Assembly of non-close-packed poly(N-isopropylacrylamide) microgels on metal halide perovskite films.

HYPOTHESIS: Poly(N-isopropylacryamide) microgels (PNIPAM MGs) are interesting model colloids with remarkable self-ordering properties. Metal halide perovskites (MHPs) are solution processable semiconductors that have attracted great interest for optoelectronics applications. Very few studies have considered complex co-crystallizing systems and their effects on structural order formation. This work examines PNIPAM MG ordering within a MHP matrix and investigates whether these pure PNIPAM MGs could form arrays on the MHP surface, passivate MHP defects and provide structurally colored films and solar cells.

EXPERIMENTS: We use a mixed cation MHP (FA0.88MA0.12PbI3) (FA and MA are formamidinium and methylammonium) and investigate the effects of MG size and PNIPAM MG/MHP composite composition on two-dimensional non-closed packed hexagonal array (2D-NCPHA) formation and structural color. SEM and angle-dependent observations of illuminated films are used to assess structural order as well as the colors of light generated within composite films and MHP solar cells. The colors observed were converted to CIE1931 values and the color trends predicted by a simple ray-tracing equation.

FINDINGS: We show that spontaneous ordering of PNIPAM MGs occurred on the MHP matrix and caused color generation from a relatively thick co-crystallizing MHP matrix. An optimum PNIPAM MG diameter is identified and the mechanisms operative are elucidated. Structurally colored solar cells are also demonstrated and their efficiency improvement is due to PNIPAM passivation of MHP defects. The results of this study indicate that PNIPAM 2D-NCPHA formation is highly tolerant to solution species and would allow, in principle, such phenomena to be achieved in other co-crystallizing matrices.