Side Chain Engineering on Polymeric Semiconductors for Optoelectronic Device Applications

Polymeric semiconductors have received great attentions for organic electronic and optoelectronic devices, such as field-effect transistors (FETs), photovoltaic cells (PVs), and memory devices. In the recent progress of polymer community, side chains are act as a crucial component in the design of novel conjugated polymers. They not only directly relate to the solubility but also affect the molecular packing motifs and thin film morphologies. The goal of this thesis is to address the effect of conjugated or alkyl side chain structures on the polymer thin film morphologies and the optoelectronic properties. In addition, the field-effect mobilities, photovoltaic, or memory characteristics are also probed to investigate the side chain engineering design on polymeric semiconductors for optoelectronic devices systematically. Three different strategies are explored in this thesis, as shown in followings: 1. Syntheses of two-dimensional branched thiophene extended octithiophene‐based conjugated polymers for field-effect transistors and photovoltaic cells: In Chapter 2, three octithiophene (8T)-based conjugated copolymers, including P8TSe, P8TT, and P8TTT, have been synthesized. The larger atomic radius selenium (Se) atom possesses higher polarizability than sulfur (T), inducing stronger intermolecular interactions in solid state. Also, 8T moiety could significantly lower the HOMO level and lead to the enhanced open circuit voltage because of its branched conformation. The hole mobilities of these 8T-based copolymers were in the range of 1.32×10-5 to 5.00×10-5 cm2V-1s-1 with on/off ratio of 104. Among them, P8TTT showed better characteristics than the other polymers due to the fused-ring TT can promote self-organization and minimize the steric interactions. The power conversion efficiencies (PCE) of the copolymers/PC71BM based photovoltaic cells were in the range of 1.28 - 2.30% under the illumination of AM 1.5G (100 mW cm-2). In particular, P8TTT showed the best PCE of 2.81%, as the blend films are prepared from the mixed solvent of o-dichlorobenzene (DCB) and 1,8-diiodoctane (DIO) (DCB/DIO = 97%:3% by volume). In Chapter 3, the synthesis, morphology and optoelectronic device applications of 2D extended quaterthiophene (4T)- and octithiophene (8T)-vinylene conjugated polymers, P4TV and P8TV, were explored. P4TV and P8TV exhibited smaller energy band gaps of 1.69 and 1.78 eV than that of parent polythiophenes, respectively, due to the reduced conformation distortion by the vinylene linkage. The highest field-effect hole mobilities of P4TV and P8TV were 0.12 and 0.0018 cm2V-1s-1, respectively, with on/off ratios around 104-105. In addition, the power conversion efficiency (PCE) of the P4TV/PC71BM based photovoltaic cells under the illumination of AM 1.5G (100 mW cm-2) was 4.04 %, which was significantly higher than that of P8TV/PC71BM with 2.69 %, due to its superior charge transport ability. However, P8TV had a better environmental stability attributed to its low-lying HOMO energy level. 2. Syntheses of main chain donor tethered side chain phenanthro[9,10-d]imidazole acceptor conjugated polymers for high performance flexible resistive memory devices: In Chapter 4, a bipolar-recorded resistive memory device consisting of a single-layer donor-acceptor conjugated polymer fabricated on plastic polyethylene naphthalate (PEN) have been developed. The newly designed conjugated polymer with a main-chain donor of fluorene and thiophene and a side-chain acceptor of phenanthro[9,10-d]-imidazole (PFT-PI) was synthesized as an active memory material. The reproducible, nonvolatile flash switching characteristics of each sandwiched PEN/Al/PFT-PI/Al memory device was demonstrated under bending. The flexible nonvolatile resistor memory devices with low threshold voltages (±2 V), low switching powers ( 100 μW cm−2), large ON/OFF memory windows (104), good retention (>104 s) and excellent endurance against electric and mechanical stimulus. The simple and facile device fabrication was obtained from a single PFT-PI memory material, without using charge injection layers or a complex multilayer structure. In Chapter 5, the synthesis and resistive memory device characteristics of new donor-acceptor conjugated poly(arylene vinylene), PVC-PI, PVT-PI, and PVTPA-PI, have been explored. The studied polymers possess similar HOMO energy levels (-5.08 ~ -5.18 eV), but with different LUMO energy levels (-2.24, -3.40, and -2.60 eV for PVC-PI, PVT-PI, and PVTPA-PI, respectively). The PVC-PI flexible memory with the sandwich configuration of PEN/Al/polymer/Al reveals the volatile static random access memory (SRAM) characteristic while the PVTPA-PI device exhibits the nonvolatile write-once-read-many-times (WORM) switching behavior. The above two devices could operate at low voltages (less than 2.5 V) with high ON/OFF current ratios (over 104) and exhibit excellent durability upon repeated bending tests. The PVT-PI device, however, only shows a diode-like electrical behavior. The polymer conformation affects the strength of D-A electrical polarization and charge trapping ability, leading to the variation on the volatility of the memory devices. 3. Effects of alkyl side chain design on charge transport: Synthesis, morphology, and stretchable transistor applications: In Chapter 6, three polymers with variant alkyl side chain structures (i.e. short linear, long linear, and branched alkyl side chains), namely P3HT, PTDPPTFT4, and PII2T, are evaluated for stretchable field-effect transistors. In addition, a facile method to efficiently identify suitable semiconducting polymers for organic stretchable transistors using soft contact lamination is described. In this method, the various polymers investigated are first transferred on elastomeric poly(dimethylsiloxane) (PDMS) slab, and subsequently stretched (up to 100 %) along with the PDMS. The polymer/PDMS matrix is then laminated on source/drain electrode-deposited Si substrates equipped with a PDMS dielectric layer. The polymer semiconductors can be repeatedly interrogated with laminate/delaminate cycles under different amounts of tensile strain, and the strain limitation of semiconductors enable different side chain structures can be derived. In Chapter 7, a series of isoindigo-based conjugated polymers (PII2F-CmSi, m=3-11) with alkyl siloxane-terminated side chains have been prepared, in which the branching point is systematically “moved away” from the conjugated backbone by one carbon atom. All soluble PII2F-CmSi (m=5-11) polymers exhibited hole charge carrier mobilities over 1 cm2V-1s-1, while the reference polymer with the same polymer backbone showed a much lower mobility of 0.13 cm2V-1s-1. PII2F-C9Si showed the highest mobility of 4.76 cm2V-1s-1, even though PII2F-C11Si exhibited the smallest π-π stacking distance at 3.379 Å. We concluded that it is beneficial that the branching site was further away from conjugated backbones to improve charge transport characteristics. The above studies demonstrate that the optoelectronic properties, charge carrier transport ability, solar cell efficiency, and memory behaviors can be manipulated using side chain engineering design. The device performances were tuned by controlling the chemical structures of conjugated side chains. Moreover, with variant alkyl side chain structures, the charge carrier mobility in stretched polymer thin films were changed, indicating the design of side chain on polymeric semiconductors plays a crucial role for next-generation electronic device application.