Abstract
The study mainly explore the effect of modified poly(3-hexylthiophene) (P3HT) by end-capping with electron-deficient moieties (EDM) on thin film transistor (TFT) and polymer solar cell (PSC) performance. On the other hand, it demonstrates to design a series of high hole mobility and excellent thermal stability thienoisoindigo (TIG)-based D-A copolymers for TFT application. This research can be divided into three parts. Firstly, for end-group modification of P3HT, all P3HTs were controlled with high regioregularity and appropriate weight-average molecular weight (~50 kD) to maintain the original absorption capability, chain packing and electric property of rr-P3HT for efficient device performance. The uncapped P3HT-end-H/Br was polymerized via GRIM method and then converted to the end-functionalized P3HTs with EDMs in chain ends by in-situ method. Since EDMs participate the conjugation with nearly linked 3-hexylthiophene units, end-functionalized P3HTs can slightly lower energy levels and improve coplanarity of successive 3-hexylthiophene units near chain ends due to its electron-withdrawing capability. However, as polymer chains arrange, compared to the interruption of interchain packing by the steric hindrance of 3,4,5-triphenyl-4H-1,2,4-triazole (TAZ) in P3HT-end-TAZ, 2,5-diphenyl-1,3,4-oxadiazole (OXD) end-groups with the planar structure can promote P3HT-end-OXD form closer packing along the side chain direction than that of P3HT-end-H/Br, leading to the enhanced crystallinity in the thin film. For this reason the hole mobility of P3HT-end-OXD-based TFT can improve from P3HT-end-H/Br with a value of 2.9 × 10-2 cm2 V-1 s-1 to 3.8 × 10-2 cm2 V-1 s-1 relative to P3HT-end-TAZ (μh = 1.8 × 10-3 cm2 V-1 s-1). Secondly, we also apply these previously used P3HTs to the PSC system with PCBM as acceptor. Similarly, end-functionalized P3HTs by electron-withdrawing capability of EDM end-groups not only can slightly lower HOMOs for the enhanced open-circuit voltage (Voc) of PSCs, but also can extend lifetime for exciton migration to the two-phase interface in the photoactive layer for efficient exciton dissociation due to the induced energy transfer by the improved coplanarity near chain ends. Furthermore, when blending with PCBM, planar OXD end-groups provide similar function as nonvolatile additives to assist P3HT-end-OXD in improving chain ordering without change of extent of phase separation in the blend film with PCBM, resulting in a promotion of absorption coefficient and charge carrier transport property for an effective contribution to short-circuit current density (Jsc) and fill factor (FF) of the resulting device. Therefore, power conversion efficiency (PCE) of P3HT-end-OXD:PCBM-based PSC can be enhanced from P3HT-end-H/Br:PCBM of 3.28 % to 4.28 %. Whereas, P3HT-end-TAZ:PCBM-based PSC has the worst PCE of 0.50 % as a result of the disrupted crystallinity and unfavorable PCBM aggregations by bulky TAZ end-groups. Thirdly, for the molecular design of TIG-based D-A copolymers, we develop a series of high mobility TIG-based D-A copolymers by the concept of extension of π-conjugation in the backbone and enhancement of interchain interaction of D-A moieties. The fused molecule, thieno[3,2-b]thiophene (TT) or (E)-2-(2-(thiophen-2-yl)vinyl)thiophene (TVT) was employed as the donor moiety in the synthesis of four new TIG-based D-A copolymers. Furthermore, thiophene molecules are introduced as end-cappers for these copolymers to eliminate the influence of end-group defects on TFT performance. The series of TIG-based copolymers not only has a broad absorption band extended up to 1300 nm, but also gives a matched HOMO level (-5.15 to -5.29 eV) with gold electrodes, which is beneficial for charge injection. Compared to C8C12TIG-TVT (μh = 0.29 cm2 V-1 s-1) and C10C14TIG-TVT (μh = 0.42 cm2 V-1 s-1), a highest hole mobility up to 0.69 cm2 V-1 s-1 among reported TIG-based PTFTs with the general device fabrication can be achieved by C8C12TIG-TT under optimum conditions. This record performance is ascribed to TT with more planar structure and more electron-donating property than TVT. Therefore, C8C12TIG-TT chains take the ordered lamellar edge-on packing structure relative to the substrate with a shortest π-π stacking distance of 3.43 Å among reported TIG-based copolymers for efficient interchain charge transport. However, it is worth noting C8C12TIG-TT at annealing temperature of 300 oC has a critical alignment transition of chain packing to form more chains with a face-on configuration, leading to a decrease in the mobility. In addition, the series of TIG-based copolymers also exhibits an outstanding thermal stability on device performance without annealing temperature-dependent on/off ratio and threshold voltage effects (Ion/Ioff > 104 and 1.5 < Vth < 2.8). The effect of end-group modification of P3HT demonstrates that the structural defect of the polymer chain ends is a non-negligible influence on device performance, but can be tailored by appropriately functional EDMs. However, the efficacy depends strongly on the structure of EDM, such as size and planarity. Hence, the success of the fine modification in P3HT might also provide a promising route for other polymers for enhancement of device performance. In the case of TIG-based D-A copolymers, the result provides further progress towards the use of TIG-based D-A copolymers for PTFT application and gives some valuable information for molecular design on this type of polymer.