Study On Fullerene Ammonium Derivatives As Cathode Interfacial Materials

Polymer solar cells (PSCs) have attracted broad interest both in academic and industry as renewable energy sources because of their unique advantages of easy fabrication, flexible and light weight. In order to enhance power conversion efficiency (PCE), researchers focus on four aspects:(1) further study of photon-to-electron conversion mechanism;(2) developing new solution processable donors and accepters with tailored energy levels;(3) developing new processing approaches to induce optimal microstructures in the active layer (layer by layer to BHJ);(4) developing new device architectures and new interfacial modification materials. Among those, developing new materials as cathode buffer layer is one of the popular research topics related to PSCs. Based on previous achievements, three fullerene ammonium derivatives were synthesized and characterized. This paper has two main parts as followed:1. Three alcohol soluble fullerene ammonium derivatives,[6,6]-phenyl-C61-butyric acid trimethylaminoethyl ester iodide (PCBANI),[6,6]-phenyl-C61-butyric acid2-((2-(trimethylammonium)ethyl)(dimethyl)ammonium)ethyl ester diiodides (PCBDANI) and [6,6]-phenyl-C61-butyric acid2-((2-(dimethylethanammonium)ethyl)(methyl)amino)ethyl ester bromide (PCBDANBr) have been synthesized. They are stable in ambient atmosphere and self n-doped, which was confirmed by electron paramagnetic resonance (ESR) measurements and conductivity measurements. The conductivities for PCBANI, PCBDANI, and PCBDANBr, were1.50×10-2,1.98×10-2, and1.05×10-2S cm-1, respectively, which are comparably high to those for common organic semi-conductor due to their self n-doped nature. These materials were able to reduce the work function of ITO and resistant to dichlorobenzene, which was confirmed by the measurement of film thickness and corresponding UV-vis absorption before and after rinsing with dichlorobenzene, indicating that all of these materials could be used as a cathode interlayer in inverted polymer solar cells (I-PSCs). Among these, PCBDANI had the best solvent resistance to dichlorobenzene. Most importantly, the inverted polymer solar cells with the structure of ITO/PCBDANI/P3HT:PCBM/MoO3/Ag retained reasonably high power conversion efficiency even at a thickness of82nm of the PCBDANI film as the cathode interlayer. Thus large-area devices via printing this interlayer or printing on this interlayer could become feasible.2. To study the mechanism of self n-doping, these fullerene ammonium derivatives were electrolyzed in deionized water and halogen anions were detected by adding AgNO3aqueous solution into the remaining solutions. Besides, the absorbance at1026nm of fullerene anion radicals were detected through UV-vis-NIR spectroscopy, meanwhile, fullerene ammonium derivatives were electrolyzed in superdry DMSO. Based on these findings together with ESR measurements, we proposed a self n-doping mechanism that demonstrates the halogen in one fullerene ammonium derivative molecule does not exist as classical halogen anion which has one negative charge but non-classical halogen atom which bears part of the negative charge. That is to say, the halogen anion shares its intrinsic electron with fullerene core. Furthermore, a conducting model was raised to explain the high conductivities of these halogen-containing fullerene ammonium derivatives. In this model, halogen atoms act as bridges linking adjacent fullerene cores for electrons to transfer. In terms of energy, halogen anions reduce the barrier for electrons to transfer intermolecularly.These three fullerene ammonium derivatives were self n-doped and highly conducting. As cathode interlayers for inverted polymer solar cells (I-PSCs), they were resistant to dichlorobenzene and tolerant of thickness. Besides, we started tentative research on self n-doping, based on which we proposed conducting model to explain the high conductivities of these materials, through electrochemical methods combined with other methods. This contribution laid the groundwork for further research on self n-doping and would shed light onto a novel strategy for electrically doping organic transport layers with high conductivity which can facilitate charge carrier transport both in organic light-emitting diodes and organic solar cells.