Ink engineering approach boosts efficiency and cuts cost of quantum dot-based photovoltaics

Colloidal quantum dots (CQDs) are tiny semiconductor particles that are just a few nanometers in size, which are synthesized in a liquid solution (i.e., colloid). These single-crystal particles, created by breaking down bulk materials via chemical and physical processes, have proved to be promising for the development of photovoltaic (PV) technologies.

Quantum dot-based PVs could have various advantages, including a tunable bandgap, greater flexibility and solution processing. However, quantum dot-based solar cells developed so far have been found to have significant limitations, including lower efficiencies than conventional silicon-based cells and high manufacturing costs, due to the expensive processes required to synthesize conductive CQD films.

Researchers at Soochow University in China, the University of Electro-Communications in Japan and other institutes worldwide recently introduced a new method that could potentially help to improve the efficiencies of quantum-dot based photovoltaics, while also lowering their manufacturing costs. Their proposed approach, outlined in a paper published in Nature Energy, entails the engineering of lead sulfide (PbS) CQD inks used to print films for solar cells.

“When people discuss colloidal quantum dots (CQDs), the first thing that comes to mind is their extremely attractive size-dependent quantum properties, as well as the compatibility with low-cost solution-based fabrication methods, which open up exciting possibilities for next-generation semiconductor materials especially in printable solar cells and optoelectronic devices,” Guozheng Shi and Zeke Liu, co-author of the paper, told Tech Xplore.

“However, these potential applications are often overshadowed by the complex and expensive synthesis and manufacturing processes required to produce conductive CQD films.”

The sophisticated and expensive processes currently used to produce conductive CQD films attain a limited yield, with the costs of CQD active layers ranging from $0.25 to $0.84/Wp, which are too high for their commercialization. Moreover, existing processes offer limited control over the quality of the materials and thus the resulting solar cells.

“Before our work, CQD solar modules exceeding 10 cm² achieved only ~1% power conversion efficiency (PCE), a stark contrast to the over 12% PCE of lab-scale devices (0.04 cm²),” said Liu. “This efficiency gap, combined with costly and complex methods involving hot injection and ligand exchange, made commercial-scale CQD photovoltaics almost impractical. The efficiency gap, along with costly methods, has made commercial-scale CQD photovoltaics impractical.”

The primary objective of the recent work by Liu and his colleagues was to facilitate the future development of PVs based on quantum dots, enabling the low-cost production of large-area and efficient solar cells. In an effort to meet this goal, they introduced a new ink engineering approach that could support the production of CQD films.

“To fabricate large-area conductive quantum dot films, these particles need to be uniformly and tightly stacked while maintaining their individual states to preserve quantum effects,” explained Liu. “Any inconsistency in size or stacking can lead to energy loss, negatively impacting semiconductor performance. This presents a delicate balance between quantum dot stacking and ligand design.”

Conventional approaches to create CQDs rely on hot injection techniques to produce quantum dots wrapped in long-chain insulating ligands, followed by a ligand exchange to shorter chains that boosts a film’s conductivity. These approaches are both expensive and complex, thus they are difficult to replicate on a large scale.

“Ligand exchange processes increase both complexity and material costs, while also causing aggregation and morphological defects, making it difficult to achieve uniformity over large areas,” said Liu. “In contrast, our approach uses a direct synthesis (DS) technique to prepare CQD inks.”

The new ink engineering method devised by Liu and his colleagues enables the synthesis of ion-capped CQDs directly in a polar solvent, thus eliminating the need for complex ligand exchange processes. Using their approach, the researchers were able to print closely packed conductive CQD films in a single step.

“To minimize aggregation and fusion, we control the chemical environment of the ink, utilizing a solution chemistry engineering (SCE) strategy for precise tuning of ionic configurations and functionality,” said Liu. “The simplified quantum dot technology and improved ink stability result in stable CQD inks with fewer defects, enabling the large-scale fabrication of quantum dot thin films and photovoltaic devices, all at a cost of less than $0.06/Wp.”

Shi, Liu and their colleagues tested their proposed approach in a series of tests and showed that it resulted in the production of highly stable quantum dot inks. In addition, they uncovered a link between surface-dominated and irreversible quantum dot interactions and the defects present in printed CQD films, as well as the performance of large-area solar cells based on these films.

“Our efforts led to the creation of the first large-area CQD solar module with a certified power conversion efficiency (PCE) exceeding 10%, marking a significant step forward toward the commercialization of CQD-based photovoltaics,” said Liu.

“In addition, we achieved a highly efficient small-area solar cell with a PCE of 13.40%, setting a new benchmark for CQD technology. These advances are crucial as they address the scalability and cost challenges that have long limited the widespread use of CQD solar cells.”

This recent study could soon contribute to the development of low-cost, large-area and highly performing CQD-based solar cells and other optoelectronic devices, such as near-infrared sensors or tools for space exploration.

As part of their next studies, Liu and his colleagues plan to further refine the inks produced using their approach, as this could result in solar cells with even better efficiencies, while also extending their possible real-world applications.

“We will explore adapting the technology for various quantum dots, including low-toxicity variants, and flexible electronics,” added Liu. “Additionally, we’ll investigate their use in fields such as short-wave infrared (SWIR) imager—critical components for advancing affordable AI technologies like autonomous vehicles, smart robots, and industrial automation.

“Ultimately, our goal is to scale this technology for commercial production, reducing both costs and the environmental impact of quantum dot electronics.”

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