Study of energy band diagrams required for intermediate band solar cells using PbS quantum dots based on photoemission yield spectroscopy measurements in air

The energy states of various thin films were evaluated in air to clarify the band diagram required for intermediate band solar cells using PbS colloidal quantum dots (CQDs). We have proposed CQD superlattice solar cells that operates by utilizing the intermediate band generated from the ground states and the conduction and valence bands generated from the excited states in CQDs. The layer configuration with the adequate band diagram ensures proper operation of the new type solar cells. The position of the valence band maximum (VBM) of the CQD layers was confirmed to shift downward with the ligands changing from oleic acid to butylamine to iodine. It was revealed how the quantum confinement energy changed on the electron and hole sides depending on the size of the CQDs. The VBM of the CQD layer was found to shift upward by at least about 0.1 eV upon superlattice formation. The optimal combination of CQD superlattice layers, hole transport layers, and electron transport layers was discussed based on the measured band diagrams, and it was shown that the CQD superlattice solar cells with one intermediate band on the electron side are the most feasible.