Nanostructured Coordination Polymers for Solid State Dye-Sensitized Solar Cells
Authors: Sasitharan Kezia , Freitag Marina 
Published: September 2023

Dye-sensitized solar cells have the potential to provide a viable source of renewable energy to help decarbonize our economy and power wearable devices. With their exceptional performance in diffused light under indoor conditions, DSCs remain competitive for powering the next digital revolution forming the internet of things.[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] Thanks to their high external surface area, aspect ratios, nanoscopic dimensions and novel electronic and optical properties, CPs have been found to outperform their precursors in a wide range of applications and are ideal for evaluation as solid-state hole transport materials in DSCs. CPs facilitate the formation of extended polymeric structure of metal ions and the coordinating atoms of the organic ligands. The systems are designed by taking the following conditions into consideration- the energy overlap of the orbitals of the metal ions and the coordinating atoms and their relative electronegativities. Incorporating redox-active metal centres into conducting polymer substrates thus creates highly efficient redox conductivity. 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] Based on these design criteriae, we explore Copper benzenetetrathiol (Cu-BTT) as an ideal candidate for hole transporting roles in solid state dye sensitized solar cells.
We show that Cu-BTT is formed by alternating Cu2+ and C6H2S4
2- units with pairs of chelating S atoms from the ligand coordinating around the metal centre. Formation of 1D chain-type nanowires is a good strategy for through bond charge transport. 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 performing, 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. Using AFM imaging studies, we show that this morphology creates the formation of highly interconnected networks of CuBTT nanowires which improves the conductivity. 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 demonstrates an efficient and complete reduction of the oxidized dye molecules therefore proving that CuBTT nanowire type 1D coordination polymers can be utilized successfully as hole transport layers in solid state dye sensitised solar cells. 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.

Figure 1