This Small Business Innovation Research Phase I project proposes to investigate the feasibility of a quantum dot (QD)-based downconverting film coated directly on a light emitting diode (LED) chip. Integration of high-efficiency QD materials as LED downconverters can enhance the efficiency and quality of the light output; however, existing QD films lack the thermal stability for on-chip application. The objective of the proposed effort is to determine the feasibility of forming QDs with inorganic ligand sets that yield solid state films with high photoluminescent (PL) external quantum efficiencies (EQEs) of greater than 80% and temperature stability that would enable them to withstand direct application onto LEDs. We will characterize cap-exchanged QDs in solution for organic content and solution quantum yield (QY). The project will determine whether the optical properties of these cap-exchanged QDs would enable their direct application to LEDs, by analyzing the photoluminescent EQE of spin-coated films as a function of temperature and time. In order to determine whether the EQE is more stable as a function of time and temperature with the inorganic ligand sets than our current organic ligands, we will run all-solid state tests on a set of control films fabricated with our standard QDs.
The broader impact/commercial potential of this project is significant. Success with this on-chip approach could be the "Holy Grail" for QDs, and for LED downconversion generally, as it provides a pathway for extremely versatile, low-cost, LED-based solutions for solid state lighting (SSL) and displays. In addition to lower cost, the reduced complexity makes this solution a true drop-in approach for lighting integrators, with high color rendering index and efficiency. These resulting benefits will drive much more rapid adoption of efficient, LED-based lighting solutions. With more than 20% of generated electricity consumed in the support of lighting applications, the opportunity to impact the demand-side of our energy budget and reduce greenhouse gases in this area is immense. Additional societal benefits include the growth of the U.S. manufacturing base and reduced dependence on foreign oil. The SSL market size is estimated to be $80 billion, and there is a compelling business opportunity to address the current barriers to market penetration. Additionally, we believe that this solid state lighting application readily transfers to backlight requirements of the display and television markets where quantum dot based films can also down-convert LED backlight units. The display market application represents an additional $100 billion opportunity.
Globally, more than 20% of electricity consumption goes to support lighting applications. With the traditional incandescent light technology at a woeful 6% efficiency, lighting offers a compelling opportunity to significantly conserve energy, reduce greenhouse gases, and minimize dependency on foreign fuel and energy sources. Phosphor down-converted light emitting diodes (LEDs) offer high efficiency as a powerful advantage over both incandescent bulbs and compact fluorescents in solid state lighting applications, but their most efficient spectral ranges result in ‘cool’ white light which consumers find objectionable. When additional phosphors are used to downconvert some of the blue wavelengths in LED emission spectra to warmer oranges and reds, substantially more power is required due to both the lower efficiency of these red phosphors at high temperatures as well as light loss to the deep red region of the visible spectrum where the human eye is not sensitive. As a result of the issues outlined above, the Phase I effort of this program focused on advancing the thermal stability of quantum dot (QD) materials to enable direct deposition onto LED chips as a replacement for current phosphors in solid state lighting systems. Traditionally, the thermal instability of the organic ligands that coordinate to the surface of inorganic semiconductor QDs was thought to be the main cause for decreased QD emission efficiency at elevated temperatures and thus limiting the commercial viability of these materials for on-chip LED applications. QD Vision, along with our partner Dmitri Talapin of the University of Chicago, replaced the organic ligands with inorganic compounds in order to study this issue. It was determined that replacement of organic ligands with inorganic compounds did not improve the external quantum efficiency of the QDs at elevated temperatures. This important finding advanced our mechanistic understanding of temperature dependent emission properties of quantum dots.
