Young Un Kim

Side-chain engineering of diketopyrrolopyrrole-based copolymer using alkyl ester group for efficient polymer solar cell

The new diketopyrrolopyrrole-based copolymer, referred to as PDPPBDTE, bearing linear alkyl ester groups, was synthesized with the objective of improving the miscibility between the donor polymer and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM). PDPPBDTA bearing ethylhexyl side-chains was also synthesized as a control polymer. The PDPPBDTE:PC71BM blend film showed a relatively small crystalline domain size compared to the PDPPBDTA:PC71BM blend film. A polymer solar cell fabricated with PDPPBDTE and PC71BM exhibited twofold higher power conversion efficiency than that employing PDPPBDTA.

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.

New fluorene-based chiral copolymers with unusually high optical activity in pristine and annealed thin films

Polyfluorene (PF)-based chiral alternating copolymers, i.e., PFPh and PFTh, were successfully synthesized to obtain high circular dichroism (CD) in pristine thin films. By tethering chiral side-chains to the fluorene moiety, the resultant PFPh and PFTh films intriguingly exhibited high CD even in pristine films, which was attributed to the relatively low steric hindrance between the repeating units and intermolecular interactions between the polymer chains. Compared to PFTh bearing a five-membered heteroaromatic thiophene monomer, PFPh containing a six-membered aromatic benzene monomer, showed significantly enhanced CD in an annealed film. Of particular importance, results of optical microscopy and atomic force microscopy can be used to assess the CD of a chiral polymer thin film after thermal annealing.

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.