The performance of perovskite solar cells (PSCs) is predominantly limited by non-radiative recombination, either through trap-assisted recombination in the absorber layer or via minority carrier recombination at the perovskite/transport layer interfaces. Here we use transient and absolute photoluminescence imaging to visualize all non-radiative recombination pathways in planar pin-type PSCs with undoped organic charge transporting layers. We find significant quasi-Fermi level splitting losses (135 meV) in the perovskite bulk, while interfacial recombination results in an additional free energy loss of 80 meV at each individual interface which limits the open-circuit voltage ( OC ) of the complete cell to ~1.12 V. Inserting ultrathin interlayers between the perovskite and transport layers allows substantial reduction of these interfacial losses at both the p and n contacts. Using this knowledge and approach, we demonstrate reproducible dopant-free 1 cm 2 PSCs surpassing 20% efficiency (19.83% certified) with stabilized power output, a high OC (1.17 V) and record fill factor (> 81%).

We quantify recombination losses in the bulk and interfaces for different perovskite compositions and popular charge transport layers.

Within the last years, perovskite semiconductors have been widely applied as active layers in thin film solar cells, as well as in many other opto-electronic devices such as light emitting diodes [1,2] and (photo) detectors. [3][4][5] Owing to their defect-tolerant nature and ease of fabrication from solution and/or vacuum deposition, [6] perovskites are the almost ideal candidate to be combined with already well-established commercial solar cell technologies such as monocrystalline silicon, [7] CIGS [8] but also with perovskite itself (all-perovskite tandem cells). [9] In the last few years, these properties enabled major research breakthroughs within a comparatively short time which has accelerated research on various PV technologies. For example, with respect to single-junction perovskite solar cells, the efficiency increased from 3.9% to 25.2% [10] within only 10 years and monolithic silicon/perovskite tandem cells reached up to 29.1% power conversion efficiency within an arguably even shorter Perovskite photovoltaic (PV) cells have demonstrated power conversion efficiencies (PCE) that are close to those of monocrystalline silicon cells; however, in contrast to silicon PV, perovskites are not limited by Auger recombination under 1-sun illumination. Nevertheless, compared to GaAs and mono crystalline silicon PV, perovskite cells have significantly lower fill factors due to a combination of resistive and non-radiative recombination losses. This necessitates a deeper understanding of the underlying loss mechanisms and in particular the ideality factor of the cell. By measuring the intensity dependence of the external open-circuit voltage and the internal quasi-Fermi level splitting (QFLS), the transport resistance-free efficiency of the complete cell as well as the efficiency potential of any neat perovskite film with or without attached transport layers are quantified. Moreover, intensitydependent QFLS measurements on different perovskite compositions allows for disentangling of the impact of the interfaces and the perovskite surface on the non-radiative fill factor and open-circuit voltage loss. It is found that potassium-passivated triple cation perovskite films stand out by their exceptionally high implied PCEs > 28%, which could be achieved with ideal transport layers. Finally, strategies are presented to reduce both the ideality factor and transport losses to push the efficiency to the thermodynamic limit.

Electromigration in interconnects continues to be an important field of study in integrated circuits as the interconnects are planned to shrink in size at comparable pace as the semiconductor functional elements. Through shrinking the interconnects approach the regime where quantum size effects become important. The observation of quantum size and shell effects is usually restricted either to low-temperatures or vacuum conditions or to chemically inert materials such as Au. Here, we show that in electromigrated Cu nanocontacts such effects can be observed at room temperature and room pressure even in the presence of oxidation. Our data provide evidence that the nanocontacts are nearly spherical objects with a triangular-cylindrical symmetry of their electronic wave functions with a stronger free-electron-like character compared to previous results. We do not observe a detrimental effect of oxygen. The presence of shell effects has implications for the technological use of Cu nanocontacts as interconnects in integrated circuits and could lead to the use of electronic wave functions of shells in such interconnects.