Degradation analysis of doped organic p-n heterojunction charge generation layers by impedance and optical spectroscopy

Abstract

Investigation of the long-term stability of charge generation layers (CGLs) provides a fundamental and an essential approach in achieving highly efficient tandem organic electronic devices. Thus, in this foremost study, the degradation mechanism of electrically aged organic p-n heterojunction CGLs has been investigated by impedance and optical spectroscopy. Rubidium carbonate (Rb2CO3)–doped 2,2,2-(1,3,5-benzinetriyl)-tris(1-phenyl-1H-benzimidazole) (TPBi) and molybdenum trioxide (MoO3)–doped 1,4-bis[N-(1-naphthyl)-N′-phenylamino]-4,4′-diamine (NPB) are used as the n-type and p-type organic semiconductors, respectively. A detailed analysis from capacitance-frequency (C–F) and capacitance-voltage (C–V) characteristics reveals reduced charge generation and 19.6% reduction in the geometric capacitance of the CGL after electrical aging. Reduced peak intensity from UV–Vis–NIR spectra of the aged CGL points to 21.4% charge transfer complex decomposition of the Rb2CO3-doped TPBi. We propose that the rate-limiting step of charge generation in the CGL is caused by the electron transport in the TPBi:Rb2CO3 layer and not the charge generation itself at the TPBi:Rb2CO3/NPB:MoO3 heterojunction. This simple, comprehensive, and non-destructive technique facilitates a crucial analysis that underpins the mechanism of device degradation and further provides a fundamental approach in developing highly stable CGLs for efficient organic electronic devices.