Monolithic integration of thin-film photodiode with the Si read-out circuit (ROIC) offers a low-cost alternative to the conventional flip-chip bonded III-V near-infrared (NIR) and short-wave infrared (SWIR) imagers. Thin film absorber materials have already enabled advanced SWIR imagers featuring the smallest pixel pitch and highest resolution. Attractive optoelectronic properties such as spectral tunability, quantum confinement, large area solution processability, ligand dependent energy band structure and electronic properties tunability make colloidal quantum-dot (CQD) as one of the most promising candidates for thin film NIR and SWIR photodetector absorber materials. Thanks to the massive advancement towards synthesis of the CQDs, these materials now include a wide range of semiconductors to choose from for applications like photodetectors, phototransistors, light-emitting diodes and solar cells.

To engineer thin-film CQD based optoelectronic devices it is crucial to obtain a complete energy band structure of CQD with various ligand types, exchange methods, and sizes of these quantum confined nanocrystals. However, the effort to design and simulate efficient devices is constrained by the lack of systematic studies and the inconsistencies found in the literature regarding reported energy band structures of CQDs. We demonstrate that the accurate characterization of energy band structure by ultraviolet photoelectron spectroscopy lies at the heart of the film preparation process and largely depends on the distribution and packing density of the deposited CQDs. Transmission electron microscopy images confirm that our proposed multi-step coating technique ensures over 90% CQD coverage (both PbS and InAs) within probing area. This is well supported by X-ray photoelectron spectroscopy, atomic force microscopy and variable angle spectroscopic ellipsometry measurements. Extracted energy band structures are further validated by fabricating SWIR PbS and InAs CQD thin film photodiodes.

Our comprehensive energy band characterization of SWIR PbS and InAs CQDs with various ligands by both solid state and liquid phase ligand exchange processes showcases an accurate and reproducible scheme to achieve complete Fermi reference energy band structure modelling of thin film photodiode stack. The insight on the overestimation of Fermi and valence band maximum |E_F-E_VBM| due to poor packing density and the summary of the energy band structures of both PbS and InAs CQDs for various ligands will advance the energy landscaping of thin film CQDs. With this methodology, we can better choose the proper ligand and size of the CQDS to efficiently design thin film devices.