Kyu Min Sim

Morphology-Driven High-Performance Polymer Transistor-based Ammonia Gas Sensor

Developing high-performance gas sensors based on polymer field-effect transistors (PFETs) requires enhancing gas-capture abilities of polymer semiconductors without compromising their high charge carrier mobility. In this work, cohesive energies of polymer semiconductors were tuned by strategically inserting buffer layers, which resulted in dramatically different semiconductor surface morphologies. Elucidating morphological and structural properties of polymer semiconductor films in conjunction with FET studies revealed that surface morphologies containing large two-dimensional crystalline domains were optimal for achieving high surface areas and creating percolation pathways for charge carriers. Ammonia molecules with electron lone pairs adsorbed on the surface of conjugated semiconductors can serve as efficient trapping centers, which negatively shift transfer curves for p-type PFETs. Therefore, morphology optimization of polymer semiconductors enhances their gas sensing abilities toward ammonia, leading to a facile method of manufacturing high-performance gas sensors.

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.

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.