Light-emitting diodes (LEDs) made from metal halide perovskites have demonstrated external electroluminescent quantum efficiencies (EQEEL) in excess of 20%. However, their poor operational stability, ...resulting in lifetimes of only tens to hundreds of hours, needs to be dramatically improved prior to commercial use. There is little consensus in the community upon which factors limit the stability of these devices. Here, we investigate the role played by ammonium cations on the operational stability. We vary the amount of phenylethylammonium bromide, a widely used alkylammonium salt, that we add to a precursor solution of CsPbBr3 and track changes in stability and EQEEL. We find that while phenylethylammonium bromide is beneficial in achieving high efficiency, it is highly detrimental to operational stability. We investigate material properties and electronic characteristics before and after degradation and find that both a reduction in the radiative efficiency of the emitter and significant changes in current–voltage characteristics explain the orders of magnitude drop in the EQEEL, which we attribute to increased ionic mobility. Our results suggest that engineering new contacts and further investigation into materials with lower ionic mobility should yield much improved stability of perovskite LEDs.
In contrast to the common conception that the interfacial energy-level alignment is affixed once the interface is formed, we demonstrate that heterojunctions between organic semiconductors and ...metal-halide perovskites exhibit huge energy-level realignment during photoexcitation. Importantly, the photoinduced level shifts occur in the organic component, including the first molecular layer in direct contact with the perovskite. This is caused by charge-carrier accumulation within the organic semiconductor under illumination and the weak electronic coupling between the junction components.
We present the synthesis of formamidinium lead tribromide (FAPbBr3) perovskite nanocrystals through a phosphine oxide route, where in comparison to more traditional syntheses oleylamine is replaced ...with trioctylphosphine oxide (TOPO). This route has previously been shown to be successful for the inorganic cesium lead tribromide perovskite nanocrystals. We examine the interactions between the precursors via nuclear magnetic resonance spectroscopy (NMR). We confirm the existence of an interaction between FA-oleate and TOPO and use this to guide the optimization of our synthesis. When the reaction is conducted at room temperature, we observe the formation of nanoparticles with high photoluminescence quantum yield (PLQY, ∼70%) at 2.39 eV (518 nm) with little ripening or size defocusing over time. Although we obtain narrow emission peaks, the crystals are irregular in shapea testament to the impact of the FA-oleate:TOPO interaction. Despite a drop in PLQY in the washed solutions, films made maintain a high PLQY of ∼50% at 2.33 eV (532 nm), which is fortuitously the ideal wavelength for the green emission channel in displays, and we demonstrate 532 nm electroluminescence in light-emitting diodes with an EQE of 3.7%.
Halide perovskite
light-emitting diodes (PeLEDs) exhibit great
potential for use in next-generation display technologies. However,
scale-up will be challenging due to the requirement of very thin ...transport
layers for high efficiencies, which often present spatial inhomogeneities
from improper wetting and drying during solution processing. Here,
we show how a thin Al
2
O
3
layer grown by atomic
layer deposition can be used to preferentially cover regions of imperfect
hole transport layer deposition and form an intermixed composite with
the organic transport layer, allowing hole conduction and injection
to persist through the organic hole transporter. This has the dual
effect of reducing nonradiative recombination at the heterojunction
and improving carrier selectivity, which we infer to be due to the
inhibition of direct contact between the indium tin oxide and perovskite
layers. We observe an immediate improvement in electroluminescent
external quantum efficiency in our p-i-n LEDs from an average of 9.8%
to 13.5%, with a champion efficiency of 15.0%. The technique uses
industrially available equipment and can readily be scaled up to larger
areas and incorporated in other applications such as thin-film photovoltaic
cells.
Halide perovskite light-emitting diodes (PeLEDs) exhibit great potential for use in next-generation display technologies. However, scale-up will be challenging due to the requirement of very thin ...transport layers for high efficiencies, which often present spatial inhomogeneities from improper wetting and drying during solution processing. Here, we show how a thin Al
O
layer grown by atomic layer deposition can be used to preferentially cover regions of imperfect hole transport layer deposition and form an intermixed composite with the organic transport layer, allowing hole conduction and injection to persist through the organic hole transporter. This has the dual effect of reducing nonradiative recombination at the heterojunction and improving carrier selectivity, which we infer to be due to the inhibition of direct contact between the indium tin oxide and perovskite layers. We observe an immediate improvement in electroluminescent external quantum efficiency in our p-i-n LEDs from an average of 9.8% to 13.5%, with a champion efficiency of 15.0%. The technique uses industrially available equipment and can readily be scaled up to larger areas and incorporated in other applications such as thin-film photovoltaic cells.
Although metal halide perovskite photovoltaics have shown an unprecedented rise in power conversion efficiency (PCE), they remain far from their theoretical PCE limit. Among the highest efficiencies ...to date are delivered when polycrystalline films are enhanced via “molecular passivation”, but this can introduce new instabilities, in particular under severe accelerated aging conditions (e.g., at 85 °C in the dark or under full spectrum simulated sunlight). Here, we utilize a benzylammonium bromide passivation treatment to improve device performance, achieving the champion stabilized power output (SPO) of 19.5 % in a p-i-n device architecture. We correlate the improved device performance with a significant increase in charge carrier diffusion lengths, mobilities, and lifetimes. Furthermore, treated devices maintain an increased performance during 120 h combined stressing under simulated full spectrum sunlight at 85 °C, indicating that enhancement from this passivation treatment is sustained under harsh accelerated aging conditions. This is a crucial step toward real-world operation-relevant passivation treatments.
Multi-junction device architectures can increase the power conversion efficiency (PCE) of photovoltaic (PV) cells beyond the single-junction thermodynamic limit. However, these devices are ...challenging to produce by solution-based methods, where dissolution of underlying layers is problematic. By employing a highly volatile acetonitrile(CH3CN)/methylamine(CH3NH2) (ACN/MA) solvent-based perovskite solution, we demonstrate fully solution-processed absorber, transport, and recombination layers for monolithic all-perovskite tandem and triple-junction solar cells. By combining FA0.83Cs0.17Pb(Br0.7I0.3)3 (1.94 eV) and MAPbI3 (1.57 eV) junctions, we reach two-terminal tandem PCEs of more than 15% (steady state). We show that a MAPb0.75Sn0.25I3 (1.34 eV) narrow band-gap perovskite can be processed via the ACN/MA solvent-based system, demonstrating the first proof-of-concept, monolithic all-perovskite triple-junction solar cell with an open-circuit voltage reaching 2.83 V. Through optical and electronic modeling, we estimate the achievable PCE of a state-of-the-art triple-junction device architecture to be 26.7%. Our work opens new possibilities for large-scale, low-cost, printable perovskite multi-junction solar cells.
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•All-perovskite two-terminal tandem solar cells fabricated via solution processing•Acetonitrile/methylamine-based solvent can process narrow band-gap perovskite•Revealing the role of excess metal ions for mixed tin/lead perovskite systems•Proof of concept of the first monolithic all-perovskite triple-junction solar cell
Silicon-based solar cells are dominating today’s solar energy market. However, their efficiencies will soon reach their maximum practical limit. Without any gains in efficiency, price reductions will become increasingly difficult to achieve. Tandem and multi-junction architectures can overcome this single-junction efficiency limit. Perovskite materials offer both band-gap tunability and solution processability. This unique combination of properties allows for fabrication of multi-junction solar cells using high-throughput deposition techniques such as blade coating, roll-to-roll, gravure coating or inkjet printing. However, these solar cells have yet to be fabricated using these deposition techniques due to difficulties in sequentially depositing these semiconductors. By utilizing an acetonitrile/methylamine-based solvent, we demonstrate the first monolithic all-perovskite multi-junction solar cells fabricated via solution processing of all active layers, apart from the electrodes.
Perovskite solar cells can be processed using solution-based methods. Furthermore, perovskite solar cells can tune their band gap to absorb different portions of the solar spectrum. This property allows for fabrication of multi-junction solar cell, which can offer higher power conversion efficiencies than single-junction architecture. Here, we combine both features to fabricate the first solution-processed, monolithic, all-perovskite tandem and triple-junction solar cells.
All‐inorganic perovskite materials are attractive alternatives to organic–inorganic perovskites because of their potential for higher thermal stability. Although CsPbI3 is compositionally stable ...under elevated temperatures, the cubic perovskite α‐phase is thermodynamically stable only at >330 °C and the low‐temperature perovskite γ‐phase is metastable and highly susceptible to non‐perovskite δ‐phase conversion in moisture. Many methods have been reported which show that the incorporation of acid (aqueous HI) or “HPbI3”—recently shown to be dimethylammonium lead iodide (DMAPbI3) —lowers the annealing temperature required to produce the black, perovskite phase of CsPbI3. Herein, the optical and crystallographic data presented show that dimethylammonium (DMA) can successfully incorporate as an A‐site cation to replace Cs in the CsPbI3 perovskite material. This describes the stabilization and lower phase transition temperature reported in the literature when HI or HPbI3 is used as precursors for CsPbI3. The Cs–DMA alloy only forms a pure‐phase material up to ≈25% DMA; at higher concentrations, the CsPbI3 and DMAPbI3 begin to phase segregate. These alloyed materials are more stable to moisture than neat CsPbI3, but do not represent a fully inorganic perovskite material.
Dimethylammonium (DMA) is incorporated in controlled, incremental amounts into the A‐site of CsPbI3 perovskite materials. Confirming that the stabilization afforded from the DMA iodide precursors in CsPbI3 perovskites comes from an alloy of the A‐site with an organic cation. The limit to DMA incorporation is ≈25%, making a Cs0.75DMA0.25PbI3 material that is more stable than neat CsPbI3.
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