Simultaneous Morphology Control and Excited-State Engineering in Organic Photovoltaics Using Volatile Solid Additives

Solid additives (SAs), especially for volatile solid additives (VSAs), have gained much attention due to the huge potential in regulating morphology of active layer, tuning the dynamics of excited states and optimizing the performance of organic solar cells (OSCs). Herein, two non-halogen isomeric VSAs, 2-BCB and 4-BCB, were screened to investigate thoroughly the effect of substitution position of hydroxyl group and ester group on the optical properties, interactions with active layers, morphology modulation and properties of excited states. Compared with 2-BCB, planar 4-BCB incorporates the intermolecular H-bond and large dipole moment, resulting in 3D-interconnected network, enhanced stacking, improved carrier transport and more highways of carriers as well as reduced recombination. The results of femtosecond transient absorption spectroscopy (fs-TAS) demonstrate the faster hole transport in 4-BCB-treated devices, which significantly affects the performance of OSCs. Consequently, the D18:L8-BO device based on 4-BCB achieved improved efficiency of 18.49 %, markedly higher than the control device (17.48 %) and device processed with 2-BCB (18.03 %), respectively. The research emphasizes the great potential of using VSAs to synergistically optimize the morphology modulation and the properties of excited states, and provides valuable insights into understanding the relationship between structure and property of VSAs.