Effect of Solvent Environment on Colloidal-Quantum-Dot Solar-Cell Manufacturability and Performance (original) (raw)
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Small, 2017
The power conversion efficiency of colloidal PbS quantum dot based solar cells is significantly hampered by lower than expected open circuit voltage (VOC). The VOC deficit is considerably higher in QD based solar cells compared to other types of existing solar cells due to in-gap trap induced bulk recombination of photogenerated carriers. Here, we report ligand exchange procedure based on a mixture of zinc iodide and 3-mercaptopropyonic acid to reduce the VOC deficit without compromising the high current density. This layer-by-layer solid state ligand exchange treatment enhanced the photovoltaic performance from 6.62% to 9.92% with a significant improvement in VOC from 0.58V to 0.66V. We further employed opto-electronic characterization, XPS and PL spectroscopy to understand the origin of VOC improvement. The mixed ligand treatment reduces the sub bandgap traps and significantly reduces the bulk recombination in the devices.
ACS Energy Letters, 2017
To realize the full potential of colloidal quantum dot (CQD) based solar cells, it is important to address the issue of large open circuit voltage (VOC) deficit which is a major roadblock in reaching higher efficiencies. The origin of the VOC deficit in these solar cells lies primarily in the presence of sub-bandgap trap states of the QDs. Here, we present a synergistic engineering framework to passivate these sub-bandgap states in PbS QDs through chemical surface passivation and remote passivation exploiting ligand and architecture engineering. In particular we form bulk nano-heterojunctions (BNH) by mixing PbS QDs with ZnO nanocrystals in conjunction with mixed ligand treatments to passivate surface traps. We employ the mixed ligand system of zinc iodide and 3-mercatopropyonic acid (MPA) to leverage the benefits of both organic and inorganic ligands for surface passivation and improved charge transport. This mixed ligand treatment in BNH architectures leads to record low Voc deficit for PbS QDs of 0.4 V-0.55 V compared to previously reported 0.6-0.8 V for the range of 1.1-1.35 eV bandgap PbS QDs.
Colloidal-quantum-dot photovoltaics using atomic-ligand passivation
Nature Materials, 2011
Colloidal-quantum-dot (CQD) optoelectronics offer a compelling combination of solution processing and spectral tunability through quantum size effects. So far, CQD solar cells have relied on the use of organic ligands to passivate the surface of the semiconductor nanoparticles. Although inorganic metal chalcogenide ligands have led to record electronic transport parameters in CQD films, no photovoltaic device has been reported based on such compounds. Here we establish an atomic ligand strategy that makes use of monovalent halide anions to enhance electronic transport and successfully passivate surface defects in PbS CQD films. Both time-resolved infrared spectroscopy and transient device characterization indicate that the scheme leads to a shallower trap state distribution than the best organic ligands. Solar cells fabricated following this strategy show up to 6% solar AM1.5G power-conversion efficiency. The CQD films are deposited at room temperature and under ambient atmosphere, rendering the process amenable to low-cost, roll-by-roll fabrication.
Advanced Materials, 2018
Developing low‐cost photovoltaic absorbers that can harvest the short‐wave infrared (SWIR) part of the solar spectrum, which remains unharnessed by current Si‐based and perovskite photovoltaic technologies, is a prerequisite for making high‐efficiency, low‐cost tandem solar cells. Here, infrared PbS colloidal quantum dot (CQD) solar cells employing a hybrid inorganic–organic ligand exchange process that results in an external quantum efficiency of 80% at 1.35 µm are reported, leading to a short‐circuit current density of 34 mA cm−2 and a power conversion efficiency (PCE) up to 7.9%, which is a current record for SWIR CQD solar cells. When this cell is placed at the back of an MAPbI3 perovskite film, it delivers an extra 3.3% PCE by harnessing light beyond 750 nm.
ACS Energy Letters, 2017
Employment of thin perovskite shells and metal halides as surface-passivants for colloidal quantum dots (CQDs) has been an important, recent development in CQD optoelectronics. These have opened the route to single-step-deposited high-performing CQD solar cells. These promising architectures employ a CQD hole-transporting layer (HTL) whose intrinsically shallow Fermi level (E F) restricts band-bending at maximum power-point during solar cell operation limiting charge collection. Here, we demonstrate a generalized approach to effectively balance band-edge energy levels of the main CQD absorber and charge-transport layer for these high-performance solar cells. Briefly soaking the CQD HTL in a solution of the metal−organic p-dopant, molybdenum tris(1-(trifluoroacetyl)-2-(trifluoromethyl)ethane-1,2-dithiolene), effectively deepens its Fermi level, resulting in enhanced band bending at the HTL:absorber junction. This blocks the back-flow of photogenerated electrons, leading to enhanced photocurrent and fill factor compared to those of undoped devices. We demonstrate 9.0% perovskite-shelled and 9.5% metal-halide-passivated CQD solar cells, both achieving ca. 10% relative enhancements over undoped baselines.