Recombination dynamics of small vs. >1 micron perovskite polycrystalline grains

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

Poster, Thomas Stergiopoulos, 451
Publication date: 5th February 2015

For perovskite heterojunction solar cells, there are two main strategies to achieve high performance: (1) having a dense-grained uniform morphology for the perovskite film with typical grain sizes being some hundreds of nm to prevent shunting paths at the uncovered surface, and (2) attaining large crystalline domains with grain sizes at least over 1 micron to assure low trap density and achieve perfect charge transport [1]. For the former, usually fast-induced perovskite crystallization is applied while in the case of the latter, solvent (or co-solvent) long annealing is preferred. In any circumstance, fast-induced perovskite crystallization seems to be the most prevailing strategy thus far to get efficiencies of more than 18% accompanied with a high Voc of 1.1 V [2]; however, it is not clear at the moment the reason behind that. Thus, in this work, we attempt to shed more light on this issue, adopting a novel route to make perovskite films of high quality, previously established by us in a very recent work as an example of fast-induced crystallization [3]. The process consists of the use of lead acetate, Pb(ac)2, as the source of Pb2+, which assists on the crystallization of the standard CH3NH3PbI3 perovskite in only a few minutes of annealing [4], resulting in the formation of ultra-smooth films with nearly perfect coverage. To have a fair comparison, we compare Pb(ac)2 with another typical experimental route, governed by much longer crystallization times of the perovskite, out of PbCl2/CH3NH3I chlorine-including precursors. In terms of the devices, we focus on the fabrication of planar heterojunction solar cells which generally suffer from more severe recombination than standard devices which employ a mesoporous blocking scaffold. Then, we study the recombination dynamics with a range of electrooptical-spectroscopic characterizations including electrochemical impedance along with time-resolved photoluminescence spectroscopies. Our findings confirm a distinct difference between the ideality factors and recombination orders of the cells along with diffusion lengths and lifetimes for the films, as expected from the different kinetics and chemistry of film formation, pointing out to a much decreased recombination rate in the case of the lead acetate route. These results refer to planar heterojunction solar cells; however, we argue that they can be also applied for record-efficient meso-structured solar cells which employ a thick capping layer of perovskite ontop of the scaffold.

[1] Stranks S. D.; Nayak, P. K.; Zhang W.; Stergiopoulos T.;Snaith H. J. Formation of thin films of organic–inorganic perovskites for high-efficiency solar cells, Angew. Chem. Int. Ed. 2015, 127, 3240–3248. [2] Heo J. H.; Han, H. J.; Kim D.; Ahn T. K.; Im S. H. Hysteresis-less inverted CH3NH3PbI3 planar perovskite hybrid solar cells with 18.1% power conversion efficiency, Energy Environ. Sci. 2015, DOI: 10.1039/c5ee00120j.

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