Design and Synthesis of Donor–Acceptor Type Small Molecules for High Performance Organic Solar Cells

Abstract

As a promising alternative to polymer solar cells, small molecular organic solar cells (OSCs) have recently been developed because of their characteristic advantages of well-defined molecular weight, synthetic reproducibility, and simple purification process. However, the photovoltaic performance is still lower than those of polymer solar cells. Thus, further works on designing molecular structure are needed for improving the performance of organic solar cell. Owing to intensive studies on the ideal donor material in bulk heterojunction OSCs for the past decade, some requirements for high performance OSCs have been established: (1) broad and strong light absorption in the visible region to harvest enough solar light, (2) high hole mobility for fast charge carrier transport to yield high short-circuit current (JSC), (3) suitable energy levels to ensure high open-circuit voltage (VOC) and efficient exciton dissociation, and (4) appropriate compatibility with the fullerene acceptor to form nano-scale bicontinuous interpenetrating networks for efficient exciton dissociation. These properties can offer high values of JSC, VOC and fill factor of the OSCs, and result in high power conversion efficiency (PCE). In order to satisfy these requirements, reasonable integration of donor and acceptor unit in conjugated small molecule (SM) should be carefully considered in terms of molecular energy levels, crystallinity, and solubility in organic solvent. In this thesis, we designed and synthesized new small molecular donors to examine the effect of structural modification on their photovoltaic properties. Firstly, a series of simple structured SMs based on diketopyrrolopyrrole (DPP) are synthesized and their photovoltaic properties are investigated in terms of the type of electron donating unit. By introducing a donor unit with different electron-donating power such as thiophene (T) and phenylene (Ph), into A–D–A type SM, the frontier orbital energy levels of SMs can effectively be tuned. The SM with a weak donor unit of Ph, Ph(TDPP)2 exhibits low-lying highest occupied molecular orbital (HOMO) energy level, nanoscale phase separation in blend film with phenyl-C71-butyric acid methyl ester (PC71BM ). Among DPP-based SMs, Ph(TDPP)2 afforded high VOC of 0.93 V with a PCE of 4.01%. While the photovoltaic performance with the DPP-based SMs have yielded the promising PCE with high VOCs, the low JSC, around 9.0 mA cm2, is a main limiting factor for high efficiency OSCs. Generally, close packing of organic materials in solid film leads to high charge carrier mobility. Therefore, we synthesized four different DPP-based SMs with A–D–A type structure, where electron-donating unit was systematically varied with different electron-donating power (thiophene vs. phenylene

thienothiophene vs. naphthalene) and different molecular planarity (bithiophene vs. thienothiophene

and biphenylene vs. naphthalene). The SMs with weak donating unit (phenylene or naphthalene) have deeper HOMO energy levels than those with strong donating unit (thiophene or thienothiophene), and thus exhibit higher VOC. When the fused aromatic ring (thienothiophene or naphthalene) with planar molecular structure is introduced in SMs, the SMs exhibit high hole mobility and thus afford high JSC. As a result, the introduction of naphthalene (weak donating power and planar structure) enhances both VOC and JSC, resulting in a promising PCE of 4.4%. Finally, two different thienopyrroledione (TPD)-based SMs with different alkyl substitution positions were synthesized, and their photovoltaic properties are measured and compared to examine the effect of the alkyl substitution position on their optical, electrochemical, and photovoltaic properties. The use of TPD as an electron-accepting unit in conjugated SMs effectively lowers the HOMO energy levels of the conjugated SMs and leads to high VOC. Two SMs with n-hexyl group substituted at different positions exhibit almost identical optical and electrochemical properties in the pristine state. However, the crystallographic and morphological characteristics of DTS(HexTPD2T)2/PC71BM blend film yield efficient charge transport. As a consequence, DTS(HexTPD2T)2 exhibits a PCE of 6.0% with a VOC of 0.94 V and a JSC of 11.8 mA cm-2.