Seongwon Yoon

Low dark current inverted organic photodiodes using anionic polyelectrolyte as a cathode interlayer

We demonstrate the effect of anionic polyelectrolyte as a cathode interlayer to enhance charge selectivity of the electrode/semiconductor junction of organic photodiodes. Poly(styrenesulfonate) (PSS) was used as a cathode interlayer to tune the energy level of an ITO/ZnO electrode, so that hole injection can be minimized while electron extraction can be maximized. Optimized photodiodes with a PSS interlayer showed lower and flatter dark current density curves compared to the reference devices, which implies that tunneling currents at the electrode/active layer interface were dramatically suppressed. Moreover, PSS as an interlayer enabled lower charge recombination yield, as confirmed by the ideality factor and linear dynamic range analysis. As a result, we could realize the near-ideal organic photodiodes with a high performance of specific detectivity up to 3.3 × 1012 Jones at −5 V.

Organic–inorganic hybrid inverted photodiode with planar heterojunction for achieving low dark current and high detectivity

In this study, the strategy of using an organic-inorganic hybrid planar heterojunction consisting of polymeric semiconductors and inorganic nanocrystals is introduced to realize a high-performance hybrid photodiode (HPD) with low dark current and high detectivity. To prevent undesired charge injection under the reverse bias condition, which is the major dark current source of the photodiode, a well-defined planar heterojunction is strategically constructed via smart solution process techniques. The optimized HPD renders a low dark current of ∼10(-5) mA cm(-2) at -5 V and ∼10(-6) mA cm(-2) at -1 V, as well as a high detectivity ∼10(12) Jones across the entire visible wavelength range. Furthermore, excellent photocurrent stability is demonstrated under continuous light exposure. We believe that the solution-processed planar heterojunction with inverted structure can be an attractive alternative diode structure for fabricating high-performance HPDs, which usually suffer from high dark current issues.

Effects of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane doping on diketopyrrolopyrrole-based, low crystalline, high mobility polymeric semiconductor

The effects of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) doping on diketopyrrolo-pyrrole-based polymeric semiconductors in terms of charge transport behavior and structural ordering are systematically investigated. Although the energy level offset between the polymeric semiconductor and the F4TCNQ acceptor was not particularly large, ultraviolet photoelectron spectroscopy analyses revealed that a low doping ratio of 1 wt. % is sufficient to tune the energy distance between the Fermi level and the HOMO level, reaching saturation at roughly 5 wt. %, which is further confirmed by the depletion mode measurements of field effect transistors (FETs). Structural analyses using grazing-incidence X-ray diffraction (GIXD) show that the overall degree of edge-on orientation is disturbed by the addition of dopants, with significant influence appearing at high doping ratios (>3 wt. %). The calculated charge carrier mobility from accumulation mode measurements of FETs showed a maximum value of 2 cm2/...

Synthesis of thiophene-based polymeric semiconductor with high aromatic density and its application in organic thin-film transistors

A new p-type polymer, poly(3,3″‴-didodecyl sexythiophene) (PST), was synthesized via FeCl3 oxidative coupling reaction. The mole ratio of FeCl3 and monomer (4:1), and keeping room temperature for polymerization were very important to obtain soluble polymer with regulated molecular weight. The photoluminescence (PL) maxima of the polymer are found at 500 and 562 nm for the solution and at 656 nm for the film. The low density of side chains enables substantial torsional deviations of the thienylene units, thereby inducing a relatively low highest occupied molecular orbital (HOMO) level (~ -5.22 eV). X-Ray diffraction analysis showed that PST forms a highly interdigitated structure due to the low density of side chains. PST were introduced as the active layers of organic thin-film transistors, and the resulting device showed field-effect mobilities of 0.12 cm2/Vs (on/off ratio=1.7×105) in air. The intrinsic mobility of PST measured by using the time of flight technique also supports a high conductance of 4×10-4 cm2/Vs.

Highly ordered bimolecular crystalline blends for low-noise and high-detectivity polymeric photodiodes

Suppressing noise current while maintaining high quantum efficiency is essential for realizing high-performance photodiodes. Solution-processed polymeric photodiodes, however, often suffer from high noise current levels because of undesired charge injection in the reverse saturation regime or localized energy states, resulting from the presence of structural imperfection. In this study, we demonstrated that such structural disorder can be avoided by constructing active layers with bimolecular crystallites. X-ray analyses of poly(2,5-bis(3-tetradecyllthiophene-2-yl)thieno[3,2-b]thiophene):phenyl-C61-butyric acid methyl ester blends confirmed low paracrystalline disorder and a high degree of orientation ordering of the active layers. The resulting optimized photodiodes with 80% PCBM content showed a high detectivity value of 1.95 × 1012 Jones, a high bandwidth of 1 kHz, and a high 94 dB linear dynamic range. This result implies that optimizing the structural perfectness of the active layers can be a decisive factor for low noise current and thus high detectivity of the solution-processed polymeric photodiodes.