2, University of Virginia, Charlottesville, Virginia, United States
3, Cornell University, Ithaca, New York, United States
Thin films based on two-dimensional metal halide perovskites (2D MHPs) have achieved exceptional performance and stability in numerous optoelectronic device applications. Simple solution processing of the 2D MHPs provides opportunities for manufacturing devices at drastically lower cost. A key to high device performance is to align the 2D MHP layers, during the solution processing, vertical to the electrodes to achieve efficient charge transport. It is, however, yet to be understood how the vertical orientations of 2D MHPs layers on substrates can be obtained. Here we report the formation mechanism of such vertically orientated 2D MHPs in which the nucleation and growth arise from the liquid-air interface, confirmed by our studies on substrate dependence, in-situ grazing incidence X-ray diffraction measurement and optical measurements. The vertical orientation originates from the anisotropic environment at the liquid-air interface. As a consequence, choice of substrates can be liberal from polymers to metal oxides depending on targeted application while still maintaining the vertical orientation for efficient charge transfer. We demonstrate control over the degree of preferential orientation of the 2D MHP layers from almost complete vertical orientation to partial random orientation, and show that the degree of orientation has a drastic impact on solar cell performance.