Trap DOS Research Articles

In Situ Measurement of Effective Gate Bias Voltage in Ionic Liquid-Gated Organic Field-Effect Transistors: Exploring Intrinsic Performance and Trap Density of States.

Electric double layer (EDL)-mediated transistors with ionic liquid (IL) gating have garnered substantial interest due to their exceptional properties, such as high transconductance and low-voltage operation, positioning them as promising candidates for organic electronics. In this study, we present an in situ measurement of effective gate bias voltage (VGS,eff) in IL-gated organic field-effect transistors (IL-OFETs) using a modified current-voltage measurement configuration. The results reveal a significant deviation between VGS,eff and the applied gate bias (VGS,app), indicating that the EDL at the gate/IL interface screens the applied voltage. It is observed that the screening effect varies depending on the specific cation and anion present in the IL. The evaluation of VGS,eff plays a pivotal role in understanding the intrinsic behavior of IL-OFETs and addresses the challenges associated with accurate performance assessment. Inherently, IL-OFETs demonstrate high transconductance, achieving values of approximately 9 mS while operating at a low threshold voltage of around 0.55 V. Through the acquisition of VGS,eff, we have successfully addressed the limitations impeding the numerical estimation of the trap density of states (trap DOS) in IL-OFETs. Remarkably, our calculations reveal an exceptionally low density of deep traps, which serves as a crucial factor contributing to the near-ideal subthreshold swing (61-68 mV dec-1) observed in IL-OFETs. Further investigations unveil the neutral electrical nature of the IL bulk during OFET operation, confirming the hypothesis that the applied gate bias voltage in electrolyte-gated OFETs drops across the EDLs formed at the interfaces. The impedance spectroscopic (IS) analysis confirms the low contact resistance (≈1 Ω·m) of IL-OFETs calculated using the transition voltage method. The IS analysis also reveals the low-transmissive nature of the IL/organic semiconductor interface. The knowledge gained from this study holds significant implications for realizing high-performance electrolyte-gated OFETs in various applications including digital electronics, energy storage, and sensing. By unraveling the factors influencing the device performance, such as VGS,eff and trap DOS, this research contributes to the advancement of organic electronics and paves the way for future developments in the field.

Low dark current density in organic photodiodes achieved by reduced trap states of conjugated polymers bearing a fluorinated moiety

A strategy to obtain low Jd in OPDs from a material point of view is presented: fluorination of the electron donor unit in a donor–acceptor type conjugated polymer is an effective way to reduce the Jd of OPDs by suppressing the trap DOS.

Ultralow-Noise Organic Transistors Based on Polymeric Gate Dielectrics with Self-Assembled Modifiers.

In this study, ultralow 1/f noise organic thin-film transistors (OTFTs) based on parylene gate dielectrics modified with triptycene (Trip) modifiers were fabricated. The fabricated OTFTs showed the lowest 1/f noise level among those of previously reported OTFTs. It is well known that 1/f noise causes degradation of signal integrity in analog and digital circuits. However, conventional OTFTs still possess high 1/f noise levels, and the factors that strongly affect 1/f noise are still ambiguous. In this work, the effect of gate dielectric surface on 1/f noise was investigated. First, by comparing OTFTs composed of various channel lengths, we revealed that contact resistance did not affect 1/f noise. Second, we compared parylene OTFTs with and without a self-assembled Trip modifier layer in terms of 1/f noise and trap density of states (Trap DOS). The experiments revealed that a specific Trip modifier layer suppresses the shallow Trap DOS in the OTFTs, leading to a low 1/f noise. Moreover, the 1/f noise level and Trap DOS of various kinds of OTFTs were comprehensively compared, which highlighted that the 1/f noise of OTFTs strongly depends on the gate dielectric surface. Finally, detailed analysis of the gate dielectric interface led us to conclude that the disorder of gate dielectrics and the crystalline quality of semiconductor films are related to shallow Trap DOS, which correlates with 1/f noise.

Broadening of Distribution of Trap States in PbS Quantum Dot Field-Effect Transistors with High-k Dielectrics.

We perform a quantitative analysis of the trap density of states (trap DOS) in PbS quantum dot field-effect transistors (QD-FETs), which utilize several polymer gate insulators with a wide range of dielectric constants. With increasing gate dielectric constant, we observe increasing trap DOS close to the lowest unoccupied molecular orbital (LUMO) of the QDs. In addition, this increase is also consistently followed by broadening of the trap DOS. We rationalize that the increase and broadening of the spectral trap distribution originate from dipolar disorder as well as polaronic interactions, which are appearing at strong dielectric polarization. Interestingly, the increased polaron-induced traps do not show any negative effect on the charge carrier mobility in our QD devices at the highest applied gate voltage, giving the possibility to fabricate efficient low-voltage QD devices without suppressing carrier transport.

Trap-limited bimolecular recombination in poly(3-hexylthiophene): Fullerene blend films

Trap-limited bimolecular recombination in poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) blend films has been investigated by using photo-induced charge extraction by linearly increasing voltage (photo-CELIV) method. The bimolecular recombination rate is strongly dependent on the photoexcitation density, the PC61BM composition and the thermal annealing process, but it slightly depends on the thickness of the blend film. The results show that the trap-limited bimolecular recombination is strongly affected by the distribution of the density of trap state (trap DOS). The higher trap-limited bimolecular recombination rate means the trap DOS centered at lower energy which is beneficial to charge carriers transportation, due to the lower activation energy and faster release rate. On the other hand, the trap-limited bimolecular recombination rate is mainly controlled by the slower species of charge carriers in the blend film when the transport of electrons and holes are strongly unbalanced, and the recombination rate will increase when the transport of electrons and holes becomes more balanced.

Device performance and density of trap states of organic and inorganic field-effect transistors

Trap states, present in any semiconductor, have a large influence on charge transport as well as various other physical processes relevant for device performance. Therefore, quantifying the density of trap states (trap DOS) in a semiconductor is a crucial step towards understanding and improving field-effect transistors (FET), organic photovoltaic cells (OPV) and organic light-emitting diodes (OLED). We present a method to determine the free vs. total charge carrier density in FETs, and therefore the trap DOS, through full normalization of the transfer curve. We apply this method to many different materials, prepared under a wide range of processing conditions, e.g. organic single crystals, thermally evaporated and inkjet printed thin-films, leading to various degrees of order/disorder. They are compared to inorganic thin-films. A quantitative analysis of the spectral density of the trap DOS reveals the trap DOS in p- and n-type semiconductors to be very similar provided they have a similar morphology. The variation by 3 orders of magnitude is dominated by the degree of crystalline order. Further, the trap DOS in organic materials is essentially the same as in inorganic materials, again, provided they have a similar morphology. Surprisingly, ink-jet printed organic polymers have a relatively low trap DOS, which is in between the one of organic single crystals and thin-films.

Approaching the Trap-Free Limit in Organic Single-Crystal Field-Effect Transistors

Crystalline organic semiconductors, bonded by weak van der Waals forces, exhibit macroscopic properties that are very similar to those of inorganic semiconductors. While there are many open questions concerning the microscopic nature of charge transport, minimizing the density of trap states (trap DOS) is crucial to elucidate the intrinsic transport mechanism. We explore the limits of state-of-the-art organic crystals by measuring single crystalline rubrene field-effect transistors that show textbook like transfer characteristics, indicating a very low trap DOS. Particularly, the high purity of the crystals and the very clean interface to the gate dielectric are reflected in an unprecedentedly low subthreshold swing of 65 ${\rm mV / decade}$, remarkably close to the fundamental limit of $58.5 {\rm mV / decade}$. From the measured subthreshold behavior we have consistently quantified the trap DOS by two different methods, yielding an exceedingly low trap density of $D_{bulk} = 1 \times 10^{13}~{\rm cm^{-3}eV^{-1}}$ at an energy of $\sim0.62~{\rm eV}$. These numbers correspond to one trap per eV in $10^8$ rubrene molecules. The equivalent density of traps located at the interface is $D_{it} = 3 \times 10^{9}~{\rm cm^{-2}eV^{-1}}$ which puts them on par with the best crystalline ${\rm SiO_2/Si}$ field-effect transistors.

Trap density of states in n-channel organic transistors: variable temperature characteristics and band transport

We have investigated trap density of states (trap DOS) in n-channel organic field-effect transistors based on N,N ’-bis(cyclohexyl)naphthalene diimide (Cy-NDI) and dimethyldicyanoquinonediimine (DMDCNQI). A new method is proposed to extract trap DOS from the Arrhenius plot of the temperature-dependent transconductance. Double exponential trap DOS are observed, in which Cy-NDI has considerable deep states, by contrast, DMDCNQI has substantial tail states. In addition, numerical simulation of the transistor characteristics has been conducted by assuming an exponential trap distribution and the interface approximation. Temperature dependence of transfer characteristics are well reproduced only using several parameters, and the trap DOS obtained from the simulated characteristics are in good agreement with the assumed trap DOS, indicating that our analysis is self-consistent. Although the experimentally obtained Meyer-Neldel temperature is related to the trap distribution width, the simulation satisfies the Meyer-Neldel rule only very phenomenologically. The simulation also reveals that the subthreshold swing is not always a good indicator of the total trap amount, because it also largely depends on the trap distribution width. Finally, band transport is explored from the simulation having a small number of traps. A crossing point of the transfer curves and negative activation energy above a certain gate voltage are observed in the simulated characteristics, where the critical VG above which band transport is realized is determined by the sum of the trapped and free charge states below the conduction band edge.

Model for determination of mid-gap states in amorphous metal oxides from thin film transistors

The electronic density of states in metal oxide semiconductors like amorphous zinc oxide (a-ZnO) and its ternary and quaternary oxide alloys with indium, gallium, tin, or aluminum are different from amorphous silicon, or disordered materials such as pentacene, or P3HT. Many ZnO based semiconductors exhibit a steep decaying density of acceptor tail states (trap DOS) and a Fermi level (EF) close to the conduction band energy (EC). Considering thin film transistor (TFT) operation in accumulation mode, the quasi Fermi level for electrons (Eq) moves even closer to EC. Classic analytic TFT simulations use the simplification EC−EF> ‘several’kT and cannot reproduce exponential tail states with a characteristic energy smaller than 1/2 kT. We demonstrate an analytic model for tail and deep acceptor states, valid for all amorphous metal oxides and include the effect of trap assisted hopping instead of simpler percolation or mobility edge models, to account for the observed field dependent mobility.

Charge transport and density of trap states in balanced high mobility ambipolar organic thin-film transistors

We report on charge transport and density of trap states (trap DOS) in ambipolar diketopyrrolopyrrole–benzothiadiazole copolymer thin-film transistors. This semiconductor possesses high electron and hole field-effect mobilities of up to 0.6 cm 2/V-s. Temperature and gate-bias dependent field-effect mobility measurements are employed to extract the activation energies and trap DOS to understand its unique high mobility balanced ambipolar charge transport properties. The symmetry between the electron and hole transport characteristics, parameters and activation energies is remarkable. We believe that our work is the first charge transport study of an ambipolar organic/polymer based field-effect transistor with room temperature mobility higher than 0.1 cm 2/V-s in both electrons and holes.

Trap density of states in small-molecule organic semiconductors: A quantitative comparison of thin-film transistors with single crystals

We show that it is possible to reach one of the ultimate goals of organic electronics: producing organic field-effect transistors with trap densities as low as in the bulk of single crystals. We studied the spectral density of localized states in the band gap (trap DOS) of small molecule organic semiconductors as derived from electrical characteristics of organic field-effect transistors or from space-charge-limited-current measurements. This was done by comparing data from a large number of samples including thin-film transistors (TFT's), single crystal field-effect transistors (SC-FET's) and bulk samples. The compilation of all data strongly suggests that structural defects associated with grain boundaries are the main cause of "fast" hole traps in TFT's made with vacuum-evaporated pentacene. For high-performance transistors made with small molecule semiconductors such as rubrene it is essential to reduce the dipolar disorder caused by water adsorbed on the gate dielectric surface. In samples with very low trap densities, we sometimes observe a steep increase of the trap DOS very close (<0.15 eV) to the mobility edge with a characteristic slope of 10-20 meV. It is discussed to what degree band broadening due to the thermal fluctuation of the intermolecular transfer integral is reflected in this steep increase of the trap DOS. Moreover, we show that the trap DOS in TFT's with small molecule semiconductors is very similar to the trap DOS in hydrogenated amorphous silicon even though polycrystalline films of small molecules with van der Waals-type interaction on the one hand are compared with covalently bound amorphous silicon on the other hand.

Calculating the trap density of states in organic field-effect transistors from experiment: A comparison of different methods

The spectral density of localized states in the band gap of pentacene (trap DOS) was determined with a pentacene-based thin-film transistor from measurements of the temperature dependence and gate-voltage dependence of the contact-corrected field-effect conductivity. Several analytical methods to calculate the trap DOS from the measured data were used to clarify, if the different methods lead to comparable results. We also used computer simulations to further test the results from the analytical methods. Most methods predict a trap DOS close to the valence band edge that can be very well approximated by a single exponential function with a slope in the range of 50-60 meV and a trap density at the valence band edge of approx. 2x10E21 eV-1cm-3. Interestingly, the trap DOS is always slightly steeper than exponential. An important finding is that the choice of the method to calculate the trap DOS from the measured data can have a considerable effect on the final result. We identify two specific simplifying assumptions that lead to significant errors in the trap DOS. The temperature-dependence of the band mobility should generally not be neglected. Moreover, the assumption of a constant effective accumulation layer thickness leads to a significant underestimation of the slope of the trap DOS.