Solar cells

Chapter 1: Introduction

Thin-film solar cells are one of the promising candidates for the next generation of solar cells among various approaches. There are many techniques to achieve thin-film solar cells, and layer-transfer processes have a promising opportunity. An epi-free method combined with reorganization of macro pores is one of these layer-transfer methods. However, the epi-free process involves optical lithography, which is still an expensive and time-consuming step. If we can replace this step by another advanced lithography process, it will save a significant amount of energy and cost. Various alternative lithography techniques are under development and, among the various approaches, nano imprint lithography reveals a higher potential than others to replace the conventional optical lithography. The different points will be discussed in this introduction chapter.

1.1 Solar cells

Developing green energies, which come from natural resources such as sunlight, rain, wind, tides and geothermal heat, is one of the best ways making the environment better.

Compared to conventional energies, green energies are cleaner, and renewable unlike fossil fuels which keep harming ther world. Moreover, oil prices reach records year by year, and will definitely not depreciate in the future. In other words, we need to focus on sustainable energies to replace fossil fuels. All of the above issues push scientists into developing a new energy, which can be inexhaustible, and affordable.

Solar cells are a promising candidate for green energy, because their energy is directly derived from the sun, which is a long lasting source of energy, available almost everywhere. Utilization of solar energy creates no pollution except in the production and maintenance of solar cells lead to some amount of pollution, and this is the most important advantage that makes it more practical than conventional energy like oil, which will release greenhouses gases into our air when it burns.

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The mechanism of solar cells can be simply expressed as following. Photons from the sunlight hit the photovoltaic (PV) cells and are absorbed. Then, electrons are released from their atoms, allowing them to flow through the cell to produce electricity.

Different cells are connected together in series and encapsulated into modules. Later, these modules can be constructed into an array to provide a larger current and voltage.

There are many different types of solar cells, and (multi-)crystalline silicon solar cells are current industry leader. The main reason is that crystalline silicon solar cell can offer higher conversion efficiency, but the high cost of fabricating them is a main issue.

As a result, scientists keep focus on researching new kinds of solar cells offering more affordable price. Based on the tradeoff between cost and efficiency, thin-film solar cells are gaining interest.

1.1.1 Thin-film solar cells (TFSC)

Thin-film solar cells (TFSC) are a promising approach to achieve a low price, and can be applied on variety of different substrates. TFSC cover a wide range of technologies, and we will focus on the issue of thin-film crystalline silicon. Except the silicon based TFSC, other two-component (binary) materials attractive for thin-film solar cells are: GaAs, CdTe, Cu₂S, Cu₂O, InP, Zn₃P₂, etc, but the scarcity of these components and even their toxicity limit their developments [1-1,1-2]. The other thin film technologies, organics and dye-sensitised cells present immense impact on the TFSC industry in the future due to their inexpensive processing and flexible applications [1-3, 1-4]. However, silicon based solar cells have their irreplaceable advantages over the other thin film technologies on its abundance of material, non-toxicity and great connection with the current semiconductor industry.

Unfortunately, the thickness of this thin crystalline layer cannot easily be obtained from current wafer techniques.

Hence, there are two ways to make a thin crystalline film; one is to directly deposit a crystalline layer on a substrate, and another is to transfer a thin crystalline layer from a thick wafer to a foreign. The first method requires the use of the epitaxy [1-5] that refers to the method of depositing single-crystalline film on a single-crystalline

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substrate. Although epitaxy is capable of producing high-quality layers, it is low-throughput and high temperature requirements for high-quality layers represent a bottleneck for industrial processes. The latter method, on the other hand, does not require epitaxial deposition and therefore presents potentially a better tradeoff between energy conversion efficiency and cost than the other techniques. A layer transfer process based on the empty-space in silicon (ESS) technique [1-6] would form a high-quality crystalline layer without the step of epitaxy. Therefore, we call this process “Epi-free” method. A plate-shaped ESS below the surface of the silicon can be fabricated by connecting the spherical empty spaces (Figure 1.1-(c)), which are formed by surface migration of Si on the patterned Si substrate upon annealing, and this surface layer above the empty-space is potentially detachable.

1.1.2 Decreasing the cost of the epi-free method

The whole solar-cell process based on the epi-free techniques is composed of the following steps (Figure 1.2). (1): Formation of regular macropores by deep-ultra-violet (DUV) lithography combined a dry etching process. (2): Annealing of these pores for formation of a detachable and singlecrystalline thin film. (3): Solar-cell processing of the first side of the film. (4): Bonding and detachment. (5): Processing of the second side.

Avoiding an expensive process like optical lithography with DUV light as source for forming regular nanostructures is the main idea of this thesis, because the cost of lithography equipment constitutes 25% to 35% of the cost of all semiconductor fabrication equipment [1-8]. DUV lithography equipment costs over $ 10 million, and the cost of lithography equipment has increased at a nearly exponential rate over the past 30 years [1-9]. The limited throughput of DUV lithography is another major disadvantage of this process. If we want to replace it, we need to solve uniformity, resolution and throughput issues. Therefore, I will discuss some possible solutions which are considered as low-cost lithography in the next section (1.2).

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