JUNCTION ADMITTANCE OF DYE SENSITIZED NANOPOROUS TiO2 SOLAR CELLS

Dye sensitized solar cells (DSSC) are a promising technological alternative to conventional pn-junction solar cells, and therefore attract considerable scientific interest. Different attempts have been made to replace the liquid electrolyte by a solid state hole conducting organic or inorganic medium. This contribution compares electrical properties of three different types of DSSCs. First, we use the standard electrolyte device with an I/I3 redox couple solved in acetonitrile. Second, we investigate a solid state device with the organic hole conductor MEO-Spiro-TAD. The third type of device under examination is a DSSC that works with a polymer gel electrolyte. This contribution investigates those three types of solar cells by admittance spectroscopy. The low-frequency conductance Glf and capacitance Clf of the liquid electrolyte DSSC increase exponentially upon increasing bias voltage V (figure 1a). We attribute this exponential increase to a diffusion admittance resulting from injection and subsequent diffusion of electrons in the conduction band of the TiO2. From the two quantities, Clf and Glf, we calculate an RC time constant τCG = Clf/Glf which in case of conventional pn-junction solar cells would give the lifetime of electrons. The values for liquid electrolyte cells are in the range of τCG ≈ 100 ms at a voltage V close to the open circuit voltage Voc ≈ 0.75 V. The time constant τCG depends on the applied bias voltage and the level of illumination. The low-frequency capacitance Clf of the solid state DSSC decreases with increasing forward voltage and for V > 0.3 V becomes even negative. This inductive behavior originates from electrons injected from the SnO2 front electrode into the TiO2. These electrons modulate the number of holes in the hole conductor and thus reduce the resistance within the pores. The time delay between electron injection and the modulation of the hole concentration leads to the observed inductive behavior. The occurrence of negative capacitance values does not allow us to calculate an RC time constant comparable to the electrolyte device. The behavior of the gel electrolyte cell appears to be right between the other two types of DSSC. For voltages V < 0.75 V, we observe the same exponential increase in the capacitance Clf as in case of the liquid electrolyte cell. Also, τCG is of similar order of magnitude. For voltages V > Voc, i.e. for high electron injection densities, the gel electrolyte DSSC changes to inductive behavior. In the liquid electrolyte DSSC, we observe the effect of inductivity if we reduce the concentration of positive charged Li ions. The concentration ratio LiI:I2 is r = 10:1 in the standard electrolyte. Upon changing this ratio to r = 1:10, we find an inductive behavior for V > 0.4 V as shown in figure 1b. Thus, we are able to control the capacitive behavior of the device by controlling the charge density in the electrolyte. We determine τCG for the liquid electrolyte DSSCs with a LiI:I2 ratio of r = 1:10 and r = 1000:1. The time constant τCG increases with decreasing I2 concentration because of the decreasing availability of I3 recombination partners for the electrons. Thus, basically all features of the junction admittance of electrolyte DSSCs are controlled by the ion concentration in the electrolyte.