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The Lead (Pb) free all inorganic perovskite solar cells (PSCs) are garnering prominent interest in place of conventional organic-inorganic lead halide-based PSCs owing to hazardous nature of Pb and volatile virtue of organic components. In that quest, novel NaSnCl3 perovskite is explored with anticipation of synergetic intermingling of high-performance, stability and cost-effectiveness. Current work elaborates numerical modeling of glass/ITO/WS2/NaSnCl3/CuO/metal-contact configured PSCs by SCAPS-1D framework where WS2 and CuO act as charge-transport layers. Performance of NaSnCl3 based PSCs is numerically optimized by tuning various input parameters. Variation in thickness of NaSnCl3 layer from 300 nm to 1500 nm leads to boost efficiency (η) of devices from 10.82% to 12.54% and short-circuit current–density (Jsc) from 24.37 to 28.94 mA/cm2. Increment in defect density from 1012 cm−3 to 1016 cm−3 induced reduction in open circuit voltage (Voc) from 552.91 to 283.96 mV. Acceptor concentration of NaSnCl3 layer played crucial role in enhancement of performance of PSCs since Voc and η are enhanced from 550 mV to 850 mV and from 12.54 to 18.57%, respectively with increment in doping density. At optimized conditions, designed PSC devices showed the elevated efficiency of 25.04% which demonstrated tremendous potential of NaSnCl3 perovskites in futuristic PV applications.
Introduction
Effective utilization of solar energy by apotheosis photovoltaic devices is admirable tactic to ensure global energy security, control global warming and associated climate changes along with sustainable economic growth across the globe. With contemplation of burning issues associated with consumption of conventional fossil fuels and induced global changes concerned, the photovoltaics could be pondered as pivotal technology for catering energy needs in environmentally friendly and cost-effective ways1,2. Due to recent swift progressions, perovskite solar cells (PSCs) have been befallen as promising contender and pathfinder in photovoltaic industry since these are based upon robust fabrication processes and lower manufacturing temperature3. The PSCs grab a great deal of attention in photovoltaic community due to their enthralling compensations viz. enhanced power conversion efficiency (PCE), superior performance even at lower intensity of sunlight, affordable price etc4,5. The mesmerizing traits like huge coefficient of absorption, crystalline nature, adjustable optical energy band gap, optimal diffusion length, higher charge carrier mobility, smaller excitation energy of perovskite layer make them promising light harvesters to develop efficient PSC devices6.
The perovskite configuration having a typical ABX3 structure offers diverse choices of light absorbing materials to realize higher efficiency and stability of PSCs. In this typical ABX3 structure, A site is acquired by monovalent cation (organic or inorganic ion), B site is taken by divalent metal cation (inorganic element/ion) and X site is corresponding to halogen atoms. In conventional PSCs, methyl ammonium ion (MA+) or formamidinium ion (FA+) is used for A site, lead ion (Pb2+) for B site and halides like I−, Cl− ions are used for X site7. Notable ameliorations on engineering of perovskite formulations along with charge transport layer optimizations and other significant efforts resulted into an impressive surge in PCE from 3.8% (in 2009) to more than 26% (till date) of PSCs8,9. Hence perovskite-based photovoltaics turn out to be direct competitor to evergreen Silicon based photovoltaics in terms of cost and efficiency10.
In the present scenario, higher efficiency of PSCs is accomplished by commonly used organic-inorganic hybrid lead halide perovskite photon harvesting layer e.g. CH3NH3PbI3 absorber1. Although such PSCs demonstrated higher PCE yet their widespread industrialization is hindered due to sets of existing issues including rapid degradation of organic component-based perovskite layer which creates instability of device11,12, Lead (Pb) toxicity etc. Hybrid perovskite layer is prone to deprivation due to volatile nature of organic components (like MA+, FA+) on the impact of environmental stresses like light, heat, moisture etc. Besides, incorporation of Pb in traditional PSCs induces severe adverse effect on human health particularly on kidney and nervous system along with lung cancer and respiratory disease13,14. Moreover, swift oxidation of Pb cation also creates instability of devices8. These significant roadblocks in pace of commercialization of PSCs indorsed substantial quest among researchers to explore alternative Pb free and stable potential perovskite absorbers. Tremendous scientific efforts demonstrated that stability issue could be effectively tackled by using inorganic perovskite absorbers i.e. by replacing organic A cation with elements like Cs, Rb, K, Na etc15. The toxicity issue could be addressed by substituting Pb with Tin (Sn) which induces no or negligible distortion in the structure of perovskite material since ionic radii of Pb and Sn are comparable and outer shell of Sn is almost similar to that of Pb2. Moreover, Sn offers higher carrier mobility and tunable band gap of perovskite material. In order to cumulatively tackle these issues, all-embracing research is being pursued to develop and identify novel functional inorganic materials having traits like narrow band gap near to optimal one, lesser defect density, higher coefficient of absorption, highly stable etc. Accordingly, assortment of inorganic perovskite structured materials viz. CsSnX3, RbSnX3, KSnX3, NaSnX3 etc. have been proposed, and associated devices have been experimentally fabricated and/or numerically simulated. Among these, alkali element-based tin halide perovskites have been least investigated while these have prospective to develop energy efficient, stable and durable PSCs.
Sumona et al.16 analyzed the performance of KSnI3 perovskite solar cell by applying variety of charge transport layers where remarkable performance viz. PCE of 20.99% is achieved with ZnO as electron and CuI as hole transport layers. Pindolia et al.17 explored the KSnCl3 PSCs using SCAPS-1D simulation and DFT approach where they took conventional TiO2 as electron and Spiro-OMeTAD as hole transport layers and exhibited optimum efficiency of 9.77%. Bouri et al.18 undertook comparative study of NaSnCl3 and KSnCl3 based PSCs wherein estimation of band gap of these materials is carried out by First principle calculations. They used conventional TiO2 and Spiro-OMeTAD as respective charge transport layers where NaSnCl3 based PSCs performed superior in terms of short circuit current density (Jsc) and PCE. Hence these studies revealed that KSnX3 and NaSnX3 have the potential for realization of efficient and stable PSCs. These studies motivated us to explore the alkali tin halide-based perovskites specifically NaSnCl3 based devices since such novel material has apt band gap of 1.04 eV, relatively better stability, apt mobilities, dielectric permittivity of 10 along with higher absorption coefficient. It is believed NaSnCl3 based devices are practically implementable and could bestow higher performance. The NaSnCl3 could be turned out as budding light harvester to PSC community in quest of potential materials for mass scale production and commercialization. Although NaSnCl3 has potential to offer superior efficiency yet it comprises Sn element. In Sn based perovskites, Sn2+ oxidation (Sn2+ → Sn4+) is generally happened under the exposure of moisture, oxygen and during device development process owing to used different solvents like dimethylsulfoxide (DMSO). This may lead to escalate the instability and rapid degradation of concerned solar cell devices. This Sn2+ oxidation process induces high density of deep defects which lead to non-radiative recombination and as well as it also leads to self-doping (p-type). Typically, this oxidation induced degradation influences the surface morphology of films and devices performance in adverse manners. In order to tackle the problem associated with Sn2+ oxidation, different additives and reducing agents like SnF2, SnCl2, SnI2, SnBr2 etc. may be used those prevent the degradation and enhances the compactness and surface morphology of films leading to better performance in terms of efficiency and stability. Moreover, through surface modifier along with cation and solvent engineering, the issue may also be minimized. Device encapsulation as well fabrication in reducing or inert conditions could also weaken the Sn2+ oxidation13,19. Hence this issue is challenging but could be addressing using different pre and post treatments.
Typically in PSCs, the light harvesting perovskite absorber layer is sandwiched between two charge transport layers namely electron transport layer (ETL) and hole transport layer (HTL). The functioning of ETL is ensure prevention of explicit contact between front contact and absorber layer along with efficient transportation and collection of photogenerated electrons from perovskite layer to front contact concerned18. The materials viz. Al2O3, ZrO2, TiO2, ZnO, WS2, CdS have been explored for role of ETL in PSCs where TiO2 is widely used but it requires higher temperature for processing and it contains higher surface defect density, hence, its alternative like WS2, CdS are being investigated3. The enrichment of quantum efficiency of devices and control on losses of photogenerated charge carriers could be ensured by selecting appropriate HTLs in PSCs. The HTL also offer low resistive path for photogenerated holes. An apt HTL should be chemically stable and must have potential for facile transportation of charge carriers in order to ensure effectual conduction of holes in PV devices. The device stability, efficiency and cost are critically affected by choice/nature of HTLs18. Generally, organic materials like PEDOT: PSS, P3HT, Spiro-MeOTAD are used to develop high efficiency PSCs but these adversely influence the stability of devices4 and hence, inorganic materials like CuO, CuI, V2O5, etc. are being inspected as alternative potential HTLs for PSCs18. The performance of perovskite solar cells is crucially governed by band offset at different interfaces viz. ETL/perovskite, perovskite/HTL etc. In order to realize the optimum performance of PSCs, it is indispensable to choose apt ETL and HTL with respect to absorber concerned and ensure proper positioning of energy band levels across the different layers3,8.
The anticipation regarding performance of PSC devices could be made by using numerical simulation and device modeling. The numerical simulation tools offer amicable strategies and theoretical approaches to attain higher efficiency of designed devices. The numerical simulation not only saves the significant time but it helps in reduction of manufacturing cost too. The theoretical calculations-based simulations offer a platform for optimization of various active and charge transport layers. Among the variety of tools, SCAPS-1D is popular platform among the researchers20 since it allows to introduce maximum seven layers in device structures and results from SCAPS-1D simulation closely matches with the experimental outcomes. In view of these perspective, present work explores the numerical modeling of all-inorganic, Pb free NaSnCl3 based PSCs where WS2 and CuO are exploited as effective charge transport layers. Since the device performance is crucially governed by numerous factors, therefore, performance of ‘glass/ITO/WS2/NaSnCl3/CuO/metal contact’ devices is optimized by exploring the effect of perovskite absorber thickness, bulk defect density, interface defect density, acceptor concentration, nature of metal contact etc. Modeled PSC devices exhibited more than 25% efficiency at optimized conditions. The novelty lies in the facts that device architecture is novel, optimized parameters are feasible and attained efficiency is experimentally realizable ensuring amalgamation of high performance, stability, cost-effectiveness and environmental friendliness nature for proposed NaSnCl3 based PSCs. Hence present study provides a detailed roadmap and encouragement to PV researchers towards exploration and subsequent development of NaSnCl3 based PSCs.
Simulation methodology and device structure designing
SCAPS-1D simulation methodology
The numerical simulations provide appropriate tactics to design the variety of solar cells and anticipate the performance prior experimental fabrication. In order to undertake the numerical simulations, different tools like SILVACO, ATLAS, wxAMPS, SCAPS-1D, COMSOL etc. are available, wherein, the SCAPS-1D gained the extensive popularity among the PV community due to advantageous traits like user friendly, easy to run, enabling to execute seven layers, resembling of attained simulation results with experimental outcomes etc21. These beneficial merits motivated us to simulate the modeled PSCs using SCAPS-1D simulation framework. The development of solar cell capacitance simulator-one dimension (SCAPS-1D) is undertaken by researchers led by Prof. Marc Burgelman at department of electronic and information system (DELIS), University of Gent22,23. The SCAPS-1D simulation framework could be accessed on personal computers having windows 95, 98, 2000, XP vista window 7 etc. The SCAPS-1D enables us to access the current density-voltage (J-V) characteristics, PCE (η), Jsc, fill factor (FF), open circuit voltage (Voc), spectral behaviour of quantum efficiency (QE) etc. in order to analyse the performance of designed solar cells24. The working of SCAPS-1D simulation tool lies in simultaneous solution of semiconductor’s fundamental equations viz. Poisson’s equation and continuity equations for charge carriers like electrons and holes in valance band and conduction bands while the boundary conditions are also taken under consideration. Poisson’s equation typically illustrates the relationship between electrostatic potential and charge density25,26. The monitoring on movement of charge carriers in semiconducting materials is carried out by continuity equations27. In order to mimic the functioning of designed devices, SCAPS-1D utilizes the SRH (Shockley-Read-Hall) recombination statistics.
The Poisson’s equation governed by semiconductors could be described as: where is corresponding electrostatic potential, e represents charge on electron, is permittivity of vacuum, is relative permittivity, p represents concentration of holes, n represents concentration of electrons, ND is donor concentration or doping density, NA is concentration of acceptors, denotes the distribution of holes and indicates the distribution of electrons28.
The continuity equation for electrons is provided as under: where Jn is current density of electrons, Gn is generation rate for electrons, Rn is recombination rate for electrons.
The continuity equation for holes is provided as under: where Jp is current density of holes, Gp is generation rate for holes, Rp is recombination rate for holes29,30.
The transportation of charge carriers (electrons and holes) could be described using drift and diffusion equations which are provided as under:
and
where is electron mobility, is diffusion coefficient for electron, is hole mobility, is diffusion coefficient for holes. The SCAP-1D solves the Poisson’s and continuity equations numerically under steady state conditions28.
Device structure designing
With consideration of baseline structure of perovskite solar cell as glass/TCO/ETL/perovskite layer/HTL/ metal contact, herein novel NaSnCl3 material is taken as active material to absorb the incident sunlight and its thickness is varied from 300 to 1500 nm. The selection of thickness range of absorber layer is exclusively made based upon the relevant literature concerned with experimental and theoretical studies of PSCs31, 32, 33, 34, 35, 36–37. For the transparent conducting oxide purpose, an ITO layer is used while n-type WS2 and p-type CuO are taken as ETL and HTL, respectively. Initially Au is taken as metal contact having work function of 5.10 eV while other metals like Ni, Pt are also explored later during optimization process. The simulations of PSCs having architecture as glass/ITO/WS2/NaSnCl3/CuO/metal contact (as shown in Fig. 1) is carried out by using SCAPS-1D having version of 3.3.10.
Fig. 1 [Images not available. See PDF.]
Device architecture of NaSnCl3 absorber-based perovskite solar cell.
The input attributes like band gap, electron affinity, dielectric permittivity etc. to different constituent layers like ITO, WS2, NaSnCl3, CuO etc. are extracted from relevant references7,18,38, 39–40 while some parameters are altered within allowed theoretical limits and these are summarized in Table 1.
Table 1. Input attributes for ETL, perovskite and HTL used during initial simulation of devices.
Parameters | Units | ITO | WS2 | NaSnCl3 | CuO |
|---|---|---|---|---|---|
Layer thickness | (µm) | 0.100 | 0.100 | Varying | 0.100 |
Dielectric permittivity | 9.00 | 11.9 | 10.0 | 18.10 | |
Electron affinity | (eV) | 4.00 | 4.3 | 4.2 | 4.07 |
Energy bandgap | (eV) | 3.5 | 1.87 | 1.04 | 1.51 |
Dielectric permittivity | 9.00 | 11.9 | 10.0 | 18.10 | |
CB density of states | (cm−3) | 1 × 1019 | 1.5 × 1018 | 1 × 1020 | 2.2 × 1019 |
VB density of states | (1/cm3) | 1 × 1019 | 1.8 × 1019 | 1 × 1020 | 5.5 × 1020 |
Electron mobility | (cm2/Vs) | 20 | 50 | 10 | 100 |
Hole mobility | (cm2/Vs) | 10 | 20 | 10 | 10 |
Electron thermal velocity | (cm/s) | 1 × 107 | 1 × 107 | 1 × 107 | 1 × 107 |
Hole thermal velocity | (cm/s) | 1 × 107 | 1 × 107 | 1 × 107 | 1 × 107 |
Shallow uniform donor density (ND) | (cm−3) | 1 × 1019 | 1 × 1016 | 1 × 1014 | – |
Shallow uniform acceptor density (NA) | (cm−3) | – | – | Varying | 1 × 1016 |
Total defect density (Nt) | (cm−3) | – | 1 × 1010 | Varying | 1 × 1010 |
Table 2. Defect attributes at different interfaces incorporated during simulation.
Interface | WS2/NaSnCl3 | NaSnCl3/ CuO |
|---|---|---|
Energy distribution | Single | Single |
Defect type | Neutral | Neutral |
Total density/ concentration (cm−2) | Varying (1010 cm−3–1014 cm−3) | Varying (1010 cm−3–1014 cm−3) |
Energy distribution | Single Et = 0.60 eV | Single Et = 0.60 eV |
Reference for defect energy level | Above the VB maximum | Above the VB maximum |
The optimization of absorber thickness with respect to modeled devices is taken place in range of 300–1500 nm. Since devices are prone to have defects during fabrication, therefore, appropriate defect densities are introduced into WS2, CuO layers as well as at CuO/NaSnCl3 and NaSnCl3/WS2 interfaces as described in Tables 1 and 2 while defect density of absorber layer is altered in range of 1 × 1012–1 × 1016 cm−3 in order to explore the impact of different defects like vacancies, dislocations, Sn oxidation centres etc. The acceptor concentration of designed devices is varied in range of 1 × 1014–1 × 1022 cm− 3 and this doping density range is taken as per relevant literature concerned with absorber and charge transport layers of PSCs41, 42–43. Moreover, since Sn2+ oxidation (Sn2+ → Sn4+) may also be happened which leads to self-doping and introduction of acceptors. So higher doping is also in-line up to show and consider the mechanism of Sn2+ oxidation in our proposed devices and undertaken simulations. Moreover, influence of defect density at ETL/perovskite and perovskite/HTL interfaces is also explored meticulously. The description of defects introduced at different interfaces during simulation is provided in Table 2. The simulations of modeled PSCs are carried out at 300 K temperature under the illumination of typical sunlight viz. AM 1.5 with intensity of 100 mW/cm2.
Results and discussion
Among the different constituent layers in PSCs, the light harvesting absorber layer is considered as core/heart of PV devices since various important processes like absorption of most of sunlight, charge carrier generation, probable recombination and efficient charge transportation are occurred inside absorbers44. The performance parameters viz. Voc, Jsc, FF, PCE, QE etc. of PSCs is crucially governed by number of factors like thickness, doping density, bulk defect density, interface defect density etc. of absorber layer along with metal contact. Hence optimization of these factors for absorber layer is crucial to analyse and explore the significant potential of NaSnCl3 perovskite material and associated PSCs. In order to simulate baseline structure having architecture as glass/ITO/WS2/NaSnCl3/CuO/metal contact, initially absorber thickness of 300 nm, bulk defect density of 1012 cm− 3, interface defect density of 1012 cm− 3 at ETL/perovskite and perovskite/HTL interface, acceptor doping density of 1014 cm− 3 are set while Au is taken as metal contact. Present comprehensive simulation study is undertaken in following steps viz. (i) optimization of thickness of NaSnCl3 layer, (ii) optimization of concentration of defects by using optimum thickness of NaSnCl3 layer, (iii) optimization of concentration of interfacial defects by using optimum thickness and bulk defect density of NaSnCl3 layer, (iv) optimization of doping density by using optimum thickness and bulk defect density of NaSnCl3 layer along with optimized interface defect density, (v) optimization of metal contact by using optimum thickness, bulk defect density, doping density of NaSnCl3 layer along with optimized interface defect density.
Optimization of thickness of NaSnCl3 perovskite absorber layer
The optimization of thickness of light harvesting absorber layer enables to ensure apt trade off among material utilization, cost, efficiency etc. The lower absorber thickness harvests insufficient number of photons yielding lesser performance while thickness beyond optimum value adversely induces recombination of charge carriers due to inappropriate diffusion length and hence leads to degradation of device performance44. Considering these facts, optimization of thickness of NaSnCl3 layer is carried out in 300–1500 nm range by analyzing the performance of glass/ITO/WS2/NaSnCl3/CuO/Au. The effect of NaSnCl3 thickness on J–V characteristics of modeled devices is illustrated in Fig. 2 which indicates the almost square J–V characteristics revealing to apt designing of devices. The J–V curves didn’t show any kind of roll-over effect which is common in experimentally fabricated devices while such roll-over could also be appeared in some modeled devices45, 46–47 Appeared huge conduction band offset at interfaces and/or back contact barrier of devices are typically attributed for this phenomenon46. In order to attain exhaustive impact of absorber thickness, performance parameters like Voc, Jsc, FF and PCE of modeled devices are computed and analysed as shown in Fig. 3.
Fig. 2 [Images not available. See PDF.]
Effect of NaSnCl3 absorber thickness on current-voltage (J-V) characteristics of designed PSC devices.
The Voc is the maximum potential which could be drawn from the photovoltaic devices. It depends upon the properties like thickness, defect density, grain boundary, doping density etc. of constituent layers. At lowest absorber thickness of 300 nm, the Voc of modeled PSCs is attained as 551.75 mV which later increased to 553.43 mV with thickness till 900 nm. The enhancement in Voc with absorber thickness could be caused by improved built-in potential along with lesser defect density which eventually increased the charge extraction and thus Voc48. However, beyond 900 nm, the Voc is gradually declined to 552.91 mV at 1.5 micron thin absorber. The decline in Voc with absorber thickness is also reported in literature49 where they simulated CH3NH3PbI3 based PSCs in thickness range of 400–1000 nm range and Voc is decreased from 1.155 volts to 1.115 volts. The Voc may be declined beyond a particular thickness owing to enhancement in dark saturation current and recombination of charge carriers18,50.
The Jsc is another crucial PV performance parameter which is related with the amount of current per unit area drawn from solar cell devices when electrodes are made shorted. It represents the maximum amount of current density which could be pulled from device under investigation. As per Fig. 3b, the Jsc is largely enhanced from 24.37 to 27.17 mA/cm2 at initial increment in absorber thickness from 300 nm to 600 nm and thereafter it increases gradually to 28.94 mA/cm2. On increasing thickness of light harvesting NaSnCl3 layer in modeled devices, more photons are absorbed which created more electrons and holes (i.e. enhanced charge carrier generation) and their apt transportation owing to the increased mobility could lead to augmentation in short circuit current density23,51.
Fig. 3 [Images not available. See PDF.]
Behaviour of (a) Voc, (b) Jsc, (c) FF, and (d) PCE of modeled devices with variation in thickness of NaSnCl3 layer.
The efficient conversion of input solar power into measurable electrical power is crucially represented by FF of devices which is measure of squareness of J-V characteristics of devices. It is described as the ratio of maximum power to the multiplication of Jsc and Voc of devices. The absorber thickness, grain boundaries, defects typically affect the FF of devices. The variation in FF of baseline devices with NaSnCl3 thickness is depicted in Fig. 3c which indicates that FF of modeled devices is monotonically declined from 80.44 to 78.38% as absorber thickness is increased from 300 nm to 1.5 micron. The enhancement in perovskite absorber thickness could induce the increment in internal resistance of devices which eventually leads to boost the series resistance which adversely affect the FF of designed devices48,52. Similar decrement in FF with absorber thickness is also attained by Ismal et al.53 wherein they varied thickness of CH3NH3SnBr3 absorber in 0.4–1.6 microns.
Table 3. Performance parameters of simulated devices having variable absorber thickness.
NaSnCl3 thickness (nm) | Voc (mV) | Jsc (mA/cm2) | FF (%) | PCE (%) |
|---|---|---|---|---|
300 | 551.75 | 24.37 | 80.44 | 10.82 |
600 | 553.22 | 27.17 | 79.97 | 12.02 |
900 | 553.43 | 28.24 | 79.41 | 12.41 |
1200 | 553.24 | 28.71 | 78.88 | 12.53 |
1500 | 552.91 | 28.94 | 78.38 | 12.54 |
The effectiveness of solar cell devices could be justified by analysing the PCE (η) which is defined as the ratio of maximum power density to input power density. The PCE of baseline devices having absorber thickness of 300 nm is found as 10.82% and it found to enhance and saturate to 12.54% at NaSnCl3 thickness of 1500 nm. Improvement in PCE with thickness could be attributed to larger density of photon absorption and higher generation of excitons which contributed in current effectively25,27 All the performance parameters of designed baseline PSCs viz. glass/ITO/WS2/NaSnCl3/CuO/Au having variable absorber thickness are listed in Table 3.
Fig. 4 [Images not available. See PDF.]
Spectral behaviour of QE of modeled devices with variation of NaSnCl3 layer thickness.
The generation and collection of charge carriers upon incidence and absorption of photons by solar cells are described in terms of QE52. It represents charge collection capacity of solar cell devices at particular wavelength or energy and it is closely related to short circuit current density. The spectral behaviour of QE with wavelength for modeled devices having variable NaSnCl3 thickness is depicted in Fig. 4. In lower wavelength region (300–380 nm), the QE is found to increase from ~ 86% to ~ 99%, and thereafter it saturated in 380–500 nm while in higher wavelength beyond 835 nm, QE abruptly vanished. In higher wavelength region, the energy of photons is considerably small and it is insufficient for apt creation electron-hole pairs which lead to decline in QE in this particular region54. In 500–830 nm spectral region, the QE of simulated devices is found to improve as absorber thickness enhances from 300 nm to 1500 nm. At higher thickness of absorber, more charge carriers are generated and their efficient separation to respective electrodes could lead to augment QE of modeled devices55.
Optimization of defect density of NaSnCl3 perovskite absorber layer
The performance of solar cells is explicitly impacted by quality of light harvesting layer which is influenced by presence of defects. These defects are inherently appeared during the development of different constituent layers of PSCs. In practical devices, defects could be appeared at surfaces and interfaces of different charge transport layers. These defects could be found in terms of grain boundaries due to development of dangling bonds and uncoordinated atoms. The surface dislocations, deviation in stoichiometric composition and sublimation of different organic component during annealing processes may also induce defects in absorber layers. Besides, bulk defect like Schottky and Frenkel defects, vacancies, interstitials, oxidation centres etc. could also adversely affect the eminence of absorbers. The existence of defects typically introduces new energy levels in the band gap of materials concerned which behaves as traps for electrons and holes. The presence of defects drastically hampers the crystallinity, morphology, microstructure, electrical and optical properties of active layers and, eventually device performance is critically altered. The presence of higher defects in absorber impacts the life time and diffusion lengths of charge carriers, and these charge carriers may be lost due to higher recombination26,56,57.
Hence recombination rate of electrons and holes in absorber layer is governed by amount of defects density in associated layers. The charge recombination function in corresponding light harvesting layer is provided as under:
Where left hand term of above equation represents the rate of recombination, ‘n’ is concentration of charge carriers like electron or holes, ‘An’ represents the Schockley Read Hall recombination (SRH), ‘Bn2’ represents the biomolecular recombination and ‘Cn3’ expresses the Auger recombination. In solar cells, performance degradation is mostly caused by trap assisted SRH phenomenon which is given by: where σ is carrier capture cross section, Nt is defect density, Et is defect energy level, Vt is thermal velocity etc26,58. Hence assessment of defect density of designed devices is critical in order to precisely predict their performance. Although, defect could be neglected during simulation but in order to ensure that simulated devices behave like realistic practical ones, defects have been introduced. These introduced defects could be considered in analogous to the deep defects induced due to probable Sn2+ oxidation which leads to non-radiative recombination. Introduced deep defects (representing Sn4+ states) are located at 0.6 eV above the valence band and varied in range of 1 × 1012–1 × 1016 cm− 3. The effect of absorber defect density on J-V characteristics of designed NaSnCl3 based PSCs is shown in Fig. 5.
Fig. 5 [Images not available. See PDF.]
Effect of defect density of NaSnCl3 absorber layer on J-V characteristics of simulated devices.
The explicit impact of absorber defect density on Voc, Jsc, FF and PCE of modeled devices is graphically presented in Fig. 6 while attained performance parameters are also listed in Table 4. It is visible from Fig. 6 that Voc is attained maximum of 552.91 mV at defect density of 1 × 1012 cm−3 and then declined negligible at defect density of 1 × 1013 cm−3. However, on further increasing defect density to 1 × 1016 cm−3, the Voc is significantly reduced to 283.96 mV. Similarly, the Jsc found almost constant in defect density range of 1 × 1012 cm−3 to 1 × 1014 cm−3 and then it considerably reduced to 24.97 mA/cm2 at higher defect density of 1 × 1016 cm−3.
Fig. 6 [Images not available. See PDF.]
Defect concentration influenced (a) Voc, (b) Jsc, (c) FF, and (d) PCE of designed devices.
Table 4. Performance parameters of simulated devices having variable absorber defect density.
Defect density (cm−3) | Voc (mV) | Jsc (mA/cm2) | FF (%) | PCE (%) |
|---|---|---|---|---|
1012 | 552.91 | 28.94 | 78.38 | 12.54 |
1013 | 539.35 | 28.93 | 67.62 | 10.55 |
1014 | 484.74 | 28.89 | 55.37 | 7.75 |
1015 | 381.94 | 28.48 | 48.11 | 5.23 |
1016 | 283.96 | 24.97 | 38.96 | 2.76 |
The FF and PCE of simulated devices is attained maximum as 78.38% and 12.54% at lowest defect density of 1 × 1012 cm− 3 and these further greatly diminished to 38.96% and 2.76% at higher defect density of 1 × 1016 cm− 3. The enhancement in defect density crucially supported to hastening of degradation, stress, microstructural, morphological changes, augmentation in trap states which hampered the effective transportation of charge carriers. All these factors eventually contributed in significant reduction in device performance of modeled devices59. The surge in defect density of NaSnCl3 absorber leads to induce higher recombination of electrons and holes, and their life time and diffusion length get reduced. These generated charge carriers could not able to reach at respective electrodes due to higher defect density and hence all the output parameters of modeled PSCs have been greatly reduced23.
In the present NaSnCl3 perovskite, different defects like vacancies, interstitials, oxidation centres due to oxidation of Sn in ambient may exist. These defects may induce different electronic states/ trap states in the band gap absorber materials. These traps states can act like SRH recombination centres where charge carriers like electrons and hole may be captured and recombined. On increasing the bulk defect density, Voc is declined due to escalated SRH recombination which also induce an enhancement in dark saturation current and decrement in quasi-Fermi level splitting. The value of Jsc is also declined as diffusion lengths of electrons and holes got reduced which critically impacted the charge collection as well. Enhancement in bulk defects of NaSnCl3 also introduced more series resistance owing higher trap assisted recombination which eventually lowered the FF of devices. Decline trend of Voc, Jsc and FF combinedly contributed in reduction in PCE of proposed devices with increment in defect density.
The influence of defect density of NaSnCl3 absorber on QE of modeled PSCs is also investigated in spectral range of 300–900 range. With respect to wavelength, QE is initially increased till ~ 370 nm, and then remained constant in 370–830 nm and later is abruptly declined. With respect to defect density, QE curves are coincided for defect levels of 1 × 1012 cm−3 and 1 × 1013 cm−3 which reveals that device quality is maintained and degradation is not initiated. However, on further enhancing defect density to 1 × 1016 cm−3, QE reduced below 50% in visible spectra as shown in Fig. 7.
Fig. 7 [Images not available. See PDF.]
Impact of defect concentration of NaSnCl3 absorber on QE of simulated PSC devices.
The analysis of above spectra reveals that QE of simulated devices is continuously decreased as defect concentration is enhanced. The defects specifically deep level act as trap centres which where charge carriers recombine non-radiatively and consequently number of electrons and holes reach at front and back contacts got reduced that lead to decline in QE of devices. Higher defect density creates more recombination which induce decrement in the quasi-Fermi level splitting and thus Voc and quantum efficiency of solar cell devices. Hence, defect density optimization suggests that better performance of NaSnCl3 based PSCs is attained at lower defect density, and therefore, for further optimization of other parameters, the defect density is set to 1 × 1012 cm−3.
Optimization of interfacial defect density of modeled devices
The PSC is multilayer devices architectures where different layers like ETL, absorber, HTL etc. are grown on one another using assortment of deposition techniques Typically interfacial defects are introduced when two separate layers come into contact and associated defect density depends upon the quality and purity of precursor materials and synthesis methods used. In PSCs, defects are generally introduced at ETL/perovskite and perovskite/HTL interfaces when absorber precursors interact with associated CLT solutions. These interface defects are consequences of mismatched crystal structures of distinct constituent layers28,39. The defects at different interfaces act as non-radiative recombination centres and trapping sites for charge carriers and degrades the crystal structure, morphology of layers and thereby degrade the device performance. The alignment of different energy levels is also influenced by interface defects which typically create injection barriers those hinder the extraction of charge carriers. Moreover, surface defects owing to crystalline indiscretions at distinct interfaces further obstruct the extraction of carriers and thereby performance48,60.
Considering the significance of interface defect density in PSCs, herein, it is equally varied in range of 1010 cm− 3-1014 cm− 3 at WS2/NaSnCl3 interface and NaSnCl3/CuO interfaces. When defect density at WS2/NaSnCl3 interface is varied then it is kept constant for NaSnCl3/CuO interfaces and vice versa. The impact of defect densities of WS2/NaSnCl3 and NaSnCl3/CuO interfaces on J-V characteristics of proposed devices is shown in Fig. 8a and b while performance parameters influenced by interfacial defect density of proposed devices are listed in Tables 5 and 6.
Fig. 8 [Images not available. See PDF.]
Effect of defect density at (a) WS2/NaSnCl3 interface and at (b) NaSnCl3/CuO interface on J-V characteristics of simulated devices.
Table 5. Performance parameters of simulated devices having variable defect density at WS2/NaSnCl3 interface.
Interface defect density (cm−3) | Voc (mV) | Jsc (mA/cm2) | FF (%) | PCE (%) |
|---|---|---|---|---|
1010 | 552.91 | 28.94 | 78.38 | 12.54 |
1011 | 552.73 | 28.94 | 78.25 | 12.52 |
1012 | 551.01 | 28.94 | 77.03 | 12.29 |
1013 | 540.50 | 28.93 | 71.59 | 11.20 |
1014 | 507.83 | 28.92 | 64.98 | 9.54 |
It could be evidenced from Table 5 that defect density at WS2/NaSnCl3 interface has negligible impact on Jsc of proposed devices whereas Jsc, FF and PCE are marginally varied in defect density range of 1010 cm− 3-1012 cm− 3. Drastic decrement in device performance is observed when defect density at WS2/NaSnCl3 interface is increased beyond 1012 cm− 3. This severe reduction in performance with defect density could be ascribed by the enhancement in recombination centres, traps states or scattering centres which impede the movement of electrons and thereby induced performance degradation of simulated devices40,48.
Table 6. Performance parameters of simulated devices having variable defect density at NaSnCl3/CuO interface.
Interface defect density (cm−3) | Voc (mV) | Jsc (mA/cm2) | FF (%) | PCE (%) |
|---|---|---|---|---|
1010 | 552.91 | 28.94 | 78.38 | 12.54 |
1011 | 552.85 | 28.94 | 78.37 | 12.54 |
1012 | 552.25 | 28.94 | 78.23 | 12.50 |
1013 | 547.91 | 28.94 | 77.11 | 12.27 |
1014 | 531.20 | 28.94 | 72.99 | 11.22 |
Table 6 indicates that Jsc of simulated devices is almost invariable irrespective of level of defect density at NaSnCl3/CuO interface. The Voc, FF and PCE are also negligibly altered when associated defect density is increased from 1010 cm− 3 to1012 cm− 3. Beyond the defect density of 1012 cm− 3, these performance parameters showed significant trend of reduction. Such decrement in performance at higher defect density of NaSnCl3/CuO interface could be ascribed by the facts that holes are being recombined, scattered or trapped and these are not efficiently extracted and collected at respective electrodes leading to the reduced performance36. Hence interface defect density analysis suggests that concentration of defects at ETL/perovskite and perovskite/HTL should be 1010 cm− 3 in order to attain higher efficiency of modeled devices. This detailed analysis showed that although influence of defects on interfaces is relatively lesser noticeable as compared to bulk defect density of absorber layer, yet passivation of these interfacial defects is indispensable in order to ensure facile injection of charge carriers and thereby improve the device performance. It is also suggested that lattice constants of constituent layers should be likewise match for weaking of impact of interface defects.
Optimization of doping concentration of NaSnCl3 perovskite absorber layer
The deliberate addition of impurities into semiconductors is a promising approach to manipulate the physical characteristics for application concerned. The doping of suitable element and concentration is crucial to maximize the performance of solar cells. Based upon the nature of impurity element, doping could be p-type and n-type to host material as well as sometimes, it is self-doping. The doping in perovskite light harvesting layer could control density of charge carriers, type of majority carriers, mobility and transportation of charges etc. The doping is found to manipulate optical and electrical properties of absorber layer and thus it eventually customizes the stability and performance of PSCs under investigation7,44,61. Hence precise adjustment of doping density is required to maximize efficiency of solar cells, and accordingly herein, optimization of acceptor doping concentration to NaSnCl3 absorber is carried out in range of 1 × 1014 cm− 3 to 1 × 1022 cm− 3 and resultant J-V characteristics of simulated PSC device are presented in Fig. 8 where voltage is found to improve with acceptor concentration.
Fig. 9 [Images not available. See PDF.]
Effect of doping concentration of NaSnCl3 layer on J-V characteristics of modeled devices.
The explicit impact of acceptor concentration to NaSnCl3 absorber on performance parameters of simulated devices is pictorially demonstrated in Fig. 10. At initial doping density range of 1 × 1014 cm− 3 to 1 × 1016 cm− 3, the Voc is marginally enhanced from 552.91 mV to 554.43 mV, thereafter it improved greatly and reached to 815.77 mV at higher acceptor concentration of 1 × 1016 cm− 3. The augmentation in doping density provides more charge carriers and lead to improve the conductivity which in turn induces higher value of built in potential (Vbi) in the devices. This Vbi assists in efficient driving to charge carriers to front and back contacts/electrodes and thus leads to increase the open circuit voltage with acceptor doping concentration7,24,62,63. An improvement in Voc of simulated devices could also be endorsed by decline in reverse saturation current with acceptor doping density23. The increment in acceptor doping also induces fluctuation in position of fermi level of holes which also lead to change in open circuit voltage of devices64.
Fig. 10 [Images not available. See PDF.]
Doping concentration influenced (a) Voc, (b) Jsc, (c) FF, and (d) PCE of designed devices.
The behaviour of Jsc with acceptor concentration is shown in Fig. 9b which indicate that it is found to decrease from 28.94 mA/cm2 to 26.64 mA/cm2 as doping density increase from 1 × 1014 cm− 3 to 1 × 1022 cm− 3. Almost monotonic reduction in Jsc is possibly due to reduction in mobility of charge carriers with doping density owing to introduction of scattering centre7,44. The reduction in Jsc for KSnI3 and CsPbIBr2 absorber-based PSCs is also observed with acceptor doping concentration by Sumona et al.16 and S Ullah et al.9, respectively.
Table 7. Performance parameters of simulated devices having variable doping density.
Acceptor Density (cm−3) | Voc (mV) | Jsc (mA/cm2) | FF (%) | PCE (%) |
|---|---|---|---|---|
1014 | 552.91 | 28.94 | 78.38 | 12.54 |
1015 | 553.08 | 28.94 | 78.44 | 12.55 |
1016 | 554.43 | 28.93 | 79.28 | 12.72 |
1017 | 566.19 | 28.88 | 81.82 | 13.38 |
1018 | 598.65 | 28.47 | 82.64 | 14.08 |
1019 | 664.1 | 27.81 | 83.56 | 14.96 |
1020 | 696.93 | 27.30 | 84.49 | 16.07 |
1021 | 752.39 | 26.95 | 85.39 | 17.31 |
1022 | 815.77 | 26.64 | 85.40 | 18.56 |
As per Table 7, the FF and PCE of NaSnCl3 absorber-based PSC devices is greatly improved from 78.38% to 85.40% and from 12.54% to 18.56% with rise in acceptor concentration. The enhancement in these performance parameters with doping is due to introduction of excessive charge carriers, their apt extraction, separation, transportation, and collection to respective front and back contacts. The reduction in series/sheet resistance with doping density also induce enhancement in FF of devices24,62.
Moreover, the acceptor concentration of NaSnCl3 perovskite absorber layer critically impacts the life time and diffusion lengths of minority charge carriers. At lower concentration of doping (till 1016 cm-3), the diffusion length and minority carrier life times are longer due to least trap assisted recombination but weak built in potential results into marginal improvement in open circuit voltage of the devices. At moderate doping level, typically minority carrier lifetime and diffusion lengths are sufficiently large and enough which induces strong built-in-potential whereas at extreme higher doping (1021-1022 cm-3), lifetime of minority carriers may be reduced due to initiation of recombination but herein performance of devices is enhanced with doping levels which indicates that minority carrier lifetime was sufficiently long. In the present study, device performance is maximized at an optimal value of acceptor concentration (1022 cm-3) and the factors which determine this value are large built-in-potential for charge separation, longer lifetime of minority carriers along with higher diffusion length than the optimal thickness of NaSnCl3 absorber layer. Hence density of acceptors into NaSnCl3 absorber crucially influenced the charge carrier dynamics and performance parameters of designed devices. Higher performance of PSCs could be attained by increasing the concentration of deliberate impurities during simulation. Similarly for experimentally fabricated devices, higher doping concentration would be required to realize the enhanced efficiency of NaSnCl3 absorber-based PSCs.
Optimization of metal back contact
The appropriate energy level alignment at different interfaces specially at semiconductor/back contact is crucial to enhance the desired charge extraction and collection efficiency and development of Ohmic contacts which in turn influence the output performance of solar cells drastically. This energy level alignment in solar cells is considerably influenced by the nature of metal back contacts since each metal behaves differently owing to diverse work function. The implementation of apt metal contact provides suitable alignment of energy levels which can enhance the built-in potential and thus open circuit voltage of devices48,59,65. Hence optimization of metal contact for simulated devices is indispensable in order to maximize the efficiency. In order to investigate the impact of rare electrodes, Gold (Au), Nickle (Ni) and platinum (Pt) metals having work functions as 5.10 eV, 5.15 eV and 5.65 eV have been explored during simulation of modeled devices66 and attained J-V characteristics are illustrated in Fig. 11 which reveals that Au and Ni contacted devices showed almost similar performance.
Fig. 11 [Images not available. See PDF.]
J–V characteristics of modeled devices having variable metal back contacts.
Table 8. Performance parameters of simulated devices having variable doping metal back contact.
Metal | Work function (eV) | Voc (mV) | Jsc (mA/cm2) | FF (%) | PCE (%) |
|---|---|---|---|---|---|
Au | 5.10 | 816 | 26.64 | 85.41 | 18.57 |
Ni | 5.15 | 825 | 26.76 | 84.95 | 18.75 |
Pt | 5.65 | 982 | 28.97 | 87.95 | 25.04 |
As per Table 8, the Voc of devices is improved from 816 mV to 982 mV, Jsc enhanced from 26.64 mA/cm2 to 28.97 mA/cm2. FF increased from 85.41% to 87.95% and PCE is drastically boosted from 18.57% to 25.04% when work function of rare contact is increased from 5.10 eV to 5.65 eV. The poor performance of simulated devices with metal contact having lower work function could be ascribed to limited movement of holes towards back contact owing to higher barrier height23,66. The metal contact like Pt with higher work function of 5.65 eV typically improves the transportation of holes to rare electrode by lowering the Schottky barrier for these charge carriers while it offers higher barrier height for electrons (resulting into blockage of electrons) due to favorable band bending with other components, and thus augment the performance of simulated devices48,67. The higher work function of rare contact (herein Pt) enhances the built-in electric filed and thus Voc is improved. The decrement in contact resistance induced by the metal leads to augmentation in FF while excellent transportation and separation of charge carriers and thereby their collection leads to enhance the Jsc and hence PCE of simulated devices59. Although Pt is noble metal and it may be costly but it would provide excellent stability since back contact like Ag may be reactive to active and charge transportation layers of solar cells. Rehman et al.68 suggested that higher work function back contact not only augments the electrical characteristics of device but also manipulates the optical properties like absorption, reflection and transmittance. Such metal contacts induce minimal reflection loss and boost light harvesting capability within the absorber layer and thus improves the performance of solar cells under investigation.
Fig. 12 [Images not available. See PDF.]
Spectral behaviour of QE of modeled devices having variable metal back contacts.
The influence of different metal back contact on QE of simulated devices is shown Fig. 12 which reveals that curves are overlapping till 420 nm while considerable enhancement in QE of simulated devices is achieved with Pt as rare contact in 450–850 nm spectral range. This behaviour is attributed to the facilitated charge extraction, separation and collection of charge carriers induced by metal with higher work function. Hence nature of metal back contact crucially governed the QE and other solar cell parameters. Implementation of Pt as rare contact could maximize performance of NaSnCl3 absorber-based PSCs with selected charge transport layers at optimized conditions.
Table 9 has been constructed which provides a comparative analysis of performance parameters of the presently simulated devices with other concerned (Sn based) PSCs devices. This table shows that results of presently simulated devices are resembled with other feasible devices and guides the researchers to realize these devices experimentally.
Table 9. Performance parameters of optimized devices with other concerned PSCs.
Device architecture | Nature | Voc (mV) | Jsc (mA/cm2) | FF (%) | η (%) | Ref. |
|---|---|---|---|---|---|---|
ITO/WS2/NaSnCl3/CuO/Pt | Simulation | 982 | 28.97 | 87.95 | 25.04 | Present work |
ITO/PCBM/CsSnI3/CFTS/Se | Simulation | 872 | 33.99 | 83.46 | 24.73 | 69 |
FTO/ZnO/CsSnI3/NiOx/Au | Simulation | 1005 | 28.02 | 81.22 | 23.84 | 70 |
FTO/TiO2/KSn(I0.5Br0.5)3/Cu2O/Au | Simulation | – | – | – | 11.31 | 71 |
AZO/TiO2/KSnBr3/FASnI3/CuSbS2/Au | Simulation | 940.6 | 31.45 | 79.60 | 22.37 | 72 |
ITO/PEDOT: PSS/CsSnI3/3-ThMAI/ICBA/BCP/Ag | Experimental | 770 | 22.68 | 69% | 12.05 | 73 |
The present simulation-based study highlighted the potential of NaSnCl3 absorber layer for PSC applications and modeled devices exhibited the elevated efficiency of more than 25% for proposed devices. However, there may be some challenges and limitations for realizing these devices experimentally. Researchers need to use high purity materials and appropriate fabrication methods as well as suitable dopants to attain the desired defect density (in bulk perovskite and at interfaces) and controlled higher dopant concentration. Uniform film deposition for scarcely materials is always challenging and optimal metal back contact may slightly influence the cost of proposed devices. Since instability due to Sn oxidation may be probable, therefore, precise use of surface modifier, encapsulation, surface additives may be required. Hence NaSnCl3 is captivating light harvesting perovskite material and associated suitable architecture has delivered higher PCE at optimal conditions through rigorous simulations. Such higher efficiency may be attained experimentally by considering the above provided suggestions.
Conclusion
The present research work crucially provided the numerical insights into advancement of photovoltaic performance of Pb free novel NaSnCl3 absorber-based PSCs. The simulations of modeled layer structured devices consisting of WS2 and CuO as charge transport layers are conducted using SCAPS-1D where ITO and different metals (Au, Ni, Pt) have been utilized as respective front and rare electrodes. The performance of glass/ITO/WS2/NaSnCl3/CuO/Au configured PSCs is analysed by optimizing the absorber parameters viz. thickness in range of 300–1500 nm, defect density in span of 1012-1016 cm−3, interface defect density of 1010-1014 cm−3 at WS2/NaSnCl3 and NaSnCl3/CuO interfaces and acceptor concentration in range 1014-1022 cm−3 along with work functions of rare metal contact. The thickness optimization suggested that Voc and PCE of designed devices are attained in span of 551.75-553.24 mV and 10.82–12.54% when thickness of NaSnCl3 is altered from 300 nm to 1500 nm where improvement in efficiency is attributed to more absorption and efficient generation of charge carriers by thicker absorber. The defects are indispensable in experimentally fabricated photovoltaic devices and consequently in order to realize simulated devices as experimental ones, analysis of defects is carried out. The enhancement in defect density of absorber layer from 1012 cm−3 to 1016 cm−3 led to performance degradation of devices as efficiency and Jsc are fallen down from 12.54% to 2.76% and 28.94 mA/cm2 to 24.97 mA/cm2, respectively. Further optimization of doping density revealed that Voc is enhanced from 552.91 mV to 815.77 mV while Jsc is undesirably declined from 28.94 mA/cm2 to 26.64 mA/cm2 on improving concentration of dopants from 1014 cm−3 to 1022 cm−3. The QE of simulated devices in most of visible spectral range is critically affected by thickness and defect density of NaSnCl3 absorber. The performance of modeled devices is also critically influenced by variation in defect density at interfaces. The implementation of Pt metal with higher work function (5.65 eV) as rare contact for simulated devices induced the excellent charge extraction and collection which lead to augment device performance. The modeled PSCs with configuration as glass/ITO/WS2/NaSnCl3/CuO/Pt exhibited preeminent performance parameters viz. Voc of 982 mV, Jsc of 28.97 mA/cm2, fill factor of 87.95% and PCE of 25.04% at optimized absorber parameters viz. thickness of 1.5 microns, defect density of 1012 cm−3, interface defect density of 1010 cm−3 at WS2/NaSnCl3 and NaSnCl3/CuO interfaces and doping concentration of 1022 cm−3. This significant numerical study revealed the remarkable potential of novel NaSnCl3 perovskite material in development of highly efficient, environmentally friendly, stable perovskite solar cells and paving the way towards sustainable development.
Acknowledgements
Authors are thankful to Prof. M. Burgelman, Department of Electronic and Information System (DELIS), University of Gent, Belgium for providing SCAPS-1D software to conduct this research work.
Author contributions
MIA, VO, RK: conceptualization, methodology, data curation, writing – original draft. GS, HR: writing – review and editing, resources, Former AnalysisDM, RM, AM: writing – review and editing, methodology, Fund acquisition AB, BBB: Project Administration, supervision, Validation, Software.
Author confirmation statement
We, the undersigned authors, confirm that we have made substantial contributions to the research and manuscript preparation. All individuals who contributed to the study have been acknowledged appropriately in the manuscript with their permission. We affirm that the list of authors is accurate, and that no other individuals who have contributed significantly to the work have been excluded.
Funding
No Funding.
Data availability
The data used to support the findings of this study are included in the article.
Declarations
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The authors declare no competing interests.
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This study was performed as a part of the employment of the authors.
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