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Tin oxide (SnO 2 ) is extensively employed as the electron transport layer (ETL) for perovskite solar cells, but the presence of unsaturated Sn dangling bonds and oxygen vacancy defects at surface hinders effective carrier transport. Herein, we present an effective strategy for constructing a hybrid ETL by doping ZnF 2 into the SnO 2 , effectively addressing the oxygen vacancy defects at both the bulk and interface of SnO 2 , thus markedly minimizing nonradiative recombination losses. Additionally, the process-induced strong bonding between F and Sn atoms facilitates the establishment of electron transfer pathways, leading to an increased electron cloud density within SnO 2 and enhanced electron transfer capability, thus further suppressing charge accumulation at the interface. The hybrid ETL strategy can be adaptable to perovskites with various cations. The MAPbI 3 and Cs 0.1 FA 0.9 PbI 3 perovskite solar cells achieved remarkable PCEs of 21.31% and 24.91%, respectively. Moreover, the hybrid ETL design significantly enhances device stability. After 600 h of ambient storage, the unencapsulated optimized devices retained approximately 90% of their initial efficiency.

An appropriate electron transport layer (ETL) or cathode buffer layer (CBL) is critical for high-performance perovskite solar cells (PVSCs). In this work, tetrabutylammonium hydroxide (TBAOH)-functionalized Ti 3 C 2 T x MXene (TBAOH-Ti 3 C 2 T x ) is developed to improve the photovoltaic performance of PVSCs. TBAOH-Ti 3 C 2 T x is synthesized by HF etching and then TBAOH intercalation, and TBAOH can effectively attach to the Ti 3 C 2 T x surface during the intercalation process. In hole transport material (HTM)-free carbon-based PVSCs with the structure of ITO/ETL/MAPbI 3 /carbon, the SnO 2 doped by TBAOH-Ti 3 C 2 T x (SnO 2 :TBAOH-Ti 3 C 2 T x ) as ETL shows decreased WF and increased conductivity and improves the growth of the perovskite film with a larger grain and significantly reduced defects, which synergistically facilitate charge transport and extraction and reduce charge recombination. The HTM-free carbon-based PVSC with SnO 2 :TBAOH-Ti 3 C 2 T x ETL exhibits a significantly higher PCE of 14.93% with enhanced device stability compared to the control device with pristine SnO 2 ETL (11.95%) and also outperforms most of the HTM-free carbon-based PVSCs with MAPbI 3 perovskite reported so far. In traditional inverted PVSCs with the structure of ITO/PTAA/MAPbI 3 /PCBM/CBL/Ag, the TBAOH-Ti 3 C 2 T x is utilized as a CBL to significantly enhance device performance with a high PCE of 21.16%, which is obviously superior than that (16.26%) of the control device without CBL. The impressive results indicate that tetrabutylammonium hydroxide-functionalized Ti 3 C 2 T x MXene possesses great application potential in different functional layers for high-performance PVSCs.

The electron transport layer (ETL) plays a critical role in efficient and stable perovskite solar cells (PSCs). The current effective method for the large-scale preparation of metal oxide ETLs is mainly based on expensive sputtering processes. Here, a screen-assisted self-spreading method is proposed as a novel approach to prepare uniformly thin and conformal TiO 2 films on a rough fluorine-doped tin oxide (FTO) substrate as an ETL in planar PSCs. The TiO 2 ETL deposited by this method exhibited good coverage and homogeneity on the rough FTO substrate, thereby minimizing interfacial recombination. The photovoltaic performance of the PSCs fabricated by this method is superior to that of the cells fabricated by spin coating, especially in terms of the fill factor. The performance enhancement can be attributed to the complete coverage of the FTO substrate by the conformal TiO 2 film, confirming the effectiveness and reliability of the proposed method for the preparation of the TiO 2 ETL. The advantages of this method lie in its scalability to prepare oxide films with a large area, eliminating the requirement of complex equipment, such as spinners, sputters, or physical vapor deposition equipment.

This study delves into enhancing the efficiency and stability of perovskite solar cells (PSCs) by optimizing the surface morphologies and optoelectronic properties of the electron transport layer (ETL) using tungsten (W) doping in zinc oxide (ZnO). Through a unique green synthesis process and spin-coating technique, W-doped ZnO films were prepared, exhibiting improved electrical conductivity and reduced interface defects between the ETL and perovskite layers, thus facilitating efficient electron transfer at the interface. High-quality PSCs with superior ETL demonstrated a substantial 30% increase in power conversion efficiency (PCE) compared to those employing pristine ZnO ETL. These solar cells retained over 70% of their initial PCE after 4000 h of moisture exposure, surpassing reference PSCs by 50% PCE over this period. Advanced numerical multiphysics solvers, employing finite-difference time-domain (FDTD) and finite element method (FEM) techniques, were utilized to elucidate the underlying optoelectrical characteristics of the PSCs, with simulated results corroborating experimental findings. The study concludes with a thorough discussion on charge transport and recombination mechanisms, providing insights into the enhanced performance and stability achieved through W-doped ZnO ETL optimization.

The spontaneous orientation polarization (SOP) of a permanent dipole moment of the molecule induces a giant surface potential (GSP) in an organic semiconductor film, and GSP is expected to be a crucial parameter for understanding the operational mechanism of organic light-emitting diodes (OLEDs). This study demonstrates that the voltage-dependent migration of a carrier recombination zone induced by a polar electron transporting layer (ETL) having a positive SOP causes a decline in the overall performance of the OLED in triplet-triplet upconversion (TTU) based on OLEDs. Specifically, the TTU efficiency in an OLED with 2,2',2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) as the ETL decreased by 20% due to the reduction of electrically generated triplet exciton density. This decrease resulted in a lower external electroluminescence (EL) quantum efficiency (EQE) of 5.4% at 1 mA cm -2 , while the OLED with a nonpolar ETL resulted in an EQE of around 8.1% at 1 mA cm -2 . We confirmed a shift in the recombination zone from the current density dependence of the EL spectra in the OLEDs. Our results indicate that the fixed carrier recombination zone near a hole transport layer and an emitting layer (HTL/EML) strongly enhanced the TTU process, while the polar EML/ETL interface induced the migration of the recombination zone depending on voltage, resulting in the decrease of triplet exciton density.

Near-infrared (NIR) organic photodetectors (OPDs) are pivotal in numerous technological applications due to their excellent responsivity within the NIR region. Polyethylenimine ethoxylated (PEIE) has conventionally been employed as an electron transport layer (hole-blocking layer) to suppress dark current ( J D ) and enhance charge transport. However, the limitations of PEIE in chemical stability, processing conditions, environmental impact, and absorption range have spurred the development of alternative materials. In this study, we introduced a novel solution: a hybrid of sol-gel zinc oxide (ZnO) and N , N '-bis( N -dimethylpropan-1-amine oxide)perylene-3,4,9,10-tetracarboxylic diimide (PDINO) as the electron transport layer for NIR-OPDs. Our fabricated OPD exhibited significantly improved responsivity, reduced internal traps, and enhanced charge transfer efficiency. The detectivity, spanning from 400 to 1100 nm, surpassed ∼5 × 10 N -dimethylpropan-1-amine oxide)perylene-3,4,9,10-tetracarboxylic diimide (PDINO) as the electron transport layer for NIR-OPDs. Our fabricated OPD exhibited significantly improved responsivity, reduced internal traps, and enhanced charge transfer efficiency. The detectivity, spanning from 400 to 1100 nm, surpassed ∼5 × 10 12 Jones, reaching ∼1.1 × 10 12 Jones at 1000 nm, accompanied by an increased responsivity of 0.47 A/W. Also, the unpackaged OPD remarkedly demonstrated stable J D and external quantum efficiency (EQE) over 1000 h under dark storage conditions. This innovative approach not only addresses the drawbacks of conventional PEIE-based OPDs but also offers promising avenues for the development of high-performance OPDs in the future.

Infrared (IR) solar cells, capable of converting low-energy IR photons to electron-hole pairs, are promising optoelectronic devices by broadening the utilization range of the solar spectrum to the short-wavelength IR region. The emerging PbS colloidal quantum dot (QD) IR solar cells attract much attention due to their tunable band gaps in the IR region, potential multiple exciton generation, and facile solution processing. In PbS QD solar cells, ZnO is commonly utilized as an electron transport layer (ETL) to establish a depleted heterostructure with a QD photoactive layer. However, band gap shrinkage of large PbS QDs makes it necessary to tailor the behaviors of the ZnO ETL for efficient carrier extraction in the devices. Herein, the characteristics of ZnO ETL are efficiently and flexibly tailored to match the QD layer by handily adjusting the postannealing process of ZnO ETL. With a suitable temperature, the well-matched energy level alignment and suppressed trap states are simultaneously achieved in the ZnO ETL, effectively reducing the nonradiative recombination and accelerating the electron injection from the QD layer to ETL. As a consequence, a high-performance PbS QD photovoltaic device with power conversion efficiencies (PCEs) of 10.09% and 1.37% is obtained under AM 1.5 and 1100 nm filtered solar illumination, demonstrating a simple and effective approach for achieving high-performance IR photoelectric devices.

In halide perovskite solar cells (PSCs), moderate lead iodide (PbI 2 ) can enhance device efficiency by providing some passivation effects, but extremely active PbI 2 leads to the current density-voltage hysteresis effect and device instability. In addition, defects distributed on the buried interface of tin oxide (SnO 2 )/perovskite will lead to the photogenerated carrier recombination. Here, rubidium chloride (RbCl) is introduced at the buried SnO 2 /perovskite interface, which not only acts as an interfacial passivator to interact with the uncoordinated tin ions (Sn 4+ ) and fill the oxygen vacancy on the SnO 2 surface but also converts PbI 2 into an inactive (PbI 2 ) 2 RbCl compound to stabilize the perovskite phase via a bottom-up evolution effect. These synergistic effects deliver a champion PCE of 22.13% with suppressed hysteresis for the W RbCl PSCs, in combination with enhanced environmental and thermal stability. This work demonstrates that the interfacial defect passivation and bottom-up excess PbI 2 management using RbCl modifiers are promising strategies to address the outstanding challenges associated with PSCs.

Recent studies have focused on exploring the potential of resistive random-access memory (ReRAM) utilizing halide perovskites as novel data storage devices. This interest stems from its notable attributes, including a high ON/OFF ratio, low operating voltages, and exceptional mechanical properties. Nevertheless, there have been reports indicating that memory systems utilizing halide perovskites encounter certain obstacles pertaining to their stability and dependability, mostly assessed through endurance and retention time. Moreover, the presence of these problems can potentially restrict their practical applicability. This study explores a resistive switching memory device utilizing MAPbBr 3 perovskite, which demonstrates bipolar switching characteristics. The device fabrication procedure involves a low-temperature, all-solution process. For the purpose of enhancing the device's reliability, the utilization of TPBI(2,2',2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) as an electron transfer material on the MAPbBr 3 switching layer was implemented for the first time. The formation and rupture of Ag filaments in the MAPbBr 3 perovskite switching layer are attributed to reduction-oxidation reactions. The TPBI is involved in the regulation of filaments during the SET and RESET processes. Hence, it can be shown that the MAPbBr 3 device incorporating TPBI exhibited about 1000 endurance cycles when subjected to continuous voltage pulses. Moreover, the device consistently maintained ON/OFF ratios above 10 7 . In contrast, the original MAPbBr 3 device without TPBI demonstrated a significantly lower endurance with only 90 cycles observed. In addition, the MAPbBr 3 device integrated with TPBI exhibited a retention time exceeding 3 × 10 3 s. The findings of this research provide compelling evidence to support the notion that electron transfer materials have promise for the development of halide perovskite memory systems owing to their favorable attributes of dependability and stability.

Semi-transparent perovskite solar cells (ST-PSCs) have attracted enormous attention recently due to their potential in building-integrated photovoltaic. To obtain adequate average visible transmittance (AVT), a thin perovskite is commonly employed in ST-PSCs. While the thinner perovskite layer has higher transparency, its light absorption efficiency is reduced, and the device shows lower power conversion efficiency (PCE). In this work, a combination of high-quality transparent conducting layers and surface engineering using 2D-MXene results in a superior PCE. In situ high-temperature X-ray diffraction provides direct evidence that the MXene interlayer retards the perovskite crystallization process and leads to larger perovskite grains with fewer grain boundaries, which are favorable for carrier transport. The interfacial carrier recombination is decreased due to fewer defects in the perovskite. Consequently, the current density of the devices with MXene increased significantly. Also, optimized indium tin oxide provides appreciable transparency and conductivity as the top electrode. The semi-transparent device with a PCE of 14.78% and AVT of over 26.7% (400-800 nm) was successfully obtained, outperforming most reported ST-PSCs. The unencapsulated device maintained 85.58% of its original efficiency after over 1000 h under ambient conditions. This work provides a new strategy to prepare high-efficiency ST-PSCs with remarkable AVT and extended stability.

Electron transport layers (ETL) based on tin(IV) oxide (SnO 2 ) are recurrently employed in perovskite solar cells (PSCs) by many deposition techniques. Pulsed laser deposition (PLD) offers a few advantages for the fabrication of such layers, such as being compatible with large scale, patternable, and allowing deposition at fast rates. However, a precise understanding of how the deposition parameters can affect the SnO 2 film, and as a consequence the solar cell performance, is needed. Herein, we use a PLD tool equipped with a droplet trap to minimize the number of excess particles (originated from debris) reaching the substrate, and we show how to control the PLD chamber pressure to obtain surfaces with very low roughness and how the concentration of oxygen in the background gas can affect the number of oxygen vacancies in the film. Using optimized deposition conditions, we obtained solar cells in the n-i-p configuration employing methylammonium lead iodide perovskite as the absorber layer with power conversion efficiencies exceeding 18% and identical performance to devices having the more typical atomic layer deposited SnO 2 ETL.

An exciplex, which is composed of electron donor and acceptor molecules and formed by intermolecular charge transfer, is an excited-state species that is able to emit light or transfer its energy to a lower-energy emitter. In reported exciplex-based organic light-emitting diodes (OLEDs), their working mechanism is to generate exciplexes either in the bulk emitting layer (bulk exciplex) or at its interface with an electron transport layer (interface exciplex); both types give promising device performance. Here, we propose a novel strategy of creating both types of exciplexes simultaneously (dual exciplexes) for the generation of more exciplexes for better device performance as indicated in the improved photoluminescence quantum yield (PLQY). Impressively, the dual exciplex-based device with blue thermally activated delayed fluorescence (TADF) emitter 9,9-dimethyl-9,10-dihydroacridine-2,4,6-triphenyl-1,3,5-triazine (DMAC-TRZ) exhibits a record-high maximum external quantum efficiency (EQE max ) of 26.7% among the solution-processed TADF blue OLEDs. By further doping with the red-emitting phosphor emitter into the EML, the white device also gives a record-high EQE max of 24.1% among the solution-processed TADF-phosphor hybrid white OLEDs (T-P WOLEDs) with the Commission Internationale de L'Eclairage (CIE) coordinates of (0.34, 0.42), color rendering index of 70, and correlated color temperature of 5198 K. Furthermore, both blue and white devices show an ultralow efficiency roll-off with external quantum efficiencies at a practical brightness value of 1000 cd m -2 (EQE 1000 ) of 25.1 and 23.9%, respectively. This is the first report of employing a dual exciplex-based OLED with excellent device performance.

Perovskite solar cells (PSCs) emerged as potential photovoltaic energy-generating devices developing in recent years because of their excellent photovoltaic properties and ease of processing. However, PSCs are still reporting efficiencies much lower than their theoretical limits owing to various losses caused by the charge transport layer and the perovskite. In this regard, herein, an interface engineering strategy using functional molecules and chemical bridges was applied to reduce the loss of the heterojunction electron transport layer. As a functional interface layer, ethylenediaminetetraacetic acid (EDTA) was introduced between PCBM and the ZnO layer, and as a result, EDTA simultaneously formed chemical bonds with PCBM and ZnO to serve as a chemical bridge connecting the two. DFT and chemical analyses revealed that EDTA can act as a chemical bridge between PCBM and ZnO, passivate defect sites, and improve charge transfer. Optoelectrical analysis proved that EDTA chemical bridge-mediated charge transfer (CBM-CT) provides more efficient interfacial charge transport by reducing trap-assisted recombination losses at ETL interfaces, thereby improving device performance. The PSC with EDTA chemical bridge-mediated heterojunction ETL exhibited a high PCE of 21.21%, almost no hysteresis, and excellent stability to both air and light.

A large number of defect states that exist at the interface between a perovskite film and an electron transport layer (ETL) are detrimental to the efficiency and the stability of perovskite solar cells (PSCs). It is still a challenge to simultaneously passivate the defects on both sides by a stable and low-cost ion compound. Herein, we demonstrate a simple and effective versatile strategy by introducing hydrochloric acid into SnO 2 precursor solution to passivate the defects in both SnO 2 and perovskite layers and simultaneously reduce the interface energy barrier, ultimately achieving a high-performance and hysteresis-free PSCs. Hydrogen ions can neutralize -OH groups on the SnO 2 surface, whereas the Cl - can not only combine with Sn 4+ in ETL but also suppress the Pb-I antisite defects formed at the buried interface. The reduced nonradiative recombination and the favorable energy level alignment result in a significantly increased efficiency from 20.71 to 22.06% of PSCs due to the enhancement of open-circuit voltage. In addition, the stability of the device can also be improved. This work presents a facile and promising approach for the development of highly efficient PSCs.

Electron transport layers (ETLs) are imperative in n-i-p structured perovskite solar cells (PSCs) because of their capability to affect light propagation, electron extraction, and perovskite crystallization, and any mismatch of optical constants, band position, and surface potential between the ETLs and the perovskites can cause unintentional optical and electrical losses. Herein, an antireflective and energetic cascade bilayer ETL with ubiquitously used SnO 2 and TiO 2 was constructed at 150 °C for PSCs, and the in-depth mechanism for performance improvement was systematically unraveled. It was revealed that the construction of an ETL with gradually increasing refractive indices can circumvent light reflection loss, resulting in enhanced photocurrent. The combined ETL forms an energetic cascade to promote electronic conductivity and facilitate electron extraction with reduced energy loss. Moreover, topologic perovskite growth with improved crystallinity and vertical orientation was preferred owing to the relative dewetting behavior, leading to reduced defect states and enhanced carrier mobility in the perovskite layer.

Developing new electron transport layers has been an effective way to fabricate high-performance bulk-heterojunction organic solar cells (OSCs). Resolving the longstanding problems associated with commonly used zinc oxide (ZnO), such as electron traps and light-induced device deterioration, however, is still a great challenge. In this study, glycerol diglycidyl ether (GDE) and 1,4-butanesultone (BS) are blended with polyethyleneimine (PEI) to produce cross-linkable PEI-based materials, PEI-GDE and PEI-GDE-BS, which can function as alternative electron transport layers to replace conventional ZnO cathode-modifying layers in inverted OSCs. PEI-GDE and PEI-GDE-BS are amendable to low-temperature annealing processes to produce cross-linked films. The inverted device structure of ITO/ETL/PM6:BTP-BO-4F:PC 71 BM/MoO 3 /Ag was used to evaluate the effects of incorporating PEI-GDE and PEI-GDE-BS as electron transport materials. Compared with ZnO-based devices, the PEI-GDE- and PEI-GDE-BS-based devices exhibit significant improvements in photovoltaic performances due to smoother surface roughness, higher charge collection and exciton dissociation efficiencies, higher electron mobilities, and stronger π-π interactions. In particular, a PEI-GDE-BS-based device shows an outstanding power conversion efficiency (PCE) of 17.55% with a V OC of 0.83 V, a J SC of 27.88 mA/cm 2 , and an FF of 75.96%, which offers great possibilities in the applications of flexible solar cells.

We report high-efficiency and long-lifetime inverted green cadmium-free (InP-based) quantum dot light-emitting diodes (QLEDs) using a stable ZnO/ZnS cascaded electron transport layer (ETL). We have successfully developed a strategy to spin-coat stable ZnS ETLs with a relatively higher conduction band minimum (CBM) and lower electron mobility than that of ZnO, which leads to balanced carrier injection and an improved device lifetime. Analysis shows that by using the ZnO/ZnS cascaded ETL, electron injection is reduced, resulting in an improved charge balance in the QD layer and suppressed exciton quenching, which preserves the emission properties of QDs. Optimized devices with ZnO/ZnS cascaded ETLs show a maximum external quantum efficiency of 10.8% and a maximum current efficiency of 37.5 cd/A; these efficiency values are an almost 2.2-fold improvement compared to those of reference devices without ZnS. The QLED devices also showed a remarkably long lifetime (LT 70 ) of 265 h at an initial luminance of 1000 cd/m 2 . The predicted half-lifetime (LT 50 ) at 100 cd/m 2 is 60,255 h, which, to our knowledge, is currently the longest lifetime yet reported for InP-based green QLEDs.

Active layer materials with silicone side chains have been broadly reported to have excellent long-term stability in harsh environments. However, the application of conjugated materials with silicone side chains in electron transport layers (ETLs) has rarely been reported. In this research, we synthesized for the first time a siloxane-modified perylene-diimide derivative (PDI-OSi) consisting of a side-chain substituent of siloxane and a conjugated group of perylene-diimide (PDI). The inserted siloxane functional groups not only can strengthen the light transmittance of PDI-OSi but also can remarkably expand its solubility and improve the film-forming ability and air stability of the material. Second, introducing siloxane-containing side chains can dramatically lower the work function and interfacial barrier of the electrode, thereby achieving a favorable ohmic contact. In addition, the moderate surface energy of siloxane functional groups makes PDI-OSi hydrophobic, which is conducive to forming excellent miscibility with hydrophobic active layers to promote charge transfer. When PDI-OSi is used as an ETL in organic solar cells (OSCs), operative exciton dissociation and more favorable surface morphology enable OSCs to realize a power conversion efficiency (PCE) of 13.99%. These results indicate that side-chain engineering with siloxane pendants is a facile strategy for constructing efficient OSCs.

An effective combination of smart materials plays an important role in charge transfer and separation for high photoelectric conversion efficiency (PCE) and stable solar cells. Black phosphorus quantum dots (BPQDs) have been revealed as a direct band gap semiconductor with ultrahigh conductivity, which have been explored in the present work as an additive component to a precursor solution of SnO 2 nanoparticles that can effectively improve the performance of SnO 2 electron transport layer (ETL)-based perovskite solar cells. Such a device can yield a high PCE of 21% with the SnO 2 /BPQDs mixed ETL, which is higher than those of perovskite solar cells based on SnO 2 single layer (18.2%), BPQDs/SnO 2 bilayer (19.5%), and SnO 2 /BPQDs bilayer (20.5%) samples. The mixed samples still possess good stability of more than 90% efficiency after 1000 h under AM 1.5G lamp irradiation and negligible hysteresis. It is found that the strong interaction of BPQDs with SnO 2 can not only modify the defects inherent to the SnO 2 layer but also inhibit the oxidation of BPQDs. This work provides a promising functional material for SnO 2 ETL-based perovskite solar cells and proves that the BPQD-based modification strategy is useful for designing other solar cells with high performance.

The growing demand for organic electronic devices warrants further development of the scalability and green solvent processibility of π-conjugated materials. Perylene diimide (PDI)-based materials have shown impressive performance as interlayers for electronic devices due to a low E LUMO energy and high charge mobility in films. The next step in the development of these materials is the transition toward scalable production and the fabrication of devices under ambient conditions. Here, we develop a green synthetic methodology to prepare a series of PDI-based electronically active materials ( X2-5 ), which can be slot-die-coated into uniform thin films from green solvents in air. Compounds X2-5 comprised a monomeric PDI core with a functional cyclic secondary amine appended to the bay region. Bromine or cyano moieties are incorporated into the molecular scaffold to systematically tune optoelectronic properties. The utility of these materials is demonstrated by slot-die coating them from ethanol to serve as cathode interlayers in prototype air-processed conventional organic photovoltaics. Using a PM6:Y6 active layer, device power conversion efficiencies reached 10%, among the best reported under these conditions.

Commercialization of organic solar cells (OSC) is imminent. Interlayers between the photoactive film and the electrodes are critical for high device efficiency and stability. Here, the applicability of SnO 2 nanoparticles (SnO 2 NPs) as the electron transport layer (ETL) in conventional OSCs is evaluated. A commercial SnO 2 NPs solution in butanol is mixed with ethanol (EtOH) as a processing co-solvent to improve film formation for spin and slot-die coating deposition procedures. When processed with 200% v/v EtOH, the SnO 2 NPs film presents uniform film quality and low photoactive layer degradation. The optimized SnO 2 NPs ink is coated, in air, on top of two polymer:fullerene-based systems and a nonfullerene system, to form an efficient ETL film. In every case, addition of SnO 2 NPs film significantly enhances photovoltaic performance, from 3.4 and 3.7% without the ETL to 6.0 and 5.7% when coated on top of PBDB-T:PC 61 BM and PPDT2FBT:PC 61 BM, respectively, and from 3.7 to 7.1% when applied on top of the PTQ10:IDIC system. Flexible, all slot-die-coated devices, in air, are also fabricated and tested, demonstrating the versatility of the SnO 2 NPs ink for efficient ETL formation on top of organic photoactive layers, processed under ambient condition, ideal for practical large-scale production of OSCs.

Interfacial quality of functional layers plays an important role in the carrier transport of sandwich-structured devices. Although the suppression of interface states is crucial to the overall device performance, our understanding on their formation and annihilation mechanism via direct characterization is still quite limited. Here, we present a thorough study on the interface states present in the electron transport layer (ETL) of blue quantum dot (QD) light-emitting diodes (QLEDs). A ZnO/ZnMgO bilayer ETL is adopted to enhance the electron injection into blue QDs. By probing the ETL band structure with photoelectron spectroscopy, we discover that substantial band bending exists at the ZnO/ZnMgO interface, elucidating the presence of a high density of interface states which hinder electron transport. By inserting a ZnO@ZMO interlayer composed of mixed ZnO and ZnMgO nanoparticles, the band bending and thus the interface states are observed to reduce significantly. We attribute this to the hybrid surface properties of ZnO@ZMO, which can annihilate the surface states of both the ZnO and ZnMgO layers. The introduction of a bridging layer has led to ∼40% enhancement in the power efficiency of blue QLEDs and noticeable performance boosts in green and red QLEDs. The findings here demonstrate a direct observation of interface states via detailed band structure studies and outline a potential pathway for eliminating these states for better performances in sandwich-structured devices.

SnO 2 is a promising material for use as an electron transfer layer (ETL) in perovskite photovoltaic devices due to its suitable energy level alignment with the perovskite, high electron mobility, excellent optical transmission, and low-temperature processability. The development of high-quality SnO 2 ETLs with a large coverage and that are pinhole-free is crucial to enhancing the performance and stability of the perovskite solar cells (PSCs). In this work, zirconium acetylacetonate (ZrAcac) was introduced to form a double-layered ETL, in which an ideal cascade energy level alignment is obtained. The surface of the resulting ZrAcac/SnO 2 (Zr-SnO 2 ) layer is compact and smooth and had a high coverage of SnO 2 , which enhances the electron extractability, improves ion blocking, and reduces the charge accumulation at the interface. As a result, the fill factor (FF, 80.99%), power conversion efficiency (PCE, 22.44%), and stability of the Zr-SnO 2 device have been significantly improved compared to PSCs with only a SnO 2 ETL. In addition, the PCE of the Zr-SnO 2 device is maintained at more than 80% of the initial efficiency after 500 h of continuous illumination.

This work demonstrates a simple yet effective method to significantly improve the power conversion efficiency (PCE) of highly efficient non-fullerene organic solar cells by mixing two electron transport materials. The new electron transport layer shows an energy level better aligned with the active layer and an improved morphology that could reduce the active layer-electrode contact. These improvements lead to enhanced charge extraction, better charge selectivity, suppressed exciton recombination, and finally a boosted PCE in the PM6:Y6-based solar cells. When applied in conjunction with the non-halogenated solvent-processed PM6:PY-IT-based active layer, the mixed ETL also gives rise to a leading result for binary all-polymer solar cells (PCE of >16%) with a concurrent increase in V OC , J SC , and FF .

The electron transport layer (ETL) exerts a dramatic influence on the power conversion efficiency (PCE) of the nonfullerene organic solar cells (NOSCs). Currently, the majority of the organic ETLs possess a relatively poor conductivity, which is not conducive to carrier transport and collection. Herein, we design and develop a novel hyperbranched conjugated polyelectrolyte (CPE) based on n-type perylene diimide (PDI) as the center core and quaternary ammonium salt as the side polar groups. The lone pair electrons of the nitrogen atoms can transfer to the electron deficient PDI core and endow the molecule with an efficient n-type self-doping effect. Moreover, the hyperbranched structure makes the molecule functionalized with more side polar groups, favoring forming more dipoles and stronger dipole moments. Therefore, the CPE PTPAPDINO possesses a high conductivity and can notably decrease the work function (WF) of the electrode, contributing to the carrier transport and collection of the device. The NOSC with PTPAPDINO as ETL delivers an excellent PCE of 15.62%, which is even superior to the device using the classical PDINO ETL. Moreover, the PCE can retain 82.6% of the optimal device when the thickness has been increased to 28 nm. These results manifest that it is a feasible strategy to design an n-type self-doping hyperbranched CPE as efficient ETL, and PTPAPDINO is a promising alternative ETL for high performance NOSCs.

The practical applications of perovskite solar cells (PSCs) are limited by further improvement of their stability and performance. Additive engineering and interface engineering are promising medicine to cure this stubborn disease. Herein, an alkali metal fluoride as an additive is introduced into the tin oxide (SnO 2 ) electron transport layer (ETL). The formation of coordination bonds of F - ions with the oxygen vacancy of Sn 4+ ions decreases the trap-state density and improves the electron mobility; the hydrogen bond interaction between the F ion and amine group (FA + ) of perovskite inhibits the diffusion of organic cations and promotes perovskite (PVK) stability. Meanwhile, the alkali metal ions (K + , Rb + , and Cs + ) permeated into PVK fill the organic cation vacancies and ameliorate the crystal quality of PVK films. Consequently, a SnO 2 -based planar PSC exhibits a power conversion efficiency (PCE) of 20.24%, while the PSC modified by CsF achieves a PCE of 22.51%, accompanied by effective enhancement of stability and negligible hysteresis. The research results provide a typical example for low-cost and multifunctional additives in high-performance PSCs.

Planar-type perovskite solar cells (p-PSCs) based on SnO 2 have garnered further attention due to their simple and low-temperature fabrication. Improving the critical properties of the electron transport layer (ETL) is an effective way to enhance the performance of p-PSC devices. Here, a brand-new method is developed to relieve the contact recombination caused by the rough fluorine-doped tin oxide (FTO) surface and further boosts the electrical concentration of the ETL. A SnO 2 -ethylene diamine tetraacetic acid (EDTA) acylamide compound (SEAC) with hydrogen bond-induced adjustable cluster size is reported for the first time. The rational choice of the SEAC cluster size is the key for obtaining the smooth interfacial morphology of the ETL on the rugged FTO substrate. In addition, the energy band gap decreases with the increasing cluster size, and consequently, results in improved electrical conductivity of the SEAC. The upshifted Fermi energy level leads to higher electron concentration, which is an important physical quantity of the ETL. The PSC devices based on the optimized SEAC achieve an improved power conversion efficiency of 21.29% with negligible J - V hysteresis due to significantly enhanced electron transport and reduced contact charge recombination at the ETL/perovskite interface. In general, this paper comes up with a unique strategy to improve the quality of the SnO 2 -based ETL.

Recently, the two-dimensional material Ti 3 C 2 T x MXene has attracted interest from researchers in perovskite solar cells (PSCs) with its great advantages in terms of high transmittance, high conductivity, tunable work function, and solution processability. However, the MXene-based PSC performance has still been inferior to that of the traditional TiO 2 - or SnO 2 -based counterpart up until now. Some critical issues regarding to the MXene/perovskite interface still have not been well addressed. Herein, we used the Ti 3 C 2 T x MXene as electron transport layer in PSCs via a room-temperature solution process followed by oxygen plasma treatment. Various characterization techniques were taken to establish the correlation between the surface properties and termination groups of MXene. We showed that oxygen plasma treatment could break parts of Ti-C bonds and generate abundant Ti-O bonds randomly distributed on MXene. The surface modification resulted in tunable work functions of MXene, as well as reduced trap states and improved electron transport close to the interface. In addition, the surface tension of MXene and corresponding perovskite morphology were thoroughly investigated by the contact angle and topography measurements. High-resolution XPS spectra indicated the Pb-O interactions between perovskite and MXene, which contributed to the device stability improvement.

Although inverted (p-i-n) structure perovskite solar cells (PSCs) have achieved high efficiency by commonly using fullerenes or their derivatives as electron transport layers (ETLs), the device stability and cost of fullerene materials are still of great concern. Herein, we demonstrate inorganic top ETLs simply composed from a family of metal oxides including In 2 O 3 and its derivative of Sn:In 2 O 3 with a gradient potential structure. For inverted PSCs, the typical film formation process of In 2 O 3 will damage or degrade perovskite materials underneath; thus, we report a low temperature synthesis approach for obtaining In 2 O 3 and Sn:In 2 O 3 nanoparticles that can form effective top ETLs without any post-treatment. The one-family oxide-based top ETL features with the enhanced built-in potential, high electron extraction, and low interfacial recombination, offering a power conversion efficiency (PCE) of 20.65% in PSCs constructed from oxide-only carrier (both hole and electron) transport layers (CTLs), which is the highest efficiency in oxide-only CTL-based inverted PSCs to the best of our knowledge. Equally important, the inverted PSCs based on the Sn:In 2 O 3 /In 2 O 3 ETL show the excellent operational stability and remain 90% of the initial value of PCE over 2000 h. Consequently, this work contributes to the robust strategy of all oxide-only CTLs in developing rigid and flexible PSCs for practical photovoltaic applications.

Organic-inorganic lead halide perovskite solar cells (PSCs) are highly efficient, flexible, lightweight, and even tolerant to radiation, such as protons, electron beams (EB), and γ-rays, all of which makes them plausible candidates for use in space satellites and spacecrafts. However, the mechanisms of radiation damage of each component of PSC [an organic hole transport material (HTM), a perovskite layer, and an electron transport material (ETM)] are not yet fully understood. Herein, we investigated the EB irradiation effect (100 keV, up to 2.5 × 10 15 cm -2 ) on binary-mixed A site cations and halide perovskite (MA 0.13 FA 0.87 PbI 2.61 Br 0.39 , MA:methylammonium cation and FA:formaminidium cation), a molecular HTM of doped SpiroOMeTAD, and an inorganic ETM of mesoporous TiO 2 . Despite the decreased power conversion efficiency of PSCs upon EB exposure, the photoconductivities of the perovskite, HTM, and ETM layers remained intact. In contrast, significant dedoping of HTM was observed, as confirmed by steady-state conductivity, photoabsorption, and X-ray photoelectron spectroscopy measurements. Notably, this damage could be healed by exposure to short-wavelength light, leading to a partial recovery of the PSC efficiency. Our work exemplifies the robustness of perovskite against EB and the degradation mechanism of the overall PSC performance.

With great research potential, the perovskite solar cells (PSCs) have been well developed in recent years, but there are still some urgent issues like efficiency and hysteresis defects that severely limit their commercialization. Interface modification is a significant measure to reduce defects and promote performance. In the article, an easy and effective strategy of modifying the electron transport layer (ETL) with NaBr is proposed to improve efficiency and reduce hysteresis. The charge carrier dynamics can be greatly optimized by diffusing NaBr on the ETL. The efficiency of the NaBr coated device can achieve 21.16%, which is extremely higher than the control one and shows low hysteresis behavior with a hysteresis index reduced from 0.135 to 0.025. The results indicate that the NaBr modification provides a novel strategy for preparing PSCs with high efficiency and low hysteresis.

The composite electron transporting layer (ETL) of metal oxide with [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) prevents perovskite from metal electrode erosion and increases p-i-n perovskite solar cell (PVSC) stability. Although the oxide exhibits protective function, an additional work function modifier is still needed for good device performance. Usually, complicated multistep synthesis is employed to have a highly crystalline film that increases manufacturing cost and inhibits scalability. We report a facile synthesis of a novel organic-molecule-capped metal oxide nanoparticle film for the composite ETL. The nanoparticle film not only has a dual function of electron transport and protection but also exhibits work function tunability. Solvothermal-prepared SnO 2 nanoparticles are capped with tetrabutylammonium hydroxide (TBAOH) through ligand exchange. The resulting TBAOH-SnO 2 nanoparticles disperse well in ethanol and form a uniform film on PCBM. The power conversion efficiency of the device dramatically increases from 14.91 to 18.77% using this layer because of reduced charge accumulation and aligned band structure. The PVSC thermal stability is significantly enhanced by adopting this layer, which prevents migration of I - and Ag. The ligand exchange method extends to other metal oxides, such as TiO 2 , ITO, and CeO 2 , demonstrating its broad applicability. These results provide a cornerstone for large-scale manufacture of high-performance and stable PVSCs.

The competition between charge recombination and extraction principally affects the fill factor (FF) and power conversion efficiency (PCE) of planar thin-film solar cells. In Sb 2 S 3 thin-film solar cells, the electrocharge recombination and extraction n transport layer (ETL) plays a significant role in electron extraction and determination of Sb 2 S 3 film absorber quality. Herein, a TiO 2 ETL is strategically modified using an inorganic salt zinc halide (i.e., ZnCl 2 , ZnBr 2 , ZnI 2 ), which simultaneously improves the electronic properties of TiO 2 and promotes the growth of Sb 2 S 3 films with larger grain size and higher crystallinity. The experimental results and theoretical calculations further reveal that the zinc halide can interact with TiO 2 and simultaneously bond strongly with the upper Sb 2 S 3 film, which creates a unique pathway for electron transfer, passivates the trap states, and alleviates the recombination losses effectively. As a result, an average PCE of 6.87 ± 0.11% and the highest PCE of 7.08% have been attained with an improved FF from 51.22 to 61.61% after ZnCl 2 introduction. Additionally, introduction of ZnCl 2 helps the unencapsulated devices to maintain 93% of their original performance after 2400 h of storage in a nitrogen-filled glovebox. This work develops an effective route for the optimization of ETLs and defect healing using simple and low-cost inorganic salts.

Great progress in modification and optimization of emission layer (EML) in perovskite light-emitting diodes (PeLEDs) results in a significant improvement in device efficiency. However, so far, less attention has been paid to the exploration of hole/electron injection and transporting layers to maximize the utilization of charge carriers for efficient and stable PeLEDs. At present, low electron mobility of electron transport layer (ETL) causes an unbalanced charge injection, and the defects at the ETL/perovskite interface limit the formation and utilization of generated excitons. Here, a series of compounds (BPBiTP, BPBiPN, and BPBiPA) flanked by diphenyl-1 H -benzo[ d ]imidazole end groups have been developed as ETL materials, where the bridging units (benzene, naphthalene, anthracene) are manipulated to achieve dual functionality, namely, the high charge carrier mobility and effective passivation of perovskite surface. The coordinating end groups effectively reduce the trap state at the interface of ETL and EML due to their strong nucleophilic quality. H-aggregation of anthracene units and large transfer integral in BPBiPA lead to its superior electron mobility of 8.4 × 10 -4 cm 2 V -1 s -1 in the solid state, over 1 order of magnitude higher than that of the typical one (TPBi). Consequently, green PeLEDs with a maximum external quantum efficiency (EQE) of 19.7%, reduced efficiency roll-off, as well as extended operational lifetime have been achieved without any outcoupling technique. Our result demonstrated that optimization of ETL materials via improving both passivation capability and electron mobility is a powerful strategy for producing high-performance PeLEDs.

In p-i-n structure perovskite solar cells (PSCs), the most prevalent electron transport layer (ETL), [6, 6]-phenyl-C 61 -butyric acid methyl ester (PC 61 BM), acts as both electron extractor and protective coverage to the underlayer perovskite. Notably, multifunctional mixed fullerene ETLs show great potential in further improving both the power conversion efficiency (PCE) and stability of PSCs compared to the single PC 61 BM ETL. In this work, we reported the mixed fullerene ETLs comprising of PC 61 BM and its two analogs with different length of fluorocarbon chains, [6, 6]-phenyl-C 61 -buryric acid 1H,1H-trifluoro-1-ethyl ester (abbreviated, CF 3 -PC 61 BM) and [6, 6]-phenyl-C 61 -buryric acid 1H, 1H-tridecafluoro-1-heptyl ester (abbreviated, C 6 F 13 -PC 61 BM). We obtained excellent PCEs of 18.37% and 17.71% for 1 wt % CF 3 -PC 61 BM- and C 6 F 13 -PC 61 BM-based PSCs (1 wt % addition of PC 61 BM) with CH 3 NH 3 PbI 3 (MAPbI 3 ) perovskites, respectively. Moreover, champion PCEs of ∼19% were obtained based on the CsFAMAPbIBr perovskites. Subsequent experiments demonstrated that the fluorocarbon chains of CF 3 -PC 61 BM and C 6 F 13 -PC 61 BM assembled at the surfaces of ETLs with the formation of thin-layer moisture-resistant protective coverage above perovskite. Results show that it significantly retarded water penetrating down to perovskite layers and led to optimal humidity stability under ambient atmosphere.

Organic-inorganic lead halide perovskites are promising materials for realization of low-cost and high-efficiency solar cells. Because of the toxicity of lead, Sn-based perovskite materials have been developed as alternatives to enable fabrication of Pb-free perovskite solar cells. However, the solar cell performance of Sn-based perovskite solar cells (Sn-PSCs) remains poor because of their large open-circuit voltage ( V ) loss. Sn-based perovskite materials have lower electron affinities than Pb-based perovskite materials, which result in larger conduction band offset (CBO) values at the interface between the Sn-based perovskite and a conventional electron transport layer (ETL) material such as TiO OC ) loss. Sn-based perovskite materials have lower electron affinities than Pb-based perovskite materials, which result in larger conduction band offset (CBO) values at the interface between the Sn-based perovskite and a conventional electron transport layer (ETL) material such as TiO 2 . Herein, the relationship between the V OC and the CBO in these devices was studied to improve the solar cell performances of Sn-PSCs. It was found that the band offset at the ETL/perovskite layer interface affects the V OC of the Sn-PSCs significantly but does not affect that of the Pb-PSCs because the Sn-based perovskite material is a p-type semiconductor, unlike the Pb-based perovskite. It was also found that Nb 2 O 5 has the CBO that is closest to zero for Sn-based perovskite materials, and the V OC values of Sn-PSCs that use Nb 2 O 5 as their ETL are higher than those of Sn-PSCs using TiO 2 or SnO 2 ETLs. This study indicates that control of the energy alignment at the ETL/perovskite layer interface is an important factor in improving the V OC values of Sn-PSCs.

The structure-dependent thermal stability of fullerene electron transport layers (ETLs) and its impact on device stability have been underrated for years. Based on cocrystallographic understanding, herein, we develop a thermally stable ETL comprising a hybrid layer of [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) and [6,6]-phenyl-C 61 -propylbenzene (PCPB). By tuning the weight ratios of PCBM and PCPB to influence the noncovalent intermolecular interactions and packing of fullerene derivatives, we obtained a champion device based on the 20PCPB (20 wt % addition of PCPB into the mixture of PCBM/PCPB) ETL and excellent thermal stability of 500 h under 85 °C thermal aging in a N 2 atmosphere in the dark. The present work exemplifies that cocrystallography can be a precise tool to probe the interaction and aggregation of fullerene derivatives in ETLs, and mixed fullerene derivatives can be sought as promising ETLs to enhance the long-term stability of perovskite solar cells under high-temperature working environments.

Amorphous NbO x electron transport layer (ETL) shows great potential for boosting the power conversion efficiency (PCE) of perovskite solar cells (PSCs) at low temperatures (<100 °C). To date, it is still a challenge to simultaneously control the hydrolysis of the NbO x precursor solution and reduce the impurities of NbO x ETLs during low-temperature solution processing under ambient conditions. Herein, for the first time, we report ozone (O 3 ) as a strong ligand to stabilize Nb salt solutions under ambient conditions. The above procedure not only enables the formation of a highly repeatable amorphous NbO x film by suppressing the hydrolysis of the solution but also reduces the OH content in the film, which decreases the defect intensity and improves the conductivity of the NbO x ETL. Thus, the formation of highly repeatable NbO x ETL-based PSCs are obtained; moreover, these PSCs have high PCE values of 19.54 and 16.42% on rigid and flexible substrates, respectively, much higher than the devices based on ETLs from a solution without an O 3 treatment.

An Nb-doped TiO 2 (Nb-TiO 2 ) film comprising a double structure stacked with a bottom compact layer and top mesoporous layers was synthesized by treating a Ti precursor-coated substrate using a one-step low-temperature steam-annealing (SA) method. The SA-based Nb-TiO 2 films possess high crystallinity and conductivity, and that allows better control over the conduction band (CB) of TiO 2 for the electron transport layer (ETL) of the perovskite solar cells by the Nb doping level. Optimization of power conversion efficiency (PCE) for the Nb-TiO 2 -based ETL was combined with the CB level tuning of the mixed-halide perovskite by changing the Br/I ratio. This band offset management enabled to establish the most suitable energy levels between the ETL and the perovskites. This method was applied to reduce the band gap of perovskites to enhance the photocurrent density while maintaining a high open-circuit voltage. As a result, the optimal combination of 5 mol % Nb-TiO 2 ETL and 10 mol % Br in the mixed-halide perovskite exhibited high photovoltaic performance for low-temperature device fabrication, achieving a high-yield PCE of 21.3%.

Titania (TiO 2 ) has wide applications in the realm of perovskite solar cells (PSCs). Because high-temperature processing severely limits the application of flexible and tandem devices, it is significant to develop a high-quality electron-transport layer (ETL) by low-temperature processing. Here, we design a new strategy by introducing a bifunctional molecule (thioacetamide, TAA) in the TiO 2 ETL. During the low-temperature annealing, the N and S atoms in TAA can bond with the Ti atom in the ETL and the Pb atom in the perovskite (PVK) layer, respectively. The formation of coordinate bonds is beneficial to increase the crystallinity and reduce the roughness of TiO 2 ETLs and PVK layers, which effectively passivate the defects. Meanwhile, the energy level matching between the ETL and PVK is optimized. The structure characterization and electrochemical measurement demonstrate the design. Compared with precursor doping, surface spin-coating is a more effective method for introducing TAA into TiO 2 . Significantly, the PSC based on the surface spin-coated TAA TiO 2 ETL achieves the best power conversion efficiency (PCE) of 21.17%. Nevertheless, the PSC fabricated with the pristine TiO 2 ETL offers a PCE of 19.52% under the same conditions. The results demonstrate a novel method for optimizing the properties of PSCs.

Organic-inorganic hybrid perovskite solar cells have attracted much attention due to their high power conversion efficiency (>25%) and low-cost fabrication. Yet, improvements are still needed for more stable and higher-performing solar cells. In this work, a series of TiO 2 nanocolumn photonic structures have been intentionally fabricated on half of the compact TiO 2 -coated fluorine-doped tin oxide substrate by glancing angle deposition with magnetron sputtering, a method particularly suitable for industrial applications due to its high reliability and reduced cost when coating large areas. These vertically aligned nanocolumn arrays were then applied as the electron transport layer into triple-cation lead halide perovskite solar cells based on Cs 0.05 (FA 0.83 MA 0.17 ) 0.95 Pb(I 0.83 Br 0.17 ) 3 . By comparison to solar cells built onto the same substrate without nanocolumns, the use of TiO 2 nanocolumns can significantly enhance the power conversion efficiency of the perovskite solar cells by 7% and prolong their shelf life. Here, detailed characterizations on the morphology and the spectroscopic aspects of the nanocolumns, their near-field and far-field optical properties, solar cells characteristics, as well as the charge transport properties provide mechanistic insights on how one-dimensional TiO 2 nanocolumns affect the performance of perovskite halide solar cells in terms of charge transport, light harvesting, and stability, knowledge necessary for the future design of higher-performing and more stable perovskite solar cells.

Tin oxide (SnO 2 ) is widely adopted as an electron transport layer in perovskite solar cells (PeSCs) because it has high electron mobility, excellent charge selective behavior owing to a large band gap of 3.76 eV, and low-temperature processibility. To achieve highly efficient SnO 2 -based PeSCs, it is necessary to control the oxygen vacancies in the SnO 2 layer, since the electrical and optical properties vary depending on the oxidation state of Sn. This study demonstrates that the performance of PeSCs may be improved by using nitrogen-doped graphene oxide (NGO) as an oxidizing agent for SnO 2 . Since NGO changes the oxidation state of the Sn in SnO 2 from Sn 2+ to Sn 4+ , the oxygen vacancies in SnO 2 can be reduced using NGO. Multiple devices are fabricated, and various techniques are used to assess their performance, including X-ray photoelectron spectroscopy, dark current analysis, and the dependence of the open-circuit voltage on light intensity. Compared with the average power conversion efficiency (PCE) of control devices, PeSCs with SnO 2 :NGO composite layers exhibit greater PCEs with less deviation. Therefore, the introduction of NGO in a SnO 2 layer can be regarded as an effective method of controlling the oxidation state of SnO 2 to improve the performance of PeSCs.

Tin oxide (SnO 2 ) is widely used in perovskite solar cells (PSCs) as an electron transport layer (ETL) material. However, its high surface trap density has already become a strong factor limiting PSC development. In this work, phosphoric acid is adopted to eliminate the SnO 2 surface dangling bonds to increase electron collection efficiency. The phosphorus mainly exists at the boundaries in the form of chained phosphate groups, bonding with which more than 47.9% of Sn dangling bonds are eliminated. The reduction of surface trap states depresses the electron transport barriers, thus the electron mobility increases about 3 times when the concentration of phosphoric acid is optimized with 7.4 atom % in the SnO 2 precursor. Furthermore, the stability of the perovskite layer deposited on the phosphate-passivated SnO 2 (P-SnO 2 ) ETL is gradually improved with an increase of the concentration. Due to the higher electron collection efficiency, the P-SnO 2 ETLs can dramatically promote the power conversion efficiency (PCE) of the PSCs. As a result, the champion PSC has a PCE of 21.02%. Therefore, it has been proved that this simple method is efficient to improve the quality of ETL for high-performance PSCs.

In regular perovskite solar cells (PSCs), the commonly used electron transport layer (ETL) is titanium oxide (TiO 2 ). Nevertheless, the preparation of a high-quality TiO 2 ETL demands an elevated-temperature sintering procedure, unfavorable for fabrication of PSCs on flexible substrates. Besides, TiO 2 -based devices often suffer from notorious photocurrent hysteresis and serious light soaking instability, limiting their potential commercialization. Herein, a novel pyridine-functionalized fullerene derivative [6,6]-(4-pyridinyl)-C 61 -ethyl acid ethyl ester (PyCEE) was synthesized and applied as an ETL to replace TiO 2 in n-i-p PSCs. PyCEE-based devices achieved a champion power conversion efficiency (PCE) of 18.27% with significantly suppressed hysteresis, superior to that of TiO 2 -based devices. PyCEE has suitable energy levels and high electron mobility, which facilitate electron extraction/transport. Besides, the pyridine moiety within PyCEE affords coordination interactions with the Pb 2+ ion within CH 3 NH 3 PbI 3 , passivating the trap states of CH 3 NH 3 PbI 3 and thus improving the device performance and suppressing hysteresis greatly. Moreover, PyCEE ETLs were applied in flexible PSCs, achieving a PCE of 15.25%. Our results demonstrated the applicability of PyCEE ETLs in flexible devices and provided new opportunity for the commercialization of PSCs.

The inherent instability of UV-induced degradation in TiO 2 -based perovskite solar cells was largely improved by replacing the anatase-phase compact TiO 2 layer with an atomic sheet transport layer (ASTL) of two-dimensional (2D) Ti 1-δ O 2 . The vital role of microscopic carrier dynamics that govern the UV stability of perovskite solar cells was comprehensively examined in this work by performing time-resolved pump-probe spectroscopy. In conventional perovskite solar cells, the presence of a UV-active oxygen vacancy in compact TiO 2 prohibits current generation by heavily trapping electrons after UV degradation. Conversely, the dominant vacancy type in the 2D Ti 1-δ O 2 ASTL is a titanium vacancy, which is a shallow acceptor and is not UV-sensitive. Therefore, it significantly suppresses carrier recombination and extends UV stability in perovskite solar cells with a 2D Ti 1-δ O 2 ASTL. Other carrier dynamics, such as electron diffusion, electron injection, and hot hole transfer processes, were found to be less affected by UV irradiation. Quantitative pump-probe data clearly show a correlation between the carrier dynamics and UV aging of perovskite solar cells, thus providing a profound insight into the factors driving UV-induced degradation in perovskite solar cells and the origin of its performance.

TiO 2 is commonly used as an electron-transporting material in perovskite photovoltaic devices due to its advantages, including suitable band gap, good photoelectrochemical stability, and simple preparation process. However, there are many oxygen vacancies or defects on the surface of TiO 2 and thus this affects the stability of TiO 2 -based perovskite solar cells under UV light. In this work, a thin (monolayer) SbI 3 modification layer is introduced on the mesoporous TiO 2 surface and the effect at the interface between of TiO 2 and perovskite is monitored by using a quartz crystal microbalance system. We demonstrate that the SbI 3 -modified TiO 2 electrodes exhibit superior electronic properties by reducing electronic trap states, enabling faster electron transport. This approach results in higher performances compared with electrodes without the SbI 3 passivation layer. CH 3 NH 3 PbI 3 perovskite solar cells with a maximum power conversion efficiency of 17.33% in air, accompanied by a reduction in hysteresis and enhancement of the device stability, are reported.

In this study, we incorporated molybdenum disulfide (MoS 2 ) nanosheets into sol-gel processing of zinc oxide (ZnO) to form ZnO:MoS 2 composites for use as electron transport layers (ETLs) in inverted polymer solar cells featuring a binary bulk heterojunction active layer. We could effectively tune the energy band of the ZnO:MoS 2 composite film from 4.45 to 4.22 eV by varying the content of MoS 2 up to 0.5 wt %, such that the composite was suitable for use in bulk heterojunction photovoltaic devices based on poly[bis(5-(2-ethylhexyl)thien-2-yl)benzodithiophene- alt-(4-(2-ethylhexyl)-3-fluorothienothiophene)-2-carboxylate-2,6-diyl] (PTB7-TH)/phenyl-C 71 -butryric acid methyl ester (PC 71 BM). In particular, the power conversion efficiency (PCE) of the PTB7-TH/PC 71 BM (1:1.5, w/w) device incorporating the ZnO:MoS 2 (0.5 wt %) composite layer as the ETL was 10.1%, up from 8.8% for the corresponding device featuring ZnO alone as the ETL, a relative increase of 15%. Incorporating a small amount of MoS 2 nanosheets into the ETL altered the morphology of the ETL and resulted in enhanced current densities, fill factors, and PCEs for the devices. We used ultraviolet photoelectron spectroscopy, synchrotron grazing incidence wide-/small-angle X-ray scattering, atomic force microscopy, and transmission electron microscopy to characterize the energy band structures, internal structures, surface roughness, and morphologies, respectively, of the ZnO:MoS 2 composite films.

ZnO nanoparticles (NPs) are widely used as the electron transport layer (ETL) in quantum dot light-emitting diodes (QLEDs) owing to their suitable electrical properties. However, because of the well-aligned conduction band levels, electrons in QDs can be spontaneously transferred to adjacent ZnO NPs, leading to severe exciton dissociation, which reduces the proportion of radiative recombination and deteriorates the device efficiency. In this work, Al-doped ZnO NPs are thoroughly investigated as a replacement of ZnO for QLEDs. The energy band structures of Al-doped ZnO are modified by adjusting the concentration of Al dopants. With the increasing Al content, the work function and the conduction band edge of ZnO are gradually raised, and thus the charge transfer at the interface of QDs/ETL is effectively suppressed. Consequently, the green QLEDs with 10% Al-doped ZnO NPs exhibit maximum current efficiency and external quantum efficiency of 59.7 cd/A and 14.1%, which are about 1.8-fold higher than 33.3 cd/A and 7.9% of the devices with undoped ZnO NPs. Our work suggests that Al-doped ZnO NPs can serve as a good electron transport/injection material in QLEDs and other optoelectronic devices.

Highly crystalline TiO 2 nanostructured films were synthesized by a simple steam treatment of a TiCl 4 precursor film under a saturated water vapor atmosphere at 125 °C, here referred to as the steam-annealing method. In a single TiO 2 film preparation step, a bilayer structure comprising a compact bottom layer and a mesoporous surface layer was formed. The mesoporous layer was occupied by bipyramidal nanoparticles, with a composite phase of anatase and brookite crystals. Despite the low-temperature treatment process, the crystallinity of the TiO 2 film was high, comparable with that of the TiO 2 film sintered at 500 °C. The compact double-layered TiO 2 film was applied to perovskite solar cells (PSCs) as an electron-collecting layer. The PSC exhibited a maximum power conversion efficiency (PCE) of 18.9% with an open-circuit voltage ( V OC ) of 1.15 V. The PCE and V OC were higher than those of PSCs using a TiO 2 film formed by 500 °C sintering.

In this study, a room-temperature and aqueous solution-processed two-dimensional (2D) transition-metal dichalcogenide TiS 2 was applied as an electron transport layer (ETL) in planar n-i-p perovskite solar cells (Pero-SCs). Upon insertion of the 2D TiS 2 ETL with UV-ozone (UVO) treatment, the power conversion efficiency (PCE) of the planar Pero-SCs was optimized to 18.79%. To the best of our knowledge, this value should be the highest efficiency to date among those PCEs of the n-i-p Pero-SCs with room-temperature-processed metal compound ETLs. More importantly, the n-i-p Pero-SCs with the UVO-treated 2D TiS 2 as an ETL also show extremely high stability, where the average PCE remained over 95% of its initial value after 816 h storage without encapsulation.