Probing the dye-semiconductor interface in NiO based p-type dye sensitized solar cells using BODIPY

Abstract

A significant portion of the scientific community is focused on developing new approaches to fulfilling the energy demands of a growing population, whilst also minimising the damage to the environment. Chemistry has played a vital role in developing new molecular systems to turn solar energy into electricity. Dye-sensitized solar cells (DSSC) have gained significant attention by offering a versatile and tuneable molecular system as a promising alternative to traditional silicon devices. Single junction devices usually based around an n-type TiO2 photoanode can be improved by pairing with a p-type photocathode to create a tandem device, which could potentially surpass the Shockley- Queisser limit of solar energy conversion over a single p-n junction. Although most of the research on DSSC focusses on TiO2 based systems, improving the performance of the p-type photocathode is required in order to approach a tandem DSSC which outperforms the TiO2 photoanode. A series of BODIPY sensitizers with small structural modifications to the core of the chromophore were prepared by various synthetic pathways to create a robust and reliable system with different photophysical and electrochemical properties with which to investigate the underlying electron transfer pathways in p-type NiO DSSC. The properties of these dyes were studied using steady state UV-Visible and Fluorescence spectroscopy and coupled with cyclic voltammetry in order to create an energy level map of the system. Although all three dyes appeared to have sufficient driving force for electron injection from the VB on NiO into the HOMO of the dye, the three dyes showed modest performance which appeared to be limited by the efficient regeneration of the dye by the redox electrolyte. BOD2 showed the most promising results when used in a working p-type device (JSC = 0.48 mA cm􀀀2) however these results did not agree with the calculated driving force for injection ( Ginj therefore the dye|semiconductor interface was studied using X-ray Photoelectron Spectroscopy (XPS) to create a new map of the energy levels inside the p-DSSC. Inspection of the valence photoelectron spectra at varied X-ray excitation energy allowed for probing of the energy levels in both the bulk semiconductor and at the dye|semiconductor interface. The HOMO energies for all three dyes adsorbed onto NiO were measured experimentally and these results confirm a shift in Fermi level for the

NiO upon dye adsorption. These results predict that BOD2 had the highest Ginj and the frontier orbitals of BOD2 were the most well placed to encourage efficient electron transfer between the dye and semiconductor. Femtosecond Transient Absorption Spectroscopy (fs-TAS) was utilized to study the kinetics of the electron transfer processes within the p-type device. Interestingly, the lifetime of the reduced dye appears to be increased in the presence of a redox electrolyte and we postulate this to result from catalytic activity of surface trap states NiO catalysing the conversion of Iodide to triiodide and deactivating a recombination pathway in the p-DSSC. This has implications on future design of dyes for p-type DSSC and outlines new methods on estimating driving forces for electron transfer within a p-DSSC.