Quantum Dots Enhance Light-to-Current Conversion

The research team at a probe station where they used simulated sunlight to characterize electronic devices. Back to front: Chang-Yong Nam, Mircea Cotlet, Prahlad Routh and Jia-Shiang Chen (Source: BNL)

The research team at a probe station where they used simulated sunlight to characterize electronic devices. Back to front: Chang-Yong Nam, Mircea Cotlet, Prahlad Routh and Jia-Shiang Chen (Source: BNL)

Harnessing the power of the sun and creating light-harves­ting or light-sensing devices requires a material that both absorbs light effi­ciently and converts the energy to highly mobile electrical current. Finding the ideal mix of properties in a single material is a challenge, so scientists have been experi­menting with ways to combine different materials to create hybrids with enhanced features.

Scientists from the US Department of Energy’s Brook­haven National Labora­tory, Stony Brook University, and the University of Nebraska describe one such approach that combines the excellent light-harvesting properties of quantum dots with the tunable electrical conductivity of a layered tin disulfide semi­conductor. The hybrid material exhibited enhanced light-harvesting properties through the absorption of light by the quantum dots and their energy transfer to tin disulfide, both in laboratory tests and when incorporated into electronic devices. The research paves the way for using these materials in opto­electronic applications such as energy-harvesting photo­voltaics, light sensors, and light emitting diodes.

According to Mircea Cotlet, the physical chemist who led this work at Brookhaven Lab’s Center for Functional Nano­materials, a DOE Office of Science User Facility, “Two-dimensional metal dichalcogenides like tin disulfide have some promising properties for solar energy conversion and photo­detector applications, including a high surface-to-volume aspect ratio. But no semi­conducting material has it all. These materials are very thin and they are poor light absorbers. So we were trying to mix them with other nano materials like light-absorbing quantum dots to improve their performance through energy transfer.”

Another fundamental study describes the hybrid quantum dot/tin disulfide material by itself. The work analyzes how light excites the quantum dots – made of a cadmium selenide core surrounded by a zinc sulfide shell, which then transfer the absorbed energy to layers of nearby tin disulfide. “We have come up with an interesting approach to discriminate energy transfer from charge transfer, two common types of inter­actions promoted by light in such hybrids,” said Prahlad Routh, a graduate student from Stony Brook University working with Cotlet. “We do this using single nanocrystal spectros­copy to look at how individual quantum dots blink when interacting with sheet-like tin disulfide. This straightforward method can assess whether components in such semi­conducting hybrids interact either by energy or by charge transfer.”

Single nanocrystal spectros­copy identifies the interaction between zero-dimensional CdSe/ZnS nano crystals (quantum dots) and two-dimensional layered tin disulfide as a non-radiative energy transfer, whose strength increases with increasing number of tin disulfide layers. Such hybrid materials could be used in optoelectronic devices such as photo­voltaic solar cells, light sensors, and LEDs. The researchers found that the rate for non-radiative energy transfer from individual quantum dots to tin disulfide increases with an increasing number of tin disulfide layers. But performance in laboratory tests isn’t enough to prove the merits of potential new materials. So the scientists incorporated the hybrid material into an electronic device, a photo-field-effect-tran­sistor, a type of photon detector commonly used for light sensing appli­cations.

The hybrid material dramatically enhanced the performance of the photo-field-effect transistors – resulting in a photo­current response that was 500 percent better than transistors made with the tin disulfide material alone. “This kind of energy transfer is a key process that enables photo­synthesis in nature,” said Chang-Yong Nam, a materials scientist at Center for Functional Nano­materials. “Resear­chers have been trying to emulate this principle in light-harvesting electrical devices, but it has been difficult particularly for new material systems such as the tin disulfide we studied. Our device demonstrates the performance benefits realized by using both energy transfer processes and new low-dimensional materials.” Cotlet concludes: “The idea of ‘doping’ two-dimensional layered materials with quantum dots to enhance their light absorbing properties shows promise for designing better solar cells and photo­detectors.” (Source: BNL)

References: Y. Huang et al.: Hybrid quantum dot-tin disulfide field-effect transistors with improved photocurrent and spectral responsively, Appl. Phys. Lett. 108, 123502 (2016), DOI: 10.1063/1.4944781  • H. Zang et al.: Nonradiative Energy Transfer from Individual CdSe/ZnS Quantum Dots to Single-Layer and Few-Layer Tin Disulfide, ACS Nano, online 31 March 2016, DOI: 10.1021/acsnano.6b01538

Links: Center for Functional Nanomaterials (M. Cotlet), Brookhaven National Laboratory, Upton, New York, USA

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