The photovoltaic performance and long-term stability of metal halide perovskite solar cells (PSC) are critically governed by the energetics and chemistry of their buried interfaces. In particular, hole-selective contacts remain a limiting factor, as their surface reactivity, defect formation pathways, and energy-level alignment with the perovskite absorber strongly influence interfacial recombination and device degradation. Here, I will present a comprehensive investigation of perovskite/HTL interface formation across both inorganic and organic contact strategies, combining soft and hard X-ray photoemission spectroscopy (XPS, HAXPES), ultraviolet photoemission (UPS), and inverse photoemission spectroscopy (IPES).

We begin by examining inorganic NiO hole transport layers in the n-i-p device configuration. Our synchrotron-based HAXPES measurements revealed that the nickel oxide film grown by atomic layer deposition (ALD) directly on top of the perovskite contained large amounts of hydroxide and oxy-hydroxide species and induced the formation of nitrogen containing and lead containing defect states inside the adjacent perovskite. These interfacial reactions were found to degrade the photovoltaic performance. Introducing an organic buffer layer in the form of a 20 nm thick PTAA film between the perovskite and the ALD-NiO suppressed these reactions and improved both device efficiency and operational stability [1].

A complementary study focused on ultraviolet ozone treated nickel oxide and the incorporation of [2-(3,6-Dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid (MeO-2PACz) as an ultrathin organic interlayer in p-i-n PSCs. Here, we find that the MeO-2PACz interlayer mitigates the formation of interface defects between the NiO film and a halide perovskite layer deposited on top, by passivating the NiO surface, which suppresses the reduction of Ni and restores favorable energetics [2].

We then extend our investigation to 2D/3D perovskite heterostructures used for surface passivation. By tracking the evolution of energetics as a function of 4‐fluoro‐phenethylammonium iodide (4-FPEAI) 2D layer thickness, we show that favorable band alignment is confined to the ultrathin regime: while a monolayer preserves efficient hole extraction, thicker 2D layers induce band-edge shifts of up to 0.2 eV, resulting in detrimental energy barriers. Subsequent deposition of the benchmark organic HTL 2,2',7,7'-Tetra(N,N-di-p-tolyl)amino-9,9-spirobifluorene (spiro-TTB) reveals how these subtle shifts propagate into the CTL/perovskite interface, altering charge-transfer pathways. Supported by DFT calculations, our findings provide direct spectroscopic evidence that the 2D/3D interfacial energetics are highly thickness-dependent.