Nonradiative Carrier Recombination Research Articles

Tailored conductive fullerenes-based passivator for efficient and stable inverted perovskite solar cells

Interfacial defects result in a limitation to the development of highly efficient and stable perovskite solar cells. The passivation of these defects by adopting various interfacial defects passivation agents is a common method for boosting device performance. However, most existing interfacial defects passivation agents form poorly conductive aggregates at the perovskite interface with the electron transport layer (ETL), hindering the transport of charge carriers. In addition, the electron mobility of passivation agents is an important factor that affects the electron communication between the adjacent layers. Herein, a fullerene-based molecular passivator, [60]fullerene-4-(1-(4-(tert-butyl)phenyl)pyrrolidin-2-yl)benzenaminium (C60-tBu-I), is designed and synthesized. This novel n-doping fullerene ammonium iodide is developed as an interfacial modification agent to accelerate charge transport from the perovskite active layer into the ETL while hindering the nonradiative charge carrier recombination. Hence, compared with the control devices (15.66%), C60-tBu-I-modified device presents a higher efficiency of 17.75%. More importantly, the tert-butyl group dramatically enhances the resistance of perovskite films to water molecular. As a result, C60-tBu-I-modified devices exhibit excellent long-term stability, remaining at more than 87% of the initial power conversion efficiency value after storage for 500 h.

A bilateral cyano molecule serving as an effective additive enables high-efficiency and stable perovskite solar cells

The existence of defects in perovskite films is a major obstacle that prevents perovskite solar cells (PSCs) from high efficiency and long-term stability. A variety of additives have been introduced into perovskite films for reducing the number of defects. Lewis base-based additive engineering has been considered as an effective way to eliminate defects, especially the defects caused by the uncoordinated Pb2+. In this work, for the first time, a bilateral cyano molecule (succinonitrile, SN) which is a commonly used plasticizer in solid electrolyte of solid-state lithium batteries was selected as an additive to modify organic–inorganic hybrid perovskite films in PSCs. SN is featured with two cyano groups (–C≡N) distributing at both terminals of the carbon chain, providing two cross-linking points to interact with perovskites crystals via coordinating with uncoordinated Pb2+ and forming hydrogen bonds with –NH2 groups in perovskite. It was found that the addition of SN into perovskite precursor solution could effectively reduce defects, particularly inhibit the appearance of Pb0 and thus suppress trap-assisted nonradiative charge carrier recombination. As a result, the efficiency of CH3NH3PbI3(Cl) (MAPbI3(Cl))-based PSCs was improved from 18.4% to 20.3% with enhanced long-term stability at N2 and humid air atmosphere. This work provides a facile and effective strategy to enhance the PCE and stability of PSCs simultaneously, facilitating the commercialization of PSCs.

Small grains as recombination hot spots in perovskite solar cells

Summary Non-radiative recombination in the perovskite bulk and at its interfaces prohibits the photovoltaic performance from reaching the Shockley-Queisser limit. While interfacial recombination has been widely discussed and demonstrated, bulk recombination and especially the influence of grain boundaries remain under debate. Most studies explore the role of grain boundaries on perovskite films rather than devices, making it difficult to link the film properties with those of the devices. Here, we systematically investigate the effects of grain boundaries on the performance of perovskite solar cells by two different methods. By combining experimental characterization with theoretical device simulations, we find that the recombination at grain boundaries is diffusion limited and hence is inversely proportional to the grain area to the power of 3/2. Consequently, the prevalence of small grains—which act as recombination hot spots—across the perovskite active layer dictates the photovoltaic performance of the perovskite solar cells.

In-situ secondary annealing treatment assisted effective surface passivation of shallow defects for efficient perovskite solar cells

The interfacial defects passivation of perovskite is one of the most effective approaches to realize highly efficient and stable perovskite solar cells (PSCs). Herein, we propose an easy in-situ secondary annealing method by using a quaternary ammonium salt tetraethylammonium hexafluorophosphate (TEAH) to enhance the photovoltaic performance and device stability. The research manifests that TEAH can homogeneously distribute on the perovskite surface, and effectively passivate the surface and boundary defects. Correspondingly, the hole extraction from perovskite to the hole transport layer (HTL) is greatly improved, hindering the non-radiative charge carrier recombination. With the introduction of TEAH treatment, the power conversion efficiency (PCE) of the PSC has been elevated from 18.57% to 20.25%, with well suppressed hysteresis. Moreover, the exposed hydrophobic ethyl-terminal groups of TEAH can protect perovskite film from degradation by ambient moisture in some extent. As a result, TEAH treated PSC exhibits good stability, retaining 74.1% of its initial PCE after 680 h aging under ambient air with a relative humidity (RH) of 55–60%.

Synergistic Effect of Fluorinated Passivator and Hole Transport Dopant Enables Stable Perovskite Solar Cells with an Efficiency Near 24%

Long-term durability is critically important for the commercialization of perovskite solar cells (PSCs). The ionic character of the perovskite and the hydrophilicity of commonly used additives for the hole-transporting layer (HTL), such as lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) and tert-butylpyridine (tBP), render PSCs prone to moisture attack, compromising their long-term stability. Here we introduce a trifluoromethylation strategy to overcome this drawback and to boost the PSC's solar to electric power conversion efficiency (PCE). We employ 4-(trifluoromethyl)benzylammonium iodide (TFMBAI) as an amphiphilic modifier for interfacial defect mitigation and 4-(trifluoromethyl)pyridine (TFP) as an additive to enhance the HTL's hydrophobicity. Surface treatment of the triple-cation perovskite with TFMBAI largely suppressed the nonradiative charge carrier recombination, boosting the PCE from 20.9% to 23.9% and suppressing hysteresis, while adding TFP to the HTL enhanced the PCS's resistance to moisture while maintaining its high PCE. Taking advantage of the synergistic effects resulting from the combination of both fluoromethylated modifiers, we realize TFMBAI/TFP-based highly efficient PSCs with excellent operational stability and resistance to moisture, retaining over 96% of their initial efficiency after 500 h maximum power point tracking (MPPT) under simulated 1 sun irradiation and 97% of their initial efficiency after 1100 h of exposure under ambient conditions to a relative humidity of 60-70%.

Reduced Thermal Conductivity in Ultrafast Laser Heated Silicon Measured by Time-Resolved X-ray Diffraction

We investigate the effect of free carrier dynamics on heat transport in bulk crystalline Silicon following femtosecond optical excitation of varying fluences. By taking advantage of the dense 500 MHz standard fill pattern in the PLS-II storage ring, we perform high angular-resolution X-ray diffraction measurements on nanosecond-to-microsecond time-scales with femtometer spatial sensitivity. We find noticeably slowed lattice recovery at increasingly high excitation intensities. Modeling the temporal evolution of lattice displacements due to the migration of the near surface generated heat into the bulk requires reduced thermal diffusion coefficients. We attribute this pump-fluence dependent thermal transport behavior to two separate effects: first, the enhanced nonradiative recombination of free carriers, and, second, reduced size of the effective heat source in the material. These results demonstrate the capability of time-resolved X-ray scattering as an effective means to explore the connection between charge carrier dynamics and macroscopic transport properties.

Multifunctional brominated graphene oxide boosted charge extraction for high-efficiency and stable all-inorganic CsPbBr3 perovskite solar cells

The perovskite film with large grain size and high charge mobility is critical for the improvement of the power conversion efficiency (PCE) and the operational stability of perovskite solar cells (PSCs). Herein, multifunctional brominated graphene oxide (Br-GO) is prepared and innovatively used as an effective additive to incorporate into CsPbBr3 to form a large-grained high-quality perovskite film and to passivate defects at grain boundaries, which remarkably reduces the trap state density and carrier non-radiative recombination. Meanwhile, the hole mobility of the homogeneous CsPbBr3 + Br-GO photoactive layer is obviously increased due to the hole acceptor properties and high carrier mobility of Br-GO, resulting in an enhanced charge separation as well as transfer and suppressed carrier radiative recombination. Moreover, the Br-GO with a downshift valence band is employed as a hole-transporting material setting at perovskite/Carbon interface to further compensate energy levels and promote hole extraction. As a consequence, a champion PCE of 10.10% with an open-circuit voltage up to 1.602 V for the optimized PSCs with a configuration of FTO/c-TiO2/m-TiO2/CsPbBr3 + Br-GO/Br-GO/Carbon is obtained, which is much higher than 6.28% efficiency for the reference device. The unencapsulated solar cells also exhibit a remarkable tolerance toward 85% RH and long-term AM 1.5G illumination in air.

1.5-μm Indium Phosphide-Based Quantum Dot Lasers and Optical Amplifiers: The Impact of Atom-Like Optical Gain Material for Optoelectronics Devices

Using quantum dot (QD) structures as active material for optoelectronics was already in focus before the development of quantum well (QW) lasers and before semiconductor lasers occupied an important place in the market. The big step in reducing laser threshold conditions by substituting bulk gain materials with QWs should find a logical continuation by further reducing the dimensionality toward full 3D carrier confinement by QDs. This was predicted by theoretical considerations at the beginning of the 1980s [1], [2]. However, no practical technologies were available at that time. Top-down approaches, as originally addressed, to produce nanoscale species on very large surfaces and interface areas resulted in the accumulation of a large defect density within the active region and the domination of nonradiative carrier recombination mechanisms. Only during the early 1990s was a major breakthrough achieved with the introduction of the bottom-up approach of strain-driven, self-organized epitaxial QD growth. The first lasers were obtained in the gallium arsenide (GaAs)-based material system [3].

Carrier Dynamics in Thin Germanium–Tin Epilayers

The Si-based mid-infrared photonics is an emerging technology in which group-IV germanium–tin (Ge1–xSnx) binary alloys can play a fundamental role in the development of a Si-compatible photonic components including monolithically integrated coherent light sources and detectors, on the same Si or SOI substrate. Although the Ge1–xSnx-on-Si lasers, at low temperatures, have already been demonstrated, the knowledge of the material properties necessary for such device optimization and real-life usage is very limited. In particular, carrier relaxation kinetics, relaxation pathways, and accompanied physical mechanisms, important for the laser’s dynamics, have not been subjected to in-depth research and understanding. In this work, we present detailed spectroscopic studies on photoinjected carrier dynamics in Ge1–xSnx epilayers, as a function of Sn content (6–12%) and temperature (20–300 K), by utilizing time-resolved differential reflectivity and photoluminescence. The latter technique allowed us to track separated electron and hole dynamics with a femtosecond time resolution, while the former experiment exploited a joined electron–hole recombination. This experimental approach allowed us to identify (i) two initial electron relaxation processes after photoexcitation; (ii) radiative electron–hole recombination on below-band gap states; (iii) nonradiative carrier recombination involving the Shockley–Read–Hall mechanism; and (iv) nonradiative recombination through the surface states. The research results significantly expand the knowledge on the initial carrier relaxation dynamics in the Ge1–xSnx epitaxial material. It provides unknown up-to-date kinetic parameters of the initial stage of electron relaxation and further carrier recombination dynamics, unveils the critical role of band gap inhomogeneity for the relaxation dynamics, and highlights the role of below-band gap states that can participate in the light generation process in Ge1–xSnx epilayers.

Heterojunction Perovskite Microrods Prepared by Remote‐Controlled Vacancy Filling and Halide Exchange

AbstractAnion exchange reaction tunes the band gap of halide perovskites, which proceeds by the migration of the ions through the halide vacancies. This study reports the preparation of the heterojunction perovskite microrods by optically controlled localized halide vacancy filling, and halide exchange leading to color tuning in the MAPbBr3 microrod crystals. The exchange reaction is homogeneously suppressed by treating the crystal with a halide precursor solution, whereas the reaction is locally inhibited at the specific site of a crystal by filling the halide vacancies using a tightly focused beam of a near‐infrared laser. By controlling the density of halide vacancies at the specific site of the crystal, the rate of nonradiative recombination of charge carriers in the crystal is controlled. This halide vacancy filling by the remote‐controlled reaction helps to locally control the crystal quality and photoluminescence for designing perovskite‐based high‐quality photovoltaic and optoelectronic devices.

Synergistic Interface Layer Optimization and Surface Passivation with Fluorocarbon Molecules toward Efficient and Stable Inverted Planar Perovskite Solar Cells.

Large-size organic halide passivation has been considered an efficient approach to enhance the perovskite solar cell (PSC) efficiency and stability. Herein, a facile posttreatment strategy was demonstrated, wherein trifluoromethyl-phenethylamine hydrobromide (CF3-PEABr) is firstly used to passivate the perovskite film surface. The CF3-PEABr surface posttreatment could coordinate with halide dangling bonds that exist at the perovskite crystal surface. Moreover, the surface treatment with CF3-PEABr could efficiently passivate the defects in the perovskite film and suppress the nonradiative carrier recombination. As a result, a high efficiency of 21.3% is obtained, and an increment of 80 mV in Voc (a large Voc of 1.15 V, with a 0.42 V voltage deficit) occurs, compared to the control device. To relieve the hydrophobic nature properties of the -CF3 functional group and the dewetting problem of PCBM layer deposition, a surfactant Triton X-100 is used to modify the PCBM layer. Furthermore, the devices with CF3-PEABr posttreatment exhibit better operational, thermal (85°C), and long storage stabilities without any encapsulation.

PbS/CdS Quantum Dot Room-Temperature Single-Emitter Spectroscopy Reaches the Telecom O and S Bands via an Engineered Stability.

We synthesized PbS/CdS core/shell quantum dots (QDs) to have functional single-emitter properties for room-temperature, solid-state operation in the telecom O and S bands. Two shell-growth methods-cation exchange and successive ionic layer adsorption and reaction (SILAR)-were employed to prepare QD heterostructures with shells of 2-16 monolayers. PbS/CdS QDs were sufficiently bright and stable to resolve photoluminescence (PL) spectra representing both bands from single nanocrystals using standard detection methods, and for a QD emitting in the O-band a second-order correlation function showed strong photon antibunching, important steps toward demonstrating the utility of lead chalcogenide QDs as single-photon emitters (SPEs). Irrespective of type, few telecom-SPEs exist that are capable of such room-temperature operation. Access to single-QD spectra enabled a direct assessment of spectral line width, which was ∼70-90 meV compared to much broader ensemble spectra (∼300 meV). We show inhomogeneous broadening results from dispersity in PbS core sizes that increases dramatically with extended cation exchange. Quantum yields (QYs) are negatively impacted at thick shells (>6 monolayers) and, especially, by SILAR-growth conditions. Time-resolved PL measurements revealed that, with SILAR, initially single-exponential PL-decays transition to biexponential, with opening of nonradiative carrier-recombination channels. Radiative decay times are, overall, longer for core/shell QDs compared to PbS cores, which we demonstrate can be partially attributed to some core/shell sizes occupying a quasi-type II electron-hole localization regime. Finally, we demonstrate that shell engineering and the use of lower laser-excitation powers can afford significantly suppressed blinking and photobleaching. However, dependence on shell thickness comes at a cost of less-than-optimal brightness, with implications for both materials and experimental design.

Influence of the Growth Substrate on the Internal Quantum Efficiency of AlGaN/AlN Multiple Quantum Wells Governed by Carrier Localization

The influence of the growth substrate on the internal quantum efficiency (IQE) of deep ultraviolet light‐emitting diodes is studied. Two nominally identical Al‐rich AlGaN/AlN multi‐quantum‐well (MQW) structures grown by metal–organic vapor phase epitaxy (MOVPE) on different substrates are investigated. The first MQW structure is grown on a native AlN substrate, whereas the second one is deposited on an AlN template on sapphire. By the combination of atomic force microscopy (AFM), photoluminescence (PL), and cathodoluminescence (CL) spectroscopy, it is demonstrated that the dislocation‐mediated spiral growth of MQWs on sapphire results in the more efficient localization of carriers. This effect helps to prevent nonradiative carrier recombination at point defects, improving the IQE of the structure.

Synergistically Enhanced Amplified Spontaneous Emission by Cd Doping and Cl‐Assisted Crystallization

AbstractOrganic−inorganic hybrid halide perovskite materials have recently been recognized as a promising optical gain medium, and the development of optically pumped amplified spontaneous emission (ASE) of perovskite materials with low threshold and high optical gain will be on the only way to go for realizing its commercial perovskite laser. In this work, enhanced‐performance ASE with lower threshold and higher optical gain is obtained in CdCl2 passivated FA0.9MA0.15Cs0.3PbI2.7Br0.3 (FAMACs) polycrystalline films coated with poly methyl methacrylate (PMMA) layer. The ASE threshold of the FAMACs polycrystalline films with CdCl2 is significantly reduced from 18.7 to 7.9 µJ cm−2, with an enhanced ASE output intensity and stability. The improved ASE performance can be attributed to the synergistic effect of Cd ion doping and Cl‐ion‐assisted crystallization in the FAMACs polycrystalline films, exhibiting remarkably enhanced photoluminescence, a prolonged decay lifetime and a low defect density, and thus leading to easily achieve the population inversion due to the reduction of nonradiative recombination of carriers in the optical gain medium.

Efficiency from hole-selective contacts

Solar Cells Perovskite/silicon tandem solar cells must stabilize a perovskite material with a wide bandgap and also maintain efficient charge carrier transport. Al-Ashouri et al. stabilized a perovskite with a 1.68–electron volt bandgap with a self-assembled monolayer that acted as an efficient hole-selective contact that minimizes nonradiative carrier recombination. In air without encapsulation, a tandem silicon cell retained 95% of its initial power conversion efficiency of 29% after 300 hours of operation. Science , this issue p. [1300][1] [1]: /lookup/doi/10.1126/science.abd4016

Monolithic perovskite/silicon tandem solar cell with >29% efficiency by enhanced hole extraction.

Tandem solar cells that pair silicon with a metal halide perovskite are a promising option for surpassing the single-cell efficiency limit. We report a monolithic perovskite/silicon tandem with a certified power conversion efficiency of 29.15%. The perovskite absorber, with a bandgap of 1.68 electron volts, remained phase-stable under illumination through a combination of fast hole extraction and minimized nonradiative recombination at the hole-selective interface. These features were made possible by a self-assembled, methyl-substituted carbazole monolayer as the hole-selective layer in the perovskite cell. The accelerated hole extraction was linked to a low ideality factor of 1.26 and single-junction fill factors of up to 84%, while enabling a tandem open-circuit voltage of as high as 1.92 volts. In air, without encapsulation, a tandem retained 95% of its initial efficiency after 300 hours of operation.

Reduction of non-radiative recombination by inserting a GaAs strain-relaxation interlayer in InGaAs/GaAsP superlattice solar cells investigated by photo-thermal spectroscopy

The role of a GaAs strain-relaxation interlayer inserted into InGaAs/GaAsP superlattice solar cells was evaluated by measuring the piezoelectric photothermal (PPT) signals in the temperature range from 100 K to a device operation temperature of around 340 K. The PPT signals caused by the non-radiative recombination of electrons photo-excited to the first quantized level were observed. The temperature-dependent PPT signal intensities were assessed using an electron carrier relaxation model comprising four processes: radiative recombination, non-radiative recombination, thermionic emission, and tunneling of carriers through the e2-miniband after thermal excitation from the e1-level. The contribution of holes in the hh1 state was also included in this model, in which e1 and e2 are the first and second electron levels in the conduction band, respectively, and hh1 is the first heavy hole level in the valence band of the quantum wells. A similar analysis was conducted using photoluminescence (PL) spectra to elucidate the carrier transition dynamics in greater detail, because PPT and PL measurements are complementary to each other in terms of non-radiative and radiative electron transitions. Consequently, although the non-radiative recombination remained dominant around room temperature, the quantum yield of the carrier tunneling process increased and became comparable to that of non-radiative recombination. This implies that the recombination loss of the photo-excited carriers is suppressed by the insertion of the GaAs interlayer. By clarifying the role of the inserted interlayer with respect to the non-radiative recombination process, the usefulness of the PPT method is demonstrated.

Perovskite Passivation Strategies for Efficient and Stable Solar Cells

Organic–inorganic halide perovskite photovoltaic devices have advanced rapidly in recent years, and the photoelectric conversion efficiency of perovskite solar cells (PSCs) has exceeded 25%. However, the defects from the crystallization process become nonradiation recombination centers and hinder the performance and the stability of PSCs. Defect passivation by tuning grain size and grain boundary (GB) is an effective strategy to reduce the defects on GBs and film surface. Herein, recent progress in the passivation strategy for perovskite films is summarized, including nonstoichiometric passivation, iodide vacancies filling, dimensional engineering, passivation with crosslink, physical passivation, and other passivation methods. These passivation strategies play an important role in improving the quality of perovskite films, adjusting the energy band structure, reducing the density of defect states, and suppressing the nonradiative recombination of carriers. Finally, this review puts forward the development direction of passivation strategies to further improve the performance of PSCs.

Stimulated emission threshold in thick GaN epilayers: interplay between charge carrier and photon dynamics

Room-temperature luminescence properties of a dense electron-hole plasma were studied in a set of GaN epilayers with thickness varying from 2 to 25 µm. The stimulated emission threshold was measured by photoluminescence and light-induced transient grating techniques under short-pulse excitation. Both techniques revealed the stimulated emission threshold increase with layer thickness despite the reduction of defect density. Numerical modeling of photoexcited carrier dynamics showed the different roles of carrier and photon populations. The stimulated emission threshold is mainly determined by the photon dynamics with weak influence of nonradiative carrier recombination. Increasing layer thickness results in the decreasing overlap of the gain layer with the optical mode, which reduces the transfer of energy from carriers to photons.

Unveiling defect-mediated carrier dynamics in few-layer MoS2 prepared by ion exchange method via ultrafast Vis-NIR-MIR spectroscopy

Defect-mediated processes in two-dimensional transition metal dichalcogenides have a significant influence on their carrier dynamics and transport properties, however, the detailed mechanisms remain poorly understood. Here, we present a comprehensive ultrafast study on defect-mediated carrier dynamics in ion exchange prepared few-layer MoS2 by femtosecond time-resolved Vis-NIR-MIR spectroscopy. The broadband photobleaching feature observed in the near-infrared transient spectrum discloses that the mid-gap defect states are widely distributed in few-layer MoS2 nanosheets. The processes of fast trapping of carriers by defect states and the following nonradiative recombination of trapped carriers are clearly revealed, demonstrating the mid-gap defect states play a significant role in the photoinduced carrier dynamics. The positive to negative crossover of the signal observed in the mid-infrared transient spectrum further uncovers some occupied shallow defect states distributed at less than 0.24 eV below the conduction band minimum. These defect states can act as effective carrier trap centers to assist the nonradiative recombination of photo-induced carriers in few-layer MoS2 on the picosecond time scale.