Solution-processed quantum dots are promising building blocks for next-generation optoelectronic devices due to their low-cost, wide tunable bandgap and solution-processibility. Phase transfer ligand exchange has been demonstrated as a promising method to prepare small-size (diameter < 3.5 nm) PbS QDs for device fabrication. However, two obstacles limit the conventional phase transfer ligand exchange method for large-size PbS QDs: densely packed organic shells and charge-neutral (100) surfaces. In this talk, we describe a new strategy providing high-quality large size PbS QD via phase transfer ligand exchange. We use lead acetate trihydrate (PbAc2·3H2O) as a precursor reducing the steric hindrance from the densely packed organic shells, which facilitates the ligand exchange. In addition, we use methylammonium acetate (MAAc) as an additive in PbI2 ligand solution forming perovskite intermediate (MAPbI2Ac) on the (100) surface, which improves the surface passivation. The resulting photodiodes using these large-size QDs without further post-treatment exhibit a near-unity internal quantum efficiency in the short wavelength infrared region.
Narrow bandgap lead sulfide (PbS) quantum dots (QDs) are solution-processed materials used for optoelectronic applications in the short-wavelength infrared (SWIR) range (1400 - 3000 nm). The PbS QDs based photodetector has achieved comparable detectivity with current commercial SWIR sensors. However, there are still obstacles towards commercialization in commonly used layer by layer (LbL) deposition, such as high material consumption and low reproducibility. Here, we developed a new ligand exchange strategy to prepare ligand exchanged QD inks for single-step PbS film deposition. Compared with LbL deposition, the EQE of PbS QD photodetector made by single-step deposition has improved from 31% to 53%. The EQE and responsivity can be further improved to 95% with IR transparent electrode.
Lead sulfide (PbS) quantum dots (QDs) exhibiting the narrow bandgap (Eg < 0.9 eV) provide a promising avenue to high performance, inexpensive shortwave infrared (SWIR) photodetector. However, most PbS-QD based SWIR photodiodes suffers from low responsivity and low external quantum efficiency (EQE) in the IR range due to insufficient ligand exchange. Here, we report a precursor engineering strategy that facilitates the commonly used Tetrabutylammonium iodide (TBAI) ligand exchange. We synthesized the PbS QDs from Lead Oxide (PbO) and lead acetate trihydrate (PbAc2·3H2O). Compared with PbS QDs photodiode based on PbO, the responsivity and EQE of PbS QDs photodiode based on PbAc2·3H2O have been improved from 0.15 to 0.586A/W and 12.35 to 47.27%.
Tandem structure provides a practical way to realize high efficiency organic photovoltaic (OPV) cells due to the limited optical absorption in organic semiconductors and tandem cells can be used to extend the wavelength coverage of the solar spectrum for light harvesting. The interconnecting layer (ICL) between subcells in a tandem solar cell plays a critical role in the reproducibility and the performance of tandem devices, and the processability of the ICL in a tandem cell has been a challenge. In this work we report on the fabrication of highly reproducible high efficiency tandem cells by employing a commercially available material, PEDOT:PSS HTL Solar (HSolar), as the hole transporting material used for the ICL. Comparing with the conventional PEDOT:PSS Al 4083, HSolar offers a better wettability on the underlying non-fullerene photoactive layers, resulting in better charge extraction properties of the ICL. When FTAZ:IT-M and PTB7-Th:IEICO-4F are used as the front cell and the back cells to fabricate the tandem solar cells, a power conversion efficiency (PCE) of 14.7% is achieved. To validate the processability of these tandem cells, three other research groups have successfully fabricated tandem cells using the same recipe and the highest PCE obtained is 16.1%. With further development of donor polymers and device optimization, our device simulation results show that a PCE < 22% can be realized in tandem cells in the near future.
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