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Acetate Salts as Non-Halogen Additives to Improve Perovskite Film Morphology for High-Efficiency Solar Cells Qiliang Wu, Pengcheng Zhou, Weiran Zhou, Xiangfeng Wei, Tao Chen, and Shangfeng Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03276 • Publication Date (Web): 02 Jun 2016 Downloaded from http://pubs.acs.org on June 7, 2016
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ACS Applied Materials & Interfaces
Acetate Salts as Non-Halogen Additives to Improve Perovskite Film Morphology for HighEfficiency Solar Cells
Qiliang Wu, Pengcheng Zhou, Weiran Zhou, Xiangfeng Wei, Tao Chen, and Shangfeng Yang*
Hefei National Laboratory for Physical Sciences at Microscale, Key Laboratory of Materials for Energy Conversion, Chinese Academy of Sciences, Department of Materials Science and Engineering, Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China (USTC), Hefei 230026, China
* Corresponding Author. E-mail:
[email protected].
ABSTRACT: Two-step method has been popularly adopted to fabricate perovskite film of planar heterojunction organo-lead halide perovskite solar cells (PSCs). However, this method often generates uncontrollable film morphology with poor coverage. Herein, we report a facile method to improve perovskite film morphology by incorporating a small amount of acetate (CH3COO-, Ac-) salts (NH4Ac, NaAc) as non-halogen additives in CH3NH3I solution used for immersing PbI2 film, resulting in improved CH3NH3PbI3 film morphology. Under the optimized NH4Ac additive concentration of 10 wt%, the best power conversion efficiency (PCE) reaches 17.02%, which is enhanced by ~23.2% relative to the pristine device without additive, whereas NaAc additive does not lead to efficiency enhancement despite of the 1 ACS Paragon Plus Environment
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improvement on CH3NH3PbI3 film morphology. SEM study reveals that NH4Ac and NaAc additives can both effectively improve perovskite film morphology by increasing the surface coverage via diminishing pinholes. The improvement on CH3NH3PbI3 film morphology is beneficial for increasing the optical absorption of perovskite film and improving the interfacial contact at the perovskite/Spiro-OMeTAD interface, leading to the increase of short-circuit current and consequently efficiency enhancement of the PSC device for NH4Ac additive only. Keywords: perovskite solar cells, additive, acetate salt, film morphology, crystallinity
Introduction Organic-inorganic hybrid perovskite solar cells (PSCs) have been attracting great attention as an emerging thin film solar cell technology due to advantages of simple fabrication, large absorption coefficients, tunable bandgaps, high carrier mobility, and especially long charge carrier diffusion lengths.1-13 Recently much effort focused on optimizing not only the composition, crystallization process and morphology of the perovskite light absorber layer but also the interfaces between perovskite and electrodes enables the power conversion efficiencies (PCEs) of PSCs ever-increasing,10,14-26 and a new certified world record PCE of 22.1% established very recently makes PSCs quite competitive to the commercialized crystalline-Si and inorganic semiconductor thin-film solar cells.27,28 For the most popular and efficient architecture employed for PSCs so far, organo-lead halide such as CH3NH3PbX3 (X= I, Br, Cl) is used as a light absorber layer, which is sandwiched between a transparent electrode and a metal electrode.13-18 In order to improve the charge transport and extraction in perovskite/electrode interfaces, interfacial layers including electron transport layers (ETLs) and hole transport layers (HTLs) are usually introduced between the perovskite layer and electrodes, which are found to be beneficial for high2 ACS Paragon Plus Environment
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efficiency PSCs.29-34 For high-efficiency PSCs with different architectures (e.g., mesoporous, planar heterojunction) and fabrication methods (e.g., one-step, two-step and thermal vapor deposition) established during the past few years,5,20,35 film quality of the perovskite light absorber layer is essential because it determines charge carrier generation.21,22,35,36 In particular, the two-step method, for which PbI2 solution was deposited first followed by dipped in a solution of CH3NH3I in isopropanol, has been popularly adopted to fabricate perovskite film of PSCs. However, this method often generates uncontrollable film morphology with pinholes and low coverage which are detrimental to device performance because of the occurence of electrical shorting which severely affects charge separation and transport.37-42 Hence, much effort has been devoted to improve perovskite film quality (morphology, crystallization size and speed) via incorporating additives during fabrication of perovskite film.16,43-49 For instance, Snaith et al. added hydroiodic acid (HI) into the stoichiometric FAI (formamidinium iodide):PbI2 (1:1) perovskite precursor solution in N, Ndimethylformamide (DMF), leading to the formation of an extremely uniform and continuous layer because the added HI helped to solubilize the inorganic component and slow down crystallization of the film upon spin-coating.16 More recently, Leung et al. incorporated another halogen acid additive, hydrochloric acid (HCl), into PbI2 precursor solution, and found that HCl additive improved the uniformity and coverage of the perovskite film fabricated by a two-step method through inhibiting the rod-shape PbI2 crystallization and promoting homogeneous nucleation and crystal growth.43 Jen et al. added 1,8-diiodooctane (DIO) into PbI2 precursor solution in DMF, leading to enhanced crystallization of solutionprocessed perovskite film fabricated by a one-step method.44 Ma and Chen et al. found that 1chloronaphthalene (CN) additive was beneficial to regulate the crystallization transformation kinetics of perovskite to form high quality perovskite films.45 Huang and Yip et al. developed several phosphonium halide salts as processing additives to modulate the morphology and crystallinity of perovskite film, which functioned also as interfacial modifiers to improve the 3 ACS Paragon Plus Environment
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electrical contact of the PCBM/Al in perovskite/fullerene planar-heterojunction PSCs.46 Noteworthy, all these additives are halogen-containing compounds without introducing alien atoms in the host matrix of CH3NH3PbX3 perovskite, while only few non-halogen additives leading to enhanced performance of PSC device have been reported so far, including H2O, 1,3-Bis[3,5-di(pyridin-3-yl)phenyl]benzene (BmPyPhB), 4-tert-butylpyridine (TBP) and poly(ethylene glycol) (PEG),50-54 and whether the introduction of alien atom/ion of nonhalogen additive would affect perovskite film quality remains intriguing. In this paper, two acetate (CH3COO-, Ac-) salts (NH4Ac, NaAc) were applied as nonhalogen additives in CH3NH3I solution used for immersing PbI2 film, resulting in improvement of CH3NH3PbI3 perovskite film morphology for both cases, while efficiency enhancement of PSC devices was achieved for NH4Ac additive only. The effects of acetate additive on the morphology, crystallinity, and optical absorption of perovskite film and interfacial contact of PSC device were investigated, and the mechanism for the efficiency enhancement upon incorporating NH4Ac additive was unveiled. Experimental Section Materials. FTO-coated glass substrates with a sheet resistance of 7 Ω.sq-1 were purchased from NSG Group, Japan. CH3NH3I was synthesized following the procedure reported in ref. 13. PbI2, lithium bis(trifluoromethylsulphonyl) imide (Li-TFSI), 4-tert-butylpyridine (tBP), hydriodic acid, methylamine solution, 1-hexanethiol, dimethyl sulfoxide (DMSO), chlorobenzene,
isopropanol
and
acetonitrile
were
purchased
from
Alfa
Aesar.
Ammonium acetate (CH3COONH4, NH4Ac) and sodium acetate (CH3COONa, NaAc) were purchased from Sinopharm. Spiro-OMeTAD was purchased from 1M company. All chemicals were used as received. Device fabrication. Our detailed fabrication procedure of the planar-heterojunction CH3NH3PbI3 PSCs has been reported previously. 25 Briefly, the FTO-coated glass substrate 4 ACS Paragon Plus Environment
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was etched with Zn powder and 6 M HCl diluted in water, then ultrasonicated in detergent, deionized water, acetone and isopropanol for 15 min every time, and subsequently dried in an oven overnight. A compact TiO2 layer was deposited onto FTO by spin-coating a mixture solution of 350 L titanium isopropoxide, 5 mL ethanol and 65 L HCl (2 mol·L-1) at 2000 rpm, followed by annealing at 500 °C for 60 minutes. CH3NH3PbI3 perovskite layer was fabricated by a two-step method reported in literatures.55 A PbI2 solution (dissolved in dimethylsulfoxide (DMSO) with a concentration of 460 mg/mL) was then spin-coated on top of the compact TiO2 layer at 4000 rpm for 30 s. The coated substrate was dipped into a solution of CH3NH3I (with or without acetate additive) in isopropanol (10 mg/mL) for 10 minutes, and then washed by isopropanol and dried by spinning at 3000 rpm. Subsequently, the as-prepared substrate was heated at 100 oC for 10 minutes. After the substrate was cooled down to room temperature, Spiro-MeOTAD was subsequently deposited by spin-coating at a speed of 3000 rpm for 30 s. The HTL solution was prepared by dissolving 73.2 mg of SpiroMeOTAD, 28.8 µL of 4-tert-butylpyridine (tBP), 18.8 µL of a 520 mg mL − 1 lithium-bis (trifluoromethanesulfonyl) imide (Li-TFSI) in acetonitrile in 1 mL of chorobenzene. Finally, the device was transferred into a vacuum chamber (~10-6 Torr), and a Au electrode (ca. 100 nm thick) were thermally deposited through a shadow mask to define the effective active area of the devices (0.10 cm2). All device fabrication procedures were carried out in a N 2-purged glovebox (
