Soo Won Heo

An effect on the side chain position of D–π–A-type conjugated polymers with sp2-hybridized orbitals for organic photovoltaics

A D–π–A-type poly[alkylidenefluorene-alt-di-2-thienyl-2,1,3-benzothiadiazole] (P1) was synthesized via Suzuki coupling reaction. Based on the positions of the spacer (π) and acceptor (A) in the polymers, a dodecyl chain (P2) and an octyloxy chain (P3), respectively, were introduced. Both chains were introduced to the spacer and acceptor of P4. The obtained polymers (P2–P4) were soluble in organic solvents such as chlorobenzene, THF and o-dichlorobenzene at room temperature. Also, the introduction of a dodecyl chain to the spacer reduced the energy of the highest occupied molecular orbital (HOMO) level (−5.5 to −5.56 eV) but increased the tilt angle (45.4–53.0°), which had prevented the main chain from π–π stacking. The orientation of the obtained polymers in thin films was confirmed by XRD measurement. P3 with only an octyloxy chain showed a face-on-rich structure with a π–π stacking distance of 3.7 A compared to other polymers. The polymer solar cells were fabricated through a solution process, and showed a power conversion efficiency (PCE) of 3.6%, with a short-circuit current density (Jsc) of 8.9 mA cm−2, an open-circuit voltage (Voc) of 0.88 V, and a fill factor (FF) of 45.7%. With the introduction of poly[9,9-bis(6′-(diethanolamino)hexyl)fluorene] (PFN-OH), a PCE of 3.9% was confirmed.

Enhanced performance in polymer light emitting diodes using an indium–zinc–tin oxide transparent anode by the controlling of oxygen partial pressure at room temperature

The amount of indium in indium zinc tin oxide (IZTO) was reduced by over 20% by manufacturing an IZTO target containing ZnO for application as the transparent conducting oxide (TCO) anode of polymer light emitting diodes (PLEDs). The IZTO target was manufactured with the composition In2O3 (70 at%)–ZnO (15 at%)–SnO2 (15 at%), and IZTO films were formed at room temperature using a pulsed DC magnetron sputtering system at oxygen partial pressures of 0–4%. The optical and electrical properties of the IZTO films and the device performance of PLEDs with IZTO film were characterized. Amorphous IZTO films prepared at an oxygen partial pressure of 3% showed the best properties. The resistivity, mobility, transmittance, figure of merit and work function of the IZTO film were 5.6 × 10−4 Ω cm, 44.59 cm2 V s−1, 81% (visible region), 3.0 × 10−3 ohm−1, and 5.56 eV, respectively. The PLEDs with the IZTO film deposited under the optimum conditions showed the maximum brightness and the maximum luminance efficiency of 23 485 cd cm−2 and 2.29 cd A−1, respectively, which showed a 21% enhancement in device performance compared to PLEDs with commercial ITO film. In addition, the stability of the fabricated device was improved.

Enhanced performance in bulk heterojunction solar cells with alkylidene fluorene donor by introducing modified PFN-OH/Al bilayer cathode

We report bulk heterojunction solar cells with benzothiadiazole-containing polyalkylidene fluorene (PAFBToBT) donors made by introducing 2,4,7,9-tetramethyldec-5-yne-4,7-diol (Surfynol 104)-doped poly[9,9-bis(6′-(diethanolamino)hexyl)-fluorene] (PFN-OH) as an interfacial layer to enhance power conversion efficiency (PCE) and stability. Surfynol 104 has amphiphilic characteristics, and it supports the formation of a homogeneous interfacial layer on the photoactive layer. In addition, the application of the Surfynol 104-doped PFN-OH interfacial layer prevented the decrease in absorption of photons in the photoactive layer and improved the photocurrent density (JSC) by increasing the electron mobility. Therefore, JSC and the fill factor increased to 10.5 mA cm−2 and 49%, respectively, and the calculated PCE was 4.8%. In addition, a modified bilayer cathode system was introduced to the PCDTBT and the P3HT-based PSCs, and the PCE improved to 5.3% and 3.9%, respectively. Moreover, the in-air stability was enhanced.