Photoconductivity

The conductivity of the P2 aggregates bridging the Pd gaps in Fig. 59 have been measured after bonding the floating electrodes at the pads and using as counter-electrode the main Pd electrode. The I-V-curves through the bridged gaps have been measured in the dark and during the irradiation with a laser of 1 mW at 632 nm wavelength and approx. 3 mm² spot size. Fig. 61 shows that the conductivity increased upon irradiation of the 400 nm gap, but almost no changes were observed at the 600 nm gap.

1.2.9 Dielectrophoresis 82

The aggregates showed a non-linear increase in conductivity above a threshold of about 10 V. A strong hysteresis was observed for the I-V measurements, which has been investigated in more detail at the 600 nm gap in the dark (b). This needed to be compensated by a longer integration time for each measurement point and 1 s/point has been chosen for the measurements (a) as compromise between measurement time and accuracy. Whereas the conductivity of the 400 nm gap increased significantly upon irradiation, that of the 600 nm gap was hardly measurable and that of the 800 nm gap, bridged by almost statistically distributed J-aggregates from Fig. 59 not measurable at all.

The 400-800 nm gaps, which have been bridged by single aggregates from the diluted dispersion in DCM, showed also no measurable conductivity increase upon irradiation with the green laser.

Fig. 62 compares the photoconductivity of the 400 nm gap, bridged by the high number of aggregates, with that of the interdigitated gold electrode from Fig. 56.

Fig. 61: Photoconductivity of P2 bridging the 400 and 600 nm Pd gaps

a) The photoconductivities across the Pd gap to the floating electrodes from Fig. 59 were measured in the dark and during the irradiation by a green laser (1 mW, 532 nm, 3 mm²). The integration time was 1s per point, which has been determined as an optimum from the series in b) recorded at the 600 nm gap in the dark. The integration time of 1 s was a compromise between the time to measure one I-V curve and the hysteresis. a) The aggregates within the 400 nm gap, with the high accumulation of P2 aggregates (Fig.

59d), showed a significant increase in conductivity upon irradiation in contrast to the 600 nm gaps. The 800 nm gaps did not show any conductivity increase upon irradiation, too (not shown).

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1.2.9 Dielectrophoresis 83

Estimation of the conductivity:

In order to estimate the conductivity of the aggregates the following parameters were used for the interdigitated gold electrode:

Width of the gap in the IDE: 50 mm (520µm·335µm/3.5µm), gap length: 1.6 µm, estimate of the number of J-aggregates bridging the gap: 10 000, dimensions of the estimated average J-aggregate within the gap:

1.6 µm long and 0.1µm·0.1µm thick.

At higher potentials (12-15 V) and under irradiation with the green laser the conductivity of the P2 J-aggregates in the direction of their long axis is estimated to be in the range of 2·10^-7 S/m. This corresponds to a resistivity of 5·10⁶ Ω·m, which is about 3 orders of magnitude higher than that of undoped silicon. In the potential range up to 5 V the conductivity under this irradiation was about 5 times lower (4·10^-8 S/m), corresponding to a resistivity of about 2.5·10^-7 Ω·m. Without irradiation the conductivity was within the potential range up to about 12 V approx. one order of magnitude lower (4·10^-9 S/m).

For the aggregates bridging the 400 nm gap between the palladium gap on the floating electrode structure (Pd-FE) the conductivity under irradiation with the green laser was, within the potential range of 12-15 V, about 25 times higher (5.6·10^-6 S/m), than that of the Au-IDE. Within the potential range up to approx. 5 V it was about 18 times lower (3·10^-7 S/m, 3·10⁶ Ω·m) than above 12 V and in the dark up to 12 V it could be approximated to 2·10^-7 S/m (5·10⁶ Ω·m).

It was estimated that the 400 nm long gap was bridged over the width of 10 µm by a densely packed array of J-aggregates with 0.1 µm · 0.1 µm cross-section, each and 0.4 µm length, thus by about 100 aggregates.

In both types of gaps the conductivity increases significantly at higher potentials. This can be explained by a space charge limited current (SCLC), which starts rising faster at a certain potential, when most trap states

Fig. 62: Photoconductivity of P2 aggregates on the interdigitated gold electrode

These measurements compare the photoconductivity of P2 aggregates within the 1.6 µm wide gaps of the interdigitated gold-electrode (IDE) from Fig. 56, measured at 3 s/point (blue and black curves) with the P2 aggregates bridging the 400 nm gap of the palladium structure (green and red curves), shown in Fig. 59d, measured with an integration time of 1s/point. The irradiation source was in both cases a green laser (1 mW, 532 nm, 3 mm² spot size). Whereas the I-V sweep of the 400 nm curves was started and finished at -15 V, that of the 1600 nm was started and finished at 0 V. The diagram on the right hand side shows the diagram of the left hand side in a logarithmic plot (only the positive current).

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100 1600 nm IDE Au-gap dark 1600 nm IDE Au-gap laser 400 nm Pd-gap dark 400 nm Pd-gap laser

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100 1600 nm IDE Au-gap dark 1600 nm IDE Au-gap laser 400 nm Pd-gap dark 400 nm Pd-gap laser

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backward

1.2.9 Dielectrophoresis 84 have been filled with charges, smoothing out the potential landscape within the aggregates. A significant difference between both types of gaps, or J-aggregate lengths, arises upon irradiation with the green laser.

Whereas the conductivity of the 400 nm long J-aggregate bridge increases by a factor of less then two upon irradiation at low potentials, that of the 1600 nm long J-aggregate bridge increases by about one order of magnitude upon irradiation at low potentials. This can be explained also by a higher number of trap states within the longer J-aggregates which shield the electric field and thus dominate the resistance within a higher potential range until the irradiation helps to release the trapped charges leading to an increases of the conductivity. The logarithmic plot of the conductivity in Fig. 62 is better suited to see the threshold potential at which the traps are filled and hence the conductivity rises faster. This diagram confirms, that the threshold potential of the SCLC is reached at about 10 V for the 400 nm short aggregates (green and red curves) and is not reached up to 15 V for the 1600 nm long aggregates (blue and black curves).

In the logarithmic plot the hysteresis between the forward and backward potential sweep is also pronounced.

It results from a relatively high capacitance in combination with a relatively low conductivity. Although the difference in the work function of the electrode materials Au and Pd is very small, it can not be excluded, that it contributed to the conductivity measurements by different charge injection rates. As the gold or Pd have their Fermi-level close to the HOMO of the aggregates it can be assumed, that this photoconductivity corresponds to a hole transport rather than to an electron transport, which may be hindered by a higher Schottky barrier. This is in agreement with additional measurements, where no occurrence of electroluminescence on the gold IDE chip could be detected, which was tried with a sensitive CCD camera through an optical microscope. This could also mean that an eventual electron transport occurs over triplet states and that the intersystem crossing into the singlet state is not likely. This would be beneficial for solar cells as it would reduce the recombination rate.

The conductivity of the aggregates might be not sufficiently high for an efficient charge transport through the entire long axis of the aggregates. However, the conductivity rose nonlinear with the decrease of the path length and hence might be significantly higher on a short length scale, which could not be investigated further with our electrode structures. However, P2 aggregates do not have such high requirements for the conductivity like dyes in organic or hybrid solar cells, because in our solar cell concept a third material takes over the part for the long-range hole transport. Using an additional hole transport material the maximum length for the hole transport within P2 aggregates would be in the range of the aggregate diameter and therefor its conductivity seems to be sufficiently high.

To study whether the photoconductivity correlates with the absorption spectrum when the irradiation wavelength is varied, a UV-Vis-NIR absorption spectrometer was used as monochromatic light source for conductivity measurements during the irradiation by a continuous wavelength sweep in the spectrometer.

Because the irradiation intensity was very low, the integration time for each current-measurement point was set to 10 s and the duration for the whole wavelength sweep was set to 1.5 hours. The spectral photoconductivity of the IDE sample with the bridged 1.6 µm gap is shown in comparison with a typical P2 J-aggregate spectrum in Fig. 63.

1.2.9 Dielectrophoresis 85

The spectral photoconductivity (dark dots) showed indeed a correlation with the absorption spectrum of P2 J-aggregates (red trace). At the beginning of the conductivity measurement in the dark a steady decrease can be seen. This trend of a decreasing baseline signal probably did not change when the wavelength sweep was started at 790 nm. When the excitation wavelength reached the first absorption band of the J-aggregates the conductivity steeply increased and decreased again after this Q band to reach a minimum between the two Q bands. Note the mirrored absorption spectrum due to the plotting in the direction of the time scale and not as usual in the direction of increasing wavelengths. The dotted vertical lines at wavelengths of 606 and 428 nm highlight the correlation at the point between the two Q bands and at the Soret band maximum. The steep decrease of the photocurrent at the beginning of the measurement in the dark shows the very slow relaxation time of the aggregate conductivity after an previous excitation. This slope in the baseline is the reason, why the spectral photoconductivity does not correlate better with the absorption spectrum. A similar slow relaxation time has been also observed by Schwab et al., who performed photoconductivity measurements on self-assemblies of meso-tetrakis(4-sulfonatophenyl) porphyrins, which bridged a 350 nm gap of a similar IDE [65]. However, the question remains, what causes this slow relaxation. It might be a high capacity.

The band at around 300 nm is the only one, which caused no change in the conductivity spectrum. This is in agreement with our hypothesis, that this band does not belong to the porphyrin's core absorption, but to the absorption of its side groups, the phenyl rings. As these are tilted by about 70° relative to the porphyrin plane, they do not belong to the extended pi-conjugated system of the porphyrin.

Fig. 63: Photoconductivity of P2 on the IDE within the UV-Vis-NIR spectrometer

The same interdigitated gold electrode bridged by the P2 aggregates, which has already be shown in Fig.

56 and on which the photoconductivity measurement from Fig. 62 has been performed, was used here again to measure the spectral photoconductivity using the monochromatic excitation source of the UV-Vis-NIR absorption spectrometer. A typical absorption spectrum of P2 J-aggregates is shown for comparison (red).

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