Promising Electron Transport Layer Research Articles

Exploring the theoretical potential of tungsten oxide (WOx) as a universal electron transport layer (ETL) for various perovskite solar cells through interfacial energy band alignment modulation

Perovskite solar cells (PSCs) play a pivotal role in advancing renewable energy to achieve United Nation's Sustainable Development Goal 7 (SDG 7), which aims to ensure universal access to affordable, sustainable, reliable and modern energy services. Aiming to enhance the performance of PSCs by replacing the typically used electron transport layers (ETLs: TiO2, SnO2, ZnO etc.), we theoretically investigated the viability of tungsten oxide (WOX) as a promising ETL for PSCs. Moreover, the effect of altering the energy levels of WOX on cell performance has also been analyzed through simulation. Initially, 12 (twelve) PSC structures having the combination of different perovskite (PSK: CsPbBr3, CsPbI3, FAPbBr3, FAPbI3) absorber layers with different organic hole transport layers (HTLs: Spiro-OMeTAD, P3HT, PEDOT:PSS) and a fixed ETL of WOX were optimized numerically for comparing their performance. As CsPbBr3-based PSCs showed the best performance, further simulations were performed by varying some WOX/CsPbBr3 interface properties such as interface defect density, conduction band offset (CBO) between WOX and CsPbBr3, energy bandgap (Eg) of WOX etc. Finally, the best-performed PSCs were found for the Eg = 3.5 eV of WOX and the CBO of - 0.5 eV confirming the conduction band minimum (CBM) of WOX is lower than that of CsPbBr3 by 0.5 eV. A properly chosen WOX layer enhanced the efficiency of CsPbBr3-based PSCs up to 14.65 %, 14.52 % and 16.09 %, aproaching the Shockley-Queisser (S-Q) limit (16.37% for CsPbBr3-based solar cell) from the initial values of 11.39 %, 11.27 %, and 12.49 %, respectively. This study ensures WOX is a promising ETL for which a proper PSC structure having a suitable PSK and an HTL can improve cell performance. Moreover, the importance of modifying energy levels of ETL material in enhancing the performance of PSCs is explored. As a result, this study opens a path for the researchers to develop WOX having suitable CBM and Eg, so that it can be well-suited with a properly matched PSK material resulting in enhanced cell performance.

Nanoscale Graded Nitrogen-Doping of TiO2 via Pulsed Laser Deposition for Enhancing Charge Transfer in Perovskite Solar Cells.

An electron transport layer (ETL) for highly efficient perovskite solar cells (PSCs) should exhibit superior electrical transport properties and have its band levels aligned with interfacing layers to ensure efficient extraction of photo-generated carriers. Nitrogen-doped TiO2 (TiO2:N) is considered a promising ETL because it offers higher electrical conductivity compared to conventional ETLs made from spray-pyrolyzed TiO2. However, the application of highly doped TiO2:N in PSCs is often limited by the misalignment of energy band levels with adjacent layers and reduced optical transparency. In this study, a novel approach is introduced to enhance the charge transport characteristics and accurately align the electronic band alignment of TiO2:N layer through nanoscale doping level grading, achieved through the pulsed laser deposition (PLD) technique. The TiO2:N ETL with a graded doping profile can combine characteristics of both highly doped and lightly doped phases on each side. Furthermore, a nanoscale doping gradation, employing an ultrathin sub-layer structure with graded doping levels, creates a smoothly cascading band-level alignment that bridges the adjacent layers, enhancing the transport of photo-generated carriers. Consequently, this method leads to a substantial increase in the power conversion efficiency (PCE), exceeding 22%, which represents a relative improvement of 11% compared to traditional spray-pyrolyzed TiO2-based PSCs.

Tin Oxide Bilayer as Effective Electron Transport Layers for Efficient and Stable Perovskite Solar Modules

Tin oxide (SnO2) is one of the most promising electron transport layers (ETL) for the commercialization of perovskite solar cells (PSCs) due to its excellent electron mobility and high transparency, along with its low processing temperature. However, the inherent defects, nonuniform coating, and poor surface morphology of SnO2 may be detrimental to the physicochemical properties, such as conductivity and electron mobility at the interface, lead to a decrease in the open‐circuit voltage (VOC) and a reduction in device stability. In this study, a method that combines atomic layer deposition and chemical bath deposition techniques to solve these issues is presented. The presence of bilayer ETLs enhances the coverage of the SnO2 film and optimizes the morphology of the buried surface of perovskite, which not only facilitates the interfacial charge transfer but also suppresses recombination reactions. As a result, a significant increase in VOC and efficiency has been achieved compared to devices with only a single layer. Additionally, the large‐area perovskite solar module (active area: 48.0 cm2) achieves a champion efficiency of 17.9%. The PSCs retain more than 93% of their initial efficiency after 700 h of continuous operation under 1‐sun illumination and 25 °C.

Enhancing efficiency and stability of perovskite solar cells through methoxyamine hydrochloride modified SnO2 electron transport layer

Perovskite solar cells (PSCs) hold promise for future photovoltaic applications due to their high performance, low cost, and simple fabrication. The electron transport layer (ETL) plays a crucial role in PSC performance, and tin dioxide (SnO2) has gained attention as a promising ETL candidate. In this study, we investigate the use of methoxyamine hydrochloride (MOACl) as a modifier for SnO2 ETLs in PSCs. MOACl improves the film quality of SnO2, resulting in improved electrical conductivity and electron mobility. MOACl also enhances perovskite crystallization, promoting the growth of larger grain sizes and more uniform film morphology. The enhanced hydrogen bond and migration of Cl within the perovskite layer aids in optimal crystallization and significantly boosts the overall PCE of the cells from 21.67 % to 24.34 %. Moreover, MOACl-SnO2 PSCs demonstrate excellent long-term stability, retaining over 80 % of their initial efficiency after 50 days in ambient air. This work highlights the practical and effective approach of MOACl modification for controlling SnO2 defects and improving perovskite crystallization, paving the way for the development of efficient and stable PSCs.

Ammonium iodide-incorporated SnO2 obtains perovskite solar cells with over 24% efficiency

Tin dioxide (SnO2) as the most promising electron transport layer (ETL) has been widely used in high-efficiency perovskite solar cells (PSCs) due to its excellent optical/electronic properties, chemical stability, and low-temperature processing. However, the surface of SnO2 ETL contains defect sites, which result in energy losses in PSCs. In order to passivate the defects of SnO2 surface and together tune the electronic properties of SnO2 ETL for getting high-performance PSCs, we herein incorporate the low-cost material ammonium iodide (NH4I) into the SnO2. After the NH4I doping, the optimized photovoltaic power conversation efficiency is significantly enhanced (the highest efficiency can reach 24.4%), the hysteresis of device is largely suppressed to a negligible level, and the stability of device is also obviously improved. The origin of these enhancements is further disclosed by the positive effects of NH4I doping on both ETL and perovskite film: the surface morphology of ETL is effectively flatten, the energy level of ETL is suitably adjusted, the electron mobility of ETL and the perovskite grain size are clearly increased, the surface defects of ETL and the trap states in the perovskite film are greatly reduced, and the PbI2 residue in the perovskite layer is obviously diminished. The study here of incorporating cheap inorganic small molecule in the ETL provides an ingenious way to enhance the performance of the planar PSCs.

High‐Performance Perovskite Solar Cells with Zwitterion‐Capped‐ZnO Quantum Dots as Electron Transport Layer and NH4X (X = F, Cl, Br) Assisted Interfacial Engineering

The systematic advances in the power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs) have been driven by the developments of perovskite materials, electron transport layer (ETL) materials, and interfacial passivation between the relevant layers. While zinc oxide (ZnO) is a promising ETL in thin film photovoltaics, it is still highly desirable to develop novel synthetic methods that allow both fine‐tuning the versatility of ZnO nanomaterials and improving the ZnO/perovskite interface. Among various inorganic and organic additives, zwitterions have been effectively utilized to passivate the perovskite films. In this vein, we develop novel, well‐characterized betaine‐coated ZnO QDs and use them as an ETL in the planar n‐i‐p PSC architecture, combining the ZnO QDs‐based ETL with the ZnO/perovskite interface passivation by a series of ammonium halides (NH4X, where X = F, Cl, Br). The champion device with the NH4F passivation achieves one of the highest performances reported for ZnO‐based PSCs, exhibiting a maximum PCE of ~22% with a high fill factor of 80.3% and competitive stability, retaining ~78% of its initial PCE under 1 Sun illumination with maximum power tracking for 250 h.

Exploring the effect of interface properties between CeO x electron transport layer and MAPbI3 perovskite layer on solar cell performances through numerical simulation

Abstract Organo-metal halide perovskite solar cells (PSCs) have received a lot of attention to the photovoltaic research community, mainly due to the rapid development of their cell performances. But industry-level production of PSCs is hindered for several reasons. At present, the use of high-temperature processed electron transport layer (ETL) such as TIO2, the use of chemically unstable ETL such as ZnO and SnO2, etc. are ETL-related obstacles behind this industrialization. Aiming to remove these problems, cerium oxide (CeO x ), one of the most Earth-rich metal oxides has been chosen as ETL for this study. In this study, the SCAPS-1D simulation package has been used for an intensive study on ETL/PSK interface for a methylammonium lead iodide (MAPbI3)-based PSC having CeO x as ETL. From this simulation, the effect of conduction band offset (CBO) between CeO x and MAPbI3 has been found as the key player behind the cell performances. Defects at this interface have also been introduced and varied for studying their effects on cell performance at different CBO values. The temperature stability of a PSC is another important issue that has been considered in this study to find the effect of operating temperature on the PSC. This study would enlighten the researchers in implying some fantastic techniques at the ETL/PSK interface for improving the cell performance that will forward the research community a few steps to use CeO x as a promising ETL in PSC.

Synthesis and Optoelectronic Applications of dπ-pπ Conjugated Polymers with a Di-metallaaromatic Acceptor.

The development of conjugated polymers especially n-type polymer semiconductors is powered by the design and synthesis of electron-deficient building blocks. Herein, a strong acceptor building block with di-metallaaromatic structure was designed and synthesized by connecting two electron-deficient metallaaromatic units through a π-conjugated bridge. Then, a double-monomer polymerization methodology was developed for inserting it into conjugated polymer scaffolds to yield metallopolymers. The isolated well-defined model oligomers indicated polymer structures. Kinetic studies based on NMR and UV/Vis spectroscopies shed light on the polymerization process. Interestingly, the resulted metallopolymers with dπ-pπ conjugations are very promising electron transport layer materials which can boost photovoltaic performance of an organic solar cell, with power conversion efficiency up to 18.28% based on the PM6:EH-HD-4F non-fullerene system. This work not only provides a facile route to construct metallaaromatic conjugated polymers with various functional groups, but also discovers their potential applications for the first time.

Synthesis and Optoelectronic Applications of dπ–pπ Conjugated Polymers with a Di‐metallaaromatic Acceptor

AbstractThe development of conjugated polymers especially n‐type polymer semiconductors is powered by the design and synthesis of electron‐deficient building blocks. Herein, a strong acceptor building block with di‐metallaaromatic structure was designed and synthesized by connecting two electron‐deficient metallaaromatic units through a π‐conjugated bridge. Then, a double‐monomer polymerization methodology was developed for inserting it into conjugated polymer scaffolds to yield metallopolymers. The isolated well‐defined model oligomers indicated polymer structures. Kinetic studies based on nuclear magnetic resonance and ultraviolet–visible spectroscopies shed light on the polymerization process. Interestingly, the resulted metallopolymers with dπ–pπ conjugations are very promising electron transport layer materials which can boost photovoltaic performance of an organic solar cell, with power conversion efficiency up to 18.28 % based on the PM6 : EH‐HD‐4F non‐fullerene system. This work not only provides a facile route to construct metallaaromatic conjugated polymers with various functional groups, but also discovers their potential applications for the first time.

Boosting the Performance of Perovskite Solar Cells through Systematic Investigation of the Annealing Effect of E-Beam Evaporated TiO2.

Electron transport layer (ETL) plays an undeniable role in improving the performance of n-i-p planar perovskite solar cells (PSCs). Titanium dioxide (TiO2) is known as a promising ETL material for perovskite solar cell. In this work, the effect of annealing temperature on optical, electrical, and surface morphology of the electron-beam (EB)-evaporated TiO2 ETL, and consequently on the performance of perovskite solar cell, was investigated. It was found that annealing treatment at an optimized temperature of 480 °C considerably improved the surface smoothness, density of grain boundaries, and carrier mobility of TiO2 film, which resulted in nearly 10-fold improvement in power conversion efficiency (11.16%) in comparison with the unannealed device (1.08%). The improvement in performance of the optimized PSC is attributed to the acceleration of charge carrier extraction, as well as suppression of the recombination at the ETL/Perovskite interface.

First-Principles Band Alignments at the Si:Anatase TiO2 Interface.

TiO2 has been identified as a promising electron transport layer in Si solar cells. Experiments have revealed that the Si:TiO2 interface undergoes structural changes depending on how it was fabricated. However, less is understood about the sensitivity of electronic properties, such as band alignments, to these changes. Here, we present first-principles calculations of band alignments between Si and anatase TiO2, investigating different surface orientations and terminations. By calculating vacuum-level alignments, we observe a large band offset reduction of 2.5 eV for the O-terminated Si slab compared to other terminations. Furthermore, a 0.5 eV increase is found for the anatase (101) surface compared to (001). We compare the band offsets obtained through vacuum alignment with four different heterostructure models. Even though the heterostructure models contain an excess of oxygen, their offsets agree well with vacuum-level alignments using stoichiometric or H-terminated slabs, and the reduction in band offsets seen for the O-terminated Si slab is not observed. Additionally, we have investigated different exchange-correlation treatments including PBE + U, postprocessing GW corrections, and the meta-GGA rSCAN functional. We find that rSCAN provides more accurate band offsets than PBE, but further corrections are still required to achieve <0.5 eV accuracy. Overall, our study quantifies the importance of surface termination and orientation for this interface.

MDACl2-Modified SnO2 Film for Efficient Planar Perovskite Solar Cells.

The electron transport layer (ETL) with excellent charge extraction and transport ability is one of the key components of high-performance perovskite solar cells (PSCs). SnO2 has been considered as a more promising ETL for the future commercialization of PSCs due to its excellent photoelectric properties and easy processing. Herein, we propose a facile and effective ETL modification strategy based on the incorporation of methylenediammonium dichloride (MDACl2) into the SnO2 precursor colloidal solution. The effects of MDACl2 incorporation on charge transport, defect passivation, perovskite crystallization, and PSC performance are systematically investigated. First, the surface defects of the SnO2 film are effectively passivated, resulting in the increased conductivity of the SnO2 film, which is conducive to electron extraction and transport. Second, the MDACl2 modification contributes to the formation of high-quality perovskite films with improved crystallinity and reduced defect density. Furthermore, a more suitable energy level alignment is achieved at the ETL/perovskite interface, which facilitates the charge transport due to the lower energy barrier. Consequently, the MDACl2-modified PSCs exhibit a champion efficiency of 22.30% compared with 19.62% of the control device, and the device stability is also significantly improved.

Enhanced performance of planar perovskite solar cells by doping the SnO2 electron transport layer with guanidinium chloride

Tin (IV) oxide is a highly promising electron transport layer (ETL) for lead halide perovskite solar cells due to its high conductivity, transparency, wide band gap, and the possibility of low-temperature processing. Nonetheless, charge carrier recombination processes at the SnO2/perovskite interface diminish the device performance. Here, we demonstrate that SnO2 doping with guanidine hydrochloride (G-SnO2) leads to efficient surface passivation and a larger band offset between the ETL and the perovskite layer, resulting in reduced voltage losses and faster electron transfer. Moreover, G-SnO2 facilitates the growth of highly crystalline perovskite layers. Consequently, a power conversion efficiency of up to 23.48% and a high open-circuit voltage of 1.18 V are obtained in solar cells incorporating the G-SnO2 ETL. These devices also exhibited negligible hysteresis and maintained more than 96% of their initial power conversion efficiency after 1,250 h exposure to the air without encapsulation.

Fabrication of TiO2/MXene Nanohybrid Electron Transport Layer by Artist Spray Gun Assisted Pyrolysis Method for Efficient Carbon Perovskite Solar Cell

AbstractA new class of 2D Ti3C2Tx (MXene) in different weight percentages (0, 1, and 2 wt.%) are hybrid with TiO2 nanopowder to form TiO2/MXene nanohybrids by artist spray gun assisted (ASG) pyrolysis method to use an electron transport layer (ETL) for carbon perovskite solar cell (PSC). The influence of Ti3C2Tx on TiO2 in light absorption, charge separation and carrier dynamics is studied in detail. The formation of TiO2/MXene is studied by X‐ray diffraction analysis. The emission spectrum, charge separation and carrier dynamics of TiO2/MXene nanohybrid are analyzed by photoluminescence studies. They exhibit an emission spectrum at 390 nm. The photoluminescence and electrochemical impedance results reveal that the Ti3C2Tx nanosheets provide a superior charge transfer path with less electron transfer resistance at the ETL/perovskite interface, which lead to produce higher photocurrent. This study reveals that TiO2/MXene nanohybrid can be used as a promising ETL to enhance the photo‐conversion efficiency of carbon PSC. These results reveal that TiO2‐ MXene (1 wt.%) nanohybrid ETL shows the highest PCE of 5.15% than that of TiO2‐ MXene (2 wt.%) nanohybrid ETL and pristine TiO2 ETL‐based PSC (4.64%).

Size-matched dicarboxylic acid for buried interfacial engineering in high-performance perovskite solar cells

Tin dioxide (SnO2) is considered to be one of the most promising electron transport layers (ETLs) in perovskite solar cells (PSCs). Considering high concentrations of defects and energy level mismatch at the ETL/perovskite interface, buried interfacial engineering aids in boosting the efficiency and stability of PSCs. Herein, three dicarboxylic acids (DAs) including oxalic acid (OA), propanedioic acid (PA) and succinic acid (SA) are selected to modify the interface between perovskite layers and ETLs. It is demonstrated that DAs can enhance interfacial interaction between SnO2 and perovskite by esterification reaction and hydrogen bonding. In particular, OA with appropriate size can passivate two hydroxyl groups on SnO2 surfaces simultaneously, resulting in more favorable energy level alignment and better defect passivation ability amongst the three dicarboxylic buffer layers. Correspondingly, the power conversion efficiency of the OA-modified PSC increases substantially from 21.20% (reference device) to 22.91%, with a fill factor up to 83.1%. This work verifies that the buried interfacial engineering can effectively improve PSC performance, and also suggests the possibility of using size-matched symmetric molecules to modify ETLs for performance enhancement.

What defines the perovskite solar cell efficiency and stability: fullerene-based ETL structure or film morphology?

A study of fullerene derivatives as electron-transport materials for perovskite solar cells revealed that their crystal structures affect charge transport and device efficiency, while the operational stability is governed by the film uniformity.

Expression of concern: Tin-zinc-oxide nanocomposites (SZO) as promising electron transport layers for efficient and stable perovskite solar cells.

Expression of concern for 'Tin-zinc-oxide nanocomposites (SZO) as promising electron transport layers for efficient and stable perovskite solar cells' by Ahmed E. Shalan et al., Nanoscale Adv., 2019, 1, 2654-2662, DOI: https://doi.org/10.1039/C9NA00182D.

An n‐n Heterojunction Configuration for Efficient Electron Transport in Organic Photovoltaic Devices

AbstractSelective electron transport and extraction are essential to the operation of photovoltaic devices. Electron transport layer (ETL) is therefore critical to organic photovoltaics (OPV). Herein, an ETL configuration is presented comprising a solution‐processed n‐n organic heterojunction to enhance electron transport and hole blocking, and boost power conversion efficiency (PCE) in OPV. Specifically, the n‐n heterojunction is constructed by stacking a narrow‐band n‐type conjugated polymer layer (PNDIT‐F3N) and a wide‐band n‐type conjugated molecule layer (Phen‐NaDPO). Based on the ultraviolet photoelectron spectroscopy measurement and numerical simulation of current density‐voltage characteristics, the formation of the built‐in potential is investigated. In three OPVs with different active layers, substantial improvements are observed in performance following the introduction of this ETL configuration. The performance enhancement arises from the combination of selective carrier transport properties and reduced recombination. Another contributing factor is the good film‐forming quality of the new ETL configuration, where the surface energies of the related materials are well‐matched. The n‐n organic heterojunction represents a viable and promising ETL construction strategy for efficient OPV devices.

Self-boosting non-hydrolytic synthesis of Cl-passivated SnO2 nanocrystals for universal electron transport material of next-generation solar cells

Recently, tin oxide (SnO2) has attracted great attention as a promising electron transport layer (ETL) material for perovskite solar cells (PSCs) due to its high electron mobility, low-temperature fabrication and superior photocatalytic stability. However, there remains room for further utilization of the full potential of this material and improvement in power conversion efficiency (PCE) of devices, including establishment of controllable synthetic process and efficient surface modification. In this study, we first report on the novel tert-butyl alcohol (t-BuOH)-mediated non-hydrolytic synthesis of SnO2 nanocrystals (NCs) with effective pre-Cl-passivation without additional treatment. In this reaction, the by-products, tert-butyl chloride and water, are readily hydrolyzed to t-BuOH and chloride ions, which can be continuously reused and boost the reaction. Consequently, Cl effectively passivates the surface of SnO2 NCs as well as facilitates the dispersion stability, leading to reduced trap density and enhanced charge extraction characteristics of the SnO2 ETL. The device fabricated using the t-BuOH-mediated SnO2 NCs exhibits improved device efficiency, excellent reproducibility, reduced hysteresis, and longer device stability compared to the device using SnO2 NCs synthesized by conventional non-hydrolytic process. For the regular type (n-i-p) PSCs, the device achieves a PCE of 20.2% and maintains 97% of initial efficiency after 70 days under ambient conditions. In addition, unnecessary high temperature annealing and additional treatment of the non-hydrolytically synthesized SnO2 NCs enable their direct deposition on the perovskite layer, thereby leading to inverted type (p-i-n) PSCs with the best PCE of 17.0%.

Efficient and Stable Quantum‐Dot Light‐Emitting Diodes Enabled by Tin Oxide Multifunctional Electron Transport Layer

AbstractZinc oxide (ZnO) based nanoparticles (NPs) are the most commonly used electron transport layer (ETL) materials in quantum dot light‐emitting diodes (QLEDs). However, numerous defects, severe quenching, and poor stability of ZnO NPs limit the commercialization of QLEDs. Herein, tin oxide (SnO2) NPs are prepared as a substitute to ZnO. The obtained SnO2 NPs exhibit high conductivity, good transparency, and excellent stability, thus enabling them to be applied as multifunctional ETLs for various structured QLEDs, e.g., 1) as a phase tuning layer to tune the microcavity effect in inverted QLEDs, 2) as a thick protection layer to improve the stability of noninverted QLEDs, 3) as a sputtering buffer layer to protect the functional layers from the ion bombardment damage in transparent QLEDs, and 4) as an internal light extraction layer to enhance the light coupling efficiency of top‐emitting QLEDs. With the multifunctional SnO2 ETL, the resultant QLEDs are highly efficient (external quantum efficiency of 27.6%) and stable (half lifetime of 323 h at 4,459 cd m−2), which are comparable and even better than ZnMgO based devices. The results suggest that SnO2 is a promising ETL material for realizing efficient and stable QLEDs.