Nitrogen soaking promotes lattice recovery in polycrystalline hybrid perovskites
alessandra alberti a, ioannis deretzis a, giovanni mannino a, emanuele smecca a, filippo giannazzo a, andrea listorti b, silvia colella b, sofia masi b, antonino la magna a
a Institute for Microelectronics and Microsystems (CNR-IMM), Zona Industriale - VIII Strada 5, Catania 95121, Italy
b Università del Salento, 1 Dipartimento di Matematica e Fisica “E. de Giorgi”, LECCE, 73100, Italy
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV19)
Roma, Italy, 2019 May 12th - 15th
Organizers: Prashant Kamat, Filippo De Angelis and Aldo Di Carlo
Oral, alessandra alberti, presentation 036
Publication date: 11th February 2019

Organic-inorganic hybrid perovskites are nowadays considered as reference materials for optoelectronics and photovoltaics. Their outstanding yields in luminescence, electroluminescence and photoelectric conversions, combined with the easy solution processing, promise large-scale production, wide dissemination and high technological throughput.

However, the well-known structural instability of hybrid perovskites under working conditions limits a real market uptake of all the related technologies. Solutions to guarantee the durability of products are mandatory and can be numerous, if the problem is addressed from different perspectives. 

Moreover, the plethora of perovskite materials produced in different laboratories, with preparation procedures changing from lab to lab and from time to time, ask for stabilising  treatments that, in a certain extent, reset the starting conditions in a convenient and reproducible manner. This is particularly needed for small grained perovskite layers, on one hand used to implement pinhole-free coverages, but, on the other hand, offering extended lattice discontinuities due to the high surface to volume ratio. In addition to morphology and environment, temperature-related effects are unavoidable during device operation. A rationalization of the phenomena needs to embody heating and thermal cycles to preserve the material integrity and/or the structural reversibility vs. temperature.

In this framework and on the basis of experiment and theory, we draw a general paradigm that reconsiders N2 not simply being an inert species but rather a small effective healing gas molecule inside a MAPbI3 layer. Nitrogen is soaked into polycrystalline MAPbIvia a post-deposition mild thermal treatment under slightly overpressure conditions in order to promote its diffusion through the whole layer. We observe a significant reduction of radiative recombination and a concurrent increase of light absorption, with a maximum benefit at 80 °C. Concomitantly, the current of holes locally drawn from the surfaces by a biased a tip with nanometric resolution has increased by a factor 3 under N2. This was framed by a reduction of the barrier for the carrier extraction. The achieved improvements were linked to a nitrogen-assisted recovery of intrinsic lattice disorder at the grain shells along with a simultaneous stabilization of under-coordinated Pb2+species and MAcations through weak electrostatic interactions. Defect mitigation under Nis further supported by the behavior of the absorption coefficient during thermal cycles in comparison to similar data under Argon. We additionally unveil that surface stabilization through Nis morphology-independent and can be thus applied after any preparation procedure.

Such simple and low-cost strategy could complement other stabilizing solutions when building perovskite solar cells or light-emitting diodes.

Paper just accepted in Advanced Energy Materials




This activity was partially supported by the national project BEYOND NANO Upgrade (CUP G66J17000350007). I.D. and A.L. acknowledge computational support from the CINECA consortium under project MD-HYPER. SC, SM and AL acknowledge the project PERSEO-“PERovskite-based Solar cells: towards high efficiency and long-term stability” (Bando PRIN 2015-Italian Ministry of University and Scientific Research (MIUR) Decreto Direttoriale 4 novembre 2015 n. 2488, project number 20155LECAJ) for funding. SC and AL acknowledge Regione Puglia and ARTI for founding FIR - future in research projects “PeroFlex” project no. LSBC6N4 and “HyLight” project no. GOWMB21. The authors also wish to thank prof. Tsutomu Miyasaka (Toin University of Yokohama) and co-workers for the kind availability in opening their laboratories to prepare textured perovskite layers as references.

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