Enhanced molecular interaction by polymer additive for efficient and stable flexible perovskite solar cells

Flexible perovskite solar cells were potential photovoltaic technologies for achieving wearable power sources with excellent power conversion efficiency (PCE) and continuous roll-to-roll processability. However, the inherent fragile property and poor crystallinity of perovskite films on flexible substrates leaded to inferior photovoltaic performance, which impeded their application in wearable devices. Herein, a biodegradable polymer macromolecule, poly(D, L-lactide) (PDLLA), was doped into perovskite precursor solution to prepare perovskite films with favorable bendable and stretchable properties as well as the corresponding flexible PSCs. Extensive theoretical calculations and experimental analysis unraveled that the carbonyl (C = O) groups of PDLLA cross-linkers and undercoordinated Pb 2+ defect sites of perovskite films were chemically coupled through C = O···Pb coordination bond and the hydroxyl (O–H) groups at the end of PDLLA molecules anchored perovskite via O–H···I hydrogen bonding interaction. The strong chemical interactions could govern perovskite crystallization to modulate the films morphology, thus prompting the photovoltaic performance of PSCs. Furthermore, this cross-linking structure was instrumental in suppressing perovskite grain boundaries defects to minimize nonradiative recombination and advancing the film flexibility to enhance environmental and mechanical stability of the devices. Consequently, the optimized devices were fabricated in open-air conditions with desirable efficiencies of 18.94% on a rigid substrate and 16.61% on the corresponding flexible substrate. Since the strong chemical interaction between PDLLA additives and perovskites, the as-fabricated flexible devices assembled on the PET/ITO substrate exhibited favorable bending property, retaining 92.1% of the initial PCE after 2000 bending cycles within a bending radius of 2 mm, suggesting the benign mechanical stability of PSCs. This provided a guide for improving the flexibility of perovskite films for the purpose of the realization of the wearable electronic device. Graphical Abstract PDLLA accumulated at the grain boundary regions cross-links the neighboring perovskite crystals via C = O···Pb coordination bond and O–H···I hydrogen bonding interaction. The suitable molecular chemical chelation can simultaneously coordinate the defects accumulated at the grain boundaries and suppress the ion migration to optimize the photovoltaic performance and the mechanical stability.