Introduction to Conversion Efficiency Improvement in GaAs Solar Cells

Abstract The finite-difference time domain (FDTD) tool is used to simulate the reflection losses of subwavelength grating (SWG) structure in GaAs solar cells. The SWG structures act as an excellent alternative antireflective (AR) coating due to its capacity to reduce the reflection losses in GaAs solar cells. The SWG structures allow the gradual change in refractive index that confirms an excellent AR coating and light trapping properties, when compared with the planar thin film structures. The nanorod (nano-grating) structure acts as a single layer AR coating, whereas the triangular (conical or perfect cone) and parabolic (i.e., trapezoidal or truncated cone) shaped nano-grating structures act as a multilayer AR coating. The simulation results show that the reflection loss of triangular (conical or perfect cone) shaped nano-grating structure having a 300 nm grating height and an 830 nm period is *2 %, which is about 28 % less than that of flat type substrates. The simulated results show that the light reflection of a rectangular shaped grating structure is *30 %, however, the light reflection becomes *2 % for a triangular (conical or perfect cone) shaped nano-grating structure, because the refractive index changes gradually in several steps and reduces the reflection losses. It is also noticed that the intermediate structures (trapezoidal and parabolic shaped), the light reflection loss is lower than the rectangular shaped nano-grating structure but higher than the triangular shaped nano-grating structure. The simulated results confirm that the reduction of light reflection losses in GaAs solar cell will increase the conversion efficiency. Therefore, this analysis confirmed that the triangular (i.e., perfect cone or conical) shaped nano-grating structures are an excellent alternative AR coating for the improvement of conversion efficiency in GaAs solar cells.


Background of Solar Energy

The World Bank statistics displayed a number for the global population which reaches almost 7 billion human lives by the end of 2011 [1]. This huge number of the human race strongly influences energy consumption on a daily basis. The International Energy Agency (IEA) recorded consumer energy usage on 2010 as 535 ExaJoule (1 eJ = 1018 J) and this number will keep increasing as time goes by [2]. However, more than 80 % of that energy has already been extracted from limited sources, such as coal, natural oil, and natural gas. If this continues, extinction and even disappearance of some resources may be faced in the short term. To avoid this critical condition, renewable energy sources such as solar cells (i.e., solar energy) need to be developed and to assist in providing energy for the demand of the world community.

The creation of the universe is such that all the planets revolve around the sun which is at the center. Rotation of each planet makes it possible for the sun’s coverage in almost all regions daily every year. The Earth, the planet where human beings live, receives 89 petaWatts (PW) of solar radiation through its atmosphere. In total, up to 3,850,000 eJ sun heat absorbed by clouds, oceans, and lands per year. This irradiation is extremely big compared to the global energy consumption [3, 4]. Thus, the sun is one of the best sources of new energy (i.e., solar cells or energy) source because of its availability and unlimited source.

Therefore, in this modern technology era, people should maximize the avail- ability of renewable energy in order to fulfill their demand for electricity which keeps increasing. Photovoltaic (PV) system is a device that can convert sunlight as renewable energy into DC voltage. This system is then connected with an inverter, so the generated DC electricity is transformed into AC source and is applicable for household usage. With the aid of solar cells, there is high possibility of decrement in the demand on fossil fuels as energy sources.

Nevertheless, the performance of solar cell these days is still far from satisfactory. Despite its high initial cost, electrical energy generation depends highly on the surrounding situation. The most common condition is during cloudy days, the cell will produce less power compared to when the sun is shining bright. The installation place of the PV module is also a consideration as there are four seasons in a country and therefore is tendency for temperature variation. It can be lower than 0 oC during the winter and may be more than 35 oC during the summer. Compared to a tropical country with an average temperature of 30 oC throughout the year, these varying seasons may decrease the cell performance, because it cannot reach its maximum power point.

There are several problems also with the PV cell itself. Because semiconductor is used as the main element of a solar cell, it can be chosen from many different materials. Nowadays, silicon is one of the most popular semiconductors because of its wide availability, making it cheaper to construct and produce. However, efficiency of the silicon solar cell is considerably low compared to Gallium Arsenide (GaAs) and others. The assembly process of the module itself can affect the performance of the device. Any manufacturer defect may create losses of energy through cells junction and dropped to earth connection.

Because of that, solar cell efficiency is required to be improved in terms of its conversion efficiency to make use of plentiful solar energy. This chapter is focused on how to optimize the cell performance, so that it can convert more sunlight into useable energy. The improvement focuses on the design of each cell and devices that might help instruct better final results. In this way, new methods on manufacturing solar cell is available to produce ample solar energy in the future for the next generation.

In history, solar energy has been defined as the radiant light and heat emitted by the sun. The PV effect was discovered approximately 173 years ago since the understanding of the PV effect by Becquerel. Hence, solar energy can be converted into electrical energy by the use of such PV panels.

The global population is increasing significantly every year, and in proportion of this situation, the request for electrical energy has increased greatly. The data from World Bank shows that the global electricity consumption has increased by around 40 % per capita over the past 40 years [1]. In order to satisfy the global high demand, natural resources such as oil, gas, coal, and others are used by the chemical power industry leaving most of the existing world assets in a critical condition. Therefore, renewable energy is crucial to take part in terms of covering the electrical energy consumption. The sun has the greatest contribution as it is very popular and almost everyone can see it daily. Fifteen thousand ExaJoule (1 Exa = 1018) of solar energy passes through our atmosphere every year. So, it is such a waste if the solar energy is not used maximally, where human consumption is still less than 2 ExaJoule [5].

The PV cell is a device that can convert sunlight into electricity using the photoelectric effect introduced by Edmond Becquerel [6]. This solar-electric- energy demand has grown consistently by 20 to 25 % annually over the past 20 years [7]. It is popular among other energy conversion devices, because it does not make any noise and is carbon-emission free during the conversion process. The PV system is also a space-saver for residential and public place purposes. It can be installed on top of the roof, where not much else can be put. The price of the converter has also been decreased compared to the initial cost, when it was introduced the first time by the government. However, the existing PV cell has the lack of conversion efficiency.

Due to wide industrialization as well as urbanization, the world is facing the danger of depletion of its existing energy resources. The alternative solution is to implement clean renewable energy for the green future. The renewable energy such as solar, wind, thermal, tidal, etc., are all taken into consideration. However, among the different types of renewable energy, solar energy research itself is the best candidate selected for the next generation source of clean energy.

Furthermore, according to the IEA, solar energy has portrayed itself as the most promising, affordable, inexhaustible, sustainable clean energy. Solar energy plays an important role in Australia and the world affecting the nations in different aspects; environmentally friendly, indigenously, socially, and economically.

The only drawback of solar energy is the low absorption rate by the PV panels. Hence, researchers have been trying to come out with different techniques to improve the conversion efficiency of solar panels. The most popular technique of all is the thin film technique. Thin film technique is the implementation of an efficient antireflection (AR) coating for solar cell applications. The AR coating reduces the reflection losses of the cells, hence, increasing the conversion efficiency of the solar cell. The initial idea of the AR coating is based on the nanostructure of a moth’s eyes. These nanostructures can reduce the light reflection losses by creating a medium where refractive index increases from one medium to another.

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