**1. Introduction**

According to the International Energy Agency, the world utilized 109613 TWh of energy in 2014, with fossil fuels accounting for 80% of that total. The depletion of fossil fuels takes 40 years for oil, 60 years for natural gas, and 200 years for coal. The use of fossil fuels and nuclear energy directly affects the environment by emitting greenhouse gases, causing climatic changes such as global warming, which cause malaria and famine [1]. If adequately developed, these sources can diversify energy supply, while wind, biomass, and geothermal cannot fully replace fossil fuels [2]. Solar energy is unique among renewable energy sources because it delivers 711019 Kcal each year, 10,000 times more than the world consumes. Solar energy is widely used in turning sunshine into useable energy for various applications such as solar water heating, solar transportation, solar ventilation, and solar electricity [3].

Photovoltaic (PV) systems convert sunlight directly into electricity, providing a practical and straightforward option to sustain the growing energy demand. Currently, solar cell research and development focuses on (i) the maturity of silicon solar cells. However, the complex manufacturing process, high energy

consumption, and high cost of classic silicon-based solar cells have slowed their progress. (ii) TFT solar cells GaAs [4]. CIGS [5], and CdTe [6] are examples of thinfilm solar cells. Although thin-film solar cells are efficient and stable, gallium and indium are scarce in the crust, and cadmium is poisonous. (iii) New solar cells A new generation of solar cells include organic photovoltaics (OPV) DSSC [7] and perovskite solar cells (PSC) [4, 8, 9]. These solar cells are light and cheap. However, various issues prevent wide-scale application.

Although PSC, photoelectric conversion efficiency (PCE) has increased from 3.8 to 23.3 per cent, device stability has remained an issue limiting applications [10]. Evaporating silicon solar cells require advanced high-temperature manufacturing, high-quality silicon, and intricate energy sources. These cells are also mechanically rigid. Polymer solar cells are a low-cost alternative to silicon solar cells since they may be made by painting or printing [11]. Polymer cells are cheap, light, and flexible [12]. Since 1992, when Sariciftci et al. [13] demonstrated effective photoinduced electron transfer from polymer semiconductors to an electron acceptor, C60 polymer solar cells have gained popularity. Utilizing conducting polymers as electron donors could achieve 6–7.9% power conversion efficiency [14]. Conjugated polymers appeal to solar cell efficiency because their bandgap's and energy levels can be tuned chemically. The discovery of new electron-donor and electronacceptor materials has recently increased OSCs' power conversion efficiencies (PCEs) to above 18% [15]. New materials, improved device processing methods and blend morphology [16, 17], and a better understanding of device physics [18] have contributed to OPV cell progress in the previous 30 years.

High-performance computing and optimization of computational chemistry codes have increased theoretical research in this area. Theoretical approaches are the best way to overcome practical synthesis problems and investigate cost-saving manufacturing and processing options. DFT can help solve these problems because it examines the electronic structure and spectroscopic properties of these materials. The only electronic structural approaches currently relevant to conjugated polymers have relied on density functional theory (DFT) [19, 20]. The highest occupied molecular orbital (HOMO), lowest occupied molecular orbital (LUMO), bandgap (Eg), ionization potential (IP), electron affinity (EA), charge mobility, open-circuit voltage (VOC), and reorganization energies (λ) are key electronic parameters that determine the optoelectronic properties of polymer solar materials. This chapter examines DFT's usage and validity in polymer solar cell research. This will be followed by a description of how the DFT method looks at polymer solar cell characteristics. In addition, to describe the DFT and TD-DFT applications for polymer solar cells characteristics is also highlighted, as is DFT's overall utility in polymer solar cell design.
