Role of Molecular Weight in Microstructural Transition and Its Correlation to the Mechanical and Electrical Properties of P(NDI2OD-T2) Thin Films

Abstract

Semiconducting polymers with high mobility and mechanical robust properties are strongly dependent on their molecular weight. However, the relationship between molecular weights and solution chain entanglements, film microstructures, charge carrier mobility, and mechanical properties for donor−acceptor conjugated polymers remains less understood. Herein, P(NDI2OD-T2) with a weight-average molecular weight (M w ) from 34.0 to 280 kDa was investigated as a model system. The polymer chain exhibited three regions in chloroform solutions: fewer entanglements (34.0−77.7 kDa), enhanced entanglements (170 kDa), and severe entanglements (280 kDa). This chain solution behavior resulted in three distinct film microstructures: (1) 34.0−77.7 kDa, liquid-crystalline-like morphologies with highly ordered chain arrangements and large crystallite lengths (l c ) yet relatively low tie-chain densities that increased with M w ; (2) 170 kDa, small fibril morphology with less ordered chain arrangements and a decreased l c of only 5.6 nm yet a high tie-chain density; and (3) 280 kDa, a seemingly amorphous film with vast wellconnected local aggregates embedded in an entangled network. The structural change in films significantly affected the electrical and mechanical performances. The electron mobility increased continuously with M w , correlating well with the tie-chain density. By contrast, the crack-onset strain was less than 3% at 34.0−77.7 kDa and then jumped to 36.4 ± 0.9 and 60.4 ± 2.1% for 170 and 280 kDa, showing a close correlation with the solution entanglement density, which could be inherited into films. This study contributes to structural development of rigid chains with M w and demonstrates that the microstructure containing vast well-connected local aggregates and adequate entanglements is promising toward mechanically robust semiconducting films.

Optimizing the Intercrystallite Connection of a Donor–Acceptor Conjugated Semiconductor Polymer by Controlling the Crystallization Rate via Temperature

“…For example, in the previous studies, the polymer chain of P(NDI2OD‐T2) always grew parallel to the long axis of the fiber. [ 2,3 ] The crystal size was large in the film prepared at 40 °C (Figure 7b 1,b2). Compared to the film prepared at 25 °C, the polymer chain order and tie‐chain arrangement showed no obvious change, as shown in Figure 7b 1,b2.…”

“…Donor-acceptor (D-A) conjugated polymers have attracted much attention because of their high device mobility despite the absence of high crystallization and long-range order. [1][2][3][4][5][6] The effective charge carrier transport in the short-range ordered region DOI: 10.1002/marc.202200084 and intercrystallite tie-chain are the contributing factors to high mobility. [7][8][9][10][11] Recently, many types of research devoted to the optimization of the electrical performance through intercrystallite tie-chain connection, which was mainly focused on two aspects: 1) molecular weight, [12,13] and 2) crystallization kinetics.…”

“…[14][15][16] The effective tie-chains can be formed when the polymer molecular weight reaches the critical molecular weight (M c ). [3,17,18] Low molecular weight conjugated polymers usually exhibit extended chains and stack into discontinuous crystalline regions. Therefore, the intercrystallite charge transfer is seriously limited.…”

“…The research of a series of poly[[N,N-bis(2octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]alt-5,5′-(2,2′-bithiophene)] (P(NDI2OD-T2)) with weight-average molecular weight (M w ) from 34 to 280 kDa showed that the mobility increased with M w (from 1.1 × 10 −4 to 1.1 × 10 −1 cm 2 V −1 s −1 ), which was consistent with the trend of tie-chain fraction. [3] The formation of tie-chain is closely related to crystallization kinetics. [31][32][33][34][35][36][37][38] First, the probability of the formation of tie-chain is closely related to the lengths of the crystal region (l c ) and the amorphous region (l a ).…”

“…The research of a series of poly[[N,N‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)] (P(NDI2OD‐T2)) with weight‐average molecular weight ( M w ) from 34 to 280 kDa showed that the mobility increased with M w (from 1.1 × 10 −4 to 1.1 × 10 −1 cm 2 V −1 s −1 ), which was consistent with the trend of tie‐chain fraction. [ 3 ]…”