Effect of Cd0.5Zn0.5s/ZnS Core/Shell Quantum Dots on Power-Conversion-Efficiency Enhancement for Silicon Solar Cells

For the past few decades, silicon solar-cells have been researched to improve PCE through surface texturing, anti-reflection coating, plasma doping, selective emitter, back contact cell, local contact cell, and metallization. Eventually, silicon solar-cells has been saturated with the maximum PCE value of ~25 %.[1] Recently, the research on the energy-down-shift via implementing quantum-dots (QDs) in silicon solar-cells has been proposed to overcome the saturation of the PCE of silicon solar−cells since QDs are able to absorb UV light and emit visible light. However, they have not reported an evident PCE improvement and clear mechanism. Thus, we implemented the energy-down-shift via Cd_0.5Zn_0.5S/ZnS core/shell ODs on silicon solar-cells.

The core/shell QDs were spin-coated on SiN_X film deposited textured p-type silicon solar-cells, as shown in Fig. 1(a). They were well coated along the SiN_X film textured surface where Al and Pt were deposited on the QDs layer to avoid the focus-ion-beam damage during the TEM sample preparation, as shown in Fig. 1(b). The core/shell QDs showed a spherical shape, were well crystallized, and well dispersed with each other, as shown in Fig. 1(c). The average size of the Cd_0.5Zn_0.5S/ZnS core/shell QDs was 6.6 nm, as shown in Fig. 1(d). The chemical composition of the core QD and shell layer coated on the core QD were Cd_0.5Zn_0.5S and ZnS, and the diameter of the core QD and thickness of the shell coated on the core QD were 4.2 and 1.2 nm, respectively, as shown in Fig. 1(e), analyzed by EDX line-scan profile. The Cd_0.5Zn_0.5S/ZnS core/shell QDs in the quartz-cuvette with 0.05 wt% absorbed 100% UV light in wavelength from 450 to 250 nm and emitted the 442 nm PL peak−signal with the quantum yield of 80%. These results evidently indicate that the Cd_0.5Zn_0.5S/ZnS core/shell QDs emitted the blue visual light when the UV light was absorbed, proving the energy-down-shift of the Cd_0.5Zn_0.5S/ZnS core/shell QDs. The dependency of the light absorption amount of the Cd_0.5Zn_0.5S/ZnS core/shell QDs on the concentration (wt %) of the QD solution was estimated as a function of the wavelength when the QD solutions were spin-coated on glass, as shown in Fig. 2(b), The absorption amount of the UV light in wavelength between 250 and 450 nm increased with the concentration of the QD solution. For p-type silicon solar-cells coated with the QDs layer, the dependency of PV performance on the average QDs layer thickness was shown in Fig. (3). The value of J_SC increased abruptly from 34.70 to 36.94 mA/cm² as the QDs layer thickness increased up to 8.8 nm, which was a 6.45 % increase compared to the reference without the QDs layer. Finally, PCE increased from 16.92 to 18.00 % as the QDs layer thickness increased up to 8.8 nm corresponds to relative 6.4 % PCE enhancement compared with that in the reference. These results indicate that the coating of Cd_0.5Zn_0.5S/ZnS core/shell QDs on the SiN_X film textured surface for p-type silicon solar-cells affect J_SC due to the energy-down-shift effect of  the QDs but it does not affect V_OCand FF.

*This work was financially supported by the Brain Korea 21 plus Project in 2014, Korea.

Fig. 1. Design of energy-down-shift via Cd_0.5Zn_0.5S/ZnS QDs coated on SiNx film textured p-type silicon solar cell.

Fig. 2. Optical characteristics for Cd_0.5Zn_0.5S/ZnS QDs.

Fig. 3. Photo-voltaic performance for p-type silicon solar-cells coated with Cd_0.5Zn_0.5S/ZnS core/shell QDs

Reference

[1] M. Tuan Trinh. et al., Nature Photonics 6, 316–321 (2012)

[2] Bae. W K. et al., Chem Master 205307-5313 (2008)