Controlling Spontaneous Orientation Polarization in Organic Semiconductors -- The Case of Phosphine Oxides

Upon film growth by physical vapor deposition, the preferential orientation of polar organic molecules can result in a non-zero permanent dipole moment (PDM) alignment, causing a macroscopic film polarization. This effect, known as spontaneous orientation polarization (SOP), was studied in the case of different phosphine oxides. We investigate the control of SOP by molecular design and film-growth conditions. Our results show that using less polar phosphine oxides with just one phosphor-oxygen bond yields an exceptionally high degree of SOP with the so-called giant surface potential (slope) reaching more than 150mV/nm in a neat BCPO film grown at room temperature. Additionally, by altering the evaporation rate and the substrate temperature, we are able to control the SOP magnitude over a broad range from 0 to almost 300mV/nm. Diluting BCPO in a non-polar host enhances the PDM alignment only marginally, but combining temperature control together with dipolar doping can result in almost perfectly aligned molecules with more than 80% of their PDMs standing upright on the substrate on average

Are the Rates of Dexter Transfer in TADF Hyperfluorescence Systems Optically Accessible?

Seemingly not, but for unexpected reasons. Combining the triplet harvesting properties of TADF materials with the fast emission rates and colour purity of fluorescent emitters is attractive for developing high performance OLEDs. In this “hyperfluorescence” approach, triplet excitons are converted to singlets on the TADF material and transferred to the fluorescent material by long range Förster energy transfer. The primary loss mechanism is assumed to be Dexter energy transfer from the TADF triplet to the non-emissive triplet of the fluorescent emitter. Here we use optical spectroscopy to investigate energy transfer in representative emissive layers. Despite observing kinetics that at first appear consistent with Dexter quenching of the TADF triplet state, transient absorption, photoluminescence quantum yields, and comparison to phosphor-sensitised “hyperphosphorescent” systems reveal that this is not the case. While Dexter quenching by the fluorescent emitter is likely still a key loss mechanism in devices, we demonstrate that – despite initial appearances - it is inoperative under optical excitation. These results reveal a deep limitation of optical spectroscopy in characterizing hyperfluorescent systems

Ambipolar Blends of Cu-Phthalocyanine and Fullerene: Charge Carrier Mobility, Electronic Structure and their Implications for Solar Cell Applications

Summary: Ambipolar transport has been realised in blends of the molecular hole conductor Cu-phthalocyanine (CuPc) and the electron conducting fullerene C 60 . Charge carrier mobilities and the occupied electronic levels have been analyzed as a function of the mixing ratio using field-effect transistor measurements and photoelectron spectroscopy. These results are discussed in the context of photovoltaic cells based on these materials

Doubly stabilized perovskite nanocrystal luminescence downconverters

Halide perovskite nanocrystals (NCs) have emerged as a promising material for applications ranging from light-emitting diodes (LEDs) to solar cells and photodetectors. Still, several issues impede the realization of the nanocrystals' full potential, most notably their susceptibility to degradation from environmental stress. This work demonstrates highly stable perovskite nanocrystals (NCs) with quantum yields as high as 95 % by exploiting a ligand-assisted copolymer nanoreactor-based synthesis. The organic ligands thereby serve a dual function by enhancing the uptake of precursors and passivating the NCs. The polymer micelles and ligands thus form a double protection system, shielding the encapsulated NCs from water-, heat- and UV-light-induced degradation. We demonstrate the optoelectronic integrability by incorporating the perovskite NCs as spectrally pure downconverters on top of a deep-blue-emitting organic LED. These results establish a way of stabilizing perovskite NCs for optoelectronics while retaining their excellent optical properties