Transformation of PbI2 into CH3NH3PbI3 and Hole Blocking Behavior of the PbI2/CH3NH3PbI3 Interface

Thomas Dittrich^a, Marin Rusu^a, Ah Reum Jeong^a, Felix Lang^a, Albert Juma^a^,^b, Voranuch Somsongkul^a^,^d, Marisa Arunchaiya^c

^aTallinn University of Technology, Deparment of Materials Science, Ehitajate tee 5, Tallinn 19086

^bDepartment of Materials Science, Faculty of Science, Kasetsart University,, Bangkok 10900

^cDepartment of Industrial Chemistry, Faculty of Applied Science, King Mongkut’s University of Technology North Bangkok, Bangkok 10800

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Poster, Felix Lang, 013
Publication date: 5th February 2015

A key technique in methylammonium lead iodide (CH₃NH₃PbI₃) thin film preparation is the phase transformation of an inorganic precursorlayer such as PbI₂, infiltrated into nanoporous TiO₂ to the final CH₃NH₃PbI₃ by dipping into an organic precursor solution containing CH₃NH₃I. The detailed transformation mechanism of this two step preparation method is believed to be an insertion reaction.^1,2Here, we present an investigation on the mass transport during transformation by Rutherford backscattering spectroscopy (RBS). Energy dispersive X-Ray (EDX) spectroscopy mapping of cross sections first, revealed a homogeneous PbI₂ infiltration into nanoporousTiO₂ before transformation and second, an accumulation of Pb and I at the surface after transformation. Quantitative depth profiles of Pb and I were obtained from RBS analysis. An instant degradation of CH₃NH₃PbI₃upon⁴He⁺ ion irradiation was found. The concentration profiles of Pb could be simulated with a one dimensional diffusion model taking into account an effective diffusion coefficient of Pb in the nanocomposite (about 1.5∙10^-11cm²/s) as well as a parameter considering frazzling at the surface due to formation of CH₃NH₃PbI₃crystallites.³ As a consequence of the complex sequential transformation mechanism PbI₂ remains between the TiO₂/CH₃NH₃PbI₃ interface. The charge selective properties of these two emerging interfaces, TiO₂/PbI₂ and PbI₂/CH₃NH₃PbI₃ were studied using surface photovoltage (SPV) spectroscopy. As cross comparison perovskite layers were deposited by solution based one step preparation and two step preparation methods. For the two step preparation on TiO₂, the SPV signal related to absorption in CH₃NH₃PbI₃ increased in comparison to the one step preparation due to possible electron transfer from CH₃NH₃PbI₃ via PbI₂ into TiO₂ whereas the SPV signal related to defect transitions decreased. In addition hole transfer from CH₃NH₃PbI₃ towards the metal oxide was blocked by the PbI₂ interlayer for the two step preparation, as investigated on MoO₃.⁴ The found mass transport, first seems to be a limiting process in the phase transformation to CH₃NH₃PbI₃, and second may alter the PbI₂/CH₃NH₃PbI₃ interface, forming a hole blocking PbI₂/CH₃NH₃PbI₃interface

(1) Liang, K.; Mitzi, D. B.; Prikas, M. T. Synthesis and Characterization of Organic−Inorganic Perovskite Thin Films Prepared Using a Versatile Two-Step Dipping Technique. Chem. Mater. 1998, 10, 403–411. (2) Ahmad, S.; Kanaujia, P. K.; Niu, W.; Baumberg, J. J.; Vijaya Prakash, G. In Situ Intercalation Dynamics in Inorganic-Organic Layered Perovskite Thin Films. ACS Appl. Mater. Interfaces 2014, 6, 10238–10247. (3) Lang F., Juma A., Somsongkul V., Dittrich T., Arunchaiya, M. Rutherford Backscattering Spectroscopy of Mass Transport by Transformation of PbI2 into CH3NH3PbI3. Hybrid Mater. 2014, accepted. (4) Somsongkul, V.; Lang, F.; Jeong, A. R.; Rusu, M.; Arunchaiya, M.; Dittrich, T. Hole Blocking PbI2/CH3NH3PbI3 Interface. Phys. status solidi - Rapid Res. Lett. 2014, 08, 763–766.

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