Chun-Chih Ho

Isoindigo-based copolymers for polymer solar cells with efficiency over 7%

A series of isoindigo-based low-band-gap copolymers (PnTI) containing an extended thiophene unit in the donor segment of the polymer were synthesized. The results show that the extended thiophene unit with centrosymmetric conformation simultaneously broadens the polymer absorption and enhances the crystallinity and, thus, hole mobility. Consequently, with additional improved solubility, the polymer P6TI exhibits the highest PCE of 7.25% (and a high Jsc of 16.24 mA cm−2) among isoindigo-based low-band-gap copolymers. This work demonstrates that by simply adjusting the donor segment and with relatively simple synthetic schemes, a material for high-performance and scalable PSCs will become available.

Enhancing the efficiency of low bandgap conducting polymer bulk heterojunction solar cells using P3HT as a morphology control agent

The development of low bandgap conducting polymers has made bulk heterojunction solar cells a viable low cost renewable energy source. The high boiling point of 1,8-diiodooctane (DIO) is usually used to control the morphology of the active layer consisting of a conducting polymer and PCBM, so that a high power conversion solar cell can be achieved. We report here an alternative approach using nonvolatile, crystalline and conducting P3HT as an effective morphology control agent. A model system of PCPDTBT/PC61BM was selected for this study. The change of optoelectronic properties with the introduction of P3HT was monitored by measuring the absorption spectra and charge carrier mobility, and the morphology change with the introduction of P3HT in the active layer was monitored by AFM, TEM, and GIXRD. The results indicate that favorable bi-continuous phase separation and appropriate domain size of each phase can be achieved to facilitate fast charge transport, and thus improve the power conversion efficiency of the solar cell. By adding 1 wt% P3HT into the blend of PCPDTBT/PC61BM, the power conversion efficiency can be improved by 20%. Moreover, with the incorporation of 1 wt% P3HT to the blend of PCPDTBT/PC61BM with DIO, the power conversion efficiency can be further increased by 17%. The strategy of this study can be expanded to other low bandgap conducting polymers for high efficiency bulk heterojunction solar cells.

Correlating interface heterostructure, charge recombination, and device efficiency of poly(3-hexyl thiophene)/TiO2 nanorod solar cell.

The charge recombination rate in poly(3-hexyl thiophene)/TiO(2) nanorod solar cells is demonstrated to correlate to the morphology of the bulk heterojunction (BHJ) and the interfacial properties between poly(3-hexyl thiophene) (P3HT) and TiO(2). The recombination resistance is obtained in P3HT/TiO(2) nanorod devices by impedance spectroscopy. Surface morphology and phase separation of the bulk heterojunction are characterized by atomic force microscopy (AFM). The surface charge of bulk heterojunction is investigated by Kelvin probe force microscopy (KPFM). Lower charge recombination rate and lifetime have been observed for the charge carriers in appropriate heterostructures of hybrid P3HT/TiO(2) nanorod processed via high boiling point solvent and made of high molecular weight P3HT. Additionally, through surface modification on TiO(2) nan,orod, decreased recombination rate and longer charge carrier lifetime are obtained owing to creation of a barrier between the donor phases (P3HT) and the acceptor phases (TiO(2)). The effect of the film morphology of hybrid and interfacial properties on charge carrier recombination finally leads to different outcome of photovoltaic I-V characteristics. The BHJ fabricated from dye-modified TiO(2) blended with P3HT exhibits 2.6 times increase in power conversion efficiency due to the decrease of recombination rate by almost 2 orders of magnitude as compared with the BHJ made with unmodified TiO(2). In addition, the interface heterostructure, charge lifetime, and device efficiency of P3HT/TiO(2) nanorod solar cells are correlated.

Molecular Structure Effect of Pyridine-Based Surface Ligand on the Performance of P3HT:TiO2 Hybrid Solar Cell

Colloid TiO(2) nanorods are used for solution-processable poly(3-hexyl thiophene): TiO(2) hybrid solar cell. The nanorods were covered by insulating ligand of oleic acid (OA) after sol-gel synthesis. Three more conducting pyridine type ligands: pyridine, 2,6-lutidine (Lut) and 4-tert-butylpyridine (tBP) were investigated respectively to replace OA. The power conversion efficiency (PCE) of the solar cell was increased because the electronic mobility of pyridine-type ligand-modified TiO(2) is higher than that of TiO(2)-OA. The enhancement of PCE is in the descending order of Lut > pyridine > tBP because of the effective replacement of OA by Lut. The PCE of solar cell can be further enhanced by ligand exchange of pyridine type ligand with conjugating molecule of 2-cyano-3-(5-(7-(thiophen-2-yl)-benzothiadiazol-4-yl) thiophen-2-yl) acrylic acid (W4) on TiO(2) nanorods because W4 has aligned bandgap with P3HT and TiO(2) to facilitate charge separation and transport. The electronic mobility of two-stage ligand exchanged TiO(2) is improved furthermore except Lut, because it adheres well and difficult to be replaced by W4. The amount of W4 on TiO(2)-tBP is 3 times more than that of TiO(2)-Lut (0.20 mol % vs. 0.06 mol %). Thus, the increased extent of PCE of solar cell is in the decreasing order of tBP > pyridine > Lut. The TiO(2)-tBP-W4 device has the best performance with 1.4 and 2.6 times more than TiO(2)-pyridine-W4 and TiO(2)-Lut-W4 devices, respectively. The pKa of the pyridine derivatives plays the major role to determine the ease of ligand exchange on TiO(2) which is the key factor mandating the PCE of P3HT:TiO(2) hybrid solar cell. The results of this study provide new insights of the significance of acid-base reaction on the TiO(2) surface for TiO(2)-based solar cells. The obtained knowledge can be extended to other hybrid solar cell systems.

Synthesis, characterization and photovoltaic properties of poly(cyclopentadithiophene-alt-isoindigo)

Isoindigo based conducting polymers have attracted extensive interest for polymer solar cell application since isoindigo is a green material and renewable from plants. We have synthesized four soluble low band gap isoindigo based polymers (PCI) with cyclopentadithiophene (CPDT) as the donor unit and isoindigo (I) as the acceptor unit, decorated with two kinds of alkyl side chains, octyl (8) and 2-ethylhexyl (e), via the Stille cross-coupling reaction denoted as PC8I8, PC8Ie, PCeI8 and PCeIe. By changing the side chain of copolymers from linear (PC8I8) to branched (PCeIe), the λmax of absorption is blue shifted from 1.37 to 1.48 eV and the HOMO level is lowered from −5.24 to −5.45 eV. The changes are due to the twist coplanarity of the polymer backbone. The density functional theory calculation revealed that the dihedral angle of copolymers has been increased from 14° to 20°. The properties of PC8Ie and PCeI8 lie between those of PC8I8 and PCeIe. The type of the side chain plays a major role in determining the photovoltaic performance of copolymers. The branched side chain improves the solubility of the polymer and increases the effective phase separation between the copolymer and PCBM. This results in favorable nanomorphology of the active layer. Thus, PCeIe with branched side chains on both donor and acceptor units exhibits the best photovoltaic properties with a Voc of 0.80 eV, Jsc of 11.6 mA cm−2 and fill factor of 43.0% and power conversion efficiency of 4.0%. The power conversion efficiency of this type of polymer could be further improved by optimizing the fabrication conditions and interlayer modification. This study offers a useful guideline for the molecular design of high efficiency isoindigo-based polymer solar cells.

Effect of rod–rod interaction on self-assembly behavior of ABC π-conjugated rod–coil–coil triblock copolymers

A new class of ABC π-conjugated rod–coil–coil triblock copolymers of poly(diethylhexyloxy-p-phenylene vinylene)-b-poly (2-vinyl pyridine)-b-polystyrene (PPV-PVP-PS) was synthesized and its self-assembly behavior was explored. Three different triblock copolymers of PPV-PVP-PS1, PPV-PVP-PS2, and PPV-PVP-PS3, each with PPV, PS, and PVP, respectively, as the major species in the copolymers, were used to study the effects of copolymer composition and rod–rod interaction between PPV blocks on their morphology. Transmission electron microscopy (TEM), polarizing optical microscopy (POM), and simultaneously measured small-angle (SAXS) and wide-angle (WAXS) X-ray scattering experiments as a function of different annealing conditions revealed the details of the copolymer morphology, molecular packing, and their phase transitions. Despite their large differences in the rod volume fraction, fPPV, from 0.43 to 0.18, all three triblock copolymers adopted a self-assembled lamellar structure, in sharp constrast with the observation of many non-lamellar structures typically exhibited by ABC coil–coil–coil triblock copolymers with similar segregation strength. For PPV-PVP-PS1 with its major species PPV rod coupled with a single-phase symmetric PVP-PS diblock precursor, PPV-PVP-PS1 self-organized to form a triple-lamellar phase with each domain corresponding to the three respective blocks. Investigation of the molecular packing of PPV rods within their domain through the analysis of the 1D electron density profile suggests the PPV rods adopted a smectic C monolayer organization below its order–disorder transition temperature (TODT). For PPV-PVP-PS2 with its PS-rich asymmetric PVP-PS diblock precursor that displayed a disordered micelle structure, PPV-PVP-PS2 with fPPV of only 0.19 still exhibited a triple-lamellar phase with PPV forming a broken lamellar layer, thus preventing the excessive chain stretching of the coil blocks on the otherwise long-range ordered PPV lamellar phase. A similar broken triple-lamellar phase can also be observed for the PVP-rich PPV-PVP-PS3 with a low fPPV of only 0.18. Simultaneous SAXS and WAXS measurements show that all three triblock copolymers undergo the ordered lamella-to-disorder transition and the smectic/isotropic transition at the same temperature, indicating that the rod–rod interaction between PPV rods plays a critical role in forming and stabilizing these lamellar structures. The observation of the phase transformations is in good agreement with a recent mean-field prediction of a rod–coil–coil triblock copolymer system.

Cylinder-to-gyroid phase transition in a rod–coil diblock copolymer

In this communication, the morphology of well-defined P3DDT-b-PMMA with a 65.2% PMMA coil volume fraction is revealed as a hexagonally packed cylinder structure by X-ray scattering experiments after thermal annealing. Upon heating, an order-to-order transition (OOT) between cylindrical and gyroidal structures is observed at temperatures above 170 °C. The evolution of the cylinder to the gyroid occurs while the crystalline structure of the P3DDT block disappears, suggesting the conformation of the P3DDT-b-PMMA at high temperatures is similar to coil–coil block copolymers. The phenomena reported here can provide a different viewpoint of the self-assembly behaviors of poly(3-alkylthiophene)-containing rod–coil block copolymers. The novel rare gyroid structure of this rod–coil copolymer is useful to fabricate long sought of bicontinuous structure for highly efficient polymer solar cells.

High refractive index transparent nanocomposites prepared by in situ polymerization

High refractive index transparent nanocomposites have been developed by in situ polymerization of a precursor that contains functional monomers and surface modified anatase TiO2 nanoparticles for optoelectronic applications. The monomers are in the liquid form, so environmentally friendly solventless precursors can be prepared. The precursor can be processed into various shapes or thick films (>50 microns) of the nanocomposite. The relationships of the chemical structure of the organic matrix, nanoparticle content and dispersity with the refractive index, transparency, mechanical and thermal properties are systematically investigated. The refractive index, and mechanical and thermal properties of the nanocomposite are increased with increasing TiO2 content and aromatic structure in the organic matrix due to their rigid characteristics. The transparency of the nanocomposite is increased with increasing TiO2 content and dispersity. At the same loading of nanoparticles, the higher dispersity and the better transparency are due to the less extent of Rayleigh scattering. At 18 vol% (60 wt%) of TiO2, the acetic acid modified TiO2/poly(4-vinyl benzyl alcohol) nanocomposite has a refractive index of 1.73 and excellent transparency (>85% from 500 nm to 800 nm). The refractive index of the nanocomposite can be further increased to 1.77 by replacing aliphatic acetic acid modified TiO2 with aromatic phenyl acetic acid modified TiO2. The results of this work provide new knowledge and a new pathway to design a polymer based high refractive index material.

Kinetically Enhanced Approach for Rapid and Tunable Self-Assembly of Rod-Coil Block Copolymers.

A facile approach is reported to process rod-coil block copolymers (BCPs) into highly ordered nanostructures in a rapid, low-energy process. By introducing a selective plasticizer into the rod-coil BCPs during annealing, both the annealing temperature and time to achieve thermodynamic equilibrium and highly ordered structures can be decreased. This process improvement is attributed to enhanced chain mobility, reduced rod-rod interaction, and decreased rod-coil interaction from the additive. The novel method is based on kinetically facilitating thermodynamic equilibrium. The process requires no modification of polymer structure, indicating that a wide variety of desired polymer functionalities can be designed into BCPs for specific applications.

Efficient spin-light emitting diodes based on InGaN/GaN quantum disks at room temperature: a new self-polarized paradigm.

A well-behaved spin-light emitting diode (LED) composed of InGaN/GaN multiple quantum disks (MQDs), ferromagnetic contact, and Fe3O4 nanoparticles has been designed, fabricated, and characterized. The degree of circular polarization of electroluminescence (EL) can reach up to a high value of 10.9% at room temperature in a low magnetic field of 0.35 T, which overcomes a very low degree of spin polarization in nitride semiconductors due to the weak spin-orbit interaction. Several underlying mechanisms play significant roles simultaneously in this newly designed device for the achievement of such a high performance. Most of all, the vacancy between nanodisks can be filled by half-metal nanoparticles with suitable energy band alignment, which enables selective transfer of spin polarized electrons and holes and leads to the enhanced output spin polarization of LED. Unlike previously reported mechanisms, this new process leads to a weak dependence of spin relaxation on temperature. Additionally, the internal strain in planar InGaN/GaN multiple quantum wells can be relaxed in the nanodisk formation process, which leads to the disappearance of Rashba Hamiltonian and enhances the spin relaxation time. Our approach therefore opens up a new route for the further research and development of semiconductor spintronics.