Optoelectronic Processes Research Articles

Coherent spin mixing at charge transfer states for spin polaron pair dissociation and energy loss in organic bulk heterojunction solar cells

Balancing the high photocurrent production and the nonradiative recombination suppression is of practical importance to design high-performance organic bulk heterojunction (BHJ) solar cells. Most efforts in this field have been directed towards the understanding of the electronic charge and photogenerated exciton related optoelectronic and transient processes. Little is known about the spin polaron pairs (PPs) dependent dissociation and nonradiative decay at charge transfer states (CTS), where the coherent spin mixing plays a key role. In this work, we combine magneto-photocurrent (MPC) and coherent spin mixing analyses for the IT- and Y-series, i.e., prototypical nonfullerene acceptors (NFA), based organic BHJ systems. We find that increasing polaron pair dissociation rates at singlet charge transfer states (CTS)S give rise to the photocurrent generation. Within the same systems, strong internal fields that include the hyperfine (HF) fields, spin-orbit coupling (SOC), and Δg value may promote the intersystem crossing (ISC) and the polaron back transfer (PBT), leading to the energy loss. By exploring the impact of the coherent spin mixing on the molecular functionality, our study opens new directions for improving photovoltaic performance of BHJ systems.

Two dimensional CuInP2S6/h-BN/MoTe2 van der Waals heterostructure phototransistors with double gate control

Ferroelectric materials enhance optoelectronic processes in CuInP2S6/h-BN/MoTe2 photodetectors, improving photocurrent, suppressing dark current, and achieving high responsivity, making them promising for high-performance optical sensing.

Enhanced Performance of a Self-Powered Au/WSe2/Ta2NiS5/Au Heterojunction by the Interfacial Pyro-phototronic Effect.

The growing need for wearable electronics and self-powered electronic devices has driven the successful development of self-powered two-dimensional (2D) photodetectors using the photovoltaic effect of Schottky and p-n junctions. However, there is an urgent need to develop multifunctional photodetectors capable of harvesting energy from different sources to overcome their limitations in efficiency and cost. While the pyro-phototronic effect has been shown to effectively influence optoelectronic processes in heterojunctions, the number of reported two-dimensional heterojunctions exhibiting interfacial pyroelectricity is still limited, and the responsivity and detectivity based on such heterojunctions tend to be low. In this study, a photodetector based on an Au/WSe2/Ta2NiS5/Au heterojunction was designed and fabricated. By harnessing the interfacial pyro-phototronic effect arising from the built-in electric fields at the Au/WSe2 Schottky junction and WSe2/Ta2NiS5 heterojunction, the photodetector exhibits a broadband response range of 405-1064 nm, with approximately 12 times enhancement in output current compared to solely relying on the photovoltaic effect. Under 660 nm light irradiation, the self-powered photodetector exhibits a responsivity of 121 mA/W, an external quantum efficiency of 22.64%, and a specific detectivity of 2 × 1012 Jones. Notably, its pyroelectric coefficient exceeds 8 × 103 μC·m-2·K-1. These findings pave the way for effectively detecting weak light and temperature variation while presenting a new strategy for developing high-performance photodetectors utilizing the interfacial pyro-phototronic effect.

Multiverse-weighted kinetic Monte Carlo and its application to modeling of optoelectronic processes in organic light-emitting diodes

Multiverse-weighted kinetic Monte Carlo and its application to modeling of optoelectronic processes in organic light-emitting diodes

Optoelectronic processes in ultrasonic spray coated organic solar cells

In this work, the role of a high boiling point green solvent additive Diphenyl ether (DPE) in device recombination processes in organic solar cell, fabricated using ultrasonic spray coating method is investigated. Without DPE and with 2.4 % DPE, the device series resistance was high of the order of 104 Ω. With 0.3 %, 0.6 %, 1.2 % DPE, series resistance was less. Transient studies show that, for these contents, the rise and decay times of photocurrent was less than 5 µs. For 0.3 % DPE, nearly 3/4th photovoltage decayed with 51.4 µs lifetime and the remaining decays with 4.6 µs lifetime. With higher DPE content (1.2 %), 97 % photovoltage decay was with a longer lifetime 46.1 µs, and remaining with 1.6 µs. With further more DPE content (2.4 %), only 1 % TPV decays with shorter lifetime of 11.6 µs and 99 % decay is in longer time scale of 672 µs. In addition, a longer lifetime of over 1000 µs was also observed for this case, indicating the presence of deep traps, in addition to the shallow traps and bimolecular recombination. Morphology studies show that for 0 % and 0.3 % DPE content, stacked structure of droplets was formed in the film, but with 0.6 % and higher content, the progressively impinging droplets intermix with each other, providing more time for the film to dry. It was shown that the contribution of trap assisted recombination for varying DPE content was due to disordered PTB7 or PC71BM phases, indicating a competition between PTB7 aggregation and PC71BM dispersion in the coated film.

Static versus dynamically polarizable environments within the many-body GW formalism.

Continuum- or discrete-polarizable models for the study of optoelectronic processes in embedded subsystems rely mostly on the restriction of the surrounding electronic dielectric response to its low frequency limit. Such a description hinges on the assumption that the electrons in the surrounding medium react instantaneously to any excitation in the central subsystem, thus treating the environment in the adiabatic limit. Exploiting a recently developed embedded GW formalism with an environment described at the fully abinitio level, we assess the merits of the adiabatic limit with respect to an environment where the full dynamics of the dielectric response are considered. Furthermore, we show how to properly take the static limit of the environment's susceptibility by introducing the so-called Coulomb-hole and screened-exchange contributions to the reaction field. As a first application, we consider a C60 molecule at the surface of a C60 crystal, namely, a case where the dynamics of the embedded and embedding subsystems are similar. The common adiabatic assumption, when properly treated, generates errors below 10% on the polarization energy associated with frontier energy levels and associated energy gaps. Finally, we consider a water molecule inside a metallic nanotube, the worst case for the environment's adiabatic limit. The error on the gap polarization energy remains below 10%, even though the error on the frontier orbital polarization energies can reach a few tenths of an electronvolt.

Imaging Valley Excitons in a 2D Semiconductor with Scanning Tunneling Microscope-Induced Luminescence.

Valley excitons dominate the optoelectronic response of transition-metal dichalcogenides and are drastically affected by structural and environmental inhomogeneities localized in these materials. Critical to understanding and controlling these nanoscale excitonic changes is the ability to correlate the imaging of excitonic states with crystalline structures on the atomic scale. Here, we apply scanning tunneling microscope-induced luminescence microscopy to image valley excitons in a semiconducting transition-metal dichalcogenide monolayer decoupled by a 10 nanometer-thick hexagonal-boron-nitride flake incorporated in a lateral homojunction on an Au electrode surface. This design enables the observation of chiral excitonic emission arising from neutral and charged valley excitons of the monolayer semiconductor at ambipolar voltages with a quantum efficiency up to ∼10-5 photon/electron. The measured light helicity demonstrates considerable circular polarization dependent on the sample voltage, reaching as much as 40%. The real-space luminescence imaging maps─at subnanometer resolution─of the valley excitons reveal striking spatial variations associated with localized inhomogeneities, including surface impurities and possibly nanoscale dielectric and/or potential disorders in the monolayer. Our study introduces a promising format for 2D materials to explore and tailor their optoelectronic processes at the atomic scale.

Features of optoelectronic processes in a molecular junction based on a fluorophore with optically active Frontier π-orbitals: electrofluorochromism in a ZnPc-based junction.

A model of the optoelectronic process in a molecular junction has been developed, in which electron transfer occurs through transmission channels associated with the filling of the π and π* orbitals of the fluorophore with transferred electrons. The contribution of each channel to the formation of current and electroluminescence (EL) is determined by the probability of the realization of those electronic states of the molecule that, at a given bias voltage, are involved in electron transfer. It is shown that in the vicinity of critical bias voltage, stepwise changes in current and EL occur, and the height of each step is controlled by kinetic processes associated with both electron transfer and intramolecular transitions. Using the obtained analytical expressions for the relative intensities of the emission lines X0 and X+ and comparing theoretical results with experimental data on STM-induced EL in a ZnPc-based junction, we showed that the method for analyzing the behavior of current and EL near critical voltages can serve as an effective tool for understanding the physical mechanisms responsible for optoelectronic processes at the single-molecule level. The method also made it possible to obtain real values of the energy of the Frontier orbitals of the ZnPc molecule embedded between the electrodes, as well as the energies of those electronic states of the neutral and charged molecules that participate in the optoelectronic process, including electrofluorochromism.

Broadband Self-Powered CdS ETL-Based MAPbI3 Heterojunction Photodetector Induced by a Photovoltaic-Pyroelectric-Thermoelectric Effect.

CdS, with a noncentrosymmetric structure, is thought as an important electron transport layer (ETL) in perovskite-based devices, but its pyroelectric effect, which can efficiently modulate the optoelectronic processes, is not well explored. In this work, a MAPbI3 heterojunction of CdS/MAPbI3/Spiro-OMeTAD with a c-axis preferred oxygen-doped CdS ETL is developed as a high-performance photodetector (PD). This PD exhibits a stable self-powered property in the spectral range of ∼360-780 nm due to its excellent photovoltaic effect. Moreover, the light-induced pyroelectric potential in the CdS ETL is demonstrated to be an efficient approach for improving the photoresponses, and different effects are observed for different laser irradiations, which can be well understood from their working mechanisms. Upon 450 nm laser irradiation, the photovoltage responsivity (R) is greatly improved from 596.9 to 6383.6 V/W with an increment of 1069.54%. In addition, the response spectrum is also extended outside the bandgap restriction of the MAPbI3 to 1550 nm due to both the pyroelectric and photothermoelectric effects, which is a big breakthrough for the perovskite heterojunction PD. Through turning the external bias voltage and ambient temperature, the coupling mechanisms of the pyroelectric and photovoltaic effects are further analyzed. This work provides an important understanding of designing the CdS ETL-based perovskite heterojunction for broadband high-performance photoelectric devices by introducing the pyroelectric effect.

Satellite Red Emission from a Single Green-Emissive MnBr42- Tetrahedron in Soft Hybrid Single Crystals.

Photoinduced self-trapped exciton emission is common in soft matter metal halide semiconductors, whereas analogous phenomena in metal halide insulators with localized emitting centers and delayed satellite emission have rarely been identified. In this study, a new zero-dimensional Mn(II) hybrid of [3DPTPP]MnBr4 (3DPTPP = (3-(dimethylamino)propyl)(triphenyl)phosphonium) with only one crystallographic Mn2+ site but dual emission is reported. The delayed red emission (∼630 nm) is successfully assigned to a satellite of the green-emissive (∼530 nm) MnBr42- tetrahedron shifted by N-H vibration (∼2500 cm-1), directly evidenced by the Raman spectra and further supported by density functional theory calculation. The photoluminescence decay curves demonstrate their same origin, but the red emission exhibits a delayed process. The temperature- and pressure-dependent PL spectra, temperature-dependent distortion of the MnBr42- tetrahedron, and light polarization spectra confirmed the consistency and distinctness of the dual emission. This study will inspire further research on self-trapped optoelectronic processes in soft metal halides.

Quantum Shell in a Shell: Engineering Colloidal Nanocrystals for a High-Intensity Excitation Regime.

Many optoelectronic processes in colloidal semiconductor nanocrystals (NCs) suffer an efficiency decline under high-intensity excitation. This issue is caused by Auger recombination of multiple excitons, which converts the NC energy into excess heat, reducing the efficiency and life span of NC-based devices, including photodetectors, X-ray scintillators, lasers, and high-brightness light-emitting diodes (LEDs). Recently, semiconductor quantum shells (QSs) have emerged as a promising NC geometry for the suppression of Auger decay; however, their optoelectronic performance has been hindered by surface-related carrier losses. Here, we address this issue by introducing quantum shells with a CdS-CdSe-CdS-ZnS core-shell-shell-shell multilayer structure. The ZnS barrier inhibits the surface carrier decay, which increases the photoluminescence (PL) quantum yield (QY) to 90% while retaining a high biexciton emission QY of 79%. The improved QS morphology allows demonstrating one of the longest Auger lifetimes reported for colloidal NCs to date. The reduction of nonradiative losses in QSs also leads to suppressed blinking in single nanoparticles and low-threshold amplified spontaneous emission. We expect that ZnS-encapsulated quantum shells will benefit many applications exploiting high-power optical or electrical excitation regimes.

Customizable 2D Covalent Organic Frameworks for Optoelectronic Applications

Comprehensive Summary2D covalent organic framework (2D‐COF), a modular three‐dimensional material, is easily influenced by the component module. The assembly of different functionalized modules gives 2D‐COF unique performance. The modular structure not only allows for customization but also allows for variety, which gives 2D‐COF a wide range of functions. Hence, many building blocks with catalytic, ligand, semiconductor, luminescent, and redox centers are integrated into the COF scaffold. The connection and assembly of such modules determine the nature of the final block material. The intra‐layer connections of the modules form a monolayer mesh chemical structure, and the subsequent stacking of the monolayer mesh structure produces the final crystalline porous material—2D‐COF. This review describes in detail the potential of COF materials as optoelectronic materials and our understanding of optoelectronic processes, starting from monolayer reticulation chemistry to final 3D stacked structures, thus establishing a new paradigm for the rational design of well‐defined novel 2D‐COF optoelectronic materials and devices.

Quantum shells versus quantum dots: suppressing Auger recombination in colloidal semiconductors.

Colloidal semiconductor nanocrystals (NCs) have attracted a great deal of attention in recent decades. The quantum efficiency of many optoelectronic processes based on these nanomaterials, however, declines with increasing optical or electrical excitation intensity. This issue is caused by Auger recombination of multiple excitons, which converts the NC energy into excess heat, whereby reducing the efficiency and lifespan of NC-based devices, including lasers, photodetectors, X-ray scintillators, and high-brightness LEDs. Recently, semiconductor quantum shells (QSs) have emerged as a viable nanoscale architecture for the suppression of Auger decay. The spherical-shell geometry of these nanostructures leads to a significant reduction of Auger decay rates, while exhibiting a near unity photoluminescence quantum yield. Here, we compare the optoelectronic properties of quantum shells against other low-dimensional semiconductors and discuss their emerging opportunities in solid-state lighting and energy-harvesting applications.

Inter-band and mid-gap luminescence in CH3NH3PbBr3 single crystal

Photoluminescence is informative to other optoelectronic processes that are less directly accessible in metal halide perovskites. In this study, we conducted optical studies on CH3NH3PbBr3 single crystals with robust cubic lattices and atomically flat surfaces. Inter-band luminescence peaking aside from the absorption cutoff was observed, and its shifts toward shorter wavelength upon cooling. Cooling the crystal below 150 K leads to the occurrence of mid-gap luminescence partly at the expense of the inter-band luminescence, which indicates bulk photoluminescence quenching by trap-mediated electronic trapping and subsequent mid-gap radiation following electronic de-trapping. Time-resolved photoluminescence probed at the mid-gap luminescence maxima shows two-stage mono-exponential traces, directly corroborating the trapping and de-trapping processes. In the presence of mid-gap luminescence at low temperatures, time-resolved inter-band luminescence demonstrates triplet decaying pathways, in sharp contrast to the doublet decaying pathways at higher temperatures in the absence of mid-gap luminescence. Time- and temperature-resolved inter-band photoluminescence gave rise to a characteristic photocarrier lifetimes evolution against temperature in a close analogy to the material's dielectric constant.

Features of gate-tunable and photon-field-controlled optoelectronic processes in a molecular junction: Application to a ZnPc-based transistor

Taking into account the fact that the transitions between the states of a molecular junction are carried out against the background of much faster relaxation processes in molecular terms and the conduction bands of electrodes, kinetic equations for integral occupancies of the molecular terms, as well as expressions for the time-dependent electronic current and radiation power of the fluorophore molecule, are obtained. Using the example of a molecular junction based on a ZnPc fluorophore, the transformation of a transient optoelectronic process into a stationary one is demonstrated. Corresponding analytical expressions are derived, including overall rates, which determine the characteristic times of establishing equilibrium current and light emission. The temporary process of the reorganization of the transmission channels dependently on the magnitude and polarity of the gate voltage as well as an external optical field is also demonstrated. The dependence of the overall rates on the elementary rates characterizing the recharge of the molecule, as well as radiation and nonradiative transitions in the molecule, is obtained. Estimates show that in a ZnPc-based transistor, the characteristic transition time is 10–100 ps if the current is in the range of 0.1–10 nA.

Real-time observation of delayed excited-state dynamics in InGaN/GaN quantum-wells by femtosecond transient absorption spectroscopy

This work employs femtosecond transient absorption spectroscopy to investigate the ultrafast carrier dynamics of bound states in In0.14Ga0.86N/GaN quantum wells. The ground state (GS) dynamics usually dominate these characteristics, appearing as a prominent peak in the absorption spectra. It is observed that the excited state also contributes to the overall dynamics, with its signature showing up later. The contributions of both the ground and excited states in the absorption spectra and time-resolved dynamics are decoupled in this work. The carrier density in the GS first increases and then decays with time. The carriers populate the excited state only at a delayed time. The dynamics are studied considering the Quantum-Confined Stark Effect-induced wavelength shift in the absorption. The relevant microscopic optoelectronic processes are understood phenomenologically, and their time constants are extracted. An accurate study of these dynamics provides fundamentally essential insights into the time-resolved dynamics in quantum-confined heterostructures and can facilitate the development of efficient light sources using GaN heterostructures.

Optoelectronic modeling of all-perovskite tandem solar cells with design rules to achieve >30% efficiency

All-perovskite tandem solar cells pave the way for increasing the efficiency of solar cells beyond the Shockley-Queisser limit imposed on single junction perovskite cells. While all-perovskite tandem solar cells with high power conversion efficiency (PCE) of 24.8% have been fabricated, the understanding of the underlying device physics during their operation still remains unclear. In this work we present an optoelectronic model for monolithic all-perovskite tandem solar cell. This coupled model incorporating transfer matrix model for optical effects, and drift diffusion model for electrical effects accurately describes the photophysical and optoelectronic processes occurring in the all-perovskite tandem cell. The results of this model have been verified with results from a recent published experimental work on all-perovskite tandem cell with 24% PCE and show good agreement. The key parameters governing the PCE of tandem solar cells, such as trap density in active layer, recombination layer resistance, carrier mobility and absorber layer thicknesses are evaluated to establish design rules to attain greater than 30% PCE for all-perovskite tandem solar cells. Even though this model has been developed for an all-perovskite tandem solar cell, the principle guiding the model is general and can be applied to any type of tandem photovoltaic technology.

A Review on the Materials Science and Device Physics of Semitransparent Organic Photovoltaics

In this review, the current state of materials science and the device physics of semitransparent organic solar cells is summarized. Relevant synthetic strategies to narrow the band gap of organic semiconducting molecules are outlined, and recent developments in the polymer donor and near-infrared absorbing acceptor materials are discussed. Next, an overview of transparent electrodes is given, including oxides, multi-stacks, thin metal, and solution processed electrodes, as well as considerations that are unique to ST-OPVs. The remainder of this review focuses on the device engineering of ST-OPVs. The figures of merit and the theoretical limitations of ST-OPVs are covered, as well as strategies to improve the light utilization efficiency. Lastly, the importance of creating an in-depth understanding of the device physics of ST-OPVs is emphasized and the existing works that answer fundamental questions about the inherent changes in the optoelectronic processes in transparent devices are presented in a condensed way. This last part outlines the changes that are unique for devices with increased transparency and the resulting implications, serving as a point of reference for the systematic development of next-generation ST-OPVs.

Generation of Free Carriers in MoSe2 Monolayers Via Energy Transfer from CsPbBr3 Nanocrystals

AbstractTransition metal dichalcogenide (TMDCs) monolayers make an excellent component in optoelectronic devices such as photodetectors and phototransistors. Selenide‐based TMDCs, specifically molybdenum diselenide (MoSe2) monolayers with low defect densities, show much faster photoresponses compared to their sulfide counterpart. However, the typically low absorption of the atomically thin MoSe2 monolayer and high exciton binding energy limit the photogeneration of charge carriers. Yet, integration of light‐harvesting materials with TMDCs can produce increased photocurrents via energy transfer. In this article, it is demonstrated that the interaction of cesium lead bromide (CsPbBr3) nanocrystals with MoSe2 monolayers results into an energy transfer efficiency of over 86%, as ascertained from the quenching and decay dynamics of the CsPbBr3 nanocrystals emission. Notably, the increase in the MoSe2 monolayer emission in the heterostructure accounts only for 33% of the transferred energy. It is found that part of the excess energy generates directly free carriers in the MoSe2 monolayer, as a result of the transfer of energy into the exciton continuum. The efficiency of the heterostructure via enhanced photocurrents with respect to the single material unit is proven. These results demonstrate a viable route to overcome the high exciton binding energy typical for TMDCs, as such having an impact on optoelectronic processes that rely on efficient exciton dissociation.

Pyro-phototronic effect enhanced self-powered photodetector

Pyro-phototronic effect has attracted much attention in fabricating self-powered high-performance photodetectors because of the significant enhancement by modulating the separation, transportation, and extraction of photo-generated electron-hole pairs in optoelectronic processes. This review highlights the advances in fabricating pyro-phototronic effect enhanced photodetectors. The fundamental research on the enhanced mechanism is discussed. Various device structures and figure of merit are summarized to demonstrate the improvement of performance by pyro-phototronic effect. Synergetic effects dominated by pyro-phototronic effect are also discussed to show their great potential and the universality of applying pyro-phototronic effect in photodetection. Finally, an outlook is delivered, and future research directions and challenges are discussed.