Organic Electron Transport Materials Research Articles

Impact of Interfaces and Laser Repetition Rate on Photocarrier Dynamics in Lead Halide Perovskites.

We studied charge carrier recombination in methylammonium lead iodide (MAPbI3) perovskite and the impact of interfaces on the charge carrier lifetime using time-resolved photoluminescence. Pristine films and those covered with organic electron and hole transport materials (ETM and HTM) were investigated at various laser repetition rates ranging from 10 kHz to 10 MHz in order to separate the bulk and interface-affected recombination. We revealed two different components in the PL decay. The fast component (shorter than 300 ns) is assigned to interfacial processes and the slow one to bulk recombination. A high repetition pulse train was shown to shorten PL decay in pristine perovskite while significantly prolonging the photocarrier lifetime in MAPbI3 covered by TMs. This effect can be qualitatively explained with a kinetic model taking interface traps into account. We demonstrate a significant influence of the excitation repetition rate on photocarrier lifetime, which should be considered when studying charge carrier dynamics in perovskites.

Effects of plasma-induced high-energy particles on the damage of organic thin films

ABSTRACTWe have investigated the effect of plasma-induced damages on the electron transport layers in organic light-emitting devices. Tris(8-hydroxyquinolinato) aluminum, tris(2,4,6-trimethyl-3-(pyridine-3yl)phenyl) borane, and 3-(4-biphenyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole (TAZ) layers were used for studying plasma-induced degradation of electron transport materials. The thicknesses, photoluminescence spectra and the current-voltage curves of organic electron transport materials were measured after exposing to Ar plasma. The high-energy species in plasma induce the sputtering of organic layers, quenching of emission sites, and the creation of defect sites in organic layers. The plasma-induced damage was strongly dependent on the glass transition temperature, exhibiting the most serious degradation in the TAZ with low glass transition temperature.

Morphology control of planar heterojunction perovskite solar cells with fluorinated PDI films as organic electron transport layer

Morphology control is critical to achieve high efficiency CH3NH3PbI3 perovskite solar cells (PSC). In this paper, fluorinated perylene diimide (FPDI) was used as novel organic electron transport material in planar heterojunction perovskite solar cells. The perovskite film was fabricated by sequential vacuum vapor deposition, and the film morphology could be controlled by optimizing the FPDI film morphology with short time solvent spin-coating or solvent vapor annealing (SVA). Dense and uniform perovskite film with high substrate coverage could be obtained when the FPDI film was treated by chloroform SVA for half an hour, and the fill factor (FF) of the perovskite solar cell increased from 30.44% to 55.20%, enhancing the power conversion efficiency (PCE) from 3.23% to 7.44%. The PCE of the best device reached 7.93%, which was comparable to that (8.25%) of the conventional ZnO electron transport layer based perovskite device prepared by the same method.

(2,2′-Binaphthyl-6,6′-diyl)bis(diphenylphosphine oxide) as a potentially simple and efficient electron-transport layer for stable organic light-emitting diodes

The synthesis of organic electron-transport materials (ETMs) for organic light-emitting diodes (OLEDs) has been intensely pursued. Herein we report an organic phosphinyl compound (2,2′-Binaphthyl-6,6′-diyl)bis(diphenylphosphine oxide) (BiNa-BiDPO) with concise synthesis and purification. BiNa-BiDPO is thermally stable up to ca. 415 °C and exhibits distinct glass transition with a Tg of 112 °C after being cooled from the melt. Upon further heating, no crystallization or melting is observed. Ultraviolet photoemission spectroscopy studies reveal that EHOMO ≈ −6.12 eV for the new compound. Consequently, the LUMO level was roughly estimated as −2.77 eV based on the onset of the film absorption spectrum. BiNa-BiDPO possesses a higher electron mobility of 1.6–8.4 × 10−5 cm2 V−1 s−1 at E = 2–5 × 105 V cm−1, relative to a common ETM 1,3,5-tris(N-phenylbenzimidazolyl)benzene (TPBi), thus providing better OLED efficiency with lower working voltage. Moreover the OLED devices involving BiNa-BiDPO as the electron-transport layer showed a half life-time of 172 h at an initial luminance of ca. 1000 cd m−2, driven at a constant current density, in contrast with the TPBi device with a shortened t1/2 of 93 h. Further device engineering as well as molecular design may provide enhanced device durability.

P‐86: Improved Performance of Quantum Dot Light Emitting Diodes by Using Charge Blocking Layer

AbstractWe fabricated highly bright and efficient inverted quantum dot (QD) light emitting by using double charge injection layer with organic electron transport material. Using organic electron blocking layer, the controlled injection of carriers was achieved by cascaded energy levels, leading to the decreasing carrier injection into QDs. The improved performance can be attributed to adjust charge injection into the QD emissive layer and reduce exciton quenching.

Efficient inverted bottom-emission blue phosphorescent organic light-emitting diodes with a ytterbium-doped electron injection layer

An efficient electron injection layer (EIL) for inverted bottom-emission organic light-emitting diodes (IBE-OLEDs) is developed by doping ytterbium (Yb) into an organic electron transport material of 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB). The Yb-doped EIL in the IBE-OLEDs having iridium(III) bis(4,6-(difluorophenyl)pyridinato-N,C2′) picolinate (FIrpic) as a blue phosphorescent dopant shows a lower turn-on voltage (∼4.5 V) and about a 6.6 times higher efficiency (∼8.6% and 15.3 cd/A at 0.15 mA/cm2) compared with the device without the Yb-doped EIL. Furthermore, the electroluminescence spectrum of the device with the Yb-doped EIL is about the same as that of the device without the Yb-doped EIL. This result indicates that the Yb-doped EIL improves the electron injection from the ITO cathode to the organic electron transport layer. Therefore, the Yb-doped EIL can be utilized as an effective electron injection layer in the OLEDs.

Metal-Interrupted Perylene Diimide: Toward a New Class of Tunable n-Type Inorganic−Organic Hybrid Semiconductors

In organic thin-film transistors (OTFTs), organic electron-transport materials (n-type semiconductors) are well behind the advances in development of hole-transport materials (p-type semiconductors). Currently, one class of organic n-type semiconductor materials that is widely utilized is N,N'-dialkyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-R) derivatives with high electron affinities (EAs), such as N,N'-dioctyl-3,4,9,10-perylenetetracarboxylic diimide with a reported EA as high as 4.4 eV. The PTCDI-R derivatives have been manipulated by adding substituents on the perylene moiety or at the amine position to afford more stable compounds and higher EAs. On the basis of these materials, we have developed metal-containing perylenediimide analogues, placing a salpen ligand for metal ion chelation between two n-isobutylnaphthalimides. We demonstrate here that the electronic properties of this class of materials can be systematically tuned in a divergent manner by simply changing the metal center. The synthesis, characterization, electrochemistry, and band-gap analysis are discussed herein.

$$\pi$$ -Conjugated Heterocyclic fused Bithiophene Materials

This article covers the developments on the synthesis and properties of heterocyclic fused π-conjugated bithiophene materials that are potentially applicable in molecular electronics and optoelectronics. This fairly young strategy to efficiently tuning the electronic properties generates materials with very narrow band gaps. The nature of the central bridging heteroatom has a significant impact on the electronic and luminescence properties of these materials leading to intriguing species that can be employed in organic light emitting diodes (OLEDs) or organic field effect transistors (OFETs). So far a variety of heteroelements of group 13–16 (B, Si, Ge, Sn, N, P, S) have been investigated and incorporated into molecular as well as polymeric systems. A significant number of these materials can potentially act as organic emitters, electron or hole transport materials in organic devices but further studies are needed to optimize the necessary properties for the utility of this young class of compound in molecular electronics.

Enhanced photoconductivity in organic single-layered photoreceptors with bipolar charge transport materials

The organic single-layered photoreceptors (SLPRs) consisting of polycarbonate (PC) matrix containing dispersed N,N′-diethyl-4-aminobenzaldehyde-1-phenyl-1′-(α-naphthyl)-hydrazone (BAH) and 2,4,7-trinitrylfluorenone (TNF) as bipolar charge (hole and electron) transport materials, as well as oxotitanium phthalocyanine (TiOPc) as charge generation materials (CGMs), were prepared. The influence of organic electron transport material (ETM) on the photosensitivity in the photoreceptors was investigated. It was found that small amount of TNF (TNF/BAH ≤0.005) could improve the photosensitivity of photoreceptors greatly, i.e. the enhanced photoconductive effect, but the further increase of TNF concentration would lead to the decline of the photosensitivity. Differential scanning calorimeter (DSC), UV-Vis and cyclic voltammograms (CV) study showed that the enhanced photoconductivity might be resulted from the improvement of the separation efficiency of electron–hole pairs, and the decline of photosensitivity was due to the formation of the charge transfer complex (CTC) of TNF–BAH.

Recent development of organic electron transport materials*

Abstract This article reviews the recent development of organic electron transport materials applied in the fields of organic photo-conductors, light-emitting diodes, field-effect transistors and solar cells. Several technologies for charge carrier mobility measurement are summarized and compared, and a series of basic principles for designing high-performance organic electron transport materials are suggested as well.

Recent development of organic electron transport materials

Recent development of organic electron transport materials

Carrier Mobilities in Organic Electron Transport Materials Determined from Space Charge Limited Current

We report the mobility in organic electron transport materials determined from current–voltage characteristics in the regime of space charge limited current. The electron mobilities obtained are highly dependent on applied electric field, similar to the cases using a conventional time-of-flight method. The electron mobilities of phenanthroline derivatives are high and comparable to hole mobilities of conventional hole transport materials. Two parameters, zero-field mobility and electric field activation parameter are discussed from the viewpoint of oxidation and reduction reactions. The electric field activation parameter increases gradually with the increase of ionization potential. These facts suggest that there is a good possibility for realizing low driving voltage in organic light-emitting diodes (OLEDs).