π-Distorted charge transfer chromophores and their materials chemistry in organic photovoltaics

This review aimed to discuss the emergence of tetracyanobutadiene (TCBD) and dicyanoquinodimethane (DCNQ)-functionalized push-pull chromophores as key active layer materials in organic solar cells (OSCs). Although TCBD/DCNQ-based pi-materials have been extensively applied in functional organic devices, their scope in OSCs is largely limited by their non-planar molecular geometry. This inherent structural feature is a barrier for effective charge carrier mobility, which considerably diminishes the overall photovoltaic efficiency. Alternatively, other appealing properties associated with TCBD/DCNQ-based chromophores have widened the space for the further screening of their OSC potential. To date, TCBD and DCNQ have been proven to be promising functional entities for the construction of tailor-made pi-semiconductors for OSC devices. Through rational molecular design, many structural cores have been appended to TCBD and DCNQ units to satisfy the requirements of OSC application. These strategies also allow for the modulation of pi-orbital properties, which not only determine their role as either a donor (p-type) or acceptor (n-type) but also affect their photovoltaic performance. Therefore, this review comprehensively discusses the various TCBD/DCNQ-based pi-semiconductors investigated for OSCs with deeper insights into the fundamental understanding of their opto-electrochemical properties contributing to the device performance. Additionally, we have highlighted the impact of processing methods on power conversion efficiencies. This review covers the chemistry and material aspects of TCBD/DCNQ-based pi-twisted charge-transfer chromophores for utilization as bulk heterojunctions in organic solar cells.