Small Molecule Organic Solar Cells
Cost reduction is the main factor that makes silicon solar cells most suitable for electricity generation. But comparatively, 50% of silicon is wasted during ingot preparation. This initiates the search for new solar harvesting materials. The main advantages of carbon technology are its transparency and lightweight properties [8]. The three factors that determine the materials used in energy devices are high efficiency, long life and low cost. But organic photovoltaic cells are found to have certain limitations in these criteria. So, to meet the criteria, an alternative option called Organic Photo Voltaic (OPV)-based oligomers is suggested, which can be normally implanted using vacuum sublimation [9].
Carbon atoms are the fundamental units of small molecule organic solar cells (OSCs) which have 100 complex atoms compared to atomic materials and have a weight of less than l,000amu. Based on van der Waals force, a solid crystal organic molecular complex is formed, and this is represented by the highest occupied molecular orbital and the lowest occupied molecular orbital, and the corresponding energy gap exists between these two orbitals [10]. Sandwiching two electrodes between two organic materials forms an OSC. In general, these devices are fabricated in the form of flat heterojunctions where the conversion of photons into electrons happens [11].
FIGURE 8.5 Different Configurations of OSC.
The various geometries of an OSC are shown in Figure 8.5. From the configurations, one can understand that the donor electrode is the light absorbing material, whereas the second electrode acts as an acceptor. By using bulk heterojunctions (BHS), efficiency can further be improved, but the thermodynamics complexity level will be increased leading to energetic and structural disorders. This is considered as a major drawback of BHJ OSCs. These problems can be resolved by increasing the interface area and using an enclosed communication channel [12].
In general, the efficiency level of a small molecule OSC is 13.2% when processed under vacuum. These molecules have certain merits in all aspects such as molecular structure, purity level, molecular weight and also morphological control. Merocyanines (MCs), phthalocyanines (Pcs), borondipyrromethenes (BODIPYs), diindenoperylene (DIP) and oligothiophenes are a few materials used for small molecule solar cells [13]. The molecular structures of various materials used in the manufacture of OSCs are depicted in Figure 8.6. MCs are high polarization and high absorption materials that possess high thermal stability and a good absorption level when phenyl-C61-butyric acid methyl ester (PCBM) is used as the active material instead of the C60 acceptor. In organic electronics, phthalocyanines (Pcs) are another important class of materials which play a vital role in terms of stability and synthetic versatility [14].
Other OSC materials include BODIPYs, which have a better absorption coefficient and photostability. These derivatives have a reasonable near infrared range that is capable of harvesting more photons even at small energy levels of the spectrum. A very simple molecular structure and enhanced mobility of charge carriers make DIP and its derivatives the most suitable candidates for organic electronic applications like OLED. At the same time, a low' extinction coefficient is a major drawback of these devices, and this results in reduced photocurrents of OSCs [15].
FIGURE 8.6 Molecular structure of (a) Merocyanines derivative HB194 (b) MPCs (c) donor molecule BDTT-BODIPY and (d) donor molecule DIP.
FIGURE 8.7 Constructional Structure of PSC.
Polymer Solar Cells
Polymer solar cells (PSCs) have a very simple construction in which an active layer is sandwiched between a cathode and an anode. The active layer is designed with an acceptor and donor combination that performs functions such as formation of an electric field internally in the device and generation of charge carriers. In the transportation of charge carriers, the following layers called the hole transport or extracting layer (HTL or HEL) and an electron transport or extracting layer (ETL or EEL) help the active layer during charge transformation [16]. The construction structure of a simple PSC is shown in Figure 8.7.
PSCs are an alternative option for OSCs and are solution processable. They contain polymers and fullerene. For both single and multiple cells, the efficiency is about 12% which helps in more production. In previous decades, solar cells were made using bilayers of polymer and buckminsterfullerene. Performance efficiencies of these devices were pretty low owing to the fact that generation of charges can occur only at the interfaces. This problem is resolved, and the efficiency is increased by using soluble fullerene derivatives (PC61BM) [17]. The fundamental principle of PSCs is that the absorption takes place through a larger transparent electrode active material, and the photons are reflected by the electrode surface when the source light falls on it [18]. Opto-electric features are fundamental properties that are mainly used in active layers for better performance.
Conjugated polymers are generally used as donor derivative materials because of their rich characteristics such as robustness, flexibility and less weight. Polythiophene (PT) derivatives are the most preferable polymers among which poly-3-hexylthiophene (P3HT) is the most used polymer in the design of PSCs [19]. The molecular structure of a poly-3-hexylthiophene is shown in Figure 8.8. The next prominent donor material is poly-p-phenylenevinylenes (PPVs) with very high solubility. These are highly processable polymers and can be made more suitable for photovoltaic applications. A PSC using a PC61BM active layer has a Power Conversion Efficiency (PCE) value range of 1.1%-1.3%. Other than this, important materials used as PSC-donor materials are D-A conjugated polymers and D-A copolymers [20]. The molecular structure of a PPV is shown in Figure 8.9.
FIGURE 8.8 Molecular structure of poly 3-hexylthiophene.
FIGURE 8.9 Molecular structure of poly-p-phenylenevinylene.
There are many acceptor materials used in designing PSCs. Among these, fuller- enes play a vital role because of their electron mobility features and efficient charge transfer mechanisms. Under ideal conditions, they have good solubility in many solvents, and these are the most suitable donor materials to increase the open circuit voltage of a device. The main drawbacks of fullerene-based derivatives are energy loss, which is about 0.6 eV, and their lower absorption power [21]. To overcome this, non-fullerene derivatives are also preferred for PSCs. Perylene diimide (PDI) is a non-fullerene derivative which is mostly preferred as an acceptor material because it can be made as small-sized crystals during the process. Other significant materials that are closely associated with PSCs are polymer/polymer blend materials [22].
