Light management for perovskite light-emitting diodes.

Perovskite light-emitting diodes (LEDs) have reached external quantum efficiencies of over 20% for various colours, showing great potential for display and lighting applications. Despite the internal quantum efficiencies of the best-performing devices already approaching unity, around 80% of the internally generated photons are trapped in the devices and lose energy through a variety of lossy channels. Significant opportunities for improving efficiency and maximizing photon extraction lie in the effective management of light. In this Review we analyse light management strategies based on the intrinsic optical properties of the perovskite materials and the extrinsic properties related to device structures. These approaches should allow the external quantum efficiencies of perovskite LEDs to substantially exceed the conventional limits of planar organic LED devices. By revisiting lessons learned from organic LEDs and perovskite solar cells, we highlight possible directions of future research towards perovskite LEDs with ultrahigh efficiencies.

Solvent-Assisted Crystallization of an α-Fe2O3 Electron Transport Layer for Efficient and Stable Perovskite Solar Cells Featuring Negligible Hysteresis.

Inorganic-organic metal halide perovskite solar cells (PSCs) show power conversion efficiency values approaching those of state-of-the-art silicon solar cells. In a quest to find suitable charge transport materials in PSCs, hematite (α-Fe2O3) has emerged as a potential electron transport layer (ETL) in n-i-p planar PSCs due to its low cost, UV light stability, and nontoxicity. Yet, the performance of α-Fe2O3-based PSCs is far lower than that of state-of-the-art PSCs owing to the poor quality of the α-Fe2O3 ETL. In this work, solvent-assisted crystallization of α-Fe2O3 ETLs was carried out to examine the impact of solvents on the optoelectronic properties of α-Fe2O3 thin films. Among the various solvents used in this study (deionized water, ethanol, iso-propanol, and iso-butanol), optimized ethanol-based α-Fe2O3 ETLs lead to champion device performance with a power conversion efficiency of 13% with a reduced hysteresis index of 0.04 in an n-i-p-configured PSC. The PSC also exhibited superior long-term inert and ambient stabilities compared to a reference device made using a SnO2 ETL. Through a series of experiments spanning structural, morphological, and optoelectronic properties of the various α-Fe2O3 thin films and their devices, we provide insights into the reasons for the improved photovoltaic performance. It is noted that the formation of a pinhole-free compact morphology of ETLs facilitates crack-free surface coverage of the perovskite film atop an α-Fe2O3 ETL, reduces interfacial recombination, and enhances charge transfer efficiency. This work opens up the route toward novel ETLs for the development of efficient and photo-stable PSCs.

Ti3C2Tx-Modified PEDOT:PSS Hole-Transport Layer for Inverted Perovskite Solar Cells.

PEDOT: PSS is a commonly used hole-transport layer (HTL) in inverted perovskite solar cells (PSCs) due to its compatibility with low-temperature solution processing. However, it possesses lower conductivity than other conductive polymers and metal oxides, along with surface defects, limiting its photovoltaic performance. In this study, we introduced two-dimensional Ti3C2Tx (MXene) as an additive in the PEDOT:PSS HTL with varying doping concentrations (i.e., 0, 0.03, 0.05, and 0.1 wt.%) to tune the electrical conductivity of PEDOT:PSS and to modify the properties of the perovskite film atop it. We noted that the grain size of the CH3NH3PbI3 (MAPI3) perovskite layer grown over an optimal concentration of MXene (0.03 wt.%)-doped PEDOT:PSS increased from 250 nm to 400 nm, reducing charge recombination due to fewer grain boundaries. Ultraviolet photoelectron spectroscopy (UPS) revealed increased work function (WF) from 4.43 eV to 4.99 eV with 0.03 wt.% MXene doping, making the extraction of holes easier due to a more favorable energy level alignment with the perovskite. Quantum chemical investigations based on density functional theory (DFT) were conducted at the ωB97XD/6-311++G(d,p) level of theory to provide more insight into the stability, bonding nature, and optoelectronic properties of the PEDOT:PSS-MXene system. The theoretical investigations revealed that the doping of PEDOT:PSS with Ti3C2Tx could cause a significant effect on the electronic properties of the HTL, as experimentally demonstrated by an increase in the electrical conductivity. Finally, the inverted PSCs employing 0.03 wt.% MXene-doped PEDOT:PSS showed an average power conversion efficiency (PCE) of 15.1%, up from 12.5% for a reference PSC employing a pristine PEDOT:PSS HTL. The champion device with a 0.03 wt.% MXene-PEDOT:PSS HTL achieved 15.5% PCE.

Photonic nanostructures mimicking floral epidermis for perovskite solar cells.

Here, we report photonic nanostructures replicated from the adaxial epidermis of flower petals onto light-polymerized coatings using low-cost nanoimprint lithography at ambient temperature. These multifunctional nanocoatings are applied to confer enhanced light trapping, water repellence, and UV light and environmental moisture protection features in perovskite solar cells. The former feature helps attain a maximum power conversion efficiency of 24.61% (21.01% for the reference cell) without any additional device optimization. Added to these merits, the nanocoatings also enable stable operation under AM 1.5G and UV light continuous illumination or in real-world conditions. Our engineering approach provides a simple way to produce multifunctional nanocoatings optimized by nature's wisdom.

Molecular materials as interfacial layers and additives in perovskite solar cells.

Solar cells based on organo-metal halide perovskites have gained unprecedented research interest over the last few years due to their low-cost solution processability, high power conversion efficiency, which has recently reached a certified value of 25.2%, and abundance of raw materials. Nevertheless, the best efficiencies remain below the Shockley-Queisser theoretical limit of 32.5% due to several losses arising from either defect traps present in the bulk of the perovskite absorber or at the device heterointerfaces. While bulk defects are detrimental for the device performance by mainly limiting the open circuit voltage, interfacial layers are also crucial. They dictate the charge transfer/transport from the perovskite layer to the collecting electrodes, hence influencing the device photocurrent, but also act as protective barriers against oxygen and moisture penetration. Molecular materials and additives are widely used to improve the bulk properties of perovskite absorbers through the formation of high-quality perovskite films with superior optoelectronic properties, and improved crystallinity, and also of electronically clean interfaces with minimum losses during charge transfer/transport. In this review, we analyze the predominant pathways that contribute to voltage and current losses due to poor interfaces and also due to non-radiative recombination losses arising from inferior perovskite morphology and its inherent polycrystalline and highly defective nature. We then discuss strategies for achieving interfacial organic and inorganic molecular materials for application as electron and hole transport layers in perovskite solar cells with ideal energy levels, high charge mobilities and improved thermal, photo, and structural stability. Moreover, the prerequisites for molecular additives to achieve dimensionality engineering, defect passivation, molecular cross-linking, interfacial energy alignment and electronic doping are thoroughly discussed. Finally, we examine prospects for future research directions and commercialization.

Suppressing the Photocatalytic Activity of Zinc Oxide Electron-Transport Layer in Nonfullerene Organic Solar Cells with a Pyrene-Bodipy Interlayer.

Organic solar cells based on nonfullerene acceptors have recently witnessed a significant rise in their power conversion efficiency values. However, they still suffer from severe instability issues, especially in an inverted device architecture based on the zinc oxide bottom electron transport layers. In this work, we insert a pyrene-bodipy donor-acceptor dye as a thin interlayer at the photoactive layer/zinc oxide interface to suppress the degradation reaction of the nonfullerene acceptor caused by the photocatalytic activity of zinc oxide. In particular, the pyrene-bodipy-based interlayer inhibits the direct contact between the nonfullerene acceptor and zinc oxide hence preventing the decomposition of the former by zinc oxide under illumination with UV light. As a result, the device photostability was significantly improved. The π-π interaction between the nonfullerene acceptor and the bodipy part of the interlayer facilitates charge transfer from the nonfullerene acceptor toward pyrene, which is followed by intramolecular charge transfer to bodipy part and then to zinc oxide. The bodipy-pyrene modified zinc oxide also increased the degree of crystallization of the photoactive blend and the face-on stacking of the polymer donor molecules within the blend hence contributing to both enhanced charge transport and increased absorption of the incident light. Furthermore, it decreased the surface work function as well as surface energy of the zinc oxide film all impacting in improved power conversion efficiency values of the fabricated cells with champion devices reaching values up to 9.86 and 11.80% for the fullerene and nonfullerene-based devices, respectively.

Enhanced Organic and Perovskite Solar Cell Performance through Modification of the Electron-Selective Contact with a Bodipy-Porphyrin Dyad.

Photovoltaic devices based on organic semiconductors and organo-metal halide perovskites have not yet reached the theoretically predicted power conversion efficiencies while they still exhibit poor environmental stability. Interfacial engineering using suitable materials has been recognized as an attractive approach to tackle the above issues. We introduce here a zinc porphyrin-triazine-bodipy donor-π bridge-acceptor dye as a universal electron transfer mediator in both organic and perovskite solar cells. Thanks to its "push-pull" character, this dye enhances electron transfer from the absorber layer toward the electron-selective contact, thus improving the device's photocurrent and efficiency. The direct result is more than 10% average power conversion efficiency enhancement in both fullerene-based (from 8.65 to 9.80%) and non-fullerene-based (from 7.71 to 8.73%) organic solar cells as well as in perovskite ones (from 14.56 to 15.67%), proving the universality of our approach. Concurrently, by forming a hydrophobic network on the surface of metal oxide substrates, it improves the nanomorphology of the photoactive overlayer and contributes to efficiency stabilization. The fabricated devices of both kinds preserved more than 85% of their efficiency upon exposure to ambient conditions for more than 600 h without any encapsulation.

Inorganic and Layered Perovskites for Optoelectronic Devices.

Organic-inorganic halide perovskites are making breakthroughs in a range of optoelectronic devices. Reports of >23% certified power conversion efficiency in photovoltaic devices, external quantum efficiency >21% in light-emitting diodes (LEDs), continuous-wave lasing and ultralow lasing thresholds in optically pumped lasers, and detectivity in photodetectors on a par with commercial GaAs rivals are being witnessed, making them the fastest ever emerging material technology. Still, questions on their toxicity and long-term stability raise concerns toward their market entry. The intrinsic instability in these materials arises due to the organic cation, typically the volatile methylamine (MA), which contributes to hysteresis in the current-voltage characteristics and ion migration. Alternative inorganic substitutes to MA, such as cesium, and large organic cations that lead to a layered structure, enhance structural as well as device operational stability. These perovskites also provide a high exciton binding energy that is a prerequisite to enhance radiative emission yield in LEDs. The incorporation of inorganic and layered perovskites, in the form of polycrystalline films or as single-crystalline nanostructure morphologies, is now leading to the demonstration of stable devices with excellent performance parameters. Herein, key developments made in various optoelectronic devices using these perovskites are summarized and an outlook toward stable yet efficient devices is presented.

Tuning optical/electrical properties of 2D/3D perovskite by the inclusion of aromatic cation.

The employment of bulky aliphatic cations in the manufacture of moisture-stable materials has triggered the development and application of 2D/3D perovskites as sensitizers in moisture-stable solar cells. Although it is true that the moisture stability increases, it is also true that the photovoltaic performance of 2D/3D PVK materials is severely limited owing to quantum and dielectric confinement effects. Accordingly, it is necessary the synthesis and deep optical characterization of materials with an adequate management of dielectric contrast between the layers. Here, we demonstrate the successful tuning of dielectric confinement by the inclusion of a conjugated molecule, as a bulky cation, in the fabrication of the 2D/3D PVK material (C6H5NH3)2(CH3NH3)n-1PbnI3n+1, where n = 3 or 5. The absence of excitonic states related to n ≥ 1 at room temperature, as well as the very low concentration of excitons after 1 ps of excitation of samples in which n ≥ 3, provide strong evidence of an excellent ability to dissociate excitons into free charge carriers. As consequence films with low n, presenting higher stability than standard 3D perovskites, improved significantly their performance, showing one of the highest short circuit current density (Jsc ≈ 13.8) obtained to date for perovskite materials within the 2D limit (n < 10).

Perovskite-Polymer Blends Influencing Microstructures, Nonradiative Recombination Pathways, and Photovoltaic Performance of Perovskite Solar Cells.

Solar cells based on organic-inorganic halide perovskites are now leading the photovoltaic technologies because of their high power conversion efficiency. Recently, there have been debates on the microstructure-related defects in metal halide perovskites (grain size, grain boundaries, etc.) and a widespread view is that large grains are a prerequisite to suppress nonradiative recombination and improve photovoltaic performance, although opinions against it also exist. Herein, we employ blends of methylammonium lead iodide perovskites with an insulating polymer (polyvinylpyrrolidone) that offer the possibility to tune the grain size in order to obtain a fundamental understanding of the photoresponse at the microscopic level. We provide, for the first time, spatially resolved details of the microstructures in such blend systems via Raman mapping, light beam-induced current imaging, and conductive atomic force microscopy. Although the polymer blend systems systematically alter the morphology by creating small grains (more grain boundaries), they reduce nonradiative recombination within the film and enhance its spatial homogeneity of radiative recombination. We attribute this to a reduction in the density of bulk trap states, as evidenced by an order of magnitude higher photoluminescence intensity and a significantly higher open-circuit voltage when the polymer is incorporated into the perovskite films. The solar cells employing blend systems also show nearly hysteresis-free power conversion efficiency ∼17.5%, as well as a remarkable shelf-life stability over 100 days.

Role of the Metal-Oxide Work Function on Photocurrent Generation in Hybrid Solar Cells.

ZnO is a widely used metal-oxide semiconductor for photovoltaic application. In solar cell heterostructures they not only serve as a charge selective contact, but also act as electron acceptor. Although ZnO offers a suitable interface for exciton dissociation, charge separation efficiencies have stayed rather poor and conceptual differences to organic acceptors are rarely investigated. In this work, we employ Sn doping to ZnO nanowires in order to understand the role of defect and surface states in the charge separation process. Upon doping we are able to modify the metal-oxide work function and we show its direct correlation with the charge separation efficiency. For this purpose, we use the polymer poly(3-hexylthiophene) as donor and the squaraine dye SQ2 as interlayer. Interestingly, neither mobilities nor defects are prime performance limiting factor, but rather the density of available states around the conduction band is of crucial importance for hybrid interfaces. This work highlights crucial aspects to improve the charge generation process of metal-oxide based solar cells and reveals new strategies to improve the power conversion efficiency of hybrid solar cells.

A Comparative Study of Light-Emitting Diodes Based on All-Inorganic Perovskite Nanoparticles (CsPbBr3 ) Synthesized at Room Temperature and by a Hot-Injection Method.

Perovskite nanoparticles (PeNPs) have been extensively studied for optoelectronic applications, owing to their extremely high photoluminescence quantum yield, tunable band gap, and exceptionally narrow emission spectra. Therefore, PeNPs are considered excellent candidates for the development of high-efficiency, low-cost, wide-gamut, and high-purity color displays. However, their synthesis typically involves multistep cumbersome processes that might hinder commercial development. Herein, green light-emitting diodes (LEDs) prepared by using all-inorganic PeNPs CsPbBr3 synthesized at room temperature (RT) are reported and their performance compared with those prepared by a traditional hot-injection method. Insights into the morphology and optoelectronic properties of RT PeNPs are provided through AFM and TEM and employing them in LEDs.

Humidity versus photo-stability of metal halide perovskite films in a polymer matrix.

Despite the high efficiency of over 21% reported for emerging thin film perovskite solar cells, one of the key issues prior to their commercial deployment is to attain their long term stability under ambient and outdoor conditions. The instability in perovskite is widely conceived to be humidity induced due to the water solubility of its initial precursors, which leads to decomposition of the perovskite crystal structure; however, we note that humidity alone is not the major degradation factor and it is rather the photon dose in combination with humidity exposure that triggers the instability. In our experiment, which is designed to decouple the effect of humidity and light on perovskite degradation, we investigate the shelf-lifetime of CH3NH3PbI3 films in the dark and under illumination under high humidity conditions (Rel. H. > 70%). We note minor degradation in perovskite films stored in a humid dark environment whereas upon exposure to light, the films undergo drastic degradation, primarily owing to the reactive TiO2/perovskite interface and also the surface defects of TiO2. To enhance its air-stability, we incorporate CH3NH3PbI3 perovskite in a polymer (poly-vinylpyrrolidone, PVP) matrix which retained its optical and structural characteristics in the dark for ∼2000 h and ∼800 h in room light soaking, significantly higher than a pristine perovskite film, which degraded completely in 600 h in the dark and in less than 100 h when exposed to light. We attribute the superior stability of PVP incorporated perovskite films to the improved structural stability of CH3NH3PbI3 and also to the improved TiO2/perovskite interface upon incorporating a polymer matrix. Charge injection from the polymer embedded perovskite films has also been confirmed by fabricating solar cells using them, thereby providing a promising future research pathway for stable and efficient perovskite solar cells.

Solid state perovskite solar modules by vacuum-vapor assisted sequential deposition on Nd:YVO4 laser patterned rutile TiO2 nanorods.

The past few years have witnessed remarkable progress in solution-processed methylammonium lead halide (CH3NH3PbX3, X = halide) perovskite solar cells (PSCs) with reported photoconversion efficiency (η) exceeding 20% in laboratory-scale devices and reaching up to 13% in their large area perovskite solar modules (PSMs). These devices mostly employ mesoporous TiO2 nanoparticles (NPs) as an electron transport layer (ETL) which provides a scaffold on which the perovskite semiconductor can grow. However, limitations exist which are due to trap-limited electron transport and non-complete pore filling. Herein, we have employed TiO2 nanorods (NRs), a material offering a two-fold higher electronic mobility and higher pore-filing compared to their particle analogues, as an ETL. A crucial issue in NRs' patterning over substrates is resolved by using precise Nd:YVO4 laser ablation, and a champion device with η ∼ 8.1% is reported via a simple and low cost vacuum-vapor assisted sequential processing (V-VASP) of a CH3NH3PbI3 film. Our experiments showed a successful demonstration of NRs-based PSMs via the V-VASP technique which can be applied to fabricate large area modules with a pin-hole free, smooth and dense perovskite layer which is required to build high efficiency devices.

Vertical TiO2 Nanorods as a Medium for Stable and High-Efficiency Perovskite Solar Modules.

Perovskite solar cells employing CH3NH3PbI3-xClx active layers show power conversion efficiency (PCE) as high as 20% in single cells and 13% in large area modules. However, their operational stability has often been limited due to degradation of the CH3NH3PbI3-xClx active layer. Here, we report a perovskite solar module (PSM, best and av. PCE 10.5 and 8.1%), employing solution-grown TiO2 nanorods (NRs) as the electron transport layer, which showed an increase in performance (∼5%) even after shelf-life investigation for 2500 h. A crucial issue on the module fabrication was the patterning of the TiO2 NRs, which was solved by interfacial engineering during the growth process and using an optimized laser pulse for patterning. A shelf-life comparison with PSMs built on TiO2 nanoparticles (NPs, best and av. PCE 7.9 and 5.5%) of similar thickness and on a compact TiO2 layer (CL, best and av. PCE 5.8 and 4.9%) shows, in contrast to that observed for NR PSMs, that PCE in NPs and CL PSMs dropped by ∼50 and ∼90%, respectively. This is due to the fact that the CH3NH3PbI3-xClx active layer shows superior phase stability when incorporated in devices with TiO2 NR scaffolds.

Mesoporous titania-vertical nanorod films with interfacial engineering for high performance dye-sensitized solar cells.

Working electrode (WE) fabrication offers significant challenges in terms of achieving high-efficiency dye-sensitized solar cells (DSCs). We have combined the beneficial effects of vertical nanorods grown on conducting glass substrate for charge transport and mesoporous particles for dye loading and have achieved a high photoconversion efficiency of (η) > 11% with an internal quantum efficiency of ∼93% in electrode films of thickness ∼7 ± 0.5 µm. Controlling the interface between the vertical nanorods and the mesoporous film is a crucial step in attaining high η. We identify three parameters, viz., large surface area of nanoparticles, increased light scattering of the nanorod-nanoparticle layer, and superior charge transport of nanorods, that simultaneously contribute to the improved photovoltaic performance of the WE developed.