Investigation On The Phase Separation And Device Performance Of Polymer/Small Molecule-Based Solar Cells

Organic solar cells(OSCs)have received extensive attention owing to their advantages of flexibility,semitransparency,solution-processable,and large-area processing.The power conversion efficiency(PCE)of organic solar cells strongly depends on the phase-separated morphology of the active layer.The ideal phaseseparated morphology is bicontinuous donor-acceptor pathways with the domain size of about 10-20 nm and high crystallinity to guatantee efficient exciton splitting and charge transport.Meanwhile,high domain purity and gradient vertical component distribution are benificial to increase charge mobility and suppress charge recombination.The interpenetrating phase-separated morphology of the active layer is controlled by thermodynamics and kinetics.During solution processing,the active layer usually forms the undesirable morphology of excessive aggregation or intimately mixing of donor and acceptor due to the fast removal of solvent,which results in serious geminate or bimolecular recombination and has an adverse effect on the performance of OSCs.In addition,among various polymer donors,polythiophenes,whose conjugated backbones are entirely composed of thiophene rings,are one of the most promising polymer donors for large-scale manufacturing and industrialization due to their advantages of low cost,compatibility with high-throughput production techniques,and high crystallinity.In this study,we take the PTB7-Th:IEICO-4F system where the acceptor is prone to excessive aggregation and the P3HT:O-IDTBR system where the donor and acceptor are intimately mixed as the model systems,respectively.Through controlling the film-formation process,the molecular packing of donor and acceptor and the phase-separated morphology of the active layer are optimized,which suppress the charge recombination and improve the device performance.The main conclusions are as follows:1.A third component NCBDT-4Cl that has good compatibility with IEICO-4F is used to shorten the crystal growth time of IEICO-4F,which suppresses the excessive aggregation of IEICO-4F and leads to the formation of refined interpenetrating network.NCBDT-4Cl has good miscibility with IEICO-4F and they can achieve a nanoscale mixing.Therefore,the addition of NCBDT-4Cl can effectively hinder the nucleation and growth of IEICO-4F.The growth time of IEICO-4F is reduced from 122 s to 45 s by incorporating 20 wt%NCBDT-4Cl,which limits the diffusion and self-assembly of IEICO-4F.As a result,the excessive aggregation of IEICO-4F in blend film is suppressed and the small IEICO-4F aggregates disperse in the PTB7-Th matrix,leading to the formation of refined bicontinuous pathways.The optimized morphology promotes the exciton splitting and charge transport and reduces the geminate recombination.Compared the PTB7-Th:IEICO-4F binary devices,the PCE of the optimized ternary devices are improved from 10.91±0.18%to 12.15±0.22%.2.A high boiling point additive dibenzyl ether(DBE)that has selected solubility for O-IDTBR is used to promote the secondary crystallization of O-IDTBR,which enhances the molecular packing order and improves the lateral and vertical phase separation of the P3HT:O-IDTBR blend.As DBE selectively dissolves O-IDTBR,partial O-IDTBR remains swollen after the removal of host solvent chlorobenzene(CB)and continues to crystallize with the slow volatilization of DBE.The crystal growth time of O-IDTBR is increased from 3 s to 60 s with the addition of 0.75%DBE,which provides more time for O-IDTBR to develop order and migrate toward the substrate.Meanwhile,DBE accelerates the emergence of the crystal nuclei of P3HT by degrading the solvent quality,favoring the formation of P3HT fibrils.As a result,the molecular packing order of P3HT and O-IDTBR is enhanced and a gradient vertical composition distribution is formed in the blend film,which greatly improve the electron mobility and suppress the bimolecular recombination.After the addition of 0.75%DBE,the PCE of the devices based on the P3HT:O-IDTBR blend are boosted from 4.29 ± 0.04%to 7.30±0.12%.3.In sequential spin-coated P3HT/O-IDTBR(DBE)film,UV irradiation and solution aging increase the aggregation of P3HT in solution and its crystallization rate,leading to the formation of dense P3HT nanofibers.This restricts the deep penetration of O-IDTBR into the P3HT matrix and improves the vertical phase separation of the film.Firstly,UV irradiation provides the necessary energy for the nucleation of P3HT in solution.Then,the aging process promotes the fiber growth of P3HT,which increases the aggregation fraction of P3HT in solution and improves its crystallization rate,inducing P3HT to form dense nanofibers.During the layer-by-layer(LbL)processing,P3HT nanofibers suppress the excessive interdiffusion of donor and acceptor layers,resulting in the formation of intertwined donor/acceptor bicontinuous network with high crystallinity and favorable vertical segregation.Such morphology greatly increases the hole mobility and reduces the bimolecular recombination.The PCE of P3HT/O-IDTBR(DBE)devices based on P3HT nanofibers are improved from 6.70±0.12% to 7.71±0.10%.