Molecular design, device function and surface potential of zwitterionic electron injection layers

A series of zwitterionic molecules were synthesized by addition of iodoalkanes to sodium tetrakis(1-imidazolyl)borate (NaBIm₄). Single substitution at the N3 site leads to compounds designated as Cn-BIm4 (n = 1, 6, 12, 16), where the subscript corresponds to the number of carbon atoms in the linear alkyl chain. Metrical parameters in the molecular structure of C ₁-BIm₄ determined by single crystal X-ray diffraction studies confirms the zwitterionic nature and were used to calculate the molecular dipole moment. Compounds C_n-BIm₄ can be used to improve the electroluminescence efficiencies of polymer light emitting diodes (PLEDs) fabricated by simple solution methods that use poly[2-methoxy-5- (2′-ethylhexyloxy-1,4-phenylenevinylene)] (MEH-PPV) as the emissive semiconducting layer and aluminum as the cathode. This improvement is primarily due to a reduction in the barrier to electron injection and is strongly dependent on the length of the alkyl substituent in C_n-BIm ₄, with C16-BIm4 yielding the best performance. Examination of the surface topography of C₁-BIm₄ and C₁₆-BIm ₄ atop MEH-PPV by atomic force microscopy (AFM) shows that the longer alkyl chains assist in adhesion to the surface, however the coverage is poor due to insufficient wetting. These studies also reveal an unexpected planarization of the C₁₆-BIm₄ layer adjacent to the electrodes after metal deposition, which is suggested to improve electrical contact. A combination of open circuit voltage measurements, surface reconstruction studies and surface potential measurements indicates that zwitterion insertion leads to the formation of a spontaneously organized dipole layer at the metal/ organic interface. The net effect is a shift in the vacuum level and concomitant improvement of electron injection.