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Investigation of Oxygen Passivation for High-Performance All-Inorganic Perovskite Solar Cells
摘要： Defect passivation using oxygen has been identified as an efficient and convenient approach to suppress non-radiative recombination and improve the photovoltaic performance of hybrid organic-inorganic halide perovskites (HHPs). However, oxygen can seriously undermine the chemical stability of HHPs due to the reaction of superoxide with protonated organic cations such as CH3NH3+ and [(NH2)2CH]+, thus hindering the deep understanding of how oxygen affects their defect properties. Here we substitute free-proton inorganic Cs+ for organic moiety to avoid the negative effect of oxygen and then systematically investigate the oxygen passivation mechanism in all-inorganic halide perovskites (IHPs) from theory to experiment. We find that, in contrast to conventional oxygen molecule passivation just through physisorption on the surface of perovskites, oxygen atom can provide better passivation effect due to its stronger interaction with perovskites. The key point to achieve O-passivated perovskites rather than O2 is the dry-air processing condition, which can dissociate the O2 into O during the annealing process. O-passivated IHP solar cells exhibit enhanced power conversion efficiency (PCE) and better air stability than O2-passivated cells. These results not only provide deep insights into the passivation effect of oxygen on perovskites, but also demonstrate the great potential of IHPs for high photovoltaic performance with simplified ambient processing.
关键词： oxygen passivation,perovskite solar cells,CsPbI2Br,trap states,photovoltaics
更新于2025-09-19 17:13:59
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Trap States, Electric Fields, and Phase Segregation in Mixeda??Halide Perovskite Photovoltaic Devices
摘要： Mixed-halide perovskites are essential for use in all-perovskite or perovskite–silicon tandem solar cells due to their tunable bandgap. However, trap states and halide segregation currently present the two main challenges for efficient mixed-halide perovskite technologies. Here photoluminescence techniques are used to study trap states and halide segregation in full mixed-halide perovskite photovoltaic devices. This work identifies three distinct defect species in the perovskite material: a charged, mobile defect that traps charge-carriers in the perovskite, a charge-neutral defect that induces halide segregation, and a charged, mobile defect that screens the perovskite from external electric fields. These three defects are proposed to be MA+ interstitials, crystal distortions, and halide vacancies and/or interstitials, respectively. Finally, external quantum efficiency measurements show that photoexcited charge-carriers can be extracted from the iodide-rich low-bandgap regions of the phase-segregated perovskite formed under illumination, suggesting the existence of charge-carrier percolation pathways through grain boundaries where phase-segregation may occur.
关键词： halide segregation,perovskites,electric fields,trap states,photovoltaic devices
更新于2025-09-16 10:30:52
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Sensitivity of Sub-bandgap External Quantum Efficiency Measurements of Solar Cells under Electrical and Light Bias
摘要： The measurement of the external quantum efficiency (EQE) for photo-current generation at photon energies below the bandgap of semiconductors has always been an important tool for understanding phenomena such as Urbach tail and trap state dynamics. The shape of the sub-gap EQE can also reveal the subtle but important physics of inter-and-intramolecular states that lay at the heart of charge photogeneration in molecular systems such as organic semiconductors. In this work, we examine the influence of optical and electrical noise on the sensitivity of EQE measurements under different electrical and optical bias conditions and demonstrate how to enhance the dynamic range to an unprecedented >100 dB. We identify and study several apparatus-and-device-related factors limiting the sensitivity including: the electrical noise floor of measurement system; flicker and pick-up noise; illumination source stray light; the photon noise of the light bias source; the electrical noise of the voltage bias source; and shunt-resistance-limited thermal and electrical shot noise of the device. By understanding and minimizing the influence of these factors we are able to detect EQE signals derived from weak sub-gap absorption features in both organic and inorganic solar cell systems at photon energies well below their bandgaps. We area also able to observe sub-gap low finesse cavity interference effects which are sometimes confused with, for example, directly stimulated charge transfer state photo-current.
关键词： Photocurrent spectroscopy,Sub-bandgap absorption,Sensitive external quantum efficiency,Trap states,Solar cells,Charge transfer states
更新于2025-09-12 10:27:22
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Organic Photodetectors and their Application in Large Area and Flexible Image Sensors: The Role of Dark Current
摘要： Organic photodetectors (OPDs) have gained increasing interest as they offer cost-effective fabrication methods using low temperature processes, making them particularly attractive for large area image detectors on lightweight flexible plastic substrates. Moreover, their photophysical and optoelectronic properties can be tuned both at a material and device level. Visible-light OPDs are proposed for use in indirect-conversion X-ray detectors, fingerprint scanners, and intelligent surfaces for gesture recognition. Near-infrared OPDs find applications in biomedical imaging and optical communications. For most applications, minimizing the OPD dark current density (Jd) is crucial to improve important figures of merits such as the signal-to-noise ratio, the linear dynamic range, and the specific detectivity (D*). Here, a quantitative analysis of the intrinsic dark current processes shows that charge injection from the electrodes is the dominant contribution to Jd in OPDs. Jd reduction is typically addressed by fine-tuning the active layer energetics and stratification or by using charge blocking layers. Yet, most experimental Jd values are higher than the calculated intrinsic limit. Possible reasons for this deviation are discussed, including extrinsic defects in the photoactive layer and the presence of trap states. This provides the reader with guidelines to improve the OPD performances in view of imaging applications.
关键词： large area image sensors,charge injection,trap states,dark current,flexible image sensors,organic photodetectors
更新于2025-09-12 10:27:22
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Simulating nanocrystal-based solar cells: A lead sulfide case study
摘要： Nanocrystal-based solar cells are promising candidates for next generation photovoltaic applications; however, the most recent improvements to the device chemistry and architecture have been mostly trial-and-error based advancements. Due to complex interdependencies among parameters, determining factors that limit overall solar cell efficiency are not trivial. Furthermore, many of the underlying chemical and physical parameters of nanocrystal-based solar cells have only recently been understood and quantified. Here, we show that this new understanding of interfaces, transport, and origin of trap states in nanocrystal-based semiconductors can be integrated into simulation tools, based on 1D drift-diffusion models. Using input parameters measured in independent experiments, we find excellent agreement between experimentally measured and simulated PbS nanocrystal solar cell behavior without having to fit any parameters. We then use this simulation to understand the impact of interfaces, charge carrier mobility, and trap-assisted recombination on nanocrystal performance. We find that careful engineering of the interface between the nanocrystals and the current collector is crucial for an optimal open-circuit voltage. We also show that in the regime of trap-state densities found in PbS nanocrystal solar cells (～1017 cm?3), device performance exhibits strong dependence on the trap state density, explaining the sensitivity of power conversion efficiency to small changes in nanocrystal synthesis and nanocrystal thin-film deposition that has been reported in the literature. Based on these findings, we propose a systematic approach to nanocrystal solar cell optimization. Our method for incorporating parameters into simulations presented and validated here can be adopted to speed up the understanding and development of all types of nanocrystal-based solar cells.
关键词： nanocrystal-based solar cells,simulation,charge carrier mobility,lead sulfide,drift-diffusion models,trap states
更新于2025-09-12 10:27:22
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Fast Electron and Slow Hole Relaxation in InP-Based Colloidal Quantum Dots
摘要： Colloidal InP-based quantum dots are a promising material for light-emitting applications as an environment friendly alternative to their Cd-containing counterparts. Especially for their use in optoelectronic devices, it is essential to understand how charge carriers relax to the emitting state after injection with excess energy and if all of them arrive at this desired state. Herein, we report on time-resolved differential transmission measurements on colloidal InP/ZnS and InP/ZnSe core/shell quantum dots. By optically exciting and probing individual transitions, we are able to distinguish between electron and hole relaxation. This in turn allows us to determine how the initial excess energy of the charge carriers affects the relaxation processes. According to the electronic level scheme, one expects a strong phonon bottleneck for electrons, while holes should relax easier as their energy levels are more closely spaced. On the contrary, we find that electrons relax faster than holes. The fast electron relaxation occurs via an efficient Auger-like electron-hole scattering mechanism. On the other hand, a small wave function overlap between core and shell states slows the hole relaxation down. Additionally, holes can be trapped at the core/shell interface leading to either slow detrapping or nonradiative recombination. Overall, these results demonstrate that it is crucial to construct devices enabling the injection of charge carriers energetically close to their emitting states in order to maximize the radiative efficiency of the system.
关键词： InP,differential transmission spectroscopy,phonon bottleneck,charge carrier relaxation,trap states,electron-hole scattering,colloidal quantum dots
更新于2025-09-11 14:15:04
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Spontaneous Enhancement of the Stable Power Conversion Efficiency in Perovskite Solar Cells
摘要： The power conversion efficiency (PCE) of lead-halide perovskite solar cells (PSCs) is reported to increase over a period of days after their fabrication while they are stored in dark. Thus far, effects underlying this spontaneous enhancement are not understood. This work investigates the phenomenon for a variety of multi-cation-halide PSCs with different perovskite compositions and architectures. The observations reveal that spontaneous enhancement is not restricted to specific charge-transport layers or perovskite compositions. The highest PCE observed in this study is the enhanced stable PCE of 19% (increased by 4% absolute). An increased open-circuit voltage is the primary contributor to the improved efficiency. Using time-resolved photoluminescence measurements, initially-present low-energy states are identified that disappear over a storage period of a few days. Furthermore, trap states probed by thermally stimulated current technique exist in pristine PSCs and strikingly decrease for stored devices. In addition, ideality factor approaches unity and X-ray diffraction analyses show a lattice strain relaxation over the same period of time. These observations indicate that spontaneous enhancement of the PSCs is based on a reduction in trap-assisted non-radiative recombination possibly due to strain relaxation. Considering the demonstrated generality of spontaneous enhancement for different compositions of multi-cation-halide PSCs, our results highlight the importance of determining absolute PCE increase initiated by spontaneous enhancement for developing high-efficiency PSCs.
关键词： spontaneous enhancement,perovskite solar cells,power conversion efficiency,trap states,strain relaxation
更新于2025-09-11 14:15:04
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Photoluminescence Decay of Colloidal Quantum Dots: Reversible Trapping and the Nature of the Relevant Trap States
摘要： Interfaces are crucial factors in shaping the properties of colloidal quantum dots (QDs), in particular the size-dependent optical properties that are a hallmark of these materials. However, the role played by the interfaces associated with QDs on the kinetics of photoluminescence (PL) decay of these nanocrystals is not fully understood even for the most extensively investigated II-VI QDs. In particular, interfaces are a hotbed of trapping sites over which control is essential for efficient performance of QD-based devices, because traps condition PL lifetimes and may be related to PL intermittency. In this work, we analyze the room-temperature PL decay of drop-cast films of CdSe/ZnS QDs varying a number of factors (casting solvent, capping ligands, core/shell interface character). We show how the use of a function that accounts for reversible trapping of photogenerated charge carriers with physically meaningful parameters (time constant, trapping and detrapping rate constants, and average number of traps per QD) can provide valuable information concerning the relevant interfaces, and therefore the nature of the trap states, involved in the recombination of those charge carriers. This approach should be applicable to QDs of a variety of compositions as well as materials beyond inorganic semiconductors.
关键词： photoluminescence decay,trap states,reversible trapping,CdSe/ZnS QDs,colloidal quantum dots
更新于2025-09-11 14:15:04
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Broadband Defects Emission and Enhanced Ligand Raman Scattering in 0D Cs <sub/>3</sub> Bi <sub/>2</sub> I <sub/>9</sub> Colloidal Nanocrystals
摘要： Excitonic 0D and 2D lead-halide perovskites have been recently developed and investigated as new materials for light generation. Here broadband (>1 eV) emission from newly synthesized 0D lead-free colloidal Cs3Bi2I9 nanocrystals (NCs) is reported. The nature of their emissive states as well as the relative dynamics which are currently hotly debated are investigated. In particular, it is found that the broadband emission is made by the coexistence of emissive excitons and sub-bandgap emissive trap-states. Remarkably, evidence of enhanced Raman scattering from the ligands is observed when attached to the NCs surface, an effect that is preliminarily attributed to strong exciton-ligands electronic coupling in these systems.
关键词： trap-states emission,enhanced Raman scattering,broadband emission,lead-free perovskites,0D nanocrystals
更新于2025-09-10 09:29:36
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Temperature dependent electrical properties of AlN/Si heterojunction
摘要： AlN is an integral part of many Si based electronic, optoelectronic, and electromechanical devices. The transport of charge carriers and their recombination at the AlN (0002)/Si (111) interface become crucial for the performance and reliability of such devices. In this work, we have studied the temperature dependent current-voltage (I-V-T) characteristics of AlN/Si heterojunctions to gain a deeper understanding. The analysis of the I-V-T characteristics interestingly suggested a temperature dependent turn-on voltage in the forward bias of the Schottky barrier. Also, the Schottky barrier itself was found to be temperature dependent as expected. We have qualitatively explained the temperature dependence of the turn-on voltage in terms of trap states at the AlN/Si heterojunction.
关键词： electrical properties,temperature dependent,AlN/Si heterojunction,Schottky barrier,trap states
更新于2025-09-10 09:29:36