Electrostatic Self-Assembled Metal Oxide/Conjugated Polyelectrolytes as Electron-Transporting Layers for Inverted Solar Cells with High Efficiency

摘要

Three conjugated polyelectrolytes (CPEs) based on poly-thiophenes bearing anionic (poly[(3-(4'-sulfonatobutyl)oxymethyl-2,5-thio-phene)-alt-2,5-thiophene] sodium salt, PTSO-Na), neutral (HT-poly[3-(6'-diethanolamino)-hexylthiophene], PTNOH) and cationic (HT-poly[3-(6'-N,N,N-trimethylammonium)-hexyIthiophene], PTN-Br) pendant groups were synthesized to improve the power conversion efficiency (PCE) of inverted polymer solar cells (I-PSCs) by deposition on the surface of ZnO to form a ZnO/CPE electron-transporting layer (ETL). Insertion of CPE to ZnO—active layer interfaces effectively lowered the energy barrier for electron transport and reduced the inherent incompatibility between the hydrophilic metal oxide and hydrophobic active layers. The I-PSCs (ITO/ ZnO/CPE/P3HT:PCBM/PEDOT:PSS/Ag) incorporating anionic PTSO-Na achieved a 16% efficiency enhancement (PCE = 3.47%) over the standard device with a ZnO monolayer ETL (PCE = 2.99%). For the deposition of neutral PTNOH and cationic PTN-Br on top of ZnO, we observed strong electrostatic interaction between cationic quaternary amines of the CPE and anionic oxygen ions of the ZnO surfaces, which obtained a uniform formation of strong dipoles across the interfaces and an intimate interfacial contact. The self-assembly formed by partial protonation in neutral PTNOH increased the PCE of I-PSC to 3.98%, whereas the stronger electrostatic self-assembly produced in ZnO/PTN-Br bilayers not only delivered the device with the best PCE (4.08%) among the three CPEs but also yielded an exceptional device lifetime without encapsulation. It is worth noting that the performance of the I-PSC with PTN-Br already surpassed that of conventional ones (ITO/PEDOT:PSS/P3HT:PCBM/PTN-Br/LiF/Al). Moreover, the PCE of the,device based on a ZnO/PTN-Br ETL was further improved to 4.45% after UV treatment with a 43% enhancement compared with the monolayer ZnO device, which is due to improved electrostatic self-assembly. These findings on electrostatic self-assembled metal oxide/CPE bilayer ETL provide a simple and easy strategy for fabrication of high-performance and long-term stable I-PSCs.