Rajiv Prakash

Organic phototransistors (OPTs) with high responsivity and low-voltage operation are crucial for future optoelectronic applications. Herein, we demonstrate a semitransparent low-voltage OPT by integrating a highly oriented organic semiconductor thin film with an ion-conducting dielectric interface. The active semiconductor layer was fabricated using the facile unidirectional floating film transfer method (UFTM), enabling long-range molecular alignment and improved charge transport pathways. Efficient low-voltage operation was achieved using a solution-processable lithium–alumina (Li–Al2O3) ion-conducting dielectric, which facilitates strong electric-double-layer (EDL) charge modulation at the semiconductor/dielectric interface. The synergistic effect of UFTM-induced molecular ordering and dielectric engineering significantly enhances the photodetection performance, resulting in a high responsivity exceeding 127 A W–1 and estimated shot-noise-limited specific detectivity (DShot*) on the order of 1013 Jones. The improved performance is attributed to enhanced photogenerated carrier transport, reduced trapping effects, and efficient interfacial charge modulation. These results demonstrate the potential of semitransparent, low-voltage organic phototransistors for future energy-efficient optoelectronic applications.

In this study, we demonstrate the direct visualization of photoinduced charge carrier redistribution by Time-Resolved Second Harmonic Generation (TRM-SHG) of enhanced photoresponse in organic phototransistors based on highly oriented PBTTT-C14 thin films fabricated via the unidirectional floating film transfer method (UFTM). The implementation of a bilayer structure, incorporating Rose Bengal (RB) dye spin-cast atop PBTTT-C14, dramatically improves device performance, with the photosensitivity enhanced up to ~106 orders of magnitude. The optimized devices exhibit high responsivity on the order of 7×102 A/W and extremely high specific detectivity on the order of 1016 Jones. Furthermore, enhanced photocarrier generation and dynamics in the bilayer device structure were directly visualized and investigated using time-resolved optical second-harmonic generation (TRM-SHG), providing additional insight into the charge transport mechanisms. The findings demonstrate an effective strategy to improve photoresponse in organic optoelectronic devices.

In this study, we demonstrate the direct visualization of photoinduced charge carrier redistribution by Time-Resolved Second Harmonic Generation (TRM-SHG) of enhanced photoresponse in organic phototransistors based on highly oriented PBTTT-C14 thin films fabricated via the unidirectional floating film transfer method (UFTM). The implementation of a bilayer structure, incorporating Rose Bengal (RB) dye spin-cast atop PBTTT-C14, dramatically improves device performance, with the photosensitivity enhanced up to ~106 orders of magnitude. The optimized devices exhibit high responsivity on the order of 7×102 A/W and extremely high specific detectivity on the order of 1016 Jones. Furthermore, enhanced photocarrier generation and dynamics in the bilayer device structure were directly visualized and investigated using time-resolved optical second-harmonic generation (TRM-SHG), providing additional insight into the charge transport mechanisms. The findings demonstrate an effective strategy to improve photoresponse in organic optoelectronic devices.

In this work, we demonstrate a highly sensitive ammonia (NH3) gas sensor based on organic field-effect transistors (OFETs) with a polymer nanocomposite of graphene oxide (GO) and the conjugated polymer Poly[N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)] (PCDTBT) as the active semiconductor channel. The nanocomposite semiconductor was fabricated using the unidirectional floating film transfer method (UFTM) at the liquid-air interface, enabling the preparation of highly oriented thin films with good molecular ordering. Our approach leverages UFTM-induced molecular orientation in a GO–conjugated polymer nanocomposite to simultaneously achieve ultrafast response, high sensitivity at low NH3 concentrations, and exceptional operational robustness under harsh conditions. The resulting OFET sensors exhibit an outstanding sensing response of approximately 80% at a ammonia concentration of 20 ppm, with response and recovery times of 3 and 5 s, respectively, under ambient conditions (~30 °C). Moreover, the devices showed superior stability under harsh environmental conditions, maintaining their sensing performance at temperatures up to 100 oC and relative humidity of 85%. The sensors also maintained their performance over an extended period of 90 days. The enhanced orientation and molecular packing of the polymer films achieved by UFTM are responsible for the high sensitivity, reliability, and stability of the fabricated OFET-based ammonia sensors.

Manipulating and understanding the crystallization processes of solution processed organic thin films have been a key challenge not only for fundamental studies but also for intrinsic charge transport properties. Among various techniques, formation of self-assembled thin films at liquid–air interface has been known for polymeric semiconductors. However, integration of this technique for organic small molecules (OSM) is scarcely explored. This sought-after exploration results in possibilities of integrating the superior charge transport properties of OSM with other fabrication advantages of liquid substrate-based growth methods. In this work, self-assembly of an OSM-based semiconductor over a liquid–air interface is thoroughly investigated using the 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8BTBT) molecule and its crystallization behaviour along with morphology is studied over different liquid base substrates (LBSs). The correlation of the morphology change due to the change in the surface tension and viscosity of the LBS is stated and the resulting films are investigated for charge transport properties of C8BTBT based field-effect transistors. The ribbon-like morphology emerges for the low surface tension LBS, which gives rise to higher charge transport among the films. This study provides a novel and easy processing technique with a better understanding of the growth mechanism for large area and highly crystalline thin films of organic small molecule semiconductor-based electronic devices.

In this work, we report the synthesis of MXene (Ti2CTX) nanobelts from its MAX (Ti2AlC) phase via hydrothermal treatment with 5 M NaOH. This is an environmentally friendly and safer alternative to traditional HF-based etching processes. This approach reduces risks to health and handling concerning HF but maintains the exfoliation quality of the MXene layers. The Ti2CTX nanobelts were then incorporated at various concentrations into poly(3-hexylthiophene) (P3HT) matrices to form nanocomposite films using the unidirectional floating film transfer method (UFTM). These aligned hybrid films showed dramatically improved charge transport properties compared to pristine P3HT. For instance, adding 3% (v/v) MXene increased the charge carrier mobility from 0.05 cm2V ${}^{-{1}}$ s ${}^{-{1}}$ (pristine P3HT) to 0.57 cm2V ${}^{-{1}}$ s ${}^{-{1}}$ with a high on/off current ratio of $10^{{5}}$ . We attribute this improvement to the template effect of the MXene nanobelts, which promotes the orientational alignment of P3HT chains, thus facilitating efficient charge transport pathways.