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Abstract
Transition metal di-chalcogenides (TMDCs) have strong potential for space photovoltaic applications since they are resilient to radiation, and hybrid alloys exhibit tunable electro-optic properties. The electronic properties of tungsten-based TMDC alloys containing sulfur, selenium and tellurium were calculated using density functional theory. Hybrid alloys have tunable direct bandgaps dependent on the chalcogen composition. A photovoltaic model consisting of pure and hybrid TMDCs was demonstrated to give an efficiency above 23% under the AM0 space solar spectrum. The non-ionizing energy loss due to high-energy radiation was investigated; it was shown that TMDCs have significantly enhanced radiation resilience than commonly used semiconductors.
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1. Introduction
Transition Metal Di-Chalcogenides (TMDCs) have gathered increasing interest for nano-electronic and photonic applications because of their promising electronic and optical properties, 2D layered structure, and tunability of properties by dopants and hybrid alloys [1]. TMDCs have the unique advantage of flexibility and tunability of their structure and composition, which can allow for controlled variations of their physical and chemical properties [2,3]. The band gap of TMDCs plays a key role in their electronic properties and device-level applications, such as field effect transistors, photodiodes, phototransistors and solar cells [4]. Tunability of the bandgap enables controlled modification of the optical absorption and electronic features, which can enhance optoelectronic device performance [5]. Bandgap modulation of TMDCs by the formation of hybrid alloys has been studied both theoretically and experimentally [6,7], with good thermodynamic stability at room temperatures [8]. Large-scale synthesis of high quality and uniform alloys of TMDCs is possible by chemical vapor deposition (CVD) to achieve bandgap tunability over a large energy range [9].
It is possible to obtain alloys of TMDCs by altering their composition to contain more than one kind of chalcogen atoms [10,11], leading to the formation of hybrid TMDCs with tunable electronic and optical properties. We have investigated tungsten-based TMDC alloys containing various combinations of sulfur, selenium and tellurium atoms, whose electronic properties are intermediate of the pure compounds tungsten disulfide (WS2), tungsten diselenide (WSe2) and tungsten ditelluride (WTe2), with tunable direct bandgaps dependent on the chalcogen atom concentrations. The electronic properties of the pure and hybrid tungsten-based TMDCs were studied using Density Functional Theory (DFT) calculations, from which the band structures and corresponding bandgaps were obtained.
Space photovoltaics is an important alternative to conventional energy sources for providing electric power in most space missions. Solar cells provide cheap, convenient and reliable power supply for extended satellite operations. Space photovoltaic systems have some key requirements such as high conversion efficiency, ultra-thin device structure, and resistance to radiation-induced degradation; the presence of large concentrations of high energy particles in space can lead to significant displacement damage in the photovoltaic materials [12].
Two-dimensional (2D) TMDCs have strong potential for future implementation in space photovoltaic technology, in addition to space-based electronic and optical devices, since they have high absorption of solar radiation in ultra-thin material structures, together with good absorptivity for high-energy photons and flexible properties [13]. TMDCs also have a high degree of resistance to radiation-induced damage in space environment; nanoelectronic devices based on TMDCs have been demonstrated to exhibit negligible performance degradation on exposure to high energy radiation [14].
Single-junction solar cells for space applications based on silicon and GaAs (gallium arsenide) have been demonstrated to exhibit efficiencies of 15% and 19% under AM0 spectrum, respectively, with higher values under different spectra [15–18]. Space solar cells composed of III-V multi-junction solar cells have been demonstrated to exhibit efficiencies around 30% [19,20], but such high efficiencies are beyond the scope of single-junction solar cells. The goal of our project is to develop a single-junction photovoltaic model composed of ultra-thin materials with tunable structure and electro-optic properties, with performance similar to current single-junction technology and added advantage of radiation-resistance.
In photovoltaic systems, light trapping arrangement is used for enhancing the optical absorption within a solar cell, and also decreasing the active layer thicknesses, which results in lightweight, high-performance devices with efficient carrier collection [21]. With an appropriate light trapping structure, an ultra-thin solar cell can retain the high absorption levels of a thicker device and also have a higher open-circuit voltage [22,23]. In solar cells, the loss of incident radiation by surface reflection can be reduced by a great extent by incorporating a suitable dielectric layer to function as an anti-reflection coating [24].
Here, we have investigated an ultra-thin solar cell model based on pure and hybrid tungsten-based TMDCs for space photovoltaic applications. The device performance parameters such as short-circuit current density (JSC), open-circuit voltage (VOC) and efficiency were obtained using the semi-classical drift-diffusion model [25,26]. Our initial photovoltaic model with a thickness of 1 µm gave an efficiency above 20%, which is comparable to current space photovoltaic technology. However, it is beneficial to have the total active layer thickness in our solar cell to be around 100-200 nm for minimizing radiation damage [27,28]. Solar cell materials with thickness around microns are prone to significant damage from high energy particles, such as protons, in the space environment [29]; while ultra-thin materials are much more resistant to radiation-induced displacement damage with less performance degradation [21]. Our modified solar cell model with total active layer thickness of 200 nm, together with light trapping and anti-reflection coating, gave an efficiency above 23%, which is very promising for the development of TMDC-based space photovoltaic modules.
There is presence of protons, electrons and heavy ions in the space environment which have the potential to contribute to radiation-induced damage in solar cells. The flux levels of heavy ions in the galactic cosmic rays (GCR) are very low compared to protons; heavy ions constitute only about 1% of GCR [30]. In addition, it has been demonstrated that the degradation of solar cell performance is primarily due to high energy protons; protons have been shown to have a much more pronounced effect than electrons [31]. The effect of electrons on power degradation of solar cells is typically 2-4% the magnitude of the effect that protons exert on it. Thus, the dominant particles resulting in solar cell damage in space are high energy protons.
We have analyzed the resistivity of tungsten-based TMDCs in presence of high-energy protons (0.1-400 MeV) in the outer space environment [32,33]. The Non-Ionizing Energy Loss (NIEL) in the TMDC materials was obtained and compared to common semiconductor materials, such as silicon and GaAs. NIEL is a physical quantity describing the amount of non-ionizing energy deposited by an incident particle passing through a material and resulting in displacement damage processes [34,35]. Displacement damage refers to the structural damage imparted on the crystal lattice of the material by high-energy particles. It involves the creation of crystal imperfections such as lattice atoms displaced to new defect locations and vacant lattice sites [36]. This effect is detrimental to solar cells because the electrical properties and structural parameters at the defect’s region get permanently altered by the introduction of new energy states (defect states) inside the semiconductor’s energy band gap region [34].
Displacement damage leads to a significant drop in the minority carrier lifetimes which causes reduction of the output power from solar cells [28]. The performance of a semiconductor device gets degraded with increasing amount of absorbed NIEL dose if the dominant damaging mechanism is caused by atomic displacements. It has been shown in previous work that the solar power degradation is a direct function of the NIEL dose deposited on the device [35]. For estimating the NIEL dose in our solar cell model, we have taken into account the proton radiation in the geostationary orbit as reference, which is around 36,000 km above the surface of the Earth [32].
2. Theory and methods
2.1 Structure of TMDCs
TMDCs have a trigonal prismatic unit cell structure in which the transition metal and chalcogen atoms are arranged in a hexagonal pattern (top-view) as shown in Fig. 1(A). A single layer TMDC consists of a plane of transition metal atoms sandwiched between two planes of chalcogen atoms [37,38], as shown in Fig. 1(B). There are strong covalent bonds between the atoms within a layer, while adjacent layers are held together by weak can der Waals’ forces. The lattice parameters of WS2, WSe2 and WTe2 are 3.16 Å, 3.29 Å and 3.6 Å, respectively [39]. Alloys of TMDC were investigated by altering the composition of a single layer to contain more than one chalcogen atom type, as shown in Fig. 1(C), leading to hybrid materials with tunable electronic and optical properties.
Fig. 1. Structure of TMDCs [40]: (A) Top view (hexagonal pattern); (B) Pure monolayer of TMDCs;
(C) Hybrid alloys of TMDCs.
2.2 Band structure calculation of pure and hybrid TMDCs
Density Functional Theory (DFT) calculations were performed to obtain the preliminary band structures of WS2, WSe2 and WTe2 using the tool Quantum ESPRESSO [41,42]. DFT is a powerful quantum mechanical method for obtaining the band structure of a material using many-body perturbation theory [43]. Quantum ESPRESSO is a quantum mechanical modeling tool used for simulating many-body systems. DFT calculations simulate individual atoms using pseudopotential files. GGA pseudopotentials (which add gradient correction to the more basic LDA ones), as described by the Perdew-Burke-Ernzerhof (PBE) scheme [44], were used to simulate the atoms.
The DFT band structure gives accurate information about the energy band features, but the bandgap is significantly underestimated [37]. This problem was solved by incorporating the GW approximation together with the DFT results; it involves the expansion of the self-energy in terms of the single particle Green's function G and the screened Coulomb interaction W to model many body systems [45,46]. The GW calculations were carried out using the GWL package in Quantum ESPRESSO [47]. The energy correction value obtained from the GW approximation was added to the DFT bands to obtain the accurate bandgap of bulk TMDCs; however, for a TMDC monolayer, it is necessary to include the effect of the exciton binding energy [48]. The bandgaps of monolayer TMDCs obtained by DFT + GW calculations are higher than experimental results because of large excitonic effect in a two-dimensional system. When the energy correction due to the excitonic effect is subtracted from the DFT + GW bands, we get very accurate bandgaps that match closely with experimental results [1,4]. All the calculations were carried out assuming a temperature of 300 K. The calculated electronic band structure and bandgaps have been validated against experimentally established results. This methodology has been implemented and discussed in detail in our previous published work [42].
TMDC alloys were modeled by introducing more than one chalcogen atom type in a supercell structure [49]. Three families of hybrid TMDCs were investigated – tungsten with sulfur and selenium, tungsten with sulfur and tellurium, and tungsten with selenium and tellurium. A cell relaxation calculation was carried out for each of alloy to obtain the lowest energy and most stable unit cell structure [50]. The unit cell structure parameters of pure TMDCs are known from existing research, however, the lattice parameters of hybrid TMDCs vary with the material composition; so, it is essential to perform a unit cell relaxation calculation for each hybrid TMDC to obtain the accurate lowest energy unit cell parameters. The band structure and corresponding bandgap of each TMDC alloy were obtained by DFT calculations together with GW corrections as explained earlier.
2.3 Photovoltaic applications of hybrid TMDCs
A solar cell model is based on a p-n junction device design for generating electron-hole pairs from incoming photons in the incident solar illumination [51]. A 3-layer device architecture was investigated for our photovoltaic model, with a thin n-type highly-doped top layer, a relatively thick intrinsic or light-doped middle layer, and a thin p-type highly-doped bottom layer, similar to a silicon HIT (hetero-junction with intrinsic thin layer) solar cell structure [52]. The electronic and optical properties of TMDCs used in the device simulations include the electronic band structure, bandgap, absorption coefficient, conduction band effective density of states NC, valence band effective density of states NV, carrier mobilities and carrier lifetimes. The electronic band structure and bandgap of pure and hybrid TMDCs were obtained using DFT calculations, as explained previously. The electronic and optical parameters for device simulations were taken from our previous published work [42].
Our initial photovoltaic model is defined as Model A, as shown in Fig. 2(A). It consists of three-layers in an n + -p-p + junction stack. The top layer is a thin layer of highly-doped n-type monolayer WSeS stack; the middle layer is a thick layer of lightly-doped (almost intrinsic) p-type bulk WS2; and the bottom layer is a thin layer of highly-doped p-type amorphous silicon (a-Si). The optimal Model A device structure is summarized in Table 1. This device is similar in architecture to our previously developed models [53]; however, instead of using pure TMDC monolayers, we have used hybrid TMDC monolayers as the top thin layer in our device in this work. The primary advantage of using hybrid TMDC alloys with composition-dependent tunable bandgaps is that it allows us to use materials with any desired bandgap as effective photovoltaic absorber layers. This has the potential for development of flexible photovoltaic devices with higher efficiencies.
Fig. 2. Photovoltaic device models. (Note: thicknesses not to scale): (A) Initial solar cell (Model A); (B) Solar cell with light trapping and anti-reflection coating (Model B).
Table 1. Summary of device structure for Model A.
Our device simulations were performed using the AM0 space solar spectrum as the incident solar illumination to predict the performance and features of our TMDC-based solar cell model in potential space photovoltaic applications [54]. The temperature used for the device model simulations was 40°, which falls within the average temperature range of most satellite orbits [12]. The device performance and current-voltage characteristics of our solar cell were calculated using the Drift-Diffusion model in the tool ADEPT (A Device Emulation Program and Tool) on nanoHUB [55].
In order to minimize radiation-induced damage, it is beneficial to limit the total active layer thickness to around 100-200 nm. Taking into consideration this requirement, we improved Model A to have a much a reduced thickness of 200 nm together with additional optical features. This modified device was defined as Model B, as shown in Fig. 2(B) and summarized in Table 2. It includes a light trapping structure and an anti-reflection coating layer to increase absorption of the incident solar illumination in an ultra-thin device structure.
Table 2. Summary of device structure for Model B.
The lowering of absorption in reducing the device thickness is compensated by these additional optical features in Model B which preserve the high JSC of Model A. The optical absorption enhancement was investigated using S4sim (Stanford Stratified Structure Solver), available on nanoHUB [56–58]. S4 is a frequency domain code to solve the linear Maxwell’s equations in layered periodic structures using Rigorous Coupled Wave Analysis (RCWA, also called the Fourier Modal Method (FMM)) and the S-matrix algorithm [59]. This tool accurately predicts optical propagation (transmission, reflection and absorption) in structures composed of periodic, patterned, or planar layers. This methodology, together with development of the optimized light trapping structure and anti-reflection coating layer, has been discussed in detail in our previous published work [53].
2.4 Estimation of non-ionizing energy loss (NIEL) in TMDCs
The geostationary satellite orbit was chosen in our work to estimate the NIEL damage dose in our solar cell model. The proton fluence over a period of one year in this orbit was obtained from SPENVIS [33] to get an understanding of the incident high-energy protons in this orbit. The proton fluence profile, which is the incident high-energy radiation in our solar cell, is shown in Fig. 3. Geostationary orbit is known to have relatively high radiation levels compared to low earth orbit, but is generally below the levels seen in certain medium earth orbits.
The NIEL for WS2 and WSe2 were calculated using the tool SR-NIEL [60] to obtain the fundamental radiation resistant properties of TMDCs; these profiles were compared to those of silicon and GaAs to understand potential improvements over commonly used semiconductors. There are several essential parameters needed for performing the NIEL calculations in SR-NIEL. It is important to accurately define the materials in terms of structural composition, molecular density, atomic masses and displacement threshold energy of the constituent elements. It is also essential to specify the incident particles on our material and the energy range of the calculation. For our work, we have defined the incident particles to be ionized hydrogen (or protons). The energy range of 0.1-400 MeV was used to calculate the NIEL profiles. The most important material parameter on which the NIEL profile depends is the atomic mass of the constituent elements. A material with a higher atomic mass will have a correspondingly higher displacement threshold energy, which would result in a lower NIEL profile. Silicon and sulfur have similar displacement threshold energies around 20 eV, since they have very similar atomic masses. Tungsten has a much higher displacement threshold energy of around 90 eV owing to its significantly higher atomic mass [61]. This is the key factor which leads to significantly lower NIEL profiles in tungsten-based TMDCs.
The NIEL spectral dose of the materials was obtained over the energy range 0.1-400 MeV as the product of the NIEL profile and proton fluence for the corresponding energies over a period of one year. This NIEL dose curve was integrated over the energy range to calculate the total NIEL dose which is absorbed by the material in one year. This analysis was carried out for WS2 and WSe2, and compared to silicon and GaAs.
3. Results and discussion
3.1 Bandgap of hybrid TMDCs
We have studied the tunable bandgaps of hybrid alloys composed of tungsten with sulfur and selenium in our previous published work [49]. Those findings are presented here in Fig. 4 for reference. Here, we have studied tungsten-based alloys composed of varying concentrations of sulfur and tellurium, and selenium and tellurium. This allows us to investigate and better understand the properties of hybrid TMDC alloys for different families of materials. The pure TMDCs studied in this work (WS2, WSe2 and WTe2) and the hybrid alloys (S-Se, S-Te and Se-Te) have direct bandgaps in monolayer configuration.
The bandgaps of pure WS2 and WSe2 are 2.15 eV and 1.67 eV, respectively. For an alloy containing tungsten with both S and Se atoms, the bandgap energy is an intermediate value dependent on the corresponding chalcogen composition. The bandgaps of the alloys decrease with inverse proportionality to increasing Se atom concentration from pure WS2 (2.15 eV) to pure WSe2 (1.67 eV). Introducing a small percentage of Se atoms to WS2 leads to a significant change in the electronic properties and decrease of bandgap of the material. Similarly, introducing a small percentage of S atoms to WSe2 leads to significant change of electronic properties and increase of bandgap. This results in a significant rise or drop in the bandgap when the dopant chalcogen atoms are initially added to the pure materials. For the hybrid alloys, there is almost linear variation of the bandgap with change in chalcogen composition. This bandgap tunability feature of hybrid TMDCs has been demonstrated experimentally in molybdenum-based S-Se alloys [10].
The bandgap energies and trend of the tungsten-based S-Te alloys are shown in Fig. 5(A). The bandgaps of pure WS2 and WTe2 are 2.15 eV and 1.25 eV, respectively. For the alloys containing various concentrations of S and Te atoms with tungsten, the bandgap energy is an intermediate value between 1.25-2.15 eV depending on the chalcogen atom composition. The bandgaps of the S-Te alloys also show inverse proportionality with increasing Te concentration from WS2 (2.15 eV) to WTe2 (1.25 eV), similar to the S-Se alloys as observed above in Fig. 4.
Fig. 5. Bandgap variation in tungsten- and tellurium-based TMDC alloys: (A) S-Te alloy; (B) Se-Te alloy.
The bandgap energies and trend of the tungsten-based Se-Te alloys are shown in Fig. 5(B). Pure WSe2 and WTe2 have bandgaps of 1.67 eV and 1.25 eV, respectively. For the tungsten-based alloys containing various concentrations of Se and Te atoms, the bandgap energy is an intermediate value between 1.25-1.67 eV depending on the corresponding chalcogen composition. Similar to the previous results with S-Se and S-Te alloys, and patterns in Figs. 4 and 5(A), the bandgaps of the Se-Te alloys show an approximately linear decrease with increasing Te concentration from WSe2 (1.67 eV) to WTe2 (1.25 eV).
3.2 Photovoltaic model performance
The photovoltaic performance parameters and features of Model A, namely, JSC, VOC and efficiency, were obtained from the device simulations on nanoHUB. The I-V plot of Model A is shown in Fig. 6(A). The efficiency is around 20.8%, with VOC 0.84 V and JSC 40.8 mA/cm2. The VOC and JSC obtained are similar to those of many conventional solar cells. The efficiency of Model A is comparable to some of the previously developed high-efficiency single junction solar cells under AM0 spectrum [18].
The device performance of Model B is summarized in Fig. 6(B). The efficiency is around 23.8%, with VOC 0.88 V and JSC 44.8 mA/cm2. We have been able to reduce the thickness of the active layers in our solar cell from 1 µm to 200 nm in Model B, but still maintained the high performance parameters due to the additional optical features. The light trapping structure in Model B compensated for the reduced layer thicknesses by increasing the total absorption of the incident solar illumination. As an additional enhancement, the anti-reflection coating layer reduces surface reflection from the photovoltaic materials, which leads to further improvement of light absorption and correspondingly higher device efficiency in the solar cell. Due to the significant reduction in the thickness of the active layers, Model B has the additional advantage of enhanced resistivity to high energy protons in the space environment than Model A.
3.3 NIEL dose of TMDCs
The NIEL profiles for WS2 and WSe2 are shown in Fig. 7. In both cases, the values have been compared with silicon and GaAs to get a firm understanding of the advantages of using TMDCs over most commonly used semiconductors. It can be seen that the NIEL is much lower for tungsten-based TMDCs than silicon and GaAs. This is due to the higher atomic mass of the TMDC materials which makes them much more resistant and resilient to high-energy particles in outer space. It is expected that the NIEL profiles for other pure and hybrid tungsten-based TMDCs would be similar to those of WS2 and WSe2, and much lower than silicon and GaAs, since they have very high atomic masses.
Fig. 7. NIEL profiles for tungsten-based TMDCs: (A) Tungsten disulfide (WS2); (B) Tungsten diselenide (WSe2).
Using the product of NIEL profile and proton fluence, the spectral NIEL dose of WS2 and WSe2 were obtained over the energy range 0.1-400 MeV, as shown in Fig. 8. The total NIEL dose was calculated for WS2 and WSe2 as integral of the spectral NIEL dose over energy, and compared with silicon and GaAs as reference. The results are summarized in Table 3. It can be seen that the TMDC materials have a much lower total NIEL dose per unit material as compared to silicon and GaAs over the energy range of high-energy protons in outer space for a period of one year. The total NIEL dose for other pure and hybrid tungsten-based TMDCs is similar to those of WS2 and WSe2, since they have comparatively high atomic masses. For additional reference, we have summarized the NIEL and the spectral NIEL dose of WS2, WSe2, GaAs and silicon in Table 4 for an incident fluence of 10 MeV protons over a one-year period.
Fig. 8. Spectral NIEL dose for tungsten-based TMDCs; (A) Tungsten disulfide (WS2); (B) Tungsten diselenide (WSe2).
Table 3. Total NIEL dose for tungsten-based TMDCs and silicon for one year.
Table 4. Summary of NIEL and Spectral NIEL Dose of WS2, WSe2, Si and GaAs for 10 MeV protons
For a better comparison, we have calculated the total displacement energy dose which would affect solar cell architectures composed of TMDCs and compared it with silicon solar cells. The density of silicon is 2.33 g/cm3, while the density of WS2 is 7.5 g/cm3. If we compare a 1 µm thick WS2-based solar cell (Model A) and a 10 µm thick silicon solar cell as reference, we can calculate that the quantity of active materials by mass in the silicon solar cell is around 3 times that of the WS2-based solar cell. Accounting for the material quantity and the NIEL dose, it can be concluded that a silicon solar cell would be exposed to about 6 times more displacement energy dose than our WS2-based solar cell over a period of one year. Furthermore, if we compare a 200 nm thick WS2-based solar cell (Model B) and a 10 µm thick silicon solar cell, we can calculate that the silicon solar cell would be exposed to about 30 times more displacement energy dose than our ultra-thin WS2-based solar cell. In addition, many silicon solar cells have thicknesses much greater than 10 µm. In such cases, the reduction of the displacement energy dose exposure in the solar cell would be even greater by using an ultra-thin TMDC-based solar cell.
Thus, we can significantly reduce the potential for displacement damage in solar cells in the space environment on account of lower displacement energy dose by using ultra-thin photovoltaic device architectures composed of TMDCs. This would allow for the development of longer lasting, more reliable and high resistance solar cells for space photovoltaic applications in satellite orbits. While the remaining displacement damage is likely to generate sulfur or selenium vacancies because of their relatively lower atomic numbers, the exact distribution of each type of vacancy has not yet been calculated, but would be appropriate for a follow-up study. Additionally, it is essential to note that if the displacement damage dose (DDD) is decreased by an order of magnitude or more, other structural and/or operational failure mechanisms may become more important. This indicates that the lifetime would not necessarily be extended by such a large factor; further study, in this regard, would be needed to accurately determine operational reliability.
4. Conclusion
In this work, we have developed a DFT-based model to calculate electronic properties of hybrid TMDC alloys based on tungsten. The electronic band structure and bandgap of hybrid tungsten-based TMDC alloys containing various combinations of S-Se, S-Te and Se-Te atoms were calculated and analyzed for electronic and photonic applications. Our results show that it is possible to engineer a hybrid 2D TMDC material with a specific target range of bandgap values suitable for any particular desired application, such as solar cells, photodetectors, and transistors. If we consider tungsten-based TMDCs containing sulfur, selenium and tellurium, it is possible to obtain a hybrid material which can have any desirable direct bandgap in the range 1.25-2.15 eV, and also exhibit the favorable electronic and optical properties of semiconducting TMDCs. This has the potential to open up new possibilities in fabrication and manufacture of tunable bandgap semiconductors and materials engineered for desired target applications.
We have developed a solar cell model based on tungsten-based TMDCs for proposed implementation in space photovoltaics. Our model consists of a solar cell composed of pure and hybrid tungsten-based TMDCs, together with a light trapping structure consisting of a 1D silver grating, and an appropriate dielectric material layer to function as an anti-reflection coating. The device performance of our final photovoltaic model was 23.8% at 40°C under the AM0 space solar spectrum. Our solar cell design shows that it is possible to obtain efficiencies above 20% with the space solar spectrum in an ultra-thin device structure. Light absorption can be enhanced in ultra-thin layers by incorporating a light trapping structure and an anti-reflection coating layer, resulting in high efficiencies.
The estimates of the NIEL dose show that TMDCs are much more resilient to high-energy protons as compared to silicon and GaAs for satellite-based applications in the space environment. The ultra-thin nature of our solar cell model makes it a strong candidate for potential space photovoltaic applications. The favorable performance of our TMDC-based photovoltaic model shows that this device can be competitive and comparable to the performance of existing single-junction solutions, while adding advantages of radiation resistance and resilience. The simulation results provide an insight towards development of an experimentally feasible solar cell model for space photovoltaic systems. Further study may aim to verify the predictions experimentally, and to look at the interactions with other failure mechanisms once the DDD failure pathway is suppressed.
We have taken measures and precautions in our simulations to propose models which are consistent with experimentally fabricable and measurable devices. We have accounted for non-idealities in our materials and device architecture wherever possible. The goal of this project was to present a radiation-resistant solar cell model which would give the predicted performance and features when tested and characterized experimentally. Thus, we refrained from incorporating near-ideal device parameters and operating conditions.
Funding
U.S. Department of Defense (W52P1J2093009); Office of Naval Research (N00014-19-S-B001); National Science Foundation (CBET-1855882, EEC-1227110, EEC1454315-CAREER); TCG-CREST (40003482).
Acknowledgments
The authors thank Mike Alles for fruitful discussions.
Disclosures
The authors declare no conflicts of interest.
Data availability
The data underlying the results presented in this article are not currently publicly available, but may be obtained from the authors upon reasonable request.
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