**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 fuelbased 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 buildingintegrated photovoltaic systems for harvesting solar power, toward the goal of selfsustainable 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 siliconbased 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.

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

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 highefficiency 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 Al0.54Ga0.46As, GaAs, and In0.37Ga0.63. 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.
