Kwan Wook Song

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.

Alkylidenefluorene–isoindigo copolymers with an optimized molecular conformation for spacer manipulation, π–π stacking and their application in efficient photovoltaic devices

D–(π)n-A-type copolymers with different thienyl spacers (n = 0–2) between alkylidenefluorene and isoindigo (P1, P2, P3) were synthesized via a Suzuki coupling reaction. As the number of spacers was increased in a polymer (P1), the UV-Vis absorption maximum (λmax) red-shifted, and the band gap decreased from 1.83 to 1.64 eV. The highest occupied molecular orbital (HOMO) levels of the polymers were increased by increasing the number of spacers. In addition, the facile intermixing due to better accessibility with the PCBM led to an increase in the hole mobility and JSC. In contrast to P1 and P2, when the P3 thin films were blended with PC70BM in the X-ray diffraction (XRD) measurement, an increased face-on structure of the crystal was observed. The power conversion efficiency (PCE) of P3 was 3.0% and it reached 4.2% for the inverted device fabricated at 1 : 2 ratio with PC70BM.

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.