Defect Formation During the Halide Perovskite Growth: Timing is the Way to Effective Passivation
Ales Vlk a, Nada Mrkyvkova b c, Vladimir Held b, Peter Nadazdy b, Riyas Subair b, Eva Majkova b c, Matej Jergel b c, Martin Ledinsky a, Antonin Fejfar a, Jianjun Tian d, Peter Siffalovic b c
a Laboratory of Thin Films, Institute of Physics, ASCR, Cukrovarnická 10, 162 00 Prague, Czech Republic
b Institute of Physics, Slovak Academy of Sciences, Dúbravská cesta 9, 845 11, Bratislava, Slovakia
c Center for Advanced Materials Application, Slovak Academy of Sciences, Dúbravská cesta 9, Bratislava 845 11, Slovakia
d Institute for Advanced Materials and Technology, University of Science and Technology Beijing, 100083 Beijing, China
International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV22)
València, Spain, 2022 May 19th - 25th
Organizers: Pablo Docampo, Eva Unger and Elizabeth Gibson
Oral, Ales Vlk, presentation 021
Publication date: 20th April 2022

Over the past decade, we are witnessing a rapid growth of power conversion efficiencies of the perovskite solar cells approaching the conventional silicon single-crystal performances. Nowadays, the short circuit currents JSC almost reached the fundamental limit. Therefore, the biggest space for improvement lies in the open-circuit voltage VOC and fill factor FF of a solar cell. Both properties are directly affected by active defects in the active material. VOC is reduced by non-radiative recombination on local defects, and FF is proportional to the charge carrier’s lifetime, which is also affected by defect density. A detailed understanding of the defect formation process is therefore crucial for its reduction and improvement of the final photovoltaic properties.

In our study, we used a simultaneous measurement of in-situ photoluminescence (PL) and grazing-incidence wide-angle X-ray scattering (GIWAXS) to observe the perovskite layer crystallization process in real-time. We show that the layer formation process can be divided into three stages:

1. The growth starts with nucleation and free growth of perovskite grains. According to the high PL intensity, the defect density remains at a very low level during this stage.

2. In the second stage, the PL intensity decreases by order of magnitude, although the perovskite GIWAXS signal proves continual film growth. It indicates the formation of defective grain boundaries, and the non-radiative charge carriers recombination takes place. This corresponds to an increase of non-radiative losses in VOC by more than 100 mV. Direct passivation of these defects during this growth stage will significantly increase the final solar cell performance.

3. During the final part, the perovskite film slowly decomposes into PbI2. This process passivates the surface, and the photoluminescence signal is partially recovered. This implies that although post-growth defect passivation is possible, it cannot cure all active recombination sites. [1]

Our study provides direct control of perovskite growth and, more importantly, defect formation and eventual passivation. The most significant advantage of such an approach is that the addition of passivating agents can be precisely timed, and the effect immediately measured. With the correct timing, the formation of defects at grain boundaries may be suppressed. This will lead to a significant improvement of VOC and FF in the finalized photovoltaic device.

We acknowledge the financial support of projects APVV-17-0352, APVV-15-0641, APVV-15-0693, APVV-19-0465, SK-CN-RD-18-0006, SK-AT-20-0006, APVV-14-0745, APVV-14-0120, VEGA 2/0059/21, VEGA 2/0041/21, ITMS 26230120002, ITMS 26210120002, and ITMS 26210120023. This work was performed during the implementation of the project Building-up Centre for Advanced Materials Application of the Slovak Academy of Sciences, ITMS project code 313021T081, supported by the Research & Innovation Operational Programme funded by the ERDF. The authors also acknowledge the support of the Operational Programme Research, Development, and Education financed by the European Structural and Investment Funds and the Czech Ministry of Education, Youth and Sports (project no. CZ.02.1.01/0.0/0.0/16_019/0000760—SOLID21, CzechNanoLab Research Infrastructure LM2018110 and CZ.02.1.01/0.0/0.0/16_026/0008382 CARAT) and the National Key Research and Development Program of China (2017YFE0119700).

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