Charge Collection Process Research Articles

Recent Advancements in Enhancing Interfacial Charge Transport for Perovskite Solar Cells

Finding the most suitable pathways to improve the interfacial charge transportation in lead halide perovskite solar cells is a highly desirable research area to enhance device performance and enable commercialization. The complexities of interfacial charge dynamics, encompassing separation, diffusion, and collection processes, pivot on the thoughtful selection of interlayers and their inherent properties. Challenges arise from nonideal interfaces characterized by mismatched energy levels and defects that hinder efficient charge transport. To address these concerns, implementing tailored interfacial engineering strategies, including interlayer modification, band alignments, and passivation techniques, can help mitigate unwanted nonradiative recombination. This review aims to elucidate the impact of trap states on suppressing charge transport in the device, along with subsequent passivation techniques designed to enhance interfacial charge transport. Following that, a comprehensive overview is presented, highlighting recent advancements in interface engineering techniques that improve interfacial properties between the electron transport layer/perovskite and perovskite/hole transport layer. Significantly, the impact of using buffer and dipole layers as interlayers on overall device performance and stability is investigated.

Simulation Study on the Charge Collection Mechanism of FinFET Devices in Single-Event Upset.

Planar devices and FinFET devices exhibit significant differences in single-event upset (SEU) response and charge collection. However, the charge collection process during SEU in FinFET devices has not been thoroughly investigated. This article addresses this gap by establishing a FinFET SRAM simulation structure and employing simulation software to delve into the charge collection process of FinFET devices during single-event upset. The results reveal substantial differences in charge collection between NMOS and PMOS, and that direct incidence of PMOS leads to the phenomenon of multiple-node charge collection causing SRAM unit upset followed by recovery.

High-Performance Wearable Organic Photodetectors by Molecular Design and Green Solvent Processing for Pulse Oximetry and Photoplethysmography.

White-light detection from the visible to the near-infrared region is central to many applications such as high-speed cameras, autonomous vehicles, and wearable electronics. While organic photodetectors (OPDs) are being developed for such applications, several challenges must be overcome to produce scalable high-detectivity OPDs. This includes issues associated with low responsivity, narrow absorption range, and environmentally friendly device fabrication. Here, an OPD system processed from 2-methyltetrahydrofuran (2-MeTHF) sets a record in light detectivity, which is also comparable with commercially available silicon-based photodiodes is reported. The newly designed OPD is employed in wearable devices to monitor heart rate and blood oxygen saturation using a flexible OPD-based finger pulse oximeter. In achieving this, a framework for a detailed understanding of the structure-processing-property relationship in these OPDs is also developed. The bulk heterojunction (BHJ) thin films processed from 2-MeTHF are characterized at different length scales with advanced techniques. The BHJ morphology exhibits optimal intermixing and phase separation of donor and acceptor moieties, which facilitates the charge generation and collection process. Benefitting from high charge carrier mobilities and a low shunt leakage current, the newly developed OPD exhibits a specific detectivity of above 1012 Jones over 400-900nm, which is higher than those of reference devices processed from chlorobenzene andortho-xylene.

Examination of Trapping Effects on Single-Event Transients in GaN HEMTs

The effect of various trapping centers on single event-transients (SETs) in GaN high-electron-mobility transistors (HEMTs) are examined via calibrated technology computer aided design (TCAD) simulations. A single-channel, Schottky-gate HEMT computational model is developed and validated with static characteristics and single-photon absorption laser data. Simulations of SETs show that a two-dimensional electron gas (2DEG) enhancement effect occurs, which leads to additional collected charge in excess of the generated charge. The origins of this effect are examined and shown to result from an increase in occupation rate of the surface donor traps as the influx of charge is developed in the charge track. Variations in trap characteristics are then introduced and their effect on internal charge collection processes examined. It is further shown the SET response is dominated by the properties of the surface donor traps (i.e., energy level and density) such that the SET drain current closely follows the transient decay of occupation rate back to the original DC level.

19.10% Efficiency and 80.5% Fill Factor Layer-by-Layer Organic Solar Cells Realized by 4-Bis(2-Thienyl)Pyrrole-2,5-Dione Based Polymer Additives for Inducing Vertical Segregation Morphology.

The morphology plays a key role in determining the charge generation and collection process, thus impacting the performances of organic solar cells (OSCs). The limited selection pool of additives to optimize the morphology of OSCs, especially for the emerging layer-by-layer (LbL) OSCs, impeding the improvements of photovoltaic performances. Herein, a new method of using conjugated polymers as the additives to optimize the morphology for improving the photovoltaic performances of LbL-OSCs is reported. Four polymers of PH, PS, PF, and PCl are developed with different side chains. These polymers exhibit poor performances as donor materials and additives in the BHJ devices, due to the unsuitable energy level alignment and unfavorable molecular interactions. By contrast, they can be served as efficient additives to optimize the PM6 fibril matrix for facilitating the penetration of BTP-eC9 and forming an intertwined D/A bicontinuous network with a vertical segregation. Such morphology is optimized by side chain engineering, which enables the progressive improvement of the charge separation and collection. As a result, adding a small amount of PCl as the additive, the optimized morphology contributes to a champion PCE of 19.10% with a high FF of 80.5%.

Signal amplification in multistage charge collection process in Si detectors in situ irradiated at superfluid helium temperature

The effect of charge multiplication in Si p+-n-n+ detectors observed in the unique conditions of the in situ irradiation with 23 GeV proton beam fragmented into 400 ms spills and the temperature of superfluid helium, 1.9 K, was analyzed. The current pulse responses of the detector irradiated to 5 × 1013 p/cm2 demonstrated a two-stage process of charge collection, in which, in the first stage, the current was induced by holes, and in the second, by electrons arising due to avalanche multiplication produced by holes. In the detector irradiated to 2.7 × 1014 p/cm2, the third stage of charge collection was observed that was related to avalanche multiplication produced by electrons reaching the n+ contact and sequential drift of generated holes back to the p+ contact. The developed procedure of data treatment allowed extracting the full set of carrier transport parameters and internal charge gain rising to 3.7 at the bias voltage of 400 V and the fluence of 5 × 1013 p/cm2. An evolution of charge multiplication at the p+ contact initiated by holes originated from a high rate of the hole trap introduction at 1.9 K described by a fivefold rise in the trapping probability at 1.9 K in comparison with its value at T = -10°C. This increases the electric field near the p+ contact and leads to the avalanche multiplication affecting the following stages of charge collection.

2D Transition Metal Dichalcogenides‐Based Electrocatalysts for Hydrogen Evolution Reaction

AbstractHydrogen is an efficient, clean, and economical energy source, owing to its huge energy density. Electrochemical water splitting is a potential candidate for inexpensive and eco‐friendly hydrogen production. Recently, the development of 2D transition metal chalcogenides (TMDs) nanomaterials with a variety of physicochemical properties has shown their potential as eminent non‐noble metal‐based nanoscale electrocatalysts for hydrogen evolution. Nanostructuring such materials induces deep modification of their functionalities, compared to their bulk counterparts. High density of different types of exposed active sites is formed, and the small diffusion paths, which enhances the electron transfer in the 2D structures, can successfully aid the charge collection process in the electrocatalytic hydrogen evolution reactions. In this review, the key parameters to improve the catalyst performance of 2D TMDs in electrochemical hydrogen evolution reaction (HER) processes are discussed in detail and the most recent developments in the field are summarized, focusing on the improvement of the electrocatalytic activity of 2D TMDs. This review delivers deep insight for the clear understanding of the potential of 2D TMDs nanoscale materials as electrocatalysts for HER, suggesting the development of new type of catalyst with efficient activity in HER as well as other renewable energy fields.

Versatile Sequential Casting Processing for Highly Efficient and Stable Binary Organic Photovoltaics.

Forming an ideal bulk heterojunction (BHJ) morphology is a critical issue governing the photon to electron process in organic solar cells (OSCs). Complementary to the widely-used blend casting (BC) method for BHJ construction, sequential casting (SC) can also enable similar or even better morphology and device performance for OSCs. Here, BC and SC methods on three representative donor:acceptor (D:A) blends are utilized, that is, PM6:PC71 BM, PM6:IT-4F and PM6:L8-BO. Higher power conversion efficiencies (PCEs) in all cases by taking advantage of beneficial morphology from SC processing are achieved, and a champion PCE of 18.86% (certified as 18.44%) based on the PM6:L8-BO blend is reached, representing the record value among binary OSCs. The observations on phase separation and vertical distribution inspire the proposal of the swelling-intercalation phase-separation model to interpret the morphology evolution during SC processing. Further, the vertical phase segregation is found to deliver an improvement of device performance via affecting the charge transport and collection processes, as evidenced by the D:A-ratio-dependent photovoltaic properties. Besides, OSCs based on SC processing show advantages on device photostability and upscale fabrication. This work demonstrates the versatility and efficacy of the SC method for BHJ-based OSCs.

Self‐Assembly Metal Chelate as Ultraviolet Filterable Interface Layer for Efficient Organic Solar Cells

AbstractInterface engineering plays a vital role in the further improvement of efficiency and stability for organic solar cells (OSCs). Herein, a self‐assembly metal chelate based on hafnium and a designed ligand, N‐(4‐(3‐oxobutanoyl)phenyl)acetamide (ACBN) is applied as both interfacial modification layer and UV‐light filter in OSCs. The strong hydrogen‐bond induced intermolecular interaction enables Hf(ACBN)4 with the prerequisite of adequate solvent resistance to work as an electron transport layer (ETL) in the inverted OSCs. The self‐assembly behavior of Hf(ACBN)4 on the SnO2 film surface via constructing compact coordination structure has been verified via systematic theory calculations. In addition to optimizing the energy level alignment, the Hf(ACBN)4 modification effectively passivates the surface defect of SnO2 films for less surface charge recombination and a more efficient charge collection process. Thus, the OSCs with Hf(ACBN)4 layer yield a maximum PCE of 18.1%, better than that based on the bare SnO2 layer. Moreover, beneficial from the reduced oxygen vacancies via coordination effect and the UV‐light filter function of Hf(ACBN)4, the OSCs based on SnO2/ Hf(ACBN)4 composite ETL exhibit preferable stabilities under UV‐light irradiation or continuous operational conditions.

The Evolution of Classical Spiro-OMeTAD: Synthesis of Arylamine Endcapped Indenone Spirofluorene.

Spiro-OMeTAD is the well-known hole transporting material (HTM) in perovskite solar cells. In this work, its derivatives, namely four D-A shaped triphenylamine or biphenylamine endcapped indenone spirofluorene (SFD-TPA, SFD-OMeTPA, SFD-TAD, and SFD-OMeTAD), were designed and synthesized. With the introduction of electron-donating moieties and the extension of conjugation length, a series of changes in photophysical and electrochemical properties could be detected. Notably, in comparison with the optical gap (2.96 eV) of the reported spiro-OMeTAD, SFD-OMeTAD presents an optical gap as low as 1.87 eV. Moreover, density functional theory simulations were employed to further investigate their geometric and electronic structures. Finally, steady-state photoluminescence measurements proved the efficient charge separation and collection processes at the perovskite/HTM interface. It can be predicted that all four compounds with enhanced sunlight absorption capability and suitable frontier energy levels can be used as hole-transporting materials for perovskite solar cells.

Highly efficient CsPbI3/Cs1-xDMAxPbI3 bulk heterojunction perovskite solar cell

<h2>Summary</h2> The open-circuit voltage (<i>V</i>_OC) of cesium-based perovskite solar cells (Cs-PSCs) is severely limited by carrier recombination both in bulk and at the interface of the perovskite layer, which mainly roots in its elevated fabrication temperature. Enhancing the quality of perovskite film and optimizing the device structure are effective strategies to improve the performance of Cs-PSCs. Here, the <i>V</i>_OC is markedly promoted by constructing CsPbI₃/Cs_1-xDMA_xPbI₃ bulk heterojunction (BHJ) structure, which was spontaneously formed by a precisely controlled spatially selective phase transition method. The perovskite BHJ structure not only facilitates the charge separation and collection process by enhancing the built-in potential but also obviously reduces the carrier recombination loss. Importantly, a maximum <i>V</i>_OC of 1.23 V was obtained as the <i>V</i>_OC deficit was 0.45 V, and the champion power conversion efficiency (PCE) reached 20.32% (certified PCE of 19.66%). Our finding indicates that junction engineering will be a new strategy for efficient perovskite devices.

High resolution and time resolved photoemission spectroscopy for developing more efficient materials to reduce energy consumption and increase renewable energy production

Due to the increase of energy consumption and the resulting ecological challenge, a collective awareness leads to the development of renewable energies and more efficient materials to increase the green energy production. Development of efficient photovoltaic materials is very closely related to their chemical and electronic properties. A better knowledge of these imbricated properties is needed, in addition to a better comprehension of their interplay with charge transport mechanisms. Exciton creation and recombination processes, charge transfer and charge collection processes take place at the surface and interface of the photoactive materials. Photoemission spectroscopy as chemical specific and surface sensitive spectroscopic technique is a method of choice on the study of physical phenomena at the origin of photoconversion efficiency. Time resolved photoemission spectroscopy has been recently renewed interest covering time scale from fs to more than seconds. It permits to probe the dynamics of relaxation of photoexcited charges and determine their lifetime. It finds application in various materials used in solar photovoltaics. In this paper, we define the physical and chemical properties determined by the combination of high resolution and time resolved photoemission spectroscopy. We show examples dealing with the development of renewable energy and energy consumption reduction in agreement with the current ecological trend for a better future.

Reconciling models of interfacial state kinetics and device performance in organic solar cells: impact of the energy offsets on the power conversion efficiency.

Achieving the simultaneous increases in the open circuit voltage (Voc), short circuit current (Jsc) and fill factor (FF) necessary to further increase the power conversion efficiency (PCE) of organic photovoltaics (OPV) requires a unified understanding of how molecular and device parameters affect all three characteristics. In this contribution, we introduce a framework that for the first time combines different models that have been used separately to describe the different steps of the charge generation and collection processes in OPV devices: a semi-classical rate model for charge recombination processes in OPV devices, zero-dimensional kinetic models for the photogeneration process and exciton dissociation and one-dimensional semiconductor device models. Using this unified multi-scale model in conjunction with experimental techniques (time-resolved absorption spectroscopy, steady-state and transient optoelectronic measurements) that probe the various steps involved in charge generation we can shed light on how the energy offsets in a series of polymer: non-fullerene devices affect the charge carrier generation, collection, and recombination properties of the devices. We find that changing the energy levels of the donor significantly affects not only the transition rates between local-exciton (LE) and charge-transfer (CT) states, but also significantly changes the transition rates between CT and charge-separated (CS) states, challenging the commonly accepted picture of charge generation and recombination. These results show that in order to obtain an accurate picture of charge generation in OPV devices, a variety of different experimental techniques under different conditions in conjunction with a comprehensive model of processes occurring at different time-scales are required.

Solution-Processed, Inverted AgBiS2 Nanocrystal Solar Cells.

AgBiS2 nanocrystals are a promising nontoxic alternative to PbS, CsPbI3, and CdS quantum dots for solution-fabricated nanocrystal photovoltaics. In this work, we fabricated the first inverted (p-i-n) structure AgBiS2 nanocrystal solar cells. We selected spray-coated NiO as the hole-transporting material and used PCBM/BCP as the electron-transporting material. Combining transient photocurrent and photovoltage measurements with femtosecond transient absorption spectroscopy, we investigated the charge collection process on metal oxide/AgBiS2 interfaces and demonstrated that the NiO/AgBiS2 NC junction in the p-i-n configuration is more efficient for charge carrier collection. The fabricated p-i-n solar cells exhibited a 4.3% power conversion efficiency (PCE), which was higher than that of conventional n-i-p solar cells fabricated using the same sample. Additionally, inverted devices showed an ultrahigh short-circuit current (JSC) over 20.7 mA cm-2 and 0.38 V open-circuit voltage (VOC), suggesting their potential for further improvements in efficiency and, eventually, for large-scale production.

Easy-processing saccharin doped ZnO electron extraction layer in efficient polymer solar cells

The performance of polymer solar cells is known to be critically dependent on the characters of the charge extraction layers. In this work, an efficient electron extraction layer (EEL) is developed by doping saccharin into precursor of ZnO. It shows that the introduction of saccharin helps to passivate defects in ZnO EEL and polish the interfacial contact, which lead to efficient charge transport and collection processes, suppressed charge recombination loss, and increased light photon utilization. In poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2–6-diyl)]:[6,6]-phenyl C71-butyric acid methyl ester (PTB7-Th:PC71BM) based cells, over 10% enhancement of power conversion efficiency (PCE) is obtained compared to that of the reference cell with pristine ZnO as EEL. This work proves that the strategy of precursor additive has potential to prepare highly efficient EEL for polymer solar cells.

Single-Event Latchup sensitivity: Temperature effects and the role of the collected charge

Single-Event Latchup (SEL) is considered as a major reliability issue for the CMOS technology due to its capability of permanently damaging electronic components. In this work, the impact of temperature variation on the SEL mechanism is investigated. As the SEL sensitivity is influenced by design and environment parameters, the temperature variation is also evaluated along the variation of three parameters related to the geometry and to the design of the component: the doping profile, the anode to cathode spacing (A-C spacing) and the substrate and well taps placement. Moreover, the charge collection process has been analyzed. The goal was to verify whether the concept of critical charge, through studying the collected charge by the source implants, can be used for SEL, as it is used for upsets. 2D TCAD simulations have been performed, using an NPNP structure based on 65 nm CMOS inverter. From these simulations, we have analyzed the threshold LET and SEL rate. Results show that temperature impact is stronger when the component is less sensitive to SEL. Moreover, charge collected has shown promising results about its usage for SEL.

Study of charge sharing effect in a GaAs:Cr-based Timepix3 detector

Hybrid pixel semiconductor detectors find more and more applications in modern experimental physical setups. In particular, pixelated detectors based on GaAs:Cr sensor and Timepix3 chip are used in R&D for a state-of-the-art Transition Radiation Detector prototype at CERN. Motivation and usage aspects for GaAs:Cr-Timepix3 device in the experiment are covered in the talk. Authors” contribution to the work of the research group is a study of charge collection and transportation processes in the sensor including fluorescence and so-called charge sharing effect. These are able to significantly affect both spatial and energy resolution of the system. Estimates for the effects are obtained via numerical modelling and then used to analyze the impact on the detector performance.

Charge collection properties of TowerJazz 180 nm CMOS Pixel Sensors in dependence of pixel geometries and bias parameters, studied using a dedicated test-vehicle: The Investigator chip

This paper contains a compilation of parameters influencing the charge collection process extracted from a comprehensive study of partially depleted Monolithic Active Pixel Sensors with small (<25µm2) collection electrodes fabricated in the TowerJazz 180nm CMOS process. These results gave guidance for the optimization of the diode implemented in ALPIDE, the chip used in the second generation Inner Tracking System of ALICE, and serve as reference for future simulation studies of similar devices. The studied parameters include: reverse substrate bias, epitaxial layer thickness, charge collection electrode size and the spacing of the electrode to surrounding in-pixel electronics. The results from pixels of 28µm pitch confirm that even in partially depleted circuits, charge collection can be fast (<10ns), and quantify the influence of the parameters onto the signal sharing and amplitudes, highlighting the importance of a correct spacing between wells and of the impact of the reverse substrate bias.

Serial Switch Only Rectifier as a Power Conditioning Circuit for Electric Field Energy Harvesting

Because traditional electronics cannot directly use the alternating output voltage and current provided by electric field energy harvesters, harvesting systems require additional regulating and conditioning circuits. In this field, this work presents a conditioning circuit, called serial switch-only rectifier (SSOR) for low-voltage electric field energy harvesting (EFEH) applications. The proposed approach consists of a tubular topology harvester mounted on the outer jacket of a 230 V three-wires electrical cable (neutral, ground, and phase), in which terminals are connected to SSOR. We compare SSOR performance with classic electronic approaches, such as a full-bridge rectifier and voltage doubler. Experimental findings showed that the gathered energy by a 1 m cylindrical harvester increased in approximately 73.3% using the SSOR as a power management circuit. Experimental findings showed that the gathered energy by a 1 m cylindrical harvester increase in approximately 73.3% using the SSOR as a power management circuit. This increase is principally due to the fact that a serial bidirectional switch disconnects the harvester from the rest of the management circuit, enhancing the charge collection process. Although simulated results disclosed that SSOR increased collected energy for smaller-scale harvesters (experimental tests obtained using a 10 cm cylindrical harvester), additional losses in bidirectional switch reduced its performance. In addition, we introduce a comprehensive analysis of EFEH systems based on SSOR according to the mains frequency for future power systems.

Solution-processed Ga-TiO2 electron transport layer for efficient inverted organic solar cells

To boost the charge transport and collection processes in inverted organic solar device, gallium (Ga) was added to conventional TiO2 film to form the Ga-TiO2 film. Compared to the pristine film, the Ga-TiO2 film showed better energy level alignment, which is in good agreement with the heightened open-circuit voltage and fill factor. Attributed to the superior electron transport, the Ga-TiO2 based solar cell achieved paramount power conversion efficiency (PCE) of 7.72%, while the pristine TiO2 based solar cell showed lower PCE of 6.65%.