Organic Solvent Dispersible MXene for Work Function and Interface Engineering of CQD Photovoltaics

Lead sulfide (PbS) colloidal quantum dots (CQDs), are promising optoelectronic materials owing to their size and shape dependent optical bandgap, high absorption coefficients, multiple exciton generation effect, and facile solution processability. With these benefits, recent remarkable advances in CQD solar cells are achieved by tailoring the CQD surfaces and device architecture. The lead iodide (PbI₂) based surface passivated CQDs (PbS-I) enabled achieving an outstanding air stable CQD devices, and the thiol-passivated CQDs (PbS-EDT) as hole transporting layer (HTL) mediated the band alignment of CQD solar cells efficiently by virtue of their electron blocking ability originating from shallow conduction band edge level (E_C). The bulk homojunction of CQDs via a cascade surface modification method enabled fabrication of thicker CQD layer and reaching the remarkable power conversion efficiency (PCE) of CQD solar cells surpassing 13%.
Despite these advances in CQD photovoltaics, some as remaining challenges hinder further improvements of CQD solar cell performances. First, the Fermi-level (E_F) misalignment between lead iodide treated-photoactive PbS CQD (PbS-I) and PbS-EDT (E_F,Iodide<E_F,EDT) made hole accumulation in CQD solar cells. In the solar cells, for efficient charge extraction, the energy level relation of E_F,ETL>E_F,active>E_F,HTL (E_F of photoactive layer should be lower than that of the electron transport layer (ETL) and higher than that of the HTL) should be achieved. In this regard, the junction of PbS-I photoactive layer and PbS-EDT HTL is undesirable. The E_F alignment of E_F,Iodide<E_F,EDT in conventional CQD solar cell forms the depletion region direction opposed to desirable hole flow. Second, the numerous surface cracks of PbS-EDT HTL formed during the solid-state ligand exchange process can permit the direct contact between PbS-I and metal electrode, which is the electron-blocking free junction.
To address these problems, this work demonstrates a work function and interface engineering strategy for CQD solar cells using polycatechol-functionalized MXene (PCA-MXene). PCA-MXene facilitates its use as a dopant and an interlayer for CQD solar cells via dispersion in organic solvents, used in the CQD solar cell fabrication process. This is not feasible with conventional water-borne pristine MXene. PCA-MXene, when used as a dopant, tailors the work function of PbS-I CQDs and resolves the chronic problem of an undesirable depletion region direction at the PbS-I/PbS-EDT interfaces. The 2D structure of the PCA-MXene interlayer, inserted in the PbS-EDT/electrode interfaces, effectively prevents metal penetration into the crack sites during the electrode deposition process. Leveraging these benefits, the PCA-MXene integrated CQD solar cells demonstrated a PCE of 13.6%, compared to the reference device PCE of 12.8%. Moreover, the hydrophobic nature of PCA-MXene on the CQD solar cells facilitated the retention of 73% of the initial PCE after 30 hours of thermal aging in 70°C air (with a relative humidity (RH) of approximately 40%), whereas conventional CQD solar cells retained only 59% of the initial PCE.