Device Stack Research Articles

Unveiling the role of carbon black in printable mesoscopic perovskite solar cells

Carbon-based perovskite solar cells (C–PSCs) have attracted significant attention from the scientific community owing to their improved stability, low-cost fabrication, and potential scalability with screen printing technology. In this study, we examine the effect of three most commonly used carbon blacks, i.e. vulcan carbon (VC), meso carbon (MC) and super P (SP), on the properties of the resultant carbon electrode and the effect on photovoltaic performance as well as device stability. It shows that VC based carbon electrode promotes decent wettability to perovskite precursor solution, excellent infiltration and strong adhesion within the device stacks due to its high porosity and high pore volume. Furthermore, counter electrodes consisting of highly conductive VC exhibit sheet resistance of as low as 11.0 Ohm/□ as compared to those of MC (15.3 Ohm/□) and SP (22.0 Ohm/□). C–PSCs fabricated using VC based carbon electrode display champion power conversion efficiency of 12.55% with almost no decrease in efficiency under ambient conditions (~75% relative humidity) for 30 days without encapsulation. This work highlights the importance of carbon paste formulation for developing highly efficient and stable perovskite solar cells.

Ambient Air Blade‐Coating Fabrication of Stable Triple‐Cation Perovskite Solar Modules by Green Solvent Quenching

Although halide perovskite solar cell (PSC) technology reaches, in few years, efficiencies greater than 25%, the cost‐ceffective perspective is achievable only if scalable processes in real manufacturing conditions (i.e., pilot line and/or plant factory) are designed and optimized for the full device stack. Herein, a full semiautomatic scalable process based on the blade‐coating technique is demonstrated to fabricate perovskite solar modules in ambient conditions. An efficient and stable triple‐cation cesium methylammonium formamidinium (CsMAFA) perovskite is deposited in ambient air with a two‐step process assisted by air and green anti‐solvent quenching. The developed industry compatible coating process enables the fabrication of several highly reproducible small‐area cells on module size substrate with an efficiency exceeding 17% and with high reproducibility. Corresponding reproducible modules (less than 2% variability) with a 90% geometrical fill factor achieve an efficiency larger than 16% and T 80 = 750 h in light‐soaking condition at maximum power point and room temperature/ambient after encapsulation. Film deposition properties are assessed by different characterization techniques, namely, scanning electron microscopy, profilometry, UV–vis and photoluminescence (PL) spectroscopy, and PL and electroluminescence imaging. The techniques confirm less defects and local coating variations of the ambient air/bladed devices with respect to the nitrogen air/spinned devices.

2D/3D perovskite engineering eliminates interfacial recombination losses in hybrid perovskite solar cells

<h2>Summary</h2> Interface engineering and design is paramount in the optimization of a multilayer device stack. This stands true for multi-dimensional (2D/3D) perovskite-based solar cells, in which high efficiency can be combined with promising device durability. However, the complex function of the 2D/3D device interfaces remains vague. Here, we provide the exact knowledge on the interface energetics and demonstrate that the 2D/3D perovskite interface forms a <i>p-n</i> junction that is capable of reducing the electron density at the hole transport layer interface and ultimately suppresses interfacial recombination. As a consequence, we demonstrate photovoltaic devices with an enhanced fill factor (FF) and open-circuit voltage (V_OC) of 1.19 V, which approaches the potential internal quasi-Fermi level splitting (QFLS) voltage of the perovskite absorber, nullifying the interfacial losses. We thus identify the essential parameters and energetic alignment scenario required for 2D/3D perovskite systems to surpass the current limitations of hybrid perovskite solar cell performances.

27‐4: Organic Light‐Emitting Diodes with Directional Polarized Light Emission

Organic light emitting diodes (OLEDs) with directional and polarized light emission have many photonic applications, and beam shaping of these devices is fundamentally challenging because they are Lambertian light sources. In this work, using OLEDs for demonstrations, by selectively diffracting the TE waveguide mode while suppressing other optical modes in a nanostructured substrate, we demonstrated highly directional light emission from a full‐area OLED with a small divergence angle less than 3°, and a TE to TM polarization extinction ratio of 13. The highly selective diffraction of only the TE waveguide mode is possible due to the planarization of the device stack by thermal evaporation and solution processing.

20‐2: Invited Paper: High‐Efficiency, Long‐Lifetime, Deep‐Blue Organic Light‐Emitting Diodes

Simultaneously achieving a high efficiency and long lifetime in deep‐blue organic light‐emitting diodes (OLEDs) is challenging because most high‐efficiency blue OLEDs suffer from a short device lifetime. We describe high‐efficiency, long‐lifetime deep‐blue thermally activated delayed fluorescence (TADF) OLEDs by designing new blue TADF materials and developing a triplet‐exciton‐recycling device protocol. A new deep‐blue TADF emitter that harvests the singlet excitons of the fluorescent emitter was embedded in the device structure of the triplet‐exciton‐distributed (TED) TADF devices built on a new device protocol. Two TADF materials, one distributing and one emitting, were doped into a host with a triplet energy between those of these two materials. Codoping the TADF materials to realize TED‐TADF in the low‐triplet‐energy host achieved a high external quantum efficiency of 28.3% and almost quadrupled the device lifetime relative to that of the reference device with only the emitting TADF material. Recycling the triplet excitons of the distributing TADF material that were quenched by the host and the singlet‐exciton‐harvested TADF emission of the emitting TADF material were the main mechanisms realizing simultaneous high efficiency and extended lifetime. The TADF devices based on this new approach combined with an optically optimized device stack demonstrated &gt;30% efficiency, a lifetime of &gt;6,000 h, and a y color coordinate &lt;0.10, which are unprecedented device metrics for deep‐blue OLEDs. This is the first work reporting such a device lifetime for the deep‐blue OLEDs with a y color coordinate below 0.09.

Three-micrometer-diameter needle electrode with an amplifier for extracellular in vivo recordings

Microscale needle-electrode devices offer neuronal signal recording capability in brain tissue; however, using needles of smaller geometry to minimize tissue damage causes degradation of electrical properties, including high electrical impedance and low signal-to-noise ratio (SNR) recording. We overcome these limitations using a device assembly technique that uses a single needle-topped amplifier package, called STACK, within a device of ∼1 × 1 mm2 Based on silicon (Si) growth technology, a <3-µm-tip-diameter, 400-µm-length needle electrode was fabricated on a Si block as the module. The high electrical impedance characteristics of the needle electrode were improved by stacking it on the other module of the amplifier. The STACK device exhibited a voltage gain of >0.98 (-0.175 dB), enabling recording of the local field potential and action potentials from the mouse brain in vivo with an improved SNR of 6.2. Additionally, the device allowed us to use a Bluetooth module to demonstrate wireless recording of these neuronal signals; the chronic experiment was also conducted using STACK-implanted mice.

Mitigating Dark Current for High-Performance Near-Infrared Organic Photodiodes via Charge Blocking and Defect Passivation.

Thin-film organic near-infrared (NIR) photodiodes can be essential building blocks in the rapidly emerging fields including the internet of things and wearable electronics. However, the demonstration of NIR organic photodiodes with not only high responsivity but also low dark current density that is comparable to that of inorganic photodiodes, for example, below 1 nA cm-2 for silicon photodiodes, remains a challenge. In this work, we have demonstrated non-fullerene acceptor-based NIR photodiodes with an ultralow dark current density of 0.2 nA cm-2 at -2 V by innovating on charge transport layers to mitigate the reverse charge injection and interfacial defect-induced current generation. The same device also shows a high external quantum efficiency approaching 70% at 850 nm and a specific detectivity of over 1013 Jones at wavelengths up to 940 nm. Furthermore, the versatility of our approach for mitigating dark current is demonstrated using a NIR photodetector utilizing different non-fullerene systems. Finally, the practical application of NIR organic photodiodes is demonstrated with an image sensor integrated on a silicon CMOS readout. This work provides new insight into the device stack design of low-dark current NIR organic photodiodes for weak light detection.

Small- and Large-Signal Dynamic Output Capacitance and Energy Loss in GaN-on-Si Power HEMTs

The origin of C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oss</sub> and the energy dissipation due to output capacitance can be roughly separated into two types: resistive loss and capacitive hysteresis loss. The resistive losses are due to the resistance of gallium nitride (GaN) buffer layers and Si-substrate. These loss components can be potentially improved by reengineering the doping concentrations in the buffer stacks and substrate layer. However, the capacitive hysteresis loss that is suspected to be due to trapping dynamics remains largely unexplained. This article presents a physics view of the trapping dynamics related to C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oss</sub> in the GaN high-electron-mobility transistor (HEMT) under small- and large-signal excitations. It is observed that, under large-signal excitation, traps with shallow energy levels (in the range of 0.5 eV above valence band) are mostly responsible for the charge imbalance and energy dissipation. Under small-signal conditions, deep traps with an energy level of around 0.98 eV from the valence band are the causes for the hysteresis at dc. By identifying dominating types of traps and relevant trap energy levels in GaN HEMT, we can gain important insights on how to optimize the device stack design for high-frequency applications.

Recent advancements and perspectives on light management and high performance in perovskite light-emitting diodes

Abstract Perovskite semiconductors have experienced meteoric rise in a variety of optoelectronic applications. With a strong foothold on photovoltaics, much focus now lies on their light emission applications. Rapid progress in materials engineering have led to the demonstration of external quantum efficiencies that surpass the previously established theoretical limits. However, there remains much scope to further optimize the light propagation inside the device stack through careful tailoring of the optical processes that take place at the bulk and interface levels. Photon recycling in the emitter material followed by efficient outcoupling can result in boosting external efficiencies up to 100%. In addition, the poor ambient and operational stability of these materials and devices restrict further commercialization efforts. With best operational lifetimes of only a few hours reported, there is a long way to go before perovskite LEDs can be perceived as reliable alternatives to more established technologies like organic or quantum dot-based LED devices. This review article starts with the discussions of the mechanism of luminescence in these perovskite materials and factors impacting it. It then looks at the possible routes to achieve efficient outcoupling through nanostructuring of the emitter and the substrate. Next, we analyse the instability issues of perovskite-based LEDs from a photophysical standpoint, taking into consideration the underlying phenomena pertaining to defects, and summarize recent advances in mitigating the same. Finally, we provide an outlook on the possible routes forward for the field and propose new avenues to maximally exploit the excellent light-emitting capabilities of this family of semiconductors.

Tuning oxygen vacancies and resistive switching properties in ultra-thin HfO2 RRAM via TiN bottom electrode and interface engineering

Resistive random access memory (RRAM) technologies based on non-volatile resistive filament redox switching oxides have the potential of drastically improving the performance of future mass-storage solutions. However, the physico-chemical properties of the TiN bottom metal electrode (BME) can significantly alter the resistive switching (RS) behavior of the oxygen-vacancy RRAM devices, yet the correlation between RS and the physico-chemical properties of TiN and HfOx/TiN interface remains unclear. Here, we establish this particular correlation via detailed material and electrical characterization for the purpose of achieving further performance enhancement of the stack integration. Two types of RRAM stacks were fabricated where the TiN BME was fabricated by physical vapor deposition (PVD) and atomic layer deposition (ALD), respectively. The HfOx layer in HfOx/PVD-TiN is more oxygen deficient than that of the HfOx/ALD-TiN because of more defective PVD-TiN and probably because pristine ALD-TiN has a thicker TiO2 overlayer. Higher concentration of oxygen vacancies induces a larger magnitude of band bending at the HfOx/PVD-TiN interface and leads to the formation of a higher Schottky barrier. Pulsed endurance measurements of up to 106 switches, with 10 μA ± 1.0 V pulses, demonstrate the potentialof the studied ultra-thin-HfOx/TiN device stack for dense, large scale, and low-power memory integration.

Role of bulk- and interface-defects in spectrum-dependent energy harvesting of CZTSSe-based indoor photovoltaic devices

In this work, we theoretically investigate spectrum dependent energy harvesting of a thin-film indoor photovoltaic (PV) device, taking into account the role of defects. By numerically solving Poisson’s equation and the continuity equation under optical generation-recombination conditions, performance characteristics of a Cu2ZnSn(S,Se)4-based thin-film PV device have been evaluated under spectrally varying white light emitting diodes (LEDs). Without any loss of generality, the results of the experimentally validated theoretical model suggest that a thin-film PV device becomes significantly tolerant to both bulk and interface defects when a fraction of blue emission in the white LED spectra remains relatively low. For a white LED having cool white emission characteristics, the efficiency of a CZTSSe-based PV device can equal the efficiency of the CZTSSe solar cell, the experimentally reported champion, while having about two orders of magnitude higher interface defect density, as well as about twenty times higher bulk defect density. In addition, for all practical densities of both types of defects, the efficiency of the indoor PV device remains at least 20% higher than the efficiency obtained under AM1.5 solar irradiation. The underlying reasons behind such observations have been traced back to the wavelength dependent carrier generation recombination dynamics of the thin-film device stack. The results of this work in effect provide guidelines for designing low-cost yet energy-efficient indoor photovoltaic devices with defect-rich thin film material systems.

Directional Polarized Light Emission from Thin-Film Light-Emitting Diodes.

Light-emitting diodes (LEDs) with directional and polarized light emission have many photonic applications, and beam shaping of these devices is fundamentally challenging because they are Lambertian light sources. In this work, using organic and perovskite LEDs (PeLEDs) for demonstrations, by selectively diffracting the transverse electric (TE) waveguide mode while suppressing other optical modes in a nanostructured LED, the authors first demonstrate highly directional light emission from a full-area organic LED with a small divergence angle less than 3° and a TE to transverse magnetic (TM) polarization extinction ratio of 13. The highly selective diffraction of only the TE waveguide mode is possible due to the planarization of the device stack by thermal evaporation and solution processing. Using this strategy, directional and polarized emission from a perovskite LED having a current efficiency 2.6 times compared to the reference planar device is further demonstrated. This large enhancement in efficiency in the PeLED is attributed to a larger contribution from the TE waveguide mode resulting from the high refractive index in perovskite materials.

High-performance methylammonium-free ideal-band-gap perovskite solar cells

Summary The development of mixed tin-lead (Sn-Pb)-based perovskite solar cells (PSCs) with low band gap (1.2–1.4 eV) has become critical not only for pushing single-junction devices toward the maximum efficiency given by the Shockley-Queisser limit, but also for enabling all-perovskite tandem devices beyond this limit. However, achieving high power-conversion efficiency (PCE) and long-term device operation stability simultaneously remains a significant challenge for Sn-Pb-based PSCs. Here, we demonstrate near ideal-band-gap (∼1.34 eV) methylammonium-free Sn-Pb-based PSCs with high efficiency (∼20%) and promising operational stability of maintaining >80% of initial PCE over 750 h under maximum-power-point tracking. The key to this success is the use of a SnCl2⋅3FACl complex additive that improves the microstructure and reduces the development of residual stress in the Sn-Pb perovskite thin films, which in turn enhances the efficiency and stability of the Sn-Pb-based ideal-band-gap PSCs.

Bipolar and Complementary Resistive Switching Characteristics and Neuromorphic System Simulation in a Pt/ZnO/TiN Synaptic Device.

In this work, a ZnO-based resistive switching memory device is characterized by using simplified electrical conduction models. The conventional bipolar resistive switching and complementary resistive switching modes are accomplished by tuning the bias voltage condition. The material and chemical information of the device stack including the interfacial layer of TiON is well confirmed by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analysis. The device exhibits uniform gradual bipolar resistive switching (BRS) with good endurance and self-compliance characteristics. Moreover, complementary resistive switching (CRS) is achieved by applying the compliance current at negative bias and increasing the voltage at positive bias. The synaptic behaviors such as long-term potentiation and long-term depression are emulated by applying consecutive pulse input to the device. The CRS mode has a higher array size in the cross-point array structure than the BRS mode due to more nonlinear I–V characteristics in the CRS mode. However, we reveal that the BRS mode shows a better pattern recognition rate than the CRS mode due to more uniform conductance update.

Buried Interfaces in Halide Perovskite Photovoltaics.

Understanding the fundamental properties of buried interfaces in perovskite photovoltaics is of paramount importance to the enhancement of device efficiency and stability. Nevertheless, accessing buried interfaces poses a sizeable challenge because of their non-exposed feature. Herein, the mystery of the buried interface in full device stacks is deciphered by combining advanced in situ spectroscopy techniques with a facile lift-off strategy. By establishing the microstructure-property relations, the basic losses at the contact interfaces are systematically presented, and it is found that the buried interface losses induced by both the sub-microscale extended imperfections and lead-halide inhomogeneities are major roadblocks toward improvement of device performance. The losses can be considerably mitigated by the use of a passivation-molecule-assisted microstructural reconstruction, which unlocks the full potential for improving device performance. The findings open a new avenue to understanding performance losses and thus the design of new passivation strategies to remove imperfections at the top surfaces and buried interfaces of perovskite photovoltaics, resulting in substantial enhancement in device performance.

Efficient and Stable Graded CsPbI3−xBrx Perovskite Solar Cells and Submodules by Orthogonal Processable Spray Coating

<h2>Summary</h2> Inorganic perovskite solar cells (PSCs) are promising for achieving long-term operational stability with good device performance. Here, we report the fabrication of inorganic CsPbI₂Br-based perovskite thin films with a well-defined CsPbI_3−xBr_x composition gradient in the surface region by a scalable, orthogonal processable spray-coating approach. The graded structure broadens absorption wavelength range and increases carrier lifetime, but it also causes electrical field redistribution within a device stack for more efficient charge separation and collection. With this approach, we obtained a power conversion efficiency of 16.81% for a 0.096-cm² PSC. We further demonstrated a monolithically integrated perovskite submodule based on the graded CsPbI_3−xBr_x by spray coating with an efficiency of 13.82% (112-cm² aperture area) and ∼9% degradation over 1,000-h continuous 1-sun light soaking.

Facile Approach for Improving Synaptic Modulation of Analog Resistive Characteristics by Embedding Oxygen Vacancies‐Rich Sub‐TaOx in Pt/Ta2O5/Ti Device Stacks

Facile Approach for Improving Synaptic Modulation of Analog Resistive Characteristics by Embedding Oxygen Vacancies‐Rich Sub‐TaO_<i>x</i> in Pt/Ta₂O₅/Ti Device Stacks

An Interface-Induced Dielectric Properties Degradation in Heterogeneous Stacked Device With P(VDF-TrFE)-Based Ferroelectric Polymers

Heterogeneous stacked electronics are one of the major topics in flexible electronics applications, which require high performances. At the same time, P(VDF-TrFE)-based polymers with unique characteristics, e.g., ferroelectric and flexible properties, also bring attention to heterogeneous stacked device applications. Since the interfacial characteristic has strong effects with performance and reliability, therefore, it becomes one of the key issues in P(VDF-TrFE)-based heterogeneous stacked devices. In this article, a mechanism of an interface trap charge-induced low-k interfacial layer was proposed and examined by experimental methods. To analyze the low-k interfacial layer effects, both general-doping (few 1015 cm−3) p-type and n-type silicon substrate were used. These experimental results showed that different types of silicon substrates have distinct interface properties. Then, different interface trap density ( $1.75\times 10^{12}$ – $4.1\times 10^{11}$ eV−1 cm−2) silicon substrates were fabricated to further analyze the low-k interfacial layer. These showed the substrate with the lowest interface trap density has less low-k interfacial effect than other substrates do. Based on these experimental verifications, the mechanism of interface trap charge-induced low-k interface characteristic was proposed. As a consequence, this interface trap–charge-induced behavior is a necessary consideration for applications of heterogeneous stacked devices.

Conduction and switching behavior of e-beam deposited polycrystalline Nb2O5 based nano-ionic memristor for non-volatile memory applications

Investigation has been done of bipolar resistive switching mechanism of Pt/Nb2O5/Pt and Bare conductive paint (BCP)/ Nb2O5/Pt memristive devices with orthorhombic phase prepared by e-beam evaporation method. The device exhibits reproducible switching with a ratio of (Roff/Ron) of 20 or higher which is good enough in read out for practical memory devices. The device exhibits reversible switching and better consistency during the switching from a high resistance state to a low resistance state and vice versa under a low operational voltage (<±1 V) due to its controlled formation and/or annihilation of conduction path. Switching mechanism is elucidated using a model invoking drift and diffusion of oxygen vacancies for the formation and rupture of conduction path during the Set and Reset process. The as deposited Nb2O5 thin film shows itself as amorphous in nature and transferred to orthorhombic phase by post annealing at 600 °C/30 min in O2 ambient. The reliability test of memristive device stack structures has been investigated by observing the retention of 600 min and reproducible switching through endurance test of over 1000 cycles which is better than the reported structures employing single layer of Nb2O5. The device does not show any performance degradation during the test.

Simulation of Monolithically Integrated Meta-Lens with Colloidal Quantum Dot Infrared Detectors for Enhanced Absorption

Colloidal quantum dots (CQDs) have been intensively investigated over the past decades in various fields for both light detection and emission applications due to their advantages like low cost, large-scale production, and tunable spectral absorption. However, current infrared CQD detectors still suffer from one common problem, which is the low absorption rate limited by CQD film thickness. Here, we report a simulation study of CQD infrared detectors with monolithically integrated meta-lenses as light concentrators. The design of the meta-lens for 4 μm infrared was investigated and simulation results show that light intensity in the focused region is ~20 times higher. Full device stacks were also simulated, and results show that, with a meta-lens, high absorption of 80% can be achieved even when the electric area of the CQD detectors was decreased by a factor of 64. With higher absorption and a smaller detector area, the employment of meta-lenses as optical concentrators could possibly improve the detectivity by a factor of 32. Therefore, we believe that integration of CQD infrared detectors with meta-lenses could serve as a promising route towards high performance infrared optoelectronics.