Terpolymer approach for controlling the crystalline behavior of naphthalene diimide-based polymer acceptors and enhancing the performance of all-polymer solar cells

Abstract

A new series of n-type D–A terpolymers (P(NDI2HD-T-S)) was synthesized from an electron-deficient naphthalene diimide (NDI)-based unit in conjugation with two electron-rich thiophene (T) and selenophene (S) units, and their performances as electron acceptors in all-polymer solar cells (all-PSCs) were compared. The crystallinity of the P(NDI2HD-T-S) terpolymers can be systematically controlled by tuning the T/S molar ratios (T/S=100/0, 80/20, 50/50, 20/80 and 0/100) in the polymer backbone. An increase in the S content induced a significant enhancement in the crystallinity of the terpolymers. Therefore, the incorporation of more S units enhanced the structural ordering of the terpolymers and the electron mobility in the all-PSCs. The power conversion efficiency of the all-PSCs based on a P(NDI2HD-T-S) terpolymer acceptor and a PTB7 polymer donor increased from 2.50 to 3.60% as the S content increased, which was primarily due to the enhanced short-circuit current. To understand the effects of the T/S compositions on the photovoltaic performances, we investigated their influence on the optical, electrical and structural properties of the n-type D–A terpolymers. A series of n-type D–A terpolymers (P(NDI2HD-T-S)) with various thiophene (T) and selenophene (S) compositions (T/S=100/0, 80/20, 50/50, 20/80 and 0/100) was successfully synthesized. To understand the effects of the T/S ratios, we studied their influence on the optical, electrical and structural properties. The insertion of more S content in the polymer backbone induced the enhanced structural ordering and electron mobility. Therefore, the all-polymer solar cells based on PTB7:P(NDI2HD-T-S) blend had greatly improved efficiency as the S content increased.