Investigation on polymer solar cells by using calcium as an Electron Transportation Layer

• Ca–S bonds result the shift of energy level and the improvement of VocVoc is consistent with XPS images.
• The accumulative error during the thermal evaporation is the main reason of IscIsc and FF decrease.
• Recombination is explored via quantum tunneling theory and it is consistent with experimental data.

This work investigated the transportation and recombination mechanisms of carriers in organic photovoltaic by using calcium (Ca 10 to 50 nm) as the Electron Transportation Layer (ETL). Compared with the sample without ETL layer, the Fermi Energy Level (EF) moved at least 0.4 eV towards the Lowest Unoccupied Molecular Orbit (LUMO) of P3HT after Ca was inserted. Ca–S bonds at the interface resulted in the shift of Highest Occupied Molecular Orbit (HOMO) which increased VocVoc to 0.58 V. Meanwhile, the shift of HOMO and low work function of Ca decreased the recombination possibility (from 1.38% to 0.403%), which resulted in 1% improvement of Fill Factor (FF). The experimental results were coincide with theoretical explanation. Additionally, the existence of accumulative error during the thermal evaporation cannot be neglected. This is the main reason for the decrease of IscIscand FF.