Photosensitive Organic Field-effect Transistors Research Articles

Overcoming the Unfavorable Effects of "Boltzmann Tyranny:" Ultra-Low Subthreshold Swing in Organic Phototransistors via One-Transistor-One-Memristor Architecture.

Organic phototransistors (OPTs), as photosensitive organic field-effect transistors (OFETs), have gained significant attention due to their pivotal roles in imaging, optical communication, and night vision. However, their performance is fundamentally limited by the Boltzmann distribution of charge carriers, which constrains the average subthreshold swing (SSave ) to a minimum of 60mV/decade at room temperature. In this study, an innovative one-transistor-one-memristor (1T1R) architecture is proposed to overcome the Boltzmann limit in conventional OFETs. By replacing the source electrode in an OFET with a memristor, the 1T1R device exploits the memristor's sharp resistance state transitions to achieve an ultra-low SSave of 18mV/decade. Consequently, the 1T1R devices demonstrate remarkable sensitivity to photo illumination, with a high specific detectivity of 3.9 × 109 cm W-1 Hz1/2 , outperforming conventional OPTs (4.9 × 104 cm W-1 Hz1/2 ) by more than four orders of magnitude. The 1T1R architecture presents a potentially universal solution for overcoming the detrimental effects of "Boltzmann tyranny," setting the stage for the development of ultra-low SSave devices in various optoelectronic applications.

Active layer and electrode thickness dependent performances of bottom contact photosensitive organic field-effect transistors

Abstract Optimizing the thicknesses of the organic active layer and source/drain electrode layer of photosensitive organic field-effect transistors (PhOFET) for high device performance is of great importance. With the increase of organic active layer thicknesses, both the photocurrent and photoresponsivity display the same variation trend of increasing sharply at first and then tending to saturate at large values. The Au source/drain electrode-layer thickness shows great impact on the performance of PhOFET. With the increase of Au source/drain electrode-layer thicknesses, both the dark current and photoresponsivity increase sharply at first and then tend to saturate at large thicknesses. For pentacene PhOFETs with bottom-gate bottom-contact geometry, the Au source/drain electrode-layer thickness should be larger than 150 nm for optimized device performance. This research may supply a theoretical basis for the development of PhOFET.

Channel-length-dependent performance of photosensitive organic field-effect transistors

Pentacene is a small molecule organic semiconductor and has high absorption in the UV and visible region. In this work, we report the channel-length-dependent performance of bottom-gate top-contact photosensitive organic field-effect transistors based on pentacene with different channel lengths ranging from 25 to 150μm. The results show that, for a given drain voltage, the photoresponsivity, external quantum efficiency, and specific detectivity decrease with increasing channel length. For a given channel length and gate voltage, the photoresponsivity, external quantum efficiency, and specific detectivity increase with drain voltages, and then tend to saturate at high drain voltages.

Effect of concentration of DH6T on the performance of photoconductor fabricated using blends of P3HT and DH6T

The influence of small molecule (SM) α, ω-di-hexyl-sexithiophene (DH6T) concentration, in the blends of conjugated polymer (CP) poly (3-hexylthiophene) (P3HT) and DH6T, was investigated in terms of barrier potential reduction and improved photoresponse of the fabricated photoconductor using these blends. Barrier potential in Au/P3HT:DH6T/Au device structure, occurring at the interface of Au (top)/P3HT:DH6Twas estimated by Fowler Nordheim (FN) tunneling model-based analysis of I-V characteristics. The barrier potential of the fabricated device was observed to decrease upon addition of a small quantity of DH6T in comparison to the pristine polymer-based device. This reduction in barrier was attributed to the improved ordering and morphology of the polymer chains upon blending it with an SM. The variation in the ordering of the polymer chains was further confirmed with Photoluminescence spectroscopy, Absorption spectroscopy, and XRD data. Subsequently, it was also observed that only up to a definite SM concentration (25% in this study) ordering of polymer chains improved causing a reduction in barrier potential and subsequent improvement in the photoresponse of the fabricated devices. Finally, it was observed that the optimized blending of CP and SM could to be useful in reducing the effect of penetration of Au inside the CP matrix in the top contact configuration thereby resulting in the reduction of a barrier for carrier injection which is generally lower in the bottom contact configurations. These studies are critical from the point of view of the development of photoconductors and photosensitive top contact organic field effect transistors (OFETs).

A comprehensive investigation of organic active layer structures toward high performance near-infrared phototransistors

Tin phthalocyanine (SnPc) is a kind of organic semiconductor material with excellent absorbing characteristic in near-infrared (NIR) region but rarely applied in photosensitive organic field-effect transistors (PhOFETs). Here, a series of SnPc based PhOFETs with different active layer structures were fabricated and characterized for the purpose of making a systematic investigation of organic active layer structure toward NIR phototransistors. Among the sample devices with different active layer structures, the tri-layer planar heterojunction structure of Device C exhibits high photoresponsivity of 2686.1 mA/W and the photosensitivity is up to 104. The tri-layer hybrid planar-bulk heterojunction configuration of Device D also possesses excellent overall performance, whose photoresponsivity reaches 1894.7 mA/W and photosensitivity is up to 5925. Meanwhile, the detectivity remains at the level of 1012 Jones. After a comprehensive investigation of different organic active layer structures, some useful design principles for NIR phototransistors are obtained. While the mobility of SnPc is relatively low, the photosensitive performance of the devices can be improved to various extent by applying planar and hybrid planar-bulk heterojunction configuration. Given the similar features of some organic NIR materials, we believe that the device design concepts we used for SnPc can be helpful to guiding and designing higher performance NIR phototransistors based on other organic materials.

High-Performance ZnPc Thin Film-Based Photosensitive Organic Field-Effect Transistors: Influence of Multilayer Dielectric Systems and Thin Film Growth Structure.

The key impact and significance of a multilayer polymer-based dielectric system on the remarkable photoresponse properties of zinc phthalocyanine (ZnPc)-based photosensitive organic field-effect transistors (PS-OFETs) have been systematically analyzed at various incident optical powers. A combination of inorganic aluminum oxide (Al2O3) and organic nonpolar poly(methyl methacrylate) (PMMA) is used as the bilayer dielectric configuration, whereas in the trilayer dielectric system, a bilayer polymer dielectric, consisting of PMMA, as the low-k dielectric polymer, on top of poly(vinyl alcohol) (PVA), the high-k polar dielectric, has been fabricated along with Al2O3 as the third layer. Before fabricating the OFETs, a systematic optimization of the nature of growth of the ZnPc molecules, deposited on PMMA-coated glass substrates at different substrate temperatures (Ts) was performed and examined by atomic-force microscopy, field-emission scanning electron microscopy, X-ray diffraction, and Raman analysis. At 90 °C, the fabricated PS-OFETs with the Al2O3/PVA/PMMA trilayer dielectric configuration showed the best p-channel behavior, with an enhanced and remarkable photoresponsivity of R ∼ 9689.39 A W–1 compared to that of the Al2O3/PMMA bilayer dielectric system (R ∼ 2679.40 A W–1) due to the polarization of the dipoles inside the polar PVA dielectric, which increases the charge transport through the channel. The charge carrier mobility of the device also improved by one order (μh ∼ 1.3 × 10–2 cm2 V–1 s–1) compared to that of the bilayer dielectric configuration (μh ∼ 3.5 × 10–3 cm2 V–1 s–1). The observed specific detectivity (D*) and NEP values of the bilayer dielectric system were 6.01 × 1013 Jones and 2.655 × 10–17 W Hz–1/2, whereas for the trilayer dielectric system, the observed D* and NEP values were 5.13 × 1014 Jones and 1.043 × 10–17 W Hz–1/2, respectively. Additionally, the operating voltage of each of the fabricated devices was also very low (−10 V) due to the influence of the inorganic high-k Al2O3 dielectric layer. The electrical stability of all of the fabricated devices was also investigated by bias stress analysis under both light and dark conditions in vacuum. To the best of our knowledge, the photoresponsivity (R) reported here with an Al2O3/PVA/PMMA trilayer dielectric configuration is the highest reported value for thin film-based PS-OFETs, at a remarkably low operating voltage of −10 V, on low-cost glass substrates without indium tin oxide or/and Si/SiO2.

Towards high performance broad spectral response fullerene based photosensitive organic field-effect transistors with tricomponent bulk heterojunctions

Recently, broad spectral response phototransistors have drawn substantial attention due to their applications in the field of industry and science. However, it is difficult to synthesize appropriate photosensitive materials, which greatly limits their development. Integrating multicomponent bulk heterojunctions with high mobility materials to form hybrid planar-bulk heterojunction is a very effective approach to overcome these shortcomings. Herein, we reported broad spectral response photosensitive organic field-effect transistors with tricomponent bulk heterojunctions (Tri-BHJ) and high mobility channel transport layer, and they were sensitive over a bandwidth from ultraviolet–visible to near infrared. The Tri-BHJ composed of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), chloroaluminum phthalocyanine (AlClPc) and lead phthalocyanine (PbPc) was used as the photosensitive layer, fullerene (C60) as the channel layer, and SiO2 as the gate dielectric. By replacing SiO2 with polyvinyl alcohol (PVA), the device performance was improved significantly, and the photosensitivity, photoresponsivity, external quantum efficiency and specific detectivity of the device built on PVA dielectric reached up to 105, 108.44 A/W, 25325% and ∼2.7 × 1012 Jones, respectively, which are comparable or even superior to those of commercially silicon and indium gallium arsenide photodetectors or other reported organic photodetectors. This work indicated new directions for the future development of high performance broad spectral response phototransistors.

Broad spectral response photosensitive organic field-effect transistors realized by the hybrid planar-bulk heterojunction composed of three molecules with complementary optical absorption

Photodetectors have a growing number of diverse applications in both military and civilian use, and improving their response bandwidth is of great importance. Broad spectral response photosensitive organic field-effect transistors (PhOFETs) are realized by adopting hybrid planar-bulk heterojunction (HPBHJ) as the active structure that containing three molecules with complementary optical absorption. PhOFETs based on the HPBHJ composed of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA), chloroaluminum phthalocyanine (AlClPc) and C60, and SiO2 as the gate dielectric exhibit photo response in the broadband wavelength range of 300–850 nm. At an incident optical intensity of 0.07 mW/cm2, the mean photoresponsivity and uniformity factor were 0.40 A/W and 0.79, respectively. Furthermore, by replacing SiO2 gate dielectric with polyvinyl alcohol (PVA), the device performance could be further improved, the mean photoresponsivity was increased to 2.44 A/W. The physical origin of the performance improvement is discussed.

Airstable near-infrared sensitive organic field-effect transistors utilizing erbium phthalocyanine as photosensitive layer

Near-infrared photosensitive organic field-effect transistors (PhOFETs) based on pentacene/erbium phthalocyanine (ErPc2) and copper phthalocyanine (CuPc)/ErPc2 heterojunction, as well as ErPc2 single-layer PhOFETs, were fabricated and characterized. The results showed that the heterojunction PhOFETs exhibit higher mobility, maximum photoresponsivity (Rmax) and maximum photo/dark current ratio (Pmax), comparing with the ErPc2 single-layer ones. Such a better performance of the heterojunction devices can be attributed to the higher mobility channel layer. And the pentacene/ErPc2 PhOFETs demonstrate an Rmax of 6498mA/W, Pmax of 2.6×103, hole mobility of 1.5×10−2cm2V−1s−1 and excellent air stability, its Rmax is 186 times larger than that of the ErPc2 single-layer PhOFETs. Therefore, ErPc2 can be used as an excellent NIR photoresponsive layer of heterojunction PhOFETs.

Effect of intrinsic polymer properties on the photo sensitive organic field-effect transistors (Photo-OFETs)

In this work, we have demonstrated how the intrinsic properties of a conjugated polymer can influence the electro-optical characteristics of photo sensitive organic field - effect transistors (Photo-OFETs). Photo-OFETs fabricated with three batches of poly[2-methoxy,5-(3′,7′-dimethyl-octyloxy)]-p-phenylene vinylene (MDMO-PPV) were investigated in the scope of our work. Photo-OFETs were fabricated with the polymers, than electrically and electro-optically characterized. It was observed that the channel current and the field-effect mobility increase with increasing polymer molecular weight. Interestingly, the electro-optical characteristics and photo switching properties of the transistors were found to depend on the polydispersity (PDI) of the polymer as well. These results are explained in terms of influences of chain packing, ordering and trap density on the FET switching properties and transistor parameters.

Photosensitive organic field effect transistors: the influence of ZnPc morphology and bilayer dielectrics for achieving a low operating voltage and low bias stress effect.

Photosensitive-organic field effect transistors (PS-OFETs) based on a morphology controlled zinc phthalocyanine (ZnPc) layer, with an inorganic-organic bilayer gate dielectric system, fabricated on a glass substrate showed remarkable efficiency as light sensors at various incident optical powers. The indium tin oxide (ITO) and Si/SiO2 free low-cost OFET devices show low bias stress and a reduced operating voltage with aluminum oxide and poly(methyl methacrylate) (Al2O3/PMMA) as bilayer gate dielectrics and copper (Cu) as a top contact. They exhibit excellent p-channel behavior with a remarkable photo-responsivity of 2679.40 A W-1 and a photo-ON/OFF current ratio of 933.56 with a very low operating voltage (0 to -8 V), which have not been observed previously. The bias stress effect of the device was investigated under both light and dark conditions in a vacuum. It was observed that the effect of the stress is extremely small in the presence of light (a decay of IDS of ∼ 20% after 30 min) compared to the dark, with a characteristic carrier relaxation time τ' ∼ 104 s. This device with high electrical stability under ambient conditions and a low threshold voltage under constant electrical bias stress is expected to have potential applications in optoelectronic devices and energy efficient sensors.

Enhanced performance of PbPc photosensitive organic field effect transistors by inserting different-thickness pentacene inducing layers

Lead phthalocyanine (PbPc) based photosensitive organic field effect transistors (PhOFETs) with different-thickness pentacene inducing layers (INLs) inserted between SiO2 and PbPc layer were fabricated and characterized. The photoelectric measurements demonstrate that the device with 2-nm-thick pentacene INL exhibits the largest photoresponsivity of 505.75mA/W and maximum photo/dark current ratio of 405.35 in all devices. For this, we give an overall explanation that different-thickness INLs display different continuity and crystallinity and thus produce strong or weak template inducing. Especially, when the INL thickness (δ) is 2nm a quasi-continuous and highly crystalline approximate-monolayer INL forms on SiO2 surface, which may play a strong role of template inducing, thus causing its upper PbPc film to demonstrate the strongest triclinic (300) line and the strongest NIR absorption in series PbPc films. When δ=1nm, pentacene does not form a continuous film. And when δ=5 or 10nm, a continuous multilayer INL with a declined crystallinity due to possible lattice mismatch forms on SiO2 surface and gives a weakened template inducing. Thereby, it can be recognized that inserting a pentacene INL can markedly enhance the performance of single layer PbPc PhOFET and the optimum INL thickness is proved ∼2nm in present conditions.

Improved Performance of Photosensitive Field-Effect Transistors Based on Palladium Phthalocyanine by Utilizing Al as Source and Drain Electrodes

In conventional photosensitive organic field-effect transistors (FETs) (OFETs) (photo-OFETs) based on p-type organic semiconductors, high work function metals such as Au are generally used as source/drain electrodes, whose photosensitivity is generally low. We report on the performance improvements of photo-OFETs based on palladium phthalocyanine (PdPc) by using Al as source and drain electrodes. It is concluded that the dark currents of the photo-OFET based on PdPc with Al as source and drain electrodes (denoted as Al-PdPc device) are about only one-thousandth of that of the photo-OFET based on PdPc with Au as source and drain electrodes (denoted as Au-PdPc device). This tremendous decrease of dark current results in about a three-order-of-magnitude increase for photosensitivity at the gate and drain voltages of -50 V. The enormous decrease of dark current is ascribed to the Schottky contacts between the Al source/drain electrodes and PdPc. In addition, the Al-PdPc devices show also larger photoresponsivity compared with the Au-PdPc devices.

High performance near infrared photosensitive organic field-effect transistors realized by an organic hybrid planar-bulk heterojunction

Compared with organic photodiodes, photoresponsive organic field-effect transistors (photOFETs) exhibit higher sensitivity and lower noise. The performance of photOFETs based on conventional single layer structure operating in the near infrared (NIR) is generally poor due to the low carrier mobility of the active channel materials. We demonstrate a high performance photOFETs operating in NIR region with a structure of hybrid planar-bulk heterojunction (HPBHJ). PhotOFETs with the structures of single layer [lead phthalocyanine (PbPc) or copper phthalocyanine (CuPc)], single planar heterojunction (PHJ) of CuPc/PbPc, double PHJ of CuPc/PbPc/3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and HPBHJ of CuPc/PbPc:PTCDA were fabricated and characterized. It is concluded that the photOFET with HPBHJ structure showed superior performance compared to that with other structures, and for NIR light of wavelength 808nm, the photOFET with HPBHJ structure exhibited a large photoresponsivity of 322mA/W, a high external quantum efficiency of around 50%, and a maximal photosensitivity of 9.4×102. The high performance of HPBHJ photOFET is attributed to its high exciton dissociation efficiency and excellent hole transport ability. For 50-nm thick CuPc layer, the optimal thickness of the PbPc:PTCDA layer is found to be around 30nm.

Control over Multifunctionality in Optoelectronic Device Based on Organic Phototransistor

Highly stable, reproducible, photosensitive organic field-effect transistors based on an n-type organic material, copper hexadecafluorophthalocyanine, and two different polymeric gate dielectrics has been reported and their performances have been compared by evaluating the surface/interface properties. The devices produced a maximum photocurrent gain (I(light)/I(dark)) of 79 at V(G) = 7 V and showed the potentiality as multifunctional optoelectronic switching applications depending upon the external pulses. The switching time of the transistor upon irradiation of light pulse, i.e., the photoswitching time of the device, was measured to be approximately 10 ms. On the basis of optical or combination of optical and electrical pulses, the electronic/optoelectronic properties of the device can be tuned efficiently. The multifunctions achieved by the single device can ensure very promising material for high density RAM and other optoelectronic applications. Furthermore, as the device geometry in the present work is not limited to rigid substrate only, it will lead to the development of flexible organic optoelectronic switch compatible with plastic substrates.