Recently, a research team led by Profs. ZHOU Huiqiong , QIU Xiaohui and ZHANG Yong from the National Center for Nanoscience and Technology (NCNST) of the Chinese Academy of Sciences (CAS) proposed a new strategy to investigate the regulation of nanoscale surface energy distribution at the interface layer of organic solar cells. The related result was published in Joule entitled with "Nanoscale Heterogeneous Distribution of Surface Energy at Interlayers in Organic Bulk-Heterojunction Solar Cells".
Surface energy (γs) plays a key role in the formation of bulk-heterojunction (BHJ) films in organic solar cells fabricated by solution process. The miscibility of BHJ films can be predicted by the difference of surface energy between donor and acceptor. The vertical distribution and the stacking orientation of BHJ films can be regulated by the surface energy in the bottom interface layer. The surface energy of thin film is usually obtained by measuring contact angle using Owens-Wendt model. However, this measurement method cannot reflect the surface energy distribution in the nanoscale range, and it cannot directly explain the nanoscale stacking and phase separation in the BHJ structure.
Prof. Zhou has been devoted to the interface research of solution-processed organic solar cells. They have carried out a series of studies on the surface energy regulation in organic solar cells. Through incorporating WOx nanoparticles in poly3, 4-ethylenedioxythiophene: polystyrene sulfonate (PEDOT:PSS) to improve the surface energy, a high fill factor of 80% was achieved in organic solar cell (Adv. Mater., 2018, 30, 1801801). Then, relationships between the stacking orientation of the active layer, the performance of organic solar cells and the surface energy of the interface layer were further explored in the next work (Adv. Mater. 2019, 31 1806921). This strategy of interfacial modification was also applied to the study of electron transporting layer in inverted devices (J. Mater. Chem. A, 2019, 7, 3570–3576). Besides, this strategy has been utilized in perovskite solar cells. Through using the biopolymer heparin sodium to modify the surface energy, the interface defect of perovskite solar cells was passivated with improvements of power conversion efficiency (PCE) and stability (Adv. Mater., 2018, 30, 1706924).
In their latest research, the AFM-based Peak-Force Quantitative Nanomechanical Mappings (PFQNM) technique was used to characterize the nanoscale surface energy distribution of hole transporting layers in organic solar cells. It was found that the surface energy distribution of PEDOT:PSS can be effectively regulated by doping MoS2 nanosheets with different lateral sizes and the heterogeneity of PEDOT:PSS distribution can be enlarged. The heterogeneous distribution of surface energy (HeD-SE) can further regulate the molecular distribution, crystal orientation and phase separation of the active layer. Due to the optimization of the active layer morphology by the HeD-SE, the performance and stability of organic solar cells were enhanced with the best PCE of 18.27% (the PCE certified by National Institute of Metrology, China was 17.80%). Besides, the enhancement ratio of PCE (ΔPCE, defined by the PCE difference between the devices with MP-2 HTLs and those with PEDOT:PSS HTLs) was proportional to the enlargement of Δγs (defined as the difference of surface energy of donor and acceptor) in the BHJ.
This work was helped by other collaborators. This research also was financially supported by the National Key Research and Development Program of China (2017YFA0206600), the National Natural Science Foundation of China (No. 21922505, 21773045), the CAS Instrument Development Project (No.YJKYYQ20190010) and the Strategic Priority Research Program of Chinese Academy of Sciences (No. XDB36000000).