Simultaneous Enhancement of Efficiency and Operational-Stability of Mesoscopic Perovskite Solar Cells via Interfacial Toughening.

The combined effects of compact TiO2 (c-TiO2 ) electron-transport layer (ETL) are investigated without and with mesoscopic TiO2 (m-TiO2 ) on top, and without and with an iodine-terminated silane self-assembled monolayer (SAM), on the mechanical behavior, opto-electronic properties, photovoltaic (PV) performance, and operational-stability of solar cells based on metal-halide perovskites (MHPs). The interfacial toughness increases almost threefold in going from c-TiO2 without SAM to m-TiO2 with SAM. This is attributed to the synergistic effect of the m-TiO2 /MHP nanocomposite at the interface and the enhanced adhesion afforded by the iodine-terminated silane SAM. The combination of m-TiO2 and SAM also offers a significant beneficial effect on the photocarriers extraction at the ETL/MHP interface, resulting in perovskite solar cells (PSCs) with power-conversion efficiency (PCE) of over 24% and 20% for 0.1 and 1 cm2 active areas, respectively. These PSCs also have exceptionally long operational-stability lives: extrapolated T80 (duration at 80% initial PCE retained) is ≈18 000 and 10 000 h for 0.1 and 1 cm2 active areas, respectively. Postmortem characterization and analyses of the operational-stability-tested PSCs are performed to elucidate the possible mechanisms responsible for the long operational-stability.

Tailoring the thermal conductivity of two-dimensional metal halide perovskites.

Proper thermal management of solar cells based on metal halide perovskites (MHPs) is key to increasing their efficiency as well as their durability. Although two-dimensional (2D) MHPs possess enhanced thermal stability as compared to their three-dimensional (3D) counterparts, the lack of comprehensive knowledge of the heat transfer mechanisms dictating their ultralow thermal conductivities is a bottleneck for further improvements in their thermal performance. Here, we experimentally and computationally study the Dion-Jacobson (DJ) and Ruddlesden-Popper (RP) phases of MHPs (n = 1) to demonstrate that the length of the organic spacers has a negligible influence on their thermal transport properties; we experimentally measure thermal conductivities of 0.19 ± 0.03 W m-1 K-1 and 0.18 ± 0.03 W m-1 K-1 for the RP and DJ phases with 13.6 Å and 6.3 Å interlayer inorganic separations, respectively. In contrast, we show that thermal conductivity is mainly dependent on the separation between the adjacent organic cations. Decreasing the intermolecular distance (by up to 40%) leads to drastically enhanced overall heat conduction (with monotonically increasing thermal conductivity by more than threefold) which is mainly driven by the vibrational hardening of the organic spacers. Although these 2D layered materials constitute a high density of hybrid organic-inorganic interfaces, our results also show that a substantial portion of heat is conducted through coherent phonon transport and that the thermal conductivity of these materials is not solely limited by incoherent interfacial scattering.

Dual-Interface-Reinforced Flexible Perovskite Solar Cells for Enhanced Performance and Mechanical Reliability.

Two key interfaces in flexible perovskite solar cells (f-PSCs) are mechanically reinforced simultaneously: one between the electron-transport layer (ETL) and the 3D metal-halide perovskite (MHP) thin film using self-assembled monolayer (SAM), and the other between the 3D-MHP thin film and the hole-transport layer (HTL) using an in situ grown low-dimensional (LD) MHP capping layer. The interfacial mechanical properties are measured and modeled. This rational interface engineering results in the enhancement of not only the mechanical properties of both interfaces but also their optoelectronic properties holistically. As a result, the new class of dual-interface-reinforced f-PSCs has an unprecedented combination of the following three important performance parameters: high power-conversion efficiency (PCE) of 21.03% (with reduced hysteresis), improved operational stability of 1000 h T90 (duration at 90% initial PCE retained), and enhanced mechanical reliability of 10 000 cycles n88 (number of bending cycles at 88% initial PCE retained). The scientific underpinnings of these synergistic enhancements are elucidated.

Time-resolved vibrational-pump visible-probe spectroscopy for thermal conductivity measurement of metal-halide perovskites.

Understanding thermal transport at the microscale to the nanoscale is crucially important for a wide range of technologies ranging from device thermal management and protection systems to thermal-energy regulation and harvesting. In the past decades, non-contact optical methods, such as time-domain and frequency-domain thermoreflectance, have emerged as extremely powerful and versatile thermal metrological techniques for the measurement of material thermal conductivities. Here, we report the measurement of thermal conductivity of thin films of CH3NH3PbI3 (MAPbI3), a prototypical metal-halide perovskite, by developing a time-resolved optical technique called vibrational-pump visible-probe (VPVP) spectroscopy. The VPVP technique relies on the direct thermal excitation of MAPbI3 by femtosecond mid-infrared optical pump pulses that are wavelength-tuned to a vibrational mode of the material, after which the time dependent optical transmittance across the visible range is probed in the ns to the µs time window using a broadband pulsed laser. Using the VPVP method, we determine the thermal conductivities of MAPbI3 thin films deposited on different substrates. The transducer-free VPVP method reported here is expected to permit spectrally resolving and spatiotemporally imaging of the dynamic lattice temperature variations in organic, polymeric, and hybrid organic-inorganic semiconductors.

Knowledge extraction and transfer in data-driven fracture mechanics.

Data-driven approaches promise to usher in a new phase of development in fracture mechanics, but very little is currently known about how data-driven knowledge extraction and transfer can be accomplished in this field. As in many other fields, data scarcity presents a major challenge for knowledge extraction, and knowledge transfer among different fracture problems remains largely unexplored. Here, a data-driven framework for knowledge extraction with rigorous metrics for accuracy assessments is proposed and demonstrated through a nontrivial linear elastic fracture mechanics problem encountered in small-scale toughness measurements. It is shown that a tailored active learning method enables accurate knowledge extraction even in a data-limited regime. The viability of knowledge transfer is demonstrated through mining the hidden connection between the selected three-dimensional benchmark problem and a well-established auxiliary two-dimensional problem. The combination of data-driven knowledge extraction and transfer is expected to have transformative impact in this field over the coming decades.

Interfacial toughening with self-assembled monolayers enhances perovskite solar cell reliability.

Iodine-terminated self-assembled monolayer (I-SAM) was used in perovskite solar cells (PSCs) to achieve a 50% increase of adhesion toughness at the interface between the electron transport layer (ETL) and the halide perovskite thin film to enhance mechanical reliability. Treatment with I-SAM also increased the power conversion efficiency from 20.2% to 21.4%, reduced hysteresis, and improved operational stability with a projected T80 (time to 80% initial efficiency retained) increasing from ~700 hours to 4000 hours under 1-sun illumination and with continuous maximum power point tracking. Operational stability-tested PSC without SAMs revealed extensive irreversible morphological degradation at the ETL/perovskite interface, including voids formation and delamination, whereas PSCs with I-SAM exhibited minimal damage accumulation. This difference was attributed to a combination of a decrease in hydroxyl groups at the interface and the higher interfacial toughness.

Linking melem with conjugated Schiff-base bonds to boost photocatalytic efficiency of carbon nitride for overall water splitting.

Developing an efficient single component photocatalyst for overall water splitting under visible-light irradiation is extremely challenging. Herein, we report a metal-free graphitic carbon nitride (g-CxN4)-based nanosheet photocatalyst (x = 3.2, 3.6, or 3.8) with melem rings conjugated by Schiff-base bonds (N[double bond, length as m-dash]C-C[double bond, length as m-dash]N). The presence of the conjugated Schiff-base bond tunes the band gap of g-CxN4 and, more importantly, serves as an electron sink to suppress electron-hole pair recombination. The projected density of states (PDOS) calculations suggest that the melem ring and Schiff-base bond act as oxidizing and reducing centers, respectively, for photocatalytic water splitting. As a result, g-CxN4, in particular g-C3.6N4, can catalyze overall water splitting without the need for any co-catalyst or sacrificial donor. Under visible light (>420 nm wavelength) irradiation, g-C3.6N4 catalyzes the overall water splitting with H2 and O2 generation rates of 75.0 and 36.3 µmol h-1 g-1, respectively. g-C3.6N4 is the most efficient single-component photocatalyst ever reported for overall water splitting. Our studies demonstrate a new approach for tuning the bandgap and the electronic structure of graphitic carbon nitride for maximizing its photocatalytic performance for water splitting, which will be important for hydrogen generation and for energy applications.

Interpenetrating interfaces for efficient perovskite solar cells with high operational stability and mechanical robustness.

The perovskite solar cell has emerged rapidly in the field of photovoltaics as it combines the merits of low cost, high efficiency, and excellent mechanical flexibility for versatile applications. However, there are significant concerns regarding its operational stability and mechanical robustness. Most of the previously reported approaches to address these concerns entail separate engineering of perovskite and charge-transporting layers. Herein we present a holistic design of perovskite and charge-transporting layers by synthesizing an interpenetrating perovskite/electron-transporting-layer interface. This interface is reaction-formed between a tin dioxide layer containing excess organic halide and a perovskite layer containing excess lead halide. Perovskite solar cells with such interfaces deliver efficiencies up to 22.2% and 20.1% for rigid and flexible versions, respectively. Long-term (1000 h) operational stability is demonstrated and the flexible devices show high endurance against mechanical-bending (2500 cycles) fatigue. Mechanistic insights into the relationship between the interpenetrating interface structure and performance enhancement are provided based on comprehensive, advanced, microscopic characterizations. This study highlights interface integrity as an important factor for designing efficient, operationally-stable, and mechanically-robust solar cells.

p-p orbital interaction via magnesium isovalent doping enhances optoelectronic properties of halide perovskites.

p-p orbital interaction through Mg(ii) isovalent doping in methylammonium lead chloride perovskite significantly enhances the electronic properties while not affecting the optical bandgap. This chemical behaviour shows promising applications to optoelectronic devices.

Anomalous 3D nanoscale photoconduction in hybrid perovskite semiconductors revealed by tomographic atomic force microscopy.

While grain boundaries (GBs) in conventional inorganic semiconductors are frequently considered as detrimental for photogenerated carrier transport, their exact role remains obscure for the emerging hybrid perovskite semiconductors. A primary challenge for GB-property investigations is that experimentally they need to be performed at the top surface, which is not only insensitive to depth-dependent inhomogeneities but also could be susceptible to topographic artifacts. Accordingly, we have developed a unique approach based on tomographic atomic force microscopy, achieving a fully-3D, photogenerated carrier transport map at the nanoscale in hybrid perovskites. This reveals GBs serving as highly interconnected conducting channels for carrier transport. We have further discovered the coexistence of two GB types in hybrid perovskites, one exhibiting enhanced carrier mobilities, while the other is insipid. Our approach reveals otherwise inaccessible buried features and previously unresolved conduction pathways, crucial for optimizing hybrid perovskites for various optoelectronic applications including solar cells and photodetectors.

Arrays of Plasmonic Nanostructures for Absorption Enhancement in Perovskite Thin Films.

We report optical characterization and theoretical simulation of plasmon enhanced methylammonium lead iodide (MAPbI 3 ) thin-film perovskite solar cells. Specifically, various nanohole (NH) and nanodisk (ND) arrays are fabricated on gold/MAPbI 3 interfaces. Significant absorption enhancement is observed experimentally in 75 nm and 110 nm-thick perovskite films. As a result of increased light scattering by plasmonic concentrators, the original Fabry-Pérot thin-film cavity effects are suppressed in specific structures. However, thanks to field enhancement caused by plasmonic resonances and in-plane interference of propagating surface plasmon polaritons, the calculated overall power conversion efficiency (PCE) of the solar cell is expected to increase by up to 45.5%, compared to its flat counterpart. The role of different geometry parameters of the nanostructure arrays is further investigated using three dimensional (3D) finite-difference time-domain (FDTD) simulations, which makes it possible to identify the physical origin of the absorption enhancement as a function of wavelength and design parameters. These findings demonstrate the potential of plasmonic nanostructures in further enhancing the performance of photovoltaic devices based on thin-film perovskites.

In situ transfer of CH3NH3PbI3 single crystals in mesoporous scaffolds for efficient perovskite solar cells.

Printable mesoscopic perovskite solar cells are usually fabricated by drop-casting perovskite precursor solution on a screen-printed mesoporous TiO2/ZrO2/carbon triple-layer followed by thermal annealing. They have attracted much attention due to their simple fabrication process and remarkable stability. However, challenges lie in how to achieve complete pore fillings of perovskites in the meso-pores and to obtain high-quality perovskite crystals. Here, we report an in situ crystal transfer (ICT) process based on gas-solid interaction to deposit perovskite CH3NH3PbI3 absorber in the scaffold. CH3NH3PbI3 single crystals are first transformed into a liquid phase via exposure to methylamine gas flow. After complete infiltration into the nano-structured scaffolds, the liquid phase is converted back to the solid phase with reduction of methylamine gas partial pressure, maintaining the high-quality of CH3NH3PbI3 single crystals. Compared with the conventional drop-casting method, the ICT method effectively leads to interconnected morphology and prolongs the charge-carrier lifetime (from ∼37.52 ns to ∼110.85 ns) of the perovskite absorber in the scaffold. As a result, the devices can deliver a power conversion efficiency of 15.89%, which is attributed to the suppressed charge recombination and correspondingly enhanced open-circuit voltage of 0.98 V.

Sub-1.4eV bandgap inorganic perovskite solar cells with long-term stability.

State-of-the-art halide perovskite solar cells have bandgaps larger than 1.45 eV, which restricts their potential for realizing the Shockley-Queisser limit. Previous search for low-bandgap (1.2 to 1.4 eV) halide perovskites has resulted in several candidates, but all are hybrid organic-inorganic compositions, raising potential concern regarding device stability. Here we show the promise of an inorganic low-bandgap (1.38 eV) CsPb0.6Sn0.4I3 perovskite stabilized via interface functionalization. Device efficiency up to 13.37% is demonstrated. The device shows high operational stability under one-sun-intensity illumination, with T80 and T70 lifetimes of 653 h and 1045 h, respectively (T80 and T70 represent efficiency decays to 80% and 70% of the initial value, respectively), and long-term shelf stability under nitrogen atmosphere. Controlled exposure of the device to ambient atmosphere during a long-term (1000 h) test does not degrade the efficiency. These findings point to a promising direction for achieving low-bandgap perovskite solar cells with high stability.

Publisher Correction: Carrier lifetime enhancement in halide perovskite via remote epitaxy.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Carrier lifetime enhancement in halide perovskite via remote epitaxy.

Crystallographic dislocation has been well-known to be one of the major causes responsible for the unfavorable carrier dynamics in conventional semiconductor devices. Halide perovskite has exhibited promising applications in optoelectronic devices. However, how dislocation impacts its carrier dynamics in the 'defects-tolerant' halide perovskite is largely unknown. Here, via a remote epitaxy approach using polar substrates coated with graphene, we synthesize epitaxial halide perovskite with controlled dislocation density. First-principle calculations and molecular-dynamics simulations reveal weak film-substrate interaction and low density dislocation mechanism in remote epitaxy, respectively. High-resolution transmission electron microscopy, high-resolution atomic force microscopy and Cs-corrected scanning transmission electron microscopy unveil the lattice/atomic and dislocation structure of the remote epitaxial film. The controlling of dislocation density enables the unveiling of the dislocation-carrier dynamic relation in halide perovskite. The study provides an avenue to develop free-standing halide perovskite film with low dislocation density and improved carried dynamics.

Highly stable and efficient all-inorganic lead-free perovskite solar cells with native-oxide passivation.

There has been an urgent need to eliminate toxic lead from the prevailing halide perovskite solar cells (PSCs), but the current lead-free PSCs are still plagued with the critical issues of low efficiency and poor stability. This is primarily due to their inadequate photovoltaic properties and chemical stability. Herein we demonstrate the use of the lead-free, all-inorganic cesium tin-germanium triiodide (CsSn0.5Ge0.5I3) solid-solution perovskite as the light absorber in PSCs, delivering promising efficiency of up to 7.11%. More importantly, these PSCs show very high stability, with less than 10% decay in efficiency after 500 h of continuous operation in N2 atmosphere under one-sun illumination. The key to this striking performance of these PSCs is the formation of a full-coverage, stable native-oxide layer, which fully encapsulates and passivates the perovskite surfaces. The native-oxide passivation approach reported here represents an alternate avenue for boosting the efficiency and stability of lead-free PSCs.

Synthetic Approaches for Halide Perovskite Thin Films.

Halide perovskites are an intriguing class of materials that have recently attracted considerable attention for use as the active layer in thin film optoelectronic devices, including thin-film transistors, light-emitting devices, and solar cells. The "soft" nature of these materials, as characterized by their low formation energy and Young's modulus, and high thermal expansion coefficients, not only enables thin films to be fabricated via low-temperature deposition methods but also presents rich opportunities for manipulating film formation. This comprehensive review explores how the unique chemistry of these materials can be exploited to tailor film growth processes and highlights the connections between processing methods and the resulting film characteristics. The discussion focuses principally on methylammonium lead iodide (CH3NH3PbI3 or MAPbI3), which serves as a useful and well-studied model system for examining the unique attributes of halide perovskites, but various other important members of this family are also considered. The resulting film properties are discussed in the context of the characteristics necessary for achieving high-performance optoelectronic devices and accurate measurement of physical properties.

Bandgap Optimization of Perovskite Semiconductors for Photovoltaic Applications.

The bandgap is the most important physical property that determines the potential of semiconductors for photovoltaic (PV) applications. This Minireview discusses the parameters affecting the bandgap of perovskite semiconductors that are being widely studied for PV applications, and the recent progress in the optimization of the bandgaps of these materials. Perspectives are also provided for guiding future research in this area.

Homogenous Alloys of Formamidinium Lead Triiodide and Cesium Tin Triiodide for Efficient Ideal-Bandgap Perovskite Solar Cells.

The alloying behavior between FAPbI3 and CsSnI3 perovskites is studied carefully for the first time, which has led to the realization of single-phase hybrid perovskites of (FAPbI3 )1-x (CsSnI3 )x (0

Methylammonium-Mediated Evolution of Mixed-Organic-Cation Perovskite Thin Films: A Dynamic Composition-Tuning Process.

Methylammonium-mediated phase-evolution behavior of FA1-x MAx PbI3 mixed-organic-cation perovskite (MOCP) is studied. It is found that by simply enriching the MOCP precursor solutions with excess methylammonium cations, the MOCPs form via a dynamic composition-tuning process that is key to obtaining MOCP thin films with superior properties. This simple chemical approach addresses several key challenges, such as control over phase purity, uniformity, grain size, composition, etc., associated with the solution-growth of MOCP thin films with targeted compositions.