Metal Oxide Charge Transport Layers Research Articles

Towards scalability: progress in metal oxide charge transport layers for large-area perovskite solar cells

In this review article, we aim to offer a focused overview of metal oxide CTLs for large-area PSCs, emphasizing recent advancements in deposition techniques, modification strategies, and their suitability for large-area device applications.

Applications of Metal Oxide Charge Transport Layers in Perovskite Solar Cells

Metal oxide (MO) charge transport layers (CTLs) are widely used for fabricating highly efficient and stable perovskite solar cells (PSCs) due to their superior stability, material and preparation cost, light transmission, and charge selection. However, the complex surface states, unbalanced carrier mobility, and variable energy band structure determined by MOs can lead to additional interfacial charge recombination and transport losses within the device, which limit further improvements in device performance. Extensive research has been conducted to address these challenges. In this review, an overview of current popular MO‐CTLs and their preparation methods for PSCs are provided. Interface regulation strategies, such as passivating interface defects, modulating interface energy level alignment, and improving interface contact are also discussed, which can enhance the performance of PSCs. Meanwhile, the commonly used dopants and doping strategies for optimizing the charge transport properties of CTLs are also discussed.

High Performing Inverted Flexible Perovskite Solar Cells via Solution Phase Deposition of Yttrium-Doped SnO2 Directly on Perovskite

Solution processing of flexible perovskite solar cells (f-PSCs) provides an avenue for scalable, high-throughput printing of lightweight, scalable, and cost-effective flexible solar cells. However, the deposition of fully solution-processed metal oxide charge transport layers on perovskites has been limited by solvent incompatibilities and high processing temperatures for metal oxide nanoparticles. In this study, we present high-performance, inverted f-PSCs from the direct deposition of yttrium-doped SnO2 nanoparticles functionalized with acetate on top of perovskite as an ink in anhydrous ethanol via blade coating. Yttrium doping improved device performance by improving the charge extraction with a decreased series resistance leading to improvements in the open-circuit voltage and fill factor. The champion power conversion efficiency for 0.1 cm2 devices increased from 14.3% for undoped SnO2 to 16.5% with 2% Y:SnO2 doping, which is unprecedented for f-PSCs on ITO-PET substrates employing SnO2 as an ETL.

Geometric Optimization of Perovskite Solar Cells with Metal Oxide Charge Transport Layers.

Perovskite solar cells (PSCs) are a promising area of research among different new generations of photovoltaic technologies. Their manufacturing costs make them appealing in the PV industry compared to their alternatives. Although PSCs offer high efficiency in thin layers, they are still in the development phase. Hence, optimizing the thickness of each of their layers is a challenging research area. In this paper, we investigate the effect of the thickness of each layer on the photoelectric parameters of n-ZnO/p-CH3NH3PbI3/p-NiOx solar cell through various simulations. Using the Sol–Gel method, PSC structure can be formed in different thicknesses. Our aim is to identify a functional connection between those thicknesses and the optimum open-circuit voltage and short-circuit current. Simulation results show that the maximum efficiency is obtained using a perovskite layer thickness of 200 nm, an electronic transport layer (ETL) thickness of 60 nm, and a hole transport layer (HTL) thickness of 20 nm. Furthermore, the output power, fill factor, open-circuit voltage, and short-circuit current of this structure are 18.9 mW/cm2, 76.94%, 1.188 V, and 20.677 mA/cm2, respectively. The maximum open-circuit voltage achieved by a solar cell with perovskite, ETL and HTL layer thicknesses of (200 nm, 60 nm, and 60 nm) is 1.2 V. On the other hand, solar cells with the following thicknesses, 800 nm, 80 nm, and 40 nm, and 600 nm, 80 nm, and 80 nm, achieved a maximum short-circuit current density of 21.46 mA/cm2 and a fill factor of 83.35%. As a result, the maximum value of each of the photoelectric parameters is found in structures of different thicknesses. These encouraging results are another step further in the design and manufacturing journey of PSCs as a promising alternative to silicon PV.

A general and facile solution approach for deposition of high-quality metal oxide charge transport layers

Metal oxide charge transport layers (CTLs) are significant components for fabricating high-performance thin film solar cells. Generally, sol-gel methods have been widely used to deposit high-quality metal oxide CTLs, but they usually suffer from the choices of solvents, ligands and metal precursors. Here we report a general and facile ionic liquid (n-butylammonium butyrate) approach to prepare V2O5, MoO3, WO3, NiO, ZnO and SnO2 thin films, which exhibits strong dissolving ability and good film-forming property. The thermal volatilization of ionic liquid is mild, which provides a slow and stable decomposition process of metal compounds and favors the formation of smooth and compact CTLs. All thin films are highly transparent in the visual range from the optical transmittance spectra and suitable to be CTLs in photovoltaic devices. Encouragingly, we have received the efficiencies of 15.06% and 13.38% based on the area of 0.09 and 1.21 cm2 (NiO as the hole transport layer), and 19.13% and 16.71% based on the area of 0.09 and 1.21 cm2 (SnO2 as the electron transport layer), which are obviously higher than those of devices prepared by traditional sol-gel methods.

Thermally Stable All‐Perovskite Tandem Solar Cells Fully Using Metal Oxide Charge Transport Layers and Tunnel Junction

All‐perovskite tandem solar cells offer a promising avenue to go beyond the efficiency limit of single‐junction devices. Their efficiencies have been increasing rapidly in the past few years; however, their commercial viability is hindered by the instability under thermal stressing. Herein, comprehensive device design strategies are proposed to achieve thermally stable all‐perovskite tandem solar cells while retaining the advantages of solution processing. Metal oxides, i.e., NiOx and SnO2, are used for the hole and electron transport layers in both wide bandgap and narrow subcells. The metal‐based recombination layer is replaced with a stable and conductive indium tin oxide nanocrystals film to fabricate an all metal‐oxide‐based tunnel junction. Based on those design strategies, the encapsulated all‐perovskite tandem solar cells retained 85% of their initial efficiency after stressing at 85 °C for 2500 h and maintained >80% of their initial performance after 900 h operation at the maximum power point and operating temperature of ≈65 °C. Achieving such thermal stability represents a crucial step toward commercial viability of all‐perovskite tandem solar cells.

Perspective on Predominant Metal Oxide Charge Transporting Materials for High-Performance Perovskite Solar Cells

Nowadays, the power conversion efficiency of organometallic mixed halide perovskite solar cells (PSCs) is beyond 25%. To fabricate highly efficient and stable PSCs, the performance of metal oxide charge transport layers (CTLs) is one of the key factors. The CTLs are employed in PSCs to separate the electrons and holes generated in the perovskite active layer, suppressing the charge recombination rate so that the charge collection efficiency can be increased at their respective electrodes. In general, engineering of metal oxide electron transport layers (ETLs) is found to be dominated in the research community to boost the performance of PSCs due to the resilient features of ETLs such as excellent electronic properties, high resistance to thermal temperature and moisture, ensuring good device stability as well as their high versatility in material preparation. The metal oxide hole transport layers in PSCs are recently intensively studied. The performance of PSCs is found to be very promising by using optimized hole transport materials. This review concisely discusses the evolution of some prevalent metal oxide charge transport materials (CTMs) including TiO2, SnO2, and NiOx, which are able to yield high-performance PSCs. The article begins with introducing the development trend of PSCs using different types of CTLs, pointing out the important criteria for metal oxides being effective CTLs, and then a variety of preparation methods for CTLs as employed by the community for high-performance PSCs are discussed. Finally, the challenges and prospects for future research direction toward scalable metal oxide CTM-based PSCs are delineated.

Effect of an Al-doped ZnO electron transport layer on the efficiency of inverted bulk heterojunction solar cells

Doping is a widely-implemented strategy for enhancing the inherent electrical properties of metal oxide charge transport layers in photovoltaic devices because higher conductivity of electron transport layer (ETL) can increment the photocurrent by reducing the series resistance. To improve the conductivity of ETL, in this study we doped the ZnO layer with aluminum (Al), then investigated the influence of AZO on the performance of inverted bulk heterojunction (BHJ) polymer solar cells based on poly [[4,8-bis [(2-ethylhexyl)oxy]benzo [1,2-b:4,5-b’]dithiophene-2,6-diyl]-[3-fluoro-2[(2-ethylhexyl)-carbonyl]-thieno-[3,4-b]thiophenediyl ]] (PTB7):[6,6]-phenyl C71 butyric acid methyl ester (PC71BM). The measured conductivity of AZO was ~10−3 S/cm, which was two orders of magnitude higher than that of intrinsic ZnO (~10−5 S/cm). By decreasing the series resistance (Rs) in a device with an AZO layer, the short circuit current (Jsc) increased significantly from 15.663 mA/cm2 to 17.040 mA/cm2. As a result, the device with AZO exhibited an enhanced power conversion efficiency (PCE) of 8.984%.

Recent progress on cesium lead/tin halide-based inorganic perovskites for stable and efficient solar cells: A review

A comparable power conversion efficiency (PCE) to that of commercial Si solar cells (over 23%) has been achieved by organic–inorganic hybrid perovskite solar cells (OIH-PSCs) within several years. As OIH-PSC materials have hygroscopic organic cations that limit their thermal and long-term stability (i.e. operational lifetime of about 1 year, much shorter than commercial Si of 20–25 years), significant research efforts have been directed to the development of all-inorganic PSCs to overcome this limitation. These studies have demonstrated that cesium lead halide (CsPbX3) and Pb-free cesium tin halide (CsSnX3) perovskites are promising materials for the fabrication of thermally stable and efficient solar cells. This work reviews recent progress on versatile CsPbX3 and CsSnX3 inorganic PSCs. Remarkable PCE values over 17% and 4% have been achieved by employing CsPbX3 and CsSnX3 perovskites, respectively, in a short development time. In addition, we evaluate the materials engineering methods and film deposition techniques for producing such inorganic perovskite materials. Several strategies including surface and interfacial passivation are discussed to alleviate hysteresis and instability of inorganic PSCs. Furthermore, future research directions including device engineering using inorganic metal oxide charge transport layers are suggested to further reinforce this innovative advances in the inorganic PSCs.

Metal Oxide Charge Transport Layers for Efficient and Stable Perovskite Solar Cells

AbstractCurrently, the efficiency of perovskite solar cells (PSCs) is ≈24%. For the fabrication of such high efficiency PSCs, it is necessary to use both electron and hole transport layers to effectively separate the charges generated by light absorption of the perovskite layer and selectively transfer the separated electrons and holes. In addition to the efficiency, the materials used for transporting charges must be resilient to light, heat, and moisture to ensure long‐term stability of PSCs; furthermore, low‐cost fabrication is required to form a charge transport layer at low temperatures by a solution process. For this purpose, metal oxides are best suited as charge transport materials for PSCs because of their advantages such as low cost, long‐term stability, and high efficiency. In this Review, the metal oxide electron and hole transport materials used in PSCs are reviewed and preparation of these materials is summarized. Finally, the challenges and future research direction for metal oxide‐based charge transport materials are described.

Electron‐Beam‐Evaporated Nickel Oxide Hole Transport Layers for Perovskite‐Based Photovoltaics

AbstractHigh‐quality charge carrier transport materials are of key importance for stable and efficient perovskite‐based photovoltaics. This work reports on electron‐beam‐evaporated nickel oxide (NiOx) layers, resulting in stable power conversion efficiencies (PCEs) of up to 18.5% when integrated into solar cells employing inkjet‐printed perovskite absorbers. By adding oxygen as a process gas and optimizing the layer thickness, transparent and efficient NiOx hole transport layers (HTLs) are fabricated, exhibiting an average absorptance of only 1%. The versatility of the material is demonstrated for different absorber compositions and deposition techniques. As another highlight of this work, all‐evaporated perovskite solar cells employing an inorganic NiOx HTL are presented, achieving stable PCEs of up to 15.4%. Along with good PCEs, devices with electron‐beam‐evaporated NiOx show improved stability under realistic operating conditions with negligible degradation after 40 h of maximum power point tracking at 75 °C. Additionally, a strong improvement in device stability under ultraviolet radiation is found if compared to conventional perovskite solar cell architectures employing other metal oxide charge transport layers (e.g., titanium dioxide). Finally, an all‐evaporated perovskite solar mini‐module with a NiOx HTL is presented, reaching a PCE of 12.4% on an active device area of 2.3 cm2.

High-efficiency, solution-processable, multilayer triple cation perovskite light-emitting diodes with copper sulfide–gallium–tin oxide hole transport layer and aluminum-zinc oxide–doped cesium electron injection layer

Light-emitting diodes with perovskite luminophores have great potential in next-generation displays because of their exceptional color purity with narrow emission bandwidth, broadband color tunability, and solution processability. However, their low luminescent efficiency is a critical drawback. Here, we report the first demonstration of a multicolor, large-area, perovskite display, which can be made flexible by using an optimized perovskite emissive layer sandwiched between inorganic metal oxide charge transport layers, all of which are coated via a facile solution process. We show that advanced interfacial engineering, especially the energy level alignment at the interface, plays a vital role in determining the device performance because of its effects on charge injection, transport, and recombination. These devices exhibit maximum current and power efficiencies of 74.25 cd A−1 and 89.72 lm/w for green emission, 21.40 cd A−1 and 25.84 lm/w for red emission, and 15.21 cd A−1 and 15.84 lm/w for blue emission, respectively. Furthermore, with the introduction of inorganic charge transport layers, these devices exhibit high environmental stability, and the encapsulated devices have operating lifetimes exceeding 450 h with an initial brightness of 1000 cd/m2.

Novel Plasma-Assisted Low-Temperature-Processed SnO2 Thin Films for Efficient Flexible Perovskite Photovoltaics

The recent evolution of solution-processed hybrid organic–inorganic perovskite-based photovoltaic devices opens up the commercial avenue for high-throughput roll-to-roll manufacturing technology. To circumvent the thermal limitations that hinder the use of metal oxide charge transport layers on plastic flexible substrates in such technologies, we employed a relatively low-power nitrogen plasma treatment to achieve compact SnO2 thin-film electrodes at near room temperature. The perovskite photovoltaic devices thus fabricated using N2 plasma-treated SnO2 performed on par with thermally annealed SnO2 electrodes and resulted in a power conversion efficiency (PCE) of ca. 20.3% with stabilized power output (SPO) of ca. 19.1% on rigid substrates. Furthermore, the process is extended to realize flexible perovskite solar cells on indium tin oxide (ITO)-coated polyethylene terephthalate (PET) substrates with champion PCE of 18.1% (SPO ca. 17.1%), which retained ca. 90% of its initial performance after 1000 bending ...

Solution Processed High Efficiency Quantum Dot Light Emitting Diode With Inorganic Charge Transport Layers

High efficiency all-inorganic quantum dot light emitting diodes are fabricated with metal oxide charge transport layers. Alloyed CdSe/ZnS quantum dots are used as the active layer with nickel oxide thin film hole transport layer and zinc oxide nanoparticle electron transport layer. The turn on voltage of the device is 5 V and the emission peak is at 555 nm. The current efficiency reached a maximum of 144 cd/A with a luminance of 116,000 cd/ $\text{m}^{2}$ at 10 V bias voltage. The peak external quantum efficiency of 11.4% is achieved at a current density of 80 mA/cm2, and it starts to decrease at higher current densities. The superior device performance can be attributed to enhanced charge transport from the adjacent transport layers into the active quantum dot emissive layer.

Versatile Device Architectures for High-Performing Light-Soaking-Free Inverted Polymer Solar Cells.

Metal oxide charge transport layers have been widely employed to prepare inverted polymer solar cells with high efficiency and long lifetime. However, the intrinsic defects in the metal oxide layers, especially those prepared from low-temperature routes, overshadow the high efficiency that can be achieved and also introduce "light-soaking" issues to these devices. In this work, we have employed polyethyleneimine (PEI) and poly(9,9-bis(6'-(N,N-diethylamino)propyl)-fluorene-alt-9,9-bis-(3-ethyl(oxetane-3-ethyloxy)-hexyl)-fluorene] (PFN-OX) to modify our low-temperature-processed TiO2 electron transport layer (ETL) and demonstrated that the light-soaking issue can be effectively eliminated by PEI modifications because of the formation of abundant dipole moments, whereas PFN-OX was ineffective as a result of deficient dipole moments at the interface. Excitingly, PEI modifications enable versatile device architectures to obtain light-soaking-free, inverted PTB7-Th:PC71BM solar cells with efficiencies of over 10%, by adding PEI either in the bulk or as an adjacent layer below or above the TiO2 ETL.

Near-infrared photodetector consisting of J-aggregating cyanine dye and metal oxide thin films

We demonstrate a near-infrared photodetector that consists of a thin film of the J-aggregating cyanine dye, U3, and transparent metal-oxide charge transport layers. The high absorption coefficient of the U3 film, combined with the use of a reflective anode and optical spacer layer, results in a zero-bias external quantum efficiency of 16.1 ± 0.1% (λ = 756 nm) for a device containing an 8.1 ± 0.3 nm-thick U3 film. The specific detectivity (D*) and response speed (f3dB) of a fully optimized device are measured to be (4.3 ± 0.1) × 1011 cm Hz1/2 W−1 and 92 kHz, respectively.

A ZnO/N-doped carbon nanotube nanocomposite charge transport layer for high performance optoelectronics

Metal oxide charge transport layers are widely used to promote the interfacial charge transport of organic optoelectronics. Nevertheless, frequently used wide-bandgap metal oxides with low electrical conductivities reveal inherent limitations in the charge transport enhancement. We present the remarkable electro-conductivity enhancement of solution processable ZnO charge transport layers upon dispersing a tiny amount (less than 0.1 wt%) of chemically doped CNTs and the corresponding device performance improvement of light-emitting diodes (OLEDs). Using various undoped or doped CNTs, whose work function was systematically tuned by substitutional doping of electron deficient B or electron rich N,N-doped CNT (N-CNT), the composite showed a lowered work function matching well with the conduction band of ZnO. Consequently, the ZnO/N-CNT nanocomposite transport layer with 0.08 wt% N-CNT showed a five-fold enhancement of electron mobility, while maintaining the intrinsic bandgap energy levels, optical transparency and solution processability of pure ZnO. The inverted OLEDs employing ZnO/N-CNT nanocomposite electron transport layers could facilitate well-balanced electron–hole injection and, thus, more than two-fold enhancement of maximum luminance (from 21 000 cd m−2 at 14.6 V to 46 100 cd m−2 at 14.0 V) and efficiency (from 6.9 cd A−1 at 13.4 V to 14.3 cd A−1 at 13.6 V). This highly effective charge mobility enhancement enabled by work function tunable, chemically doped CNTs would be beneficial for various organic and inorganic charge transport materials with different energy levels.

Selection of Metal Oxide Charge Transport Layers for Colloidal Quantum Dot LEDs

We investigate the effect of the electronic energy level positioning, conductivity, and morphology of metal oxide charge transport layers on the performance of light emitting devices (LEDs) that consist of a colloidally synthesized quantum dot (QD) luminescent film embedded between electron and hole injecting ceramic layers. We demonstrate that understanding of these material properties and their effect on charging processes in QDs enables the systematic design of higher efficiency QD-LEDs and excitation of QDs with different emission colors using the same device structure.

Colloidal quantum-dot light-emitting diodes with metal-oxide charge transport layers

Colloidal quantum dots, with their tunable luminescence properties, are uniquely suited for use as lumophores in light-emitting devices for display technologies and large-area planar lighting1,2,3,4,5,6,7,8,9,10. In contrast to epitaxially grown quantum dots, colloidal quantum dots can be synthesized as highly monodisperse colloids and solution deposited over large areas into densely packed, solid-state multilayers, which have shown promise as efficient optical gain media11. To be a viable platform for colour-tunable electrically pumped lasers, the present-generation quantum-dot LEDs must be modified to withstand the extended, high-current-density operation needed to achieve population inversion. This requirement necessitates a quantum-dot LED design that incorporates robust charge transport layers. Here we report the use of sputtered, amorphous inorganic semiconductors as robust charge transport layers and demonstrate devices capable of operating at current densities exceeding 3.5 A cm−2 with peak brightness of 1,950 Cd m−2 and maximum external electroluminescence efficiency of nearly 0.1%, which represents a 100-fold improvement over previously reported structures8,10. The authors show that metal oxide and colloidal quantum dots can be combined to fabricate monochrome LEDs with a brightness that matches that of the best organic-based quantum-dot LEDs, but with the advantage of improved shelf-life robustness. The reported maximum external electroluminescence efficiency is nearly 0.1%, which represents a 100-fold improvement over previously reported structures