Comparative Study of the N-Type Doping Efficiency in Solution-processed Fullerenes and Fullerene Derivatives

Molecular doping of organic semiconductors and devices represents an enabling technology for a range of emerging optoelectronic applications. Although p-type doping has been demonstrated in a number of organic semiconductors, efficient n-type doping has proven to be particularly challenging. Here, n-type doping of solution-processed C60, C70, [60]PCBM, [70]PCBM and indene-C60 bis-adduct by 1H-benzimidazole (N-DMBI) is reported. The doping efficiency for each system is assessed using field-effect measurements performed under inert atmosphere at room temperature in combination with optical absorption spectroscopy and atomic force microscopy. The highest doping efficiency is observed for C60 and C70 and electron mobilities up to ≈2 cm2/Vs are obtained. Unlike in substituted fullerenes-based transistors where the electron mobility is found to be inversely proportional to N-DMBI concentration, C60 and C70 devices exhibit a characteristic mobility increase by approximately an order of magnitude with increasing dopant concentration up to 1 mol%. Doping also appears to significantly affect the bias stability of the transistors. The work contributes towards understanding of the molecular doping mechanism in fullerene-based semiconductors and outlines a simple and highly efficient approach that enables significant improvement in device performance through facile chemical doping