Interface Recombination Velocity Research Articles

Open-Circuit-Voltage Improvement Mechanism of Perovskite Solar Cells Revealed by Operando Spin Observation.

Organic-inorganic hybrid perovskite solar cells have attracted much attention as important next-generation solar cells. Their solar cell performance is known to change during operation, but the root cause of the instability remains unclear. This report describes an investigation using electron spin resonance (ESR) to evaluate an improvement mechanism for the open-circuit voltage, VOC, of inverted perovskite solar cells at the initial stage of device operation. The ESR study revealed electron transfer at the interface from the perovskite layer to the hole-transport layer not only under dark conditions but also under light irradiation, where electrons are subsequently trapped in the hole-transport layer. An electron barrier is enhanced at the perovskite/hole-transport-layer interface, improving field-effect passivation at the interface. Thereby, the interface recombination velocity is reduced, and thus the VOC improves. These findings are crucially important for elucidating the mechanisms of device performance changes under operation. They reveal a relation between charge transfer and performance improvement, which is valuable for the further development of efficient perovskite solar cells.

Computational Investigation of c-GaN/GaAs1–x N x /GaAs Heterojunction Solar Cell

GaN-based electronics have witnessed an increase in both research and industrial activities, first spurred by the successful demonstration of GaN LEDs, and are now expanding into transistors and photovoltaic cells. In addition, GaN/GaAs heterojunction devices are currently receiving much interest. In this study, we conduct rigorous optoelectronic computational analysis of cubic phase GaN (c-GaN)/GaAs heterojunction solar cells for a comprehensive understanding of the cell. We utilize a compositionally graded GaAs1–xNx buffer layer to reduce defect states at the heterojunction interface caused by a significant lattice mismatch between c-GaN and GaAs. Furthermore, we enhance the performance of the cell by optimizing GaAs absorber layer thickness and c-GaN buffer layer doping concentration. Moreover, we examine the effects of GaAs1–xNx/GaAs interface recombination velocity (IRV) on the cell. Overall, we achieve ~23% power conversion efficiency within 1.25- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> thin-film GaAs at low GaAs1–xNx/GaAs IRV. The analyses and results presented in this study demonstrate the vast application potential of c-GaN/GaAs heterojunction in high-efficiency solar cells.

High minority carrier mobility and electronic diffusion length in annealed GaInAsSb/GaSb active layer in Vertical Cavity Surface Emitting Laser (VCSEL)

Owing to its important role as an active layer for Vertical Cavity Surface Emitting laser (VCSEL). We studied the transport properties in annealed GaInAsSb /GaSb by means of PTD technique. We remark that annealing time improve significantly the diffusion length and minority carrier mobility. For an annealed time duration of 60 min the diffusion length reach 25 µm (3.9%) and the minority carrier mobility reach 3673 cm2/V. (3.4%).We have observed that surface recombination velocity (SRV) decreased in respect of annealing time and was found to be 1450 m / s (± 3.6). Unfortunately, interface recombination velocity (IRV) is highly increased from 1196 m / s (± 5) to 6000 m/s (5%) for GaInAsSb as grown to that annealed during 60 min.

Large‐Area (Ag,Cu)(In,Ga)Se2 Thin‐Film Solar Cells with Increased Bandgap and Reduced Voltage Losses Realized with Bulk Defect Reduction and Front‐Grading of the Absorber Bandgap

The 1.24 eV bandgap, 18.8% power conversion efficiency Ag‐alloyed chalcopyrite (Ag,Cu)(In,Ga)Se2 (ACIGS) solar cells are characterized to relate voltage and efficiency improvements to electro‐optical (EO) characteristics. Shockley–Read–Hall recombination center defect density, identified and characterized through deep level transient spectroscopy and time‐resolved photoluminescence (TRPL), is reduced through potassium and copper treatment optimization. Concomitantly, longer minority carrier lifetimes are achieved, which increases open‐circuit voltage (VOC). Near‐conduction band defects associated in earlier studies with light‐induced current instability are also mitigated. Analysis of charge‐carrier dynamics after single‐ and two‐photon excitation is used to separate recombination at the front interface and in the absorber bulk. From TRPL decay simulations, the authors estimate ranges of key solar cell material characteristics: bulk carrier lifetime τbulk = 110–210 ns, charge‐carrier mobility μ = 110–160 cm2 V−1 s−1, and front interface recombination velocity Sfront = 700–1050 cm s−1. This lowest‐reported Sfront for ACIGS absorbers originates from the notched conduction band grading, which also makes the impact of the back interface recombination negligible. It is suggested in the results that solar cell performance enhancements can be made most readily with two distinct strategies: improving device architecture and reducing semiconductor defect densities. Using these approaches, power conversion efficiency in large‐area solar cells is improved by 1.1% absolute.

Characterization of Interface Recombination Velocity in GaAs/InGaP Heterojunction Solar Cells Using Dark Curve Measurements

In this article, a method for obtaining the interface recombination velocity in a solar cell from a measurement of the dark curve is presented. The front interface recombination <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S_f</i> between window and emitter of a GaAs-InGaP heterojunction cell passivated by an AlInP window is used to demonstrate the method. By choosing a proper emitter thickness, the equation for the dark saturation current <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> reduces to two terms, one related to the bulk recombination and the other to the interface recombination. The growth direction (either upright or inverted) is found to have a huge influence on <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S_f</i> as a result of the formation of an interfacial layer in the inverted grown cells. It is also shown that <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S_f</i> and, therefore, the optimum cell design strongly depend on the emitter doping level. Using the obtained <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S_f</i> data, an optimized cell configuration on the substrate and in a thin film has been calculated, leading to a 1% (absolute).

Optimization of inverted planar perovskite solar cells with environment-friendly MAPb0.25Ge0.75I3 absorber approaching Shockley Queisser limit for efficiency

The search for an environmentally benign substitution of toxic lead (Pb) in MAPbI3 –based perovskite solar cells has so far resulted in a compromised performance like poorer efficiency, which is more pronounced in inverted planar (p-i-n) structure. A few theoretical studies have revealed that mixed cation perovskites with low Pb-content, MAGe0.75Pb0.25I3, can have an optical bandgap between 1.33 and 1.41 eV, a bandgap range that can attain maximum efficiency indicated by Shockley Quiesser (SQ) limit. With the motivation of finding an environmental friendly as well as high performing perovskite solar cells, an inverted planar cell configuration is simulated utilizing MAGe0.75Pb0.25I3 as the absorber with commonly used transport layers. The simulation model is optimized based on MAGe0.75Pb0.25I3 material parameters (bandgap, Eg and electron affinity, χ) and film properties (thickness, bulk, and interface defect densities). It is observed that landmark efficiency over 30% is obtainable with Eg = 1.4eV at typical film thicknesses of 500–600 nm with absorber bulk defect density 1014 cm−3 and interface recombination velocity 100 cms−1. Moreover, the cell displays significant efficiency of 20–24% even at higher defect densities, emulating degraded absorber film quality. Thus from the comprehensive study presented here, it is quite evident that MAGe0.75Pb0.25I3 indicates a pathway to approach the SQ limit in terms of efficiency for perovskite solar cells.

Efficiency limits of perovskite solar cells with n-type hole extraction layers

Inorganic materials, such as MoOx and V2Ox, are increasingly explored as hole transport layers for perovskite based solar cells. Due to their large work function and n-type nature, hole collection mechanisms with such materials are fundamentally different, and the associated device optimizations are not well elucidated. In addition, prospects of such architectures against the challenges posed by ion migration are yet to be explored—which we critically examine in this contribution through detailed numerical simulations. We find that, for similar ion densities and interface recombination velocities, ion migration is more detrimental for perovskite solar cells with n-type hole transport layers with much lower achievable efficiency limits (∼21%). The insights shared by this work could be of broad interest to critically evaluate the promises and prospects of n-type materials as hole transport layers for perovskite solar cells.

Distinguishing bulk and surface recombination in CdTe thin films and solar cells using time-resolved terahertz and photoluminescence spectroscopies

Understanding the nature of recombination and its dependence on defects and interfaces is essential for engineering materials and contacts for a higher open-circuit voltage (Voc) and power conversion efficiency in photovoltaic (PV) devices. Time-resolved photoluminescence (TRPL) has conventionally been used to evaluate recombination, but carrier redistribution often dominates the response at short times. Here, we report on the quantification of carrier dynamics and recombination mechanisms by complementary use of both time-resolved terahertz spectroscopy and TRPL combined with numerical modeling of the continuity equations and Poisson's equation. We have demonstrated this approach using CdTe thin films. A thin-film stack with CdTe fabricated by vapor transport deposition and treated with CdCl2 exhibited a bulk lifetime of 1.7 ± 0.1 ns, a negligible CdTe/CdS interface recombination velocity, and a back surface recombination velocity of 6.3 ± 1.3 × 104 cm/s. In contrast, a film stack without CdCl2 treatment had a bulk lifetime of only 68 ± 12 ps and a higher interface recombination velocity of 4 ± 2 × 108 cm/s. By determining the locus and mechanisms of performance-limiting recombination, we can accelerate the development of thin-film PVs with higher Voc and efficiency. While the method has been demonstrated here using CdTe, it is also applicable to perovskites, Cu(InGa)Se2, Cu2ZnSn(S,Se)4, and emerging technologies.

Is a passivated back contact always beneficial for Cu (In,Ga)Se2 solar cells?

AbstractUltrathin CIGSe solar cells on TCOs (transparent conductive oxides) have many promising potential applications due to their transparency. However, their performance is strongly constrained due to the back contact, often referred to acting as a Schottky junction. In this work, we show that the current limitation due to a back Schottky junction can be responsible for the apparent photocurrent loss and suppression of the forward current, which deteriorates cell performance and leads to the S‐shaped J‐V characteristics, respectively. However, due to the non‐negligible photocurrent in the back Schottky diode and the current amplification effect of a transistor, cells with a reduced absorber thickness overall exhibit a relieved current suppression, resulting in a light J‐V curve closer to the one with an Ohmic back contact. Notably, a higher back interface recombination velocity is capable of increasing the forward current of the Schottky diode by recombination, which mitigates the limitation of current flow through the back Schottky junction and thus improves the cell performance. The results show strong implications that introducing high density defects at the back interface is favorable for CIGSe solar cells on TCOs, rather than employing point‐contact structures for interface passivation. This suggests a different path to improve ultrathin CIGSe solar cells on TCOs compared to the devices on Mo with an Ohmic back contact.

Improvement of quantum efficiency of field-assisted GaN monolayer photocathode

In this paper, in order to explore quantum efficiency of field-assisted GaN monolayer photocathode, uniform-doping and exponential-doping reflection-mode GaN monolayer photocathode model with an external electric field have been established, and quantum efficiency formulas have been derived. We simulate the quantum efficiency of photocathode with uniform-doping and exponential-doping structure under different external electric fields, number of monolayers in emission layer, thickness of buffer layer and interface recombination velocity. The results show that quantum efficiency enhances with increase of external electric field. As the thickness of emission layer increases, the quantum efficiency decline, while as the thickness of buffer layer increases, the quantum efficiency improves. At the same time, an appropriate interface recombination velocity can improve the quantum efficiency of photocathode. When back surface recombination velocity and GaN ML/GaN ML interface recombination velocity are 8 × 106 cm/s and 108 cm/s, respectively, the optimal quantum efficiency can be obtained.

Extraction of PEDOT:PSS/c‐Si Junction Properties by Modeling of Injection‐Dependent Lifetime Measurements Including Depletion Region Modulation

The carrier lifetime of n‐type silicon wafers coated with the conducting polymer PEDOT:PSS as a function of the excess carrier concentration Δp within the wafer is characterized using the quasi‐steady‐state photoconductance (QSSPC) method, and a drastic increase in the measured apparent lifetime τapp with decreasing Δp is observed. The observed increase with the depletion region modulation (DRM) effect is explained, as PEDOT:PSS‐coated p‐type silicon wafers do not show any increase in lifetime toward low injection levels. By modeling the measured τapp(Δp) curves on n‐type silicon including interface recombination as well as the DRM effect, the interface recombination velocity as well as the band bending Ψs within the silicon induced by PEDOT:PSS are able to be extracted. The impact of adding sorbitol to the PEDOT:PSS dispersion on the τapp(Δp) curves is examined, and it is demonstrated that the admixture of sorbitol improves chemical interface passivation but leaves the band bending within the silicon bulk toward the PEDOT:PSS/c‐Si interface unaffected.

Materials requirements for improving the electron transport layer/perovskite interface of perovskite solar cells determined via numerical modeling

Perovskite solar cells continue to garner significant attention in the field of photovoltaics. As the optoelectronic properties of the absorbers become better understood, attention has turned to more deeply understanding the contribution of charge transport layers for efficient extraction of carriers. Titanium oxide is known to be an effective electron transport layer (ETL) in planar perovskite solar cells, but it is unlikely to result in the best device performance possible. To investigate the importance of band energy alignment between the electron transport layer and perovskite, we employ numerical modeling as a function of conduction band offset between these layers, interface recombination velocity, and ETL doping levels. Our simulations offer insight into the advantages of energy band alignment and allow us to determine a range of surface recombination velocities and ETL doping densities that will allow us to identify novel high performance ETL materials.

Study of surface photovoltage spectrum in p+-GaAs/p-GaAlAs/p-GaAs structures

Surface photovoltage (SPV) in p+-GaAs/p-GaAlAs/p-GaAs has been studied by establishing a multilayer model and measuring the SPV at room temperature. The model mainly considers surface recombination velocity, interface recombination velocity and the space charge region (SCR) at the surface of p+-GaAs. The SPV of the multilayer structure is shown to originate predominantly from the minority carrier diffusion, which caused photovoltage between the surface and bottom. Subsequently, the minority carrier diffusion lengths in p+-GaAs and in p-GaAs are obtained from fitting experimental data to the theoretical model. At the same time, the minority carrier diffusion length in p-GaAs is obtained by illuminating the backside (illuminating on p-GaAs) of the p+-GaAs/p-GaAlAs/p-GaAs. The p+-GaAs in p+-GaAs/p-GaAlAs/p-GaAs structure with different thickness are measured to show the variation of SPS with different thickness, but the experimental parameters are not affected. In multi-layer structure, the SPV contributed by different layers has a great difference with different dark saturation current density.

Impact of Carrier Concentration and Carrier Lifetime on MgZnO/CdSeTe/CdTe Solar Cells

The performance impact of multiple parameters related to the use of MgZnO (MZO) as the emitter for bilayer CdSeTe/CdTe solar cells has been investigated in detail through numerical simulations. Such a comprehensive study is particularly important, because while cell fabrication using MZO has been highly successful in some cases, it has been less so in others, and it has not been clear which combinations of parameter values are most effective. The parameters considered here include the recombination velocity at the emitter/absorber interface, bulk recombination lifetime, and the carrier concentrations of the emitter and absorber. The ranges chosen for the simulation parameters are those most likely to be found experimentally. The primary finding is that independent of the interfacial recombination velocity and bulk recombination lifetime, the MZO carrier density should be $>$ 1017 cm–3 and in any case greater than that of the absorber to reduce interface recombination. At the same time, a shallow dopant the order of 50 meV or less should reduce Shockley-Read-Hall recombination in the bulk and enable $V_{\text{OC}} >$ 1 V.

Analyzing Interface Recombination in Lead‐Halide Perovskite Solar Cells with Organic and Inorganic Hole‐Transport Layers

AbstractThe interfaces between absorber and transport layers are shown to be critical for perovskite device performance. However, quantitative characterization of interface recombination has so far proven to be highly challenging in working perovskite solar cells. Here, methylammonium lead halide (CH3NH3PbI3) perovskite solar cells are studied based on a range of different hole‐transport layers, namely, an inorganic hole‐transport layer CuOx, an organic hole‐transport layer poly(triarylamine) (PTAA), and a bilayer of CuOx/PTAA. The cells are completed by a [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM)/bathocuproine/Ag electron contact. Energy levels are characterized using photoelectron spectroscopy and recombination dynamics by combining steady‐state photoluminescence and transient photoluminescence with numerical simulations. While the PTAA‐based devices hardly show any interface recombination losses and open‐circuit voltages &gt;1.2 V, substantial losses are observed for the samples with a direct CuOx/perovskite interface. These losses are assigned to a combination of energetic misalignment at the CuOx/perovskite interface coupled with increased interface recombination velocities at the perovskite/PCBM interface.

Carrier recombination dynamics in Ga0.51In0.49P double-heterostructures up to 500 K

The bulk minority carrier lifetime and interface recombination velocity (IRV) in GaInP double-heterostructures (DHs) lattice matched to GaAs are extracted using time-resolved photoluminescence (PL) measured between 300 and 500 K. Effective lifetimes show a strong dependence on temperature for samples with insufficiently strong confinement potentials due to significant thermionic emission losses out of the DHs. An increased PL signal from the underlying GaAs layer relative to GaInP’s PL at high temperatures supports this hypothesis. The impact is a shorter recombination lifetime which can be wrongly interpreted as a high IRV of up to 4500 cm s−1 at 500 K. These effects are investigated experimentally using samples of different barrier heights based on the Al-content in (AlxGa1–x)0.51In0.49P. A larger barrier height is shown to inhibit thermionic emission out of the DH, thus revealing a more accurate IRV of 300 cm s−1 at 500 K. The results are then used to develop a correction procedure to extract a more accurate IRV at the barriers of the DH. Optoelectronic device simulations are used to gain insight into carrier dynamics as a function of temperature and the DH’s barrier height, and confirm a strong inhibition of the thermionic emission losses as the Al-content is maximized in the barrier.

The Critical Role of AlInP Window Design in III–V Rear-Emitter Solar Cells

This article highlights the critical role of window design on short circuit carrier collection in rear-emitter solar cells, as demonstrated through modeling and experiment using metamorphic GaAs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-y</sub> . Ultimately, if the window design is not carefully considered, surface depletion caused by Fermi level pinning at the window/air interface can extend into regions of active collection resulting in a large increase in effective window/base interface recombination velocity. This was experimentally shown here to result in a potential AM1.5G photocurrent loss of 8.1 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , or nearly 50%, based on integrated internal quantum efficiency (IQE). Associated IQE modeling and curve fitting indicate that the effective IRV at the GaAs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.75</sub> P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.25</sub> /Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.64</sub> In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.36</sub> P interface is increased by multiple orders of magnitude when the window is not sufficiently thick or doped to fully contain the surface depletion to within the window layer. Calculations of the surface depletion depth as a function of doping and the surface Fermi pinning energy level provides insight into the fundamental limits of window thickness. This allows the estimation of the Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.64</sub> In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.36</sub> P surface pinning level to be at least 1.25 eV below the conduction band edge, or in the bottom half of the bandgap.

Design of Ultrathin InP Solar Cell Using Carrier Selective Contacts

Most recently, III–V based ultrathin solar cells have attracted considerable attention for their inherent advantages, such as increased tolerance to defect recombination, efficient charge carrier separation, photon recycling, flexibility, and reduced material consumption. However, so far, almost all reported devices make use of conventional doped p-i-n kind of structures with a wide-bandgap III–V lattice-matched epitaxial window layer, for passivation and reduced contact recombination. Here, we show that a high-efficiency device can be obtained utilizing an InP thin film of thickness as low as 280 nm, without the requirement of a conventional p-n homojunction or epitaxial window layer. This is achieved by utilizing a wide-bandgap electron and hole selective contacts for electrons and holes transport, respectively. Under ideal conditions [assuming interface recombination velocity (IRV) = 103 cm/s and bulk lifetime = 1 μs], the proposed solar cell structure can achieve efficiency as high as 28%. Although, in the presence of bulk and interface Shockley–Read–Hall recombination, the efficiency reduces, still for bulk minority carrier lifetime as low as 2 ns and an IRV as high as 105 cm/s, an efficiency of ∼22% can be achieved with InP thickness as low as 280 nm. The proposed device structure will be beneficial in cases where the growth of controlled p-n homojunction and window layer can be tedious as in case of low-cost deposition techniques, such as thin-film vapour–liquid–solid and close-spaced vapour transport.

Research on quantum efficiency of exponential-doping GaN monolayer reflection-mode photocathode with ultra-thin emission layer

In this paper, we present an exponential-doping GaN monolayer reflection-mode photocathode whose emission layer is composed of few GaN monolayers with different doping concentrations. To understand its optoelectronic emission performance, the quantum efficiency formula of exponential-doping GaN photocathode with ultra-thin monolayers as the emission layer is obtained. Then, we simulate the impact of recombination velocity of AlGaN/GaN ML interface, recombination velocity of GaN ML/GaN ML interface, thickness of emission layer with GaN monolayers, surface escape probability and surface reflectivity on quantum efficiency based on the formula, respectively. The results imply that interface recombination velocity made a significant contribution to quantum efficiency of photocathode. When interface recombination velocity in AlGaN/GaN ML and GaN ML/GaN ML is appropriate, they will promote electrons’ escape to the cathode surface and achieve higher quantum efficiency finally. Through our simulation results, a helpful reference can be given for design of exponential-doping GaN monolayer photocathode.

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.