Nano-Alignment in Semiconducting Polymer Films: A Path to Achieve High Current Density and Brightness in Organic Light Emitting Transistors

Organic light emitting field effect transistors (LEFETs) integrate light emission of a diode with logic functions of a transistor into a single device architecture. This integration has the potential to provide simplified displays at low costs and access to injection lasing. However, the charge carrier mobility in LEFETs is a limiting factor in realizing high current densities along with a trade-off between brightness and efficiency. Herein, we present a technique controlling the nanoscale morphology of semiconducting polymers using nanoscale grooved substrates and dip-coating deposition to achieve high current density. We then applied this approach to heterostructure LEFETs and demonstrated brightness exceeding 29000 cd m–2 at an EQE of 0.4% for a yellow emitter and 9600 cd m–2 at an EQE of 0.7% for a blue emitter. These results represent a significant advancement in organic optoelectronics and are an important milestone toward the realization of new applications in displays and electrically pumped lasing

Efficient and Stable Solution-Processed Organic Light Emitting Transistors using a High-k Dielectric

We report the development of highly efficient and stable solution-processed organic light emitting transistors (OLETs) that combine a polymer heterostructure with the transparent high-k dielectric poly(vinylidenefluoride0.62-trifluoroethylene0.31-chlorotrifluoroethylene0.7) (P(VDF-TrFE-CTFE)). The polymer heterostructure comprises of the poly[4-(4,4- dihexadecyl-4H-cyclopenta[1,2-b:5,4-b’]dithiophen-2-yl)-alt-[1,2,5]thiadiazolo[3,4- c]pyridine] (PCDTPT) and Super Yellow as charge transporting and light emitting layers, respectively. Device characterization shows that the use of P(VDF-TrFE-CTFE) leads to larger channel currents (≈2 mA) and lower operating voltages (-35 V) than for previously reported polymer based OLETs. Furthermore, the combined transparency of the dielectric and gate electrode, results in efficient bottom emission with external quantum efficiency of ≈0.88 % at a luminance L ≥ 2000 cd m−2. Importantly, the resulting OLETs exhibit excellent shelf life and operational stability. The present work represents a significant step forward in the pursuit of all-solution-processed OLET technology for lighting and display applications

Nanostructured Channel for Improving Emission Efficiency of Hybrid Light-Emitting Field-Effect Transistors

We report on the mechanism of enhancing the luminance and external quantum efficiency (EQE) by developing nanostructured channels in hybrid (organic/inorganic) light-emitting transistors (HLETs) that combine a solution-processed oxide and a polymer heterostructure. The heterostructure comprised two parts: (i) the zinc tin oxide/zinc oxide (ZTO/ZnO), with and without ZnO nanowires (NWs) grown on the top of the ZTO/ZnO stack, as the charge transport layer and (ii) a polymer Super Yellow (SY, also known as PDY-132) layer as the light-emitting layer. Device characterization shows that using NWs significantly improves luminance and EQE (≈1.1% @ 5000 cd m–2) compared to previously reported similar HLET devices that show EQE < 1%. The size and shape of the NWs were controlled through solution concentration and growth time, which also render NWs to have higher crystallinity. Notably, the size of the NWs was found to provide higher escape efficiency for emitted photons while offering lower contact resistance for charge injection, which resulted in the improved optical performance of HLETs. These results represent a significant step forward in enabling efficient and all-solution-processed HLET technology for lighting and display applications

Large Area Emission in p-Type Polymer-Based Light-Emitting Field-Effect Transistors by Incorporating Charge Injection Interlayers

Organic light-emitting field-effect transistors (LEFETs) provide the possibility of simplifying the display pixilation design as they integrate the drive-transistor and the light emission in a single architecture. However, in p-type LEFETs, simultaneously achieving higher external quantum efficiency (EQE) at higher brightness, larger and stable emission area, and high switching speed are the limiting factors for to realise their applications. Herein, we present a p-type polymer heterostructure-based LEFET architecture with electron and hole injection interlayers to improve the charge injection into the light-emitting layer, which leads to better recombination. This device structure provides access to hole mobility of ~2.1 cm2 V−1 s−1 and EQE of 1.6% at a luminance of 2600 cd m−2. Most importantly, we observed a large area emission under the entire drain electrode, which was spatially stable (emission area is not dependent on the gate voltage and current density). These results show an important advancement in polymer-based LEFET technology toward realizing new digital display applications

Large Area Emission in p-Type Polymer-Based Light-Emitting Field-Effect Transistors by Incorporating Charge Injection Interlayers

Organic light-emitting field-effect transistors (LEFETs) provide the possibility of simplifying the display pixilation design as they integrate the drive-transistor and the light emission in a single architecture. However, in p-type LEFETs, simultaneously achieving higher external quantum efficiency (EQE) at higher brightness, larger and stable emission area, and high switching speed are the limiting factors for to realise their applications. Herein, we present a p-type polymer heterostructure-based LEFET architecture with electron and hole injection interlayers to improve the charge injection into the light-emitting layer, which leads to better recombination. This device structure provides access to hole mobility of ~2.1 cm2 V−1 s−1 and EQE of 1.6% at a luminance of 2600 cd m−2. Most importantly, we observed a large area emission under the entire drain electrode, which was spatially stable (emission area is not dependent on the gate voltage and current density). These results show an important advancement in polymer-based LEFET technology toward realizing new digital display applications

Effect of Gate Conductance on Hygroscopic Insulator Organic Field-Effect Transistors

Hygroscopic insulator field‐effect transistors (HIFETs) are a class of low‐voltage‐operation organic transistors that have been successfully demonstrated for biosensing applications through modification of the gate electrode. However, modification of the gate electrode often leads to nonideal transistor characteristics due to changes in its intrinsic electrical properties. This work investigates the effect of gate conductance in HIFETs using poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) as a model gate electrode. It is revealed that a reduction in gate conductance results in a reduction in the effective gate voltage and plays an important role in defining HIFET characteristics. Key figures of merit, including ON/OFF ratio, threshold voltage, transconductance, and saturation mobility increase with increasing gate conductance and reach a plateau once sufficient gate conductance is attained. This effect is attributed to a decrease in the effective gate voltage along the gate electrode arising from its resistivity when a gate leakage current is present. These results are widely applicable and serve as design rules for HIFET device optimization

Organic Light‐Emitting Transistors: Advances and Perspectives

The rapid development of charge transporting and light‐emitting organic materials in the last decades has advanced device performance, highlighting the high potential of light‐emitting transistors (LETs). Demonstrated for the first time over 15 years ago, LETs have transformed from an optoelectronic curiosity to a serious competitor in the race for cheaper and more efficient displays, also showing promise for injection lasers. Thus, what is an LET, how does it work, and what are the current challenges for its integration into mainstream technologies? Herein, some light is shed on these questions. This work also provides the fundamental working principle of LETs, materials that have been used, and device physics and architectures involved in the progression of LET technology. The state‐of‐the‐art development of LETs is also explored as prospect avenues for the future of research and applications in this area

On the Operational, shelf life and degradation mechanism in polymer field effect transistors

Organic Field Effect Transistors (OFETs) have shown great potential for future electronic technologies due to their low-cost solution processing, mechanical flexibility and potential applications for large area displays. One of the big obstacles in the realization of the practical applications is the inherent poor ambient stability of the OFETs. Here we report on the aging dependent degradation mechanism in the Poly[2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl)thieno [3,2-b]thiophene)] (DPPDTT) based OFETs in the ambient conditions. These polymer OFETs showed the charge carrier mobility, threshold voltage and current on/off ratios in the range of 0.2 cm2V−1s−1, -15 V and 106 respectively. The device parameters showed variations in their values initially and then became stable with aging after ∼20% initial degradations in the ambient. We have correlated the degradation in the OFET performance parameters with the degradation in the polymer channel layer that is confirmed with a time dependent FTIR spectra. Our findings are thus important to understand and achieve stability in OFET devices by aging them

Mobility Evaluation of [1]Benzothieno[3,2‑b][1]benzothiophene Derivatives: Limitation and Impact on Charge Transport

Amongst contemporary semiconductors many of the best performing materials are based on [1]benzothieno[3,2-b][1]benzothiophene (BTBT). Alkylated derivatives of these small molecules not only provide high hole mobilities but can also be easily processed by thermal vacuum or solution deposition methods. Over the last decade numerous publications have been investigating molecular structures and charge transport properties to elucidate what makes these molecules so special. However, the race towards ever higher mobilities resulted in significantly deviating values, which exacerbates linking molecular structure to electronic properties. Moreover, a recently arisen debate on overestimation of organic field-effect transistor mobilities calls for a revaluation of these numbers. We synthesised and characterised four BTBT derivatives with either one or two alkyl chains (themselves consisting of either eight or ten carbon atoms), and investigated their spectroscopic, structural and electrical properties. By employing two probes, gated 4-point probe and gated van der Pauw measurements we compare field effect mobility values at room and low temperatures and discuss their feasibility and viability. We attribute mobility changes to different angles between molecule planes and core-to-core double layer stacking of asymmetric BTBT derivatives and show higher mobilities in the presence of more and longer alkyl chains. A so called “zipper effect” brings BTBT cores in closer proximity promoting stronger intermolecular orbital coupling and hence higher charge transport

Low-Voltage Solution-Processed Hybrid Light-Emitting Transistors

We report the development of low operating voltages in inorganic–organic hybrid light-emitting transistors (HLETs) based on a solution-processed ZrOx gate dielectric and a hybrid multilayer channel consisting of the heterojunction In2O3/ZnO and the organic polymer “Super Yellow” acting as n- and p-channel/emissive layers, respectively. Resulting HLETs operate at the lowest voltages reported to-date (<10 V) and combine high electron mobility (22 cm2/(V s)) with appreciable current on/off ratios (≈103) and an external quantum efficiency of 2 × 10–2% at 700 cd/m2. The charge injection, transport, and recombination mechanisms within this HLET architecture are discussed, and prospects for further performance enhancement are considered