Polymer-modified halide perovskite films for efficient and stable planar heterojunction solar cells

Co-encapsulation of collagenase type I and silibinin in chondroitin sulfate coated multilayered nanoparticles for targeted treatment of liver fibrosis

Components of the extracellular matrix (ECM) are overexpressed in fibrotic liver. Collagen is the main component of the liver fibrosis stroma. Here we demonstrate that chondroitin sulfate coated multilayered 50-nm nanoparticles encapsulating collagenase and silibinin (COL + SLB-MLPs) break down the dense collagen stroma, while silibinin inhibits activated hepatic stellate cells. The nanoparticles were taken up to a much greater extent by hepatic stellate cells than by normal hepatocytes, and they down-regulated production of type I collagen. In addition, chondroitin sulfate protected the collagenase from premature deactivation. COL + SLB-MLPs were delivered to the cirrhotic liver, and the collagenase and silibinin synergistically inhibited fibrosis in mice. Immunofluorescence staining of liver tissues revealed that CD44, mediated by chondroitin sulfate, delivered the nanoparticles to hepatic stellate cells. This strategy holds promise for degrading extracellular stroma and thereby facilitating drug penetration into fibrotic liver and related diseases such as liver cirrhosis and liver cancer.

Engineering bandgap of CsPbI3 over 1.7 eV with enhanced stability and transport properties

Potential multijunction application of CsPbI₃ perovskite with silicon solar cells to reach efficiencies beyond the Shockley-Queisser limit motivates tremendous efforts to improve its phase stability and further enlarge its band gap between 1.7 and 1.8 eV. Current strategies to increase band gap via conventional mixed halide engineering are accompanied by detrimental phase segregation under illumination. Here, ethylammonium (EA) in a relatively small fraction (x < 0.15) is first investigated to fit into three-dimensional CsPbI₃ framework to form pure-phase hybrid perovskites with enlarged band gap over 1.7 eV. The increase of band gap is closely associated with the distortion of Pb-I octahedra and the variation of the average Pb-I-Pb angle. Meanwhile, the introduction of EA can retard the crystallization of perovskite and tune the perovskite structure with enhanced phase stability and transport properties.

Gentle Materials Need Gentle Fabrication: Encapsulation of Perovskites by Gas-Phase Alumina Deposition

Metal halide perovskites have attracted tremendous attention as promising materials for future-generation optoelectronic devices. Despite their outstanding optical and transport properties, the lack of environmental and operational stability remains a major practical challenge. One of the promising stabilization avenues is metal oxide encapsulation via atomic layer deposition (ALD); however, the unavoidable reaction of metal precursors with the perovskite surface in conventional ALD leads to degradation and restructuring of the perovskites' surfaces. This Perspective highlights the development of a modified gas-phase ALD technique for alumina encapsulation that not only prevents perovskites' degradation but also significantly improves their optical properties and air stability. The correlation between precise atomic interactions at the perovskite-metal oxide interface with the dramatically enhanced optical properties is supported by density functional theory calculations, which also underlines the widespread applicability of this gentle technique for a variety of perovskite nanostructures unbarring potential opportunities offered by combination of these approaches.

Ambient-Air-Stable Lead-Free CsSnI3 Solar Cells with Greater than 7.5% Efficiency

Black orthorhombic (B-γ) CsSnI₃ with reduced biotoxicity and environmental impact and excellent optoelectronic properties is being considered as a promising eco-friendly candidate for high-performing perovskite solar cells (PSCs). A major challenge in a large-scale implementation of CsSnI₃ PSCs includes the rapid transformation of Sn²⁺ to Sn⁴⁺ (within a few minutes) under an ambient-air condition. Here, we demonstrate that ambient-air stable B-γ CsSnI₃ PSCs can be fabricated by incorporating N,N'-methylenebis(acrylamide) (MBAA) into the perovskite layer and by using poly(3-hexylthiophene) as the hole transporting material. The lone electron pairs of -NH and -CO units of MBAA are designed to form coordination bonding with Sn²⁺ in the B-γ CsSnI₃, resulting in a reduced defect (Sn⁴⁺) density and better stability under multiple conditions for the perovskite light absorber. After a modification, the highest power conversion efficiency (PCE) of 7.50% is documented under an ambient-air condition for the unencapsulated CsSnI₃-MBAA PSC. Furthermore, the MBAA-modified devices sustain 60.2%, 76.5%, and 58.4% of their initial PCEs after 1440 h of storage in an inert condition, after 120 h of storage in an ambient-air condition, and after 120 h of 1 Sun continuous illumination, respectively.

Light-Promoted Electrostatic Adsorption of High-Density Lewis Base Monolayers as Passivating Electron-Selective Contacts

Achieving efficient passivating carrier-selective contacts (PCSCs) plays a critical role in high-performance photovoltaic devices. However, it is still challenging to achieve both an efficient carrier selectivity and high-level passivation in a sole interlayer due to the thickness dependence of contact resistivity and passivation quality. Herein, a light-promoted adsorption method is demonstrated to establish high-density Lewis base polyethylenimine (PEI) monolayers as promising PCSCs. The promoted adsorption is attributed to the enhanced electrostatic interaction between PEI and semiconductor induced by the photo-generated carriers. The derived angstrom-scale PEI monolayer is demonstrated to simultaneously provide a low-resistance electrical contact for electrons, a high-level field-effect passivation to semiconductor surface and an enhanced interfacial dipole formation at contact interface. By implementing this light-promoted adsorbed PEI as a single-layered PCSC for n-type silicon solar cell, an efficiency of 19.5% with an open-circuit voltage of 0.641 V and a high fill factor of 80.7% is achieved, which is one of the best results for devices with solution-processed electron-selective contacts. This work not only demonstrates a generic method to develop efficient PCSCs for solar cells but also provides a convenient strategy for the deposition of highly uniform, dense, and ultra-thin coatings for diverse applications.

Moisture-Resistant FAPbI3 Perovskite Solar Cell with 22.25 % Power Conversion Efficiency through Pentafluorobenzyl Phosphonic Acid Passivation

Perovskite solar cells (PSCs) have shown great promise for photovoltaic applications, owing to their low-cost assembly, exceptional performance, and low-temperature solution processing. However, the advancement of PSCs towards commercialization requires improvements in efficiency and long-term stability. The surface and grain boundaries of perovskite layer, as well as interfaces, are critical factors in determining the performance of the assembled cells. Defects, which are mainly located at perovskite surfaces, can trigger hysteresis, carrier recombination, and degradation, which diminish the power conversion efficiencies (PCEs) of the resultant cells. This study concerns the stabilization of the α-FAPbI₃ perovskite phase without negatively affecting the spectral features by using 2,3,4,5,6-pentafluorobenzyl phosphonic acid (PFBPA) as a passivation agent. Accordingly, high-quality PSCs are attained with an improved PCE of 22.25 % and respectable cell parameters compared to the pristine cells without the passivation layer. The thin PFBPA passivation layer effectively protects the perovskite layer from moisture, resulting in better long-term stability for unsealed PSCs, which maintain >90 % of the original efficiency under different humidity levels (40-75 %) after 600 h. PFBPA passivation is found to have a considerable impact in obtaining high-quality and stable FAPbI₃ films to benefit both the efficiency and the stability of PSCs.

Nanoscale Architecture of Polymer-Organolead Halide Perovskite Films and the Effect of Polymer Chain Mobility on Device Performance

The integration of polymer chains with organolead halide perovskite (MAPbI₃) films, leading to enhanced stability and electro-optical performance, is critically affected by the molecular weight of chains. The molecular weight determines the mobility and volume of the chains, which affects the crystallization kinetics and, hence, perovskite grain size. The insulating nature of the chains is another critical factor that affects both ion migration and conduction of electronic charge. The combined effect of these factors leads to optimal performance with the use of medium-length chains. A simple model integrating the two effects accurately fits the response of the polymer-perovskite composite. Further characterization results show that the polymer-perovskite films have a three-layer architecture consisting of nanoscale polymer-rich top and bottom layers. These combined results show that the optimization of performance in polymer-perovskite devices depends critically on the size of the chains due to their multiple effects on the perovskite matrix.

Interfacial engineering of CuSCN-based perovskite solar cells via PMMA interlayer toward enhanced efficiency and stability

We report a new interfacial engineering strategy to improve the photovoltaic performance of CuSCN-based perovskite solar cells.

Nature of the different emissive states and strong exciton-phonon couplings in quasi-two-dimensional perovskites derived from phase-modulated two-photon micro-photoluminescence spectroscopy

Quasi two-dimensional perovskites have attracted great attention for applications in light-emitting devices and photovoltaics due to their robustness and tunable highly efficient photoluminescence (PL). However, the mechanism of intrinsic PL in these materials is still not fully understood. In this work, we have analysed the nature of the different emissive states and the impact of temperature on the emissions in quasi two-dimensional methyl ammonium lead bromide perovskite (q-MPB) and cesium lead bromide perovskite (q-CPB). We have used spatially resolved phase-modulated two-photon photoluminescence (2PPL) and temperature-dependent 2PPL to characterize the emissions. Our results show that at room temperature, the PL from q-MPB is due to the recombination of excitons and free carriers while the PL from q-CPB is due to the recombination of excitons only. Temperature-dependent measurements show that in both materials the linewidth broadening is due to the interactions between the excitons and optical phonons at high temperatures. Comparing the characteristics of the emissions in the two systems, we conclude that q-CPB is better suited for light emitting devices. With a further optimization to reduce the impact on the environment, q-CPB-based LEDs could perform as well as OLEDs.

Enhanced Photovoltaic Properties of Perovskite Solar Cells by Employing Bathocuproine/Hydrophobic Polymer Films as Hole-Blocking/Electron-Transporting Interfacial Layers

In this study, we improved the photovoltaic (PV) properties and storage stabilities of inverted perovskite solar cells (PVSCs) based on methylammonium lead iodide (MAPbI3) by employing bathocuproine (BCP)/poly(methyl methacrylate) (PMMA) and BCP/polyvinylpyrrolidone (PVP) as hole-blocking and electron-transporting interfacial layers. The architecture of the PVSCs was indium tin oxide/poly(3,4-ethylenedioxythiophene):polystyrenesulfonate/MAPbI3/[6,6]-phenyl-C61-butyric acid methyl ester/BCP based interfacial layer/Ag. The presence of PMMA and PVP affected the morphological stability of the BCP and MAPbI3 layers. The storage-stability of the BCP/PMMA-based PVSCs was enhanced significantly relative to that of the corresponding unmodified BCP-based PVSC. Moreover, the PV performance of the BCP/PVP-based PVSCs was enhanced when compared with that of the unmodified BCP-based PVSC. Thus, incorporating hydrophobic polymers into BCP-based hole-blocking/electron-transporting interfacial layers can improve the PV performance and storage stability of PVSCs.

Accessing Highly Oriented Two-Dimensional Perovskite Films via Solvent-Vapor Annealing for Efficient and Stable Solar Cells

Accessing vertical orientation of two-dimensional (2D) perovskite films is key to achieving high-performance solar cells with these materials. Herein, we report on solvent-vapor annealing (SVA) as a general postdeposition strategy to induce strong vertical orientation across broad classes of 2D perovskite films. We do not observe any local compositional drifts that would result in impure phases during SVA. Instead, our experiments point to solvent vapor plasticizing 2D perovskite films and facilitating their surface-induced reorientation and concomitant grain growth, which enhance out-of-plane charge transport. Solar cells with SVA 2D perovskites exhibit superior efficiency and stability compared to their untreated analogs. With a certified efficiency of (18.00 ± 0.30) %, our SVA (BDA)(Cs_0.1FA_0.9)₄Pb₅I₁₆ solar cell boasts the highest efficiency among all solar cells with 2D perovskites (n ≤ 5) reported so far.

Interface and grain boundary passivation for efficient and stable perovskite solar cells: the effect of terminal groups in hydrophobic fused benzothiadiazole-based organic semiconductors

The defects at the interface and grain boundaries (GBs) of perovskite films limit the performance of perovskite solar cells (PSCs) seriously. Herein, organic semiconductors with different terminal groups including a ladder-type electron-deficient-core-based fused structure (DAD) fused core with 2-(3-oxo-2,3-dihydro-1H-inden-1 ylidene)malononitrile (BTP-4H), DAD with 2-(5,6-dichloro-3-oxo-2,3-dihydro-1H-inden-1 ylidene)malononitrile (BTP-4Cl), and DAD with 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1 ylidene)malononitrile (BTP-4F) are introduced into perovskite films to study the effects of the terminal groups on the PSC performance. A physical model is proposed to understand the effects of the terminal groups on the perovskite growth and energy level alignment of devices. Compared with BTP-4H and BTP-4Cl, BTP-4F can more effectively delay the crystallization rate and increase the crystal sizes due to hydrogen bonding of F and FA. BTP-4F can also provide more efficient charge transport channels due to the optimal energy level alignment. Most importantly, BTP-4F can promote charge transport from the perovskite film to spiro-OMeTAD and to SnO₂, thus realizing simultaneous up-bottom passivation of perovskite films. Finally, the BTP-4F passivated PSCs exhibit a remarkable PCE of 22.16%, and the device can maintain ∼86% of the initial PCE after 5000 h. Therefore, this work presents significant potential of organic semiconductors in PSCs toward high efficiency and high stability due to the terminal groups.

Nicotinamide as Additive for Microcrystalline and Defect Passivated Perovskite Solar Cells with 21.7% Efficiency

Passivation of electronic defects on the surface and at grain boundaries (GBs) of perovskite films has become one of the most effective tactics to suppress charge recombination in perovskite solar cells. It is demonstrated that trap states can be effectively passivated by Lewis acid or base functional groups. In this work, nicotinamide (NTM, commonly known as vitamin B3 or vitamin PP) serving as a Lewis base additive is introduced into the PbI₂ and/or FAI: MABr: MACl precursor solution to obtain NTM modified perovskite films. It has been found that the NTM in the perovskite film can well passivate surface and GBs defects, control the film morphology and enhance the crystallinity via its interaction with a lone pair of electrons in nitrogen. In the presence of the NTM additive, we obtained enlarged perovskite crystal grain about 3.6 μm and a champion planar perovskite solar cell with efficiency of 21.72% and negligible hysteresis. Our findings provide an effective route for crystal growth and defect passivation to bring further increases on both efficiency and stability of perovskite solar cells.

Traps in metal halide perovskites: characterization and passivation

Metal halide perovskites (MHPs) have become a research focus in the field of optoelectronics due to their excellent optoelectronic properties and simple and cost-effective thin film manufacturing processes. In particular, the power conversion efficiency (PCE) of solar cells (SCs) and external quantum efficiency (EQE) of light-emitting diodes (LEDs) based on perovskite materials have reached 25.2% and 21.6%, respectively, in a short period, making perovskites especially promising for fabricating next-generation optoelectronic devices. Despite these inspiring results, obtaining high-performance, high-stability MHP-based devices still faces many challenges, among which the defects and the consequent traps in MHPs are key factors. Defect-induced traps can trap charge carriers or even act as non-radiative recombination centers, seriously degrading the device performance, causing hysteresis and deteriorating the stability of MHP-based devices. Thus, understanding the chemical/physical nature of traps and adopting appropriate strategies to passivate traps are important to enhance the device performance and stability. Herein we present a review in which the knowledge and understanding of traps in MHPs are considered and discussed. Moreover, the latest efforts on passivating traps in MHPs for improving device performance are summarized, with the hope of providing guidance to future development of high-performance and high-stability MHP-based devices.

Identifying the functional groups effect on passivating perovskite solar cells

Many organic molecules with various functional groups have been used to passivate the perovskite surface for improving the efficiency and stability of perovskite solar cell (PSCs). However, the intrinsic attributes of the passivation effect based on different chemical bonds are rarely studied. Here, we comparatively investigate the passivation effect among 12 types of functional groups on para-tert-butylbenzene for PSCs and find that the open circuit voltage (V_OC) tends to increase with the chemical bonding strength between perovskite and these passivation additive molecules. Particularly, the para-tert-butylbenzoic acid (tB-COOH), with the extra intermolecular hydrogen bonding, can stabilize the surface passivation of perovskite films exceptionally well through formation of a crystalline interlayer with water-insoluble property and high melting point. As a result, the tB-COOH device achieves a champion power conversion efficiency (PCE) of 21.46%. More importantly, such devices, which were stored in ambient air with a relative humidity of ≃45%, can retain 88% of their initial performance after a testing period of more than 1 year (10,080 h). This work provides a case study to understand chemical bonding effects on passivation of perovskite.

A Critical Review on Crystal Growth Techniques for Scalable Deposition of Photovoltaic Perovskite Thin Films

Although the efficiency of small-size perovskite solar cells (PSCs) has reached an incredible level of 25.25%, there is still a substantial loss in performance when switching from small size devices to large-scale solar modules. The large efficiency deficit is primarily associated with the big challenge of coating homogeneous, large-area, high-quality thin films via scalable processes. Here, we provide a comprehensive understanding of the nucleation and crystal growth kinetics, which are the key steps for perovskite film formation. Several thin-film crystallization techniques, including antisolvent, hot-casting, vacuum quenching, and gas blowing, are then summarized to distinguish their applications for scalable fabrication of perovskite thin films. In viewing the essential importance of the film morphology on device performance, several strategies including additive engineering, Lewis acid-based approach, solvent annealing, etc., which are capable of modulating the crystal morphology of perovskite film, are discussed. Finally, we summarize the recent progress in the scalable deposition of large-scale perovskite thin film for high-performance devices.

Enhanced Device Performances of MAFACsPb(IxBr1-x) Perovskite Solar Cells with Dual-Functional 2-Chloroethyl Acrylate Additives

Perovskite photovoltaics (PePVs) tend to suffer from a high density of defects that restrict the device in terms of performances and stability. Therefore, defect passivation and film-quality improvement of perovskite active layers are crucial for high-performance PePVs. In this work, 2-chloroethyl acrylate (CEA) with C═O and -Cl groups in Cs_0.175FA_0.750MA_0.075Pb (I_0.880Br_0.120) precursor solutions is introduced as a novel bifunctional additive to act as both a defect passivator and perovskite-growth controller. With the aid of CEA, the perovskite crystallinity and average grain size are improved, and perovskite defects are effectively reduced, thus increasing the representative efficiency (PCE = 19.32%). PePVs with CEA also maintain their initial efficiency of 85% even after about 500 h under air conditions with a humidity of 40 ± 5%. As a result, this study proves that the novel additive CEA can produce higher PePV efficiency and more stable devices.

Color-pure red light-emitting diodes based on two-dimensional lead-free perovskites

It remains a central challenge to the information display community to develop red light-emitting diodes (LEDs) that meet demanding color coordinate requirements for wide color gamut displays. Here, we report high-efficiency, lead-free (PEA)₂SnI₄ perovskite LEDs (PeLEDs) with color coordinates (0.708, 0.292) that fulfill the Rec. 2100 specification for red emitters. Using valeric acid (VA)-which we show to be strongly coordinated to Sn²⁺-we slow the crystallization rate of the perovskite, improving the film morphology. The incorporation of VA also protects tin from undesired oxidation during the film-forming process. The improved films and the reduced Sn⁴⁺ content enable PeLEDs with an external quantum efficiency of 5% and an operating half-life exceeding 15 hours at an initial brightness of 20 cd/m² This work illustrates the potential of Cd- and Pb-free PeLEDs for display technology.

Grain Boundary Defect Passivation of Triple Cation Mixed Halide Perovskite with Hydrazine-Based Aromatic Iodide for Efficiency Improvement

Perovskites have been unprecedented with a relatively sharp rise in power conversion efficiency in the last decade. However, the polycrystalline nature of the perovskite film makes it susceptible to surface and grain boundary defects, which significantly impedes its potential performance. Passivation of these defects has been an effective approach to further improve the photovoltaic performance of the perovskite solar cells. Here, we report the use of a novel hydrazine-based aromatic iodide salt or phenyl hydrazinium iodide (PHI) for secondary post treatment to passivate surface and grain boundary defects in triple cation mixed halide perovskite films. In particular, the PHI post treatment reduced current at the grain boundaries, facilitated an electron barrier, and reduced trap state density, indicating suppression of leakage pathways and charge recombination, thus passivating the grain boundaries. As a result, a significant enhancement in power conversion efficiency to 20.6% was obtained for the PHI-treated perovskite device in comparison to a control device with 17.4%.

Polysilane-Inserted Methylammonium Lead Iodide Perovskite Solar Cells Doped with Formamidinium and Potassium

Polysilane-inserted CH3NH3PbI3 perovskite photovoltaic devices combined with potassium and formamidinium iodides were fabricated and characterized. Decaphenylcyclopentasilane layers were inserted at the perovskite/hole transport interface and annealed across a temperature range of 180–220 °C. These polysilane-coated cells prevented PbI2 formation, and the conversion efficiencies were improved over extended periods of time.

Towards commercialization: the operational stability of perovskite solar cells

Recently, perovskite solar cells (PSCs) have attracted much attention owing to their high power conversion efficiency (25.2%) and low fabrication cost. However, the short lifetime under operation is the major obstacle for their commercialization. With efforts from the entire PSC research community, significant advances have been witnessed to improve the device operational stability, and a timely summary on the progress is urgently needed. In this review, we first clarify the definition of operational stability and its significance in the context of practical use. By analyzing the mechanisms in established approaches for operational stability improvement, we summarize several effective strategies to extend device lifetime in a layer-by-layer sequence across the entire PSC. These mechanisms are discussed in the contexts of chemical reactions, photo-physical management, technological modification, etc., which may inspire future R&D for stable PSCs. Finally, emerging operational stability standards with respect to testing and reporting device operational stability are summarized and discussed, which may help reliable device stability data circulate in the research community. The main target of this review is gaining insight into the operational stability of PSCs, as well as providing useful guidance to further improve their operational lifetime by rational materials processing and device fabrication, which would finally promote the commercialization of perovskite solar cells.

An Efficient Trap Passivator for Perovskite Solar Cells: Poly(propylene glycol) bis(2-aminopropyl ether)

Perovskite solar cells (PSCs) are regarded as promising candidates for future renewable energy production. High-density defects in the perovskite films, however, lead to unsatisfactory device performances. Here, poly(propylene glycol) bis(2-aminopropyl ether) (PEA) additive is utilized to passivate the trap states in perovskite. The PEA molecules chemically interact with lead ions in perovskite, considerably passivate surface and bulk defects, which is in favor of charge transfer and extraction. Furthermore, the PEA additive can efficiently block moisture and oxygen to prolong the device lifetime. As a result, PEA-treated MAPbI3 (MA: CH3NH3) solar cells show increased power conversion efficiency (PCE) (from 17.18 to 18.87%) and good long-term stability. When PEA is introduced to (FAPbI3)1-x(MAPbBr3)x (FA: HC(NH2)2) solar cells, the PCE is enhanced from 19.66 to 21.60%. For both perovskites, their severe device hysteresis is efficiently relieved by PEA.

Surface chelation of cesium halide perovskite by dithiocarbamate for efficient and stable solar cells

Surface engineering has been shown critical for the success of perovskite solar cells by passivating the surface enriched defects and mobile species. The discovery of surface modulators with superior interaction strength to perovskite is of paramount importance since they can retain reliable passivation under various environments. Here, we report a chelation strategy for surface engineering of CsPbI2Br perovskite, in which dithiocarbamate molecules can be coordinate to surface Pb sites via strong bidentate chelating bonding. Such chelated CsPbI2Br perovskite can realize excellent passivation of surface under-coordinated defects, reaching a champion power conversion efficiency of 17.03% and an open-circuit voltage of 1.37 V of CsPbI2Br solar cells. More importantly, our chelation strategy enabled excellent device stability by maintaining 98% of their initial efficiency for over 1400 h in ambient condition. Our findings provide scientific insights on the surface engineering of perovskite that can facilitate the further development and application of perovskite optoelectronics.

Efficient and Stable MAPbI3-Based Perovskite Solar Cells Using Polyvinylcarbazole Passivation

Hybrid perovskite solar cells attract a great deal of attention due to the feasibility of their low-cost production and their demonstration of impressive power conversion efficiencies (PCEs) exceeding 25%. However, the insufficient intrinsic stability of lead halides under light soaking and thermal stress impedes practical implementation of this technology. Herein, we show that the photothermal aging of a widely used perovskite light absorber such as MAPbI₃ can be suppressed significantly by using polyvinylcarbazole (PVC) as a stabilizing agent. By applying a few complementary methods, we reveal that the PVC additive leads to passivation of defects in the absorber material. Introducing an optimal content of PVC into MAPbI₃ delivers a PCE of 18.7% in combination with a significantly improved solar cell operational lifetime: devices retained ∼70% of the initial efficiency after light soaking for 1500 h, whereas the control samples without PVC degraded almost completely under the same conditions.

Broad-Band Photodetectors Based on Copper Indium Diselenide Quantum Dots in a Methylammonium Lead Iodide Perovskite Matrix

Low-temperature solution-processed methylammonium lead iodide (MAPbI₃) crystalline films have shown outstanding performance in optoelectronic devices. However, their high dark current and high noise equivalent power prevent their application in broad-band photodetectors. Here, we applied a facile solution-based antisolvent strategy to fabricate a hybrid structure of CuInSe₂ quantum dots (CISe QDs) embedded into a MAPbI₃ matrix, which not only enhances the photodetector responsivity, showing a large on/off ratio of 10⁴ at 2 V bias compared with the bare perovskite films, but also significantly (for over 7 days) improves the device stability, with hydrophobic ligands on the CuInSe₂ QDs acting as a barrier against the uptake of environmental moisture. MAPbI₃/CISe QD-based lateral photodetectors exhibit high responsivities of >0.5 A/W and 10.4 mA/W in the visible and near-infrared regions, respectively, partly because of the formation of a type II interface between the respective semiconductors but most significantly because of the efficient trap-state passivation of the perovskite grain surfaces, and the reduction in the twinning-induced trap density, which stems from both CISe QDs and their organic ligands. A large specific detectivity of 2.2 × 10¹² Jones at 525 nm illumination (1 μW/cm²), a fast fall time of 236 μs, and an extremely low noise equivalent power of 45 fW/Hz^1/2 have been achieved.

22% Efficiency Inverted Perovskite Photovoltaic Cell Using Cation-Doped Brookite TiO2 Top Buffer

Simultaneously achieving high efficiency and high durability in perovskite solar cells is a critical step toward the commercialization of this technology. Inverted perovskite photovoltaic (IP-PV) cells incorporating robust and low levelized-cost-of-energy (LCOE) buffer layers are supposed to be a promising solution to this target. However, insufficient inventory of materials for back-electrode buffers substantially limits the development of IP-PV. Herein, a composite consisting of 1D cation-doped TiO2 brookite nanorod (NR) embedded by 0D fullerene is investigated as a top modification buffer for IP-PV. The cathode buffer is constructed by introducing fullerene to fill the interstitial space of the TiO2 NR matrix. Meanwhile, cations of transition metal Co or Fe are doped into the TiO2 NR to further tune the electronic property. Such a top buffer exhibits multifold advantages, including improved film uniformity, enhanced electron extraction and transfer ability, better energy level matching with perovskite, and stronger moisture resistance. Correspondingly, the resultant IP-PV displays an efficiency exceeding 22% with a 22-fold prolonged working lifetime. The strategy not only provides an essential addition to the material inventory for top electron buffers by introducing the 0D:1D composite concept, but also opens a new avenue to optimize perovskite PVs with desirable properties.

Nitrobenzene as Additive to Improve Reproducibility and Degradation Resistance of Highly Efficient Methylammonium-Free Inverted Perovskite Solar Cells

We show that the addition of 1% (v/v) nitrobenzene within the perovskite formulation can be used as a method to improve the power conversion efficiency and reliability performance of methylammonium-free (CsFA) inverted perovskite solar cells. The addition of nitrobenzene increased power conversion efficiency (PCE) owing to defect passivation and provided smoother films, resulting in hybrid perovskite solar cells (PVSCs) with a narrower PCE distribution. Moreover, the nitrobenzene additive methylammonium-free hybrid PVSCs exhibit a prolonged lifetime compared with additive-free PVSCs owing to enhanced air and moisture degradation resistance.

Micro- and Nanopatterning of Halide Perovskites Where Crystal Engineering for Emerging Photoelectronics Meets Integrated Device Array Technology

Tremendous efforts have been devoted to developing thin film halide perovskites (HPs) for use in high-performance photoelectronic devices, including solar cells, displays, and photodetectors. Furthermore, structured HPs with periodic micro- or nanopatterns have recently attracted significant interest due to their potential to not only improve the efficiency of an individual device via the controlled arrangement of HP crystals into a confined geometry, but also to technologically pixelate the device into arrays suitable for future commercialization. However, micro- or nanopatterning of HPs is not usually compatible with conventional photolithography, which is detrimental to ionic HPs and requires special techniques. Herein, a comprehensive overview of the state-of-the-art technologies used to develop micro- and nanometer-scale HP patterns, with an emphasis on their controlled microstructures based on top-down and bottom-up approaches, and their potential for future applications, is provided. Top-down approaches include modified conventional lithographic techniques and soft-lithographic methods, while bottom-up approaches include template-assisted patterning of HPs based on lithographically defined prepatterns and self-assembly. HP patterning is shown here to not only improve device performance, but also to reveal the unprecedented functionality of HPs, leading to new research areas that utilize their novel photophysical properties.

Progress in Materials Development for the Rapid Efficiency Advancement of Perovskite Solar Cells

The efficiency of perovskite solar cells (PSCs) has undergone rapid advancement due to great progress in materials development over the past decade and is under extensive study. Despite the significant challenges (e.g., recombination and hysteresis), both the single-junction and tandem cells have gradually approached the theoretical efficiency limit. Herein, an overview is given of how passivation and crystallization reduce recombination and thus improve the device performance; how the materials of dominant layers (hole transporting layer (HTL), electron transporting layer (ETL), and absorber layer) affect the quality and optoelectronic properties of single-junction PSCs; and how the materials development contributes to rapid efficiency enhancement of perovskite/Si tandem devices with monolithic and mechanically stacked configurations. The interface optimization, novel materials development, mixture strategy, and bandgap tuning are reviewed and analyzed. This is a review of the major factors determining efficiency, and how further improvements can be made on the performance of PSCs.

Improved photoemission and stability of 2D organic-inorganic lead iodide perovskite films by polymer passivation

2D organic-inorganic lead iodide perovskites hold great promise for functional optoelectronic devices. However, their performances have been seriously limited by poor long-term stability in ambient environment. Here, we perform a systematic study for the stability improvement of a typical 2D organic-inorganic lead iodide perovskite (PEA)2PbI4. The degradation of the (PEA)2PbI4 films can be attributed to the interaction with the humidity in environment, which leads to decomposition of the perovskite components. Then, we demonstrate that polymer passivation provides an effective approach for improving the crystal quality and stability of the (PEA)2PbI4 films. Correspondingly, the photoemission of the polymer-passivated (PEA)2PbI4 films has been enhanced due to the decreased trap states. More importantly, a hydrophobic polymer (Poly(4-Vinylpyridine), PVP) will protect the (PEA)2PbI4 films from humidity in ambient environment, which can greatly improve the physical and chemical stability of the 2D perovskite films. As a result, the PVP-passivated (PEA)2PbI4 films can produce a bright emission even after long-term (>15 d) exposure to ambient environment (25 °C, 80% RH) and continuous UV illumination. This work provides a convenient and effective approach for improving the long-term stability of 2D organic-inorganic lead iodide perovskites, which shows great promise for fabricating large-area and versatile optoelectronic devices.

Efficient and stable tin perovskite solar cells enabled by amorphous-polycrystalline structure

Tin perovskite solar cells (TPSCs) have triggered intensive research as a promising candidate for lead-free perovskite solar cells. However, it is still challenging to obtain efficient and stable TPSCs because of the low defects formation energy and the oxidation of bivalent tin; Here, we report a TPSC with a stable amorphous-polycrystalline structure, which is composed of a tin triple-halide amorphous layer and cesium-formamidinium tin iodide polycrystals. This structure effectively blocks the outside oxygen, moisture and also suppresses the ion diffusion inside the devices. In addition, its energy level benefits the charge extraction and transport in TPSCs. This design enabled us to obtain the certified quasi-steady-state efficiency over 10% for TPSCs from an accredited certification institute. The cell was stable, maintaining 95% of the initial PCE after operation at the maximum power point under AM 1.5 G simulated solar light (100 mWcm⁻²) for 1000 hours.

Tin based perovskite solar cells could be promising replacements for lead based counterparts due to their lower toxicity, but their efficiencies are much lower. Here, Liu et al. report stable tin based perovskite solar cells with efficiency over 10% by designing amorphous-polycrystalline nanostructures.

Exfoliated Fluorographene Quantum Dots as Outstanding Passivants for Improved Flexible Perovskite Solar Cells

Flexible perovskite solar cells (PSCs) are currently one of the most attractive flexible thin-film photovoltaic technologies. Despite achieving remarkable progress in power conversion efficiencies (PCEs), flexible PSCs have not yet kept pace with rigid PSCs. Defect passivation is of crucial importance to further enhance the PCEs of flexible PSCs. Here, highly dispersed fluorographene quantum dots (FGQDs) are exfoliated from graphite fluoride with the aid of stirring and sonication and used to passivate the grain boundaries and surface of the perovskite films for high-performance flexible PSCs. Photoluminescence spectroscopy (PL) and time-resolved PL decays indicate that the FGQDs are beneficial for suppressing carrier recombination. Space-charge-limited current measurements prove that the passivated perovskite film exhibits reduced trap densities. As a result, a best PCE of 20.40% is achieved from the flexible PSCs, owing to significantly reduced charge recombination. Moreover, the champion device delivers an outstanding steady-state PCE exceeding 20%. The flexible PSCs with the FGQDs also exhibit enhanced thermal stability and environmental stability. Our work not only highlights the importance of passivating the defects within the perovskite films for high-efficiency flexible PSCs but also offers a promising future for the commercialization of flexible PSCs.

Electronic Coordination Effect of the Regulator on Perovskite Crystal Growth and Its High-Performance Solar Cells

The rapid growth of perovskite crystals leads to excessive grain boundaries and surface defects, which have a negative effect on the performance of solar cells (PSCs). Passivating defects by controlling the crystal growth rate becomes a crucial research hotspot for preparing high-crystallinity perovskite films. In this work, phenylacetonitrile (PA) and 2-naphthylacetonitrile (2-NA) serving as crystal growth regulators are introduced into the perovskite precursor. The coordination effect of lone-pair electrons (n-electrons) and π-electrons in the regulator molecule with Pb²⁺ on the nucleation and growth of FA_0.80MA_0.15Cs_0.05Pb(I_0.85Br_0.15)₃ perovskite crystal along with the passivation of surface defects and grain boundaries are systematically investigated. The n-electrons of N atom form a coordination bond with Pb²⁺, and the π-electrons in the aromatic ring generate a cation-π interaction with Pb²⁺. This combined effect efficiently delays the crystallization rate of the perovskite crystal and then promotes the grain growth and reduces the grain boundaries, which is favorable for the dissociation of more excitons to carriers. The PA-optimized PSCs shows an increase of power conversion efficiency (PCE) from 18.01 to 21.09%, with an unencapsulated device retaining 91.2% of its initial efficiency for 60 days in 40 ± 5% relative humidity under dark conditions.

Secondary Grain Growth in Organic-Inorganic Perovskite Films with Ethylamine Hydrochloride Additives for Highly Efficient Solar Cells

The grain boundaries of perovskite polycrystalline are regarded as a defect region that not only provides carrier recombination sites but also introduces device degradation pathways. Efforts to enlarging the grain size of a perovskite film and reducing its grain boundary are crucial for highly efficient and stable perovskite solar cells (PSCs). Some effective methods that facilitate grain growth are postdeposition thermal annealing and solvent vapor annealing. However, a detailed understanding of grain growth mechanisms in perovskite films is lacking. In this study, perovskite films were prepared by adding ethylamine hydrochloride (EACl) to the precursor solution. This additive strategy promotes a new grain growth mode, secondary grain growth, in perovskite films. Secondary grain growth leads to much larger grains with a high crystallographic orientation. These excellent properties lead to reduced grain boundaries and the densities of boundary defects. The improved film quality results in a prolonged charge-carrier lifetime and a significantly enhanced power conversion efficiency (PCE). Compared with the 18.42% PCE of the control device, the PCE of the device with EACl additives reaches 21.07%.

Using Soft Polymer Template Engineering of Mesoporous TiO2 Scaffolds to Increase Perovskite Grain Size and Solar Cell Efficiency

The mesoporous (meso)-TiO₂ layer is a key component of high-efficiency perovskite solar cells (PSCs). Herein, pore size controllable meso-TiO₂ layers are prepared using spin coating of commercial TiO₂ nanoparticle (NP) paste with added soft polymer templates (SPT) followed by removal of the SPT at 500 °C. The SPTs consist of swollen crosslinked polymer colloids (microgels, MGs) or a commercial linear polymer (denoted as LIN). The MGs and LIN were comprised of the same polymer, which was poly(N-isopropylacrylamide) (PNIPAm). Large (L-MG) and small (S-MG) MG SPTs were employed to study the effect of the template size. The SPT approach enabled pore size engineering in one deposition step. The SPT/TiO₂ nanoparticle films had pore sizes > 100 nm, whereas the average pore size was 37 nm for the control meso-TiO₂ scaffold. The largest pore sizes were obtained using L-MG. SPT engineering increased the perovskite grain size in the same order as the SPT sizes: LIN < S-MG < L-MG and these grain sizes were larger than those obtained using the control. The power conversion efficiencies (PCEs) of the SPT/TiO₂ devices were ∼20% higher than that for the control meso-TiO₂ device and the PCE of the champion S-MG device was 18.8%. The PCE improvement is due to the increased grain size and more effective light harvesting of the SPT devices. The increased grain size was also responsible for the improved stability of the SPT/TiO₂ devices. The SPT method used here is simple, scalable, and versatile and should also apply to other PSCs.

Improving the Performance and Stability of Perovskite Light-Emitting Diodes by a Polymeric Nanothick Interlayer-Assisted Grain Control Process

CsPbBr₃ is a promising light-emitting material due to its wet solution processability, high photoluminescence quantum yield (PLQY), narrow color spectrum, and cost-effectiveness. Despite such advantages, the morphological defects, unsatisfactory carrier injection, and stability issues retard its widespread applications in light-emitting devices (LEDs). In this work, we demonstrated a facile and cost-effective method to improve the morphology, efficiency, and stability of the CsPbBr₃ emissive layer using a dual polymeric encapsulation governed by an interface-assisted grain control process (IAGCP). An eco-friendly low-cost hydrophilic polymer poly(vinylpyrrolidone) (PVP) was blended into the CsPbBr₃ precursor solution, which endows the prepared film with a better surface coverage with a smoothened surface. Furthermore, it is revealed that inserting a thin PVP nanothick interlayer at the poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS)/emissive layer interface further promotes the film quality and the performance of the derived LED. It is mainly attributed to three major consequences: (i) reduced grain size of the emissive layer, which facilitates charge recombination, (ii) reduced current leakage due to the enhanced electron-blocking effect, and (iii) improved color purity and air stability owing to better defect passivation. As a result, the optimized composite emissive film can retain the luminescence properties even on exposure to ambient conditions for 80 days and ∼62% of its initial PL intensity can be preserved after 30 days of storage without any encapsulation.

Commercially available jeffamine additives for p-i-n perovskite solar cells

Commercially available Jeffamines (polyetheramine) with average molecular weights of 2000 and 3000 g mol^-1; one (M2005), two (D2000), and three (T3000) primary amino groups end-capping on the polyether backbone; and propylene oxide (PO) and ethylene oxide (EO) functionality were explored as additives for application in MAPbI₃ perovskite solar cells (PSCs). The results indicated that the embedding of Jeffamine additives effectively passivates the defects in the grain boundaries of perovskite through the coordination bonding between the nitrogen atom and the uncoordinated lead ion of perovskite. We fabricated p-i-n PSC devices with the structure of glass/indium tin oxide (ITO)/NiO_x/CH₃NH₃PbI₃ (with and without Jeffamine)/PC₆₁BM/BCP/Ag. We observed the interaction between the Jeffamine and perovskites. This interaction led to increased lifetimes of the carriers of perovskite, which enabled the construction of high-performance p-i-n PSCs. For the Jeffamine-D2000-derived device, we observed an increase in the power conversion efficiency from 14.5% to 16.8% relative to the control device. Furthermore, the mechanical properties of the perovskite films were studied. The interaction between the additive and perovskite reinforced the flexibility of the thin film, which may pave the way for stretchable optoelectronics.

Roadmap on halide perovskite and related devices

Since the first report on solid-state perovskite solar cells (PSCs) with ∼10% power conversion efficiency (PCE) and 500 h-stability in 2012, tremendous effort has been being devoted to develop PSCs with higher PCE, longer stability and recycling hazardous lead waste. As a result, PCE over 23% was recorded in 2018 and stability over 10 000 h was reported. Beyond photovoltaics, lead halide perovskite materials demonstrated superb properties when they were applied to flat-panel x-ray detectors and non-volatile resistive switching memory. In this review, the progress of the lead halide perovskite in photovoltaics, x-ray imaging and memristors is investigated. Pb-based PSCs and non-Pb-based PSCs are compared, where technologies of non-Pb-based PSCs are not matured for commercialization. Pb-based PSCs were found to be highly suitable for both terrestrial and space photovoltaics. Higher sensitivity under low dose rate observed from the lead halide perovskite suggests a bright future for perovskite x-ray imaging systems. Moreover, high on/off ratio and low energy consumption observed in resistive switching enables perovskite to be a promising candidate for high density memristors.

Prolonged Lifetime of Perovskite Solar Cells Using a Moisture-Blocked and Temperature-Controlled Encapsulation System Comprising a Phase Change Material as a Cooling Agent

Although the power conversion efficiency of perovskite solar cells (PSCs) reached up to 25% that made them comparable to the commercial solar cells, they are facing issues toward commercialization, especially their short lifetime. Remarkably, the most important key factors that regulate the durability of the devices are moisture, light, and heat. In this work, prolonging the device lifetime is focused by designing a flexible moisture-blocked and temperature-controlled encapsulation system. In this regard, a thermally adjusted phase change material is embedded in a polymer encapsulation layer to avoid the moisture diffusion, rapid temperature fluctuation, and undesired crystalline phase change of the perovskite layer in the PSCs under the operation condition. As a result, a 2 year stable device is achieved, whereas the reference device loses more than 50% of its performance after 10 days. Surprisingly, the charge transport resistance and recombination rate show no significant change during 450 days of storage, which confirms no increase in the defect density.

Grain Enlargement and Defect Passivation with Melamine Additives for High Efficiency and Stable CsPbBr3 Perovskite Solar Cells

The preparation of high-quality perovskite films with low grain boundaries and defect states is a prerequisite for achieving high-efficiency perovskite solar cells (PSCs) with good environmental stability. An effective additive engineering strategy has been developed for simultaneous defect passivation and crystal growth of CsPbBr₃ perovskite films by introducing 1,3,5-triazine-2,4,6-triamine (melamine) into the PbBr₂ precursor solution. The resultant melamine-PbBr₂ film has a loose, large-grained structure and decreased crystallinity, which has a positive effect on the crystallization process of the perovskite as it retards the crystallization rate as a result of the interaction between melamine and lead ions. Additionally, the passivation by melamine gives a high-quality CsPbBr₃ perovskite film with fewer grain boundaries, lower defect densities, and better energy level matching is achieved by multistep liquid-phase spin-coating, which greatly suppresses the nonradiative recombination resulting from the defects and promotes charge extraction at the interface. A champion power conversion efficiency as high as 9.65 % with a promising open-circuit voltage of 1.584 V is achieved for PSCs with an architecture of fluorine-doped tin oxide/c-TiO₂ /m-TiO₂ /melamine-added CsPbBr₃ /carbon-based hole-transporting layer. Furthermore, the unencapsulated melamine-added CsPbBr₃ PSC shows superior thermal and humidity stability in ambient air at 85 °C or 85 % relative humidity over 720 h.