Felix Sunjoo Kim

Highly Efficient Solar Cells Based on Poly(3-butylthiophene) Nanowires

Poly(3-butylthiophene) (P3BT) nanowires, prepared by solution-phase self-assembly, have been used to construct highly efficient P3BT/fullerene nanocomposite solar cells. The fullerene/P3BT nanocomposite films showed an electrically bicontinuous nanoscale morphology with average field-effect hole mobilities as high as 8.0 x 10(-3) cm2/Vs due to the interconnected P3BT nanowire network revealed by TEM and AFM imaging. The power conversion efficiency of fullerene/P3BT nanowire devices was 3.0% (at 100 mW/cm2, AM1.5) in air and found to be identical with our similarly tested fullerene/poly(3-hexylthiophene) photovoltaic cells. This discovery expands the scope of promising materials and architectures for efficient bulk heterojunction solar cells.

High‐mobility Ambipolar Transistors and High‐gain Inverters from a Donor–Acceptor Copolymer Semiconductor

Ambipolar organic field-effect transistors (OFETs), which are capable of both p- and n-channel operations, are gaining attention as an alternative approach to mimicking complementary metal-oxide semiconductor (CMOS) digital integrated circuits for achieving high-performance and cost-effective circuits in organic electronics. [1‐13] Low power dissipation and high performance are some of the major advantages of CMOS technology over non-complementary ones. [14] Power consumption is minimized in CMOS circuits because the component transistors are selectively turned on only when the circuit is switching, otherwise they are off at the steady state. The better performance of a CMOS circuit in terms of sharp switching and high noise immunity arises because every elemental transistor actively contributes to the function of the circuit. [14] Most efforts towards CMOS-like circuits in organic electronics have focused on utilizing distinct p- and n-type semiconductors. [1,15] However, the necessity of lateral patterning of semiconductors in CMOS circuits makes device fabrication on a common substrate a very complex process. Ambipolar OFETs represent an approach to high-performance CMOS-like circuits that minimize patterning and complex fabrication processes. [1] Ambipolar transistors are also of interest in fundamental studies of charge transport in organic semiconductors [1,6,16] as well as the development of efficient light-emitting transistors. [8,17‐21]

Phthalimide-Based Polymers for High Performance Organic Thin-Film Transistors

The synthesis and characterization of two new thiophene copolymers with backbone phthalimide units is reported. Thin-film optical and wide-angle X-ray diffraction measurements indicate extended electronic conjugation and close intermolecular pi-stacking for both polymers. Ambient carrier mobility of thin-film transistors prepared from these polymers is as high as 0.28 cm(2)/(V s) with an on/off ratio greater than 10(5).

Polymer Nanowire/Fullerene Bulk Heterojunction Solar Cells: How Nanostructure Determines Photovoltaic Properties

We report studies of bulk heterojunction solar cells composed of self-assembled poly(3-butylthiophene) nanowires (P3BT-nw) as the donor component with a fullerene acceptor. We show that the nanostructure of these devices is the single most important variable determining their performance, and we use a combination of solvent and thermal annealing to control it. A combination of conductive and photoconductive atomic force microscopy provides direct connections between local nanostructure and overall device performance. Films with a dense random web of nanowires cause the fullerene to aggregate in the interstices, giving a quasi-ordered interpenetrating heterojunction with high short-circuit current density (10.58 mA/cm(2)), but relatively low open circuit voltage (520 mV). Films with a low density of nanowires result in a random bulk heterojunction composed of small crystalline PCBM and P3BT phases. Fewer nanowires result in higher open circuit voltage (650 mV) but lower current density (6.02 mA/cm(2)). An average power conversion efficiency of 3.35% was achieved in a structure which balances these factors, with intermediate nanowire density. The best photovoltaic performance would be realized in a material structure which maintains the interpenetrating network of nanowires and fullerene phases (high current density), but avoids the device bridging we observe, and the recombination and shunt losses associated with it (high open-circuit voltage).

High-Mobility n-Type Conjugated Polymers Based on Electron-Deficient Tetraazabenzodifluoranthene Diimide for Organic Electronics

High-mobility p-type and ambipolar conjugated polymers have been widely reported. However, high-mobility n-type conjugated polymers are still rare. Herein we present poly(tetraazabenzodifluoranthene diimide)s, PBFI-T and PBFI-BT, which exhibit a novel two-dimensional (2D) π-conjugation along the main chain and in the lateral direction, leading to high-mobility unipolar n-channel transport in field-effect transistors. The n-type polymers exhibit electron mobilities of up to 0.30 cm(2)/(V s), which is among the highest values for unipolar n-type conjugated polymers. Complementary inverters incorporating n-channel PBFI-T transistors produced nearly perfect switching characteristics with a high gain of 107.

Enhanced thermoelectric properties of PEDOT:PSS nanofilms by a chemical dedoping process

We report that a simple chemical dedoping treatment of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) nanofilms enhances the thermoelectric properties of the polymer nanofilms. The dedoping process was done by over-coating a mixture of dimethyl sulfoxide (DMSO) and hydrazine (HZ), a strong chemical reducing agent, onto the PEDOT:PSS nanofilms. This additional step led to the removal of excess PSS chains and the formation of neutral states of PEDOT chains, resulting in an improvement in the Seebeck coefficient, from 30 μV K−1 to 142 μV K−1, and a decrease in the electrical conductivity from 726 S cm−1 to 2 S cm−1. By controlling the concentration of HZ, we obtained an optimized power factor of 112 μW m−1 K−2 at 0.0175 wt% of HZ in DMSO at room temperature. The corresponding electrical conductivity and Seebeck coefficient under optimized conditions were 578 S cm−1 and 67 μV K−1, respectively. We expect that this simple dedoping process can be applied to general thermoelectric nanofilms based on chemically doped polymers in order to enhance the power factor.

Efficient solar cells based on a new phthalimide-based donor–acceptor copolymer semiconductor: morphology, charge-transport, and photovoltaic properties

Bulk heterojunction solar cells based on blends of the new low band gap donor–acceptor copolymer, poly(N-(dodecyl)-3,6-bis(4-dodecyloxythiophen-2-yl)phthalimide) (PhBT12), and fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) were systematically investigated. The PhBT12/fullerene blend films were found to exhibit a crystalline nanoscale morphology with space-charge-limited mobility of holes as high as 4.0 × 10−4 cm2/Vs without thermal annealing, leading to moderately efficient devices. The performance of the solar cells varied significantly with PhBT12/fullerene composition, reaching a power conversion efficiency of 2.0% with a current density of 6.43 mA/cm2 and a fill factor of 0.55 for the 1:1 PhBT12/PC71BM blend devices. However, thermally annealed (120 °C) PhBT12/fullerene blend devices had negligible photovoltaic properties due to micrometer scale phase separation of the blends which is attributed to the long side chains. We expect that better photovoltaic performance can be achieved by modifying the polymer side chain length and the device processing as well. These results show that phthalimide-based donor–acceptor copolymer semiconductors, exemplified by PhBT12, are promising low band gap materials for developing efficient bulk heterojunction solar cells.

Effects of one- and two-dimensional carbon hybridization of PEDOT: PSS on the power factor of polymer thermoelectric energy conversion devices

We investigated the thermoelectric properties of polymer composites based on a conducting polymer and carbon materials with various dimensionalities. PEDOT:PSS as a conducting polymer matrix was successfully hybridized with graphene sheets and multi-walled carbon nanotubes through in situ polymerization of 3,4-ethlyenedioxythiophene monomers in an aqueous solution in the presence of the carbon materials dispersed by using a polymeric dispersant. The hybrid structures of PEDOT:PSS, graphene, and carbon nanotubes in the composite showed an electrical conductivity, Seebeck coefficient, and power factor of 689 S cm−1, 23.2 μV K−1, and 37.08 μW mK−2, respectively, values that are much higher than those of pristine PEDOT:PSS, PEDOT:PSS/graphene, or PEDOT:PSS/carbon-nanotube composites. The thermoelectric figure of merit increased from 0.017 in the pristine PEDOT:PSS to 0.031 in the composite, corresponding to an 80% enhancement. We believe that the enhanced thermoelectric performance comes from the synergic effects of multi-component systems with excellent electrical bridging and electronic coupling between PEDOT and carbon materials.

n-Channel polymer thin film transistors with long-term air-stability and durability and their use in complementary inverters

We report an investigation of the stability and durability of p- and n-channel polymer thin film transistors in air over a 4-year period. All-polymer p/n complementary inverters fabricated from an n-channel poly(benzobisimidazobenzophenanthroline) (BBL) transistor and a p-channel poly(3-hexylthiophene) (P3HT) transistor showed excellent switching characteristics and a large voltage gain. The electrical parameters (electron mobility, on/off current ratio, and threshold voltage) of the n-channel BBL transistors in air were found to be constant over the 4 years. The performance of the p-channel P3HT transistors deteriorated dramatically after only 2 weeks in air. The excellent stability/durability of the BBL transistors in air is explained by the closely-packed crystalline morphology which creates a kinetic barrier against diffusion of extrinsic molecules and its high electron affinity that provides energetic stability against chemical/electrochemical reactions. The results demonstrate the longest air-stability and durability of non-encapsulated organic electronic devices to date while offering insights for the design of more environmentally rugged organic semiconductors.

Air-stable ambipolar field-effect transistors and complementary logic circuits from solution-processed n/p polymer heterojunctions.

We demonstrate the use of n/p polymer/polymer heterojunctions deposited by sequential solution processing to fabricate ambipolar field-effect transistors and complementary logic circuits. Electron and hole mobilities in the transistors were ∼0.001-0.01 cm(2)/(V s) in air without encapsulation. Complementary circuits integrating multiple ambipolar transistors into NOT, NAND, and NOR gates were fabricated and shown to exhibit sharp signal switching with a high voltage gain.