Dye-sensitized solar cells (DSSC) have attracted considerable attention in recent years as they offer the possibility of low-cost conversion of photovoltaic energy. However, there are various aspects that should be taken into account to reach higher improvement. The better understanding of the contribution of each of the components to the working of the solar cell is very crucial. One thing is about the recombination process between electron transferred to the photo-electrode (i.e. TiO2) and either the oxidized redox species or electron acceptor in the electrolyte which consists of reduced iodide [1]. This experiment used Nano-porous TiO2 layer deposited on the ITO substrate by doctor blade method, continued by sintering in an oven for 30 minutes at 450 oC to remove the organic fillers in the TiO2 paste [2]. The films then were dipped into both dyes (N719 and Z907) 0.05 mM in concentration which this concentration is the optimum value that the dye covered in maximum area of nano-porous TiO2.[3] Then the prepared films exposed into the electrolyte contains of iodine 0.3 mM. The UPS and MIES Spectrum have been taken in both before and after exposing into the electrolyte. To ensure the stability of the dye layer, the two blank UPS and MIES spectrums have been taken before exposing into the electrolyte.
In this study, a further investigation of the UV Photoelectron Spectroscopy (UPS) and Meta-stable Induced Electron Spectroscopy techniques has been conducted in DSSC layer. It clearly can be seen that the rinsing and non-rinsing treatments between two different dyes affected in both intensity and spectrum shifting on UPS and MIES spectrum.
The results (Fig. 1 and 2) show that the UPS and MIES Spectrum for Z907 dye shifted to lower binding energy. However, this shifting did not occur in UPS and MIES Spectrum for the N719 dye (Fig. 3 and 4). The shift is estimated because the dipole layer formed on the top of dye surface. The tailing of the negatively charged electron cloud into vacuum makes the vacuum side negative, while the lack of electrons inside the surface makes the bulk side positive [4].
The XPS analysis of both dyes (Z907 and N719) could be done to see how the percentage of each element influences the configuration of the electronic state of the dye molecule affected by the presence of iodine atom.
References :
[1] Boschloo, G. and A. Hagfeldt, Characteristics of the Iodide/Triiodide Redox Mediator in Dye-Sensitized Solar Cells. Accounts of Chemical Research, 2009. 42(11): p. 1819-1826.
[2] Gratzel, M. and J.R. Durrant, Dye Sensitised Mesoscopic Solar Cells, in Nanostructured and Photoelectrochemical System for Solar Photon Conversion, MD Archer and A.J. Nozik. 2007: Imperial College Press.
[3] Ellis-Gibbings, L., et al., Formation of N719 Dye Multilayers on Dye Sensitized Solar Cell Photoelectrode Surfaces Investigated by Direct Determination of Element Concentration Depth Profiles. Langmuir, 2012. 28(25): p. 9431-9439.
[4] H. Ishii, et.al. Energy Level Alignment and Interfacial Electronic Structures at Organic/metal and Organic/Organic Interfaces, Adv. Mater., 1999. 11 (8) : p 605-625.
PhD Student : Herri Trilaksana. Supervisor : Prof. Gunther Andersson.