Introduction
All-polymer solar cells (all-PSCs) hold great promise for future advancements in the field of organic solar cells thanks to their excellent film-formation and excellent properties.[1-11] To date, the highest PCE achieved in all-PSCs has surpassed 18%[12-14]thanks to the rapid development of polymer acceptors, especially the recent strategy of polymerizing high-performance small molecule acceptors.[15-25] Nevertheless, the development of these polymerized small molecule acceptors (PSMAs) is so rapid that there is a gap between the exhibited performance and the highest achievable performance of the materials due to the relatively slow pace of device engineering. For instance, Yang et al. recently reported a PSMA named PJ1r, whose frontier orbital energy levels are uplifted compared to the popular PSMA such as PY-IT and PYF-T-o, which should promote the voltage of the device in principle. However, the best PCE reported so far of this PSMA was only about 16.1%,[26] achieved using a polymer donor (JD40) with an uplifted LUMO and HOMO levels than the workhorse donor, PM6. This could benefit exciton dissociation but it undermined the potential of the PSMA to achieve high Voc.
Furthermore, the best efficiency of most recent high-performance all-PSCs was achieved using chloroform as the processing solvent including PJ1r.[12-13, 27-37] In addition to its hazardous properties, chloroform has a high vapor pressure at temperatures relevant to device fabrication, making it unsuitable for large-scale fabrication techniques such as slot-die coating and blade-coating.[38-46] To take advantage of the superior film-formation properties and mechanical properties of all-PSCs in future large-scale productions, chloroform must be replaced processing solvent with a high boiling point at standard pressure, preferentially non-halogen solvents.
In this paper, we combined the sequential processing (SqP) technique and a solid additive strategy to address the aforementioned issues, namely, promoting the performance of existing PSMAs and using hydrocarbon solvent to replace chloroform. Compared to the conventional blend-casting (BC) method that relies on their inherent crystallization kinetics to achieve vertical phase separation, the SqP technique separates the preparation of the donor and acceptor solutions and the deposition of them, which utilizes the mutual solubility of the upper solution with the underlying film to form a bicontinuous interpenetrating network structure known as the bulk heterojunction (BHJ). On the basis of SqP, we further mix a tiny amount of PC71BM into the active layer as a solid additive (SA) to tune the morphology and electrical properties of all-PSCs in this study. Specifically, we first employ PM6 as the donor polymer and PJ1as the acceptor to fabricate binary all-PSC devices. We demonstrate a 17.4% PCE for the SqP PM6:PJ1-r device compared to a 15.8% PCE for the BC device. Subsequently, when 0.5% PC71BM is introduced as a solid additive into the active layer, the PCE is further increased to 18.0%. Film-depth-dependent light absorption spectra (FLAS) reveals that the addition of the SA improves the vertical phase segregation of the active layer, leading to enhanced charge carrier lifetime, reduced carrier extraction time, increased electron mobility, and more balanced electron/hole transport, which in turn contributes to the high photocurrent. Moreover, we carry out the same experiments using another PSMA (PYF-T-o ) and provide side-by-side comparison between the devices with these two acceptors. We show that the addition of the SA also increases the performance of the PYF-T-o devices. Finally, the maximum power point tracking (MPPT) technique is used to study the device stability. We show that both the incorporation of SA and the SqP method improve the photo-stability of the device.
Results and Discussion