5 Changes in ZnO-based DSCs after storage in the dark
5.2 Changes in the chemical capacitance and transport properties by a storage in the dark
5. Changes in ZnO-based DSCs after storage in the dark
128
5.2 Changes in the chemical capacitance and transport properties by a storage
of the shift on the sensitizer. For cell [43], EIS measurements performed at red LED illumination showed a very similar shift of the conduction band edge upon ageing of the cell (not shown). A downward shift of the conduction band edge as observed here during storage of the cells in the dark gives an explanation for the increase in ISC which is observed during the storage.
A downward shift in Ec as observed for the storage of the cells in the dark should also decrease VOC by the same amount. For most cells, however, the change in VOC upon storage in the dark is either in a different direction or smaller than expected from ΔEc/q (see Table 12). It can be concluded that the shift in the conduction band edge affects the open-circuit voltage for some of the cells, but other factors also influencing the open-circuit voltage are (over)compensating this effect 17,60. Some possible effects influencing the open-circuit voltage are discussed below.
A possible reason for the observed ΔEc/q could be the adsorption or desorption of polar molecules. For the oxidic semiconductor ZnO prepared from solution, a change by adsorption or desorption of water is probable; the observed downward shift would mean for example desorption of OH- or H2O facing with the oxygen atom to the ZnO from the ZnO surface into the electrolyte. This desorption would also be a possible reason for the observed increase in Zd during the storage in the dark. The association with adsorbed water is further corroborated by independent measurements by Felix Fiehler 131, where deposited ZnO films (EosinY already desorbed) were stored in water for ~100 days.
Table 11 – Values determined from EIS measurements, from measurements of VOC vs. intensity, from the recombination current under illumination and from IV-curves. Table adapted from 259.
Sample Measure-ment
β β α Rs/Ω Zd/Ω Rpt/Ω Nt/Nt,ref
from EIS (at red LED
light for one cell)
from VOC vs.
intensity
from EIS (at red LED light for one cell)
at -0.54 V reference cell [61]
from section 3.3.1 D14915minLCA
[p25]
directly 0.54 0.84 0.39 5.9 1.7 3.5 3.33
after 4 weeks 0.49 0.84 0.35 5.8 4.9 4.9 3.33
DN9115minLCA
[35]
directly 0.69 0.95 0.52 5.6 1.7 3.3 0.69
after 4 weeks 0.67 0.96 0.48 5.8 3.9 3.7 0.69
DN9115minLCA
[43]
directly 0.64 (0.86) 0.92 0.51 (0.48) 5.8 1.4 3.6 0.45
after 4 weeks 0.67 (0.79) 0.84 0.53 (0.46) 5.7 1.6 3.5 0.43 DN21615minLCA
[p29]
directly 0.54 0.93 0.36 5.4 1.6 3.5 1.43
after 4 weeks 0.48 0.90 0.40 5.6 2.6 4.0 1.43
DN21615minLCA [p30]
directly 0.60 0.95 0.40 4.8 2.4 3.6 1.67
after 4 weeks 0.57 0.94 0.38 5.3 4.9 3.8 1.67
DN21615minLCA [p31]
directly 0.53 0.86 0.39 5.0 1.7 3.2 0.67
after 4 weeks 0.49 0.86 0.40 5.3 3.2 3.2 0.65
DN28515minLCA [34]
directly 0.71 - 0.47 5.5 2.2 4.5 0.62
after 4 weeks 0.68 0.93 0.48 5.5 3.2 5.8 0.65
DN28515minLCA [p28]
directly 0.57 0.90 0.41 5.3 3.3 3.4 0.62
after 4 weeks 0.51 0.86 0.42 5.5 5.6 3.2 0.65
0.0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -5
0 5 10
Storage of film in water prior to cell preparation
Cell after preparation:
D14915minLCA [p25]
Cell after storage:
D14915minLCA [p25] 4 weeks Cells from films stored in water:
D14915minLCA [p09]
D14915minLCA [p04]
Current density / mA cm-2
Potential / V
Storage of completed cell in the dark
Figure 49 – Influence of water on the current-voltage characteristics at AM1.5 illumination and in the dark. Comparison the measurement of cell [p25] directly after cell preparation (red full line), the same cell after a storage in the dark for 4 weeks (red dashed line), and two cells, [p09] and [p04] prepared from films which were stored for ~100 days in water prior to cell preparation (cyan and blue line). Dotted lines indicate measurements in the dark. Adapted from 259.
After this cells were prepared as usual (see also section 2.4) from these water-treated films. A sensitization with D149 and a coadsorbate yielded dark films with a high light harvesting efficiency, however IV-curves of these cells showed a very low ISC, even though the dark current was considerably decreased compared to a reference cell [p25], see Figure 49. For one cell prepared with a water-treated ZnO film, cell [p09], EIS measurements were performed to find possible reasons for such a marked change. For the water-treated cell [p09], Cµ corrected for Nt showed a large shift of Ec
of about 300 mV to higher energies, see Figure 50(a), and additionally, Rrec at a given DOS decreased for this cell (Figure 50(b)). The very high position of Ec slows down injection into the ZnO film, as the DOS of the film is considerably reduced at a given energy and thus also at the energy of excited state of the dye. This explains the low ISC observed for cell [p09].
-0.2 -0.3 -0.4 -0.5 -0.6 -0.7
10-5 10-4 10-3
Storage of film in water prior to cell preparation Cell after preparation:
D14915minLCA [p25]
Cell after storage:
D14915minLCA [p25] 4 weeks Cell from film stored in water
D14915minLCA [p09]
Cµ (normalized by nSC) / F cm-2
Vf / V
Storage of completed cell in the dark
1017 1018 1019
100 101 102 103
Storage of film in water prior to cell preparation
Storage of completed cell in the dark
Rrec / cm2
DOS / eV-1 cm-3
Figure 50 – Comparison of the influence of water on (a) Cµ and (b) Rrec. (a) for a standard cell directly after preparation and after a storage in the dark for 4 weeks, and a cell prepared from a film stored in water for ~100 days prior to film preparation.
Adapted from 259.
(a) (b)
For the long storage time in water, an adsorption of water or OH- onto the ZnO surface is expected, and such an adsorption also thought to be the most probable explanation for the observed shift of Ec. As this change is exactly the opposite of what is observed during the storage of already prepared cells in the dark (lower Ec, higher Rrec, see also Figure 50), a desorption of water or OH- from the ZnO surface might be the cause for the changes discussed in this chapter xxiv:
For cells sensitized with the different indoline dyes, the transport time τtr (Figure 51) was very similar after a normalization with Nt/Nt,ref and only affected in the slope by the different α, indicating that the transport properties were similar even for the relatively different Cµ. Trap states and their occupancy have a high influence especially at low illumination intensities 210, and in this region the cells show a very good overlap. The dye molecules adsorbed to the semiconductor surface should have only a minor effect on the transport time 260, as the surface is not directly participating in the conduction of electrons. Thus no direct dependence on the sensitizer can be found for τtr. The storage of the cells in the dark slightly increased τtr probably due to the shift of Ec to lower energies, however the values were still within the range of the different cells.
10-2 10-1 100 101
10-4 10-3 10-2
tr (normalized by nSC)/ s
Short circuit current density / mA cm-2
Figure 51 – Transport times of the different dye-sensitized cells determined from IMPS measurements, normalized by the relative total trap density Nt/Nt,ref, see Table 11. Measurements taken directly after the cell assembly are indicated by filled symbols while measurements taken after 4 weeks storage in the dark are indicated by open symbols. The different cells are indicated by different colors, for the exact designation see the legend in Figure 87. Adapted from 259.
xxiv A change of the electrolyte potential would also lead to a seeming shift of Ec, however a comparison of Cµ
for a higher concentration of electrolyte species in section 9.1.8 showed that the position of the redox potential is