Organic heterojunctions as a charge generation layer in tandem organic light-emitting diodes: The effect of interfacial energy level and charge carrier mobility

A homologous series of p-type thiophene organic semiconductors NaTn (naphthyl end-capped oligothiophenes, n = 2-6 represents the number of thiophene units) were employed to form organic heterojunctions with n-type organic semiconductor C₆₀ (C₆₀/NaTn), applied for high-performance tandem organic light-emitting diodes (OLEDs). The effect of organic heterojunctions as a charge generation layer on the performance of tandem OLEDs has been well demonstrated. We found that not only are the highest occupied molecular orbital levels of NaTn close to the lowest unoccupied molecular orbital of C₆₀, but also that a high charge carrier mobility is very important for constructing an effective charge generation layer to achieve high power efficiency in tandem OLEDs. Our results offer a design/selection rule for organic semiconductors used to construct effective organic heterojunction charge generation layers, which will be useful in future high-performance tandem OLEDs.