the amorphous silicon benchmark of 1 cm 2 Vs −1 , application of organic FETs in displays and sensors have now become a reality. Organic FETs differ from metal oxide semiconductor FET (MOSFET) in several ways; most organic FETs operate in the accumulation region compared to the inversion operating region of MOSFETs. The metal-semiconductor and the semiconductor-dielectric interfaces play a vital role in charge transport properties. In particular, the dielectric interface is notorious for charge trapping. As a result, achieving intrinsic transport in the organic semiconductor layer in FET architectures is very challenging. Using the same organic semiconductor film (either evaporated or solution processed) but different dielectric layers may yield order of magnitude differences in FET carrier mobilities. On the other hand, ultrapure organic single crystals such as rubrene, grown from vapor phase, have shown intrinsic FET mobilities higher than 20 cm 2 Vs −1 . [2] Since the charge accumulation is directly proportional to the dielectric capacitance (C), where; κ being the dielectric constant, ε 0 the permittivity of free space, A the area of the capacitor, and d the thickness of the dielectric, a high value of the dielectric capacitance is required for lowering the operating voltage of FETs. Low-operating voltage FETs, therefore, demand high κ dielectrics, which are more difficult to achieve with polymeric materials compared to inorganic dielectric materials due to their inherently low κ values. Facile methods of preparing polymer dielectrics by appropriate choice of solvents result in thin (well below 100 nm) and pinhole-free films for low-operating voltage FETs. [3,4] Polymer ferroelectrics with higher values of κ compared to non-ferroelectric polymers allow an alternate route toward boosting the capacitance values in FETs. Poly(vinylidene fluoride) (PVDF) ferroelectric polymer and its copolymer such as PVDF trifluorethylene (PVDF-TrFE) with κ > 8 at room temperature have been extensively used in memory and pressure sensing applications. [5][6][7][8][9] Naturally, such dielectrics also provide a route toward low-operating voltage FETs. The vast range of work has utilized PVDF and its copolymers as a gate dielectric in organic FETs. [10][11][12][13] Design of PVDF with carbon quantum dots has opened applications in nanogenerators where the mechanical energy may be efficiently converted to electricity. [14] More recently, PVDF copolymers have been used with charge-modulated organic FETs for multimodal Polymer ferroelectrics are playing an increasingly active role in flexible memory application and wearable electronics. The relaxor ferroelectric dielectric, poly(vinylidene fluoride trifluorethylene (PVDF-TrFE), although vastly used in organic field-effect transistors (FETs), has issues with gate leakage current especially when the film thickness is below 500 nm. This work demonstrates a novel method of selective poling the dielectric layer. By using solutionprocessed 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-p...

The use of polymer ferroelectric dielectrics in organic field-effect transistors (FETs) for nonvolatile memory application was demonstrated more than 15 years ago. The ferroelectric dielectric polyvinylidene fluoride (PVDF) and its copolymers are most widely used for such applications. In addition to memory applications, polymer ferroelectrics as a dielectric layer in organic FETs yield insights into interfacial transport properties. Advantages of polymer ferroelectric dielectrics are their high dielectric constant compared to other polymer dielectrics and their tunable dielectric constant with temperature. Further, the polarization strength may also be tuned by externally poling the ferroelectric dielectric layer. Thus, PVDF and its copolymers provide a unique testbed not just for investigating polarization induced transport in organic FETs, but also enhancing device performance. This article discusses recent developments of PVDF-based ferroelectric organic FETs and capacitors with a focus on tuning transport properties. It is shown that FET carrier mobilities exhibit a weak temperature dependence as long as the dielectric is in the ferroelectric phase, which is attributed to a polarization fluctuation driven process. The low carrier mobilities in PVDF-based FETs can be enhanced by tuning the poling condition of the dielectric. In particular, by using solution-processed small molecule semiconductors and other donor–acceptor copolymers, it is shown that selective poling of the PVDF-based dielectric layer dramatically improves FET properties. Finally, the prospects of further improvement in organic ferroelectric FETs and their challenges are provided.

Hybrid organic-inorganic photodiode interfaces have gained significant interest due to their unique physical properties such as mechanical flexibility and high photosensitivity. Two diketopyrrolopyrrole (DPP)-based donor-acceptor copolymers with different backbone conformations are characterized in an inverted non-fullerene photodiode architecture using ZnO nano-patterned films as the electron transport layer. The DPP copolymer with a thienothiophene unit (PBDT-TTDPP) is more planar and rigid compared to the DPP system with a thiophene unit connecting the donor and acceptor moieties within the monomer (PBDT-TDPP). The hybrid interfaces were optimized by using poly(3-hexylthiophene) (P3HT) as the p-type layer for monitoring the critical thickness and morphology of the ZnO layer. The maximum photoresponsivity from a P3HT:ZnO photodiode was found to be 56 mA/W. The photoresponsivity of PBDT-TTDPP:ZnO photodiodes were found to be more than two orders of magnitude higher than PBDT-TDPP:ZnO photodiodes, which is attributed to an enhanced transport of carriers due to the planar backbone conformation of the PBDT-TTDPP copolymer. Capacitance-voltage measurements from hybrid Schottky barrier interfaces further shed light into the nature of photocarriers and device parameters. Firstprinciples time-dependent density-functional theoretical calculations yield a higher absorptivity for the PBDT-TTDPP dimer compared to PBDT-TDPP.

Copolymers based on diketopyrrolopyrrole (DPP) cores have attracted a lot of attention because of their high p-type as well as n-type carrier mobilities in organic field-effect transistors (FETs) and high power conversion efficiencies in solar cell structures. We report the structural and charge transport properties of n-dialkyl side-chain-substituted thiophene DPP end-capped with a phenyl group (Ph-TDPP-Ph) monomer in FETs which were fabricated by vacuum deposition and solvent coating. Grazing-incidence X-ray diffraction (GIXRD) from bottom-gate, bottom-contact FET architectures was measured with and without biasing. Ph-TDPP-Ph reveals a polymorphic structure with π-conjugated stacking direction oriented in-plane. The unit cell comprises either one monomer with a = 20.89 Å, b = 13.02 Å, c = 5.85 Å, α = 101.4°, β = 90.6°, and γ = 94.7° for one phase (TR1) or two monomers with a = 24.92 Å, b = 25.59 Å, c = 5.42 Å, α = 80.3°, β = 83.5°, and γ = 111.8° for the second phase (TR2). The TR2 phase thus signals a shift from a coplanar to herringbone orientation of the molecules. The device performance is sensitive to the ratio of the two triclinic phases found in the film. Some of the best FET performances with p-type carrier mobilities of 0.1 cm/V s and an on/off ratio of 10 are for films that comprise mainly the TR1 phase. GIXRD from in operando FETs demonstrates the crystalline stability of Ph-TDPP-Ph.

Organic–inorganic interfaces in photodiodes have recently gathered significant interest due to the realization of intrinsic p–n junctions and unique physical properties. Nanopatterned sol–gel ZnO films provide an alternate path for fullerene-free organic photodetectors. However, naturally occurring oxygen vacancies in ZnO often act as trap sites and can degrade device performance if left unchecked. Here, we focus on the role of UV–ozone treatment for filling oxygen vacancies in sol–gel processed ZnO for improving the hybrid interface with thienothiophene linked diketopyrrolopyrrole (DPP) films. The ZnO films are characterized by X-ray diffraction, ultraviolet photoelectron spectroscopy (UPS), cross-sectional electron microscope images, and electron energy loss spectroscopy (EELS). UV–ozone treatment shows no change in the crystal structure, but UPS indicates that the treated films are more resistive and have a higher oxygen concentration at the surface. The EELS spectra show gradual passivation of oxygen vacancies within the bulk of the ZnO films. Fullerene-free photodetectors fabricated from ZnO:DPP interfaces show dark currents reduced by half and photoresponsivities nearly doubled, on average, when the ZnO surface is UV–ozone treated compared to nontreated ZnO films, indicating this simple technique to be excellent for improving photodiode performance when ZnO is used as an electron transport layer.