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Partial disorder is an inherent property of self-assembled organic semiconductors that complicates their rational design, because electronic structure, self-assembling properties, and stability all have to be accounted for simultaneously. Therefore, the understanding of charge transport mechanisms in these systems is still in its infancy. A theoretical study of charge transport in organic semiconductors was performed on self-assembled layers of [1]benzothieno[3,2-b]benzothiophene functionalized with alkyl side chains. Analysis showed that semiclassical dynamics misses static (on time scales of charge transport) disorder while the solution of the master equation combined with the high-temperature limit Marcus theory for charge transfer rates does not take into account molecular dynamic modes relaxing on a time scale of charge hopping. A comparison between predictions based on a perfectly ordered and a realistic crystal structure reveals the strong influence of static and dynamic disorder. The advantage of two-dimensional charge transporting materials over one-dimensional ones is clearly shown. The Marcus theory-based prediction of 0.1 cm 2 V -1 s -1 is in good agreement with our FET mobility of 0.22 cm 2 V -1 s -1, which is an order of magnitude lower than that reported in the literature [Ebata, H.; et al.

The blend films of small-molecule semiconductors with insulating polymers exhibit not only excellent solution processability but also superior performance characteristics in organic thin-film transistors (OTFTs) over those of neat small-molecule semiconductors. To understand the underlying mechanism, we studied triethylsilylethynyl anthradithiophene (TESADT) with small amounts of impurity formed by weak UV exposure. OTFTs with neat impure TESADT had drastically reduced field-effect mobility (<10(-5) cm(2)/(V s)), and a disappearance of the high-temperature crystal phase was observed for neat impure TESADT. However, the mobility of the blend films of the UV-exposed TESADT with poly(α-methylstyrene) (PαMS) is recovered to that of a fresh TESADT-PαMS blend (0.040 cm(2)/(V s)), and the phase transition characteristics partly return to those of fresh TESADT films. These results are corroborated by OTFT results on "aged" TIPS-pentacene. These observations, coupled with the results of neutron reflectivity study, indicate that the formation of a vertically phase-separated layer of crystalline small-molecule semiconductors allows the impurity species to remain preferentially in the adjacent polymer-rich layer. Such a "zone-refinement effect" in blend semiconductors effectively removes the impurity species that are detrimental to organic electronic devices from the critical charge-transporting interface region.

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To utilize thermally activated delayed fluorescence (TADF) technology for future displays, it is necessary to develop host materials which harness the full potential of blue TADF emitters. However, no publication has reported such hosts yet. Although the most popular host for blue TADF, bis[2‐(diphenylphosphino)phenyl]ether oxide (DPEPO) guarantees high‐maximum external quantum efficiency (EQEmax) TADF devices, they exhibit very short operational lifetimes. In contrast, long‐lifespan blue TADF devices employing stable hosts such as 3′,5‐di(9H‐carbazol‐9‐yl)‐[1,1′‐biphenyl]‐3‐carbonitrile (mCBP‐CN) exhibit much lower EQEmax than the DPEPO‐employed devices. Here, an elaborative approach for designing host molecules is suggested to achieve simultaneously stable and efficient blue TADF devices. The approach is based on engineering the molecular geometry, ground‐ and excited‐state dipole moments of host molecules. The engineered hosts significantly enhance delayed fluorescence quantum yields of TADF emitters, as stabilizing the charge‐transfer excited states of the TADF emitters and suppressing exciton quenching, and improve the charge balance. Moreover, they exhibit both photochemical and electrochemical stabilities. The best device employing one of the engineered hosts exhibits 79% increase in EQEmax compared to the mCBP‐CN‐employed device, together with 140% and 92‐fold increases in operational lifetime compared to the respective mCBP‐CN‐ and the DPEPO‐based devices.

The degradation mechanisms of PHOLEDs were reported in terms of the intrinsic degradation of the weak material, external impurity, charge imbalance, and bimolecular interaction. [3] Among these origins, the interaction between excitons and polarons in the EML was critical to the materials degradation because the molecular dissociation of the host and dopant directly affects the luminance of the device. [4] Therefore, the kinetic paths and their specific rates of excitons and polarons should be investigated to understand the nature of device stability.The bimolecular processes of organic light-emitting diodes have been analyzed in the steady-state via external quantum efficiency (EQE) roll-off characteristic and the transient state via transient electroluminescence (EL) and photoluminescence (PL). [5] In the PHOLEDs, it was shown that the EQE roll-off was significantly caused by the triplet-triplet annihilation (TTA) and triplet-polaron annihilation (TPA) because of high triplet exciton density in the EML. [6] In the TADF OLEDs, the contribution of singlet-triplet annihilation (STA) and TTA increased the EQE roll-off due to the conversion of triplet excitons into singlet excitons via reverse intersystem crossing (RISC). [7]