Combining the electronic properties of graphene(1,2) and molybdenum disulphide (MoS2)(3-6) in hybrid heterostructures offers the possibility to create devices with various functionalities. Electronic logic and memory devices have already been constructed from graphene-MoS2 hybrids(7,8), but they do not make use of the photosensitivity of MoS2, which arises from its optical-range bandgap(9). Here, we demonstrate that graphene-on-MoS2 binary heterostructures display remarkable dual optoelectronic functionality, including highly sensitive photodetection and gate-tunable persistent photoconductivity. The responsivity of the hybrids was found to be nearly 1 x 10(10) A W-1 at 130 K and 5 x 10(8) A W-1 at room temperature, making them the most sensitive graphene-based photodetectors. When subjected to time-dependent photoillumination, the hybrids could also function as a rewritable optoelectronic switch or memory, where the persistent state shows almost no relaxation or decay within experimental timescales, indicating near-perfect charge retention. These effects can be quantitatively explained by gate-tunable charge exchange between the graphene and MoS2 layers, and may lead to new graphene-based optoelectronic devices that are naturally scalable for large-area applications at room temperature.

Graphene-MoS2 hybrid structures for multifunctional photoresponsive memory devices

While organic electronics is mostly dominated by light-emitting diodes, photovoltaic cells and transistors, optoelectronics properties peculiar to organic semiconductors make them interesting candidates for the development of innovative and disruptive applications also in the field of light signal detection. In fact, organic-based photoactive media combine effective light absorption in the region of the spectrum from ultraviolet to near-infrared with good photogeneration yield and low-temperature processability over large areas and on virtually every substrate, which might enable innovative optoelectronic systems to be targeted for instance in the field of imaging, optical communications or biomedical sensing. In this review, after a brief resume of photogeneration basics and of devices operation mechanisms, we offer a broad overview of recent progress in the field, focusing on photodiodes and phototransistors. As to the former device category, very interesting values for figures of merit such as photoconversion efficiency, speed and minimum detectable signal level have been attained, and even though the simultaneous optimization of all these relevant parameters is demonstrated in a limited number of papers, real applications are within reach for this technology, as it is testified by the increasing number of realizations going beyond the single-device level and tackling more complex optoelectronic systems. As to phototransistors, a more recent subject of study in the framework of organic electronics, despite a broad distribution in the reported performances, best photoresponsivities outperform amorphous silicon-based devices. This suggests that organic phototransistors have a large potential to be used in a variety of optoelectronic peculiar applications, such as a photo-sensor, opto-isolator, image sensor, optically controlled phase shifter, and opto-electronic switch and memory.

Organic light detectors: photodiodes and phototransistors.

Polymer electronic memories: Materials, devices and mechanisms

Organic photoresponse materials and devices are critically important to organic optoelectronics and energy crises. The activities of photoresponse in organic materials can be summarized in three effects, photoconductive, photovoltaic and optical memory effects. Correspondingly, devices based on the three effects can be divided into (i) photoconductive devices such as photodetectors, photoreceptors, photoswitches and phototransistors, (ii) photovoltaic devices such as organic solar cells, and (iii) optical data storage devices. It is expected that this systematic analysis of photoresponse materials and devices could be a guide for the better understanding of structure–property relationships of organic materials and provide key clues for the fabrication of high performance organic optoelectronic devices, the integration of them in circuits and the application of them in renewable green energy strategies (critical review, 452 references).

Organic photoresponse materials and devices

Functional organic field-effect transistors (OFETs) have attracted increasing attention in the past few years due to their wide variety of potential applications. Research on functional OFETs underpins future advances in organic electronics. In this review, different types of functional OFETs including organic phototransistors, organic memory FETs, organic light emitting FETs, sensors based on OFETs and other functional OFETs are introduced. In order to provide a comprehensive overview of this field, the history, current status of research, main challenges and prospects for functional OFETs are all discussed.

Functional organic field-effect transistors.

Memory operations in polymer phototransistors have been demonstrated. A fraction of light- induced drain current in the depletion mode of a polythiophene- based field- effect transistor persists after switching off the photoexcitation, and can be erased by reversing the gate voltage (V g ). Write, store, read, and erase operations can be performed by applying a combination of gate voltages and incident light over a wide temperature range.

Gate‐Voltage Control of Optically‐ Induced Charges and Memory Effects in Polymer Field‐Effect Transistors

Conductive wires of sub-micrometer width made from platinum-carbonyl clusters have been fabricated by solution-infilling of microchannels as in microinject molding in capillaries (MIMIC). The process is driven by the liquid surface tension within the micrometric channels followed by the precipitation of the solute. Orientation of supramolecular crystalline domains is imparted by the solution confinement combined with unidirectional flow. The wires exhibit ohmic conductivity with a value of 0.2 S/cm that increases, after thermal decomposition of the platinum-carbonyl cluster precursor to Pt, to 35 S/cm.

Conductive sub-micrometric wires of platinum-carbonyl clusters fabricated by soft-lithography.

We report detailed studies of the slow relaxation of the photoinduced excess charge carriers in organic metal-insulator-semiconductor field effect transistors consisting of poly(3-hexylthiophene) as the active layer. The relaxation process cannot be physically explained by processes, which lead to a simple or a stretched-exponential decay behavior. Models based on serial relaxation dynamics due to a hierarchy of systems with increasing spatial separation of the photo-generated negative and positive charges are used to explain the results. In order to explain the observed trend, the model is further modified by introducing a gate voltage dependent coulombic distribution manifested by the trapped negative charge carriers.

Nonexponential relaxation of photoinduced conductance in organic field effect transistors

We demonstrate solution-processed zinc tin oxide thin-film transistors (TFTs) with a patterned-gate configuration. High-k and solution-processed zirconium oxide (ZrO2) was employed to lower down the operating voltage. Devices with recessed and nonrecessed gate electrodes were compared to study the influence of gate surface relief on the performance of the solution-processed thin films. The TFTs with the recessed gate electrode operate at 5 V and have a threshold voltage of ~ 1 V, subthreshold slope of ~ 0.23 V/dec, saturation mobility of 2.5 cm2/V ·s, and on/off current ratio of > 106.

Solution-Processed ZTO TFTs With Recessed Gate and Low Operating Voltage

A thin film transistor (TFT) using zinc tin oxide (ZTO), deposited from solution, as the active material is demonstrated. A sol–gel method is used to deposit the film, followed by post-deposition annealing at the temperatures from 400 to 600 °C. The field-effect mobility is found to be dependent on post-annealing temperature and becomes as high as 6 cm 2  V −1  s −1 when annealed at 600 °C. The presence of interfacial traps and the surface morphology are studied using capacitance–voltage ( C – V ) measurement and atomic force microscopy (AFM), respectively, and are likely to be responsible for the annealing temperature-dependent mobility. The transistor is employed as a chemical vapor sensor, operating at room temperature. The sensor performance is optimized by controlling the post-annealing temperature. A considerable increase in drain current is observed in the film corresponding to annealing temperature range 400–500 °C upon delivering polar chemical analytes. The high chemical stability of such inorganic semiconducting oxides makes this a promising approach for developing chemical sensors.

Soumya Dutta

Papers

Gate‐Voltage Control of Optically‐ Induced Charges and Memory Effects in Polymer Field‐Effect Transistors

Conductive sub-micrometric wires of platinum-carbonyl clusters fabricated by soft-lithography.

Nonexponential relaxation of photoinduced conductance in organic field effect transistors

Solution-Processed ZTO TFTs With Recessed Gate and Low Operating Voltage

Zinc tin oxide thin film transistor sensor