Jae-Chol Lee

Dopant-free polymeric hole transport materials for highly efficient and stable perovskite solar cells

We report a dopant-free polymeric hole transport material (HTM) that is based on benzo[1,2-b:4,5:b′]dithiophene and 2,1,3-benzothiadiazole, which results in highly efficient and stable perovskite solar cells (∼17.3% for over 1400 h at 75% humidity). The HTM comprises a random copolymer (RCP), which is characterized using UV-vis absorption spectroscopy, cyclic voltammetry, space-charge-limited current, and grazing-incidence wide-angle X-ray scattering. The RCP-based perovskite solar cell exhibits the highest efficiency (17.3%) in the absence of dopants [lithium bis(trifluoromethanesulfonyl)imide and tert-butylpyridine]. The observed efficiency is attributed to a deep HOMO energy level and high hole mobility. In addition, the long-term stability of the device is dramatically improved by avoiding deliquescent or hygroscopic dopants and by introducing a hydrophobic polymer layer. RCP devices maintain their initial efficiency for over 1400 h at 75% humidity, whereas devices made of HTMs with additives fail after 900 h.

A feasible random copolymer approach for high-efficiency polymeric photovoltaic cells

Two random copolymers based on (2,5-difluorophenylene)dithiophene and dialkoxybenzothiadiazole with benzodithiophene (P1) or thiophene (P2) as the third conjugated bridge having sulfur and fluorine (S⋯F) and/or oxygen (S⋯O) non-covalent intramolecular interaction are synthesized and characterized. In spite of a molecular weight difference over three times between both polymers, P1 and P2 possess similar solubility in organic solvents and thermal stability (Td ∼ 320 °C), which means probably due to that P1 with bulky alkylthiophene substituted benzodithiophene as a third conjugated bridge has less non-covalent intramolecular interaction than that of P2 with thiophene as a bridge. Both polymers were used as electron donors in bulk heterojunction organic photovoltaics (BHJ OPV) with PC71BM as an acceptor. From the photovoltaic measurements it was revealed that P2 shows higher power conversion efficiency (PCE) of up to 6.82% than that of P1 (2.44%). After 1,8-diiodooctane (DIO) treatments as a processing additive, the P1 and P2 devices show a significantly improved PCE of 5.95% for P1 and 7.71% for P2. The surface morphology analysis of the blend films using the atomic force microscope (AFM) reveals that the P1:PC71BM film shows macrophase separation, while the P2 film has a smooth morphology. After DIO treatments, the morphology of both polymer blend films is improved with better bi-continuous nanoscale networks. Charge carrier mobilities through the space charge limited current (SCLC) method demonstrate that P2 with the thiophene bridge has higher charge carrier mobilities than P1. In particular, an inverted structured BHJ OPV with P2 exhibits a PCE of 8.50%, which is the highest PCE reported in the literature regarding random copolymers.

High Performance Solution Processed Organic Field Effect Transistors with Novel Diketopyrrolopyrrole-Containing Small Molecules

The donor-acceptor (D-A)-type diketopyrrolopyrrole (DPP)-based small molecules (LGC-D117 and LGC-D118) were synthesized and used as the active layer of solution-processable organic field-effect transistors (OFETs). Both LGC-D117 and LGC-D118 contain silaindacenodithiophene as electron-donor units with DPP as an electron-accepting linker, and octylrhodanine as the electron-accepting end group. The molecules were functionalized with different side chains to study their effects on OFET characteristics. LGC-D117 has a simple branched alkyl side chain, whereas LGC-D118 features a bulky siloxane-terminated hybrid alkyl chain. The siloxane side chains of LGC-D118 account for its better crystallinity, leading to significantly high field-effect mobility (max 3.04 cm2 V−1 s−1). In particular, LGC-D118 is well soluble and sustains the high mobility in the environmentally friendly 2-methyltetrahydrofuran solvent with low temperature annealing at 100 °C due to the bulky siloxane-terminated alkyl side chain.

A novel random terpolymer for high-efficiency bulk-heterojunction polymer solar cells

A new random terpolymer, coded LGC-D013, based on N-alkylthieno[3,4-c]pyrrole-4,6-dione (TPD) as the acceptor and benzodithiophene (BDT) and terthiophene as the donor units, has been synthesized and characterized as a donor material in bulk-heterojunction (BHJ) organic photovoltaic (OPV) applications. The thermal, optical, and electrochemical properties of the LGC-D013 were characterized. The polymer has a deep highest occupied molecular orbital (HOMO) level of −5.56 eV and an optical band gap of 1.84 eV in film. The OPV based on LGC-D013:PC71BM blend film demonstrated a power conversion efficiency (PCE) of 6.09%, with a relatively high fill factor (FF) of 72.55%. After 1,8-diiodooctane (DIO) treatment with a 3% volume ratio, the short circuit current density (Jsc) was improved from 9.48 to 11.29 mA cm−2, and FF was improved from 72.55 to 73.13%, resulting in PCE improvement from 6.09 to 7.22%. The BHJ OPV using LGC-D013 as the donor polymer based on TPD and BDT with terthiophene showed an improved fill factor as high as 72%, which is higher than those of conventional TPD–BDT based polymers.