Impact of Solvent Additive on Morphology of High-Efficiency Ternary Blend Solar Cell

Background: It is well known the PTB7:PC71BM based photovoltaic cells have drawn great attention because the devices now hold the record in PSC performance. However, the crystallinity level of the used materials was reported very low, and this was tested with X-ray diffraction. It was reported that crystalline portions of the component enable a high absorption and a balanced charge transportation. As regards the phenomenon, the impact of the additive 1,8-diiodooctane (DIO) on the device performance has been reported in many references. In the work, the impact of solvent additive DIO, on the morphology formation and performance of PTB7:PC71BM:F8BT (poly[(9,9-di-noctylfluorenyl- 2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)]) ternary PSCs was investigated. Methods: Researches related to ternary PSCs were reviewed, and many repeatable experiments have been done. To analyze the effect of DIO on the PTB7:PC71BM:F8BT based ternary cells, we did many measurements such as absorption spectral, PL spectral, device performance, X-ray Diffraction (XRD) and Atomic Force Microscopy (AFM). Results: It was found that the additive plays an important role in controlling the active layers morphology of the ternary devices. When 3% described solvent additive was added to solvent CB, an increasing nearly 36% in JSC and a relatively modest 41% gain in the FF were found, which resulting in a nearly doubling of the PCE. In order to analyze the influence of additive on the device performance, morphology characterization of PTB7:PC71BM:F8BT ternary layers was elucidated using AFM and energy transport progress of the devices was probed from photoluminescence spectra. Conclusion: In summary, we have successfully constructed the polymer ternary BHJ devices based on PTB7:PC71BM:F8BT with a nearly doubling of the PCE compared to CB fabricated ternary devices. By analyzing the absorption spectral, surface morphology, energy transfer progress and external quantum efficiency, we found that DIO as solvent additive can favor the charge separation into the charge transfer state after exciton dissociation. In more detail, an effective morphology in CB+DIO film can be obtained that provides a plentiful interfacial area for charge generation and a balanced percolation way for charge transfer, which resulting in a nearly doubling of the PCE. This study provides method to improve morphology of active films, and thus improve the performance for ternary PSCs. Keywords: Additive, bulk heterojunction (BHJ), polymer solar cells (PSCs), ternary blends.