**3. Organic-inorganic photovoltaic solar cells (hybrid)**

Organic and organic-inorganic photovoltaics (PVs) (third generation solar cells) follows the second generation (thin film inorganics such as amorphous silicon (a-Si), cadmium telluride (CdTe), and copper indium gallium selenide (CIGS)) and first generation (semiconducting, crystalline) PVs. Third generation solar cells continues to attract great attention from the PV community, due to their promising features such as low fabrication cost, flexibility and light weight. The organic PVs include devices with flat and bulk heterojunction between the various types of conjugated polymers, small molecules, fullerene derivatives, and carbon nanotubes. Organic PVs are still unable to overcome the high 6–7% barrier of conversion efficiency, despite considerable progress in solar cell architecture, design and rational choice of the donor-acceptor materials, [19].

The heterojunction in organic-inorganic hybrid solar cells is formed between inorganic semiconductors and organic compounds (small molecules, oligomers, polymers, carbon nanotubes). The hybrid PVs has higher carrier mobility of the semiconductor and the light absorption at longer wavelengths than for organic compounds. Whereas, the organic component allows hybrid solar cells to be superior over conventional semiconducting PVs in terms of cost efficiency, scalable wet processing, the variety of organic materials (mismatch between inorganic components can be minimized or prevented), light weight, and flexibility. The progress in advanced semiconducting nanostructures in combination with organic nanomaterials (fullerenes and carbon nanotubes) opens the way to overcome the 10% barrier of conversion efficiency for hybrid solar cells. Although the band engineering of the hybrid solar cell is not as facile as for semiconducting PVs, but it is a useful instrument in the design of the hybrid solar cell architecture. For example, the chemical functionalizing of the organic component effects on the band gap energy and position of the Fermi level for conducting polymers and small molecules. **Figure 4** illustrates the types of hybrid PVs depending on the nature of organic and inorganic component and the morphology of the devices [20].

One potential alternative to crystalline silicon PV cells is cells made from thin films (<1 μm) of conjugated (semiconducting) polymers, which can easily be cast onto flexible substrates over a large area using wet-processing techniques. Polymer or hybrid solar cells often utilize a nanostructured interpenetrating network of electron-donor and electron-acceptor materials. The hybrid polymer solar cells using blends of the conjugated polymer and inorganic materials to convert sunlight into electricity. These devices will combine the advantages of two materials, high electron mobility and photosensitivity of inorganic semiconductors, and high hole

**417**

*Mechanism for Flexible Solar Cells*

**Figure 4.**

*Classification of hybrid solar cells [20].*

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

mobility of conjugated polymers. Due to the poor interfacial junction between the organic and inorganic materials, the power conversion efficiency of the hybrid photovoltaic devices is still very low. Improving the heterojunction between two materials is concentrated by many researchers. Therefore, an important method is to use two materials having complementary operation of the p-type and n-type

In bulk heterojunction, electron accepting nanoparticles are mixed with the electron donating polymer and the exciton created in the polymer material diffuse to the donor-accepter interface for charge separation. Bulk heterojunction solar cell are preferred than multilayer or heterojunction polymer solar cells, because of the binding energy of the polymeric excitons, which is in the range of 0.2 eV-0.4 eV and that is considerably higher than the binding energy for inorganic semiconductor materials. Also, the life time of the exciton in the conjugated polymer is about sub-nanoseconds and the small diffusion range which is about (5–10) nm. After absorption of light and for efficient charge generation, each exciton has to find a donor – acceptor interface within femtoseconds within few nano-meter, otherwise it will be lost without charge generation. Because of these properties, a poor efficiency results by the heterojunction or multilayer organic photovoltaic devices. To solve this problem the semiconducting nanoparticles are incorporated into the polymer matrices since polymer materials phase separate on a nanometer dimension. The Junctions throughout the bulk of the material are created due to the mixing of the p and n type materials that ensure quantitative dissociation of photo generated excitons irrespective of the thickness. In such hybrid materials, organic polymer acts as electron donor and the inorganic nanoparticles as an electron acceptor. So that the positive charges move by hole hopping, and the negative charges by electron hopping via charge transfer between molecules. It is obvious from this mechanism; that both the organic and inorganic materials contribute to

Organic-inorganic hybrid solar cells are typically thin film devices consisting out of photoactive layer(s) between two electrodes of different work functions. The anode which is often with high work function is a conductive and transparent indium tin oxide (ITO) on a flexible plastic or glass substrate. The photoactive light absorbing thin film consists out of a conjugated polymer as organic part and

electronic properties for the hybrid polymer solar cell [21].

the photocurrent in hybrid bulk heterojunction cells [22].

*Solar Cells - Theory, Materials and Recent Advances*

organic thin film solar cells as shown in **Table 1** [18].

**3. Organic-inorganic photovoltaic solar cells (hybrid)**

on casting conditions [16, 17].

the donor-acceptor materials, [19].

devices at a maximum wavelength reached more than 70% for P3HT:PC61BM solar cells, compared to ~50% for MDMOPPV: PC61BM solar cells [15]. Because of the semicrystalline nature of P3HT spin-cast films, it has among the highest reported field-effect transistor mobilities for a conjugated polymer. The morphology and the mobility of pure rr-P3HT and blended rr-P3HT: PCBM films are highly dependent

The flexible side chain of the P3HT molecule introduces a good solubility in organic solvents in spite of its kind as stiff polymers. The long and narrow fibrils which are produced by P3HT crystalizes are from a network that is able to create a good percolation paths for the charge carriers' transportation. This leads to high carrier mobility. Differing from the P3HT, PCBM is a derivative of fullerene; it has a high electron affinity which makes it a qualified electron acceptor material in the

Organic and organic-inorganic photovoltaics (PVs) (third generation solar cells) follows the second generation (thin film inorganics such as amorphous silicon (a-Si), cadmium telluride (CdTe), and copper indium gallium selenide (CIGS)) and first generation (semiconducting, crystalline) PVs. Third generation solar cells continues to attract great attention from the PV community, due to their promising features such as low fabrication cost, flexibility and light weight. The organic PVs include devices with flat and bulk heterojunction between the various types of conjugated polymers, small molecules, fullerene derivatives, and carbon nanotubes. Organic PVs are still unable to overcome the high 6–7% barrier of conversion efficiency, despite considerable progress in solar cell architecture, design and rational choice of

The heterojunction in organic-inorganic hybrid solar cells is formed between inorganic semiconductors and organic compounds (small molecules, oligomers, polymers, carbon nanotubes). The hybrid PVs has higher carrier mobility of the semiconductor and the light absorption at longer wavelengths than for organic compounds. Whereas, the organic component allows hybrid solar cells to be superior over conventional semiconducting PVs in terms of cost efficiency, scalable wet processing, the variety of organic materials (mismatch between inorganic components can be minimized or prevented), light weight, and flexibility. The progress in advanced semiconducting nanostructures in combination with organic nanomaterials (fullerenes and carbon nanotubes) opens the way to overcome the 10% barrier of conversion efficiency for hybrid solar cells. Although the band engineering of the hybrid solar cell is not as facile as for semiconducting PVs, but it is a useful instrument in the design of the hybrid solar cell architecture. For example, the chemical functionalizing of the organic component effects on the band gap energy and position of the Fermi level for conducting polymers and small molecules. **Figure 4** illustrates the types of hybrid PVs depending on the nature of

organic and inorganic component and the morphology of the devices [20].

One potential alternative to crystalline silicon PV cells is cells made from thin films (<1 μm) of conjugated (semiconducting) polymers, which can easily be cast onto flexible substrates over a large area using wet-processing techniques. Polymer or hybrid solar cells often utilize a nanostructured interpenetrating network of electron-donor and electron-acceptor materials. The hybrid polymer solar cells using blends of the conjugated polymer and inorganic materials to convert sunlight into electricity. These devices will combine the advantages of two materials, high electron mobility and photosensitivity of inorganic semiconductors, and high hole

**416**

#### **Figure 4.** *Classification of hybrid solar cells [20].*

mobility of conjugated polymers. Due to the poor interfacial junction between the organic and inorganic materials, the power conversion efficiency of the hybrid photovoltaic devices is still very low. Improving the heterojunction between two materials is concentrated by many researchers. Therefore, an important method is to use two materials having complementary operation of the p-type and n-type electronic properties for the hybrid polymer solar cell [21].

In bulk heterojunction, electron accepting nanoparticles are mixed with the electron donating polymer and the exciton created in the polymer material diffuse to the donor-accepter interface for charge separation. Bulk heterojunction solar cell are preferred than multilayer or heterojunction polymer solar cells, because of the binding energy of the polymeric excitons, which is in the range of 0.2 eV-0.4 eV and that is considerably higher than the binding energy for inorganic semiconductor materials. Also, the life time of the exciton in the conjugated polymer is about sub-nanoseconds and the small diffusion range which is about (5–10) nm. After absorption of light and for efficient charge generation, each exciton has to find a donor – acceptor interface within femtoseconds within few nano-meter, otherwise it will be lost without charge generation. Because of these properties, a poor efficiency results by the heterojunction or multilayer organic photovoltaic devices. To solve this problem the semiconducting nanoparticles are incorporated into the polymer matrices since polymer materials phase separate on a nanometer dimension. The Junctions throughout the bulk of the material are created due to the mixing of the p and n type materials that ensure quantitative dissociation of photo generated excitons irrespective of the thickness. In such hybrid materials, organic polymer acts as electron donor and the inorganic nanoparticles as an electron acceptor. So that the positive charges move by hole hopping, and the negative charges by electron hopping via charge transfer between molecules. It is obvious from this mechanism; that both the organic and inorganic materials contribute to the photocurrent in hybrid bulk heterojunction cells [22].

Organic-inorganic hybrid solar cells are typically thin film devices consisting out of photoactive layer(s) between two electrodes of different work functions. The anode which is often with high work function is a conductive and transparent indium tin oxide (ITO) on a flexible plastic or glass substrate. The photoactive light absorbing thin film consists out of a conjugated polymer as organic part and an inorganic part out of e.g. semiconducting nanocrystals [23]. A large number of various semiconductors have been investigated for organic–inorganic hybrid solar cells. For example, titanium dioxide (TiO2) nanoparticles have been extensively employed due to their high power conversion efficiency. Also, Zinc sulfide (ZnS) is another promising semiconductor, but it has been less studied. A top metal electrode (e.g. Al, LiF/Al, Ca/Al) is vacuum deposited onto the photoactive layer finally. **Figure 5** is a schematic illustration of a typical device structure. For photoactive layers there are two different structure types, the bilayer structure and the bulk heterojunction structure. The bulk heterojunction structure is usually realized by just blending the donor and acceptor materials and depositing the blend on a substrate [24].

The charge separation process at the donor- acceptor interface in a hybrid solar cell is shown in **Figure 6**. The major photovoltaic steps include: photo-excitation

#### **Figure 5.**

*A schematic illustration of typical device structures for hybrid solar cells [24].*

#### **Figure 6.**

*A schematic diagram of the charge separation process at the donor-acceptor interface in a hybrid solar cell [25].*

**419**

**Author details**

Ghaida Salman Muhammed

provided the original work is properly cited.

Department of Physics, College of Science, University of Baghdad, Baghdad, Iraq

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

\*Address all correspondence to: ghaidasalman7@gmail.com

*Mechanism for Flexible Solar Cells*

used to get rid of this dependency.

diffusion of these technologies [2].

**4. Conclusions**

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

reduction of the photovoltaic conversion efficiency [25].

remarkable properties, functionality and phenomena.

into excitons (1), excitons migration to interfaces (2), charge transfer from the donor to the acceptor at the interface (3), charge migration to electrodes (4) and charge injections into electrodes. The recombination of the excitons in the donor and separated charges at the interfaces are the processes which resulting in the

Nano-science is defined as the study of small dimensions materials that exhibit

Nanotechnology controls current progress in chemistry, physics, material science, biotechnology and electronics to create novel materials that have unique properties because their structures are determined on the nanometer scale [26]. The world needs to curb CO2 emissions soon and reduce our dependence on expensive hydrocarbons; thus, a renewable materials and solar energy on a massive scale are required. Therefore, flexible photovoltaic modules will be among the main tools

This chapter has highlighted the advancements that have been made for the fabrication of flexible solar cell and the progress in this field, aimed at facilitating into excitons (1), excitons migration to interfaces (2), charge transfer from the donor to the acceptor at the interface (3), charge migration to electrodes (4) and charge injections into electrodes. The recombination of the excitons in the donor and separated charges at the interfaces are the processes which resulting in the reduction of the photovoltaic conversion efficiency [25].
