Kyu Cheol Lee

Benzodipyrrolidone (BDP)-Based Polymer Semiconductors Containing a Series of Chalcogen Atoms: Comprehensive Investigation of the Effect of Heteroaromatic Blocks on Intrinsic Semiconducting Properties

In order to determine the effects of actual chalcogen atoms on semiconducting properties for application in a variety of optoelectronic devices, a class of donor (D)-acceptor (A) polymer semiconductors, namely PBDP-Fu, PBDP-Th, and PBDP-Se, containing the recently formulated benzodipyrrolidone (BDP) accepting unit and furan (Fu), thiophene (Th), or selenophene (Se) as a donating unit has been synthesized, characterized, and used in an active layer of organic field-effect transistors (OFETs). With the LUMO levels being comparatively consistent for all three polymers (-3.58 to -3.60 eV) due to the dominant BDP contribution to the polymer backbone, the HOMO energies are somewhat sensitive to the structurally distinctive feature of the donor counits used. Utilizing a combination of X-ray diffraction (XRD) and atomic force microscopy (AFM), it is apparent that further crystalline domains occur with edge-on orientation for the polymers (PBDP-Th and PBDP-Se) with relatively heavier chalcogen atoms such as Th and Se, compared with PBDP-Fu which has a rather amorphous nature. Investigation of their OFET performance indicates that all the polymers show well balanced ambipolar operations. The desirable morphological structures of both the PBDP-Th and PBDP-Se result in higher mobilities in OFETs than those of PBDP-Fu. In particular, 200 °C annealed PBDP-Se OFETs results in ambipolarity being mobile for both holes of up to 1.7 × 10(-2) cm(2)/V·s and electrodes of up to 1.9 × 10(-2) cm(2)/V·s. In addition, OFETs with PBDP-Th show nearly equivalent charge carrier mobilities for both holes (μ(h) = 1.2 × 10(-2) cm(2)/V·s) and electrons (μ(e) = 1.1 × 10(-2) cm(2)/V·s). Consequently, we systematically demonstrate how the manipulation of existing heteroaromatics can modulate the electronic properties of conjugated D-A polymers, elucidating structure-property relationships that are desirable for the rational design of next generation materials.

A Roundabout Approach to Control Morphological Orientation and Solar-Cell Performance by Modulating Side-Chain Branching Position in Benzodithiophene-Based Polymers

To be meaningful to guide the rational design of novel high-performance conjugated semiconductors, we prepared three benzo[1,2-b:4,5-b]dithiophene (BDT)-based polymers by systematically moving the branching point of the alkyl chain. The effect of side-chain engineering was thoroughly investigated by a range of techniques. We demonstrate that a subtle change in the branching position in the BDT core can have a critical impact on polymer packing and preferential backbone orientation in thin films; copolymers made from BDT and thieno[3,4-c]pyrrole-4,6-dione units (TPD) adopt more of a face-on orientation as the branching point is shifted closer to the backbone, which can be correlated with a dramatic difference in solar-cells performance. The high short-circuit current density (11.6 mAu2009cm(-2) ) for the copolymer with one carbon atom between the alkoxylated oxygen atom and the branching point results from its predominantly face-on orientation and smoother surface in thin films, which results in power conversion efficiencies as high as 4.56u2009%.

A dithienodisilacyclohexadiene (DTDS)-based conjugated model semiconductor: understanding unique features and monitoring structural transition

To enable a superior σ*–π* conjugation, we present a dithienodisilacyclohexadiene (DTS) analogue of DTS(FBTTh2)2 – namely, DTDS(FBTTh2)2 – by replacing dithienosilole (DTS) with a dithienodisilacyclohexadiene (DTDS) ring in the main backbone, where DTDS possesses a double silicon-bridged bithiophene (Si–Si). With this replacement, a blue shift of the absorption and a high-lying LUMO are observed. Disclosed herein is a structural change of DTDS(FBTTh2)2 (DTDS to ox-DTDS skeleton as the corresponding oxidation structure) occurring under ambient conditions, which is monitored by real-time 1H NMR and UV absorption methods. This work not only provides a full understanding of the nature of DTDS, but also uses unique DTDS chemistry as a new toolbox to develop systems as novel functionality materials.

Medium bandgap copolymers based on carbazole and quinoxaline exceeding 1.0 V open-circuit voltages

Open-circuit voltage (VOC) is an important parameter in determining the performance of polymer solar cells (PSCs). Given the desire for superior VOC values in PSCs, we have designed and synthesized a series of ‘medium bandgap’ donor–acceptor (D–A) copolymers containing carbazole (Cz) and quinoxaline (Qx) (PCzDT-Qx, PCzDT-fQx, and PCzDT-ffQx). As a result of their deep-lying HOMO levels (−5.45 to −5.61 eV), high VOC values are achieved in PSCs with the resulting copolymers, despite the expense of short-circuit current density (JSC) and fill factor (FF) parameters. In this study, in addition to the best power-conversion efficiency (PCE) of up to 4.03% from PCzDT-fQx-based on PSCs, we have demonstrated a VOC value exceeding 1.0 V with PSCs of PCzDT-ffQx, which is among the highest VOC values achieved to date. Moreover, a comprehensive investigation on the mechanism of charge recombination and transport characteristics can determine a clear structure–property correlation in this class of molecules, which is helpful for designing better materials with maximum VOC without scarifying other key photovoltaic parameters.

Ultrafast Channel II process induced by a 3-D texture with enhanced acceptor order ranges for high-performance non-fullerene polymer solar cells

To achieve efficient non-fullerene polymer solar cells (NF-PSCs), an in-depth understanding of the key properties that govern the power output is necessary. Herein, three trialkylsilyl substituted benzodithiophene-based polymer donors (PJ1, PJ2, and PJ3) were synthesized with fine-tuning of the highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) and optical absorption. Using the polymer series paired with absorption-complementary small molecular acceptors (SMAs), namely, m-ITIC, IDIC, and AIDIC, we systematically studied the performance of a 3 × 3 matrix of NF-PSCs. An increasing open-circuit voltage with deepening HOMOs of the polymer donors, and the enhanced short-circuit current (JSC) and fill factor (FF) were ascribed to the well-intermixed blend morphology containing enhanced SMA order ranges with mixed face-on and edge-on orientations, the so-called 3-D texture. Such an optimal microstructure is best exemplified in the PJ2:IDIC combination, affording a highest efficiency of 12.01% with a simultaneously high JSC of 17.0 mA cm−2 and FF of 75.3%. The devices with an active layer thickness of 300 nm still maintain an impressive efficiency approaching 10% with a decent FF of 60.0%. Moreover, the Channel II process, i.e., photoinduced hole transfer through acceptor excitation, was demonstrated to be crucially important for photocurrent generation. This study highlights the importance of optimizing the trade-off between charge separation/transport and domain size to achieve high-performance NF-PSCs.

An efficient lactone-to-lactam conversion for the synthesis of thiophene Pechmann lactam and the characterization of polymers thereof

Despite having unique structural features, e.g., high co-planarity and a strong polar bicyclic lactam structure, thiophene bipyrrolylidene-2,2′(1H,1′H)-dione (TBPD) has been less explored as a dye, mainly due to the quite low yield in its synthesis via lactone-to-lactam conversion. We reported an efficient methodology for synthesizing TBPD in high yield using p-toluenesulfonic acid monohydrate and a catalytic amount of 4-dimethylaminopyridine in the chloroform solvent. From the newly synthesized series of TBPD-based donor–acceptor-type polymers, we fabricated organic field-effect transistors (OFETs), which were subjected to a systematic study on the relationship between film microstructure and charge transport. Among them, the annealed PTBPD-Th film revealed a more ordered lamellar packing with the highest number of interlayers and preferential edge-on orientation, yielding the best hole mobility (up to 0.46 cm2 V−1 s−1). The improved synthesis of TBPD and our findings concerning related polymers could promote further research and development associated with the TBPD unit.