Thermoelectric (TE) materials based on conjugated polymers have received much attention due to their great advantages of solution processibility, light weight, flexibility, and low thermal conductivity. These advantages make them potential candidates for large-area, low-cost and low-power TE applications. Both efficient p-type and n-type conjugated polymers with high and comparable thermoelectric performance are required for practical TE applications. However, due to the inefficient n-doping efficiency and unstable electron transport of most n-type conjugated polymers, the TE performance of n-type polymers is much poorer than that of their p-type counterparts, impeding the development of polymer TE materials. Great efforts have been made to address the low n-doping efficiency and TE performance of n-type polymers, including the chemical modification of traditional n-type polymers, the design of new n-type conjugated polymers, and the development of more efficient n-dopants, as well as doping engineering. Nowadays, the TE performance of n-type polymers has been greatly improved, indicating a bright future for polymer TE materials. In this review, we summarize the recent progress made on n-type polymer TE materials, mainly focusing on the structure-performance relationships based on promising n-type polymers for TE applications. This review aims to provide some guidelines for future material design.

A thickness-insensitive cathode interlayer (CIL) is necessary for large-area polymer solar cells (PSCs), in which thickness variation is unavoidable. These CIL materials are typically based on n-type conjugated polymer/molecule backbones, which show strong light absorption in the visible/near-infrared (NIR) region. This interferes with the sunlight absorption by the active layer and deteriorates device efficiency. In this study, we developed graphene quantum dots functionalized with ammonium iodide (GQD-NI) at the edge as a thickness-insensitive CIL with high optical transparency. The peripheral ammonium iodide groups of GQD-NI formed the desired interfacial dipole with the cathode to decrease the work function. The graphene basal planes of GQD-NI with a lateral size of ca. 3 nm demonstrated a good conductivity of 3.56 × 10^–6 S·cm^–1 and high transparency in the visible/NIR region (λ_max^abs = 228 nm). Moreover, GQD-NI was readily soluble in polar organic solvents, e.g., methanol, which enabled multilayer device fabrication with orthogonal solvent processing. As a result, the PSC device with GQD-NI as the CIL exhibited a power conversion efficiency (PCE) of 7.49%, which was much higher than that of the device without the CIL (PCE = 5.38%) or with calcium as the CIL (PCE = 6.72%). Moreover, the PSC device performance of GQD-NI was insensitive to the GQD-NI layer thickness in the range of 2–22 nm. These results indicate that GQD-NI is a very promising material for application as a CIL in large-area printed PSCs.