Advancing inverted (p–i–n) perovskite solar cells (PSCs) is key to further enhance the power conversion efficiency (PCE) and stability of flexible and perovskite-based tandem photovoltaics. Yet, the presence of defects at grain boundaries and in particular interfacial recombination at the perovskite/electron transporting layer interface induce severe non-radiative recombination losses, limiting the open-circuit voltage (VOC) and fill factor (FF) of PSCs in this architecture. In this work, we introduce a dual passivation strategy using the long chain alkylammonium salt phenethylammonium chloride (PEACl) both as an additive and for surface treatment to simultaneously passivate the grain boundaries and the perovskite/C60 interface. Using [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz) as a hole transporting layer and a methylammonium (MA)-free Cs0.18FA0.82PbI3 perovskite absorber with a bandgap of ∼1.57 eV, prolonged charge carrier lifetime and an on average 63 meV enhanced internal quasi-Fermi level splitting are achieved upon dual passivation compared to reference p–i–n PSCs. Thereby, we achieve one of the highest PCEs for p–i–n PSCs of 22.7% (stabilized at 22.3%) by advancing simultaneously the VOC and FF up to 1.162 V and 83.2%, respectively. Using a variety of experimental techniques, we attribute the positive effects to the formation of a heterogeneous 2D Ruddlesden–Popper (PEA)2(Cs1−xFAx)n−1Pbn(I1−yCly)3n+1 phase at the grain boundaries and surface of the perovskite films. At the same time, the activation energy for ion migration is significantly increased, resulting in enhanced stability of the PSCs under light, humidity, and thermal stress. The presented dual passivation strategy highlights the importance of defect management both in the grain boundaries and the surface of the perovskite absorber layer using a proper passivation material to achieve both highly efficient and stable inverted p–i–n PSCs.

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Two birds with one stone: dual grain-boundary and interface passivation enables >22% efficient inverted methylammonium-free perovskite solar cells

Metal halide perovskites are the first solution processed semiconductors that can compete in their functionality with conventional semiconductors, such as silicon. Over the past several years, perovskite semiconductors have reported breakthroughs in various optoelectronic devices, such as solar cells, photodetectors, light emitting and memory devices, and so on. Until now, perovskite semiconductors face challenges regarding their stability, reproducibility, and toxicity. In this Roadmap, we combine the expertise of chemistry, physics, and device engineering from leading experts in the perovskite research community to focus on the fundamental material properties, the fabrication methods, characterization and photophysical properties, perovskite devices, and current challenges in this field. We develop a comprehensive overview of the current state-of-the-art and offer readers an informed perspective of where this field is heading and what challenges we have to overcome to get to successful commercialization.

Roadmap on organic–inorganic hybrid perovskite semiconductors and devices

The past decade has witnessed rapid development of perovskite solar cells (PSCs), the record power conversion efficiency (PCE) of which has been rapidly boosted from the initial 3.8% to a certified 25.5%, approaching the Shockley–Queisser (SQ) limit for single-junction solar cells. Tandem solar cells (TSCs) have gradually attracted more attention due to their great potential to break the SQ limit by reducing the thermalization losses. Among various kinds of perovskite-based tandems, all-perovskite TSCs offer great promise with the advantages of solution processability, low cost, and flexibility. As top cells for TSCs, the wide-bandgap (wide-Eg, >1.7 eV) PSCs play an essential role in harvesting the high-energy photons and providing high open-circuit voltage (Voc) for the multi-junction TSCs, which is helpful for maintaining the stability of the bottom cell by filtering harmful ultraviolet radiation. However, both organic–inorganic hybrid perovskites and all-inorganic perovskites encounter several problems, such as the phase instability and the large Voc deficits (Eg/q − Voc), which result in inferior performance of wide-Eg PSCs. Many efforts have been made to overcome the issues and researchers have already made encouraging progress. In this review, we summarize the recent progress on wide-Eg PSCs based on organic–inorganic hybrid perovskites and all-inorganic perovskites, as well as their application in all-perovskite TSCs. Finally, the main challenges and perspectives are discussed.

Wide-bandgap organic–inorganic hybrid and all-inorganic perovskite solar cells and their application in all-perovskite tandem solar cells

The performance of perovskite solar cells with inverted polarity (p-i-n) is still limited by recombination at their electron extraction interface, which also lowers the power conversion efficiency (PCE) of p-i-n perovskite-silicon tandem solar cells. A MgFx interlayer with thickness of ~1 nanometer at the perovskite/C60 interface favorably adjusts the surface energy of the perovskite layer through thermal evaporation, which facilitates efficient electron extraction and displaces C60 from the perovskite surface to mitigate nonradiative recombination. These effects enable a champion open-circuit voltage of 1.92 volts, an improved fill factor of 80.7%, and an independently certified stabilized PCE of 29.3% for a monolithic perovskite-silicon tandem solar cell ~1 square centimeter in area. The tandem retained ~95% of its initial performance after damp-heat testing (85°C at 85% relative humidity) for >1000 hours. Description A fluoride boost The wide-bandgap perovskite layer in perovskite-silicon tandem solar cells is still limited by high interface recombination at the electron extraction interface. Liu et al. show that adding an ultrathin magnesium fluoride interlayer between the perovskite and C60 electron transport layer during growth facilitates mitigated nonradiative recombination. An analysis of electronic structural data showed that conduction band bending of the perovskite and C60 facilitated electron extraction. A monolithic perovskite-silicon tandem solar cell with a certified power conversion efficiency of 29.3% retained about 95% of its initial performance for 1000 hours. —PDS The surface energy of the perovskite layer at the C60 interface is favorably adjusted with a magnesium fluoride interlayer.

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Efficient and stable perovskite-silicon tandem solar cells through contact displacement by MgFx

Apparent Defect Densities in Halide Perovskite Thin Films and Single Crystals

Non-radiative recombination processes are the biggest hindrance to approaching the radiative limit of the open-circuit voltage for wide-band gap perovskite solar cells. In addition, to high bulk quality, good interfaces and good energy level alignment for majority carriers at charge transport layer-absorber interfaces are crucial to minimize non-radiative recombination pathways. By tuning the lowest-unoccupied molecular-orbital of electron transport layers via the use of different fullerenes and fullerene blends, we demonstrate open-circuit voltages exceeding 1.35 V in CH3NH3Pb(I0.8Br0.2)3 device. Further optimization of mobility in binary fullerenes electron transport layer can boost the power conversion efficiency as high as 18.6%. We note in particular that the Voc-fill factor product is > 1.085 V, which is the highest value reported for halide perovskites with this band gap.

Interface Optimization via Fullerene Blends Enables Open-Circuit Voltages of 1.35 V in CH3NH3Pb(I0.8Br0.2)3 Solar Cells

Selecting suitable charge transport layers and suppressing non-radiative recombination at interfaces to the absorber layer are vital to maximize the efficiency of halide perovskite solar cells. In this work, high-quality perovskite thin films and devices are fabricated with different fullerene-based electron transport layers and different self-assembled monolayers as hole transport layers. We then perform a comparative study of a significant variety of different electrical, optical and photoemission-based characterization techniques to quantify the properties of the solar cells, the individual layers and importantly the interfaces between them. In addition, we highlight the limitations and problems of the different measurements, the insights gained by combining different methods and the different strategies to extract information from the experimental raw data.

Johanna Siekmann

Papers

Apparent Defect Densities in Halide Perovskite Thin Films and Single Crystals

Interface Optimization via Fullerene Blends Enables Open-Circuit Voltages of 1.35 V in CH3NH3Pb(I0.8Br0.2)3 Solar Cells

Interface Optimization via Fullerene Blends Enables Open-Circuit Voltages of 1.35 V in CH3NH3Pb(I0.8Br0.2)3 Solar Cells

Characterizing the Influence of Charge Extraction Layers on the Performance of Triple‐Cation Perovskite Solar Cells