**1. Introduction**

Flexible photovoltaics are including the way to cheap electricity. The preparation of organic, inorganic and organic–inorganic solar cells over flexible substrates by often roll-to-roll printing technologies is to provide lightweight and economic solar modules that can be incorporated in various surfaces. Under standard conditions the current conversion efficiencies are in the 3–15% range for potentially flexible, thin film devices. Meanwhile heavy, stiff, and fragile inorganic materials can exceed 30% efficiency [1, 2], but in real applications the overall productivity is high. In 2012, Luke F. Lester et al. [1] discussed the preliminary studies on the integration of GaAs-based InAs/InGaAs quantum dots-in-a-well (DWELL) solar cells onto flexible surfaces such as Kapton and nanopaper including weight considerations for all the integrated materials. The 2 billion people who lack access to the grid as well as to energy-eager companies and families in the industrialized world facing the increasing costs of electricity generated using fossil fuel resources can get cheap and clean electricity by these new photovoltaic technologies [2].

Progress of flexible and lightweight solar cell is fascinating for many terrestrial and space applications that require a very high specific power. More than 2 kW\kg specific power can be produced by thin-film solar cells on polymer films. They are

**Figure 1.** *Picture of a solar cell fabricated on a flexible substrate [4].*

important for the development of novel products, such as lightweight and portable sources of power for emergencies and recreational use, photovoltaics incorporated buildings (roof and facades), consumer electronics (smart cards, data and telecommunication products), and solar boats and cars, etc. [3]. **Figure 1** shows a solar cell fabricated on a flexible substrate [4].

Flexible solar cells are proposed to accelerate a numerous of possibilities for providing new applications in consumer electronics and space satellites. Organic and amorphous semiconductors are very important materials to achieve flexible and light-weight solar cells, essentially due to their strong light absorption properties, process temperature compatibility with flexible substrates and potentially cheap processing cost. Due to the highly disordered and defective crystalline structure in these materials, the poor minority carrier lifetime prevent their use for making high efficiency and reliable solar cells [5].

The replacement of the traditional rigid glass plate substrate with plastic or metallic foils has been concentrated by the recent research in thin-film electronics. Metallic materials, stainless steel and molybdenum foils have been used as substrates in the fabrication of thin-film transistors. A number of plastic materials (organic polymers) also have been verified successfully in a variety of thin-film applications [6]. The glass substrate may contribute to more than 90% of the total weight of the solar cells. The glass substrate should be substituted with a lightweight and flexible thin substrate, such as metal or polymer foils to maximize the high specific power. This gives flexibility to the solar panels to change to any kind of shape for incorporation in buildings, and for application in a variety of products. Flexible solar modules can help low cost and easily deployable power generators in space. Solar cells with AM1.5 efficiency of 11.4% on foils (highest efficiency recorded for flexible CdTe cell) have been developed. A comparison of the cells prepared on different polyimides is presented by A. Romeo et al. [7]. Plastic substrates solar cells can also well used in the solar car because of those characteristic [4]. Inexpensive solar cells would help game reserve the environment. Coating existing roofing materials with its plastic photovoltaic cells which are inexpensive enough to cover a home's entire roof with solar cells, then enough energy could be captured to power almost the entire house. Then, the dependence on the electric grid (fossil fuels) would decrease and help to reduce pollution [8].
