energy is another form of energy that can be utilized for generating electricity and is practiced in some countries. It is relatively a clean energy, most concentrated energy and does not require big places/areas. The disadvantage is potential accidental hazards due to control deficiencies, management failures, and possible leaks (nuclear fission process).
Energy from SUN is the most abundant form of renewable energy reaching the earth and is known as Solar Energy. Radiant light and heat emanating from SUN and reaching the earth can be utilized for various purposes. Solar energy is renewable, clean, and green and is a secured type of energy reaching earth 200,000 times more than the electrical energy generated in a day on the earth. Earth receives 174 peta Watts of sun radiation, in the form of 8% UV radiation, 46% visible light, and 46% infrared radiations, and is absorbed by earth atmosphere, oceans, and land mass. Because of the clean, green, and abundant renewable energy coming from sun is found to be more attractive for the researchers to work on the conversion of light energy in to electricity; presently, this has become a globally attractive and potential research domain to devise solar energy trapping units in to usable form of electricity, which can serve global energy requirements. Solar cell or photovoltaic (PV) cell is a device or unit which converts light in to electricity and globally research scientists are making all out efforts to prepare an efficient solar cell with an excellent photo-conversion efficiency, such that it can be developed as viable technology for society. Silicon solar cells are already in the hands of citizens/public having ~26% efficiency with some limitations, and this situation placed research on organic solar cells the most demanding and desirable field.
1.2 Classification of Solar Cells
The possible classification of solar cells is given in Figure 1.1. Among the many (Figure 1.1), organic solar cells attracted rigorous attention because of various advantages like simple preparation of organic solar materials, light weight (low density), low cost, flexibility of the PV modules, semitransparency, easy integration in to other products, low environmental impact, easy adoption of printing technology, and large area of fabrication.
Figure 1.1 Classification of solar cells.
Thin film solar cells are further put in to four categories. The two categories involving fullerenes have found limitations in due course of research investigations, although research was conducted on fullerene-based OSC over two decades. The other two categories, non-fullerene polymer smallmolecule and non-fullerene all small-molecule OSC, based on present scenario, are intensively investigated. Therefore, this chapter will be focused only on these two non-fullerene–based polymer-small-molecule and all small-molecule OSC.
1.3 Solar Cell Structure
Fundamental steps occurring in a schematic representation of a typical solar cell device and its functioning are schematically provided in Figures 1.2 and 1.3. (a) Typical OSC devices based on donor-acceptor in bulk hetero-junction configuration, another way it is the sandwich of active organic blend material in between anode and cathode electrodes with light absorbing property. (b) Donor-acceptor hetero-junction solar cells with basic steps involved: 1) Photo-excitation of the donor-acceptor blend to generate an exciton/excited state [radicalanion/electron–radicalcation/ hole pair bound by ionic and radical (Coulomb) interactions]. 2) Exciton/ excited state diffusion to the donor-acceptor interface. Excitons/excited states that do not reach the inter-face, they recombine and do not contribute to the photocurrent (longer diffusion length, LD). 3) Dissociation of bound excitons at the donor-acceptor interface to form a geminate radical-anion (electron)–radical-cation (hole) pair [increased interfacial charge separation requires optimal energy offset between LUMO (lowest unoccupied molecular orbital) of the donor and LUMO of the acceptor material]. 4) Free charge carrier transport and collection at the external electrodes (require high charge-carrier mobility). (c) Fundamental processes (light illumination, exciton formation, charge separation, charge migration, and charge collection) of bulk-heterojunction solar cells (p = donor material, n = acceptor material).
Figure 1.2 Typical solar cell.
Figure 1.3 Possible events present in BHJOSCs.
1.4 Photovoltaic Parameters or Terminology Used in BHJOSCs
1.4.1 Open-Circuit Voltage Voc
The voltage at which no current flows through a solar cell is called open circuit voltage Voc and it is the maximum voltage available from solar cell. Several studies have demonstrated a strong dependence of Voc on the energy difference ΔE between the HOMO (highest occupied molecular orbital) of donor material and LUMO of acceptor material of an organic solar cell.
1.4.2 Short-Circuit Current Jsc
For V = 0, only the short-circuit current (Jsc) flows through the solar cell. Jsc represents the maximum current that could be obtained in a solar cell. This current depends on the number of absorbed photons, surface area of the photo active layer, device thickness, and charge transport properties of active material, which play important role.
1.4.3 Incident-Photon-to-Current Efficiency (IPCE)
The incident-photon-to-current efficiency is defined as the ratio of the number of incident photons Nphoton and the number of photo induced charge carriers Ncharge which can be extracted out of the solar cell.
1.4.4 Power Conversion Efficiency ηp (PCE)
It is a measure of the quality of the cell which provides evidence of how much power the cell will generate per incident photon. The efficiency ηp is the maximum electrical power Pmax per light input PL.
1.4.5 Fill Factor (FF)
The FF, which determines the quality of solar cell can be obtained from the ratio of the maximum power output to the product of its Voc and Jsc and is always < 1.
1.5 Some Basic Design Principles/Thumb Rules Associated With Organic Materials Required for BHJOSCs
The donor and acceptor molecules to be employed in BHJOSCs must have light absorption property matching the solar region, with high molar absorption coefficients and excellent width at half height of absorption spectrum. It would be best if absorption ranges of donor and acceptor
