Rutherford Backscattering Spectroscopy of Mass Transport by Transformation of PbI2 into CH3NH3PbI3 within np-TiO2

Abstract

The hybrid perovskite methylammonium lead iodide (CH 3 NH 3 PbI 3 ) combines a band gap between 1.5 eV and 1.6 eV [1,2], a high absorption coefficient of α ~ 10 4 cm -1 at 600 nm [3] and ambipolar diffusion lengths of around 100 nm [4,5]. The combined properties allowed solar cell efficiency boosting from e.g. 3 % in 2009 [6] to over 15 % in 2013 [7,8] and 16.2 % [9] / 17.9 % [10] in 2014. Perovskite thin film preparation can be drawn back to four major techniques: 1 st a co-evaporation of organic and inorganic precursors [8], 2 nd spin coating from dissolved perovskite [11][12][13][14][15][16][17], 3 rd in a sequential deposition process [7,[18][19][20] and 4 th using spray deposition [21,22]. Individual advantages enabling high efficiency and cost-effective processing are still under debate. Discussed effects range from reliable control over morphology [7,19,20,23], influence on material properties [24,25] to the formation of passivated interfaces [26][27][28].The key reaction in the sequential deposition is the transformation reaction of an inorganic precursor layer such as PbI 2 to the final CH 3 NH 3 PbI 3 by dipping into an organic precursor solution containing CH 3 NH 3 I. Providing high surface to volume ratio, observed reaction kinetics are fast (tens of seconds) [7]. The underlying mechanism is believed to be an insertion reaction, accelerated by the layered PbI 2 crystal structure [7,18,29]. Recently a model of the intercalation process on a molecular level was proposed based on in-situ photoluminescence (PL) studies of the intercalation depending on precursor concentration (3 -15 mM), solvent polarity and size of the organic component [29]. The proposed model starts with transformation of the first PbI 2 layer to an incomplete PbI 4 2-network, while created interstices allow organic intercalation and consecutive conversion of underlying layers [29]. Intercalation kinetics matched the JohnsonMehl-Avrami-Kolmogorov (JMAK) model [29].The objective of this work was the investigation of the Abstract: Mass transport during transformation of PbI 2 infiltrated in nanoporous TiO 2 into CH 3 NH 3 PbI 3 has been investigated by Rutherford backscattering spectroscopy (RBS). Fast initial reaction kinetics were confirmed using optical ex-situ and in-situ measurements. Mapping with energy dispersive X-ray spectroscopy of the cross section of samples revealed a homogeneous PbI 2 infiltration in nanoporous TiO 2 before transformation but an accumulation of Pb and I at the surface after transformation, in accordance with a depletion of Pb and I in a near surface region. Quantitative depth profiles of Pb and I were obtained from RBS analysis. An instant degradation of CH 3 NH 3 PbI 3 to PbI 2 and volatiles upon ion radiation 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 -11 cm²/s) as well as a parameter considering frazzling at the surface due to formation of crystallites.

Organometal halide perovskite solar cells: degradation and stability

“…The in situ techniques are specifically of great importance. X-ray photoelectron spectroscopy (XPS), [244][245][246] Scanning Kelvin probe microscopy (SKPM), 112,185 Rutherford backscattering (RBS), 247 X-rays for reflectometry, Surface Enhanced Raman spectroscopy (SERS), and Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) are some of the surface sensitive techniques.…”

Radio Frequency Magnetron Sputtering Deposition of TiO2 Thin Films and Their Perovskite Solar Cell Applications

“…(PbI 2 ) 1-x (MAPbI 3 ) x films are not converted completely when the immersing time is less than 10 min, and longer immersing time (more than 60 min) leads to fracture of the device. The transformation process of PbI 2 into CH 3 NH 3 PbI 3 has been investigated by Lang et al 22 . They proposed that the diffusion of Pb allows the formation of CH 3 NH 3 PbI 3 from stacked PbI 2 and CH 3 NH 3 I by inter-diffusion, and the perovskite crystallites at the surface are associated with the diffusion.…”

A review of printable, flexible and tissue equivalent materials for ionizing radiation detection

“…The first report of radiation induced damage on perovskites was observed during characterization methods that relied on beam probing with ionizing radiation sources. MAPbI 3 perovskites were observed to be prone to electron (100 keV to 300 keV) and helium ion (1.4 MeV) beam induced damage as detected by microscopy techniques, exhibiting degradation to PbI 2 [187,188]. Electron degradation in perovskites has been observed for energies as low at 1.5 keV during electron beam-induced current profiles, with a significant drop in the peak intensity towards zero after three measurements [189].…”