ABSTRACT
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 J-V 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 Alq3 alone.
ABSTRACT
In order to grow semi-transparent organic photovoltaic cells (OPVs), multilayer dielectric/metal/dielectric (D/M/D) structures are used as a transparent top electrode in inverted OPVs. Two different electrodes are probed, MoO3/Ag/MoO3 and MoO3/Ag/Cu:Ag/ZnS. Both of them exhibit high transmission in visible and small sheet resistance. Semi-transparent inverted OPVs using these electrodes as the top anode are probed. The active organic layers consist in the SubPc/C60 couple. The dependence of the OPV performances on the top electrode was investigated. The results show that far better results are achieved when the top anode MoO3/Ag/MoO3 is used. The OPV efficiency obtained was only 20% smaller in comparison with the opaque OPV, but with a transparency of nearly 50% in a broad range of the visible light (400-600 nm). In the case of MoO3/Ag/Cu:Ag/ZnS top anode, the small efficiency obtained is due to the presence of some Cu diffusion in the MoO3 layer, which degrades the contact anode/organic material.
