# Overview on Different Types of Solar Cells: An Update
## 2 Organic Solar Cells
### Materials for Organic Solar Cells  
Organic materials for OSC can be derivative of small molecules or semiconducting polymers [28]. Organic semiconductors have conjugated organic moieties such as dendrimers, pigments, dyes, polymers, and small molecules, which can be a good choice for OSC. Any OSC requires two types of materials: an electron pair donor and an electron pair acceptor. Conjugated polymers are employed as donor materials. Initially, poly(1,4-phenylenevinylene) (PPV) and its derivatives gained considerable attention due to their appropriate optoelectronic properties [29; 30]. Later, polythiophene and its derivative, especially poly(3-hexylthiophene), P3 HT, began to be used widely (as a donor material) owing to its stability, high carrier mobility, and solubility [31; 32; 33]. The structures of some important donor materials for OSC technology are listed in Figure 1. Although fullerene and its derivatives dominated OSC technology in the initial phase [34; 35; 36; 37; 38], small molecules such as 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(4 hexylphenyl))-dithieno [2,3-d:2\({}^{\prime}\),3\({}^{\prime}\)-d\({}^{\prime}\)]-s-indaceno [1,2-b:5,6-b\({}^{\prime}\)]dithiophene (ITIC), IHIC, and PCDTBT (termed as non-fullerene acceptors, NFA) have attracted considerable attention recently due to their superior properties [39; 40; 41; 42]. Moreover, donor-acceptor conjugated polymers such as poly [2,6-(4,4-bis-(2-ethylhexyl)-4\(H\)-cyclopenta [2,1-b;3,4-b\({}^{\prime}\)]dithiophene)-_all_-4,7(2,1,3-benzothiadiazole)], PCPDTBT [43], and poly [2,7-(9,9-dioctylcarbazole)-alt-4,7-bis(thiophene-2-yl)benzo-2,1,3-thiadiazole], PCzDTBT [44] have been employed. The structures of some important fullerene and non-fullerene acceptor materials are depicted in Figure 1.  
The engineering of the input donor and acceptor materials can help in improving the optoelectronic properties of these materials, which, in turn, improve the performance of OSC. One of the major limitations of the donor materials for OSC is their limited capacity of absorbing the whole wavelength range of the solar spectrum. The absorption efficiency can be improved by increasing the HOMO level, decreasing the LUMO level, or by compressing the band gap between HOMO-LUMO levels [45; 46]. Polymers have higher molar mass and possess longer conjugation lengths, and hence the energy band gap is lower [47; 48; 49]. Voltametric p-type doping can also help in improving the optoelectronic properties through broadening of the absorption spectrum [50; 51; 52; 53]. Regioregularity of polymers facilitates the  
Figure 1: The structures of some important donor and acceptor materials of OSC. mobility of excitons and thus improve the performance of OSC [54; 55; 56]. Physiochemical properties of OSC materials can be easily tuned by changing their chemistry, which is a unique advantage compared with conventional inorganic solar cells. For instance, the introduction of flexible side-chains such as alkyl or alkoxy leads to an improvement of solubility while the introduction of electron-donating or -withdrawing substituents can help in the alignment of electronic energy levels. The improvement of the optoelectronic properties of polymers by chemical modifications is shown for several polymers such as poly(p-phenylene vinylene) [57] and P3 HT [58; 59; 60]. Electron-withdrawing groups such as Fluorine on the polymer chain improve the charge transference by tuning of the LUMO level [61], while electron-donating substituents on the polymer chain enhance charge-carrier mobility by minimizing the structural defects due to an increase in the HOMO energy level [62; 63; 64]. Moreover, large donor-acceptor interfaces as provided by nanostructures render effective exciton dissociation and efficient charge transport [65; 66; 67; 68; 69; 70; 71; 72]. Conjugated polymer-based metallic nanoparticles can be prepared by several methods [73; 74; 75].