All-Polymer Solar Cells Incorporating Readily Accessible Naphthalene Diimide and Isoindigo Acceptor Polymers for Improved Light Harvesting

The development of readily accessible polymer acceptors is imperative to preserve the guiding principles of all-polymer solar cells as a low-cost and sustainable technology for alternative energy production. In this study, we report a computationally guided design and facile synthesis of new acceptor polymers comprising the alternating copolymer, PNIT, and the corresponding random copolymer, r-PNIT, which incorporate structurally simple naphthalene diimide (NDI) and isoindigo (IID) units. PNIT was prepared via direct arylation polymerization (DArP), which proceeds via C–H activation and avoids the use of toxic reagents and extended synthetic pathways for the monomer synthesis. PNIT has broad optical absorption in the 300–850 nm range and an enhanced absorption coefficient (47 × 103 cm–1) compared to r-PNIT (300–850 nm; 40 × 103 cm–1). When incorporated in all-polymer solar cells (APSCs) using PBDB-T as the donor polymer, PBDB-T:PNIT provides greater than two times the average (maximum) power conversion efficiency (PCE) of 5.18 ± 0.08 (5.32)% compared to that of PBDB-T:r-PNIT, 2.38 ± 0.16 (2.57)%, due to increased Jsc (10.36 vs 5.45 mA cm–2) and fill factor (FF) (0.58 vs 0.50) metrics. The PBDB-T:PNIT PCE is among the highest reported for an IID-based APSC and demonstrates the viability of DArP for the synthesis of new APSC acceptor polymers. Detailed morphological and microstructural investigations using atomic force microscopy (AFM) and grazing-incidence wide-angle X-ray scattering (GIWAXS), respectively, reveal enhanced texturing for PNIT, which enhances charge transport properties as supported by space-charge-limited current (SCLC) mobility measurements.