Surface initiated oxidative crosslinking of a polymeric hole transport material for improved efficiency and lifetime in soluble organic light-emitting diodes

A surface initiated oxidative coupling method was developed as a crosslinking approach of hole transport materials for solution processed organic light-emitting diodes. The surface initiated crosslinking method was better than bulk oxidative crosslinking method in terms of quantum efficiency and lifetime of the organic light-emitting diodes by suppressing exciton quenching by the hole transport layer. Doubled efficiency and quadrupled lifetime were obtained using the new crosslinking approach without any chemical modification of the hole transport material.     

Color stable and low driving voltage white organic light-emitting diodes with low efficiency roll-off achieved by selective hole transport buffer layers

White organic light-emitting diodes (WOLEDs) showing high color stability, low operating voltage, high efficiency and low efficiency roll-off by adopting different hole transport buffer layers which also behaves as electron/exciton blocking layers (EBL) have been developed. The characteristics of WOLEDs based on blue–green and orange phosphors could be easily manipulated by hole transport buffer layer, which tailors charge carrier transportation and energy transfer. Our WOLEDs show low operating voltages, 100 cd/m² at 3.2 V, 1000 cd/m² at 3.7 V and 10000 cd/m² at 4.8 V, respectively, and achieve a current efficiency of 35.0 cd/A, a power efficiency of 29.0 lm/W at a brightness of 1000 cd/m², and a low efficiency roll-off 8.7% calculated from the maximum efficiency value to that of 5000 cd/m².     

Synthesis and characterization of a new perylene bisimide (PBI) derivative and its application as electron acceptor for bulk heterojunction polymer solar cells

A symmetrical perylene bisimide derivative (PBI) with 2-(4-nitrophenyl)acrylonitrile groups at the 1,7 bay positions of perylene and solubilizing cyclohexyl units was synthesized and characterized. The absorption spectrum of PBI was broad with the most prominent peak at 655 nm and optical band gap of 1.72 eV. The electrochemical investigation indicates that PBI has a LUMO energy level of −3.9 eV which is similar to that of PCBM or PC₇₀BM. Bulk heterojunction solar cell fabricated using a blend of poly(3-hexylthiophene) (P3HT) and PBI (1:1 w/w) as active layer cast from THF exhibited power conversion efficiency (PCE) at 1.56%. However, the device with P3HT:PBI blend deposited from mixed solvent (DIO/THF) improved the PCE to 2.78% which further increased to 3.17% on using the thermal annealed active layer. The improvement in the PCE is attributed to the enhanced crystallinity of the blend (particularly P3HT) and increase in hole mobility leading to balanced charge transport.     

Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate films with low conductivity and low acidity through a treatment of their solutions with probe ultrasonication and their application as hole transport layer in polymer solar cells and perovskite solar cells

Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) has been widely used as the hole transport material in optoelectronic devices. To avoid the cross talk among different crossbars, PEDOT:PSS with low conductivity is required. It thus has a high loading of the non-conductive PSSH. The PSSH-to-PEDOT weight ratio is 6 for Clevios P VP Al 4083 that is the most popular polymer as the hole transport layer. However, the acidic PSSH brings severe problems to the device stability and performance. Here, PEDOT:PSS solutions with low acidity can be prepared through a facile treatment of PEDOT:PSS solution by probe ultrasonication. Two grades of PEDOT:PSS, Clevios PH1000 and Clevios P, with a PSSH-to-PEDOT weight ratio of 2.5 were treated by probe ultrasonication. The ultrasonication can lower the viscosity and the colloidal sizes of PEDOT:PSS solutions and conductivity of PEDOT:PSS films. The pH value of probe-ultrasonicated Clevios P was 2.12, higher than that (1.77) of pristine Clevios P VP Al 4083. The ultrasonication-treated PEDOT:PSS solutions were used as hole transport layer in polymer solar cells and perovskite solar cells. The photovoltaic performances of these solar cells are comparable to that of control devices employing Clevios P VP Al 4083 PEDOT:PSS as the hole transport layer.