LAUSR

Halide perovskite solar cells (PSC) have the potential to trigger a revolution in the photovoltaic sector due to their lowcost production and outstanding efficiencies. The material combines exceptional properties such as a high absorption coefficient, panchromatic light absorption,[1] long carrier diffusion lengths,[2,3] shallow trap energy levels,[4] and astonishingly high (external) photoluminescence (PL) yields (up to 66%[5]), rendering its optoelectronic quality comparable to that of GaAs.[6] Generally, all these properties allow for a high photocurrent collection and low nonradiative recombination losses. However, despite the continuous advance of the scientific community in increasing the power conversion efficiencies (PCEs), perovskite solar cells are still limited by the open-circuit voltage (VOC). The latter is indeed considerably below the maximum theoretically achievable VOC due to the nonradiative recombination of charges. In order to fully exploit the thermodynamic potential of this material, a deeper understanding of these recombination processes has to be accomplished. Through the years, several studies spotlighted the perovskite surface[7–9] and the grain boundaries[9,10] as main recombination centers in the perovskite absorber. More recently, the perovskite/transport layer (TL) junctions have been identified as the main source of free energy losses in several efficient devices due to significant nonradiative recombination taking place across these internal interfaces.[11–14] However, only a few studies aimed at identifying the interplay and the relative importance of the recombination losses in the perovskite bulk, at the interfaces and/or at the metal contacts.[15–20] One of the most popular approaches to assess the dominant recombination mechanism is the measurement of the ideality factor (nid). This figure of merit describes the deviation from the ideal diode behavior where only bimolecular recombination is considered as recombination process. An elegant and already well-established approach to determine the nid is to measure the VOC as a function of the light intensity (I). This avoids the issue of poor transport properties and related voltage losses which become problematic when extracting the nid from dark current–voltage characteristics.[23,24] Commonly, nid = 1 is assumed to be representative of a second-order (bimolecular) radiative recombination of free charges, whereas nid = 2 is attributed to a first-order (monomolecular) nonradiative recombination process, e.g., trap-assisted recombination through The measurement of the ideality factor (nid) is a popular tool to infer the dominant recombination type in perovskite solar cells (PSC). However, the true meaning of its values is often misinterpreted in complex multilayered devices such as PSC. In this work, the effects of bulk and interface recombination on the nid are investigated experimentally and theoretically. By coupling intensity-dependent quasi-Fermi level splitting measurements with drift diffusion simulations of complete devices and partial cell stacks, it is shown that interfacial recombination leads to a lower nid compared to Shockley–Read– Hall (SRH) recombination in the bulk. As such, the strongest recombination channel determines the nid of the complete cell. An analytical approach is used to rationalize that nid values between 1 and 2 can originate exclusively from a single recombination process. By expanding the study over a wide range of the interfacial energy offsets and interfacial recombination velocities, it is shown that an ideality factor of nearly 1 is usually indicative of strong first-order nonradiative interface recombination and that it correlates with a lower device performance. It is only when interface recombination is largely suppressed and bulk SRH recombination dominates that a small nid is again desirable.