Nanostructured coordination polymers for high performing solar cells Kezia Sasitharan and Marina Freitag Newcasle University, UK Dye-sensitized solar cells (DSCs) are a potential renewable energy technology to help decarbonize our economy and power wearable devices. [1] Conventional DSCs use a liquid electrolyte; however, a lot of research is going into replacing it with a solid hole transporter material to make the technology better suited for scale-up and commercialization. We work with an emerging class of materials called nanostructured coordination polymers (CP) and explore their application as a hole transport material aimed at creating high performing monolithic solid- state DSCs. Nanostructured CPs possess the highly ordered structure of inorganic materials combined with the chemically tailorable properties and low cost of organics .[2] CPs facilitate the formation of extended polymeric structure of metal ions and the coordinating atoms of the organic ligands. Metal centres can provide efficient sites for redox conductivity but can also act as thermodynamic sinks that trap/localize charges due to their low-lying energetic states. [3] Therefore, Copper benzene tetrathiol (Cu-BTT) is an ideal candidate for hole transporting roles in solid state dye sensitized solar cells.
We show that Cu-BTT forms a 1D coordination polymer formed by alternating Cu 2+ and C 6 H 2 S 4 2- units with pairs of chelating S atoms from the ligand coordinating around the metal centre. The as-prepared pristine Cu-BTT coordination polymers were found to exhibit a conductivity of the order 10 -6 S cm -1 . Upon altering the technique used for thin film preparation, the conductivity was found to increase by up to an order of a magnitude. Layer by layer assembly of the coordination polymer was found to be the best giving a fairly uniform epitaxial growth of Cu- BTT over large areas and showing conductivities of the order 10 -3 S cm -1 without additives. We also demonstrate that epitaxially grown Cu-BTT also shows better performance as a hole transport material in the DSSC devices as compared to Cu-BTT films prepared using drop casting or spin-coating technique. This is because during epitaxial growth the precursors get a better chance to infiltrate the pores in the mesoporous layer and the polymeric HTM forms direct contact with the dye molecules. This is corroborated by photoinduced absorption spectroscopy (PIA) measurements where we observe that the ground state bleach and absorption peaks of the dye (Y123) disappear upon introduction of the epitaxially grown CuBTT HTM. This work shows that 1D coordination polymers hold significant potential as solid state hole transport materials to create monolithic solid state DSSCs with improved performance. References 1. M. Freitag et al. , “Dye-sensitized solar cells for efficient power generation under ambient lighting,” Nat. Photonics , vol. 11, no. 6, pp. 372–378, 2017, doi: 10.1038/nphoton.2017.60. 2. K. Sasitharan et al. , “Metal‐Organic Framework Nanosheets as Templates to Enhance Performance in Semi‐Crystalline Organic Photovoltaic Cells,” Adv. Sci. , vol. 2200366, p. 2200366, 2022, doi: 10.1002/advs.202200366. 3. A. J. Clough et al. , “Room Temperature Metallic Conductivity in a Metal − Organic Framework Induced by Oxidation,” 2019, doi: 10.1021/jacs.9b06898.
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© The Author(s), 2021
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