Synthesis, Structural Characterization, and Field-Effect Transistor Properties of n-Channel Semiconducting Polymers Containing Five-Membered Heterocyclic Acceptors: Superiority of Thiadiazole Compared with Oxadiazole

Abstract

Five-membered 1,3,4-oxadiazole (OZ) and 1,3,4-thiadiazole (TZ) heterocycle-based copolymers as active layer have long been ignored in solution-processable n-channel polymer field-effect transistors (PFETs) despite the long history of using OZ or TZ derivatives as the electron-injecting materials in organic light-emitting devices and their favorable electron affinities. Herein, we first report the synthesis and PFETs performance of two n-channel conjugated polymers bearing OZ- or TZ-based acceptor moieties, i.e., PNOZ and PNTZ, where simple thiophene units are utilized as the weak donors and additional alkylated-naphthalenediimides units are used as the second acceptors. A comparative study has been performed to reveal the effect of different heterocyclic acceptors on thermal properties, electronic properties, ordering structures, and carrier transport performance of the target polymers. It is found that both polymers possess low-lying LUMO values below -4.0 eV, indicating high electron affinity for both heterocycle-based polymers. Because of strong polarizable ability of sulfur atom in TZ heterocycle, PNTZ exhibits a red shift in maximal absorption and stronger molecular aggregation even in the diluted chlorobenzene solution as compared to the OZ-containing PNOZ. Surface morphological study reveals that a nodule-like surface with a rough surface morphology is observed clearly for PNOZ films, whereas PNTZ films display highly uniform surface morphology with well interconnected fiber-like polycrystalline grains. Investigation of PFETs performance indicates that both polymers afford air-stable n-channel transport characteristics. The uniform morphological structure and compact π-π stacking endow PNTZ with a high electron mobility of 0.36 cm V s, much higher than that of PNOZ (0.026 cm V s). These results manifest the feasibility in improving electron-transporting property simply by tuning heteroatom substitutes in n-channel polymers; further demostrate that TZ derivatives possess much superior potential for developing high-performance n-channel polymers compared to OZ derivatives.

The Photoelectric Properties of Polymer Acceptors Containing Oxadiazole and Thiadiazole

“…PNOZ and PNTZ were synthesized according to the literature . The M n and polydispersity index (PDI) are 20.9 kg mol −1 and 1.7 for PNOZ, 14.3 kg mol −1 and 1.8 for PNTZ, respectively.…”

“…As depicted in the energy diagram (Figure ), PNOZ, PNTZ, and PTB7 have the LUMO/highest occupied molecular orbital (HOMO) levels of −4.05 eV/−5.82 eV, −4.02 eV/−5.70 eV, and −3.31 eV/−5.15 eV, respectively. The up‐lying LUMO levels of the polymer acceptors are in favor of providing high V oc of the all‐PSCs.…”

“…Both polymer acceptors possess deeply lowest unoccupied molecular orbital (LUMO) energy levels below −4.0 eV, which are sufficiently low as the acceptor materials for most of donor polymers. In addition, PNOZ and PNTZ exhibit high electron mobilities of 0.026 and 0.36 cm 2 V −1 s −1 in organic field‐effect transistors, respectively . The characteristics suggested a great potential of PNOZ and PNTZ as the acceptors in all‐PSCs.…”

Imide‐Functionalized Polymer Semiconductors

“…The highly electron‐deficient imine containing 1,3,4‐oxadiazole and 1,3,4‐thiadiazole enabled the resulting polymers to have high electron affinity and good backbone planarity, making them promising for building n‐type semiconductors. With a low‐lying LUMO level below −4.0 V and strong aggregation characteristics, 38 b exhibited an air‐stable μ e,OTFT as high as 0.36 cm 2 V −1 s −1 . In another report, Liu and co‐workers introduced the thiophene/selenophene flanked benzothiadiazole into the NDI‐based polymer 39 .…”

“…With al ow-lying LUMO level below À4.0 Vand strong aggregation characteristics, 38 b exhibited an air-stable m e,OTFT as high as 0.36 cm 2 V À1 s À1 . [51] In another report, Liu and co-workersi ntroduced the thiophene/ selenophene flankedb enzothiadiazole into the NDI-based polymer 39.A ni mpressive m e,OTFT as high as 8.5 cm 2 V À1 s À1 was attained for the OTFTsc ontaining 39 b as the active layer,i n which the benzothiadiazole unit was flanked with two selenophene units. Thus, as trongi nterchain interaction with ac lose p-p stacking distance was obtained because of the larger p-orbital of the selenophene rings in 39 b.…”