Metal Oxide Transport Layers Research Articles

Interface-Engineered Organic Near-Infrared Photodetector for Imaging Applications.

We report a high-speed low dark current near-infrared (NIR) organic photodetector (OPD) on a silicon substrate with amorphous indium gallium zinc oxide (a-IGZO) as the electron transport layer (ETL). In-depth understanding of the origin of dark current is obtained using an elaborate set of characterization techniques, including temperature-dependent current-voltage measurements, current-based deep-level transient spectroscopy (Q-DLTS), and transient photovoltage decay measurements. These characterization results are complemented by energy band structures deduced from ultraviolet photoelectron spectroscopy. The presence of trap states and a strong dependency of activation energy on the applied reverse bias voltage point to a dark current mechanism based on trap-assisted field-enhanced thermal emission (Poole-Frenkel emission). We significantly reduce this emission by introducing a thin interfacial layer between the donor: acceptor blend and the a-IGZO ETL and obtain a dark current as low as 125 pA/cm2 at an applied reverse bias of -1 V. Thanks to the use of high-mobility metal-oxide transport layers, a fast photo response time of 639 ns (rise) and 1497 ns (fall) is achieved, which, to the best of our knowledge, is among the fastest reported for NIR OPDs. Finally, we present an imager integrating the NIR OPD on a complementary metal oxide semiconductor read-out circuit, demonstrating the significance of the improved dark current characteristics in capturing high-quality sample images with this technology.

Co-evaporated p-i-n perovskite solar cells with sputtered NiOx hole transport layer

Metal oxides have long been used as charge transport layers in thin film solar cells due to their suitable energy band levels, high charge carrier mobilities, and excellent long-term stability. One example is non-stoichiometric nickel oxide (NiOx), a hole transport material which can be deposited in a conformal, scalable, and solvent-free manner via sputtering. However, sputtered NiOx has only ever been combined with spin-coated perovskite films in perovskite solar cells (PSCs). Here we report the fabrication of the first co-evaporated PSCs with sputtered NiOx as the hole transport layer. We reveal how the amount of argon gas used during sputtering affects important physical properties such as hole conductivity, visible light transmittance, and suitability of the NiOx surface for perovskite nucleation and growth. Optimized sputtering conditions led to CH3NH3PbI3 (MAPbI3) PSCs with an 18.6% power conversion efficiency (PCE), which was further increased to 19.5% by adding a spiro-TTB interlayer between NiOx and MAPbI3. We also show that removing the NiOx annealing step only reduced the device PCE by 1%, paving the way toward low-temperature deposition of metal oxide transport layers. This work exhibits the versatility of NiOx sputtering and perovskite evaporation as a powerful combination for solvent-free and low-temperature fabrication of highly efficient PSCs.

Charge carrier dynamics across the metal oxide/BaTiO3 interfaces toward photovoltaic applications from the theoretical perspective

Transport layers in perovskite solar cells are the crucial parts in charge carrier extraction and transfer in the solar cell circuit. The physical and chemical properties of their interfaces with neighbouring layers, such as buffer layers, govern the dynamics of electrons across the interface boundaries. Using ab initio calculations, we study in detail the interface properties of metal oxide/BaTiO3 heterostructures. The structural changes near the metal oxide/BaTiO3 interfaces are detailed for the first time. The electronic properties are used to assess the quantities of interest for use in perovskite solar cells (band gap, effective mass, carrier mobility, plane averaged electrostatic potential, band offsets). The analysed results introduce several competing quantities which influence the charge dynamics: the band gap character versus the charge carrier effective mass and the averaged electrostatic potential difference versus the unfavourable conduction band offset. While experimental studies are unable to distinguish the relevance of the competing factors, our results point to the fact that BaTiO3 interlayer morphology plays a more important factor in the cell's performance than its intrinsic properties. Based on the results, general insights are given on the structural requirements for improved buffer layer materials in combinations with metal oxide transport layer used as substrates in the solar cell architecture.

Efficient and stable organic solar cells based on all-solution-processed metal oxide transport layers

Doping of solution-processed molybdenum oxide hole transport layers enabled the efficient and stable organic solar cells.

Power conversion efficiency optimization of LaFeO3 Mott insulator based solar cell with metal oxide transport layers using SCAPS

Mott insulator-based heterostructures have been proposed as good candidates for highly efficient solar cells. Mott insulating transition metal oxide (LaFeO3) containing earth-abundant elements have long-term stability and the strong electron–electron correlation found in them enhances the quantum efficiency of the device. This paper represents a simulation work to access the energy conversion ability of Mott insulator-based solar cell with LaFeO3 as its absorbing layer using a solar cell capacitance simulator (SCAPS). This work explores the influence of the electron transport layer (ETL), hole transport layer (HTL), active perovskite layer thickness, and defect density on device performance. The simulation showed that In2O3 and MoO3 are the most competent materials for ETL and HTL layers. Additionally, the impact of other optoelectronic parameters of different layers is also investigated and discussed. An efficiency of 9.11 % (with Voc = 1.997 V, Jsc = 6.75 mA/cm2, and fill factor of 67.55 %) has been achieved with Nb:STO/In2O3/LaFeO3/MoO3/Au solar cell architecture. The simulated model has great potential for making stable solar cell devices by using Mott insulator perovskite materials.

Metal Oxide-Induced Instability and Its Mitigation in Halide Perovskite Solar Cells.

Halide perovskite solar cells (PSCs) have emerged as a promising photovoltaic technology for sustainable energy solutions because of their impressive power conversion efficiency and a path to be manufactured by low-cost, high-throughput methods. To reach PSCs' full potential for practical implementation, it is crucial to solve the issues related to long-term operational stability. Given that PSCs consist of many layers of dissimilar materials which form multiple internal interfaces, it is prudent to examine whether there exist interfacial interactions, most importantly between transport layers and perovskite absorbers, that can trigger instability and affect device performance. In this Perspective, we bring to the attention of the PSC research community the lesser-known interfacial degradation of halide perovskites promoted by contact with metal oxide transport layers and highlight the deleterious effects on the PSCs' performance and stability. We also discuss various mitigation strategies that have shown promise for achieving high-performing and stable PSCs.

Passivation of the Buried Interface via Preferential Crystallization of 2D Perovskite on Metal Oxide Transport Layers.

The open-circuit voltage (Voc ) of perovskite solar cells is limited by non-radiative recombination at perovskite/carrier transport layer (CTL) interfaces. 2D perovskite post-treatments offer a means to passivate the top interface; whereas, accessing and passivating the buried interface underneath the perovskite film requires new material synthesis strategies. It is posited that perovskite ink containing species that bind strongly to substrates can spontaneously form a passivating layer with the bottom CTL. The concept using organic spacer cations with rich NH2 groups is implemented, where readily available hydrogens have large binding affinity to under-coordinated oxygens on the metal oxide substrate surface, inducing preferential crystallization of a thin 2D layer at the buried interface. The passivation effect of this 2D layer is examined using steady-state and time-resolved photoluminescence spectroscopy: the 2D interlayer suppresses non-radiative recombination at the buried perovskite/CTL interface, leading to a 72% reduction in surface recombination velocity. This strategy enables a 65mV increase in Voc for NiOx based p-i-n devices, and a 100mV increase in Voc for SnO2 -based n-i-p devices. Inverted solar cells with 20.1% power conversion efficiency (PCE) for 1.70eV and 22.9% PCE for 1.55eV bandgap perovskites are demonstrated.

Photonic Curing of Nickel Oxide Transport Layer and Perovskite Active Layer for Flexible Perovskite Solar Cells: A Path Towards High-Throughput Manufacturing

High-throughput roll-to-roll (R2R) manufacturing of perovskite solar cells (PSCs) is currently limited by thermal processes that take tens of minutes each, translating to impractically long annealing tools at high web speeds. In addition, PSCs are usually made with metal oxide transport layer materials that require high temperatures for thermal annealing. Here, we demonstrate the fabrication of PSCs using photonic curing, instead of thermal annealing, to convert NiOx directly from sol-gel precursors for hole transport layers and to crystallize methylammonium lead iodide (MAPbI3) active layers on flexible Willow® Glass substrates. Photonic curing uses short, intense pulses of light to process materials at a high speed, hence it is compatible with R2R manufacturing. We achieved power conversion efficiencies (PCEs) of 11.7% in forward-scan and 10.9% in reverse-scan for PSCs made with photonic cured NiOx and MAPbI3 films. Furthermore, both NiOx and MAPbI3 films could be processed with a single photonic curing pulse, with a web speed of 5.7 m/min, and still produce PCEs comparable to thermally annealed control samples. Based on the single-pulse photonic curing condition for each film, we project a web speed of 26 m/min, laying a pathway to high-throughput production of perovskite solar modules.

Unusual Hole Transfer Dynamics of the NiO Layer in Methylammonium Lead Tri-iodide Absorber Solar Cells.

Nickel oxides (NiO) as hole transport layers (HTLs) in inverted-type perovskite solar cells (PSCs) have been widely studied mainly because of their high stability under illumination. Increases in the power conversion efficiency (PCE) with NiO HTLs have been presented in numerous reports, although the photoluminescence (PL) quenching behavior does not coincide with the PCE increase. The dynamics of the charge carrier transport between the NiO HTLs and the organic-inorganic halide perovskite absorbers is not clearly understood yet and quite unusual, in contrast to organic/polymerics HTLs. We deposited NiO HTLs with precisely controlled thicknesses by atomic layer deposition (ALD) and studied their photovoltaic performances and hole transfer characteristics. Ground state bleaching (GSB) recovery was observed by ultrafast transient absorption spectroscopy (TAS), which suggested that backward hole injection occurred between the perovskites and NiO HTLs, so that the uncommon PL behaviors can be clearly explained. Backward hole injection from the NiO HTL to the perovskite absorber originated from their similar valence band (VB) energy positions. The thickness increase of the NiO HTLs induced VB sharing, which caused a red-shift of the photoinduced hole absorption spectrum in near-infrared (NIR) femtosecond TAS and a decrease in the PL intensity. Our studies on inorganic metal oxide transport layers, NiO in this work, with a thickness dependence and the comparison with organic layers provide a better understanding of the interfacial carrier dynamics in PSCs.

Performance analysis of an efficient and stable perovskite solar cell and a comparative study of incorporating metal oxide transport layers

As poor stability is the primary constraint for the commercialization of perovskite solar cells, improving stability has been the primary focus of recent research works regarding solar cells that make use of perovskite materials. Different metal oxide transport layers are being used with the aim of fabricating stable perovskite solar cells. A stable and efficient solar cell with both metal oxide transport layers (ZnO and N i O X ) and a perovskite (methyl ammonium lead iodide) absorber layer is simulated in this work, and a comparison of performance parameters is made with other transport layers from the literature. The issue of optimization regarding the thickness of the absorber layer and the doping concentration in the absorber and transport layers has been addressed, and the effect of defect concentration at the interface has been investigated. Optimum performance is achieved with an absorber layer of thickness 800 nm. Linear grading is also introduced in the absorber layer by varying the concentration of different halides, which increases the efficiency by approximately 8% owing to the increase in the short circuit current density.

Enhancing the stability of perovskite solar cells through functionalisation of metal oxide transport layers with self-assembled monolayers

A systematic study of the application of self-assembly monolayers (SAMs) onto electron and hole transporting layers for perovskite solar cells (PSCs) stability is reported. Cs0.05FA0.83MA0.17Pb(I0.87Br0.13)3 (FMC) perovskite films were deposited onto tin oxide (SnO2) and nickel oxide (NiOx) layers that were functionalized with ethylphosphonic acid (EPA) and 4-bromobenzoic acid (BBA) SAMs. X-ray diffractometry measurements were performed on these films shortly after they were deposited. The diffractograms agree with the positions reported in the literature for the crystal structure of the FMC. The results show that the deposition of SAMs on the metal oxide layers yields positive improvements in the FMC film stability and in the device stability when using FMC as the active layer. The work shows that by adopting SAMs, the long-term stability of PSCs cells under accelerated test conditions can be enhanced, and this provides one step on the way to making this technology a commercial reality.

Altered Stability and Degradation Pathway of CH3NH3PbI3 in Contact with Metal Oxide

Degradation in CH3NH3PbI3 (MAPbI3), when in contact with commonly used metal oxide transport layer materials in optoelectronic devices, is examined experimentally and theoretically. On the basis of...

Bulk and interfacial decomposition of formamidinium iodide (HC(NH2)2I) in contact with metal oxide

Interfacial reaction between formamidinium iodide and metal oxide transport layer triggers degradation and lower the intrinsic stability which is dictated by the oxide, rather than the perovskite.

All-Inorganic Perovskite Solar Cells Based on CsPbIBr2 and Metal Oxide Transport Layers with Improved Stability.

Despite the successful improvement in the power conversion efficiency (PCE) of perovskite solar cells (PSCs), the issue of instability is still a serious challenge for their commercial application. The issue of the PSCs mainly originates from the decomposition of the organic–inorganic hybrid perovskite materials, which will degrade upon humidity and suffer from the thermal environment. In addition, the charge transport layers also influence the stability of the whole devices. In this study, inorganic transport layers are utilized in an inverted structure of PSCs employing CsPbIBr2 as light absorbent layer, in which nickel oxide (NiOx) and cerium oxide (CeOx) films are applied as the hole transport layer (HTL) and the electron transport layer (ETL), respectively. The inorganic transport layers are expected to protect the CsPbIBr2 film from the contact of moisture and react with the metal electrode, thus preventing degradation. The PSC with all inorganic components, inorganic perovskite and inorganic transport layers demonstrates an initial PCE of 5.60% and retains 5.56% after 600 s in ambient air at maximum power point tracking.

All-solution-processed perovskite light-emitting diodes with all metal oxide transport layers.

All-solution-processed perovskite light-emitting diodes (PeLEDs) with all metal oxide transport layers were successfully realized based on an ITO/NiOx/CsPbBr3/ZnMgO/Al conventional device structure. A unique perovskite-polymer composite method enables the deposition of solution-processed ZnMgO nanoparticles on the perovskite film. As a result, we achieved highly efficient PeLEDs with a maximum luminance of 17 017 cd m-2, and the efficiency showed little roll-off with increasing current density.

All inorganic quantum dot light emitting devices with solution processed metal oxide transport layers

ABSTRACTAll inorganic quantum dot light emitting devices with solution processed transport layers are investigated. The device consists of an anode, a hole transport layer, a quantum dot emissive layer, an electron transport layer and a cathode. Indium tin oxide coated glass slides are used as substrates with the indium tin oxide acting as the transparent anode electrode. The transport layers are both inorganic, which are relatively insensitive to moisture and other environmental factors as compared to their organic counterparts. Nickel oxide acts as the hole transport layer, while zinc oxide nanocrystals act as the electron transport layer. The nickel oxide hole transport layer is formed by annealing a spin coated layer of nickel hydroxide sol-gel. On top of the hole transport layer, CdSe/ZnS quantum dots synthesized by hot injection method is spin coated. Finally, zinc oxide nanocrystals, dispersed in methanol, are spin coated over the quantum dot emissive layer as the electron transport layer. The material characterization of different layers is performed by using absorbance, Raman scattering, XRD, and photoluminescence measurements. The completed device performance is evaluated by measuring the IV characteristics, electroluminescence and quantum efficiency measurements. The device turn on is around 4V with a maximum current density of ∼200 mA/cm2 at 9 V.

Improved air stability of perovskite solar cells via solution-processed metal oxide transport layers.

Lead halide perovskite solar cells have recently attracted tremendous attention because of their excellent photovoltaic efficiencies. However, the poor stability of both the perovskite material and the charge transport layers has so far prevented the fabrication of devices that can withstand sustained operation under normal conditions. Here, we report a solution-processed lead halide perovskite solar cell that has p-type NiO(x) and n-type ZnO nanoparticles as hole and electron transport layers, respectively, and shows improved stability against water and oxygen degradation when compared with devices with organic charge transport layers. Our cells have a p-i-n structure (glass/indium tin oxide/NiO(x)/perovskite/ZnO/Al), in which the ZnO layer isolates the perovskite and Al layers, thus preventing degradation. After 60 days storage in air at room temperature, our all-metal-oxide devices retain about 90% of their original efficiency, unlike control devices made with organic transport layers, which undergo a complete degradation after just 5 days. The initial power conversion efficiency of our devices is 14.6 ± 1.5%, with an uncertified maximum value of 16.1%.

Root-cause failure analysis of photocurrent loss in polythiophene:fullerene-based inverted solar cells.

Metal oxide transport layers have played a crucial role in recent progress in organic photovoltaic (OPV) device stability. Here, we measure the stability of inverted and encapsulated polythiophene:fullerene cells with MoO3/Ag/Al composite anode in operational conditions combining solar radiation and 65 °C. Performance loss of over 50% in the first 100 h of the aging is dominated by a drop in the short-circuit current (Jsc). We reveal a concurrent loss in reflectance from 85% to 50% above 650 nm, which is below the optical gap of the used photoactive materials, hence, excluding any major degradation in the bulk of this layer. Correlating the responses of aged devices to a series of test structures comprised of ITO/ZnO cathode, MoO3/Ag, and MoO3/Ag/Al anodes and their combinations with the active layer allowed us to identify that the presence of Al causes the reduced reflectance in these devices, independent of the presence of the active layer. Systematic single-stress aging on the test structures further indicates that elevated heat is the cause of the reflectance loss. Cross-section transmission electron microscopy coupled with elemental analysis revealed the unsuspected role of Al; notably, it diffuses through the entire 150 nm thick Ag layer and accumulates at the MoO3/Ag interface. Moreover, XRD analysis of the aged MoO3/Ag/Al anode indicates the formation of Ag2Al alloy. Depth profiling with X-ray photoelectron spectroscopy advanced our understanding by confirming the formation of Ag-Al intermetallic alloy and the presence of oxidized Al only at the MoO3/Ag interface suggesting a concomitant reduction of MoO3 to most probably MoO2. This latter compound is less reflective than MoO3, which can explain the reduced reflectance in aged devices as proven by optical simulations. On the basis of these results, we could estimate that 20% of the loss in Jsc is ascribed to reduction of MoO3 triggered by its direct contact with Al.