In an article recently published in the journal Polymers, researchers proposed a novel approach to expand the photodetectors’ spectral responsivity by integrating cadmium selenide/zinc sulfide (CdSe/ZnS) quantum dots and poly(2-methoxy-5(2’-ethyl) heroxyphenylenevinylene) (MEH−PPV) polymer composite.
(a) Transmission electron microscope (TEM) images of MEH−PPV and MEH−PPV:CdSe/ZnS composites (QDs concentration is 5%), and (b) schematic diagram of MEH−PPV:CdSe/ZnS-based PD. Image Credit: https://www.mdpi.com/2073-4360/16/16/2371
Background
Although zinc oxide (ZnO) holds huge potential for photodetector applications, its absorption, primarily in the ultraviolet spectrum, limits its applicability in a wider range of optical sensing tasks. Materials like composite structures, polymers, or nanoparticles with high absorbance in the visible region are incorporated to extend the optical absorption range of ZnO into the visible spectrum.
Organic-inorganic nanocomposites have recently gained significant attention for several applications in electronic and optical fields. In optoelectronic and photonics applications, the MEH-PPV polymer is often selected as an active layer component owing to its substantial absorbance in the visible spectrum.
Additionally, CdSe/ZnS quantum dots are suitable for photo-electrochemical applications as these inorganic semiconductor materials possess high thermal and chemical stability and absorption coefficients. For instance, the photodetector performance can be enhanced by fabricating controllable layer-by-layer CdSe/ZnS quantum dot structures.
The Study
In this work, researchers fabricated an organic-inorganic composite by incorporating CdSe/ZnS core-shell quantum dots into the MEH-PPV polymer. Then, a MEH-PPV: CdSe/ZnS core-shell quantum dot composite-based photodetector was created and analyzed.
They investigated the energy transfer mechanism between the organic polymer MEH-PPV and CdSe/ZnS quantum dots. To gain deeper insights, a hybrid ZnO-based photodetector was fabricated using the MEH-PPV: CdSe/ZnS composite.
CdSe/ZnS quantum dots could facilitate enhanced charge separation due to a type-II heterojunction formation with MEH-PPV and their high surface area. This can enhance charge transport and decrease recombination losses. During the charge transfer process, the quantum dots can act as electron acceptors to contribute to a higher photocurrent and improve the photo-generated excitons' separation.
MEH-PPV possesses a wide absorption spectrum, with a peak around 500-550 nm, which makes it suitable for visible light absorption, while CdSe/ZnS quantum dots possess size-tunable emission and absorption properties, allowing complementary absorption to polymer MEH-PPV. Moreover, the ZnS and CdSe quantum dots display high absorbance in the ultraviolet range.
Significance of the Study
The Förster resonance energy transfer (FRET) from CdSe/ZnS quantum dots to MEH-PPV polymer molecules allowed efficient excitation energy transfer from the quantum dots/energy donors to the polymer/energy acceptor. Thus, the FRET mechanism improved the separation and generation of charge carriers, resulting in a higher photocurrent and an overall increased device efficiency.
Additionally, the combination of MEH-PPV and CdSe/ZnS core-shell quantum dots facilitated a broad spectral response in photodetectors, spanning from the ultraviolet to the visible range. Specifically, photodetectors with quantum dots in the composite demonstrated excellent photosensitivity to both ultraviolet light (365 nm) and visible light (505 nm).
After exploring the impact of different quantum dot concentrations in polymer/quantum dot composites on photodetector performance, an optimal quantum dot concentration of 5% was selected. The current-voltage characteristics of photodetectors based on ZnO without/with MEH-PPV: CdSeZnS (5%) under illuminated and dark conditions were evaluated.
Results showed that the photocurrent generated under illumination at 505 nm and 365 nm wavelengths displays a significant increase compared to other wavelengths. This rise in photocurrent at specific wavelengths signified the photosensor's pronounced sensitivity to light in the visible and ultraviolet spectral regions.
Moreover, a substantial improvement in photosensitivity was observed at 505 nm and 365 nm wavelengths when using the MEH-PPV: CdSe/ZnS composite in the photodetector compared to the photodetector without the composite.
For instance, the MEH-PPV: CdSe/ZnS-based photodetector's photosensitivity increased 5-fold at 365 nm, marking a significant 2.5-fold increase compared to the ZnO-only-based photodetector.
Similarly, the MEH-PPV: CdSe/ZnS-based photodetector's photosensitivity reached 3-fold at 505 nm, while the ZnO-only-based photodetector demonstrated no noticeable response. These findings indicated the superior detection and photosensitivity capabilities realized due to MEH-PPV: CdSe/ZnS composite incorporation.
Overall, the critical role of the FRET mechanism that allowed the energy transfer from CdSe/ZnS to MEH-PPV, coupled with the combination of high absorbance materials in visible and ultraviolet wavelengths, contributed to the superior performance of the wide-spectra photodetector devices.
In conclusion, this work effectively demonstrated the feasibility of integrating MEH-PPV and CdSe/ZnS quantum dots in the photodetector to expand its spectral responsivity to realize a broader detection range. Thus, hybrid organic-inorganic photodetectors represent a logical choice for combining the distinct properties of organic polymers and inorganic quantum dots.
Journal Reference
Tran, T. T., Nguyen, H. T., Sharma, A., Cho, Y., Kumar, M., Yun, J. (2024). Expanding the Spectral Responsivity of Photodetectors via the Integration of CdSe/ZnS Quantum Dots and MEH−PPV Polymer Composite. Polymers, 16(16), 2371. DOI: 10.3390/polym16162371, https://www.mdpi.com/2073-4360/16/16/2371
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