The unstable PbSe quantum dot (QD) surface requires tedious and complicated synthetic protocols and renders them substantially underdeveloped compared to PbS QDs. Here, we describe a direct synthesis of PbSe QD inks at room temperature. In comparison to the conventional three-step synthesis, our strategy simplifies the fabrication process to one step and reduces the preparation cost by a factor of eight. A photovoltaic device based on these PbSe QD inks has achieved a photovoltaic conversion efficiency (PCE) of 10.38% with high device stability, which is one of the highest PCEs for all reported PbSe QD solar cells. More importantly, the obtained ink has demonstrated the best colloidal stability by far compared with all the reported lead chalcogenides (PbX) QD inks for photovoltaic application. This simple and low-cost synthesis will facilitate ink storage and transport and may ignite a new round of research efforts on the optoelectronic applications of PbSe QDs.

Almost all surfaces sensitive to the ambient environment are covered by water, whereas the impacts of water on surface-dominated colloidal quantum dot (CQD) semiconductor electronics have rarely been explored. Here, strongly hydrogen-bonded water on hydroxylated lead sulfide (PbS) CQD is identified. The water could pilot the thermally induced evolution of surface chemical environment, which significantly influences the nanostructures, carrier dynamics, and trap behaviors in CQD solar cells. The aggravation of surface hydroxylation and water adsorption triggers epitaxial CQD fusion during device fabrication under humid ambient, giving rise to the inter-band traps and deficiency in solar cells. To address this problem, meniscus-guided-coating technique is introduced to achieve dense-packed CQD solids and extrude ambient water, improving device performance and thermal stability. Our works not only elucidate the water involved PbS CQD surface chemistry, but may also achieve a comprehensive understanding of the impact of ambient water on CQD based electronics.

The direct-synthesis of conductive PbS quantum dot (QD) ink is facile, scalable, and low-cost, boosting the future commercialization of optoelectronics based on colloidal QDs. However, manipulating the QD matrix structures still is a challenge, which limits the corresponding QD solar cell performance. Here, for the first time a coordination-engineering strategy to finely adjust the matrix thickness around the QDs is presented, in which halogen salts are introduced into the reaction to convert the excessive insulating lead iodide into soluble iodoplumbate species. As a result, the obtained QD film exhibits shrunk insulating shells, leading to higher charge carrier transport and superior surface passivation compared to the control devices. A significantly improved power-conversion efficiency from 10.52% to 12.12% can be achieved after the matrix engineering. Therefore, the work shows high significance in promoting the practical application of directly synthesized PbS QD inks in large-area low-cost optoelectronic devices.

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