We report high-efficiency blue-violet quantum-dot-based light-emitting diodes (QD-LEDs) by using high quantum yield ZnCdS/ZnS graded core-shell QDs with proper surface ligands. Replacing the oleic acid ligands on the as-synthesized QDs with shorter 1-octanethiol ligands is found to cause a 2-fold increase in the electron mobility within the QD film. Such a ligand exchange also results in an even greater increase in hole injection into the QD layer, thus improving the overall charge balance in the LEDs and yielding a 70% increase in quantum efficiency. Using 1-octanethiol capped QDs, we have obtained a maximum luminance (L) of 7600 cd/m(2) and a maximum external quantum efficiency (ηEQE) of (10.3 ± 0.9)% (with the highest at 12.2%) for QD-LEDs devices with an electroluminescence peak at 443 nm. Similar quantum efficiencies are also obtained for other blue/violet QD-LEDs with peak emission at 455 and 433 nm. To the best of our knowledge, this is the first report of blue QD-LEDs with ηEQE > 10%. Combined with the low turn-on voltage of ∼2.6 V, these blue-violet ZnCdS/ZnS QD-LEDs show great promise for use in next-generation full-color displays.

We report full-color quantum-dot-based light-emitting diodes (QLEDs) with high efficiency and long-lifetime by employing high quantum-yield core/shell QDs with thick shells. The increased shell thickness improves the confinement of excitons in the QD cores, and helps to suppress Auger recombination and Förster resonant energy transfer among QDs. Along with optimizing the QD emitting layer thickness and hole transport materials, we achieved significant improvements in device performance as a result of increasing the QD shell thickness to above 5 nm. By using poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine] (TFB) as a HTL with a 38 nm thick QD layer, these QLEDs show maximum current efficiencies of 18.9 cd A, 53.4 cd A, and 2.94 cd A, and peak external quantum efficiencies (EQEs) of 10.2%, 15.4%, and 15.6% for red, green, and blue QLEDs, respectively, all of which are well maintained over a wide range of luminances from 10 to 10 cd m. To the best of our knowledge, this is the first report of blue QLEDs with η > 15%. Most importantly, these devices also possess long lifetimes with T (the time at which the brightness is reduced to 70% of its initial value) of 117 h (red, with an initial luminance of 8000 cd m), 84 h (green, 6000 cd m) and 47 h (blue, 420 cd m). With further optimization of QD processing and device structures, these LEDs based on thick-shell QDs show great promise for use in next-generation full-color displays and lighting devices.

Thin-film optoelectronic devices based on polycrystalline organolead-halide perovskites have recently become a topic of intense research. Single crystals of these materials have been grown from solution with electrical properties superior to those of polycrystalline films. In order to enable the development of more complex device architectures based on organolead-halide perovskite single crystals, we developed a process to form epitaxial layers of methylammonium lead iodide (MAPbI) on methylammonium lead bromide (MAPbBr) single crystals. The formation of the MAPbI layer is found to be dominated by the diffusion of halide ions, leading to a shift in the photoluminescence and absorption spectra. X-ray diffraction measurements confirm the single-crystal nature of the MAPbI layer, while carrier transport measurements show that the converted layer retains the high carrier mobility typical of single-crystal perovskite materials. Such heterostructures on perovskite single crystals open possibilities for new types of devices.

It is commonly believed that large dielectric constants are required for efficient charge separation in polymer photovoltaic devices. However, many polymers used in high‐performance solar cells do not possess high dielectric constants. In this work, the effect of polymer–fullerene interactions on the dielectric environment of the active layer blend and the device performance for several donor–acceptor conjugated polymer systems is investigated. It is found that, while none of the high‐performing polymers studied has a dielectric constant value larger than 3, all polymer–fullerene blends have a significantly larger dielectric constant compared to their pristine constituents. Additionally, it is found that the blend dielectric constant reaches a maximum value in fully optimized devices. Using PTB7:PC71BM blends as an example, it is showed that, in addition to a small increase in the dielectric constant, devices fabricated using the optimum processing additive concentration exhibit almost 3X larger excited state polarizability. This large increase in excited state polarizability results in a substantial difference in short‐circuit current and ultimately device performance. The results show that the excited state polarizability critically depends on polymer–fullerene interactions, and can be a leading indicator of device performance for a given material system.

Three stereochemically pure isomers and two isomeric mixtures of a solutionprocessable diketopyrrolopyrrole-containing oligothiophene ( SMDPPEH ) have been used to study the effect of 2-ethylhexyl solubilizing group stereochemistry on the fi lm morphology and bulk heterojunction (BHJ) solar cell characteristics of small molecule organic photovoltaics. The different SMDPPEH stereoisomer compositions exhibit nearly identical optoelectronic properties in the molecularly dissolved state, as well as in amorphous fi lms blended with PCBM. However, for fi lms in which SMDPPEH crystallization is induced by thermal annealing, signifi cant differences in molecular packing between the different stereoisomer formulations are observed. These differences are borne out in photovoltaic device characteristics for which unannealed devices show very similar behavior, while after annealing the RRand SS-SMDPPEH enantiomers show blue-shifted peak EQE relative to the SMDPPEH isomer mixtures. Unannealed devices made from the most crystalline stereoisomer, meso RS -SMDPPEH , are not completely amorphous, and show improved photocurrent generation as a result. Unlike the other compounds, after thermal annealing the RS -SMDPPEH devices show reduced device performance. The results reveal that the chirality of commonly used 2-ethylhexyl solubilizing chains can have a signifi cant effect on the morphology, absorption, and optimum processing conditions of small molecule organic thin fi lms used as photovoltaic device active layers.