Abstract
Abstract Polymer solar cells (PSCs) are a promising alternative for low-cost renewable energy due to their solution-process ability, flexibility and large-area fabrication. However, the power conversion efficiency (PCE) of PSCs depends on complicated mechanism of carrier generation and transport. In general, the morphology of active layer in PSCs plays a very important role in PCE. Many reports have studied on effective control of morphology but its characteristics are depending on active polymer, making it difficult to establish a general method for controlling the morphology. Multi-fluoro-substituted benzene has the nature of electron-depleted ring center since the electron withdrawing characteristic of the F-substituents makes the electron density of benzene ring moving to F atoms in the periphery. Thus, multi-fluoro-substituted benzene has the capability to strongly interact with electron-rich species such as double bonds and aromatic rings. Based on this concept, we propose two methods to improve the device performance by controlling morphology of the active layer. The first method is to add a multi-fluorobenzene substituted polymer, poly(pentafluorophenyl methacrylate）(PFPPA), into active layer as additive. The multi-fluorobenzene can attract the [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and make it align along with the main chain of the PFPPA. The aligned PCBM makes the electron transport easier and thus the PCE is improved from 3.43 % to 3.74 %. The second method is to modify ZnO surface by incorporating 2,3,5,6- Tetrafluoro-1,4-benzenedicarboxylic acid (TFTPA) on top of it. Upon coating a layer of fullerene derivative on top of it, the fullerene molecules can be physically absorbed via coulombic interaction and facilitate a promoted electron collection from the bulk. Significant enhancement in PCE is observed for the devices with the active layer P3HT:PC61BM (or PC71BM) by promoting from 3.20 to 4.03% (or from 3.27 to 4.04%); and with the active layer PTB7: PC71BM from 6.03 to 6.90 %. This method should be also applicable to other types of active layer.