Organic Semiconductor Devices

We report on the influence of localized surface plasmon resonance excitation of Au@SiO 2 core−shell nanoparticles on the amplified spontaneous emission of a semiconductor polymer composite (F8BT/MEH-PPV). Au@SiO 2 nanoparticles are compatible with the donor−acceptor polymer matrix and get uniformly distributed within the whole polymer film. The plasmon resonance band of the nanoparticles correlates with both the emission and excitation spectra of the polymer composite, as well as with the donor emission and acceptor excitation spectra. We demonstrate that resonantly excited Au@SiO 2 nanoparticles enhance the amplified spontaneous emission and the modal gain of the polymer films. The measurement of influential factors reveals that the emission is enhanced predominantly by the increase of acceptor excitation rate, which is accompanied by depletion of the FRET efficiency and increase of quantum yield. The enhancement factor is increased by both introducing a higher loading of plasmonic nanoparticles in the polymer film and increasing the excitation energy. This work shows that these plasmonic nanoantennas are able to enhance the stimulated emission of semiconductor polymers by improving the size mismatch between the excitation light and the emitting polymer.

— We have investigated the stability of key electro-optical properties (Conductivity, Work Function, Transmittance) of several commonly used electrode materials for organic optoelectronics (Indium-Tin Oxide, Gold, Silver, Aluminum) in different media (Air, DI water, Phosphate Buffered Saline and Dulbecco's Modified Eagle Medium). By comparing the electrode materials side by side, we aimed to identify their advantages and drawbacks for use in solid/liquid devices. Since many applications of such devices involve the direct contact with biological species, we also investigated their biocompatibility by evaluating cytotoxicity of HEK293 cells cultured on the candidate materials.

Current transport mechanisms (CTMs) of Au/Ca3Co4Ga0.001Ox/n-Si/Au (MIS) type structures were investigated using current–voltage (I–V) characteristics in the temperature range of 80–340 K. Semilogarithmic I–V plots show two linear regions corresponding to low (0.075–0.250 V) and moderate (0.27–0.70 V) biases, respectively. Zero-bias barrier height (ΦB0) was observed to increase with increase in the temperature, whereas opposite behaviour was observed for ideality factor (n). ΦB0 and (n−1–1) vesus q/2kT and ΦB0 versus n plots were drawn to get an evidence of Gaussian distribution (GD) of the barrier heights. These plots, too, show two linear regions corresponding to low (80–160 K) and high (200–340 K) temperature ranges (LTR and HTR), respectively. Mean value of barrier height (B0) and standard deviation (σs) were extracted from the intercept and slope of ΦB0 versus q/2kT plots for two linear regions as 0.382 eV, 0.0060 V for LTR and 0.850 eV, 0.135 V for HTR at low biases and 0.364 eV, 0.0059 V for LTR and 0.806 eV, 0.132 V for HTR at high biases, respectively. B0 and Richardson constant (A*) values were also obtained from the intercept and slope of the modified Richardson (Ln(Io/T2)−(q2σs2/2k2T2) versus q/kT) plots as 131.81 Acm−2 K−2, 0.381 eV for LTR and 129.35 Acm−2 K−2, 0. 854 eV for HTR at low biases and 148.01 Acm−2 K−2, 0.377 eV for LTR and 143.77 Acm−2 K−2, 0.812 eV for HTR at high biases, respectively. In conclusion, CTM in the structure can be successfully explained in terms of thermionic emission theory with the double GD of barrier heights.

Pulsed Laser Deposition (PLD) technique is applied to produce organic semiconductor (OS) rubrene thin film for
spintronics applications. The use of organic material for spintronics is motivated by the advantages such as long spin
diffusion length due to low spin-orbit and hyperfine coupling, chemical tuning of electronic functionality, easy
structural modifications, ability of self-assembly and mechanical flexibility etc. However, a major drawback of OS is its low mobility compared to inorganic semiconductors. The PLD growth of rubrene aims on fabricating OS films under
more controlled environment to achieve higher crystalline order to improve its mobility and spin coherence length.
Among organic materials, rubrene reveals the highest hole mobility - up to 40 cm2/(V∙s) and can be exploited in organic
light-emitting diodes (OLEDs) or field-effect transistors (OFETs) 4. In this work the rubrene thin films are produced
from hardened pellets in vacuum using Nd:YAG pulsed laser operated at 1064 nm, 2 Hz and energy fluence around 0.2
J/cm2. For the reference rubrene samples on SiO2 glass the AFM data reveal continuous 5-7 nm thick films. The
amorphous structures are confirmed by XRD measurements and also Raman spectra which show signatures of both
tetracene and phenyl bands and a broadband at 1373 cm-1. The obtained results indicate that continuous, defect-free
rubrene films can be prepared by means of PLD for investigation of the spin polarization properties of organic-inorganic hybrids. Further studies are on the way to improve crystalline qualities of the rubrene films for less grain boundary
related defects and improved mobility and spin diffusion length.

Organic semiconductor devices and materials have matured sufficiently to be limited by intrinsic degradation processes which are as yet not understood well. We use high quality Alq 3 based organic light emitting diodes to study the rate processes involved in degradation due to electrical stressing and its auto-recovery. The method involves interspersing degradation due to electrical pulsing with variable relaxation windows to monitor time evolution of loss and recovery of luminescence. The corresponding rate processes for permanent and auto-recoverable degradation is discussed on the basis of charging and discharging of traps, and a phenomenological model based on metastability in configuration-coordinate diagram is proposed.