Insights into electron and hole extraction layers for upright and inverted vacuum-deposited small molecule organic solar cells

Vacuum-deposited small molecule organic solar cells (OSCs) with various electron extraction layers (EELs) and hole extraction layers (HELs) are studied. Upright OSCs are examined for their efficiencies, and for their photo- and thermal-stabilities. It is shown that the EEL conducts only because of the subsequent top electrode metal deposition, and that exciton blocking characteristics of the EEL alone are not sufficient to grant good solar cell characteristics. To this end, wide bandgap organic HELs cannot be placed between the bottom electrode and active organic layers to achieve the same effect as wide bandgap organic EELs at the opposite end of the device. Inverted vacuum-deposited OSCs, which place the HEL adjacent to the top metal anode, can make use of a wide bandgap organic HEL in combination with a thin MoO3 layer to achieve good device performance. Thick MoO3 HELs alone are also shown to be suitable for inverted vacuum-deposited OSCs, thus simplifying device fabrication. For these inverted devices, the thickness of the HEL required to achieve good device performance is found to be substantially higher than the equivalent EEL thickness in a standard upright device.