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Physics of degradation in organic, nanocrystal, and hybrid solar cells – PROCES

Colloidal solar cells and perovskite solar cells: What factors control their stability? Can one improves it?

«Third-generation« solar cells based on colloidal nanocrystals and perovskites are emerging technologies. While their efficiencies are being improved, the Achilles heel lies in their instability. Fundamental understandings on their degradation is indispensable for their further development.

Understanding and mitigating the stability issues in colloidal quantum dot (QD) solar cells and perovskite solar cells

Solar cells based on colloidal nanocrystal QDs and hybrid perovskite have shown remarkable progress in terms of power-conversion efficiency over the past decade. Yet the Achilles heel of these devices lies in their environmental instability. This project thus aims to develop fundamental understanding of the physicochemical processes leading to their degradation. Different fundamental and technical aspects related to their stability were studied: (i) The relationship between the ion migration-induced electrode degradation and the operational stability of MAPbI3-based perovskite solar cells; (ii) On triple-cation perovskite solar cells, the impact of the excess interfacial PbI2 on solar cell stability. (iii) On PbS QD solar cells, the degradation origins in PbS QD solar cells under oxygenated environments. (iv) On triple-cation perovskite solar cells, the engineering solution to apply TiO2 nanocolumn electron transport layers (ETLs) for stability/performance improvements. This project strongly boosted French-Germany collaboration between LPEM and TU-Dresden establishing a research platform to tackle the complex and multi-domain challenges to maximize the stability of colloidal and perovskite solar cells. The results obtained provided fundamental knowledge necessary for the future development of these technologies.

The obtention of the results in this project was based on the various methods grouped in the following two areas: (1) Material synthesis, modification, and physical properties; and (2) Photovoltaic device fabrication & controlled degradation. Specifically, (1) includes colloidal synthesis, the synthesis of different perovskite precursors, the optimization of QD ligand exchange methods, the fabrication of thickness-tunable thin film from solution, the application of interfacial modification chemical treatments, and the detailed characterizations on structural, optical, thermal, and spectroscopic aspects. (2) includes the fabrication of colloidal and perovskite solar cells, the experiments of different device architectures, the exploration of charge transport layer engineering, the application of different solar cell degradation conditions, the examination of photovoltaic parameters and efficiencies, the correlation between these parameters and results from impedance spectroscopy, and the examining the degraded thin film and solar cells by different structural, optical and spectroscopic techniques to reveal the physical processes during solar cell degradation and the effectiveness of a specific approach against degradation.

This 42-month ANR-DFG France Germany collaboration project was carried out by a tight collaboration between two partners from Heidelberg University/TU Dresden (Germany) and the LPEM (Laboratoire de Physique et d'Etude des Matériaux, a UMR research unit of CNRS/ESPCI-ParisTech/Sorbonne Université/Université PSL). It was built on the strength and expertise of these two teams to allow for a multidisciplinary investigation. Different fundamental knowledge and technical approaches related to the stability of solar cells base on colloidal nanocrystals and perovskite solar cells were obtained/explored: (i) The investigation of ion migration-induced electrode degradation on the operational stability of MAPbI3-based perovskite solar cells. We found that blocking the ion migration to the electrode did not necessarily improve the device stability significantly under operational conditions, suggesting other physical mechanisms (than ion migration to electrode) being the culprit of degradation; (ii) On triple-cation perovskite (Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3) solar cells, we demonstrated different surface chemical treatment routes to the remove the excess interfacial PbI2 leading to a significant improvement of device stability of under continuous illumination. (iii) On PbS QD solar cells, we demonstrated that the degradation processes in PbS QDs which are exposed to oxygenated environments are tightly related to the choice of nanocrystal ligands. (iv) On triple-cation perovskite solar cells, we explored the application of a series of TiO2 nanocolumn photonic ETLs. Improvement on both the power conversion efficiency of the perovskite solar cells together with a prolonged solar cell storage shelf life were achieved thanks to the application of TiO2 nanocolumn ETL. The initial objectives of the project have been well realized. Key results have been published in 18 international peer-reviewed journals and 11 conference and workshop presentations. This project strongly boosted the research capability of each team establishing a research platform to tackle the complex and multi-domain challenges to maximize the environmental stability in solar cells based on colloidal inorganic nanocrystals and hybrid perovskite halides. These results obtained provided fundamental knowledge necessary for the future design of more-performing and more-stable perovskite and colloidal QD solar cells.

Various open questions on different aspects are raised at the end of this project: For example, as the culprit of the colloidal PbS QD solar cells was identified as the ethanedithiol-capped PbS QDs, can one find an effective and more robust replacement, with suitable energetic levels and hole transport property, to obtain degradation-free colloidal solar cells? On perovskite solar cells, we found that the device degradation was not associated with ion-migration electrode degradation but rather the diffusion of volatile products out of the perovskite active layer and into the PCBM (Phenyl-C61-butyric acid methyl ester) layer. Does it exist a device architecture and/or suitable charge transport layers where the diffusion of such volatile products can be blocked? We found that the excess PbI2 at the perovskite/HTL (hole transport layer) interface is detrimental on device stability under continuous illumination. How about the excess PbI2 inside the perovskite film (non-interfacial)? Can one obtain a trade-off between performance (which requires slight excess of PbI2) and stability (where excess PbI2 is detrimental) ? Can the engineering approaches developed in this project (e.g. removal of interfacial PbI2 by the treatments of organic salt solutions, application of photonic nanocolumn array ETLs) be applied into other form of solar cells, e.g. semi-transparent perovskite solar cells or tandem perovskite/Si solar cells? On this final point, to follow up this PROCES project, we submitted a new collaborative project («TESSERAE«, ANR-DFG 2022 call) to consolidate our collaboration with TU-Dresden team.

This PROCES Project has produced 18 publications in international peer-reviewed journals. Among these 18 publications, there are 4 co-authored by the French (LPEM) and the German (TU Dresden) partner, 6 mono-partner publications by the French team (LPEM), and 8 mono-partner publications by the German team (TU Dresden). The list of publications in international peer-reviewed journals can be found below.
(i) B. Rivkin et al., ACS Omega, 3, 10042-10047 (2018);
(ii) M. Schoenauer Sebag et al., ACS Appl. Energy Mater., 1, 3537-3543 (2018);
(iii) D. Becker-Koch et al., Journal of Physics: Condensed Matter, 31 124001 (2019);
(iv) A. Weu et al., ACS Appl. Energy Mater. 2 (3), 1943 (2019);
(v) D. Becker-Koch et al., Sustainable Energy & Fuels, 4, 108 (2020);
(vi) Z. Hu et al., ACS Appl. Mater. Interfaces, 12, 49, 54824-54832 (2020);
(vii) L. Kuai et al., ACS Energy Lett., 5, 1, 8-16 (2020);
(viii) Y. J. Hofstetter et al., Front. Chem. 66 (2020);
(ix) A. Weu et al., Adv. Funct. Mater. 30 (5), 1907432 (2020);
(x) L. M. Falk et al., Energy Tech. 8 (4), 1900737 (2020);
(xi) Z. Hu et al., ACS Appl. Mater. Interfaces, 12, 5, 5979-5989 (2020);
(xii) Z. Hu et al., Journal of Applied Physics, 127, 125113 (2020);
(xiii) C. Xin et al., Materials Today Energy, 22, 100859 (2021);
(xiv) M. Albaladejo-Siguan et al., Adv. Energy Mater. 11 (12), 2003457 (2021);
(xv) D. Becker-Koch et al., ACS Appl. Mater. Interfaces 13, 16, 18750–18757 (2021);
(xvi) Z. Hu et al., Nanoscale Advances, 4, 7, 1786-1792 (2022);
(xvii) D. Becker-Koch et al., Nanoscale 14, 3020-3030 (2022);
(xviii) O. Telschow et al., J. Mater. Chem. A, 2022, Advance Article (https://doi.org/10.1039/D1TA10566C).

Facing the rising energy usage worldwide, we urgently need to increase the proportion of electricity generated from clean and renewable energy sources. Organic, colloidal nanocrystal quantum dots (QDs), and hybrid organic-inorganic perovskites are highly promising solution-processable material candidates for "third-generation" solar cells. Their unique material characteristics can lead to flexible, light-weight, low-cost and high-performance solar cells and enable non-conventional solar cell products. While the efficiencies are being improved constantly by intensive research, the Achilles heel of these devices seems to be their environmental instability. So far only limited research has been done to study the fundamental causes and mechanisms leading to the environmental instability in devices based on organic, nanocrystal and hybrid perovskite materials. Developing a clear understanding of the physicochemical processes of degradation would aid the integration of these devices into industrial applications, guiding both material and device engineering to improve device lifetimes.

Therefore, in this "PROCES" project we aim to (1) identify the fundamental causes of degradation of organic, inorganic nanocrystal and hybrid organic-inorganic thin films; (2) understand the physical origin of degradation, i.e. the formation of degradation products; (3) correlate the changes in device characteristics to the causes identified; and (4) develop strategies to improve material and device stability.

It can be anticipated that through this study we will gain fundamental understanding of how different choices of materials (organic, nanocrystal, or hybrid components), their synthetic and surface chemistry, and different device architectures, impact on the device degradation mechanisms. Understanding these aspects will not only lead to organic, quantum dot and hybrid solar cells with improved device lifetimes, but also offer material and device design guidelines for further optimization of future third-generation photovoltaics.

This 3-year ANR-DFG project will be carried out by a tight collaboration between two research teams from Heidelberg University (Germany) and the LPEM (Laboratoire de Physique et d'Etude des Matériaux, a research unit of CNRS/ESPCI-ParisTech/Université Pierre et Marie Curie). The project will build on the strength and expertise of these two teams to allow for a multidisciplinary investigation. The results of this collaborative approach will boost the research capability of each team surpassing its current level and allowing an ideal platform to tackle the complex and multi-domain challenges to maximize the environmental stability in organic, inorganic nanocrystal and hybrid organic-inorganic material systems.

Project coordinator

Madame Zhuoying CHEN (Laboratoire de Physique et d'Etude des Matériaux)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

LPEM Laboratoire de Physique et d'Etude des Matériaux
Heidelberg University Kirchhof Institute for Physics, Heidelberg University

Help of the ANR 170,314 euros
Beginning and duration of the scientific project: September 2018 - 36 Months

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