Quantum Dots Coupled to an Oriented Two-Dimensional Crystalline Matrix for Solar Cell Application

Abstract

Colloidal quantum dots (QDs) have emerged as promising materials to harness panchromatic solar light, owing to their size-tunable optoelectronic properties. Advancements in surface passivation strategy and processing technique have contributed immensely to their developments in photovoltaic applications. Recently, surface passivation using halometallate ligands was shown to form a protective shell layer, which reduced the structural and energetic disorder in the QD solid. Here, we report lead sulfide (PbS) QDs coupled to an oriented two-dimensionally (2D) confined crystalline matrix by using a halometallate ligand. The QDs undergo surface reconstruction during the ligand treatment process, which leads to change in their shape, size, and axis length. We show that the 2D matrix is a combination of two distinct crystalline layers consisting of a crystalline Pb–amine complex and a 2D perovskite layer. The thickness of the matrix layer is modulated further by adjusting counter cations, which results in the enhancement in charge carrier mobility, carrier recombination lifetime, and diffusion length in the QD solid. 2D passivated QDs are implemented to fabricate photovoltaic devices with high power conversion efficiency of 9.1%.

Reduction of Hydroxyl Traps and Improved Coupling for Efficient and Stable Quantum Dot Solar Cells

“…The J – V characteristics for different scan directions for −I passivated and optimized hybrid passivated (10 mM CPT) solar cells are shown in Figure d, and the photovoltaic parameters are given in Table S2. Only −I passivated solar cells show strong hysteresis, while the hybrid passivated solar cells show negligible hysteresis in their J – V characteristics. The average PCE of hybrid passivated solar cells (PCE average : 10.45%) increases by 26% compared to only −I passivated solar cells (PCE average : 8.26%).…”

“…We posit that there could be competitive recombination pathways inside the solar cells other than the carrier recombination via surface traps. We determine the recombination lifetime of photocarriers inside the solar cells using the transient open-circuit method. , In the transient open-circuit technique, the photocarriers are subjected to recombine inside the solar cells, and the decay of the photovoltage is measured across a high resistance at the output circuit. This technique allows us to determine the time duration that the photocarriers spend before they recombine inside the solar cell.…”

“…This suggests that the special arrangement of the active QD layer inside the solar cell and the interfacial recombination centers may limit the photocarrier lifetime in hybrid passivated solar cells. We measure the charge carrier mobility inside the solar cells employing the transient current technique reported elsewhere. ,, From the slope of the vs applied bias plot (where d is solar cell thickness and τ tr is the carrier transit time) (shown in Figure S8), the mobilities of the solar cells are compared in Figure f. The charge carrier mobility of hybrid passivated QD solids decreases systematically when the CPT concentration increases from 4 to 300 mM.…”

“…PbS QDs capped with OAs are synthesized from a previously reported method . For halide ligand precursor preparation, 75 mM NH 4 I, 75 mM PbI 2 , and 25 mM PbBr 2 are mixed in 5 mL of DMF to obtain a clear transparent yellow solution.…”

“…Replacing lead oxide with lead acetate reduces the −OH ligands but could not eliminate them. , The use of a PbCl 2 precursor in synthesizing PbS QDs is shown to replace the −OH ligands with Cl ligands; however, this has caused insulating PbCl x shell layer to impede their electrical transport. , Further, during the purification of QDs from the reaction mixture, methanol is the preferred choice of solvent due to its high solubility toward the reaction byproducts. , However, methanol cleaning is shown to detach the oleate ligands, leading to the hydroxylation of the QD surface. , Several efforts have been made to remove the hydroxyl ligands from the QD surface, including precursor engineering, surface reconstruction, solvent engineering, and heat treatment . Advanced solution-phase surface passivation using lead halide ligands is found to be inefficient in removing the −OH ligands from the QD surface. ,, Additionally, the lead halide treatment is shown to grow a thick crystalline matrix layer in QD films, which limits the electrical transport in solar cells. − …”