Confinement Controlled Phase‐Pure 2D‐Layered Perovskites for Band‐Edge Engineering

Controlling the number n of connected octahedra layers in 2D metal halide perovskites enables fine control of the semiconductor bandgap that is highly advantageous for optoelectronic and light emission applications. Phase purity in terms of n is crucial toward narrow emission bands and the design of band structure alignment for charge carrier funneling. We present a simple and scalable synthesis method using cesium and butylammonium as A‐cations to fabricate microcrystal powders of 2D lead bromide perovskites that results in samples with very high n ‐phase purity. The obtained microcrystals show a sharp onset in absorbance and single narrow emission peak in UV‐blue spectral range, and we confirm their structure and composition with X‐ray diffraction and energy dispersive spectroscopy. We show that we can tailor the spectral absorption of microcrystals samples by controlled mixing of the n ‐phase pure powders, and we demonstrate this capability with spectral photocurrent measurements using pellets pressed from pure and mixed powders. Both the spectral and dynamic response of the photocurrent of the pellets depend strongly on the composition of the powders, highlighting the design potential of our approach toward band‐edge engineering.