Universität zu Köln

Heshmati, Niusha ORCID: 0009-0000-4310-7439 (2025). Ecofriendly Processing of Stable Lead Halide Perovskite Solar Cells with Green Solvents. PhD thesis, Universität zu Köln.

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Abstract

Lead halide perovskites have been widely studied as potential materials in the field of photovoltaics, owing to their exceptional optoelectronic properties, low fabrication cost, and solution-processability. Despite their rapid progress, two major challenges continue to hinder their large-scale commercialization: the toxicity and the intrinsic instability of the perovskite structure. In this thesis, both issues have been addressed through the development of alternative processing routes and compositional engineering strategies. In the first part of this work, a green chemistry approach was employed to eliminate the toxic polar aprotic solvent dimethylformamide (DMF), which is conventionally used in the fabrication of perovskite films. A biodegradable, cellulose-based solvent system dihydrolevoglucosenone (Cyrene™) and 2-Methyltetrahydrofuran (2-MeTHF) with addition of co-solvent Dimethyl sulfoxide (DMSO) was leading to the complete removal of DMF. The solar cells fabricated using green solvent and additives achieved 95% of the performance of the DMF-based perovskite solar cells, demonstrating the feasibility of green solvent processing without performance loss. The second part of this thesis focused on the structural stability of lead halide perovskites. Particularly, bromide-based perovskites due to their enhanced structural and operational stability as well as their high open-circuit voltage (~ 1.4-1.5 V) compared with silicon solar cells (~ 0.6-0.7 V) and MAPbI3 perovskites (~ 1.1 V). These properties make bromide perovskites suitable for applications such as photoelectrochemical (PEC) water splitting. Therefore, MAPbBr3 solar cells were fabricated under ambient conditions and integrated into a PEC system without any encapsulation for the first time. A photocurrent density of 5.3 mA/cm2 at 1.23 V (vs. RHE) with stability over two hours were achieved. Encouraged by these results, A-site cation engineering was performed to further enhance the structural order and optoelectronic properties of the bromide perovskites. Multiple cations, including Methylammonium (MA+), Formamidinium (FA+), Guanidinium (GA+), and cesium (Cs+), were introduced into the perovskite lattice. Counterintuitively, increased compositional complexity reduced the lattice disorder and improved the dynamic lattice coherence and optoelectronic properties. To gain deeper insights into the nature of the improved structural stability, high-quality single crystals of the optimized composition were grown. Temperature-dependent single crystal X-ray diffraction (XRD) and Raman spectroscopy revealed uniquely stable cubic phase maintained down to 80 K which is never seen before. These effects were attributed to a more favorable lattice configuration facilitated by the mixed-cation system. In summary, this thesis contributes novel solutions for enhancing the sustainability and stability of lead halide perovskites, offering promising pathways for the industrial deployment of perovskite-based solar cells. Lead halide perovskites have been widely studied as potential materials in the field of photovoltaics, owing to their exceptional optoelectronic properties, low fabrication cost, and solution-processability. Despite their rapid progress, two major challenges continue to hinder their large-scale commercialization: the toxicity and the intrinsic instability of the perovskite structure. In this thesis, both issues have been addressed through the development of alternative processing routes and compositional engineering strategies. In the first part of this work, a green chemistry approach was employed to eliminate the toxic polar aprotic solvent dimethylformamide (DMF), which is conventionally used in the fabrication of perovskite films. A biodegradable, cellulose-based solvent system dihydrolevoglucosenone (Cyrene™) and 2-Methyltetrahydrofuran (2-MeTHF) with addition of co-solvent Dimethyl sulfoxide (DMSO) was leading to the complete removal of DMF. The solar cells fabricated using green solvent and additives achieved 95% of the performance of the DMF-based perovskite solar cells, demonstrating the feasibility of green solvent processing without performance loss. The second part of this thesis focused on the structural stability of lead halide perovskites. Particularly, bromide-based perovskites due to their enhanced structural and operational stability as well as their high open-circuit voltage (~ 1.4-1.5 V) compared with silicon solar cells (~ 0.6-0.7 V) and MAPbI3 perovskites (~ 1.1 V). These properties make bromide perovskites suitable for applications such as photoelectrochemical (PEC) water splitting. Therefore, MAPbBr3 solar cells were fabricated under ambient conditions and integrated into a PEC system without any encapsulation for the first time. A photocurrent density of 5.3 mA/cm2 at 1.23 V (vs. RHE) with stability over two hours were achieved. Encouraged by these results, A-site cation engineering was performed to further enhance the structural order and optoelectronic properties of the bromide perovskites. Multiple cations, including Methylammonium (MA+), Formamidinium (FA+), Guanidinium (GA+), and cesium (Cs+), were introduced into the perovskite lattice. Counterintuitively, increased compositional complexity reduced the lattice disorder and improved the dynamic lattice coherence and optoelectronic properties. To gain deeper insights into the nature of the improved structural stability, high-quality single crystals of the optimized composition were grown. Temperature-dependent single crystal X-ray diffraction (XRD) and Raman spectroscopy revealed uniquely stable cubic phase maintained down to 80 K which is never seen before. These effects were attributed to a more favorable lattice configuration facilitated by the mixed-cation system. In summary, this thesis contributes novel solutions for enhancing the sustainability and stability of lead halide perovskites, offering promising pathways for the industrial deployment of perovskite-based solar cells.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Heshmati, Niusha niusha.heshmati@uni-koeln.de orcid.org/0009-0000-4310-7439 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-788110
Date:	August 2025
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Chemistry > Institute of Inorganic Chemistry
Subjects:	Physics Chemistry and allied sciences
Uncontrolled Keywords:	Keywords Language Lead halide perovskites English Green solvent English Wide bandgap solar cells English
Date of oral exam:	18 August 2025
Referee:	Name Academic Title Mathur, Sanjay Prof. Dr. Dr. (h.c.) Lindfors, Klas Prof. Dr
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/78811