4 Influence of the coadsorbate
4.3 Role of the coadsorbate for transport and recombination in DSCs
0.0 -0.2 -0.4 -0.6 10-6
10-5 10-4 10-3
AM1.5 | red LED | dark
D14915min [47]
D14915minLCA [61]
DN9115min [48]
DN9115min [66]
DN9115min LCA [60]
C µ / F cm-2
Vf / V
0.0 -0.2 -0.4 -0.6
10-6 10-5 10-4 10-3
D149 (15 min) [47]
D149 + CA (15 min) [61]
DN91 (15 min) [48]
DN91 (15 min) [66]
DN91 + LCA (15 min) [60]
DN216 (15 min) [49]
DN216 + LCA (15 min) [52]
DN285 (15 min) [51]
DN285 + LCA (15 min) [63]
C µ (normalized by n SC) / F cm-2
Vf / V
0.0 -0.2 -0.4 -0.6
10-6 10-5 10-4 10-3
AM1.5 | red LED | dark
DN21615min [49]
DN216
15min LCA [52]
DN285
15min [51]
DN28515minLCA [63]
C µ / F cm-2
Vf / V
0.0 -0.2 -0.4 -0.6
10-6 10-5 10-4 10-3
D149 (15 min) [47]
D149 + CA (15 min) [61]
DN91 (15 min) [48]
DN91 (15 min) [66]
DN91 + LCA (15 min) [60]
DN216 (15 min) [49]
DN216 + LCA (15 min) [52]
DN285 (15 min) [51]
DN285 + LCA (15 min) [63]
Cµ (normalized by nSC) / F cm-2
Vf / V
0.0 -0.2 -0.4 -0.6
10-6 10-5 10-4 10-3
AM1.5 | red LED | dark
DN216
1h [67]
DN216
1h CA [68]
DN2161hLCA [70]
C µ / F cm-2
Vf / V
0.0 -0.2 -0.4 -0.6
10-6 10-5 10-4 10-3
DN216 (1 h) [67]
DN216 + CA (1 h) [68]
DN216 + LCA (1 h) [70]
Cµ (normalized by nSC) / F cm-2
Vf / V
Figure 41 – Chemical capacitance Cµ of ZnO-based DSCs sensitized with different indoline dyes in the presence or in the absence of a coadsorbate. Lighter colors indicate cells with an adsorption without coadsorbate, and darker colors indicate an adsorption with coadsorbate. (a,b,c,d) Cells sensitized for 15 min and (e,f) cells sensitized for 1 h. (a,c,e) Cµ and (b,d,f) Cµ
normalized by a measure of the total trap density, nSC, with cell [61] as a reference. Filled symbols indicate measurements at AM1.5, open symbols indicate measurements at red LED illumination and at VOC, and half-filled symbols indicate measurements in the dark.
As also observed in the previous chapter, α decreases for lower illumination intensities, which was ascribed partly to a lower temperature and partly to a changed trap distribution caused by the intense light at AM1.5 illumination, see section 3.3.1. Even so, α is higher than usually found in the literature 16,17,18,19
(and for smaller cells in chapter 5), which was already in the previous chapter ascribed to differences in the details of the electrodeposition. Deep monoenergetic traps states,
(e) (f)
(a) (b)
(c) (d)
around -0.3 V for the measurement of cell [66] in the dark, for other cells the measurement in the dark was not yet established as a part of the standard set of experiments at the time of the measurements.
However, as such a shoulder is observed for all measurements in the dark and also for one cell without coadsorbate, it is probable that also for these cells deep monoenergetic trap states are present in the dark. For measurements at AM1.5, no additional capacitance from deep monoenergetic trap states around -0.3 V is observed, confirming thus that the trap distribution changes by the high-intensity at AM1.5.
For cells sensitized for 1 h with DN216 in the presence or absence of a coadsorbate, the Cµ curves also have a similar shape and a similar slope at intermediate voltages, see Figure 41(e). The trap distribution parameter α (Table 9) is higher than for cells sensitized for 15 min regardless of the presence of a coadsorbate, similar to the findings in section 3.3.1. The relative position of the conduction band edge, see the horizontal position of the Cµ curves in Figure 41(f), for all cells with a sensitization for 1 h is higher than for a sensitization for 15 min, however it is not changed by the presence or absence of a coadsorbate. This is probably the case because for both the sensitizer and the coadsorbate the binding to the ZnO surface is supposed to occur via the carboxylic anchor group present in both, and thus the influence of the binding on the electronic structure of ZnO will be similar.
A change in the deposition of ZnO, as applied for cell [70], leads to a lower position of the conduction band edge by around 30 mV, and this observed shift is also one of the causes for the higher ISC found in Figure 39.
Table 9 – Different cell values for ZnO-based DSCs sensitized with different indoline dyes with or without a coadsorbate.
The values were determined from EIS measurements at different measurement modes, from current transients and from measurements of VOC against intensity. Values for other sensitization times for comparison are listed in Table 7. Bold values are values more relevant for the use of the solar cell (determined at AM1.5 illumination).
Value Sample
Nt/Nt,ref α α α β β β β (1/m)
Measurement (plot)
current transient (nSC vs. ISC)
EIS (Cµ vs. Vf)
EIS (Cµ vs. Vf)
EIS (Cµ vs. Vf)
EIS (Rrec vs. Vf)
EIS (Rrec vs. Vf)
EIS (Rrec vs. Vf)
V_oc vs.
int.
Illumination red LED AM1.5 red LED dark AM1.5 red LED dark red LED
Reference cell or temperature
cell [61] ~325 K ~298 K ~298 K ~325 K ~298 K ~298 K ~298 K
D14915min [47] 0.80 0.62 0.50 - 0.68 0.87 - 0.91
D14915minCA [61] 1 (Ref.) 0.67 0.45 0.46 0.42 0.67 0.60 0.77
DN9115min [48] 0.83 0.64 0.47 - 0.62 0.82 - 0.92
DN9115min [66] 0.45 0.65 0.68 0.64 0.69 1.03 0.94 1.15
DN9115minLCA [60] 1.61 0.66 0.40 0.41 0.38 0.69 0.61 0.78
DN21615min [49] 0.80 0.66 0.40 - 0.58 0.82 - 0.86
DN21615minLCA [52] 1.30 0.58 0.42 - 0.63 0.83 - 0.90
DN2161h [67] 0.80 0.76 0.61 0.58 0.65 0.83 0.76 0.90
DN2161hCA [68] 0.54 0.68 0.65 0.65 0.64 0.82 0.75 0.86
DN2161hLCA [70] 0.54 0.73 0.68 0.63 0.62 0.81 0.77 0.89
DN28515min [51] 1.11 0.60 0.44 - 0.63 0.77 - 0.87
DN28515 minLCA
[63] 0.98 0.70 0.44* 0.48 0.48 0.71 0.63 0.79
* No maximum of alpha values, thus this value could be higher.
Transport properties for films sensitized with and without coadsorbate
From IMPS measurements (measured at red LED illumination) values of the effective transport time τtr were determined, see Figure 42(a). Similar to the measurements in the previous chapter, the transport time decreases with increasing light intensity or with increasing ISC, and the absolute values of τtr differ by more than a factor 5 for the different cells without apparent dependence on the sensitizer. From Figure 42(b), which shows τtr corrected by Nt/Nt,ref (Table 9, p. 115), it becomes clear that these variations are predominately caused by the differences in total trap density, and not by differences in the transport time, as the curves almost overlap at low ISC. No difference in τtr is expected for the same ZnO films, since the rate of charge transport in the ZnO network should be widely independent of the sensitizer. The different slopes for the different cells (depending on the trap distribution parameter α, see 1.3.3.4) lead to a variation of τtr at higher illumination intensities.
Especially the cells sensitized for 1 h show a bending of the τtr curve with intensity, probably due to a higher α found for these cells (Table 9). The similarity of the τtr curves shows that the “bulk” (or non-surface) ZnO is not influenced by the slight, unavoidable variations in electrodeposition 127, at least not to an extent that the transfer of electrons through the ZnO matrix is affected.
Also for the film where difficulties in electrodepositions occurred and where the position of Ec is considerably higher, film [66], the transport times are similar, and even the change of the electrodeposition for film [70] does not lead to very different τtr compared to films [67] or [68]. This probably means that even when Ec is higher, and thus a lower DOS at a given energy is present, this DOS is still sufficient to maintain an efficient transport of electrons through the ZnO network. And even when the pore/grain structure is changed, as it is supposed from the lower light scattering of film [70], the transport through the film is not influenced by this change, probably because the electrodeposition is for the larger part of the time almost identical.
0.01 0.1 1
10-4 10-3 10-2
D14915min [47]
D14915minCA [61]
DN9115min [48]
DN9115min [66]
DN9115minLCA [60]
DN21615min [49]
DN21615minLCA [52]
D14915min [47]
D14915minCA [61]
DN9115min [48]
DN9115min [66]
DN9115minLCA [60]
DN21615min [49]
DN21615minLCA [52]
Transition time tr / s
Short circuit current density / mA cm-2
0.01 0.1 1
10-4 10-3 10-2
DN28515min [51]
DN28515minLCA [63]
DN2161h [67]
DN2161hCA [68]
DN2161hLCA [70]
tr (normalized by nSC) / s
Short circuit current density / mA cm-2 Figure 42 – Transport time τtr of ZnO-based DSCs sensitized with different indoline dyes in the presence or in the absence of a coadsorbate. (a) Transport times as determined from the measurement, and (b) transport times normalized by the relative total trap density Nt/Nt,ref (determined from current transients with cell [61] as reference, listed in Table 9). Lighter colors indicate sensitization without coadsorbate, while darker colors indicate cells sensitized with coadsorbate. Circles indicate
(a) (b)
4.3.2 Recombination depending on the use of a coadsorbate
Information about recombination is given especially by recombination resistance Rrec or the electron lifetime τn. As mentioned also in section 3.3.2, a plot of Rrec or τn vs. the voltage Vf gives recombination of the cells, see also Figure 100, p. 208 and Figure 101, p. 209. In this representation for example the higher VOC of cell [66] compared to other cells sensitized for 15 min is explained, as Rrec or τn at a certain voltage is higher for this cell. However this representation contains influences like a possibly different DOS, which strongly influences the recombination. The representation of Rrec
or τn vs. DOS in Figure 43 and Figure 44, respectively, allows the interpretation of the recombination without such influence, thus focusing more on the influence of the sensitization procedure.
Influence of the coadsorbate for a sensitization for 15 min
A comparison of Rrec at the same DOS for cells sensitized for 15 min with or without a coadsorbate in Figure 43(a,b) clearly shows a decrease in Rrec for a sensitization without coadsorbate for D149, DN91 and DN285, while for DN216 Rrec remains unchanged, resulting in the following order of Rrec for intermediate DOS: DN285+coads., DN91+coads., D149+coads. > DN216+coads., DN216 > DN285, D149 > DN91. The same sequence is also found for τn vs. DOS in Figure 44, p. 119 and in Figure 102, p. 209, where the lifetime from IMVS was plotted against nOC determined from charge extraction (both at VOC). An increased recombination compared to a sensitization with coadsorbate was also found before for ZnO sensitized with D149 for different times without coadsorbate for intermediate and lower DOS 16. The increased aggregation without coadsorbate for D149 and DN285 and the approximately constant aggregation for DN216 (as observed in the normalized absorbance in Figure 38, p. 108) fits well with this observation, as dye aggregates induce additional recombination centers. However for cell DN9115min [48], the aggregation is only slightly higher than for DN9115min
LCA
[60], but without coadsorbate the cell has a clearly lower Rrec. It is possible that the normalization of the absorbance does not give an exact measure of the aggregation, see discussion in section 9.1.1, but for most cells it corresponds well with the trends in recombination. Also for cell [66], the very low Rrec
is explained by the highest aggregation in comparison with a cell with a similar sensitization, see Figure 38, p. 108. The trends observed for the recombination are also reflected upon VOC (Table 8, p.
111) especially as Ec is similar, because an increase in recombination decreases the accumulated charge in the ZnO film and thus VOC. Thus mostly a higher VOC is found for a sensitization with a coadsorbate, where recombination is not as much favored by dye aggregates as for a sensitization without coadsorbate. For most cells the linear curve shape near 0 V for τn from OCVD measurements (Figure 101, p. 209) indicates a good blockinglayer, whereas for cells [47] and [48] τn is decreased compared to the other cells. As this decrease in τn is small enough to not affect VOC at low light intensities (Figure 103), it could be caused by higher aggregation rather than by a defective blocking
layer. The difference in recombination for measurements at high and low illumination intensity allows an estimation of the regeneration efficiency. The difference in Rrec is smallest for DN28515min [51], and successively higher for DN9115min [48,66], for D14915min [47] and for DN21615min [49]. With coadsorbate, the difference of Rrec is similar for all sensitizers, and comparable to the relatively high difference found for DN216 without a coadsorbate.
From Figure 100, p. 208, the differential values of β were calculated and an overall β determined (Table 9) as discussed in section 3.3.2. The recombination parameter β determined at AM1.5 illumination is mostly higher for cells sensitized without a coadsorbate (except for DN216, where recombination is very similar), whereas differently deposited ZnO sensitized with D149 an increase of β was observed for the addition of a coadsorbate 16. The FF (Table 8, p. 111) shows the respective trend with (slight) increase for a sensitization without coadsorbate, due to the dependence in equation (35). Similar as in the previous chapter, β determined from measurements in the dark and at red LED illumination is similar even when determined from different measurements (EIS, Figure 100, p. 208 or VOC vs. illumination intensity, Figure 103, p. 210). For the higher Ec for cell [66], β is ~1, which indicates that recombination occurs mainly via the conduction band.
1017 1018 1019
100 101 102 103 104 105 106
107 AM1.5 | red LED | dark
D14915min [47]
D14915min CA [61]
DN9115min [48]
DN91
15min [66]
DN91
15min LCA [60]
Rrec / cm2
DOS / eV-1 cm-3
1017 1018 1019
100 101 102 103 104 105 106
107 DN21615min [49]
DN216
15min LCA [52]
DN285
15min [51]
DN28515minLCA [63]
AM1.5 | red LED
Rrec / cm2
DOS / eV-1 cm-3
1017 1018 1019
100 101 102 103 104 105 106 107
AM1.5 | red LED | dark
DN216
1h [67]
DN216
1h CA [68]
DN216
1h LCA [70]
Rrec / cm2
DOS / eV-1 cm-3
Figure 43 – Recombination resistance Rrec vs. the density of states DOS for different ZnO-based DSCs sensitized with different indoline dyes in the presence or absence of a coadsorbate. (a,b) Cells sensitized for 15 min and (c) cells sensitized
(a) (b)
(c)
At high illumination intensity β is lower than at low illumination intensity or in the dark, which is ascribed to a different recombination due to a slightly lower conduction band edge and additional trap states generated by the high illumination intensity (oxidized dye molecules due to inefficient regeneration and light-induced trap states in the ZnO film).
From these results it is concluded that if the cells had the same position of the conduction band edge, the use of a coadsorbate would increase the performance of the cell by reducing recombination via dye aggregates, provided that other parameters are similar. However the lower amount of adsorbed dye for sensitization with coadsorbate and a probably lower regeneration efficiency lead to lower short-circuit currents, which reduces the gain from lower recombination for cells sensitized with coadsorbate, and thus a higher cell efficiency is found for most cells adsorbed without a coadsorbate.
1017 1018 1019
10-3 10-2 10-1 100
AM1.5 | red LED
D14915min [47]
D14915minCA [61]
AM1.5 | red LED | dark
DN9115min [48]
DN9115min [66]
DN9115minLCA [60]
Electron lifetime / s
DOS / eV-1 cm-3
1017 1018 1019
10-3 10-2 10-1 100
AM1.5 | red LED
D14915min [47] D14915minCA [61]
DN9115min [48]
DN9115min [66]
DN9115minLCA [60]
DN21615min [49]
DN21615minLCA [52]
DN28515min [51]
DN285
15min LCA [63] DN21615min [49]
DN21615minLCA [52]
DN285
15min [51]
DN28515minLCA [63]
Electron lifetime / s
DOS / eV-1 cm-3
1017 1018 1019
10-3 10-2 10-1
100 AM1.5 | red LED | dark
DN2161h [67]
DN2161hCA [68]
DN2161hLCA [70]
Electron lifetime / s
DOS / eV-1 cm-3
Figure 44 – Electron lifetimes vs. the DOS, determined for ZnO-based DSCs sensitized with different indoline dyes with and without a coadsorbate, for (a,b) 15 min and (c) 1 h. Measurements performed at AM1.5 illumination are designated by filled symbols, while measurements at red LED illumination are indicated by open symbols, and measurements in the dark by half-filled symbols.
(a) (b)
(c)
Sensitization for 1 h with or without coadsorbate
For an adsorption time of 1 h, Rrec (Figure 100, p. 208) and τn (Figure 101, p. 209) vs. Vf does not directly depend on the presence of a coadsorbate. Recombination at a given DOS (Rrec or τn vs. DOS in Figure 43 and Figure 44) however is higher for the cell adsorbed without coadsorbate. This is also in line with the higher aggregation seen in the normalized absorbance, Figure 38, p. 108. For film [70], for which the ZnO deposition was altered, a very similar aggregation and recombination was observed as for the standardly deposited and similarly sensitized film [68]. Probably the change in deposition conditions has little or no effect on the final ZnO surface, as the change in deposition was an increased deposition voltage only during the first few seconds of ZnO/EosinY deposition, however the seed layer for this deposition is very likely changed by the higher voltage 258, which influences the grain density and the size of particles and thus the scattering of the films, see also section 4.1. The difference in recombination in the dark and at AM1.5, which can give information about regeneration, is approximately the same for the cell sensitized without coadsorbate and for cell [70], but smaller for cell [68]. This indicates that the regeneration is more efficient for cell [68], at least when compared with cell [67] where recombination in the dark is very similar. This difference could be caused by the lower ISC for cell [68] (Table 8, p. 111), which requires less diffusion of the electrolyte and thus might not be influenced so much by the diffusion limit of the electrolyte. The equal relative position of Rrec
and τn vs. DOS shows that a correct area normalization was performed for Rrec.
For the very similar position of Ec , β is very similar for all cells sensitized for 1 h with or without coadsorbate, see Table 9, p. 115. This is also expected for the very similar curves of Rrec or τn, and indicates that also the recombination order is similar, not influenced by the additional recombination without coadsorbate (cell [67]) of the already aggregated samples (compare results in section 3.3.2).
The linear dependence of τn from OCVD measurements with voltage near 0 V (Figure 101, p. 209) indicates a good quality of the blockinglayer, which effectively hinders recombination via the substrate.
Recombination and regeneration from recombination currents
The recombination currents in the dark and under illumination for the different cells sensitized with or without coadsorbate are shown in Figure 45, with a voltage correction for voltage losses at the series resistance and for relative shifts of the conduction band edge. Similar to Rrec and τn, recombination is enhanced for the measurements under AM1.5 illumination, and mostly cells without coadsorbate show a higher recombination, also similar to earlier findings 17. For the plot of recombination currents, a difference in dark and illuminated currents was ascribed especially to recombination with oxidized electrolyte species for intense illumination 17, and thus inefficient regeneration. Thus also these results support that inefficient regeneration occurs at higher illumination efficiency.
-0.3 -0.4 -0.5 -0.6 10-1
100 101
102 D14915min [47]
D14915minCA [61]
DN9115min [48]
DN9115min [66]
DN9115minLCA [60]
| Idark rec | and Ilight rec / mA cm-2
Vc / V
-0.3 -0.4 -0.5 -0.6
10-1 100 101 102
DN21615min [49]
DN21615minLCA [52]
DN28515min [51]
DN28515minLCA [63]
| Idark rec | and Ilight rec / mA cm-2
Vc / V
-0.3 -0.4 -0.5 -0.6
10-1 100 101 102
DN2161h [67]
DN2161hCA [68]
DN2161hLCA [70]
| Idark rec | and Ilight rec / mA cm-2
Vc / V
Figure 45 – Recombination current density in the dark and under AM1.5 illumination for DSCs sensitized with different indoline dyes in the presence or absence of a coadsorbate. The graphs are grouped after different dyes and adsorption times, (a) D149 and DN91 adsorbed for 15 min, (b) DN216 and DN285 adsorbed for 15 min, and (c) DN216 adsorbed for 1 h. Light colors indicate an adsorption without coadsorbate, while dark colors indicate an adsorption with coadsorbate, see also the legends. Dashed lines indicate measurements in the dark, while full lines indicate recombination currents at AM1.5 illumination.
Also the comparison of the shape of the curves in the dark and under regeneration speaks for recombination, as for a voltage range where ISC is high (0 to around -0.45 V) the difference is larger, and for lower and reverse currents, where the diffusion limit of the electrolyte is probably not yet reached, this difference is smaller, and the same is observed for cells where ISC (Table 8, p. 111) is smaller (cells [66], [61], [51], [63]). This finding is different from the comparison of Rrec or τn at different light intensities, where especially cells with coadsorbate show a larger difference than cells without coadsorbate. This indicates that either EIS measurements or recombination currents yet focus on different processes.
The graphs of the recombination currents show different slopes and even a crossing of the recombination current in the dark and under illumination is observed. This is very likely caused by the correction of the voltage by ΔEc even though α and β are different. If the DOS would be available for the recombination currents, a plot vs. the DOS would probably give better results due to these different
(a) (b)
(c)
parameters, and this is also the reason why for Rrec and τn the representation vs. DOS was preferred.
Recombination at short circuit is not regarded in this approximation of recombination currents, regeneration can be even more pronounced than found in Figure 45, and is supposed to be one of the main loss mechanisms which lead to comparatively low ISC values for even though the light harvesting efficiency is ~1 in the wavelength range where the sensitizers absorb light.