Metal-organic framework nanosheets: A new dimension in solar cell research
Kezia Sasitharan a b
a Centre for Advanced Photovoltaics, Faculty of Electrical Engineering, Czech Technical University in Prague, CZ, Czech Republic
b Department of Chemistry, University of Sheffield, Robert Dainton Building, Hounsfield Road, Sheffield, S3 7RH, United Kingdom
Proceedings of 13th Conference on Hybrid and Organic Photovoltaics (HOPV21)
Online, Spain, 2021 May 24th - 28th
Organizers: Marina Freitag, Feng Gao and Sam Stranks
Poster, Kezia Sasitharan, 196
Publication date: 11th May 2021

In recent years, metal-organic framework nanosheets (MONs) have emerged as novel two-dimensional materials with enormous potential for use in advanced electronic devices. MONs comprise of organic linkers that are linked with metal ions or clusters in two-dimensions. They are most notable for their high surface area, nanoscopic dimensions, physical flexibility, and diverse chemical functionalities. In this work, MONs are explored as potential materials for enhancing the performance of a range of organic photovoltaic (OPV) devices. Zn2(ZnTCPP) MONs (where TCPP = tetracarboxyphenyl porphyrin) were synthesised using liquid exfoliation. The MONs were found to approach monolayer thicknesses and their optoelectronic properties were found to be ideally suited for incorporation into the active layer a polythiophene-fullerene based OPVs. P3HT-MON-PCBM ternary blend bulk heterojunctions were therefore developed. Upon optimisation, the ternary OPV devices were found to outperform the reference devices with the champion MON devices at 5.2% PCE as compared to the references at 2.6%. Detailed mechanistic investigations were carried out to probe their performance enhancement. The incorporation of MONs was found to lead to creation of highly crystalline P3HT domains in the films, which resulted in improved light absorption, higher hole mobility and reduced grain sizes. This work provides the first example of incorporation of MONs into the active layer of an OPV device and demonstrates their potential as additives for enhancing the performance of OPVs. Following the success of this investigation, the effect of different metal ions and ligands on the energy level alignment of MONs was explored. The aim was to identify the key structural and electronic features that should be considered while designing MONs for OPV applications. Upon comparing Cu2(CuTCPP) and Cu2(ZnTPyP), (where TCPP = tetracarboxyphenyl porphyrin and TPyP = tetraphenyl porphyrin) as additives in P3HT-PCBM system, the device performance doubled  with  Zn2(ZnTCPP), remained unaffected with Cu2(ZnTPyP) and halved with Cu2(CuTCPP). We found that he choice of metal ions was found to only have a small impact on the ionization energies of the MON and was not sufficient to explain the variations in device performances observed. The size of the nanosheets was found to play a significant role in influencing the device performance. Large sized nanosheets were detrimental to the power conversion efficiencies because they reduce the interface between the donor P3HT and acceptor PCBM. This work identified the key structural and electronic features to be considered while choosing MONs for OPV applications.

KS thanks the University of Sheffield for a Faculty of Science Scholarship, and a DAAD Rise professional fellowship. 

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