**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 Ltop, Lbot, Dtop, and Dbot 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].*

*Solar Energy Conversion Efficiency, Growth Mechanism and Design of III–V Nanowire-Based… DOI: http://dx.doi.org/10.5772/intechopen.105985*

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].

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

#### **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].*

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 Voc 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 [101–103] 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 latticemismatch 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 In0.2Ga0.8As 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.

*Solar Energy Conversion Efficiency, Growth Mechanism and Design of III–V Nanowire-Based… DOI: http://dx.doi.org/10.5772/intechopen.105985*

#### **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 Voc 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 Al0.54Ga0.46As, GaAs, and In0.37Ga0.63As 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.

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 [115–118]. 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)

#### **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].*

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 [118–120] 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.

Lightweight and flexible solar cells are necessarily important for designing highefficiency 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

*Solar Energy Conversion Efficiency, Growth Mechanism and Design of III–V Nanowire-Based… DOI: http://dx.doi.org/10.5772/intechopen.105985*

**Figure 14.** *(a) Dimensions of a single branched nanowire tree. (b) Array of branched NW trees [121].*

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.
