Effective End Group Modification of Poly(3-hexylthiophene) with Functional Electron-Deficient Moieties for Performance Improvement in Polymer Solar Cell

A series of end-functionalized poly­(3-hexylthiophene)­s (P3HTs) were synthesized by end-capping with electron-deficient moieties (EDMs, oxadiazole (OXD) and triazole (TAZ)) to prevent the negative influence of bromine chain ends in the common uncapped P3HT in polymer solar cell (PSC) applications. On the basis of the electron-withdrawing capability of the planar OXD end groups, P3HT-end-OXD relative to the uncapped P3HT exhibits a raised absorption coefficient, extended exciton lifetime, and increased crystalline order in the blend with PCBM, leading to an effectual improvement in photovoltaic parameters. However, P3HT-end-TAZ has an opposite result even worse than that of the uncapped P3HT, arising from bulky TAZ end groups. As a consequence, P3HT-end-OXD gives a power conversion efficiency (PCE) of 4.24%, which is higher than those of the uncapped P3HT (3.28%) and P3HT-end-TAZ (0.50%). The result demonstrates that the EDM modification is a valuable method to tailor the structural defect of polymer chain ends. However, the efficacy is dependent on the structure of EDM

Electric-Field-Induced Excimer Formation at the Interface of Deep-Blue Emission Poly(9,9-dioctyl-2,7-fluorene) with Polyelectrolyte or Its Precursor as Electron-Injection Layer in Polymer Light-Emitting Diode and Its Prevention for Stable Emission and Higher Performance

Conjugated polyelectrolytes and their precursors as electron-injection layer (EIL) in polymer light-emitting diode have attracted extensive attention because they allow the use of environmentally stable high work function metals as cathode with efficient electron injection. Here, for the first time, we find that an undesirable green emission component (470–650 nm) in the electroluminescence spectra is observed during continuous operation of deep-blue emission β-phase poly­(9,9-dioctyl-2,7-fluorene) (β-PFO) device upon introducing polyelectrolyte poly­[9,9-bis­(6′-(18-crown-6)­methoxy)­hexyl fluorene] chelating to potassium ion (PFCn6:K⁺) as EIL. This phenomenon also happens to nonchelating PFCn6, poly­[(9,9-bis­(3′-(<i>N</i>,<i>N</i>-dimethylamino)­propyl)-2,7-fluorene)-<i>alt</i>-2,7-(9,9-dioctylfluorene)], or even nonemissive poly­[4-((18-crown-6)­methoxy)­methyl styrene] chelating to K⁺ (PSCn6:K⁺). It can be ascribed to electric-field induction accompanied by thermal motion of a highly polar side chain in the polyelectrolyte leading to local segmental alignment of PFO main chains at the emitting layer (EML)/EIL interface and thus formation of green emission excimer, which is supported by the following observations: appearance of green emission component using nonemissive PSCn6:K⁺ as EIL, absence of green emission component as the device is operated at low-temperature (78 K) at which molecular thermal motion are frozen, and absence of green emission upon introducing 2,2′,2″-(1,3,5-phenylbenzenetriyl)­tris­[1-phenyl-1<i>H</i>-benzimidazole] as buffer layer in between EML and EIL for the prevention of direct contact of EML with polyelectrolyte or its precursor EIL

Multiple Functionalities of Polyfluorene Grafted with Metal Ion-Intercalated Crown Ether as an Electron Transport Layer for Bulk-Heterojunction Polymer Solar Cells: Optical Interference, Hole Blocking, Interfacial Dipole, and Electron Conduction

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–C₆₀ bisadduct (ICBA) or indene–[6,6]-phenyl-C₆₁-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 × 10⁵ 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

Enhancing Shielding of Triplet Energy Transfer to Poly(<i>p</i>-phenylene)s from Phosphor Dopant by Addition of Branched Alcohol for Highly Efficient Electrophosphorescence

To obtain an efficient electrophosphorescent device, one needs to consider quenching of phosphor phosphorescence brought by the low triplet energy of the host because the exothermic energy transfer can effectively quench phosphor phosphorescence and markedly lower the device efficiency. Here, a facile approach of adding a branched alcohol (3-tert-butyl-2,2,4,4-tetramethylpentan-3-ol, ROH) into green emission phosphor-doped dialkoxyl-substituted poly(para-phenylene)s (PPPs) is demonstrated to effectively enhance shielding of triplet energy transfer to PPPs from the phosphor, resulting from a formation of self-assembly structure that block direct contact between phosphor and the main chains. The green electrophosphorescent device performance can be improved from 7.1 and 32.2 cd/A to 25.1 and 42 cd/A for PPP with dioctoxyl substituents (dC8OPPP) and with carbozole (Cz)-capped dialkoxyl-substituents (CzPPP), respectively. The latter result 42 cd/A is the highest record for green emission in polymer light emitting diode. This finding suggests that promotion of specific electro-optical properties for small molecule and polymer can be obtained through a self-assembling interaction in addition to chemical structure modification

Excimer Formation by Electric Field Induction and Side Chain Motion Assistance in Polyfluorenes

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 × 106 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

Effective Shielding of Triplet Energy Transfer to Conjugated Polymer by Its Dense Side Chains from Phosphor Dopant for Highly Efficient Electrophosphorescence

To examine the quenching of a triplet exciton by low triplet energy (ET) polymer hosts with different chain configurations for high ET 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 ET phosphor guest to a low ET 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 ET 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

Design of Deep Blue Electroluminescent Spiro-Polyfluorenes with High Efficiency by Facilitating the Injection of Charge Carriers through Incorporation of Multiple Charge Transport Moieties

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 (η_ext) of 7.28%, which is the highest value among the db polymer fluorescent diodes ever documented