Gi Eun Park

Bis(thienothiophenyl) Diketopyrrolopyrrole-Based Conjugated Polymers with Various Branched Alkyl Side Chains and Their Applications in Thin-Film Transistors and Polymer Solar Cells

New thienothiophene-flanked diketopyrrolopyrrole and thiophene-containing π-extended conjugated polymers with various branched alkyl side-chains were successfully synthesized. 2-Octyldodecyl, 2-decyltetradecyl, 2-tetradecylhexadecyl, 2-hexadecyloctadecyl, and 2-octadecyldocosyl groups were selected as the side-chain moieties and were anchored to the N-positions of the thienothiophene-flanked diketopyrrolopyrrole unit. All five polymers were found to be soluble owing to the bulkiness of the side chains. The thin-film transistor based on the 2-tetradecylhexadecyl-substituted polymer showed the highest hole mobility of 1.92 cm2 V(-1) s(-1) due to it having the smallest π-π stacking distance between the polymer chains, which was determined by grazing incidence X-ray diffraction. Bulk heterojunction polymer solar cells incorporating [6,6]-phenyl-C71-butyric acid methyl ester as the n-type molecule and the additive 1,8-diiodooctane (1 vol %) were also constructed from the synthesized polymers without thermal annealing; the device containing the 2-octyldodecyl-substituted polymer exhibited the highest power conversion efficiency of 5.8%. Although all the polymers showed similar physical properties, their device performance was clearly influenced by the sizes of the branched alkyl side-chain groups.

Perylene diimide isomers containing a simple sp3-core for non-fullerene-based polymer solar cells

In order to investigate the effect of the geometries of perylene diimide (PDI)-based small molecules, five different isomers were synthesized by using a cyclohexane core as a simple sp3-σ core. Diaminocylohexane is such an effective core for the systematic development of many kinds of isomers via geometric tuning as well as for reducing the self-aggregation tendency of PDIs. Depending on the anchoring position of the PDI units on the cyclohexane core (ortho-, meta- and para-), isomers exhibited differences in solubility and crystallinity. Among the studied isomers, ortho-substituted t-OCP was found to have a highly twisted molecular structure which minimizes the strong tendency towards crystallization due to individual PDI moieties. The unique geometrical nature of the t-OCP isomer led to the highest power conversion efficiency (PCE = 6.23%) of bulk heterojunction (BHJ) polymer solar cells (PSCs) with a higher short-circuit current density (Jsc) and fill factor (FF). It is mainly ascribed to the formation of a nanophase interpenetrating network with well-balanced carrier mobility in the blend film.

Excellent Long-Term Stability of Power Conversion Efficiency in Non-Fullerene-Based Polymer Solar Cells Bearing Tricyanovinylene-Functionalized n-Type Small Molecules

New small molecules having modified acceptor strength and π-conjugation length and containing dicyanovinylene (DCV) and tricyanovinylene (TCV) as a strongly electron-accepting unit with indacenodithiophene, IDT(DCV)2, IDT(TCV)2, and IDTT(TCV)2, were synthesized and studied in terms of their applicability to polymer solar cells with PTB7-Th as an electron-donating polymer. Intriguingly, the blended films containing IDT(TCV)2 and IDTT(TCV)2 exhibited superior shelf life stabilities of more than 1000 h without any reduction in the initial power conversion efficiency. The low-lying lowest unoccupied molecular orbital energy levels and robust internal morphologies of small TCV-containing molecules could afford excellent shelf life stability.

(D)n–σ–(A)m type partially conjugated block copolymer and its performance in single-component polymer solar cells

We synthesized two types of novel poly(3-alkylthiophene)-free (D)n-b-(A)m conjugated block copolymers: PTQI-block-PNDISs. PTQI-b-PNDIS is the fully conjugated block copolymer, in which the donor block is connected to the acceptor block via a π-conjugated unit. In addition, the donor block was connected to an acceptor block through a non-conjugated alkylene spacer, yielding PTQI-b-PNDISL, which is a partially conjugated block copolymer. The power conversion efficiency (PCE) was 1.54% with an open-circuit voltage of 0.79 V in the single-component polymer solar cell (PSC) based on PTQI-b-PNDISL, better performance than that of the PSC containing PTQI-b-PNDIS (PCE ∼ 0.36%). These results reveal the potential for improving the photovoltaic performance in PSCs by controlling the internal morphology with exploitation of the self-segregating behavior of p- and n-blocks in the film state.

Regular terpolymers with fluorinated bithiophene units for high-performing photovoltaic cells

We demonstrate effective structural control of various electron-donating moieties containing bithiophene (BT) and naphthalene derivatives with 3,3′-difluoro-2,2′-bithiophene in a regular terpolymer system and compare the properties of these polymers with those of the three binary copolymers PDPPNp, PDPPBT, and PDPPFBT. PDPPBF and PDPPNF were successfully synthesized to be diketopyrrolopyrrole (DPP)-based terpolymers. The optical and electronic properties of these regular terpolymers could be precisely controlled by introducing three different components in the repeating units. A polymer solar cell (PSC) made of PDPPNF, which contains naphthalene as a weak donating unit, had a high open-circuit voltage (Voc) of 0.77 V, resulting from a relatively deep HOMO energy level. In contrast, PDPPBF bearing BT, which is an example of a relatively strong electron-donating unit, had a higher short-circuit current density (Jsc = 13.44 mA cm−2) than that of PDPPFBT. Therefore, improved power conversion efficiency (PCE) was possible in a PSC made of PDPPBF containing the electron-donating bithiophene unit in the terpolymer backbone. However, the improved internal morphology, as characterized by AFM and TEM, was the primary contributor to the enhanced PCEs (i.e., 6.38% and 6.39% for PDPPBF and PDPPNF, respectively).

A new n-type semiconducting molecule with an asymmetric indenothiophene core for a high-performing non-fullerene type organic solar cell

Herein, a new asymmetric n-type semiconducting molecule (PhITBD) with an indenothiophene core was designed, synthesized using tethering 2-(benzo[c][1,2,5]-thiadiazol-4-ylmethylene)-malononitrile (BM) as terminal groups, and applied to polymer solar cells (PSCs). The PSCs with asymmetric PhITBD displayed improved power conversion efficiencies (PCE) of 6.57% as compared to those with the symmetric molecule IDT-2BM. The higher PCE value of the PhITBD-based PSC was mainly attributed to the enhanced photovoltaic properties, the Voc, Jsc, FF induced by complementary light absorption (550–600 nm range), morphological improvement, and balanced charge carrier transport in the active layer. Due to the effective morphological control and absorption enhancement, asymmetric structured n-type-conjugated molecules are potential candidates for improving the performance of bulk heterojunction non-fullerene type PSCs.

Structural optimization of large acceptor–donor–acceptor-type molecules for improved performance of fullerene-free polymer solar cells

To control the molecular energy levels of highly π-extended n-type molecules, we synthesized two acceptor–donor–acceptor (A–D–A)-type molecules with indacenodithiophenes (IDTs) or IDT–benzodithiophene (BDT)–IDT as donating cores and 2-(2,3-dihydro-3-oxo-1H-inden-1-ylidene)propanedinitrile (IM) as terminal accepting units. These molecules showed different optical and electrochemical properties, indicating that the energy levels can be easily tuned by changing the structure of the donating core. Among two molecules, IM-BDTIDT2 showed a relatively blue shifted absorption spectrum and low-lying highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels. Although IM-IDT3 and IM-BDTIDT2 have a highly π-extended conjugated structure, no clear crystalline behaviour was observed in their thin films. When applied to polymer solar cells (PSCs), the device based on IM-BDTIDT2 displayed a higher PCE (5.33%) than the device bearing IM-IDT3 owing to the lower-lying energy levels of IM-BDTIDT2. Thus, the use of BDT as a donating core unit is favorable for limiting high-lying energy levels in highly π-extended A–D–A-type molecules.

Effect of a methyl thiophene-3-carboxylate bridge in an indacenodithiophene-based acceptor–donor–acceptor-type molecule on the performance of non-fullerene polymer solar cells

A new A-b-D-b-A-type n-type small molecule, IDT-3MT, was synthesized bearing a weak acceptor thiophene-3-carboxylate bridge (3MT = b) between indacenodithiophene as a donating core and 2-(3-oxo-2,3-dihydroinden-1-ylidene)-malononitrile as the accepting end groups. Compared to IDT-T bearing a neat thiophene bridge, IDT-3MT displayed a red-shifted absorption spectrum in the film state, which is a more effective complementary absorption behavior with PBDB-T as the donor material. The highest occupied molecular orbital and lowest unoccupied molecular orbital levels of IDT-3MT became correspondingly lower. Based on the results of grazing-incidence wide-angle X-ray scattering, the blend film of PBDB-T:IDT-3MT exhibited a more prominent face-on orientation and fine surface morphology compared with PBDB-T:IDT-T, which can facilitate charge transportation in the vertical direction. Among the two acceptors, the polymer solar cell based on a solvent additive-free as-cast PBDB-T:IDT-3MT blend film exhibited the highest power conversion efficiency of 8.40% with a high open-circuit voltage of 0.95 V and a short-circuit current density of 14.43 mA cm−2 due to the more prominent face-on orientation and favorable morphology of the blend film. According to the simple structural modification of the acceptor molecule, the ester group in the thiophene bridge played an important role toward achieving high-performance polymer solar cells.

Unconventional Three-Armed Luminogens Exhibiting Both Aggregation-Induced Emission and Thermally Activated Delayed Fluorescence Resulting in High-Performing Solution-Processed Organic Light-Emitting Diodes

In this work, three-armed luminogens IAcTr-out and IAcTr-in were synthesized and used as emitters bearing triazine and indenoacridine moieties in thermally activated delayed fluorescence organic light-emitting diodes (OLEDs). These molecules could form a uniform thin film via the solution process and also allowed the subsequent deposition of an electron transporting layer either by vacuum deposition or by an all-solution coating method. Intriguingly, the new luminogens displayed aggregation-induced emission (AIE), which is a unique photophysical phenomenon. As a nondoped emitting layer (EML), IAcTr-in showed external quantum efficiencies (EQEs) of 11.8% for the hybrid-solution processed OLED and 10.9% for the all-solution processed OLED with a low efficiency roll-off. This was evident by the higher photoluminescence quantum yield and higher rate constant of reverse intersystem crossing of IAcTr-in, as compared to IAcTr-out. These AIE luminogens were used as dopants and mixed with the well-known host material 1,3-bis( N-carbazolyl)benzene (mCP) to produce a high-efficiency OLED with a two-component EML. The maximum EQE of 17.5% was obtained when using EML with IAcTr-out doping (25 wt %) into mCP, and the OLED with EML bearing IAcTr-in and mCP showed a higher maximum EQE of 18.4% as in the case of the nondoped EML-based device.

Synthesis of Conjugated Wide-Bandgap Copolymers Bearing Ladder-Type Donating Units and Their Application to Non-Fullerene Polymer Solar Cells

Two new wide-bandgap conjugated copolymers, T-3MT and 2T-3MT, containing methyl-3-thiophenecarboxylate (3MT) as a weak accepting unit and indacenodithiophene (IDT) or indacenodithienothiophene (IDTT) as ladder-type donating units, were synthesized through Stille coupling polymerization. The effects of the IDT and IDTT units on the physical, optical, and electrochemical properties of these copolymers were investigated. Polymer solar cells (PSCs) were constructed using these new copolymers and different acceptor molecules for inter-comparative analysis of their performance. The PSCs bearing the as-cast active layers with 2,2′-((2Z,2′Z)-((4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b′] dithiophene-2,7-diyl)bis(methanylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (IM-IDT) displayed the highest PCEs of 3.78−3.96%, mainly owing to the predominant face-on orientation of blend film, and the relatively high open circuit voltage exceeding 1.0 V.