Ahmad
Murad
a,
Mohamed
Alaasar†
*c,
Ahmed F.
Darweesh
b and
Alexey
Eremin†
*a
aInstitute of Physics, Otto von Guericke University, 39106 Magdeburg, Germany. E-mail: alexey.eremin@ovgu.de; Tel: +49 391 6750099
bDept. of Chemistry, Faculty of Science, Cairo University, Giza, Egypt
cMartin Luther University, Halle-Wittenberg Kurt Mothes Str. 2, 06120 Halle (Saale), Germany. E-mail: mohamed.alaasar@chemie.uni-halle.de
First published on 2nd July 2024
We explored the electrical conductivity and photovoltaic properties of a complex polycatenar liquid crystal that contains a 5,5′-diphenyl-2,2′-bithiophene core and exhibits a nanostructured cubic bicontinuous phase. Owing to their three-dimensional network structures, such materials give rise to continuous conduction channels in three dimensions, minimising the distortions induced by structural defects observed in other liquid crystal phases. Although the pure material doesn't show any photovoltaic behaviour, we found that doping it with fullerenes significantly increases the photocurrent. We also examined the electrical conductivity and charge mobility in the hybrid compound.
The majority of recently reported Cubbi phases are exhibited by polycatenar molecules having an unsymmetrical distribution of terminal chains at both ends of the extended aromatic rod-like backbone.9–16 One of the core units used in designing such Cubbi phases to be applied as semiconductors or photovoltaic cells is the π-conjugated 5,5′-diphenyl-2,2′-bithiophene core.15,16,18,19 In almost all of these materials, two types of Cubbi phases with different symmetries were reported to differ in the number of the three-way junctions helical networks forming the cubic phase.
In the first type, two networks of opposite chirality sense are involved, leading to the achiral double gyroid phase with space group Iad (Cubbi/Ia
d) (see Fig. 1b).9 On the other hand, the second type is a chiral one composed of three helical networks with a space group I23 (
, Fig. 1a).12 Chirality was even observed in the isotropic phase
exhibited by some of these polycatenars.7,9,19,20
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Fig. 1 The two different types of helical network structures of the Cubbi phases exhibited by the nonsymmetric polycatenars (a) the achiral double-gyroid Cubbi/Ia![]() ![]() |
Recently, we reported the design and synthesis of LC polycatenars derived from the π-conjugated rod-like 5,5′-diphenyl-2,2′-dithiophene unit, in which one end has three alkoxy chains, and the other end has one variable thioalkyl chain (compounds Am/n, Fig. 2).15
Self-assembly behaviour in these materials was investigated in detail. They were found to exhibit Cubbi phases in extensive temperature ranges >200 K, which in some cases are stable at room temperature, making them excellent candidates for technological applications. Depending on the length of the thioalkyl chain, the two different types of the Cubbi phases, either Cubbi/Iad or
were observed in addition to the
phase and tetragonal phase in some derivatives.14
This paper investigates the electrical conductivity, photovoltaic properties, and charge mobility in a complex polycatenar liquid crystal containing a 5,5′-diphenyl-2,2′-bithiophene core and exhibiting the cubic bicontinuous phase. Although the pure material does not show any photovoltaic behaviour, we find that doping it with fullerenes significantly increases the photocurrent already at a concentration as low as 2 wt%. Molecular doping using chiral and electronically active acceptor/donor dopants enables the tuning of materials optical, mechanical, and electronic properties. This extends beyond liquid crystal systems to include organic semiconductors, photoluminescent materials, and photochemical and catalytic systems.21–26
The phase transition of compound 1 is as the following: heating: Cr 116 °C [40.6] 190 °C [2.5] Iso cooling: Iso 187 °C [<0.1]
182 °C [1.9]
57 °C [22.2] Cr.
The numbers in brackets provide the transition enthalpies in kJ mol−1. The designations Cr, and Iso correspond to the crystalline, cubic bicontinuous and isotropic phases, respectively. The structure of the phase is shown in Fig. 1.
As a dopant, we used [6,6]-phenyl C61 butyric acid methyl ester purchased from Merck. Four mixtures were investigated as listed in Table 1.
Sample | S1 | S2 | S3 | S4 |
---|---|---|---|---|
Concentration of PCPM (mol%) | 2.0 | 4.5 | 8.7 | 10.8 |
We used a Keithley source meter 2635B (SMU) to perform the conductivity measurements and the characterisation of the photovoltaic effect. The software Kickstart (Keithley) was used to control the SMU. To avoid the relatively high hysteresis of the measured dark current, we opted for a pulsed measuring mode with a dwelling time of 15 seconds between the measuring pulses.
The tungsten light source was used to illuminate and measure the photocurrent, while a Newport Corp. power meter 918D-UV-OD3R was used to measure the light power.
Time-of-flight measurements were made using Q-switched sapphire PDSS laser Helios (Coherent, Germany) at λ = 532 nm, pulse width τ < 690 ps, and the pulse energy 20 μJ. For the signal acquisition, we used a broadband oscilloscope Tektronix 3032b (Tektronix, USA).
In the crystalline phase, however, the J–V characteristic has a saturating behaviour in sufficiently high fields exhibiting nonlinearities. Nevertheless, ohmic behaviour was observed in all phases in the voltage range below 0.5 V. The electric conductivity is weakly temperature-dependent below 100 °C and increases continuously with increasing temperature (Fig. 3b). This growth can be attributed to the thermal activation of the charge transfer mechanism and increasing role of the ionic transport facilitated by the decrease in viscosity. The pure compound behaves as an insulator due to the deficiency of charge carriers and shows no photovoltaic response in the visible spectral range.
In order to solve the problem, we introduced fullerene as a dopant which acts as an electron acceptor (as shown in Fig. 2b). Fullerene is a commonly used material in organic semiconductors due to its bandgap of 1.5 eV.8,28–30 This property makes it suitable for applications in electronics and photovoltaics. We prepared four different mixtures with varying concentrations of the dopant, which are listed in Table 1.
Admixing such a small amount of C61 dopant to the liquid crystal does not significantly affect the dark current. Fig. 3b shows the temperature dependence of the conductivity under dark conditions. The dark current exhibited a strong temperature dependence, increasing with increasing temperature. No aggregation of fullerenes was observed in the mixture S1, as was examined using polarising microscopy (see Fig. 2c–e). The cubic phases have optically isotropic texture and appear dark between crossed polarisers (Fig. 2c). Without polarisers, the texture is uniform suggesting that the aggregation is suppressed (Fig. 2d). Mixtures S2–S4 showed a weak aggregation when exposed to the multiple cycles of Iso-LC transition.
At the same time, the current becomes very susceptive to the light. The pronounced photovoltaic effect was observed in the whole range of the LC phase. Fig. 4a shows the photocurrent in S1 in response to the illumination of various light intensities. The measurements were performed under the bias of 1 V. The current is superimposed with the ionic contribution, which was particularly strong at high temperatures and showed a pronounced time dependence. A slow current decrease with time reflects the development of the space charge double layer. The ionic contribution became small at low temperatures, and the time dependence slowed down. The photocurrent demonstrated only a weak temperature dependence and increased linearly with rising light intensity, as shown in Fig. 4b.
In mixtures with a higher concentration of C61, the temperature dependence of the photocurrent became more pronounced. Fig. 4 shows the photocurrent for the mixtures S2 and S3. The photocurrent increases by more than an order of magnitude in the mixture S3. At the same time, the intensity dependence remained linear in the measured intensity range.
Interestingly, there is a clear dependence of the photocurrent on the concentration of C61 up to 8.7 mol%. This can be attributed to the saturation of the C61 mixtures and the building of aggregates in S4, resulting in a nearly constant bulk concentration of C61. The dark current, however, did not show any markable dependence on the dopant concentration, and conductivity remained the same for all three mixtures.
The charge mobility was measured with ToF technique using excitation at 532 nm. Fig. 5a shows the typical ToF signal. The signal has a dispersive character. The corresponding mobilities are shown in Fig. 5b.
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Fig. 5 (a) Photocurrent in time of flight experiments recorded in mixture S3 at T = 100 °C (dark current subtracted), (b) temperature dependence whole mobilities in S3. |
The mixtures exhibit hole mobilities, which are nearly temperature-independent in the cubic phase suggesting the hopping transfer mechanism. The mobility values are comparable with those of the SmC and SmB phases in typical 2-phenyl naphthalene and terthiophene derivatives.31 At high temperatures, the mobility starts increasing with increasing temperature. Such an increase of the charge mobility with increasing temperature suggests a temperature-activated distributed density of states responsible for charge transport.
Footnote |
† These authors contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2024 |