Assessment of Carrier Diffusivity in Organic-Inorganic Perovskite 3D and quasi-2D Layers Using LITG Technique
Ramūnas Aleksiejūnas a, Patrik Ščajev a, Toshinori Matsushima b, Chihaya Adachi b, Saulius Juršėnas a
a Institute of Photonics and Nanotechnology, Vilnius University, Sauletekis Av. 3, LT-10257 Vilnius, Lithuania
b Center for Organic Electronics and Photonics Research (OPERA), Kyushu University 744 Motooka, Nishi, Fukuoka 819-0395, Japan
Poster, Ramūnas Aleksiejūnas, 104
Publication date: 25th November 2019

Organic-inorganic perovskites have earned the reputation of materials suitable for cheap and scalable production of not only the solar cells, but also the thin film transistors, light emitting diodes, and even lasers. Mobility and lifetime of charge carriers play a crucial role in these devices; however, some internal processes governing these parameters remain understudied. In particular, little is known about the diffusivity of holes and its dependence on carrier density or carrier localization parameters, mostly due to experimental difficulties.

In this presentation, we demonstrate the applicability of all-optical light-induced transient grating (LITG) technique for measurements of carrier diffusion coefficient in various perovskite samples. This method has several advantages, like no need for electrical contacts, the possibility to extract the mobility of holes in both doped and intrinsic semiconductor, and the ability to study the dependence of carrier diffusivity and lifetime on carrier density, especially at high excitations. 

Based on LITG results in bulk CH3NH3Pb(Sn)X3 layers deposited by vapor deposition or spin-coating techniques, we demonstrate two distinct regimes of carrier transport in the perovskites, namely band-like or localization-limited carrier diffusion [1]. Band-like diffusion with typical ambipolar diffusion coefficient of ~1 cm2/s takes place in high structural quality layers. This value is determined by the fundamental material properties and is controlled by polar-optical electron-phonon scattering. It increases weakly with carrier density due to degeneracy of carriers and is limited by screened electron-hole scattering at high carrier densities. In contrast, the trap-limited diffusion varies a lot from 10-4 to 1 cm2/s, due to strong dependence on densities of traps and charge carriers. In the latter case, the diffusivity is very sensitive to layer deposition technology. It can be enhanced, e.g., by using chemical additives during MAPbX3 layer formation [2]. On the other hand, wet cast MASnI3 layers show intrinsically high diffusivity of carriers comparable to that of vapor deposited layers or crystalline MAPbX3 samples, despite very high background p-type doping [3].

Finally, we show that diffusion coefficient in quasi-2D PEA2FAn-1PbnBr3n+1 samples depends on the number n of perovskite layers. We observe that diffusion coefficient is by order of magnitude larger in the thinnest layers with n = 1 and 2 at lower carrier densities, which points out to the two-dimensional nature of these structures. The thicker layers with more periods demonstrate the trap-limited diffusion regime, quite resembling that in bulk CH3NH3PbX3.

Vilnius University team acknowledges the financial support provided by Research Council of Lithuania under the project no. S-MIP-17-71. 

© Fundació Scito
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