2, Arizona State University, Tempe, Arizona, United States
The rapid progress in metal halide perovskites has generated great interest in the fabrication of tandems on silicon to enable the next generation of solar cells. Optimized light harvesting is essential to achieve the highest current density and efficiency in tandems. Specifically, high infrared transmission through the perovskite top cell is critical. We show here that texturing the top surface of the perovskite considerably reduces coherent reflections in the infrared.
The antisolvent method we use to deposit perovskite films results in a film with a rippled surface that micron-wide ridges with 300-nm-deep trenches. For comparison, we fabricate smooth perovskites using a two-step interdiffusion method. The smooth perovskites appear shiny, indicative of highly specular reflection. In contrast, the textured perovskites are grey on top and angle resolved transmission measurements confirm a significant increase in haze. To understand the mechanism behind the formation of this textured perovskite surface, we performed stress measurements and found that this morphology is caused by wrinkling that occurs because a compressive stress of ~20 MPa arises after the antisolvent drip. However, perovskites fabricated without an antisolvent are shown to remain in tension throughout processing, and no texturing is observed in the films. Experiments are underway to understand the factors that determine whether there is compressive or tensile stress in the films. We will point out examples in the literature in which scanning electron microscope images revealed the same buckling pattern we have observed, but the mechanism of the patterns formation was not identified.
To evaluate the effect of the textured perovskite surface, we fabricated 4-terminal perovskite-silicon tandems. When comparing perovskites with smooth and textured surfaces, we observed a ~1 mA/cm2 increase in current density in the silicon bottom cell under the textured surfaces. Reflectance measurements confirm a decrease in infrared reflections by the textured perovskite, leading to higher transmittance and an increased EQE in the silicon. With this higher current density, we achieve >25% efficient perovskite-silicon tandems.