Nonradiative Carrier Recombination Research Articles

The detrimental effect of AlGaN barrier quality on carrier dynamics in AlGaN/GaN interface

Carrier recombination and scattering at the semiconductor boundaries can substantially limit the device efficiency. However, surface and interface recombination is generally neglected in the nitride-based devices. Here, we study carrier recombination and diffusivity in AlGaN/GaN/sapphire heterointerfaces with AlGaN barriers of different quality. We employ the light induced transient grating and time-resolved photoluminescence spectroscopy techniques to extract carrier lifetime in different depths of the GaN buffer as well as in the AlGaN barrier, and to evaluate the carrier diffusion coefficient in the buffer. Moreover, we assess interface recombination velocity, Shockley-Read-Hall and radiative recombination rates. We reveal the adverse barrier influence on carrier dynamics in the underlying buffer: AlGaN barrier accelerates the nonradiative carrier recombination in the GaN buffer. The interface recombination velocity in the GaN buffer increases with decreasing AlGaN barrier quality, and the dominating recombination mechanism switches from Shockley-Read-Hall to interface recombination. These phenomena are governed by a cumulative effect of various interface-deteriorating barrier defects. Meanwhile, the carrier diffusivity in the GaN buffer is not affected by the AlGaN barrier. We conclude that barrier-accelerated interface recombination can become a major carrier loss mechanism in AlGaN/GaN interface, and may substantially limit the efficiency in nitride-based UV LEDs.

Mechanistic Investigation of the Defect Activity Contributing to the Photoluminescence Blinking of CsPbBr3 Perovskite Nanocrystals.

Exploration of the full potential of the perovskite nanocrystals (NCs) for different applications requires a thorough understanding of the pathways of recombination of the photogenerated charge carriers and associated dynamics. In this work, we have tracked the recombination routes of the charge carriers by probing photoluminescence (PL) intermittency of the immobilized and freely diffusing single CsPbBr3 NCs employing a time-tagged-time-resolved method. The immobilized single CsPbBr3 NCs show a complex PL time-trace, a careful analysis of which reveals that nonradiative band-edge recombination through trap states, trion recombination, and trapping of the hot carriers contribute to the blinking behavior of any given NC. A drastically suppressed PL blinking observed for the NCs treated with a tetrafluoroborate salt indicates elimination of most of the undesired recombination processes. A fluorescence correlation spectroscopy (FCS) study on the freely diffusing single NCs shows that enhanced PL and suppressed blinking of the treated particles are the outcome of an increase in per-particle brightness, not due to any increase in the number of particles undergoing "off"-"on" transition in the observation volume. The mechanistic details obtained from this study on the origin of blinking in CsPbBr3 NCs provide deep insight into the radiative and nonradiative charge carrier recombination pathways in these important materials, and this knowledge is expected to be useful for better design and development of bright photoluminescent samples of this class for optoelectronic applications.

Efficient Perovskite Solar Cells through Suppressed Nonradiative Charge Carrier Recombination by a Processing Additive.

It has been reported that nonradiative charge carrier recombination in hybrid perovskite materials restricts the device performance of perovskite solar cells. In this study, we report efficient perovskite solar cells through suppressed nonradiative charge carrier recombination by a processing additive, aminopropanoic acid. It is found that aminopropanoic acid not only modulates the crystal growth processes but also minimizes the defects of CH3NH3PbI3 thin films. Moreover, the CH3NH3PbI3 thin films processed with the addition of aminopropanoic acid exhibit both enhanced photoluminescence and electroluminescence and elongated charge carrier lifetime, indicating that nonradiative charge carrier recombination within the CH3NH3PbI3 thin films is drastically suppressed. As a result, perovskite solar cells fabricated using the CH3NH3PbI3 thin films processed with the addition of aminopropanoic acid exhibit approximately 15% enhanced efficiency as compared with those made with pristine CH3NH3PbI3 thin films. All of these results demonstrate that our findings provide a facile way to improve the efficiency of perovskite solar cells.

Non-radiative recombination at dislocations in InAs quantum dots grown on silicon

We study the impact of misfit dislocations on the luminescence from InAs quantum dots (QDs) grown on Si substrates. Electron channeling contrast imaging is used together with cathodoluminescence mapping to locate misfit dislocations and characterize the resulting nonradiative recombination of carriers via near-infrared light emission profiles. With a 5 kV electron beam probe, the dark line defect width due to a typical misfit dislocation in a shallow QD active layer is found to be approximately 1 μm, with a 40%–50% peak emission intensity loss at room temperature. Importantly, we find that at cryogenic temperatures, the dislocations affect the QD ground state and the first excited state emission significantly less than the second excited state emission. At the same time, the dark line defect width, which partially relates to carrier diffusion in the system, is relatively constant across the temperature range of 10 K–300 K. Our results suggest that carrier dynamics in the QD wetting layer control emission intensity loss at dislocations, and that these defects reduce luminescence only at those temperatures where the probability of carriers thermalizing from the dots into the wetting layer becomes significant. We discuss the implications of these findings toward growing dislocation-tolerant, reliable quantum dot lasers on silicon.

Highly Efficient and Stable Planar Perovskite Solar Cells with Modulated Diffusion Passivation Toward High Power Conversion Efficiency and Ultrahigh Fill Factor

2D/3D perovskite heterostructures or composites are recognized as efficient strategies to improve the stability of perovskite solar cells. Herein, a novel solution process to develop 2D/3D perovskites with modulated diffusion passivation by introducing phenylethylammonium iodide (PEAI) and N,N‐dimethylformamide (DMF) additive, which could effectively enhance device performance and long‐term stability, is demonstrated. Compared with conventional devices, the device with PEAI and DMF solvent additive treatment exhibit enhanced charge transport, improved charge extraction, and suppressed nonradiative carrier recombination. The solar cells with an optimal 2D/3D perovskite passivation treatment exhibit an extremely high fill factor of 83.6% and an average power conversion efficiency of 21.4% (21.3% using integrated photocurrent from the incident photon‐to‐current efficiency spectra) based on the NiOx hole transport layer. Furthermore, the unencapsulated device exhibits excellent stability under continuously simulated sunlight illumination and outstanding air stability after 1000 h of storage under ambient air conditions.

Ultra‐Thin Perovskite Solar Cells Analytical Model Involving Radiative and Nonradiative Carrier Recombination Mechanisms

Understanding the reasons for the high efficiencies and open‐circuit voltages of perovskite solar cells is fundamental for achieving performance improvements in the future. Therefore, herein, a perovskite cell model is developed in which both radiative and nonradiative recombination currents are included by two diode equations with ideality factors of 1 and 2, respectively, and then the conversion efficiency dependence on carrier lifetime, perovskite material bandgap, and device thickness is studied, considering these two recombination mechanisms. The capability of the model for the evaluation and prediction of performances of perovskite solar cells is also shown, agreeing very well with current experimental efficiencies for these cells. It avoids complicated numerical solutions for the transport equations and makes use of well‐known analytical expressions for developing a real novel model. This model allows the estimation of the optimum perovskite thickness, and it also helps in determining what should be the required device carrier lifetimes for having increased efficiencies, above the current 23.7% record. It is predicted that (nonradiative) lifetimes larger than 10 μs in the perovskite cell material will not cause any efficiency improvement above the expected limit of 29%. The model agrees well with reported experimental results.

Bismuth Telluride Interlayer for All‐Inorganic Perovskite Solar Cells with Enhanced Efficiency and Stability

To solve the thermal instability issue of organic–inorganic hybrid perovskites, all‐inorganic perovskite solar cells (PSCs) have been featured in the spotlight. However, their power conversion efficiencies (PCEs) are far from satisfactory due to the substantially radiative and nonradiative recombination of charge carriers in the common‐structured devices. Herein, bismuth telluride (Bi2Te3) nanoplates are designed as an interlayer between cesium lead halide (CsPbBrI2) and 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,90‐spirobifluorene (Spiro‐OMeTAD) to reduce the notorious trap states and charge recombination. Confirmed by systematic electrochemical and photoelectrical techniques, the Bi2Te3 interlayer optimizes hole extraction and transport efficiency because of the matched band level structure and drastically reduces trap defect densities. Prolonged effective lifetime and shorter diffusion time induced by the Bi2Te3 interlayer reveal less electron–hole recombination and more efficient carrier transport, which lead to a larger photocurrent and less open circuit voltage loss of PSCs. The all‐inorganic PSCs with the optimal Bi2Te3 interlayer exhibit a highly enhanced PCE of 11.96%. Moreover, Bi2Te3 also acts as a blocking layer for the migration of iodide ions, silver, and moisture, resulting in a considerable device stability of more than 70% of initial PCE after 50 days without extra encapsulation. This low‐cost and facile method for efficient and stable all‐inorganic PSCs offers great promise as a next‐generation renewable energy source.

Interconnected SnO2 Nanocrystals Electron Transport Layer for Highly Efficient Flexible Perovskite Solar Cells

This study reports on interconnected SnO2 electron transport layers (composed of presynthesized SnO2 nanocrystals interconnected by amorphous phase SnOx) processed at low temperature (120 °C) for highly efficient flexible perovskite solar cells. Herein, the amorphous phase SnOx serves as an effective binder to connect the SnO2 nanocrystals to obtain ultra‐smooth electron transport layers. Further characterization of the charge carrier kinetics at the perovskite/electron transport layer interface confirms that the interconnected SnO2 nanocrystals layer facilitates electron extraction and retards nonradiative charge carrier recombination. Consequently, a power conversion efficiency of 16.29% is achieved for flexible perovskite solar cells using the interconnected SnO2 electron transport layer on indium tin oxide/polyethylene terephthalate substrates.

Supramolecular Coordination of Pb2+ Defects in Hybrid Lead Halide Perovskite Films Using Truxene Derivatives as Lewis Base Interlayers.

Truxene derivatives, due to their molecular structure and properties, are good candidates for the passivation of defects when deposited onto hybrid lead halide perovskite thin films. Moreover, their semiconductor characteristics can be tailored through the modification of their chemical structure, which allows-upon light irradiation- the interfacial charge transfer between the perovskite film and the truxene molecules. In this work, we analysed the use of the molecules as surface passivation agents and their use in complete functional solar cells. We observed that these molecules reduce the non-radiative carrier recombination dynamics in the perovskite thin film through the supramolecular complex formation between the Truxene molecule and the Pb2+ defects at the perovskite surface. Interestingly, this supramolecular complexation neither affect the carrier recombination kinetics nor the carriers collection but induced noticeable hysteresis on the photocurrent vs voltage curves of the solar cells under 1 sun illumination.

On the effect of surface recombination in thin film solar cells, light emitting diodes and photodetectors

Radiative and non-radiative charge carrier recombination in thin-film diodes plays a key role in determining the efficiency of electronic devices made of next generation semiconductors such as organic, perovskite and nanocrystals. In this work, we show that lowering the bulk recombination does not necessarily result in enhanced performance metrics of electronic devices. From the perspective of charge carrier extraction and injection, the radiative limit of the open-circuit voltage of solar cells, noise current of photodetectors and lasing threshold of injection lasers cannot be improved if the contacts are not perfectly selective. A numerical drift-diffusion model is used to investigate the interplay between bulk recombination and surface recombination of minority carriers at the contacts in bipolar thin diode devices based on low-mobility semiconductors. The surface recombination becomes prominent in case of reduced bulk recombination strengths when non-selective contacts, i. e. contacts that are either metallic or have imperfect charge-selective interlayer, are employed. Finally, we derive analytical approximations for the case when diffusion-limited surface recombination of minority carriers at Ohmic contacts dominates the dark current. These results indicate that having perfectly selective contacts becomes crucial in systems with suppressed bulk recombination - a challenging requirement for future state-of-the-art thin-film solar cells, light-emitting devices and photodetectors made of next generation semiconductors.

Highly efficient nanocrystalline CsxMA1−xPbBrx perovskite layers for white light generation

Perovskite light converting layers optimization for cost-efficient white light emitting diodes (LED) was demonstrated. High excitation independent internal quantum efficiency (IQE) of 80% and weakly excitation dependent PL spectra suitable for white light generation were obtained in the mixed cation CsxMA1−xPbBr3 perovskite nanocrystal layers with optimal x = 0.3 being determined by effective surface passivation and phase mixing as revealed by x-ray diffraction. Enhancement of the PL homogeneity and the external quantum efficiency (EQE) were secured when using 2,2′,2″-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole (TPBi) additive in the layer preparation process. Excitation dependent PL intensity, decay time, and IQE revealed that the high emission efficiency of the layers originates from a dominant radiative localized exciton recombination (130 ns) weakly influenced by the nonradiative free carrier recombination (750 ns). Warm and cool white LEDs with correlated color temperature 3000 K and 5600 K, and color rendering index 82 and 74, respectively, were realized by using the optimized perovskite layers, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) red emitter and a blue LED.

Ultrahydrophobic 3D/2D fluoroarene bilayer-based water-resistant perovskite solar cells with efficiencies exceeding 22.

Preventing the degradation of metal perovskite solar cells (PSCs) by humid air poses a substantial challenge for their future deployment. We introduce here a two-dimensional (2D) A2PbI4 perovskite layer using pentafluorophenylethylammonium (FEA) as a fluoroarene cation inserted between the 3D light-harvesting perovskite film and the hole-transporting material (HTM). The perfluorinated benzene moiety confers an ultrahydrophobic character to the spacer layer, protecting the perovskite light-harvesting material from ambient moisture while mitigating ionic diffusion in the device. Unsealed 3D/2D PSCs retain 90% of their efficiency during photovoltaic operation for 1000 hours in humid air under simulated sunlight. Remarkably, the 2D layer also enhances interfacial hole extraction, suppressing nonradiative carrier recombination and enabling a power conversion efficiency (PCE) >22%, the highest reported for 3D/2D architectures. Our new approach provides water- and heat-resistant operationally stable PSCs with a record-level PCE.

Morphology control of CsPbBr3 films by a surface active Lewis base for bright all-inorganic perovskite light-emitting diodes

All inorganic perovskites are regarded as promising candidates for light-emitting diodes (LEDs) due to their excellent luminescent properties and material stability. However, the film morphology of the perovskite emitting layer has a substantial effect on the device performance of perovskite LEDs (PeLEDs). Herein, we developed a facial method to optimize the film morphology of solution-processed CsPbBr3 films by incorporating a Lewis base of caprylyl sulfobetaine (SB3-10) into the perovskite precursor. The interaction between SB3-10 molecules and metal ions in perovskites could retard the crystallization of the CsPbBr3 film, producing a uniform film with a smooth surface morphology. In addition, the addition of SB3-10 could passivate the defects and traps in the perovskite layer to suppress the nonradiative recombination of the charge carriers. Consequently, the resulting CsPbBr3:SB3-10 based PeLEDs presented a maximum external quantum efficiency of 2.3% with a bright luminance of 23 240 cd m−2 and a maximum current efficiency of 8.45 cd A−1. The finding in this work provides a better understanding of the role of SB3-10 based Lewis base in controlling the surface morphology of the CsPbBr3 films.

Comprehensive Computational Study of Partial Lead Substitution in Methylammonium Lead Bromide

Impurities in semiconductors, for example, lead-based hybrid perovskites, have a major influence on their performance as photovoltaic (PV) light absorbers. While impurities could create harmful trap states that lead to nonradiative recombination of charge carriers and adversely affect PV efficiency, they could also potentially increase absorption via midgap energy levels that act as stepping stones for subgap photons or introduce charge carriers via doping. To unearth trends in impurity energy states, we use first principles density functional theory calculations to extensively study partial substitution of Pb in methylammonium lead bromide (MAPbBr3), a representative lead-halide perovskite. Investigation of the density of states and energy levels related to the transition of the substitutional defect from one charge state to another reveals that several elements create midgap energy states in MAPbBr3. We machine learned trends and design rules from the computational data and discovered that a few easily computed properties of the bromide compounds of any element can be used to predict the energetics and energy levels of the substitutional defect related to that element. The calculated Fermi level-dependent defect formation energies lead to the observation that substitution by transition metals, Zr, Hf, Nb, and Sc, and group V element Sb can shift the equilibrium Fermi level and change the perovskite conductivity, as determined by the dominant intrinsic point defects. Finally, metal-substituted MAPbBr3 compounds of Bi, Sc, Ni, and Zr were experimentally investigated, and while there was an improvement in the thin-film morphology and an enhancement in charge-carrier lifetimes, no clear evidence of subgap absorption features owing to the substituent being incorporated in the MAPbBr3 lattice could be seen.

Perovskite Solar Cells Yielding Reproducible Photovoltage of 1.20 V

High photovoltages and power conversion efficiencies of perovskite solar cells (PSCs) can be realized by controlling the undesired nonradiative charge carrier recombination. Here, we introduce a judicious amount of guanidinium iodide into mixed-cation and mixed-halide perovskite films to suppress the parasitic charge carrier recombination, which enabled the fabrication of >20% efficient and operationally stable PSCs yielding reproducible photovoltage as high as 1.20 V. By introducing guanidinium iodide into the perovskite precursor solution, the bandgap of the resulting absorber material changed minimally; however, the nonradiative recombination diminished considerably as revealed by time-resolved photoluminescence and electroluminescence studies. Furthermore, using capacitance-frequency measurements, we were able to correlate the hysteresis features exhibited by the PSCs with interfacial charge accumulation. This study opens up a path to realize new record efficiencies for PSCs based on guanidinium iodide doped perovskite films.

Mixtures of Dopant-Free Spiro-OMeTAD and Water-Free PEDOT as a Passivating Hole Contact in Perovskite Solar Cells.

Doped spiro-OMeTAD at present is the most commonly used hole transport material (HTM) in n-i-p-type perovskite solar cells, enabling high efficiencies around 22%. However, the required dopants were shown to induce nonradiative recombination of charge carriers and foster degradation of the solar cell. Here, in a novel approach, highly conductive and inexpensive water-free poly(3,4-ethylenedioxythiophene) (PEDOT) is used to replace these dopants. The resulting spiro-OMeTAD/PEDOT (SpiDOT) mixed films achieve higher lateral conductivities than layers of doped spiro-OMeTAD. Furthermore, combined transient and steady-state photoluminescence studies reveal a passivating effect of PEDOT, suppressing nonradiative recombination losses at the perovskite/HTM interface. This enables excellent quasi-Fermi level splitting values of up to 1.24 eV in perovskite/SpiDOT layer stacks and high open-circuit voltages ( VOC) up to 1.19 V in complete solar cells. Increasing the amount of dopant-free spiro-OMeTAD in SpiDOT layers is shown to enhance hole extraction and thereby improves the fill factor in solar cells. As a consequence, stabilized efficiencies up to 18.7% are realized, exceeding cells with doped spiro-OMeTAD as a HTM in this study. Moreover, to the best of our knowledge, these results mark the lowest nonradiative recombination loss in the VOC (140 mV with respect to the Shockley-Queisser limit) and highest efficiency reported so far for perovskite solar cells using PEDOT as a HTM.

Organic Monomolecular Layers Enable Energy-Level Matching for Efficient Hole Transporting Layer Free Inverted Perovskite Solar Cells.

High-efficiency hole transport layer free perovskite solar cells (HTL-free PSCs) with economical and simplified device structure can greatly facilitate the commercialization of PSCs. However, eliminating the key HTL in PSCs results usually in a severe efficiency loss and poor carrier transfer due to the energy-level mismatching at the indium tin oxide (ITO)/perovskite interface. In this study, we solve this issue by introducing an organic monomolecular layer (ML) to raise the effective work function of ITO with the assistance of an interface dipole created by Sn-N bonds. The energy-level alignment at the ITO/perovskite interface is optimized with a barrier-free contact, which favors efficient charge transfer and suppressed nonradiative carrier recombination. The HTL-free PSCs based on the ML-modified ITO yield an efficiency of 19.4%, much higher than those of HTL-free PSCs on bare ITO (10.26%), comparable to state-of-the-art PSCs with a HTL. This study provides an in-depth understanding of the mechanism of interfacial energy-level alignment and facilitates the design of advanced interfacial materials for simplified and efficient PSC devices.

Perovskite Solar Cells Yielding Reproducible Photovoltage of 1.20 V.

High photovoltages and power conversion efficiencies of perovskite solar cells (PSCs) can be realized by controlling the undesired nonradiative charge carrier recombination. Here, we introduce a judicious amount of guanidinium iodide into mixed-cation and mixed-halide perovskite films to suppress the parasitic charge carrier recombination, which enabled the fabrication of >20% efficient and operationally stable PSCs yielding reproducible photovoltage as high as 1.20 V. By introducing guanidinium iodide into the perovskite precursor solution, the bandgap of the resulting absorber material changed minimally; however, the nonradiative recombination diminished considerably as revealed by time-resolved photoluminescence and electroluminescence studies. Furthermore, using capacitance-frequency measurements, we were able to correlate the hysteresis features exhibited by the PSCs with interfacial charge accumulation. This study opens up a path to realize new record efficiencies for PSCs based on guanidinium iodide doped perovskite films.

Effect of Coulomb interaction with barrier carriers in the waveguide region on multiple quantum well laser inversion threshold

A mechanism of nonradiative recombination of nonequilibrium carriers in semiconductor quantum wells is suggested and discussed. For a studied Auger recombination process the energy of localized electron-hole pair is transferred to barrier carriers due to Coulomb interaction. It is shown that the Auger coefficient is a weak function of temperature and nonmonotonically depends on the quantum well width.

Photoluminescence Flickering and Blinking of Single CsPbBr3 Perovskite Nanocrystals: Revealing Explicit Carrier Recombination Dynamics.

To obtain an in-depth understanding of the dynamics and mechanism of carrier recombination in CsPbBr3 nanocrystals (NCs), we have investigated the photoluminescence (PL) of this material at the single-particle level using the time-tagged-time-resolved method. The study reveals two distinct types of PL fluctuations of the NCs, which are assigned to flickering and blinking. The flickering is found to be due to excess surface trap on the NCs, and the flickering single particles are transformed into blinking ones with significant enhancement of PL intensity and stability on postsynthetic surface treatment. Intensity-correlated lifetime analysis of the PL time trace reveals both trap-mediated nonradiative band-edge carrier recombination and positive trion recombination in single NCs. Dynamical and statistical analysis suggests a diffusive nature of the trap states to be responsible for the PL intermittency of the system. These findings throw light on the nature of the trap states, reveal the manifestation of these trap states in PL fluctuation, and provide an effective way to control the dynamics of CsPbBr3 NCs.