Squaraine dye: Promising strategies to boost power conversion efficiencies of OPV cells

Organic photovoltaic cells (OPVs) are one of the most promising technologies in the field of renewable energy. Bulk heterojunction (BHJ), dye-sensitized solar cells (DSSCs) and Perovskite solar cells (PKSCs) are recent OPVs that have attracted a lot of attention in the sustainable energy field. Squaraine is an important class of organic dyes with efficient absorption and stability in the visible and near infrared regions. Because of this ability, squaraine is a vital candidate for its photovoltaic applications in organic solar cells (OSCs). Both symmetrical and unsymmetrical squaraine dyes (SQ & USQ) could be used as a donor or acceptor unit in different layers of these solar cells as a light absorption moiety. In this article, we've thoroughly discussed the recent advances of a squaraine dye in BHJ solar cells, ternary organic solar cells (TOSCs), quaternary organic solar cells (QOSCs), tandem organic solar cells (TDOSCs), DSSCs and PKSCs. We've mainly focused on the power conversion efficiency (PCE) of these devices that are greatly affected by varying the skeleton of squaraine such as side chain, aryl groups, end-cappers, anchoring groups and phenylene linkers along with other processing conditions. These factors improve the photovoltaic properties of organic solar cells such as open-circuit voltage (V_oc), short circuit current (J_sc), fill factor (FF), charge carriers' mobility and light harvesting capability, ultimately boost up the PCE. In this review, we've elaborated all the factors that will be helpful for a researcher community to construct an efficient, low-cost, eco-friendly and reliable OPVs.