The commercialization of solar fuel devices requires the development of novel engineered photoelectrodes for water splitting applications which are based on redundant, cheap, and environmentally friendly materials. In the current study, a combination of titanium dioxide (TiO2) and zinc oxide (ZnO) onto nanotextured silicon is utilized for a composite electrode with the aim to overcome the individual shortcomings of the respective materials. The properties of conformal coverage of TiO2 and ZnO layers are designed on the atomic scale by the atomic layer deposition technique. The resulting photoanode shows not only promising stability but also nine times higher photocurrents than an equivalent photoanode with a pure TiO2 encapsulation onto the nanostructured silicon. Density functional theory calculations indicate that segregation of TiO2 at the ZnO surfaces is favorable and leads to the stabilization of the ZnO layers in water environments. In conclusion, the novel designed composite material constitutes a promising base for a stable and effective photoanode for the water oxidation reaction.

Amorphous small-molecule organic materials are utilized in organic light emitting diodes (OLEDs), with device performance relying on appropriate chemical design. Due to the vast number of contending materials, a symbiotic experimental and simulation approach would be greatly beneficial in linking chemical structure to macroscopic material properties. We review simulation approaches proposed for predicting macroscopic properties. We then present a library of OLED hosts, containing input files, results of simulations, and experimentally measured references of quantities relevant to OLED materials. We find that there is a linear proportionality between simulated and measured glass transition temperatures, despite a quantitative disagreement. Computed ionization energies are in excellent agreement with the ultraviolet photoelectron and photoemission spectroscopy in air measurements. We also observe a linear correlation between calculated electron affinities and ionization energies and cyclic voltammetry measurements. Computed energetic disorder correlates well with thermally stimulated luminescence measurements and charge mobilities agree remarkably well with space charge–limited current measurements. For the studied host materials, we find that the energetic disorder has the greatest impact on the charge carrier mobility. Our library helps to swiftly evaluate properties of new OLED materials by providing well-defined structural building blocks. The library is public and open for improvements. We envision the library expanding and the workflow providing guidance for future OLED material design.

The electronic density-of-states (DOS) plays a central role in controlling the charge-carrier transport in amorphous organic semiconductors, while its accurate determination is still a challenging task. We have applied the low-temperature fractional thermally stimulated luminescence (TSL) technique to determine the DOS of pristine amorphous films of OLED host materials. The DOS width is determined for two series of hosts, namely, (i) carbazole-biphenyl (CBP) derivatives: CBP, mCBP, and mCBP-CN, and (ii) carbazolephenyl (CP) derivatives: mCP and mCP-CN. TSL originates from radiative recombination of chargecarriers thermally released from the lower energy part of the intrinsic DOS that causes charge trapping at very low temperatures. We find that the intrinsic DOS can be approximated by a Gaussian distribution, with a deep exponential tail accompanying this distribution in CBP and mCBP films. The DOS profile broadens with increasing molecular dipole moments, varying from 0 to 6 Debye, in a similar manner within each series, in line with the dipolar disorder model. The same molecular dipole moment, however, leads to a broader DOS of CP compared to CBP derivatives. Using computer simulations, we attribute the difference between the series to a smaller polarizability of cations in CP-derivatives, leading to weaker screening of the electrostatic disorder by induction. These results demonstrate that the low-temperature TSL technique can be used as an efficient experimental tool for probing the DOS in small-molecule OLED materials.

Organic light emitting diodes (OLEDs), utilize small organic molecules or polymers, in order to achieve an emissive electroluminescent layer. Ultra-thin, lightweight, and flexible characteristics offer a highly enticing substitute, in comparison to their inorganic counterpart. It is therefore understandable why there has been a large amount of research, focused on enhancing the efficiency and stability of OLEDs. At present, efficient and long-lasting, red [1] and green [2] OLEDs are achievable, with the weakest link being the blue OLED. Achieving a blue OLED, which is both efficient and stable, has proven to be problematic. The challenge originating with the limitations of the blue emitter: if operational lifetime is prioritized, stable fluorescent emitters can be used. Their efficiency, however, is limited by unfavorable spin statistics. To improve efficiency, phosphorescent emitters can be used. Their large coupling between the exciton spin and the orbital angular momentum allows for radiative decay from the triplet state to the ground state. Additional to this, the spin-orbit coupling allows for intersystem crossing to occur, such that the singlet excited state can also populate the triplet state, helping to achieve almost 100% internal quantum efficiency. The drawback of this highly efficient system is the long lifetime of the triplet state, typically in the order of several microseconds, much longer than the fluorescence lifetime, leading to degradation of the organic material. [3] Since stability is most important to achieve a long-lived consumer product, fluorescent emitters are the chosen source of blue OLEDs. But, with battery life on portable devices being the cost of this inefficiency, it is vital that the blue OLEDs become more efficient.Thermally activated delayed fluorescence (TADF) [4][5][6][7][8][9] is one of the existing approaches targeting the OLED efficiency, where a reverse intersystem crossing, from triplet to singlet, is achieved. Combination of TADF and conventional fluorescence emitters, in a sensitizing approach is also a possibility. [10,11] However, the decay times of TADF systems are similar to that of a phosphorescent only system, [12,13] meaning that a short-lived OLED is inevitable.A phosphor-sensitized fluorescence approach, [14][15][16][17][18] offers an alternative to TADF OLEDs, by utilizing a donor-acceptor concept with a phosphorescent donor and a fluorescent acceptor. In Unicolored phosphor-sensitized fluorescence (UPSF) is a dual emitting concept proposed for improving efficiencies and operational lifetimes of blue organic light emitting diodes (OLEDs). To overcome the limitations of the individual emitters, it uses a phosphorescent donor to sensitize a fluorescent acceptor. To quantify the potential of the concept, a multiscale model of a UPSF OLED is developed. It starts from atomistic morphologies, the rates of all processes on the available experimental data are parameterized, and the respective master equation is solved with the help of the kinetic Monte Carlo algorith...

Glass transition temperature, Tg, is the key quantity for assessing morphological stability and molecular ordering of films of organic semiconductors. A reliable prediction of Tg from the chemical structure is, however, challenging, as it is sensitive to both molecular interactions and analysis of the heating or cooling process. By combining a fitting protocol with an automated workflow for forcefield parameterization, we predict Tg with a mean absolute error of ~20 °C for a set of organic compounds with Tg in the 50–230 °C range. Our study establishes a reliable and automated prescreening procedure for the design of amorphous organic semiconductors, essential for the optimization and development of organic light-emitting diodes.