Direct x-ray detectors using thermally evaporated pentacene thin-films are fabricated in Schottky and coplanar configurations and are analysed for low x-ray dose rates. In both configurations, the x-ray induced photocurrent is found to be five orders of magnitude greater than the theoretically evaluated threshold value that may reflect possible internal gain mechanism. Coplanar detectors showed unstable x-ray photocurrent characteristics; on the other hand, Schottky photodiode structure showed stable response and thus allowed to proceed for x-ray sensitivity measurements. Pentacene-based Schottky detector presented a decent volume sensitivity of 162.3 μC/mGy/cm3. The high x-ray sensitivity of pentacene Schottky detector can be due to the complete depletion of the thin-film at the operating reverse bias, revealed by transfer characteristics of fabricated pentacene MESFET. Such a reasonably good x-ray photoconversion in low-Zorganic semiconducting materials uncovers the possibility of implementing them in x-ray medical dosimetry applications and in wearable electronic technology. 

A theoretical study was carried out to understand the geometrical, electronic and optical properties of bipheylyl/thiophene based molecular system. For that, density functional theory along with classical Marcus charge transfer theory and time-dependent potentials were used. The results indicate that the structure-property relationship between the molecular parameters and ionization energy, electron affinity, reorganization energy, charge transfer integrals, mobility, energy gap, optical absorption and transition dipole moments. Low lying HOMO values and lower bandgaps of the molecules offer a higher charge transport in the systems and 5,5″-bis(4-biphenylyl)-2,2′:5′,2″-terthiophene (BP3T) molecule shows the maximum hole and electron mobility, 0.81 cm2 V−1 S−1 and 0.17 cm2 V−1 S−1, respectively. The results have shown that all the title compounds can be fabricated as ambipolar organic semiconductor materials and can be considered for the fabrication of efficient organic light emitting transistors. 

Biphenylyl/thiophene systems are known for their ambipolar behavior and good optical emissivity. However, often these systems alone are not enough to fabricate the commercial-grade light-emitting devices. In particular, our recent experimental and theoretical analyses on the three-ring-constituting thiophenes end capped with biphenylyl have shown good electrical properties but lack of good optical properties. From a materials science perspective, one way to improve the properties is to modify their structure and integrate it with additional moieties. In recent years, furan moieties have proven to be a potential substitution for thiophene to improve the organic semiconductive materials properties. In the present work, we systematically substituted different proportions of furan rings in the biphenylyl/thiophene core and studied their optoelectronic properties, aiming toward organic light-emitting transistor applications. We have found that the molecular planarity plays a vital role on the optoelectronic properties of the system. The lower electronegativity of the O atom offers better optical properties in the furan-substituted systems. Further, the furan substitution significantly affects the molecular planarity, which in turn affects the system mobility. As a result, we observed drastic changes in the optoelectronic properties of two furan-substituted systems. Interestingly, addition of furan has reduced the electron mobility by one fold compared to the pristine thiophene-based derivative. Such a variation is interpreted to be due to the low average electronic coupling in furan systems. Overall, systems with all furan and one ring of furan in the center end capped with thiophene have shown better optoelectronic properties. This molecular architecture favors more planarity in the system with good electrical properties and transition dipole moments, which would both play a vital role in the construction of an organic light-emitting transistor. 

Modern computational chemistry methods aid experimentalists in scrutinizing molecular systems for organic optoelectronic device fabrication. They are also helpful for device engineering, such as choosing metals for contact fabrication without sophisticated instrumentation. The B3LYP functional with 6-31G(d,p) or 6-311G(d,p) basis set is a widely used method in this regard. Recently, many upgraded methods, such as the three-fold corrected composite scheme (3C), have been reported to offer cost-effectiveness and better accuracy than the B3LYP. This is particularly interesting from a materials engineering point of view as these methods offer the possibility of rapidly screening the materials prior to the experimental analysis. In order to evaluate their suitability towards organic optoelectronic systems, in this work, we have used three composite schemes, namely B97-3C, PBEh-3C and HF-3C, along with B3LYP and the experimental data. The well-studied tetracene system is considered for the analysis, and different optoelectronic properties such as bandgap, reorganization energy, and absorption maximum are calculated theoretically and compared with the experimental data. The results indicate that though slightly expensive, B3LYP performs better than the 3C methods. Of the three, only PBEh-3C performs on par with B3LYP in accuracy and time. A note of caution is that these results should be inferred as this level of theory is adequate for the screening and modeling optoelectronic systems rather than a potential computational chemistry method. 

A detailed understanding of charge transport at the interface is necessary to explore the potential of organic field-effect transistors. This can be realized by adequately analyzing the trap states at the interface. In the present work, rubrene-based organic field-effect transistors have been fabricated with three different interfaces. The device properties are used along with a technology computer-aided design to deduce the interface trap density quantitatively. The transfer characteristics are simulated using a double Gaussian density of states and fitted with experimental data by adding traps to the semiconductor dielectric interface. A typical transistor with SiO2 interface has shown an interface trap density of about ≈1016 cm−2, and it is reduced to 1014 cm−2 when poly(methyl)methacrylate or polystyrene is coated on SiO2 interface, attributed to the surface passivation. This approach provides a simple and accurate way to estimate the interfacial traps and offers the possibility to tune the device architecture and performance. 

Organic semiconductors are attractive candidates for the development of flexible and efficient optoelectronic devices. In this work, we have studied the pyrene end-capped thiophene derivatives with different number of oligomeric rings and substituted with their oxygen analogue furan. Results indicate that the molecular planarity varies with the substitution of furan rings and the number of rings play a role on optoelectronic property of the systems. All the systems have shown ambipolar behavior, but the high charge barrier in the case of electrons could limit the device performance. Further, in photovoltaic analysis the increment in oligomeric ring has substantially reduces the open circuit voltage and the fill factor. However, the introduction of furan has improved the overall light harvesting efficiency. With the proper choice of material and device engineering the shorter systems can be considered for photovoltaics, where as the longer derivatives can be considered for light emitting device applications.