Jea Woong Jo

Fluoro‐Substituted n‐Type Conjugated Polymers for Additive‐Free All‐Polymer Bulk Heterojunction Solar Cells with High Power Conversion Efficiency of 6.71%

Fluorinated n-type conjugated polymers are used as efficient electron acceptor to demonstrate high-performance all-polymer solar cells. The exciton generation, dissociation, and charge-transporting properties of blend films are improved by using these fluorinated n-type polymers to result in enhanced photocurrent and suppressed charge recombination.

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.

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 ]

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.

Efficiency enhancement of P3HT/PCBM bulk heterojunction solar cells by attaching zinc phthalocyanine to the chain-end of P3HT

A new solution processable zinc phthalocyanine dye (ZnPc), as an interface modifier between poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in bulk heterojunction solar cells, was successively synthesized and linked to the chain-end of P3HT through the formation of a coordination complex. ZnPc dye molecules do not aggregate but preferentially locate at the interface between P3HT and PCBM, and thus contribute to the photocurrent generation by both direct photo-excitation and enhancement of charge transfer between P3HT and PCBM. To localize the zinc phthalocyanine dyes at the donor–acceptor interface more effectively, another new organic dye molecule, fullerene-functionalized zinc phthalocyanine (ZnPc-C60) was also synthesized and linked to the chain-end of the P3HT, where ZnPc-C60 contributes not only to the photocurrent generation by direct photo-excitation, but also lowers the interfacial tension, resulting in the reduction of the domain size and the suppression of the macrophase separation of the P3HT/PCBM blend for prolonged thermal annealing. This leads to higher device efficiency with 20% enhancement of the short circuit current and to enhancement of long-term thermal stability of device performance as compared to that of the reference P3HT/PCBM device.

Growth and characterization of silicon-nitride films by plasma-enhanced chemical vapor deposition

Thin films of silicon nitride were deposited on Si wafers by plasma-enhanced chemical vapor deposition (PECVD). For deposition we designed and made hot wall capacitively coupled PECVD equipment which has a radial flow reactor. Using an RF generator of frequency 13.56 MHz and SiH4 (5% SiH4 in N2) + NH3 and N2 as reactive gases and the carrier gas, respectively, we systematically varied the substrate temperature (240–360°C), the partial pressure of reactive gases (0.35 <PSiH3PSiH4<1.32, 5% of S iH4 in N2) and the RF power (20–160 W), as deposition parameters. The characteristics of the films such as composition, deposition rate, refractive index and hydrogen content were investigated by AES, ellipsometry, FTIR spectrometry, nanospec and spectroscopic ellipsometry. As a result of these measurements, well-known characteristics were observed as a function of the substrate temperature and the partial pressure of the reactive gases. However, in our investigation of the RF power dependence of the refractive index of the film, we found that the refractive index increases and then decreases as we increase the RF power. To explain this, we considered the RF power-dependent heating effect in the glow discharge process and the amount of NH radicals which increases with the RF power.

Synthesis of a low bandgap polymer based on a thiadiazolo-indolo[3,2-b]carbazole derivative for enhancement of open circuit voltage of polymer solar cells

For the purpose of synthesizing an electron donor polymer with a low-lying highest occupied molecular orbital (HOMO) energy level and thus to achieve high open circuit voltage (VOC) of polymer solar cells, an indolocarbazole derivative, 3,8-bis-(4-octyl-thiophene-2-yl)-5,6-didodecyl-thiadiazolo-indolo[3,2-b]carbazole, was synthesized and used as a building block of a low bandgap alternating copolymer. An alternating copolymer, poly(3,8-bis-(4-octyl-thiophene-2-yl)-5,6-didodecyl-thiadiazolo-indolo[3,2-b]carbazole-alt-2,1,3-benzothiadiazole) (PTICThBT), was synthesized via the Suzuki coupling reaction, and the copolymer exhibited a deep HOMO level of −5.4 eV. When a blend of PTICThBT and PC71BM was used as an active layer in bulk heterojunction polymer solar cells, a high VOC of 0.84 V was obtained with a power conversion efficiency of 2.62% under AM 1.5G condition.

Synergistic effects of solvent and polymer additives on solar cell performance and stability of small molecule bulk heterojunction solar cells

We developed p-DTS(FBTTh2)2:PC71BM-based small molecule bulk heterojunction solar cells using 1,8-diiodooctane (DIO) and small amounts of PCDTBT polymer. In the film, PCDTBT effectively suppresses the over aggregation of the p-DTS(FBTTh2)2 donor phase and promotes formation of percolating networks between the donor and acceptor phases. Moreover, the portion of p-DTS(FBTTh2)2 crystallites with the face-on orientation in the blend film is significantly increased and phase separation is decreased, which enables efficient charge generation and transport. Consequently, these solar cells consistently exhibit high fill factors and photocurrent densities and high efficiencies in the range 6.68–8.13% regardless of the DIO content (0.4–3 v/v%). In contrast, a large variation was found in the performance of the solar cells with blend films processed with DIO alone, with efficiencies of 2.75–6.68% depending on the DIO content. More importantly, the PCDTBT-processed solar cells exhibit remarkably improved stability under heating and 65 °C/85% RH. Thus, processing photoactive layers utilizing a combination of DIO and PCDTBT is an effective way of preparing promising small molecule solar cells: DIO promotes the crystallinity of the donor phase, and the intermolecular interactions between the polymer and the push–pull moiety in p-DTS(FBTTh2)2 induce distribution of donor crystallites to form percolating networks by suppressing donor over-segregation.