Since 2011, Dow has hosted the Dow Korea Award to support research activities by science and engineering students and recognize outstanding papers in electronic materials.
The award is open to all Master’s or Ph.D. students at four-year institutions, or post-doctoral researchers in Korea. Entries are reviewed by a panel of judges consisting of Dow Korea officials, as well as professors and researchers in the field of electronic materials. Winners are recognized at a ceremony and receive a cash award.
Effect of π-Conjugated Bridges on Organic Photovoltaic Cells
Dow Korea Award Winners 2015. Author, Ji-Hoon Kim, a Ph.D. degree candidate at Chemistry Department, Pusan National University, is pictured in the first row, second from the left.
The 2015 Dow Korea Award program attracted 184 papers and nine papers were selected as winners by a panel of five judges. The first-place award went to Ji-Hoon Kim, a Ph.D. degree candidate in the Chemistry Department of Pusan National University for his paper “Effect of π-conjugated bridges of TPD-based medium bandgap conjugated copolymers for efficient tandem organic photovoltaic cells.” When making the award, the judging panel stated, “A high-efficiency bottom cell is expected to be a key technology to improve the efficiency of a tandem organic solar cell. This research shows that a reduction in efficiency that occurs in the course of high temperature processing can be overcome to some degree.”
Summary: Organic semiconductors have attracted extensive scientific interest for applications in organic electronics and optoelectronics, including for organic lighting-emitting diode (OLED), organic thin film transistors (OTFT), and organic photovoltaic cells (OPVs). As impressive and important renewable energy sources, OPVs have particular advantages such as simple device structure, light-weight, flexibility and low fabrication cost using simple ambient condition solution or the roll-to-roll process. Compared to inorganic-based solar cells, however, they still do not achieve the targeted 15% efficiency or satisfy lifetime requirements, which are required for widespread commercialization.
Increasing the PCE by simultaneously improving the short circuit current density (Jsc) and open circuit voltage (Voc) of low bandgap polymers in single-junction OPVs remains a challenge, as it requires a trade-off between high voltage and high current, which limits the use of the full solar spectrum. As a result, multi-junction devices with a tandem structure, made by stacking two or more cells with bulk heterojunction (BHJ) layers having complementary absorption spectra, are considered a promising alternative to enhance the performance of OPVs. Generally, a double junction tandem cell consists of a front cell with a medium bandgap material, an interconnecting layer, and a rear cell with a low bandgap material. As such, medium bandgap polymers with excellent photovoltaic properties are highly desirable in addition to high performance low bandgap polymers. However, compared to the extensive efforts put into developing low bandgap donor polymers, the exploration of medium bandgap polymers with suitable photovoltaic properties for tandem cells has been largely overlooked.
Recently, medium bandgap copolymers were synthesized by several research groups. However, despite the high power conversion efficiencies (PCEs) reported, studies regarding medium bandgap materials for tandem OPVs have remained relatively non-existent, probably because the formation of the recombination layer which connects the top and bottom cells in tandem OPV cells requires thermal treatment (100–150 oC). In other words, the following criteria are required for medium bandgap polymers used in the bottom cell in tandem OPVs: (1) strong UV-visible absorption between 300 and 700 nm, (2) high PCE in single cells, and (3) high thermal stability.
In this work, a series of medium bandgap D–π–A system copolymers has been designed and successfully tested in tandem cells as bottom-cell materials. Using the 6-alkylthieno[3,2-b]thiophene π-bridge in place of the thiophene π-bridge led to an improvement in the devices' fill factor (FF), Jsc, and PCE. A tandem photovoltaic device comprising the inverted PBDTT-ttTPD cell and a PTB7-based cell as the bottom and top cell components, respectively, showed a maximum PCE of 9.35% The obtained PCE of the bottom cell and FF of the tandem cell are, to the best of our knowledge, the highest reported to date for a tandem OPV device.
This study opens up a new direction for polymer chemists to pursue in the design of new materials for tandem OPVs, and also represents an important step forwards towards their commercialization
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