Influence of Molecular Weight on the Organic Electrochemical Transistor Performance of Ladder‐Type Conjugated Polymers

Abstract

Organic electrochemical transistors (OECTs) hold promise for developing a variety of high‐performance (bio‐)electronic devices/circuits. While OECTs based on p‐type semiconductors have achieved tremendous progress in recent years, n‐type OECTs still suffer from low performance, hampering the development of power‐efficient electronics. Here, it is demonstrated that fine‐tuning the molecular weight of the rigid, ladder‐type n‐type polymer poly(benzimidazobenzophenanthroline) (BBL) by only one order of magnitude (from 4.9 to 51 kDa) enables the development of n‐type OECTs with record‐high geometry‐normalized transconductance (gm,norm ≈ 11 S cm−1) and electron mobility × volumetric capacitance (µC* ≈ 26 F cm−1 V−1 s−1), fast temporal response (0.38 ms), and low threshold voltage (0.15 V). This enhancement in OECT performance is ascribed to a more efficient intermolecular charge transport in high‐molecular‐weight BBL than in the low‐molecular‐weight counterpart. OECT‐based complementary inverters are also demonstrated with record‐high voltage gains of up to 100 V V−1 and ultralow power consumption down to 0.32 nW, depending on the supply voltage. These devices are among the best sub‐1 V complementary inverters reported to date. These findings demonstrate the importance of molecular weight in optimizing the OECT performance of rigid organic mixed ionic–electronic conductors and open for a new generation of power‐efficient organic (bio‐)electronic devices.

On the Origin of Seebeck Coefficient Inversion in Highly Doped Conducting Polymers

“…Interestingly, aggregate sizes in the range (3-5) 3 nm 3 match well to the values extracted from GIWAXS of pristine BBL: the (010) π-π stacking coherence length is about 35 Å, while the (100) coherence length is a bit less than 20 Å. [31] Furthermore, a polaron is primarily delocalized over three monomer units in a BBL chain, [16] indicating that the coherence lengths and polaron delocalization length match extremely well with our estimated range for the unit cell size.…”

Green Synthesis of Lactone‐Based Conjugated Polymers for n‐Type Organic Electrochemical Transistors

“…To date, there are only four classes of n-type conjugated polymers for OECTs: naphthalene-1,4,5,8-tetracarboxylic-diimide-bithiophene (NDI)-based D-A conjugated polymers, [22][23][24][25][26][27] ladder-type poly(benzimidazobenzophenanthroline) (BBL) polymers, [22,28] fused electron-deficient lactam backbone PgNaN, [29] and, most recently, fused bithiophene imide dimer (f-BTI2)-based polymers, [30] and BBL152. [31] Among these, BBL152, through molecular weight engineering, exhibits the best reported performance, with a μC* of 25.9 F cm −1 V −1 s −1 and a g m,norm of 11.1 S cm −1 , slightly higher than f-BTI2 (μC* = 15.2 F cm −1 V −1 s −1 and g m,norm = 4.60 S cm −1 ), and one order of magnitude higher than PgNaN-based polymers (μC* = 0.78 F cm −1 V −1 s −1 and g m,norm = 0.21 S cm −1 ) and BBL (μC* = 0.60 F cm −1 V −1 s −1 and g m,norm = 0.3 S cm −1 ).…”

Cyano‐Functionalized n‐Type Polymer with High Electron Mobility for High‐Performance Organic Electrochemical Transistors

“…Since the pioneering work of p(gNDI-gT2) in 2016, [38] there are only limited number of reports on n-type materials for OECTs (please see summary in Table S1, Supporting Information). They mainly include donor-acceptor (D-A) polymers such as naphthalene diimde (NDI)-based P90, [39,40] p(C6-gNDI-gT2), [41] P(NDIMTEG-T), [42] P(C6-T2), [43] bithiophene diimide-based f-BTI2TEG-FT, [44] isoindigo-based AIG-BT, [45] lactone-based p(C-T), [46] and diketopyrrolopyrrolebased P(gPzDPP-CT2), [47] all-acceptor (A-A) ladder-type polymers BBL [48,49] and PgNaN, [50] small molecule C60-TEG. [51] n-Type organic mixed ionic-electronic conductors (OMIECs) with high electron mobility are scarce and highly challenging to develop.…”

“…Promising μC* has been obtained for BBL 152 with a highest reported value of 25.9 F cm −1 V −1 s −1 . [48] These performances, however, are still much inferior to their p-type counterparts, necessitating further investigation of high-performance n-type polymers and OECTs for applications such as biosensors [52][53][54] and low-power complementary circuits. [55][56][57] Based on the reported C* and μ values for n-type OECTs (Table S1, Supporting Information), it is quite clear that the electron mobility μ is the limiting factor in obtaining high μC*.…”

“…[46] Therefore, it is highly challenging to simultaneously yield high electron mobility and ion transport capability to improve the μC* for n-type polymers. In order to improve μ e,OECT , various strategies have been employed such as side-chain engineering and backbone structure optimization, [41][42][43][44]46,47] increasing polymer molecular weight, [48] lowering device contact resistance. [58] To this end, utilizing the rich library of electrondeficient building blocks, which have been successful in building a great number of high mobility n-type polymers for organic thin-film transistors (OTFTs), [59][60][61][62] is considered as an effective approach to construct n-type OECT materials.…”