Tandem solar cells that pair silicon with a metal halide perovskite are a promising option for surpassing the single-cell efficiency limit. We report a monolithic perovskite/silicon tandem with a certified power conversion efficiency of 29.15%. The perovskite absorber, with a bandgap of 1.68 electron volts, remained phase-stable under illumination through a combination of fast hole extraction and minimized nonradiative recombination at the hole-selective interface. These features were made possible by a self-assembled, methyl-substituted carbazole monolayer as the hole-selective layer in the perovskite cell. The accelerated hole extraction was linked to a low ideality factor of 1.26 and single-junction fill factors of up to 84%, while enabling a tandem open-circuit voltage of as high as 1.92 volts. In air, without encapsulation, a tandem retained 95% of its initial efficiency after 300 hours of operation.

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

Perovskite solar cells combine high carrier mobilities with long carrier lifetimes and high radiative efficiencies. Despite this, full devices suffer from significant nonradiative recombination losses, limiting their VOC to values well below the Shockley–Queisser limit. Here, recent advances in understanding nonradiative recombination in perovskite solar cells from picoseconds to steady state are presented, with an emphasis on the interfaces between the perovskite absorber and the charge transport layers. Quantification of the quasi‐Fermi level splitting in perovskite films with and without attached transport layers allows to identify the origin of nonradiative recombination, and to explain the VOC of operational devices. These measurements prove that in state‐of‐the‐art solar cells, nonradiative recombination at the interfaces between the perovskite and the transport layers is more important than processes in the bulk or at grain boundaries. Optical pump‐probe techniques give complementary access to the interfacial recombination pathways and provide quantitative information on transfer rates and recombination velocities. Promising optimization strategies are also highlighted, in particular in view of the role of energy level alignment and the importance of surface passivation. Recent record perovskite solar cells with low nonradiative losses are presented where interfacial recombination is effectively overcome—paving the way to the thermodynamic efficiency limit.

photoluminescence yields (>20%). [5] In principle, this would allow open-circuit voltages (V OC ) very close to the radiative limit (≈1.3 V for a bandgap of 1.6 eV) using already existing perovskites. However, despite the tremendous effort devoted by the scientific community on the improvement of this solar cell technology, the experimental efficiencies are still far from the Shockely-Queisser (S.Q.) theoretical predictions of power conversion efficiency (PCE) up to 30%. [6] Specifically, in order to further improve the PCE, the effort must be focused on increasing the V OC and the fill factor (FF) through the reduction of nonradiative recombination losses. Moreover, a better understanding on the predominant energy loss mechanisms in the working device has to be accomplished.Perovskite solar cells generally consist of a 300-500 nm layer of photoactive absorber, sandwiched between two charge transporting layers that have the function of selectively transporting the photogenerated electrons (holes) to the cathode (anode). In an ideal solar cell, all photons are absorbed in the perovskite films, generating electrons and holes with unity efficiency, and-under open-circuit conditions-the only recombination channel is the radiative recombination of free electrons and holes in the same layer where they are generated. Commonly, reported values for V OC are much lower due to unwanted nonradiative recombination. During the past years, many studies have evaluated recombination in perovskites layers and suggested that defects at the perovskite surface or at grain boundaries as possible reasons Today's perovskite solar cells (PSCs) are limited mainly by their open-circuit voltage (V OC ) due to nonradiative recombination. Therefore, a comprehensive understanding of the relevant recombination pathways is needed. Here, intensity-dependent measurements of the quasi-Fermi level splitting (QFLS) and of the V OC on the very same devices, including pin-type PSCs with efficiencies above 20%, are performed. It is found that the QFLS in the perovskite lies significantly below its radiative limit for all intensities but also that the V OC is generally lower than the QFLS, violating one main assumption of the Shockley-Queisser theory. This has far-reaching implications for the applicability of some well-established techniques, which use the V OC as a measure of the carrier densities in the absorber. By performing drift-diffusion simulations, the intensity dependence of the QFLS, the QFLS-V OC offset and the ideality factor are consistently explained by trap-assisted recombination and energetic misalignment at the interfaces. Additionally, it is found that the saturation of the V OC at high intensities is caused by insufficient contact selectivity while heating effects are of minor importance. It is concluded that the analysis of the V OC does not provide reliable conclusions of the recombination pathways and that the knowledge of the QFLS-V OC relation is of great importance.       J J qV n k T radiative recombination current J rad...

High fill factor, large area perovskite solar cells are realized with undoped organic transport layers by optimizing the charge carrier transit through PTAA.

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 non‐radiative 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.

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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.