Carrier Recombination Lifetime Research Articles

Extending the Charge Carrier Recombination Lifetime by Octahedral Rotations in Ruddlesden-Popper Ba3Zr2S7 Perovskites.

Intentional distortions of [BX6] octahedra within perovskite structures have been recognized as a potent strategy for precise band gap adjustments and optimization of their photovoltaic properties, yet information regarding charge carrier dynamics linked to octahedral distortion under ambient conditions for chalcogenide perovskites remains limited. In this study, we utilize ab initio nonadiabatic molecular dynamics to explore the dynamics of photogenerated carriers in a representative two-dimensional Ba3Zr2S7 material in the Ruddlesden-Popper phase. The theoretical results highlight the influence of octahedral rotation on the materials' stability and carrier recombination lifetime of the system. Specifically, the octahedrally rotating P42/mnm phase exhibits a prolonged nonradiative carrier recombination lifetime attributed to the stabilized electron-phonon coupling. These findings offer valuable insights into the fundamental physical characteristics of imposed octahedral distortion and its potential for optimizing the optoelectronic performance of 2D Ruddlesden-Popper Ba-Zr-S chalcogenide materials.

Bicyclopentadithiophene-Based Organic Semiconductor for Stable and High-Performance Perovskite Solar Cells Exceeding 22.

Well-performing organic-inorganic halide perovskites are susceptible to poor efficiency and instability due to their various defects at the interphases, grain boundaries (GBs), and surfaces. In this study, an in situ method is utilized for effectively passivating the under-coordinated Pb2+ defects of perovskite with new non-fullerene acceptors (NFAs) (INXBCDT; X = H, Cl, and Br) through their carbonyl and cyano functional groups during the antisolvent dripping process. It reveals that the bicyclopentadithiophene (BCDT) core with highly electron-withdrawing end-capping groups passivates GBs and boosts perovskite grain growth. This effective defect passivation decreases the trap density to increase the carrier recombination lifetime of the perovskite film. As a result, bromo-substituted dicyanomethylene indanone (INBr)-end-capped BCDT (INBrBCDT-b8; 3a)-passivated devices exhibit the highest power conversion efficiency (PCE) of 22.20% (vs those of 18.09% obtained for perovskite films without passivation) upon an optimized film preparation process. Note that devices treated with more soluble 2-ethylhexyl-substituted compounds (1a, 2a, and 3a) exhibit higher PCE than those treated with less soluble octyl-substituted compounds (1b, 2b, and 3b). It is also worth noting that BCDT is a cost-effective six-ring core that is easier to synthesize with a higher yield and therefore much cheaper than those with highly fused-ring cores. In addition, a long-term stability test in a glovebox for 1500 h reveals that the perovskite solar cells (PSCs) based on a perovskite absorber treated with compound 3a maintain ∼90% of their initial PCE. This is the first example of the simplest high-conjugation additive for perovskite film to achieve a PCE greater than 22% of the corresponding lead-based PSCs.

Interfacial Charge Transfer Bridge Prolongs Carrier Recombination Lifetimes of CoFe Metal‐Thiolate Framework/Hematite Photoanode for Water Oxidation

AbstractConstructing heterostructural photoanodes is attractive for elevating the photoelectrochemical (PEC) performance, however, it is a long‐standing challenge to achieve highly efficient interfacial charge transfer. Herein, a CoFe metal‐thiolate framework (CoFe MTF)/Fe2O3 photoanode connected by an interfacial Fe─O─N/S bond is designed to modulate the behavior of charge carriers and improve water oxidation performance. It is disclosed that this interfacial bond functions as a direct charge transfer bridge between shallow trap states of Fe2O3 and CoFe MTF, leading to prolonged carrier recombination lifetimes (85 ns for CoFe MTF/Fe2O3 compared to 37 ns for Fe2O3) and enhanced charge transfer efficiency. Alternatively, a robust interfacial electric field is established in the CoFe MTF/Fe2O3 p–n heterojunction, facilitating efficient charge transfer. As expected, the CoFe MTF/Fe2O3 photoanode exhibits significant enhancement in water oxidation, resulting in a three‐fold increase in photocurrent density compared to pristine Fe2O3. This study highlights the significance of designing interfacially bonded heterostructural photoelectrodes to regulate the transfer characters of charge carriers.

Validation of minority carrier recombination lifetimes in low-dimensional semiconductors found by analytical photoresponses

It is a formidable challenge to find the minority carrier recombination lifetime in low-dimensional devices as low-dimensionality increases the surface recombination rate and often reduces the recombination lifetime to a scale of picoseconds. In this work, we demonstrated a simple but powerful method to quantitatively probe the minority carrier recombination lifetime in silicon nanowires or microwires by fitting the experimental photoresponses with our recently established analytical photoresponse principle of photoconductors. The nanowires were passivated with small molecules and Al2O3 to suppress surface recombination, which will increase the minority recombination lifetimes. As expected, the minority carrier recombination lifetime found by this approach increases by orders of magnitude. These wires were also made into PIN diodes, the leakage of which was reduced at least 1 order of magnitude after surface passivation by Al2O3. The minority recombination lifetime found from the leakage current of these devices is largely consistent with what we found from our analytical photoresponse principle. As a further step, we performed scanning photocurrent microscopy to find the minority diffusion length from which we found that the minority recombination lifetime is close to what we found from the analytical photoresponses. In short, this work validated that our analytical response principle is a reliable method to find the minority recombination lifetime in low-dimensional semiconductors.

Modeling the performance of perovskite solar cells with inserting porous insulating alumina nanoplates

Peng et al. [Science 379 683 (2023)] reported an effective method to improve the performance of perovskite solar cells by using thicker porous insulator contact (PIC)-alumina nanoplates. This method overcomes the trade-off between the open-circuit voltage and the fill factor through two mechanisms: reduced surface recombination velocity and increased bulk recombination lifetime due to better perovskite crystallinity. From arguments of drift-diffusion simulations, we find that an increase in mobility and carrier recombination lifetime in bulk are the key factors for minimizing the resistance-effect from thicker PICs and achieving a maximum power conversion efficiency (PCE) at approximately 25% reduced contact area. Furthermore, the partially replacement of perovskite films with thicker PICs would result in a reduction in short-current density, but the relative low refractive index of the PICs imbedded into the high refractive index perovskite creates light trapping structures that compensate for this loss.

New equations to calculate carrier recombination lifetime of silicon epitaxial layer, based on open circuit voltage decay method

Abstract New equations for recombination lifetime calculation based on the open circuit voltage decay (OCVD) method were proposed. The new equations can yield accurate recombination lifetime values of the i-layer of PiN diodes in which a silicon epitaxial layer is employed, by eliminating the effects of carrier diffusion into p+ and n+ layers from the i-layer of PiN diodes, carrier injection into the i-layers from the depletion layer of PiN diodes, and surface recombination on the side wall of PiN diodes. To verify the effectiveness of the new equations, OCVD measurements were performed by employing technology computer-aided design (TCAD) simulation and actual PiN diodes. Although the calculated values implied the effect of epitaxial layer thickness, they were consistent with the results obtained under assumed lifetime conditions for TCAD simulation, the theory of recombination lifetime, and results in other reports.

Crystallization Retardation and Synergistic Trap Passivation in Perovskite Solar Cells Incorporated with Magnesium-Decorated Graphene Quantum Dots.

One of the encouraging strategies for enhancing the efficiency of perovskite solar cells (PSCs) is to reduce defects, trap states of pinholes, and charge recombination rate in the light absorber layer of perovskite, which can be addressed by increasing the perovskite grain size. The utilization of Mg-decorated graphene quantum dots (MGQD) or graphene quantum dots (GQDs) into a perovskite precursor solution for further crystal modification is introduced in this study. Studies on the crystalline structure and morphology of MGQD generated from GQDs demonstrate that MGQD has a greater crystal size than GQD. Therefore, higher light absorption in the whole UV-vis spectrum and a larger grain size for the perovskite/MGQD layer compared to the perovskite/GQD sample are achieved. Moreover, more photoluminescence peak quenching of perovskite/MGQD and extended carrier recombination lifetime (from 3 to 40 ns) verify the surface and grain boundary trap passivation compared to pristine perovskite. Consequently, PSCs in an n-i-p configuration containing perovskite/MGQD show a higher performance of 10.2% in comparison to the pristine perovskite at 7.2%, attributed to the enhanced JSC from 13.2 to 19.1 mA cm-2. Thus, incorporating MGQDs into the perovskite layer is a hopeful approach for obtaining a superior perovskite film with impressive charge extraction and decreased nonradiative charge recombination.

A Review of Minority Carrier Recombination Lifetime Measurements

Abstract: The recombination lifetime of minority carriers is a critical parameter in semiconductor devices such as photovoltaic cells since it controls the efficiency of such devices. Many techniques have been developed to accomplish recombination measurements and thereby test semiconductor devices' efficiencies. Recombination lifetime average values differ according to semiconductor device type; thus, choosing an appropriate technique is important. This paper studies the concept of excess minority carrier lifetime and its calculations. It also investigates the advantages, limitations, and capabilities of the most common recombination lifetime measurement techniques. A chart was drawn with all known measurement methods to make it easier to understand the relation between these techniques.

Carrier dynamics in blue, cyan, and green InGaN/GaN LEDs measured by small-signal electroluminescence

We study the carrier dynamics for c-plane InGaN/GaN light-emitting diodes (LEDs) with various emission wavelengths near the green gap using a small-signal electroluminescence method. The LEDs were grown by Lumileds using state-of-the-art growth conditions. Radiative and non-radiative recombination rates are numerically separated, and the carrier recombination lifetime and carrier density are obtained. Experiment shows that the causes of efficiency reduction at longer wavelength in the present structures are injection efficiency decrease, radiative recombination rate decrease, and imbalance of the increase in Auger–Meitner and radiative terms due to the interplay between the carrier–current density relationship and the quantum-confined Stark effect (QCSE). The effects of QCSE, phase-space filling, and the carrier–current density relationship on efficiency reduction at longer wavelengths are examined separately with experimental data and Schrödinger–Poisson calculations. In addition, we confirm the scaling law between Cn and Bn under electrical injection and find that the increase in carrier density at a given current density is the primary cause for lower radiative efficiency at high current density in longer wavelength LEDs. Conversely, we do not observe a significant efficiency reduction at longer wavelengths from extrinsic material degradation.

Utility of Shockley–Read–Hall analysis to extract defect properties from semiconductor minority carrier lifetime data

The semiconductor minority carrier lifetime contains information about several important material properties, including Shockley–Read–Hall defect levels/concentrations and radiative/Auger recombination rates, and the complex relationships between these parameters produce a non-trivial temperature-dependence of the measured lifetime. It is tempting to fit temperature-dependent lifetime data to extract the properties of the Shockley–Read–Hall recombination centers; however, without a priori knowledge of the distribution of the Shockley–Read–Hall states across the bandgap, this fit problem is under-constrained in most circumstances. Shockley–Read–Hall lifetime data are not well-suited for the extraction of Shockley–Read–Hall defect levels but can be used effectively to extract minority carrier recombination lifetimes. The minority carrier recombination lifetime is observed at temperatures below 100 K in a Si-doped n-type InGaAs/InAsSb superlattice, and deviation from its expected temperature-dependence indicates that the capture cross section of the defect associated with Si-doping has an activation energy of 1.5 meV or a characteristic temperature of 17 K. This lower temperature regime is also preferrable for the analysis of the physics of defect introduction with displacement-damage-generating particle irradiation.

Investigation of defects in melt and solution grown perovskite CsPbBr3 single crystals

Recently, there has been considerable interest in x-ray and gamma ray detectors with large volume and high energy resolution that operate at room temperature. To improve detector energy resolution, the carrier mobility-lifetime product needs to be increased, and the electronic trap state concentration needs to be minimized. Defect concentrations in the part per billion range can alter the charge transport and carrier recombination lifetime. In this work, thermally stimulated current spectroscopy measurements were systematically carried out in bulk halide perovskite single crystals of CsPbBr3 over a temperature range of 80–320 K. The origins and trap parameters of CsPbBr3 crystals from the solution growth and melt growth procedures were determined and compared. Trap concentrations were ranged from 1 × 1011 to 1 × 1016 cm−3. Appreciable detector performance was observed for CsPbBr3 crystals with trap concentrations less than 1 × 1014 cm−3. The comparison of spectral responses of crystal samples grown using two different methods shows that, after purification, solution-grown crystals are comparable to melt-grown crystals in terms of low defect concentration and improved detector performance. For an improved mobility-lifetime product and enhanced spectral response to high energy radiation from fissile materials, trap states in either type of a crystal ingot must be reduced closer to 1011 cm−3.

Strain-Induced Modification of Photoluminescence in Quasi-2D Perovskite Thin Films

Quasi-2D perovskites are recognized as a promising material for lightweight photovoltaic applications. For these applications, it is critical to understand how mechanical stresses affect the perovskite’s optoelectronic properties. Here, we discuss how applied stress impacts the photoluminescence spectrum and lifetime of quasi-2D perovskite thin films and how these effects depend on interlayer cation choice (butylammonium or phenethylammonium) and film processing (hot-casting versus postdeposition annealing). We find that compressive stress induces an enhancement and spectral redshift (>10 meV per percent strain) in the emission from bulk-like grains in the film. Moreover, the charge carrier recombination lifetime is greatly enhanced (up to a factor of 14 with only 1% strain). We show that the phenethylammonium-based perovskites are more robust against strain-induced modifications compared to those containing the butylammonium cation. These results highlight how strongly strain can modify the optoelectronic properties of quasi-2D perovskite thin films and how cation choice and film processing can have important practical implications for developing lightweight photovoltaics.

Dual-side interfacial passivation of FAPbI3 perovskite film by Naphthylmethylammonium iodide for highly efficient and stable perovskite solar cells

Interfacial passivation has been considered as an effective strategy to improve the performance of perovskite solar cells (PSCs) by modifying defect trap states between perovskite and charge transport layers. Here, we apply Naphthylmethylammonium Iodide (NMAI) salt to passivate the double-side interfaces from the perovskite/electron transport layer (ETL) and perovskite/hole transport layer (HTL). It demonstrates that using NMAI to suppress the interface defects from perovskite film can effectively decrease non-radiative recombination, extend the carrier recombination lifetime (from 4 to 113 ns), and boost PSCs device performance. Additionally, the judicious control of the interaction between perovskite and NMAI molecules can further optimize the homogenous morphology and crystallinity of the perovskite absorber layer. The double-side NMAI passivated device achieved a decent 22.97 % power conversion efficiency (PCE) with 1.11 V of open-circuit voltage (VOC), 80.35 % fill factor (FF) and short-circuit current (JSC) of 25.77 mA cm−2, compared to the control device (19.56 % PCE in conjunction with an VOC of 1.01 V, FF of 79.67 %, and JSC of 24.37 mA cm−2). Moreover, the unencapsulated devices with double-side NMAI passivation layers maintain 73 % of initial PCE after 432 h, at room temperature, under a nitrogen atmosphere, and in dark conditions. This study proposes a facile and effective method to passivate the perovskite/ETL and perovskite/HTL interfaces via NMAI spin-coating process.

Nonlinear optical properties and carrier recombination lifetime of GaN.

Nonlinear optical properties of a selection of gallium nitride samples have been measured using picosecond and nanosecond duration laser pulses at 532nm. The values of the two-photon absorption coefficient, free carrier absorption cross section, and free carrier refraction cross section are determined along with the recombination lifetime of photogenerated carriers. The effect of hot isostatic pressing on these properties in samples with linear absorption at the band edge due to defects is explored.

Hydrogen Bonds Strengthened Metal‐Free Perovskite for Degradable X‐ray Detector with Enhanced Stability, Flexibility and Sensitivity

AbstractMetal‐free perovskites (MFPs) with flexible and degradable properties have been adopted in flexible X‐ray detection. For now, figuring out the key factors between structure and device performance are critical to guide the design of MFPs. Herein, MPAZE‐NH4I3 ⋅ H2O was first designed and synthesized with improved structural stability and device performance. Through theoretical calculations, the introducing methyl group benefits modulating tolerance factor, increases dipole moment and strengthens hydrogen bonds. Meanwhile, H2O increases the hydrogen bond formation sites and synergistically realizes the band nature modulation, ionic migration inhibition and structural stiffness optimization. Spectra analysis also proves that the improved electron‐phonon coupling and carrier recombination lifetime contribute to enhanced performance. Finally, a flexible and degradable X‐ray detector was fabricated with the highest sensitivity of 740.8 μC Gyair−1 cm−2 and low detection limit (0.14 nGyair s−1).

Hydrogen Bonds Strengthened Metal-Free Perovskite for Degradable X-ray Detector with Enhanced Stability, Flexibility and Sensitivity.

Metal-free perovskites (MFPs) with flexible and degradable properties have been adopted in flexible X-ray detection. For now, figuring out the key factors between structure and device performance are critical to guide the design of MFPs. Herein, MPAZE-NH4 I3 ⋅ H2 O was first designed and synthesized with improved structural stability and device performance. Through theoretical calculations, the introducing methyl group benefits modulating tolerance factor, increases dipole moment and strengthens hydrogen bonds. Meanwhile, H2 O increases the hydrogen bond formation sites and synergistically realizes the band nature modulation, ionic migration inhibition and structural stiffness optimization. Spectra analysis also proves that the improved electron-phonon coupling and carrier recombination lifetime contribute to enhanced performance. Finally, a flexible and degradable X-ray detector was fabricated with the highest sensitivity of 740.8 μC Gyair -1 cm-2 and low detection limit (0.14 nGyair s-1 ).

Ultrafast Optomechanical Terahertz Modulators Based on Stretchable Carbon Nanotube Thin Films

For terahertz wave applications, tunable and rapid modulation is highly required. When studied by means of optical pump–terahertz probe spectroscopy, single-walled carbon nanotube (SWCNT) thin films demonstrated ultrafast carrier recombination lifetimes with a high relative change in the signal under optical excitation, making them promising candidates for high-speed modulators. Here, combination of SWCNT thin films and stretchable substrates facilitated studies of the SWCNT mechanical properties under strain and enabled the development of a new type of an optomechanical modulator. By applying a certain strain to the SWCNT films, the effective sheet conductance and therefore modulation depth can be fine-tuned to optimize the designed modulator. Modulators exhibited a photoconductivity change of approximately 2 times of magnitude under the strain because of the structural modification in the SWCNT network. Stretching was used to control the terahertz signal with a modulation depth of around 100% without strain and 65% at a high strain operation of 40%. The sensitivity of modulators to beam polarization is also shown, which might also come in handy for the design of a stretchable polarizer. Our results give a fundamental grounding for the design of high-sensitivity stretchable devices based on SWCNT films.

SLJCompact: A semiconductor-liquid junction solver for rapid band diagram insights into photoelectrochemical devices

SLJCompact is a Matlab based program that computes the band diagram and current-voltage characteristics of semiconductor photoanodes. To enable rapid insights, corresponding to various semiconductor-liquid junction (SLJ) properties and observable current-voltage characteristics, an abridged set of semi-analytical conservation equations are employed in the semiconductor region coupled with Gouy-Chapman screening within the liquid region. Electrostatic interactions between these regions across the junction are solved self-consistently through a variation on the Scharfetter-Gummel method. In this manner, the model is able to provide key band diagram insights regarding the correlated operation of photoanodes, both under illumination and in the dark, with respect to: current-voltage trends, quasi-Fermi level splitting, hole transfer rates, electron transfer rates, depletion region screening, hole inversion screening, and carrier recombination lifetimes. Motivated by Kroemer's lemma, SLJCompact is intended to further the adoption of band diagram methods within the photoelectrochemical device literature, by providing a computationally inexpensive approach residing between computationally intensive full continuity equation solvers and more approximate analytical expressions. Program summaryProgram Title: SLJCompactCPC Library link to program files:https://doi.org/10.17632/by4ddckrpt.1Developer's repository link:https://www.physics.mcgill.ca/~bevankh/Codes/SLJCompact/Licensing provisions: CC BY NC 4.0Programming language: MatlabNature of problem: Providing rapid self-consistently computed insights into the operational and band diagram characteristics of photoelectrochemical devices.Solution method: A semi-analytical approach for solving carrier transfer and recombination in the depletion region coupled self-consistently with Poisson's equation across a semiconductor-liquid junction, solved together through a variation on the Scharfetter-Gummel method.Additional comments including restrictions and unusual features: The current version is only applicable to photoanodes and forward bias trends at/beyond flat band conditions.

Vacancy-Regulated Charge Carrier Dynamics and Suppressed Nonradiative Recombination in Two-Dimensional ReX2 (X = S, Se).

Point defects in semiconductors usually act as nonradiative charge carrier recombination centers, which severely limit the performance of optoelectronic devices. In this work, by combining time-domain density functional theory with nonadiabatic molecular dynamics simulations, we demonstrate suppressed nonradiative charge carrier recombination and prolonged carrier lifetime in two-dimensional (2D) ReX2 (X = S, Se) with S/Se vacancies. In particular, a S vacancy introduces a shallow hole trap state in ReS2, while a Se vacancy introduces both hole and electron trap states in ReSe2. Photoexcited electrons and holes can be rapidly captured by these defect states, while the release process is slow, which contributes to an elongated photocarrier lifetime. The suppressed charge carrier recombination lies in the vacancy-induced low-frequency phonon modes that weaken electron-phonon coupling, as well as the reduced overlap between electron and hole wave functions that decreases nonadiabatic coupling. This work provides physical insights into the charge carrier dynamics of 2D ReX2, which may stimulate considerable interest in using defect engineering for future optoelectronic nanodevices.

Double-side modification strategy for efficient carbon-based, all-inorganic CsPbIBr2 perovskite solar cells with high photovoltage

CsPbIBr2 has attracted great attention due to its balanced bandgap and stability features. However, one-step prepared CsPbIBr2 films are generally of poor quality, hindering the performance improvement of the resulting perovskite solar cells (PSCs). Herein, we report the fabrication of high-performance carbon-based, all-inorganic CsPbIBr2 PSCs with a double-side modification strategy using PEAI/PEABr. We tune the crystallization behavior and passivate the defects of CsPbIBr2 films with modifications of their bottom and top surfaces with PEAI and PEABr, respectively. This causes the PEA cation to form a double layer of armor on both sides of the CsPbIBr2 precursor film and can provide the anions of the required I and Br. The collaborative strategy of crystallization and defect passivation for CsPbIBr2 films is exceptionally effective. It produces a fully covered CsPbIBr2 film with an average grain size increase of more than 50%, few grain boundaries, and high crystallinity. Moreover, this strategy also suppresses pinhole formation, reduces the charge trap density, and prolongs the carrier recombination lifetime. Hence, carbon-based all-inorganic PSCs with the desired CsPbIBr2 films yield an optimized efficiency of 9.96% with a particularly high photovoltage of 1.32 V. Our work provides guidance for simultaneous crystallization control and defect passivation to further improve the performance of PSCs.