Power Conversion Efficiency Improvement of Planar Organic Photovoltaic Cells Using an Original Hybrid Electron-Transporting Layer.

In organic photovoltaic (OPV) cells, besides the organic active layer, the electron-transporting layer (ETL) has a primordial role in transporting electrons and blocking holes. In planar heterojunction-OPVs (PHJ-OPVs), the ETL is called the exciton blocking layer (EBL). The optimum thickness of the EBL is 9 nm. However, in the case of inverted OPVs, such thickness is too high to permit efficient electron collection, due to the fact that there is no possibility of metal diffusion in the EBL during the top metal electrode deposition. In the present work, we show that the introduction of a thin potassium layer between the indium tin oxide (ITO) cathode and the EBL increases dramatically the conductivity of the EBL. We demonstrate that K not only behaves as a simple ultrathin layer allowing for the discrimination of the charge carriers at the cathode/organic material interface but also by diffusing into the EBL, it increases its conductivity by 3 orders of magnitude, which allows us to improve the shape of the - characteristics and the PHJ-inverted OPV efficiency by more than 33%. Moreover, we also show that PHJ-inverted OPVs with K in their EBLs are more stable than those with Alq alone.