Tzu-Hao Jen

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.

High-efficiency polymer light-emitting diodes based on poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] with plasma-polymerized CHF3-modified indium tin oxide as an anode

We demonstrate that introducing a thin CFx film formed by plasma polymerization of CHF3 on an indium tin oxide (ITO) anode surface for a polymer light-emitting diode with the structure, ITO∕CFx∕poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene](MEH-PPV)∕Ca∕Al, can lead to a high device performance (5.1cd∕A and 24000cd∕m2). The high device performance can be attributed to a better balance between hole and electron fluxes, resulting from a formation of interfacial dipole at the CFx∕MEH-PPV interface to provide a hole blocking effect and an enhancement of electron/hole recombination.

Solution processable self-doped polyaniline as hole transport layer for inverted polymer solar cells

Polymer solar cells (PSCs) are a promising alternative for low-cost renewable energy due to their solution-process, flexibility and large-area fabrication. To improve power conversion efficiency (PCE) of PSC, a hole transport layer (HTL) is usually inserted between anode and active layer to enhance hole carrier collection. In this contribution, the utilization of solution processable self-doped sulfonic acid ring-substituted polyaniline (SPAN) thin film as the HTL in inverted PSC (i-PSC) composed of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction is demonstrated. The i-PSC with SPAN as the HTL exhibits an improvement over that with PEDOT:PSS (from 3.30 ± 0.14% to 3.54 ± 0.11%) due to higher conductivity and better film quality of SPAN relative to poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS), suggesting that SPAN can be a promising alternative to PEDOT:PSS as the effective HTL for i-PSC.

Measurements of charge mobility and diffusion coefficient of conjugated electroluminescent polymers by time-of-flight method

Charge mobilities (μ) and diffusion coefficients (D) of hole (μh, Dh) and electron (μe, De) of the conjugated electroluminescent polymers, poly(phenylene vinylene)s and polyfluorenes, have been measured by fitting of a theoretical photocurrent transient equation to time-of-flight photocurrent transients. The μ so obtained are in agreement with those from inflection points of photocurrent transients. The D value lumps all factors together that cause the dispersion of carriers, and the parameter Dq/μkT can be used as an indicator of the degree of dispersion. This fitting method allows extracting μ and D from highly dispersive photocurrent transients, even for the case in which no inflection point appears.

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.

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.