A universal tactic of using Lewis-base polymer-CNTs composites as additives for high performance cm2-sized and flexible perovskite solar cells

Lewis-base polymers have been widely utilized as additives to act as a template for the perovskite nucleation/crystal growth and passivate the under-coordinated Pb2+ sites. However, it is uncovered in this work that the polymer on the perovskite grain boundaries would significantly hinder the charge transport due to its low conductivity, which brings about free carrier recombination and photocurrent losses. To circumvent this issue while fully exploiting the benefits of polymers in passivating the trap states in perovskite, we incorporate highly conductive multiwall carbon nanotubes (CNTs) with Lewis-base polymers as co-additives in the perovskite film. Functionalizing the CNTs with -COOH group enables a selective hole-extraction and charge transport from perovskite to the hole transporting materials (HTM). By studying the charge transporting and recombination dynamics, we revealed the individual role of the polymer and CNTs in passivating the trap states and facilitating the charge transport, respectively. As a result, the perovskite solar cells (PSCs) with polymer-CNTs composites exhibit an impressive PCE of 21.7% for a small-area device (0.16 cm2) and 20.7% for a large-area device (1.0 cm2). Moreover, due to the superior mechanical flexibility of both polymer and CNTs, the polymer-CNTs composites incorporation in the perovskite film encourages the fabrication of flexible PSCs (f-PSCs) with an impressive PCE of 18.3%, and a strong mechanical durability by retaining 80% of the initial PCE after 1,000 times bending. In addition, we proved that the selection criteria of the polymers can be extended to other long-chain Lewis-base polymers, which opens new possibilities in design and synthesis of inexpensive material for this tactic towards the fabrication of high performance large-area PSCs and f-PSCs.