PREFACE
1.4. Applications of perylene derivatives in organic electronics 1. Perylene based LCs for organic solar cells
Schmidt-Mende et al. first utilized a perylene dye in conjunction with a Col LC, hexaphenyl-substituted hexabenzocoronene to fabricate an organic photovoltaic device.69 Thin films formed by a mixture of these two components led to the vertically separated perylene and HBC assemblies, with a large interfacial surface area. When these films are integrated into diode structures, they exhibited an excellent photovoltaic response (external quantum efficiencies > 34 % and power efficiencies of up to ≈2% near 490 nm). This is the result of the efficient photoinduced charge transfer between the HBC and perylene, as well as the effective charge transport through the vertically segregated perylene and HBC π-systems. It is interesting to note that the PBI (3a) utilized, was a crystalline material.
Kim et al. fabricated an OSC, where the LC 3b acts as an electron acceptor and a zinc phthalocyanine (ZnPc) acts as a donor.70 The cell performance under illumination was studied with respect to the change in the thickness of organic layers. A cell comprising 25 nm thick organic layers exhibited a short circuit photocurrent (ISC) of 1.6 mA/cm2, an open circuit photovoltage (VOC) of 0.6 V, and a fill factor (FF) of 0.4 with a white Xe light illumination (wavelength P400 nm, intensity:100 mW/cm2) (Figure 1.28).
Figure 1.28. (a) Dark current potential characteristics for ZnPc:3b films of 100:100 nm (black trace);
50:50 nm (red trace); and 25:25 nm (blue trace); Inset: Current-potential characteristics for the device with configuration as ITO/ZnPc (25 nm)/3b (25 nm)/Ga; (b) device configuration (Reproduced from reference [70] with permission of Elsevier.
Bock et al. recently reported a homeotropically aligned bilayer heterojunction
formed by a pair of DLCs, which were aligned from their isotropic liquid state71 (Figure 1.29b). These materials exhibited properties like selective solubility, a low degree
of miscibility, adjusted transition temperatures and RT Colh phase. The two materials they used was a PTE (regioisomeric mixture, acceptor, PT-1) and a pyrene tetraester (donor, PY-2). This represented the first proof of principle of an organic heterojunction that is based on two oriented columnar LC layers.
Kelly et al. reported four different PBI based acceptors (A1-A4) with similar electron affinities but different LC phases. These compounds were blended with nematic LC electron-donors comprising a fluorene-thiophene structure (D1 and D2) to form single layer OSCs (Figure 1.29a). Superior results were attained when the nematic donor (D2) is mixed with an amorphous acceptor (A1 or A4). This gave a supercooled nematic glass at RT. Atomic force microscopy images revealed the phase separation on a nanometer scale of varying domain sizes. The OSC devices with the A1 exhibited the best performance. A power conversion efficiencies (PCE) of 0.9 % was found for the D1, A1 combination. The morphologies were correlated with the device performances and PCEs of 0.9% was achieved at 470 nm. D2 is a polymerizable mesogen that can be polymerized thermally or
by UV light irradiation to give a polymer network. The polymerization of an electron–donating reactive mesogen mixed with a non polymerizable PBI acceptor was
expected to change the extent of phase separation and also the composition of the
phase-separated domains.45a They have also demonstrated the solution-processed bilayer OSCs, where the lower electron-donating layer was photopolymerised prior to the deposition of a perylene-based acceptor layer on top. It was interesting to note that the performance of the polymerized and non-polymerized blended devices was similar;
dispelling fears that photopolymerization might lead to photodegradation.45b They have also utilized a LC composite approach to preparing a nematic LC polymer network from D2 with a porous surface and electron-donating properties. This is infilled with an electron-acceptor A1 to form a bilayer device. The diffused interface created allowed the excitons to dissociate and move towards the respective electrodes72. The monochromatic power conversion efficiency was seen to vary between 0.8% and 0.3% with input irradiance.
Figure 1.29. (a) Molecular structures of perylene based oligomers (D1-D2 and A1-A4); (b) Schematic structure of the two compounds PT-1 (where only one of the two regioisomers is sketched) and PY-2. Their low degree of miscibility is shown by a contact preparation between coverslip and glass slide in the isotropic liquid phase (Reproduced from reference [71] with permission of American Chemical Society).
1.4.2. Perylene based LCs for organic light emitting diodes
Bock et al. reported a red light emitting diode from perylene tetraester T-2 with a very simple device structure with single emissive layer (Figure 1.30). They prepared LEDs consisting of a ITO coated glass anode layer, one to three organic layers, and an aluminum cathode layer. All the PTEs T1-T6, T-8 and TEH exhibited a strong electroluminescence even in a single layer device. A single-layer light-emitting diode of T-2 with a film thickness of about 50 nm, exhibited an emission above a threshold of 7-8V, and the
maximum luminance was 100 cd cm-2 at a voltage of 20 V. They have also shown that color-tuning towards white emission, the color stability, and the lifetime of the device may be improved by simultaneous co-deposition of several kinds of emissive materials based on triphenylene or pyrene derivatives of very similar electro- and photochemical stability.13 They have also reported red light emitting diodes with a device configuration of ITO-coated glass/triphenylene hexaether/PTE/aluminum.73
Figure 1.30. Photograph of a light-emitting diode that contains compound T-2, without (left) and with (right) applied voltage (Reproduced from reference [13] with permission of WILEY VCH).
Bechtold et al. compared the electrical responses of Col LC 15 in a device that was fabricated either by spin coating or by thermal evaporation.74 For the spin-coated film, homeotropic alignment was obtained by thermal annealing, which was correlated with the enhanced charge carrier mobility. For the evaporated films, such alignment could not be obtained by thermal annealing. However, a three orders higher degree of rectification and more luminance was accomplished, even without annealing. The electrical response was similar to the response of the aligned spin-coated film. Spin coated thermally aligned film shows a good homeotropic alignment, while the thermally evaporated film exhibited planar alignment suitable for OFETs. However, the superior molecular packing in the evaporated films can be utilized either in OLEDs75 or OFETs, showing an acceptable electrical response.
1.4.3. Perylene based LCs for organic field effect transistors
Organic field effect transistors form an important component for emerging low-cost, large-area and flexible electronics. There number of organic p-type semiconductors with hole mobilities more than 0.5 cm2 V-1 s-1 are available in large numbers and they are utilized in OFETs with outstanding performance under ambient conditions, but n-type counterparts
are still scarce. Among the investigated n-type organic semiconductors, the ones based on naphthalene tetracarboxylic bismides (NBIs) and PBIs appear so far as the most encouraging ones. This is because of their performance under ambient conditions is comparable with that of the p-type materials used in OFETs.76 Figure 2d and 2e depict usually used OFET device configurations. Most of the OFETs are tested with bottom gate top contact device structure and the Col LC should be aligned with an ‘edge on’ or planar orientation with respect to the substrate to exhibit good charge carrier mobility (Figure 2f).
The research on n-type semiconductors is lagging behind due to the more difficult electron injection, because of the large energy barrier between the Fermi level of the source and drain contacts and the LUMO level of the n-type semiconductor. Additionally, a poor stability of radical anions and their vulnerability towards water and oxygen have been deliberated as a great hurdle. PBIs usually exhibit low lying LUMO levels, good stability and reversible oxidation and reduction cycles in cyclic voltammetry. Further, various possibilities of substitution in the core provide opportunities to tailor the HOMO and LUMO energy levels. The advantage of PBIs is that their electronic properties can be tuned by core substitution, while their solubility can be tuned by the variation of alkyl groups at imide position. Usually, these PBI derivatives are fabricated into device either by vacuum deposition or solution processing technique. The best performing devices are achieved by vacuum deposition, but this is very slow and time-consuming method. It requires the optimization of several parameters to control the type of growth and the crystallinity of the organic semiconductor. Several factors like substrate temperature, surface modification by SAM and deposition rate are considered in achieving a superior deposition on the substrate.
Solution processing methods like spin coating or ink jet printing are promising from the viewpoint of cost and large scale manufacturing. But in spite of their fast and cheap processing ability they suffer from the formation of less ordered amorphous films in comparison to those obtained by vacuum deposition. They often require additional steps like thermal annealing to obtain stable devices with good performance.
Facchetti and coworkers reported several dialkyl PBIs substituted with two cyano groups in the core. In particular, one cyano-substituted PBI with -CH2C3F7 chain atimide positionshown a better performance and stability (inert condition: 0.15 cm2 V-1 s-1; after 20 days in the air: 0.08 cm2 V-1 s-1).77 Though the present focus is on the area of solution processed devices, it is directed toward the rational synthesis of polymeric materials, which offer improved morphological stability over a long period. Thelakkat et al. reported LC diblock copolymers of polystyrene and poly-(perylene acrylate), which were spin coated to
prepare the devices. However, the mobilities were found to be low.78 Marder et al.
described a solution-processable, narrow band gap, high-mobility, alternating copolymer of PBI and dithienothiophene building blocks. They have shown broad absorption through the visible region of the absorption spectra extending into the near-IR region.79 Device measurements show that alternating D-A polymers of this type are encouraging electron-transport materials for n-channel OFETs, promising sensitizers and electron-transport materials for all-polymer OSCs. Recently, Troshin et al. did a systematic
study on a series of dialkyl PBIs by varying the chain length from n-butyl to n-dodecyl.80 The deposition of these PBIs was carried out by thermal evaporation in a vacuum. They found that the increase in the chain length improves both the charge carrier mobility and the on-off current ratio of the OFETs.81 The branched chain derivative with 2-ethylhexyl side chains exhibited inferior performance in comparison to the isomeric PBI with n-octyl chains. This may be due to the increased disorder in the case of branched chain derivative.
Although some of these compounds are liquid crystalline in nature, nothing has been mentioned on the effect of liquid crystalline organization on the device efficiency in many of the reports. The larger homologues like terrylene and quaterrylene bisimides are yet to find a foothold in OTFTs with comparable performance as their smaller homologues like NBI or PBI. However as mentioned earlier, terrylene tetracarboxdiimide (18), is known to form crystalline, large and highly ordered domains, with molecules arranged in an edge-on orientation.33 For more information on PBI based OFETs, the readers are advised to refer reviews by Würthner76 and Marder.81