Solar Energy Conversion Efficiency, Growth Mechanism and Design of III–V Nanowire-Based Solar Cells: Review

Abstract

Nanowires (NWs) are 1D nanostructures with unique and wonderful optical and electrical properties. Due to their highly anisotropic shape and enormous index of refraction, they behave as optical antennae with improved absorption and emission properties, and thus better photovoltaic cell efficiency compared to a planar material with equivalent volume. Implying important advantages of reduced material usage and cost as well as due to its direct bandgap and its flexibility for designing solar cells, we choose to review III–V NWs. Their bandgap can easily be tunable for growing on the cheapest Si substrate. The recent developments in NW-based photovoltaics with attractive III–V NWs with different growth mechanisms, device fabrication, and performance results are studied. Recently, III–V NW solar cells have achieved an interesting efficiency above 10%. GaAsP NW has achieved 10.2%; InP NW has achieved 13.8%; GaAs NW has achieved 15.3%; and moreover the highest 17.8% efficiency is achieved by InP NW. While the III–V NW solar cells are much more vital and promising, their current efficiencies are still much lower than the theoretically predicted maximum efficiency of 48%. In this review, the chapter focused on the synthesis processes of III–V nanowires, vapor-liquid-solid growing mechanisms, solar light harvesting of III–V nanowire solar cells, and designing high-efficiency and low-cost III–V nanowire solar cells.

1. Introduction

Energy can be added to the basic need of humans to live on the earth’s planet. We need energy in our daily life and economic development, but there is an insufficient energy demand in our world especially for developing countries [1]. The demand for energy is increasing exponentially due to the global population growth and economic development. As the United Nations Department of Economic and Social Affairs (UNDESA) has reported, population size is predicted to extend by two billion within the next 30 years. The expansion rate of the world population indicates that the present world population could jump from currently 7.7 billion to 8.5 by 2030, 9.7 billion by 2050, and 10.9 billion by 2100 [2]. For this population expansion, enormous energy will be required. However, fulfilling this energy demand is a key challenge and a huge obstacle for dreaming of continuous green earth [3]. Currently, fossil fuel-based energy is dominating worldwide, which is meant since it is not replaceable it is running out very fast. In addition to this, to control the amount of CO₂ within the air, it is necessary to reduce the energy demand from fossil fuels and increase the supply of the energy from renewable energy sources [4]. As an alternative to fuel energy, and to minimize CO₂ emission, solar cells, among all the renewable energy resources, can provide an efficient and environmentally friendly solution, for a sustainable green earth, which converts sunlight directly into electricity [5, 6]. The amount of energy humans use annually is about 4.6 × 10²⁰ joules, and this amount of energy is delivered to Earth by the Sun in 1 hour [7]. The largest power that the sun unceasingly delivers to earth is 1.2 × 10⁵ terawatts, which is bigger than each different energy supply, either renewable or nonrenewable [8]. It dramatically exceeds the speed at which human civilization produces and uses energy currently about 13 TW [2, 8, 9]. Depending on the estimation of the population growth rate; the global energy demand is predicted to exceed 30 terawatts by 2050, about double the current energy [2].

Solar cells have been widely utilized in different replaceable energy generation projects including roof-top installations, solar farms, spacecraft, and portable solar battery banks [10]. More importantly, solar cells have been also utilized in building-integrated photovoltaic systems for harvesting solar power, toward the goal of self-sustainable modern infrastructures, such as glass-greenhouses, bus stops, and smart building components, that is, energy generating and saving PV glass [11]. Although the resource potential of photovoltaic (PV) is gigantic, it currently constitutes a little fraction of the worldwide energy supply. One among the factors limiting the widespread adoption of PV is its low-energy density, low efficiency, and comparatively high-cost as compared to other energy technologies [12, 13]. In order to widely apply PV, scientists and researchers around the world are still conducting research on this area, including the event of varied sorts of solar cells that specialize in improving the conversion efficiency also [14]. One among the foremost relevant metrics for PV devices is that the power conversion efficiency (PCE), that is, the efficiency with which sunlight is often converted to electric power. There are several factors, from structural defects to resistance to shading effects, which affect the conversion efficiency, also as the overall performance of solar cells.

A significant effort in photovoltaic research today is objectively to enhance PCE, while simultaneously reducing cost [15]. The overwhelming majority of today’s PV market consists of three types of generation [16]: the first-generation PV is silicon-based solar cell modules, which currently dominate the solar power market due to their low-cost and long-term reliability, but only convert about 8–19% of the available solar power [17]. Second-generation PVs are thin-film solar cells that aim to decrease cost by utilizing less material and depositing on inexpensive substrates, such as metal foil, glass, and plastic. This type of PVs includes cadmium telluride (CdTe), amorphous Si, and copper indium gallium diselenide (CIGS), all of lower material quality and PCE compared to first-generation cells [18]. In order to overcome these shortage, third-generation PVs [19] are recently being pursued that aim to strike the Shockley-Queisser efficiency limit of ~30% (1 Sun) for one p-n junction [20], while keeping or reducing cost.

The III–V multi-junction planar solar cells are included under third-generation PV and have attracted several interests in the recent candidate of the solar cells that have terribly high efficiencies larger than 40% grown on Ge substrates [21]. Nevertheless, planar III–V materials and Ge substrates needed for these devices are too rare and expensive for widespread use [22, 23]. Since the main barriers to the large-scale uses of solar energy are due to the difficulties in balancing the cost and efficiency of existing devices, innovations are needed to reap solar power with greater efficiency and economic viability. The right resolution is to form the high-efficiency III–V solar cells onto the cheap mature Si platform and develop III–V/Si two-junction cells [22, 24]. It has been foreseen that III–V/Si are able to achieve an efficiency of above 40%, nevertheless, the lattice and thermal expansion coefficient mismatches between III–V layers and Si substrates are still preventing the effective implementation of this idea [22].

By reducing the size of materials from bulk to nanoscale and developing the cheap growth method can solve the problem in III–V multi-junction thin-film solar cells. Recently solar cells in one dimension and zero dimensions geometry materials have got attention. Different materials in nanowire geometries, such as Si, III–V compounds (e.g., GaAs, InP, and III-nitride-based), II-VI compounds (e.g., CdS/CuS₂ and CdS/CdTe), and most recently perovskites have been studied for solar energy harvesting [25]. NW-based solar cells are forest or single of one dimensional (1D) rods, wires, or pillars having lengths typically on the order of microns and diameters on the order of tens to several nanometers. They have unique and wonderful optical and electrical properties and they also offer flexibility to create heterojunctions in both axial and radial directions. Due to their highly anisotropic shape and enormous index of refraction, they behave as optical antennae with improved absorption and emission properties, and thus better photovoltaic cell efficiency compared to a planar material with equivalent volume [26]. The theoretical efficiency of an NW array solar cell can reach approximately 32.5% for bandgap at approximately 1.34 eV under AM 1.5 solar spectrum, exceeding that of a planar bulk solar cell (31%) with the same bandgap, implying an important advantage of reduced material usage and cost [27, 28]. The theoretical power conversion efficiency of 48% is also reported using Al_0.54Ga_0.46As, GaAs, and In_0.37Ga_0.63. As NWs arrays grown on a silicon substrate [29].

The NW arrays could also provide substantial reductions in material consumption as well as production costs for III–V-based solar cells, in part because they can be grown on low-cost substrates, such as silicon [30]. Among III–V-based solar cells, GaAs and InP are of specific interest for photovoltaic cell applications due to their direct bandgaps, which are close to the ideal value for maximizing PCE under AM 1.5G spectrum [31]. For the first, the best-reported efficiency above 10% for III–V NW is an InP nanowire cell with 13.8% efficiency [30]. The highest power conversion efficiency of the III–V NWs record is caught by InP NWs, which is 17.8% [32]. This value is approached to a planar solar cell that has been reported for silicon radial junction with vertically aligned tapered microwires achieving power conversion efficiency of 18.9% [33]. The principal goal of third-generation PVs is not only the continual increase of power conversion efficiencies but also the reduction of solar cell development costs; novel hybrid materials can provide a practical solution [27]. The array nanowire solar cells are much more important and promising. However, their current efficiencies are much lower than their theoretical prediction. NW synthesis, characterization, and device fabrications are the challenges to achieve the theoretical efficiency predicted theoretically [34].

In this review, we have focused on the synthesis process of III–V nanowires, solar energy harvesting, photon-generated carriers, different design of nanowire solar cells, and ultimately the mostly achieved power conversion efficiency for some of III–V NWs. III–V NW solar cells have gained attention, especially since 2009 and many papers have been published. Depending on these published papers, we have discussed the papers published since 2010 for each aforementioned focused area in this review.

2. Nanowires synthesis

Nanowire can be synthesized through three approaches: i) top-down approaches, ii) bottom-up approaches, and iii) the combination of top-down and bottom-up approaches.

2.1 Top-down approach

The top-down approach begins with a bulk material (microscopic materials), which will be by selection removed to create NWs through lithography patterning and wet/dry etching method [35]. From epitaxially grown-up thin films, they provide the advantage of fabricating NWs with exactly controlled doping profile and layer thickness [23]. If this NW structure has a p-n junction, it will be incorporated as an axial p-n junction after the NWs are formed. To create a radial p-n junction, ion implantation and molecular monolayer doping (MLD) can be used [31, 36, 37]. In the fabrication of nanowires, numerous lithographic styles are used with controllable exposure, size, and distance for dependable light-trapping and latterly high-effectiveness solar cells [38]. The traditional optical lithography can offer a high result, but its essential dimension is confined by the optical phenomenon restriction of the sun wavelength [38, 39]. On the other hand, traditional electron-beam lithography (EBL) has a veritably high resolution but suffers from high-cost and low throughput [40, 41]. Nanoimprint lithography (NIL) [42, 43] can be used in order to obtain both high throughput and resolution, and self-powered parallel electron lithography may be used [44, 45]. The NIL technology avoids light diffraction in optical lithography and can fabricate the nanowires with fabricating accuracy up to several nanometers [46]. The process includes lithography patterning, and then dry etching to obtain nanowires with vertical and smooth sidewalls [36, 47]. Subsequently, wet etching processes are also conducted to first etch the remaining etching masks followed by the removal of physically damaged and nonstoichiometric oxidized surface layers [48]. The top-down have disadvantages when compared with bottom-up approaches. It does not offer any material saving and also lack freedom in material design. Furthermore, the etching process could introduce surface defects that adversely affect the nanowire’s optical and electrical properties, and thus lead to much-degraded device performance [49].

2.2 Bottom-up approach

Bottom-up approach NW synthesis is supported by gas-phase epitaxial growth technique to supply detached NW ensembles with or without order. There are many techniques employed under the bottom-up approach for nanowire growth, such as chemical vapor deposition (CVD) [46, 50, 51], chemical-beam epitaxy (CBE) [52, 53], laser ablation [54], and hybrid vapor-phase epitaxy [55, 56]. Nevertheless, III–V semiconductor nanowires are mainly grown by either metal–organic vapor-phase epitaxy (MOVPE) [57, 58, 59, 60] or molecular-beam epitaxy (MBE) technique [61, 62] with and without catalysis assistance. Catalyzed growth involves the use of metal nanoparticles, such as Au, Al, and other metals [34].

The catalysts that are used as an assist in the growth of NWs can be external or from the elements of materials used to grow NWs, which are called seed particles. In general, we can classify the NWs growth mechanisms into four as shown in Figure 1: (i) homoparticle growth, (ii) heteroparticle growth, (iii) non-catalyst growth, and (iv) oxide-assisted growth. The seed particles can be homoparticle growth (Figure 1c); in this case, a seed particle is formed consisting of one or all elements used for wire growth or it can be simply a self-assisted growth. As the seed particle size varies during growth both length and diameter increase. The seed particles can also be heteroparticle growth (Figure 1d) and in this case, a seed particle (typically Au) is deposited prior to growth, in simple words, it is a foreign metal-assisted growth technique. Throughout heating to increase temperature the seed particle alloys with the substrate and/or material forms the gas phase. In this case, particle size during growth is constant. Noncatalyzed growth includes selective area epitaxy (SAE) where growth occurs on a prepatterned substrate [64, 65, 66]. In selective area epitaxy, an epitaxial layer nucleates in openings of a mask layer and continuously grows in height; its lateral growth is restricted by low-energy facets (Figure 1a). [63, 67]. Oxide-assisted growth (OAG) is additionally a mechanism used in crystal growth with the aid of the semiconductor substance’s oxides as a passivating shell to suppress the subsequent growth [68]. During OAG growth, the semiconductor and its oxide are adsorbed on the substrate, where the semiconductor produces nucleation centers, which also create the semiconductor nanowires, while the oxide forms a passivating shell (Figure 1b).

Figure 1.

Schematic representation of four basic nanowire growth mechanisms: (a) selective area epitaxy, (b) oxide-assisted growth, (c) homoparticle growth, and (d) heteroparticle growth [63].

For the non-catalyzed growth method, Maoqing Yao et al. [69] fabricated arrays of GaAs nanowires solar cell with an axial p-i-n junction, which are grown by selective area growth (SAG) method, using mass production compatible metal-organic chemical vapor deposition (MOCVD) technique. This growth method is free of the metal catalyst. The fabrication process of their axial junction GaAs nanowire solar cell is shown in Figure 2a. Their fabrication steps are (1) electron-beam lithography is applied to form hole array in silicon nitride mask, (2) SAG of p-i-n GaAs nanowire using MOCVD, (3) BCB infiltration, (4) reactive ion etching (RIE) to expose nanowire tips, and (5) transparent conductive indium tin oxide (ITO) deposition. The SEM images for as-grown vertical GaAs nanowire array (Figure 2b), after nanowires are embedded in BCB and etched by RIE to expose short tips (Figure 2c) and after coating of ITO film by sputtering (Figure 2d) is displayed.

Figure 2.

Solar cell fabrication process and SEM images. (a) Fabrication steps of GaAs nanowire array solar cells with axial junction, (b) 30° tilted SEM image of as grown vertical GaAs nanowire array on GaAs (111) B substrate, (c) SEM image after nanowires are embedded in BCB and etched by RIE to expose short tips, and (d) SEM image after coating of ITO film by sputtering. A conformal dome-like cap is formed on the tips of nanowires [69].

3. Vapor-liquid-solid growth mechanism

NWs are grown from the vapor-liquid-solid (VLS) method during which an NW is grown from the vapor phase employing a metal seed particle, like Au as shown in Figure 3a, that is usually liquid at the expansion temperature (after alloying with the substrate and/or growth species) [56, 70, 71]. The VLS growth technique as its name suggests is the growth method from the combination of the three phases (vapor phase, liquid phase, and solid phase). The vapor phase is the gas phase precursor, the liquid phase is the catalyst and the solid phase is the final grown nanowire. A foreign metal can be used to promote NW growth by forming a liquid eutectic with the desired NW material through this mechanism (Figure 3a and b). In such NW synthesis, the chemical precursor’s vapors are transported into the hot zone by an inert carrier gas and react on a substrate with metal catalyst nanoparticles [72]. With the proper choice of substrates, catalysts, precursors, and growth conditions, various types of vertical NWs, as well as planar NWs can be achieved [73]. While VLS synthesis is the most common with the seed particle in a liquid state, a mechanism is also possible if the catalysts remain solid and do not form a eutectic with the NW material [74]. Such solid-phase diffusion mechanism happens below the catalyst’s eutectic point with the metal seeds remaining as solid [75]. Ingvar Aberg et al. [76] have grown GaAs NW array solar cells by the VLS growth Au-assisted method, which demonstrated a 1-sun independently verified solar energy conversion efficiency of 15.3%.

Figure 3.

Schematics for three typical nanowire growth mechanisms: (a) foreign metal–catalyzed growth, (b) self-catalyzed growth, and (c) selective area epitaxy [31].

As explained in the previous section the VLS method can also be use self-assisted. The self-assisted method uses a component of the NW itself as shown in Figure 3b, which avoids the utilization of foreign metal elements [41]. A foreign particle which found on the top of NWs may cause harmful effects like contamination of the NWs, increased contact resistance, or reflection of sunshine from a photovoltaic device [77]. These foreign metallic particles can be etched after device processing, but etching might cause problem and will have a negative effect on the performance of device. Figure 3c shows the oxide-supported growth mechanisms.

Mandl et al. [63] have grown InAs NW by a VLS mechanism employing a liquid In droplet and they identified that the presence of the oxide layer is vital to immobilize In droplets on the surface, restricting the particle size and NW nucleation formed. They have concluded that NWs can be grown in the sequence, as illustrated in Figure 4: (a) in the deposited SiO_x openings form during heating the substrate; (b) when the trimethylindium (TMI) precursor is activated, the In atoms adsorbed on the surface diffuse into immobile In droplets formed in the openings of the SiO_x layer; (c) at the interface between indium particles and the substrate underneath, NW growth is began by enhancing the rate of growth in one direction; and (d) after deactivation of the TMI supply, the droplet forms InAs NW.

Figure 4.

The growth mechanism illustration of nanowires: (a) layer before growth, (b) layer during heating and creation of hole, (c) formation of In droplets, (d) growth of the NWs under the In droplets, and (e) droplet solidification during cooling [63].

4. Vapor-liquid-solid growth mechanism

The main challenge regarding the performance of thin-film photovoltaic cell structure is the sunshine reflection losses, for instance, with no treatment materials, around 30% of the light illuminated at the Si surface can be lost due to the reflection at the interface between air and Si [78]. During the illumination of sunlight to the surface thin-film cell, some of the light is converted into energy, others will be transmitted, whereas some parts reflect [79]. The loss due to reflectance can be reduced by using techniques, such as coating with anti-reflection and light trapping materials [80]. To reduce this loss, the most commonly used is dielectric antireflection coatings; however, it is difficult to hide the whole absorption wavelength range. Broadband antireflection methods will be achieved by light trapping schemes, such as inverted pyramid structures, but these boost the cost due to their complicated fabrication method. Contrary, NW arrays have a strong antireflection ability with superior wavelength, polarization, and angle-dependent properties compared to planar structures because NWs can form graded-refractive index layers [80]. Consequently, it will reduce the light reflectance at the interface of the two media by avoiding abrupt changes in the refractive index [81].

Wu et al. [82] have presented a model for effective and fast design of both squarely and hexagonal InP NW arrays to achieve the highest light-harvesting for PV application, achieving the maximal short-circuit current density of 33.13 mA/cm². They have investigated the geometrical dimensions for vertically aligned single, double, and multiple diameters of NW arrays. NWs and nanorods have almost the same properties and solar cells can also grow as nanorods morphology to harvest highly efficient sunlight by reducing reflection. Diedenhofen et al. [50] grew layers of GaP nanorods on AlInP/GaAs substrates. They found that nanorods can greatly reduce the reflection and increase the sunshine transmission into the substrate overbroad spectral and angular ranges due to the graded index of refraction. Strudley et al. [58] studied the sunshine transport inside an NW mat. They found that due to mesoscopic transport the high-density semiconductor NW mats exhibit huge interference contributions. From their statistical analysis of intensity oscillations, they linked that transport for focused illumination is governed by a minimum of around three open transmission modes, which is a record low value for light in a 3D medium.

Additionally, semiconductor NW is a 1D nanostructure, which is usually on the order of the sunshine wavelength. Due to their high refractive index, they behave as optical antennae that can modify the absorption and emission properties [80]. The absorption properties of NWs when they are vertically standing are determined by the waveguide modes [82]. InP NW arrays, which are vertically aligned and grown on a semi-infinite SiO₂ substrate are schematically shown in Figure 5 with either squarely or hexagonal arrangement. Repeatable unit cells in Figure 6a and b insets show respective characterization dimensions for each arrangement. Such morphology and topology of the NW arrays are in accord with the majority of the InP NW-based photovoltaic cell structures. Within each of the unit cells, the NWs own identical or different diameters as D_i (where i = 1, 2, 3,…). Periodicity P is the core to core spacing of a pair of adjacent NWs that has an analogous value for squarely arranged NWs, whereas fully different values for hexagonal NW clusters.

Figure 5.

Schematics of vertically aligned InP NW arrays. (a) Squarely, and (b) hexagonal NW arrays with insets explaining their respective unit cells [82].

Figure 6.

(a) Schematic drawing of the periodic GaAs NWs structure, (b) absorptance, (c) reflectance, and (d) transmittance of GaAs NW array with different fill factors [83].

NWs are more efficient in light absorption compared to thin-film materials of an equivalent volume. Krogstrup et al. [84] have observed a remarkable increase in absorption in single-NW solar cells, which is related to the vertical configuration of the NWs and to a resonant increase in the absorption cross-section, and the results obtained opened a new route to third-generation PVs cells. Their short-circuit current result of 180 mAcm⁻² is higher than that predicted by the Lambert-Beer law.

On other hand, when the NW is lying horizontally, the absorption properties are determined by leaky-mode resonances, which provide a chance to engineer the light absorption in NWs by controlling their physical dimensions [85]. Once the resonant modes are supported by the NWs leaky, the overlap between the incident electromagnetic attraction field and the guided mode profile is maximized, facilitating enough coupling with incident light.

Due to their outstanding advantages, NW arrays have advanced light trapping ability and hence strongly enhanced optical absorption in comparison with the thin-film [86]. This can significantly enhance the broadband light absorption over a good range of incident angles, especially the near and below bandgap absorption [81, 87]. With the same thickness as thin-film layers, the NWs short-circuit current can reach high results [88].

4.1 Effect of NW diameter and period on absorption

Nanowire diameter and separation are typically on the order of the wavelength of sunlight, where interference effects are dominant, therefore, the reflectance, absorptance, and transmittance of nanowire arrays must be determined using wave optics [89]. Long Wen et al. [83] have simulated to evaluate the efficiency limits of GaAs NW array solar cells and determined the requirements of the optical design for improving the efficiency Figures 6a–d. They have suggested that the optimized design NW might absorb 90% of above bandgap sunlight. Their combined optoelectronic simulation results reveal that optimization of optical geometry can lead to an attainable photovoltaic efficiency of 22%.

By fixing the filling factor, which is given by D/P, where D is the diameter of the nanowire and P is the separation between the grown nanowires, the effect of NWs diameter can be determined by varying it. Figure 7a shows the optical characteristics of the GaAs NW array with different diameters at a fixed D/P of 0.5 is plotted. The absorptance of a 2.2 $\mu$ m GaAs thin-film is also plotted for comparison. In short wavelengths, it can be observed that the absorptance spectra for all NW arrays are kept above 90%, which is much higher than the planar case due to the lower effective refraction index, and thus lower reflection at the top of NW arrays. In the long-wavelength regime, the absorption spectra show a significant increase when increasing diameter from 60 to 180 nm. The electromagnetic field can be coupled efficiently into the NWs at resonances, due to the large refractive index contrast between the NWs and surrounding air. For small diameter GaAs NW arrays, with fewer supporting modes, most of the incident light cannot be guided into the NWs. The absorbance of the NW array with D = 180 nm is high above the band gap wavelength, whereas when D is increased to 240 nm the absorbance of the NW array decreased (Figure 7a) due to the increased reflection and the insufficient field concentration at longer wavelength.

Figure 7.

(a) Absorptance of NWA with different diameters, and (b) Photogeneration profiles calculated by FDTD simulations [83].

Figure 7b shows plots of the vertical cross-section of the photogeneration profiles. The NWs with D = 60, 180 nm and D/P = 0.5 under 1mWcm⁻² sunshines at different wavelengths for photogeneration rates are revealed. At λ = 400 nm, it is concentrated near the highest sides of the NW for both diameters. Only a little fraction of the incident wave is transmitted onto the substrate, this will be explained by the short absorption length of GaAs at this wavelength. At 600 nm and above, for a NW, the photogeneration rates are focused on several lobes that form along the NWs for a NW array with 180 nm diameter, indicating strong guided modes confined within the NWs. In contrast, for the case of D = 60 nm, the optical generation becomes more homogeneously covered by the NWs with a longer wavelength. Clearly, the 180 nm diameter NW array induces a much larger optical concentration than the 60 nm diameter one. From both Figures 6 and 7 one can easily understand the effect of NW diameter on photon energy harvesting.

4.2 Effect of NW length on absorption

Photocurrent density is often further bettered by adding nanowires length (L). Figure 8 shows the reckoned donation to all photocurrent from the NWs and the substrate during a GaAs NW PV device for an NW periphery of 180 nm and a period of 350 nm (90). Due to the proliferation of NW length, the donation from the nanowire to all photocurrent rises, while the GaAs substrate donation similarly decreases. The uttermost photocurrent of 27.3 mAcm⁻² is obtained at 5 μm length, is on the brink of the perfect photocurrent density of 29.9 mAcm⁻² (calculated by integrating the AM1.5G spectrum above the GaAs bandgap). At the optimum NW diameter, spacing, and length of the harvesting properties of III–V NWs can be improved.

Figure 8.

Theoretical contributions from a GaAs nanowire array and the GaAs substrate to the total photocurrent density in a PV device versus nanowire length obtained at a nanowire diameter of 180 nm and period of 350 nm [90].

In 2015 Nicklas Anttu [28] compared the effectiveness of InP NW assemblage solar cells with the classical InP bulk solar cells. They accounted an NW assemblage of 400 nm periods, 4 μm length, and 170 nm periphery, which may produce 96 of the short-circuit current accessible within the impeccably taking up InP bulk cell. Also, the NW solar cells cast smaller photons than the bulk cell at the identical occasion, which allows for a more open-circuit voltage. They consequently found that NWs longer than 4 μm can really show, despite producing a lower short-circuit current, an efficiency limit of up to 32.5% that is above the bulk cells.

They have predicted the unborn capabilities in affecting both the emission and absorption characteristics of the NW assemblages, for instance, by (1) varying NWs shape, (2) varying the period of NWs, (3) sheeting the NWs with a nonabsorbing dielectric shell, (4) fitting a dielectric material between the NWs, and (5) by introducing optical antireflection layers on top of the NW. Such improvement of the NW array could conceivably further accelerate its effectiveness limit.

5. Absorption depth and diffusion length for III–V NWs

Based on the axis of charge carrier separation, an axial and a radial junction device are the two broadly classifying NW solar cells. The charge carrier separation happens along the length of the nanowire and the radial axis, in axial junction, and radial junction solar cells, respectively. Figure 9a and b display sunlight absorption and charge carrier separation in both axial and radial junctions NW solar cells correspondingly. In a solar cell, the minimum length needed to attain ample absorption is characterized by absorption depth. The absorption depth explains how deeply light penetrates the NW semiconductor or every type of solar cell device before being absorbed. At the same time, diffusion length describes the maximum length that the minority charge carrier can travel before making recombination non-radiatively [91]. For solar cells, in order for them to efficiently operate, the diffusion length should be higher than the absorption depth, as schematically shown in Figure 9. Radial junction is preferable for the fabrication of large-efficiency devices by connecting the light absorption and charge carrier separation axes. In a radial junction PV cell, sunlight absorption is along the main axis of the NW, while the charge carrier separation takes place within the radial direction, which is in nm-scale thickness. In other words, to realize the optimum performance of the NW photovoltaic cell in a radial junction photovoltaic cell, both charge carrier separation, and light absorption can separately be optimized.

Figure 9.

Nanowires solar cells schematic representation of (a) an axial p-n junction, and (b) a radial p-n junction. α denotes the absorption coefficient of the active material and L_n and L_p denote the electrons and holes diffusion lengths, respectively [91].

Yao et al. [69] have carried out an optical simulation to predict the optimized axial junction and radial NW array for maximum light absorption and have compared the merits and demerits of these NWs. They have also synthesized GaAs NWs solar cells with an axial p-i-n junction by selective area growth method, which is compatible with MOCVD technique, and they observed that low filling ratio NWs are highly absorbed. They have also studied the effect of the diameter and revealed that thicker NWs are favorable because of the high surface recombination velocity on the bare GaAs NW surface. They identified that by decreasing junction depth to around 100 nm and maintaining diameter at 320 nm, able to achieve efficiencies as high as 7.58%. Their results demonstrated that GaAs NWs are good candidates for high-efficiency and low-cost solar energy conversion and open up great opportunities for the next generation photovoltaic based on multi-junction devices composed of lattice-mismatched material systems.

6. Photogenerated carriers of III–V NWs

For solar cells, one of the key needs is to realize efficiency that keeping a huge optical thickness to facilitate high light absorption and a tiny low electrical thickness to facilitate high photogenerated carrier assortment at the contacts. The gathering of high photogenerated carriers depends powerfully on the diffusion length of minority carriers, which decline quickly with the rise in density of defect [22]. Generated carriers are going to be wasted when they are quite one diffusion length far away from the space charge region [92]. The diffusion length, L_d, of electrons or holes in a semiconductor is defined by the mean distance the relevant charge moves within the semiconductor. It is influenced by the mean distance the relevant charge moves within the semiconductor and recombination/extraction from the semiconductor. Diffusion is the movement of charge carriers directed by a concentration gradient. The diffusion coefficient (D) and additionally the equivalent term among the presence of a field, mobility (μ), are associated with one another by the relation [92]:

$$\text{D}=\frac{\mu kT}{\text{q}}$$E1

and

$$L_d={\left(D\tau \right)}^{1/2}$$E2

where τ is the charge lifetime.

When the cell is not operational at open-circuit voltage, that is, the charge is extracted, and then the lifespan can clearly be less due to the removal of the charge extracted. This is no longer an intrinsic property of the absorbing semiconductor itself, however, depends on the interfaces that exist between the semiconductor and charge extraction phases. The lifespan of charge refers to the minority charge carrier lifespan for semiconductors that are obviously either n-type or p-type. Differentiation into majority and minority carrier lifetimes is not obvious for an intrinsic semiconductor, such as the intrinsic semiconductor in a p-i-n cell [93].

In a conventional thin-film device, the gathering path of the generated carriers is parallel to the solar photon traveling path. Thus, thick enough absorption materials are in high demand on the quality of the crystal, in order that the carriers can easily undergo without any substantial recombination. The morphological anisotropy of nanowires provides the advantage of decoupling the optical and electrical thickness of PV cells by using the co-axial contact structure [91]. It can absorb sunlight along the entire nanowire, while the generated carriers are frequently separated within the radial direction. The radial distance that carriers need to travel (in the 100 s nm range) is generally much lower than, or similar to the minority carrier diffusion length. So far, the orthogonally severed sunshine and carrier separation paths can cause low bulk recombination, and hence high effectiveness. Also, the NWs have a high surface-to-volume ratio, which offers a large junction area that will further enhance the charge separation effectiveness.

The study showed that the influence of adjusting the diffusion length under radial junction may be a smaller amount than in planar junction, that is the utmost efficiency of both radial p-n junction geometry and planar geometry can increase with increasing diffusion length, but the planar geometry increases more [94, 95]. The difference in the performance between the planar and radial structures for III–V semiconductors with a high carrier diffusion length is, not as clear as that for Si [94]. However, NWs have a large surface-to-volume ratio and hence, a large density of surface state [93, 96]. All these merits allow using lower-purity, less expensive materials with low minority carrier diffusion lengths to make high-efficiency solar cells. Consequently, the use of the NW structure can enormously decrease the device cost. Due to these advantages, NWs are promising high-efficiency and less expensive solar cells and have the potential to revolutionize solar power harvesting technology.

7. III–V NWs design for high-efficiency and low-cost solar cells

The distinctive structure and advanced properties of NWs provide additional freedom in constructing novel solar cells with high-efficiency and low-cost. That is solar cells can be designed in different architectural such as tandem solar cells, axial tandem solar cells, multi-terminal solar cells, inorganic nanowire/organic hybrid solar cells, branched solar cells, and flexible solar cells, in which III–V nanowires can also be designed.

Tandem solar cell [97] is one type of design in order to have high efficiencies in solar cells, which is to use multiple semiconductors epitaxially grown on top of each other. Figure 10 shows the system with two different semiconductor materials, where one material is used as top materials and different materials are used as bottom cell materials. In this figure L_top, L_bot, D_top, and D_bot illustrate the length of the top cell, length of the bottom cell, the diameter of the top cell, and diameter of the bottom cell, respectively. It is to absorb high-energy light in a large bandgap top cell in such a tandem solar cell. Compared to the single junction cell, the thermalization loss of the high-energy light is decreased in the top cell. Then, the lower energy light continues to the bottom cell where these energies are absorbed. The bottom cell has a lower bandgap and due to the lower bandgap than in the single-junction cell, more photons are absorbed in the bottom cell. Consequently, the tandem solar cell can absorb more photons than single-junction cells and also can have reduced thermalization loss. However, in planar cells, the crystal lattice constant should be matched in adjacent subcells to offer high-quality materials [97]. Due to this lattice mismatch, they cannot grow on Si substrate, which is the second most abundant earth element and cheap. Moreover, the III–V multi-junction cells in the conventional thin-film structure can give high-efficiency but need to use Ge as substrates, which is expensive. The blending of III–V solar cells on Si substrates can greatly reduce the value, which is extremely challenging.

Figure 10.

(a) Schematic representation of a dual-junction NW array on the inactive substrate, and (b) illustration of the electrical design of NWs with axially configured p-i-n junction in which a tunnel junction connects the bottom and the top subcell [97].

Nanowire structures give an obvious advantage for multi-junction solar cells compared with thin-film cells. NWs have efficient strain relaxation, which permits for the fabrication and combination of dislocation-free and highly lattice-mismatched materials. In another word, III–V nanowire arrays can be grown on top of a Si substrate, giving the prospect of using the Si substrate as the bottom cell. Figure 11a shows the growing of III–V NWs on Si substrates consisting of a bottom Si cell and a top III–V nanowire cell [99].

Figure 11.

Model geometry of III–V NW on a Si substrate (a) side view showing doped layers, and (b) top view showing a hexagonal NW of diameter D arranged in a square array of period P [98].

The optimum structure needs the absolute stylish NW cell to have a direct bandgap of near 1.7 eV, which can be achieved by employing a number of III–V emulsion semiconductor material systems. The optimum structure also requires equal current from each sub-cell, videlicet a current-corresponding condition. This may be realized by conforming the periphery, length, and period of the NW array. Thus, NW solar cells have further degrees of freedom compared with thin-film solar cells, whose current-matching is achieved by conforming to the consistency of the absorbing subcaste in each subcell. The optimum building needs the absolute stylish NW cell to possess a direct bandgap of closer to 1.7 eV, which can be attained by engaging a number of III–V semiconductor materials. The optimum building also requires equal current from each subcell, namely a current-matching condition. This may be realized by conforming to the length, period, and diameter of the NW array. Thus, NW solar cells have more degrees of freedom compared with thin-film solar cells, whose current-matching is achieved by adjusting the thickness of the absorbing layer. Hu et al. [98] designed the current matching 1.7 eV III–V NW top and 1.1 eV Si planar bottom cell by tuning the NW diameter and period (Figure 11b). They obtained the best photocurrent density of 17.8 mAcm⁻² at NW diameter of 180 nm, period of 350 nm, and length of 5 μm, which result in 89.4% absorption of the AM1.5G spectrum and a promising efficiency above 30% under one sun illumination. Yao et al. [100] have reported the growth of III–V NW on Si tandem cells with the GaAs nanowire top cell and the Si bottom cell with a circuit voltage V_oc of 0.956 V and a high-efficiency of 11.4%. Their simulation showed that the current-matching condition plays a crucial role in the overall efficiency of the device. They also have characterized that GaAs NW arrays were grown on lattice-mismatched Si substrates, which are less expensive. They concluded that tandem solar cells supported top GaAs nanowire array solar cells grown on bottom planar Si solar cells, open up great opportunities for high-efficiency and low-cost multi-junction solar cells.

Axial and radial tandem solar cells [101, 102, 103] are another form of solar cell designing. In axial tandem solar cells, because the photogeneration events happen most often in the middle of NWs, they cannot intrinsically block the generated carriers from reaching the surface and recombining like the radial junctions. Due to a similar reason, the radial tandem solar cell faces a challenge of inefficient absorption for the cell junctions away from the core of NWs. Thus, a composite structure that combines the advantages of the axial and radial structures would provide much higher efficiency compared with homogeneous ones [104].

Furthermore, NWs have a little cross-section, which allows them to accommodate big strains axially and laterally and this may greatly facilitate the blending of materials with large lattice mismatch, providing more freedom within the structure design compared with thin-film devices [105].

An axial NW heterojunction structure with lattice mismatch can be created from the results that the axial junction will distribute the strain across the interface, which will relax the straining step by step and elastically. Regardless of the length, there exists a critical diameter below which no interface dislocation is often introduced. Dislocation-free NWs heterojunctions, such as GaAs/GaP [106], InAs/InSb [107], and InAs/InP [108] have been realized even with large lattice-mismatch. For example, Ercolani et al. [107] have reported the Au-assisted CBE growth of defect-free zincblende structure InSb NWs. InSb NW was grown on the upper sections of InAs/InSb heterostructures on the InAs (111) B substrates. They have also observed that zincblende structure InSb is often grown without any crystal defects.

With the same concept, the nanowire core has advantages with regard to lattice-mismatch strain in that it can share the nearest mismatch strain, which results in a drastically reduced strain within the shell [109]. NW core-shell structure can thus accommodate larger lattice mismatch compared with thin-film structures [110]. V. Nazarenko et al. [111] have reported the growth of core-shell InGaAs/GaAs nanopillars by MOCVD on Si substrates. They demonstrated that a shell thickness around 160 nm defect-free GaAs grown on In_0.2Ga_0.8As core NWs despite a large lattice mismatch amounts to 2% for the 20%. Their TEM characterization showed an outstanding crystal quality in the entire pillar without defects. Wang et al. [112] have grown a novel NW structure for solar cells that axially connects core-shell p-n junctions (Figure 12a) with different bandgaps. In order to evaluate the performance of this NW, they have used a coupled 3D optoelectronic simulation and their simulation results revealed a high conversion efficiency of 16.8% at a low filling ratio of 0.196. After an outstanding current matching, a promising efficiency of 19.9% was achieved at a low filling ratio of 0.283, which is much higher than the tandem axial p-n junction under the same conditions. Figure 12b illustrates vertically aligned NW arrays of axially connected core-shell structures.

Figure 12.

(a) 3D illustration of axially connected core-shell p-n structure with different III–V materials is axially connected by the tunnel diode in a NW, and (b) Schematic drawing of vertically aligned NW arrays [112].

The unique structure of NW p-n junctions enables substantial light absorption along the NW length and efficient carrier separation and collection within the radial direction. Heurlin et al. [113] demonstrated the growth of tandem junction InP NWs on a Si substrate. By applying in situ etching for total control over axial and radial growth they connected two photocurrents having p-n junctions in series by a tunnel junction. They observed a rise up of V_oc by 67%. They also believed that this provides the best way toward realizing high-efficiency multi-junction solar cells that can be fabricated on a large area and low-cost Si substrates.

Multi-terminal NW solar cell is also another promising design of nanowires. Introducing multiple bandgap concepts into NW solar cell designs has high promise for maximum solar conversion efficiency [114]. Dorodnyy et al. [29] have proposed a multi-terminal NWs solar cell design as shown in Figure 13. Their NW design resulted in theoretical power conversion efficiency of 48% utilizing an efficient lateral spectrum splitting between three different III–V material NW arrays grown on a flat silicon substrate. These authors used Al_0.54Ga_0.46As, GaAs, and In_0.37Ga_0.63As NWs with bandgap 2.01, 1.42, and 0.93 eV, respectively. However, the main challenge would be the matter of growing different NW groups with different lengths required for device fabrication.

Figure 13.

(a) Design concept illustration of the triple-junction NW array on a Si substrate, (b) Working principle of the design, and (c) Contacting scheme of the multiterminal device [29].

The mixing of inorganic NW and organic will give the opportunity to have hybrid solar cells and is also another design of the solar cells to offer high-efficiency materials [115, 116, 117, 118]. These two materials have their own advantages. Inorganic materials commonly possess high carrier mobility and affinity, whereas organic polymers commonly possess low carrier mobility and a short lifetime, which leads to low device efficiency. However, organic polymers are low in cost; as a result, researchers attempt to mix together the advantages of the two material systems. Due to the fast and efficient charge separation or collection, a greatly enhanced efficiency is hence expected for the inorganic NW/polymer combination. H Bi and R R LaPierre have fabricated hybrid solar cells consisting of GaAs NW arrays and poly(3-hexylthiophene) or P3HT. They have been fabricated by spin-coating poly(3-hexylthiophene) (P3HT) polymer onto vertically aligned n-type GaAs NW arrays synthesized by MBE and reached an efficiency of 1.04% (2.6 sun) [106].

NWs can also be fabricated and designed as branch cells. Branched NWs solar cells [118, 119, 120] can also be referred to as nanotrees or nanoforests. These nanowires have a tunable 3D morphology, homo or heterogeneous junction, and interface electronic alignment represent a unique system for applications in energy conversion and storage devices. 3D branched nanowires have merits, including structural hierarchy, high surface areas, and direct electron transport pathways and it is an attractive recent research area on energy. Lundgren et al. [115] simulated a high absorption structure branched nanowire (BNW) (Figure 14). They found that BNW tree configurations achieved a maximum absorption of over 95% at 500 nm wavelength. There has been great progress in fabricating branched NWs [115]. Wang et al. [122] have reported the branched and hyperbranched NW synthesized by a multistep nanocluster-catalyzed VLS approach. They have demonstrated the growth of branched Si and GaN NWs with multi-generation branches.

Figure 14.

(a) Dimensions of a single branched nanowire tree. (b) Array of branched NW trees [121].

Lightweight and flexible solar cells are necessarily important for designing high-efficiency solar cells [123]. Lightweight and flexibility are two of the desired properties, which can substantially reduce the facility weight, minimize the transportation cost, and cause the assumption of smart solar cells, such as integrating flexible cells into clothing. NWs provide unique merits in realizing these advanced functions, as they are going to be buried into polymers and then easily peeled away from the substrates. Han et al. [124] fabricated flexible GaAs NW solar cells with NWs lying horizontally and achieved high efficiency of 16% under atmosphere 1.5 global illuminations. All the above discussed novel designs are very important for providing highly promising III–V NWs to greatly reduce the worth and boost the efficiency of solar cells, which may revolutionize the current solar cell technologies.

8. Recent efficiencies of III–V NWs solar cells

A solar cell is characterized by parameters, such as filling factor (FF), open-circuit voltage (V_oc), short-circuit current (J_sc), and power conversion efficiency (η). Table 1 summarizes some of the fabricated III–V NWs solar cells since 2010. Values for each parameter and their growth mechanisms are also summarized. The main hindrance for commercializing III–V NWs solar cells is their low power conversion efficiency. Therefore, researchers around the world are trying to increase the efficiency of these materials by using novel designs, improving growth mechanisms, and device fabrication methods. The highest efficiency of III–V NW solar cells above 10% is reported by Holm et al. using GaAsP NWs with radial p-i-n junctions, which is 10.2% [133]. Next to this report, many III–V NWs with efficiency above 10% are reported. An efficiency of 19.6% using InP nanopillars is achieved by Son Ko et al. [145]. Krogstrup et al. even reported a high experimental efficiency of 40% using GaAs NWs [84].

III–V nanowires	Growth methods	Substrates	catalysts	Geometry	FF (%)	Voc (V)	η (%)	Jsc (mA/cm^2)	Ref.
InGaAs	SA-MOVPE	n-GaAs (111) B		axial p-i-n	72.1	0.544	7.14	18.2	[125]
InP	SA-MOCVD	InP (111) A		p-i-n single	74	0.69	2.81	5.52	[126]
InP	VLS	InP (111) B		axial p-i-n	68	0.61	7.6	18.2	[127]
GaAs	VLS	Si (111)	Ga	ZB crystal structure	40	0.45	4.1	22.8	[128]
InAs	MBE	p-Si (111)		p-n heterojunction	32	0.31	1.4	14	[129]
AlGaAs/GaAs heterojunctions	3-D optoelectronic simulation	^aNI	NI	axial and radial with AlGaAs passivation	NI	NI	8.42	NI	[130]
GaAs	MBE	p-Si (111)	Au	axial p-n	NI	NI	6.3	NI	[131]
GaAs/InGaP/GaAs	MOVPE	p-GaAs (111) B	Au	core-multishell	52	0.5	4.7	18.1	[132]
GaAsP	MBE	p-Si (111)	Ga	core-shell p-i-n	77	0.9	10.2	14.7	[133]
In0.3Ga0.7As	MOVPE	p-Si (111)	free	n-InGaAs/p-Si	50	0.37	2.4	12	[134]
InN	MBE	n-Si (111)	free	axial p-i-n	30.24	0.13	0.51	12.91	[135]
InP	Top-down	InP		axial p-n	79.4	0.765	17.8	29.3	[32]
InP	MOVPE	p-InP (111) B	Au	axial p-i-n	72.4	0.906	13.8	24.6	[30]
InP	VLS	InP (111) B	Au	axial p-n junction	73	0.73	11.1	21	[136]
InP	SA-MOVPE	p-InP (111) A	free	ITO/p-InP heterojunction	68.2	0.436	7.37	24.8	[137]
InP	Top down	p-InP (100)		n-S-SAM/p-InP heterojunction	60	0.54	8.1	25	[36]
InP	SA-MOVPE	InP (111) A	free	NI	59.6	0.457	6.35	23.4	[138]
InP	SA-MOVPE	p-InP (111) A	free	radial p-i-n	58.5	0.674	4.23	11.1	[57]
InAs hetero	MBE	p-Si (111)		n-InAs/p-Si	32	0.31	1.4	14	[129]
GaAs	MOVPE	p-GaAs (111) B	Au	axial p-i-n	79.2	0.906	15.3	21.3	[76]
GaAs	SSCVD	Non-crystalline	Au	Schottky contact	61	0.39	16	67	[124]
GaAs tandem	SAE-MOVPE	Si (111) n,p	free	axial n-i-p/n-p Si	57.8	0.956	11.4	20.64	[100]
GaAs	SAG-MOCVD	p-GaAs (111) B	free		63.65	0.565	7.8	21.08	[69]
GaAs	SA-MOVPE	n-GaAs (111) B	free	core-shell p-n	62	0.44	6.63	24.3	[139]
GaAs	MBE	p-Si (111)	Ga	core-shell p-i-n	52	0.43	40 (field concentration)	180 (field concentration)	[84]
GaAs/InGaP	SA-MOVPE	p-GaAs (111) B	free	Core-multishell p-n	65	0.5	4.01	12.7	[140]
GaAs	VLS	Si (111)	Ga	radial p-i-n	46.5	0.39	3.3	18.2	[141]
GaAs	SSCVD	Si/SiO2	Au-Ga	Schottky contact	42	0.6	2.8	11	[142]
GaAs	SA-MOCVD	p-GaAs (111) B	free	core-shell p-n	37	0.39	2.54	17.6	[143]
GaAs hybrid	MOCVD	GaAs	free	GaAs/P3HT hybrid	43	0.2	1.44	18.6	[144]

Table 1.

Some of the III–V NWs studied since 2010 and their efficiency achievement.

^*

Power conversion efficiency was measured at 1 Sun, AM1.5G illumination; ^aNI represents not identified values.

Furthermore, the highest efficient III–V NWs large-area solar cells are rapidly developing [111]. The first large-area solar cell with high-efficiency higher than 10%, which is an InP NW array solar cell with an efficiency of 13.8%, is reported by Wallentin et al. [30]. Afterward, an efficiency of 11.1% with InP NW arrays is achieved by Cui et al. [136]. Then, an efficiency of 15.3% is achieved using GaAs NW arrays by Åberg et al. [76]. Ultimately, the III–V NW with the highest efficiency and world record is 17.8% reported by Dam et al. [32].

9. Conclusion and outlook

In order to convert solar energy into electrical energy, harvesting solar energy is required and solar photovoltaic is the most promising device for this purpose. Despite the excess of sunlight reaching the earth’s planet, the current percentage of solar energy is much smaller than both renewable and nonrenewable energies. This is due to low energy density, low efficiency, and relatively high-cost materials compared to other types of energy technologies. Therefore, novel materials that can enormously harvest sunlight are important and they are the current issues attracting research interest. Due to its unique properties from bulk materials, III–V NWs can be used as high-performance solar cells because of their attractive advantages, such as unique optical and electrical properties, direct band, and fewer solar light reflections. In constructing novel solar cells with high-efficiency and low-cost, the distinctive structure and advanced properties of NWs provide more freedom. Today’s solar cell market is dominated by the thin film of Si, which has the lowest efficiency, but low cost. By combining the advantages of III–V NWs and Si by growing III–V NWs on Si substrate tandem solar cells, enormously improved performance of the solar cells can be achieved. By controlling the III–V NW morphology and its geometry with optimum diameter, period, and length it is possible to get high-efficiency solar cell materials. Furthermore, III–V NW solar cells can be designed as tandem solar cells, axial and radial tandem solar cells, multiterminal solar cells, inorganic nanowire/organic hybrid solar cells, branched solar cells, and flexible solar cells.

Future works can be focused on the optimum design that can overcome all the limitations of III–V NW solar cells in order to achieve high-performance and low-cost III–V NW-based solar cells. Thus, one of the best aspects of III–V NWs commercialization with high power conversion efficiency may be achieved by designing it in a way that it can absorb solar light enormously with reduction of materials used and low-cost substrates. According to our understanding, all the designs of III–V NWs that are mentioned in this review are beneficial for future commercialization; however, it is good to identify the one that is more attractive than the others by conducting research on each design. All types of design have their own advantages in case of reducing materials used for the fabrications of solar cells and cost reductions. So far, it is good if this area will be researched more, especially on the architecture of III–V NWs due to its infinite advantages. Despite the challenges of achieving high efficiency in these NWs, they are the hope of the next-generation solar cells due to their flexibility for designing it even in a multi-junction of different NWs, which can absorb the different wavelengths of solar light for harvesting huge solar light. Furthermore, to advance III–V NW-based solar cells toward possible commercialization the power conversion efficiency should be increased, for which the tandem architecture is highly interesting by growing on Si substrate, which is cost-effective.

Acknowledgments

We acknowledge Adama Science and Technology University and Oda Bultum University for their financial support.

Conflicts of interest

There is no conflict of interest.