Thermal Co-Evaporation of Lower-Dimensional Interlayers for Efficient Perovskite Solar Cells

Ruddlesden-Popper (R-P) metal halide perovskite-derived interlayer structures have been successfully used in a variety of optoelectronic applications such as solar cells, photodetector and LEDs. Here , the n-value refers to the number of conjoined lead halide octahedral sheets between organic spacer cations. However, fast reactions during solution processing prevent accurate control on crystallization dynamics and limit the processing window to a narrow set of solution concentrations, spin conditions and annealing methods. This may result in uncontrolled reconstruction of the 3D/2D interface as well as poor control on layer thickness, which limits device performance and stability.<br/>In this work, we first report the deposition of 2D (n = 1) Ruddlesden-Popper (PEA₂PbI₄ where PEA⁺ is phenethylammonium) structures using solvent-free and industrially compatible thermal co-evaporation routes. Using synchrotron-based structural characterization, we characterize the structural properties of the 2D film and find them to be comparable to films deposited using solution-processing. Upon coating a 3D (CsFAPbI₃where FA⁺ is formamidinium) with the R-P layer, we use X-ray photoelectron spectroscopy, X-ray diffraction and hyperspectral photoluminescence imaging to show that the excess FA⁺ on the surface of the 3D layer reacts with the R-P layer to form a quasi-2D (n = 2) phase. Once the FA⁺ concentration depletes with increasing R-P thickness, the 2D phase (n = 1) forms. The development of this heterostructure has implications on charge-carrier dynamics with carrier lifetimes increasing in the presence of the n = 2 phase and decreasing thereafter as the n = 1 phase forms. Finally, the interfacial R-P layer is used in devices to demonstrate n-i-p solar cells with efficiency approaching 22%, showing a 2% absolute gain in performance from control devices with no interlayers. The gain in performance, shown to occur over a 40 nm R-P layer thickness window, occurs as a result of improvements in carrier transport across the perovskite/HTL interface, resulting in gains in the V_oc and FF. Taken together, we show a deposition pathway to achieve high efficiency solar cell devices prepared using R-P interlayers with a large thickness window developed using a scalable deposition method for the interlayer that can be translated to future commercial devices.