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Abstract
In this work, a dual-heterojunction (DH) thin film solar cell of notable efficiency has been designed and simulated where p-type CuInSe2 (CIS) has been employed as the base layer in combination with an n-type CdS window and a p + -type GeSe back surface field (BSF) layer. The influences of each layer have been revealed using the SCAPS-1D simulator. While the n-CdS/p-CIS single heterojunction (SH) structure acting alone has been found to be resulted with 24.86% of photoconversion efficiency (PCE) with the JSC = 42.80 mA/cm2, VOC = 0.70 V, and FF = 83.44%, an enhancement to PCE of 30.52% is observed with the corresponding JSC of 44.10 mA/cm2, VOC of 0.86 V, and FF of 80.30% owing to the addition of GeSe as BSF layer in the proposed structure with optimized parameters. Because of the enormous built-in potential of the CIS/GeSe interface, increased VOC mostly contributes to the efficiency enhancement. These findings suggest that the CIS absorber layer with GeSe BSF layer is a promising choice for solar energy harvesting in the near future.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The need for energy has emerged as one of the most urgent problems in the developing countries due to an ever-increasing population, economic extension, and more industrialization. The majority of energy demands have so far been met by fossil fuels. But, due to their limited availability, traditional energy sources won't be able to meet future energy demand on a worldwide scale. Moreover, the burning of fossil fuels releases CO2, which has the consequence of causing the greenhouse effect and environmental damage. Therefore, at present, one of the biggest challenges facing academics is to produce renewable resources of energy to take the place of fossil fuels. Owing to the appearance of sun being almost totally above the Earth, solar energy is the most prominent renewable source to meet the global energy demands. The most efficient method to utilize solar energy is the straight transformation of it into electrical energy using photovoltaic (PV) technology.
Whereas, 90% of the worldwide solar market is being controlled by silicon (Si) solar cells, the highest efficiency for such cells has been recorded to be 26.33% in laboratory [1]. Moreover, in the theoretical approach, the highest PCE of 38% had been acquired for Si solar cells by employing PEDOT:PSS as the window layer and In3Se4 as the back surface field (BSF) layer [2]. Recently, a study demonstrates the effects of multilevel impurities on Si solar cells and estimates the PCE to be 35.4% through the absorption of longer wavelength light [3]. Si solar cells, however, have a number of weak points, including the indirect bandgap behavior they display with a relatively low absorption coefficient and the demand for an extremely high temperature of roughly 1400 ◦C during the production process [4].
Meanwhile, thin film photovoltaic cells have recently attracted a significant amount of attention owing to their low production costs, increased adaptability, stability, and perceived PCE, preparing them a competitive option to silicon solar cells [5–8]. For instance, thin film-based chalcogenide CdTe, Cu(InGa)Se2 (CIGS), and Sb2Se3 solar cells already yield 22.1, 22.6, and 10.12% of PCE, respectively [9–11].
Currently, CuInSe2 (CIS) thin film based photovoltaic devices are receiving rapid research attention because of their stability, flexibility, strong radiation resistance, and affordable manufacturing cost [12]. CuInSe2 is a ternary chalcogenide semiconductor compound belonging to the group of I-III-VI2 [13]. It sets out a direct bandgap of 1.04 that can provide the advantage of absorbing 90% of solar radiation using 1 to 2 µm thick films [14]. Many deposition methods could be employed to synthesize CIS-based solar cells including chemical-bath deposition (CBD), Electron Beam evaporation, chemical-ion exchange method, three stage co-evaporation, and electro-deposition [14]. However, an 8.2% efficiency has been attained from solution-processed CIS-based solar cells [15]. Additionally, the ZnO/CdS/p-CuInSe2 thin film solar cell also exhibits a PCE of 15% [16]. Until now, the maximum cell efficiency of 19.2% was achieved in 2019 for CIS solar cells [17]. More recently, theoretical research demonstrates the PCEs of 13.31 to 14% for CIS solar cells [14,18]. Hence, more research is needed to fully utilize the potential of CIS solar cells and increase the efficiency.
In this attempt, we develop a novel dual-heterojunction solar cell based on CIS that uses GeSe as the BSF layer and CdS as the window layer. Having a wider bandgap of 2.4 eV, n-type CdS window allows most of the visible light to pass through it [19]. It can be therefore employed as a window layer with a variety of absorber layers, such as Si, CIGS, CdTe, and others [20–22]. The usage of CdS as a buffer layer with a CIS absorber layer has already been studied [16,18,23]. Therefore, in this current study, CdS has been utilized as a window layer with a CIS absorber to reveal the potentiality of this structure.
On the contrary, GeSe is a type of phosphorene analogue that belongs to the binary group IV-VI monochalcogenide family. It is a very fantastic material due to its numerous usages in the field effect transistor, gas sensor, water splitting, photodetectors, and other devices [24–27]. On account of its acceptable bandgap of 1.1 to 1.2 eV and absorption coefficient > 104 cm-1, numerous studies consider it as a potential absorber for photovoltaic implications. The PCE has already reached 17-21.47% for the GeSe-based solar cells [28–31]. In this work, GeSe plays a role of strong contender for the BSF layer as it has the ability to create a favorable pp+ junction with the CIS compound due to its electron affinity of -4.07 eV and ionization potential of -5.21 eV.
Therefore, this endeavor has explored extensively the unique n-CdS/p-CIS/p + -GeSe thin film solar cell structure to highlight the prospective performance of CIS solar cells with GeSe as a BSF material.
2. Design of CIS-based photovoltaic device and numerical modeling
2.1 Proposed structure
In the modeled n-CdS/p-CIS/p + -GeSe DH photovoltaic cell, photons penetrates via indium tin oxide (ITO), which serves as the substrate and CdS window layer, as illustrated in Fig. 1(a). However, the properties of CdS i.e. an ionization potential of 6.8 eV and an electron affinity of 4.4 eV make CuInSe2 absorber able to create a possible n/p junction at window because of having -4.3 and -5.34 eV of EC and EV, respectively, as demonstrated in the energy band layout of Fig. 1(b) [2,32]. Nevertheless, CIS is a tetragonal structured chalcopyrite compound that contains lattice parameters a = b = 6.1944 and c = 12.4157 Å [33]. Whereas, CdS has a cubic (5.8320 Å) or hexagonal crystal structure (a = b = 4.160 and c = 6.756 Å) [19]. As a result, few dislocation densities will appear in the n-CdS/p-CIS junction which will produce structural defects in the interface. This is why the simulation has been performed with a substantial number of interface defects into account. On the other hand, GeSe owns an orthorhombic crystal structure having lattice parameters a = 4.40, b = 3.83, and c = 10.84 Å and band edge energies, EC = -4.07 eV and EV = -5.21 eV [34]. Therefore, despite a few interface defects, which have also been accounted for in our simulation, CIS could also form a favorable p/p+ junction with GeSe. It is also seen in the Fig. 1(b) that the quasi-Fermi energy level of electrons EFn enters into below the CV edge of the n-type CdS window layer whereas, the quasi-Fermi level of holes EFp enters above the VB edge of the p-type GeSe BSF layer. Therefore, photo-generated electrons in p-absorber layer are transferred to the n-window layer and are inhibited by the p + -BSF layer. Similarly, photo-generated holes in p-absorber are also transferred to the p + -BSF and are obstructed by the n-window. As a consequence, the anode can easily capture the photogenerated holes, and the cathode can easily collect the electrons. For this purpose, Al and Ni metals are employed as anode and cathode, respectively in this simulation.
Fig. 1. (a) The sketches of n-CdS/p-CIS/p + -GeSe device structure on ITO substrate and (b) lighted energy band layout of the proposed PV cell.
2.2 Numerical simulation and materials parameters
The entire computation of the proposed CIS-based heterojunction solar cell was carried out on SCAPS-1D software considering AM 1.5 G standard spectrum, single sun (100 mW/cm2), 300 K working temperature and ideal series and shunt resistances [35]. SCAPS 1D software solves Poisson's equation, continuity equations for free holes and electrons, and the drift-diffusion equation to simulate solar devices [35]. The Poisson’s equation can be written as follows:
The hole and electron continuity equations can also be expressed as follows:
Here, Jp and Jn denote the current density for hole and electron, respectively, Gop is the carrier generation rate, and R presents recombination rate.
The following equations can be used to describe how carriers are transported in semiconductors by drift and diffusion:
where, EFp and EFn are the quasi Fermi levels of p-type and n-type carrier, respectively, and $\mu $p presents the hole mobility and $\mu $n indicates the electron mobility.
Table 1. Physical parameters for different constituent layers employed in this device modeling.
This simulation did not account for the radiative recombination co-efficient. The bulk defects of 1 × 1014 cm-3 were considered for each device layer. The electron/hole thermal velocity were taken as 107 cm/s for each layer. The default values of capture cross-section of electron/hole were set to 10−15 cm-2 for each layer, except the hole capture cross-section of GeSe which was 10−17 cm-2. The optical absorption coefficient data for the CdS window, CIS absorber, and GeSe BSF layer were collected from experimental works [19,37,38]. The physical characteristics for the various layers stated in Table 1 were gathered from a variety of reported literatures. In this simulation, the thickness, doping concentration, and bulk defects for various layers were varied to explore the maximum performances of the proposed CIS solar cell.
3. Results and discussions
3.1 Influence of absorber layer on the performance of CIS solar device
This section evaluates the photovoltaic response of the designed DH thin film solar cell concerning width, concentration of dopant atoms, and bulk defects of the CIS absorber layer.
Figure 2 displays the contour plots of the photovoltaic parameters in relation to the width and carrier density of the CuInSe2 absorber layer. As shown in Fig. 2(a), the short circuit current (JSC) rises with the addendum of the CIS base width. For thickness 0.4 to 1.2 µm, it rises from 42.28 to 46.53 mA/cm2. Since an extended absorber can utilize more photons leading to creation of additional electron-hole pairs and photocurrent as a result, it seems sensible that JSC increases with thickness [39,40]. Additionally, it can also be demonstrated that it is independent of the explored limit of carrier concentrations because free carrier absorption might not be the major phenomenon there.
Fig. 2. The dependency of PV performance of proposed DH thin film solar cell on thickness and doping of CIS absorber.
Figure 2(b) shows how the open circuit voltage (VOC) is affected by absorber width and doping density of the proposed cell structure. At 0.4 µm thickness with 1 × 1014 cm-3 doping concentration, the VOC results with 0.87 V and stands for the maximum value. Such a large VOC is developed by the built in potential at both n/p and p/p+ heterojunctions with window and BSF, respectively [41]. However, VOC falls to 0.79 V at a rise in thickness to 1.2 µm with doping concentration to 1 × 1018 cm-3. Such a slight reduction of VOC is reasonable due to the increment of dark current with the thicker absorber layer [42].
In Fig. 2(c), the fill factor (FF) is seen to decrease with thickness and lower doping concentration of the CIS layer. At 1.2 µm thickness with 1 × 1017 cm-3 carrier concentration, the FF drops to 74.27% from 82.05% at 0.4 µm thickness with 1 × 1014 cm-3 carrier concentration. However, at higher >1 × 1018 cm-3 doping concentration, the FF is noticed to increase with the absorber width. The ascent in FF to 84.37 from 83% at 1.2 to 0.4 µm thickness at 1 × 1018 cm-3 doping concentration may have been caused by a reduction in series resistance [2,43].
Although the PCE declines with lower doping concentrations as a reduction in the fill factor of the CIS absorber layer, it improves with thickness owing to an enhancement in JSC as described in Fig. 2(d). However, at 1 × 1018 cm-3 doping concentration, it enhances from 29.05 to 31.13% for a change of thickness from 0.4 to 1.2 µm.
Figure 3 visualizes the contour plots of photovoltaic response with width and defect density of CIS absorber. In Fig. 3(a), the JSC is found to increase with thickness but it is independent of the CIS defect densities. In Fig. 3(b), the VOC is found to exhibit constant behavior with thickness and up to bulk defects of 1013 cm-3. However, with large defects, it considerably drops with thickness as a result of the dominant Shockley-Read-Hall (SRH) recombination over others. Consequently, the reverse saturation current increases resulting the degradation of the device's VOC [36,42]. The FF in Fig. 3(c) also shows a similar trend as like as VOC. Finally, the PCE in Fig. 3(d) is found to decrease drastically at higher defect orders. The utmost PCE of 34% was achieved at thickness >1 µm and at bulk defects < 1013 cm-3 of the CIS absorber.
Fig. 3. Performance dependency of proposed DH thin film solar cell on thickness and bulk defects of CIS absorber.
Hence, for future computations, thickness of 0.6 µm, doping concentration of 1016 cm-3, and bulk defects of 1014 cm-3 have been taken into consideration as the optimal value for the CIS absorber layer.
3.2 Absorber layer dependent quantum efficiency of CIS thin film PV cell
Quantum efficiency (QE) is the amount of the number of charge carriers captured by solar devices to the number of photons incident on the cell [36]. Figure 4 illustrates the modification in QE for addendum of CIS absorber layer thickness in the designed DH solar structure. The QE is seen to climb with increasing thickness of the CIS layer because of photon absorption rises with increasing absorber thickness and enhances short circuit current [22,36]. When the energy of photons (hν) falls beneath the energy of CIS band gap (Eg), all the curves corresponding with different thicknesses begin to decline toward 0% at a certain higher wavelength.
3.3 Impression of CdS layer on the performance of CIS solar cell
Figure 5 represents how the device parameters of the designed CIS DH solar cell are affected by thickness, doping concentration, and bulk defects of the CdS window layer.
Fig. 5. The dependency of performance of CIS thin film solar cell on (a) thickness, (b) doping concentration and (c) volume defects of CdS layer.
Figure 5(a) shows the change in short circuit current density concerning the thickness of the window layer. All the parameters are seen to be constant at 44.09 mA/cm2, 0.86 V, 80.30%, and 30.52%, for JSC, VOC, FF, and PCE respectively with thickness of CdS layer in the range of 0.05 to 0.30 µm. Due to its large band gap, CdS window does not take part in photon absorption, therefore, have negligible impact on cell performance up to the varied thickness.
Figure 5(b) demonstrates the PV response of n-CdS/p-CIS/p + -GeSe solar cell depending on the doping concentration of CdS within the limit of 1016 to 1020 cm-3. Here, JSC is unaffected by the carrier density of the window CdS. However, the VOC is discovered to have slightly increased from 0.86 to 0.87 V, which may have been resulted by the production of higher built-in potential in the n/p window-absorber interface, which lowers the reverse saturation current [34]. The FF is also noticed to increase with doping concentration from 78.57 to 80.42% as a consequence of mitigation in series resistance [44]. Such enhancement in VOC and FF makes the PCE of the proposed cell to rise from 29.82 to 30.92% with gradual increment of doping concentration within the indicated range.
The consequences of increase in volume defects of the CdS layer within the limit of 1012 to 1016 cm-3 have been depicted with Fig. 5(c). Throughout the defect density range of the CdS layer, all PV parameters are nearly constant. Hence, the bulk defects of the window layer have ignorable effects on the n-CdS/p-CIS/p + -GeSe DH solar cell.
Therefore, 0.1 µm thickness, 1018 cm-3 carrier concentration, and 1014 cm-3 bulk defects have been used as optimal values for the CdS window layer for other calculations.
3.4 Impression of GeSe BSF layer on the performance of the designed CIS PV cell
Figure 6(a) visualizes how the cell performance of proposed solar device are influenced by the width of the GeSe layer within the range of 0.1 to 0.5 µm. When a 0.1 µm GeSe BSF layer is added to a CIS solar cell, the JSC increases from 42.80 to 43.62 mA/cm2. Additionally, as GeSe is made thicker, the JSC rises as well, peaking at 44.95 mA/cm2 at only thickness of 0.5 µm. This is happened because of the mitigation of velocity of surface recombination at the interface of p-CIS/p + -GeSe due to the insertion of the BSF layer [2].
Fig. 6. The dependency of output performance of CIS thin film photovoltaic cell on (a) thickness, (b) doping concentration, and (c) volume defects of GeSe layer.
Besides, the VOC is found to increase from the pristine case of 0.70 to 0.86 V due to the addition of only a 0.1 µm thick GeSe BSF layer. The embodiment of the GeSe layer yields a significant built-in potential at the absorber-BSF heterointerface, which enhances the VOC [2,36].
The VOC is maintained almost at a constant value by the structure with further rise in width of the BSF layer. However, the inclusion of the 0.1 µm GeSe layer is found to reduce the FF from 83.44 to 80.27%, which may have occurred because of an enhancement in series resistance within the cell. After that, the thickness of GeSe has negligible impacts on FF. Finally, the PCE is found to increase significantly from 24.86 to 30.18% at only a 0.1 µm thick GeSe layer. Then, with increasing thickness, the PCE also increases to 31.16 at 0.5 µm thickness depending on the JSC.
Figure 6(b) shows the dependency of the performance of n-CdS/p-CIS/p + -GeSe solar cell concerning doping concentration of GeSe BSF layer in the range of 1016 to 1021 cm-3. As can be noticed in the figure, with doping concentrations of 1016 to 1021 cm-3, the JSC marginally falls from 44.11 to 43.68 mA/cm2, which has occurred as a result of the free carrier recombination of photon induced electron-hole pairs [45]. The higher built-in potential in the p-CIS/p + -GeSe interface is also observed to cause the VOC to increase from 0.85 to 0.88 V with high carrier concentration [46]. At a rise doping from 1016 to 1018 cm-3, the FF increases from 78.35 to 80.58% before declining as the doping concentration rises. Depending on JSC and FF, the PCE is shown to increase until the doping concentration reaches 1019 cm-3, at which it begins to decline.
Figure 6(c) manifests that the defects of the GeSe BSF layer have very little influence on any of the PV parameters. However, at higher order of defect densities, a reduction in PCE is observed because GeSe BSF layer could have an impact on the JSC and VOC.
Hence, 0.2 µm, 1019 cm-3, and 1014 cm-3 have been selected as the optimal thickness, doping, and defect density, respectively of GeSe BSF layer.
3.5 Role of device resistances on n-CdS/p-CIS/p + -GeSe PV cell
The series (RS) and shunt (RSh) resistances of a PV device are caused by the contacts among various layers, manufacturing defects, and metal contacts at the anode and cathode [47]. The high series resistance has a notable dominance on the PCE of a solar cell. It mainly affects the FF of solar cells.
In Fig. 7(a), the values of series resistance have been varied from 1 to 5 Ω.cm2. The JSC and VOC are noticed independent of the variation of series resistance. The FF does, however, abruptly decline with series resistance. It decreases from 80.3 to 75.67% when series resistance increases by only 1 Ω.cm2 and it further decreases to 58.86% when it jumps to 5 Ω.cm2. The PCE also drops to 22.39% at a series resistance of 5 Ω.cm2 due to the decrement of FF.
Fig. 7. Performance dependency of proposed CIS thin film solar cell on (a) series, and (b) shunt resistance.
Figure 7(b) depicts how the shunt resistance affects the performances of n-CdS/p-CIS/p + -GeSe DH solar cell. The shunt resistance values ranged from 1 to 5 kΩ.cm2. The JSC and VOC are observed to be constant with shunt resistance. The FF is seen to drop negligibly at 1 kΩ.cm2 from the ideal case. Following that, it marginally rises as the shunt resistance increases. The PCE exhibits the same behavior depending on FF.
3.6 Role of operating temperature on CIS solar cell
Figure 8 depicts the role of operating temperature on the performance of CIS solar cell. The JSC shows the constant behavior with operating temperature which was varied from 300 to 450 K as depicted in the figure. However, the VOC decreases drastically with increasing temperature. It falls to 0.69 V at 450 K from 0.86 V at 300 K due to the increase in dark current with operating temperature [45]. The FF is also observed to decrease from 80.30 to 72.73% at a rise of operating temperature from 300 to 450 K. Finally, as a result of the reduction in VOC and FF, the PCE also falls from 30.52 to 21.96% within the operating temperature range. Moreover, at greater temperatures, there would be a higher chance of photocarriers and vibrating atoms colliding, which might cause a loss of power in the solar cell [43].
3.7 J-V characteristics and QE of CIS-based single junction and DH structure
Figure 9 compares the simulated light-dependent J-V and QE curve of CIS solar cells in absence and presence of the GeSe BSF layer. The n-CdS/p-CIS single heterojunction is shown to have an efficiency of 24.86% with JSC = 42.80 mA/cm2, the VOC = 0.67 V, and the FF = 83.44%, respectively as shown in Fig. 9(a). However, when only a 0.2 µm thick p + -type GeSe is employed as BSF layer in n-CdS/p-CIS solar device, the efficiency enhances to 30.52% with JSC of 44.10 mA/cm2, VOC of 0.86 V, and FF of 80.30%. Due to the addition of the GeSe BSF layer, the PCE of the CIS-based DH PV cell has been increased because of the increment of VOC and JSC of the device. The formation of a high built-in potential at the back interface with absorber causes the VOC to rise and the JSC is increased due to the decrement of velocity of surface recombination at the p-CIS/p + -GeSe heterojunction [2,36]. But, the decrement of FF has happened due to the increases in series resistance in n-CdS/p-CIS/p + -GeSe solar cell.
Fig. 9. The (a) light-dependent output characteristic and (b) QE curves for CuInSe2 thin film solar cell.
Figure 9(b) compares the corresponding QE curves for the proposed structure without and with GeSe BSF layer. The QE for dual-heterojunction is shown to marginally improve due to the incorporation of the BSF layer, indicating that the GeSe layer lowers surface carrier recombination at CIS [43].
As a result, due to the incorporation of the GeSe BSF layer, the overall performance of the designed CIS-based thin film solar cell significantly improves.
4. Conclusion
The SCAPS-1D simulator has been utilized to simulate the designed n-CdS/p-CIS/p + -GeSe dual-heterojunction solar cell. The thicknesses, doping concentration, and bulk defects of the absorber, window and BSF layers have been adjusted to achieve the best cell performance. The optimized PCE of the n-CdS/p-CIS single-heterojunction device has been found to be 24.86% with JSC of 42.80 mA/cm2, and VOC of 0.70 V. The JSC and VOC increase by 1.3 mA/cm2 and 0.16 V, respectively owing to the insertion of GeSe back surface layer in the structure, resulting in an enhanced PCE of 30.52%. The decrease in surface recombination velocity and production of high built-in potential in the p-CIS/p + -GeSe interface is the cause of the increase in JSC and VOC. Hence, this study indicates the potentiality of GeSe as a BSF layer in the CIS solar cells and open the way for experimental work on CIS thin film solar cells with GeSe BSF layer.
Acknowledgments
Dr. Marc Burgelman, of the University of Gent in Belgium, is much appreciated by the authors for donating the SCAPS 1D simulator.
Disclosures
The authors declare no competing financial interest.
Data availability
Simulation details and associated data are available free of charge from authors upon reasonable request.
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