Dominant Recombination Mechanism Research Articles

Unravelling the electrochemical impedance spectroscopy of hydrogenated amorphous silicon cells for photovoltaics

Abstract This research reveals the application of electrochemical impedance spectroscopy (EIS) in analyzing and improving the performance of hydrogenated amorphous silicon (a-Si: H) based photovoltaic cells. As a non-destructive technique, EIS provides deep insight into the electrochemical characteristics of photovoltaic cells, including series resistance, layer capacitance, recombination mechanisms, and charge transport. The impedance data is obtained and analyzed using small AC potential signals at various frequencies via Nyquist diagrams and Bode plots. This analysis allows the identification of resistive and capacitive elements as well as the evaluation of the quality of the interface between the active layer and the electrode. The results show that EIS can identify internal barriers that reduce the efficiency of a-Si: H solar cells, such as dominant recombination mechanisms and inefficient charge transport. Using equivalent circuit models, electrochemical parameters are extracted to reveal cell behavior and performance. In addition, these results also confirm that EIS is an important tool in design optimization and performance improvement of a-Si: H photovoltaic cells, providing a solid scientific basis for the development of more efficient and sustainable solar cell technology. These findings contribute to efforts to increase solar energy efficiency, supporting broader and more effective use of photovoltaic technology in meeting global sustainable energy needs.

Comprehensive study of photoluminescence and device properties in Cu2Zn(Sn1−xGex)S4 monograins and monograin layer solar cells

Ge-alloying of Cu2ZnSnS4 is a promising strategy for producing wide-bandgap absorber materials suitable for use in tandem structures or indoor solar cell applications. Incorporating Ge can suppress Sn-related defects while maintaining the kesterite structure and p-type conductivity throughout the entire range of composition. In this study, Cu2Zn(Sn1−xGex)S4 monograins were synthesized in molten salt by the synthesis-growth method, where value x was varied from 0 to 1, with a step of 0.2. The inclusion of Ge into the crystals was confirmed by energy-dispersive X-ray spectroscopy, Raman spectroscopy, and X-ray diffraction analysis. The bandgaps of Cu2Zn(Sn1−xGex)S4 solid solutions were determined as Eg = 1.50–2.25 eV by external quantum efficiency measurement. A detailed study of temperature and laser power-dependent photoluminescence (PL) for powder crystals with x = 0, x = 0.2 and x = 0.4 revealed that the dominant recombination mechanisms originate from defect clusters involving a shallow acceptor and deep donor defect. Ge-alloying helped to suppress the harmful SnZn donor defects, but at the same time shifted the defects' energy levels deeper into the bandgap. The obtained activation energies indicate that the acceptor defect becomes shallower with the inclusion of Ge. At the mid-temperature range (T = 60–200 K), the presence of two different recombinations was revealed in the system, originating from very closely located PL emission bands.

Effective Lifetime of Nonequilibrium Carriers in Perovskite‐Inspired Cu2AgBiI6

Perovskite‐inspired materials (PIMs) are potential alternatives to lead halide perovskites, as they not only inherit the benign optoelectronic properties but also diminish the stability and toxicity issues of lead halide perovskites. As a newly discovered PIM, Cu2AgBiI6 has exhibited promising potential for photovoltaic applications. However, studies on its fundamental properties related to photovoltaic performance are scarce, particularly from a theoretical perspective. Herein, the effective lifetime of nonequilibrium carriers (photo‐excited charge carriers) is systematically investigated, a critical property affecting the photovoltaic performance of Cu2AgBiI6, based on the nonadiabatic molecular dynamics simulations. It is found that under the standard solar spectrum illumination, the dominant recombination mechanism affecting the effective lifetime can be band‐to‐band nonradiative decay, band‐to‐band radiative decay, or Shockey–Read–Hall defect‐assisted decay. The specific mechanism is highly dependent on the radiative recombination coefficient and the density of defect recombination levels. The effective lifetime can vary from 0.1 ms to 10 ns. When considering different illumination conditions (generation rates), Auger decay can also become the dominant recombination mechanism, with the effective lifetime varying from 0.1 s to 0.1 ns. These findings can be vital for further experimental researches aimed at enhancing the power conversion efficiency of Cu2AgBiI6‐based solar devices.

Surface recombination in Pyrex in oxygen DC glow discharges: mesoscopic modelling and comparison with experiments

Surface recombination in an oxygen DC glow discharge in a Pyrex (borosilicate glass) tube is studied via mesoscopic modelling and comparison with measurements of recombination probability. A total of 106 experimental conditions are assessed, with discharge current varying between 10 and 40 mA, pressure values ranging between 0.75 and 10 Torr, and fixed outer wall temperatures ( Tw ) of −20, 5, 25 and 50∘C . The model includes O+O and O+O2 surface recombination reactions and a Tw dependent desorption frequency. The model is validated for all the 106 studied conditions and intends to have predictive capabilities. The analysis of the simulation results highlights that for Tw=−20∘C and Tw=5∘C the dominant recombination mechanisms involve physisorbed oxygen atoms ( OF ) in Langmuir–Hinshelwood (L-H) recombination OF+OF and in Eley–Rideal (E-R) recombination O2+OF , while for Tw=25∘C and Tw=50∘C processes involving chemisorbed oxygen atoms ( OS ) in E-R O+OS and L-H OF+OS also play a relevant role. A discussion is taken on the relevant recombination mechanisms and on ozone wall production, with relevance for higher pressure regimes.

Ultrafast photocarrier dynamics in InAs/GaAs self-assembled quantum dots investigated via optical pump-terahertz probe spectroscopy

Optical pump-terahertz probe (OPTP) spectroscopy was performed to measure the lifetime of photogenerated carriers in the barrier and the wetting layer (WL) regions of an indium arsenide on gallium arsenide (InAs/GaAs) single-layer self-assembled quantum dot (QD) sample. A modified rate equation model of carrier dynamics was proposed where possible state-filling in both QD and WL is considered. Drude model fitting was also performed to extract the time-dependent plasma frequency and phenomenological scattering time from the terahertz transmission spectra. The results of the OPTP experiment show two prominent recombination processes that occur at different timescales after photoexcitation. These two processes were attributed to carrier recombination in the GaAs barrier and the quantum well-like states of the WL based on the fitted lifetimes. Calculations using the coupled differential rate equations were also able to replicate the experimental trend at low fluence. The lack of agreement between experimental data and numerical calculations at high optical fluence was mainly attributed to the possible saturation of the GaAs density of states. Lastly, the results of the parameter fitting for the plasma frequency and scattering time indicate a transition from the barrier to the WL recombination as the dominant carrier recombination mechanism within the time scale of the OPTP scan. This further lends credence to the proposed model for carrier dynamics in SAQD systems under state-filling conditions.

Carrier recombination in highly Al doped 4H-SiC: dependence on the injection conditions

We investigate carrier recombination mechanisms in heavily aluminum (Al) doped p-type 4H-SiC, a material crucial for power devices. The recombination mechanisms in Al-doped p-type 4H-SiC have remained unclear, with reports suggesting various possibilities. To gain insights, we employ photoluminescence (PL) measurements, particularly time-resolved PL (TR-PL), as they are well-suited for studying carrier lifetimes in heavily Al-doped p-type 4H-SiC. We examine the temperature and excitation intensity dependencies of TR-PL and PL spectra and discuss the underlying recombination mechanisms. We observe that the dominant recombination mechanism varies with injection conditions for the samples with Al concentration less than 1019 cm−3. Under low injection conditions, recombination via the Al acceptor level appears dominant, exhibiting weak temperature dependence. However, under high injection conditions, Shockley–Read–Hall recombination takes precedence, leading to shorter carrier lifetimes with increasing temperature. This temperature dependence implies that presences of the deep recombination centers with the small capture barrier for holes.

Asymmetric and symmetric interfaces in type II MWIR InAs/GaSb superlattices

We report on the comprehensive investigations of InAs/GaSb type-II superlattice (SL) with asymmetric and symmetric interfaces (IFs) formed using a migration-enhanced epitaxy (MEE) and an interrupted growth. The best structural and optical properties were observed for strain-balanced SL with asymmetric IFs grown using MEE at the bottom IF and continuous growth at the upper IF. The analysis of IF-related lines in Raman spectra of investigated SLs allowed for the evaluation of the quality of the InSb-like and GaAs-like interfaces. The thermal dependency of band gap energy was established from photoluminescence measurements according to the Varshni formula. The activation energies E1 and E2 for the dual-channel non-radiative recombination process were calculated. Linear characteristics of PL intensity vs. excitation power density were fitted using the relation IPL ∼ Eexck with different k. This allowed for the indication of dominant recombination mechanisms: radiative with a share of Auger process (k ≈ 0.8), pure radiative (k ≈ 1.0), and radiative with a slight share of Shockley–Read–Hall (k ≈ 1.1). Different behaviours of PL peak energy depending on the increase of excitation power density were observed: it was either blue-shifted, red-shifted, and/or a combination of both.

Carrier generation and collection in Zn3P2/InP heterojunction solar cells

Zinc phosphide (Zn3P2) has been lauded as a promising solar absorber material due to its functional properties and the abundance of zinc and phosphorous. In the last 4 decades, there has not been any significant improvement in the efficiencies of Zn3P2-based solar cells. This is vastly due to the limited understanding of how to tune its optoelectronic properties. Recently, significant progress has been made in the growth and characterization of the material, which has shed light on its potential. In this study, we report an energy conversion efficiency as high as 4.4% for a solar cell based on a polycrystalline Zn3P2/InP heterojunction. This device presents an open circuit voltage of 528.8 mV, 7.5% higher than the Zn3P2 homojunction record. We investigate the dominant recombination mechanisms within the film using different techniques to identify the key factors underpinning our device efficiency. In particular, we pinpoint that reduced carrier collection at the front of Zn3P2 is responsible for the reduced collection of high-energy photons. This allows us to suggest the design rules for next-generation Zn3P2-based heterojunction solar cells, which should allow us to go beyond the current conversion values.

Charge Carrier Lifetime Determination in Graded Absorber Solar Cells Using Time‐Resolved Photoluminescence Simulations and Measurements

Thin‐film photovoltaic device efficiencies are limited by carrier recombination, thus understanding recombination mechanisms is critical for performance improvements. Bulk minority carrier lifetime (τ bulk) is a critical parameter for solar cells but is difficult to determine in P–N junction devices, especially for high doping. As doping ≥1016 cm−3 is required for efficient drift‐charge‐carrier‐collection devices, a method for τ bulk determination in doped P–N junction devices is necessary. This work utilizes time‐resolved photoluminescence (TRPL) simulations to quantify bulk and interface recombination properties in highly doped, graded absorber CdSeTe structures. The two methods developed here for τ bulk determination include utilization of an instantaneous lifetime representation to guide TRPL fitting and direct comparison between measured and simulated decays. Simulations verified that both methods are valid for state‐of‐the‐art device architectures which include graded bandgap absorbers, graded doping, and graded lifetimes. Shifts in the dominant recombination mechanism are identified for sufficiently long τ bulk, where front and back interface quality plays a more prominent role. Evaluation of surface recombination velocities and conduction band offset illustrate electro‐optical advantages of a positive conduction band offset and highlight the necessity of improved interfaces as bulk quality in photovoltaic devices improves.

Evaluating Recombination Mechanisms in RbF Treated Cu(In${}_\mathrm{x}$Ga$_\mathrm{1-x}$)Se$_{2}$ Solar Cells

Rubidium fluoride (RbF) postdeposition treatment (PDT) has been shown to improve the performance of Cu(In <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${}_\mathrm{x}$</tex-math></inline-formula> Ga <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_\mathrm{1-x}$</tex-math></inline-formula> )Se <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{2}$</tex-math></inline-formula> (CIGS) photovoltaic devices. In this study, temperature-dependent current voltage (JVT) and time-resolved photoluminescence (TRPL) experiments were combined with modeling using the solar cell capacitance simulator (SCAPS) computer code to investigate the effect of the RbF PDT. Two devices, one as-deposited and one with RbF PDT, were deposited by a three stage coevaporation process. JVT measurements suggest the dominant recombination mechanism may be tunneling-enhanced recombination via bandtail states, but that defect states in the bandgap can also be important. RbF PDT is shown to decrease the characteristic energy of the bandtails. TRPL data show an increase in the minority carrier lifetime after RbF PDT, leading to an improved open-circuit voltage. SCAPS modeling indicates that the dominant recombination mechanism is dependent on the specific defect makeup of a device, suggesting that small changes in processing conditions can impact device behavior. This explains the observation that, for some devices, defect states in the gap dominate while others, as is the case here, appear to be dominated by bandtails.

Identifying dominant recombination mechanisms in spiro-based conventional perovskite solar cells: Roles of interface and bulk recombination

Perovskite solar cells (PSCs) have been popular in the photovoltaic (PV) community, owing to their ease of processing and high efficiency. In order to improve their device performance, the interfaces are believed to play a vital role. In this regard, the interfacial energies and the associated physical processes such as charge transport, accumulation and recombination have been the focus of the research community. Despite the rise in power conversion efficiency (PCE) and all development, PSCs still suffer from the charge carrier recombination. As a result, highly efficient PSCs cannot meet the theoretical values of fill factor (FF) and open-circuit voltage ( VOC). Therefore, to reach out to the theoretical limit of performance for PSCs, carrier recombination needs to be controlled both in the bulk and at the corresponding interfaces. Herein, we have systematically studied the recombination in PSCs by establishing the role of different layers of PSCs using electrochemical impedance spectroscopy (EIS). To get insights into the role of different layers in recombination we fabricated PSCs with n-i-p configuration and changed the thickness of TiO 2 electron transport layer (ETL), absorber layer, and spiro-OMeTAD hole transport layer (HTL). Our results suggest that the recombination is reduced in case of devices with thicker HTL and is trap-assisted recombination. These results are in good agreement with impedance spectroscopy analysis. The value of recombination resistance (extracted by fitting of Nyquist response) is higher at VOC, which is consistent with higher VOC in these two cases. In case of the increased ETL thickness, the performance of the device is dropped since the increased interfacial resistance encourages the recombination. Our study presents a simple and effective approach to elucidate the dominating recombination sites in PSCs and improve the device performance.

Revealing fundamentals of charge extraction in photovoltaic devices through potentiostatic photoluminescence imaging

Revealing fundamentals of charge extraction in photovoltaic devices through potentiostatic photoluminescence imaging

Reduced Auger Coefficient through Efficient Carrier Capture and Improved Radiative Efficiency from the Broadband Optical Cavity: A Mechanism for Potential Droop Mitigation in InGaN/GaN LEDs.

Efficiency droop at high carrier-injection regimes is a matter of concern in InGaN/GaN quantum-confined heterostructure-based light-emitting diodes (LEDs). Processes such as Shockley-Reed-Hall and Auger recombinations, electron-hole wavefunction separation from polarization charges, carrier leakage, and current crowding are identified as the primary contributors to efficiency droop. Auger recombination is a critical contributor owing to its cubic dependence on carrier density, which can not be circumvented using an advanced physical layout. Here, we demonstrate a potential solution through the positive effects from an optical cavity in suppressing the Auger recombination rate. Besides the phenomenon being fundamentally important, the advantages are technologically essential. The observations are manifested by the ultrafast transient absorption pump-probe spectroscopy performed on an InGaN/GaN-based multi-quantum well heterostructure with external DBR mirrors of varying optical confinement. The optical confinement modulates the nonlinear carrier and photon dynamics and alters the rate of dominant recombination mechanisms in the heterostructure. The carrier capture rate is observed to be increasing, and the polarization field is reducing in the presence of optical feedback. Reduced polarization increases the effective bandgap, resulting in the suppression of the Auger coefficient. Superluminescent behavior along with enhanced spectral purity in the emission spectra in presence of optical confinement is also demonstrated. The improvement is beyond the conventional Purcell effect observed for the quantum-confined systems.

Photoelectrical properties of flexible quasi-interdigitated back-contact perovskite solar cells

Perovskites are a promising class of solution-processed semiconductor materials that are intensively investigated for their application in flexible solar cells and other optoelectronic devices. In this work, the fabrication and characterization of flexible quasi-interdigitated back-contact perovskite solar cells (FQIBC PSCs) are presented. The performance of fabricated devices on illumination direction (front-side or rear-side) and illumination intensity are studied. The current–voltage characteristics of the devices reveal that the performance of devices is better when illuminated from the rear-side. The quantum efficiency measurements show that the front-side illumination allows to avoid the light transmission losses and achieve a wide-spectral response in the devices. The dependence of the device open-circuit voltage on the illumination intensity is investigated to determine the dominant charge carrier recombination mechanism in the devices.

Illumination Intensity Dependence of the Recombination Mechanism in Mixed Perovskite Solar Cells.

Recombination mechanisms in solar cells are frequently assessed through the determination of ideality factors. In this work we report an abrupt change of the value of the "apparent" ideality factor (nAP ) in high-efficiency FA0.71 MA0.29 PbI2.9 Br0.1 based mesoscopic perovskite solar cells as a function of light intensity. This change is manifested as a transition from a regime characterized by nAP ∼1.8-2.5 at low light intensities (<10 mWcm-2 ) to one characterized by nAP ∼1. This transition is equally observed in the recombination resistance extracted from open-circuit impedance measurements. We use drift-diffusion simulations with explicit consideration of ion migration to determine the origin of this transition. We find that a change ofrecombination mechanism concurrent with a modification of the concentration of ionic vacancies is the most likely explanation of the observed behaviour. In the drift-diffusion simulations we show that the apparent ideality factor is in fact affected by the ion vacancy concentration so it is not the optimal parameter to assess the dominant recombination mechanism. We argue that a procedure based on a recently derived "electronic" ideality factor obtained from the high frequency feature of the impedance spectrum is better suited to determine the recombination route that dictates the photovoltage.

Immediate and Temporal Enhancement of Power Conversion Efficiency in Surface-Passivated Perovskite Solar Cells.

This work reports strategies for improving the power conversion efficiency (PCE) by capitalizing on temporal changes through the storage effect and immediate improvements by interface passivation. It is demonstrated that both strategies can be combined as shown by PCE improvement in passivated perovskite solar cells (PSCs) upon ambient storage because of trap density reduction. By analyzing the dominant charge recombination process, we find that lead-related traps in perovskite bulk, rather than at the surface, are the recombination centers in both as-fabricated and ambient-stored passivated PSCs. This emphasizes the necessity to reduce intrinsic defects in the perovskite bulk. Furthermore, storage causes temporal changes in band alignment even in passivated PSCs, contributing to PCE improvement. Building on these findings, composition engineering was employed to produce further immediate PCE improvements because of defect reduction in the bulk, achieving a PCE of 22.2%. These results show that understanding the dominant recombination mechanisms within a PSC is important to inform strategies for producing immediate and temporal PCE enhancements either by interface passivation, storage, composition engineering, or a combination of them all to fabricate highly efficient PSCs.

Optical Studies and Transmission Electron Microscopy of HgCdTe Quantum Well Heterostructures for Very Long Wavelength Lasers.

HgTe/CdHgTe quantum well (QW) heterostructures have attracted a lot of interest recently due to insights they provided towards the physics of topological insulators and massless Dirac fermions. Our work focuses on HgCdTe QWs with the energy spectrum close to the graphene-like relativistic dispersion that is supposed to suppress the non-radiative Auger recombination. We combine various methods such as photoconductivity, photoluminescence and magneto-optical measurements as well as transmission electron microscopy to retrofit growth parameters in multi-QW waveguide structures, designed for long wavelengths lasing in the range of 10–22 μm. The results reveal that the attainable operating temperatures and wavelengths are strongly dependent on Cd content in the QW, since it alters the dominating recombination mechanism of the carriers.

Photon recycling in perovskite solar cells and its impact on device design

Abstract Metal halide perovskites have emerged in recent years as promising photovoltaic materials due to their excellent optical and electrical properties, enabling perovskite solar cells (PSCs) with certified power conversion efficiencies (PCEs) greater than 25%. Provided radiative recombination is the dominant recombination mechanism, photon recycling – the process of reabsorption (and re-emission) of photons that result from radiative recombination – can be utilized to further enhance the PCE toward the Shockley–Queisser (S-Q) theoretical limit. Geometrical optics can be exploited for the intentional trapping of such re-emitted photons within the device, to enhance the PCE. However, this scheme reaches its fundamental diffraction limits at the submicron scale. Therefore, introducing photonic nanostructures offer attractive solutions to manipulate and trap light at the nanoscale via light coupling into guided modes, as well as localized surface plasmon and surface plasmon polariton modes. This review focuses on light-trapping schemes for efficient photon recycling in PSCs. First, we summarize the working principles of photon recycling, which is followed by a review of essential requirements to make this process efficient. We then survey photon recycling in state-of-the-art PSCs and propose design strategies to invoke light-trapping to effectively exploit photon recycling in PSCs. Finally, we formulate a future outlook and discuss new research directions in the context of photon recycling.

Recombination of photo-generated charge carriers in H-terminated and (photo-)oxidized silicon nanoparticles

The mechanisms of recombination of photo-generated charge carriers and the origin of luminescence in silicon nanocrystals, quantum dots or nanoparticles (Si-NPs) are essential for the application of these materials in future nano-optoelectronics. In the last two decades, these issues have been lively debated in the literature, with different investigations reporting seemingly contradicting results. With the study presented here, we clarify the dominant electron-hole recombination mechanisms and the origin of light emission in Si-NPs with different forms of surface passivation, namely H-terminated and (photo-)oxidized. We find that, independently of surface passivation, at low excitation powers recombination of photo-carriers is dominated by monomolecular channels via defect states, while at relatively high excitation electron-hole bimolecular recombination dominates. Moreover, we establish that in H-terminated Si-NPs, light emission is mainly due to recombination involving sub-band gap electronic states at the Si core of the NPs, whereas in surface (photo-)oxidized Si-NPs a two-component luminescence is observed, originating from Si core and oxide-related states, respectively. Further, we discuss the critical sensitivity of H-terminated Si-NPs to photo-oxidation even under low oxygen and water molecule environment and its consequences for Si-NP applications. Photo-oxidation is described within the Cabrera-Mott chemical reactions theory.

An Open Circuit Voltage Decay System for a Flexible Method for Characterization of Carriers’ Lifetime in Semiconductor

Among all the material parameters of a semiconductor, the lifetime of the carriers is one of the most complex, as it is a function of the dominant recombination mechanism, the number of carriers, the structural parameters and the temperature. Nevertheless, the lifetime of the carriers is a very useful and fundamental parameter to be determined for the qualification of the semiconductor in order to allow the improvement of the manufacturing process and the optimization of the operation of the semiconductor device. Thus being strongly linked to many physical and electronic parameters, the lifetime of the carriers cannot be provided only with a theoretical average value and an experimental measured value must be obtained. In the case of semiconductor junctions, precise measurements of the open-circuit voltage decay, OCVD, make it possible to trace the lifetime of the carriers through the device. An automated method for OCVD measurements presented in this contribution overcomes the main limitations that arise in the standard method when used for the characterization of the lifetime of carriers as it achieves the "open circuit conditions" of the device under test and reduces inherent noise of the differential operation mode of the method.