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Understanding performance losses in all‐perovskite tandem photovoltaics is crucial to accelerate advancements toward commercialization, especially since these tandem devices generally underperform in comparison to what is expected from isolated layers and single junction devices. Here, the individual sub‐cells in all‐perovskite tandem stacks are selectively characterized to disentangle the various losses. It is found that non‐radiative losses in the high‐gap subcell dominate the overall recombination in the baseline system, as well as in the majority of literature reports. Through a multi‐faceted approach, the open‐circuit voltage (VOC) of the high‐gap perovskite subcell is enhanced by 120 mV. Employing a novel (quasi) lossless indium oxide interconnect, this enables all‐perovskite tandem solar cells with 2.00 V VOC and 23.7% stabilized efficiency. Reducing transport losses as well as imperfect energy‐alignments boosts efficiencies to 25.2% and 27.0% as identified via subcell selective electro‐ and photo‐luminescence. Finally, it is shown how, having improved the VOC, improving the current density of the low‐gap absorber pushes efficiencies even further, reaching 25.9% efficiency stabilized, with an ultimate potential of 30.0% considering the bulk quality of both absorbers measured using photo‐luminescence. These insights not only show an optimization example but also a generalizable evidence‐based optimization strategy utilizing optoelectronic sub‐cell characterization. [ABSTRACT FROM AUTHOR]

Mixed tin‐lead halide perovskites are promising low‐bandgap absorbers for all‐perovskite tandem solar cells that offer higher efficiencies than single‐junction devices. A significant barrier to higher performance and stability is the ready oxidation of tin, commonly mitigated by various additives whose impact is still poorly understood for mixed tin‐lead perovskites. Here, the effects of the commonly used SnF2 additive are revealed for FA0.83Cs0.17SnxPb1−xI3 perovskites across the full compositional lead‐tin range and SnF2 percentages of 0.1–20% of precursor tin content. SnF2 addition causes a significant reduction in the background hole density associated with tin vacancies, yielding longer photoluminescence lifetimes, decreased energetic disorder, reduced Burstein–Moss shifts, and higher charge‐carrier mobilities. Such effects are optimized for SnF2 addition of 1%, while for 5% SnF2 and above, additional nonradiative recombination pathways begin to appear. It is further found that the addition of SnF2 reduces a tetragonal distortion in the perovskite structure deriving from the presence of tin vacancies that cause strain, particularly for high tin content. The optical phonon response associated with inorganic lattice vibrations is further explored, exhibiting a shift to higher frequency and significant broadening with increasing tin fraction, in accordance with lower effective atomic metal masses and shorter phonon lifetimes. [ABSTRACT FROM AUTHOR]

Despite rapid advancements in power conversion efficiency in the last decade, perovskite solar cells still perform below their thermodynamic efficiency limits. Non-radiative recombination, in particular, has limited the external radiative efficiency and open circuit voltage in the highest performing devices. We review the historical progress in enhancing perovskite external radiative efficiency and determine key strategies for reaching high optoelectronic quality. Specifically, we focus on non-radiative recombination within the perovskite layer and highlight novel approaches to reduce energy losses at interfaces and through parasitic absorption. By strategically targeting defects, it is likely that the next set of record-performing devices with ultra-low voltage losses will be achieved. [ABSTRACT FROM AUTHOR]

This paper investigates the impact of microstructure on the degradation rate of methylammonium lead triiodide (MAPbI3) perovskite films upon exposure to light and oxygen. By comparing the oxygen induced degradation of perovskite films of different microstructure-fabricated using either a lead acetate trihydrate precursor or a solvent engineering technique-it is demonstrated that films with larger and more uniform grains and better electronic quality show a significantly reduced degradation compared to films with smaller, more irregular grains. The effect of degradation on the optical, compositional, and microstructural properties of the perovskite layers is characterized and it is demonstrated that oxygen induced degradation is initiated at the layer surface and grain boundaries. It is found that under illumination, irreversible degradation can occur at oxygen levels as low as 1%, suggesting that degradation can commence already during the device fabrication stage. Finally, this work establishes that improved thin-film microstructure, with large uniform grains and a low density of defects, is a prerequisite for enhanced stability necessary in order to make MAPbI3 a promising long lived and low cost alternative for future photovoltaic applications. [ABSTRACT FROM AUTHOR]

The article offers information on the hysteresis in the current–voltage characteristics of vapor‐deposited perovskite solar cells. Topics discussed include usage of interface engineering in producing crystalline perovskite material; information on planar heterojunction solar cells; and the chemical vapor deposition process.

The rapid pace of development for hybrid perovskite photovoltaics has recently resulted in promising figures of merit being obtained with regard to device stability. Rather than relying upon expensive barrier materials, realizing market-competitive lifetimes is likely to require the development of intrinsically stable devices, and to this end accelerated aging tests can help identify degradation mechanisms that arise over the long term. Here, oxygen-induced degradation of archetypal perovskite solar cells under operation is observed, even in dry conditions. With prolonged aging, this process ultimately drives decomposition of the perovskite. It is deduced that this is related to charge build-up in the perovskite layer, and it is shown that by efficiently extracting charge this degradation can be mitigated. The results confirm the importance of high charge-extraction efficiency in maximizing the tolerance of perovskite solar cells to oxygen. [ABSTRACT FROM AUTHOR]

Highest reported efficiency cesium lead halide perovskite solar cells are realized by tuning the bandgap and stabilizing the black perovskite phase at lower temperatures. CsPbI2Br is employed in a planar architecture device resulting in 9.8% power conversion efficiency and over 5% stabilized power output. Offering substantially enhanced thermal stability over their organic based counterparts, these results show that all‐inorganic perovskites can represent a promising next step for photovoltaic materials. [ABSTRACT FROM AUTHOR]

The photoluminescence, transmittance, charge-carrier recombination dynamics, mobility, and diffusion length of CH3NH3PbI3 are investigated in the temperature range from 8 to 370 K. Profound changes in the optoelectronic properties of this prototypical photovoltaic material are observed across the two structural phase transitions occurring at 160 and 310 K. Drude-like terahertz photoconductivity spectra at all temperatures above 80 K suggest that charge localization effects are absent in this range. The monomolecular charge-carrier recombination rate generally increases with rising temperature, indicating a mechanism dominated by ionized impurity mediated recombination. Deduced activation energies Ea associated with ionization are found to increase markedly from the room-temperature tetragonal ( Ea ≈ 20 meV) to the higher-temperature cubic ( Ea ≈ 200 meV) phase adopted above 310 K. Conversely, the bimolecular rate constant decreases with rising temperature as charge-carrier mobility declines, while the Auger rate constant is highly phase specific, suggesting a strong dependence on electronic band structure. The charge-carrier diffusion length gradually decreases with rising temperature from about 3 μm at −93 °C to 1.2 μm at 67 °C but remains well above the optical absorption depth in the visible spectrum. These results demonstrate that there are no fundamental obstacles to the operation of cells based on CH3NH3PbI3 under typical field conditions. [ABSTRACT FROM AUTHOR]

Organic-inorganic perovskites are currently one of the hottest topics in photovoltaic (PV) research, with power conversion efficiencies (PCEs) of cells on a laboratory scale already competing with those of established thin-film PV technologies. Most enhancements have been achieved by improving the quality of the perovskite films, suggesting that the optimization of film formation and crystallization is of paramount importance for further advances. Here, we review the various techniques for film formation and the role of the solvents and precursors in the processes. We address the role chloride ions play in film formation of mixed-halide perovskites, which is an outstanding question in the field. We highlight the material properties that are essential for high-efficiency operation of solar cells, and identify how further improved morphologies might be achieved. [ABSTRACT FROM AUTHOR]

Reversible photo-induced performance deterioration is observed in mesoporous TiO2-containing devices in an inert environment. This phenomenon is correlated with the activation of deep trap sites due to astoichiometry of the metal oxide. Interestingly, in air, these defects can be passivated by oxygen adsorption. These results show that the doping of TiO2 with aluminium has a striking impact upon the density of sub-gap states and enhances the conductivity by orders of magnitude. Dye-sensitized and perovskite solar cells employing Al-doped TiO2 have increased device efficiencies and significantly enhanced operational device stability in inert atmospheres. This performance and stability enhancement is attributed to the substitutional incorporation of Al in the anatase lattice, 'permanently' passivating electronic trap sites in the bulk and at the surface of the TiO2. [ABSTRACT FROM AUTHOR]

Organometal trihalide perovskite based solar cells have exhibited the highest efficiencies to-date when incorporated into mesostructured composites. However, thin solid films of a perovskite absorber should be capable of operating at the highest efficiency in a simple planar heterojunction configuration. Here, it is shown that film morphology is a critical issue in planar heterojunction CH3NH3PbI3- xCl x solar cells. The morphology is carefully controlled by varying processing conditions, and it is demonstrated that the highest photocurrents are attainable only with the highest perovskite surface coverages. With optimized solution based film formation, power conversion efficiencies of up to 11.4% are achieved, the first report of efficiencies above 10% in fully thin-film solution processed perovskite solar cells with no mesoporous layer. [ABSTRACT FROM AUTHOR]

Thin (approximately 10 nm) oxide buffer layers grown over lead-halide perovskite device stacks are critical for protecting the perovskite against mechanical and environmental damage. However, the limited perovskite stability restricts the processing methods and temperatures (≤110 °C) that can be used to deposit the oxide overlayers, with the latter limiting the electronic properties of the oxides achievable. In this work, we demonstrate an alternative to existing methods that can grow pinhole-free TiO x (x ~ 2) films with the requisite thickness in <1 min without vacuum. This technique is atmospheric pressure chemical vapor deposition (AP-CVD). The rapid but soft deposition enables growth temperatures of ≥180 °C to be used to coat perovskites with or without a protective layer of PC 61 BM. This is ≥ 70 °C higher than achievable by current methods and results in more conductive TiO x films, boosting solar cell efficiencies by >2%. Likewise, when AP-CVD SnO x (x ~ 2) is grown directly on the perovskite, there is also minimal structural damage to the underlying perovskite layer. The SnO x layer is pinhole-free and conformal. When used to cover perovskite devices with a PC 61 BM electron transport layer, shunting due to the pinholes in the spin-coated PC 61 BM is reduced, resulting in increases in the steady-state efficiency from 16.5% (no SnO x) to 19.4% (60 nm SnO x), with fill factors reaching 84%. This work shows AP-CVD to be a versatile technique for growing oxides on thermally-sensitive materials. Pinhole-free TiO 2 and SnO 2 films are grown within minutes under atmospheric conditions from the vapor-phase onto lead-halide perovskite/PC 61 BM, improving photovoltaic performance through reduced shunting in the electron transport layer. The short processing times allows oxides to be grown at ≥180 °C onto the perovskite device stack, which is ≥ 70 °C larger than achievable by current methods. Oxides grown at higher temperature are more conductive, resulting in higher efficiencies. Image 1 • Vapor-based technique demonstrated for growing pinhole-free oxide buffer layers in open-air orders of magnitude faster than existing vacuum-based methods. • Rapid deposition allows TiO 2 to be grown at up to 180 °C on thermally-unstable lead-halide perovskites. • Higher growth temperatures result in less resistive TiO 2 films, boosting absolute perovskite solar cell efficiencies by >2%. • Technique also demonstrated for SnO 2 , and perovskite solar cells with dense SnO 2 overlayer improve in efficiency from 16.5% (no SnO 2) to 19.4% (60 nm SnO 2). [ABSTRACT FROM AUTHOR]

In article number 1700977, Yana Vaynzof and co‐workers investigate how the MAPbI3 perovskite layer microstructure is affecting its stability upon exposure to oxygen and light. The authors show that the degradation process is initiated at the grain boundaries, demonstrating that devices whose active layer consists of larger, more uniform grains exhibit enhanced photovoltaic stability. [ABSTRACT FROM AUTHOR]

Hybrid metal-halide perovskites are promising new materials for use in solar cells; however, their chemical stability in the presence of moisture remains a significant drawback. Quasi two-dimensional (2D) perovskites that incorporate hydrophobic organic interlayers offer improved resistance to degradation by moisture, currently still at the cost of overall cell efficiency. To elucidate the factors affecting the optoelectronic properties of these materials, we have investigated the charge transport properties and crystallographic orientation of mixed methylammonium (MA)-phenylethylammonium (PEA) lead iodide thin films as a function of the MA-to-PEA ratio and, thus, the thickness of the "encapsulated" MA lead-halide layers. We find that monomolecular charge-carrier recombination rates first decrease with increasing PEA fraction, most likely as a result of trap passivation, but then increase significantly as excitonic effects begin to dominate for thin confined layers. Bimolecular and Auger recombination rate constants are found to be sensitive to changes in electronic confinement, which alters the density of states for electronic transitions. We demonstrate that effective charge-carrier mobilities remain remarkably high (near 10 cm²V−1s−1) for intermediate PEA content and are enhanced for preferential orientation of the conducting lead iodide layers along the probing electric field. The trade-off between trap reduction, electronic confinement, and layer orientation leads to calculated charge-carrier diffusion lengths reaching a maximum of 2.5 μm for intermediate PEA content (50%). [ABSTRACT FROM AUTHOR]

Reversible photo-induced performance deterioration is observed in mesoporous TiO2-containing devices in an inert environment. This phenomenon is correlated with the activation of deep trap sites due to astoichiometry of the metal oxide. Interestingly, in air, these defects can be passivated by oxygen adsorption. These results show that the doping of TiO2 with aluminium has a striking impact upon the density of sub-gap states and enhances the conductivity by orders of magnitude. Dye-sensitized and perovskite solar cells employing Al-doped TiO2 have increased device efficiencies and significantly enhanced operational device stability in inert atmospheres. This performance and stability enhancement is attributed to the substitutional incorporation of Al in the anatase lattice, 'permanently' passivating electronic trap sites in the bulk and at the surface of the TiO2. [ABSTRACT FROM AUTHOR]