The impact of metal ions doping on the defect chemistry of methylammonium lead iodide
Nga Phung a, Antonio Abate a, Daniele Meggiolaro b c, Filippo De Angelis b c, Roberto Felix Duarte a, Marcus Bär a
a Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany, Berlin, Germany
b Istituto Italiano di Tecnologia (IIT), Genova, Italy, Via Morego, 30, Genova, Italy
c Computational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), CNR-ISTM, Perugia, Italy
Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics
Proceedings of International Conference on Perovskite and Organic Photovoltaics and Optoelectronics (IPEROP19)
Kyōto-shi, Japan, 2019 January 27th - 29th
Organizers: Hideo Ohkita, Atsushi Wakamiya and Mohammad Nazeeruddin
Oral, Nga Phung, presentation 039
Publication date: 23rd October 2018

To date, halide perovskites are extraordinary photovoltaic materials that enabled to prepare solar cells of more than 23% power conversion efficiency which surpasses commercialised PV technologies based on CIGS and CdTe [1]. Nonetheless, the operational long-term stability is one of the most pressing challenges that may hinder their commercialization. The devices lose initial efficiency because of the convolution of external factors such as moisture, heat, light, oxygen, as well as intrinsic instability of the perovskite. Recent studies have shown that many degradation pathways start from the defects of the material. The films with a large defect concentration are more prone to degradation. Hence, a key pathway towards the long-term stability and high efficiency of halide perovskite solar cells is to reduce the defects concentration presented in the material. Incorporating small amounts of metal cations into the perovskite precursors has been shown to improve device performance and stability. For example, monovalent cations such as rubidium, and potassium are particularly effective to boost efficiency in state-of-the-art devices [2].

More recently, divalent cations especially alkaline earth metals are attracting growing attention. It has been speculated that the divalent cations such as strontium can passivate the surface of perovskite and significantly improves device’s performance [3]. It seems that the small amount of foreign ions in the precursor has a different working mechanism compared to the classical doping of inorganic semiconductor in which the charge carrier concentration is changed due to dopants.

Herein, we propose a metal cation doping mechanism of halide perovskite to passivate defects. In this work, we explore the effect of strontium and magnesium on methylammonium lead iodide. We use advanced characterisation technique based on synchrotron high energy X-ray source to understand the impact of foreign ions on the chemical and electronic environment of the material. Furthermore, X-ray Diffraction patterns refinement creates an opportunity to identify the interaction of the dopants with methylammonium lead iodide lattice. From the experimental results, we speculate that the dopants affect the defect chemistry of perovskite which is supported by the theoretical calculation. Then, we prepare perovskite solar cells to prove the mechanism in the state-of-the-art devices. As a result, we are able to demonstrate more than 1.15 V open circuit voltage for doped methylammonium lead iodide which is much higher than commonly reported 1 V for this type of perovskite solar cells.

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