We present a novel electron transport (ET) polymer composed of polyfluorene grafted with a K(+)-intercalated crown ether involving six oxygen atoms (PFCn6:K(+)) for bulk-heterojunction polymer solar cells (PSCs) with regioregular poly(3-hexylthiophene) (P3HT) as the donor and indene-C(60) bisadduct (ICBA) or indene-[6,6]-phenyl-C(61)-butyric acid methyl ester (IPCBM) as the acceptor in the active layer and with Al or Ca/Al as the cathode. A remarkable improvement in the power conversion efficiency (PCE) (measured in air) was observed upon insertion of this ET layer, which increased the PCE from 5.78 to 7.5% for a PSC with ICBA and Ca/Al (5.53 to 6.63% with IPCBM) and from 3.87 to 6.88% for a PSC with ICBA and Al (3.06 to 6.21% with IPCBM). This ET layer provides multiple functionalities: (1) it generates an optical interference effect for redistribution of light intensity as an optical spacer; (2) it blocks electron-hole recombination at the interface with the cathode; (3) it forms an interfacial dipole that promotes the vacuum level of the cathode metal; and (4) it enhances electron conduction, as evidenced by (1) the increase in total absorption of 1:1 w/w P3HT:ICBA by a factor of 1.3; (2) the reduction in the hole-only current density profile by a factor of 3.3 at 2.0 × 10(5) V/cm; (3) the decrease of 0.81 eV in the work function of Al from 4.28 to 3.47 eV, as determined by UV photoelectron spectroscopy; and (4) the decrease in the series resistance of PSCs with ICBA and Al by a factor of 4.5, as determined by the current-voltage characteristic under dark conditions; respectively. The PSC of 7.5% is the highest among the reported values for PSC systems with the simplest donor polymer, P3HT.
For poly (9,9-di(6-(2-(3-oxetanyl)butoxyl)hexyl)-2,7-fluorene) (POBOHF), measurements on field induction-thermally stimulated current (FI-TSC) and electroluminescence (EL) under a wide temperature range demonstrate that electric field induction (FI) accompanied by side chain motion can lead to a formation of excimers, which contribute to a growth of a green component in the EL spectrum. This phenomenon also happens to poly(9,9-di-n-octyl-2,7-fluorene) (PFO), especially under long-term operations with higher electric fields (1 × 10 6 V/cm), copolymers of OBOHF and FO (PF-1/1 and PF-1/3), and even cross-linked POBOHF. The higher polarity of the side chain in the polyfluorenes (PFs) can cause a more pronounced FI effect. For POBOHF, the green component can even dominate after a few cycles of device operation. Lowering the content of cross-linkable commoner in the copolymers from 50 to 25 mol % only moderately suppresses the formation of FI excimers.
To examine the quenching of a triplet exciton by low triplet energy (E(T)) polymer hosts with different chain configurations for high E(T) phosphor guests, the quenching rate constant measurements were carried out and analyzed by the standard Stern-Volmer equation. We found that an effective shielding of triplet energy transfer from a high E(T) phosphor guest to a low E(T) polymer host is possible upon introducing dense side chains to the polymer to block direct contact from the guest such that the possibility of Dexter energy transfer between them is reduced to a minimum. Together with energy level matching to allow charge trapping on the guest, high device efficiency can be achieved. The extent of shielding for the systems of phenylene-based conjugated structures from iridium complexes follows the sequence di-substituted (octoxyl chain) in the para position (dC8OPPP) is greater than monosubstituted (mC8OPPP) and the PPPs with longer side chains are much higher than a phenylene tetramer (P4) with two short methyl groups. Further, capping the dialkoxyl-susbstituents with a carbazole (Cz) moiety (CzPPP) provides enhanced extent of shielding. Excellent device efficiency of 30 cd/A (8.25%) for a green electrophosphorescent device can be achieved with CzPPP as a host, which is higher than that of dC8OPPP as host (15 cd/A). The efficiency is higher than those of high E(T) conjugated polymers, poly(3,6-carbazole) derivatives, as hosts (23 cd/A). This observation suggests a new route for molecular design of electroluminescent polymers as a host for a phosphorescent dopant.
The development of polymer light-emitting diodes (PLEDs) has been of long-standing interest for their simple fabrication process and potential use in large-area displays. In spite of rapid progress in PLED technology after its discovery, [1] there is still an important open question about how to design high performance light-emitting conjugated polymers (LEPs) as a blue light source for full-color display applications. The major difficulties for most blue LEPs are imbalance in charge carrier fluxes because of a high barrier for hole injection and discrepancy in charge carrier mobilities. Currently, two strategies are adopted to overcome such difficulties, namely, chemical structure tuning and device structure design. By the former strategy, the incorporation of charge transport moieties on a main chain, [2] on a side chain with a flexible spacer, [3][4][5] and at the chain ends [6,7] have been studied for polyfluorene to promote hole injection and thus balanced bipolar fluxes, by invariably using triphenylamine (TPA) or carbazole (Cz) derivatives. The latter strategy also includes the introduction of additional hole transporting layers [8][9][10] and a self-assembling monolayer.[11] However, none of the above approaches is adequate to diminish the hole injection barrier (D h ) and, therefore, there remains room for improvement for the performance of a blue emitting device, with the best external quantum efficiency h ext of less than 5% reported so far. To diminish the hole injection barrier, the modification of an anode with multiple deposition layers to create stepped and graded highest occupied molecular orbital (HOMO) levels has been proposed, which includes utilization of protonic aciddoped conjugated polymers with various doping levels (which achieved h ext of 6% for a green emitting device), [12] crosslinkable hole transporting layers (h max 2.7 cd A À1 for a blue emitting device), [13] and self assembling multilayers (h ext 0.65% for a green OLED), [14] although the procedures to prepare such hole injection layers are complicated. No report so far, however, has attempted to apply such a concept in molecular design, i.e., the creation of stepped and graded HOMO levels in a single conjugated polymer.Here, for the first time, we integrate the ideas of chemical structure tuning by incorporating a hole transport moiety [2][3][4][5][6][7] and establishing gradient HOMO levels in the hole injection layer to design single polymers with a graded electronic profile for hole injection by grafting polyspirofluorene (poly-spiro) with dual hole-transporting moieties. The levels of ionization potential (I p ) of these moieties are located sequentially in between the work function of the anode and the HOMO level of the conjugated polymer (poly-spiro) main chain. By finely tuning the logical spatial sequence of these transport moieties to establish an efficient graded HOMO route for hole injection, a dramatically improved hole injection near the interface can be realized between the anode and LEPs by a factor of 10 3 -10 4...
For polymer light-emitting diodes, developing highly efficient, stable, and saturated blue emitting polymer is essential in display and lighting applications and has long been a challenge. Here we report a concept for designing highly efficient electroluminescent polymers by introducing multiple charge transport moieties for efficient injection of charge carriers into spiro-polyfluorene (sPF). We integrate the triphenylamine (TPA) and carbazole (Cz) in the same side chain of sPF with logical spatial and energetic sequence of these moieties to establish graded route for more efficient hole injection and incorporate the electron transport moiety with strong electron-withdrawing capability, triazole (TAZ), on both chain ends to give favorable arrangement in space and energy for electron injection. These two factors allow the corresponding single layer device to exhibit deep blue (db) emission with external quantum efficiency (ηext) of 7.28%, which is the highest value among the db polymer fluorescent diodes ever documented.
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