Development of Efficient OLEDs from Solution Deposition

The use of highly efficient organic emitters based on the thermally activated delayed fluorescence (TADF) concept is interesting due to their 100% ...

University of Aveiro

Organic photovoltaic (OPV) devices, made with semiconducting polymers, have recently attained a power conversion efficiency (PCE) over 14% in single junction cells and over 17% in tandem cells. These high performances, together with the suitability of the technology to inexpensive large-scale manufacture, over lightweight and flexible plastic substrates using roll-to-roll (R2R) processing, place the technology amongst the most promising for future harvesting of solar energy. Although OPVs using non-fullerene acceptors have recently outperformed their fullerene-based counterparts, the research in the development of new fullerenes and in the improvement of the bulk-heterojunction (BHJ) morphology and device efficiency of polymer:fullerene solar cells remains very active. In this review article, the most relevant research works performed over the last 3 years, that is, since the year 2016 onwards, in the field of fullerene-based polymer solar cells based on the copolymers PTB7, PTB7-Th (also known as PBDTTT-EFT) and PffBT4T-2OD, are presented and discussed. This review is primarily focused on studies that involve the improvement of the BHJ morphology, efficiency and stability of small active area devices (typically < 15 mm²), through the use of different processing strategies such as the use of different fullerene acceptors, different processing solvents and additives and different thermal treatments.

Recent Developments in the Optimization of the Bulk Heterojunction Morphology of Polymer: Fullerene Solar Cells.

To achieve significant efficiency and low roll-off in thermally activated delayed fluorescence (TADF) material organic light-emitting diodes (OLEDs), it is essential to choose a host that has suitable high triplet energy (T) and bipolar character to boost the TADF characteristics as a dopant and avoid exciton annihilation. Herein, we present the effect of different host materials on the efficiency of organic light-emitting diodes (OLEDs) based on bis[4-(3,6 dimethoxycarbazole)phenyl]sulfone (DMOC-DPS) deep-blue emitter. The devices with 10 wt.% of an emitter in different electron types of host bis[2-(diphenylphosphino) phenyl] ether oxide (DPEPO), and hole types of host 1,3-bis(-carbazolyl)benzene (mCP), were fabricated to study the effect on device performance. The results show that an external quantum efficiency (EQE) of 4% and maximum current efficiency (ƞ) up to 5.77 cd/A with high luminescence (l) 8185 cd/m in DPEPO was achieved, compared to 2.63% EQE, ƞ 4.12 cd/A with l 5338 cd/m in mCP in a very simple device structure. As a remarkable result, the roll-off is suppressed at 1000 cd/m, and for maximum brightness, the roll-off is less than 50%. Further general applications are discussed.

Effect of the Host on Deep-Blue Organic Light-Emitting Diodes Based on a TADF Emitter for Roll-Off Suppressing.

Novel C and C -methyl-fulleropyrrolidine derivatives, containing both electron withdrawing and electron donating substituent groups, were synthesized by the well-known Prato reaction. The corresponding highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy levels were determined by cyclic voltammetry, from the onset oxidation and reduction potentials, respectively. Some of the novel fullerenes have higher LUMO levels than the standards PCBM and PCBM. When tested in PffBT4T-2OD based polymer solar cells, with the standard architecture ITO/PEDOT:PSS/Active-Layer/Ca/Al, these fullerenes do not bring about any efficiency improvements compared to the standard PCBM system, however they show how the electronic nature of the different substituents strongly affects the efficiency of the corresponding organic photovoltaic (OPV) devices. The functionalization of C yields a mixture of regioisomers and density functional theory (DFT) calculations show that these have systematically different electronic properties. This electronic inhomogeneity is likely responsible for the lower performance observed in devices containing C derivatives. These results help to understand how new fullerene acceptors can affect the performance of OPV devices.

PffBT4T-2OD Based Solar Cells with Aryl-Substituted -Methyl-Fulleropyrrolidine Acceptors.

Organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence emitters (TADF) in simple device structures fabricated by solution processing are strongly dependent on a suitable host molecular conformation and morphology. Herein, we report the fabrication of highly efficient yellow-red TADF-based OLEDs via solution processing in a simple, two-organic-layer device structure. The devices were fabricated at different weight concentrations of 5%, 8%, and 10% emitter in an n-/p-type mixed host matrix, and their characteristics were studied. The device performance was compared with different thickness parameters for both the emitting layer (EML) and the electron transport layer (ETL) in various solvents, including chlorobenzene, dichlorobenzene, and chloroform. By optimizing the mixed ratio of EML, yellow-red OLEDs of 2-[4 (diphenylamino)phenyl]-10,10-dioxide-9-thioxanthen-9-one (TXO-TPA) emitter in an n-/p-type host matrix of poly(-vinylcarbazole):1,3-Bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene (PVK:OXD-7) as a blend for the active layer were fabricated. In the best results, the device exhibited a lower turn-on voltage at around 6 V, with an external quantum efficiency (EQE) of 18.44%, current efficiency of 36.71 cd/A, and power efficiency of 14.74 Lm/W for the 8% emitter concentration. The importance of solvent for improving the electrical properties, together with organic layer thickness and host effect for the charge carrier's transport and device characteristics are also discussed.

Towards Highly Efficient TADF Yellow-Red OLEDs Fabricated by Solution Deposition Methods: Critical Influence of the Active Layer Morphology.

Charge balance, concentration quenching, and exciton confinement are the most important factors for realizing the use of thermally activated delayed fluorescence (TADF) emitters for organic light-emitting diodes. Red-orange organic light-emitting diodes of a TADF emitter 2-[4 (diphenylamino)phenyl]-10,10-dioxide-9-thioxanthen-9-one (TXO-TPA) have been reported by doping in a mixed p-type host system of poly(-vinylcarbazole) (PVK) and 1,3-bis(-carbazolyl)benzene (mCP) via solution-processed. We have demonstrated the peak external quantum efficiency of 9.75%, maximum current efficiency of 19.36 cd/A, and power efficiency of 12.17 lm/W along with a CIE coordinate of (0.45, 0.51). The devices were compared with different doping concentrations of TXO-TPA, and a comparative investigation on the effect of the thickness electron transport layer was studied. The results clearly indicated that this solution-processed TXO-TPA device structure is a promising strategy to develop highly efficient but simple OLED structures.

Mixed-Host Systems with a Simple Device Structure for Efficient Solution-Processed Organic Light-Emitting Diodes of a Red-Orange TADF Emitter.

The impact of fullerene side chain functionalization with thiophene and carbazole groups on the device properties of bulk-heterojunction polymer:fullerene solar cells is discussed through a systematic investigation of material blends consisting of the conjugated polymer poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3‴-di(2-octyldodecyl)-2,2';5',2″;5″,2‴-quaterthiophen-5,5‴-diyl)] (PffBT4T-2OD) as donor and C or C fulleropyrrolidines as acceptors. The photovoltaic performance clearly depended on the molecular structure of the fulleropyrrolidine substituents although no direct correlation with the surface morphology of the photoactive layer, as determined by atomic force microscopy, could be established. Although some fulleropyrrolidines possess favorable lowest unoccupied molecular orbital levels, when compared to the standard PCBM, they originated OPV cells with inferior efficiencies than PCBM-based reference cells. Fulleropyrrolidines based on C produced, in general, better devices than those based on C, and we attribute this observation to the detrimental effect of the structural and energetic disorder that is present in the regioisomer mixtures of C-based fullerenes, but absent in the C-based fullerenes. These results provide new additional knowledge on the effect of the fullerene functionalization on the efficiency of organic solar cells.

Thiophene- and Carbazole-Substituted -Methyl-Fulleropyrrolidine Acceptors in PffBT4T-2OD Based Solar Cells.

Molybdenum oxide thin films were deposited on stiff and flexible substrates by reactive DC magnetron sputtering. Two sets of samples were prepared. The first with different O/Ar flow rate ratios and the second, fixing the oxygen content, with different time of deposition. As the O/Ar flow rate ratio varies from 0 up to 0.56, a threshold was found, ranging from crystalline to amorphous nature, and from a nontransparent appearance with metallic-like electrical conductivity to transparent and dielectric behaviour. From the second set, all transparent, the MoO films present a compact/dense and featureless morphology with thickness from 190 up to 910 nm, depending on the time of deposition. Their structure was corroborated by XPS and Rutherford Backscattering Spectrometry (RBS) and density measurements were performed by RBS and X-ray reflectivity (XRR), revealing a value of 2.4 g/cm. The surface roughness is in the order of a few nanometers and the maxima optical transmission, in the visible range, is around 89%. Electrochemical cyclic voltammograms showed noticeable color reversibility and reproducibility on the flexible substrates opening new framework possibilities for new electrochomic devices.

Molybdenum Oxide Thin Films Grown on Flexible ITO-Coated PET Substrates.

The impact of several solvent processing additives (1-chloronaphthalene, methylnaphthalene, hexadecane, 1-phenyloctane, and p-anisaldehyde), 3% / in o-dichlorobenzene, on the performance and morphology of poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3‴-di(2-octyldodecyl)-2,2',5',22033,5″,2‴-quaterthiophen-5,5‴-diyl)] (PffBT4T-2OD):[6,6]-phenyl-C71-butyric acid methyl ester (PCBM)-based polymer solar cells was investigated. Some additives were shown to enhance the power conversion efficiency (PCE) by ~6%, while others decreased the PCE by ~17-25% and a subset of the additives tested completely eliminated any power conversion efficiency and the operation as a photovoltaic device. Grazing-Incidence Wide Angle X-ray Scattering (GIWAXS) revealed a clear stepwise variation in the crystallinity of the systems when changing the additive between the two extreme situations of maximum PCE (1-chloronaphthalene) and null PCE (hexadecane). Small-Angle Neutron Scattering (SANS) revealed that the morphology of devices with PCE ~0% was composed of large domains with correlation lengths of ~30 nm, i.e., much larger than the typical exciton diffusion length (~12 nm) in organic semiconductors. The graded variations in crystallinity and in nano-domain size observed between the two extreme situations (1-chloronaphthalene and hexadecane) were responsible for the observed graded variations in device performance.

Graded Morphologies and the Performance of PffBT4T-2OD:PCBM Devices Using Additive Choice.

Herein, we report the synthesis and characterization of two Pt(II) coordination compounds, the new platinum(II)[,'-bis(salicylidene)-3,4-diaminobenzophenone)] ([Pt(sal-3,4-ben)]) and the already well-known platinum(II)[,'-bis(salicylidene)-o-phenylenediamine] ([Pt(salophen)]), along with their application as guests in a poly [9,9-dioctylfluorenyl-2,7-diyl] (PFO) conjugated polymer in all-solution processed single-layer white organic light-emitting diodes. Completely different performances were achieved: 2.2% and 15.3% of external quantum efficiencies; 2.8 cd A and 12.1 cd A of current efficiencies; and 3103 cd m and 6224 cd m of luminance for the [Pt(salophen)] and [Pt(sal-3,4-ben)] complexes, respectively. The Commission Internationale de l'Eclairage (CIE 1931) chromaticity color coordinates are (0.33, 0.33) for both 0.1% mol/mol Pt(II):PFO composites at between approximately 3.2 and 8 V. The optoelectronic properties of doped and neat PFO films have been investigated, using steady-state and time-resolved photoluminescence. Theoretical calculations at the level of relativistic density functional theory explained these results, based on the presence of the Pt(II) central ion's phosphorescence emission, considering spin-orbit coupling relationships. The overall results are explained, taking into account the active layer morphological properties, along with the device's electric balance and the emitter's efficiencies, according to deep-trap space-charge models. Considering the very simple structure of the device and the ease of synthesis of such compounds, the developed framework can offer a good trade-off for solution-deposited white organic light-emitting diodes (WOLEDs), with further applications in the field of lighting and signage.

Luiz Pereira

Department of Physics and i3N,
Institute for Nanostructures,
Nanomodulation and Nanofabrication,
Department of Physics and i3N, Institute for Nanostructures, Nanomodulation and Nanofabrication

University of Aveiro

Development of Efficient OLEDs from Solution Deposition

Luiz Pereira

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Department of Physics and i3N,Institute for Nanostructures,Nanomodulation and Nanofabrication,Department of Physics and i3N, Institute for Nanostructures, Nanomodulation and Nanofabrication

University of Aveiro

Development of Efficient OLEDs from Solution Deposition

Department of Physics and i3N,
Institute for Nanostructures,
Nanomodulation and Nanofabrication,
Department of Physics and i3N, Institute for Nanostructures, Nanomodulation and Nanofabrication