Influence of Lithium Ion Concentration on Electron Injection, Transport, and Recombination in Dye-Sensitized Solar Cells

Abstract

Varying Li+ concentration in the electrolyte of dye-sensitized solar cells equipped with compact TiO 2 blocking layers is found to alter the mean slopes of semilogarithmic open-circuit photovoltage-intensity and dark current-voltage plots. Almost identical values of ideality factor or transfer coefficient are required to fit data in the dark and under illumination for each Li + concentration. It is found that cell characteristics become progressively more "ideal" as Li + concentration is increased, with a transfer coefficient of ca.1 for 1 M Li + in the electrolyte. We find that trends in photovoltage-intensity data are well fitted using a model which assumes that electron transfer to acceptor species in the electrolyte occurs from both the conduction band of the TiO 2 and an exponential distribution of band gap surface states. Changes in the mean ideality factor and linearity of semilogarithmic photovoltage-intensity plots can be rationalized by considering the variation in overlap between occupied donor states (conduction band and surface states) with electron acceptor states in the electrolyte, as the conduction band edge is shifted positive by increasing Li + concentration. In accordance with previous studies, this positive shift in conduction band edge is also found to cause a dramatic increase in the photocurrent generation efficiency of the cells, especially in the long-wavelength region of the photocurrent action spectrum. It is argued that this improvement in photocurrent is predominantly due to an increase in wavelength-dependent electron injection efficiency, as opposed to an increase in electron collection efficiency.

Simulation of dye solar cells: through and beyond one dimension

“…As known from literature, the recombination term is not linear in the electron density, with an exponent in the range of 0.5-0.7 [59]. Recently, several experimental and theoretical papers have addressed the problem showing that in fact the non-linearity seems to be related to charge transfer via a distribution of surface states [60][61][62]. It is also shown to be in agreement with the measurements of electron lifetime [63].…”

Effect of molecular filtering and electrolyte composition on the spatial variation in performance of dye solar cells

“…The addition of Li + ions has been shown to shift the peak position towards longer wavelengths. 22,33,35,42,43 Considering that the data in Table 2 …”

Titanium Dioxide Modifications for Energy Conversion: Learnings from Dye-Sensitized Solar Cells

“…For iodide/triiodide most commonly employed additives are Li + , GuNCS and 4-tBP, whereas one electron redox shuttles mainly employ Li + and 4-tBP only. Generally speaking, cationic additives charge the TiO 2 surface positively thus lowering the conduction band ( Table 1, entry 1) [56,146]. Electron rich or nitrogen containing additives on the other hand charge the TiO 2 surface negatively or increase electron density thus raising the conduction band, blocking the recombination and resulting in higher V oc .…”