% Fabricated via Lewis Base Adduct of Lead(II) Iodide. J. Am. Chem. Soc. 2015, 137, 8696-8699. (15) Zhou, Y.; Yang, M.; Wu, W.; Vasiliev, A. L.; Zhu, K.; Padture, N. P. Room-Temperature Crystallization of Hybrid-Perovskite Thin Films via Solvent–Solvent Extraction for HighPerformance Solar Cells. J. Mater. Chem. A 2015, 3, 8178-8184. (16) Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395-398. (17) Lin, Q.; Armin, A.; Nagiri, R. C. R.; Burn, P. L.; Meredith, P. Electro-Optics of Perovskite Solar Cells. Nat. Photonics 2014, 9, 106-112. (18) Bansode, U.; Naphade, R.; Game, O.; Agarkar, S.; Ogale, S. Hybrid Perovskite Films by a New Variant of Pulsed Excimer Laser Deposition: A Room-Temperature Dry Process. J. Phys. Chem. C 2015, 119, 9177-9185. (19) Bansode, U.; Ogale, S. On-Axis Pulsed Laser Deposition of Hybrid Perovskite Films for Solar Cell and Broadband Photo-Sensor Applications. J. Appl. Phys. 2017, 121, 133107. (20) Chen, C.-W.; Kang, H.-W.; Hsiao, S.-Y.; Yang, P.-F.; Chiang, K.-M.; Lin, H.-W. Efficient and Uniform Planar-Type Perovskite Solar Cells by Simple Sequential Vacuum Deposition. Adv. Mater. 2014, 26, 6647-6652. (21) Pate, R.; Stiff-Roberts, A. D. The Impact of Laser-Target Absorption Depth on the Surface and Internal Morphology of Matrix-Assisted Pulsed Laser Evaporated Conjugated Polymer Thin Films. Chem. Phys. Lett. 2009, 477, 406-410. (22) Barraza, E. T.; Dunlap-Shohl, W. A.; Mitzi, D. B.; Stiff-Roberts, A. D. Deposition of Methylammonium Lead Triiodide by Resonant Infrared Matrix-Assisted Pulsed Laser Evaporation. J. Electron. Mater. 2017. https://doi.org/10.1007/s11664-017-5814-0 (23) Ge, W.; Atewologun, A.; Stiff-Roberts, A. D. Hybrid Nanocomposite Thin Films Deposited by Emulsion-Based Resonant Infrared Matrix-Assisted Pulsed Laser Evaporation for Photovoltaic Applications. Org. Electron. 2015, 22, 98-107. 16 ACS Paragon Plus Environment

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ACS Energy Letters

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