Jae Woong Jung

A Fluorinated Phenylene Unit as a Building Block for High‐Performance n‐Type Semiconducting Polymer

Copolymers composed of diketopyrrolopyrrole and phenylene units with different numbers of fluorine subsitution are synthesized. When the effect of the number of fluorine substitution on the n-channel transporting property is investigated, the polymer with four fluorine substitutions exhibits the best n-type charge-transporting behavior with an electron mobility of 2.36 cm(2) V(-1) s(1).

Fabrication of Highly Conductive and Transparent Thin Films from Single-Walled Carbon Nanotubes Using a New Non-ionic Surfactant via Spin Coating

Oligothiophene-terminated poly(ethylene glycol) was synthesized and used as a non-ionic and amphiphilic surfactant for fabricating high-quality single-walled carbon nanotube (SWCNT) films by a simple spin coating method. The absence of charge repulsion between SWCNT/surfactant complexes successfully leads to formation of a dense network of SWCNTs on the substrate through a single deposition of spin coating. When the SWCNT film was treated with nitric acid and thionyl chloride after washed with dichloromethane and water, a high-performance SWCNT film with the sheet resistance of 59 ohm/sq and the transparency of 71% at 550 nm was successfully obtained. Since the SWCNT film exhibits a high value of σ(dc)/σ(ac) (∼17) and excellent dimensional stability after releasing from the substrate, the film can be used as a transparent electrode in flexible optoelectronic devices.

Semi-crystalline random conjugated copolymers with panchromatic absorption for highly efficient polymer solar cells

An effective approach to extend the light absorption range of conjugated polymers for high performance photovoltaics is synthesis of copolymers composed of at least two different chromophores with a complementary absorption range. For this purpose, we synthesized random conjugated copolymers consisting of DPP and isoindigo as co-electron accepting units in donor–acceptor type conjugated copolymers. The random copolymers exhibited both broad light absorption and low-lying HOMO level, which contribute to enhancement of JSC and VOC, respectively. Furthermore, the predominant face-on orientation of the random copolymers on the substrate is beneficial for charge transport in the device. The polymer solar cell (PSC) based on the random copolymer has shown a promising efficiency of 6.04% which is the highest value among PSCs based on random copolymers.

Fluorination on both D and A units in D–A type conjugated copolymers based on difluorobithiophene and benzothiadiazole for highly efficient polymer solar cells

Fluorination of conjugated polymers is one of the effective strategies to tune the frontier energy levels for achieving high efficiency polymer solar cells. In this study, three fluorinated D–A polymers, consisting of 3,3′-difluoro-2,2′-bithiophene and 2,1,3-benzothiadiazole (BT) with different numbers of fluorine substitution, were synthesized in order to investigate the effect of fluorination on their photovoltaic properties. The polymers with fluorinated BT show lower frontier energy levels, improved polymer ordering, and a narrower fibril size in the blend with PC71BM. The polymer with mono-fluorinated BT exhibits a superior PCE of 9.14% due to a high SCLC hole mobility, mixed orientation of polymer crystals in the active layer, and low bimolecular recombination. This result demonstrates that the fluorine content in conjugated polymers should be controlled for optimizing optoelectrical and photovoltaic properties of fluorinated conjugated polymers.

Enhanced Performance and Air Stability of Polymer Solar Cells by Formation of a Self‐Assembled Buffer Layer from Fullerene‐End‐Capped Poly(ethylene glycol)

morphology control, [ 3 ] and device optimization. [ 4 ] To date, a power conversion effi ciency (PCE) over 5% has been obtained by using poly(3-hexylthiophene) (P3HT) [ 5 ] or other low-bandgap polymers [ 6 ] as a donor and [6,6]-phenyl-C 60 -butyric acid methyl ester (PCBM) as an acceptor. One of the most important issues of PSCs is that the effi ciency is lower than conventional Si-based solar cells or dyesensitized solar cells. [ 7 ] Various approaches for morphology control such as thermal annealing, [ 8 ] solvent annealing, [ 9 ] solvent mixture, [ 10 a–e] and microwave annealing [ 10 f ] have been proposed to increase the effi ciency. These methods have been very effective to give nanoscale phase-separated morphology in the horizontal direction (parallel to the fi lm surface). However, the methods have limited control over the vertical distribution of the components in active layer, although the vertical distribution is very critical for effective transport of charge carriers. [ 11 ]

Synthesis of C60-end capped P3HT and its application for high performance of P3HT/PCBM bulk heterojunction solar cells

A new C60-end capped poly(3-hexylthiophene) (P3HT-C60) was synthesized via a simple three-step process, and used as a compatibilizer for P3HT/PCBM composite for the purpose of controlling the morphology of P3HT/PCBM composite film, and thus improving the long-term thermal stability of solar cell performance. When a small amount of P3HT-C60 was added to P3HT/PCBM, the bicontinuous and nanometre-scale film morphology was developed and preserved for 2 h of annealing at 150 °C. Furthermore, the addition of P3HT-C60 as a compatibilizer suppressed large-scale phase separation of P3HT/PCBM composite even after prolonged annealing time (8 days), and as a result, the P3HT/PCBM/P3HT-C60 bulk heterojunction solar cells exhibited the excellent long-term thermal stability of device performance.

Degradation and stability of polymer-based solar cells

Stability of polymer solar cells (PSCs) is critically important for PSCs to be commercialized. The performance deterioration of PSCs arises mainly from macrophase separation of the finely tuned nanoscale morphology of donor–acceptor blends, photo-degradation of active layer materials, and oxidative degradation of donor polymers due to diffusion of oxygen and water molecules from the interlayer/electrode. In this article, the degradation mechanisms of various types of active layer materials are discussed and the methods how to protect the active layer materials from degradation to stabilize the device performance of PSCs are extensively discussed based on recent publications.

Morphology control of a polythiophene-fullerene bulk heterojunction for enhancement of the high-temperature stability of solar cell performance by a new donor-acceptor diblock copolymer.

A well defined diblock copolymer (P3HT-b-C(60)) based on regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) was synthesized via two controlled polymerization steps and used as a compatibilizer for the P3HT/PCBM blend, which has widely been used as an active layer in bulk heterojunction polymer solar cells. The addition of a small amount of P3HT-b-C(60) results in not only the reduction of phase size of P3HT/PCBM blend but also the suppression of macrophase separation for long-time thermal annealing owing to the preferential location of the diblock copolymers at the interface between P3HT and PCBM phases. The morphology change with the annealing time is closely related to the change of the power conversion efficiency (PCE) of solar cells: the PCE of P3HT/PCBM greatly decreases with increasing annealing time while the addition of P3HT-b-C(60) significantly reduces the decrease of PCE for long-time thermal annealing.

A low band-gap copolymer composed of thienyl substituted anthracene and diketopyrrolopyrrole compatible with multiple electron acceptors for high efficiency polymer solar cells

A low band-gap copolymer consisting of 9,10-thienyl-substituted anthracene (TA) as an electron-donating unit and diketopyrrolopyrrole (DPP) as an electron-withdrawing unit has been synthesized and applied to the donor material in polymer solar cells. The weak electron-donating characteristic of TA, which has 2-D extended conjugation by thiophene side groups, makes the copolymer exhibit a low-lying HOMO level of −5.5 eV with a low band-gap of 1.65 eV, which affords both high photo-voltage and photo-current of the polymer solar cells. As a result, the solar cell device fabricated from the blend of the copolymer and PC71BM exhibits a promising power conversion efficiency of 7.02% (VOC = 0.80 V, JSC = 13.1 mA cm−2, FF = 0.67). Moreover, the polymer solar cell as fabricated from the blend of the copolymer and di-perylene bisimide (di-PBI) as a non-fullerene electron acceptor exhibits a promising power conversion efficiency of 4.23% with VOC = 0.77 V, JSC = 9.8 mA cm−2, and FF = 0.56. This photovoltaic performance of the copolymer not only demonstrates that the TA is a promising electron-donating building block for a high performance low band-gap copolymer, but also the copolymer is compatible with both PC71BM and a non-fullerene acceptor for high efficiency polymer solar cells.