Qingshuo Wei

Tailoring organic heterojunction interfaces in bilayer polymer photovoltaic devices

In an ideal model, a p-n junction is formed by two stacked slabs of semiconductors. Although the construction of actual devices is generally more complex, we show that such a simple method can in fact be applied to the formation of organic heterojunctions. Two films of the organic semiconductors poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) can be connected by a simple film-transfer method without disturbing their flat surfaces. Each film can further be modified with a surface-segregated monolayer to tune the strength and direction of the surface dipole moment. Using this method, we fabricated bilayer organic photovoltaic devices with interfacial dipole moments that were selected to align the energy levels at the heterojunction. The open-circuit voltages of the P3HT/PCBM devices could be tuned over a wide range between 0.3 and 0.95 V, indicating that, even if the same combination of bulk materials is used, the interfacial properties drastically alter the performance of organic photovoltaic devices.

Morphological Change and Mobility Enhancement in PEDOT:PSS by Adding Co-solvents

Adding ethylene glycol (EG) to a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) solution improves the crystallinity of the PEDOT and the ordering of the PEDOT nanocrystals in solid films. The carrier-mobility enhancement is confirmed by using ion-gel transistors combined with in situ UV-vis-NIR spectroscopy.

Independent Tuning of the Band Gap and Redox Potential of Graphene Quantum Dots

The band gap and redox potential of semiconductor nanocrystals are two quantities of primary importance for their applications in energy conversion devices. Herein, we report on covalent functionalization of colloidal graphene quantum dots through a solution-chemistry approach and studies of their band gaps and redox potentials. We show that their band gaps and redox potentials can be independently controlled, the former by size and the latter by functionalization. The size and the functionalization dependence of the properties can be numerically reproduced with tight-binding calculations, which thus provides a simple theoretical tool to guide the design of graphene QDs with desired properties.

Additive-driven assembly of block copolymer-nanoparticle hybrid materials for solution processable floating gate memory.

Floating gate memory devices were fabricated using well-ordered gold nanoparticle/block copolymer hybrid films as the charge trapping layers, SiO(2) as the dielectric layer, and poly(3-hexylthiophene) as the semiconductor layer. The charge trapping layer was prepared via self-assembly. The addition of Au nanoparticles that selectively hydrogen bond with pyridine in a poly(styrene-b-2-vinyl pyridine) block copolymer yields well-ordered hybrid materials at Au nanoparticle loadings up to 40 wt %. The characteristics of the memory window were tuned by simple control of the Au nanoparticle concentration. This approach enables the fabrication of well-ordered charge storage layers by solution processing, which is extendable for the fabrications of large area and high density devices via roll-to-roll processing.

Recent Progress on PEDOT-Based Thermoelectric Materials

The thermoelectric properties of poly(3,4-ethylenedioxythiophene) (PEDOT)-based materials have attracted attention recently because of their remarkable electrical conductivity, power factor, and figure of merit. In this review, we summarize recent efforts toward improving the thermoelectric properties of PEDOT-based materials. We also discuss thermoelectric measurement techniques and several unsolved problems with the PEDOT system such as the effect of water absorption from the air and the anisotropic thermoelectric properties. In the last part, we describe our work on improving the power output of thermoelectric modules by using PEDOT, and we outline the potential applications of polymer thermoelectric generators.

Bilayer Ambipolar Organic Thin-Film Transistors and Inverters Prepared by the Contact-Film-Transfer Method

Ambipolar organic thin-film transistors with a bilayer structure of poly(3-hexylthiophene) and [6,6]phenyl C(61) butyric acid methyl ester were fabricated using a simple solution-based, contact-film-transfer method. The transistors exhibited balanced electron and hole mobilities of 2.1 x 10(-2) and 1.1 x 10(-2) cm(2) V(-1) s(-1), respectively. Complementary inverters based on two identical ambipolar transistors showed good performance with a gain of 14.

Surface-Segregated Monolayers: A New Type of Ordered Monolayer for Surface Modification of Organic Semiconductors

We report a new type of ordered monolayer for the surface modification of organic semiconductors. Fullerene derivatives with fluorocarbon chains ([6,6]-phenyl-C(61)-buryric acid 1H,1H-perfluoro-1-alkyl ester or FC(n)) spontaneously segregated as a monolayer on the surface of a [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) film during a spin-coating process from the mixture solutions, as confirmed by X-ray photoelectron spectroscopy (XPS). Ultraviolet photoelectron spectroscopy (UPS) showed the shift of ionization potentials (IPs) depending on the fluorocarbon chain length, indicating the formation of surface dipole moments. Surface-sensitive vibrational spectroscopy, sum frequency generation (SFG) revealed the ordered molecular orientations of the C(60) moiety in the surface FC(n) layers. The intensity of the SFG signals from FC(n) on the surface showed a clear odd-even effect when the length of the fluorocarbon chain was changed. This new concept of the surface-segregated monolayer provides a facile and versatile approach to modifying the surface of organic semiconductors and is applicable to various organic optoelectronic devices.

Polymer thermoelectric modules screen-printed on paper

We report organic thermoelectric modules screen-printed on paper by using conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and silver paste. Our large-area devices provided sufficient power to illuminate light-emitting diodes. This is the first example of thermoelectric modules containing conducting polymers being used to power practical devices. The stability of this proof-of-concept module was tested at 100 °C for over 100 h without any encapsulation. We showed that the decrease in device performance was caused not by the deterioration of the materials but by degradation of the interface between the conducting polymers and silver paste. These results suggest that organic thermoelectric modules could be used to harvest heat energy at low temperature, although the stability of the interface must be improved.

Thermoelectric power enhancement of PEDOT:PSS in high-humidity conditions

We report an increase in the thermoelectric power factor of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) from 23 ± 5 to 225 ± 130 µW/(m·K2) in high-humidity conditions. This enhancement was caused mainly by an increase in the apparent Seebeck coefficient, which could be related to morphological change after water absorption or electrochemical reaction of PEDOT in air. Our results demonstrate a positive effect of water in the PEDOT:PSS system and indicate the need for well-controlled measurement conditions, particularly humidity, in evaluating the performance of conducting organic materials.

Enhanced Charge Transport in Polymer Thin-Film Transistors Prepared by Contact Film Transfer Method

The charge-carrier transport in the structures formed at the surface of various conjugated polymer films is investigated by constructing organic thin-film transistors using a novel and simple contact film transfer method. Thin-film transistors prepared by this transfer process have higher field-effect mobility values compared with conventional spin-coated devices for all the studied polymers. In contrast to previous reports, the hole mobility in regioregular poly(3-alkylthiophene)s does not depend on the length of the alkyl chain when the contact film transfer method is used. These results suggest that the thiophene rings adopt a highly ordered edge-on orientation and strong interchain pi-pi interactions spontaneously form at the polymer/air interface during the spin coating.