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-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.

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 (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.

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 ß-phase poly(9,9-dioctyl-2,7-fluorene) (ß-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'-( N, N-dimethylamino)propyl)-2,7-fluorene)- alt-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 H-benzimidazole] as buffer layer in between EML and EIL for the prevention of direct contact of EML with polyelectrolyte or its precursor EIL.

Singlet Exciton Fraction in Electroluminescence from Conjugated Polymer.

The efficiency of electrofluorescent polymer light-emitting diodes is determined by singlet exciton fraction (χS) formation and its value still remains controversial. In this work, χS in spiropolyfluorene (SPF) is determined by analyzing transient emission of phosphor-dopant probe. The χS is found to range from 50% to 76%, depending on applied voltage. Higher applied voltage gives larger χS. Besides, more rapid increment in χS with applied voltage is observed in the higher-molecular-weight polymer. The voltage or molecular weight dependence of χS suggests the probability of singlet exciton (SE) generation through triplet-triplet annihilation (TTA) is enhanced due to higher triplet exciton (TE) concentration at higher applied voltage or accommodation of more TEs in a polymer chain with high molecular weight, thereby increasing probability of TTA. At lower applied voltage, χS is contributed by charge recombination. Its value (χS ~50%) higher than the statistical limit 25% is in agreement with efficient interconversion between triplet and singlet polaron pairs (PP) and with larger formation rate of SE relative to that of TE.

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.

Traps in regioregular poly(3-hexylthiophene) and its blend with [6,6]-phenyl-C61-butyric acid methyl ester for polymer solar cells.

Thermally stimulated current (TSC) technique is used to characterize traps in the regioregular poly(3-hexylthiophene) (rr-P3HT) and its blend with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). A hole trap in bulk rr-P3HT and an interfacial hole trap located at indium tin oxide (ITO)/rr-P3HT interface are revealed from the TSC measurement. Besides, molecular oxygen (O2) can form a deep electron trap with an onset of detrapping temperature at 225 K in rr-P3HT, in which O2 is located at the main chain region and the detrapping process is induced by chain motions under elevated temperature. In the blend of rr-P3HT:PCBM (1:1 w/w), additional hole trap states are generated in this blend system as compared to those of pure rr-P3HT and PCBM; however, these hole trap states can be reduced by thermal or solvent annealing approaches. Similar to rr-P3HT, a deep electron trap with an onset of detrapping temperature at 250 K can be formed in the blend after O2 exposure. In the case of low PCBM content in the blend (rr-P3HT:PCBM weight ratio of 4:1), an additional electron trap is generated.

Enhancing shielding of triplet energy transfer to poly(p-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 (dC(8)OPPP) 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.