*Mechanism for Flexible Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.93818*

*Solar Cells - Theory, Materials and Recent Advances*

fabricated on a flexible substrate [4].

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

**Figure 1.**

efficiency and reliable solar cells [5].

**2. Organic semiconductors**

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

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

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].

The toleration of the ability to develop a long-term technology that is economically active for large-scale power generation based on environmentally safe

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materials with unlimited availability is caused by the organic materials. The organic semiconductors are less expensive materials than the inorganic semiconductors like Si; they have high optical absorption coefficients which offer the opportunity for the production of very thin solar cells. Also, thin flexible devices can be fabricated using high throughput and low temperature approaches that employ well established printing techniques in a roll-to-roll process. The electronic structure of all organic semiconductors is based on conjugated π-alternation between single and double carbon-carbon bonds. Single bonds are known as σ-bonds and are associated with localized electrons, and double bonds contain a σ-bond and a π-bond. The π-electrons are much more mobile than the σ-electrons; they can jump from site to site between carbon atoms thanks to the mutual overlap of π orbital's along the conjugation path, which causes the wave functions to delocalize over the conjugated backbone. The π-bands are either empty (called the Lowest Unoccupied Molecular Orbital (LUMO)) or filled with electrons (called the Highest Occupied Molecular Orbital (HOMO)). The band gap of these materials ranges from 1 to 4 eV. This π-electron system has all the essential electronic features of organic materials: light absorption and emission, charge generation and transport [9]. Also, molecular orbitals which form σ and π bonds represent the energy levels for organic semiconductor materials [10].

The denoted bonding molecular orbitals (σ and π) form the highest molecular orbital (highest energy levels) where the denoted anti-bonding molecular orbitals (σ\* and π\*) form the lowest molecular orbitals (lowest energy levels). These molecular orbitals are similar to energy bands levels in inorganic materials. **Figure 2** shows the method of creating energy gap levels in organic semiconductor [11].

The anti-bonding π\* molecular orbitals (conduction band) joined the π bonding molecular orbitals (valance band) to create the Lowest Unoccupied Molecular Orbital (LUMO) and Highest Occupied Molecular Orbital (HOMO). The gap between the (LUMO) and (HOMO) is the energy gap where the conductivity in organic semiconductor depends on. Thus, from **Figure 2** it is clear that σ bonds are extremely filled with electrons where π bonds are empty. On the other hand, if the energy band gap becomes as small as possible the tolerance of electrons to move from the (HOMO) to (LUMO) increases. Some organic semiconductors have a very small band gap of <2 eV, which mean that it is good materials compared to some inorganic semiconductors, which haves a large energy band gap [10].

**Figure 2.** *The energy levels in organic semiconductors [11].*

In organics the small radius excitons generated as a result of photon absorption is the source of photocarriers. Excitons in organics have a binding energy in the range 200–400 meV, which is significantly higher than the binding exciton energy for semiconductor materials ~2–40 meV. The exciton dissociation should occur at the interface between the donor and acceptor materials with suitable (HOMO) and (LUMO) energy levels because that thermal energy at room temperature is not sufficient (~25 meV) for exciton dissociation to hole and electron in the bulk medium. The mechanism of exciton dissociation is not completely known, however, the charge transfer process between the donor and acceptor components is the major factor controlling the charge separation at the interface. After charge separation, holes and electrons move to the opposite electrodes because of drift and diffusion [12].

The working principle of a polymer bulk heterojunction (BHJ) solar cell as shown in **Figure 3** is summarized by the creation of an exciton in the active layer, due to light absorption, and then this exciton will separate into two charge carriers at the interfaces between the species that constitute the active layer (typically, a binary blend of a polymer and a fullerene or two polymers, which act as the donor phase and the acceptor phase), with subsequent collection by the electrodes. To get efficient steps, all of these steps must follow very strict limitations. At first, the generated excitons must hop between the molecules reaching an interface between the two phases before recombining (radiatively or non-radiatively). This means that the two phases should be mixed in an optimal structure, with phase domains usually in the order of (10–30) nm (the average exciton diffusion length in polymers). Then, the position of the energy levels at the interface must be favorable for a fast exciton dissociation followed by charge separation (*i.e.*, the electron in the acceptor phase and the hole in the donor phase without successive recombination). After that, the charges must travel inside the respective phases, reaching the collecting electrodes again without a charge recombination: at this point, the energetic level structure at the electrode interfaces plays an essential role, ideally the interface being an ohmic contact [13].

The conjugated polymer-PCBM bulk heterojunction is currently the best conjugated polymer-based PV cell. One significant improvement to this device structure was made recently by many researchers, who found that the morphology of the blend could be optimized by casting the polymer and PCBM from a solvent that prevents long-range phase separation and enhances the polymer chain packing. This

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**Table 1.**

*Organic molecular structure [18].*

*Mechanism for Flexible Solar Cells*

conditions [14].

*DOI: http://dx.doi.org/10.5772/intechopen.93818*

avoids the formation of isolated regions of polymer and PCBM in the film and gives the polymer increased hole mobility. This fabricates a device with more than double the EQE of the previous best device and with 2.5% power efficiency under AM1.5G

For organic solar cells, the magnitude of JSC, VOC, and FF depends on parameters such as: light intensity, temperature, composition of the components, thickness of the active layer, the choice of electrodes used, and the solid state morphology of the film. A clear understanding of the device operation and photocurrent Jph generation and its limitations in these devices are required for their optimization and maximization. In order to allow for further design of new materials that can improve the efficiency of this type of solar cells, the relation between the experimental Jph and material parameters (charge-carrier mobility, band gap, molecular energy levels, or

Two intrinsic issues can be resolved by the bulk heterojunction (BHJ) structure, charge separation and charge transport, in organic layers. These are representative structures; though, they have a fundamental limitation in terms of open circuit voltage (VOC) that is basically determined by the offset energy between the highest occupied molecular orbital (HOMO) of p-type organic semiconductors and the lowest unoccupied molecular orbital (LUMO) of n-type organic semiconductors, even though the work functions of electrodes often affect VOC. A tandem structure can be applied to maximize the power conversion efficiency in organic solar cells by increasing VOC, because the overall voltage becomes the sum of individual VOC values in each sub cell (front and back cells). In addition, the selecting complementary BHJ layers enhances the overall short circuit current density (JSC) of tandem cells, because these layers have different absorption ranges for maximizing solar light harvesting. An adverse effect is also present owing to marginally increased electrical resistances by the presence of additional interfaces and active layers in series connection. As illustrated previously, both normal- type and inverted-type structures are possible by placing suitable electrodes with appropriate work functions on each side. To date, the most popular bulk heterojunction structure is made with the composites of conjugated polymers (p-type) and fullerene derivatives (n-type), leading to polymer:fullerene solar cells. Hence, P3HT polymers have been introduced because they can absorb visible light up to 650 nm and their glass transition temperature approaches ~110C°. The external quantum efficiency (EQE) of

**Material Molecular structure HOMO (eV) LUMO (eV) Carrier mobility** 

P3HT 5.2 3.2 μh = 2 × 10−4

PCPDTBT 4.9 3.5 μh = 2 × 10−2

PCBM 6 4.2 μe = 3 × 10−3

**(cm2 /Vs)\***

relative dielectric constant) needs to be understood and controlled [9].
