Abstract
Photovoltaic (PV) technology could be a promising candidate for clean and green source of energy. The nanowire technology provides extra mileage over planar solar cells in every step from photon absorption to current generation. Indium Gallium Nitride (InxGa1-xN) is a recently revised material with such a bandgap to absorb nearly whole solar spectrum to increase the conversion efficiency copiously. One of the major technological challenge is in-built polarization charges. This chapter highlights the basic advantageous properties of InxGa 1−xN materials, its growth technology and state-of-the-art application towards PV devices. The most important challenges that remain in realizing a high-efficiency InxGa 1−xN PV device are also discussed. III-Nitride nanowires are also explored in detail to overcome the challenges. Finally, conclusions are drawn about the potential and future aspect of InxGa 1−xN material based nanowires towards terrestrial as well as space photovoltaic applications.
Keywords
- III-nitride
- polarization charges
- efficiency
- InGaN/GaN
- nanowires
- stress
- strain
1. Introduction
Photovoltaic (PV) technology is the most emerging way of harnessing huge amount of energy from sun light as compared to solar thermal and photo electrochemical cells [1]. PV devices convert incident photons from sunlight to electricity upon exposed to light. PVs are popular because of its compactness and can be used anywhere for different application [2]. Additionally, involvement of nanostructures further boost the performance of solar cell. Over the past decade, nanostructured solar cell has become hot topics within research community due to its potential to enhance the spectral response of cell. Although, first generation silicon wafer based solar cell leads the current global PV market, however this conventional technology do not have any further scope to improve efficiency and reduce cost [3]. Additionally, it is also not recommended to use Silicon based solar cell for space application due to its low radiation tolerance. Second generation thin film technology such as hydrogenated amorphous silicon (a-Si: H), CIGS, and CdTe could not line-up with wafer-based silicon due to use of rare earth elements and low stability [4, 5]. Furthermore, highly efficient compound semiconductors based third generation solar cell have a demerit of high cost which limits its use in terrestrial applications. Hence, the hunt for low cost high performance solar cell are still unachievable. In the meantime, involvement of nanotechnology could bring a ray of hope for future generation solar cell. Nanowire (NW) geometry has remarkable advantages over planner geometry due to optical, electrical, and mechanical effects. New charge separation mechanisms, low defects and low cost also add more mileage to this journey. Looking towards the current scenario, existing PV technologies aren’t the solid foundation for the future projection of the renewable energy generation. None of the existing technology can satisfy global energy demand in future [2, 5]. Moreover, if the material or technological limitations restrict the future roadmap of PV technology, then the incorporation of new efficient materials and transpose of technology will be an assurance against high cost and low efficiency solar cells. Newly explored InxGa1-xN material brings an bunch of opportunities for future PV technology, having capability to absorb full solar spectrum using a single absorber material. One of the major properties of InGaN material is its tunable bandgap from 0.6 to 3.4 eV by changing ‘In’ content [6, 7, 8, 9, 10]. It also has easy growth of nanowire and nanorod structures with proven technology [11, 12, 13, 14]. It is a direct bandgap material where photon absorption and direct interband transition can be occurred without interference of phonons to conserve momentum. Additionally, high absorption coefficient of 105 is an additional benefit for good absorption with thin layer. Hence the cost can be minimized as well as recombination rate can also be minimized. InGaN also possess a high saturation velocity and a low effective mass of charge carriers, which ensures the more carrier separation along the junction. High radiation tolerance of InGaN are always appreciated for harsh environments. Moreover, InGaN solar cell do not contain any toxic elements such as arsenic, cadmium or phosphorous as used in MJ solar cell. Thus, it is evident that In
2. Planar, nanodisk and nanowire III-nitride solar cells
In the present context mainly different geometry of III-Nitride GaN/InGaN material based solar cell are considered, such as planar, nanodisk and nanowire types. Theoretically, it is anticipated that power conversion efficiency more than 40% could be achievable with GaN/InGaN junctions [19, 20]. However, practically achievable efficiency is quite low [21, 22]. One of the major challenges is the association of in-house defects with InGaN layer at higher ‘In’ content. Which in turn leads to stuck of immobile charges (known as polarization charges) along interfaces and reduces minority lifetime. Recently III-Nitride nanowires (NWs) are proposed as stand-alone PV devices due to enhanced light trapping, defect and stress-free growth [23, 24]. In general, two widely used structure for nanowire solar cell are (i) axial junction and (ii) radial junction. Axial junction is also known as nanodisk (ND) whereas radial junction is known as core-shell-shell (CSS) solar cell. Vapor–liquid–solid (VLS) technique is one of the popular methods to grow the GaN/InGaN nnaowires [23]. The fabricated NWs have hexagonal cross-section with {0001} orientation of top facet and {1–100}/ {10–10} orientation alongside walls of NWs [23]. Literature shows the comparative analysis of planar and circular NWs reported by Y. Zeng
Where
All the material parameters for GaN and InGaN with different compostion can be found from [30]. The interpolation method is used to calculate all parameters for InxGa1-xN composite alloys. Defect density in the order of 1017, 1014 and 1012 are incorporated for planar, ND and CSS-NW SCs respectively [31].
The model developed by Romanov
Where
Vertical illumination of light source on the front surface of the device is consider during all analysis as shown in Figure 1. Transport of carriers and separation mechanisms in the device depends solely on its geometrical structure. Here, carrier transport in planar, ND and NW type solar cell are explored with the help of energy band diagram. Figure 2(a) shows that tilt of energy band along
Thus, with increase in bias voltage, photogenerated carriers accumulate inside the potential valley rather than traveling in
Figure 3 shows dependency of short-circuit current density (
Where volume of the absorption region is,
Thus
Where, active absorption layer is,
Figure 4 shows the current density-voltage-power density (J-V-P) curve of n-GaN/i-In0.1Ga0.9N/p-GaN planar, ND and NW type solar cell. The optimized thickness of 100 nm, 150 nm and 180 nm i-InGaN is considered for all performance analysis. It shows a higher Jsc in CSS-NW as compared to ND and planar structure. It is anticipated that higher current in CSS-NW solar cell is mainly due to higher active absorption region and efficient carrier separation. It is important to highlight that planar and ND type solar cells shows a stair-case type
It is also observed that due to high degree of strain relation, low defects density and more active area of absorption, CSS-NW structure can accommodate higher thickness of active InGaN layer (W) region. Moreover, higher thickness of absorber enhances the probability of more absorption of photons from sunlight. In other hand, active region of planar and ND type solar cells cannot be increased due to are limitation of surface recombination arte, polarization induced electric field, low degree of strain relaxation and defect density. Voc of ND type solar cell is seen to be higher than CSS-NW type solar cell which is may be due to the recombination rate along the junction. However, the rate of increase in Jsc of CSS-NW structure is comparatively higher than ND and planar solar cell. Similarly, planar solar cell possesses a low as compared to ND and NW solar cell. It is mainly due to appearance of staircase type
W (nm) | Jsc (mA/cm2) | Voc (V) | FF (%) | Efficiency η (%) | |
---|---|---|---|---|---|
Planar | 100 | 0.90 | 2.41 | 82.7 | 1.8 |
Nanodisk (ND) | 150 | 1.93 | 2.52 | 87.6 | 4.28 |
CSS-Nanowire (NW) | 180 | 2.84 | 2.47 | 91.6 | 6.46 |
3. Conclusion
In this chapter, the importance of nanowire solar cell with III-Nitride material is explored in a detailed manner. A comparative analysis is carried out with planar, nanodisk and nanowire type solar cell and concluded that nanowire type structure shows a better performance as compared to others. Additionally, it is found that nanowires in InGaN materials are grown either in triangular or hexagonal orientation. The strain relaxation is more in CSS-nanowires which in turns leads to low in-house defect density along the interfaces. CSS-NWs are also able to accommodate higher thickness of intrinsic material due to high carrier diffusion length. Radial separation of carriers also provides more surface area and better control on carrier separation mechanisms. Hence, it is concluded that radial growth of nanowire provides a broad range of opportunity for performance enhancement of solar cell. The similar type of observation are also applicable to LASER and light emitting diodes, where III-Nitride materials are used.
Acknowledgments
The authors acknowledge Dept. of Science and Technology, Govt. of India for financial support for providing the instrumental facility under DST-FIST in Dept. of ECE, SRM Institute of Science and Technology, Chennai, India.
The authors also acknowledge Visvesvaraya PhD Fellowship of MeitY, Govt. of India and TEQIP-II for procurement of Silvaco TCAD tool in Department of ECE, NIT Silchar for carrying out the research work.
Conflict of interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this chapter.
Appendices and nomenclature
indium gallium nitride gallium nitride copper indium gallium selenide cadmium telluride
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