Ji Hwang Lee

Control of the electrode work function and active layer morphology via surface modification of indium tin oxide for high efficiency organic photovoltaics

Indium tin oxide (ITO) substrates modified with self-assembled monolayers (SAMs) were used to control the anode work function and active layer morphology of organic solar cells based on poly(3-hexylthiophene)/[6:6]-phenyl-C61 butyric acid methyl ester heterojunctions. By using SAMs with the terminal groups –NH2, –CH3, and –CF3, the authors were able to control the hole injection barrier of the ITO closer to the highest occupied molecular orbital level of active layer and surface energy of the ITO substrate. A solar cell device with CF3 SAM treated ITO was found to exhibit high efficiency performance, about 3.15%.

High‐Efficiency Organic Solar Cells Based on End‐Functional‐Group‐Modified Poly(3‐hexylthiophene)

[*] Prof. K. Cho, Prof. J. K. Kim, Y. Lee, J. H. Park Department of Chemical Engineering Polymer Research Institute Pohang University of Science and Engineering Pohang, 790-784 (Korea) E-mail: kwcho@postech.ac.kr; jkkim@postech.ac.kr Dr. J. S. Kim, Dr. J. H. Lee School of Environmental Science and Engineering Polymer Research Institute Pohang University of Science and Engineering Pohang, 790-784 (Korea)

Bulk heterojunction solar cells based on preformed polythiophene nanowires via solubility-induced crystallization

Here, we report the preparation of well-controlled nanoscale morphologies in photoactive thin films. The fabrication of bulk heterojunction structures in blend films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) employed two steps to achieve first the in situ formation of self-organized P3HT nanowires using a marginal solvent, and second, phase separation via mild thermal annealing. Morphological changes in the active layers that had been spin-cast from a marginal solvent, with varying annealing temperatures, were systematically studied and compared to the morphologies of films spin-cast from a good solvent. The interpenetrating nanowire structure yielded power conversion efficiencies as high as 4.07% due to the enhanced charge transport. Hole and electron mobilities increased substantially to 1.6 × 10−3 cm2 V−1 s−1 and 1.4 × 10−3 cm2 V−1 s−1, respectively, due to the two step process of P3HT crystallization by nanowire formation and subsequent phase separation. Photovoltaic performances improved with increasing film thickness up to 300 nm as a result of the interpenetrating donor/acceptor network structure.

Enhanced device performance of organic solar cells via reduction of the crystallinity in the donor polymer

A new π-conjugated polymer, poly(2,5-dioctyloxyphenylene vinylene-alt-3,3-dioctylquater thiophene) (PPVQT-C8), consisting of alternating p-divinylene phenylene and 3,3-dialkylquater thiophene units, was synthesized for use in organic solar cell devices. The crystallinity of poly(quaterthiophenes) (PQTs) was reduced by introducing p-divinylene phenylene units between pairs of quaterthiophene units. A well-mixed nanoscale morphology of PPVQT-C8:PCBM photoactive layer resulted from this modification, which increased the power conversion efficiency relative to devices based on highly crystalline PQTs. Bulk heterojunction solar cells based on PPVQT-C8:PC60BM blends with a 1:3 ratio presented the best photovoltaic performances, with a short-circuit current density (Jsc) of 6.7 mA cm−2, an open-circuit voltage (Voc) of 0.67 V, a fill factor (FF) of 0.62 and a power conversion efficiency (PCE) of 2.8% under illumination of AM 1.5 with light intensity of 100 mW cm−2. The charge carrier mobility and morphology studies indicated that an optimized interpenetration network composed of PPVQT-C8: PCBM could be achieved in a blend ratio of 1:3.