Effect of the incorporation of poly(ethylene oxide) copolymer on the stability of perovskite solar cells†
The incorporation of a well-established copolymer based on poly(ethylene oxide) was investigated towards maximizing the stability of perovskite solar cells based on methylammonium lead iodide (MAPbI3). Poly(ethylene oxide-co-epichlorohydrin) P(EO/EP) was chosen as an additive to the precursor solution to minimize the degradation process under ambient conditions. A power conversion efficiency of 17.04% was obtained for the standard solar cell, and the efficiency was gradually decreased as the concentration of P(EO/EP) was increased. In spite of the efficiency loss, the stability of the films against aggressive humidity and illumination conditions was investigated and the perovskite films containing the copolymer showed enhanced stability. Through H-NMR spectroscopy, it was possible to verify the existence of hydrogen bonding between the polymer and methylammonium cation. This interaction is responsible for retaining the cation in the structure, thus conferring stability to the film. It was also observed that the polymer incorporation delays the perovskite crystallization, which was accompanied by in situ grazing incidence wide angle X-ray scattering. The incorporation of P(EO/EP) has also decreased the average grain size and passivated the surface defects of the perovskite layer. The device without P(EO/EP) and the device containing 1.5 mg mL−1 P(EO/EP) retained 47% and 68% of the starting PCE values, respectively, after 480 hours (20 days) exposed to environmental conditions (relative humidity ∼53%), indicating that the incorporation of P(EO/EP) in the perovskite active layer provided greater stability to the perovskite film.