Studies on Molecular Design of Vinylene and Cyano-Vinylene Containing Conjugated Polymers for High Performance Organic Solar Cells and Organic Field-Effect Transistors

Abstract

Conjugated polymers have been extensively studied because of their conducting and semiconducting properties since their discovery in 1977. Their charge transport properties enable their use in various electronic applications, such as in organic solar cells (OSCs) and organic field-effect transistors (OFETs). These applications require semiconductors that have two characteristics: a proper energy level and high charge mobility. The key design for realizing these properties requires efficient π-π stacking of the conjugated backbone and utilization of appropriate electron-monomer and electron-accepting monomer to match the desired energy level. In this study, I have focused on vinylene-containing conjugated polymers and their deverivatives, cyanovinylene-containing conjugated polymers. While vinylene group has attractive features, such as ability to make coplanar backbones and the easy tunability of their energy levels by substitution with cyano group, they have received little attention as semiconductors for OSCs and OFETs, because of the low device performance arising from their poor charge transport properties. This poor charge transport properies are be attributed to the structural disorders of the vinylene group, which is disadvantageous for π-π stacking. In this study, high performance conjugated polymers containing vinylene and cyanovinylene for OSC and OFET applications have been designed by controlling the structural disorders First, vinylene containing conjugated polymer has been designed for OSC applications. Poly(phenylenevinylene) (PPV) is a representative vinylene polymer. However, PPV has a poor charge transport properties. Recently, thiophene introduced PPV type polymer (PPVTV) has showed relatively high hole mobility, and thus achieved relatively high efficiency (3.5%) for OSCs as compared to PPVs. However, I have thought that the polymer contains a possibility of further improvement of performance by using the well known good moiety. In this respect, the new vinylene polymer (PBDS) has beed designed. This polymer design was implemented by introducing a two-dimensional benzo[1,2-b:4,5-b]dithiophene (2D-BDT), which is known to be a good charge transport moiety, into the highly coplanar and trans-stereoregular PPVTV. Interestingly, it was found that 2D-BDT plays a role in not only in improving the space charge limited current (SCLC) hole mobility but also in lowering the HOMO energy via a comparative analysis with polymer without 2D-BDT. Consequently, PBDS:PC71BM-based solar cell exhibited an enhanced open-circuit voltage (VOC) of 0.81 V and a short-circuit current (JSC) of 10 mA cm–2 compared to that without 2D-BDT, resulting in a power conversion efficiency (PCE) of 5%. This is a large PCE considering that the polymer has a wide bandgap of 2.0 eV, and it is, thus, suitable for tandem solar cell and ternary blend solar cell applications. (Chapter 2) Next, for further improvement of the PCE, PBDS has been modified by substitution with cyano group. While, cyanovinylene containing polymers have showed good optical properties, they have not showed high performances in OFET and OPV applications, due to their trans to cis photoisomerization reaction. In this work, cyano group was substitutied on beta site of vinylene, because beta cyanovinylene molecules are known to have coplanar structure and strong intermolecular interaction. Interestingly, the resulting polymer (PBDCS) exhibited 100% trans-stereoregularity and no trans-to-cis photoisomerization on exposure to UV light, unlike other cyanostyrylbenzene molecules. Besides, PBDCS has a much deeper FMOs and a lower band gap compared to PBDS, and shows storng aggregation properties in solution. Subsequently, an OSC based on PBDCS:PC71BM was found to have a high short-circuit current (JSC) of 15 mAcm-2 and a high open-circuit voltage (VOC) of 0.95 V, resulting in a relatively low Eloss of 0.80 eV, high EQEmax of 82.7%, and a high PCE of 8.75%. Furthermore, OSC based on PBDCS:ITIC exhibited a high PCE of 7.81% with a remarkably high VOC of 1.08 V and high JSC of 15.9 mAcm-2. In addition, the OSC has an extremely small Eloss of 0.49 eV despite the high EQEmax of 74.2%. This Eloss is the smallest value in an efficient OSC. From these results, we found that the cyanovinylene group provides strong intermolecular interactions and strongly stabilized FMOs (Chapter 3). Finally, we have used the cyanovinylene group to design an ambipolar semiconducting polymer. In general, the design of semiconductors with ambipolar charge transport properties is very limited due to their restrictive requirement of HOMO and LUMO energy levels, which must be close to the electrode work function. Donor-acceptor alternating polymer is the most widely used design. Howere, these polymers shows unipolar, especially p-type, because sufficiently strong accepting unit is very few, compared with donating units. In this study, ambipolar semiconducting polymer (PBDCS) has been designed by introducing the trans-cyanovinylene group into conventional donor-acceptor alternating polymers, p-type semiconductor. As a result, the polymer showed highly balanced hole and electron mobilities of μh,max ~ 0.2 cm2 V-1s-1 and μe,max ~ 0.2 cm2 V-1s-1, respectively, besides, which are virtually independent of the annealing temperature over the range of 80°C to 250°C. (chapter 4)