Improving the performances of photovoltaic (PV) devices by suppressing nonradiative energy losses through surface defect passivation and enhancing the stability to the level of standard PV represents one critical challenge for perovskite solar cells. Here, reported are the advantages of introducing a tetrapropylammonium (TPA + ) cation that combines two key functionalities, namely surface passivation of CH 3 NH 3 PbI 3 nanocrystals through strong ionic interaction with the surface and bulk passivation via formation of a type I heterostructure that acts as a recombination barrier. As a result, nonencapsulated perovskite devices with only 2 mol% of TPA + achieve power conversion efficiencies over 18.5% with higher V OC under air mass 1.5G conditions. The devices fabricated retain more than 85% of their initial performances for over 1500 h under ambient conditions (55% RH ± 5%). Furthermore, devices with TPA + also exhibit excellent operational stability by retaining over 85% of the initial performance after 250 h at maximum power point under 1 sun illumination. The effect of incorporation of TPA + on the structural and optoelectronic properties is studied by X-ray diffraction, ultraviolet-visible absorption spectroscopy, ultraviolet photon-electron spectroscopy, time-resolved photoluminescence, and scanning electron microscopy imaging. Atomic-level passivation upon addition of TPA + is elucidated employing 2D solid-state NMR spectroscopy.
Perovskite‐based photovoltaics (PVs) have garnered tremendous interest, enabling power conversion efficiencies exceeding 25%. Although much of this success is credited to the exploration of new compositions, defects passivation and process optimization, environmental stability remains an important bottleneck to be solved. The underlying mechanisms of thermal and humidity‐induced degradation are still far from a clear understanding, which poses a severe limitation to overcome the stability issues. Herein, in situ X‐ray diffraction (XRD), in operando liquid‐cell transmission electron microscopy (TEM) and ex situ solid‐state (ss)NMR spectroscopy are combined with time‐resolved spectroscopies to reveal new insights about the degradation mechanisms of methylammonium lead halide (MAPbI3) under 85% relative humidity (RH) at different length scales. Liquid‐cell TEM enables the live visualizations from meso‐to‐nanoscale transformation between the perovskite particles and water molecules, which are corroborated by the changes in local structures at sub‐nanometer distances by ssNMR and longer range by XRD. This work clarifies the role of surface defects and the significance of their passivation to prevent hydration and decomposition reactions.
In the last few years, perovskite solar cells (PSCs) became one of the most advanced technology in photovoltaics (PVs), reaching 25.5% certified power conversion efficiency (PCE) in single-junction cells. [1] This high-performance level, exceeding 80% of the thermodynamic bandgap limit of the perovskite, stems from the maximization of the solar spectrum absorption capability and reduction of the open-circuit voltage deficit through well-adjusted interfacial band level alignment and bulk/interface defects passivation to abate nonradiative recombination processes. [2] The reduction of interfacial energy losses due to energy misalignment is facilitated given the large richness of cationic and anionic substitution possibilities starting from the prototypical CH 3 NH 3 PbI 3 (MAPbI 3 ) composition, affording a very precise control of the optoelectronic characteristics. Through a wide exploration of composition, triple cation/double halide formulation Cs 0.05 (MA 0.17-FA 0.83 ) 0.95 Pb(Br 0.17 I 0.83 ) 3 (CsMAFA) rapidly emerged as one of the most efficient compositions for single-junction PSC [3,4] and also as a top cell in a monolithic perovskite/silicon tandem architecture reaching above 29% certified PCE with a slightly bromide richer composition. [5] However, the performance enhancement has progressed more rapidly than improving the stability, inhibiting the technology transfer to larger scale. Many efforts are now turned toward this objective through device encapsulation, [6,7] hydrophobic interfacial layers, [8] nanoscale 2D/3D structuration, [9] and defects passivation. [10][11][12][13] Topdown approaches that give further insights into the degradation pathways of the perovskite absorber and device stacks under operational conditions are highly desirable to propose rational ways to improve the stability at different scales from bulk and surface of the materials, interfaces, and finally on the entire device. Given the complexity in deciphering all possible contributions involved during the degradation, the first step ex situ investigations provide already relevant trends about material weaknesses. For instance, exposure of the conventional MAPbI 3 composition to a humid atmosphere showed rapid decomposition into PbI 2 and gas releases. [14] Depending on the relative humidity (RH)
The time and annealing temperature for the film crystallization in perovskite solar cells (PSCs) is critical and is at the stake of device optimization. It governs the crystallization process, the film’s morphorlogy and texture and the level of non-radiative defects, which in whole control the power conversion efficiency (PCE). However, deciphering each of these parameters in the device cell characteristics remains not totally clear. In this work, we led a holistic study considering temperature and time for the MAPbI3 crystallization as a free parameter to study how the latter is impacting on the film’s characteristics and how the device figure of merit is affected. The results suggest that the crystallinity level of the grains plays an important role in the photo-current value whereas the morphology and PbI2 impurities resulting from the onset of thermal decomposition of MAPbI3 penalizes the cell photovoltage and the fill factor values. Based on this study, it is highlighted that flash high temperature annealing is beneficial to limit out-of-plane substrate grain boundaries, resulting in a device exhibiting 18.8% power conversion efficiency compared to 18.0% when more standard post-annealing procedure is employed.
As a promising solar energy harvesting technology, solution-processed metal halide perovskites (MHPs) are of great current interest in developing low-cost and efficient photovoltaic cells. Despite their excellent optoelectronic properties and the nascent advancements in compositional tailoring and interfacial engineering to develop high-performance MHPs, issues associated with the long-term environmental stability of these materials are yet to be addressed. Here we examine the moisture-induced cascade degradation reactions over a year for methylammonium l e a d i o d i d e ( M A P b I 3 ) -a n d f o r m a m i d i n i u m -r i c h [Cs 0.05 (MA 0.17 FA 0.83 ) 0.95 Pb(Br 0.17 I 0.83 ) 3 ] formulations at 40 and 85% relative humidity (RH) in the air. The transformative reactions at 85% RH lead to chemical degradation process in both MA-rich and FA-rich perovskites, yielding to the different organic and inorganic byproducts within a few hours, but the exposure to 40% RH retains the longevity of these materials up to several months. The defect passivation by the tetrapropylammonium cation (TPA + ) imparts enhanced stability of MAPbI 3 particles, irrespective of the exposure conditions to water vapor. By resolving thin-film morphology at sub-nanometer to nanometer resolution using solid-state (ss)NMR spectroscopy and X-ray diffraction techniques, kinetics of degradation reactions and structural insights into the inorganic/organic interfaces and degradation products are obtained and compared. Our findings provide mechanistic details into the cascade degradation reactions in pristine and defect-passivated MHPs, enabling guidance for novel passivating and interfacial engineering strategies to further improve the robustness of the MHPs with respect to environmental stressors.
<p><b>This work presents a novel and comprehensive approach to predict and understand the stabilisation mechanisms of dispersions of nanoparticles in ionic liquids which </b><b>is at present unpredictable. This opens up applications with new materials combining the properties of both nanoparticles and ionic liquids.</b></p>
<p><b>This work presents a novel and comprehensive approach to predict and understand the stabilisation mechanisms of dispersions of nanoparticles in ionic liquids which </b><b>is at present unpredictable. This opens up applications with new materials combining the properties of both nanoparticles and ionic liquids.</b></p>
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