Sub-nanometer conformal TiO₂ blocking layer for high efficiency solid-state perovskite absorber solar cells

Progress and Application of Halide Perovskite Materials for Solar Cells and Light Emitting Devices

Halide perovskite materials have attracted worldwide attention in the photovoltaic area due to the rapid improvement in efficiency, from less than 4% in 2009 to 26.1% in 2023 with only a nanometer lever photo-active layer. Meanwhile, this nova star found applications in many other areas, such as light emitting, sensor, etc. This review started with the fundamentals of physics and chemistry behind the excellent performance of halide perovskite materials for photovoltaic/light emitting and the methods for preparing them. Then, it described the basic principles for solar cells and light emitting devices. It summarized the strategies including nanotechnology to improve the performance and the application of halide perovskite materials in these two areas: from structure-property relation to how each component in the devices affects the overall performance. Moreover, this review listed the challenges for the future applications of halide perovskite materials.

Charge transport materials for mesoscopic perovskite solar cells

An overview on recent advances in the fundamental understanding of how interfaces of mesoscopic perovskite solar cells (mp-PSCs) with different architectures, upon incorporating various charge transport layers, influence their performance.

Lattice Strain Relaxation and Grain Homogenization for Efficient Inverted MAPbI3 Perovskite Solar Cells

The lattice strain of a perovskite film is vital to the controllable growth and charge transport in perovskite solar cells (PSCs). In this work, a lead chloride (PbCl₂) assisted crystallization (LCAC) protocol is introduced for releasing the strain across the interface of a NiO_x/perovskite, which induces a preferred (h00) crystal plane growth and grain homogenization. PSCs with LCAC show a facilitated charge extraction and suppressed nonradiative recombination. Thanks to the controlled film growth and strain-released interface, the inverted MAPbI₃ (MA = methylammonium) PSC devices with LCAC deliver a power conversion efficiency (PCE) over 20% with a short-circuit current density (J_sc) of 23.60 mA cm^-2, which is obviously higher than that of the control device with a PCE of 18.36% and a J_sc of 21.74 mA cm^-2. Meanwhile, the LCAC devices maintain 80% of their initial efficiency after being exposed to an ambient atmosphere with a relative humidity of 40% over 1000 h in the dark.

Solid-State Solar Cells Based on TiO2 Nanowires and CH3NH3PbI3 Perovskite

Perovskite inorganic-organic solar cells are fabricated as a sandwich structure of mesostructured TiO2 as electron transport layer (ETL), CH3NH3PbI3 as active material layer (AML), and Spiro-OMeTAD as hole transport layer (HTL). The crystallinity, structural morphology, and thickness of TiO2 layer play a crucial role to improve the overall device performance. The randomly distributed one dimensional (1D) TiO2 nanowires (TNWs) provide excellent light trapping with open voids for active filling of visible light absorber compared to bulk TiO2. Solid-state photovoltaic devices based on randomly distributed TNWs and CH3NH3PbI3 are fabricated with high open circuit voltage Voc of 0.91 V, with conversion efficiency (CE) of 7.4%. Mott-Schottky analysis leads to very high built-in potential (Vbi) ranging from 0.89 to 0.96 V which indicate that there is no depletion layer voltage modulation in the perovskite solar cells fabricated with TNWs of different lengths. Moreover, finite-difference time-domain (FDTD) analysis revealed larger fraction of photo-generated charges due to light trapping and distribution due to field convergence via guided modes, and improved light trapping capability at the interface of TNWs/CH3NH3PbI3 compared to bulk TiO2.

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.

Atomic/molecular layer deposition for energy storage and conversion

Energy storage and conversion systems, including batteries, supercapacitors, fuel cells, solar cells, and photoelectrochemical water splitting, have played vital roles in the reduction of fossil fuel usage, addressing environmental issues and the development of electric vehicles. The fabrication and surface/interface engineering of electrode materials with refined structures are indispensable for achieving optimal performances for the different energy-related devices. Atomic layer deposition (ALD) and molecular layer deposition (MLD) techniques, the gas-phase thin film deposition processes with self-limiting and saturated surface reactions, have emerged as powerful techniques for surface and interface engineering in energy-related devices due to their exceptional capability of precise thickness control, excellent uniformity and conformity, tunable composition and relatively low deposition temperature. In the past few decades, ALD and MLD have been intensively studied for energy storage and conversion applications with remarkable progress. In this review, we give a comprehensive summary of the development and achievements of ALD and MLD and their applications for energy storage and conversion, including batteries, supercapacitors, fuel cells, solar cells, and photoelectrochemical water splitting. Moreover, the fundamental understanding of the mechanisms involved in different devices will be deeply reviewed. Furthermore, the large-scale potential of ALD and MLD techniques is discussed and predicted. Finally, we will provide insightful perspectives on future directions for new material design by ALD and MLD and untapped opportunities in energy storage and conversion.

Inorganic Materials by Atomic Layer Deposition for Perovskite Solar Cells

Organic–inorganic hybrid perovskite solar cells (PSCs) have received much attention with their rapid progress during the past decade, coming close to the point of commercialization. Various approaches in the process of PSC development have been explored with the motivation to enhance the solar cell power conversion efficiency—while maintaining good device stability from light, temperature, and moisture—and simultaneously optimizing for scalability. Atomic layer deposition (ALD) is a powerful tool in depositing pinhole-free conformal thin-films with excellent reproducibility and accurate and simple control of thickness and material properties over a large area at low temperatures, making it a highly desirable tool to fabricate components of highly efficient, stable, and scalable PSCs. This review article summarizes ALD’s recent contributions to PSC development through charge transport layers, passivation layers, and buffer and recombination layers for tandem applications and encapsulation techniques. The future research directions of ALD in PSC progress and the remaining challenges will also be discussed.

Perovskite Films with Reduced Interfacial Strains via a Molecular-Level Flexible Interlayer for Photovoltaic Application

Interface strains and lattice distortion are inevitable issues during perovskite crystallization. Silane as a coupling agent is a popular connector to enhance the compatibility between inorganic and organic materials in semiconductor devices. Herein, a protonated amine silane coupling agent (PASCA-Br) interlayer between TiO₂ and perovskite layers is adopted to directionally grasp both of them by forming the structural component of a lattice unit. The pillowy alkyl ammonium bromide terminals at the upper side of the interlayer provide well-matched growth sites for the perovskite, leading to mitigated interface strain and ensuing lattice distortion; meanwhile, its superior chemical compatibility presents an ideal effect on healing the under-coordinated Pb atoms and halogen vacancies of bare perovskite crystals. The PASCA-Br interlayer also serves as a mechanical buffer layer, inducing less cracked perovskite film when bending. The developed molecular-level flexible interlayer provides a promising interfacial engineering for perovskite solar cells and their flexible application.

Effect of Annealing Temperature on Spatial Atomic Layer Deposited Titanium Oxide and Its Application in Perovskite Solar Cells

In this study, spatial atomic layer deposition (sALD) is employed to prepare titanium dioxide (TiO2) thin films by using titanium tetraisopropoxide and water as metal and water precursors, respectively. The post-annealing temperature is varied to investigate its effect on the properties of the TiO2 films. The experimental results show that the sALD TiO2 has a similar deposition rate per cycle to other ALD processes using oxygen plasma or ozone oxidant, implying that the growth is limited by titanium tetraisopropoxide steric hindrance. The structure of the as-deposited sALD TiO2 films is amorphous and changes to polycrystalline anatase at the annealing temperature of 450 °C. All the sALD TiO2 films have a low absorption coefficient at the level of 10-3 cm-1 at wavelengths greater than 500 nm. The annealing temperatures of 550 °C are expected to have a high compactness, evaluated by the refractive index and x-ray photoelectron spectrometer measurements. Finally, the 550 °C-annealed sALD TiO2 film with a thickness of ~8 nm is applied to perovskite solar cells as a compact electron transport layer. The significantly enhanced open-circuit voltage and conversion efficiency demonstrate the great potential of the sALD TiO2 compact layer in perovskite solar cell applications.

Low-Temperature Solution-Processed Amorphous Titania Nanowire Thin Films for 1 cm2 Perovskite Solar Cells

The development of solution-processed inorganic amorphous electron-transporting layers (ETLs) is important for the future commercialization of perovskite solar cells (PSCs). The formation of such ETLs using low-temperature processing techniques will lower potential production costs and accommodate diverse substrate materials. Herein, a low-temperature (<150 °C) solution process forms amorphous titania nanowire (Am-TNW) thin films on fluorine-doped tin oxide conducting glass substrates. When applied as an ETL in PSCs, the Am-TNW layer achieves a higher average power conversion efficiency (18.3%) relative to that of a nanocrystalline anatase TNW (ATNW) layer obtained after high-temperature (500 °C) heating (16.7%). Compared to the ATNW counterparts, the Am-TNW-based PSCs exhibit inferior charge extraction across the TNW/CH₃NH₃PbI₃ interface but more effectively suppress interfacial charge recombination. The insertion of a fullerene layer between the Am-TNW and CH₃NH₃PbI₃ improves the charge extraction. The Am-TNW-based bilayer ETL gave optimal power conversion efficiencies of 20.3% and 19.0% for PSCs with 0.16 cm² and 1.00 cm² apertures, respectively. This is due to the concurrent advantages of enhanced light absorption, facilitated charge extraction, and reduced charge recombination. The use of the Am-TNW as an ETL in PSCs provides a facile, efficient way to increase the effectiveness of PSCs.

Realizing Stable Artificial Photon Energy Harvesting Based on Perovskite Solar Cells for Diverse Applications

As the fastest developing photovoltaic device, perovskite solar cells have achieved an extraordinary power conversion efficiency (PCE) of 25.3% under AM 1.5 illumination. However, few studies have been devoted to perovskite solar cells harvesting artificial light, owing to the great challenge in the simultaneous manipulation of bandgap-adjustable perovskite materials, corresponding matched energy band structure of carrier transport materials, and interfacial defects. Herein, through systematic morphology, composition, and energy band engineering, high-quality Cs_0.05 MA_0.95 PbBr_x I_{3- x} perovskite as the light absorber and Nb_y Ti_{1- y} O₂ (Nb:TiO₂ ) as the electron transport material with an ideal energy band alignment are obtained simultaneously. The theoretical-limit-approaching record PCEs of 36.3% (average: 34.0 ± 1.2%) under light-emitting diode (LED, warm white) and 33.2% under fluorescent lamp (cold white) are achieved simultaneously, as well as a PCE of 19.5% (average: 18.9 ± 0.3%) under solar illumination. An integrated energy conversion and storage system based on an artificial light response solar cell and sodium-ion battery is established for diverse practical applications, including a portable calculator, quartz clock, and even environmental monitoring equipment. Over a week of stable operation shows its great practical potential and provides a new avenue to promote the commercialization of perovskite photovoltaic devices via integration with ingenious electronic devices.

Chlorine capped SnO2 quantum-dots modified TiO2 electron selective layer to enhance the performance of planar perovskite solar cells

SnO₂ quantum dots (QDs) ended with chlorine ions are introduced at the interface of spin-coated TiO₂ electron selective layer (ESL)/perovskite to fill the pinholes in the layer and passivate the trapping defects. As a result of the increased interface electron collection and reduced bulk recombination, the planar perovskite solar cell with the QDs modified ESL gives the large power conversion efficiency enhancement from 14.9% to 17.3% and greatly improved stability under the continuous light irradiation.

Atomic layer deposition for efficient and stable perovskite solar cells

Extended understandings of perovskite solar cells by recent ALD application studies as well as challenges toward enhancing the efficiency and stability will be addressed.

Deposition of zinc oxide as an electron transport layer in planar perovskite solar cells by spray and SILAR methods comparable with spin coating

CH₃NH₃PbI₃ planar-structure perovskite solar cells were fabricated with the configuration FTO/ZnO/CH₃NH₃PbI₃/Au. ZnO nanoparticles were synthesized by the precipitation method. Three different deposition methods including spin-coating, spraying and successive ionic layer adsorption and reaction (SILAR) were applied to fabricate the ZnO films as electron transport layers. Certain analyses, such as XRD, SEM, FESEM, UV-visible and I–V measurements, were carried out to evaluate the performance of the cells. The best cell performance was achieved for the perovskite solar cell with a ZnO film coated by the spin method. The average efficiency was 7% without using any hole transport materials and 10.25% using spiro-OMeTAD as a hole transport material. The average efficiencies of the cells coated by the spraying and SILAR methods using spiro-OMeTAD, were found to be 8.64% and 7.7% respectively. This study demonstrates the versatility of the spray and SILAR coating methods and their potential for fabricating low-cost, large scale, flexible and mass produced perovskite solar cells.

We have examined the versatility of spray and SILAR coating methods in fabricating mass produced low cost efficient planar perovskite solar cells.

Laser Annealing of TiO2 Electron-Transporting Layer in Perovskite Solar Cells

Solution-processed TiO₂ and other metal-oxide electron-transporting layers (ETLs) for perovskite solar cells commonly require high-temperature annealing (>450 °C), causing the underlying indium-tin oxide (ITO) to degrade and inhibiting the use of flexible plastic substrates, such as poly(ethylene naphthalate). Laser-based solar cell manufacturing is attracting increased interest and can enable rapid and low-temperature fabrication of perovskite solar cells. By using novel pulsed ultraviolet laser processing on the solution-processed TiO₂, we demonstrate a champion 17.1% efficient flexible perovskite solar cell. We can independently control the annealing of the ETL without affecting the underlying ITO or substrate due to the shallow absorption depth and short pulse duration of the laser. Ellipsometry and X-ray photoelectron spectroscopy verify that the laser-annealed TiO₂ thin film is stoichiometric and relatively denser than the thermally annealed control sample. The efficiencies of the laser-processed devices exceeded those fabricated via hot plate, but with the added benefit of a high-throughput, low-temperature, and flexible-substrate-friendly process.

Mesoporous Titanium Dioxide: Synthesis and Applications in Photocatalysis, Energy and Biology

Mesoporous materials are materials with high surface area and intrinsic porosity, and therefore have attracted great research interest due to these unique structures. Mesoporous titanium dioxide (TiO₂) is one of the most widely studied mesoporous materials given its special characters and enormous applications. In this article, we highlight the significant work on mesoporous TiO₂ including syntheses and applications, particularly in the field of photocatalysis, energy and biology. Different synthesis methods of mesoporous TiO₂-including sol⁻gel, hydrothermal, solvothermal method, and other template methods-are covered and compared. The applications in photocatalysis, new energy batteries and in biological fields are demonstrated. New research directions and significant challenges of mesoporous TiO₂ are also discussed.

A Novel Conductive Mesoporous Layer with a Dynamic Two-Step Deposition Strategy Boosts Efficiency of Perovskite Solar Cells to 20

Lead halide perovskite solar cells (PSCs) with the high power conversion efficiency (PCE) typically use mesoporous metal oxide nanoparticles as the scaffold and electron-transport layers. However, the traditional mesoporous layer suffers from low electron conductivity and severe carrier recombination. Here, antimony-doped tin oxide nanorod arrays are proposed as novel transparent conductive mesoporous layers in PSCs. Such a mesoporous layer improves the electron transport as well as light utilization. To resolve the common problem of uneven growth of perovskite on rough surface, the dynamic two-step spin coating strategy is proposed to prepare highly smooth, dense, and crystallized perovskite films with micrometer-scale grains, largely reducing the carrier recombination ratio. The conductive mesoporous layer and high-quality perovskite film eventually render the PSC with a remarkable PCE of 20.1% with excellent reproducibility. These findings provide a new avenue to further design high-efficiency PSCs from the aspect of carrier transport and recombination.

TiO2 Phase Junction Electron Transport Layer Boosts Efficiency of Planar Perovskite Solar Cells

In the planar perovskite solar cells (PSCs), the electron transport layer (ETL) plays a critical role in electron extraction and transport. Widely utilized TiO2 ETLs suffer from the low conductivity and high surface defect density. To address these problems, for the first time, two types of ETLs based on TiO2 phase junction are designed and fabricated distributed in the opposite space, namely anatase/rutile and rutile/anatase. The champion efficiency of PSCs based on phase junction ETL is over 15%, which is much higher than that of cells with single anatase (9.8%) and rutile (11.8%) TiO2 as ETL. The phase junction based PSCs also demonstrated obviously reduced hysteresis. The enhanced performance is discussed and mainly ascribed to the excellent capability of carrier extraction, defect passivation, and reduced recombination at the ETL/perovskite interface. This work opens a new phase junction ETL strategy toward interfacial energy band manipulation for improved PSC performance.

Solution-Processed Ultrathin TiO2 Compact Layer Hybridized with Mesoporous TiO2 for High-Performance Perovskite Solar Cells

The electron transport layer (ETL) is a key component of perovskite solar cells (PSCs) and must provide efficient electron extraction and collection while minimizing the charge recombination at interfaces in order to ensure high performance. Conventional bilayered TiO₂ ETLs fabricated by depositing compact TiO₂ (c-TiO₂) and mesoporous TiO₂ (mp-TiO₂) in sequence exhibit resistive losses due to the contact resistance at the c-TiO₂/mp-TiO₂ interface and the series resistance arising from the intrinsically low conductivity of TiO₂. Herein, to minimize such resistive losses, we developed a novel ETL consisting of an ultrathin c-TiO₂ layer hybridized with mp-TiO₂, which is fabricated by performing one-step spin-coating of a mp-TiO₂ solution containing a small amount of titanium diisopropoxide bis(acetylacetonate) (TAA). By using electron microscopies and elemental mapping analysis, we establish that the optimal concentration of TAA produces an ultrathin blocking layer with a thickness of ∼3 nm and ensures that the mp-TiO₂ layer has a suitable porosity for efficient perovskite infiltration. We compare PSCs based on mesoscopic ETLs with and without compact layers to determine the role of the hole-blocking layer in their performances. The hybrid ETLs exhibit enhanced electron extraction and reduced charge recombination, resulting in better photovoltaic performances and reduced hysteresis of PSCs compared to those with conventional bilayered ETLs.

TiO2 Electron Transport Bilayer for Highly Efficient Planar Perovskite Solar Cell

In planar perovskite solar cells, it is vital to engineer the extraction and recombination of electron-hole pairs at the electron transport layer/perovskite interface for obtaining high performance. This study reports a novel titanium oxide (TiO₂ ) bilayer with different Fermi energy levels by combing atomic layer deposition and spin-coating technique. Energy band alignments of TiO₂ bilayer can be modulated by controlling the deposition order of layers. The TiO₂ bilayer based perovskite solar cells are highly efficient in carrier extraction, recombination suppression, and defect passivation, and thus demonstrate champion efficiencies up to 16.5%, presenting almost 50% enhancement compared to the TiO₂ single layer based counterparts. The results suggest that the bilayer with type II band alignment as electron transport layers provides an efficient approach for constructing high-performance planar perovskite solar cells.

Room-Temperature Processing of TiOx Electron Transporting Layer for Perovskite Solar Cells

In order to realize high-throughput roll-to-roll manufacturing of flexible perovskite solar cells, low-temperature processing of all device components must be realized. However, the most commonly used electron transporting layer in high-performance perovskite solar cells is based on TiO₂ thin films processed at high temperature (>450 °C). Here, we demonstrate room temperature solution processing of the TiO_x layer that performs as well as the high temperature TiO₂ layer in perovskite solar cells, as evidenced by a champion solar cell efficiency of 16.3%. Using optical spectroscopy, electrical measurements, and X-ray diffraction, we show that the room-temperature processed TiO_x is amorphous with organic residues, and yet its optical and electrical properties are on par with the high-temperature TiO₂. Flexible perovskite solar cells that employ a room-temperature TiO_x layer with a power conversion efficiency of 14.3% are demonstrated.

Modulated CH3NH3PbI3-xBrx film for efficient perovskite solar cells exceeding 18

The organic-inorganic lead halide perovskite layer is a crucial factor for the high performance perovskite solar cell (PSC). We introduce CH₃NH₃Br in the precursor solution to prepare CH₃NH₃PbI_3−xBr_x hybrid perovskite, and an uniform perovskite layer with improved crystallinity and apparent grain contour is obtained, resulting in the significant improvement of photovoltaic performance of PSCs. The effects of CH₃NH₃Br on the perovskite morphology, crystallinity, absorption property, charge carrier dynamics and device characteristics are discussed, and the improvement of open circuit voltage of the device depended on Br doping is confirmed. Based on above, the device based on CH₃NH₃PbI_2.86Br_0.14 exhibits a champion power conversion efficiency (PCE) of 18.02%. This study represents an efficient method for high-performance perovskite solar cell by modulating CH₃NH₃PbI_3−xBr_x film.

Atomic Layer Deposition of Titanium Oxide on Single-Layer Graphene: An Atomic-Scale Study toward Understanding Nucleation and Growth

Controlled synthesis of a hybrid nanomaterial based on titanium oxide and single-layer graphene (SLG) using atomic layer deposition (ALD) is reported here. The morphology and crystallinity of the oxide layer on SLG can be tuned mainly with the deposition temperature, achieving either a uniform amorphous layer at 60 °C or ∼2 nm individual nanocrystals on the SLG at 200 °C after only 20 ALD cycles. A continuous and uniform amorphous layer formed on the SLG after 180 cycles at 60 °C can be converted to a polycrystalline layer containing domains of anatase TiO₂ after a postdeposition annealing at 400 °C under vacuum. Using aberration-corrected transmission electron microscopy (AC-TEM), characterization of the structure and chemistry was performed on an atomic scale and provided insight into understanding the nucleation and growth. AC-TEM imaging and electron energy loss spectroscopy revealed that rocksalt TiO nanocrystals were occasionally formed at the early stage of nucleation after only 20 ALD cycles. Understanding and controlling nucleation and growth of the hybrid nanomaterial are crucial to achieving novel properties and enhanced performance for a wide range of applications that exploit the synergetic functionalities of the ensemble.

Current status of electron transport layers in perovskite solar cells: materials and properties

Methyl ammonium lead halide-based hybrid perovskite solar cells (PSCs) have been intensively studied in recent years because of their high efficiency and low processing costs.

Atomic Layer Deposition of TiO2 for a High-Efficiency Hole-Blocking Layer in Hole-Conductor-Free Perovskite Solar Cells Processed in Ambient Air

In this study we design and construct high-efficiency, low-cost, highly stable, hole-conductor-free, solid-state perovskite solar cells, with TiO2 as the electron transport layer (ETL) and carbon as the hole collection layer, in ambient air. First, uniform, pinhole-free TiO2 films of various thicknesses were deposited on fluorine-doped tin oxide (FTO) electrodes by atomic layer deposition (ALD) technology. Based on these TiO2 films, a series of hole-conductor-free perovskite solar cells (PSCs) with carbon as the counter electrode were fabricated in ambient air, and the effect of thickness of TiO2 compact film on the device performance was investigated in detail. It was found that the performance of PSCs depends on the thickness of the compact layer due to the difference in surface roughness, transmittance, charge transport resistance, electron-hole recombination rate, and the charge lifetime. The best-performance devices based on optimized TiO2 compact film (by 2000 cycles ALD) can achieve power conversion efficiencies (PCEs) of as high as 7.82%. Furthermore, they can maintain over 96% of their initial PCE after 651 h (about 1 month) storage in ambient air, thus exhibiting excellent long-term stability.

Using a low-temperature carbon electrode for preparing hole-conductor-free perovskite heterojunction solar cells under high relative humidity

We demonstrate the application of a low-temperature carbon counter electrode with good flexibility and high conductivity in fabricating perovskite solar cells. A modified two-step method was used for the deposition of nanocrystalline CH3NH3PbI3 under high relative humidity. The carbon counter electrode was printed on a perovskite layer directly, with different sizes of graphite powder being employed. The interfacial charge transfer and transport in solar cells were investigated through photoluminescence and impedance measurements. We find that the existence of nano-graphite powder in the electrode has a noticeable influence on the back contact and cell performance. The prepared devices of hole-conductor-free perovskite heterojunction solar cells without encapsulation exhibit advantageous stability in air in the dark, with the optimal power conversion efficiency reaching 6.88%. This carbon counter electrode has the features of low-cost and low-temperature preparation, giving it potential for application in the large-scale flexible fabrication of perovskite solar cells in the future.

Nano-structured electron transporting materials for perovskite solar cells

Organic-inorganic hybrid perovskite solar cells have been developing rapidly in the past several years, and their power conversion efficiency has reached over 20%, nearing that of polycrystalline silicon solar cells. Because the diffusion length of the hole in perovskites is longer than that of the electron, the performance of the device can be improved by using an electron transporting layer, e.g., TiO2, ZnO and TiO2/Al2O3. Nano-structured electron transporting materials facilitate not only electron collection but also morphology control of the perovskites. The properties, morphology and preparation methods of perovskites are reviewed in the present article. A comprehensive understanding of the relationship between the structure and property will benefit the precise control of the electron transporting process and thus further improve the performance of perovskite solar cells.

Solution-processed carrier selective layers for high efficiency organic/nanostructured-silicon hybrid solar cells

The reduction of interface minority carrier recombination is regarded as a key performance indicator in improving the power conversion efficiency (PCE) of organic-inorganic hybrid solar cells. In this study, we chose two kinds of carrier-selective layers to be applied in a hybrid solar cell device. A hole selective transporting layer of N,N'-bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD) was added to the interface between Si nanohole structures and

PEDOT

PSS, and the electron selective layer cesium carbonate (Cs2CO3) was added to the interface between the backside Si wafer and the rear Ti/Ag electrode. The main process used a clean and low-cost solution process, and the annealed temperature was under 140 °C. In addition, after we inserted these two carrier selective layers, the minority carrier lifetime was prolonged from 29.98 μs to 140.81 μs, indicating its significance in reducing the recombination rate. Eventually, we demonstrated that the PCE of Si/organic heterojunction solar cells can be improved to 13.23%.

Recent progress and challenges of organometal halide perovskite solar cells

We review recent progress in the development of organometal halide perovskite solar cells. We discuss different compounds used to construct perovskite photoactive layers, as well as the optoelectronic properties of this system. The factors that affect the morphology of the perovskite active layer are explored, e.g. material composition, film deposition methods, casting solvent and various post-treatments. Different strategies are reviewed that have recently emerged to prepare high performing perovskite films, creating polycrystalline films having either large or small grain size. Devices that are constructed using meso-superstructured and planar architectures are summarized and the impact of the fabrication process on operational efficiency is discussed. Finally, important research challenges (hysteresis, thermal and moisture instability, mechanical flexibility, as well as the development of lead-free materials) in the development of perovskite solar cells are outlined and their potential solutions are discussed.

In situ growth of SnO2 nanosheets on a substrate via hydrothermal synthesis assisted by electrospinning and the gas sensing properties of SnO2/polyaniline nanocomposites

In situ preparation of SnO₂ nanosheet/polyaniline composite on a substrate and its applications as a high performance gas sensor.

Light absorption enhancement by embedding submicron scattering TiO2nanoparticles in perovskite solar cells

With optimized amount of the embedded scattering NPs, obviousJ_scincrement and consequent PCE improvement over 5% can be accomplished when comparing with the regular mesostructured PSCs.

Multifunctional Inverse Opal-Like TiO2 Electron Transport Layer for Efficient Hybrid Perovskite Solar Cells

A novel multifunctional inverse opal-like TiO₂ electron transport layer (IOT-ETL) is designed to replace the traditional compact layer and mesoporous scaffold layer in perovskite solar cells (PSCs). Improved light harvesting efficiency and charge transporting performance in IOT-ETL based PSCs yield high power conversion efficiency of 13.11%.

Selective Deposition of Insulating Metal Oxide in Perovskite Solar Cells with Enhanced Device Performance

We report a simple methodology for the selective deposition of an insulating layer on the nanoparticulate TiO2 (np-TiO2) mesoporous layer of perovskite solar cells. The deposited MgO insulating layer mainly covered the bottom part of the np-TiO2 layer with less coverage at the top. The so-called quasi-top-open structure is introduced to act as an efficient hole-blocking layer to prevent charge recombination at the physical contact of the transparent conducting oxide with the perovskite. This leads to an open-circuit voltage higher than that of the reference cell with a compact TiO2 hole-blocking layer. Moreover, such a quasi-top-open structure can facilitate the electron injection from perovskite into the np-TiO2 mesoporous layer and improve the spectral response at longer wavelength because of the less covered insulating layer at the top. This work provides an alternative way to fabricate perovskite solar cells without the need to use a conventional compact TiO2 layer.

The tailored inner space of TiO2 electrodes via a 30 second wet etching process: high efficiency solid-state perovskite solar cells

We fabricated a perovskite solar cell with enhanced device efficiency based on the tailored inner space of the TiO2 electrode by utilizing a very short chemical etching process. It was found that the mesoporous TiO2 photoanode treated with a HF solution exhibited remarkably enhanced power conversion efficiencies under simulated AM 1.5G one sun illumination. The controlled inner space and morphology of the etched TiO2 electrode provide an optimized space for perovskite sensitizers and infiltration of a hole transport layer without sacrificing its original electron transport ability, which resulted in higher JSC, FF and VOC values. This simple platform provides new opportunities for tailoring the microstructure of the TiO2 electrode and has great potential in various optoelectronic devices utilizing metal oxide nanostructures.

High-Performance Planar Perovskite Optoelectronic Devices: A Morphological and Interfacial Control by Polar Solvent Treatment

Highly efficient planar perovskite optoelectronic devices are realized by amine-based solvent treatment on compact TiO2 and by optimizing the morphology of the perovskite layers. Amine-based solvent treatment between the TiO2 and the perovskite layers enhances electron injection and extraction and reduces the recombination of photogenerated charges at the interface.

Titanate and titania nanostructured materials for environmental and energy applications: a review

The state-of-the-art development of fabrication strategies of multi-dimensional titanate and titania nanostructures is reviewed first. This is followed by an overview of their potential applications in environmental, energy, and biomedical sectors.

Electrodeposition of PbO and its in situ conversion to CH3NH3PbI3 for mesoscopic perovskite solar cells

The perovskite CH₃NH₃PbI₃ is deposited on the mesoscopic TiO₂ film, and used as a light absorber for perovskite solar cells, exhibiting a high PCE of 12.5% under standard AM 1.5 conditions.

Cesium carbonate as a surface modification material for organic–inorganic hybrid perovskite solar cells with enhanced performance

Cs₂CO₃was used as surface modification material for organic–inorganic perovskite solar cells to retard the charge recombination at the mp-TiO₂surface, leading to enhanced performance.

Rutile TiO2 nanowire-based perovskite solar cells

Different lengths of rutile TiO₂ nanowires (NW) with wide-open space for effective material filling were used as photoanodes for perovskite solar cells.