Strong-bonding hole-transport layers reduce ultraviolet degradation of perovskite solar cells.

The light-emitting diodes (LEDs) used in indoor testing of perovskite solar cells do not expose them to the levels of ultraviolet (UV) radiation that they would receive in actual outdoor use. We report degradation mechanisms of p-i-n-structured perovskite solar cells under unfiltered sunlight and with LEDs. Weak chemical bonding between perovskites and polymer hole-transporting materials (HTMs) and transparent conducting oxides (TCOs) dominate the accelerated A-site cation migration, rather than direct degradation of HTMs. An aromatic phosphonic acid, [2-(9-ethyl-9H-carbazol-3-yl)ethyl]phosphonic acid (EtCz3EPA), enhanced bonding at the perovskite/HTM/TCO region with a phosphonic acid group bonded to TCOs and a nitrogen group interacting with lead in perovskites. A hybrid HTM of EtCz3EPA with strong hole-extraction polymers retained high efficiency and improved the UV stability of perovskite devices, and a champion perovskite minimodule-independently measured by the Perovskite PV Accelerator for Commercializing Technologies (PACT) center-retained operational efficiency of >16% after 29 weeks of outdoor testing.

Towards linking lab and field lifetimes of perovskite solar cells.

Metal halide perovskite solar cells (PSCs) represent a promising low-cost thin-film photovoltaic technology, with unprecedented power conversion efficiencies obtained for both single-junction and tandem applications1-8. To push PSCs towards commercialization, it is critical, albeit challenging, to understand device reliability under real-world outdoor conditions where multiple stress factors (for example, light, heat and humidity) coexist, generating complicated degradation behaviours9-13. To quickly guide PSC development, it is necessary to identify accelerated indoor testing protocols that can correlate specific stressors with observed degradation modes in fielded devices. Here we use a state-of-the-art positive-intrinsic-negative (p-i-n) PSC stack (with power conversion efficiencies of up to approximately 25.5%) to show that indoor accelerated stability tests can predict our six-month outdoor ageing tests. Device degradation rates under illumination and at elevated temperatures are most instructive for understanding outdoor device reliability. We also find that the indium tin oxide/self-assembled monolayer-based hole transport layer/perovskite interface most strongly affects our device operation stability. Improving the ion-blocking properties of the self-assembled monolayer hole transport layer increases averaged device operational stability at 50 °C-85 °C by a factor of about 2.8, reaching over 1,000 h at 85 °C and to near 8,200 h at 50 °C, with a projected 20% degradation, which is among the best to date for high-efficiency p-i-n PSCs14-17.

Mechanism of the Anomalous Dependence between Spin-Orbit Coupling and Dimensionality in Lead Halide Perovskites.

The spin-orbit coupling (SOC) effect of lead (Pb) atoms is a consequential attribute of the unique optoelectronic and defect properties of lead halide perovskites (LHPs). It has been found that the SOC effect varies significantly as the structural dimensionality changes with an anomalous dependence; i.e., while the SOC strength monotonically decreases as structural dimensionality decreases from three-dimensional (3D) to two-dimensional (2D) and then to one-dimensional (1D), the zero-dimensional (0D) SOC strength is greater than the 1D SOC strength. The underlying mechanism of such a SOC dimensionality dependence anomaly remains elusive. In this work, we show that Pb 6p energy splitting increases from 3D to 2D and to 1D LHPs due to the increased degree of distortion, leading to a reduced SOC strength. However, the degree of distortion decreases for the 1D to 0D transformation, resulting in reverse SOC enhancement. The mechanism described in this work can be employed to regulate the SOC effect in the design of perovskite materials.

Rational design of Lewis base molecules for stable and efficient inverted perovskite solar cells.

Lewis base molecules that bind undercoordinated lead atoms at interfaces and grain boundaries (GBs) are known to enhance the durability of metal halide perovskite solar cells (PSCs). Using density functional theory calculations, we found that phosphine-containing molecules have the strongest binding energy among members of a library of Lewis base molecules studied herein. Experimentally, we found that the best inverted PSC treated with 1,3-bis(diphenylphosphino)propane (DPPP), a diphosphine Lewis base that passivates, binds, and bridges interfaces and GBs, retained a power conversion efficiency (PCE) slightly higher than its initial PCE of ~23% after continuous operation under simulated AM1.5 illumination at the maximum power point and at ~40°C for >3500 hours. DPPP-treated devices showed a similar increase in PCE after being kept under open-circuit conditions at 85°C for >1500 hours.

Compositional texture engineering for highly stable wide-bandgap perovskite solar cells.

The development of highly stable and efficient wide-bandgap (WBG) perovskite solar cells (PSCs) based on bromine-iodine (Br-I) mixed-halide perovskite (with Br greater than 20%) is critical to create tandem solar cells. However, issues with Br-I phase segregation under solar cell operational conditions (such as light and heat) limit the device voltage and operational stability. This challenge is often exacerbated by the ready defect formation associated with the rapid crystallization of Br-rich perovskite chemistry with antisolvent processes. We combined the rapid Br crystallization with a gentle gas-quench method to prepare highly textured columnar 1.75-electron volt Br-I mixed WBG perovskite films with reduced defect density. With this approach, we obtained 1.75-electron volt WBG PSCs with greater than 20% power conversion efficiency, approximately 1.33-volt open-circuit voltage (Voc), and excellent operational stability (less than 5% degradation over 1100 hours of operation under 1.2 sun at 65°C). When further integrated with 1.25-electron volt narrow-bandgap PSC, we obtained a 27.1% efficient, all-perovskite, two-terminal tandem device with a high Voc of 2.2 volts.

Surface reaction for efficient and stable inverted perovskite solar cells.

Perovskite solar cells (PSCs) with an inverted structure (often referred to as the p-i-n architecture) are attractive for future commercialization owing to their easily scalable fabrication, reliable operation and compatibility with a wide range of perovskite-based tandem device architectures1,2. However, the power conversion efficiency (PCE) of p-i-n PSCs falls behind that of n-i-p (or normal) structure counterparts3-6. This large performance gap could undermine efforts to adopt p-i-n architectures, despite their other advantages. Given the remarkable advances in perovskite bulk materials optimization over the past decade, interface engineering has become the most important strategy to push PSC performance to its limit7,8. Here we report a reactive surface engineering approach based on a simple post-growth treatment of 3-(aminomethyl)pyridine (3-APy) on top of a perovskite thin film. First, the 3-APy molecule selectively reacts with surface formamidinium ions, reducing perovskite surface roughness and surface potential fluctuations associated with surface steps and terraces. Second, the reaction product on the perovskite surface decreases the formation energy of charged iodine vacancies, leading to effective n-type doping with a reduced work function in the surface region. With this reactive surface engineering, the resulting p-i-n PSCs obtained a PCE of over 25 per cent, along with retaining 87 per cent of the initial PCE after over 2,400 hours of 1-sun operation at about 55 degrees Celsius in air.

Electron Delocalization and Structure Coupling Promoted π-Conjugated Charge Transport in a Novel [Ga-Tpy2]PbI5 Perovskite-like Single Crystal.

Multidimensional perovskite techniques are of intense research interest since they are proved to be advantageous to enhance the perovskite stability. Thereinto, the structure engineering strategy has been widely used to regulate the low dimensional (LD) perovskite structures and obtain expected optoelectronic properties. In this work, we intercalate a thus far unreported metallic coordination compound [Ga-Tpy2]3+ (Tpy: 2,2';6',2″-terpyridine) to the inorganic Pb-I building block as the A-site organic group, and the zero dimensional (0D) [Ga-Tpy2]PbI5 perovskite-like single crystal is obtained. This material displays suitable band edge levels, which enable its potential application as light absorber in solar cells. The DFT calculations manifest delocalized charge distribution on Tpy ligands that can facilitate electron transport, which is attributed to the formation of a double hybrid coordinate bond, i.e., σ bonds and π bonds, between Ga3+ ions and Tpy ligands. These coordinate bonds make metallic complexes promising molecules to regulate structure-associated optoelectronic performances of the LD perovskites.

Architecturing 1D-2D-3D Multidimensional Coupled CsPbI2 Br Perovskites toward Highly Effective and Stable Solar Cells.

Despite the rapid development of CsPbIx Br3- x (0 ≤ x ≤ 3) inorganic perovskite solar cells, associated with their superior thermal stability, their low moisture stability limits their commercial deployment. In this study, 1D-2D-3D multidimensional coupled perovskites are prepared by means of an in situ self-integration approach. This pioneering method allows incorporating thus far unreported 1D-Tpy2 Pb3 I6 and 2D-TpyPb3 I6 (Tpy; terpyridine) perovskites. Heterojunction perovskites demonstrate superior stability against water in comparison with control 3D CsPbI2 Br, which is related to the hydrophobicity of low-dimension (LD) perovskites. Remarkably, the spontaneous involvement of LD perovskites can adjust/reconstruct the interfacial structure. This modification allows releasing the residual strain, establishing effective charge transfer channels that increase the carrier transport ability. Accordingly, 1D-2D-3D hybrid CsPbI2 Br perovskite solar cells demonstrate a stabilized power conversion efficiency as high as 16.1%, which represents a very significant improvement, by a factor of 43%, with respect to control 3D CsPbI2 Br perovskite solar cell. Equally importantly, the multidimensional coupled perovskite solar cells exhibit extraordinary stability, well above 1000 h in ambient atmosphere.

Terpyridine-derived perovskite single crystals with tunable structures and electronic dimensionality.

Dimensionality engineering has proved to be a reliable strategy for addressing the issue of perovskite stability. In this study, a series of previously unreported low-dimensional organic-inorganic hybrid perovskite single crystals were designed and grown by following a simple hydrothermal approach involving solution processing. The as-prepared terpyridine-derived perovskite single crystals displayed tunable structures and electronic dimensionality, which was closely associated with the crystal growth conditions. The performed DFT calculations suggested that the fluctuating conduction band edge demonstrates obvious charge delocalization associated with the π-conjugation effect, a feature promoting efficient charge transport by means of coupling structural dimensionality and electronic dimensionality. This study has provided new ideas for the design of new materials to be used in fields involving photovoltaic devices.

Engineered Electronic Structure and Carrier Dynamics in Emerging Cs2AgxNa1-xFeCl6 Perovskite Single Crystals.

Lead-free double perovskites have attracted noteworthy attention due to their compositional flexibility and electronic diversity. In this study, we hydrothermally grow a new class of Cs2AgxNa1-xFeCl6 (0 ≤ x ≤ 1) perovskite single crystals with high thermal stability. The substitution of B-site cation allows to regulate the crystallographic and band structure, which gives rise to enlarged band absorbance close to the near-infrared region (∼800 nm) via composition engineering. Ultrafast transient absorption spectroscopy (TAS) certifies that the decay time of excited-state absorption is 5.02 and 2450 ps in the case of Cs2NaFeCl6 and Cs2AgFeCl6, respectively. The corresponding charge carrier diffusion length accordingly enhances from 3.7 to 311 nm by means of increasing Ag dopant concentration. Structurally, the primitive cell shrinks due to the partial replacement of [NaCl6]5- octahedra by [AgCl6]5- octahedra. It is proved theoretically as well as experimentally that the introduction of Ag species can effectively enhance the electron mobility (from 1.06 to 15.3 cm2 V-1 s-1) by ∼15 times through realizing stronger orbital coupling of the conductive ions, which enables such a novel double perovskite to be a potential candidate for the optoelectronic and photovoltaic applications.

Controllable Cs xFA1- xPbI3 Single-Crystal Morphology via Rationally Regulating the Diffusion and Collision of Micelles toward High-Performance Photon Detectors.

Cs xFA1- xPbI3 single crystals are expected to provide more excitement in optoelectronic applications, including photodetector, laser, light-emitting diode, etc. Herein, we aim to gain an in-depth understanding of the growth mechanisms of perovskite single crystal with various morphologies in view of microscopic dynamics by the combination of component, structure, and solvent engineering. A sequence of Cs xFA1- xPbI3 (0 ≤ x ≤ 0.14) perovskite single crystals with a dodecahedron morphology and tunable aspect ratio can be obtained by means of a solution-processed uniform-cooling approach. The optimized Cs0.1FA0.9PbI3 single crystals prepared in γ-butyrolactone mixed with dimethyl sulfoxide are theoretically and experimentally demonstrated to have superior performances, e.g., extremely higher long-term stability, lower trap density, and higher mobility. The broadband absorption, i.e., 300-910 nm, enables its application in near-infrared detection (880 nm), and the corresponding detector demonstrates higher responsivity at different light intensities and a fast photocurrent response (τ1 = 11 µs, τ2 = 10 µs). Equally important, we also explore the application of optimized Cs0.1FA0.9PbI3 single crystals with a tunable aspect ratio in an X-ray detector and the extremely high sensitivity (2772.1 µC Gyair-1 cm-2 under a bias of 150 V) demonstrates their good potential for radiation detection.