Effect of Thermal Annealing on the Charge Carrier Selectivity of Ultra-Thin Organic Interface Dipoles in Silicon Organic Heterojunction Solar Cells
Interfacial layers consisting of organic molecules with a permanent dipole moment exhibit enhanced charge carrier selectivity when applied as electron-selective contacts in crystalline silicon (c-Si) heterojunction solar cells. It is found that thermal annealing has a detrimental effect on the charge carrier selectivity of dipole materials based on the amino acid l-histidine mixed with a fluorosurfactant. Although, the implied open-circuit voltage (iVoc) increases with annealing, the Voc decreases significantly which is accompanied by a decrease in the built-in voltage (Vbi) and increase in the specific contact resistivity (rc). Based on numerical device simulations, it is concluded that the tunneling of electrons through the dipole layer becomes less effective with increasing annealing temperature due to the decomposition of the dipole materials. The decomposition leads to a more ""resistive"" interfacial layer and to a gradient in the electron quasi-Fermi potential and, thus, a decrease in Voc. Furthermore, storage under ambient air at room temperature degraded the electron-selective contacts substantially, limiting the potential of the dipole material for the application in silicon organic heterojunction solar cells.