Comparative Study of the Mechanical Properties of All-Polymer and Fullerene-Polymer Solar Cells: The Importance of Polymer Acceptors for High Fracture Resistance

Kim, Wansun,Choi, Joonhyeong,Kim, Jae-Han,Kim, Taesu,Lee, Changyeon,Lee, Seungjin,Kim, Mingoo,Kim, Bumjoon J.,Kim, Taek-Soo American Chemical Society 2018 Chemistry of materials Vol.30 No.6

<P>High fracture resistance of polymer solar cells (PSCs) is of great importance to ensure long-term mechanical reliability, especially considering their potential in roll-to-roll printing processes and flexible devices. In this paper, we compare mechanical properties, such as the cohesive fracture energy, elastic modulus, and crack-onset strain, of all-polymer solar cells (all-PSCs) and fullerene-based solar cells (PCBM-PSCs) based on the same, representative low-bandgap polymer donor (PTB7-Th) as a function of acceptor content. The all-PSCs exhibit higher fracture energy (2.45 J m<SUP>-2</SUP>) than PCBM-PSCs (0.29 J m<SUP>-2</SUP>) at optimized device conditions. Additionally, a 15-fold higher crack-onset strain is observed in all-PSCs than in PCBM-PSCs. Dramatically different mechanical compliances observed for all-PSCs and PCBM-PSCs are investigated in detail by analysis of the blend morphologies as a function of acceptor content (either P(NDI2HD-T) or PCBM acceptors). The superior fracture resistance of all-PSCs is attributed to the more ductile characteristics of the polymer acceptor and the large degree of plastic deformation during crack growth, in contrast to the brittle nature of PCBM and the weak interaction between the polymer-rich phase and highly aggregated PCBM-rich domains. Therefore, this work demonstrates that replacing a small-molecule acceptor (i.e., PCBM) with polymeric materials can be an effective strategy toward mechanically robust PSCs.</P> [FIG OMISSION]</BR>

고분자 태양 전지의 억셉터 종류에 따른 파괴 거동에 대한 연구

김완선(Wansun Kim),최준형(Joonhyeong Choi),김재한(Jae-Han Kim),김태수(Taesu Kim),이창연(Changyeon Lee),이승진(Seungjin Lee),김민구(Mingoo Kim),김범준(Bumjoon J. Kim),김택수(Taek-Soo Kim) 대한기계학회 2018 대한기계학회 춘추학술대회 Vol.2018 No.4

Regioregular-<i>block</i>-Regiorandom Poly(3-hexylthiophene) Copolymers for Mechanically Robust and High-Performance Thin-Film Transistors

Park, Hyeonjung,Ma, Boo Soo,Kim, Jin-Seong,Kim, Youngkwon,Kim, Hyeong Jun,Kim, Donguk,Yun, Hongseok,Han, Junghun,Kim, Felix Sunjoo,Kim, Taek-Soo,Kim, Bumjoon J. American Chemical Society 2019 Macromolecules Vol.52 No.20

<P>In this work, we develop mechanically robust and high-performance organic thin-film transistors (OTFTs) based on poly(3-hexylthiophene) (P3HT) regioblock copolymers (<I>block</I>-P3HTs). These <I>block</I>-P3HTs consist of regioregular (<I>rre</I>) and regiorandom (<I>rra</I>) P3HTs, where the highly crystalline <I>rre</I> block allows efficient charge transport while the amorphous <I>rra</I> block provides mechanical robustness and interdomain connection. To examine the effects of the molecular architecture on the OTFT performance and stretchability, we prepare a series of <I>block</I>-P3HTs having different number-average molecular weight (<I>M</I><SUB>n</SUB>) values of <I>rra</I> blocks (from 0 to 32 kg mol<SUP>-1</SUP>) and a fixed <I>M</I><SUB>n</SUB> of <I>rre</I> blocks (11 kg mol<SUP>-1</SUP>). Thin films of all of the <I>block</I>-P3HTs exhibit a high charge-carrier mobility due to the formation of well-developed edge-on crystallites from the <I>rre</I> blocks confined within the <I>rra</I> domains, leading to a hole mobility of 1.5 × 10<SUP>-1</SUP> cm<SUP>2</SUP> V<SUP>-1</SUP> s<SUP>-1</SUP>, which is superior to that of the <I>rre</I> P3HT homopolymer. In addition, the mechanical toughness of <I>block</I>-P3HT thin films is remarkably enhanced by the <I>rra</I> block. While the <I>rre</I> P3HT homopolymer thin film shows a brittle behavior with an elongation at break of only 0.3%, the elongation at break of the <I>block</I>-P3HT thin films increases by a factor of 100, yielding 30.2% with increasing <I>M</I><SUB>n</SUB> of the <I>rra</I> block, without sacrificing the electrical properties. In particular, a noticeable enhancement of both elongation at break and toughness is observed between <I>M</I><SUB>n</SUB> values of the <I>rra</I> block of 8 and 20 kg mol<SUP>-1</SUP>, indicating that the critical molecular weight of <I>rra</I> P3HT plays an important role in determining the mechanical response of the <I>block</I>-P3HT thin films. This study provides guidelines and strategies to improve the mechanical properties of organic electroactive materials without the disruption of optoelectrical properties, which is critical to fabricate high-performance soft electronics.</P> [FIG OMISSION]</BR>

High-Crystalline Medium-Band-Gap Polymers Consisting of Benzodithiophene and Benzotriazole Derivatives for Organic Photovoltaic Cells

Kim, Ji-Hoon,Song, Chang Eun,Shin, Nara,Kang, Hyunbum,Wood, Sebastian,Kang, In-Nam,Kim, Bumjoon J.,Kim, BongSoo,Kim, Ji-Seon,Shin, Won Suk,Hwang, Do-Hoon American Chemical Society 2013 ACS APPLIED MATERIALS & INTERFACES Vol.5 No.24

<P>Two semiconducting conjugated polymers were synthesized via Stille polymerization. The structures combined unsubstituted or (triisopropylsilyl)ethynyl (TIPS)-substituted 2,6-bis(trimethylstannyl)benzo[1,2-<I>b</I>:4.5-<I>b</I>′]dithiophene (BDT) as a donor unit and benzotriazole with a symmetrically branched alkyl side chain (DTBTz) as an acceptor unit. We investigated the effects of the different BDT moieties on the optical, electrochemical, and photovoltaic properties of the polymers and the film crystallinities and carrier mobilities. The optical-band-gap energies were measured to be 1.97 and 1.95 eV for PBDT-DTBTz and PTIPSBDT-DTBTz, respectively. Bulk heterojunction photovoltaic devices were fabricated and power conversion efficiencies of 5.5% and 2.9% were found for the PTIPSBDT-DTBTz- and PBDT-DTBTz-based devices, respectively. This difference was explained by the more optimal morphology and higher carrier mobility in the PTIPSBDT-DTBTz-based devices. This work demonstrates that, under the appropriate processing conditions, TIPS groups can change the molecular ordering and lower the highest occupied molecular orbital level, providing the potential for improved solar cell performance.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/aamick/2013/aamick.2013.5.issue-24/am401926h/production/images/medium/am-2013-01926h_0013.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/am401926h'>ACS Electronic Supporting Info</A></P>

Sequentially Fluorinated PTAA Polymers for Enhancing <i>V</i>_OC of High-Performance Perovskite Solar Cells

Kim, Youngwoong,Jung, Eui Hyuk,Kim, Geunjin,Kim, Donguk,Kim, Bumjoon J.,Seo, Jangwon Wiley (John WileySons) 2018 ADVANCED ENERGY MATERIALS Vol.8 No.29

Aqueous Soluble Fullerene Acceptors for Efficient Eco-Friendly Polymer Solar Cells Processed from Benign Ethanol/Water Mixtures

Kim, Youngkwon,Choi, Joonhyeong,Lee, Changyeon,Kim, Youngwoong,Kim, Changkyun,Nguyen, Thanh Luan,Gautam, Bhoj,Gundogdu, Kenan,Woo, Han Young,Kim, Bumjoon J. American Chemical Society 2018 Chemistry of materials Vol.30 No.16

<P>We present a new series of fullerene derivatives that exhibit solubility in ethanol/water solvent mixtures and implement these materials to fabricate polymer solar cells (PSCs) using environmentally benign solvents. In order to simultaneously optimize the processability of the fullerenes in ethanol/water solvent mixtures and device performance, different fullerene mono-adducts were designed by introducing oligoethylene glycol side chains with different lengths and number of branches. As a result, we achieved power conversion efficiencies up to 1.4% for PSCs processed from benign ethanol/water mixtures in air. Significantly, the new alcohol/water-soluble fullerene derivatives displayed electron mobilities up to 1.30 × 10<SUP>-4</SUP> cm<SUP>2</SUP> V<SUP>-1</SUP> s<SUP>-1</SUP>, 150 times higher than those of a previously reported alcohol-soluble fullerene bis-adduct, owing to efficient packing of the fullerenes. Femtosecond transient absorption spectroscopy revealed the acceptor side chain to markedly impact geminate and/or nongeminate charge recombination in the PSCs. In addition, side chain optimization of these fullerenes produced well-intermixed morphologies with high domain purity when blended with p-type polymer to provide hole and electron transport pathways. Our results provide important guidelines for the design of electroactive materials for safe and environmentally benign fabrication of PSCs and other organic electronic devices.</P> [FIG OMISSION]</BR>

Effects of Solubilizing Group Modification in Fullerene Bis-Adducts on Normal and Inverted Type Polymer Solar Cells

Kim, Ki-Hyun,Kang, Hyunbum,Kim, Hyeong Jun,Kim, Pan Seok,Yoon, Sung Cheol,Kim, Bumjoon J. American Chemical Society 2012 Chemistry of materials Vol.24 No.12

<P>Structural control of solubilizing side groups in fullerene-based electron acceptors is critically important to optimize their performance in bulk heterojunction (BHJ)-type polymer solar cell (PSC) devices. The structural changes of fullerene derivatives affect not only their optical and electrochemical properties but also their solubility and miscibility with electron donor polymers. Herein, we synthesized a series of <I>o</I>-xylenyl C<SUB>60</SUB> bis-adduct (OXCBA) derivatives with different solubilizing side groups to systematically investigate the effects of fullerene derivative structures on the photovoltaic properties of PSCs. The xylenyl side groups on the OXCBA were modified to produce several different OXCBA derivatives in which the xylenyl groups were functionalized with fluorine (FXCBA), nitro (NXCBA), methoxy and bromine (BMXCBA), and phenyl groups (ACBA). End group modifications of OXCBA dramatically affect photovoltaic performance in blend films with poly(3-hexylthiophene) (P3HT), resulting in power conversion efficiencies (PCEs) ranging from 1.7 to 5.3%. We found that this large range in PCE values is mainly due to differences in the blend morphology and interfacial area of the P3HT:OXCBA derivative films caused by changes in the hydrophobicity of the OXCBA derivatives and their interaction with P3HT. The trend in photovoltaic performance of the different OXCBA derivatives agrees well with those of the interfacial tension, PL quenching, and exciton dissociation probability, which suggests that changes in the interaction with P3HT are largely responsible for their photovoltaic performances. Finally, the OXCBA derivatives were applied in inverted type PSC devices. We note that P3HT:OXCBA blend devices exhibited more than 5% PCE with excellent air stability, which is one of the best inverted type devices based on the P3HT polymer in a simple device architecture without any extra interlayers.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/cmatex/2012/cmatex.2012.24.issue-12/cm3010369/production/images/medium/cm-2012-010369_0005.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/cm3010369'>ACS Electronic Supporting Info</A></P>

Tuning Mechanical and Optoelectrical Properties of Poly(3-hexylthiophene) through Systematic Regioregularity Control

Kim, Jin-Seong,Kim, Jae-Han,Lee, Wonho,Yu, Hojeong,Kim, Hyeong Jun,Song, Inho,Shin, Minkwan,Oh, Joon Hak,Jeong, Unyong,Kim, Taek-Soo,Kim, Bumjoon J. American Chemical Society 2015 Macromolecules Vol.48 No.13

<P>While the regioregularity (RR) of conjugated polymers is known to have a strong influence on their inherent properties, systematic study of the RR effect has been limited due to the lack of a synthetic methodology. Herein, we successfully produced a series of poly(3-hexylthiophene)s (P3HTs) having a wide range of RR from 64 to 98%. Incorporation of controlled amounts of head-to-head (H-H) coupled dimer in modified Grignard metathesis polymerization allows a facile tuning of the RR of the P3HTs with comparable molecular weight and low polydispersity. Then, we investigated the effect of RR on structural, electrical, and mechanical properties of P3HTs in which a higher content of H-H regio-defects, namely lower RR, systematically lowered the degree of crystallinity. Although high RR P3HT (98%) had higher charge carrier mobility (1.81 x 10(-1) cm(2) V-1 s(-1)), its strong crystallinity induced high brittleness and stiffness, resulting in device failure under a very small strain, as shown in tensile and bending tests. The tensile modulus was reduced significantly from 287 MPa (RR 98%) to 13 MPa (RR 64%), and also the RR 64% P3HT film had much better mechanical resilience with an order of magnitude higher elongation at break than that of the RR 98% polymer. Our findings suggest that the mechanical and electrical properties of conjugated polymers can be systematically tuned by controlling the RR to meet the purposes of various organic electronic applications, i.e., flexible portable devices vs high-performance panels.</P>

Modulating Regioregularity of Poly(3-hexylthiophene)-based Amphiphilic Block Copolymers To Control Solution Assembly from Nanowires to Micelles

Kim, Youngkwon,Kim, Hyeong Jun,Kim, Jin-Seong,Yun, Hongseok,Park, Hyeonjung,Han, Junghun,Kim, Bumjoon J. American Chemical Society 2018 Chemistry of materials Vol.30 No.21

<P>In selective solvents, poly(3-hexylthiophene) (P3HT)-based block copolymers (BCPs) assemble into one-dimensional nanowires (NWs) due to strong π-π stacking interactions of the P3HT block. Herein, we report the effect of P3HT regioregularity (RR) on the assembly of P3HT-based amphiphilic BCPs in solution. We synthesized a series of P3HT-<I>block</I>-poly(2-vinylpyridine) (P3HT-<I>b</I>-P2VP) copolymers with similar molecular weights and P3HT volume fractions but with different RRs, ranging from 55% to 95%, and studied their assembly in tetrahydrofuran/<I>n</I>-butanol mixtures. P3HT-<I>b</I>-P2VP copolymers with high RR (>80%) crystallized into well-ordered NWs with core widths consistent with fully extended P3HT chains. In BCP nanostructures with decreasing RR, more flexible P3HT chains produced gradual increases in the width of the NWs, from 12 to 24 nm. Eventually, a morphological transition to spherical micelle structures was observed at 55% RR. The structural differences were visualized by incorporating Au nanoparticles, which locate at the interface of P3HT and P2VP blocks, onto the NWs and imaging the resulting hybrid nanostructures by transmission electron microscopy. In addition, the crystalline behaviors of the assembled nanostructures were determined using differential scanning calorimetry and grazing incidence X-ray scattering. We elucidated important relationships between the solution assembly behaviors and the crystalline interactions of conjugated semicrystalline-coil BCPs, which will guide design of future versatile BCP nanostructures.</P> [FIG OMISSION]</BR>

Architectural Effects on Solution Self-Assembly of Poly(3-hexylthiophene)-Based Graft Copolymers

Kim, Youngkwon,Kim, Hyeong Jun,Kim, Jin-Seong,Hayward, Ryan C.,Kim, Bumjoon J. American Chemical Society 2017 ACS APPLIED MATERIALS & INTERFACES Vol.9 No.3

<P>While solution assembly of conjugated block copolymers has been widely used to produce long 1-D nanowires (NWs), it remains a great challenge to provide a higher level of control over structure and function of the NWs. Herein, for the first time, we report the solution assembly of graft copolymers containing a conjugated polymer backbone in a selective solvent and demonstrate that their self assembly behaviors can be manipulated by the molecular structures of the graft copolymers. A series of poly(3-hexylthiophene)-graft-poly(2-vinylpyridine) (P3HT-g-P2VP) copolymers was designed with two different architectural parameters: grafting fraction (f(g)) and molecular weight of P2VP chains (M-n,M-P2VP) on the P3HT backbone. Interestingly, crystallization of the P3HT-g-P2VP copolymers was systematically modulated by changes in f(g) and M-n,M-P2VP, thus allowing for control of the growth kinetics and curvatures of solution-assembled NWs. When M-n,M-P2VP (4.4 to 15.1 kg/mol) or fg (2.8 to 9.2%) of the P3HT-g-P2VP polymers was increased, the crystallinity of the copolymers was reduced significantly. Steric hindrance from the grafted P2VP chains apparently modified the growth of NWs, leading to shorter NWs with a greater degree of curvature for graft copolymers with more hindrance. Therefore, we envision that such conjugated chain-based graft copolymers can be versatile building blocks for producing NWs with controlled length and shape, which can be important for tailoring the optical and electrical properties of NW-based devices.</P>