Full Length ArticleInfiltration of methylammonium metal halide in highly porous membranes using sol-gel-derived coating method
Graphical abstract
CH3NH3PbI3 perovskite layer coated via a facile sol-gel-derived solution process shows excellent pore filling and full coverage on various nanostructures.
Introduction
Organic–inorganic halide perovskites (OIHPs) have been spotlighted as promising optoelectronic materials since methylammonium lead iodide (CH3NH3PbI3; MAPbI3), one of the OIHPs that has a perovskite crystal structure, was adopted as a light-absorbing (and/or charge-transport) material of organic-inorganic hybrid solar cells [1]. MAPbI3 has large light-absorption coefficients (∼105 cm−1 at 550 nm) owing to its direct bandgap (∼1.55 eV) [2], where intramolecular charge transfer occurs between high-density orbitals in valence (Pb s and I p orbitals) and conduction (Pb p orbitals) band edges [3], [4]. Moreover, MAPbI3 easily produces free charges because of its weak exciton binding energy and has good ambipolar charge-transport characteristics, with electron and hole mobilities of ∼7.5 and ∼12.5 cm2 V−1 s−1, respectively [5]. The outstanding optoelectronic properties of OIHPs enable their successful application in perovskite solar cells (PSCs) [6], [7], solar-water splitting [8], lasing [9], and light-emitting diodes [10].
For most of these applications, nanometer-scale OIHP layers, such as nanoshells over nanoparticles (NPs) [11] or thin films filling mesoporous (mp) structures [12], are necessary. Especially in PSCs, the full surface coverage and jam-full pore filling with an OIHP over/into nanostructured electron transport layer is of great importance for the device performance [13], [14]. Such nano-OIHP layers are mainly fabricated via solution processes. Various spin-coating-based solution processes, such as two-step [14], [15], [16], anti-solvent treatment [17], [18], and ion-exchange methods [19], [20], have been developed. However, these methods are optimized for PSCs and not for other nanostructures, such as one-dimensional nanorods (NR) or deep pores with a high aspect ratio, which may be useful for other applications in the near future. Besides, there are several vapor deposition methods, including evaporation [21] and atomic layer deposition (ALD) [22]. However, evaporation can only be used for making planar-type films, owing to the non-conformal step coverage and the shadow effect, and ALD requires considerable time and cost to form an OIHP film with a sufficient thickness, although it can produce high quality OIHP nanoshells on nanostructures. Therefore, well-established solution processes are presently considered to be the best methods for achieving high-performance devices at a low cost.
To fabricate nano-OIHP layers on nanostructured membranes via a solution process, sufficient infiltration of precursor materials into the nanostructures must be achieved before the reaction among the materials. In this aspect, the sol-gel method, a facile classical solution process, is a suitable process to produce a full-filling OIHP layer. The sol-gel method is a low-cost process and is suitable for mass production via large-area coating [23], [24], [25], [26]. Moreover, large variety of choice for solute and solvent guarantees the successful setup of an efficient route.
Herein, we report a sequential solution route, starting from a sol-gel-processed PbO coating, to fabricate a uniform MAPbI3 film that fully covers and completely fills various nanostructured membranes, such as a TiO2 mp film, TiO2 NRs, and porous anodic aluminum oxide (AAO). Excellent pore filling of the sol-gel-processed films compared with films fabricated via other solution processes is confirmed by compositional and morphological analyses.
Section snippets
Chemicals and materials
Lead acetate trihydrate (99%) and 2-methoxyethanol (2ME, ≥99%) were purchased from Duksan and Acros, respectively. Ethanol (99.5%) and 2-propanol (99.5%) were purchased from Dychemi and Sigma-Aldrich, respectively. Monoethanolamine (MEA, ≥98%), hydroiodic acid (HI, 57 wt% in H2O) and titanium tetra-isopropoxide (TTIP, 97%) were purchased from Sigma–Aldrich. TiO2 NP (20 nm) paste for the mp-TiO2 film was purchased from Dyesol and diluted with ethanol before use. Methylamine iodide (MAI, 99.5%) and
Results and discussion
The full-filling MAPbI3 layer was formed on various mesoporous membranes using the aforementioned sol-gel-derived process. In short, a Pb-acetate solution is coated on the surface of the nanostructure to form a PbO shell via condensation and hydrolysis, followed by heating to crystallize the oxide shell. Then, PbO is sequentially transformed to PbI2 and CH3NH3PbI3 by ion exchange (O2− to 2I−) and reaction with methylammonium iodide (MAI), respectively. Among the processes, the first step (PbO
Conclusion
In summary, we successfully developed a sol-gel-derived sequential solution route for the formation of an MAPbI3 layer. With this sol-gel-based method, MAPbI3 can be better incorporated into highly porous nanostructures—such as TiO2 NP-based mesoporous films, TiO2 NRs, and nanopores of AAO—than with the other conventional methods. The superior infiltration characteristic of the lead-acetate solution is attributed to the strong hydrogen bonding between the carboxyl groups of lead acetate and the
Conflict of interest
The authors declare no competing financial interest.
Acknowledgement
This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2012M3A7B4049967, NRF-2014R1A4A1008474 and NRF-2016R1C1B2013087).
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