CHARACTERIZATION OF ELECTROLYTE-BASED CELLS
4.1. Determination of optimal preparation conditions of DSSCs
4.1.2. The choice of the electrolyte: The influence of tBP and Lithium on the performance of DSSCs
oc k T d d
q d d
dV 1 1
d
n p
n p
W W W
⋅ − −
=⎛ ⎞⎛ + ⎞
⎜ ⎟
⎜ ⎟⎝ ⎠
⎝ ⎠ 4.5
as the explicit formula for the change of output voltage with change of layer thickness W, based on the consideration of the entropy reduction with compression. Since the derivatives of particle densities with respect to thickness, such as dn/dW, are negative, the open-circuit voltage Voc does increase with shrinking W, and so does the efficiency, since the current remains constant.
The charge recombination in DSSC can be estimated by the magnitude and onset of the dark current, which arises from the reduction of I3− ions by the electrons in the conduction band. The dark current potential scans for photoelectrodes are plotted in Figure 4.1(b). The dark current onset is shifted to a lower potential with increasing thickness, and a thinner film produces a smaller dark current at the same potential above 0.25 V. These observations reflect a higher recombination rate between transported electrons and I3− ions in thicker films. The increase of the dark current with film thickness results in a loss in Voc. Thus, Voc decreases with increasing thickness [41].
From this investigation, a 7 µm thickness of TiO2 turned out to generate device with good performance. If not otherwise mentioned 7 µm is retained as thickness of TiO2 inDSCCs in this work, since all dye investigated have an extinction coefficient lying in the same range.
In the following sub-section the influence of electrolyte composition on the performance of DSSCs will be scrutinized.
4.1.2. The choice of the electrolyte: The influence of tBP and Lithium on the
with the amine function of the tBP [123]. The presence of these two additives and their influence on the performance DSSCs were investigated by photovoltaic experiments.
Characterisation of modified devices
In this experiment a series of electrolyte-based cells with electrolyte consisting of lithium on one hand and with tBP on the other hand were compared in terms of their influence on photovoltaic parameters. Table 4.1 compares the photovoltaic output for varying cell types.
Electrolyte A containing 0.6 M 1-butyl-3-methylimidazolium iodide, 0.1 M I2 in 3-methoxypropionitrile solvent is considered as the reference electrolyte for this investigation.
The lithium ion was introduced by dissolving LiClO4 salt in the electrolyte.
Tableau 4.1. Change in device performance at 100% Sun and 54% Sun light intensity for various cell type.
100% Sun Electrolyte and
Additives Voc
[mV]
Jsc
[mA/cm2]
FF
[%] η
[%]
A 548 0.247 0.544 0.074
A + 0.5 M tBP 569 0.222 0.627 0.079
A + 0.05 M Li 403 6.975 0.588 1.653
A + 0.5 M tBP
+ 0.05 M Li 508 1.641 0.697 0.581
54% Sun Electrolyte and
Additives Voc
[mV] Jsc
[mA/cm2] FF
[%] η
[%]
A 499 0.137 0.569 0.075
A + 0.5 M tBP 534 0.090 0.639 0.057
A + 0.05 M Li 390 3.456 0.641 1.599
A + 0.5 M tBP
+ 0.05 M Li 481 0.788 0.724 0.508
0,0 -0,2 -0,4 -0,6 -0,8 -2
0 2 4 6 8
a)
Voltage / V
Current density / mA.cm-2
A A + tBP A + Li A + Li + tBP
0,0 -0,2 -0,4 -0,6 -0,8 -1,0
-10 -8 -6 -4 -2 0
A A + tBP A + Li A + Li + tBP
b)
Current density / mA.cm-2
Voltage / V
400 500 600
0 20 40 60 80
c)
W a ve le ng th / n m
IPCE / %
A A + tB P A + L i A + L i + tB P
Figure 4.3. Current-voltage characteristics at illumination intensity of 1 Sun (a) and in dark (b); IPCE for different device types (c).
Figure 4.3 shows the current voltage characteristics and the incident photon to current conversion efficienecy (IPCE) of modified cells. While introducing tBP in the electrolyte A results in a clearly enhanced open circuit voltage from 548 to 569 mV, the tendency for the short circuit current is the opposite, it varies from 0.25 to 0.22 mA cm-2 at 1 Sun (Table 4.1, Figure 4.3(a)). The same tendency is observed at 0.54 Sun. In the presence of tBP additive the fill factor increases independently of light intensity. This increase of the fill factor and the open circuit voltage is due to the fact that the presence of tBP in the electrolyte shifts the conduction band edge of TiO2 and thus leads to the suppression of the dark current at the semiconductor electrolyte junction. In fact, triiodide, due to its relatively small size, either crosses the dye layer or has access to nanometer-sized pores [41] into which the P1 cannot penetrate, i.e., where the surface TiO2 is bare and exposed to the redox electrolyte. The effect of tBP is to decrease the rate of the reduction of triiodide. This is clearly noticeable in Figure 4.3(b), where the onset of the dark current in the presence of tBP is shifted to higher voltage with respect to a device based on electrolyte A. The efficiency is nearly the same at 1 Sun and
tends to decrease at 0.54 Sun. The presence of tBP in the electrolyte turns out to be more beneficial for open circuit voltage and fill factor.
After introducing lithium in the electrolyte A, (A + 0.05 M Li), Voc decreases while the current increases strikingly from 0.25 to 6.98 mA cm-2 at 1 sun (Table 4.1, Figure 4.3(a)). A slight increase in fill factor is observed and efficiency increases remarkably from 0.07 to 1.65%. All these parameters behave quite identically independently of low or high incident light intensity except the fill factor that increases at 0.54 Sun in presence of lithium.
Noticeable is the striking improvement of efficiency in the presence of lithium.
After combining both tBP and lithium in the electrolyte (A + 0.5 M tBP + 0.05 M Li) a so called co-modified cell is obtained. Its Voc, Jsc and η are 508 mV, 1.64 mA cm-2 and 0.98%, respectively. All parameters of this co-modified cell lie between those obtained in the presence of tBP and lithium separately (Table 4.1, Figure 4.3(a)). However most remarkable is the decrease in Jsc and efficiency with respect to cell containing only lithium ion independently of light intensity. While the fill factor is increased when tBP and lithium are added at 1 Sun, it increases more at low light intensity. The dark current in this co-modified cell is less pronounced with respect to lithium-based cell (Figure 4.3(b)).
Figure 4.3(c) depicts the evolution of IPCE of the modified devices. When tBP is added to the electrolyte A the IPCE decreases from 14.1% to 1.7%. In contrast after adding lithium ions to the electrolyte A the IPCE increases strikingly to 63.7%. Combining both additives leads to IPCE of 21.3% at 475 nm.
Out of this, it can be noticed that in the presence of additives in the corresponding device behaves differently with respect to the unmodified device. The presence of tBP in the electrolyte provokes an increasing Voc and decreasing Jsc while the presence of lithium ions leads to higher Jsc and lower Voc. The divergent effects of these additives were rationalised in terms of their influence on the energetics of the conduction band/ trap states and therefore on the kinetics of charge recombination: While lithium causes a band edge shift away from the vacuum level, tBP provokes a band edge shift in the other direction by deprotonation of the TiO2 surface [69,78,204]. The adsorption of tBP respectively pyridine on metal oxides has been studied in detail by FTIR, since pyridine serves as an analytical tool to detect surface states acting as Lewis acids [205-207]. From this study it is known that pyridine can bind physically or chemically via the nitrogen to the oxide surface. While the physical coordination is rather weak and will not stand higher temperatures, the chemical coordination of the pyridine shows high temperature stability [206]. Since all devices where prepared and characterised in standard conditions, the physisorbed tBP could probably less evaporate but
can react with free lithium ions in the cell. The shift of the TiO2 band edge away from the vacuum level after adding lithium increases the driving force for electron injection from the excited dye while the tBP acts conversely. That is why low Jsc is generated by the device containing tBP with respect to the device containing only lithium ions. This observation shows that the presence of lithium in the cell is beneficial, while the presence of tBP seems to be disadvantageous in term of efficiency generated at this level of investigation. The impact of lithium ions on the performance of the device will be discussed in detail in section 6.4.
The investigation of the effect of tBP in DSSC was also extended to other dyes used in this work. The results obtained are shown in Table 2. All dyes exhibit similar behaviour than P1-based cells. Voc and FF generated by all dye types are higher than their corresponding tBP-free cells. The Jsc and the efficiency also decrease considerably. These observations suggest that the presence of tBP in electrolyte decreases the performance of device independently of the chemical structure of the dye.
From this study one can observe that device based on electrolyte A + 0.05 M Li exhibits better performance while tBP containing electrolyte turns out to be disadvantageous.
Therefore, unless otherwise mentioned electrolyte A + 0.05 M Li will be used as mediator for a comparative study of DSSCs based on various dye type.