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. 2022 Dec 21;8(1):1632–1642. doi: 10.1021/acsomega.2c07211

24% Efficient, Simple ZnSe/Sb2Se3 Heterojunction Solar Cell: An Analysis of PV Characteristics and Defects

Raman Kumari †,‡, Mamta Mamta †,‡, Rahul Kumar †,‡, Yogesh Singh †,‡, Vidya Nand Singh †,‡,*
PMCID: PMC9835802  PMID: 36643481

Abstract

graphic file with name ao2c07211_0017.jpg

In this work, a new wide-band-gap n-type buffer layer, ZnSe, has been proposed and investigated for an antimony selenide (Sb2Se3)-based thin-film solar cell. The study aims to boost the Sb2Se3-based solar cell’s performance by incorporating a cheap, widely accessible ZnSe buffer layer into the solar cell structure as a replacement for the CdS layer. Solar Cell Capacitance Simulator in One Dimension (SCAPS-1D) simulation software is used to thoroughly analyze the photovoltaic parameters of the heterojunction structure ZnSe/Sb2Se3. It includes open circuit voltage (VOC), short-circuit current density (JSC), fill factor (FF), power conversion efficiency (PCE), and external quantum efficiency (EQE). The absorber layer (Sb2Se3) thickness is adjusted from 0.5 to 3.0 μm to perfect the device. In addition, the influence of cell resistances, radiative recombination coefficient, acceptor and donor defect concentration in the Sb2Se3 layer, and interface defects of the ZnSe/Sb2Se3 layer on overall device performance are investigated. The ZnSe buffer layer and the Sb2Se3 absorber layer are designed to have optimal thicknesses of 100 nm and 1.5 μm, respectively. The proposed device’s efficiency with optimized parameters is calculated to be 24%. According to the simulation results, it is possible to build Sb2Se3-based thin-film solar devices at a low cost and with high efficiency by incorporating ZnSe as an electron transport layer.

1. Introduction

In thin-film solar cells, high bulk lifespan, effective carrier collection efficiency, sufficient photon absorption, and superior junction interface contribute to high device performance. Sb2Se3 has demonstrated excellent potential as an absorber material in thin-film solar cell (TFSC) technology.1−4 It is a binary compound having less-toxic and earth-rich elements. It has decent carrier mobility (electron mobility of >16.9 cm2 V–1 s–1 and hole mobility of >0.69 cm2 V–1 s–1),5 a high absorption coefficient over the visible spectrum (>105 cm–1), and a band gap of 1.2 eV.1 In 2009, Messina et al. achieved 0.66% power conversion efficiency (PCE) with CdS as a buffer layer.6 After 5 years, in 2014, Zhou et al. and Choi et al. achieved 2.26 and 3.21% power conversion efficiency (PCE) with TiO2 as a buffer layer, respectively.2,7 Wang et al. and Chen et al. gained 5.93 and 6.5% PCEs with ZnO and CdS as buffer layers, respectively, in 2017.3,4 In 2018, Wen et al. achieved 7.6% efficiency using a vapor-transport-deposited Sb2Se3 film and a CdS buffer layer.8 The highest efficiency obtained by Sb2Se3 thin-film solar cells was 9.2% in 2019, with CdS as a buffer layer.1 Many simulations were done to determine the theoretical PCE of Sb2Se3 solar cells. Mamta et al. achieved 27.84% efficiency for the Sb2Se3/CdS/ZnO solar device with a CdS buffer layer using SCAPS-1D.9 Ahmed et al. achieved 29.35% efficiency in Al/FTO/CdS/Sb2Se3/BaSi2/Mo solar devices using SCAPS-1D.10 Baig et al. used In2S3 as a buffer layer and achieved 13.20% efficiency using SCAPS-1D.11

Defects in the Sb2Se3 layer lead to carrier recombination and low device performance. The dangling bonds at grain boundaries act as recombination centers in a 3D crystal structure.12 Intrinsic point defects such as substitution/antisite (SbSe, SeSb), vacancies (VSe, VSb), and interstitials (Sei), which can be donor/acceptor/amphoteric, could act as both hole and electron traps and reduce the lifespan of the minority carrier lifetime and the solar cell performance.13

An ideal buffer layer should satisfy the fundamental criteria of having good carrier mobility, high electrical conductivity, and an appropriate energy band alignment with minimum conduction band offset (CBO).14 In Sb2Se3 TFSC, CdS is a commonly used buffer layer. The primary issue with the CdS thin film is its toxicity, which raises concerns for plants, the environment, and human health and is generally classified as a carcinogen.15−17 Besides its toxicity, Cd and S diffuse into the Sb2Se3 layer, which decreases the device performance, and CdS has a high lattice mismatch with the Sb2Se3 layer. Because of its narrow band gap of 2.4 eV, the CdS layer parasitically absorbs blue light, which causes photocurrent and conversion loss.18,19 The wide-band-gap buffer material provides high UV absorption and optical transparency in visible and IR regions.3,20 ZnSe having an energy gap of 2.7 eV is a direct wide-energy-gap semiconductor with a higher energy gap than CdS. It has permeability through a broad range of the visible spectrum and has a significant nonlinear optical coefficient value.21 ZnSe can be synthesized by thermal evaporation, spray pyrolysis, chemical bath deposition, electrodeposition, etc.22−25

In this work, the ZnSe/Sb2Se3 device analysis is performed by tuning the thickness of absorber and buffer layers, radiative recombination coefficient, shallow acceptor density, absorber defects, interface defect density, back contact work function, and series and shunt resistance with the aid of SCAPS-1D software. The performance of a solar cell is optimized by considering each of these factors. The many junction properties, including current density–voltage (J–V), built-in voltage by the Mott–Schottky plot, and energy band formation, have also been simulated.

2. Device Structure and Parameters of Materials Used in Modeling

2.1. Device Structure

The solar device structure is Mo/Sb2Se3/ZnSe/Ag, as shown in Figure 1, where Sb2Se3 is the absorber layer, ZnSe is the buffer layer, and Mo and Ag are the rear and front contacts, respectively.

Figure 1.

Figure 1

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Schematic of the proposed Mo/Sb2Se3/ZnSe/Ag solar cell.

2.2. Solar Cell Modeling Parameters

The performance of TFSCs can be investigated using a variety of applications, including COMSOL,26 SILVACO ATLAS,27 SCAPS,28 wxAMPS,29 and AMPS.30 SCAPS is a 1D (Version 3.3.07) solar cell simulation software created by the Department of Electronics and Information Systems at Gent University in Belgium. It was utilized in the current study to model the capability of solar cells and their many deciding parameters. This software has several benefits, including the potential to execute performance analysis across up to seven levels, in-depth and batch analyses, and simple results to understand and analyze.31 Three fundamental equations—Poisson’s equation, the carrier continuity equation, and the drift-diffusion equation—forming the foundation of the theoretical calculation are as follows

2.2. 1
2.2. 2
2.2. 3

where q is the charge on electrons, p is the concentration of holes, ε is the dielectric constant, n is the concentration of electrons, ϕ is the electric potential, R is the carrier recombination rate, G is the carrier generation rate, and Jp is the current density due to holes. Jn is the current density due to electrons. Dn and Dp are coefficients of electron and hole diffusion, respectively, while μn and μp are the mobilities of electrons and holes, respectively.

Each layer of the structure must contain several material parameters for SCAPS-1D software to simulate device design. The material and interface parameters used for the simulation analysis are shown in Tables 1 and 2, respectively. These parameters were based on the literature.32,33 The simulation was done under AM 1.5 G, 1000 W m–2 illumination intensity, at 300 K. For the absorber and buffer layers, thermal velocity was taken as 107 cm s–1.

Table 1. Material Parameters of the n-ZnSe/p-Sb2Se3 Solar Cell Used in the Simulationa.

parameters p-Sb2Se3 n-ZnSe
thickness (μm) 0.5–3.0 0.1
band gap (eV) 1.2 2.7
EA (eV) 4.04 4.09
ε (relative) 18 10
CB DOS (cm–3) 2.4 × 1018 1.5 × 1018
VB DOS (cm–3) 1.8 × 1019 1.8 × 1019
electron thermal velocity (cm s–1) 1 × 107 1 × 107
hole thermal velocity (cm s–1) 1 × 107 1 × 107
μn(cm2 V–1 s–1) 15 50
μp(cm2 V–1 s–1) 5.1 20
ND (cm–3) 0 1 × 1018
NA (cm–3) 1 × 1013 0
type of defect neutral acceptor
capture cross section of electrons (cm2) 1 × 10–15 1 × 10–15
capture cross section of holes (cm2) 1 × 10–17 1 × 10–15
energetic distribution Gaussian single
reference for the defect energy level, Et above EV above EV
energy level with respect to a reference (eV) 0.6 1.350
characteristic energy (eV) 0.1 0.1
Nt total (cm–3) 1 × 1014 1 × 1013
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a

EA is the electron affinity, ε is the dielectric permittivity, CB DOS is the conduction band density of states, VB DOS is the valence band density of states, μn is the electron mobility, μp is the hole mobility, ND is the shallow uniform donor density, NA is the shallow uniform acceptor density, EV is the valence band maximum, and Nt is the total defect density.

Table 2. Interface Parameters of the n-ZnSe/p-Sb2Se3 Layer.

parameters ZnSe/Sb2Se3
type of defect neutral
capture cross section of electrons (cm2) 1 × 10–19
capture cross section of holes (cm2) 1 × 10–19
energetic distribution single
reference for the defect energy level, Et above the highest EV
energy level with respect to a reference (eV) 0.6
characteristic energy (eV) 0.1
total density 1 × 1010
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3. Results and Discussion

The prime goal of this investigation is to examine the results of varying numerous absorber (Sb2Se3) and buffer (ZnSe) layer parameters on solar device photoconversion efficiency.

3.1. Influence of the Varying Absorber Layer (Sb2Se3) and Buffer Layer (ZnSe) Thicknesses on Device Performance

The effect of varying the thickness of the Sb2Se3 and ZnSe layers on VOC, JSC, fill factor (FF), and PCE is studied in this simulation. The absorber layer thickness is varied in the range from 0.5 to 3.0 μm. It can be seen from Figure 2a that VOC extends to an optimal value of 0.65 V (Table S1) at a thickness of 1.5 μm and remains constant when the thickness is further increased. In contrast, JSC first increases to a value of 38 mA cm–2 at 2.0 μm and remains almost constant when Sb2Se3 thickness is increased. The saturation of JSC occurs with an increase in absorber thickness because of an increase in the carrier recombination rate with the rate of carrier production.5 The saturation of VOC occurs because the absorber is thick enough to absorb nearly all incident light, which strengthens the electric field of the p–n junction. It probably reaches a plateau because of the nonradiative losses linked to point defect, carrier lifetime, and interface recombination.

Figure 2.

Figure 2

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Variation of (a) VOC and JSC and (b) FF and PCE as a function of Sb2Se3 absorber layer thickness.

Figure 2b shows that FF increases to 81.83% at 1.0 μm thickness and then decreases when the thickness of the Sb2Se3 layer is further increased. In contrast, the PCE value increases to 19.99% at 1.5 μm (Table S2) and then decreases. FF represents the ratio of maximum power output to VOC and JSC, i.e., Inline graphic, and PCE represents the ratio of maximum power to incident light, i.e., Inline graphic. An increase in the values of JSC and VOC at lower thickness leads to an increase in FF. Still, at higher absorption layer thickness, the depletion is accelerated and the internal resistance increases, decreasing the fill factor. The PCE of a solar cell depends on light absorption and carrier transport. At low absorber layer thickness, light absorption determines the PCE, as a carrier having high diffusion length than thickness can reach the electrode efficiently. At more increased thickness, carrier transport dominates, as light absorption becomes saturated. Thus, the optimum thickness value of Sb2Se3, i.e., the absorber layer, is taken as 1.5 μm.

The buffer layer improves the device’s performance by charge extraction and collection. In this device, ZnSe is used as a buffer layer, and the influence of varying the thickness on photovoltaic parameters is studied. The thickness value is varied from 50 to 300 nm. From Figure 3a,b, it is perceived that on increasing the thickness of the buffer layer, VOC and FF remain constant, whereas JSC and PCE decrease to 6.81 mA cm–2 and 19.24%, respectively. A thick buffer layer will absorb more photons, lowering efficiency, since it prevents them from reaching the absorber layer. It causes a decrease in JSC and PCE. A very thin buffer layer leads to a high leakage current.34 Thus, the optimal thickness value of the ZnSe buffer layer is 0.1 μm.

Figure 3.

Figure 3

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Variation of (a) VOC and JSC and (b) FF and PCE as a function of ZnSe buffer layer thickness.

3.2. Influence of Varying Shallow Acceptor Doping Densities of the Absorber Layer (Sb2Se3) on Device Performance

Acceptor density is a crucial parameter for regulating the solar cell’s performance. In this study, the acceptor density changes from 1013 to 1018 cm–3. From Figure 4a, it can be observed that VOC increases with increases in doping density, whereas JSC decreases. From Figure 4b, FF and PCE increase to 85.5 and 23.5% (Table S3), respectively. The decrease in the value of JSC is due to increased recombination in bulk. VOC and PCE increase with the increase in doping density, whereas FF slightly decreases to 1016 cm–3 and increases further with an increase in doping concentration. So, the optimal acceptor density is determined to be 1018 cm–3. The excessive doping density above 1018 cm–3 disrupts the crystal structure by forming a shunt path in the solar cell, thereby decreasing efficiency.

Figure 4.

Figure 4

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Variation of (a) VOC and JSC and (b) FF and PCE as a function of shallow acceptor doping density.

3.3. Influence of Varying Radiative Recombination Coefficients

Radiative recombination is the process of direct transition of an electron from the conduction band (CB) to the valence band (VB), and a photon (excess energy) is released. The following expression gives the radiative recombination coefficient

3.3. 4

where B is the radiative recombination coefficient (RRC), np is the concentration of electron–hole in trap states, and ni is the intrinsic carrier concentration. Recombination is the primary obstruction in creating high-efficiency solar cells.

In this study, the value of RRC is changed in the range of 10–1–10–15 cm3 s–1. Figure 5a,b shows that the maximum values of VOC, JSC, FF, and efficiency are achieved when RRC is 10–9 cm3 s–1 and remain constant (Table S4) for values lower than that. The optimal value at which the maximum value of all of the parameters is obtained is 10–11 cm3 s–1.

Figure 5.

Figure 5

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Variation of (a) VOC and JSC and (b) FF and PCE with RRC.

3.4. Influence of Varying Total Defect Densities in the Absorber Layer on Performance Parameters

Defect density is another significant property that has a direct influence on the efficiency of solar cells. Defects regulate interfacial recombination rate, carrier lifetime, and material doping level.35 The photogenerated current is mainly produced in the absorber layer. Therefore, as the defect density increases, carrier recombination also increases, reducing device efficiency. Intrinsic point defects in Sb2Se3 single crystal and solar cells are similar.36 Three intrinsic defects are vacancy defects, interstitial defects, and antisite defects. Vacancy defects (VSe, VSb) occur when Sb or Se atoms are missing from one of its sites. Both antimony vacancies (VSb1, VSb2) exhibit an acceptor nature and are present above the VBM. They have a high enthalpy of formation, which indicates that their concentration in the material will not matter.36 Selenium vacancies have a low enthalpy of formation and are very deep donor defects.36 Interstitial defects (Sbi, Sei) occur when an atom occupies an interstitial site in the crystal lattice. They are deep defects and will hardly impact the material’s electronic conductivity.

Antisite defects (SeSb, SbSe) occur when Se atoms occupy Sb sites and vice versa. Sb2Se3 exhibits p-type conductivity due to an antisite defect, SbSe.36 According to the first-principle calculation, antisite defects are dominant in Sb2Se3 due to its 1D structure.37 Antisite SeSb is a deep donor defect, whereas SbSe is a shallow acceptor defect. VTD-fabricated Sb2Se3 has two hole traps at 0.48 and 0.71 eV above the valence band (VB) and an electron trap at 0.61 eV below the conduction band (CB). The hole trap at 0.48 and 0.71 eV is due to VSb and SeSb acceptor defects, respectively, while the electron trap at 0.61 eV is due to SbSe antisite donor defects.8 Here, in this simulation, the effect of antisite defect density is investigated, and vacancy defects are not included, as they have a high enthalpy of formation and are less likely to happen.

3.4.1. Varying Donor Defect Densities

This simulation is based on considering only donor-type (SbSe antisite) defects in the Sb2Se3 layer, which are present at 0.61 eV below CB. Here, the donor defect concentration is changed from 1012 to 1017 cm–3. Figure 6a,b shows that all of the parameters decrease with increasing defect concentration. An optimum value of 1013 cm–3 is obtained for determining the I–V characteristics, where photovoltaic parameters are 0.64 V, 37.60 mA cm–2, 82.09%, and 19.75% (able ST5).

Figure 6.

Figure 6

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Variation of (a) VOC and JSC and (b) FF and PCE as a function of donor defect density in the absorber layer.

3.4.2. Varying Acceptor Defect Densities

Here, the simulation is carried out under consideration when the absorber layer contains only acceptor-type (SeSb antisite) defects at 0.71 eV above VB. It is seen from Figure 7a,b that when the defect density of the absorber layer is increased, FF and PCE decrease. In contrast, VOC is constant at 0.64 V up to 1015 cm–3 concentration and then decreases, and JSC remains constant at 37.60 mA cm–2 till 1013 cm–3 and then decreases. So, the optimum value of 1013 cm–3 is considered for the I–V curve, where PCE, VOC, FF, and JSC are 19.75%, 0.64 V, 82.08%, and 37.60 mA cm–2 (Table S6), respectively.

Figure 7.

Figure 7

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Variation of (a) VOC and JSC and (b) FF and PCE as a function of acceptor defect density in the absorber layer.

There is a common observation in both types of defects that the device is defect tolerant up to 1015 cm–3 defect concentration, i.e., there is a very slight change in the values of photovoltaic parameters.

3.5. Influence of Varying Interface Defect Densities on PV Parameters

Interface states are a crucial factor that considerably affects solar cell performance. Sb2Se3 has an orthorhombic structure (unit cell parameters, a = 11.7808 Å, b = 3.9767 Å, c = 11.6311 Å), and ZnSe crystallizes in a hexagonal structure (unit cell parameters, a = b = c = 5.668 Å). So, a lattice mismatch contributes to interface states and recombination. Interface recombination is one of the main reasons for the loss in VOC, FF, and efficiency. At the ZnSe/Sb2Se3 interface, defect concentration is changed from 1012 to 1017 cm–3.

3.5.1. Varying Neutral Defect Densities

It is seen in Figure 8a,b that with the increase in the concentration of neutral defects of the ZnSe/Sb2Se3 interface, the value of VOC remains unaffected, but JSC, FF, and PCE decrease with an increase in defect density (Table S7).

Figure 8.

Figure 8

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Variation of (a) VOC and JSC and (b) FF and PCE with neutral interface defect density.

3.5.2. Varying Acceptor Defect Densities

It is seen in Figure 9a,b that with the increase in the concentration of acceptor defects, FF decreases by 20%, and PCE decreases by 37% for 1017 cm–3. Still, there is a slight increase in VOC and JSC initially and then both decrease at higher defect concentrations (Table S8).

Figure 9.

Figure 9

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Variation of (a) VOC and JSC and (b) FF and PCE with acceptor interface defect density.

3.5.3. Varying Donor Defect Densities

It is seen from Figure 10a,b that there is a very minute change in JSC, FF, and efficiency for donor defect concentration ranging from 1012 to 1015 cm–3, while at a high defect concentration, there is a decrease in efficiency. JSC changes by 7%, FF changes by 5%, and PCE changes by 12% from their respective values at 1012 cm–3. It is observed that there is almost no effect on VOC (1% decrease from the value at 1012 cm–3; Table S9) with increasing donor defect concentration of the interface.

Figure 10.

Figure 10

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Variation of (a) VOC and JSC and (b) FF and PCE with donor interface defect density.

3.6. Influence of Varying Series and Shunt Resistances on PV Parameters

Series and shunt resistances are parasitic parameters that reduce the fill factor and, thus, solar cell efficiency. The metallic contacts and interconnections, carrier transit via the top diffused layer, the bulk resistance of the semiconductor material, and the contact resistance between semiconductor and metallic contacts significantly influence series resistance (Rs). The impurities and nonidealities decrease the shunt resistance (Rsh) at the p–n junction, which partially shortens the junction, especially close to cell boundaries.38 In this study, Rs varies from 1.0 to 30 Ω cm2, and Rsh ranges from 100 to 1500 Ω cm2. From Figure 11a, it can be observed that VOC remains unaffected at a value of 0.64 V (Table S10) with an increase in series resistance, and JSC decreases abruptly after 15 Ω cm2 to a value of 20.6 mA cm–2 at 30 Ω cm2. Figure 11b shows that the FF value decreases from 76.53 to 37.34% with an increase in series resistance from 1 to 10 Ω cm2. The efficiency decreases from 18.40 to 8.96% with an increase in series resistance from 1 to 10 Ω cm2.

Figure 11.

Figure 11

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Variation of (a) VOC and JSC and (b) FF and PCE with series resistance.

From Figure 12a,b, it is observed that with the increase in shunt resistance value, JSC and VOC remain constant at 37.59 mA cm–2 and 0.64 V, respectively. In contrast, FF and PCE increase rapidly to 77.67 and 18.61%, respectively, in the initial increase in shunt resistance and then increase slightly to reach maximum values of 81.04 and 19.47% at 1500 Ω cm2 (Table S11), respectively. The low value of shunt resistance creates power losses in solar cells by giving the current generated by the light a short-circuit channel between the buffer and the metal. The voltage generated by the solar cell decreased due to this diversion. It also reduced the current flowing through the junction of the solar cell. The current density and the voltage at the maximum powerpoint decreased due to a reduction in FF. The optimized series and shunt resistance values are 1 and 1000 Ω cm2, respectively.

Figure 12.

Figure 12

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Variation of (a) VOC and JSC and (b) FF and PCE with shunt resistance.

3.7. Optimized Device Parameters, EQE Curve, and Energy Band Diagram

Figure 13a shows the I–V curve under AM 1.5 G (1000 W m–2, 300 K) illumination of the device with the optimized simulated parameters. The optimized parameters of the device are as follows: a 1.5 μm thin Sb2Se3 layer, a 0.1 μm thin ZnSe layer, 1018 cm–3 shallow acceptor density of the absorber layer, a recombination coefficient of 10–11 cm3 s–1, donor and acceptor defects of 1013 cm–3 concentration in the absorber layer, and a neutral interface defect of 1013 cm–3. The maximum efficiency obtained for Sb2Se3/ZnSe solar cells using these optimized parameters is 24.0%. Other parameters of the device, such as VOC, JSC, and FF, are 0.85 V, 34 mA cm–2, and 83.6%, respectively.

Figure 13.

Figure 13

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(a) I–V curve, (b) EQE curve, (c) energy band diagram for the ZnSe/Sb2Se3 solar cell with optimized simulation parameters.

Figure 13b shows that the EQE covers the entire visible spectrum and has a strong spectral response between 300 and 1000 nm. Additionally, the ZnSe/Sb2Se3 solar cell converts more photons into charge carriers in the shorter wavelength region than the commonly used CdS buffer layer. The parasitic absorption of the CdS layer is responsible for reducing the shorter wavelength region.34

The energy band structure of the ZnSe/Sb2Se3 solar cell is shown in Figure 13c. When photogenerated carriers separate at the intersection of the buffer and absorber layers, the built-in voltage induced by the strong electric field of the p–n junction accelerates the charge carriers, causing them to be quickly drawn apart from the p–n junction. As the holes pass through the absorber layer and electrons through the buffer layer, they are collected by the back and front contacts, respectively. The charge carriers are well suited to the band offsets and can reach the metal contact without recombination.

3.8. Influence of Varying Rear Metal Work Functions on PV Parameters

Back contact functions as a contact for transferring carriers to the load and as an optical reflector. A variety of materials have been suggested as back contacts, such as aluminum (4.28 eV), silver (4.3 eV), tungsten (4.5 eV), copper (4.61 eV), molybdenum (5.0 eV), gold (5.1 eV), etc. Mo and Au are the most typical material utilized for the rear contact. It is due to the low contact resistance of molybdenum to the absorber layer and its stability at processing temperatures. These can reduce atom diffusion, minimizing harmful reactions during Sb2Se3 development.39 Gold is an inert metal and does not have undesirable interfacial interactions with the absorber. Both Mo (5.0 eV) and Au (5.1 eV) metals have a high work function, which is necessary to avoid a high energy hurdle for hole extraction at the rear interface in absorbers of p-type conductivity and to produce an ohmic contact.40 Here, the rear metal contact work function varies from 4.2 to 5.1 eV. Figure 14a shows that VOC and JSC increase (Table S12) as the back contact work function increases. Figure 14b illustrates that FF and PCE also increase gradually with the increase of back contact work function. It can be seen that when the value is below 4.8 eV, the solar cell’s performance suffers significantly. It is due to the potential for the Schottky junction to form at the Sb2Se3/metal interface, which may happen due to Fermi-level pinning at the interface.41 So, to achieve better device performance, the rear metal work function should be above 4.8 eV.

Figure 14.

Figure 14

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Variation of (a) VOC and JSC and (b) FF and PCE with the back contact work function.

3.9. Capacitance–Voltage Characteristics

The C–V studies are done to discover more fundamental characteristics of the solar device. The essential parameters generated from the capacitance–voltage curve are built-in potential (Vbi) and doping density (Na), which are shown by the following two relations42,43

3.9. 5
3.9. 6

where C is the measured capacitance, q is the charge of an electron, Na is the doping density, ε0 is the permittivity of the free space (8.85 × 10–14 F cm–1), εs is the dielectric constant, A is the area of the cell (1 cm2), Vbi is the built-in potential, and V is the applied potential.

A linear area with a slope equal to the doping concentration is produced by eq 5. The cell’s built-in voltage is obtained by the intersection of 1/C2 with the voltage axis, which is 1.2 V. The C–V studies are done at a frequency of 1 MHz.

A Mott–Schottky plot, obtained from C–V characteristics, is shown in Figure 15.

Figure 15.

Figure 15

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Mott–Schottky plot for the ZnSe/Sb2Se3 device.

By putting all of the values of parameters and slope (−22.19) in eq 6, Na is found to be 3.5 × 1017 cm–3. Since the plot has a negative slope, it is likely that the majority of charge carriers are holes and that the p-type Sb2Se3 layer contains most of the space charge area. Vbi and Na are much higher than those of CdS–Sb2Se3 heterojunction solar cells.43,44

4. Conclusions

In this study, SCAPS-1D was used to simulate the Mo/Sb2Se3/ZnSe/Al solar cell to obtain optimized parameters of the solar device. The solar cell’s efficacy is tuned for various controllable factors, including the thickness of the buffer and absorber layers, acceptor density and defect, recombination, shunt and series resistance, and rear metal work function. Results show that other properties of the solar device, such as efficiency, are notably influenced by the absorber layer’s thickness. The buffer and absorber layers’ thicknesses are optimized to 100 nm and 1.5 μm, respectively. The RRC value obtained is 10–11 cm3 s–1. For Sb2Se3-based solar cells to work at their best, the simulation findings further highlighted the significance of managing radiative recombination, defect, and shallow acceptor density. The most outstanding efficiency for the ZnSe/Sb2Se3 solar cell after adjusting all of the parameters above is 24%. The Mott–Schottky plot showed that the built-in voltage of the device was 1.2 V, and the cell had a carrier concentration of 3.5 × 1017 cm–3. The results of this study will help the researchers to develop highly efficient Sb-chalcogenide-based solar cells with less-toxic and earth-abundant materials.

Acknowledgments

R. Kumari highly acknowledges University Grants Commission (UGC), India, for the Senior Research Fellowship (SRF) grant. Mamta, R. Kumar, and Y.S. highly acknowledge the Council for Scientific and Industrial Research (CSIR), India, for the Senior Research Fellowship (SRF) grant. The authors would like to thank their institute and Professor Burgelman of Gent University in Belgium, who provided SCAPS software.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c07211.

  • Various tables presenting the influence of Sb2Se3 thickness; ZnSe thickness; Sb2Se3 shallow acceptor doping density; varying radiative recombination coefficients; varying donor defect densities of the Sb2Se3 layer; acceptor defect density of the Sb2Se3 layer; varying neutral interface defect densities of ZnSe/Sb2Se3; varying acceptor interface defect densities of ZnSe/Sb2Se3; varying donor interface defect densities of ZnSe/Sb2Se3; series resistance; varying shunt resistances; and varying back contact metal work function on device performance (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao2c07211_si_001.pdf (51.1KB, pdf)

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