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

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 Voc 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 Iscand FF.